Carbonate Precipitates: Precambrian Analogs for Paleozoic Cementstones
GROTZINGER, JOHN P.
Patterns of early diagenesis in carbonate rocks strongly influenced fluid pathways for later diagenesis and, consequently, reservoir quality. Prior to the advent of calcifying nannoplankton in the Jurassic, there appear to have been distinct times when great quantities of pore-filling marine cements were precipitated; in some cases these precipitates also directly encrusted the sea floor as a primary depositional component. Late Permian reefs of the Capitan complex, west Texas; the Magnesian Limestone, England; Chuenmuping reef, south China; and elsewhere contain anomalously large volumes of aragonite and calcite marine cements and sea-floor crusts, as well as abundant microbial precipitates. These components strongly influenced reef growth and may have been responsible for the construction of rigid, open reefal frames in which bryozoans and sponges became encrusted and structurally reinforced. In some cases, such as the upper biostrome of the Magnesian Limestone, precipitated microbiolites and inorganic crusts were the primary constituents of the reef core. These microbial and inorganic reefs do not have modern marine counterparts; on the contrary, their textures and genesis are best understood through comparison with the older rock record, particularly that of the early Precambrian.
Early Precambrian reefal facies are interpreted to have formed in a stratified ocean with anoxic deep waters containing high concentrations of dissolved inorganic carbon. Upwelling mixed deep and surface waters, resulting in massive sea-floor precipitation of aragonite and calcite. The anomalous late Permian sea-floor precipitates are interpreted as the product of several important processes. Massive carbonate precipitation was favored by (1) reduced shelf space for carbonate precipitation, (2) increased flux of Ca to the oceans during increased continental erosion, (3) deep basinal anoxia that generated upwelling waters with elevated alkalinites, and (4) further evolution of ocean water in the restricted Delaware, Zechstein, and other basins. Temporal coincidence of these processes resulted in surface seawater that was greatly supersaturated by Phanerozoic standards and whose only precedents occurred in Precambrian oceans.
AAPG Search and Discovery Article #90938©1997-1998 AAPG Distinguished Lecturers