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Towards the standardization of sequence stratigraphy

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Morphologic and facies trends through the fluvial–marine transition in tide-dominated depositional systems: A schematic framework for environmental and sequence-stratigraphic interpretation

The recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply, which correspond to sequences in the rock record. These cycles may be symmetrical or asymmetrical, and may or may not include all types of systems tracts that may be expected within a fully developed sequence. Depending on the scale of observation, sequences and their bounding surfaces may be ascribed to different hierarchical orders. Stratal stacking patterns combine to define trends in geometric character that include upstepping, forestepping, backstepping and downstepping, expressing three types of shoreline shift: forced regression (forestepping and downstepping at the shoreline), normal regression (forestepping and upstepping at the shoreline) and transgression (backstepping at the shoreline). Stacking patterns that are independent of shoreline trajectories may also be defined on the basis of changes in depositional style that can be correlated regionally. All stratal stacking patterns reflect the interplay of the same two fundamental variables, namely accommodation (the space available for potential sediment accumulation) and sediment supply. Deposits defined by specific stratal stacking patterns form the basic constituents of any sequence stratigraphic unit, from sequence to systems tract and parasequence. Changes in stratal stacking patterns define the position and timing of key sequence stratigraphic surfaces. Precisely which surfaces are selected as sequence boundaries varies as a function of which surfaces are best expressed within the context of the depositional setting and the preservation of facies relationships and stratal stacking patterns in that succession. The high degree of variability in the expression of sequence stratigraphic units and bounding surfaces in the rock record means ideally that the methodology used to analyze their depositional setting should be flexible from one sequence stratigraphic approach to another. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture. The purpose of this paper is to emphasize a standard but flexible methodology that remains objective.

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Sequence stratigraphy: methodology and nomenclature

Stratigraphic and sedimentological data from New South Wales and Queensland, eastern Australia, indicate that the Late Palaeozoic Ice Age comprised at least eight discrete glacial intervals (each 1–8 Ma in duration, here termed ‘glaciations’), separated by nonglacial intervals of comparable duration. These events spanned an interval from mid-Carboniferous ( c . 327 Ma) to the early Late Permian ( c . 260 Ma), and illustrate a pattern of increasing climatic austerity and increasingly widespread glacial ice from initial onset until an acme in the late Early Permian, followed by an opposite trend towards the final demise of glaciation in the Late Permian. The alternating glacial–nonglacial motif suggests that the Late Palaeozoic Ice Age was considerably more dynamic than previously thought. These patterns are remarkably consistent with recent interpretations of palaeofloral change, eustatic sea-level fluctuations and CO 2 –climate–glaciation relationships for this interval of time. The detailed record of alternating glacial and nonglacial climate mode disclosed herein may facilitate more closely resolved evaluations of stratigraphic records elsewhere, notably in far-field, ice-distal, northern hemisphere successions.

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Stratigraphic imprint of the Late Palaeozoic Ice Age in eastern Australia: a record of alternating glacial and nonglacial climate regime

Despite the historical assumption that the bulk of marine “shelf” mud is deposited by gradual fallout from suspension in quiet water, recent studies of modern muddy shelves and their associated rivers show that they are dominated by hyperpycnal fluid mud. This has not been widely applied to the interpretation of ancient sedimentary fluvio-deltaic systems, such as dominate the mud-rich Cretaceous Western Interior Seaway of North America. We analyze two such systems, the Turonian Ferron Sandstone Member of the Mancos Shale Formation, in Utah, and the Cenomanian Dunvegan Formation in Alberta. Paleodischarge estimates of trunk rivers show that they fall within the predicted limits of rivers that are capable of generating hyperpycnal plumes. The associated prodeltaic mudstones match modern hyperpycnite facies models, and suggest a correspondingly hyperpycnal character. Physical sedimentary structures include diffusely stratified beds that show both normal and inverse grading, indicating sustained flows that waxed and waned. They also display low intensities of bioturbation, which reflect the high physical and chemical stresses of hyperpycnal environments. Distinct “mantle and swirl” biogenic structures indicate soupground conditions, typical of the fluid muds that represent the earliest stages of deposition in a hyperpycnal plume. Hyperpycnal conditions are ameliorated by the fact that these rivers were relatively small, dirty systems that drained an active orogenic belt during humid temperate (Dunvegan Formation) to subtropical (Ferron Sandstone Member) “greenhouse” conditions. During sustained periods of flooding, such as during monsoons, the initial river flood may lower salinities within the inshore area, effectively “prepping” the area and allowing subsequent floods to become hyperpycnal much more easily. Although shelf slopes were too low to allow long-run-out hyperpycnal flows, the storm-dominated nature of the seaway likely allowed fluid mud to be transported for significant distances across and along the paleo-shelf. Rapidly deposited prodeltaic hyperpycnites are thus considered to form a significant component of the muddy shelf successions that comprise the thick shale formations of the Cretaceous Western Interior Seaway.

