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