STAGE BY STAGE DEVONIAN PALEOGEOGRAPHY IN EAST-CENTRAL IDAHO WITH A FAMENNIAN FOCUS: CHALLENGES FACING REGIONAL STRATIGRAPHIC CORRELATION

The establishment of a system-wide sequence stratigraphy and nomenclature for the Devonian shelf will improve the nomenclature for the 300 to 1600 meters of representative shelf strata that span many mountain ranges in Idaho and Montana. Major limiting factors include the need for detailed biostratigraphic study and an underdeveloped correlation between the distal and proximal shelf. Complicating the latter is the uplift and removal of the outer shelf rock record during the Famennian. Whereas the Lower to Early Late Devonian rocks show easily recognizable subtidal to supratidal facies stacking patterns, very cryptic deposits of the Famennian include thick sedimentary and post-depositional brecciated intervals precluding simple interpretation. The latter are associated with artifacts after evaporates and restricted subtidal brines trapped on the expansive, but breaking-up Devonian platform. We model a peri-cratonic "sub-basin-in-motion" with multiple bathymetric/island nuclei for progradational shallow shelf environments.

Specifically of interest in this report is: 1) Lower Devonian coastal systems and incised channels of the long-term 1st order Tippecanoe/Kaskaskia lowstand on the outer margin, 2) Middle to Early Late Devonian shelf-wide (Idaho only) depositional sequences, and 3) "Transitional" Late Devonian time when multiple interpreted diastems and Famennian lowstand events must compete with thick accumulation rates associated with increased subsidence. The Famennian represents a 2nd order interim in the 1st order Kaskaskia supersequence and as previous authors have suggested, it represents a period of regional, even global tectonic and climatic readjustment. Here provided are graphics showing a restricted Famennian sub-basin on the shelf (Central Idaho Trough).

As in Nevada, we appeal to local tectonically driven stratigraphic sequences ("Antler pulses") with very localized uplifts, unconformities and down-dropped hybrid basins. Loaded Early Mississippian foredeeps (of transtensional-compressional character) were coupled with differential accommodation and the effects were first felt by restriction-exposure-hiatus episodes on the Famennian shelf. Glacio-eustasy may also have contributed to these complex stratigraphic successions.