18. The Mesa Roldรกn reef
Above the lower sedimentary unit there is an erosion surface, and, above it, a new sedimentary unit is present. The latter, also Messinian in age, but younger (around 5.5 million years) is again constructed from coral reefs along with some particulate carbonate sediments that are called oolites (lower figure). Here the reefs have only a small dimension, patches a few metres wide with a height of 1 or 2 metres, formed by corals of the genera Porites (Photo D) and well-developed micritic coatings of microbial origin (Photo E). These reefs grew surrounded by oolitic sediments. This type of oolitic sediment consist of small carbonate particles with a spherical shape and an internal structure of concentric laminations (Photo F). At present, ooliths, as the particles are known, are formed in shallow, agitated, tropical seas. The oolitic carbonates as much as the coral reefs testify that in the western part of the Mediterranean, on the southeast Iberian peninsula, during the Messinian around 5.5 million years ago, at the end of the Miocene, a tropical climate prevailed, similar to that now found in lower latitudes, closer to the equator.
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Since that period until the Present, the climate in the region has followed a general trend of cooling, although this tendency has suffered strong fluctuations, especially during the last 2 million years.
Ooliths
2m
UPPER SEDIMENTARY UNIT THE PATCH REEFS
Reef
Ooliths Breccias Previous Reef
D
Patch reefs and oolitic limestones appear in the upper unit.
E
F
Coral colonies of the genera Porites (vertical sticks), surrounded by micritic carbonate coatings (white hue), that constitute the patch reefs of the upper unit.
Micritic carbonate coatings encrusting the upper part of patch reefs at the same time as the coral colonies were periodically living.
Microscopic image of oolitic sediments. These particles, ooliths, constitute the bulk of sediments in the upper unit.