http://basin.earth.ncu.edu.tw/download/lab%20reading/Hyperpycnal%20flows%20and%20fluid%20mud%20deposition/Bhattacharya,%20J.P.,%20MacEachern,%20J.A.%20(2009)%20Hyperpycnal%20rivers%20and%20prodeltaic%20shelves%20in%20the%20Cretaceous%20seaway%20of%20north%20America.pdf

Hyperpycnal Rivers and Prodeltaic Shelves in the Cretaceous Seaway of North America

Analyses of deltaic facies successions highlight recurring ichnological patterns that reflect a variety of physicochemical stresses imposed upon infaunal organisms by the interaction of various delta-front processes. Analysis of numerous ancient deltaic deposits in Canada, the United States, Australia, and offshore Norway persistently show reductions in bioturbation intensity and impoverishment in ichnological diversity, compared to those of nondeltaic shorelines. Some facies locally demonstrate sporadic colonization and recolonization of substrates left denuded by episodic depositional conditions. Deltaic ichnological suites also locally display size reductions of ichnogenera and a paucity of suspension-feeding ethologies. Resulting trace-fossil suites are overwhelmingly dominated by depositfeeding behaviors, even in sandy facies. Such ichnological characteristics largely reflect increased sedimentation rates and heightened fluvial discharge, which serve to impede infaunal colonization. River-derived stresses are profound: salinity changes, hypopycnal-flow-induced water turbidity, distributary flood discharges with accompanying phytodetrital (comminuted plant debris) pulses, hyperpycnal-flow-induced sediment gravity flows, and fluid-mud deposition all conspire to produce the overall depauperate nature of the ichnological assemblage. Freshet-discharge events during river floods, accompanied by hyperpycnal conditions, may lead to the episodic introduction of reduced-salinity waters immediately above the sediment–water interface in delta-front and prodelta depositional settings. Such conditions may facilitate development of synaeresis cracks and promote reductions in infaunal populations. Wave energy generally buffers fluvial effects, by dispersing suspended sediment offshore and encouraging the thorough mixing of waters of contrasting salinity. High mud concentrations near the delta front damp wave energy, however, limiting its effectiveness in remediating the benthic ecosystem, particularly immediately following distributary flood discharges and storm events. In wavedominated settings, strong alongshore drift also operates to extend river-derived stresses considerable distances downdrift from distributary mouths. Where asymmetric deltas are formed, markedly different ichnological expressions are expressed on either side of distributary-channel mouths. Updrift settings typically retain classic shoreface assemblages, whereas downdrift environments commonly acquire markedly stressed suites. Storm energy may be effective in dispersing mud and mixing waters, but it also results in erosion and episodic sediment deposition. Concomitant precipitation induces river floods, returning river-derived stresses to the delta front. Tidal energy and its effects on the infaunal communities of deltas are poorly documented. Tidal flux may trap mud plumes against the delta front, elevating water turbidity. Pronounced mud flocculation coupled with increased settling velocity associated with tidal mixing also leads to rapid deposition of thick fluid muds, particularly in low-lying areas, hampering or precluding colonization. Tidal energy also leads to marked changes in energy and salinity near the sediment–water interface at several time scales. Deltaic ichnological suites are characterized by structures of opportunistic trophic generalists, though mainly those of facies-crossing deposit feeders. High water turbidity, particularly near the sea floor, precludes most suspension-feeding behavior, and suppresses the development of the Skolithos ichnofacies, even in many proximal delta-front deposits. Ichnological characteristics record the dynamic interplay and relative importance of these different processes, both temporally and spatially. on delta systems. River Deltas—Concepts, Models, and Examples SEPM Special Publication No. 83, Copyright © 2005 SEPM (Society for Sedimentary Geology), ISBN 1-56576-113-8, p. xxx–xxx. INTRODUCTION Although general concepts of deltaic sedimentation are generally well understood, the specific details of the accompanying ichnology are not. Many recent studies of modern deltaic systems, including many in this volume, emphasize the physical sedimentology. Measured sections through cores typically depict sedimentary structures, textures, grain size, body fossils, microfossils, geochemical analyses, and age dating. Although most descriptions note that burrows are present, detailed analysis of the ichnology, such as assessment of the Bioturbation Index (BI; Figure 1) and its distribution, identification of ichnospecies, ichnogenera, and general information on the diversity, abundance, and ethological groupings of the trace-fossil suite are almost totally lacking. Consequently, there is a paucity of work addressing the potentially