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

https://doi.org/10.1016/j.earscirev.2006.10.002Get rights and content

Abstract

Most tide-dominated estuarine and deltaic deposits accumulate in the fluvial-to-marine transition zone, which is one of the most complicated areas on earth, because of the large number of terrestrial and marine processes that interact there. An understanding of how the facies change through this transition is necessary if we are to make correct paleo-environmental and sequence-stratigraphic interpretations of sedimentary successions. The most important process variations in this zone are: a seaward decrease in the intensity of river flow and a seaward increase in the intensity of tidal currents. Together these trends cause a dominance of river currents and a net seaward transport of sediment in the inner part of the transition zone, and a dominance of tidal currents in the seaward part of the transition, with the tendency for the development of a net landward transport of sediment. These transport patterns in turn develop a bedload convergence within the middle portion of all estuaries and in the distributary-mouth-bar area of deltas. The transport pathways also generate grain-size trends in the sand fraction: a seaward decrease in sand size through the entire fluvial–marine transition in deltas, and through the river-dominated, inner part of estuaries, but a landward decrease in sand size in the outer part of estuaries. A turbidity maximum (i.e., a zone of significantly elevated suspended-sediment concentrations) is developed within estuaries and the delta-plain region of deltas as a result of flocculation and density-driven water-circulation patterns. This leads to an area within the estuary or delta plain where the abundance and thickness of the mud drapes are greatest, including the potential for the development of fluid-mud deposits (i.e., structureless mud layers more than 0.5–1 cm thick that were deposited in a single slack-water period). A monotonic seaward increase in salinity characterizes both estuaries and deltas. The brackish-water conditions in the transition zone, accompanied by the high turbidity and physically harsh conditions, produce a biologically stressed environment, in which bioturbation is generally not pervasive. The ichnofossil assemblage in this zone is characterized by the low diversity of ichnogenera, small size of the individual burrows (typically smaller than their open-marine counterparts), and highly variable population densities, ranging from unbioturbated to very high-density mono-specific assemblages in local areas.

This review begins with a survey of how and why each depositional process varies through the fluvial-to-marine transition and then examines the sedimentological responses to these processes, focussing on the observable, longitudinal variations in the development and/or abundance of each deposit characteristic (e.g., sand grain size, paleocurrent patterns, mud drapes, and biological attributes). The review ends with a summary of the characteristics of each major facies zone through the transition, with separate discussions for both estuaries and deltas. It must be noted that any attempt to generalize, as is done here, will undoubtedly contain statements that are inappropriate for individual deposits or systems. Exceptions must be expected and the concepts must be applied with due consideration for the local context.

Introduction

The correct interpretation of ancient sedimentary deposits, whether for academic or applied purposes, requires knowledge about two separate, but inter-related aspects of sedimentary successions: interpretation of the original depositional environments, using the techniques of facies analysis, as illustrated by the popular textbook “Facies Models” (Walker and James, 1992); and subdivision of the stratigraphic succession into genetically related units using the principles of sequence stratigraphy (e.g., Van Wagoner et al., 1988, Posamentier and Allen, 1999, Catuneanu, 2006). The integration of these two lines of investigation allows the construction of realistic paleogeographic reconstructions that show how the depositional facies are related in space and time. From this, it is possible to develop more precise depositional histories, and to predict more accurately the location and geometry of hydrocarbon reservoir facies.

The sequence-stratigraphic analysis of sedimentary successions, including the identification of sequence boundaries and maximum flooding surfaces, is based on the identification of sequential (i.e., progressive) changes in the nature of the deposits. Thus, progradational successions, in which more proximal deposits overlie those formed in more distal settings, characterize the falling-stage, lowstand, and highstand systems tracts, whereas retrogradational facies stacking (i.e., more distal over more proximal deposits) occurs in the transgressive systems tract. Facies stacking patterns are also important for the correct identification of some environments. For example, estuaries, as defined by Dalrymple et al. (1992; see also Boyd et al., 2006, Dalrymple, 2006), form only under transgressive conditions and thus are represented primarily by transgressive successions, whereas deltas are progradational (Dalrymple et al., 2003).

