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Des Peres, MO, United States

Myrow P.M.,Colorado College | Hanson A.,Colorado College | Phelps A.S.,Colorado College | Creveling J.R.,California Institute of Technology | And 3 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology

Integrated analysis of the sedimentology, stratigraphy, and chemostratigraphy of the uppermost Devonian Chaffee Group of Colorado reveals the presence of two regionally extensive unconformity surfaces associated with globally recognized extinction/eustatic events. The contact between semi-restricted, marginal marine, mixed siliciclastic-carbonate deposits of the Parting Formation and open marine carbonate of the Dyer Formation is a major marine flooding surface across western Colorado. This flooding surface rests at the top of an ~5m thick, transgressive, cross-bedded, shoreline sandstone unit that locally overlies a 2.5-m-thick paleokarst breccia. δ13C values shift lighter across the formation contact, in some cases by as much as 5‰. Oxygen isotopic values are extremely variable between measured stratigraphic sections, in cases invariant across the contact, and in other cases covarying with the δ13C values. At Ouray, CO, δ18O covaries with δ13C throughout the section, and reaches extreme values (<-30‰) below the unconformity. An isotopic shift in rocks of this age in Utah, coined ALFIE, was previously correlated to the Parting-Dyer contact. This study demonstrates that the carbon and oxygen isotopic record of ALFIE is highly variable across western Laurentia, and that important carbonate chemostratigraphic variations result from diagenesis that is clearly linked to a regional unconformity and associated relative sea-level fall. This lowstand may be a signal of eustatic fall associated with the Dasberg Event, a late Famennian marine extinction event. Similar isotopic patterns exist for strata below and above a paleokarst breccia in the upper Dyer Formation that we link to the globally significant latest Famennian Hangenberg Event, which includes a eustatic lowstand and subsequent transgression. Similar to the Parting-Dyer contact, both carbon and oxygen isotopes in strata below this regional unconformity surface show the variable nature of diagenetic alteration of carbonate units during lowstand conditions. Our data also suggest that correlatable δ13C chemostratigraphic shifts can be diagenetically produced during lowstands across a regionally widespread (western U.S.) basin, and that these δ13C shifts may be expressed within outcrops that show no macroscopic sedimentological signature of subaerial exposure. This has broad implications for the evaluation of δ13C data in the rock record, particularly the assumption that extensive correlatable isotopic anomalies reflect global marine signatures. © 2013 Elsevier B.V. Source

Myrow P.M.,Colorado College | Taylor J.F.,Indiana University of Pennsylvania | Runkel A.C.,University of Minnesota | Ripperdan R.L.,1417 Fairbrook Drive
Journal of Sedimentary Research

High-resolution sedimentological and biostratigraphic data recently recovered from Upper Cambrian strata in the northern Rocky Mountain and central Appalachian regions reveal that meter-scale cycles of very different character developed synchronously during deposition of the basal subzones of the Ibexian Series in both the carbonate facies belt and the distal part of the inner detrital, mixed siliciclastic-carbonate facies belt of the Laurentian paleocontinent. A typical mixed siliciclastic- carbonate cycle in the Snowy Range Formation of Wyoming and Montana consists of the following lithofacies in ascending order: shale, shale with very thin grainstone interbeds, grainstone with subordinate thin shale interbeds, flat-pebble conglomerate, and (in some cycles) thrombolitic bioherms. Internal transitions between the constituent lithofacies are mostly gradational, but cycle boundaries are sharp with shale directly overlying flat-pebble conglomerate and/or thrombolitic boundstone. We postulate that these mixed siliciclastic-carbonate cycles developed in response to different depositional dynamics than meter-scale cycles developed in carbonate-belt facies. Specifically, cycles in the carbonate belt were produced largely by temporal variation in accommodation space, and associated filling of that space through progradation of the carbonate system and aggradation to sea level. In contrast, cyclic deposition within the inner detrital belt was controlled by variations in terrigenous sediment input and resulting effects on carbonate productivity. In this case, enhanced delivery of siliciclastic mud by rivers was triggered by regression and/or a shift to more humid climatic conditions. An upward increase of carbonate in the main part of the cycles reflects enhanced carbonate production in response to the reduction of terrigenous clay supply during shoreline retreat. Widespread cementation of fine grainstone substrates led to deposition of extensive beds of flat-pebble conglomerate, possibly in association with elevated sea-surface temperatures and enhanced stormintensity. Thrombolites formed at times of maximum transgression as microbial communities thrived in the absence of turbidity, colonizing the irregular surfaces of flat-pebble conglomerate beds.We thus interpret these wholly subtidal cycles to record full changes in paleobathymetry with the bulk of the cycles recording upward deepening, rather than shoaling associated with regression. Comparison with ageequivalent shoaling cycles of the Conococheague Formation of the carbonate belt of the Appalachian Mountains highlights the different depositional dynamics in the mixed siliciclastic-carbonate systems, and underscores the peril in extrapolation of depositional models derived from carbonate systems to mixed systems, regardless of age or paleogeographic setting. Copyright © 2012, SEPM. Source

Miller J.F.,Missouri State University | Evans K.R.,Missouri State University | Ethington R.L.,University of Missouri | Freeman R.L.,University of Kentucky | And 4 more authors.

