Abstract
ABSTRACTThe Pleistocene deposits exposed in the Amapá Coastal Plain (onshore portion of the Foz do Amazonas Basin, northeastern South America) were previously interpreted as Miocene in age. In this work, they were named as "Itaubal Formation" and were included in the quaternary coastal history of Amazonia. The study, through facies and stratigraphic analyses in combination with optically stimulated luminescence (single and multiple aliquot regeneration), allowed interpreting this unit as Late Pleistocene tidal and fluvial deposits. The Itaubal Formation, which unconformably overlies strongly weathered basement rocks of the Guianas Shield, was subdivided into two progradational units, separated by an unconformity related to sea-level fall, here named as Lower and Upper Units. The Lower Unit yielded ages between 120,600 (± 12,000) and 70,850 (± 6,700) years BP and consists of subtidal flat, tide-influenced meandering stream and floodplain deposits, during highstand conditions. The Upper Unit spans between 69,150 (± 7,200) and 58,150 (± 6,800) years BP and is characterized by braided fluvial deposits incised in the Lower Unit, related to base-level fall; lowstand conditions remained until 23,500 (± 3,000) years BP. The studied region was likely exposed during the Last Glacial Maximum and then during Holocene, covered by tidal deposits influenced by the Amazon River.
Highlights
The Cenozoic siliciclastics deposits exposed in the Amapá Coastal Plain (ACP) have traditionally been interpreted as alluvial fans, fluvial and lacustrine deposits representative of the Miocene Barreiras Formation, a characteristic unit mainly exposed along the northern and northeastern Brazilian coast, and sand and muddy terraces deposits formed by sediments derived from the Amazon River, during Holocene (Lima et al 1974, 1991; Mendes 1994; Silveira 1998; IBGE 2003; CPRM 2004; Santos 2006; Guimarães et al 2013a, 2013b)
The presence of the Pleistocene Itaubal Formation just above the underlying strong weathering basement rocks suggests the absence of the Barreiras Formation in the ACP, and probably western border of the Marajó Graben being an uplifted area during the Miocene-Pliocene
The Itaubal Formation was subdivided into two progradational units, separated by an erosional unconformity
Summary
The Cenozoic siliciclastics deposits exposed in the Amapá Coastal Plain (ACP) have traditionally been interpreted as alluvial fans, fluvial and lacustrine deposits representative of the Miocene Barreiras Formation, a characteristic unit mainly exposed along the northern and northeastern Brazilian coast, and sand and muddy terraces deposits formed by sediments derived from the Amazon River, during Holocene (Lima et al 1974, 1991; Mendes 1994; Silveira 1998; IBGE 2003; CPRM 2004; Santos 2006; Guimarães et al 2013a, 2013b). Souza (2010) suggests tidal influence in Miocene deposits and, based on low-resolution optically stimulated luminescence — OSL (multiple aliquot regeneration), indicates Late Pleistocene age for the older rocks. Pleistocene deposits exposed in the ACP, onshore portion of the Foz do Amazonas Basin, northeastern South America, are included in the Quaternary coastal history of Amazonia
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