THE ORIGIN AND DIAGENESIS OF GRAIN-COATING SERPENTINE-CHLORITE IN TUSCALOOSA FORMATION SANDSTONE, US GULF-COAST

Randomly interstratified serpentine-chlorite (Sp-Ch) is ubiquitous in sandstones of the subsurface Tuscaloosa Formation. The Sp-Ch occurs as grain coatings, pore fillings, peloids, and infillings. The presence of peloids and infillings suggests that the Sp-Ch originated as odinit e, a 7 Angstrom mineral that forms at the sediment-seawater interface in shallow, tropical marine sediments. We conclude that during the ver y earliest stages of diagenesis, odinite peloids and infillings transf ormed to mixed-layer Sp-Ch, forming grain coatings with preservation o f some of the original odinite textures. The formation of grain-coatin g Sp-Ch during early diagenesis apparently preserved high primary poro sity in medium-grained specimens. Fine-grained and silty sandstones, h owever, 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%, indicating that Sp layers transform to Ch layers with incr easing diagenetic grade. The polytype of the Sp-Ch is essentially Ibb at depths <2000 m. At depths >2000 m, the polytype is randomly interst ratified Ibb-Iaa, and the proportion of Ibb layers decreases from simi lar to 100 to 49% over the interval studied. At depths <2000 m, orthoh exagonal Sp transforms to Ibb Ch, but at depths >2000 m, orthohexagona l Sp transforms to Iaa Ch. There is essentially no change in Sp-Ch che mical composition with depth, implying that temperature is the dominan t control on the structural transformations. Mean crystallite thicknes s also remains relatively constant with increasing depth, indicating t hat diagenetic transformations in Sp-Ch do not involve crystal-growth mechanisms typical of most clay minerals. Rather, the mineralogic tran sformations appear to proceed on a layer-by-layer basis, with individu al layers dissolving and reprecipitating within the confines of the cr ystallites.