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Impact of River Mouth Migration on Continental Margin Sedimentation

David M. Bush, Orrin H. Pilkey, Jr., Rafael W. Rodriguez

The migration of river mouths from one location to another on the high-energy, steep, narrow north shelf of Puerto Rico affects continental margin sedimentation in two major ways: (1) it results in vertical stacking of carbonate and terrigenous facies, and (2) it spreads sediment from small rivers over a large area. Additionally, it may explain the large number of submarine canyon tributaries that indent the shelf.

River mouth migration thus appears to be a central factor of sedimentation in this dynamic, storm-dominated environment. Terrigenous sediment is introduced by small rivers only during storm-induced floods. This facies is characterized by dark, muddy, fine sand, mixed with a small percentage of continuously produced biogenic shelf carbonates. The terrigenous contribution forms thick "pods" of dark sediment directly off river mouths. This sediment contrasts with the thin veneer of light-colored carbonate sediment that slowly accumulates on shelf area away from river influence.

In the Manati River pod, a small, extensively studied region of the north shelf, cores reveal an expected sediment sequence: coarse carbonate sand overlain by terrigenous material. Five kilometers away, in a separate sediment pod not presently adjacent to the active river, the inverse is true. In cores there, the lower unit is terrigenous sand and the upper unit is calcareous. This relationship is evidence of river mouth migration. The "relict pod" of river sediments has been gradually covered by biogenic shelf carbonates deposited since the Manati River mouth migrated to its present location.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.