NIGSCon 1998 Abstract Volume by NIGSCON

ABSTRACTS AND PROGRAM

1998 NATIONAL INSTITUTE OF GEOLOGICAL SCIENCES' RESEARCH SYMPOSIUM

26 November 1998 Audio Visual Room National Institute of Geological Sciences University of the Philippines Diliman

National Institute of Geological Sciences' 1998 Research Symposium Program of Activities 08:00 - 08:40 Registration Welcome Address - Joselito P. Duyanen - Director, NIGS Opening Remarks - Claro T. Llaguno - Chancellor, UPD Oral Presentations Petrology and Tectonics 08:40 - 09:10 International cooperation study in geology in the Philippines - T. M Santos 09:10 - 09:30 Non-cogenetic magmas in island arc volcanoes - the rule or the exception? Insights from some Philippine examples - E. L. Listanco 09:30 - 09:50 Mantle structure beneath Luzon: constraints from petrology and seismic wave attenuation - C. Arcilla 09:50 - 10:10 Tectonic significance of melange complexes: southeast Bohol example - J. V. de Jesus, D. V. Faustino, J. A. L. Barretto, C. B. Dimalanta, E. J. Marquez, K. L. Queano, F. A. Jimenez, and G. P. Yumul, Jr. 10:10 - 10:30 Petrology of basic volcanic rocks from the southeast Bohol Ophiolite Complex, Central Philippines - D. V. Faustino, J. A. L. Barretto, J. V. de Jesus, C. B. Dimalanta, E. J. Marquez, K L. Queano, F. A. Jimenez, and G. P. Yumul, Jr. 10:30 - 11:00 Break - Coffee, Cookies and Posters Economic Geology 11:00 - 11:20 High salinity fluid inclusion in the Uwamuki No. 4 Kuroko orebody, Akita Prefecture, Japan - V. B. Maglambayan 11:20 - 11:40 Chemical reaction path modelling of mineralization in the Tongonan geothermal field, Leyte - M R. I. Balangue-Tarriela 11:40 - 12:00 The metallogeny of Zamboanga region - F. A. Jimenez, J. V. de Jesus, Jr., E. J. Marquez, J. A. L. Barretto, D. V. Faustino, K. L. Queano, and G. P. Yumul, Jr. 12:00 - 12:20 The Malusok volcanogenic massive sulfide deposits: axial versus off-axis deposits: hydrothermal activities - C. L. Querubin 12:20 - 01:10 Break - Lunch and Posters 01:10 â&#x20AC;&#x201D; 01:20 Message from the Dean

Danilo M. Yanga Dean, College of Science

Sedimentation and Biogenic Componentsln Open Oceans 01:20 - 02:00 Particle influx in the South China Sea - M Weisner 02:00 - 02:20 The Braarudosphaeraproblem in the Oligocene, South Atlantic - A. PeleoAlampay, G. A. Mead and W. Wei 02:20 - 02:40 Calcareous nannofossil distribution on the eastern South China Sea - A.G. Fernando and A. Peleo-Alampay

02:40 - 03:00 Distribution of Recent foraminifera from Panata and Pagasa Islands, Kalayaan Group of Islands, South China Sea - R. Berdin, M J. M Villanoy and F. P. Siringan 03:00 - 03:30 Break Coffee, Cookies and Posters -

Coastal and Environmental Geology 03:30 - 03:50 Neogene stratigraphy of south-central Ifugao - N. Macalalad, lfugao Geology 170 Class, N. Aguda, H. Cabria, C. Pascua and J. M Foronda 03:50 - 04:10 Subsidence and the worsening floods in the Pampanga and Bulacan areas - F. P. Siringan and K. S. Rodolfo 04:10 - 04:30 Sediment dispersal in the Pampanga Bay - C. L. Ringor and F. P. Siringan 04:30 - 04:50 Effects of delta and river switching to coastal morphology and evolution: Examples from the Agno River System, Lingayen Gulf - Z. R P. Mateo and F. P. Siringan 04:50 - 05:10 Heavy metal pollution of the Pasig River - R. R. Amores and J. P. Duyanen 05:10 - 05:20 Closing Ceremonies

Poster Presentations 1. A study of the calcareous nannoplankton and foraminiferal assemblages of the pre- and post1991 Mt. Pinatubo eruption sediments of Pampanga Bay - M M de Leon and N. C. Agapay, Jr. 2. Eastern South China Sea: volcanism and foraminiferal occurrence E. J. Marquez, P. J. M Macias, M M de Leon, J. V. de Jesus, D. V. Faustino, J. A. L. Barretto, K L. Queano, C. B. Dimalanta, F. A. Jimenez, and G. P. Yumul, Jr. 3. Preliminary investigation in predicting groundwater occurrence and identifying geologic hazards, Mati-Northern Pujada Peninsula C. S. Pascua, J. Cagas, C. Gonzales, F. Jimenez., and G. P. Yumul, Jr. 4. Maceral Composition of coals from the Maniguin-X Well and its relevance to paleomire conditions - F. E. Casareo and M. T. Forbes 5. Permian microflora of the Upper Liddel Coal, Wittingham Coal Measures, NSW Australia - N N. Casareo 6. Oceanic crust-mantle sequence: what do we know - G. P. Yumul, Jr., C. B. Dinalanta, D. V. Faustino, J. L. Barretto, J. V. de Jesus, E. J. Marquez, K L. Queano, F. A. Jimenez, Jr., F. T., Jumawan 7. Gravity, Magnetics and Ophiolite Complexes - J. A. L. Barretto, C. B. Dimalanta, D. V. Faustino, J. V. de Jesus, E. J. Marquez, K. L. Queano, , F. A. Jimenez, and G. P. Yumul, Jr. 8.Mechanism of formation of the present-day sand dunes along the west coast of Ilocos Norte, Luzon, Philippines - F. P. Siringan and A. L. Pataray 9. Geomorphological and sedimentological indicators of nearshore and offshore sediment dispersal in Lingayen Gulf Philippines - F. P. Siringan andZ R. P. Mateo 2

10. Multiple approaches to coastal sediment transport studies, Cagayan de Oro, Philippines - F. P. Siringan, R. D. Berdin, J. S. Doruelo, C M B. Jaraula, Z R. P. Mateo, J. B. 7'. Ong, and M. Y. E Yacat 11. Predicting sediment dispersal patterns in Manila Bay using geomorphological-based parameters - F. P. Siringan and C. L. Ringor 12. Alpha-spectrometric 230Th/234U dates of Pleistocene corals from the Philippine Islands and their tectonic implications - A. Omura, T. Kawana, Y. Maeda, and F. P. Siringan 13. Records of Holocene high-sea level stands in the Philippine Islands - Y. Maeda, F. P. Siringan, M Cano, A. Omura, T. and K. Sasaki 14. Lithofacies and architecture of Quaternary deposits and pulses of uplift in the Cagayan de Oro area, northern Mindanao - F. P. Siringan, R. D. Berdin, C. M B. Jaraula, Z R P. Mateo, T A. Sandoval, M J. M Villanoy 15. Proposed geochemical baselines for the sediment-based monitoring of pollution in Manila Bay — J. P. Duyanen and N. A. Aguda 16. Emplacement of large-volume ignimbrites: an example from Laguna Caldera — S. G. Catane* and M C. B. Arpa 17. Isotopic study of basement rocks from Shatsky Rise, Northwestern Pacific Ocean — M. L. G. Tejada, J. J. Mahoney, W. W. Sager and the Shipboard Scientific Party (Cruise TNO37, R/V Thompson) 18. Revisiting the pyroclastic flow deposits of the Laguna Formation at Bagongbayan, Teresa, Rizal - Eligio C. Obille Jr.

ABSTRACTS OF ORAL PRESENTATIONS (Arranged according to order of presentation)

INTERNATIONAL COOPERATION STUDIES IN GEOLOGY IN THE PHILIPPINES Teodoro M. Santos National Institute of Geological Sciences, University of the Philippines

This paper presents the experience of the Philippines in four developmental international cooperation projects in Geology with the view of extracting lessons that can serve as input into the planning and implementation of similar projects in the future. The four projects and their main objectives are: (1) the USAID-MSA Geology Project in the 1950s which was designed to develop Geology to aid the country in its economic recovery and development. Specifically, it aimed to put up a Department of Geology (and Geography) which is fully staffed and run by Filipinos and institute a professional B.S. Geology Program. (2) The UNDP-UNTAB Institute of Applied Geology Project in the early 1960s. Its main objective was to train mining engineers as exploration geologists by immersing them in a one year training course. A secondary objective was to put up a graduate school in Geology to take the place of the Institute of Applied Geology upon termination of the Project. (3) The German Exchange Professor Project in Geology (and Geography) in the 1970s which aimed to help invigorate the new graduate programs. (4) The RP-German Geology Project in the 1980s. Its main objective is to upgrade the U.P. Department of Geology into a national institution which provides international quality, practice oriented instruction, research and extension. In order to draw lessons from the projects a graphical analysis was done in tandem with the analyses of the management aspects. Some of the lessons learned from the international cooperation projects, among others, are: (1) the most effective project is characterized by the following elements: (I) well - thought plan that embodies the needs and visions shared by beneficiaries and donors; (ii) intimate involvement of personnels of recipient and donor organizations in all aspects of the project; (iii) adequate managerial, manpower, physical and financial support; (iv) adequate institutional support from both the donor and recipient's ends, and (v) adequate time to achieve the objectives. (2) Given the four projects, much higher level of development could have been attained if the gains in each project were adequately maintained. Furthermore, the same high level of development could have been accomplished if the projects were phased - in at the proper time. (3) Creation of a sustainable training and research institution in a less developed country setting requires much more time than five years, perhaps 15 - 20 years of sustained efforts. Any interruption would extend the process much longer and increase costs. (4) Each of the technical assistance contributed incrementally towards the development of the sustainable training and research institute in the geological sciences. (5) The prime difficulty of the NIGS in receiving development assistance is its inability to maintain laboratory and equipment facilities in good working condition. It is, moreover, impossible to replace worn out or obsolete equipment.

