Foraminiferal Paleoecology of the Grayson Formation (Upper Cretaceous) of North-Central Texas

ABSTRACT The Grayson Formation (Upper Cretaceous) of north-central Texas is characterized by a diverse and well-preserved microfauna predominated by neritic taxa. The most common benthic foraminifers are Valvulineria loetterlei (Tappan), Lenticulina gaultina (Berthelin), Gavelinella plummerae (Tappan), Lingulogavelinella asterigerinoides (Plummer), Fursenkoina minuta (Cushman), Gaudryinella delrioensis Plummer, and Textularia rioensis Carsey. Planktonic foraminiferal populations are comprised chiefly of Hedbergella delrioensis (Carsey), Heterohelix moremani (Cushman), and Gubkinella graysonensis Tappan. The predominant ostracodes are Cytherella comanchensis Alexander, Paracyprideis graysonensis (Alexander), cythereids, and cytherelloidids. Microfaunal composition and diversity change laterally and vertically in the formation. The basal, middle, and upper beds are predominated by hyaline benthic foraminifers. The lower claystone has a high percentage of agglutinated and miliolid benthic foraminifers. The middle claystone has the highest number of cytherellid ostracodes. Locally, the uppermost claystone is rich in planktonic foraminifers. These microfaunal changes probably reflect variations in water depth, sedimentation rate, substrate, and extent of circulation with the open ocean.