Tracking the Eurydesma Fauna transgression across southwestern Gondwana

Abstract

The cold water Eurydesma Fauna, consisting of bivalves, brachiopods, and scarce gastropods, has been associated in several basins of Gondwana with the terminal phase of the Late Paleozoic Ice Age (LPIA). The Eurydesma Fauna-bearing transgression has been identified in basins of South America and southern Africa. In western Karoo Basin of South Africa four phases of ice growth and decay have been proposed for the Dwyka Group on the basis of interstratified glaciogenic diamictites overlain by non-glaciogenic, ice rafted debris free mudstones. Each of these phases is assigned to deglaciation sequences (sensu Visser, 1997) and has been proposed to be incomplete, third order sequences made up of a basal lowstand systems tract (LST) followed by a transgressive systems tract (TST) or exclusively by a thick TST. The correlation of these deglaciation sequences has been extended to the adjacent Aranos-Kalahari basin and to the Paraná Basin (Itararé Group) in South America. Recent high-precision U–Pb ages on single crystal chemical abrasion, isotope dilution-thermal ionization mass spectrometry (CA-IDTIMS) have helped to refine the diachronous demise of the LPIA and the age of the Eurydesma transgression. Tentative correlations with the glaciogenic section (Sauce Grande Formation) in the Ventana Fold belt (VFB) and Malvinas/Falkland Islands (Fitzroy Tillite Formation) are supported by radiometric ages, and similar facies associations respectively. This interbasinal correlation of deglaciation sequences and radiometric ages allowed to constrain the occurrence of this fauna in southwestern Gondwana to the Asselian. The postglacial nature of the Eurydesma Fauna transgression in South American basins, particularly in the VFB-Claromecó Basin of Argentina, is in contrast with its interglacial character in the Southern African basins, where the Eurydesma transgression caps the deglaciation sequence 3 and underlies the deglaciation sequence 4.