Abstract

Slope and basin deposits in the Cow Head Group of early Paleozoic age contain a wide variety of carbonate clasts. Among the most common, largest, and most persistent (middle Middle Cambrian to lower Middle Ordovician) are blocks of fine-grained white limestone. The origin of these clasts is unknown, as no limestones of similar lithology have been found in contemporaneous shallow-water carbonate deposits anywhere in the northern Appalachians.

Petrographic study of these enigmatic limestones reveals that, no matter what their age, they are of similar composition: calcified algae, cement, and fine-grained sediment. The predominant algae are Epiphyton spp., occurring as numerous masses of tiny shrublike and clumplike growths, and Girvanella spp., developed as numerous thin strands and arcuate sheets. Renalcis spp. and stromatolitic algae, although locally common, are not persistent. The algae are surrounded by fibrous calcite cement interpreted to be synsedimentary, while the numerous small cavities between algae are partly filled with geopetal marine sediment.

These blocks are interpreted to be redeposited fragments of reef mounds and buildups which grew along the shallow, submerged margin of the North American craton during early Paleozoic time. These buildups are now probably buried beneath the allochthonous Cow Head strata. The mounds were composed of calcareous algae, because there were no large reef-building metazoans during this time in geologic history. Recognition of similar clasts in analogous settings in the Quebec and southern Appalachians, as well as in the Cordillera, suggests that this facies was both widespread and persistent for more than 70 m.y.

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