Linking Climate with Laramide Tectonism, Valley Incision, and Sedimentary Fill: New Insights from Upper Cretaceous and Paleocene Examples

Anton F.-J. Wroblewski
Northeastern Illinois University, Chicago, IL

Upper Cretaceous and Paleocene paleovalley fills of the Medicine Bow, Ferris, and Hanna formations in southern Wyoming's Hanna and Carbon basins reflect tectonic influence of surrounding Laramide uplifts which supplied sediment and directly influenced 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 a variety of signatures, ranging from exquisitely preserved to barely discernable, in the sand-rich 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 incision and sedimentary filling in the Ferris and Hanna formations to be accurately compared with chronologies of sea-level and climate change derived from study of passive margin environments such as the New Jersey Coastal Plain and deep sea deposits. Paleovalley genesis and fill occurred in the lower Ferris Formation (Maastrichtian: ~67-65 Ma), the middle Ferris (65-64 Ma) and the Hanna Formation (Danian-Selandian: 62-58 Ma) and seems to have been controlled primarily by tectonic quiescence and/or uplift of the Hanna and Carbon basins.

The cyclic nature (approximately 0.5-1 m.y. apart in the Cretaceous and 3 m.y. apart in the Paleocene) of these incision and fill events correlates with episodes of global warming as indicated by stable isotope data, indicating the possibility of a previously unrecognized link between climate and Laramide tectonism. The sand-dominated fills of the paleovalleys in 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 local sea level that may have been generated by autocylcic processes.

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