| Abstract: | Benthic and planktic foraminifera in the shales and mudrocks of the Cenomanian-Turonian Greenhorn Cycle are excellent proxies for sea level change in the Late Cretaceous Western Interior Seaway. By examining the foraminiferal assemblages, one can track the transgressive and regressive phases of the Greenhorn Cycle as well as gain detailed information on salinity, benthic oxygenation, productivity, and stratification of ancient water masses (Eicher and Worstell, 1970; Eicher and Diner, 1985; Leckie et al., 1991; West et al., 1998). In the Greenhorn Sea, oceanographic variables fluctuated through time due to seasonality, climatic cyclicity, and mixing of Boreal and Tethyan water masses. Percent carbonate is another useful proxy of water mass affinity and sea level change. This is because calcareous nannoplankton and planktic foraminifera thrived in the warm, normal marine Tethyan (southern) waters compared with the cooler, less saline waters of Boreal (northern) affinities. Leithold (1994) recognized six fourth-order depositional sequences within the third-order Greenhorn Cyclothem based in part on carbonate content. Foraminiferal assemblages can be used to define and correlate the fourth-order sea-level cycles superimposed on the Greenhorn Cycle.
In this study, foraminiferal assemblages and their response to sea level change were documented for two different sites: a drill core through the Tropic Shale in Escalante, Utah, and an outcrop section through the upper Belle Fourche Shale, Greenhorn Formation, and Carlisle Shale in Billings, Montana. Assemblage analyses are based on sample split counts of the >63 mm and >150 mm size fractions. Chronostratigraphy and correlation of the sections are based on molluscan biostratigraphy, percent carbonate, and widely dispersed bentonites.
The first appearance of planktic foraminifera is diachronous from south (Escalante) to north (Billings). At both sites the planktic foraminifera appear suddenly and abundantly within the sections, accounting for 60-80% of the total assemblage. The proportions of planktics to benthics fluctuate during third-order transgression (fourth-order Cycles 1-3) but stabilize during highstand (Cycle 4). Planktic foraminiferal diversity is low throughout the Greenhorn Cycle (1-6 species, including biserial Heterohelix globulosa, and trochospiral Hedbergella delrioensis, H. planispira, and Whiteinella spp.). Planktic foraminifera became very rare along the western side of the seaway during third-order regression (Cycle 6).
In the >63 micron size fraction, the calcareous benthic, Neobulimina albertensis, dominates during Cycles 2, 3 and 4 at both sites, corresponding with peak transgression and highstand of the Greenhorn Sea. During peak transgression and highstand, the southern portion of the seaway was dominated by a warm, oxygen-poor Tethyan intermediate water mass. Fourth-order sea level change resulted in pulses of Gavelinella abundance due to shifting water masses, improved benthic oxygenation, and increased organic matter deposition. This yields a succession of Gavelinella followed by Neobulimina-dominated assemblages, due to this important interplay of food availability vs. oxygen level with changing sea level. Agglutinated benthics characterize the seaway during regression (middle Turonian), which reflects the displacement of Tethyan waters by cooler, less saline water masses from the North.
Patterns of planktic and benthic foraminifera in the Escalante (UT) core and the Billings (MT) section correlate broadly to Leithold's six carbonate cycles. The rapid and isochronous increase in CaCO3 at the Cenomanian/Turonian boundary at these two widely separated sites and the abrupt carbonate decrease in the basal middle Turonian (C. woollgari biozone, above bentonite TT5), suggest that water mass movement within the seaway responded to widespread changes in relative sea level. Eustasy is the likely explanation based on the roughly 400 kyr cyclicity in the carbonate data (e.g., Meyers et al., 2001; Tibert et al., in press) and the observed forced regression represented by Cycle 5 (Leithold, 1994; West et al., 1998). Neobulimina tracks carbonate, and both are proxies for the warm, oxygen poor water mass(es) of Tethyan affinity. Tethyan waters persisted longer in the vicinity of the southern Escalante site. |