--> --> Abstract: New Insights into the Chrono and Sequence Stratigraphy of the Milk River/Eagle and Pakowki/Claggett Formations in Southern Alberta and Montana, by Tobias H. D. Payenberg and Andrew D. Miall; #90914(2000)

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Tobias H.D. Payenberg1, Andrew D. Miall1
(1) University of Toronto, Toronto, ON

Abstract: New insights into the chrono and sequence stratigraphy of the Milk River/Eagle and Pakowki/Claggett Formations in Southern Alberta and Montana

The overall regressive Milk River Formation in southern Alberta and the Eagle Formation in Montana have traditionally been correlated lithostratigraphically. The name difference for the same rocks stems from the Canadian/U.S.A. border across which the formation spans. U-Pb zircon geochronology from the Legend Bentonite Zone in the overlying, transgressive Pakowki Formation in southeast Alberta gives an age of 80.7 ± 0.2 Ma, and correlates well with Montana’s Ardmore Bentonite K-Ar age of 79.5 Ma, and 40Ar/39Ar date of 80.7 ± 0.55 Ma. The Pakowki/Claggett transgression was therefore synchronous across southern Alberta and Montana. It was also a very rapid transgression, as the maximum flooding surface identified in over 500 well logs lies very close to the lower contact of the formation.

Despite the synchronous transgression in the study area, there is a considerable time gap of approximately 3 Ma between the Milk River Formation and Pakowki Formation in southern Alberta. During this hiatus, the Alderson member was deposited as a lowstand wedge to the east, putting a sequence boundary between the Milk River Formation and the Alderson Member. This sequence boundary can be traced into north central Montana, where is seems to subdivide the Eagle Formation into two parts. The western part stratigraphically correlates with the Milk River Formation in southern Alberta, while the eastern, gas bearing rocks correlate with the Alderson member in southeast Alberta. The location of the sequence boundary in Montana and Alberta has possible control over hydrocarbon migration and trapping, especially in the Tiger Ridge area.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana