| Summary |
The Ste. Genevieve Limestone of Washington County, Virginia, is a mixed carbonate/siliciclastic unit. Lithologies vary from detritus- free limestones to rocks with virtually equal parts of primary carbonate and siliciclastic detritus to clean sandstones. The Ste. Genevieve documents continued carbonate production with high terrigenous influx, a condition typically thought to preclude production of carbonate. Observed carbonate lithologies include wackestones, packstones, grainstones, oolites and oncolites. Siliciclastic lithologies are mudrocks, siltstones and sandstones. Siliclastic and carbonate components are commonly present in roughly equal abundances, either due to reworking of carbonates with terrigenous sediments or to carbonate production accompanied by terrigenous influx. In such cases it is difficult to classify the rock as carbonate or siliciclastic. This exemplifies a nearly complete mixing of carbonate and siliciclastic detritus. Less complete mixing is characterized by frequent interlamination and/or interbedding of carbonate and siliciclastic lithologies. Depositional environments recognized include; 1) a beach characterized by sandstones, grainstones and oolites which exhibit extensive mixing; 2) a back bar, semi-restricted, subtidal zone marked by the presence of calcarenaceous mudrocks often interlaminated with siltstone; 3) a carbonate sand bar of skeletal grainstones and oolites; and 4) a subtidal, open marine, low energy environment represented by packstones and wackestones. These environments are transitional and arranged in an overall regressive sequence. Diagenesis is extensive and varied in the Ste. Genevieve. The most ubiquitous features are related to compaction/pressure solution and silicification. Other features which are independent of depositional environment are pyritization, equant calcite cement, allochem recrystallization, dolomitization and X-type matrix. Remaining diagenetic features are controlled, at least to some degree, by depositional environment. These include the formation of iron oxides and fibrous, bladed and syntaxial calcite cements; neomorphism of unstable constituents; sericitization; vacuolization; chloritization; authigenic clay formation; corrosion of quartz and replacement of detrital grains by calcite and/or dolomite. Penecontemporaneous diagenesis began in the marine environment followed by freshwater infiltration and pervasive calcite cementation that continued through deep burial. As cementation occluded porosity, overburden stresses were dispersed, alleviating pressure solution and initiating reduction of rock volume through the formation of stylolites. Freshwater was introduced during a Late Mississippian regression followed by uplift during the Alleghanian orogeny. |
