Clay diagenesis and Holocene sediments of Galveston Bay and the adjacent continental shelf



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Clay mineral abundance and diagenesis were investigated in association with sediment distribution of Galveston Bay, Texas and the adjacent continental shelf. The source area for the sediment is the tertiary shales and sands limiting the sediment spectrum of the bay to secondary material. Fine to very fine quartz sand constitutes the terrigneous sediments coarser than silt in the bay and offshore. Oyster valve fragments are the only other sand sized material in the bay. Pelecypod, gastropod, and other shell hash were distributed in all offshore sand sized material. Heavy mineral distribution was used to identify sediment source on the continental shelf. The Mississippi, Trinity, and Colorado Rivers contain heavy mineral suites which contrast sufficiently to identify their respective deposits on the shelf. This information was used to identify the source of clay minerals associated with the heavy minerals. Trinity River clays, in deposits from upper Galveston Bay to forty miles offshore, were studied for changes in the relative abundance of the clay minerals. Four micron and one micron fractions were analyzed using x-ray powder diffraction techniques. Expandable clays, consisting mostly of mica derived montmorillonite, were systematically replaced by illite as upper bay sediments are transported on a traverse from the upper bay to the continental shelf. The smaller size fraction experienced the greatest loss in expandables. Kaolinite content showed an insignificant variation in the bay but decreased in the Trinity River sediments offshore, particularly in the coarser fraction. The expandable-illite exchange was attributed to diagenesis preferentially affecting the more reactive fine fraction. The loss of kaolinite in the offshore area was attributed to preferential flocculation of this mineral.