Geology of the Devonian-Carboniferous Boundary Beds in Alberta

Abstract New biostratgraphic evidence in Alberta indicates that the base of the Carboniferous succession occurs above the black shale unit of the Exshaw Formation. The black shale unit overlies a basal transgressive deposit composed of carbonates (the Upper Devonian Big Valley Formation) which in turn overlie a slightly erosional diastem that separates these carbonates from more restricted carbonates and evaporites below. Information from conodonts suggests that the Big Valley Formation and the lower part of the Exshaw Formation are condensed successions of marine shallow and deep water deposits. In the Peace River region, the upper part of the Big Valley Formation and the lower part of the black shale unit of the Exshaw Formation onlap a basal unconformity across the crestal part of the pre-Devonian Peace River highland. Both the carbonates and the shales change laterally into a thin, interbedded succession of black shale and feldspathic sandstone that constitutes more nearhore sediments.