Abstract:

Mudstone-dominated shallow-marine rocks of the Kaskapau Formation were deposited on a low-gradient ramp that spanned the foredeep of the Western Canada foreland basin during the late Cenomanian to middle Turonian. Organic-, clay-rich, and silt-rich mudstone accumulated on the flank of the forebulge, > 200 km from the western shoreline. Within this mudstone, a large proportion of the clay minerals are organized into silt- and very fine sand-size aggregate grains. These aggregates were produced both in the water column as marine snow and phytodetritus and also through reworking of previously deposited cohesive mud to form intraclasts. The latter, along with siliceous silt grains, form wave- and combined-flow ripples, graded beds, scour-fills, and ripple-tail lamination. Where sand-size sediment (comprising detrital siliceous, calcareous bioclastic, or phosphatic grains) is present, it is molded into combined-flow ripples, HCS, gutter casts, and lags. Thus all sediment grades indicate storm- wave and current reworking of the sea floor at a distance of > 200 km offshore. The common occurrence of clay minerals in the form of aggregate grains, organized into combined-flow ripples and parallel lamination, implies advective transport of clay minerals as bedload, driven by combined flows across a very low-gradient ramp. On the distal part of this ramp, latest Cenomanian rocks include thin tongues of SW-prograding quartz-rich sandstone that was derived from an emergent forebulge. This source was drowned during the early Turonian eustatic rise when the sediment abruptly changed to organic-rich mudstone dominated by clay-mineral aggregates. This compositional change was mainly a response to a sudden increase in distance to detrital sources in the west, rather than a dramatic increase in water depth. Throughout much of early to middle Turonian time, the sea floor in the forebulge region lay above effective storm wave base for silt, estimated at ∼ 70 m. At sea-level lowstands, wave winnowing and erosion of the sea floor concentrated bioclastic lags at the top of siltier-upward sequences; lags are interpreted to correspond to falling-stage, lowstand, and early transgressive systems tract deposits on the western margin of the basin. Previous studies may have substantially overestimated water depth for organic- and clay-rich calcareous mudstones in the Western Interior Seaway.

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