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Estimating Rates of Subsidence Using Sedimentation Over the Trinity River Incised Valley, Galveston Bay, Texas, and the Potential Implications for Geohazards

Abstract

Parts of Galveston Bay are situated over the Trinity River Incised Valley (TRIV), an incised Pleistocene channel, up to 30 m deep, filled with unconsolidated Holocene sediment. Previous studies of the area have revealed high rates of localized subsidence. The Galveston Pier 21 tidal gauge has a 100 year record which has recorded a relative sea level rise rate 3 times the global average, and the highest relative sea level rise recorded for any tidal gauge In the US. This rate has been considered the regional rate. However, Pier 21 resides over the TRIV, which likely has experienced differential compaction of the underlying sediment. If differential compaction has enhanced the sea level rise rate, then this rate only represents a localized phenomenon, creating an overestimation of the regional rate for Galveston Island and the upper Texas Coast. This project investigates the role differential compaction plays in relative sea level rise along the Texas coast by quantifying localized subsidence along the TRIV due to Holocene sediment compaction. Data will include 210Pb and 239/240Pu geochronology and geotechnical analyses of 13 sediment cores collected from both within and outside of the area overlying the TRIV, as well as analyses of a grid of CHIRP seismic lines crossing over the TRIV. Variations in sedimentation rates will be used as a proxy to determine rates of subsidence. The TRIV is only one of several valleys that exist along the coast and in some cases, significant infrastructure and industry assets reside over these channels. Rapid subsidence has previously been identified as a major geohazard. Determining if localized subsidence is enhanced within the TRIV is the first step in addressing potential geohazards stemming from compaction of shallow Holocene sediments in coastal regions of the Gulf of Mexico.