『Abstract
Miocene and Pliocene deposits from the Ecuadorian, Bolivian and
Peruvian Amazonian foreland basin (north and south Amazonian foreland
basin, NAFB and SAFB) were analyzed for major and trace element
geochemistry (97 and 104 samples respectively). Neogene deposits
are well homogenized and have an overall PAAS-like composition.
They are mainly derived from a granodioritic crust. In the NAFB,
chemical weathering increases from Miocene to Pliocene to CIA
values 〜100. Physical sorting seems to have played a minor role
in the chemical composition of these sediments. Chemical ratios
such as Cr/Th and Th/U and Eu anomaly vary within each formation
and from one formation to another. In the SAFB, sediments have
a PAAS-like composition with recycling increasing toward the distal
part of the foreland basin. In the NAFB, the Neogene formations
have various provenances. Basic/andesitic sediments originated
from the Ecuadorian Andes. These sediments were trapped in the
subsiding Ecuadorian foredeep during the Miocene and reached the
Iquitos forebulge to join the paleo-Amazon near Iquitos (Amazon
formation) during the Pliocene. Other Miocene formations have
PAAS-like affinities but are enriched in Zr and Hf relative to
the PAAS suggesting that they are derived from a recycled and
differentiated upper continental crust. Uplift of the Iquitos
forebulge may have been responsible for dividing drainage networks
originating from the Andes and the craton. The Quaternary floodplain
deposits of the Amazon River resemble PAAS with depletion in trace
elements and enrichment in Zr and Hf, which indicate that they
are derived from a recycled differentiated upper continental crust.
Overall, the transition from tidal sedimentation in the Miocene
to fluvial sedimentation in the Late Miocene-Pliocene related
to a change with foreland basin dynamics have caused increasing
silicate weathering that could have had consequences on climate
and global carbon cycle during the Neogene.
Keywords: Major and trace elements; Weathering; Provenance; Amazonian
foreland basin; Neogene』
『要旨
エクアドル、ボリビア、ペルーのアマゾン前地流域（北アマゾン前地流域および南アマゾン前地流域、NAFBおよびSAFB）からの中新世と鮮新世の堆積物が、主要元素と微量元素の地球化学性のために分析された（それぞれ、97と104試料）。新第三紀堆積物は十分に均質化されており、全体としてPAAS様の組成をもつ。それらは主に花崗閃緑岩質地殻から由来している。NAFBでは、化学風化は中新世から鮮新世にかけてCIA値約100に増加している。物理的分級は、これらの堆積物の化学組成に小さな役割しか演じなかったと思われる。Cr/Th
や Th/U や Eu 異常のような化学比は、それぞれの地層内部および地層間で変動する。SAFBでは、堆積物は前地流域の末端部に向かって再循環が増加するようなPAAS様の組成をもつ。NAFBでは、新第三紀層はさまざまな起源をもつ。塩基性／安山岩質堆積物はエクアドル・アンデス山脈から生じている。これらの堆積物は中新世の間に沈下するエクアドル前地に捕えられ、鮮新世の間にイキトス近くの古アマゾン（アマゾン層）とつながりイキトス・フォアバルジに達した。他の中新世の地層はPAAS様類似点をもつが、PAASに比べてZrとHfに富み、これはそれらが再循環し分化した上部大陸地殻から由来することを示す。イキトス・フォアバルジの隆起は、アンデス山脈とクラトンから生じている分岐流域網の原因であったかもしれない。アマゾン川の第四紀氾濫原堆積物は微量元素に欠けZrとHfに富むPAASに似ており、このことはそれらが再循環され分化された上部大陸地殻から由来することを示す。全体として、前地流域ダイナミックスにともなう変化に関連した、中新世の潮汐性堆積作用から中新世後期−鮮新世の河川性堆積作用への変遷は、新第三紀の間の気候と世界的炭素循環に結果を与えた珪酸塩風化を進めた原因であった。』
1. Introduction
2. Geological setting
3. Stratigraphy of western Amazonia and sampling strategy
3.1. Sampling and stratigraphy of the north Amazonian foreland
basin (NAFB)
3.2. Sampling and stratigraphy of the south Amazonian foreland
basin (SAFB)
4. Analytical methods
5. Results
5.1. Major elements (SiO2, Al2O3, MnO, MgO, CaO, Na2O, K2O, TiO2, P2O5), LILE (Rb, Ba, Cs, Sr, Th, U), HFSE (Y, Zr,
Hf, Nb), and TTE (Cr, Co, V)
5.1.1. South Amazonian foreland basin (SAFB)
5.1.2. North Amazonian foreland basin (NAFB)
5.1.2.1. Miocene tidal formations
5.1.2.2. Late Miocene-Pliocene formations
5.2. Rare earth elements (REE)
5.2.1. South Amazonian foreland basin (SAFB)
5.2.2. North Amazonian foreland basin (NAFB)
5.2.2.1. Miocene tidal formations
5.2.2.2. Late Miocene-Pliocene formations
6. Discussion
6.1. Source area weathering
6.2. Sedimentary processes and sorting
6.3. Provenance
6.3.1. Provenance of the SAFB sediments
6.3.2. Provenance of the NAFB sediments
6.3.2.1. Miocene tidal sediments
6.3.2.2. Late Miocene-Pliocene fluvial sediments
6.3.3. Foreland basin dynamics and chemical characteristics
of Neogene Amazonian sediments
7. Conclusions
Acknowledgements
References