Nowadays, the great interest in carbonates is justified by the fact that about 50% of
world oil production is associated with this type of rock. In the Amazon basin,
hydrocarbon exploration has been done through several surveys since 1930. The
study sequences have been mainly the aeolian sandstones of the Monte Alegre
Formation and subordinately the Itaituba Formation carbonates, of the Lower
Pennsylvanian. Both are included into the Tapajos Group which represents the last
transgressive-regressive Paleozoic cycle of the Amazon basin. Of neocarboníferapermian
age, this cycle is characterized by association with significant climatic
change from cold to hot-arid. The Itaituba Formation represents itself a cyclic unit,
heterogeneous, abundantly fossiliferous and composed of limestones with high
content of marine bioclasts, dolomites, shales, siltstones, sandstones and evaporites.
This study was carried on the samples from the CALMINAS company quarry, located
on the right margin of the Tapajos river about 3 km to east of Itaituba-PA city, at the
southern edge of the Amazon basin. Important isotopic studies have been conducted
in the Itaituba Formation, but a refinement of the deposition age, as well as the
estimation of paleoenvironmental conditions based on the use of isotopes of Sr, C e
O was necessary. For this, the samples were submitted to petrographic, X-ray
diffraction and cathodoluminescence (CL) analysis in order to select the best
samples for application of isotopic analyses. Therefore samples PC-0.9, PC-7.0 and
PC-14.8, classified as packstones, and that proved to be well preserved, with high
content of calcite, reaffirmed by luminescence patterns observed in CL images were
selected for Sr isotopic analyses. Stable isotopes (C and O) analyses were
performed on all samples collected. The carbonates presented positive values of
δ13C ranging from 2.21 to 6.22‰, negative values of δ18O ranging from -7.66 to -
0.31‰ and 87Sr/86Sr ratios between 0.708754 and 0.710115. Some negative and
positive trends in the values of δ13C may indicate probable changes in sea level, and
small fluctuations in the δ18O values may reflect short variations in paleotemperature
and paleossalinity. The high reasons 87Sr/86Sr ratios suggest radiogenic anomalies of
Sr.
|