The Global boundary Stratotype Section and Point (GSSP) for the base of the Ordovician System is at the First Appearance Datum (FAD) of the conodont Iapetognathus fluctivagus at Green Point in Newfoundland, Canada. Strata there are typical graptolitic facies that were deposited near the base of the continental slope. We propose establishing an Auxiliary boundary Stratotype Section and Point (ASSP) at the FAD of I. fluctivagus at the Lawson Cove section in the Ibex area of Millard County, Utah, USA. There, strata consist of typical shelly facies limestones that were deposited on a tropical carbonate platform and contain abundant conodonts, trilobites, brachiopods, and other fossil groups. Cambrian and Ordovician strata in this area are ∼5300m thick, with the Lawson Cove section spanning 243m in three overlapping segments. Six other measured and studied sections in the area show stratigraphic relationships similar to those at Lawson Cove. Faunas have been used to divide these strata into 14 conodont and 7 trilobite zonal units. The widespread olenid trilobite Jujuyaspis occurs ∼90cm above the proposed boundary at Lawson Cove; this genus is generally regarded as earliest Ordovician. Rhynchonelliform and linguliform brachiopods are common to abundant and are useful for correlation. The FAD of Iapetognathus fluctivagus and occurrences of Jujuyaspis and the Lower Ordovician planktonic graptolite Anisograptus matanensis all occur within a 2.4m interval of strata at a nearby section. Non-biological correlation tools include a detailed sequence stratigraphic classification and a detailed carbon-isotope profile. Especially useful for correlation is a positive δ13C excursion peak ∼15cm below the proposed boundary horizon. All of these correlation tools form an integrated framework that makes the Lawson Cove section especially useful as an ASSP for global correlation of strata with faunas typical of shallow, warm-water, shelly facies. Source

Miller J.F.,Missouri State University | Ripperdan R.L.,1417 Fairbrook Drive | Loch J.D.,University of Central Missouri | Freeman R.L.,University of Kentucky | And 3 more authors.
Annales de Paleontologie

The lowest occurrence of the conodont Eoconodontus notchpeakensis (Miller, 1969) has been proposed as the base of Cambrian Stage 10. The horizon is recognized in three sections in the House Range, western Utah, USA in the lower part of the Red Tops Member of the Notch Peak Formation. This horizon fits within a tightly integrated framework that includes conodont, trilobite, and brachiopod biozonations, as well as carbon-isotope stratigraphy and sequence stratigraphy. The proposed horizon is the base of the Eoconodontus conodont Zone. This horizon is in the lower part of the Saukiella junia Subzone of the Saukia trilobite Zone and is near the top of the Billingsella brachiopod Zone. The HERB Event is a high-amplitude, negative carbon-isotope excursion that has been identified in Laurentia, Australia, China, and Argentina. The start of the excursion is at a negative carbon-isotope peak that is less than half a metre above the base of the E. notchpeakensis Subzone, and the highest-amplitude peak of the HERB Event is near the middle of that relatively thin subzone. The HERB Event has been identified in strata with minimal faunal data, providing the possibility of identifying the base of Stage 10 in nearly unfossiliferous strata. The Notch Peak Formation has been divided into a detailed sequence-stratigraphic framework within a lithostratigraphic context, and some of the sequence boundaries have been identified in Australia and China. Conodonts diagnostic of the Eoconodontus Zone have been identified at 54 localities around the world, including in a succession of Cambrian deep-ocean radiolarian cherts. The Utah conodont zonation has been identified across Laurentia and in other parts of the world in facies ranging from continental slope to nearshore sandstone deposits. The variety of correlation tools and the integration of diverse data produce a superior framework for correlation of the proposed base of Stage 10 and for correlation of many horizons within Stage 10. © 2015 Elsevier Masson SAS. All rights reserved. Source

Miller J.F.,Missouri State University | Evans K.R.,Missouri State University | Freeman R.L.,University of Kentucky | Ripperdan R.L.,1417 Fairbrook Drive | Taylor J.F.,Indiana University of Pennsylvania

The Stage 10 Working Group of the International Subcommission on Cambrian Stratigraphy is tasked with recommending a stratotype section and horizon for the base of Stage 10, the uppermost stage of the Cambrian System. We identify three sections in the House Range in western Utah, USA, for consideration as locations for defining and characterizing the base of the proposed stage. We also propose a boundary horizon at the base of the Eoconodontus conodont Zone combined with a distinctive negative carbon-isotope excursion named the HEllnmaria-Red Tops Boundary (HERB) event. These and other biological and nonbiological tools can be used for correlating the proposed stage. © 2014 © 2014 Taylor & Francis. Source

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