Siliciclastic deposits from Diamantino Formation represent the final stage of
sedimentation in Alto Paraguai Group, a sequence of sedimentary rocks with
Neoproterozoic-Cambrian age, also composed by Raizama and Sepotuba
formations. The study area is at river banks of Serragem II River, nearby
Nobres city- MatoGrosso, and holds the record of turbiditic deposits as a basal
succession of Diamantino Formation. Were identified two facies association:
AF1- Turbiditic fans compounds by laminated and massive mudstones (Pl),
massive sandstone (Am), sandstone with parallel lamination (Alp), sandstone
with climbing ripples cross-lamination (Alc) and tangencial cross-bedding
(Atg);AF2-Turbiditic channel characterized by laminated and massive mudstone
(Pl), sandstones with convolute lamination (Aco), sandstones with trough crossbedding
(Aa), and sandstone with tangencial cross-bedding (Atg). This facies
association were generated due to variation of gravitational flow and current.
Among the facies produced by gravitational flow highlights the Pl, Am, Alp and
Alcfacies, composing a set of facies that harrow vertically and laterally in the
same layer, showing constant flow transformations, composing cyclic patterns
characteristic of intermediate to distal turbidite fansdeposits(AF1). Futhermore,
the presence of coarse grained sandstones with cross-bedding and erosional
features in the base (gutter cast) can also be product of the same processes
came from hydrodynamic currents generated by the dissipation of turbulent
flows related to turbiditic channel deposits (AF2). Studies of heavy minerals
made in basal Diamantino Formation deposists indicated the presence of
zircon, tourmaline, rutile, actnolita, hornblende, garnet, apatite, fluorite, epidote,
kyanite and silimanite. The occurrence of grains with high sphericity and
rounding associated with angular grains and sub-rounded, such as zircon,
garnet and rutile corroborates an origin from a source with lithological diversities
and suggests mixing of sediments with diferent histories. Previous works have
suggested an age of about 541 Ma indicating probable source areas of
sediments of Diamantino Formation from Brasilia Belt rocks (790-600 Ma). The
data collected in this work when added to others in the literarture corroborate
the idea that the Diamantino Formation was deposited in an environment of
restricted sea (lake), inserted into a context of the foreland basin and
foredeepsub-basin, due the likely closure of the Clymeneocean at the end of
Brasiliana/Pan-African tectonic.
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