Paleozoic carbonate platforms

Overall, the Late Ordovician extinction appears to be the result of purely terrestrial phenomena. High sea-level stands of the early Paleozoic allowed for animal diversification on shallow-water, epicontinental carbonate platforms that proved, however, to be highly sensitive to glacio-eustatic effects on ecospace availability and lateral shifts in the oxic-anoxic interface. Tectonic activity facilitated the establishment of Gondwanan ice sheets which robbed the shallow seas of water, leading to extinction, establishment of recovery ecosystems, and then the destruction of these as the ice sheets melted, perhaps catastrophically. 

The sedimentary section of the U.S. southern midcontinent consists in large part of Paleozoic platform carbonate rocks interbedded with shale and subordinate sandstone. As documented below, a large volume of the Paleozoic carbonate rocks in this region is enriched in arsenic, as well as lead, zinc, copper, cobalt, molybdenum, and nickel compared to surrounding carbonate strata (Erickson et al., 1981 Lee and Goldhaber, 2001). In plan view this trace-mineralized area extends over thousands of square kilometers. The widespread metal enrichment process was related to the formation of much more spatially localized ore districts. These districts consist of economic concentrations of the ore elements zinc and lead, and to a lesser extent copper, cobalt, and barium that occur in the platform... 

Fig. 7. Schematic north—south cross-section through San Antonio and the Muil Field (see Fig. 1). Jurassic and Lower Cretaceous sediments (the aquifer ) are shown by the limestone Pattern since most are carbonates. They are overlain by Upper Cretaceous clay and chalk. The northern-most outcrops are associated with the Balcones Fault zone near San Antonio. The shallow part of the section is underlain by Paleozoic basement , or by Triassic rocks further south. Jurassic salt is shown by solid hatchures. Darkened areas at A, B and C depict the three producing zones, the Stuart City Reef Trend (the carbonate platform margin), the Karnes Trough and the Atascosa Trough, respectively. Note the faulting associated with them. The temperature axis is speculative below 5 km.

Dickens GR, Castillo MM, Walker JCG (1997) A blast of gas in the latest Paleocene simulating first-order effects of massive dissociation of oceanic methane hydrate. Geology 25 259-262 Dunham JB (1977) Depositional environments and paleogeography of the Upper Ordovician, Lower Silurian carbonate platform of central Nevada. In Stewart JH, Stevens CH, Fritsche AE (eds) Paleozoic Paleogeography of the Western United States. Pac Sect Soc Econ Paleo Mineral 7 157-164. 

The majority of the oils in the southern Paleozoic province were formed during the Paleozoic when burial of the source rocks exceeded 2-3 km. However, the Paleozoic traps were destroyed by the Hercynian erosion and the hydrocarbons then were able to dissipate. During the Mesozoic, throughout the subsidence of the northern and northeastern parts of the platform, the burial depths again reached the conditions necessary for the generation of hydro carbons. During the Cretaceous (post-Aptian) the depth of burial of the source rocks reached values of about 3 km which favoured mainly the formation of gas. 

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