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Tidal and Storm Wave Deposition in Lower Silurian Shelf Sequences, East Tennessee

Steven G. Driese, Mark W. Fischer, Kurt A. Easthouse, Amy E. Schoner

Lower Silurian sequences exposed across east Tennessee consist of an easterly quartzarenite (Clinch Sandstone) that grades westward into heterolithic sandstone, siltstone, and shale (Rockwood Formation). Farther west, the sequence intertongues with a cratonic carbonate-shale succession (Brassfield Formation).

The Clinch Sandstone represents deposition in shoreface and inner shelf environments. Inferred depositional processes include (1) ebb-dominated tidal currents, (2) wave-induced currents (longshore and shore-oblique), and (3) wave-orbital currents. Strong tidal influence is indicated by abundant medium to large-scale, variably stacked sets of ebb-oriented (west-northwest) cross-strata. These strata exhibit tidal bundling, double clay drapes, reactivation/pause surfaces containing concentrations of shale intraclasts and/or biogenic structures, and shale intraclasts lining deep scours. The cross-strata were produced by migration of large-scale, low-amplitude bed forms (class II, III, and IV sand wave of Allen), and densely bioturbated sequences are interbedded with the cross-strata domin ted by Skolithos-assemblage traces.

The Rockwood Formation represents deposition in middle to outer shelf environments where storm-induced currents dominated. Possible depositional mechanisms include (1) storm-surge-ebb (relaxation) flows associated with coastal set-up conditions, which may have evolved into geostrophic flows; and (2) turbidity flows initiated by storm-induced liquefaction of shoreface sand. Features suggest deposition above fair-weather wave base (high-angle cross-strata), below fair-weather wave base but within storm wave base (hummocky cross-stratification, micro-hummocky cross-stratification and wave ripples), and below storm wave base (Bouma bce turbidites). Westerly sand transport was most prevalent, and bioturbation is extensive and dominated by Cruziana-assemblage traces.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.