NON-COGENETIC MAGMAS IN ISLAND ARC VOLCANOES--- THE RULE OR THE EXCEPTION?: INSIGHTS FROM SOME PHILIPPINE EXAMPLES Eddie L. Listanco National Institute of Geological Sciences, University of the Philippines

In magmatic processes modelling, comagmatic relation among samples is usually implicitly assumed especially for suites taken from a single volcanic edifice. Plots of incompatible element ratios may filter genetically-related magmas from those that are not because the ratio of two highly incompatible elements cannot be changed by neither partial melting nor crystallization. When applied to some Philippine volcanoes, selected incompatible element ratios suggest non-cogenetic magmatism in some individual edifices. This signifies that 1) these volcanoes have tapped genetically unrelated magmas, 2) the configuration of their magma plumbing systems might have been favorable to such process, and 3) either the magma source is heterogeneous with respect to these elements and/or there are variations in the amount and/or composition of fluids introduced during melting. These observations in turn raise the question whether tapping of genetically unrelated magmas is an island-arc rule or exception.

MANTLE STRUCTURE BENEATH LUZON: CONSTRAINTS FROM PETROLOGY AND SEISMIC ATTENUATION Arcilla, C.A1,2., Besana, G.M.,3,4, Flower, M.F.J1., and Ando, M3. 'Dept. of Earth & Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607; National Institute of Geological Sci., University of the Philippines, Quezon City, Philippines; 3 Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan ; 4 Philippine Institute of Volcanology & Seismology, Quezon City, Philippines 2

Primitive lava compositions from selected Luzon volcanoes, calibrated with published experimental work permit estimations of pressure and temperature conditions of magma segregation and source fertility. Thus, empirical P,T and compositional sections can be compared with model slab/wedge thermal structures involving hydration-dehydration reactions, slab-derived fluid and mantle melt migration, and decompression melting. In addition, attenuation structures derived from inversion of earthquake intensity data delineate locations of subducted slabs (low attenuation) and high-attenuation zones, which could be due to melt/fluid infiltration. Combined petrological-seismic attenuation transects show: 1) correspondence of mantle amphibole and phlogopite breakdown depths with high attenuation beneath frontal and back-side volcanoes (Pinatubo-Arayat transect at 15oN) 2) evidence for transtensive rifting as shown by decreasing depth of magma segregation away from the slab in the Macolod Corridor transect (14oN), coincident with a large zone of high attenuation 3) conformity of high attenuation zones with areas overlain by active volcanism in contrast to those by extinct volcanoes and 4) high attenuation zones beneath forearcs not overlain by active volcanism could be due to massive slab dehydration. Conformity between hypocenter distributions, hydrous breakdown curves and tomographic data suggest that part of the arc lithosphere mantle is weakened by massive slab dehydration, making it susceptible to entrainment by the slab and corner flow. "Converted" refractory lithosphere mantle could be the source of frontal arc volcanoes which have more refractory sources than "back-arc" volcanoes.

TECTONIC SIGNIFICANCE OF MELANGE COMPLEXES: SOUTHEAST BOHOL EXAMPLE De Jesus, J.V. 1 , Faustino, D.V. 1 , Barretto, J.L. 1 , Dimalanta, C.B. 1 . 2 1 , Queario, K.L. 13, Jimenez, F.A. Jr. and Yumul, G.P.Marquez,E.J Jr. 1 Rushurgent Working Group, National Institute of Geological Sciences College of Science, University of the Philippines, Diliman, Quezon City, PHILIPPINES 2 Ocean Research Institute, University of Tokyo, Nakano-ku, Minami-dai, 1-15-1, Tokyo 164, JAPAN 3 Department of Petroleum Geosciences, Universiti Brunei Darussalam, Bandar Seri Begawan, BRUNEI

A mappable chaotic unit composed predominantly of ophiolite-derived clasts was recognized in the southeastern portion of Bohol Island. This unit was identified as a melange following the definition proposed by Raymond (1984) and designated as the Cansiwang Melange. These melange assemblages are noted along the Cansiwang-Labo Road, east of Guindulman and northwest of Alicia. The exposures are NE-SW trending linear bodies and are parallel to and in thrust contact with the harzburgites of the Southeast Bohol Ophiolite Complex (SEBOC). These contacts are mainly east-verging thrust faults. The rock units are characterized by the dominance of ophiolite-derived clasts such as sheared harzburgites near the top and pillow basalts with minor clasts of sandstones and cherts near the base. No Alicia metamorphic rocks were noted in the melange. The lithologies and their distribution in the melange points to a harzburgite source at the top and a deep-water volcanic source for the base of the melange. Structural as well as stratigraphic evidences show that these melange units are underthrusted with respect to the harzburgites of the Southeast Bohol Ophiolite Complex and thrusted above the Alicia Schist. This configuration provides a structural as well as stratigraphic support to the notion that the formation of the Alicia Schist is not in any way related to the emplacement of the ophiolite complex. The absence of circular gravity low anomalies (Barretto, 1998) indicate that these melange units were not diapiric in nature. In addition, the geochemical character of the volcanic rocks of the related ophiolite suggests formation in an incipient subduction setting (Faustino, et al. 1998). The Cansiwang Melange was produced in a sediment-starved subduction zone during the evolution of an accretionary prism. The tectonic history of southeastern Bohol in particular and the Visayan Region in general is further constrained by this study. Financial and laboratory support from NIGS, DOST-PCIERD and Plantation Mining are acknowledged

PETROLOGY OF BASIC VOLCANIC ROCKS FROM THE SOUTHEAST BOHOL OPHIOLITE COMPLEX, CENTRAL PHILIPPINES D.V. Faustinol , J.L. Barretto l , J.V. de Jesus", C.B. Dimalanta l2 E.J. Marquez', K.L. Queario3 , F.A. Jimenez' and G.P. Yumul, Jr.' 1 Rushurgent Working Group, National Institute of Geological Sciences College of Science, University of the Philippines, Diliman, Quezon City, PHILIPPINES 2 Ocean Research Institute, University of Tokyo, Nakano-ku, Minami-dai, 1-15-1, Tokyo 164, JAPAN 3 Department of Petroleum Geoscience, Universiti Brunei Darussalam Bandar Seri Begawan, BRUNEI

A complete ophiolite suite was recognized in southeast Bohol with a minimum thickness of about 2,000 m, from the residual harzburgite up to the pillow basalts. The volcanic section of the rock suite (Southeast Bohol Ophiolite Complex (SEBOC)) is composed of pillow basalts and sheeted dikes with intercalated sediments of both pelagic and tuffaceous origin. Samples taken from this volcanic section were analyzed for their major and trace element signatures. In general, four populations can be distinguished from the sample set: island arc tholeiites (IAT), mid-ocean ridge (MOR)-like basalts, back-arc basin (BAB)-like basalts and boninitic samples. Most samples exhibit negative Nb anomalies; in varying intensities, suggesting a subduction zone signature. MOR-like basalts and BAB-like samples exhibit negative Nb trends but to a lesser degree than the IAT samples. Elevated light rare earth element (LREE) trends are exhibited by samples that plot in island arc (IA) fields in tectonic discrimination diagrams. Rocks affected by ocean island basalt (OIB) enrichment or samples that come from a continental source region or enriched-type mid-ocean ridge basalt (E-MORB) source characteristically exhibit elevated LREE. The enrichment of LREE in the samples cannot be attributed to an EMORB since Nb and Ti are depleted. They cannot also be attributed to a continental source region since there are no geochemical or field evidence to support this idea. The closest rationalization would be that these IA samples were formed by subduction processes with probable involvement of an OIB. Another set of samples that can be classified as high magnesian basalts (MgO>10%) display characteristics that points to its boninitic composition (Zr/Y<1, very large negative Nb anomalies and depleted REE trends). These samples may have been formed in an environment characterized by either incipient subduction or subduction in its waning stage coupled with the start of marginal basin formation. The presence of a melange unit in the area, the Cansiwang Melange, favors the idea that the boninitic rocks are associated with an incipient subduction environment. These geochemical signatures in combination with field evidence suggests that the SEBOC was formed in relation with subduction processes in a marginal basin environment. Financial and laboratory support from NIGS, DOST-PCIERD, Plantation Mining, JSPS, University of Tokyo and University of Hong Kong are acknowledged. 8

HIGH SALINITY FLUID INCLUSIONS IN THE UWAMUKI NO. 4 KUROKO OREBODY, AKITA PREFECTURE, JAPAN 'Victor B. Maglambayan, 2Daizo lshiyama, 2Toshio Mizuta and 2Yohei Ishikawa 'National Institute of Geological Sciences, University of the Philippines Akita University

The Uwamuki No. 4 orebody is a typical Kuroko-type deposit which consists of five ore zones. They are from below in ascending order, (1) stockwork siliceous ore (Zone 1); (2) massive siliceous ore (Zone 2); (3) massive yellow ore (Zone 3); (4) massive black ore (Zone 4); and (5) massive barite ore (Zone 5). Zone 1 ores consist of pyrite, chalcopyrite, galena, and sphalerite which occur as fracture-filling grains in veinlets and as disseminations in tuffaceous breccias. These ores formed apparently in the conduits where hydrothermal solutions deposited the overlying zones. Zones 2, 3, 4, and 5 occur as dense disseminations and massive aggregates of pyrite, chalcopyrite, galena, sphalerite, and tetrahedrite-tennantite in brecciated volcanic rocks. The common primary textures in ore zones 2 to 5 are microcrystalline, colloform-like, pisolitic, euhedral, and concentric growth zoning, and rare framboidal and dendritic textures. The dominant secondary ore textures are recrystallization, replacement, chalcopyrite disease in sphalerite, clastic and dissolution textures. A distinct coarsening of sphalerite due to recrystallization in Zones 3, 4 and 5 was observed. This can be related to the post-mineralization intrusion of the Takaderayama rhyolite on the hanging wall of the orebody. Homogenization temperatures of 15 primary and secondary fluid inclusions in quartz, barite, and sphalerite in the ores (Zones 1, 2, 3, and 4) range from 161° to 387° C (n=11). Their salinities range from 2.4 to 14.1 wt % NaCI equivalent (n=14). Fluid inclusions in the footwall rocks were homogenized from 346° C to 361° C (n=2) and have salinity from 0.2 to 6.2 wt. % NaCI equivalent (n=2). Fluid inclusions in hanging wall tuffaceous rocks were homogenized from 265° to 393° C (n=6) and have salinity with a range from 3.8 to 46.3 wt. % NaCI equivalent (n=6). The very high salinity, secondary fluid inclusions (>28 wt. % NaCI equivalent) are halite-bearing (3-phase) at room temperature. The presence of these very high salinity secondary fluid inclusions can be related to the post-mineralization circulation of phase-separated fluids. The low salinity fluid inclusions occurring alongside the very high salinity fluid inclusions are possibly the condensed vapors which separated from highly saline fluid. The intrusion of the post-ore Takaderayama rhyolite may have caused the intense heating of the post-ore convecting fluids.

