Fácies deposicionais, estratigrafia e aspectos estruturais da cobertura sedimentar paleoproterozoica na serra do Tepequém, Escudo das Guianas, Estado de Roraima

Abstract

In the northern South America, occurs the largest continuous exposure of Paleoproterozoic sedimentary deposits related to the Roraima Supergroup, forming the Pacaraima Block with 73,000 km2. This siliciclastic succession of more than 2 km thick is inserted in the Guyana Shield, northern Amazon Craton, extending from in the border of Brazil, Venezuela, Guyana and Suriname. Near of the Pacaraima Block occur isolated mountains with little-known siliciclastic successions of the faciologic and stratigraphic point view, which does not allow a better correlation with the Roraima Supergroup and, consequently, hinders the paleogeographic and paleoenvironmental reconstitution of the Paleoproterozoic in this part of Amazonia. Outcrop-based stratigraphic and facies studies in the Serra do Tepequém and Uiramutã region, State of Roraima, northern Brazil, allowed redefined and redescribed the lower unit of Roraima Supergroup as Arai Group. The Arai Group of approximately 400 m thick overlies acid volcanic rocks of the Surumu Group, is covered by siliciclastic deposits of the Suapi Group and has been subdivided into two formations. The lower one, similar in both areas, generally consists of polymictic conglomerates, cross-bedded sandstones with microplacers of hematite, and subordinate mudstones interpreted as braided stream deposits. In contrast, the upper formation, in the Uiramutã region, is composed of silty fine-grained sandstones with medium-scale trough cross bedding and convolute lamination deposited in a more distal braided stream environment. This unit, in the Serra do Tepequém region, consists of fine to medium-grained cross-bedded sandstones with mudstones, sandstone/mudstone rhythmites and subordinate conglomerates and breccias interpreted as coastal, tide-influenced deposits. The top of the Arai Group is marked by an expressive unconformity covered with diamond-bearing conglomerates and pebbly sandstones of the basal Suapi Group, interpreted as braided stream deposits. This study confirms the previous interpretation of a big braided stream system migrating to southwest in the central part of the Guyana Shield. In addition it could be shown that the fluvial system in its distal part (Serra do Tepequém) was influenced by tidal processes. The stratigraphy of the Arai Group has established the basis for a regional correlation included deposits of isolated occurrences in the Guyana Shield and points to the presence of a large intracratonic Paleoproterozoic basin connected to the open sea. The structural framework of Serra do Tepequém indicates that major sinistral oblique, normal and reverse NE-SW fault zones bound domains whose bedding dips mainly towards SE and NW. Regional scale forced folds are represented by kilometer scale kink bands and chevron folds compatible with upper-to-middle crustal level. These findings differ from previous regional models based on folding under ductile conditions related to collisional tectonics and evidence the importance of Guiana Shield early basement structures, reactivated probably during the K'Mudku event (~ 1.2 Ga).