EARLY CAMBRIAN ALGAL, ARCHEOCYATHAN, TABULACONUS MOUNDS, SEKWI FORMATION, SELWYN BASIN, MACKENZIE MOUNTAINS, CANADA

Algal mounds that contain archeocyathans and Tabuloconus, corallomorphs first described by Handfield (1969) are well-exposed in the Early Cambrian Sekwi Formation, near Intga River, Selwyn Basin, Mackenzie Mountains, Canada. The mounds are found in a distinctive ~25 m thick horizon about 250 m above the base of the section. The mounds are underlain and overlain by interbedded brown shale and calcisiltite in the Nevadella biozone. Individual mounds commonly are circular to ovoid in shape being up to five meters high and eight meters wide. Most of the mounds are upright, but some appear to be tilted from the horizontal. Rudstone containing mound clasts occurs locally between the mounds. Renalcis and Epiphyton algae are in all the mounds. The abundance of algae varies from 10-60% in individual mounds. Renalcis and Epiphyton algae are preserved as solitary bushes and as fragments with archeocyathans and Tabuloconus. Archeocyathans in the mounds are found as interlocking networks or solitary units. Tabulaconus is preserved as solitary units among the other fossils, most commonly algae. The matrix between the organisms in the mounds ranges from carbonate mudstone to carbonate siltstone. Marine cement commonly fills the archeocyathans and Tabulaconus. Unlike coeval(?) Tabulaconus mounds in Alaska that occur as olistostromes, these mounds grew in place within a subtidal depositional environment.