• Harvey Thorleifson, Chair
    Minnesota Geological Survey
  • Carrie Jennings, Vice Chair
    Minnesota Geological Survey
  • David Bush, Technical Program Chair
    University of West Georgia
  • Jim Miller, Field Trip Chair
    University of Minnesota Duluth
  • Curtis M. Hudak, Sponsorship Chair
    Foth Infrastructure & Environment, LLC


Paper No. 9
Presentation Time: 4:15 PM


JAYKO, A.S.1, MCGEEHIN, John P.2, PACES, James B.3, FIRBY, James R.4 and REHEIS, Marith C.3, (1)Earth Surface Processes Team, U.S. Geological Survey, 3000 East Line St, Bishop, CA 93514, (2)US Geological Survey, 12201 Sunrise Valley Drive, Reston, VA 20192, (3)United States Geological Survey, Denver Federal Center, MS-980, Denver, CO 80225, (4)University of Nevada, Reno, NV 89512,

Mollusk assemblages are locally associated with OIS-2 and spillway limited OIS-6 strandlines of paleo Lake Russell. They provide links between the climate record in the Wilson Creek Fm. and related deposits, and the late Pleistocene shorelines. The assemblages near the LGM strandline occur in sandy surface deposits, bedded shallow water lacustrine deposits, and nearshore crossbedded-sandy tufa cemented deposits between 2152 and 2161 m elevation. They include bivalves Sphaerium sp, Pisidium sp. and gastropods Helisoma (Carinifex) newberri, Vorticifex (Parapholyx) cf. gesteri, V. (P.) cf. effusa (rare), Valvata humeralis and Amnicola longinqua. Four sites have yielded calibrated 14C ages: (1) 27,750 ± 250 ka on indurated carbonate cemented coquina interbedded with lacustrine silt and ash layers (Mono Craters ?), (2) 31,960 ± 430 ka on shelly-ostracod bearing-oolitic sand, (3) 34,120 ± 560 ka on Sphaerium sp. shell, and (4) 43,200±1,700 ka on Valvata humeralis and Heliosoma newberri shell fragments with internal secondary carbonate. Glass tephra shards are locally cemented in fossil-bearing tufa and rare, well-rounded, oblate pumice clasts, presumably from Mono Craters, are locally present in the beach lag deposits. Assuming the fossils at these sites were endemic to the lake at the time of deposition and not reworked from older deposits, then the 14C ages indicate the 2155 m highstand was occupied during the early OIS-2 as well as late OIS-3(?), concurrent with formation of prominent wave cut benches inset onto Tioga and Tahoe moraines.

The spillway-limited highstand at ~2205 m has eluded direct dating but is constrained by inset strandline relations with dated moraines. U-series dating on tufa yields post OIS-6 interglacial ages interpreted as minimum ages, making OIS-6 the likely most recent breaching event. Sample EL95-116 (2195 m) yielded a two-point isochron age of 80-100 ka on gastropod-bearing tufa within a beach deposit, and a single, detritus-corrected age of 94 ± 17 ka. WHSJ-U2-08 (2198 m) has an isochron age of 69-76 ka on tufa encrusting a beach boulder. EL95-112 (2218 m) ~10 m above the spillway yielded two reliable dates of 359 ± 35 ka and 400 ± 39 ka. The mollusk assemblage at ~2205 m is comparable to OIS-2 and locally includes articulated Pisidium clam in wetland deposits that are unlikely to have been reworked.

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