Tundra Rodents in a Late Pleistocene Fauna from the Tofty Placer District, Central Alaska

Abstract

The Sullivan Pit is the principal mining excavation in the Tofty placer district. It lies at an altitude of about 600 ft. near the centre of the Tanana A-2 quadrangle in central Alaska. The Tofty mining district is in the zone of discontinuous permafrost where the Pleistocene sediments are perennially frozen to depths of more than 100 ft. Four stratigraphic units are recognized. Unit A has 10 to 30 ft. of stratified silt and peaty silt with interbeds of peat. Unit B is a series of channel and pond deposits of sand and fine gravel locally 7 ft. thick but more typically 1 or 2 ft. thick. Unit C consists of 5 to 20 ft. of silt and peaty silt similar to unit A. Unit D is a nearly continuous layer of well-sorted pebble gravel 10 to 20 ft. thick in most places. Units A and B are very young and were deposited less than 200 and more than 45 years ago (between A.D. 1760 and 1916). Units C and D are of late Pleistocene, probably Wisconsin age. Evidently A and B record an episode between 1760 and 1915 when material was excavated or eroded from older sediments higher on the slopes and redeposited in and near the Sullivan and Tofty pits. Fossil fauna recovered from A and B include Mollusca (Pisidium, Valvata lewisi, Lymnaea, Succinea strigata), Insecta (Pterostichus haematopus, Lepidophora lineaticollis, Dytiscidae), small fish, a proboscidean, Equus, cervids, a hypsodont bovid, Bison and rodents (Spermophilus undulatus, a castorid, Dicrostonyx torquatus, Lemmus sibiricus, Synaptomys, Clethrionomys, Microtus miurus, Microtus sp.). The 4 rodents identified to species indicate a tundra environment at the time they were fossilized. ABSTRACT AUTHORS: W. W. Judd