LOWER AND MIDDLE DEVONIAN FISH-BEARING ESTUARINE-TO-BIOSTROMAL MARINE DEPOSITS IN THE LOST RIVER RANGE, IDAHO: AN ILLUSTRATION OF WIDESPREAD REGIONAL STRATIGRAPHIC SEQUENCES

Before the Late Devonian paleogeographic transition to active sub-basins and highs (Antler orogeny) Lower and Middle Devonian sediment accumulation on the Idaho shelf occurred at rates less than 25 m/m.y., much lower than Famennian or Early Mississippian rates (100 &1000 m/m.y. respectively). Whereas deposits of quartz arenite and muddy carbonates at this time generally support a passive-margin setting, previous studies rarely show or predict the effects and extent of base-level changes. We describe regionally correlative unconformity-bounded sequences illustrated by sections at Meadow Peak and Borah Peak in the Lost River Range (LRR). Stacking patterns there show environmental changes over a broad ramp and involve the Laketown and Carey dolomites and the Beartooth Butte and Jefferson formations.

In the LRR, as in the Lemhi Range (LR) 25 miles to the NE, transition occurs between the Silurian and Devonian systems with sharp transgressive contacts above karst artifacts. Deepening-upward Lower and Middle Devonian ramp cycles below ubiquitous outer shelf Frasnian deposits consist primarily of sand-sized, estuarine clastics overlain by peritidal dolostones and open marine, dark biostromal units (very common in the LLR). Similar rocks also back-filled incised paleovalley deposits, which are spectacularly exposed in the LR. Non-Walterian decameter deepening-upward successions were built by Walterian meter-scale, shallowing-upward cycles; bed contacts at all scales are sharp. Four large-scale depositional sequences are identified and can be easily viewed in cirque walls near Borah Peak.

Although far offshore and subject to long-term Kaskaskia transgression, disconformable sequences in the LLR were subject to lowstand subaerial exposure and clastic influx. Basal estuarine deposits in the LRR consist of amalgamated cross-bedded channelized units with sometimes-abundant fish remains. Field observations suggest that sequence boundaries can be extended from the LR to the LRR and are also identifiable on the distal shelf (Spar Canyon area). We suggest that sediment accumulation was dominated by the interaction of subsidence with low-amplitude eustatic influences typical of the global greenhouse climate. Further biostratigraphic resolution will test this hypothesis.