Influence of continental fraction in recent biogenic carbonate sedimentation in the oriental shelf of Northeastern Brazil

Abstract

The influence of continental sediments on formation of recent marine sediments, has been min-eralogically and geochemically studied using shelf-floor whole sediments (WS). The studied area corresponds to the oriental shelf of Northeastern Brazil between the 10 to 80m isobaths, adjacent to a continental area dominated by semi-arid to humid tropical climate. 47 representative samples from the dominating facies in this shelf have been analyzed by X-ray diffraction (XRD) and chemical analisis and results allowed identification of two types of floor sediments of: (a) quartz-terrigenous dominance and (b) carbonate dominance, whose fraction present distinct mineralogical and geochemical characteristics. In the terrigenous facies, that occur from 10 to 25-55m depths, quartz, potassic feldspar and high Mg-calcite predominate, with Sr generally <2,200 ppm. In the carbonate facies, high Mg-calcite and aragonite coexist and predominate, with Sr concentrations up to 6,400 ppm and ranging >2,600 ppm. In both cases, low-Mg calcite is absent. Even when the terrigenous fraction is found in accessory amounts (< 3% WS), geochemical signatures, marked by strong intercorrelations SiO2-Al2O3-K2O-Rb, exhibit correlation coefficients with r ≥ 0.9, reflecting a continental felsic source. The statistical analyses (co-variation between Mg/Ca and SiO2), chemistry and mineralogy of the sediments indicate that textural and mineralogical nature of the substratum is the main factor controlling the biota (domi-nantly benthonic in this shelf) and that, in these conditions, quartzous terrigenous facies favor species that are preserved in high Mg-calcite instead of those preserved in aragonite. This tendency showed to be independent of depth of sampling between 10 and 55 m.