John Swartz, Patricia Standring , JOHN A GOFF , Sean Gulick, Christopher Michael Lowery

Modern lowland river systems show transitions in flow characteristics near coastlines that lead to systematic changes in sediment deposition and stratigraphic architecture. Sensitivity of fluvial morphodynamics to base-level has important implications for the prediction and interpretation of fluvial stratigraphy, particularly in deposits formed during periods of relative sea-level rise such as the early Holocene. Improving our understanding of how fluvial stratigraphy is created and preserved in such environments is crucial to paleoenvironmental reconstruction, sand resource estimation, and mapping subsurface facies distributions. A significant challenge has been capturing the spatiotemporal evolution of such systems on long timescales (centuries to millennia), often due to poor data coverage and resolution. Here we investigate the offshore stratigraphic architecture of the Trinity coastal river system, Texas. The paleo-valley is interpreted to have formed during the last lowstand and filled by backstepping fluvial-deltaic and estuarine sediments over the Holocene. However, the nature of this transition and resulting stratigraphy is unclear. We present unprecedented imaging of the incised valley and fluvial stratigraphy using a combination of over 500km2 of 3D seismic and nearly 700km of full waveform chirp data with 250m line spacing, combined with sediment cores and geotechnical borings. Our chirp processing technique creates images of strata on the decimeter scale, allowing for near outcrop scale mapping and interpretation. Archival industry 3D seismic data show that the basal valley fill is comprised of a highly-amalgamated fluvial channel belt with numerous avulsions and loop cutoffs. The chirp data show the transition of individual fluvial channels from sand rich, laterally migrating systems to muddy channels that depict high rates of vertical aggradation and little lateral movement. We observe the interaction of these channels with the paleo-floodplain and show that aggradation and infilling of the incised valley is dominated by fluvial processes before a transition to bay and estuarine conditions. Our work illustrates that traditional models of incised valley filling and fluvial response to transgression fail to fully capture the morphodynamics of coastal river systems.