PAYENBERG, TOBIAS H.D., University of Toronto, Dept. of Geology, Toronto, ON, Canada

ABSTRACT: Stratigraphic Relationships of the Diachronous Upper Cretaceous EagleMilk River Formation in Montana and Southern Alberta

The Upper Cretaceous EagleMilk River Formation is an overall regressive, clastic wedge in Montana, southern Alberta and southwestern Saskatchewan. Despite name differences across the international border, the formation is thought to be in lithostratigraphic continuity. Recent U-Pb geochronology on zircons from bentonite beds in the overlying, transgressive ClaggettPakowki Formation, however, indicate a hiatus of 2.5 million years between the Milk River and the Pakowki Formation in southern Alberta. Such a hiatus is not thought to be present throughout central and southern Montana. While the coarser clastic, coastal rocks of the Milk River Formation in southern Alberta are of upper Santonian age, the similar, nearshore clastics of the Eagle Formation in southern Montana are of lower Campanian age.

Large reserved of shallow biogenic gas can be found in the more distal rocks of the EagleMilk River Formation throughout Montana, Alberta and Saskatchewan. A thorough understanding of the stratigraphic relationships within the EagleMilk River depositional system is vital for field re-evaluation and future shallow gas exploration. To investigate the stratigraphic relationships within the EagleMilk River Formation detailed sedimentological sections in outcrops throughout southern Alberta and north and central Montana are integrated with subsurface wireline correlations. Preliminary results suggests more continuous, marine sedimentation in the upper part of the Eagle Formation towards central Montana, while a hiatus in deposition was occurring in the proximal parts of the Milk River Formation and northern parts of the Eagle Formation.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid