ABSTRACT: Tidal Influence in Fluvial Strata--a Key Element in High-Resolution Sequence Stratigraphic Correlation

Keith W. Shanley, Peter J. McCabe

Recognition of tidal influence in fluvial, estuarine, and alluvial strata is critical to establishing of high-resolution sequence stratigraphic correlations between marine and nonmarine strata. In Turonian through Campanian strata of the Kaiparowits Plateau of southern Utah, tidally influenced facies were deposited during at least two periods of relatively rapid base level rise. These facies form part of transgressive systems tracts. Landward of the transgressive maxima, these tidal facies are present in channels that are otherwise encased in alluvial strata. Recognition of subtle sedimentary structures resulting from tidal processes allows an accurate chronostratigraphic framework to be developed between marine deposits and strata deposited up to 50 km inland.

Tidally influenced channel deposits near the shoreline are sandstone dominated. They have well-developed sigmoidal bedding, thin mudstone drapes, and contain multiple reactivation surfaces and scattered marine trace fossils. Thirty kilometers inland, channel deposits contain wavy to lenticular bedded fine-grained sandstones and mudstones interbedded with thin sigmoidal cross-stratified sandstones. Teredolites, bidirectional current orientations, and the heterolithic nature suggest a tidally influenced system. Twenty kilometers farther inland lateral accretion deposits consisting of alternating ripple cross-laminated sandstones and mudstones are interpreted as meandering river deposits. The persistence of many mud drapes from the base to the top of lateral accretion surfaces, the prese ce of lenticular bedded sandstones, and trace fossil evidence suggest a tidal influence.

Tidal facies landward of transgressive maxima are correlated to condensed sections representing periods of maximum flooding in the marine shales. The interpretation of tidal influence in alluvial strata and the correlation to maximum flooding surfaces and condensed intervals within marine strata allow an accurate chronostratigraphic framework to be extended from the marine to nonmarine environments over a distance of at least 50 km.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990