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

Abstract

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 km 2 ) 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 middlelate 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. Extrabasinal 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 h...

Evolution of the northern Cordilleran foreland basin during the middle Cretaceous

“…8 and 11). Unit I shows a wedge shape, thinning from ~110 m in northeastern British Columbia to ~10 m in northwestern Alberta (Varban and Plint, 2005). The Upper Belle Fourche Formation in central Alberta thins eastward from ~70-80 m near the foothills to 4-6 m near the Alberta-Saskatchewan border (Tyagi et al, 2007) (Figs.…”

“…2 and 8). The Dunveganoceras ammonite zone, indicative of a late Cenomanian age, was found in the Pouce Coupe and Unit I units in northwestern Alberta (Warren and Stelck, 1940;Varban and Plint, 2005). The A-X unit exhibits a wedge shape, thinning from ~50 m in northeastern British Columbia to ~10 m in central northern Alberta (Plint, 2000).…”

“…However, continuous northeast-verging thrusting and shortening of the Rocky Mountains resulted in cannibalization of the late Cenomanian foredeep depozone in southeastern British Columbia and the northern United States (Price and Sears, 2000;Sears, 2001;Ross et al, 2005). The marine shale interbedded with shoreline and delta front sandstone of the Lower Kaskapau Formation in on 23 July 2009 gsabulletin.gsapubs.org Downloaded from northeastern British Columbia, with a thickness of ~350 m, represents deposition in the foredeep depozone (Plint, 2000;Varban and Plint, 2005;Kreitner and Plint, 2006) (Fig. 8).…”

Biostratigraphy and Inoceramus survival across the Cenomanian–Turonian (Cretaceous) boundary in the Ram River section, Alberta, Canada

“…1). In that basin-margin region, sandstonerich shoreface and inner shelf facies are stacked vertically, forming the principal lithology through ~ 550 m of strata in the most westerly part of the basin (Varban and Plint 2005).…”

Migration and stratigraphic fill of an underfilled foreland basin: Middle–Late Cenomanian Belle Fourche Formation in southern Alberta, Canada

“…2A, 3). The Late Cenomanian lower Kaskapau Formation in northeastern British Columbia was deposited in the foredeep zone (Plint, 2000;Varban and Plint, 2005) and allomember B-E (upper Belle Fourche Formation) in southern Alberta in the forebulge and backbulge zones (Yang and Miall, 2009) (Figs. 2, 3).…”