The Role of Ichnology in Refining Shallow Marine Facies Models
James A. MacEachern, Kerrie L. Bann, 2008. "The Role of Ichnology in Refining Shallow Marine Facies Models ", Recent Advances in Models of Siliciclastic Shallow-Marine Stratigraphy, Gray J. Hampson, Ronald J. Steel, Peter M. Burgess, Robert W. Dalrymple
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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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Siliciclastic shallow-marine deposits record the interface between land and sea, and its response to a variety of forcing mechanisms: physical process regime, the internal dynamics of coastal and shelfal depositional systems, relative sea level, sediment flux, tectonic setting, and climate. These deposits have long been the subject of conceptual stratigraphic models that seek to explain the interplay between these various forcing mechanisms, and their preservation in the stratigraphic record. This volume arose from an SEPM research conference on shoreline–shelf stratigraphy that was held in Grand Junction, Colorado, on August 24–28, 2004. The aim of the resulting volume is to highlight the development over the last 15 years of the stratigraphic concepts and models that are used to interpret siliciclastic marginal-marine, shallow-marine, and shelf deposits.