Wroblewski, Anton F.¹ 
(1) Northeastern Illinois University, Chicago, IL

ABSTRACT: Relative Influence of Eustacy and Tectonism on Valley Incision and In-Filling in the Latest Cretaceous and Paleocene, Southern Wyoming

Upper Cretaceous and Paleocene paleovalley fills of the Ferris and Hanna formations in southern Wyoming’s Hanna and Carbon basins are excellent examples of incised valley systems in tectonically active regions. Regional tectonic influence from the Sevier thrust-belt and Laramide uplift of surrounding basement-cored structures supplied sediment, but also affected local base level, promoting valley incision during episodes of lowered base level. Time-equivalent shorelines were close enough that tidal processes and brackish-water faunas left abundant signatures in the valley fills. Valley filling was at least partially influenced by sea level rise (eustatic or relative) rather than being a simple function of down-stream progradation of sediment waves in isolation of marine influence, as has been proposed for some Quaternary valley fills. Biostratigraphic data allow timing of paleovalley genesis and sedimentary filling in the Ferris and Hanna formations to be accurately compared with chronologies of sea level change derived from study of passive margin environments such as the New Jersey Coastal Plain. In the lower Ferris Formation (Maastrichtian: ~67-65 Ma), paleovalley entrenchment and fill is approximately correlative with a sea level fall depicted on the Exxon Production Research (EPR) sea level curve. Sea level fall cannot be ruled out as a primary driving mechanism of valley incision in the Ferris Formation simply because the Hanna Basin area was tectonically active. In contrast, paleovalley incision in the Hanna Formation (Danian-Selandian: 62-58 Ma) does not correlate with significant sea level falls on the EPR or New Jersey Coastal Plain sea level curves, but seems to be controlled primarily by tectonic quiescence and/or uplift of the Hanna and Carbon basins. Valley fills of the Hanna Formation are composed of fluvial and estuarine channel belts that exhibit strong cyclic stacking patterns. The alternating periods of fluvial- and estuarine-dominated valley fill represent high-frequency alternations in sea level caused either by tectonic influence or eustacy.

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