Seismic expression and stratigraphic preservation of a coastal plain fluvial channel belt and floodplain channels on the Gulf of Mexico inner continental shelf

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Abstract
Subsurface fluvial deposits in coastline‐proximal settings record the spatiotemporal evolution of the coastal landscape and may be viable repositories of sediment for future coastal restoration projects. However, quantitative linkages between the geomorphic form and stratigraphic expression of coastal plain fluvial elements remain lacking, complicating coastal stratigraphic interpretations and subsurface resource assessment. This study explores the expression and preservation of fluvial coastal plain geomorphic features through outcrop‐scale seismic stratigraphic analysis of ultra‐high‐resolution chirp acoustic reflection data from the north‐western Gulf of Mexico inner continental shelf, offshore the Brazos River, Texas, USA. The chirp data exhibit decimetre‐scale vertical resolution within the upper 35 m of the shelf, evincing the preservation of two distinct types of coexisting fluvial channel‐forms: Type 1, a 1 km wide and 9 m deep single‐storey channel belt filled with sand‐rich lateral accretion deposits and channel fills; and Type 2, numerous 10 to 800 m wide and 1 to 20 m deep incisional floodplain channels filled with extensive drapes of overbank‐derived sediments. The chirp data resolve fluvial geomorphic elements including lateral accretion surfaces, levées, floodplain deposits and abandoned channel fills. Rollover of lateral accretion surfaces, widespread draping of seismic reflectors within channel fills, and quantitative comparison between the interpreted stratigraphic forms and analogous features on the nearby Texas Coastal Plain suggest near‐complete stratigraphic preservation of the geomorphic form of both channel belt and floodplain channel elements. Rapid aggradation of the coastal plain by the Brazos River in this region during the Holocene transgression is hypothesized as a mechanism for the high degree of preservation achieved. This study proposes the first recognition criteria for the seismic stratigraphic expression of coastal floodplain channels and provides direct linkages between the geomorphic and stratigraphic expression of a coastal plain fluvial landscape, promoting improved coastal stratigraphic interpretations and assessment of subsurface lithofacies distributions.

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