Late Quaternary Shift of Mixed Siliciclastic/Carbonate Environments Induced by Sea Level Fluctuations, Northern Part of the Southern Shelf Lagoon, Belize

Elmer C. Ferro, Andre W. Droxler, John B. Anderson

In the northern part of the Southern Shelf Lagoon, siliciclastic sands are deposited along the coastal area, marls infill the shelf lagoon, and pure carbonates are restricted to the offshore barrier reef platform.

High resolution seismic profiles show, in the sub-seafloor of the northern part of the Southern Shelf Lagoon, two prominent drainage systems of buried incised valleys, partitioned at the latitude of Dangriga. One system drained the area north of Dangriga into the partially buried incised canyon of the English Cay Channel. At the mouth of the channel, the seismic survey images a thick fan-like sedimentary body. It is characterized by stacked and prograding lobes, unconformably separated by an erosive surface at 112 m below sea level from an overlying thinner sedimentary package with low angle progradation.

During Last Glacial Maximum (18 k.y. ago), sea level dropped an estimated 120 m exposing the Southern Shelf Lagoon. A system of incised valleys drained the shelf lagoon north of Dangriga, at that time a siliciclastic fluvial plain, into a narrow basin west of Turneffe Island. Sediment bypassed the exposed and karstified barrier platform, deposited during a previous highstand, through the incised canyon of English Cay Channel. At the mouth of the channel, a lowstand shelf margin delta was deposited, similar in size to the modern highstand delta on which Belize City is located. In the ensuing transgression, the top of the lowstand delta experienced ravinement and the lower part of English Cay Channel was only partially infilled. As the rate of sea level rise slowed, incised valleys furt er upstream and in the Southern Shelf Lagoon became first infilled with fluvial to estuarine sediments and then buried under marine marls. Simultaneously, coral reefs reestablished themselves once sea level flooded the karstified barrier platform.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995