Anatomy of a late Cenomanian transgressive shelf system: The influence of high-frequency eustasy and crustal flexure on stratigraphy and paleogeography, basal Kaskapau Formation, Western Canada Foreland Basin

Abstract

The late Cenomanian Kaskapau Formation records transgression of the Greenhorn Sea over deltaic strata of the Dunvegan Formation. However, stratigraphic and paleogeographic details of the initial stages of this profound reorganization of basin geography have not previously been determined. The basal Kaskapau strata above the Dunvegan Formation are assigned to the informal allostratigraphic ‘A-X unit’. Subsurface and outcrop correlation shows that five regionally-mappable allomembers, bounded by marine flooding surfaces, can be mapped within the A-X unit which, as a whole, forms a wedge that thickens south-westward from <5 to 60 m over approximately 300 km. Within each allomember, three main depositional environments can be distinguished. Sandy heterolithic facies in the north and west form few-metre scale, upward-shoaling successions that contain abundant brackish-water molluscs and are capped by paleosols and dinosaur-trampled surfaces. These rocks represent river-dominated deltas that prograded into a low-energy embayment, about 200 x 200 km, and open to the SE. The central part of the embayment accumulated mud-dominated heterolithic successions with a restricted fauna of lingulid brachiopods and inoceramids, suggestive of turbid, low-energy and reduced salinity conditions. The muddy facies enclose isolated units of well-sorted fine- to very fine-grained sandstone that form NE-SW elongate bodies up to 170 km long, 50 km wide and 11 m thick. Each sandstone body is interpreted to represent the shoreface of one or more strandplains and/or wave-dominated deltas that developed across the mouth of the embayment at sea-level lowstand. Isopach mapping shows that the A-X unit, as a whole, forms a prismatic wedge, thickest adjacent to the fold and thrust belt in the SW. This geometry indicates that accommodation was created by spatially uniform flexural subsidence in response to a linear tectonic load striking NW-SE. Isopach maps of individual allomembers, however, show that subsidence took place in a more complex pattern, with localized depocentres of approximately 100 km radius that suggest brief periods of subsidence before being superseded by a new depocentre along-strike. This pattern may reflect temporally and spatially discontinuous deformation in the adjacent orogenic wedge. During each relative sea-level cycle, the shoreline oscillated by about 150–200 km in a NW-SE direction. Shoreline movement was perpendicular to the direction that would be expected (i.e. SW-NE), if flexural subsidence had been the dominant control on relative sea-level change. This geometric relationship suggests that allomembers were generated by high-frequency, high rate eustatic changes that were superimposed on a lower rate of flexural subsidence. Isopach maps spanning Dunvegan allomembers C, A+B, and the Kaskapau A-X unit show a near 90° anticlockwise rotation of isopleths. This abrupt re-orientation of the axis of flexure marks a new phase of subsidence linked to the onset of NE-directed thrusting that was driven by dextral transpression across a restraining bend in the Northern Rocky Mountain Trench fault. This new stress regime was in turn linked to a change in the convergence direction between the North American and Farallon plates.