Elsevier

Earth-Science Reviews

Volume 192, May 2019, Pages 214-235
Earth-Science Reviews

Invited review
Fluvio-tidal transition zone: Terminology, sedimentological and ichnological characteristics, and significance

https://doi.org/10.1016/j.earscirev.2019.03.001Get rights and content

Abstract

The fluvio-tidal transition zone (FTZ) is sedimentologically and ichnologically complex, encompassing variable sedimentary facies. Published research on the FTZ implies a wide range of physical and chemical conditions as well as resultant sedimentary structures. Variable definitions and terms have been applied to this transition zone for decades. This paper aims to refine the existing models of fluvio-tidal sedimentation, provide an inventory of sedimentological and ichnological characteristics collected from modern and ancient datasets, and produce useful criteria to facilitate its paleogeographic recognition.

The usage of the descriptive term “fluvio-tidal transition zone” (abbreviated FTZ) is recommended. The zone is defined by the effective tidal limit at its upper end and the salinity limit at its lower end. The length and position of the FTZ can change with time depending on the fluvial flux and tidal force. However, the averaged position of the FTZ should be determined for the rock record based on a number of physical and biogenic sedimentary structures. Lithologically, the FTZ is represented by medium-grained sandstone to pebble conglomerate in smaller systems, and very fine- to fine-grained sandstone in middle- to large-scale depositional systems. The typical sedimentary structures constitute: unidirectional and bidirectional trough cross-stratification and ripple cross-lamination, abundant carbonaceous debris, coal beds, mudstone and peat rip-up clasts, massive bedding, tidal bundles and rhythmites, inclined heterolithic stratification (IHS), soft-sediment deformation, herringbone bedding, sigmoidal cross-bedding, and reactivation surfaces. Ichnologically, the FTZ is subdivided into the lower or tidal-fluvial part characterized by the Skolithos and depauperate Cruziana ichnofacies passing upstream into the upper or fluvio-tidal part, represented by the Scoyenia and Mermia ichnofacies.

Introduction

The transition zone between the river and the estuary or delta is variably expressed and sedimentologically complex. It encompasses sediment deposited between the inner estuary or distributary delta channels and non-tidal fluvial settings, and therefore exhibits profound changes on preserved sedimentary facies, particularly inclined heterolithic stratification (IHS). Published modern and ancient examples of the fluvio-tidal transition zone (FTZ) imply a wide range of depositional conditions and resultant sedimentary structures, in part, because the FTZ has been demarcated in estuaries and deltas alike. The latest review paper on the zonation of the estuaries and the proximal–distal changes in the facies characteristics of tidal environments was published a decade ago (Dalrymple and Choi, 2007), and much work has been conducted since then. Despite a significant body of research that has been done on fluvio-tidal systems in the past ten years (Choi, 2010; Martinius and Van den Berg, 2011; Johnson and Dashtgard, 2014; La Croix and Dashtgard, 2015; Díez-Canseco et al., 2015; Olariu et al., 2015; Webb et al., 2015; Broughton, 2016; Shchepetkina et al., 2016a, Shchepetkina et al., 2016c), the FTZ is still referred to by an array of terms and presently lacks a unified definition. The concept of the FTZ has to be refined in order to facilitate the identification of this zone in both modern and ancient settings.

The ability to distinguish facies trends is essential for hydrocarbon exploration: tide-influenced systems tend to host considerable amounts of oil and gas reserves. Heavy oil from marginal-marine deposits has been widely explored in Canada and is contained within the McMurray Formation, Lower Cretaceous, Alberta (e.g., Stewart, 1963, Stewart, 1980; Stewart and MacCallum, 1978; Pemberton et al., 1982; Wightman et al., 1987; Beynon et al., 1988; Ranger and Pemberton, 1988; Hein et al., 2001; Langenberg et al., 2002; Ranger and Gingras, 2006; Gingras et al., 2016; Shchepetkina et al., 2016b); the Bluesky Formation, Lower Cretaceous, Alberta (Hubbard et al., 1999, Hubbard et al., 2002, Hubbard et al., 2004; Botterill et al., 2015; Campbell et al., 2016); the Gething Formation, Lower Cretaceous, Alberta and British Columbia (e.g., Hubbard et al., 2000; MacKay and Dalrymple, 2005; Gingras et al., 2010); in Venezuela within the Oficina Formation, Miocene, Faja del Orinoco belt (Dusseault, 2001; Martinius et al., 2012; Solórzano et al., 2017; Rodríguez et al., 2018); in the USA within the Griggs and Robins formations, Upper Carboniferous, Lawrence Field, Illinois (Webb et al., 2015); and in Russia within the Vikulov Formation, Lower Cretaceous, Kamennoe field area, western Siberia (Medvedev, 2010), Pokurskaya Formation, Upper Cretaceous, Van-Yogan field area, western Siberia (Bether et al., 2014), and Urna, Ust-Teguss and Tyamskay field areas, Middle Jurassic, western Siberia (Dolson et al., 2014).

