EARLY PROTEROZOIC (APHEBIAN) PLAN-VIEW ALGAL MATS IN THE LOWER ALBANEL BLACK SHALE ON LAKE MISTASSINI, CENTRAL QUEBEC

Precambrian algal mats are ordinarily seen as thin dark laminae in or between stromatolites cut in vertical or thin-section. Unusual fossils seen along Lake Mistassini may be such mats but preserved in plan view.

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.