Water Resources of the Caribbean
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Geology and Hydrogeology of the Caribbean Islands Aquifer System of the Commonwealth of Puerto Rico and the U.S. Virgin Islands ABSTRACTPoorly 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 Rico's South Coast ground-water province, more than 1,000 meters of clastic 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 eustasy. 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 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 Rico's 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. Deep well data indicates that it is possible to vertically separate and group a highly complex and irregular-bedded detrital sequence that underlies distal parts of the fan-delta plain into discrete water-bearing units if correlated with 30- to 40-meter thick, eustatically-controlled depositional cycles. Lithofacies maps show that greatest hydraulic conductivity within the fan-delta plain is generally associated with proximal fan and midfan areas. Distal and interfan areas are least permeable. Alluvial valley aquifers located in the western part of the South Coast ground-water province are important local sources of water supply and appear to contain some of the same physical and hydraulic characteristics as the South Coast aquifer. Older sedimentary rocks within the basin are poor aquifers; conglomeratic beds are well-cemented, and carbonate beds do not contain well-developed solution features, except locally where the beds are overlain by alluvium. Ground-water occurs under unconfined conditions in proximal and midfan areas. Confined conditions within deeper parts of the system and in interfan and some midfan areas are created largely by the intercalated nature of discontinuous fine-grained beds that retard vertical ground-water movement.
The development of water resources in southern Puerto Rico has modified the hydrologic system of the South Coast aquifer considerably. Under predevelopment conditions, the South Coast aquifer was recharged in the unconfined, proximal fan and some midfan areas by infrequent rainfall and seepage from streams near the fan apex. Discharge occurred as seabed seepage, baseflow discharge along the lower coastal reach of streams, seepage to coastal wetlands, or evapotranspiration in areas underlain by a shallow water table. Under development conditions, seepage from irrigation canals and areal recharge from furrow irrigation represented a principal mechanism for recharge to the aquifer. Increased ground-water withdrawals in the 1960's and 1970's resulted in declines in the water table to below sea level in some places and intrusion of salt water into the aquifer. By the middle 1980's, a reduction in ground-water withdrawals and a shift from furrow irrigation to drip-irrigation techniques resulted in the recovery of water levels. Under present-day (1986) conditions, regional ground-water flow is coastward but with local movement to some well fields. In addition to the discharge mechanisms described above, ground-water discharges also to coastal canals.
The North Coast limestone aquifer system consists of limestone, lesser amounts of dolomite, and minor clastic detritus of Oligocene to Pliocene age that form an unconfined upper aquifer and a confined lower aquifer; these aquifers are separated by a clay, mudstone, and marl confining unit. Topographic relief and incision of carbonate coastal plain rocks by streams are the principal factors controlling the direction of ground-water flow. The North Coast limestone aquifer system is recharged principally by precipitation that enters the upper and lower aquifers where they crop out. Regional groundwater movement from the upper aquifer is to the major rivers, wells, coastal wetlands, coastal, nearshore, and offshore springs, or as seabed seepage. Regional discharge from the lower aquifer is to the major rivers along its unconfined parts or where the confining unit has been breached by streams. Discharge from the lower aquifer also occurs in the San Juan area where the Mucarabones Sand provides an avenue for diffuse upward ground-water flow. Transmissivity within the upper limestone aquifer appears to be largely regulated by the thickness of the freshwater lens. The lens is thickest and transmissivity is greatest in interstream areas that lie in a zone that closely corresponds to the landwardmost extent of the underlying saltwater wedge. Hydraulic conductivity of the upper aquifer generally increases in a coastward direction and reflects lithologic control, karstification in the upper 30 to 100 meters of the section, and enhanced permeability in a zone of freshwater and saltwater mixing. Transmissivity of the lower aquifer is an order of magnitude smaller than that of the upper aquifer; highest transmissivities in the lower aquifer largely correspond to a coarse grainstone-packstone and coral-patch-reef depositional facies contained within the outcropping parts of the Montebello Limestone Member and its subsurface equivalents. Porosity within the North Coast limestone aquifer system is high in grainstone-packstones and low in wackestone and marl. Dolomitized zones and moldic grainstone-packstone strata are the most porous carbonate rocks, but occur in thin beds that usually are only a few meters thick. Processes of karstification that include the development of caverous zones and large vugs, and dissolution along possible regional fracture sets has enhanced permeability within the upper part of the aquifer system. Stratigraphic and lithologic control play an important role controlling permeability within the lower part of the system.
