Elevated Modern Sedimentation Rates over the Buried Trinity River Incised Valley Suggests Elevated, Localized Subsidence Rates, Galveston Bay, TX, USA

Abstract

This study investigates sedimentation rates within Galveston bay, TX, and their utility in estimating differential compaction in Texas coastal plain estuaries. Most modern Texas estuaries are situated over incised Pleistocene paleo-channels up to 30 m deep, filled primarily with unconsolidated mud-dominated Holocene fluvial and estuarine deposits. 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 this rate is considered a regional rate. However, Pier 21 is situated over the Trinity River incised paleo-valley (TRIV). A CHIRP sub-bottom profile and multiple transects of vibra-cores were collected across the TRIV to investigate if sedimentation resulting from enhanced subsidence has increased over the TRIV compared to the TRIV flanks. Profiles of Hg and textural changes were used alongside sedimentation rates derived from excess ²¹⁰Pb activity to determine the recent sedimentary history of the bay. Sedimentation rates ranged from 0.34 – 1.48 cm yr¯¹, with the highest rate being found directly over the TRIV. Correlations in Hg and textural data reveal a general deepening and extension of strata within cores collected directly over the TRIV. Analyses of the CHIRP line also reveals a localized thickening of individual stratigraphic layers within Holocene estuarine fill across the TRIV, with a thinning on the flanks. Taken together, the evidence suggests that differential compaction of the unconsolidated Holocene estuarine fill within the TRIV has produced localized, elevated sedimentation rates, suggesting enhanced localized subsidence up to four times as great within Trinity Bay over the TRIV, than in areas adjacent to it.