unique characteristics of those ichnological suites, which may be critical in identifying various deltaic subenvironments. Some of the ichnological terms used in this paper are included in a glossary following the references. Many sedimentary processes (e.g., formation of ripples or dunes) are not especially sensitive to the environment of deposition. Dunes, for example, can be formed in deserts, rivers, JAMES A. MACEACHERN, KERRIE L. BANN, JANOK P. BHATTACHARYA, AND CHARLES D. HOWELL, JR. 2 shorefaces, and shelves, and observation of cross bedding is not diagnostic of any particular environment. Organisms, in contrast, are extremely sensitive to the environment (Pemberton et al., 1992a, 2001). Environmental factors such as energy conditions, substrate consistencies, depositional rates, oxygenation, salinity, and other physicochemical conditions all lead to discrete biological communities (Ekdale et al., 1984; Bromley, 1990; Pemberton et al., 1992a). These faunal communities yield recurring, strongly facies-controlled groupings of trace fossils that reflect specific combinations of organism behaviors (ethology). Such recurring groupings have been designated “ichnofacies”. The environmental factors that control organism distributions and their traces only tend to change in response to changes in water depth, and consequently, trace-fossil groupings typically display, at most, a passive relationship to paleobathymetry (Frey et al., 1990). This paradigm lies at the heart of ichnology and ichnofacies analysis. Various general models and approaches for using trace fossils in the identification and interpretation of specific sedimentary environments have been developed (e.g., Seilacher, 1967; Howard and Frey, 1984; Savrda and Bottjer, 1989; MacEachern and Pemberton, 1992; Pemberton et al., 1992a; Pemberton et al., 1992b; Pemberton and Wightman, 1992; Gingras et al., 1999; MacEachern et al., 1999a, Taylor et al., 2003; Bann et al., 2004, among others). Animal–sediment interactions are well documented from the more uniform conditions associated with strandplain shorefaces, and the shoreface ichnofacies model is robust (Fig. 2) (e.g., Howard and Reineck, 1981; Howard and Frey, 1984; Pemberton and Frey, 1984; Frey, 1990; Vossler and Pemberton, 1989; MacEachern and Pemberton, 1992; Pemberton and MacEachern, 1995; MacEachern et al., 1999b; Bann and Fielding, 2004). Likewise, ichnological models are well established for inshore tidal settings, on the basis of studies of modern environments (e.g., German and Dutch North Sea; Schäfer, 1956; 1962; Reineck, 1958; Reineck et al., 1967; et al., 1968; Dörjes, 1970; Hertweck, 1970; the Normandy coast of France; Larsonneur, 1975; the Georgia coast of the United States; Frey and Howard, 1972; Hertweck, 1972; Dörjes and Howard, 1975; Howard and Frey, 1973, 1975; Howard et al., 1975; and Majou and Howard, 1975; the tidal flats of Taiwan; Dörjes, 1978; Reineck and Cheng, 1978; and Willapa Bay, Washington; Gingras et al., 1999). This paper reviews and integrates what is known from the ichnological and biological literature with sedimentological studies of both modern and ancient delta deposits, in order to develop a more robust framework for understanding the ichnology of deltaic depositional systems. In deltaic settings, the effects of faunal responses and therefore of animal-sediment characteristics are typified by the dynamic interplay of fluvial influx, fluvial discharge types, tidal energy, wave action, and storms (see Tables 1 and 2). Such interactions, at least as they affect infaunal diversity, abundance, feeding strategy and overall behaviors, are poorly understood. Moslow and Pemberton (1988) presented a preliminary assessment of deltaic ichnology, based on a single core each of the Lower Cretaceous Cadotte Member and Bluesky Formation of the Western Interior Seaway of Alberta. Raychaudhuri and Pemberton (1992) and Raychaudhuri (1994) summarized the ichnological characteristics of the Lower Cretaceous Bow Island Formation of south-central Alberta. Gingras et al. (1998) conducted a preliminary ichnological study of the waveand riverinfluenced delta lobes in the Upper Cretaceous Dunvegan Formation in west-central Alberta, building on the detailed stratigraphic analysis and mapping of the depositional systems of Bhattacharya (1989) and Bhattacharya and Walker (1991a, 1991b). This preliminary ichnological work was expanded upon by Coates and MacEachern (1999, 2000), Coates (2001), and MacEachern and Coates (2002), who also included a comparison with the more mixed-influence (fluvial–wave) deltaic deposits of the Upper Cretaceous Basal Belly River Formation. Siggerud and Steel (1999) presented an integrated ichnofabric and sedimentological assessment of the coarse-grained Eocene Sant Llorenç del Munt fan-delta complex of SE Ebro Basin, Spain. The ichnology of ancient deltaic intervals has become increasingly studied in recent years. Howell et al. (2004) and Gani et al. (2004) have concentrated on ichnological signals in deltaic strata of the Upper Cretaceous Wall Creek Member, Powder River Basin, Wyoming. Soegaard and MacEachern (2003) carried out a det

