Subsurface stratigraphy of the cretaceous-neogene sedimentary sequence of Manaus and Itacoatiara regions, Central Amazon

Abstract

The post-Paleozoic stratigraphic setting of Manaus and Itacoatiara area (central Amazonia) was developed based on four coross-sections containing geophysical and lithological data from 15 wells, up to 300 m deep, and outcrop description. Gamma ray (GR) and electric resistivity (ER) patterns allowed the separation into lower, intermediate and upper sedimentary units, and their contacts (S1 and S2 surfaces). The lower unit, starting at -130 m, is composed by carbonatic shale, with characteristic funnel and serrated GR and ER patterns. The intermediary unit occurs between 20 and -180 m, and is composed by sandstones with intercalated silt and claystone, with scattered layers of conglomerate. These intercalations generate cyclic repetitions of GR and ER values, creating a serrated and bell-type patterns. The upper unit occurs from -30 m to the surface and is composed by sandstone with scattered silt- and claystone, generating symmetrical (GR) and box-type (ER and GR) patterns. The stratigraphic position, lithological composition and the contact surfaces of the lower, intermediate and upper units allow us to associate them with the Nova Olinda-Andirá (Paleozoic), Alter do Chão (Cretaceous), and Novo Remanso (Neogene) formations of the Amazon Sedimentary Basin. Weathering and vegetation are always a challenge for geologic mapping the Amazon. Thus, the geophysical patterns and lithological aspects described in this paper may well serve as a good starting point for determining post-Paleozoic formations and their contact surfaces in the Amazon Sedimentary Basin