Glacial Interpretation for the Rainstorm Member of the Johnnie Formation, Eastern California
Tracy L. Fenger and Dawn Y. Sumner
2001 GSA Cordilleran Section Annual Meeting, Abstracts 33:A48
Facies containing diamictite deposits and negative d13C isotope values suggest that the Rainstorm Member of Neoproterozoic Johnnie Formation, Eastern California represents a glacial sequence. In the Southern Nopah Range, 5 to 50 m above the Johnnie oolite, the stratigraphic sequence contains HCS, current and wave ripples, and rip-up clasts in mixed siliciclastics and carbonates, suggesting a nearshore, shallow marine environment. Above 50 m, deposition shifts to interbedded sandstones and siltstones with wavy and planar lamination, implying an offshore, deeper depositional environment. An unsorted breccia unit overlies the sandstones and siltstones and contains up to 1 m diameter clasts composed of sandstone, pisolitic sandstone, shale, oolite, and dolomicrite. Siltstones overlie the breccia and contain several isolated clasts more than 2 m in diameter. These features suggest a glacial origin for the clasts.
The petrographic features of the carbonate rocks from the limestone-dolostone facies show both preservation of primary sedimentary features and evidence of alteration and diagenesis after deposition. The carbon and oxygen analyses show d13C of -10.9 to -8.8 %o and d18O of -15.5 to -12.7 %o, implying some diagenetic alteration. However, negative d13C values are commonly associated with Neoproterozoic glacial deposits. Thus, the limestone-dolostone facies is interpreted as being deposited during early deglaciation. Then as deglaciation continued, diamictites were deposited from ice rafted debris and debris flows composed of glacial sediments.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616