Sedimentology and diagenesis of Lower to Middle Cambrian carbonate platform, Shady Dolomite, Virginia

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Virginia Polytechnic Institute and State University

Drill cores through the Lower to Middle Cambrian Shady Dolomite carbonate platform (600 to 1200 m thick) in the Austinville, Virginia, region allow the evolution of the carbonate platform from a gently sloping ramp, to a high relief, rimmed shelf to be documented. The Shady Dolomite forms the initial carbonate foundation for the overlying Cambrian-Ordovician carbonate shelf sequence, which persisted for about 30 m.a. until it was destroyed by incipient collision during the Early Ordovician.

The Shady Dolomite records several episodes of dolomitization during burial, coeval with late Paleozoic deformation. Rare relict corcs of zone 1 dolomite were replaced and overgrown by zone 2A dolomite, the dominant replacement phase. After replacement dolomitization, the sequence was subjected to dissolution and fracturing, followed by Pb-Zn mineralization, zones 2B, 3 and 4 dolomite cement, sphalerite, quartz, and calcite.

Zone 1 dolomite apparently has similar isotopic and trace element composition as zone 2A dolomite. Zone 2A dolomite (δ¹⁸O = -10.2 to -7.0 °/oo PDB; δ¹³C = +1.0 to +1.6 °/oo PDB) is depleted in ¹⁸O and enriched in ¹³C relative to marine cements (δ¹⁸O = -7.5 to -6.1; δ¹³C = +0.2 to +0.8), reflecting precipitation at elevated temperatures from fluids in equilibrium with the host limestone. Zone 2B dolomite cement has identical δ¹⁸O values as zone 2A dolomite, indicating precipitation from similar fluids for the two dolomite generations. The Mn and Fe contents of zones 2A and 2B dolomite likely reflect a pH control over the fluid Mn and Fe chemistry; their similar low total Sr and nonradiogenic ⁸⁷Sr/⁸⁶Sr, imply that Sr was largely derived from the limestone precursor.

Zones 3 and 4 dolomite cements (δ¹⁸O = -13.8 to -11.3; δ¹³C = -0.7 to +0.9) are depleted in δ¹⁸O relative to previous dolomites, recording hotter fluids. Zone 3 dolomite is depleted in Fe, due to pyrite precipitation whereas zone 4 dolomite cement has relatively high Mn and Fe contents. Zones 3 and 4 dolomite cements and later calcite are enriched in total Sr and have high ⁸⁷Sr/⁸⁶Sr, indicating late radiogenic Sr-enriched brines.

Fluid inclusions indicate that zones 2A and 2B dolomite precipitated from warm (100-175°C), saline (23-26 wt.% NaC1 equiv.) fluids, followed by later hotter (175-225°C) more saline (30-33 wt.% NaCl equiv.) fluids. Pressure solution of the Knox Group dolomites during overthrusting provided much of the Mg²⁺ for dolomitization, this Mg²⁺ was transported by regional gravity-driven fluid flow that developed in response to tectonic uplift.