INVITED: DISSOCIATING THE PLACEMENT OF HIATUSES FROM LITHOLOGICAL CRITERIA IN NONMARINE SUCCESSIONS, ALBERTA FORELAND BASIN

The accepted axiom is that major unconformities occur at the base of regionally extensive high energy sandstone/conglomeratic units in fluvial systems. Two examples from the foredeep of central Alberta demonstrate that the major, sequence-bounding, unconformities frequently occur within fine-grained clastic and coal-bearing, low energy depositional units somewhat below the lithologically conspicuous base of high energy rock units. The stratigraphically lowest of these is the Upper Maastrichtian to Lower Paleocene Coalspur/Scollard sequence. Proximal to the orogen the conventional placement of the lower bounding unconformity has been at the base of the Coalspur “Entrance” conglomerate. Palynologically, the time gap occurs within the fine clastics of the underlying Brazeau Formation. More distally, the base of the amalgamated basal Scollard sandstone has been considered as overlying a regional unconformity. Instead, the most extensive hiatus has been demonstrated palynologically to be at the base of the underlying lacustrine mudstones of the Battle Formation, which directly overlie laminated claystones of the Whitemud Formation. Overlying the Upper Maastrichtian to Lower Paleocene sequence is the upper Paleocene Paskapoo sequence. The basal, sometimes conglomeratic, amalgamated sandstone of the Paskapoo Formation has been considered to unconformably overlie the fine-grained sediments of the upper Coalspur/Scollard formations. Now, in the most proximal preserved portion of the foredeep, the Early/Late Paleocene hiatus is found to be 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 Paskapoo sandstones and the underlying coal-bearing upper Scollard Formation.

Sequence boundaries positioned within fine-grained clastic units are interpreted to separate sediments deposited during the waning phase of a prior sequence and similar lithologies deposited during the initial stages following orogenic uplift/basin subsidence event prior to the entry of high energy sediments into the foredeep. The less usual occurrences of sequence- bounding unconformities at the base of regionally extensive coarse units may be related to the proximity to the peripheral bulge.