|Institution:||Louisiana State University|
|Department:||Geology & Geophysics|
|Keywords:||gssp; turonian; cenomanian; taphonomy|
|Full text PDF:||http://etd.lsu.edu/docs/available/etd-04282010-151225/|
Recent paleoenvironmental and geochemical studies indicate that benthic conditions during Oceanic Anoxic Event II (OAE II) (93.5 Mya) in the Cretaceous Interior Seaway (CIS) were dysoxic rather than anoxic. In this thesis, dysoxic benthic conditions were tested for by observing patterns in taphonomic alteration of the calcite portions of inoceramid bivalve shells during and after OAE II. Inoceramid valves were extracted from beds that spanned OAE II at the Cenomanian/Turonian Global Boundary Stratotype Section and Point (GSSP) in Pueblo, Colorado: beds 62 (Hartland Shale), 72 (Bridge Creek Limestone), and 115 (Bridge Creek Limestone). Degree of alteration was tallied using categorical data for three taphonomic characteristics (recrystallization, fragmentation, and partial dissolution) using four categories: high (> 60%), medium (59 - 30%), low (29 - 11%), and none (< 10%). The expected taphonomic expression for dysoxic benthic conditions is medium alteration for each taphonomic characteristic. Valve calcite taphonomic characteristics from beds deposited during OAE II (Bed 62 and Bed 72), and after OAE II (Bed 115) were tested for dissimilarities using the Chi-squared test of independence. Primary and retrial test results indicated a statistically significant similarity in alteration of calcite among the three beds: P(χ2 > 19.963) = 0.003 (d.f. = 6, α = 0.01, n = 480). These results match expectations for preservation during dysoxic benthic conditions for the duration of the CIS OAE II signal. Trace element data from the Cenomanian/Turonian GSSP verify taphonomic data, and point to benthic dysoxia through the interval. Thus, both taphonomic and geochemical data corroborate recent studies that infer that the CIS was dysoxic during OAE II.