Abstract

The Muskiki and Mistanusk formations (Coniacian-Santonian) crop out in the deformed belt of the Foothills in northwestern Alberta and northeastern British Columbia. Nine outcrop sections were examined; five cores and 1100 well logs provide supplementary subsurface data. Outcrop sections consist of several coarsening-upward sequences (parasequences) within an overall upward coarsening, prograding clastic shoreline system. Parasequences may be capped by a pebble bed. Comparable parasequences are recognizable in logs. Subsurface correlation of these parasequences demonstrates that they can be traced basinward for many tens of kilometres.

The Bad Heart type section in the Plains consists of peloidal grainstones and oolitic sandstones. Detailed correlation of this unit into the subsurface demonstrates that it pinches out toward the west and lies stratigraphically above the ‘Bad Heart Formation’ of the Foothills. The latter, which pinches out toward the east, is therefore genetically distinct from the Bad Heart type section. Consequently, the Foothills units are informally renamed as the Mistanusk formation of the Smoky Group.

Detailed correlation of parasequences reveals four important aspects of Mistanusk history: 1) shelf sediments form a series of up to four coarsening-upward parasequences, each of which probably represents shelf aggradation following a minor relative sea-level rise, 2) shoreface sandstones prograded toward the east-northeast for at least 30 kilometres, 3) locally, shoreface sandstones rest on eroded muddy shelf sediments, and 4) the top of the Mistanusk has an erosional topography, with a relief of about 10 m.

These data suggest that: 1) the Muskiki and Mistanusk formations represent a major transgressive - regressive package, within which minor relative sea-level changes are recognized, 2) toward the end of Mistanusk deposition, relative sea-level fall promoted rapid shoreface progradation over, and local scouring into, a shallow, muddy shelf, 3) continued relative sea-level fall, and possible concurrent tectonic uplift, resulted in subaerial erosion to the west and seaward movement of the shoreline tens of kilometres into the basin, and 4) erosional shoreface retreat during subsequent transgression carved an erosional topography into the Mistanusk Formation, forming a regional unconformity. In outcrop, this unconformity is overlain by a pebble veneer. The stratigraphic position and lithology of the Bad Heart Formation suggest that it represents deposition on a relatively starved shelf during lowstand and the earliest phase of transgression. Parasequence thicknesses indicate that subsidence was greatest in the west near the thrust belt. From south to north, the Muskiki and Mistanusk thin from 75 to 45 m, before pinching out altogether. This suggests that the Peace River Arch may have been a positive tectonic element at this time.