Litho-, chrono- and allostratigraphy of the Santonian to Campanian Milk River and Eagle Formations in southern Alberta and north-central Montana: Implications for differential subsidence in the Western Interior foreland basin

The Santonian to Campanian succession of southern Alberta and northern Montana contains the Milk River/Eagle clastic wedge, encased in marine Colorado and Claggett/Pakowki shales. Litho- and chronostratigraphic analysis across the international border showed that the Santonian Telegraph Creek Member/Formation and the Virgelle and Deadhorse Coulee members are in lithostratigraphic continuity, and of similar age along the paleoshoreline. The Earliest Campanian saw clastic sedimentation of the Upper Eagle Member in north-central Montana, while a hiatus of up to 2.5 Ma developed in southern Alberta around Writing-On-Stone Provincial Park. Only the Pakowki/Claggett transgression during the Early Campanian was again synchronous across the study area.; The Santonian Virgelle shoreline is marked by a strong dominance of storm generated sedimentary structures in an unusually thick shoreface succession exceeding 30 m. Campanian Upper Eagle sedimentation took place in a more restricted, deltaic environment that was sourced from the southwest.; Based on an allostratigraphic approach, high-resolution sequence analysis enabled the subdivision of the study interval into seven allomembers. The first three allomembers, Virgelle 1, 2 and 3, are each seaward stepping from another. Following a regional transgression was the deposition of the Upper Eagle A, B and C allomembers, also in seaward stepping shingles. Upper Eagle D Allomember is a transgressive unit topped by the regional “Eagle Shoulder” sequence boundary.; The hiatus developed in southern Alberta during the Earliest Campanian is reflected in the distribution of the Upper Eagle allomembers. Projection of the Upper Eagle allomembers onto structural elements of the Sweetgrass Arch shows the coastal sandstones curving around the Kevin Sunburst Dome element of the Sweetgrass Arch and paralleling its structural contours.; Despite often cited basement control for the Sweetgrass Arch, aeromagnetic data does not show any direct correlation of basement lineaments with the anticlinal crest of the Kevin Sunburst Dome. However, a magnetically heterogeneous block of crest underneath the Kevin Sunburst Dome, surrounded by very highly magnetic crustal material, might have buckled under stress due to different rheological properties. This could explain the up and down movement observed in the hiatus and distribution patterns of the Milk River/Eagle interval.