The steep, narrow, northern shelf of Puerto Rico is a mixed-sediment, storm-dominated environment. Siliciclastic muddy silt and sand is introduced directly onto the shelf by river floods, and accumulates off river mouths in lens-shaped deposits up to 35 m thick. Clean biogenic shelf carbonate sand and gravel accumulates slowly on shelf areas away from river mouths. Changing river mouth locations control which portion of the shelf is actively receiving siliciclastic sediment at a given time. In addition, switching of river mouth location results in vertical stacking of shelf-derived and river-derived sediments.

Frequent river floods introduce siliciclastic sediment; tropical cyclones and swell from extratropical storms quickly rework sediment. Two end-member type storm units are identified. Storm units in cohesive sediments are thick, have basal lags of “fresh-looking” skeletal carbonates (or river gravel or coarse sand near river mouths), have high mud content, high degrees of bioturbation, and show less degrees of amalgamation. Storm units in noncohesive sediments typically have basal lags composed of skeletal carbonates and coralline algae (or river gravels near rivers), and amalgamation is common. Bioturbation can range from none to intense. Carbonate grain physical condition is typically much less degraded in clean than in cohesive sediments. Storm units on the outer shelf in noncohesive sediments often consist of thick rhodolith basal lags grading upward to a massive rhodolith/Halimeda coarse sand layer that is rarely amalgamated and rarely contains more than a few percent mud. Storm units are not laterally extensive and often could not be correlated in cores even over short distances.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah