Tracking Barrier Island Response to Early Holocene Sea-level Rise: High Resolution Study of Estuarine Sediments in the Trinity River Paleovalley

Abstract:

Understanding how paleoshorelines responded to factors such as variations in sediment supply, relative sea-level rise (RSR), and accommodation is a valuable analog for preparing coastal communities for future impacts of climate change. Here we examine the stratigraphic framework of the Trinity River paleovalley, offshore Galveston, Texas. The youngest paleovalley was incised during the last low-stand and infilled during the Holocene as rising sea levels flooded the valley, forming an estuarine system protected from direct marine incursion by discontinuous, landward-stepping barrier island systems. We present high-resolution imaging of the Trinity incised valley fill using over 1200 km² of 3D seismic, <700 km of 2D full waveform chirp data, along with 9 cores, 5 platform borings, with associated grain size, foraminiferal, and Carbon-14 data. 3D seismic data outline the geographic extent of the incised valley, as well as show the amalgamated, sinuous fluvial channel belt comprising the basal valley fill. The transition from fluvial to estuarine conditions is preserved, with over half of the overall valley fill comprised of estuarine stratigraphy. The ground-truthed chirp data reveal that the estuarine section is composed of three main stratigraphic units recording the upper estuary to barrier-proximal evolution: 1) the central bay unit, consisting of low-amplitude, laminated, muddy sediments deposited during quiescent conditions, 2) the tidal ravinement and fill unit, consisting of erosional, cut-and-fill channels associated with a paleo-tidal inlet, and 3) the final outer bay unit, composed of aggradational interbedded muds and sands interpreted as washover deposits associated with a proximal barrier island. These units thus record a gradually increasing proximity of the barrier system within our studied section of the incised valley fill. Previous authors have focused on discrete flooding events associated with episodic, rapid RSR that drive abrupt, landward jumps in the barrier island system. Our work challenges this model, as seismic stratigraphic, foraminiferal, and carbon-14 data suggest that the paleoestuary within our study area was influenced by a more gradual landward migration of the barrier system, and that factors other than RSR alone may have been key to barrier island stability.