Sources and dispersal of late Quaternary silt on the northern Gulf of Mexico continental shelf

Abstract

The surficial sediments of the northern Gulf of Mexico continental shelf consist of large volumes of silt and clay and lesser amounts of sand of late Quaternary age. A study of the roundness and surface textures of quartz silt grains in these surficial sediments was conducted to determine the sources and dispersal patterns of this size fraction and to gain a better understanding of late Quaternary sedimentation on this shelf. The results indicate that there are six distinct silt provinces within the study area that can be distinguished on the basis of grain roundness and surface textures. Three of the provinces are characterized by relatively angular quartz silt grains (on the average) and an abundance of first-cycle crystalline quartz grains; the remaining three provinces are characterized by relatively rounded quartz silt grains and an abundance of multicyclic (sedimentary) rounded grains. The dispersal patterns of these six provinces are partly controlled by the late Wisconsinian paleogeography. Thus, a comparison of the dispersal patterns with the locations of the modern and ancient fluviodeltaic depocenters allows the identification of these six provinces as deposits of the Rio Grande, the coastal plain streams of South Texas, the Brazos and Colorado River, the Mississippi, the Mobile River and the rivers of the southeastern United States. However, the dispersal patterns have also been modified by the prevailing shelf and longshore currents, and there is up to 400 km of downcurrent displacement of silt from its Late Wisconsinian sites of deposition on some parts of the shelf.