Ichnofossils and Ichnofabrics in Rhythmically Bedded Pelagic/Hemi-Pelagic Carbonates: Recognition and Evaluation of Benthic Redox and Scour Cycles

  1. P. L. de Boer3 and
  2. D. G. Smith4
  1. C. E. Savrda1 and
  2. D. J. Bottjer2

Published Online: 29 APR 2009

DOI: 10.1002/9781444304039.ch15

Orbital Forcing and Cyclic Sequences

Orbital Forcing and Cyclic Sequences

How to Cite

Savrda, C. E. and Bottjer, D. J. (1994) Ichnofossils and Ichnofabrics in Rhythmically Bedded Pelagic/Hemi-Pelagic Carbonates: Recognition and Evaluation of Benthic Redox and Scour Cycles, in Orbital Forcing and Cyclic Sequences (eds P. L. de Boer and D. G. Smith), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304039.ch15

Editor Information

  1. 3

    Utrecht, The Netherlands

  2. 4

    London, UK

Author Information

  1. 1

    Department of Geology, Auburn University, Auburn, AL 36849-5305, USA

  2. 2

    Department of Geological Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA

Publication History

  1. Published Online: 29 APR 2009
  2. Published Print: 28 JAN 1994

ISBN Information

Print ISBN: 9780632037360

Online ISBN: 9781444304039

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Keywords:

  • ichnofossils and ichnofabrics - pellgic/hemi-pellgiccarbonates;
  • level of benthic oxygenation;
  • dissolution cycles;
  • dilution cycles;
  • productivity cycles;
  • scour cycles

Summary

Provided that their production and preservation are understood, discrete trace fossils and general ichnofabrics within pelagic and hemi-pelagic strata may aid in the recognition of depositional cyclicity and the interpretation of the palaeoenvironmental mechanisms responsible. Trace fossils are particularly sensitive to periodic benthic redox variations and scour events.

Variations in benthic oxygenation levels are indicated by systematic changes in the diversity, diameter and penetration depth of burrows. These parameters can be employed to recognize oxygen-related ichnocoenoses (ORI), the vertical stacking patterns of which can be translated to palaeo-oxygenation curves. As demonstrated by applications to the lower Bridge Creek Limestone Member (Cenomanian–Turonian) of the Greenhorn Limestone (Colorado), such curves can help to refine reconstructions of cyclicity. Curves for the Bridge Creek, constructed on the basis of vertical disposition of laminated strata and four recurring ORI, exhibit short-term redox cycles arranged in bundles of four to six cycles. Bundling patterns and estimated cycle periodicities suggest forcing by axial precession modulated by orbital eccentricity. Although palaeoceanographic implications can be drawn from trace-fossil-derived redox histories, the complex feedback relations among bottom-water oxygenation, bioturbation, physical and chemical properties of substrates, and other parameters necessitate caution in interpretation.

Scour cycles, reflecting periodic fluctuations in bottom current strength, are indicated by the ichnological record of the response of organisms to changes in substrate consistency. Intervals characterized by normal softground ichnofabrics alternate with intervals upon which firmground prelithification omission suites dominated by Thalassinoides and, possibly, hardground postlithification omission suites of borings are superimposed. Rhythmic occurrences of hardgrounds, nodular limestones and/or omission surfaces that are suggestive of orbital control are common in European Late Cretaceous chalks but are typically lacking in North American examples.