Evolução sedimentar miocênica nos estados do Pará e Maranhão

Abstract

Miocene deposits, lithostratigraphically designated as the Pirabas and Barreiras formations, are well exposed in northeastern State of Pará and along the coastline of the State of Maranhão, where they are represented by a variety of sedimentary facies ranging from limestones to siliciclastics. The majority of the limestones and mixed limestone to siliciclastic deposits were formed in environments ranging from marine transitional to shelf. These deposits are internally organized into three stratigraphic successions defined by regional discontinuity surfaces. Analysis of these surfaces, combined with facies characteristics and stratal patterns, suggests an incised valley estuarine system with discontinuous deposition due to multiple episodes of high-frequency relative sea level fluctuations superimposed on an overall transgression to highstand. The establishment and evolution of this incised valley estuarine system occurred after an interval of up to 40 Ma of non-deposition and/or erosion, and probably resulted from eustatic fluctuations combined with tectonic reactivation. Based on the results obtained for the Miocene deposits exposed in the states of Pará and Maranhão, it is proposed that studies combining facies analysis, ichnology, stratal architecture, and mapping of key stratigraphic surfaces might be profitably applied to similar deposits elsewhere in Brazil.