Alluvial coasts form under the influence of marginal-marine processes on sedimentary deposits of fluvial origin. These may be alluvial fans, alluvial plains, or terraces of low slope and elevation. These coasts are neither strongly outbuilding of retreating, and are not strongly influenced by tectonic of isostatic uplift or subsidence.
Alluvial-plain coasts are systems “formed where the broad alluvial slope at the base of a mountain range is built out into a lake or the sea” and are part of a “neutral” coast (Johnson 1919, p. 188). The term “neutral coast” is little used in modern publications, but is still a useful classification of coastlines built by interaction of river and coastal processes where sediment input and relative sea level change are morphodynamically balanced, producing a coastline that neither progrades nor retreats rapidly. Alluvial-plain coasts may be closely associated with deltas, estuaries, beach-ridge plains, barrier-lagoon systems, and in some settings glacial outwash plains. Variations in geomorphology and process responses among these coastal environments are determined by the rate of relative sea level change and the abundance or paucity of sediment supply. This sediment supply may come from local bluff erosion, alongshore from fluvial or other sources, or from offshore. Climate, wave and tidal energy, and underlying geologic controls such as relief, complexity of coastline shape, and tectonic setting are other controlling factors.
Tectonic setting starts from a plate tectonics viewpoint. Inman and Nordstrom (1971) provide a broad-brush classification of the tectonic activity and morphology of the world’s coastlines. Alluvial-plain coasts are found primarily on passive margin coasts. In sequence stratigraphic terminology, alluvial-plain coasts are expected within a highstand systems tract (Posamentier and Vail 1988) or falling-stage systems tract (Plint and Nummedal 2000). These systems tracts form during rising to highstand sea level and highstand to falling sea level, respectively. However, the use of these interpretations clearly requires an understanding of rates of sediment influx and the influence of local basin subsidence or tectonic uplift. Boyd et al. (1992, Fig. 2) show a summary model of coasts of both outbuilding and retreat, with the neutral, alluvial plain coasts an intermediate between the two.
Modern coastal environments are graded to the late stages of Holocene sea level change, which varies around the globe, but has been rising at a gradually slowing rate in much of the Northern Hemisphere over the past 6000 years. However, during times of slow change, such as the last interglacial highstand alluvial-plain coasts may have been more predominant. Blum and Price (1994) show examples of this setting on extensive alluvial fans created at highstand on the Texas coast. A similar argument can be made for lowstand systems.
Low-relief alluvial plains are built by fluvial processes such as meandering stream pointbar accretion, deposition on levees, over-bank flood deposition, and a variety of channel avulsion processes that create oxbow lakes, flood chutes, and crevasse-splay deposits. In higher-relief coastal settings fan deltas may develop. Fan deltas are sloping alluvial sediments deposited in a sweeping arc where mountain streams flow out onto lowlands and into a sea or lake, with a substantial subaqueous extent (Prior and Bornhold 1989). Their subaerial portions are alluvial fans, dominated by braided stream and sediment-gravity flows in irregular flow conditions. Their subaqueous portions include fan-shaped deltaic and prodelta facies, usually with only a narrow shelf. Alluvial-plain coasts are influenced by littoral processes, either by the reworking by waves and tides of fluvial sediments brought into the coastal setting or by the gradual infill of preexisting embayments.
Alluvial-plain coasts associated with large rivers may grade into delta systems, representing an inactive lobe of a delta cycle (Coleman 1988), where the coast is distant from the primary river influx. The Huanghe (Yellow River) entering the Bo Hai Sea of northeastern China has an active modern birdsfoot delta, but there are also broad stretches of alluvial-plain coast on the Bo Hai coast that formed by coastal reworking of former fluvial/deltaic environments (Saito et al. 2001). Marsh and swamp environments, precursors to the coals found in ancient sequences (e.g., Fielding 1987), are commonly associated with delta plain and alluvial-plain coasts.
Recognition of alluvial-plain coasts in ancient rock sequences may be challenging, particularly when attempting to distinguish them from delta plain and barrier-lagoon systems. However, the Cretaceous Mesaverde Group littoral facies provide clear examples of alluvial-plain coasts on the margin of the interior seaway of Colorado and Wyoming (Hollenshead and Pritchard 1961; Kraft and Chrzastowski 1985). This succession includes transitions from fluvial delta plain (Menefee Fm.) with associated coal and carbonaceous shales, to the Lookout Point Fm. sandstone, in a regressive succession, overlain by the Cliff House Fm. Sandstone, in a transgressive succession. Determining the relative influences of eustatic sea-level fluctuations and rates of sediment supply, as well as climate, subsidence, and local process variations, can be even more challenging in the ancient rocks than in Holocene systems, where geomorphology and geologic setting are more directly traced and more completely understood.
Summary and Conclusions
Alluvial-plain coasts may be underrepresented in the literature because they are combined with delta, barrier, or estuarine environments. They represent intermediate conditions of balance between sediment accumulation and sea-level change. They are neither strongly prograding, like deltas, nor transgressed like estuaries. They reflect primary fluvial processes of deposition, but also significant reworking by littoral processes. They may be recognized by the lack of distinct barrier and lagoon environments and close juxtaposition of alluvial and beach systems.
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