Abstract
The Hammett Shale (Lower Cretaceous) represents the offshore marine equivalent of overlying carbonate beach (Cow Creek Limestone) and alluvial (Hensel Sandstone) deposits of Aptian age which prograded shelfward off the southeast margin of the Llano uplift in central Texas. Interbedded and intermixed dolomites and limestones compose most of the upper part of the Hammett Shale and, within this section, dolomites decrease in abundance upward. Dolomites are primarily echinoid-oyster wackestones with clay-rich, medium crystalline dolospar matrix. Limestones are mollusk packstones-wackestones with clay-poor microspar and pseudospar matrix. The dolomites were most likely deposited on a grass or algally stabilized seafloor, whereas the limestones represent units deposited in high r energy environments. Dolomitization probably took place during shallow burial as the beach sequence prograded eastward, and the regional, fresh-groundwater flow system invaded the marine sediments. Carbonate packstones resisted dolomitization because of original differences in mineralogic composition, and because they were semilithified. They were fractured prior to dolomitization, and have sharp contacts with dolomite. Carbonate wackestones underwent dolomitization because they initially contained more magnesium (high-magnesian calcite, mixed-layer illites, and chlorite), and fine detrital dolomite which acted as seed crystals. Dolomites often display flow-aligned bioclasts parallel with their contacts with limestones, indicating that they were somewhat fluid at the time of limestone lithification, thus llowing the dolomitizing waters to pass through more effectively. Although the marine interstitial water, the fresh water draining Llano uplift Paleozoic dolomites, and the hydrodynamics of the zone of water-mixing provided the means of dolomitization, original sedimentologic differences were a key factor as well. End_of_Article - Last_Page 472------------
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