FAMENNIAN AND TOURNAISIAN BIOSTRATIGRAPHY OF THE BIG VALLEY, EXSHAW AND BAKKEN FORMATIONS, SOUTHEASTERN ALBERTA AND SOUTHWESTERN SASKATCHEWAN

This biostratigraphic study of the conodont faunas from the Devonian-C arboniferous boundary beds in southeastern Alberta and southwestern Sa skatchewan suggests that there is a significant hiatus between the Big Valley and Bakken formations. The hiatus may span the Upper trachyter a to Upper postera zones. It divides the Kaskaskia sequence of the Wil liston and Western Canada Sedimentary basins into lower and upper part s. The Devonian Big Valley Formation is composed of interbedded calcar eous claystones and shallow marine carbonates. The uppermost limestone bed is overlain with an erosional contact by a rubble deposit that gr ades upward into a silty and fossiliferous claystone. This limestone b ed contains conodonts of the Uppermost marginifera to Lower trachytera zones. The claystone bed is sharply overlain, by means of a pyritized lag deposit, by the euxinic, ''offshore'' shale facies of the lower s hale members of the Bakken and Exshaw formations, which contain conodo nts of the Famennian lower to Upper expansa zones. The middle (Colevil le Sandstone) member of the Bakken Formation and the siltstone unit of the Exshaw Formation include fossils and sedimentary structures typic al of elastic, shallow marine deposits. These units, composed of sands tone, siltstone and claystone, did not yield diagnostic, short-ranging conodonts but probably include the Devonian-Carboniferous boundary. T he sandy and silty beds are overlain by another black, euxinic ''offsh ore'' shale facies: the upper shale member of the Bakken Formation. Th is black shale unit contains an early Tournaisian conodont fauna and c hanges upward and westward into a greenish-grey marine shale facies th at forms the basal part of the Carboniferous Banff and Lodgepole forma tions. The greenish-grey shale facies is locally calcareous and yielde d early Tournaisian conodonts of the sulcata to Lower crenulata zones.