CHEMICAL REACTION PATH MODELLING OF MINERALIZATION IN THE TONGONAN GEOTHERMAL FIELD, LEYTE ROSANA BALANGUE-TARRIELA National Institute of Geological Sciences, University of the Philippines

A study of well discharges in the Tongonan geothermal field (Leyte) suggests that the chemistries of the geothermal fluids can be explained by boiling and mixing with a steam-heated end member. Using chemical speciation and reaction path calculations, boiling and mixing of the recalculated "primary" geothermal water of the Tongonan system (Leyte) were simulated. The main purpose of the computer simulation was to ascertain which of these two processes would be the better precipitating mechanism for ore minerals. Primary water was assumed to be represented by the least diluted well discharge (Well 410) as suggested by chloride-enthalpy diagram. Adiabatic boiling done from 300 to 100°C resulted to the precipitation of a sequence containing paragenetically early traces of gold with quartz; followed by quartz, acanthite, chalcocite and late quartz, sphalerite, galena, acanthite, bornite. Boiling the solution could precipitate traces of gold. Interestingly, the pH of the residual boiled solution decreased instead of the commonly accepted increase with the release of volatiles in the steam phase. Steam-heated end-member used for mixing was generated by titrating the vapor phase at 100°C after resulting from the boiling of the primary water and mixing with a sample of the groundwater. Mixing of this steam-heated water with the boiled primary water at 300°C resulted to the formation of the ore minerals: acanthite, bomite, chalcocite, covellite, galena, pyrite, and sphalerite. Gangue minerals include anhydrite, Mg-chlorite, alunite, kaolinite, muscovite, and quartz. Observed vein minerals in cores and well cuttings include base metals (sphalerite, galena, chalcopyrite), pyrite, anhydrite, quartz, calcite, epidote-clinozoisite, chlorite with minute specks of Au-Ag electrum. The results of the chemical modelling suggest that both boiling and mixing processes and the combination of these processes could precipitate the ore minerals in a Tongonan-like geothermal system. However, the absence of some minerals predicted by the simulation (e.g. acanthite) and the presence of other minerals in the veins (e.g., chalcopyrite) suggest that physico-chemical conditions other than those assumed in the simulations could be prevailing in the system.

THE METALLOGENY OF ZAMBOANGA REGION F.A. Jimenez Jr., J.V. de Jesus, E.J. Marquez, J.A.L. Barretto, D.V. Faustino, K.L.Queano, and G.P. Yumul Jr. National Institute of Geological Sciences, College of Science

Precious and base-metal mineralizations recognized in the Zamboanga Region generally contain the same essential features related to mineralization (e.g. plutonic body, volcaniclastic hosts, unique tectonic setting) as shown by other metallogenic provinces in the Philippines. Gold-silver and copper-lead-zinc mineralizations in the Zamboanga Region are characterized by both volcanic and plutonic association. Mineralizations vary from massive sulfide replacement type to disseminated to vein-type deposits occurring in both the volcanic and sedimentary host rocks. In most of the deposits found in the Zamboanga Region, characteristic features are the presence of dioritic bodies intruded to the volcano-sedimentary host rocks of Lower Miocene to Upper Miocene age. Majority of the deposits in the Zamboanga Region are also characterized by northwest to north-northwest trends. The more prominent mineralized zones vary from less than a meter to few meters in thickness and few meters to tens of meters in length measured along the strike. These altered/mineralized zones show close proximity to major northwest-trending structures (e.g. Lipawan Fault) and generally exhibit secondary mineral assemblages typical of hydrothermal-induced alteration such as quartz-pyritesericite, sericite-chlorite-clay, and quartz-pyrite assemblages. A recognition of the regional geological and tectonic setting of Zamboanga Peninsula will give an idea if there is a causal relationship between tectonic events that transpired here and the observed mineralizations.

THE MALUSOK VOLCANOGENIC MASSIVE SULFIDE DEPOSITS: AXIAL VERSUS OFF-AXIS HYDROTHERMAL ACTIVITES Cliff L. Querubin National Institute of Geological Sciences, University of the Philippines

The Malusok VMS deposits comprise two ore bodies which have been designated as the Main Malusok and the Malusok Southeast ore bodies. These deposits are confined within a thick metamorphic sequence which is in fault contact with serpentinized peridotites towards the northwest and overlain to the southeast by marine clastic sediments. The structural and metamorphic overprint combined with pervasive alteration undergone by the lithologies comprising Malusok rendered the recognition of primary textures and lithologies difficult. In establishing the stratigraphic relationship of the Main Malusok Southeast ore bodies, rock types were initially classified according to dominant mineral assemblages whereas the main lithologic units were segregated based on the position of each sequence relative to the ore zone. A fine-grained, finely-laminated chlorite/epidote â&#x20AC;&#x201D;rich based located at the interface between the hanging wall and the host sequence served as a distinct stratigraphic marker from which lithologies were referred to. Lithogeochemical analysis was employed in identifying the populations which compromise Malusok. These populations were later subdivided into subtypes based on alteration trends generated from plots of samples on a Zr-TiO2 binary diagram. Alteration gradients, on the other hand, were quantified through mass change calculations. Through these mass change calculations, the approximate localities of conduits for hydrothermal fluids were determined. Based on findings generated from field evidences and lithogeochemical analysis, it is inferred that axial hydrothermal activity led to the formation of the Main Malusok VMS deposit whereas the Malusok Southeast ore bodies were formed as a result of off-axis hydrothermal activity.

PARTICLE FLUX IN THE SOUTH CHINA SEA Martin G. Wiesner Ins...ute of Biogeochemistry and Marine Chemistry, University of Hamburg, Germany

The ocean contain some 50 times as much carbon dioxide as the atmosphere. Consequently, minor variations in the oceanic carbon reservoir can cause noticible changes in the atmospheric carbon pool. Such changes are recorded in ice cores during glacial-intergalcial cycles and have been explained by fluctuations in ocean productivity (Sarnthein et al., 1988) and, fluctuations in the quantity of carbon stored in the deep sea, which in turn are related to ocean circulation changes (Curry et al., 1988). They may have also important consequences during the climate changes hat are predicted to occur in the next 50-100 years, as a result of the rapidly rising levels of atmospheric CO2 and other greenhouse gases (Houghton et al., 1996). Models indicate that the oceans are currently taking up at least one third of the anthropogenic CO2 (Sarmiento et al., 1998). The uptake capacity depends on the CO2 concentration in the surface layer, and therefore the enormous chemical absorption potential of the world oceans can only come into force when CO2 is transferred or "pumped" from the surface to the deep sea. Relevant in the context of short-term removal fromthe atmosphere are three processes: (i) the formation of organic mater during photosynthesis, and (ii) the formation of carbonate by biomineralizing organisms (the so-called biological pumps) and (iii) the transport of these components downdepth which can be accelerated by abiogenic matter introduced to the surface from terrestrial sources (Ittekkot et al., 1990). Part of the biologically fixed carbon is oxidized to CO2 in the upper ocean which exchanges gas with the atmosphere; the remaining fraction settles to the deep sea where it is either oxidized or dissolved, or becomes part of the sediment. The CO2 stored in the deep water may return to the surface via upwelling and is released to the atmosphere depending on the solubility equilibrium. The net uptake of CO2 by the ocean is thus a function of the ratio of degassing and the combined effect of the tow carbon pumps.

THE BRAARUDOSPHAERA PROBLEM IN THE OLIGOCENE SOUTH ATLANTIC Alyssa M. Peleo-Alampay l , Gregory A. Meade and Wuchang Wei 3 'National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City 1101 Department of Geology, University of Florida, Gainesville, FL 32611 U.S.A. 3 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093 U.S.A. 2

The origin of the Oligocene Braarudosphaera layers in the South Atlantic has remained a mystery since its discovery almost three decades ago during Deep Sea Drilling Project Leg 3. Results from a detailed micropaleontologic study coupled with oxygen and isotope data from Sites 363 and 522 in the South Atlantic have shed some new light on this question. The first quantitative Braarudosphaera abundance data collected in this study correlate with oxygen isotope curves and suggest that Braarudosphaera preferred cold waters. Variations in the relative abundance of sphenoliths (warm water taxa) and longer C. pelagicus placoliths indicate colder water conditions during times of elevated Braarudosphaera deposition. Upwelling and mid-ocean overturn of cold, nutrient-rich, low salinity water are inferred as possible paleoceanographic mechanisms for the deposition of Braarudosphaera layers.

PRELIMINARY STUDY ON THE CALCAREOUS NANNOPLANKTON DISTRIBUTION OF THE EASTERN PORTION OF SOUTH CHINA SEA Allan Gil S. Fernando, Alyssa M. Peleo-Alampay and Sonne 132 Shipboard Scientific Party National Institute of Geological Sciences, University of the Philippines

Core samples from the South China Sea were taken during the Sonne Cruise 132 this year. One of the general objectives of the cruise is to characterize and reconstruct short-term changes in the Quaternary sediments of the South China Sea as a result of monsoonal variability. This particular study aims to characterize calcareous nannoplankton components of core top sediments from South China Sea. Calcareous nannoplanktons (coccoliths) are haptophyte algae capable of secreting calcareous tests and are one of the major components of deep-sea sediments. Although there are only few published studies about nannoplanktons in this area, this study was able to come up with a characterization of calcareous nannoplankton assemblages from marine coastal environment to open-ocean conditions. Results show that Emiliania huxleyi, Gephyrocapsa oceanica and Florisphaera profunda are the three (3) dominant taxa.

