Neoproterozoic reef microstructures from the Little Dal Group, northwestern Canada

Basinal facies in the Neoproterozoic Little Dal Group contain large reefs up to 300 m in height and 8 km in diameter. In contrast to the stromatolitic composition typical of earlier Proterozoic buildups, Little Dal reefs consist mainly of three different calcareous microfossils. A tubule-thread microfossil, analogous to the calcimicrobe Girvanella in Paleozoic reefs, formed laminar-reticulate structures, like those in Ordovician bioherms. A clotted to saccate microfossil, comparable morphologically to Paleozoic Renalcis, constructed botryoidal to encrusting masses. An enigmatic component of multicellular appearance that has attributes of an early calcified metaphyte exhibits the encrusting sheetlike habit characteristic of middle to late Paleozoic algae. These reefs represent an intermediate stage between Proterozoic stromatolitic buildups and Paleozoic metazoan-calcimicrobial reefs and biogenic mounds.