Aminostratigraphy, geochronology and geochemistry of fossils from Late Cenozoic marine units in southeastern Virginia
Date
1990
Authors
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Publisher
University of Delaware, Newark
Abstract
Paired Amino Acid Racemization (AAR) and Electron Spin Resonance (ESR) data have been obtained from individual mollusc valves (g. Mercenaria) collected from stratified nearshore deposits at three sites in southeastern Virginia (Gomez Pit, Norris Bridge and Yadkin Pit). AAR and ESR data are used here to define aminozones in the region, and to infer age relationships among these superposed aminozones. These data are compared to U-Th analyses of corals from both sites (Szabo, 1985). Age differences among aminozones are estimated using models of racemization kinetics.
Previous workers have generally concluded that the lithologic section at Gomez Pit represents a single transgression followed by minor regression. However, AAR and ESR data indicate at least one unconformity in this transgressive sequence, representing approximately 100 ka. The uppermost aminozone lla is defined by a mean ALLO/ISO values of 0.141+/-0.026 (n = 55). Aminozone lla directly overlies aminozone llc (ALLO/ISO = 0.335+/-0.028; n = 19). Molluscs representing a third, lowest aminozone (lld; ALLO/ISO = 0.471+/-0.038; n = 9) are recognized, but taphonomic and ESR data suggest that lld molluscs in Gomez Pit may be reworked. At Norris Bridge, only aminozone lld (ALLO/ISO = 0.473+/-0.020; n = 5) is recognized.
ESR age estimates for aminozone lla range from 93-125 ka (n = 4), in general agreement with a Stage 5 estimate of 75+/-5 ka from U-Th analyses of corals from the same unit. ESR age estimates of 220 and 262 ka were calculated from 2 shells showing ALLO/ISO = 0.33; these data support a Stage 7 or Stage 9 interpretation for aminozone llc. AAR kinetic models suggest an age $>$400 ka for aminozone lld shells, inconsistent with a 187+/-20 ka age from a solitary coral at Norris Bridge. Attempts to obtain age estimates using ESR have been unsuccessful.
Shells representing excellent, good, fair and poor preservation conditions from Gomez Pit aminozone lla were compared to determine the effects of condition on precision of amino acid data. These data were used to test the hypothesis that leaching by groundwater selectively removes the most extensively epimerized (free) amino acids from the mollusc shell, thus lowering the ALLO/ISO value. No statistically significant difference in amino acid concentrations, amino acid fractions or ALLO/ISO values was found among these Gomez Pit shells representing different preservation characteristics.
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Keywords
Earth sciences, Marine fossils