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Uncoupling the Position of Sequence-Bounding Previous HitUnconformitiesNext Hit from Lithological Criteria in Fluvial Systems

Arthur Richard Sweet1, Octavian Catuneanu2, and John F. Lerbekmo2
1 Natural Resources Canada, Calgary, AB
2 University of Alberta, Edmonton, AB

The accepted axiom is that major Previous HitunconformitiesNext Hit occur at the base of regionally extensive sandstone/conglomeratic units. Tested palynologically, this axiom may be more the exception than the rule.

An Upper Maastrichtian to Lower Paleocene Coalspur/Scollard depositional sequence occurs across the foredeep of central Alberta. Proximal to the orogen the conventional placement of the lower bounding unconformity has been at the base of the Coalspur “Entrance” conglomerate. Palynologically, the hiatus occurs within the fine clastics of the underlying Brazeau Formation. More distally, the base of the amalgamated Scollard sandstone has been considered as overlying a regional unconformity. Instead, this hiatus has been demonstrated palynologically to be at the base of the underlying lacustrine mudstones of the Battle Formation. Similarly, the basal sandstone of the Upper Paleocene Paskapoo Formation has been considered to unconformably overlie the fine-grained sediments of the upper Coalspur/Scollard formations. Now, in the proximal region, the Early/Late Paleocene hiatus is found within the uppermost part of the underlying coal-bearing upper Coalspur Formation. However, a more conventional pattern occurs distally, where the major hiatus is between the basal amalgamated Paskapoo sandstones and the underlying coal-bearing upper Scollard Formation.

Sequence boundaries within fine clastics are interpreted to separate sediments deposited during the waning phase of a prior sequence and similar lithologies deposited outboard from the orogen subsequent to renewed subsidence and before coarse sediments spill over the foredeep. Exceptional occurrences of sequence bounding Previous HitunconformitiesTop at the base of regionally extensive coarse units may be related to proximity to the peripheral bulge.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005