This study uses the postmortem ages of Mulinia lateralis shells from surface sediments and shallow cores (3-m-long) to quantify net sediment accumulation and reworking rates in the estuary and delta platform of the Aransas River delta, Copano Bay, Texas. The scale of time averaging is governed by two factors: condensation, which determines the minimum possible degree of time averaging in a given increment of stratigraphic thickness, and reworking, which causes further time averaging post-depositionally. The net sediment accumulation rates of the estuary and delta platform are 0.29 mm/yr and 0.73 mm/yr, respectively. In both locations, the top 20 cm of sediment contain a distinct, essentially modern assemblage. Although the estuary is capable of preserving a temporal resolution of ~88 years per 10 cm, shells within 10-cm intervals at depths of 20 to 70 cm have age ranges spanning between 393.92 and 743.89 years. In contrast, the delta platform is capable of preserving a temporal resolution of ~35 years per 10 cm, but shells within 10-cm intervals at depths from 20 to 225 cm have age ranges spanning between 78.37 and 1031.69 years. These results indicate that the rate and depth of reworking, not condensation, primarily govern the magnitude of time averaging in the sediment column of the Aransas River deltaic system as it progrades into Copano Bay.
