ZONNEVELD, JOHN-PAUL¹, HENDERSON, CHARLES M.¹, and MOSLOW, THOMAS F.^2
1Applied Stratigraphy Research Group, Department of Geology & Geophysics, University of Calgary, Calgary, Alberta
²Ulster Petroleums, Calgary, Alberta

Abstract: The Sequence Stratigraphic Significance Of Bioclastic Accumulations In Siliciclastic Systems: Examples From The Triassic Of Western Canada

The Lower-Middle Triassic Montney, Doig and Halfway formations in the Western Canada Sedimentary Basin comprise dominantly siliciclastic units in which bioclastic accumulations (coquinas) form significant reservoirs and exploration targets.

Mixed lingulid-bivalve concentrations, commonly associated with the mid-Montney (Dienerian-Smithian) sequence boundary form important reservoir units in west-central Alberta. Strata immediately above boundary are characterized by restricted, stressed estuarine/incised valley systems dominated by diminutive lingulid brachiopods. During early transgression, these lingulids were reworked, mixed with shoreface-derived bivalves and redeposited in a series of ridges and bars proximal to the estuary mouths.

Middle Triassic mixed siliciclastic-bioclastic accumulations are more common and considerably more diverse. Brachiopod and bivalve dominated bioclastic tidal inlet channel fill, deposited during lowstand and early transgression within a barred barrier island shoreface succession, comprise a significant reservoir lithology within the Doig/Halfway interval in northeastern British Columbia.

Terebratulid-echinoderm biostromes, common within outcrop of Halfway equivalent shoreface sandstones in British Columbia, may represent potential reservoir units if identified in the subsurface. Storm-generated skeletal concentrations provided islands of stable substrate, which were subsequently colonized by terebratulid brachiopods, cidaroid echinoids and articulate crinoids. These units accumulated during periods of low siliciclastic input to the shoreface, within the later stages of a highstand systems tract.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas