Isotopic - Textural - Chemical Studies of Mid-Cretaceous Limestones: Implications for Carbonate Diagenesis and Paleooceanography

256 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1986.Skeletal and nonskeletal components from Mid-Cretaceous limestones have been investigated using coordinated textural, isotopic, and chemical methods to delineate their diagenetic histories and to provide information on the temperatures and salinity of contemporaneous seawater. The degree of alteration of each component was determined by means of cathodoluminescence, light microscopy, and scanning electron microscopy, along with inferences from isotopic and chemical results.Oxygen isotopic values of nonluminescent parts of a pectenid, chondrodontids, and some oysters (-3.0 to -0.5('o)/oo vs PDB) appear to reflect the temperature, salinity, and isotopic composition of contemporaneous Mid-Cretaceous surface seawater. In contrast, the oxygen isotopic composition (-4.0 to -2.5('o)/oo) of the primary calcite outer layer of three rudistid families and of some oysters suggests that these well preserved specimens were not precipitated at isotopic equilibrium with seawater. These samples are also significantly depleted in (delta)('18)O relative to altered portions of the same specimens, i.e., luminescent regions, calcitized inner layers of requieniids and void-fill calcite within skeletal molds. This apparent isotopic "reversal" supports the conclusion that non-equilibrium oxygen isotope effects were involved in the secretion of calcite layers in some rudists and oysters.Mid-Cretaceous limestones studied here have undergone marine and meteoric diagenesis at least two times in their early post-depositional history. All the radiaxial fibrous calcites analyzed contain less than 0.7 mole % MgCO(,3). Apparent retention of the primary (delta)('18)O marine signature and absence of microdolomite inclusions within the well-preserved samples suggest that RFC formed as primary low-Mg calcite cements in the shallow marine environments during the Mid-Cretaceous. Chemical data from altered skeletal components and blocky calcite suggest that, the diagenetic system was "rock-dominated" or closed and, with respect to Sr, Mg, and (delta)('18)O, it was "water-dominated" or open.Assuming a Cretaceous surface seawater temperature equal to or slightly higher than today, (delta)('18)O values of well preserved skeletal and nonskeletal components suggest that surface water in the ancestral Gulf of Mexico was highly saline, and a possible source of "warm saline bottom water" during the Cretaceous time. Also, the (delta)('18)O increase in bivalves from the Albian to Maastrichtian indicate that the temperature of surface seawater became colder from Middle to Late Cretaceous. (Abstract shortened with permission of author.)U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD