--> ABSTRACT: Differential Subsidence and the Distribution of Sequence Boundaries in the Shallow Gas Bearing Eagle/Milk River Formation in Southern Alberta and Northern Montana - Lessons From Outcrop, by Tobias H. D. Payenberg, Andrew D. Miall, Dennis R. Braman, and Donald W. Davis; #90906(2001)

Datapages, Inc.Print this page

Tobias H.D. Payenberg1, Andrew D. Miall1, Dennis R. Braman2, Donald W. Davis3

(1) University of Toronto, Toronto, ON
(2) Royal Tyrrell Museum of Palaeontology, Drumheller, AB
(3) Royal Ontario Museum, Toronto, ON

ABSTRACT: Differential Subsidence and the Distribution of Sequence Boundaries in the Shallow Gas Bearing Eagle/Milk River Formation in Southern Alberta and Northern Montana - Lessons From Outcrop

The Upper Cretaceous Eagle Formation in northern Montana and the lithostratigraphic equivalent Milk River Formation of southern Alberta are producing shallow gas from conventional shallow marine rocks and from unconventional silty and shaley rocks. A detailed knowledge of regional stratigraphic relationships within these formations are essential for future development and exploration. Detailed chrono- and sequence-stratigraphic analysis of the Eagle/Milk River formation revealed major stratigraphic breaks, which are responsible for reservoir distribution.

U-Pb geochronology confirmed a gap of 2.5 Ma, or 5 ammonite zones in the early Campanian between the Milk River Formation and the overlying Pakowki Formation around Writing-On-Stone Provincial Park in southern Alberta. During this hiatus, shallow marine sedimentation took place in northern and central Montana. Palynological assemblages indicate these rocks to be of early Campanian age, while the presence of dinoflagellates confirms deposition in a marine environment.

The stratigraphic relationship of the hiatus and shallow marine sedimentation mean a sequence boundary is sitting below these marine rocks within the upper part of the Eagle Formation. The top of the Eagle/Milk River Formation is also marked by a sequence boundary. Chert pebbles and the presence of reworked sands from the underlying Eagle Formation indicate the Eagle/Milk River Shoulder to be a transgressive surface of erosion. These sequence boundaries, especially the lower one, are evidence for the active tectonic nature of the basement underlying the Western Interior Basin. Differential subsidence along-strike might reflect the movement of individual thrust sheets in the building Rocky Mountains.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado