REGIONAL SCALE PALEOSOL CORRELATION AND ITS SEQUENCE STRATIGRAPHIC SIGNIFICANCE, CRETACEOUS WESTERN INTERIOR BASIN, MID-CONTINENT, US

Paleosols can be regionally correlatable, and therefore can be used as important sequence stratigraphic marker horizons. In this study, a paleosol that formed at the Albian-Cenomanian boundary in the Western Interior Basin of North America is shown to exist in non-marine strata deposited along the eastern margin of the basin and in offshore marine shales. The recognition of this cross-basin paleosol correlation provides an important chronostratigraphic marker for establishing the framework in which paleoclimatic records from the paleosols can be understood.

A globally recognized eustatic sea-level lowstand occurred at the Albian-Cenomanian boundary. This regression caused the through going Western Interior Seaway of North America to partially withdraw from the continental platform, separating the seaway into northern and southern arms. During this lowstand, large parts of the basin were subaerially exposed and subjected to pedogenesis.

Highstand and transgressive systems tracts deposited along the eastern margin of the Seaway contain a stack of amalgamated paleosols. White et al. (2005, Geology, 33(1), 13-16) determined that extreme depletion in d18O values from paleosol sphaerosiderites marked the Albian-Cenomanian boundary in the Dakota Formation in northwestern Iowa. Here we correlate the Iowa paleosols to amalgamated paleosols in the Davison core that penetrated the Dakota Formation in southeastern South Dakota, 200 km offshore, and to a single paleosol observed in a core and outcrop on the western flank of the Black Hills, northeastern Wyoming, 700 km offshore in the basin. In the most distal locales, the paleosol exists between the late Albian Newcastle Sandstone and early Cenomanian Mowry Shale. The paleosols were identified by the presence of root structures, cutans, and sphaerosiderites. These observations indicate that: 1) the stratigraphic position of the Albian-Cenomanian boundary in this region of the basin can be accurately placed at the Newcastle/Mowry Formation boundary; 2) the heretofore-interpreted-to-be transgressive Newcastle Sandstone must have formed during regressive conditions, and 3) widespread conditions of intensified precipitation affected the basin during formation of the paleosols.