The Kingshill aquifer of St. Croix, in large part, is composed of deepwater limestone that contains only microscopic pores and is poorly permeable; however, the upper part of the aquifer, a shallow-water skeletal and reef limestone, is fairly permeable, but restricted in areal extent. Permeability within these uppermost beds of the aquifer has been enhanced by meteoric leaching, dissolution within a mixing zone of saltwater and fresh water, and dolomitization. However, most large-yield wells completed in the Kingshill aquifer are also screened in alluvium that overlies or infills incised channels. The alluvial deposits serve as a temporary storage zone for rainfall, runoff, and ground water slowly entering the Kingshill aquifer.
CONTENTS
Forward Abstract Introduction Physiographic setting Puerto Rico Puerto Rico's offshore islands Isla de Vieques Isla de Culebra Isla Desecheo Isla Mona U.S. Virgin Islands Previous investigations Puerto Rico Puerto Rico's offshore islands U.S. Virgin Islands Method of investigation Geology of the South Coast ground-water province of Puerto Rico Stratigraphy Juana Díaz Formation Unnamed pelagic carbonate rocks Ponce Limestone Subsurface stratigraphic relations Pleistocene to Holocene fan-delta and alluvial deposits Configuration of basal Quaternary contact and thickness of deposits Lithofacies Vertical profile Sand and gravel percentage The South Coast Tertiary Basin: Controls on clastic deposition Structural features and evidence of tectonic movement in the South Coast ground-water province Changes in base level: Its record in the stratigraphic sequence Climate Geology of the West Coast ground-water province of Puerto Rico Geology of the East Coast ground-water province of Puerto Rico Geology of the Interior ground-water province of Puerto Rico Geology of the North Coast ground-water province of Puerto Rico Introduction Geologic setting of the North Coast Tertiary Basin Stratigraphy Methods of correlation Lithostratigraphic nomenclature San Sebastián Formation ("middle" to upper Oligocene) Outcrop Subsurface Lares Limestone (upper Oligocene to lower Miocene) Outcrop Subsurface Mucarabones Sand (upper Oligocene to lower Miocene) Outcrop Subsurface Cibao Formation (lower Miocene) Outcrop Subsurface Montebello Limestone Member Mudstone unit Río Indio/Quebradas Arenas Limestone Members Undifferentiated Cibao Formation Aguada (Los Puertos) Limestone (lower to middle Miocene) Outcrop Subsurface Aymamón Limestone (middle Miocene) Outcrop Subsurface Quebradillas Limestone (uppermost Miocene to Pliocene) Outcrop Subsurface Depositional history Introduction Middle to late Oligocene Late Oligocene to early Miocene Early Miocene Late early Miocene to early middle Miocene Middle Miocene Latest Miocene-Pliocene Effects of structural movements on deposition of upper Oligocene to lower Miocene rocks Sequence stratigraphy of Oligocene to middle Miocene rocks Introduction Sequence boundaries in northern Puerto Rico Geology of central St. Croix, U.S. Virgin Islands Introduction Stratigraphy and sedimentology of the central limestone plain region Jealousy Formation Lithology, facies, and depositional environment Structure and distribution Age Kingshill Limestone La Reine (lower) Member of the Kingshill Limestone Lithology, facies, and depositional environment Structure and distribution Age Mannings Bay (upper) Member of the Kingshill Limestone Lithology, facies, and depositional environment Structure and distribution Age Blessing Formation Lithology, facies, and depositional environment Structure and distribution Age Dolomitization and diagenesis Alluvial deposits Sedimentary and structural setting of the Kingshill Basin Hydrogeologic framework of the U.S. Caribbean Islands The South Coast ground-water province The South Coast aquifer The vertical sequence—major water-bearing units Hydraulic conductivity and the lateral continuity of water-bearing units