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...

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The Kaskapau Formation spans Late Cenomanian to Middle Turonian time and was deposited on a low‐gradient, shallow, storm‐dominated muddy ramp. Dense well log control, coupled with exposure on both proximal and distal margins of the basin allows mapping of sedimentary facies over about 35 000 km2. The studied portion of the Kaskapau Formation is a mudstone‐dominated wedge that thins from 700 m in the proximal foredeep to 50 m near the forebulge about 300 km distant. Regional flooding surfaces permit mapping of 28 allomembers, each of which represent an average of ca 125 kyr. More than 200 km from shore, calcareous silty claystone predominates, whereas 100 to 200 km offshore, mudstone and siltstone predominate. From about 30 to 100 km offshore, centimetre‐bedded very fine sandstone and mudstone record along‐shelf (SSE)‐directed storm‐generated geostrophic flows. Five to thirty kilometres from shore, decimetre‐bedded hummocky cross‐stratified fine sandstone and mudstone record strongly oscillatory, wave‐dominated flows whereas some gutter casts indicate shore‐oblique, apparently mostly unidirectional geostrophic flows. Nearshore facies are dominated by swaley cross‐stratified or intensely bioturbated clean fine sandstone, interpreted as recording, respectively, areas strongly and weakly affected by discharge from distributary mouths. Shoreface sandstones grade locally into river‐mouth conglomerates and sandstones, including conglomerate channel‐fills up to 15 m thick. Locally, brackish lagoonal shelly mudstones are present on the extreme western margin of the basin. There is no evidence for clinoform stratification, which indicates that the Kaskapau sea floor had extremely low relief, lacked a shelf‐slope break, and was probably nowhere more than a few tens of metres deep. The absence of clinoforms probably indicates a long‐term balance between rates of accommodation and sediment supply. Mud is interpreted to have been transported >250 km offshore in a sea‐bed nepheloid layer, repeatedly re‐suspended by storms. Fine‐grained sediment accumulated up to a ‘mud accommodation envelope’, perhaps only 20 to 40 m deep. Continuous re‐working of the sea floor by storms ensured that excess sediment was redistributed away from areas that had filled to the ‘accommodation envelope’, being deposited in areas of higher accommodation further down the transport path. The facies distributions and stratal geometry of the Kaskapau shelf strongly suggest that sedimentary facies, especially grain‐size, were related to distance from shore, not to water depth. As a result, the ‘100 to >300 m’ depth interpreted from calcareous claystone facies for the more central parts of the Interior Seaway, might be a significant overestimate.

The Lower Colorado Group (Late Albian-earliest Cenomanian) has been allostratigraphically divided on the basis of regional unconformities and transgressive surfaces, resulting in recognition of an informal Lower Colorado allogroup, comprising the Paddy, Joli Fou, Viking, Westgate and Fish Scales alloformations. The Paddy alloformation forms an eastward-thinning wedge up to 125 m thick, composed of nine allomembers that progressively onlap the basal unconformity PE0 from west to east. Paddy rocks are mainly alluvial in the west, grading into marginal marine in the east and north. Paleo valley-fills are present at the tops of most allomembers. The Joli Fou alloformation transgressively overlies nonmarine Mannville Group rocks, and forms a relatively sheet-like blanket (average 20 m) of marine mudstone that coarsens in the north, where it is assigned to the lithostratigraphic Viking Formation, and in the far south, where it is assigned to part of the lithostratigraphic Bow Island Formation. The Viking alloformation erosively overlies the Joli Fou alloformation at surface VE0 and consists of regional allomembers VA, VB and VD, separated by unconformities VE0, VE1, VE3 and VE4. Allomembers VA and VB (mean 30 m thick) are intensely bioturbated shallow marine silty fine sandstones whereas allomember VD is weakly bioturbated, and includes sandy shoreface deposits in the SW and thick (>50 m) offshore marine mudstone in the north (part of the lithostratigraphic Hasler Formation). Allomember VC is confined to paleovalley deposits below VE3. All regional Viking allomembers can be traced into the lower Bow Island Formation in the south. Marine mudstones of the Westgate alloformation form a wedge thinning from >600 m in the NW to <40 m in the far south, comprising informal units WA, WB and WC from base to top. Westgate strata onlap southward onto VE4 such that only unit WC persists to southern Alberta, where it passes into marginal marine facies of the middle and upper Bow Island Formation. The Fish Scales alloformation (earliest Cenomanian) erosively overlies the Westgate alloformation at surface FE1 and comprises two units FA and FB, the former being confined to a depocentre in the NW. The Fish Scales alloformation is characterized by abrupt introduction of fine sand into the basin and sea-floor erosion which formed uranium-enriched phosphatic lags which give a characteristic highly radioactive log signature. An absence of benthic fauna and high organic content indicate deposition below anoxic water. The top of the Fish Scales alloformation is the Fish Scales Upper (FSU) marker which is a highly radioactive condensed section and downlap surface below prograding clinothems of the early-mid Cenomanian Dunvegan alloformation. Allomember FB is locally coarse-grained in the far south, forming the lower part of the Barons Sandstone, whereas the upper Barons is fine grained and equivalent to allomember C of the Dunvegan Formation.The Paddy alloformation is entirely, or almost entirely older than the Joli Fou alloformation, and henc...

