Fabrics of Allochthonous Reefal Blocks, Shady Dolomite (Lower to Middle Cambrian), Virginia Appalachians

Abstract

ABSTRACT Studies of reefal limestones in the Lower to Middle Cambrian Shady Dolomite, southwest Virginia, have determined their allochthonous character, defined the various reefal facies present, and determined the influence of diagenesis on the reefal fabrics. Three generations of geopetal fills occur in boundstone cavities in the allochthonous blocks. Generation 1 geopetals of lime mud on algal frame formed while reefs were in place, and were rotated following displacement of the reefs downslope. Generation 2 geopetals of lime mud on marine cement formed after displacement of reef masses into foreslope-slope settings. Generation 3 geopetals of dolomite silt, which rests on earlier calcite cement and is postdated by saddle dolomite, formed during deep burial and late Paleozoic deformation of the sequence. Reefal blocks are composed of Epiphyton and Girvanella boundstone, thrombolites, and stromatolitic carbonates. Epiphyton boundstone were constructed by Epiphyton that generally grew sideways from mounds, and lesser Girvanella and minor Renalcis. Girvanella boundstones were constructed by crusts and tufts of Girvanella. Thrombolites generally lack calcified algae, and are composed of tiny digitate structures, wackestone/mudstone and irregular fills of pellet sediment. Stromatolites include domes and laterally linked heads, with laminae of pellet sediment and mud. Although all these facies are associated in the blocks, the sequence of Epiphyton to Girvanella boundstone to thrombolite to stromatolite probably reflects ncreasing restriction and shallowing of the shelf edge periodically. Synsedimentary marine cements are abundant. and are mainly isopachous, banded neospar with local radial fibrous habit. The cements resemble modern acicular aragonite, micrite and high-Mg calcite cements found in platform margin reefs. The blocks of algal and cryptalgal limestone in the Shady Dolomite exhibit a greater variety of fabrics than have been recorded from shelf edges of equivalent Cambrian age. Furthermore, the association of cryptalgal deposits along with calcified-algal reefs in a shelf edge setting is significant, because most geologists consider cryptalgal thrombolites and stromatolites to be restricted to inner platform settings in Cambrian and younger sequences.