A variety of names has been assigned to the FTZ by different researchers. The FTZ has been referred to as tidal-fluvial transition - TFT (Dalrymple et al., 1990a, Dalrymple et al., 1992; Allen, 1991; Martinius and Van den Berg, 2011; Dashtgard et al., 2012; Johnson and Dashtgard, 2014; La Croix and Dashtgard, 2015); fluvio-estuarine transition (Buatois et al., 1998a, Buatois et al., 1998b, Buatois et al., 2005; Lanier and Tessier, 1998; Doy, 2004; Mángano and Buatois, 2004; Choi, 2010; Greb et al., 2011; Archer and Greb, 2012; Jalon-Rojas et al., 2015); fluvially affected tidal (FAT) channels (Dashtgard et al., 2013); tidally influenced fluvial (Thomas et al., 1987; Smith, 1988; La Croix and Dashtgard, 2014; Prokocki et al., 2014); tide-influenced fluvial reach (Sisulak and Dashtgard, 2012); fluvio-tidal transition zone or reach (FTZ) (Gietlink, 1973; Hughes et al., 1998; Hovikoski et al., 2008; Gingras et al., 2010; Gingras et al., 2012a; Webb et al., 2015; Broughton, 2016; Shchepetkina et al., 2016a, Shchepetkina et al., 2016c; Timmer et al., 2016); fluvial-tidal transition (Dalrymple and Makino, 1989; Tessier, 1993; Van den Berg et al., 2007; Dalrymple and Choi, 2007; Choi, 2011); river-dominated portion of the fluvial-marine transition (Dalrymple and Choi, 2007); estuarine or fluvial transition (Chappell, 1993); inner estuary (Dorjes and Howard, 1975; Howard and Frey, 1975; Hauck et al., 2009); riverine estuary (Jouanneau and Latouche, 1981); upper estuary channel (Allen, 1991); and fluvio-deltaic zone (Amos and Zaitlin, 1985; Ghosh et al., 2005; Gani et al., 2007; Bhattacharya and MacEachern, 2009; Guo et al., 2015). Despite the abundance of designated names, the FTZ has few clear definitions.

As noticed above, there is at least a mild degree of chaos in the fluvio-tidal terminology. Herein, the authors would like to provide a clarification on two widely used terms and their relationship to the FTZ: the fluvial to marine transition (FMT or FMTZ) and tidal rivers. A major confusion exists between the terms FTZ and FMT or FMTZ as they have been interchangeably used in the scientific papers. However, these terms are not exactly synonyms. The fluvial to marine transition has been defined as an area of coastal rivers that occurs both in tide-dominated deltas and estuaries (Dalrymple and Choi, 2007; Longhitano et al., 2012; Gugliotta et al., 2017). The inner, fluvial-dominated part of the FMT is characterized by a seaward decrease in river flow, increase in tidal currents, and a net seaward sediment transport. The outer, tidal-dominated part is defined by the dominance of tidal currents, net landward sediment transport, and a landward decrease in grain size (Dalrymple and Choi, 2007). Brackish-water conditions with non-pervasive bioturbation have been reported to be predominant in the FMT (Dalrymple and Choi, 2007). Sedimentation in the FMT is controlled by the interaction of fluvial and marine processes, which include tides and waves (Gugliotta et al., 2017). The salinity boundary has not been well defined for the fluvial to marine concept, and the tidal limit is considered a zone rather than a specific point in the estuary or delta (Dalrymple and Choi, 2007; Gugliotta et al., 2017). Review of the relevant literature shows that: 1) the definition of a FTZ does not take into consideration the influence of waves, and only tidal action is regarded; 2) the FMT or FMTZ is a more extensive zone that encompasses the whole delta or estuary from its pure fluvial part to prodelta and shelf, whereas the FTZ forms a subdivision in the whole system (e.g., fluvial/riverine part – FTZ – inner estuary – middle estuary – outer estuary); as such, the FTZ constitutes part of the FMT or FMTZ; and 3) it has been also noticed from the literature review that the term FMT or FMTZ has been preferrably used for the deltaic systems, such as the Fly River delta by Dalrymple and Choi (2007) and Mekong River delta by Gugliotta et al. (2017), and not as much for the estuaries.

