ABSTRACT: Sequence Stratigraphy of Early-Middle Devonian Carbonate Shelf to Basin Transitions, Northeast British Columbia

MCILREATH, IAN A., Petro-Canada, Calgary, Alberta, Canada, and BARRY D. SHADE, B. D. Shade Geological Services, Calgary, Alberta, Canada

The Early Devonian carbonate near margin shelf exposed in the Rocky Mountains, from the Williston Reservoir northwards to Prophet River, can be explained in terms of two unconformity bounded sequences, corresponding to the Muncho-McConnell and Stone formations. The near margin shelf systems tracts in both sequences are characterized by mixed carbonate-siliciclastic lithofacies that were deposited in very shallow subtidal to peritidal settings. During the lacuna between the two sequences, a thin calcareous sandstone unit (Wokkpash Formation) accumulated in reciprocal fashion to the northwest and possibly in the adjacent basin. Similarly during the Ebbutt unconformity, which terminated Stone deposition, a thick unit of mixed quartz sandy calcarenites and peloidal quartz sandstones accum lated in the Lady Laurier Embayment. These two, mainly quartz-rich sand units interrupt basinal shales, lime mudstones, and siltstones of the Road River Group.

The overlying Middle Devonian carbonate succession, which is equivalent to the Keg River-Slave Point interval in the subsurface, is nearly devoid of quartz sand. The Watt Mountain-equivalent erosional event effectively divides this succession into two sequences. Shelf and margin systems tracts include platform interior parasequences and stromatoporoid and coral-dominated reefal assemblages. Large differences in stratigraphic thickness of the total succession at

various outcrop localities reflect significant "backstepping" at margins. The Middle Devonian succession was terminated by "drowning" related to the south and east transgression of overlying Frasnian basinal shales.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)