In the Western Amazon Craton, specifically in Western Parecis Basin, Rondônia State,
carbonate rocks exposed on border of Pimenta Bueno and Colorado Grábens are considered to
be part of the eopaleozoic basin fill. The facies and microfacies analysis together with
chemostratigraphy of theses rocks in Chupinguaia and Pimenta Bueno Region, confirmed the
occurrence of pinkish dolostone that overlie glaciogenic diamictite, previously interpreted as
alluvial fan. Previous works reported δ13C negative excursions, confirmed in this work as
well, ranging from -4.6 e -3,8‰VPDB in Chupinguaia, and average of -3,15‰VPDB in Pimenta
Bueno. This sedimentation and chemostratigraphic pattern, uncommon in paleozoic rocks, is
widely found in the anomalous neoproterozoic carbonates. In the Southern Amazon Craton,
Mato Grosso State, rocks with the same features were described as cap carbonates related to
the Marinoan Glaciation (635 Ma). Therefore this work considers this dolostones at the same
context of the cap carbonate in Mato Grosso. Additionally we stand out the sharp and loaded
contact between dolostone and diamictite, which happens in both occurrences, and is
seemingly a typical feature of cap carbonates in the Amazon Craton. This paradoxal
relationship has been interpreted as rapid change from icehouse to greenhouse conditions, and
the loaded contact is attributed to isostatic rebound. The Rondônia cap carbonate presents two
facies associations (FA2 and FA3) that overlie glaciomarine deposits (FA1) subdivided in two
facies: Polymitic paraconglomerates (Pp) and laminated pebbly sandstone (Asl). The FA2
consists into: peloidal dolomudstone/dolopackstone with planar to quasi-planar laminations
and low-angle truncations (Dp), megarriple bedding (Dm) and wave truncated laminations.
This association is interpreted as shallow platform deposits wave influenced. This coastal
succession is overlaid by FA3, which comprises the facies: dolomudstone/dolopackstone and
dolomudstone/ dolograinstone with shale partition (Df) and laminated shaly siltstone (Sl). Df
comprises 6m-thick of dolomite with parting shale, showing laterally continuous laminations
of fibrous calcite (pseudomorph of gypsum) and dolomite with current wavy lamination. The
Sl comprises 5m-thick of planar-laminated shaly siltstone. This association is interpreted as
shallow platform deposits tide influenced. Finally, this inner platform succession is overlaid
unconformably, in angular contact, by eopaleozoic glaciogenic diamictite. The isotopic values
of C and O are negative and reflect the primary signal of C, however it can be considered a
slight influence of meteoric diagenesis in the signal. The main shifts in negative signals are
associated with meteoric influences, expressed by replacement and pores filling by calcite,
and also by its proximity of stratigraphic surfaces, which reflect some patterns of diagenetic
alteration, represented by the most negative signals. Differently from Mato Grosso cap
carbonate, the Rondônia occurrence presents levels of pseudomorph of evaporites and
dolomite with parting shale (rhythmites), order in succession of shallow marine facies, where
the dolomites of wavy influenced shallow platform pass up-section to rhythmites and shaly
siltstone of tide influenced shallow platform, setting up a retrogradational succession. This
new occurrence of cap carbonate has strong implications to the stratigraphy of the base of
Parecis Basin, since it excludes these carbonate rocks from the eopaleozoic sequence.
Moreover, it provides information that allows reconstruct the coastal paleogeography of
neoproterozoic basin that accumulated deposits of Araras Platform, as well extends the postmarinoan
events of the Snowball/Slushball Earth hypothesis to the southwesternmost Amazon
Craton, exposed in the Rondônia State.
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