Identifying the Regressive-Transgressive Turnaround in Shoreline Sequences from West Spitsbergen, Norway, and Wyoming, U.S.A.

Carlos A. Uroza and Ron Steel
The University of Texas, at Austin, Austin, TX

The regressive-transgressive turnaround in a stratigraphic sequence is the level at which the regressive depositional system (eg deltas, strandplains) became transgressive (eg barrier/lagoons, estuaries). We suggest that this turnaround can be miss-picked, and the volume of transgressive deposits underestimated if the turnaround is picked too high. Transgression does not necessarily proceed swiftly across the shelf surface (leaving meager deposits) particularly where (a) alluvial input is high, (b) previous base-level fall created low topography along the axis of the prior delta system, leading to thick estuarine backfill, or (c) the shelf steepens (eg near the shelf edge) causing the topography to be transgressed more slowly. An example of thick transgressive deposits above this turnaround level, in a shelf-edge location, is illustrated from Eocene deposits in West Spitsbergen. Here the turnaround between shelf-edge delta strata (high-energy, wave-influenced mouth bars) and overlying thick estuarine dunes and bars is recognized by the striking landward (flood tidal) orientation of the latter. We think that the great thickness of the transgressive deposits (50% of each sequence) results from a combination of (a), (b) and (c) above. In a second case, where transgression probably proceeded more rapidly because of low gradient of the transgressive surface (McCourt Tongue of the Rock Springs Fm, WIS) we again recognize tidally-influenced transgressive deposits (above wave-dominated shelf deltas) by the prominence of landward-directed 2D and 3D dunes within the transgressive tract. Here it would be easy to mistake the transgressive deposits for regressive upper-shoreface deposits, except for their thickness and flood-tidal orientation.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005