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Gani, M. Royhan1, Janok P. Bhattacharya1, James A. MacEachern2
(1) University of Texas at Dallas, Richardson, TX
(2) Simon Fraser University, Burnaby, BC

ABSTRACT: Using Ichnology to Determine Relative Influence of Waves, Storms, Tides, and Rivers in Deltaic Deposits: Examples From Cretaceous Delta Complexes in the Western Interior Seaway, Wyoming-Utah, U.S.A

Ichnological variations within parasequences of several Cretaceous delta complexes are correlated with both short and long-term changes in depositional processes. These changes reflect various proportions of river-, flood-, wave-, storm-, and tide-influence. Storm and river-flood deposits tend to show a low Bioturbation Index (BI) of 0-2, owing to high instantaneous accumulation rates, although this also depends on storm/river-flood frequencies. Top surfaces of individual storm/flood beds may show BI values of 4-5, reflecting the transition to longer-lived fair-weather conditions. Fair-weather waves facilitate persistent agitation, buffering environmental stresses. Wave-dominated deposits not affected by storms, thus yield the most robust ichnofacies signatures with diverse suites reflecting uniform and high BI values averaging 4.
River-dominated intervals show the least uniform trends because conditions vary widely (e.g. hyperpycnal to hypopycnal). BI values vary from 0 to 4, with generally low ichnogenera diversities. These alternations likely record seasonal fluctuations in sedimentation rate (river discharge) and water turbidity, influencing substrate conditions near distributary mouths.
Tide-dominated deposits tend to show the most ‘stressed’ conditions, with non-uniform and low BI values of 0-2. This reflects salinity fluctuations, heightened water turbidity, rapidly shifting substrates, and narrow colonization windows associated with daily and monthly changes in tidal periodicity.
Ichnological signatures change significantly across parasequence bounding discontinuities, generally showing a marked increase in BI as delta lobes are abandoned and transgressed. We suggest that different parts of a delta may experience differences in river, wave, and tide influence over time, reflecting the enormous complexity of operative processes at various time scales.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.