--> Abstract: Surface Expression of Cyclicity and Biofacies Transitions in the Middle Tertiary Lares and Montebello Limestones, Northern Puerto Rico, by J. J. Galluzzo and W. R. Ramirez; #90937 (1998)

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Abstract: Surface Expression of Cyclicity and Biofacies Transitions in the Middle Tertiary Lares and Montebello Limestones, Northern Puerto Rico

GALLUZZO, JAMES J. and WILSON R. RAMIREZ, Tulane University

Across northern Puerto Rico, the Upper Oligocene-Lower Miocene Lares and Montebello Limestones form a confined aquifer which makes up an important part of the island's North Coast Aquifer System. These rocks represent carbonate sediments accumulated on a ramp in the central part of the North Coast Tertiary Basin. The recent construction of a major north-south thoroughfare (PR-10) across north-central Puerto Rico has opened up a highly vegetated, karsted area where previously the geology was poorly exposed and existing outcrops were inaccessible. The new highway exposes a spectacular 9-kilometer-long dip section through the Lares and Montebello Limestones and reveals for the first time details of the geology in the area of maximum thickness of the Middle Tertiary carbonate platform.

Field investigation has identified cyclicity in the distribution patterns of coral-dominated, foraminifera-dominated, and red algae/rhodolite-dominated units within the Montebello Limestone. Observed biofacies patterns most likely were in response to tectonic activity and/or high-order sea-level fluctuations across the gently-dipping carbonate platform. The depositional cycles seen at the surface can be correlated with subsurface data from downdip cores. These new surface exposures provide an excellent opportunity to study the characteristic lithological heterogeneities of these aquifer limestones, particularly in terms of lithofacies controls on porosity and permeability.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah