Cretaceous Strata at the West Edge of the Rockies — A Wedge-Top Remnant of the Western Canada Foreland Basin

Abstract

Deformation and uplift of the Canadian Rocky Mountain Fold and Thrust Belt (FTB) in the Late Cretaceous and Tertiary has removed the record of the Western Canada Foreland Basin (WCFB) almost everywhere west of the Foothills or Front Ranges. Cretaceous strata at two localities along the western edge of the Rocky Mountains in the western Stanford Range (WSR) of southeast British Columbia provide unique insights on the western part of the WCFB. Both localities occur in down-dropped, fault bounded slices along the west side of the Redwall Fault and are in the oil window. Two main lithologic packages were exposed at the first locality: 1) Black limestone with ostracods, gastropods, shaley organic material and minor sandstone thought to be the age of the Blairmore calcareous member and probably Middle Albian in age; 2) Dark mudstone, siltstone and sandstone with one band of chert and quartz sandstone pebble conglomerate of Late to possibly Middle Albian age based on well preserved palynomorph assemblages. Strata at a second locality consist of soft grey siltstone, mudstone, very fine-grained sandstone and minor carbonaceous shale. Resistant bands of poorly sorted, somewhat stratified chert and quartz sandstone clast conglomerate occur in the lower part of the succession. Palynomorph assemblages indicate a Late Albian to Early Cenomanian age. Detrital zircons indicate a maximum depositional age of Early Cenomanian. Preliminary thermal modelling of AFT and VR data indicates that these sedimentary rocks were heated relatively quickly shortly after deposition and then cooled from about 90° C to near surface conditions in the early Tertiary. Cretaceous strata at both WSR localities were never deeply buried by the Maastrichtian-Paleocene foreland basin clastic wedge. Structural, lithostratigraphic, chronologic and provenance relationships suggest the following history: 1)Shortening and deformation of western part of FTB in Barremian-Aptian? with kilometres of uplift and erosion west of the Redwall Fault. 2) Eroded WSR blanketed by Blairmore calcareous member and nonmarine upper Blairmore strata (Middle to Upper Albian). 3) Deposition in WSR of nonmarine lower Cenomanian strata (wedge-top); thrusting and orogenic loading east of Stanford Range; nondeposition and erosion in Foothills. 4) Maximum 2-3 km burial of WSR by Paleocene as basin is piggy-backed by Late Cretaceous-Paleocene shortening. 5) Oligocene-Miocene normal faulting down-dropping Cretaceous blocks in WSR.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016