The El Cajon Mountain Rock Avalanche and Debris Flow

The El Cajon Mountain Rock Avalanche and Debris Flow San Diego County, California

Mike Hart, mwHart40@gmail.com he El Cajon Mountain Rock Avalanche and resulting debris flow (EMRA) is located 6.5 miles east of the community of Lakeside and approximately 1,000 feet west of El T Diego River (Figure 1). Hungr and Evans (2004) describe similar failure mechanisms for other large rock avalanche/debris flows in the Canadian Rockies. The toe of the western two-thirds of Capitan Reservoir in San Diego County, California. The EMRA the EMRA has a steep, somewhat bulbous front that is locally occurred on the east side of a finger-like ridge that projects near vertical as a result of erosion by the San Diego River. The southward from the peak of El Cajon Mountain (el. 3,675 ft.) toe of the eastern third has been partially modified and buried composed of Corte Madera Monzogranite (Todd, 2004). Interby modern debris flows resulting from re-establishment of the pretation of topographic maps and aerial images indicates that creek bed that existed prior to the avalanche and ensuing runa large slab of granitic rock measuring approximately 3,400 out onto the valley floor. feet in width detached from the steep flank of El Cajon MounEvidence for a rock avalanche/debris flow origin for the tain along a master joint system. The height of the resulting debris lobe instead of a more mundane alluvial fan resulting scarp is nearly 1,500 feet. As the rapidly moving avalanche primarily from fluvial action is as follows: impacted the adjacent valley it incorporated and liquefied nearly 1. The ridge on which the avalanche occurred is strikingly saturated valley colluvium and alluvium into the debris to enable asymmetrical with the east side (the scarp area) being relaa nearly one-mile runout into the San Diego River channel. The tively planar and much steeper than the west side of the resulting 4,100-foot-wide debris lobe nearly dammed the San ridge (Figure 1).

Figure 1.

El Cajon Mountain Rock Avalanche and Debris Flow (high-angle oblique image looking north). Avalanche likely initiated along a northeast trending joint system. Top of headscarp shown by yellow dotted line. The avalanche transitioned to a debris flow by incorporating wet colluvial and alluvial soils from the canyon slopes and valley bottom. A smaller rock avalanche deposit (Qra) occurs 1000 ft. west of the EMRA. Blue Star: location of rock quarry; Yellow Star: location of Figure 3.

LEGEND

Qra: Rock Avalanche deposit Qdf: El Cajon Mountain Rock Avalanche and debris flow (EMRA) Kcm: Corte Madera Monzogranite Klb: Tonalite of Los Bancos

2. The presence of a master joint system at the upper edge of the steep scarp. The joint system provides a low strength discontinuity in otherwise very strong rock. This is an essential condition for the slab failure. 3. Alluvial fans, even when made up of a combination of fluvial and debris flow action, generally have very low gradients of 2 to 6 degrees and a concave up profile.

Debris flow deposits typically have steeper gradients, have flat profiles, and usually a steep rounded front. The

EMRA has an average gradient of 10 degrees. 4. Most of the boulders exposed on the surface of the debris flow are huge, some exceeding 20 feet in the longest dimension. The catchment area would seem too small to support a fluvial system that could transport such large “clasts.”

One feature of the EMRA that was at first puzzling was the generally round to sub-round appearance of the large boulders exposed on the surface of the debris lobe. It seemed that a rock avalanche origin for the deposit would demand that the large rocks making up the debris lobe should be more angular. This could be explained by rounding in place by exfoliation that is occurring on many of the boulders (Figure 2). Such weathering is even occurring in the subsurface where a road cut for a transmission line has exposed completely grussified boulders.

The age of the EMRA is suggested by an approximately 2.5-foot-thick soil profile consisting of a well-developed A horizon underlain by a stone zone containing numerous 1.1- to 1.8-cm-diameter pisoliths or ironstone concretions overlying a weak, slightly argillic B horizon (Figure 3). Pisoliths in the soil are interpreted to have formed in San Diego County under coniferous forest conditions in the Pleistocene when the climate was cool and wet.

Acknowledgements

Thanks to Tom Rockwell, professor of Geology, San Diego State University, for information on the environment required for formation of pisoliths and to Bill Elliott; a classmate, and colleague who many years ago suggested the possibility of landsliding on El Cajon Mountain.

Figure 2. 8-foot-diameter (2.4 m) boulder in debris lobe undergoing exfoliation

Figure 3. Soil profile in road cut for power line road at approximate midpoint of debris lobe showing A-Horizon, stone zone with pisoliths at base (arrows) underlain by a weak B-Horizon

References

Hungr, 0., Evans, S.G., 2004, Entrainment of debris in rock avalanches—an analysis of long run-out mechanisms: Geological Society of America Bulletin, v. 116, p. 1240-1250. Todd, V.R., 2004, Preliminary geologic map of the El Cajon 30’ X 60’ quadrangle, southern California: U.S. Geological Survey Open-File Report 2004- 1361, scale 1:100,000.