Abstract

At the Exshaw type section on Jura Creek the uppermost Devonian and lowermost Carboniferous are represented by a sequence comprising the upper Costigan Member of the Palliser Formation and the overlying 47 m thick Exshaw Formation. Open marine, upper Famennian carbonates of the upper Costigan disconformably overlie restricted marine carbonates and lie within the Lower to Middle expansa conodont biozones. The upper Costigan records transgression and deepening that culminated with deposition of the overlying lower Exshaw black shale at moderate depth (below storm wave base) in the oxygen-deficient water of the Prophet Trough. The abrupt Costigan/Exshaw contact is interpreted to be a minor submarine disconformity. The Exshaw, of latest Famennian to early middle Tournaisian (early Tn2) age, consists of a 9.3 m thick, shale-dominated member gradationally overlain by a 37.4 m thick siltstone member. Three units form the lower member: a thin (1 to 6 cm) basal bed of sandstone and conglomerate; a middle 6.8 m thick noncalcareous black shale unit; and an upper, 2.4 m thick calcareous black shale unit. Conodonts of the uppermost Devonian Upper expansa to Upper praesulcata biozones occur in the middle shale unit. Siphonodella cooperi, first appearing in the lower Tournaisian (upper Tnlb) Upper duplicata Biozone and continuing upward into the middle Tournaisian (Tn2) Upper crenulata-isosticha Biozone, occurs in the upper 76 cm of the upper shale unit.The Carboniferous/Devonian boundary probably lies at the sharp contact between the two shale units. Lowermost Carboniferous strata (within the sulcata and duplicata biozones) have not been unequivocally identified, however, and are either absent or in a condensed sequence. At Jura Creek the lower middle Tournaisian (lower Tn2) siltstone member, comprising bioturbated siltstone and limestone, records shallowing, improved water circulation and deposition at moderate water depth.Shallowing culminated with submarine erosion and the resulting minor disconformity is overlain by deep water shale, siltstone and silty limestone of the middle Tournaisian (Tn2) basal Banff Formation.