Ichnology of Deltas: Organism Responses to the Dynamic Interplay of Rivers, Waves, Storms, and Tides

The Ichnofacies Paradigm: High-Resolution Paleoenvironmental Interpretation of the Rock Record

The Role of Ichnology in Refining Shallow Marine Facies Models

:
The ichnofacies paradigm endures as the elegant, unifying framework within which accurate ichnological observations and reliable environmental interpretations can be derived from the rock record. These temporally and geographically recurring, strongly facies-controlled groupings of trace fossils reflect specific combinations of organism behavior (ethology), and constitute the benchmark animal-sediment responses to optimum environmental conditions. Seilacherian ichnofacies are distinctive, archetypal associations of traces. Ichnofacies are part of the total aspect of the rock, and consist of primary biogenic structures imparted by organisms responding in predictable ways to variations in energy conditions, deposition rates, food resource types, substrate consistency, water salinity, oxygenation, subaerial exposure, substrate moisture, and temperature, among others. Like lithofacies, ichnofacies are subject to Walther's Law, have lateral continuity, display predictable vertical successions, and lead to mappable constructs. Like all facies analyses, interpretations of ichnofaunas are improved substantially when evaluated in the context of the host rocks and their sedimentologic and stratigraphic implications.

CHAPTER 4 – The Ichnofacies Paradigm: A Fifty-Year Retrospective

The ichnofacies paradigm has evolved over a six-decade period since its original inception by Dolf Seilacher. It is a multidimensional framework underpinned by recurring, facies-controlled groupings of biogenic structures that reflect animal responses to paleoenvironmental conditions. These constitute spatially and temporally extensive associations commonly regarded as “Seilacherian Ichnofacies.” The marine realm hosts five recurring softground ichnofacies (Psilonichnus, Skolithos, Cruziana, Zoophycos, and Nereites), generally distributed in a proximal–distal trend reflecting a passive response to increasing water depth (i.e., controlled by depositional factors that progressively change with bathymetry). There are three substrate-controlled ichnofacies (Trypanites, Teredolites, and Glossifungites), recording organism occupation of palimpsest substrates. Finally, there are six continental ichnofacies (Scoyenia, Mermia, Coprinisphaera, Termitichnus, Celliforma, and Octopodichnus–Entradichnus), mainly recording organism responses to temperature and the availability of moisture (i.e., climate-driven associations) in terrestrial settings or oxygenation, depositional energy, and substrate consistency in subaqueous settings. The Seilacherian ichnofacies operate as facies models, built through the distillation of ichnological characteristics derived from numerous modern and ancient case studies. Like lithofacies models, they serve as a norm for comparison, a framework for observations, a predictor in new situations, an integrated basis for interpretation, and a basis for teaching and communication.

Kerrie L. Bann

Papers

Ichnology of Deltas: Organism Responses to the Dynamic Interplay of Rivers, Waves, Storms, and Tides

The Ichnofacies Paradigm: High-Resolution Paleoenvironmental Interpretation of the Rock Record

The Role of Ichnology in Refining Shallow Marine Facies Models

CHAPTER 4 – The Ichnofacies Paradigm: A Fifty-Year Retrospective

The Ichnofacies Paradigm