Genesis And Environmental Position Of The Algal Mounds Of The Stonehenge Formation

Abstract

Carbonate mounds of the Lower Ordovician (Canadian) Stone­henge Formation contain evidence of algal origin in the form of convex upward growth structures, stromatolites, and filamentous algae. Facies distribution and thickness relationships of the Stonehenge show that the mounds formed near the margins of a distinct sedimentary basin on the Cambro-Ordovician carbonate platform of Pennsylvania. Interpretations of the sedimentary environments of the major lithologies deposited in the seaway indicate that the Stonehenge beds were deposited in facies mosaic style. Five major lithologic associations have been described and interpreted in this framework. 1. dolostone cycles ... supratidal facies, 2. siliceous dolomitic pelmicrite ... subtidal flat facies, 3. intrasparudite-intrasparite ... subtidal channel facies, 4. massive calcilutite ... algal mound facies, 5. cryptalgal calcarenite ... mound debris facies. The positions of these lithologies in the stratigraphic succession of the Stonehenge indicate that the mounds formed in the base of subtidal channels at the climax of a regional transgression.