Paper No. 8
Presentation Time: 10:15 AM
INTEGRATING BIO-, CHEMO-, CHRONO-, GAMMA-RAY, LITHO-, AND SEQUENCE STRATIGRAPHY IN THE UPPER CAMBRIAN AND LOWER ORDOVICIAN: PROGRESS TOWARD A COMPREHENSIVE STRATIGRAPHIC FRAMEWORK
MILLER, James F., Geography, Geology, & Planning Dept, Missouri State University, Springfield, MO 65897, DATTILO, Benjamin F., Department of Geosciences, Indiana University Purdue University, Fort Wayne, IN 46805, ETHINGTON, Raymond L., Geological Sciences Dept, University of Missouri-Columbia, Columbia, MO 65211, EVANS, Kevin Ray, Geography, Geology, and Planning, Missouri State University, 901 S. National Ave, Springfield, MO 65897, FREEMAN, Rebecca L., Earth & Environmental Science, University of Kentucky, Lexington, KY 40506, LOCH, James D., Biology & Earth Sciences Dept, University of Central Missouri, Warrensburg, MO 64093, REPETSKI, John E., U. S. Geological Survey, 926A National Center, Reston, VA 20192, RIPPERDAN, Robert L., 1417 Fairbrook Drive, Des Peres, MO 63131, RUNKEL, Anthony C., Minnesota Geological Survey, 2642 University Avenue W, St. Paul, MN 55114 and TAYLOR, John F., Geoscience Dept, Indiana University of Pennsylvania, Indiana, PA 15705, jimmiller@missouristate.edu
Chronostratigraphic classification of the Laurentian Upper Cambrian and Lower Ordovician is based on strata in Utah, Nevada, Missouri, Minnesota, Wisconsin, and Alberta. Biozonations are based on strata there as well as in Texas and Oklahoma. Regional trilobite zonations vary somewhat because of facies influence and periods of regional endemism; the zonation is quite different in slope deposits. Conodont zonation begins in the middle Upper Cambrian; many taxa occur in a variety of facies and are cosmopolitan, so zones can be correlated across Laurentia and globally. Calcitic brachiopod zones established in Oklahoma have been identified in other areas where faunas have been studied. Phosphatic brachiopod zones were established first in the lower Upper Cambrian, and the zonation is being extended into the Lower Ordovician.
Carbon-isotope profiles of Upper Cambrian and Lower Ordovician strata from several parts of Laurentia are used for precise correlation, especially in conjunction with trilobite and conodont data. Distinctive profile peaks have been correlated among several areas in Laurentia and to Australia, China, and Kazakhstan.
Stratigraphic sequences identified in Utah, Nevada, and Texas range in scale from four supersequences within the Sauk Megasequence to individual meter-scale cycles. In Utah a series of highstand strata (mostly deep-subtidal lime mudstones) alternate with well-preserved lowstand strata (shallow-subtidal lime grainstones). Lowstand strata have higher concentrations of fine quartz sand compared with highstand strata; insoluble residue data are from hundreds of conodont samples. Nearly all sequence boundaries are conformable in thick carbonate strata in Utah and Nevada but may correlate to hiatuses in cratonic areas. Several lowstand intervals can be identified on other continents if sufficient conodont or carbon-isotope data exist there. Outcrop gamma-ray logs allow correlation among exposures and into the subsurface. Lithostratigraphic boundaries in various areas of Laurentia often equate to sequence boundaries, as do some zonal boundaries.
Cooperative research using all of these approaches is producing a comprehensive stratigraphic framework for Upper Cambrian and Lower Ordovician strata in Laurentia. More work is needed to fill in parts of the entire framework.
