Stabilization and lithification of originally deposited sediments began early during subaerial exposure and subsequent freshwater diagenesis. Primary intergranular and intrabiotic porosity is best developed and preserved in biograinstones along the shallow flanks of the constructional mound. This is evident in micritization, submarine cements, and effects attributed more » to binding and encrusting habits of algae, particularly Archaeolithoporella and Tubiphytes. ![]() Early diagenesis occurred after deposition of the sediments in the marine environment. Five distinctive limestone facies, designated according to their most outstanding characteristics are (1) constructional phylloid algal buildups, (2) crestal boundstones, (3) flanking bed packstone-wackestone, (4) foremound pelloidal grainstone, and (5) capping grainstones. ![]() Eleven cores in Fisher and Jones Counties have been subjected to megascopic and petrographic analysis with special interest given to the Alkali Creek SW field in Fisher County. The Flippen carbonate (Wolfcampian) was deposited during a high stillstand of sea level under subtidal to intertidal conditions. Intensive pressure solution is observed only in packstone units not affected by early marine cementation. Cementation alone is responsible for destruction of all of the original porosity in these rocks. Pressure solution contacts among grains in grainstones are generally uncommon. Grainstone units lack any intergranular porosity. Postdepositional diagenesis encompasses extensive dissolution of originally aragonite and magnesian calcite components, precipitation of a late unzoned ferroan calcite, and minor amounts of saddle dolomite. Early diagenesis of the formation is dominated by marine cementation. Ooids in sections to the west of Mountain more » View are all calcitic. These ooids exhibit low strontium and high magnesium concentration. The overlying lagoonal facies are composed of originally calcite ooids that have retained their original radial fabrics. These ooids are replaced by neomorphic calcite and exhibit elevated strontium and low magnesium concentrations. In a section south of Mountain View, Arkansas, the lower marine shoal and shoreface facies are composed of originally aragonitic ooids. Ooids formed on the Pitkin carbonate shelf show lateral and vertical variations in original mineralogy. The Pitkin formation is a marine oolitic-bioclastic limestone exhibiting shoal, lagoonal, beach, and tidal-channel facies. Saddle dolomite is frequently replaced (both partially and wholly) by mildly ferroan calcite (dedolomitization). ![]() Saddle dolomite precipitated from ferroan waters derived from compacting shales within deeper parts of the basins. Saddle dolomite more » was emplaced at a late stage during deeper burial and at temperatures well in excess of 55/sup 0/C. ![]() Microrhombic dolomite represents an early phase of mixing zone dolomitization. Saddle dolomite occurs within primary pores, secondary intergranular and intragranular pores, as intergranular displacive crystals, within fractures, and locally replacing ooids and ferroan calcite cements. Dolomite occurs in two forms: (1) as finely crystalline rhombs replacing original micrite and (2) as weakly ferroan saddle (or baroque) dolomite. In contrast to its outcrop character, the Great Oolite in the subsurface is often extensively dolomitized. Temporary, and some extensive, phases of exposure led to the generation of early meteoric diagenetic features. The Great Oolite shelf prograded as two major regressive limestone units, each separated by a minor but regionally extensive transgressive unit. Minor coral-rich thickets occur sporadically in back-shoal settings. Southward, below south England, grainstones give way to oncolitic packstones and eventually to basinal mudstones. The latter facies provide major reservoir targets in south England. Both at outcrop and in the subsurface, complete transitions are seen from lignitic terrestrial facies, through muddy restricted shelf to open-water grainy associations. The oolitic and skeletal grainstones and packstones of the Great Oolite accumulated as a southward-prograding ramp sequence adjacent to both the emergent London-Brabant Massif and the rift-related swamps of the North Sea.
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