The origin and diagenesis of grain-coating serpentine-chlorite in Tuscaloosa Formation sandstone, U.S. Gulf Coast

摘要

Randomly interstratified serpentine-chlorite (Sp-Ch) is ubiquitous in sandstones of the subsurface Tuscaloosa Formation. The Sp-Ch occurs as grain coatings, pore fillings, pel-oids, and infillings. The presence of peloids and infillings suggests that the Sp-Ch originated as odinite, a 7 A mineral that forms at the sediment-seawater interface in shallow, tropical marine sediments. We conclude that during the very earliest stages of diagenesis, odinite peloids and infillings transformed to mixed-layer Sp-Ch, forming grain coatings with preservation of some of the original odinite textures. The formation of grain-coating Sp-Ch during early diagenesis apparently preserved high primary porosity in medium-grained specimens. Fine-grained and silty sandstones, however, contain poorly formed grain coatings and are tightly cemented by quartz, implying that grain size also has some effect on authigenic quartz growth. With increasing burial depth between 1702 and 6216 m, the proportion of Sp layers in Sp-Ch decreases from 20.7 + - 1.1 to 1.3 + - 0.1 percent, indicating that Sp layers transform to Ch layers with increasing diagenetic grade. The polytype of the Sp-Ch is essentially Ibb at depths <2000 m. At depths >2000 m, the polytype is randomly interstratified Ibb-laa, and the proportion of Ibb layers decreases from approx 100 to 49 percent over the interval studied. At depths <2000 m, orthohexagonal Sp transforms to Ibb Ch, but at depths >2000 m, orthohexagonal Sp transforms to Iaa Ch. There is essentially no change in Sp-Ch chemical composition with depth, implying that temperature is the dominant control on the structural transformations. Mean crystallite thickness also remains relatively constant with increasing depth, indicating that diagenetic transformations in Sp-Ch do not involve crystal-growth mechanisms typical of most clay minerals. Rather, the mineralogic transformations appear to proceed on a layer-by-layer basis, with individual layers dissolving and reprecipitating within the confines of the crystallites.