The Kaskapau and Cardium Formations span Late Cenomanian to Early Coniacian time and were deposited on a low-gradient foredeep ramp. The studied portion of the Kaskapau Formation spans ca 3AE5 Myr and forms a mudstone-dominated wedge thinning from 700 to <50 m from SW to NE over ca 300 km. In contrast, the Cardium Formation spans about 2AE1 Myr, is about 100 m thick, sandstone-rich and broadly tabular. The Kaskapau and Cardium Formations are divided, respectively, into 28 and nine allomembers, each bounded by marine flooding surfaces. Kaskapau allomembers 1 to 7 show about 200 km of offlap from the forebulge, accompanied by progradation of thin sandstones from the eroded forebulge crest. In contrast, Kaskapau allomembers 8 to 28 and Cardium allomembers C1 to C9 show overall onlap onto the forebulge of about 350 km, and contain no forebulge-derived sandstones. This broad pattern is interpreted as recording a latest Cenomanian pulse of tectonic loading which led to shoreline back-step in the proximal foredeep and coeval uplift of the forebulge, leading to erosion. The advance of the sediment wedge after Kaskapau allomember 7 is attributed primarily to the isostatic effect of a distributed sediment load; the advance of the orogenic wedge had a subordinate effect on subsidence of the forebulge. For Kaskapau allomembers 1 to 6, isopachs trend north to south, suggesting a load directly to the west; allomembers 7 to 28 show an abrupt rotation of isopachs to NW-SE, suggesting that the load shifted several hundred kilometres to the south. This re-orientation might be related to a change from an approximately orthogonal to a dextral transpressive stress regime.Within the longer-term offlap-onlap cycle recorded by the Kaskapau and Cardium Formations, individual allomembers are grouped into packages reflecting higher-frequency onlap-offlap cycles, each spanning ca 0AE5 to 0AE7 Myr. Offlap from the forebulge tends to be accompanied by more pronounced transgression in the foredeep, whereas onlap onto the forebulge is accompanied by progradation of tongues of shoreface sandstone. This relationship suggests that changes in deformation rate in the orogenic wedge modulated proximal subsidence rate, enhancing or suppressing shoreline progradation, and also causing subtle uplift or subsidence of the forebulge region. Wedge-shaped allomembers in the Kaskapau Formation contain shoreface sandstone and conglomerate that prograded, respectively, <40 and 1 Present address: Imperial Oil Resources, 395 <25 km from the preserved basin margin; progradation of coarse clastics was limited by rapid flexural subsidence. Tabular allomembers of the Cardium Formation imply a low flexural subsidence rate and contain sandy and conglomeratic shoreface deposits that prograded up to ca 180 km from the preserved basin margin. This relationship suggests that low rates of flexural subsidence promoted steeper alluvial gradients, more vigorous gravel transport and more extensive shoreface progradation. Overall, observed stratal geometry and facies distributio...