Palaeoenvironments, palaeogeography, and physiography of a large, shallow, muddy ramp: Late Cenomanian‐Turonian Kaskapau Formation, Western Canada foreland basin

Abstract

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.

Sedimentology of the early Neoproterozoic Morar Group in northern Scotland: implications for basin models and tectonic setting

“…Beaumont 1981). The gradual and systematic facies changes (both laterally and vertically) exhibited by the Morar Group and the lack of sharp influxes of coarse deposits are consistent with the genesis of the basin under a suite of conditions such that the rate of subsidence exceeded the rate of sediment flux, a pattern not uncommon in foreland settings (Tankard 1986;Varban & Plint, 2008;Yang & Miall 2010). The palaeocontinental reconstructions also indicate that the Moine basin was located relatively near to the periphery of Rodinia (Li et al 2008; see also Cawood et al 2010), so that the evolving basin may also have been affected by far-field plate boundary effects and sea-level changes, similar to those which would have affected correlative successions in East Greenland and Svalbard (Cawood et al 2010).…”

“…Cretaceous Interior Seaway basins that fringed the North American Cordillera from Canadian to Mexico (Beaumont, 1981;Flemings & Jordan 1989;Cant & Stockmal 1989;Flemings & Jordan 1990;Jordan & Flemings 1991;Underschultz & Erdmer 1991;Plint et al 2001;Varban & Plint 2008;Yang & Miall 2010). There, episodic loading events are known to have driven km-scale progradational-retrogradational depositional cycles, not dissimilar to those described above for the Morar Group.…”

Wave-enhanced sediment-gravity flows and mud dispersal across continental shelves: Reappraising sediment transport processes operating in ancient mudstone successions

“…To reconcile these observations with theoretical constraints, mechanisms have been proposed to explain observed cross-shelf distributions of mud. These include storm-driven underfl ows (Abbott, 2000) and combined fl ows oriented oblique to shore because of Coriolis steering (e.g., Varban and Plint, 2008). Due to the perceived shortcomings of these solutions, Dalrymple and Cummings (2005) argued that cross-shelf transport occurred mostly during periods when accommodation availability was low, due to base level fall and basinward shift of the marginal marine mud belt.…”

Clay fabric of fluid-mud deposits from laboratory and field observations: Potential application to the stratigraphic record

“…The depositional processes and environments of muds and mudstones have recently been reconsidered in terms of energetic wave and current conditions in coastal and shallow-marine environments, based on detailed studies in modern environments (Wells and Coleman, 1981;Rine and Ginsburg, 1985;Kuehl et al, 1986;Kineke and Sternberg, 1995), experimental studies (Baas and Best, 2002;Schieber et al, 2007;Baas et al, 2009;Schieber and Southard, 2009), and examinations of stratigraphic records (Varban and Plint, 2008;Bhattacharya and MacEachern, 2009;Ichaso and Dalrymple, 2009;Ghadeer and Macquaker, 2011;Mackay and Dalrymple, 2011). This recent interpretation contrasts with the 'traditional' idea that shallow energetic environments are typified by sandy deposits.…”