Sistemas deposicionais e petrologia da sucessão siliciclástica do Gráben de Jaibaras (cambro-ordoviciano), Província Borborema, NW do Ceará

Abstract

An integrated faciologic and petrographic studies of the depositional sequences (Cambrian-Ordovician) in the Jaibaras Trough, northwest of Borborema Province (NW of Ceará State) is presented for first time. The sedimentary basins’s fill includes lithofacies of upper flow regime deposited during episodic sedimentation events. The sedimentary succession includes two sequences made in three evolution stages of the basin: a lower sequence composed by Massapê and Pacujá formations, which represent the earlier and intermediate sin-rift stage; and an upper sequence represented by the Aprazível Formation, developed in the final sin-rift. The Massapê Formation includes conglomerates and sandstones of alluvial-fans with intercalations of volcanic rocks of the Parapuí Suite. This stratigraphic interval gradates to Pacujá Formation composed by sandstones, siltstones, mudstones and breccias accumulated in fluvial-deltaic to marine environments. This delta progradates into shallow water, leading to the development of slope failures, debris-flow, and, in places, growth faults. Icnofossils in the transitional and prodeltaic turbidites deposits suggest marine influence. The Aprazível Formation consists of alluvial fans and high-flow braided channels deposits, which occur unconformably with Massapê and Pacujá formations. Detrital sediments source to Massapê and Pacujá formations were plutonic and metamorphic rocks of the adjacent basement. The sediments sources for Aprazível Formation include plutonic rocks (Meruóca Suite), also the Massapê and Pacujá formations, and metamorphic rocks. The framework characteristics of sandstones indicate low-burial. However, petrographic features suggest eo-, meso- to telodiagenetic regimes. Chlorite grains are detrital and diagenetic in origin, and don’t indicate ankimetamorphism. The accumulation and quickly burial of these sedimentary sequences in a rift basin, as well as sedimentary supply from adjacent sources, the low mineralogical and textural maturity, indicate tectonic and climatic controls. Furthermore, the preservation of sediments was favorable due to paleogeographic context, once in Lower Paleozoic, the Gondwana Supercontinent was located at high latitudes (next to the South Pole).