CHRONOSTRATIGRAPHY OF ALBIAN-SANTONIAN WESTERN INTERIOR BASIN FILL

During the middle part of the Cretaceous the Western Interior foreland basin subsided and filled as the Cordilleran Mountains were uplifted. The Albian-Santonian mixed siliciclastic and pelagic carbonate strata record five widespread sea-level and shoreline fluctuations separated by regional unconformities that represent five major, long-term depositional sequences. The chronostratigraphic framework is composed of high-precision biostratigraphy based on ammonites, inoceramids, nannofossils, and dinoflagellates constrained by numerous radiometrically dated bentonites.

The precision of previous correlations is here tested by interpolating numerical ages by graphic correlation of the megafossils and microfossils. Graphic correlation is an objective experimental technique that compares fossil ranges in given sections to a database of ranges derived from many sections in diverse geographic regions. The global database used here, CRET1, is based on more than 150 sections and more than 3400 fossils, geochemical events, magnetochrons, and marker beds.

Four Late Albian to Late Turonian depositional cycles varied in duration from 1 myr to 7 myr. The Early-Middle Cenomanian flooding of the western shore extended more than 500 km in 1.6 myr, a rate of 100 km in 320 ka. The Cenomanian-Turonian maximum flooding during OAE2 deposited organic-rich shale in the basin center and was a hiatus nearshore. The Juana Lopez Member was a lowstand wedge. During the latest Turonian-Coniacian westward flooding marl deposition began abut 1.4 myr after basin-center chalk deposition.