PLIO-PLEISTOCENE SEDIMENTS INDICATE A PLEISTOCENE UPLIFT OF GRAND TETONS

Lacustrine sediments of the Shooting Iron Formation in the south end of Jackson Hole, yield a highly modern conifer-dominated pollen flora, more similar to the modern pollen rain than the Miocene Teewinot Formation. The Shooting Iron deposits contain an array of vertebrates of Blancan age, including the rodent Ophiomys meadensis, Pliophenacomys primaevus and a horse identified by A. Barnosky. The highly diverse gastropod fauna (with many specimens retaining the original shell colors) has been described by Dwight Taylor as also of Blancan or Plio-Pleistocene age. The astonishing feature of these Neogene units is that they lack rock types from the high Teton Range. Field mapping by J. David Love and by Jack Reed (Teton Park map) indicate that the rock types of the high Teton Range (the Grand Tetons) have special diagnostic characteristics: they are pre-Cambrian igneous and metamorphic rocks containing no marble and no quartzite. Appearance of these distinctive rock types in valley deposits at Jackson Hole, Wyoming, provide an indication of timing for uplift of the high Teton Range. Love’s mapping indicates two phases of uplift in the Teton Range: one is probably during the early Tertiary interval , and the other is very recent, probably after the Plio-Pleistocene transition. Evidence for the latter uplift comes from the Neogene and Pleistocene formations in the valley. The distinctive rock lithologies of the Grand Tetons are absent in the Neogene formations at Jackson Hole, e.g. Teewinot Formation and gravels (late Miocene ~ 400O ft. thick) , Conant Creek Tuff (~200-250 ft thick; Pliocene) of welded tuff and pumicite; Shooting Iron Formation (Blancan) and Huckleberry Tuff (Plio-Pleistocene ca 2.02 Ma ). Glacial Gravels that overlie the Shooting Iron and/or the Huckleberry Tuff represent at least four phases of early glacial activity. It is in the fourth gravel that the cobbles and gravels of the high Teton Range first appear and are the dominant the rock type. The findings suggest that the last major uplift of the high Tetons, the tectonic movement that exposed the distinctive Cambrian igneous and metamorphic types lacking in marble and quartzite probably did not occur until after the Shooting Iron Formation and the Huckleberry Tuff were deposited ca 2.02 million years ago.