ABSTRACT: A Structural and Depositional Framework for Fan Deltas in Southern Puerto Rico

Robert A. Renken

Puerto Rico's southern plain consists of six coalescing fan deltas of Holocene to Miocene age that are bordered to the north by the steep-faced mountains of the Cordillera Central. These fan deltas extend 70 km along the Caribbean Sea and form a narrow plain that ranges from 3 to 8 km in width. A narrow, transitional marine zone separates the subaerial part of the fan from the coast. Possibly due to their poor preservation or subsequent erosion, deeply buried marginal marine fan-delta deposits have not been identified in well core data. Subaerial fan-delta deposits are separated at outcrop and in the subsurface into a fine-grained facies of bedded silt sheet-flow deposits and a coarse-grained facies of boulder to free sand streamflow deposits.

The six fan deltas form an elongate depositional basin nearly parallel to the west-northwest structural trend of the great southern Puerto Rico fault zone. Subsurface mapping indicates that this fault zone, which contains numerous sinistral and high-angle cross faults, probably extends beneath the fan-delta plain Movement along these faults has resulted in pull-apart basin structures beneath the Salinas fan delta and a releasing bend structure beneath the Ponce-Capitanejo fan deltas. Percentage sand and gravel lithofacies maps show that the trunk streams concentrate coarse-grained material as a narrow proximal channel near the apex or as a midfan lobe. Bedded silt deposits dominate the distal and interfan areas. Thickening-upward sand and gravel beds and thinning-upward silt beds in t e distal fan subsurface areas indicate that several fan deltas are part of a coarsening-upward prograding sequence.

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