EVOLUÇÃO DIAGENÉTICA DOS ARENITOS DA FORMAÇÃO MORRO DO CHAPÉU, GRUPO CHAPADA DIAMANTINA, NA REGIÃO DE MORRO DO CHAPÉU, BAHIA

Résumé

The Mesoproterozoic Chapada Diamantina Group crops out in the central part of Bahia State, coveringthe São Francisco Craton. It is composed, from the base to the top, of the following formations: Tombador,Caboclo and Morro do Chapéu. The lower contact of the Chapada Diamantina Group is characterized byonlap with the Archean basement or by an unconformity with the Mesoproterozoic rocks of ParaguaçuGroup. The upper contact with the Una Group is unconformable and characterized by a depositional hiatus.A depositional model, a rock classification and a diagenetic evolution of the Morro do Chapéu Formation arehere presented based on stratigraphic profiles, outcrop description and microscopic studies in 45 thin sections.The rocks were deposited by transitional and fluvial systems. They show high mineralogical and texturalmaturities, and can be classified as quartz-arenites. Besides quartz and chert, which usually make up to 95 %of the rock framework, feldspars, micas, heavy minerals as well as mud and carbonate intraclasts also occur.The carbonate intraclasts are frequently silicified. The presence of these intraclasts, which come from theunderlying Caboclo Formation consists an important record of a depositional hiatus between both units.Nine diagenetic events could be recognized in the fluvial sandstones of the Morro do Chapéu Formation:mechanical compactation, mechanical clay infiltration, early hematite cementation, quartz overgrowths,pressure solution, secondary porosity generation, clay authigenesis, late diagenetic phases and telodiageneticprocesses. The transicional sandstones, display, aditionally, four other events: a second quartz overgrowthphase, an incipient dolomitization, antraxolite and gipsite authigenesis.