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Book Chapter

The Role of Ichnology in Refining Shallow Marine Facies Models

By
James A. MacEachern
James A. MacEachern
Earth Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada e-mail: jmaceach@sfu.ca
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Kerrie L. Bann
Kerrie L. Bann
Earth Sciences, Simon Fraser University, Burnaby, British Columbia, V5A 1S6, Canada e-mail: jmaceach@sfu.ca
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Published:
January 01, 2008

Abstract

The facies characteristics of the shallow marine environment are largely determined by the complex interplay and relative importance of wave energy, tidal flux, storm magnitudes and frequencies, and river-sediment input. Ichnology is a valuable tool in constraining these processes when integrated with sedimentological and stratigraphic analysis. Trace fossils are biogenic sedimentary structures, strongly facies controlled, and generally temporally long ranging, making them ideal for facies analysis. Ichnofossils are also readily observable at outcrop and core scales, making their identification and interpretation as routine as that of primary physical sedimentary structures.

The ichnofacies paradigm is exceptionally well poised to offer critical information about the conditions operating during deposition (e.g., softground ichnofacies) or during development of stratigraphic discontinuities (e.g., substrate-controlled ichnofacies and palimpsest softground suites). The ichnofacies concept stands as one of the most elegant but also most widely misunderstood and misused concepts in ichnology. Softground ichnofacies have been refined to include proximal, archetypal, and distal expressions, permitting high-resolution subdivision of depositional environments such as strandline shoreface complexes. Models addressing brackish-water-induced stresses, substrate consistency changes, reduced oxygen levels, and energy variations on bioturbation have enhanced the identification and subdivision of estuarine incised-valley, embayment, and interdistributary-bay deposits. The ichnological characteristics of these brackish-water settings include: (1) suites characterized by reductions in the numbers and diversities of ichnogenera, corresponding to impoverished marine assemblages; (2) traces that are generally diminutive compared to their fully marine counterparts; (3) a predominance of simple opportunistic structures of inferred trophic generalists; (4) suites comprising elements that record variations in substrate consistency and depositional rates; and (5) successions showing locally high degrees of bioturbation, as well as monogeneric trace suites. Ongoing work concentrates on the effects of hypopycnal-induced water turbidity, hyperpycnal discharge, freshets, fluid-mud deposition, heightened depositional rates, and storm events on infaunal behavior, and helps to identify the deltaic ichnological signal, elucidating the relative importance of waves, tides, and fluvial discharge. Generally reduced and sporadically distributed bioturbation intensities, common unburrowed and mud-draped event beds, abrupt juxtaposition of fully marine suites with impoverished suites, predominance of facies-crossing deposit-feeding structures adapted to sandy substrates, and a paucity of dwelling structures attributable to suspension-feeding organisms constitute the recurring ichnological deltaic signal that has been elucidated to date.Ultimately, such models could be applied to along-strike variations in linked depositional systems, enhancing paleogeographicreconstructions.

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Contents

SEPM Special Publication

Recent Advances in Models of Siliciclastic Shallow-Marine Stratigraphy

Gray J. Hampson
Gray J. Hampson
Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
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Ronald J. Steel
Ronald J. Steel
Department of Geosciences, Jackson School, University of Texas at Austin, Austin, Texas 78712, U.S.A.
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Peter M. Burgess
Peter M. Burgess
Shell International Exploration and Production, Kessler Park 1, P.O. Box 60, 2280 AB Rijswijk, The NetherlandsPresent address: Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK
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Robert W. Dalrymple
Robert W. Dalrymple
Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
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SEPM Society for Sedimentary Geology
Volume
90
ISBN electronic:
9781565763180
Publication date:
January 01, 2008

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