DISTRIBUTION OF RECENT FORAMINIFERA FROM PANATA ISLAND AND PAGASA ISLAND AND SHOAL OF THE KALAYAAN ISLANDS GROUP, SOUTH CHINA SEA Rose Berdin, Marilou Justina Mamaril-Villanoy and Fernando Siringan National Institute of Geological Sciences, University of the Philippines

The thanatocoenoses of foraminifera from two islands, Panata Island and Pag-asa Island and shoal, of the Kalayaan Islands Group in the South China Sea are examined. Twenty-two sediment samples, collected from various locations and depths around the islands, were analyzed. Dominance rather than diversity characterize the foraminiferal assemblage may have resulted from: 1) the restricted subenvironments in a carbonate island system; 2) limited sources of species because of open sea conditions; 3) stresses brought by events such as storms and El Nino; and 4) possible combinations of the above factors. The assemblage distribution, using test and species counts of some major faunal groups as variables, defined a zonation that parallels the elongation of Pagasa Island and shoal. Low test and species counts typify the northern part of the island and shoal while higher test and species counts occur at the southern part. This is probably due to the transport of foraminiferal tests rather than being reflective of biotopes. Regression analysis of depth versus diversity, depth versus number of tests and depth versus number of species show poor correlation probably because of the limited depth range of the samples collected (17 meters). Cluster analysis using TWINSPAN yielded four major thanatofacies and three major thanatotopes for the foraminiferal assemblage. The defined thanatotopes indicate that water depth and position in the reef influence the assemblage distribution.

GEOLOGY OF THE SOUTH CENTRAL IFUGAO, NORTHERN LUZON Geol 170 class 1998, N.R. Aguda*, H. Cabria*,C. Pascua * and J. Foronda* Presnt:NilMacd National Institute of Geological Sciences, University of the Philippines

The UP Geol 170 class of 1998 conducted a two-week field mapping exercise that focused on the lithologic and structural character of south central Ifugao. The area covered includes the municipalities of Kiangan, Asipulo and Lagawe. Supplementary data are derived from topographic maps, aerial photos, and SAR images. Three formational units were encountered, namely, the Ibulao Formation, Lubuagan Formation, and the Balbalan Formation. These formations are in part covered with the most recent Terrace Gravel Deposits. The oldest unit is the Ibulao Limestone primarily composed of biostromal to biohermal limestone exposed in its type locality in Ibulao Gate. It forms north-south trending fold ridges. It is characterized by a gradual decrease in the bed thickness from the base to the topmost portion. It has an assigned age of Early Miocene. The Lubuagan Formation conformably overlies the Ibulao Formation as observed near the Ibulao Gate in Lagawe. It consists mainly of sequences of clastic and volcaniclastics rocks that are largely exposed on the western portion of the study area. Micropaleontologic dating yielded an Early Miocene age. Clasts in the rudites include diorite, andesite, granodiorite and syenite. The youngest unit, the Balbalan Formation, is a clastic unit separated by an unconformity from the Lubuagan Formation. It is dated early Middle Miocene based on calcareous nannoplanktons. Fine to medium-grained sandstones, siltstones, and mudstone sequences comprise the basal portion. The upper part of this unit is composed of bedded medium to coarse-grained sandstones, breccias and conglomerates. Marine fossils found in the topmost part include mollusks and delicate finger corals. The sequences of this unit are characteristic of shallow marine to bathyal deposits. Fault array analysis of strike-slip and reverse faults from the Lower and Middle Miocene units indicate a NE-SW to E-W maximum stress direction. The axes of plunging inclined folds in the Lower Miocene units also indicate an E-W maximum stress direction. Quaternary strike slip and reverse faults can be explained by NW-SE directed compression.

* National Institute of Geological Sciences, College of Science, University of the Philippines, 1101 Diliman, Quezon City, Philippines

SUBSIDENCE AND THE WORSENING FLOODS IN PAMPANGA AND BULACAN Fernando P. Siringan l and Kelvin S. Rodolfo2 National Institute of Geological Sciences, University of the Philippines of Illinois at Chicago, Chigaco, Illinois, USA

2 University

At the north end of Manila Bay, the heavily populated Pampanga delta and its highly developed agri- and aquaculture are extremely vulnerable to even minor changes in local sea level, which translate to large horizontal saltwater invasions along its very gentle surface and are greatly amplified by the numerous estuaries along which they occur. Estimates of mean global sea level rise during the last century range from 0.12 to 0.18 cm/yr. Accelerating rise over the next century, owing to global warming, is estimated at about a meter, considering only thermal expansion of surface ocean waters and increased glacial melting. Local geological factors may amplify or even overwhelm the global trend. The north end of Manila Bay, being deltaic, undergoes subsidence, its thick deposit of shallow-water and coastal sediments cause the loaded crust to subside. The sediments themselves compact slowly under their own accumulating weight of overburden. Tectonic and volcanic factors are likely to affect local sealevel significantly. Manila Bay is surrounded by volcanoes, including two very active ones: Pinatubo and Taal. Some regional lineaments on satellite images might be faults experiencing creep with great vertical displacement; certainly, deltaic accumulations could contribute gravitationally to such motion. Groundwater withdrawal may also be contributing to local subsidence. Our field reconnaissance has documented active subsidence on the scale of centimeters per year at some well-sites. The degree to which groundwater withdrawal is a contributory factor to subsidence at these locales needs to be evaluated. Pumpage and associated subsidence can be expected to accelerate as the population grows.

SEDIMENT DISPERSAL PATTERNS IN THE PAMPANGA BAY REGION, MANILA BAY Cherry L. Ringor and Fernando P. Siringan National Institute of Geological Sciences, University of the Philippines

Pampanga Bay is a semi-enclosed basin in the northwestern part of Manila Bay. It receives 50% of the total annual freshwater budget of Manila Bay and the largest river sediment input. Thus, determination of sediment transport pathways in this part of the bay is very important for coastal planning and monitoring. This study attempts to delineate the sediment dispersal patterns in Pampanga Bay using water depth and shoreline changes, sediment distribution patterns, and Landsat Thematic Mapper imageries. A net westward shift of river mouths along the Pampanga coast indicates the predominance of westward longshore currents. Along the Bataan coast, based on shoreline change and delta morphology, a predominant southward longshore drift occurs in the Kalaguiman River Delta area. To the south, in the Talisay River Delta area, the net longshore drift is to the north. In the more offshore areas, along-shore transport as defined by grain size parameters is predominantly westward off the Pampanga coast and southward off the Bataan coast. Offshore turbidity patterns, captured by Landsat imageries from four different periods, show the occurrence of westsouthwest and southward along-shore transport directions off the Pampanga coast. The west-southwest drift occurs when winds in the bay come from the northwest and southeast; southward sediment drift is associated with northeast and northwest winds. Off the Bataan coast, sediment drift is southwards regardless of wind directions. Despite the large volume of sediment discharged into Pampanga Bay, water depth changes indicate that deepening in the order of a meter has occurred nearshore while shoaling in the order of a few meters has occurred offshore. These water depth changes may indicate an efficient offshore transport of sediments. The relatively high concentrations of coarse-grained sediments coinciding with the zones of deepening tend to support this. However, high rates of subsidence in the Pampanga Delta might also explain the observed water depth changes.

EFFECTS OF DELTA AND RIVER SWITCHING TO COASTAL MORPHOLOGY AND EVOLUTION: EXAMPLES FROM THE AGNO RIVER SYSTEM, LINGAYEN GULF Zenon Richard P. Mateo and Fernando P. Siringan National Institute of Geological Sciences, University of the Philippines

The effects of river and delta switching to coastal morphology, are examined in the wave-dominated bayhead coast of Lingayen Gulf. The short-term changes - within tens of years - in shoreline position and bathymetry and the grain size distribution of surface sediments in the nearshore areas were established. The longer-term â&#x20AC;&#x201D; hundred to thousands of years - coastal changes are defined using the beach-ridge and drainage patterns recognized in remotely-sensed images. Re-routing of the Agno River for flood control purposes, from the Labrador outlet to the Lingayen outlet, during the late 1930's resulted in the formation of the present-day Agno-Lingayen Delta and erosion of the Agno-Labrador Delta. The shoreline in Lingayen prograded by as much as 1050 m while retreat of as much as 500 m occurred in Labrador. Shoreline erosion at Labrador is accompanied by the disappearance of a 6 km by 2 km subaqueous delta lobe in the offshore area. Based on paleochannel and beach ridge patterns, Agno River used to connect with the Dagupan River. The old Agno River appears to have occupied this site for a long time resulting in a deltaic progradation of at least 5000 m. Shoreline retreat of as much as 1000 m may have occurred when the old Agno River shifted to Labrador. In the offshore area, erosion of the corresponding subaqueous delta lobe is manifested by a relatively high concentration of sand in a zone of deepening off Dagupan. The channel shift originated 30 km landward of the present coastline and may have been due to a relative movement along a western splay of the Philippine Fault.

HEAVY METAL POLLUTION OF THE PASIG RIVER Roderick R. Amores and Joselito P. Duyanen, PhD National Institute of Geological Sciences, University of the Philippines

Past toxicity studies in the Pasig River entail heavy metal analysis in water with very few reference to river sediment which are natural sinks for metals. Weakly-bonded or adsorbed metals are very potent environmental pollutants because they are easily released to the water column and participate readily in nutrient recycling. This research provides an inventory of seven (7) environmentally-relevant heavy metals determined from the bottom sediments of the Pasig and Marikina Rivers. Quantitative measure of pollution of the Pasig River sediments was done using the Index of Geoaccumulation (I ge() which standardizes heavy metal concentrations from background sources and corrects for possible lithogenic variations. This index consists of seven grades (0-6) where the highest grade (6) reflects a 100-fold enrichment of metal levels from background values. From this transformation, three environmentally-critical zones are identified in the study area: the discharge point in Manila Bay at one kilometer radius from the mouth of the river; the lower Pasig River from the mouth to Sta Ana (Panaderos), Manila; and the lower Marikina River from the confluence in Napindan to the City of Pasig. Samples taken from the bay yield metal values with I geo's equivalent to 6 (excessively polluted) for Ni; and, 3 (moderately to strongly polluted) for Cu, Pb and Zn. This area represents compounded metal input from the metropolis and discharges from the nearby South Harbor. The Lower Pasig River is the most polluted portion of the river with I geo's reaching high values equivalent to classes 5 and 6 for Cd; 2 to 3 for Zn; and 2 for Cu and Pb. Highest concentrations of Zn, Pb, Cd and, to some extent, Cu for the entire study site were obtained from samples taken at the junction of the Pasig and San Juan Rivers. The San Juan River is found to be a major pollutant contributor to the Pasig River and is the most polluted sub-basin of the Pasig River system. Although less polluted than the Pasig River, Marikina River nevertheless contains heavy metal load with corresponding high I geo values, particularly for Cd (5-6), Zn (2-3), Cu (12) and Pb (1-3). These classes are measured from sediments taken at the confluence of the Pasig and Marikina Rivers, and this geochemical pattern persists upstream to Pasig City. Samples taken near a stagnant portion of the river, where estero drain effluents coming from a petrochemical storage facility, yield highest values of Cd for the entire Marikina River and is moderately polluted with respect to Cu and Zn.

