Sedimentology and stratigraphy of Neoproterozoic-lower Paleozoic carbonate-siliciclastic succession of the southwesternmost Amazon Craton, state of Rondônia, Brazil

ABSTRACT: Facies and stratigraphic analysis were carried out in Neoproterozoic-Lower Paleozoic carbonate-siliciclastic deposits of Cacoal and Pimenta Bueno formations exposed on basement rocks and into the Pimenta Bueno Graben, northwestern portion of Parecis Basin, southwesternmost Amazon Craton. The redescription and redefinion of this succession confirmed the previous interpretation for the Cacoal Formation as a Marinoan (~ 635 Ma) cap carbonate. The Cacoal Formation is subdivided here in two units separate by sharp contact found exclusively overlying Mesoproterozoic crystalline basement rocks: 1) a homonymous formation characterized by diamictites, sandstones and siltstones with dropstones interpreted as glacio-marine deposits; and 2) the Espigão d’Oeste Formation that consists of dolostone, dolomitic stromatolites, dolostone-siltstone rhythmite and siltstone interpreted as shallow to moderately deep platform deposits. The Ordovician to Silurian Pimenta Bueno Formation is a filling of Pimenta Bueno graben and overlies locally the Meso and Neoproterozoic rocks. This unit consists in diamictites, sandstones, siltstones and pelites interpreted as glacial-marine and tide- to storm-influenced platform deposits, recording a glacio-eustatic regressive-transgressive event. This new stratigraphic proposal modify the current stratigraphy for the Parecis Basin and suggest, at least, two levels of glaciation exposed in the sothwesternmost Amazon Craton related to the Marinoan and Late Ordovician-Early Silurian events.