Abstract

The late Coniacian Bad Heart Formation in the northwest Alberta Plains is dominated by very fine, variably ooidal sandstone, overlying a dark laminated mudstone. The formation is organised into two principal, sandier-upward sequences, bounded by erosion surfaces. The Bad Heart Formation passes laterally into mudstone both towards the southwest and the northeast. The base of the Bad Heart is a pebble-veneered erosion surface that truncates the underlying Marshybank and Muskiki formations. The Bad Heart is up to about 32 m thick, but shows dramatic lateral variations in thickness from east to west. Thickness changes take place laterally over a few kilometres, and zones of change can be mapped as linear, northwest-southeast-trending features. Progressively older strata subcrop against the basal erosion surface from southwest to northeast. At least 30 m of strata have been bevelled off in the study area at a rate of about 30 cm/km. We attribute regional bevelling to marine (and possibly subaerial) erosion over a rising forebulge, which also appears to have induced uplift of the Peace River Arch. Erosional bevelling (over ? <100 000 yrs.), extends from about 200 km to at least 300 km from the Coniacian orogenic front. Following regional bevelling, the Bad Heart Formation was deposited, probably in <300 000 yrs. Sediment was derived primarily from the rising forebulge/Peace River Arch in the north and northeast. During deposition of the Bad Heart, the area underwent no further regional tilting. Four principal, linear zones of rapid thickness and/or facies change in the Bad Heart are interpreted to record minor stratal drape of 5-20 m, over deeper horst and graben structures. These blocks are bounded by faults that are interpreted to flatten at depths of several km into low-angle decollement zones separating Proterozoic terranes. We postulate that subtle alternation between compression and extension on the decollement zones, related in turn to activity in the Cordillera, could explain, respectively, the observed alternation between regional warping and periods of block faulting as indicated by evidence from the Bad Heart and adjacent formations.

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