1. Foraminiferal Assemblages of the Basal Tropic Shale (Uppermost Cenomanian-Lowermost Turonian)
in Relation to the Pliosaur Brachauchenius lucasi, South-Central Utah
Elizabeth Freeman and R. Mark Leckie
Wright State University & University of Massachusetts AmherstIntroduction Results & Discussion
Conclusions
Methods
The southwestern side of the Cretaceous Western Interior Sea underwent dramatic
oceanographic fluctuations during the latest Cenomanian-early Turonian transgression. The
basal Tropic Shale near Big Water, Utah records the westward expansion of the seaway and a
deepening upwards succession of mud-dominated lithofacies.
Two nearly complete specimens of the pliosaur Brachauchenius lucasi were described from the
basal Turonian (Albright et al., 2007; Journal of Vertebrate Paleontology). The Tropic Shale
section studied here was located about 100 miles (175 km) from the western shoreline. The
area is of diverse scientific importance based on a shallow open marine record of Oceanic
Anoxic Event 2 (OAE 2), and the occurrence of numerous large marine vertebrate fossils.
Eight shale samples spanning 7 m of the Tropic Shale collected in the section
where the B. lucasi specimens were excavated , were processed, and analyzed
for planktic and benthic foraminifera in order to document the nature of the
environment in which these large marine vertebrates lived.
Intern Elizabeth
prepping samples.
Planktic/benthic ratios display :
• Cyclical fluctuations of up to 30%
indicating the dynamics of this neritic
environment
• Possible changes in water mass
conditions, including temperature,
salinity, and/or turbidity.
• Fluctuations may have been driven by
climate cycles, which are well-developed
in more distal, correlative facies.
• Foraminiferal assemblage data indicate cycles of relative transgression and regression, or
fluctuating water mass conditions and low oxygenated water across the Cenomanian- Turonian
boundary interval, and low diversity assemblages.
• Changes in planktic foram assemblages suggest a deepening water column. The pliosaur
Brachauchenius lucasi survived OAE2 across the Cenomanian-Turonian boundary and thrived
despite fluctuating oceanographic conditions.
0
1
2
3
4
5
6
7
8
0.00 20.00 40.00 60.00
% Planktic
Benthic foram assemblages, dominated by the infaunal Neobulimina suggest conditions
on the seafloor were oxygen stressed due to stratification and/or an abundance of organic
matter. Gavelinella was an epifaunal taxon.
0
1
2
3
4
5
6
7
8
0.00 20.00 40.00 60.00 80.00 100.00
% Neobulimina
0
1
2
3
4
5
6
7
8
0.00 10.00 20.00 30.00 40.00
% Gavelinella
Acknowledgments: The late Dr. Randolph Bromery and Mrs. Bromery, UMass micropaleo lab
crew (Renata de Mello, Andy Fraass, Serena Dameron, and Khalifa Elderbak), Dr. Nick Venti and
Dr.Rebecca Teed. We also thank Dr. Alan Titus for the samples from Big Water.
Western Interior
Seaway at peak
transgression in the
early Turonian
Study site near
Big Water,Utah
Map courtesy of Ron Blakey, Colorado
Plateau Geosystems, Arizona USA.
http://www.oceansofkansas.com/pliosaur.html