Abstract: Diversity of Incised Valley Fills: A Comparative Study with Examples from the Cretaceous of Arizona and Wyoming

Darren Box, Trevor Elliott

Incised valley fill deposits are important elements of stratigraphic sequences however, we still have a restricted view of their variability. Several key problems in the development and infilling of incised valleys remain, such as, (1) to what extent are the varying fluvial/tidal-estuarine depositional systems preserved?; (2) what are the dominate systems tracts within valley fills?; (3) what differences exist between valley fills associated with high and low order sequence boundaries?; and (4) to what extent do autocycline events contribute to valley fill architecture? These and other problems will be discussed with reference to an extensive database (158 measured sections) of the incised valley components of five stratigraphic units, namely, the Ericson, Newcastle, Teapot and Foxhil s Sandstones of Wyoming and the Toreva formation of Arizona.

Overall, the studied examples display a comparable range of depositional systems however, facies diversity within valley fills can be significant and is largely a function of the precise location within the valley fill in terms of depositional dip. Facies diversity increases down dip due to the presence of the estuarine facies component initiated with a relative sea level rise.

A substantial amount of the sediment within incised valley fills is entrapped during periods of rising relative sea level. Discrimination between sediments deposited during the early lowstand systems tract and the late lowstand systems tract is interpreted from the inception of estuarine facies in the valley dill. Within the studied sections there appears to be a predominance of deposits assignable to the late lowstand systems tract whereas fluvial deposits ascribed to the early and late lowstand systems tract arise further updip where the initial effects of a steady increase in relative sea level may not be encountered.

It is clear from the examples studied that notable differences exist between low and high order incised valley fills. Low order incised valley fills display relatively little facies diversity compared to high order incised valley fills. Both valley geometry and interfluves are better defined in low order valley fills.

In the updip, fluvially dominated sector of incised valley fill complexes autocycline events such as channel migration and switching govern the architecture of the fill. However, further down dip the effects of high frequency relative sea level fluctuations are dominate.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994