ABSTRACTS OF POSTER PRESENTATIONS (Arranged according to number in the program)

A STUDY OF THE CALCAREOUS NANNOPLANKTON AND FORAMINIFERAL ASSEMBLAGES OF THE PRE-AND POST-1991 MT. PINATUBO ERUPTION SEDIMENTS OF PAMPANGA BAY, PHILIPPINES Marietta M. De Leon and Nelson C. Agapay, Jr. National Institute of Geological Sciences, University of the Philippines

This paper aims to study the calcareous nannoplankton and foraminiferal assemblages in the Pampanga Bay sediments and determine how they were affected by the 1991 eruption of Mt. Pinatubo. A total of 38 samples taken from eight gravity cores collected last 29 April 1997 off the coast of Malabon - Navotas municipalities were utilised and studied. Analysis of the distribution of bioclast, a general term used in this paper to describe calcareous nannoplankton, foraminifera and other marine organism remains, and lithoclast, pertaining to sedimentary materials having a composition rich in quartz, calcite and volcanic glass, shows a varying pattern from core to core. Generally, an increase of marine micro faunal and nannofloral abundance with increasing distance from the shoreline was observed. Conversely, lithoclasts were noted to decrease farther from land. Bioclasts were generally observed to have an inversely proportional relationship with the lithoclasts. Such relationships may be attributed to the sudden burial of marine organisms by flooding events or sediment influx causing unfavorable living conditions to the marine micro fauna or nannoflora. Preliminary study of the vertical distribution of bioclast from bottom to top of each core shows a decreasing trend. This pattern is also attributed to flooding events. The very low diversity of both the foraminifera and calcareous nannoplankton is probably due to the closed sea system of the area.

EASTERN SOUTH CHINA SEA: VOLCANISM AND FORAMINIFERAL OCCURRENCE Edanjarlo J. Marquez', Priscilla J. Militante-Matias l , Marietta M. de Leon', Joel V. de Jesus', Decibel V. Faustino l ,Jenny Anne L. Barretto l , Karlo L. Queano1 '2 , Carla B. Dimalanta l '3 , Francisco A. Jimenez' and Graciano P. Yumul, Jr. 1 National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City Department of Petroleum Geoscience-Universiti Brunei Darussalam, Bandar Seri Begawan, Brunei Darussalam 3 Ocean Research Institute, University of Tokyo, Nakano-ku, Minami-dai, 1-15-1, Tokyo 164, Japan 1

Six stations along the eastern margin of the South China Sea show a unique assemblage of benthonic foraminifera in the 1991 Mount Pinatubo ash layer. The following observations were established: (1) the total counts of benthonic foraminifera in the ash layer is low; (2) below the ash layer, the assemblage is diverse and abundant while diversity in the ash layer is quite low with high abundance of Quinqueloculina sp., Reophax sp. and Rhabdammina sp.; (3) the percent distribution of the different wall structures shows an almost equal distribution of the three forms in the top part of the ash layer while dominance of the textulariids was noted in the bottom part of the layer; and, (4) re-appearance of assemblages begins to develop in the post-eruption layers. These findings suggest that the disturbance created by the large amounts of sediments that originated from the eruption of Mt. Pinatubo may have exerted a significant control in the occurrence of foraminifera in Eastern South China Sea. The information generated from this type of studies will help provide insights, not only on the relationship between Recent foraminiferal distribution and the deep-sea environment but also the effects of the eruption on the foraminiferal communities. In effect, knowledge on the distribution and ecology of Recent foraminifera makes it easier to interpret foraminiferal associations in relation to paleoenvironmental studies.

Support from the Department of Petroleum Geoscience-Universiti Brunei Darussalam, the National Institute of Geological Sciences-University of the Philippines and the RN Sonne are acknowledged.

PRELIMINARY INVESTIGATION IN PREDICTING GROUNDWATER OCCURRENCE AND IDENTIFYING GEOLOGIC HAZARDS, MATI-NORTHERN PUJADA PENINSULA Pascua, C. S., Geol 215 '98 and Yumul, G.P., Jr. National Institute of Geological Sciences, University of the Philippines

Six formational units define the geology of Mati-Northern Pujada Peninsula, these are (1) the Pujada Ophiolite, (2) the Iba Formation, (3) Sanghay Formation, (4) Sigaboy Formation, and (5) the Bitaogan Metamorphics. These formational units are differentially covered with the (6) Quaternary Deposits comprised of quaternary alluvium, river terrace gravels, and coralline limestones. The Pujada Ophiolite is composed of peridotites, gabbros and basalts with an assigned age of Middle Cretaceous. The Iba Formation is composed of cherts, limestones and alternating sandstone, siltstone and mudstone sequences exposed in Brgy. Iba, its type locality. The Bitaogan River Metamorphics are distributed as patches of phyllite and schist associated with the Iba Formation and Pujada Ophiolite. It follows a regional foliation grain that trends northwest. The Sanghay Formation is composed of arkosic sandstone, siltstone-sandstone sequences, and limestones unconformably overlying the Iba Formation. The Sigaboy Formation composed of tuffaceous sandstone and conglomerate unconformably overlies the Sanghay Formation. Unconformably overlying the Iba Formation, the Sanghay Formation is composed of arkosic sandstone, siltstone-sandstone sequences, and limestones. Tuffaceous sandstone and conglomerate characterize the Sigaboy Formation, which unconformably overlie the Sanghay Formation. Groundwater occurrences are sporadically and locally distributed on the different lithologies present. Confined aquifers may be present in the sandstones of the Sanghay and Sigaboy Formation and the Quaternary limestones exposed along the Guanguan Peninsula. Other occurrences are in unconfined aquifers present on the capping sediments of the other lithologies, and the extensive distribution of unconsolidated deposits. Most of the subsurface water is derived from shallow wells drilled on the unconsolidated deposits along rivers, pediment and coasts. Geologic hazards present in these areas include flooding, land and slope instability, liquefaction, earthquakes, and mass movement. Associated hazards in the event of an earthquake include land and slope instability, liquefaction, mass movement, subsidence, seches and tsunamis. Other geologic hazards may be initiated and triggered by human activity include accelerated erosion, flooding, mass movements, land and slope instability and ground subsidence.

MACERAL COMPOSITION OF COALS FROM THE MANIGUIN-3 WELL AND ITS RELEVANCE TO PALAEOMIRE CONDITIONS Francis Casareo and Monina Forbes National Institute of Geological Sciences, University of the Philippines

The Maniguin-3 Well yielded minor amounts of hydrocarbons at depths whereby coal seams are intersected. These seams belong to the Miocene ❑ Late Oligocene Semirara Formation. Coal maceral analyses of isolated coal particles (from well-cuttings obtained from depths at which coal seams of significant thickness occur) reveal vertical variations in the concentration of vitrinite , liptinite, and inertinite macerals respectively. Coal facies likewise vary from a &fry forest-swampti- to a owet forest-swampO with an irregular pattern of increase ❑ decrease in mire owetnessO from Early Miocene (?) thru to Late Miocene. Moreover, the Tissue Preservation Index (TPI) ❑ Gelification Index (GI) scatter plots suggest palaeomires that are telmatic to limno-telmatic. The presence of relatively high amounts of sclerotinite in certain coal seam intervals probably suggests palaeomire conditions that are dry and with sufficient oxygen supply. Moreover, relatively high percentage values of fusinite/semifusinte macerals are likewise seen in these intervals. There is no clear relationship between the occurence of oil (albeit in low amounts) and the presence of high amounts of hydrogen-rich liptinite macerals, although there is a possibility that resinite is a major oil-contributor.

SOME FOSSIL MICROFLORA FROM THE LATE PERMIAN WITTINGHAM COAL MEASURES OF THE NORTHERN SYDNEY BASIN, N. S. W., AUSTRALIA Nina Navarro-Casareo National Institute of Geological Sciences, University of the Philippines

The Wittingham Coal Measures is part of the extensively-developed Gondwanan coals associated with glacigene deposits of Carboniferous to Late Permian age. Fossil microflora associated with these Coal Measures are typical of the Australian Permian. The fossil assemblage consists of the following: (1) Spores: Granulatisporites trisinus Balme & Hennelly, 1956, G. micronodosus Balme & Hennelly, 1956, Calamospora sp. Schopf, Wilson & Bentall 1944, Leiotriletes directus Balme & Hennelley 1956, Phaselisporites cicatricosus (Balme & Hennelly) Price 1983, Microbaculispora tentula Tiwari 1965; (2) Disaccate, non-taeniate pollen: Scheuringipollenites ovatus (Balme & Hennelly) Foster 1975, S. maximus (Hart) Tiwari 1973; (3) Disaccate, taeniate pollen: Protohaploxypinus amplus (Balme & Hennelly) Hart 1964, P. limpidus (Balme & Hennelly) Balme & Playford 1967, Striatoabietes multistriatus (Balme & Hennelly) Hart 1964 and Striatopodocarpites cancellatus (Balme & Hennelly) Hart 1964; and (1) Colpate pollen: Cycadopites cymbatus (Balme & Hennelly) Segroves 1970, C. folliculan's Wilson & Webster 1946, Marsupipollenites triradiatus Balem & Hennelly 1956 and Praecolpatites sinuosus (Balme & Hennelly) Bharadwaj & Srivastava 1969.

