MID-CRETACEOUS PALEOCEANOGRAPHY AND CHEMOSTRATIGRAPHY OF THE EASTERN MARGIN, WESTERN INTERIOR SEAWAY

In the center of the Western Interior Basin, the Bridge Creek Limestone Member of the Greenhorn Formation is of late Cenomanian to early Turonian age and consists of alternating limestone and marl beds. The Bridge Creek Limestone represents transgression, with the upper half representing maximum Greenhorn high-stand. As transgression proceeded, ocean water from the Tethys Sea progressed north, west and east into the seaway. Many central and western stratigraphic sections have been extensively studied, but much less is known about the eastern margin of the Greenhorn Sea.

Recently we obtained core samples from the Iowa Geologic Survey/United States Geologic Survey, cored in northwestern Iowa, near Hawarden and Grant City. Few biostratigraphically useful macrofossils have been obtained from the cores, making correlation problematic. Recently conducted d13C-carbonate analysis has aided correlation. The well known Oceanic Anoxic Event II carbon excursion, which begins near the base of the Sciponoceras zone has been identified at Harwarden. Due to missing core, at Grant City only the upper portion of the excursion has been recovered.

Planktonic foraminifera are known to occur from the central portion of the seaway at Pueblo, as early as the deposition on the Granaros Shale. They became abundant near the X bentonite at the base of the Greenhorn Formation. As transgression progressed, planktonic foraminifera geographic range expanded. Heterohelix globulosa are the first planktonic foraminifera to migrate into Iowa. They do not appear until the middle Hartland and are initially very rare and small. At Harwarden, near the base of the Sciponoceras zone,Heterohelix is joined by rare and small specimens of Hedbergella delrioensis and an assortment of calcareous benthic foraminifera. Above this level the planktonic assemblages become somewhat more diverse and the individuals become larger in size. In the Iowa cores, calcareous nannofossils appear earlier and in greater abundance and diversity than do the planktonic foraminifera.