The Albian deposits comprise ca. 70% of sedimentary fill of the São Luís-Grajaú
Basin, with 500 m thick in their main depocenters. The origin and sedimentary evolution
of these deposits is related to the breakup of Africa and South America which led to the
connection of the formerly separated Central and South Atlantic oceans, in Mesozoic
time.
Albian exposures in the southern border of the São Luís-Grajaú Basin, Grajaú
region, represent a fluvial-deltaic succession whith six depositional environments,
including: delta front (mouth) bar, distal bar/prodelta, upper shoreface/foreshore,
interdistributary bay/crevasse, fluvial channel and distributary channel. They consist,
mainly, of fine- to medium sandstones, as well as siltstones, mudstones and
intraformational conglomerate. Palaeocurrent study, petrography and geochemistry of
sandstones and heavy minerals, and detrital zircon ages were utilized to investigate the
provenance of this sedimentary unit.
This study was performed on sandstone samples whose modal compositions plot
in the quartzarenite field indicating origin from recycled orogen and continental blocks.
High proportion of quartz grains, monocrystaline and polycrystalline, as well as quartzite
fragments, and rounded grains of zircon and tourmaline with a wide variety of
mechanical surface textures, suggest a provenance from sedimentary rocks, and that at
least part of the studied sediments are multicyclic. Chemical weathering processes is
little evident as indicated by absence of solution features in these mineral grains.
Therefore, the modal composition can be related to the sedimentary recycling or strong
reworking on depositional environment. High CIA (Al2O3/Al2O3+K2O+Na2O+CaO*x100)
values suggest transportation and recycling from sources located far away from the
depositional basin, which is, in turn, consistent with a provenance from the recycled
orogens and continental blocks.
Based on palaeocurrent patterns and RuZi the Albian succession in the south
region of the São Luís-Grajaú Basin was defined four heavy mineral zones, generically
named A, B, C and D, in ascending stratigraphic order. Geochemistry of tourmaline grains indicate provenance from metapelites and metapsammites, with few contributions
from granite and pegmatite. Zircon Hf patterns suggest changing of the source of these
zones: Zone A is characterized by an Hf unimodal distribution, while in the Zones B, C
and D, is bimodal. The staurolite shows a relatively limited amount of compositional
variations. Involvement of metasediments is inferred from the presence of staurolite and
kyanite in the sandstones, as well as by predominance of dravite in the tourmaline
populations. Discriminant function analysis using major element compositions show that these
deposits were deposited in the passive continental margin (PM). PM sediments are
mainly quartz-rich, sourced from craton interiors or stable continental regions, which
were deposited in intra-cratonic sedimentary basins or on passive continental margin.
The Albian sediments are characterized by LREE enrichment, depletion in HREE, and
negative Eu-anomaly. This REE pattern, measured to infer the provenance of sediments
and their relationship with average post-Archean upper continental crust, is very
consistent with this interpretation. Enriched HREE concentration in the some samples
may be attributed to the presence of REE bearing heavy minerals, supported by the fact
that these samples have higher concentration of Th, U and Zr, reflecting natural
concentration of zircon grains.
Pb-Pb geochronological analyses of 238 detrital zircon grains show a direct
fingerprint of Precambrian terrains (Archean to Proterozoic) in the source. Three major
zircon populations were detected: Archean (3103-2545 Ma), Paleoproterozoic (2460-
1684 Ma) and Neoproterozoic (993-505 Ma); small groups of Mesoproterozoic (1570-
1006 Ma), Paleozoic (440-540 Ma) and Mesozoic (141-314 Ma) grains are also present.
The Neoproterozoic component shows an increase upwards with main peaks
between 550 Ma and 650 Ma. A similar pattern is shown by the Archean interval, which
exhibits a strong relative increase upwards, peaking between 2725 Ma and 2926 Ma,
while Paleoproterozoic component has a distinct behavior, showing an evident decrease
upwards.
The potential source regions were deduced on the basis of palaeocurrent patterns
and correlations of detrital zircon age from the sandstones studied with U-Pb and Pb-Pb
zircon data from the basement. Our data suggest that the Albian deposits, specially those of the Zone A, were preferentially sourced from the northern and northeastern
regions, including São Luís Craton, Gurupi Belt and northwestern portion of Borborema
Province. Paleoproterozoic and Neoproterozoic zircon ages as those found in zircons
from this zone, are very common in these basement. In contrast, the sediments of zones
B, C e D were supplied from the areas located to the south, southwest and, possibly
east, involving the eastern portion of the Amazonian Craton/Araguaia Belt, and
Borborema Province. Metassedimentary and igneous rocks with similar zircon ages
(mainly Archean) have been described in these regions. The ca. 1.0 Ga detrital zircon
ages show a correspondence with the Cariris Velhos Event, widely recognized in the
central portion of the Borborema Province. In summary, this study demonstrates the
effectiveness of an integrated approach to provenance evaluation of Cretaceous
sedimentary deposits using petrography, heavy minerals and bulk sediment chemistry,
zircon ages, and palaeocurrent data.
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