Abstract
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 Espigao 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.
Highlights
The more ancient sedimentary cover in the Southwesternmost Amazon Craton is represented by a carbonate-siliciclastic succession which overlies Precambrian crystalline basement rocks and fill the Pimenta Bueno Graben (PBG), a geotectonic feature linked to the evolutive history of the Paleozoic Parecis Basin (Fig. 1)
Conglomerates and dolostones included in Cacoal Formation were described and interpreted, respectively, as alluvial fans and deltaic depositional system developed in rift-phase of the Paleozoic Parecis Basin (Bahia et al 2006)
Gaia et al (2017) based on facies analysis, carbon isotope and correlation with previously carbonate occurrences in the State of Mato Grosso, Mid-western Brazil, the Cacoal Formation was reinterpreted as glacial and shallow to moderately deep platform deposits, which represent a Neoproterozoic cap carbonate linked to the Marinoan glacial event (~ 635 Ma)
Summary
The more ancient sedimentary cover in the Southwesternmost Amazon Craton is represented by a carbonate-siliciclastic succession which overlies Precambrian crystalline basement rocks and fill the Pimenta Bueno Graben (PBG), a geotectonic feature linked to the evolutive history of the Paleozoic Parecis Basin (Fig. 1). The inference of Neoproterozoic age for Cacoal Formation by Gaia et al (2017), related to the Marinoan glacial event (~ 635 Ma), basically indicates the ages of glacial levels in the studied region (Fig. 2) Following this new conception, it is important to verify which of the more ancient units of the Parecis Basin are exposed only in the PBG and Colorado Graben. We conclude that the PBG and Colorado Graben are uplifted features with Precambrian basement rocks (crystalline rocks and Neoproterozoic cap carbonate) underlaid by Paleozoic siliciclastic deposits, in part glaciogene, that fill this depressions
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