TRANSITIONAL UPPER DEVONIAN STRATIGRAPHY IN IDAHO—A RELATIVE SEA-LEVEL GEOHISTORY SUGGESTS COMPLEX DISCONFORMITIES ON A LOW LATITUDE WESTERN EURAMERICAN MARGIN

Devonian shelf evolution is organized into two successions using facies stacking patterns, sequence boundaries and limited conodont data. Siliciclastic and carbonate ramp strata of the Jefferson Formation consist of 1000+ m of a Lochkovian to late Frasnian (rhenana Zone) lower succession and a very thick to locally thin (500-70m) Famennian upper succession capped by widespread fossiliferous limestones and shales (Three Forks Fm.), and overlain by Mississippian turbidites. Jefferson lithofacies consist primarily of laminated, bioturbated, muddy fossiliferous carbonates and quartz arenites, but their accumulation geometry changed. Eastern strandlines connected to a conformable, western distal shelf wedge (lower succession) switched to a confused Famennian paleogeography (upper succession) where open marine biota disappeared and cryptic unconformities and stromatolitic shallow water facies occur across the entire shelf.

Famennian rocks are separated from lower Devonian rocks by widely accommodated latest Frasnian (rhenana-linguiformis zones?) transgressive sequences. These include “Nisku” equivalent Thamnopora and Amphipora biostromes. Lithofacies above these rocks include sandy brecciated carbonates (east) and shoaling peloidal-intraclastic rocks (west). Proximal to distal shelf lithofacies are cyclic at meter to 10m scales, but larger (3^rd-order?) 50-100m sequences are observed in only pre-Famennian rocks. Quartzose sands were throughout the Devonian derived from the craton.

The lower stratigraphic succession was deposited on a relatively quiet margin during 1st-order Kaskaskia sea level rise. The upper stratigraphic succession was deposited during global(?) 2nd-order regression and climatic-eustatic effects. Gross stratigraphic geometry suggests an intra-shelfal basin formed in the early Famennian, but by middle Famennian time, a well dated, more homogenous blanket of the Three Forks Formation was draped across the entire shelf. Prior to extreme Mississippian drowning, upper Devonian strata in Idaho suggest incipient, regional extensional foreland break-up of the margin with discontinuous Jefferson facies followed by a radical change in depositional environments, cyclicity and hiatus during Three Forks time.