Latest Ordovician‐earliest Silurian chitinozoans from northwestern Argentina, Western Gondwana

DE LA PUENTE, G.S.; ASTINI R.A.,; RUBINSTEIN, C.V.; OVIEDO, N. DEL V.

Kentucky

Congreso; 45th Annual Meeting of AASP ‐ The Palynological Society and Meeting of the CIMP ‐ Commission Internationale de la Microflore du Paléozoïque Subcommissions; 2012

AASP THE PALYNOLOGICAL SOCIETY

The Ordovician‐Silurian boundary remains undetermined in northwestern Argentina, at the southern end of the Central Andean Basin on the western Gondwana margin. The Zapla Range exposes Ordovician and Silurian deposits in the eastern part of this basin. Glacial deposits of the Zapla Formation are traditionally assigned to the Hirnantian, and postglacial deposits of the Lipeón Formation are assigned to the early Silurian. Previous palynological studies by the authors in the area constrain the lower part of the Lipeón Formation to the Aeronian‐Telychian. A detailed study of chitinozoan assemblages is being carried out in additional outcrops of the uppermost Zapla Formation and lowermost Lipeón Formation in order to constrain the ages of these units, and to contribute to the knowledge of the glacial and post‐glacial depositional events. Massive and stratified matrix‐rich diamictites within the Zapla ?glacial horizon? have a maximum thickness of 58 m along the type area. Because of the pervasive synsedimentary flow features and ductile deformation it is interpreted as redeposited till, probably along a proglacial slope in subaqueous environments with no indication of wave or storm action. The unit truncates the underlying stratigraphy at different levels across sharp boundaries, implying deep erosion within the substrate. No striated pavements have been observed. Hence, erosion may be related to a major sea‐level drop coeval with the waxing stage of the Hirnantian glaciations, whereas most of the unit records the glacial retreat. The flooding surface in the base of the Lipeón Formation initiates the deposition of several basin‐wide oolitic ironstones that record high‐frequency sea‐level fluctuations. These economic deposits are interpreted as related to repeated transgressive‐ravinement surfaces that truncate Fe+2‐saturated estuaries after the glacial waning stage and glacioisostatic rebound.