Abstract

The Stoddart Group was studied in northwestern Alberta and northeastern British Columbia within the eastern and subsurface portion of the Peace River Embayment. The Stoddart Group comprises three formations: the Golata, Kiskatinaw and Taylor Flat (in ascending order).

Golata Formation shales form the base of a thick, clastic-dominated depositional sequence of interbedded sandstone, shale and limestone of the Stoddart Group; a sequence representing a major clastic pulse overlying a thick sequence of Rundle Group carbonates and equivalent units. The Golata Formation can be subdivided into a thin basal transition unit, a thick medial unit of black shale, and a thin upper unit of coloured shale.

The contact between Golata Formation shales and overlying Kiskatinaw Formation sandstones superficially resembles a disconformity and has the appearance of a depositional sequence boundary. The Golata Formation has an eroded upper surface with a relief of up to tens of metres, deposits of variegated shales (Coloured Shale unit) with related development of a subaerial exposure surface imprinted on the marine Black Shale unit, and a facies contrast with the overlying Kiskatinaw Formation. The Kiskatinaw package represents a tidal-estuarine-fluvial complex that, where basal sandstones occur, filled valleys eroded into the Golata Formation Black Shale unit. However, correlations indicate that basal Kiskatinaw sandstones may be laterally equivalent to the uppermost Golata Coloured Shale unit and, since the Coloured Shale unit grades downward to and is conformable with the Black Shale unit, the erosional Kiskatinaw/Golata Black Shale unit contacts likely represent localized erosion rather than a regional disconformity. This relationship indicates that the erosion surface above the Golata Formation is a localized cut and fill feature developed during channel sandstone deposition of the basal Kiskatinaw, occurring simultaneously with off-channel variegated shale deposition. An east-northeast valley trend and north-northwest shifts in valley trends are parallel to faults related to the Peace River Arch, suggesting that the valley system was fault controlled. An understanding of valley trends may be useful in predicting Kiskatinaw sandbody locations and assist exploration for stratigraphically trapped gas in Kiskatinaw channel-fill sands.