Estuarine and Incised-Valley Facies Models

Abstract Modern estuaries and incised valleys are important depositional settings that have widespread significance for human land use. The deposits of these environments are economically important for hydrocarbon exploration and production. Estuaries and incised valleys are a complex and possibly unique environmental grouping, inasmuch as they represent creation of depositional space by one process (mainly fluvial erosion) and fill of that space by a range of other processes (fluvial, estuarine, and marine deposition). Early investigations of valleys began slowly in Greek and Roman times, but increased in the nineteenth century, when they were used to develop ideas on the age of the earth in uniformitarian debates. Gradual progress was made throughout the nineteenth and twentieth centuries with the introduction of ideas on river grade, fluvial equilibrium profiles, and base level, followed by the development of fluvial facies models in the 1960s. Studies on estuaries began in earnest much later than those on valleys, and major advances were not made until the mid-twentieth century, with development of the first comprehensive facies model in the 1990s. Research on estuaries and incised valleys was energized in the 1980s by the concept of sequence stratigraphy, and work in the field has mushroomed since then. Indeed, the currently used facies models for estuaries and incised valleys were among the first to explicitly take into account the external control on the creation of accommodation and to be presented in a sequence-stratigraphic framework. In line with other sedimentary environments, the facies models for estuary and incised-valley environments have also proliferated, leading to the need for fundamental advances in how facies models are conceived. Estuaries, as defined geologically here, are transgressive in nature. They receive sediment from both fluvial and marine sources, commonly occupy the seaward portion of a drowned valley, contain facies influenced by tide, wave, and fluvial processes, and are considered to extend from the landward limit of tidal facies at their heads to the seaward limit of coastal facies at their mouths. Estuaries can be divided, on the basis of the relative power of wave and tidal processes, into two main types, wave-dominated estuaries and tide-dominated estuaries. Estuarine facies models exhibit generally retrogradational stacking of facies and a tripartite zonation reflecting the interaction of marine and fluvial processes. All estuaries and incised valleys have a fluvial input by definition, but estuarine facies models reflect the balance between wave and tidal processes. Valleys form because the transport capacity of a river exceeds its sediment supply. An incised-valley system is defined as a fluvially eroded, elongate topographic low that is characteristically larger than a single channel, and is marked by an abrupt seaward shift of depositional facies across a regionally mappable sequence boundary at its base. The fill typically begins to accumulate during the next baselevel rise, and it may contain deposits of the following highstand and subsequent sea-level cycles if the accommodation is not filled during the first sea-level cycle. Incised valleys may be formed by either a piedmont or a coastal-plain river and can exhibit a simple or compound fill. The erosion that creates many incised valleys is thought to be linked to relative sea-level fall, although climatically produced changes in discharge and/or sediment supply may independently cause incision, even in areas far removed from the coast. In the case of valleys in coastal areas, fluvial deposition typically begins at the mouth of the incised-valley system when sea level is at its lowest point and expands progressively farther up the valley as the transgression proceeds, producing depositional onlap in the valley. Based on the longitudinal distribution of broad depositional environments, the length of an incised valley can be divided into three segments. Ideally, the fill of the seaward portion of the incised-valley (segment 1) is characterized by backstepping (lowstand to transgressive) fluvial and estuarine deposits, overlain by transgressive marine deposits. The middle reach of the incised valley (segment 2) consists of the drowned-valley estuarine complex that existed at the time of maximum transgression, overlying a lowstand to transgressive succession of fluvial and estuarine deposits similar to those present in segment 1. The innermost reach of the incised valley (segment 3) is developed headward of the transgressive estuarine-marine limit and extends to the point where relative sea-level changes no longer controlled fluvial style (i.e., to the landward limit of sea-level-controlled incision). This segment contains only fluvial deposits; however, the fluvial style changes systematically due to changes in the rate of change of base level. The effect of base-level change decreases inland until eventually climatic, tectonic, and sediment-supply factors become the dominant controls on the fluvial system. In valleys far removed from the sea, the fill consists entirely of terrestrial deposits, but shows changes in fluvial style that are similar to those in segment 3, even though the stacking patterns are controlled more by local tectonics and climate. Recent and future development of estuarine and incised-valley facies models has emphasized the use of ichnology to recognize brackish-water deposits and the ability to subdivide compound valley fills on the basis of sediment composition. Imaging the valley and its fill has been greatly improved with 3D and 4D seismic techniques. Seabed mapping of modern estuaries has enabled detailed distributions of facies and morphology to be compiled, enhancing the ability to predict these features in ancient rocks. Our current set of facies models represents the early classification stage in the development of depositional models. The appropriate way forward appears to be a transformation from qualitative approaches to empirical and quantitative computer-based models with predictive capability, based on a thorough understanding of the dominant processes operating in each environment.

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