Fernando Gómez-Gómez

Geology and hydrogeology of the Caribbean islands aquifer system of the Commonwealth of Puerto Rico and the U.S. Virgin Islands

Poorly lithified to unconsolidated carbonate and clastic sedimentary rocks of Tertiary (Oligocene to Pliocene) and Quaternary (Pleistocene to Holocene) age compose the South Coast aquifer and the North Coast limestone aquifer system of Puerto Rico; poorly lithified to unlithified carbonate rocks of late Tertiary (early Miocene to Pliocene) age make up the Kingshill aquifer of St. Croix, U.S. Virgin Islands. The South Coast aquifer, North Coast limestone aquifer system, and Kingshill aquifer are the most areally extensive and function as the major sources of ground water in the U.S. Caribbean Islands Regional Aquifer-System Analysis (CI-RASA) study area. In Puerto Ricos South Coast ground-water province, more than 1,000 meters of elastic and carbonate rocks of Oligocene to Pliocene age infill the South Coast Tertiary Basin. The pattern of lithofacies within this basin appears to have been controlled by changes in base level that were, at times, dominated by tectonic movement (uplift and subsidence), but were also influenced by eustiisy. Deposition of the 70-kilometer long and 3- to 8-kilometer wide fan-delta plain that covers much of the South Coast ground-water province occurred largely in response to glacially-induced changes in sea level and climate during the Quaternary period. Tectonic movement played a much less important role during the Quaternary. The North Coast ground-water province of Puerto Rico is underlain by a homoclinal coastal plain wedge of carbonate and siliciclastic rocks that infill the North Coast Tertiary Basin and thicken to more than 1,700 meters. A thin basal siliciclastic sequence of late Oligocene age is overlain by a thick section of mostly carbonate rocks of Oligocene to middle Miocene age. Globigerinid limestone of late Miocene to Pliocene age crops out and lies in the shallow subsurface areas of northwestern Puerto Rico. Oligocene to middle Miocene age rocks tentatively can be divided into five depositional sequences and associated systems tracts; these rocks record carbonate and minor siliciclastic deposition that occurred in response to changes in relative sea level. The Cibao Formation represents the most complex of these sequences and contains a varied facies of carbonate, mixed carbonate-siliciclastic, and siliciclastic rocks that reflect differential uplift, subsidence, and transgression of the sea. Uplift, graben formation, and gradual shallowing of the sea are reflected within the bathyal-dominated sedimentary facies of the Kingshill Limestone in St. Croix, U.S. Virgin Islands. Reef-tract limestone beds of Pliocene age were subject to exposure, resubmergence, and meteoric leaching of aragonitic skeletal debris; these beds contain patchy lenses of dolomite that are restricted to a small, structurally-controlled embayment. The South Coast aquifer, the principal water-bearing unit of Puerto Ricos South Coast ground-water province, consists of boulder- to silt-size detritus formed by large and small coalescing fan deltas of Pleistocene to Holocene age.

Structure and Depositional Patterns and Their Influence on the Hydraulic Conductivity of Fan-Deltas in Southern Puerto Rico

Subsurface data and results of surface geologic mapping indicate that areal variations in hydraulic conductivity of water-bearing strata in the semiarid alluvial plain of southern Puerto Rico may be controlled by geologic structure and depositional patterns. Six fan-deltas coalesce to form a narrow, 3- to 8-km-wide alluvial plain that extends eastward 70 km from Ponce to Patillas. These unconsolidated coastal alluvial fan deposits consist of a moderately thick (50 to 1,000 m) clastic sequence of Holocene to Miocene(?) age. The fan-deltas are bordered to the north by foothills and mountains of the Cordillera Central and to the south by the Caribbean Sea. Bedrock hills protrude through thinner parts of the alluvial plain, and northwest-trending strike-slip faults and northeast-trending cross faults of the great southern Puerto Rico fault zone extend beneath the alluvial cover.

Hydrologic, water-quality, and biological assessment of Laguna de las Salinas, Ponce, Puerto Rico, January 2003-September 2004

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