[Throughout this review, the terms “estuary” and “estuarine” refer only to transgressive coastal areas and not to those areas with brackish-water! Indeed, as will be noted later, brackish-water conditions also occur in deltas and even in some shelf environments, whereas some transgressing coastal areas have either fully fresh or fully marine salinity. However, the use of “estuary” here differs slightly from that proposed by Dalrymple et al. (1992) and instead follows the revised definition proposed by Dalrymple (2006) in that we do not restrict the term to incised-valley systems. Thus, the abandoned portions of delta plains that are undergoing transgression (i.e., the “destructive phase” of the delta cycle) are here considered to be estuaries (Fig. 1). In this context, the term “delta” is applied only to the actively prograding portion of the larger deltaic system.]

The paragraphs above show that the ability to distinguish proximal facies from more distal deposits is an essential element of most sedimentary interpretations. However, the distinction of proximal from distal facies is not equally easy in all environmental settings. Wave-dominated coastal zones (i.e., the beach-shoreface-shelf suite of environments) display a simple and well-understood decrease in wave-energy level as the water depth increases (Fig. 2). As a result of this monotonic trend in wave energy, there is a predictable correlation between water depth and facies that is represented by an upward-coarsening succession (Fig. 3A, C) that passes from mudstones (“offshore”), through deposits with thin, discrete sandstone beds with wave ripples and hummocky cross stratification (HCS) (offshore transition), into amalgamated sandstones with HCS (lower shoreface) and eventually into sandstones with swaley cross stratification (SCS) and cross bedding (upper shoreface) (e.g., Walker and Plint, 1992). In fact, this vertical succession is so predictable that deviations from the expected succession can be used to infer such things as forced regressions (Fig. 3B).

By comparison, the proximal–distal changes in processes and facies that occur in tide-dominated environments (sensu Boyd et al., 1992; see the “General considerations” section below for a discussion of what is meant by “tide dominated”) are not well known because of their inherent complexity. At least two fundamental factors account for this. First, tidal energy does not vary in a simple (i.e., monotonic) way with onshore-offshore position. Studies in many modern environments show that tide-dominated environments are generally hypersynchronous. This means that the tidal range increases landward because of the funnel-shaped geometry of the channel systems comprising the estuary or delta (Fig. 4, Fig. 5). This in turn means that there are two areas with relatively weak tidal currents (at the mouth and at the head), separated by an area with stronger tidal currents. Thus, it might be possible to get similar tidal deposits in two very different parts of the fluvial–marine transition, leading to confusion and potential mis-interpretation of the depositional environment. Secondly, tidal environments are characterized by complex networks of tidal channels and bars. This causes the architecture of the deposits to be complex because of the migration and stacking of successive channels and the presence of erosion surfaces of several different orders (Fig. 6, Fig. 7). Furthermore, there are vertical changes in tidal current speeds within a single channel that mimic the longitudinal changes in tidal energy. The erosional juxtaposition of channel bodies also makes it difficult to recognize any larger-scale stratigraphic trends that may exist.

The task of interpreting ancient tidal deposits is made even more challenging by the fact that there is a global dominance of transgressive coastlines in the modern world. Consequently, almost all of the well-studied modern, tide-dominated systems are transgressive (i.e., estuaries such as the Bay of Fundy — Dalrymple et al., 1990, Dalrymple et al., 1991, Dalrymple and Zaitlin, 1994; and the Severn Estuary — Harris and Collins, 1985, Allen, 1990). By comparison, there are very few well-documented modern (Dalrymple et al., 2003) or ancient (e.g., Mutti et al., 1985, Maguregui and Tyler, 1991, Martinius et al., 2001) examples of progradational (i.e., deltaic) tide-dominated successions, and some well-respected sedimentologists have even suggested that tide-dominated deltas do not exist (Walker, 1992, Bhattacharya and Walker, 1992), a view that is not universally accepted (Dalrymple, 1999, Harris et al., 2002, Dalrymple et al., 2003, Willis, 2005). This bias in the availability of analogues leads to a tendency for workers to assume that ancient tide-dominated deposits were also formed during transgressions.

