PALEOCEANOGRAPHIC INTREPRETATIONS, WESTERN INTERIOR SEAWAY, FROM PLANKTONIC FORAMINIFERA AND CALCAREOUS NANNOFOSSILS

The foraminiferal and calcareous nannofossil assemblages from samples above the latest Cenomanian Neocardioceras bentonite indicate the paleoceanographic importance of the vast coastal waters that occurred within the Western Interior Seaway. Most of the foraminiferal samples contain greater than normal percentages of Heterohelix globulosa. Leckie and others (1998) reported a Heterohelix increase, that begins in the Sciponoceras zone and extends upward throughout the latest Cenomanian, that is attributed to a widespread oxygen minimum zone incursion. On our time-line, the greatest abundance of Heterohelix occurs in the central axis of the seaway and along the eastern coast. Their distribution suggests that they lived at shallow depths both along the eastern coast and above the oxygen minimum zone.

Percentages of Guembelitria harrisi are highest along the edges of the seaway and especially in the Four Corners region. Guembelitria is generally considered to be a shallow water genus and is commonly associated with the shallow dwelling Heterohelix globulosa. An assumed modern day counterpart of Guembelitria is associated with low oxygen waters and upwelling zones.

Hedbergella planispira is negatively correlated with Guembelitria harrisi. Oxygen and carbon isotopic evidence suggests that although Hedbergella planispira was a shallow water foraminifer, it lived at greater depth than Heterohelix globulosa and Hedbergella delrioensis. Hedbergella planispira occurs in greater numbers in the open/deeper waters of the seaway, whereas, Guembelitria harrisi occurs in greater numbers in the shallower, near shore waters.

High fertility nannofossil assemblages occur along the edges of the seaway, where nutrients entered. These high fertility nannofossil assemblages tend to co-occur with greater numbers of Guembelitria harrisi. This co-occurrence reinforces the suggestion that Guembelitria, like its modern counterparts, was an organism adapted to high productivity conditions. Oceanic calcareous nannofossil assemblages occur in the central seaway, where Heterohelix globulosa and Hedbergella planispira are more abundant. Calcareous nannofossils appear unaffected by the oxygen minimum zone.