ABSTRACT: Sequence Stratigraphy of the Fall River Sandstone, Northeastern Powder River Basin, Wyoming and Montana

James P. Haerter, Frank G. Ethridge

Complex associations of interbedded sandstones, siltstones, and shales make up the Lower Cretaceous (Albian) Fall River Formation in the northeastern Powder River basin and adjacent Black Hills. Episodes of shoreline progradation produced three coarsening-upward cycles; each of these parasequences is bounded above and below by a marine flooding or correlative surface resulting from rapid transgression. Typical cycles contain, from bottom to top: (1) marine/lagoonal shale and siltstone; (2) marginal marine (foreshore and shoreface) siltstone and sandstone; (3) coastal plain deposits, including fluvial, estuarine, crevasse splay, and marsh facies; and (4) destructional (shoreface) sandstones, overlain by marine siltstones and shales or paleosol horizons.

Black Hills outcrops and cores from southeastern Montana provide evidence for an intraformational unconformity. Marginal marine facies in the middle parasequence are deeply incised and replaced by fluvial and estuarine valley-fill deposits. Blocky to fining-upward fluvial intervals with large-scale trough and tabular cross-bedded sandstones grade upward into bioturbated, estuarine sandstones and siltstones. Correlation of similar facies from 583 electric logs within the adjacent Powder River basin reveals a northward-directed paleodrainage pattern. Paleovalleys range from less than a mile in the south, to nearly 10 mi wide at the northern limit of the study area.

Identification of this sequence boundary within deposits previously interpreted as fluvial and deltaic requires a substantial drop in relative sea level prior to late Fall River deposition. This event induced channel incision across at least 50 mi (north-south) of coastal plain. Resumed transgression subsequently preserved thick fluvial and estuarine sandstones encased in marine and lagoonal mudstones.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990