Devonian Stratigraphic Framework in Western Montana and East-Central Idaho: Paleogeographic Highs and Shared Intrashelf Basin Depositional Patterns
We are assembling a regional sequence stratigraphic framework for Devonian rocks, up to 1.5 km thick (Beartooth Butte, Maywood, Jefferson and Three Forks formations) of east-central Idaho and southwestern Montana using facies stacking patterns, sequence boundaries, and limited conodont data to better understand reservoir potential. The Idaho (down-dip) portion of the margin records a much longer period of deposition with Lochkhovian to late Frasnian dolostone and quartz arenite deposited on a passive margin carbonate ramp unconformably overlain by active margin, late Frasnian to Famennian dolostone, limestone, breccia, quartz arenite, and shale. Similar up-dip Frasnian-Famennian deposits in Montana are significantly thinner. Paleohighs and lows across the margin produced a segmented (faulted?) ramp along the Antler borderland during the Famennian.
Local tectonic adjustments, significant unconformities, loss of biostromal community members, and changes in the diagenetic environment near and above the Frasnian-Famennian boundary fundamentally changed stratigraphic architecture. The lower Devonian succession was deposited on a relatively quiescent passive margin during 2nd-order Kaskaskia sea level rise in a global greenhouse climate mode. Conversely, the upper succession was deposited during the 2nd-order Kaskaskia fall as this margin was transitioning into a compressive margin with coincident change to global icehouse conditions.
In Idaho major sequence boundaries in the lower succession have well-defined incised paleovalleys and gradual deepening-upward stacking patterns with low-amplitude peritidal cyclicity. The upper stratigraphic succession is more complex and includes development of restricted intra-shelf basins east and west of the Lemhi Arch (Idaho-Montana border area). Stratigraphic stacking patterns, influx of siliciclastics, and hiatuses on ramps adjoining the paleohighs are similar before regional Mississippian flooding suggesting coherent eustatic signals and mixed silicicilastic and carbonate facies shifts. Locally, evaporite units formed in the upper Jefferson and lower Three Forks formations, followed by strong influx of siliciclastics from the east, localized unconformities and irregular deposition of black shales (Bakken equivalent).
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009