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congresos y reuniones científicas
Latest Ordovician-earliest Silurian chitinozoans from northwestern Argentina, Western Gondwana
Simposio; 45th Annual Meeting of the American Association of Stratigraphic Palynologists; 2012
Institución organizadora:
45th Annual Meeting of the American Association of Stratigraphic Palynologists -University of Kentucky-Kentucky Geological Survey
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 deposits are interpreted as related to repeated transgressive-ravinement surfaces that truncate Fe+2-saturated estuaries after the glacial waning stage and glacioisostatic rebound. Palynological samples, taken from the Zapla Formation outcropping in the Capillas River, come from a thin interval of dark green to black shales (15.3 m below the contact of Zapla-Lipeón formations) present between graded sandy beds above the massive glacially derived diamictite, and also from a highly organic-rich black shale at the very top of the unit. The chitinozoan assemblages mainly contain Desmochitina minor, Spinachitina cf. S. oulebsiri, Hercochitina spp, and Belonechitina spp. The Lipeón Formation begins with a 1 m thick transgressive conglomerate, which truncates the underlying unit. In this area, 0.3-0.5 m of greenish silty-shales gradationally overlie the basal conglomerate of the Lipeón Formation, containing Spinachitina fragilis, Cyathochitina caputoi, Ancyrochitina cf. A. nodifera, and Belonechitina cf. B. postrobusta. This succession continues with regionally and laterally continuous oolitic ironstone beds (~2.2 m, 1.2 m and 0.3 m) overlain by greenish to yellowish silty-sandy pervasively bioturbated shales. Thin finer-grained partitions between the major two lower beds (2.7 m above the contact) contain Cy. caputoi, Spinachitina maennili, Pogonochitina djalmai, Ancyrochitina udayanensis, and Sphaerochitina silurica. Silty sandy beds above the ironstone horizons contain Cy. caputoi, P. djalmai and A. udayanensis (5 m above the contact). The rest of the pervasively bioturbated silty-sandy shales (~340 m thick), exposed above and along the Los Matos Creek, contain Telychian-Sheinwoodian chitinozoan assemblages. These preliminary results from the studied chitinozoan assemblages corroborate a Hirnantian age for the complete Zapla Formation up to its very top, including an interval immediately above the glacially derived diamictites. The chitinozoan assemblages constrain an earliest Silurian age for the base of the Lipeón Formation, hence the unconformity below the ravinement conglomerate may be under chitinozoan biostratigraphic resolution. A Rhuddanian age is interpreted for the shale atop the conglomerate, representing a condensed interval coeval with the flooding surface that occurred after the Hirnantian glacial waning stage. The shale intervals within and immediately above the major ironstone beds are interpreted to be an Aeronian age, though for the rest of the Lipeón Formation cropping in this region a Telychian-Sheinwoodian age is suggested. This biostratigraphic constrain indicates that the ironstones are also quite condensed, along with the peculiar oolite-rich ironstones that need an extremely low sedimentation rate, which is compatible with maximum flooding surfaces. The age for the Lipeón Formation largely agrees with the acuminatus and atavus Zones recorded from this unit in the region (Rickards et al., 2002), however the detailed sampling herein allows constraining gaps and condensed intervals as represented by postglacial omission, flooding and maximum flooding surfaces. Integration of high resolution palynological and sedimentary studies contributes to the unraveling of key surface hierarchy and intervening hiatuses in stratigraphy. In peri-Gondwanan regions, ironstones and hot shales characterize the postglacial stratigraphy and overlie Hirnantian glacial intervals. Age constrains within these postglacial intervals allow enhancing correlations of such important global events and their better understanding.