EARLY PROTEROZOIC (APHEBIAN) PLAN-VIEW ALGAL MATS IN THE LOWER ALBANEL BLACK SHALE ON LAKE MISTASSINI, CENTRAL QUEBEC
These probable fossils, discovered by DCM, occur along the NW shore of Lake Mistassini, 30 km/18 mi NNW of Mistissini village, 585 km/365 mi N of Montreal. Their horizon is the black shale (dated to 2.1 or 1.8 Ga) low in the Lower Albanel Formation. Cree and Canadian/Quebecois collecting restrictions force reliance on field photos for documentation.
The best-preserved specimen seen in the field is 5 cm long by 3 cm wide, and consists of 39 semicircular, nearly flat lobes 5-13 mm wide, 3-9 mm long, and 1-3 mm thick, arranged like fish scales, with the rounded edge being the apparent growth direction. This suggests a life history starting with a single cell in the center of each semicircle, and proceeding by cell divisions both distally and laterally pushing out the semicircular edge. Each lobe consists of dark gray-black homogeneous claystone; the rounded edge is enhanced by a thin quartz-filled fissure.
At the earliest-formed end of this specimen, the lobes grade into squarer blocks. Other specimens along shore consist entirely of such small square to rectangular blocks (up to 200/specimen), 4-19 mm wide by 3-11 mm long, arranged in a concentric segmented pattern, reminiscent of septarian concretions 9-30 cm long by 6-11 cm wide, but thin (<1 cm), suggesting diagenetic alteration obscuring their organic origins.
All these fossils are developed on top of very thinly laminated, black shale, composed of alternating dark clay and light quartz-silt layers, indicating quiet-water conditions. The black shales overall are both carbon- and pyrite-rich, and could represent shallow coastal lagoons or off-shore deep water.
Because of their macroscopic size, these fossils might be argued as being multicellular, colonial, or eukaryotic; however, no evidence suggests such advanced affinities here. Their early (Paleoproterozoic) age supports best considering them as prokaryotic blue-green algae (Cyanobacteria) in radially organized mats, previously unknown but related to the stromatolites in the overlying dolomites.