OCEANIC CRUST - MANTLE SEQUENCES: WHAT DO WE KNOW? G. P. Yumul,Jr.', C.B. Dimalanta 1.2 , D.V. Faustino l , J.L. Barretto l , J.V. De Jesus', E.J. Marquez', K.L.Queafio l '3 , F.A. Jimenez,Jr. 1 , R.A. Tamayo,Jr. F.T. Jumawan 1 Rushurgent Working Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, PHILIPPINES 2 Ocean Research Institute, University of Tokyo, Tokyo, JAPAN 3 Department of Petroleum Geoscience, Universiti Brunei Darussalam, BRUNEI 4 UMR Domains Oceaniques, University of Bretagne Occidentale, Brest, FRANCE 5 Extar Resource Development Corporation, San Juan, Metro Manila, PHILIPPINES

NOTES Samples recovered from ocean drilling activities, theoretical calculations of fluxes and forward/ reverse experimental (petrological/ geochemical) works showed results which confirm what we know about the ocean crust-mantle sequence. What is more significant, however, are the departures from the norms in petrological and geochemical modeling. Some of these are: SUBDUCTION PROCESSES: Recent numerical calculations showed that water expelled from the subducting slab is hosted by serpentine and chlorite which is dragged down to 150 km. Breakdown at this depth results into the water being transported upwards. Very young slabs (<< 10 my with subduction velocity of ca. 6 cm/yr) were found to be capable of melting in the forearc region (Iwamori, 1998); The global subducting sediment (GLOSS) composition is dominated by terrigenous material (76 wt% terrigenuous, 7 wt% calcium carbonate, 10% opal, 7 wt% mineral-bound H20+) similar to the upper continental crust composition (Plank and Langmuir, 1998); MID-OCEAN RIDGE SETTING: The calcic nature of plagioclase in MORB may be due to abnormally high CaO/Na20 values (>8) (Panjasawatwong et al., 1995). Nevertheless, mantle - melt interaction can produce this, too (Kamenetsky et al., 1998); Dunite - troctolite - olivine gabbro are products brought about by mantle - melt interaction in the oceanic regime. Primary hydrous minerals (phlogophites and pargasites) found as inclusions in these rocks confirm the presence of water in the oceanic mantle (Arai and Matsukage, 1998; Matsukage and Arai, 1997); Two models exist concerning the presence of melt in mid-ocean ridge regimes. Melt can form in a narrow zone beneath the mid-ocean ridge or that it can be present over a much wider area of the mantle around the ridge. Mantle Electromagnetic and Tomography (MELT) experiments in the East Pacific Rise favor the wider distribution of melt. The region of slower velocities centered to the portion of the ridge moving faster and containing more melt (Levi, 1998). 27

MINERALIZATIONS: Chromitites can form in oceanic mantle regime but following a different evolutionary path as compared to those formed in supra-subduction zones (Arai and Matsukage, 1998); Below a volcanic-associated massive sulfide deposit, a stratified zone of highgrade copper-rich replacement mineralization is present as observed by the ODP Leg 169 in the northern Juan de Fuca spreading center (Zierenberg et al., 1998). Off-axis magmatisn and mixing of seawater and hydrothermal fluid are confirmed to be important in the alteration and mineralization process (e.g. Eddy et al., 1998; Alt et al., 1998). STRUCTURES: Studies made on exposed oceanic plateaus and oceanic crusts showed the significance of crustal thickness, transfer faulting and imbrication in the evolution, preservation and even mineralization of ocean crust -mantle suites (Kerr et al., 1998; Yumul et al., 1998). These information are related to the results we have generated from our study of several exposed oceanic crust - mantle sequences (Amnay in Mindoro, Antique, Tacloban, Cebu, Bohol, Zamboanga, Pujada) with the aim in view of constraining their generation and evolution. Support from the National Institute of Geological Sciences, Department of Science and Technology and a Lepanto Consolidated Co. Professorial Chair in Geology are acknowledged. Analyses were done at the University of Hong Kong (ICP-MS), University of Bretagne Occidentale (ICP-AES), University of Tokyo (XRF; EPMA) and the UP - NIGS (AAS). References cited: Alt, J.C. et a/., 1998. J. Geophys. Res. 103, 12,365-12,380. Arai, S., and Matsukage, K., 1998. Lithos 43, 1-14. Eddy, C.A. et al., 1998. Tectonophysics 292, 189-210. Iwamori, H., 1998. Earth Planet. Sc!. Lett. 160, 65-80. Kamenetsky, V.S. et a/., 1998. Earth Planet. Sci. Lett. 160, 115-132. Kerr, A.C. et at., 1998. Tectonophysics 292, 173-188. Levi, B.G., 1998. Physics Today, July issue, 17-19. Matsukage, K. and Arai,S., 1998. Contrib. Mineral. Petrol. 131, 111-122. Panjasawatwong, Y. at a/., 1995. Contrib. Mineral. PetroL 118, 420-432. Plank, T. and Langmuir, C.H., 1998. Chem. Geol. 145, 325-394. Yumul, G.P.,Jr. et a/., 1998. Tectonophysics 293, 255-272. Zierenberg, R.A. at a/., 1998. Nature 392, 485-488.

GRAVITY, MAGNETICS AND OPHIOLITE COMPLEXES Jenny Anne L. Barretto', Carla B. Dimalanta l '2 , Decibel V. Faustino', Joel V. de Jesus', Edanjarlo J. Marquez, Karlo L. Queatio 3 , Francisco J. Jimenez', and Graciano P. Yumul, Jr.", Rush urgent Working Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, PHILIPPINES 2 0cean Research Institute, University of Tokyo, Nakano-ku, Minami-dai, 1-15-1, Tokyo 164, JAPAN 3 Universiti Brunei Darussalam, Bandar Seri Begawan, BRUNEI 1

Ophiolites are ocean-crustâ&#x20AC;&#x201D;upper-mantle sequences consisting of (from top to bottom): pillow basalts, sheeted dike/sill complex, isotropic gabbros, layered mafic and ultramafic cumulates, and residual peridotites. Based on their geochemistry, these rock suites are classified into two types: Mid-Ocean Ridge (MOR) and Supra-Subduction Zone (SSZ) ophiolites. Ophiolites can be emplaced by subduction prism accretion, onramping, and large-scale strike-slip faulting. In terms of geophysical characteristics, ophiolite provinces can either be characterized with high gravity anomalies from those with low gravity anomalies. The Troodos Massif in Cyprus and the Papuan Ultramafic Belt in New Guinea are of the first type which are associated with broad and intense Bouguer anomalies having peak to trough amplitudes of more than 100 mgals. The East Halmahera Ophiolite, on the other hand, is of the second type which is characterized by complex gravity variations of generally low (<50 mgals) amplitudes. The Southeast Bohol Ophiolite Complex (SEBOC) in Bohol Island, Central Philippines is of the second type. Gravity data along the SEBOC show an 83 mgal difference between maximum and minimum values. The low gravity values suggest that the ophiolite is fragmented and is not presently rooted in the mantle. The Bouguer anomaly highs trend NE-SW and are coincident with the SEBOC units, the Alicia Schist and the intrusive Jagna Andesite body. Gravity values generally decrease toward the coast except for an E-W low gravity zone over sedimentary rocks in the vicinity of Duero and a linear gravity low over the Cansiwang Melange in its type locality. The linear gravity low over Cansiwang suggests that this serpentinite melange is not diapiric in nature since serpentinite diapirs are characterized by circular gravity low contours. The SEBOC, Alicia Schist and the Cansiwang Melange comprise the basement complex of southeastern Bohol. Magnetic data over southeastern Bohol agree with field geologic data that the basement rocks are relatively shallow. Short-wavelength magnetic anomalies characterize the study area except for the long wavelengths over the Alicia area. These long-wavelength anomalies obviously do not arise from deep basement sources since the basement rocks are actually exposed in this area. The shallow features are spatially coalesced and have registered long instead of short wavelengths. Financial and logistic support from DOST-PCIERD, PGI, McDougald Group, TVI and the UP-NIGS are acknowledged.

MECHANISM OF FORMATION OF PRESENT-DAY /EOLIAN SAND DUNES ALONG THE WESTERN COAST OF !LOCOS NORTE Fernando P. Siringan and Alex L. Pataray National Institute of Geological Sciences, University of the Philippines

A 2 to 4 kilometer wide by 40 kilometer long coastal stretch of western Ilocos Norte in Luzon Island, Philippines, is covered with generally linear, coast parallel .%olian dunes with superimposed parabolic dunes along the interdune areas. The linear dunes are almost symmetrical in cross-section and normally exhibit ridge heights varying from less than a meter to greater than 30 meters, ridge lengths ranging from several meters to kilometers, and ridge spacings of approximately 100 m to 300 m. Associated embryonic dunes along the present day backshore are a few centimeters to slightly greater than 2 meters in height with lengths from several centimeters to tens of meters and display a steep northeast facing slope and a much gentler southwest facing slope. These dunes consist mainly of moderately well-sorted to well-sorted medium- to fine-grained sand resting ontop of foreshore deposits. The embryonic dunes, based on their morphology and internal architecture, meteorological and field data, are longitudinal dunes. Dune formation and growth appears to be active during dry months when strong northerly winds continuously blow through the area. Nucleation occurs on the wind shadow of debris or vegetation forming sand mounds elongated along the wind direction. These mounds grow through vertical accretion and lateral extension in all directions; the highest lateral growth is exhibited by the downwind side. With the aid of vegetation at the crest serving as anchor, the windward side is able to slowly migrate upwind. A growing mound may coalesce with other mounds in all directions but alongwind amalgamation is favored. Some of the smaller mounds alongside can be eroded. Further growth of the dune occurs as new sprouts of vegetation allow more sand to be trapped. During the rainy season some of the dunes are destroyed or degraded. Bigger dunes, those anchored on vegetation and are farther removed from the shoreline have the best preservation potential. Coastal progradation provides new space for dune formation and removes the relatively older growing dunes from the reach of storm waves. Once a continuous line of still relatively low but already interconnected dunes is formed, it becomes more resistant to wipe-out during storms or tsunamis and is able to grow further. Repetition of this cycle may have produced the dune field along the west coast of !locos Norte.

