PROGRESS REPORT ON GEOLOGIC MAPPING OF TRIASSIC–JURASSIC VOLCANIC AND SEDIMENTARY ROCKS AT PITTSBURG LANDING, IDAHO: A TALE OF TWO PLATES

Triassic–Jurassic volcanic and sedimentary rocks are superbly exposed in Hells Canyon at Pittsburg Landing, Idaho. These rocks contain a record of Mesozoic sedimentation, magmatism, and tectonics during the origin and accretion(?) of Cordilleran island arc terranes in the western U.S. Local Mesozoic stratigraphy is divided by the Kurry Creek thrust fault into upper- and lower-plate assemblages. The lower plate includes Upper Triassic–Upper Jurassic rocks, whereas the upper plate likely includes only Middle Jurassic strata. In the lower plate, Upper Triassic volcanic rocks (Wild Sheep Creek Fm.) may be in fault or depositional contact with calcareous sandstones and oolitic grainstones of the Late Triassic Kurry unit. At the top of the Kurry unit, we map limestone beds and low-relief bioherms(?) which contain a Late Triassic fauna, indicating affinity with the underlying Kurry unit. An overlying, Early Jurassic (ca. 197 Ma), red tuffaceous sandstone-conglomerate and welded tuff unit is, in-turn, overlain by Late Jurassic (ca. 160 Ma) fluvial- to deep-marine rocks of the Coon Hollow Fm. Lower plate Coon Hollow Fm. includes fluvial conglomerate–mudstone and thin-interbedded tuff gradationally overlain by sparsely fossiliferous black shale. These marine rocks contain faunas of probable late Middle to early Late Jurassic age, such as the latest Callovian or early Oxfordian ammonite Cardioceras sp. aff. C. scarburgense/praecordatum. In contrast to the lower plate assemblage, fluvial rocks in the upper plate are interbedded with a single 20–30 meter thick tuff bed and the transition to marine deposits consists of 5–10 parasequence-scale lithofacies cycles. Marine fossils in the upper plate include the ammonite Cadoceras(?) or Lilloettia(?)and the bivalves Myophorella aff. devexa (Eichwald), Myophorella aff. montanaensis (Meek) and Scaphotrigonia naviformis (Hyatt) suggesting that Middle Jurassic, late Bathonian to Middle Callovian ages, at least, are present. Detailed sedimentary provenance analysis is underway to address possible region-wide correlations of Middle Jurassic rocks and to understand if the newly recognized Late Jurassic depositional interval is related to the evolving Jurassic western North American plate margin or an intra-oceanic system.