ABSTRACT

New data suggest a revised picture of middle Holocene sea-level change for the Texas Gulf of Mexico coast, and suggest reevaluation of coastal evolution. First, brackish marsh facies with calibrated radiocarbon ages of 7.7 to 7.8 ka have been recovered from depths of -8.5 to -9 m in a core from the ancestral Colorado River delta, and are interpreted to represent a sea-level pinning point. Second, a series of ridges along the Copano Bay margin farther south consist of shelly mud and fine sand with subtidal foram assemblages, occur at elevations of 1.95 m above the modern intertidal zone, and have produced calibrated radiocarbon ages on foram tests of ca. 6.8 to 4.8 ka. These ridges are interpreted to represent relict shallow subtidal to intertidal spits that provide minimum sea-level positions for the middle Holocene, and are now emergent because of later sea-level fall. In aggregate, these data show rates of sea-level rise during this time period that are very comparable to, or even lower than, published eustatic rates, and suggest a middle Holocene sea-level highstand for this non-uplifting, non-rebounding, and very slowly subsiding part of the North American coastline.