MICROFITÓLITOS ASSOCIADOS A CONSTRUÇÕES ESTROMATOLÍTICAS DO GRUPO BAMBUÍ, PROTEROZÓICO SUPERIOR, NA REGIÃO DE ARCOS - MG

The Neoproterozoic Bambui Group consists in the Arcos region of a sucession of 120m of carbonate rocks withouth associated siliciclastic rocks. Four main shallowing upward intervals were identified in the area: (1) a basal interval represented by a carbonate ramp with beach-barrier-lagoon in the inner ramp and evidence of subaerial exposure in the upper part of the interval; (2) stromatolitic interval consisting of several, few meters thick, shallowing upward cycles, with common intercalations of detritic carbonate horizons with cross-bedded microphytolitic grainstones-to-packstones up to l .5 m thick and few horizons of rudstones composed of intraformational dolomite flakes derived from the stromatolites. The stromatolites are mostly stratiform to pseudocolumnar; biostromes are dominant, with rare bioherms of columnar or dome-shaped stromatolites. The entire interval is dolomitized and the dolomite texturally preserves crypto-to-fme crystalline; anhydrite occurs scattered with no particular concentration zones. Intervals 3 and 4 are very similar to each other and resemble Oolite-Grainstone Cycles; the upper facies of Interval 3 is similar to Lime-mud-Sabkha Cycles. Microphytolites, as used in the paleontological literature, refer to a great variety of carbonatic grains such as oncoids, microbial lumps and catagraphs. They are restricted in time and were fnost important in Proterozoic times. They constitute an especial kind of grain with a complex, polygenetic origin, resulting from interaction between microbial activity and sedimentary/diagenetic processes, and are considered as paleoenvironmentai indicators reflecting intermediate, moderate, energy conditions. In the Arcos region, microphytolites are found in Interval 2, and consist of catagraphs, most of them similar to Vesicularites and Vermiculites. They are usually associated with columnar stromatolites or constitute cross-bedded dolarenite. Wherever related to stromatolites, they compose part of the laminations and the walls, and exibhit some morphological differences as compared to the detrital ones. They also may fill the intercolumnar space between stromatolites. The microphytolites of Mina da Bocaina afford new insights into Proterozoic paleoenvironments and reflect moderate energy conditions.