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ABSTRACT: Depositional History of Oligocene-Miocene Carbonate Rocks of Northeastern Puerto Rico

Richard Scharlach

The apparent tectonic stability of the northern Puerto Rico platform during the late Oligocene and early Miocene allows for the depositional history of subsurface carbonate rocks of northeastern Puerto Rico to be related to major changes in eustatic sea level. During a late Oligocene north to south transgression of sea level, fluvial/deltaic to shallow marine terrigenous sediments (San Sebastian Formation) and, subsequently, open-ramp carbonates (Lares Limestone) accumulated in the central basin. Following a minor regression (third-order cycle?), a more extensive early Miocene(?) transgression resulted in deposition of deeper ramp carbonate mudstone and marl (Mudstone unit) in an apparent trough in the central basin, and open-ramp reefal carbonate (upper Lares) was deposi ed over a wider area of the basin. The San Sebastian Formation/Lares Limestone/Mudstone Unit sequence was most likely deposited during the second-order supercycle, TB1.

An early Miocene relative fall in sea level resulted in deposition of interfingering inner-ramp limestone and terrigenous sediments (Cibao Formation) and the development of subaerial crusts, especially in the upper part of the unit. During a sea level rise, terrigenous deposition decreased and gave way to inner- and middle-ramp carbonate sediments (Los Puertos Limestone). A middle Miocene highstand in sea level brought basin-wide deposition of open-ramp carbonate sediments (Aymamon Ls). The Cibao Formation/Los Puertos Limestone/Aymamom Limestone sequence may correspond to the second-order supercycle, TB2. During the late middle Miocene(?), the carbonate platform was exposed and extensively karsted, possibly in an event related to the sea level drop at the end of TB2.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990