Given these inherent difficulties with the interpretation of tide-dominated deposits, which are of increasing economic importance given the large number of important petroleum reservoirs hosted by tidal deposits (e.g., the McMurray Oil Sands, Alberta, Canada), the purpose of this report is to synthesize the available information on the proximal–distal changes in the facies characteristics of tidal environments, from the limit of tidal action within fluvial systems, through the coastal zone, and out onto the shelf. In addition, we examine changes in facies as a function of water depth, both within channels in the inshore zone (i.e., landward of the main coast) and with increasing water depth in the offshore zone. Our approach is based on theoretical considerations, supplemented by what information there is from modern estuaries and deltas. Our objective is to produce a set of criteria that can be applied to ancient tide-dominated deposits in order to facilitate their environmental and sequence-stratigraphic subdivision and interpretation.

Section snippets

General considerations

The transition zone between terrestrial (river) environments and the open-marine shelf (i.e., the coastal zone sensu lato) represents one of the most profound spatial changes in depositional conditions that can be found anywhere on earth. Many factors that influence the nature of the deposits change dramatically across this zone. The most fundamental of these are (Fig. 8):

  • (1)

    the bathymetry and geomorphology — from relatively shallow-water, channelized environments landward of the coast, to deeper,

Process variations

In the following sections, we examine the longitudinal (from land to sea) and depth-related variation of the physical, chemical, and biological processes that directly influence the nature of the deposits. Because estuaries and deltas have important differences in some regards, they are considered separately.

Sedimentological consequences

The operation of the above processes produces a variety of observable sedimentological consequences that can be used to determine the relative location at which a given deposit formed in the fluvial–marine transition.

Environmental summaries

The foregoing material has examined the fluvial–marine transition zone in some detail, considering each of the several depositional processes and responses separately. This approach highlights the fundamental processes that are responsible for the facies gradients that exist, but makes it difficult to appreciate the facies characteristics of each part of the proximal–distal transition that result from the combined influence of all processes. Therefore, we provide here a summary of the deposits

Concluding remarks

The transition between the land and the sea in tide-dominated coastal environments is among the most complex on Earth, because of the interaction of numerous physical, chemical and biological processes. The resulting deposits are also complex and consist predominantly of channel deposits: most of the tidal bars that occur in these environments produce lateral-accretion deposits because of lateral migration of the adjacent channel. The complex architecture of the resulting succession makes

Acknowledgements

The authors thank the sponsors (Norsk Agip A/S, BP Norge AS, DONG Norge as, Esso Norge AS, Fortum Petroleum AS, Phillips Petroleum Company Norway, A/S Norske Shell, Statoil ASA, and TotalFinaElf Exploration Norge) of the FORCE (FOrum for Reservoir Characterization and reservoir Engineering, a branch of the Norwegian Petroleum Directorate) “Tidal Signatures” project for the financial support that enabled the writing of this review, and for granting permission to publish it. Much of Dalrymple's

References (142)

  • S.J. Culver

    Differential two-way sediment transport in the Bristol Channel and Severn Estuary, U.K.

    Marine Geology

    (1980)
  • R.W. Dalrymple et al.

    Estuarine dunes and bars

  • K.R. Dyer

    Sediment transport processes in estuaries

  • H. Fenies et al.

    Facies and geometry of tidal channel-fill deposits (Arcachon Lagoon, SW France)

    Marine Geology

    (1998)
  • R.J. Gibbs et al.

    Coagulation and settling of Amazon River suspended sediment

    Continental Shelf Research

    (1986)
  • P.T. Harris

    Large scale bedforms as indicators of mutually evasive sand transport and the sequential infilling of wide-mouthed estuaries

    Sedimentary Geology

    (1988)
  • P.T. Harris et al.

    A preliminary study of sedimentation in the tidally dominated Fly River Delta, Gulf of Papua

    Continental Shelf Research

    (1993)
  • P.T. Harris et al.

    Sediment transport in distributary channels and its export to the pro-deltaic environment in a tidally-dominated delta: Fly River, Papua New Guinea

    Continental Shelf Research

    (2004)
  • K. Hori et al.

    Architecture and evolution of the tide-dominated Changjiang (Yangtze) River delta, China

    Sedimentary Geology

    (2002)
  • J.M. Huthnance

    On one mechanism forming linear sand banks

    Estuarine, Coastal Shelf Science

    (1982)
  • J.M. Jaeger et al.

    Tidal controls on the formation of fine-scale sedimentary strata near the Amazon River mouth

    Marine Geology

    (1995)
  • S.A. Kuehl et al.

