Paper No. 3
Presentation Time: 8:30 AM
ESTIMATING PALEO-BOTTOMWATER DEGREE-OF-ANOXIA (DOA) USING FAUNAL, ICHNOFABRIC, AND GEOCHEMICAL DATA: EXAMPLES FROM CORE SHALES OF UPPER PENNSYLVANIAN CYCLOTHEMS OF THE EASTERN MIDCONTINENT SHELF (KANSAS), U.S.A
Bottomwater oxygen levels, an important control on the preservation of sedimentary organic matter, can be estimated on the basis of faunal, ichnofabric, and geochemical data. Three core shales (Hushpuckney, Stark, and Muncie Creek) of Missourian Stage cyclothems from the Eastern Midcontinent Shelf provided an excellent case study. The inorganic and organic geochemistry of these 1- to 2-m-thick shales was characterized using XRF, Leco, Rock Eval, and GC-MS, and cm-scale sampling permitted detailed study of vertical compositional trends. The shales are divisible (from the base upward) into four zones: (1) a high-TE, laminated lower black shale, (2) a low-TE, weakly bioturbated upper black shale, (3) a moderately bioturbated, non-fossiliferous lower gray shale, and (4) a fully bioturbated, fossiliferous upper gray shale. Within the black shale, TOC falls only slightly from zone 1 to zone 2 but redox-sensitive TEs (U, Mo, V) fall abruptly, suggesting crossing of a critical O2 threshold. Bioturbation is limited to a few macroburrows (Planolites, Zoophycos) in zone 2 but includes complex, tiered ichnocoenoses in zone 3. Preliminary organic geochemical data suggest substantial variation in the type (marine algal vs. terrestrial plant) and quality of organic matter between zones as well. We infer a fairly steady increase in bottomwater O2 levels throughout core shale deposition. While the black shale is commonly described as "anoxic," reduced TE concentrations in zone 2 suggest merely dysoxic conditions (ca. 0.1-0.2 ml O2 l-1 H2O). Intensification of bioturbation in zone 3 reflects a further rise in O2 levels, to weakly dysoxic conditions (ca. 0.2-0.5 ml O2 l-1 H2O) that supported a bioturbating, probably worm-dominated, infauna.