CHARACTERIZATION OF PARASEQUENCES WITHIN THE NEOPROTEROZOIC TO LOWER CAMBRIAN UPPER UNICOI FORMATION, JAMES RIVER, VIRGINIA

GLEDHILL, H.M.; and NOTO, R.C.; Dept. of Phy. Sci., Kutztown Uni., Kutztown, PA 19530; HARRISON, M.J.; Dept. Geol., Bowling Green State Uni., Bowling Green, OH 43403; DILLIARD, K.A.; and SIMPSON, E.L.; Dept. of Phy. Sci., Kutztown Uni. Kutztown, PA 19530.

The Neoproterozoic to Lower Cambrian Unicoi Formation of the Chilhowee Group can be subdivided into a lower rift-related, fluvial unit dominated by conglomerates and sandstones and an upper passive margin-related marine unit characterized by quartz sandstones. Three-dimensional exposures of the upper Unicoi Formation along the James River in Virginia provide an opportunity to reconstruct depositional processes active along the Laurentian shelf. upper Unicoi parasequences characterize a transgressive systems tract which is associated with the initiation of the Sauk Sequence.

Upper Unicoi parasequences are composed of three facies associations. Upper shoreface (US) facies association is characterized by through cross bedding of different scales, opposed, bidirectional bedforms and exposure features that together indicate the influence of tides and longshore currents generated by waves. Trough, tabular-planar and compound, cross-bedded sandstones and preserved symmetrical wave ripples characterize the lower shoreface to inner shelf (LSIS) facies association. The outer shelf (OS) facies association is characterized by thin Bouma sequence beds and mudstones deposited below the storm wave base. From oldest to youngest, the three parasequences (P1 through P3) are made up of the following facies association transitions: P1 - LSIS ->US; P2 - OS ->LSIS->US; P3 - OS->LSIS ->US. Vertically within the parasequence set, deeper-water facies progressively overlie the flooding surface of younger parasequences. Below the flood surface at the base of P2 is the shallowest facies that consist of opposed bidirectional bedforms, a preserved on the top of P3 and reflect a deeper-water setting than the top of P1. This parasequence stacking pattern is characteristic of a retrogradational parasequence set that records the development of a transgressive systems tract. This sequence continues into the upper part of the Chihowee Group. Preliminary results indicate correlation of these parasequences to other sections is problematic; other Unicoi Formations sections only have two parasequences developed.

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D.J. Moore
Oct. 2002
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