The Late Devonian Jefferson Formation in the Lemhi Range is composed of large scale sequences of cyclic dolostone and sandstone deposits. Jefferson rocks thin to the southeast from approximately 1068 meters in the Gilmore area to 110 meters at Black Canyon. Geomorphology and “hinge-line” sedimentation along the western Laurussian shelf over a period of about 30 million years includes 1. Accommodation effects of incipient Antler foreland loading, 2. Stratigraphic modification by persistent paleo-high and autocyclic controls, and 3. Major fluctuations in relative sea level (glacio-eustacy?). Coarsely resolved Lemhi sections are coupled with meter scale paleoenvironmental interpretations and previous studies to produce 4 paleogeographic maps for southeast Idaho.

Discontinuous estuarine deposits of the Middle Devonian channel sandstones and associated pre-Jefferson Beartooth Butte Formation (Emsian-Eifelian) record the initial foundering of the karstified, incised and planed Lower Paleozoic shelf. Thick conglomerate debris flows occur near Badger Creek. Mappable lithologies of the Jefferson Formation follow. These are divided into six members (D1 to D6 — after Hair, 1965).

400+ meter thick Givetian to Frasnian peritidal (“Banded Dolomite” — D1 and D2) and off-shore biostromal deposits of the (“Dark Dolomite” — D3) record strong marine transgression. However, after the Frasnian/Famennian boundary, sedimentation style changed: 350+ meter thick deposits beginning with regressive shallow water and strandline deposits of the Grandview Dolomite (Famennian — D4) are followed by minor inversion or transgression and deposition of the sparsely fossiliferous “False Birdbear” member — D5). Thick carbonate breccias and extensive bleached limestones of the D6 member (up to 200 meters) are correlated to the Logan Gulch member of the Three Forks Formation. Absent at Black Canyon, these later rocks represent regressive lowstand deposits on a differentially subsiding shelf.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah