Overprinting of headwater detrital provenance signatures by coastal plain bedrock erosion

Modification of headwater provenance signals in distal fluvial sediment is a known concern in source-to-sink (S2S) provenance analysis. While “buffering” or “shredding” of signals due to temporal lags in transport or reworking of older channel-belt deposits from the same S2S system is well-described, the modification of headwater signatures due to new erosional inputs from bedrock sources along low-gradient coastal plain regions is less well-quantified. Provenance results from U-Pb geochronology, bulk geochemistry and luminescence sensitivity data from regional rivers along the NW Gulf of Mexico indicate a marked change in provenance signatures in fluvial sediments of modern, Holocene and late-Pleistocene age above, within, and below an outcrop belt of antecedent Cenozoic strata along the coastal plain. Detrital zircon mixture modeling suggests that up to 20% of detrital zircons in the lowermost Colorado River are likely derived from coastal plain bedrock sources. In the Guadalupe River, 100% of detrital zircons are likely derived from coastal plain bedrock erosion. Moreover, results indicate that sediment derived from coastal plain bedrock erosion is not well mixed with headwater sediment until tens to less than 100 km below the coastal plain outcrop belt. Basin-margin exposures of antecedent strata that were deposited under different geologic conditions, with different catchment extents, are common along the low-gradient coastal plains of long-lived sedimentary basins with high total subsidence. Results from this study indicate that coastal plain erosion of new bedrock material can significantly overprint headwater provenance signatures in this setting.