GEOMORPHOLOGICAL AND SEDIMENTOLOGICAL INDICATORS OF NEARSHORE AND OFFSHORE SEDIMENT DISPERSAL IN LINGAYEN GULF, PHILIPPINES F. P. Siringan and Z. R. P. Mateo National Institute of Geological Sciences

Predominant nearshore and offshore dispersal patterns in Lingayen Gulf are established using coastal geomorphic features, changes in shoreline position and bathymetry, granulometry of surface sediments, satellite images and regional and local meteorological data. Coastal geomrphic features and changes in shoreline position are derived from topographic and nautical maps, Landsat and Synthetic Aperture Radar (SAR) images. East of Lingayen Gulf, the assymetry of Bauang and Aringay Deltas and the southward growth of Damortis Spit indicate the predominance of southward Iongshore currents along this segment of the coastline. The extensive carbonate development along the western sector of Lingayen Gulf implies possible absence of northward transport of sediment from Agno River distributaries. Along the bayhead coast geomorphological elements such as the asymmetry of deltas and mouthbar growth indicate the net transport direction of sediments and possible occurence of several diverging and shifting longshore current cells. The 1991 SAR image implies a southwestward longshore current direction from Bued-Patalan River mouth that veers to a westerly direction towards Agno-Lingayen and Labrador River Deltas. However, shifting of transport direction off Agno-Labrador Delta is indicated by the periodic changes in the orientation of mouthbar spits. Convergence of the two oppositely directed Iongshore currents within the vicinity of Agno-Lingayen River mouth results in a north-northeasterly offshore directed plume and the development of a shoreperpendicular submarine bar at the center of the bayhead coast. Other similar offshore features are associated with deposition of rive borne sediments.

MULTIPLE APPROACHES TO COASTAL SEDIMENT TRANSPORT STUDIES, CAGAYAN DE ORO, PHILIPPINES Fernando P. Siringan, Rose D. Berdin, Julius S. Doruelo, Caroline Marie B. Jaraula, Zenon Richard P. Mateo, John B. T. Ong and Maria Yvainne. Y. Yacat National Institute of Geological Sciences, University of the Philippines

Sediment transport and dispersal patterns along Macajalar Bay and the Cagayan de Oro River estuary were derived using grain size parameters, wave refraction modeling, and interpretation of geomorphic indicators from topographic maps and aerial photographs. Results show that sediments coming from the Iponan River generally follow a dominant westward transport along the western segment of the Iponan River Delta. At the opposite side of the delta, sediments are transported eastward all the way to the Cagayan de Oro River. Along the northern flank of the Tagaloan River Delta, sediment drift is towards the south while sediments originating from the south are moved northwards to the delta. Along the Cagayan de Oro River estuary, grain size parameter-derived sediment pathways generally show an upstream-directed transport along the thalweg probably due to salt water intrusion and a downstream course for sediments coming from shallower parts of the river. The results from the different methodologies are in good agreement and when combined yield a better understanding of coastal sediment transport; net and seasonal variations in transport pathways can be established, thus environmental monitoring and predictions can be enhanced.

PREDICTING CONTAMINANT DISPERSAL IN MANILA BAY USING GEOMORPHOLOGICAL-BASED PARAMETERS Fernando P. Siringan and Cherry L. Ringor National Institute of Geological Sciences, University of the Philippines

Sediment dispersal, nearshore and offshore, in Manila Bay is established using coastal geomorphological features, changes in shoreline and bathymetry, and meteorological data. Manila Bay's coastal geomorphology is examined using Synthetic Aperture Radar (SAR) images and 1:50,000 scale maps. The predominant offshore sediment dispersal pathways and lateral variations in sedimentation rates in Manila Bay, specifically off Cavite, Manila and in the Pampanga Bay, are determined using mainly sediment distribution patterns and water depth changes within the period 1901 to 1950. Highresolution reflection seismic profiles and piston cores provided information on longerterm sedimentation. Manila Bay is fringed by a variety of coastal subenvironments, due to variations in the balance of fluvial, wave, and tidal processes. Along the northern coast, the broad fluvial-tidal delta plain indicates the relative importance of tides while along the eastern coast, the predominantly sandy strandplains, spits, and wave-dominated deltas attest to the importance of waves. Very gentle coastal gradients amplify tidal processes along the northern coast. Waves from South China Sea intruding into Manila Bay are the primary cause of wave-dominance along the eastern coast. The predominant nearshore sediment drift along the eastern coast is to the northeast, along Manila to Pampanga is to the northwest, and along Bataan is to the north. Wave refraction and eddy formation at the tip of the Cavite Spit result in a southwest sediment drift along the Zapote-Kawit coast. The flared and relatively stable river mouths fronting the northern fluvial-tidal delta plain indicate the greater importance of onshore-offshore sediment transport along this segment of the coast. Off the eastern portions of Manila Bay, waters move in a general northerly direction. Major sediment sinks occur north of the Cavite Spit and west northwest of the Pasig River mouth. Here, sedimentation rates can be as high as 9 cm/y. Variation in the magnitude of shallowing occurs across the three study areas; Pampanga Bay shows the least amount of shallowing while the Pasig River area shows the greatest. The apparent low rates of sedimentation in Pampanga Bay could be due to high subsidence rates in this part of Manila Bay. A general increase of sedimentation rate in the offshore direction is also indicated by the bathymetric changes. This trend implies low retention of sediments near the coast, which might be due primarily to a relative sea level rise in the bay. The seismic lines indicate that the relatively high sedimentation rates along the deeper central portions are not a recent trend. However, this long-term trend is probably controlled by the bay's morphology rather than sea level fluctuations.

ALPHA-SPECTROMETRIC 23Â°TH/234 U DATES OF PLEISTOCENE CORALS FROM THE PHILIPPINE ISLANDS AND THEIR TECTONIC IMPLICATIONS 'Akio Omura, 2Toshio Kawana, 3Yasuo Maeda, and 3 Fernando Siringan 'Department of-Earth Sciences, Faculty of Science, Kanazawa University, Japan Laboratory of Geography, College of Education, University of the Ryukyus, Japan 3 National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines 2

Alpha-spectrometric U-series dates are reported for Pleistocene corals collected from reefal limestones at some localities (e.g. Punta Cruz in Bohol Island, Doljo Point, Biking, and San Isidro in Panglao Island, Bohol; Tuturingen Point in Palawan) in the Philippine Islands. Corals dated in this study can be assigned to two oxygen isotope stages, stages 5e (ca. 80 to 130 ka) and stage 7 (ca. 190 to 280 ka). Some dates are, however, not reliable because they are thought not to have been preserved as a closed system for U and Th isotopes through their diagenetic history. According to the critical evaluation of reliability of the U and Th isotopic composition, only three dates which are correlative to the stage 5e, are fully reliable, although reefal limestones of stages 5a and 7 seem to be distributed in the Philippine Islands. The paleo-shoreline at the isotope stage 5e is geomorphologically estimated to be 6 to 7 m high at Bohol and Panglao Islands, which is almost the same as that in Palawan Island. Such an elevation of paleo-sea level at the stage 5e does not differ so much as those in tectonically stable areas since the last interglacial maximum, such as Bahamas, Bermuda, and Oahu Islands, western Australia and Jamaica, among others. This fact suggests that our study areas have not been uplifted, even though the Philippine Islands are situated in a tectonically active region.

RECORDS OF HOLOCENE HIGH SEA-LEVEL STANDS AND LOCAL TECTONIC MOVEMENTS IN THE PHILIPPINE ISLANDS Yasuo Maedal, Fernando P. Siringanl, Miguel Cano 2 , Akio Omura 3 3 andKichSsk 'National Institute of Geological Sciences of Science and Mathematics Education Development University of the Philippines, Quezon City, Philippines 1101 3 Department of Earth Sciences, Faculty of Science, Kanazawa University, Japan 2 lnstitute

Emerged tidal notches, shore platforms, coral reefs and beach ridges along the coasts of the Philippine Islands are used to estimate higher than present relative paleosea levels of possible Holocene age. Estimates of paleo sea levels range from 0.1 to 3.9 m above the present mean sea level. Available radiocarbon age controls for paleosea levels range from approximately 8 kyrs BP to 4 kyrs BP. Adjacent coastal areas show different paleosea level histories, both in the level and number of relative stillstands. Local tectonics appear to explain these variations best. However, the widespread occurrence of higher than-present Holocene paleo sea levels in the Philippine Islands is also consistent with the hydro-isostasy driven mid-Holocene emergence in the region. Greater elevation and number of recorded stillstands along the north west coast of Luzon is probably due to the more compressional regime of the Philippine Fault in this region. Lower elevations in other regions possibly result from purer strike-slip motions. In the islands of Palawan, paleosea level data indicate east northeast tilting of northern Palawan, and of southern Palawan to the south southwest. Tilting could be due to relative movements along a local fault and or to differential uplift caused by hydoisostasy.

LITHOFACIES AND ARCHITECTURE OF LATE QUATERNARY DE-t-ul IS AND PULSES OF BASE LEVEL CHANGES IN THE CAGAYAN DE ORO AREA, NORTHERN MINDANAO Fernando P. Siringan, Rose D. Berdin, Caroline Marie B. Jaraula, Zenon Richard P. Mateo, Teodorico Sandoval and Marilou Justina M. Villanoy National Institute of Geological Sciences, University of the Philippines

Not much work has been done in the Philippines addressing base level changes during the Quaternary. The occurrence of well-defined terraces and the relative abundance and continuity of outcrops in Cagayan de Oro provide an opportunity to contribute in filling-in this knowledge gap. Lithofacies mapping and logging of late Quaternary deposits in the Cagayan de Oro area were conducted. Digital 3-dimensional rendering of the topography enhanced the identification, delineation and correlation of terraces within the study area. The results of this study indicate that high amplitude base level changes occurred during the late Quaternary in the Cagayan de Oro area. Relative to global eustatic sea level changes, net uplift in the area is at least 170 m. Our mapping indicates that the stratigraphic unit referred to as Cagayan Terrace Gravels or Cagayan Terrace Deposits by previous workers consists of two sequences. The older sequence, at least 180 m thick, overlies the lndahag Limestone. It is composed of Gilbert-type marine deltas deposited during an overall lowering of base level thereby forming the series of terraces at elevations 170 m, 140 m, 120 m, 100 m, 80 m and 20 m. The deltas are comprised mainly by steeply-dipping foresets (maximum of 20 Â°) of conglomerates and sandstones which grade into fine sandstone and mudstone towards the basin. These downstepping deltas are capped by very thickly bedded, clast-supported fluvial conglomerates. The younger sequence, which is at least 60 m thick, unconformably overlies the delta deposits. This package consists of at least 9 shallowing-up cycles. Each cycle, anywhere from 2 m to10 m thick, is typically characterized by organic-rich mudstones grading-up to intensely bioturbated, typically medium- to thick-bedded, well-sorted medium- to coarse-grained sandstones with pebbly layers and occasional interlaminations of heavy minerals. Wave and current ripples and cross-bedding (e.g. swaley and herring-bone) and Ophiomorpha burrows are very common in the sandstone layers. The mudstone-dominated portions of the cycles are interpreted as lower shoreface deposits; the sand-dominated portions are upper shoreface deposits. Each cycle represents a progradation, separated by episodes of base level rise accompanied by a backstepping of the shoreline. Terraces at elevations of 60 m, 40 m and 20 m, within the younger package, are delineated with the aid of the 3-D rendered topography. The terraces at 40 m and 20 m are capped by clast-supported pebbly conglomerates which are interpreted as fluvial deposits. These terraces were formed by a subsequent fall of base level.