    Sediment deposition, accumulation, and seabed dynamics in an energetic fine-grained coastal environment

    Continental Shelf Research

    (1996)
  • P. Lesueur et al.

    Shelf mud fields formation within historical times: examples from offshore the Gironde estuary, France

    Continental Shelf Research

    (1996)
  • C. Li et al.

    Late Quaternary incised-valley fill of the Yangtze delta (China): its stratigraphic framework and evolution

    Sedimentary Geology

    (2002)
  • A.W. Martinius et al.

    Sedimentology of the heterolithic and tide-dominated Tilje Formation (Early Jurassic, Halten Terrace, offshore nid-Norway)

  • K. Muylaert et al.

    Dissolved organic carbon in the freshwater tidal reaches of the Schelde estuary

    Estuarine, Coastal and Shelf Science

    (2005)
  • G.P. Allen

    Sedimentary processes and facies in the Gironde estuary: a recent model for macrotidal estuarine systems

  • G.M. Ashley

    Classification of large-scale subaqueous bedforms: a new look at an old problem

    Journal of Sedimentary Petrology

    (1990)
  • J.H. Barwis

    Sedimentology of some South Carolina tidal-creek point bars, and a comparison with their fluvial counterparts

  • D.J. Beets et al.

    The Holocene evolution of the barrier and the back-barrier basins of Belgium and the Netherlands as a function of late Weichselian morphology, relative sea-level rise and sediment supply

    Geologie en Mijnbouw

    (2000)
  • R.H. Belderson et al.

    Bedforms

  • S. Berné et al.

    Internal structure of subtidal sandwaves revealed by high-resolution seismic reflection

    Sedimentology

    (1988)
  • J. Bhattacharya et al.

    Deltas

  • R. Boyd et al.

    Estuary and incised valley facies models

  • L.A. Buatois et al.

    The paradox of nonmarine ichnofacies in tidal rhythmites: integrating sedimentologic and ichnologic data from the late Carboniferous of eastern Kansas, USA

    Palaios

    (1997)
  • L.A. Buatois et al.

    Colonization of brackish-water systems through time: evidence from the trace-fossil record

    Palaios

    (2005)
  • P.-Y. Burban et al.

    The flocculation of fine-grained sediments in estuarine waters

    Journal of Geophyical Research

    (1989)
  • Calverley, E.A., 1984. Sedimentology and geomorphology of the modern epsilon cross-stratified point bar deposits in the...
  • O. Catuneanu

    Principles of Sequence Stratigraphy

    (2006)
  • E. Chaumillon et al.

    Spatial variability of modern incised-valleys on the French Atlantic Coast: Comparison between the Charente and the Lay–Sèvre incised-valleys

  • K.S. Choi et al.

    Sedimentology of modern, inclined heterolithic stratification (IHS) in the macrotidal Han River delta, Korea

    Journal of Sedimentary Research

    (2004)
  • J.D. Collinson

    Bedforms of the Tana River, Norway

    Geographisca Annaler

    (1970)
  • J.D. Collinson

    Current vector dispersion in a river of fluctuating discharge

    Geologie en Mijnbouw

    (1971)
  • R.W. Dalrymple

    Morphology and internal structure of sand waves in the Bay of Fundy

    Sedimentology

    (1984)
  • R.W. Dalrymple

    Tidal depositional systems

  • R.W. Dalrymple

    Tide-dominated deltas: do they exist or are they all estuaries?

  • R.W. Dalrymple

    Incised valleys in time and space: introduction to the volume and an examination of the controls on valley formation and filling

  • R.W. Dalrymple et al.

    Sediment transport by tidal currents

  • R.W. Dalrymple et al.

    High-resolution sequence stratigraphy of a complex, incised valley succession, the Cobequid Bay–Salmon River estuary, Bay of Fundy, Canada

    Sedimentology

    (1994)
  • R.W. Dalrymple et al.

    Dynamics and facies model of a macrotidal sand bar complex

    Sedimentology

    (1990)
  • Cited by (760)

    View all citing articles on Scopus
    1

    Present address: Faculty of Earth Systems and Environmental Sciences, Chonnam National University, Gwangju 500-757, Korea. Tel.: +82 62 530 3473; fax: +82 62 530 3469.

    View full text