ABSTRACT

Five types of noncalcareous (fleshy) algae - probable red, brown, and green macrophytes - comprise abundant fragments along two bedding planes in a core taken from basinal argillaceous limestones (Ireton Formation) that abruptly overlie a stepped-back margin along the northwest side of Sturgeon Lake (Leduc) platform. These fossils are flattened and brown; organic detritus of similar colour, and possibly similar origin, is common in the Ireton core and in green shale of the Leduc strata near the Peace River Arch. Fossil algae have been found throughout core at twelve well sites in central Alberta, including east of the Rimbey-Meadowbrook reef trend where algae also occur in Duvernay strata. Algal fragments indicate substantial off-bank or downslope transport. If some fragments are in situ, then Frasnian slopes were within the photic zone. Fleshy algae appear to have formed an integral part of the biomass in slope and platform settings during the Frasnian but have received little attention due to poor preservation. Oceanographic events and sea level chanes may have caused changes in platform water quality allowing proliferation of fleshy algae at the expense of typical calcareous biota and leading to the formation of intraplatform marine hardgrounds. Geographic and temporal differences in macrophytic abundance in basin sediments calls for caution in interpretation of petroleum source and migration using biomarkers, because sterane (C_27-29) proportions derived from macrophytes versus planktonic organic matter can differ.

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