Processes, facies and architecture of fluvial distributary system deposits

doi:10.1016/j.sedgeo.2006.07.004

Sedimentary Geology

Volume 195, Issues 1–2, 15 February 2007, Pages 75-90

https://doi.org/10.1016/j.sedgeo.2006.07.004 Get rights and content

Abstract

There is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down-flow and transitions from proximal, channelised to distal, unconfined flow. These deposits form fan-shaped bodies several tens of kilometres in radius, and their stratigraphic architecture is aggradational, with no evidence of deep incision driven by base-level fall. The fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today: an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation. A conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation. Proximal areas are characterised by amalgamated coarse, pebbly and sandy channel deposits with little preservation of overbank facies. Channel dimensions are generally smaller in the medial areas, but sizes are variable: deposits are of braided, meandering and simple channels which show varying degrees of lateral migration. The channel-fills may be mud or sand, with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion. The proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow. Interconnection of sandstone bodies is poor in the distal areas because channel-fill bodies are sparse, small and are not deeply incised. The radial pattern of the sediment body forms by the repeated avulsion of channels: active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas. The term ‘fluvial distributary system’ is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand.

Introduction

Friend (1978) pointed out that some ancient river systems seem to show features that distinguish them from many modern-day river systems. He recognised three distinctive characteristics that could be recognised in ancient fluvial stratigraphic units: (1) a downstream decrease in river depth, (2) an absence of alluvial incision and (3) a convex-upwards, lobate topography of the river systems. He suggested that these features indicated deposition by a distributive river system which formed a ‘terminal fan’. Subsequent work in one of the areas of Friend's original studies, the Ebro Basin, established more details of the sedimentology of two ‘fluvial distributary systems’ of Miocene age (Hirst and Nichols, 1986, Nichols, 1987, Friend, 1989, Hirst, 1991). The concept of terminal fans was also expanded by Kelly and Olsen (1993) with reference to some Devonian examples. In this paper we summarise the architectural characteristics of fluvial systems of this type and consider the tectonic, climatic and related base-level controls on their formation and preservation. Examples from the Miocene of the northern Ebro Basin are used to illustrate the characteristics of these systems, which are typically tens of kilometres in radius and are comprised of fluvial channel and overbank deposits which vary in relative abundance and character between proximal and distal areas. Comparison is also made with other subaerial fan deposits, including alluvial fans, fluvial fans and megafans, and the usage of different terminology considered. The term ‘fluvial distributary system’ is used in preference to ‘terminal fan’ for reasons which are discussed in later sections.

Section snippets

Characteristics of a fluvial distributary system

Conceptual models for a fluvial distributary system are shown in Fig. 1, Fig. 2. These have been developed from earlier models presented in Friend (1978), Nichols, 1987, Nichols, 1989, Kelly and Olsen (1993) and Stanistreet and McCarthy (1993) with the addition of data from other sources (e.g. Graham, 1983, MacCarthy, 1990, Sadler and Kelly, 1993, Williams, 2000, Nichols, 2004, Nichols, 2005, Fisher et al., 2006-this volume) and new observations in the Ebro Basin. The locations of the examples

Fluvial channel and overbank processes in a distributary system

The fluvial channel and overbank facies described from the examples of fluvial distributary systems in the stratigraphic record can be used to determine the depositional processes which are acting in different parts of a system. These may be used to develop a conceptual model for an active fluvial distributary system (Fig. 1, Fig. 2).

Fluvial distributary systems and lakes

The conceptual model for fluvial distributary systems presented in Fig. 1, Fig. 2 is valid for conditions where the river channels do not reach a basin centre lake. During periods of high discharge, unconfined flow from the terminal splays may spread out onto the alluvial plain and deposit suspended sediment out of a temporary standing body of water (Fisher et al., 2006-this volume). This body of water may be considered to be transitional to an ephemeral lake, depending on where the distinction

Conditions for the formation of a fluvial distributary system: tectonic and climatic setting

Basins of internal drainage can form in a variety of tectonic settings, from extensional rift basins, such as the modern basins in the East African Rift Valley, to transtensional settings, such as the modern Dead Sea and the Devonian Orcadian Basin in Scotland, to foreland basins such as the Ebro Basin, Spain in the mid-Cenozoic. Accumulation of a thick succession of fluvial deposits would be facilitated by subsidence and a high basin margin sill, which prevents the development of an external

Alluvial fans, fluvial fans, megafans, humid fans

The problem of terminology to be used in the description of fan-shaped bodies of alluvium has exercised a number of geologists and geomorphologists over the years. Blair and McPherson (1994) consider that the term ‘alluvial fan’ should be restricted to steep, debris flow and sheetflood dominated deposits, and draw a distinction between the processes and products of these alluvial fans and those of rivers. Such a distinction is not recognised by all workers in this field, and there are many

The stratigraphic architecture of fluvial distributary system deposits

The short and long-term preservation potential of fluvial distributary system deposits in endorheic basins is good. To preserve thick successions of fluvial strata the area of deposition must have a long history of relative base-level rise. This condition is met in basins of internal drainage which are underfilled (Bohacs et al., 2000) and have rates of evaporation exceeding water supply. Accumulation of strata in the basin will continue until the spill point is reached, and the total thickness

Conclusions

Fluvial distributary systems are a distinctive style of continental sedimentation that occur mainly in basins of internal drainage. They are characterised by a down-flow decrease in channel dimensions caused by a loss of discharge downstream due to evaporation and soak-away. They are typically fan-shaped bodies tens of kilometres in radius built up by repeated avulsion of river channels on a very low gradient alluvial plain.

Terminal splays are considered to be elements of these systems, being

Acknowledgements

GJN would like to acknowledge the contribution of colleagues who have contributed to discussions about these systems, particularly Peter Friend, Philip Hirst, Colin North and Ed Williams. JAF acknowledges the support of an NERC studentship. Kevin Bohacs and Anne Mather are thanked for their constructive reviews.

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Processes, facies and architecture of fluvial distributary system deposits

Abstract

Introduction

Section snippets

Characteristics of a fluvial distributary system

Fluvial channel and overbank processes in a distributary system

Fluvial distributary systems and lakes

Conditions for the formation of a fluvial distributary system: tectonic and climatic setting

Alluvial fans, fluvial fans, megafans, humid fans

The stratigraphic architecture of fluvial distributary system deposits

Conclusions

Acknowledgements

Sedimentary Geology

Marine and Petroleum Geology

Sedimentary Geology

Sedimentary Geology

Sedimentary Geology

Sedimentary Geology

Sedimentary Geology

Palaogeomorphological controls on the distribution and sedimentary styles of alluvial systems, Neogene of the NE of the Madrid Basin, (central Spain)

Ebro Basin continental sedimentation associated with late compressional Pyrenean tectonics (north-eastern Iberia): controls on basin margin fans and fluvial systems

Basin Research

A flexural model for the Paradox Basin: implications for the tectonics of the Ancestral Rocky Mountains

Basin Research

Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes and facies assemblages

Journal of Sedimentary Research. Section A, Sedimentary Petrology and Processes

Lake basin type, source potential and hydrocarbon character: an integrated sequence-stratigraphic-geochemical framework

Processes, bar morphology and sedimentary structures in braided outwash fans, north eastern Gulf of Alaska

Proglacial braided outwash: a model for humid alluvial-fan deposits

Alluvial sediments

Unconfined flow deposits in distal sectors of fluvial distributary systems: examples from the Miocene Luna and Huesca Systems, northern Spain

Sedimentary Geology

Vegetation-precipitation interactions drive palaeoenvironmental evolution

Eos Transactions of the American Geophysical Union

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