Geological and taphonomic significance of electron spin resonance (ESR) ages of Middle-Late Pleistocene marine shells from barrier-lagoon systems of Southern Brazil

摘要

The Coastal Plain of Rio Grande do Sul state (CPRS), southern Brazil, encompasses Quaternary barrier-lagoon depositional systems developed in response to successive marine transgressions driven by glacial-interglacial cycles. As an attempt to refine he chronostratigraphy of the Pleistocene depositional systems, electron spin resonance (ESR) ages were obtained in 42 disarticulated shells of the marine bivalve Amiantis purpurata preserved in deposits in the southern CPRS, consisting of 35 specimens from five boreholes made during the construction of wind farms, plus another 7 shells from an irrigation channel. The shells were obtained from depths between 5.5 and 14 m, in two chronostratigraphically distinct depositional units (Barrier-Lagoon Systems II and III). Although the ages in each group of samples vary widely, the youngest age from the Barrier II (similar to 247 ka) is consistent with the timing of the transgression of the marine isotope stage (MIS) 7 inferred from other luminescence and ESR ages of sediments and shells. The ages obtained in shells from the Lagoon III deposits located landward of the Barrier III indicate deposition by the sea-level highstand of the substage MIS 5e, despite being older than the estimated age of similar to 120 ka of the latter. The younger shells from the seaward side of the Barrier III dated as of similar to 100 ka indicate deposits formed by the low-amplitude MIS 5c highstand, in agreement with luminescence ages of similar to 109, 104 and 101 ka obtained in outcrops on the shoreline. One shell dated as similar to 87 ka could indicate another highstand during MIS 5a. The presence of shells much older than the depositional systems indicate that marine deposits older than MIS 7 existed in the CPRS but were totally eroded by subsequent transgressions, and that shells can be preserved for periods of about 100-200 kyrs under certain conditions. Moreover, the ages younger than the highstand of MIS 5e on the seaward side of the Barrier III indicate that one (and possibly two) smaller amplitude highstands previously unrecognized in the CPRS left geological and fossil records in the study area.