DUNN, THERESE, Shell Offshore Inc., New Orleans, LA

ABSTRACT: Calcareous Nannofossil Biostratigraphy and Paleoecology of the Fort Hays Limestone Member (Upper Cretaceous), Niobrara Formation, from Jewell County, Kansas, Wolcott, Colorado, and Wagon Mound, New Mexico

The calcareous nannofossils present in the Fort Hays Limestone Member in Jewell County, Kansas, Wolcott, Colorado, and Wagon Mound, New Mexico were examined to document the nannofossil assemblages, determine the biostratigraphic zonation and examine the paleoenvironmental conditions of the member. The distribution of nannofossil species indicates placement of the basal Fort Hays Limestone Member in Jewell County in Perch-Nielsen's Lithastrini's septenarius Subzone (CC 13b, lower Coniacian) of Sissingh's Marthasterites furcatus Zone (CC 13). The top of the Jewell County section isplaced in Sissingh's Lucianorhabdus cayeuxii Zone (CC 16, upper Santonian). The basal Fort Hays in Wolcott falls within Sissingh's Micula decussata Zone (CC 14, upper Coniacian and lower Santonian). In Wagon M und, the base of the section is placed within Subzone CC 13b. The upper part of the Fort Hays Limestone Member examined in this study in the Wolcott and Wagon Mound sections falls within Sissingh's Reinhardtites anthophorus Zone (CC 15, upper part of the lower Santonian).

These stage assignments for the basal Fort Hays Limestone Member corroborate a foraminiferal study suggesting placement of the Turonian/Coniacian boundary below the first limestone bed in Wolcott Regional correlation of the nannofossil zones in these sections is difficult due to the rare occurrences of indicator species. Two zones, CC 14 and CC 15, enabled correlations among the three stratigraphic sections examined in this study.

The relative abundance pattern of Eprolithus floralis indicates unstable water conditions. The paucity of the shallow water indicators Lucianorhabdus cayeuxii and Calculites ovalis, and the rare occurrences of the cosmopolitan indicator species Lithasrnnus grillii and Micula decussata, even in the well preserved Jewell County section, suggest the presence of some unusual environmental condition.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.