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 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.
Forward
Abstract
Introduction by Robert A. Renken
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 by Robert A. Renken
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 by W.C. Ward, R.A. Scharlach, and J.R. Hartley
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 by I.P. Gill, D.K. Hubbard, P.P. McLaughlin, and C.H. Moore
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 by Robert A. Renken, Fernando Gómez-Gómez, and Jesús Rodríguez-Martínez
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
Middle confining unit
Lower aquifer
Basal confining unit
Distribution of transmissivity and hydraulic conductivity
Controls on porosity and permeability in carbonate aquifers of northern Puerto Rico by W.C. Ward
Stratigraphic control
Lithologic and diagenetic control
Upper aquifer
Middle confining unit
Lower aquifer
Basal confining unit
Fracture control
U.S. Virgin Islands by Robert A. Renken
Distribution and types of porosity within the Kingshill aquifer by I.P. Gill, D.K. Hubbard, P.P. McLaughlin, and C.H. Moore
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 by Robert A. Renken
Weathered mantle-bedrock aquifer
Vieques Island
Summary
References
PLATE 1. Geology, hydrogeology, and hydrology of the South Coast ground-water province between Ponce and Patillas, Puerto Rico
Maps showing:
PLATE 2. Geology and hydrogeology of the South Coast ground-water province between Cabo Rojo and Ponce, southwestern Puerto Rico
Maps showing:
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
Maps showing:
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