EMPLACEMENT OF LARGE-VOLUME IGNIMBRITES: AN EXAMPLE FROM LAGUNA CALDERA Sandra G. Catane* and Ma. Carmencita B. Arpa Philippine Institute of Volcanology and Seismology (PHIVOLCS) C.P. Garcia Ave., UP Campus, Diliman, Quezon City

Detailed stratigraphic studies in Metro Manila reveal the existence of several pyroclastic flow units related to large-scale calderagenic eruptions. At least two major events can be discerned: a mixed pumice-scoria flow deposit which was emplaced at about 47,000 ybp and a younger fine-grained pumice flow unit with an age of about 27,000 ybp. The older unit, the Binangonan scoria flow, occurs mainly in the Rizal Province, particularly in areas of Binangonan, Teresa, and Pililia and distributes within a NW-NE azimuth from Laguna de Bay. Correlative units are found in Diliman, Marikina, Caloocan and may reach as far as Bustos, Bulacan, about 70 km from the postulated source. The younger pyroclastic unit, the Metro Manila ignimbrite, extends laterally towards the east, in the general direction of Quezon City, Mandaluyong, Pasig, Makati and Taytay, Rizal. Isopach and isopleth maps of both these pyroclastic flow deposits suggest that their probable source vent(s) may be found within the Laguna de Bay.

*Present address: National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City

ISOTOPIC STUDY OF BASEMENT ROCKS FROM SHATSKY RISE, NORTHWESTERN PACIFIC OCEAN Marissa L G Tejada, 2J J Mahoney, 3W W Sager and the Shipboard Scientific Party (Cruise TNO37, RN Thompson) 1

7 NIGS,

University of the Philippines, Diliman, Quezon City, 1101 Philippines University of Hawaii, Honolulu, HI, 96822 3Department of Oceanography, Texas A&M University, College Station, TX 77843-3146 2 SOEST,

Dredging of the Late Jurassic to Early Cretaceous Shatsky Rise in the northwest Pacific Ocean recovered basement rocks from two of the three large volcanic edifices that make up the rise and from seamounts that flank them. The rocks dredged from the Central High and South High contrast with those from the seamounts in their Nd, Sr, and Pb isotopic characteristics; also, samples from the seamounts are generally vesicular and aphyric; those from the highs are medium- to fine-grained-phyric, mostly non-vesicular, and dense. Isotopic compositions of the basement rocks of the main highs generally fall in a range overlapping normal Pacific MORB and "plume-type" compositions roughly similar to those seen in the Early to Mid-Cretaceous Ontong Java Plateau in the southwestern ospb/204 c j 17.60Pacific: 87Sr/86Sr)t = 0.70269-0.70315; eNd(t) = +9.7 to +5.9; pb,t = These results are 18.17; (20 p b /204p j t 15.45-15.50; and (208pbfioa pbsj t 37.66-37.97. consistent with geophysical data suggesting the role of a plume in the origin of Shatsky Rise (e. g., Sager and Han, Nature 364, 610, 1993; Sager, IUGG XXI, A465, 1995). The most MORB-like sample is from the South High, which represents the first (or presumed plume-head) phase of eruption, coinciding with the initiation or capture of a ridge-ridge-ridge triple junction. The isotopically least MORB-like sample is from Earthwatch Seamount on the North Arm of the rise. Accepting an age progression across the rise from south to north based on magnetic lineations, there appears to be a contrast in composition between the earlier (-145-140 Ma) volcanism which formed the main highs and the later (-130 Ma), less voluminous volcanism which formed the northern extension of Shatsky Rise. The presence of much more MORB-like isotopic compositions among the Shatsky Rise lavas, relative to Ontong Java Plateau basalts (e.g. Tejada et al., J. Petrol. 37, 361, 1996), may reflect the emplacement of the rise at a triple junction, together with a much smaller and/or heterogeneous plume. A sample from Cooperation Seamount, between the Central High and the South High, +7.6; cospb/204pbâ&#x20AC;˘ t = 18.99; has isotopic compositions ((87Sr/86Sr)t = 0.70260; ONd(t) = j co7pb/204p b.j t = 15.54; and (208pbpoapbsj t = 38.66 suggesting the influence of a HIMUtype source. Although gravity anomalies suggest that this seamount formed near the time of emplacement of Shatsky, it is not clear whether this seamount is related to the rise.

ISOTOPIC STUDY OF BASEMENT ROCKS FROM SHATSKY RISE, NORTHWESTERN PACIFIC OCEAN 1 Marissa

L G Tejada, 2J J Mahoney, 2W W Sager and the Shipboard Scientific Party (Cruise TNO37, RN Thompson) NIGS, University of the Philippines, Diliman, Quezon City, 1101 Philippines University of Hawaii, Honolulu, HI, 96822 3 Department of Oceanography, Texas A&M University, College Station, 7X 77843-3146 I

2 SOEST,

Dredging of the Late Jurassic to Early Cretaceous Shatsky Rise in the northwest Pacific Ocean recovered basement rocks from two of the three large volcanic edifices that make up the rise and from seamounts that flank them. The rocks dredged from the Central High and South High contrast with those from the seamounts in their Nd, Sr, and Pb isotopic characteristics; also, samples from the seamounts are generally vesicular and aphyric; those from the highs are medium- to fine-grained-phyric, mostly non-vesicular, and dense. Isotopic compositions of the basement rocks of the main highs generally fall in a range overlapping normal Pacific MORB and "plume-type" compositions roughly similar to those seen in the Early to Mid-Cretaceous Ontong Java Plateau in the southwestern Pacific: (87Sr/86Sr)t = 0.70269-0.70315; ENd(t) = +9.7 to +5.9; cos p bfioap jt = 17.60= 18.17; (nip bfioa pb) t = 15.45-15.50; and (208pbem pbsj t 37.66-37.97. These results are consistent with geophysical data suggesting the role of a plume in the origin of Shatsky Rise (e. g., Sager and Han, Nature 364, 610, 1993; Sager, IUGG XXI, A465, 1995). The most MORB-like sample is from the South High, which represents the first (or presumed plume-head) phase of eruption, coinciding with the initiation or capture of a ridge-ridge-ridge triple junction. The isotopically least MORB-like sample is from Earthwatch Seamount on the North Arm of the rise. Accepting an age progression across the rise from south to north based on magnetic lineations, there appears to be a contrast in composition between the earlier (-145-140 Ma) volcanism which formed the main highs and the later (-130 Ma), less voluminous volcanism which formed the northern extension of Shatsky Rise. The presence of much more MORB-like isotopic compositions among the Shatsky Rise lavas, relative to Ontong Java Plateau basalts (e.g. Tejada et al., J. Petrol. 37, 361, 1996), may reflect the emplacement of the rise at a triple junction, together with a much smaller and/or heterogeneous plume. A sample from Cooperation Seamount, between the Central High and the South High, has isotopic compositions (( 87Sr/86Sr)t = 0.70260; â?&#x2018; Nd(t) = +7.6; cospb/204pbst) 18.99; (207 p b/204 p b . t j 15.54; and (208p b/204 p jt 38.66 suggesting the influence of a HIMUtype source. Although gravity anomalies suggest that this seamount formed near the time of emplacement of Shatsky, it is not clear whether this seamount is related to the rise.

REVISITING THE PYROCLASTIC FLOW DEPOSITS OF THE LAGUNA FORMATION AT BAGONGBAYAN, TERESA, RIZAL Eligio C. Obille Jr. National Institute of Geological Sciences, University of the Philippines

A continuous exposure of the pyroclastic flow deposits of the Laguna Formation at Sitio Canlibot and Sitio Cubangbaan, Bagongbayan, Teresa, Rizal offers an excellent crosssection revealing vertical changes within. From bottom to top, the 15-m exposure is comprised of 1) an ignimbrite dominated by pumice, overlain by 2) a mixed pumice transition zone whose distinctive feature is the presence of banded or streaky pumice, which in turn is overlain by 3) an upper portion dominated by scoria. There is a gradual change in color from pinkish pale-gray at the base to pinkish or reddish brown at the top. The deposits are characteristically poorly sorted and massive. Welding is present only in the topmost portion, where the scoria-rich ignimbrite displays columnar jointing. No deposits of a different origin interrupt the sequence. The scoria-rich ignimbrite and pumice-rich ignimbrite contain respectively scoria and pumice that are crystal-poor. This may indicate that crystallization had barely begun when it was interrupted by an eruption. Phenocrysts in the pumice and scoria are mostly plagioclase crystals. Two plagioclase populations occur in the pumice and scoria as well as in the matrix: 1) clear and elongate plagioclase crystals which are almost always resorbed; and 2) smaller plagioclase crystals with euhedral outlines. Lower horizons of the pumice-rich ignimbrite contain pumice clasts with few plagioclase crystals or none at all. Succeeding tephra contain pumice fragments with more plagioclase crystals, suggesting eruption from a zoned magma chamber. A zoned magma chamber could also explain the successive eruption of juvenile material that is continuously changing color, and the presence of banded or streaky pumice. The abundance of banded pumice in the deposits could be the result of mingling of magma from different levels during eruption.