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.