Abstract: The Late Albian Lowstand Event, Central Montana and Southern Alberta

PORTER, KAREN W., THADDEUS S. DYMAN, DAVID A. LOPEZ, GERRY E. REINSON, W. A. COBBAN, and GARY G. THOMPSON

The late Albian (98 m.y.) lowstand unconformity that is well known in the Skull Creek Shale-Muddy Formation interval of Wyoming and in the Joli Fou Shale-Viking Formation interval of southern Alberta is now also recognized in central Montana. In south-central (Pryor Mountains) and central (Grassrange area; Judith Mountains) Montana, a chert pebble and bone lag deposit marks the unconformity at the base of marine mudrocks and poorly developed sandstones comprising the informal sandy member of the Thermopolis Shale. Farther north, in the Little Rocky Mountains, the Muddy-equivalent Cyprian Sandstone is a shoreline-channel deposit underlain and overlain by marine shales of the Skull Creek and upper Thermopolis, respectively; the unconformity here may correlate with the capping chart-pebble bed or may be unexposed below the sandstone. In the Bears Paw Mountains, the unconformity is tentatively placed at the chert-pebble lag at the top of the upper of three coarsening-upward marine cycles. In the eastern Sweet Grass Hills, the lowstand unconformity is placed at a pebble lag at the top of the upper of six coarsening-upward cycles, the Bow Island-Viking interval, and from there it is correlated north in the subsurface at the Viking sandstone level where it is thought to underlie valley-fill deposits as it does in northern Wyoming. Sedimentologic and palynologic data for this late Albian interval in central Montana suggest that this part of the Western Interior sedimentary basin remained marine while well-documented subaerial incisement was occurring on the basin margins.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado