Estuarine evolution in response to Holocene sea-level rise: The Trinity-Sabine River paleovalley system on the East Texas shelf

Estuaries are ecologically, commercially, and culturally important habitats that provide numerous ecosystem services. Throughout the Holocene, estuarine transgression has occurred in former river valleys. These paleovalleys provide ample accommodation that preserves estuary evolution in the stratigraphic record. Localized processes, such as changes in tides, storms, winds, and waves, impact transgression and evolution of coastal environments. However, these have not been well studied within paleovalley stratigraphy due to the lack of high-resolution data needed to image these detailed processes. Using high resolution chirp subbottom data, we aim to understand the dominant localized and regional processes responsible for preservation of estuarine, bayhead delta, and paleo-barrier island deposits on the east Texas shelf. Within the Trinity-Sabine River paleovalley, estuarine units display higher amplitude seismic facies and more erosional truncations in the SW portion of the valley than in the NE, which displays passively infilling, draping reflections of varying amplitudes. This transition suggests a shift from a tide-dominated estuary when the paleovalley was first inundated in the lower reaches, to a wave-dominated estuary later in the Holocene as accommodation became limited. We also recognize two bayhead delta units deposited near the confluence of tributaries, which acted as pinning points during transgression. We attribute the drowning of the bayhead deltas to a reduction of sediment supply and paleovalley slope. Additionally, we observed seaward dipping shoreface deposits. The preservation of these deposits suggests that even under conditions of high sediment supply, the paleo-barrier island drowned under high rates of sea-level rise and a quiescent wave climate. Our results show that both regional and localized processes are important for the evolution of the shelf during inundation. Estuarine deposits record more local conditions, like wave and tide processes, while bayhead deltas evolve due to more regional changes in paleovalley slope, sediment supply, and sea-level rise.