USE OF KEY SEQUENCE BOUNDARIES IN STAGE-BY-STAGE RECONSTRUCTION OF PASSIVE TO ACTIVE MARGIN DEVONIAN STRATIGRAPHY IN EAST-CENTRAL IDAHO

Evolution of the Devonian shelf in east-central Idaho (Beartooth Butte, Carey, Jefferson, Three Forks, Picabo Fms.) is assembled using facies stacking patterns, sequence boundaries and limited conodont data. Mixed siliciclastic and carbonate ramp strata of Idaho are divided into two stratigraphic successions separated by major late Frasnian/Famennian (F/F) sequence boundaries. The lower succession consists of 1000m(+/-) of Lockhovian to late Frasnian (Rhenana Zone) quartz arenites and dolostones. The upper succession (late F/F) consists of ~500m or less of the same lithologies, plus limestone, breccias, and shales.

The lower stratigraphic succession was deposited on a relatively quiet margin during 1st-order Kaskaskia sea level rise (greenhouse climate mode). Major sequence boundaries occur with incised paleovalleys and gradual deepening-upward stacking patterns with low-amplitude peritidal cyclicity. The upper stratigraphic succession was deposited during the 2nd-order regressive limb of the Euramerican Johnson et al. sea level curve, transitional climatic conditions, and increased regional tectonic activity. An intra-shelfal basin formed at this time. Depositional history of the late F/F through early Mississippian is complex and reconstructed using two different interpretations. These models diverge in how sparse data in the Milligen Basin to the west are correlated with eastern shelfal strata.

Axiomatic model #1 explains stratigraphic evolution through Antler tectonic controls with distal shelf uplift resulting in clastics being shed from the west towards the shelf. This model requires a “flexural forebulge” (or “yo-yo” tectonics?) to explain end-Devonian distal shelf uplift followed by intense earliest Mississippian subsidence. Model #2 explains late regional F/F sequences and hiatuses in terms of gradual, incipient shelfal subsidence with higher frequency swings in relative sea level. Widespread sequence boundaries, “missing conodont zones,” and shoreline environments with arenites derived from the east support this. Sands could also have bypassed the shelf and accumulated during lowstands in the attached Milligen Basin (still unclear). Either model speaks to strike-slip, hybrid foreland basin dynamics described for overlying strata by Wilson et al. (1994).