Newton Souza Gomes

Carbonate cementation patterns and diagenetic reservoir facies in the Campos Basin cretaceous turbidites, offshore eastern Brazil

Massive turbidite arkoses contain more than 80% of the oil reserves of the Campos Basin, the main petroleum province of Brazil. The porosity and permeability distribution in the Namorado (Albian-Cenomanian) and Carapebus (Turonian-Santonian) sandstones is

The occurrence of microdiamonds in Mesoproterozoic Chapada Diamantina intrusive rocks: Bahia / Brazil

The origin of diamonds from Serra do Espinhaço in Diamantina region (State of Minas Gerais) and in Chapada Diamantina, Lençóis region (State of Bahia) remains uncertain, even taking into account the ample research carried out during the last decades. The lack of typical satellite minerals in both districts makes a kimberlitic source for these diamonds uncertain. In mid 18th century the occurrence of a metamorphosed igneous rock composed of martite, sericite and tourmaline was described in Diamantina region and named hematitic phyllite, considered by some researchers as a possible diamond source. Similar rocks were found in Lençóis and examined petrographically and their heavy mineral concentration was investigated by means of scanning electron microscopy (SEM). Petrographic analyses indicated an igneous origin for these rocks and SEM analyses showed the discovery of microdiamonds. Geochronological studies using the Ar/Ar technique in muscovites yielded minimum ages of 1515+/-3 Ma, which may correlate with 1710+/-12 Ma from U-Pb method in igneous zircons from the hematitic phyllites. Both rock types also have the same mineral and chemical composition which leads to the conclusion that the intrusive rocks were protolith of the hematitic phyllites. This first discovery of microdiamonds in intrusive rocks opens the possibility of new investigation models for diamond mineralization in Brazilian Proterozoic terrains.

Domínios geomorfológicos na área de ocorrência dos depósitos de espongilito da região de João Pinheiro, Minas Gerais, Brasil

Important erosive processes that took place after the Cretaceous Era were responsible for the evolution of the landscape in the Joao Pinheiro region where spongillite deposits occur. The progression of these erosive processes on the carbonate rocks led to the development of negative karstic features where the ponds present formed the spongillite deposits. Based on field studies and analysis of multispectral images, four geomorphological domains were identified: i) domain 1, represented by a plateau related to sandstones from the Areado Group that present the highest altitudes in the area; ii) domain 2, that constitutes a desiccated area related to pelites from the Areado Group; iii) domain 3, comprising a karstic planation surface associated with carbonate rocks from the Bambui Group and Pre-Bambui unit covered by Cenozoic sediments where the ponds occur; iv) domain 4, composed of gutter shaped valleys enclosing meandering of underfit streams (of the Prata and Paracatu rivers). These valleys cut through the karstic planation surface (domain 3) where Pleistocene sediments occur and characterize the more recent morphological domain.

PETROGRAFIA E DIAGÊNESE DA FORMAÇÃO UBERABA, CRETÁCEO SUPERIOR DA BACIA DO PARANÁ NO TRIÂNGULO MINEIRO

The paper characterizes the depositional model and the diagenetic evolution of the Uberaba Formation, the lowermost unit of Bauru Group, Upper Cretaceous of the Parana Basin. The Uberaba Formation is geographically restricted to the Triângulo Mineiro region and consists of medium-grained sandstones, conglomerates, pebbly sandstones and siltstones. Its framework consists of quartz, feldspars, rock fragments (basalt, quartzites, schists, sandstones) and sandstone intraclasts. These rocks can be classified as lithic sandstones, sublithic sandstones and feldspathic lithic sandstones. The presence of thick conglomeratic sequences — consisting of sandstone intraclasts — near Serra do Verissimo, is interpreted as due to tectonic activity in this area during the deposition of the Uberaba Formation. The diagenetic evolution may be divided into eodiagenetic and mesodiagenetic stages. The eodiagenesis is represented by the following aspects: a) presence of large volumes of volcanic dust due to the volcanic activity in the Alto Paranaiba region, which were mechanically infiltrated in the intergranular pores of rocks; b) autigenesis of zeolite and silica; c) autigenesis of iron oxides, dolomites, calcretes, silcretes, attapulgite and sepiolite. The mesodiagenesis is marked by formation of calcite and barite, the former being the most important phase in volume. The textural relationships as well as the reddish colors of cathodoluminescence due to the high Mn content suggest the mesodiagenetic origin of calcite. The sedimentary environments characterized by braided river complex controlled the development of the autigenic minerals, during the eodiagenesis. The high permeability of these rocks is related to the textural heterogeneity, which allowed the influx of meteoric water. The framework constituents of these rocks were the main source of material for the diagenetic phases.