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Allostratigraphy of the Kaskapau Formation (Cenomanian–Turonian) in the subsurface and outcrop: NE British Columbia and NW Alberta, Western Canada Foreland Basin

Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, gplint{at}uwo.ca

The Kaskapau Formation is a major, mudstone-dominated syntectonic wedge deposited in Late Cretaceous time in the foredeep of the Western Canada foreland basin. This study addresses the middle and upper parts of the formation (Late Cenomanian–Middle Turonian), embracing the uppermost part of the Sunkay, Vimy, Haven, and lower part of the Opabin members, which thin eastward from 700 to about 50 m over 250 km. A regional (35,000 km²) subsurface allostratigraphic framework based on 756 well logs and six cores has been extended into 16 major outcrop sections in the British Columbia Foothills and the Peace River valley in Alberta. The lower and upper boundaries of the study interval are defined allostratigraphically at two regional erosion surfaces, the K1 (base) and Granular Siderite (top).

In the west, the Kaskapau Formation is divided into five units (I–V), each defined by a major transgressive surface. Each unit contains a major tongue of stacked shoreface sandstones that are named, in ascending order the Dickebusch, Trapper, Tuskoola, Wartenbe and Mount Robert sandstones. All the shoreface sandstones grade eastward over a distance of about 20 to 40 km into heterolithic and muddy facies. Each unit comprises five to seven informal allomembers, each of which shoals upward and is bounded by a flooding or ravinement surface. Most of the 28 allomembers have a wedge shape, thinning to the east, although allomembers 1 and 4 pinch out to the west. Westward-thickening wedges of nonmarine deposits up to 40 m thick are present in units IV and V in the far northwest.

In the northeastern part of the study area, three, 3 to 5 m thick sandstone bodies are present at the tops of allomembers 1, 4 and 6. These are named the Erin Lodge, Howard Creek and Josephine Creek sandstones. Their distribution suggests that each sandstone was supplied from the northeast. In the eastern part of the study area, a mudstone package approximately 40 m thick, characterized by high radioactivity forms the basin-wide log marker known as the Second White Speckled Shale. Log correlation shows that this organic-rich mudstone is a diachronous facies which formed during rapid proximal subsidence when sand and silt were trapped in the western foredeep.

Units I to III of the Kaskapau Formation taper dramatically to the east whereas units IV and V taper less strongly. These geometric changes are interpreted to record a progressively decreasing rate of flexural subsidence during middle-late Kaskapau time. The limited progradation of shorelines (<20 km) in units I to III and somewhat more extensive progradation (<40 km) in units IV and V also suggest a slow decrease in the ratio of accommodation to supply. Extra-basinal conglomerate is an insignificant component of units I to III, but is abundant in units IV and V. This vertical distribution is interpreted as a response to an initially high accommodation rate that resulted in lower alluvial gradients, rapid alluvial aggradation, and gravel storage on the coastal plain. As accommodation rate diminished, rivers may have been able to construct steeper alluvial plains, permitting gravel transport to the shoreline. This trend of a decreasing ratio of accommodation to supply is maintained in the overlying Cardium Formation, which is a broadly tabular unit that records at least 150 km of shoreline progradation. The Kaskapau Formation spans most of the Greenhorn eustatic cycle. Although unequivocal evidence for an eustatic component of accommodation cannot be discerned on the western margin of the basin, it probably finds expression in deepening over the forebulge in units II and III.

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