An in situ shelly fauna from the lower Paleozoic Zapla glacial diamictite of northwestern Argentina: implications for the age of glacial events across Gondwana

BENEDETTO, J.L.; HALPERN, K.; DE LA PUENTE, S.; MONALDI, C.R.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2015 vol. 64 p. 166 - 182

0895-9811

A shelly fauna from the upper part of the Zapla Formation consists of the lingulate brachiopod Orbiculoidea radiata Troedsson, the rhynchonelliforms Dalmanella cf. testudinaria (Dalman), and Paromalomena sp., the bivalve Modiolopsis? sp. and the trilobite Dalmanitina subandina Monaldi. Both taphonomic and paleoecologic attributes indicate that fossils lack of transport reflecting the original community. The assemblage is closely comparable to the widespread latest Ordovician Hirnantia-Dalmanitina fauna. The Hirnantian age of the Zapla diamictite is further corroborated by the record of the northern Gondwana chitinozoans Spinachitina cf. oulebsiri, and Desmochitina minor typica. The graptolites and chitinozoans from the overlying Lipeón Formation indicate that the posglacial transgression took place in the earliest Llandovery (P. acuminatus Biozone). The Early Silurian age of the Cancañiri and San Gabán diamictites of north Bolivia and south Peru indicated by their palynological content is more likely the age of the gravity flows containing glacial boulders rather than the age of glaciation. According to such tecto-sedimentary hypothesis, the weakly lithified glacigenic deposits of Hirnantian age were reworked and redistributed by high-energy marine processes during the posglacial transgression and then transferred to the adjacent deep-marine trough. Iron-rich horizons have been recognized in almost all southern South American basins reflecting eustatic and paleoclimatic fluctuations. Most of them formed during the early stages of the postglacial transgression around the Ordovician/Silurian transition and are associated with low sedimentation rates and condensed intervals. The mild maritime posglacial climate, the increasing atmospheric CO2, and possibly the presence of incipient vegetated areas led to extensive weathering of glacigenic sediments supplying iron into the marine system to form ferruginous deposits. The sea level fall related to the peak of glaciation is recorded by both paleovalley incision and a sharp subaerial to subglacial unconformity. The transgressive systems tract starts with fluvio-estuarine deposits within the valleys or with a regional transgressive surface onlaping the basement rocks followed by subtidal to open marine shales. The recognition of key stratigraphic markers such as the sequence boundary, the subsequent flooding surface, and the development of ferriferous beds, alongside biostratigraphic evidence, allow establishing a more accurate correlation between the Central Andean basin of Peru, Bolivia and NW Argentina, W Puna region, Paraná basin of Paraguay and Brazil, Precordillera of W Argentina and Cape basin of South Africa.