Paleoambientes de depósitos conglomeráticos no município de Ourém, NE do Pará

Abstract

This paper presents a facies analysis about a conglomeratic deposit throughout Ourém city, northeastern Pará State. Seven sedimentary facies has been identified, divided into two facies associations: Facies Association A and B. Facies association A comprises arenoconglomerátics deposits, composed of pebbles of quartz and quartzite, sub-rounded, moderate sorted, low sphericity, immersed in coarse quartz matrix, which is abruptly interrupted by layers of quartz sandstone and lenticular mudstones. These deposits fill concave and flat geometry, arranged inner finning upward cycles, presenting tabular cross stratification, agradacional and massive texture, interpreted as a braided fluvial channel system interdigitated to a system of meandering river channel. The association of facies B, superimposed, consists of conglomerate pebbles of quartz and quartzite, sub-rounded, low sphericity, poorly sorted, immersed in coarse quartz sand matrix under slightly flat sigmoidal geometry, organized inner fining upward cycles, with planar cross-bedding, slotted and sigmoidal cross-stratification, which has represented by a river braided channels system. According to this interpretation, is considered that Ourém city depositional history has developed over a river system scheme of meandering channel interdigitated by a braided channels (Facies Association A) superimposed by a river braided in nature (facies B Association). The base and top, marked by Ipixuna and Pós-Barreiras Formations, respectively, allowed to relate conglomerate deposits of Ourém city into a Barreiras Formation.