Loch Macumber (early Carboniferous) of Atlantic Canada

Abstract

Following the Acadian Orogeny, Atlantic Canada accommodated several, large, relatively deep lakes within a wrench-fault basin complex called the Maritime Basin. Late Devonian and Tournaisian lakes were hydrologically open, shallow to deep, mainly fresh water bodies. Middle Visean lakes, here collectively called Loch Macumber, were closed, deep, and meromictic. Their deposits comprise the first and thickest of five sequences in the Maritime Basin. Salinity in the loch increased with time from restricted marine or penesaline, to saline. Basin-centre facies consist of a thin, but extensive, sheet of black, peloidal laminated lime mudstones and an overlying thick evaporite complex. The carbonate sheet grades laterally into both laminated to thinly bedded marlstones, siliciclastic sandstones, and microbial, biocementstone mounds. Laminae consist of alternating carbonate and either silty carbonaceous shale or siliciclastic clay and silt. The mudstone and marlstone are locally interbedded with siliciclastic and carbonate turbidites, resedimented (?deep water) breccias, and olistostromes. Seasonal changes in anoxia and/or carbonate production produced rhythmic laminae of carbonate and carbonaceous shale. Carbonate grains consist of silt-sized microbial clots and rare arthropod carapaces and brachiopod shells. The mounds originated as tufa precipitated around subaqueous hydrothermal springs that supported chemosynthetic communities. Resedimentation processes including incipient brecciation, sliding, slumping, debris flows, and turbidity currents were common. The mounds trapped hydrocarbons from the surrounding laminite and sulphides from underlying hydrothermal vents. Increasing salinity with time resulted in sulphate and chloride precipitation that filled the basins and ended the life of Loch Macumber. After the deposition of thick evaporites the topography became less accentuated, the seas less saline, and the faunas more normal marine.