Source and Dispersal of Silt on Northern Gulf of Mexico Continental Shelf

Mark Peterson, Jim Mazzullo

The surficial sediment on the continental shelf of the northern Gulf of Mexico is characterized by abundant silty clay which was deposited during the late Pleistocene lowstand and reworked during and after the Holocene transgression. The purposes of this study were to determine the sources of the silt fraction in this surficial sediment by quartz grain roundness and surface texture analysis, and to determine the effects of modern shelf currents upon the distribution of silt.

Areal variations in quartz grain roundness and surface texture define six silt provinces on the northern Gulf of Mexico continental shelf. The Mississippi province is the largest province and stretches from the Chandeleur Islands to Matagorda Bay. It is characterized by a mixture of rounded grains that were derived from the sedimentary rocks of the Gulf coastal plain and the Mid-Continent, and angular, fractured grains that were derived from glacial deposits in the northern United States. The Rio Grande and Brazos-Colorado provinces were found on the northern and southern parts of the South Texas shelf, and the Mobile province is found on the central part of the MAFLA shelf. They are all characterized by a mixture of angular and subangular grains with fractures and crystalline nodes t at were derived from crystalline rocks and first-cycle sedimentary rocks within the drainage basins of their respective river systems. The Guadalupe province is found on the central part of the South Texas shelf, and the Eastern Gulf province is found on the eastern part of the MAFLA shelf. They are both characterized by a mixture of rounded and angular, fractured grains that were derived from the Gulf coastal plain.

A comparison of the areal distribution of these six provinces with the late Pleistocene paleogeography of the continental shelf shows evidence for varying degrees of shore-parallel transport of silt by modern shelf currents. Silt is most mobile in the central part of the study area, where it has been transported up to 400 km from its original site of deposition. It is less mobile in the eastern and western parts of the study area, where it has been transported less than 125 km from its original site of deposition.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.