A large volume of existing literature uses another synonym for the FTZ, namely, the tidal river (or backwater zone). Literature review on the tidal rivers shows some consensus on the usage of the term (Woodroffe et al., 1985; Nowacki et al., 2015; Hoitink and Jay, 2016; Devkota and Fang, 2015; Ralston and Geyer, 2017). The concept has been predominantly applied to the lower river reaches in estuaries and deltas, especially to their freshwater, tidally varying parts (Nowacki et al., 2015; Devkota and Fang, 2015). It has been noted that tidal rivers are separate entities from deltas and estuaries, although they can intergrade (Nowacki et al., 2015; Hoitink and Jay, 2016). Woodroffe et al. (1985) defined the tidal river as the area between the tide reversal limit (where upstream flow is prevented by the freshwater discharge during rising tide) and the tide limit. This definition includes exclusively the freshwater part of the river with the permanent seaward flow and the water surface rising and falling tidally. Hoitink and Jay (2016) and Ralston and Geyer (2017) defined the tidal river as a lowland region between the landward limit of salinity intrusion at low flow and the tidal intrusion limit. Therefore, only the freshwater part of the river is taken into consideration (it is also referred to as a “tidal freshwater river” and “tidal river reach”). According to the aforementioned information, the tidal river concept accounts for a somewhat different region than the FTZ, which is bounded at the seaward side by the salinity limit (similar to the tidal river concept, and defined as a limit between normal-marine and brackish (0.5–30‰) or freshwater) and by the effective tidal limit (approximately, the tide reversal limit) on its upper end (dissimilar to the tidal river). The concept of the FTZ also takes into consideration the high and moderate flow in the river, allowing for the brackish-water conditions to exist within its lower reaches.

The objectives of this review are to: 1) present current models of the FTZ; 2) provide an inventory of the sedimentological and ichnological characteristics of modern and ancient examples of fluvio-tidal deposits; 3) produce a set of the most useful criteria that can be applied to ancient fluvio-tidal deposits in order to facilitate their paleogeographic interpretation; and 4) refine the concept of FTZ and suggest a unifying term for this complex zone.

Section snippets

Existing models of FTZ

A number of published papers have been of paramount importance for providing a synthesis of the FTZ characteristics and its predominant processes (e.g., Dalrymple and Choi, 2007; Van den Berg et al., 2007; Díez-Canseco et al., 2015). Other research papers captured characteristics of the FTZ within individual estuarine and, to a lesser degree, deltaic systems (e.g., Buatois et al., 1997, Buatois et al., 1998a; Dashtgard and La Croix, 2015; Shchepetkina et al., 2016a, Shchepetkina et al., 2016c).

Sedimentary characteristics of modern fluvio-tidal systems

Sixteen case studies on the distribution and characteristics of modern and ancient estuarine and deltaic systems have been consulted to better understand the predominant FTZ bedforms, their lithology and constituent sedimentary structures. For the modern tidally influenced depositional systems, the collected information is recounted in Table 1 and includes: 1) mean tidal range at the estuary or delta mouth and, if available, at the FTZ; 2) salt-water limit at high and low river flow, and median

Ichnological characteristics of modern fluvio-tidal systems

Ichnological data from sixteen modern tidally influenced estuaries and deltas have been collected. Given that, the data on the ichnology of the modern systems are sparse. Even less data are available for the fluvio-tidal zone. Modern sedimentologists still heavily rely on the physical sedimentary characteristics to define the estuarine and deltaic zones, and barely employ the wealth of the ichnological data. Ichnological data of the modern fluvio-tidal systems are presented in Table 1, which

Conclusions

  • 1)

    A large number of terms and a lesser number of definitions have been applied by different authors to the zone, located between the river and the tide-influenced estuary or delta. In order to unify the descriptions of this zone, usage of the term “fluvio-tidal transition zone” (“FTZ”) is recommended.

  • 2)

    The upper edge of the FTZ is defined by the effective tidal limit and the outer edge lies at the seaward salinity limit. The fluvio-tidal zone length and position can change with time depending on

Acknowledgements

The authors would like to thank the editor Dr. A. Strasser, Dr. M.Gugliotta and an anonymous reviewer for their valuable comments on an earlier draft of the manuscript.

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