Sediment provenances and drainage evolution of the Neogene Amazonian foreland basin

During the Middle-Late Miocene, tidal sedimentation was the rule in the Amazon foreland basin. The Amazon foreland basin became emerged in the Late Miocene-Pliocene times. In this study, trace element chemistry (n=104) and Nd-Sr isotopic systematic (n=30) are used to constrain the provenance of the Miocene tidal sediments and of the Late Miocene-Pliocene fluvial sediments. Neogene Amazonian foreland basin sediments are the result of a mixing between Andean andesitic volcanic rocks and cratonic shield rocks. The south Amazonian foreland basin (SAFB) sediments are the result of long-term weathering, recycling, and erosion of the Brazilian shield and Andean Paleozoic/Mesozoic rocks also Brazilian shield in provenance. Compared with north Amazonian foreland basin (NAFB) sediments, SAFB sediments are more felsic. NAFB sediments have 3 distinct signatures: i) an upper crust signature with trace element characteristics similar to PAAS and UCC and with epsilon Nd(0) values between -8 and -11.9; ii) an arc andesitic rock signature with high Cr/Th ratios, low Eu anomalies, low Th/Sc ratios and epsilon Nd(0) values between -3 and -5; and iii) a cratonic signature with high Eu anomalies and Zr/Sc ratios and with a very low epsilon Nd(0) value (-15.5). Our data indicate that most of the basic detritus came from the Ecuadorian Andes. In the distal part of the NAFB, the successive uplifts of the Iquitos forebulge were responsible for Late Miocene to Pliocene divide between Andean and cratonic drainage systems. The modem Amazon River drainage network is no younger than the Pliocene. (c) 2005 Elsevier B.V. All rights reserved.