HR: 0800h
AN: PP41A-0580    [Abstracts]
TI: Preliminary $\delta$$^{13}$C and $^{87}$Sr/$^{86}$Sr Curves for Late Mississippian (Chesterian) Carbonates and Siliciclastics in East-Central Idaho
AU: * Batt, L
EM: batt6553@uidaho.edu
AF: University of Idaho Geology Department, PO Box 443022, Moscow, ID 83844 United States
AU: Isaacson, P
EM: isaacson@uidaho.edu
AF: University of Idaho Geology Department, PO Box 443022, Moscow, ID 83844 United States
AU: Monta$\~{n}ez, I
EM: montanez@geology.ucdavis.edu
AF: University of California Geology Department, 1 Shields Ave, Davis, CA 95616 United States
AU: Pope, M
EM: mcpope@wsu.edu
AF: Washington State University Department of Geology, Webster 1228, Pullman, WA 99164 United States
AB: Upper Mississippian (Chesterian) carbonate and siliciclastic rocks across east-central Idaho and southwest Montana were measured and sampled for biostratigraphy and chemostratigraphy to derive the regional sequence stratigraphy. The resulting $\delta$$^{13}$C profile displays high frequency perturbations that, in conjunction with a well defined $^{87}$Sr/$^{86}$Sr curve, may provide insight into the relationship between Chesterian seawater chemistry and relative sea level history. A collection interval of 0.5 meters was applied to a $>$500 meter composite section from Idaho, and whole rock analysis of the 700 resulting samples was augmented by component analysis of brachiopods and marine cements. Sample staining and cathodoluminescence of select thin sections attest to the integrity of the data set. The average maximum and minimum values for the resulting $\delta$$^{13}$C curve are +3.0$\permil$ (PDB) and -3.0$\permil$ (PDB), respectively. A number of long term trends are evident in the curve. Except for an initial abrupt decline from +2$\permil$ to -2$\permil$, lower Chesterian values average +1.5$\permil$ until a significant negative excursion to -1$\permil$ and -2$\permil$ in the middle Chesterian. This boundary coincides with a sea level fall, recognized stratigraphically as an abrupt transition from carbonate to calcareous siltstone. Uniform cyclic fluctuations remain between -1 and +1$\permil$ until truncated by a rise to +3$\permil$. The resulting increased amplitude persists into latest Chesterian oscillations; values fall from +3 to -2.5$\permil$ and then increase to an concluding average of +2$\permil$. Superimposed on the longer term curve are multiple smaller fluctuations that appear, in some cases, to correspond with shallowing upward depositional sequences. Overall per mil values are lower than published Chesterian averages, but trends in the curve appear similar to those in the $\delta$$^{13}$C curve established for the North American mid-continent (Mii et al, 1999). $^{87}$Sr/$^{86}$Sr values of thick shelled brachiopods were obtained by laser ablation (MC)-ICP-MS. The resulting curve delineates a rise in $^{87}$Sr/$^{86}$Sr ratios that is in accord with published trends for the Carboniferous, and that may be used in establishing a chronologic framework for the observed $\delta$$^{13}$C signature.
DE: 4825 Geochemistry
DE: 4870 Stable isotopes
DE: 3344 Paleoclimatology
DE: 4556 Sea level variations
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting