ABSTRACT: Tectonic Control of Facies and Stratal Geometry in the Bad Heart Formation of NW Alberta

DONALDSON, W. STEVEN, and A. GUY PLINT, University of Western Ontario, London, Ontario, Canada

The Bad Heart Formation of northwestern Alberta has been studied in 24 outcrop sections and traced in over 1500 well logs. The Bad Heart is a thin (<20m) package of marine mudstones, siltstones, and oolitic sandstones deposited during a Late Cretaceous (early Santonian) third-order sea level lowstand. The Bad Heart rests unconformably on the underlying Muskiki Formation. The contact is marked by a pebble bed (chert plus phosphatized intraclasts) and the surface has tens of meters of erosional relief. Within the Bad Heart, an abrupt westward transition from sandy to muddy facies can be mapped along a northwest-southeast line. The Bad Heart consists of two major sandier-upward cycles; in outcrop, each cycle culminates in an oolitic ironstone and/or phosphatic horizon and each is capp d by a thin chert/intraclast pebble bed. These are interpreted as fourth-order cycles that record progressive shoaling and clastic starvation. Subaerial emergence and subsequent transgression may be recorded by the pebble beds. Log markers in the underlying Muskiki Formation diverge to the east, whereas in the overlying Puswaskau Formation markers diverge to the west. This suggests a significant change in regional subsidence patterns during Bad Heart time. The thickness of the Bad Heart varies along broadly northwest-southeast-trending lines as a result of differential erosion at the top of the formation; as well, a local graben structure is present in the Puskwaskau. These thickness and facies changes may have been controlled by syndepositional movement along faults.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)