LONGITUDINAL AND TEMPORAL COMPARISONS OF RIO GRANDE RIVER SANDS IN THE RIO GRANDE DELTA SYSTEM

A century of dam construction, water removal for irrigation and municipal use, and land use modifications have combined to dramatically reduce the sediment load of the modern Rio Grande delta. In a previous study, we measured detrital zircon U-Pb age distributions from modern sands within the Rio Grande delta. We compared these results with previously reported data acquired upstream of Falcon Dam (Laredo, Texas) and from sandstones from older strata that represent previous shoreline deposits of southern Texas. These results led us to conclude that damming had limited delivery of upstream sediment to a sufficient extent that locally eroded Cenozoic sandstones dominated the provenance signal of modern sands within the Rio Grande delta. Specifically, the previous provenance studies indicate that modern Rio Grande river sand upstream of Falcon Dam was highly enriched in zircon derived from Oligocene volcanic fields within the southern Rocky Mountains and the Sierra Madre Occidental. Modern Rio Grande delta sand lacked this strong enrichment in Oligocene zircon and exhibited detrital zircon U-Pb age distributions that could be accounted for by more local erosion of Cenozoic strata downstream of Falcon Dam.

To further test this conclusion, we have sampled: (1) Quaternary sandstone structures within the Rio Grande delta near Boca Chica Beach; and (2) Lissie Formation (lower Quaternary) and Beaumont Formation (upper Quaternary) point bar sands deposited by Rio Grande distributaries in Cameron County, Texas. Detrital zircon U-Pb results obtained from these samples are enriched in Oligocene zircon. These new results support the previous conclusion that the change in detrital zircon provenance signature has occurred since the Quaternary and is likely the result of extensive human modifications to the Rio Grande river and its delta.