Chitinozoans throughout the Ordovician/ Silurian boundary in northwestern Argentina, Western Gondwana

DE LA PUENTE, G. SUSANA

Mendoza

Congreso; 4th International Palaeontological Congress. The history of life: A view from the Southern Hemisphere; 2014

IPA-INTERNATIONAL PALAEONTOLOGICAL ASSOCIATION, CCT-CONICET Mendoza

Chitinozoans across the Ordovician/Silurian boundary from different regions of Northwestern Argentina, at the southern end of the Central Andean Basin, are analysed. The unit containing the uppermost Ordovician deposits is traditionally described as the glacial Zapla Formation, and locally named as Caspalá Formation or Mecoyita Formation. The Silurian deposits are described as the Lipeón Formation, and record the postglacial event in its basal part. The glacial deposits of the Zapla, Caspalá and Mecoyita formations have been related to the Late Ordovician glaciation. Chitinozoans from the glacial Caspalá Formation of the Sierra de Zenta, in the middle part of the basin, are reworked. The youngest indentified species is Tanuchitina fistulosa, an index species for the middle Katian of Northern Gondwana. Chitinozoans from the overlaying Lipeón Formation in this area are abundant and indicate a Telychian age for this postglacial deposit. In the Sierra de Zapla, to the eastward, the Zapla Formation glacial horizon contains Spinachitina cf. oulebsiri. S. oulebsiri is assigned to the late Hirnantian in Northern Gondwana. This glacial unit is succeeded by a transgressive basal conglomerate of the lowermost Lipeón Formation. The conglomerate is overlain by a condensed section composed of bioturbated greenish shales, and several basin-wide oolitic ironstones that record high-frequency sea-level fluctuations. This 5m thick condensed section contains chitinozoans that allow identifying, for the first time in the area, Rhuddanian, Aeronian and Telychian ages. The sequence continues with pervasively bioturbated siltysandy shales containing abundant Telychian and probably Sheinwoondian chitinozoans in its uppermost part. In the Puna region, to the west of the basin, the Salar del Rincón Formation probably spans the Ordovician/Silurian limit, although glacial deposits are not observed. This unit is deposited between two regional unconformities which separate it from the Lower Ordovician and the Carboniferous deposits. The chitinozoan associations, with common species of Spinachitina, Ancyrochitina and Cyathochitina, could indicate the postglacial event of the latest Hirnantian-earliest Silurian. Chitinozoans from Western Gondwana indicate that the glacial and postglacial intervals of the Ordovician/Silurian boundary could have been deposited in similar conditions as contemporaneous peri-Gondwanan regions, where ironstones and hot shales characterize the postglacial stratigraphy, and the presence of the earliest Silurian deposits evidence that they were proximal to the Hirnantian ice-cap. Chitinozoans are demonstrated to be a useful biostratigraphic tool for the Central Andean Basin, allowing a high resolution in the determination of the ages, and the better understanding of the regional and global events.