Alluvial valley aquifers of the South Coast The Ponce-Juana Díaz aquifer The West Coast ground-water province Alluvial valley aquifers of the East Coast ground-water province Alluvial valley aquifers of the Interior ground-water province North Coast ground-water province: The North Coast limestone aquifer system Occurrence and movement of ground water Alluvial valley aquifers and local confining units Upper aquifer Lower aquifer Basal confining unit Distribution of transmissivity and hydraulic conductivity Controls on porosity and permeability in carbonate aquifers of northern Puerto Rico Stratigraphic control Lithologic and diagenetic control Upper aquifer Middle confining unit Lower aquifer Basal confining unit Fracture control U.S. Virgin Islands Distribution and types of porosity within the Kingshill aquifer Kingshill Limestone in the Central and Northern Basin Mannings Bay Member and the Blessing Formation along the southern coastline Dissolution and karsting Dolomitization Alluvial aquifers Weathered mantle-bedrock aquifer Vieques Island Summary References
PLATES PLATE 1. Geology, hydrogeology, and hydrology of the South Coast ground-water province between Ponce and Patillas, Puerto Rico
Maps showing: A. Geology of the South Coast B. Configuration of bedrock surface underlying fan-delta deposits of Quaternary age C. Thickness of Holocene to Pleistocene fan-delta and alluvial deposits D. Percentage of sand and gravel in the fan-delta deposits E. Interpretive structure and major lineaments and/or faults underlying the fan-delta plain F. Estimated potentiometric surface of the South Coast aquifer, February, 1968 G. Estimated potentiometric surface of the South Coast aquifer, March, 1986 H. Estimated thickness of freshwater lens contained within the South Coast aquifer, 1986–87 I. Distribution of hydraulic conductivity within the South Coast aquifer
PLATE 2. Geology and hydrogeology of the South Coast ground-water province between Cabo Rojo and Ponce, southwestern Puerto Rico
Maps showing: A. Geology of the South Coast ground-water province B. Configuration of bedrock surface underlying alluvial-valley and fan-delta sediments of Quaternary age C. Percentage of sand and gravel contained within alluvial-valley and fan-delta areas
PLATE 3. Cyclic depositional sequences within the southern fan-delta plain
PLATE 4. Geologic map and sections showing principal stratigraphic units and lithofacies of rocks of Oligocene to Pliocene age in the North Coastal limestone aquifer system
PLATE 5. Hydrogeology and hydrology of the North Coast limestone aquifer system
A. Section A–A' showing principal aquifers and confining units of the North Coast limestone aquifer system
Maps showing:
B. Estimated potentiometric surface of the upper aquifer showing average conditions between 1980 and 1990 C. Estimated predevelopment potentiometric surface of the lower aquifer D. Potentiometric surface of the lower aquifer in area of greatest development (Río Grande de Arecibo to Río Cibuco) showing average conditions during 1987 E. Estimated thickness of the freshwater lens within the upper aquifer F. Top of the middle confining unit G. Distribution of transmissivity in the upper aquifer H. Distribution of hydraulic conductivity in the upper aquifer I. Distribution of transmissivity in the lower aquifer
¹U.S. Geological Survey ²Department of Geology and Geophysics, University of New Orleans, 2000 Lakeshore Drive, New Orleans, LA 70148 (retired) ³Department of Geology, University of Puerto Rico, Mayaguüez, PR 00681
The citation for this report, in USGS format, is as follows: Renken, A., Ward, W.C., Gill, I.P., Gómez-Gómez, F., Rodriguez-Martínez, J., and others, Geology and Hydrogeology of the Caribbean Islands Aquifer System of the Commonwealth of Puerto Rico and the U.S. Virgin Islands: U.S. Geological Survey Professional Paper 1419, 139 p., 5 pls. |
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