Abstract
The Rinconada Formation is a mélange that crops out in the eastern margin of the Argentine Precordillera, an exotic terrane accreted to Gondwana in Ordovician times. Its gravity-driven deposits have been studied by means of conodont and graptolite biostratigraphy, and complemented with stratigraphic analyses. 46 rock samples (85 kg total weight) were obtained from blocks of limestones and of carbonate-cemented quartz-arenites, and from limestone clasts included in conglomerate blocks and debrites. 16 of these samples were productive after standard laboratory acid procedures, yielding 561 conodont elements. The specimens occur in variable number per sample and are frequently fragmented, but they reveal the occurrence of phantom stratigraphic units in the Darriwilian of the Precordillera. Lithological and fossil evidence from the Rinconada Formation provide new constraints on the biostratigraphy, palaebiogeography and tectonostratigraphic history of the southwestern margin of Gondwana during the Ordovician to Lower Devonian times.
Highlights
Sedimentary mélanges are mappable units displaying a chaotic internal structure and containing exotic blocks
Thereby, the fossils and rocks included in a mélange may reveal important aspects on the biostratigraphy, palaebiogeography and the tectonostratigraphic history of a basin
This is the case of the Rinconada Formation, a thick, mainly siliciclastic mélange containing large limestone blocks that overlies the lower Palaeozoic carbonates of the Argentine Precordillera (Heim, 1948), an allochthonous terrane accreted to Gondwana during the Ordovician (Astini et al, 1995)
Summary
Sedimentary mélanges are mappable units displaying a chaotic internal structure and containing exotic blocks They occur in different tectonic settings and derive from a broad range of sedimentary and deformational, gravitationally driven processes (see a review in Festa et al, 2016). Thereby, the fossils and rocks included in a mélange may reveal important aspects on the biostratigraphy, palaebiogeography and the tectonostratigraphic history of a basin This is the case of the Rinconada Formation, a thick, mainly siliciclastic mélange containing large limestone blocks that overlies the lower Palaeozoic carbonates of the Argentine Precordillera (Heim, 1948), an allochthonous terrane accreted to Gondwana during the Ordovician (Astini et al, 1995). We analyse new Ordovician conodont and graptolite records from the Rinconada Formation They provide high-resolution biostratigraphic constraints on the mobilised blocks that compose the mélange, and reveal the occurrence of Ordovician phantom units in the Rinconada Formation, currently unknown in other sectors of the Precordillera, and have important palaeogeographic implications
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