HIGH-RESOLUTION SEQUENCE STRATIGRAPHY OF MARINE MUDSTONES REVEALS FOREBULGE RESPONSE TO LOADING, AND SHIFTING THRUST LOADS, WESTERN CANADA FORELAND BASIN

A high-resolution sequence stratigraphic study of the 130-700 m thick mudstone-dominated Cenomanian-Turonian Blackstone Formation and its laterally-equivalent Kaskapau Formation demonstrates the importance of regional scale study of offshore mudstones. This study encompasses an area of about 235,000 km2 in Alberta and British Columbia and integrates data from 33 outcrops, 10 cores, 2800 wells, and biostratigraphy. An allostratigraphic approach integrating subsurface and outcrop data has provided a rare insight into the stratigraphic evolution of the Western Canada Foreland Basin. Allomembers, defined by flooding surfaces or bentonites represent duration of 100-400 ky. Isopach maps of successive allomembers show that along the 500 km orogenic margin, depocenters shifted laterally on length scales of hundreds of km, and time scales ranging from 100-400 ky. These maps show that the deformation front did not advance as a line load but rather as a series of intermittent ‘point loads'. The planform shape of depocentres also reveals subtle to strong basement control on subsidence. A regionally extensive diachronous forebulge unconformity developed during the late Middle Cenomanian. It is expressed as a subtle erosion surface mantled by a bioclastic lag. This unconformity dies out towards the foredeep. This surface is interpreted to be the result of a major loading event manifest as an isopach thickening of late Middle Cenomanian deltaic strata along the SW margin of the basin. Towards the north, over the Peace River Arch, a younger, Late Cenomanian forebulge was emergent and eroded to supply sand from the NE. The location of this forebulge was influenced by suture zones between underlying Precambrian basement terranes that acted as zones of weakness, controlling the location of the forebulge.