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NEW SEDIMENTARY ASPECTS OF THE GONGO SOCO BASIN – QUADRILÁTERO FERRÍFERO: UNCOVERING THE COAL LAYERS

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The Gongo Soco Basin, in the northeastern of Quadrilátero Ferrífero (QF), is one of the many Cenozoic basins known in the region. It was first described in the late 20th century, based on a few mining exposures. Ongoing excavations unveiled a great thickness of the units and their basement contacts. Geological mapping and sampling were used to describe the sedimentary units, based on their composition, grain size, and sedimentary structures. Two main units were described, being the base river and lacustrine sediments, and the upper debris sediments. The basin’s base is marked by laterally intermittent metric gravel layers with an erosive contact, as paleochannel features. The gravel is overlaid with red clay and silt and fine-grained sand, interbedded with two distinguished coal layers, potentially organic fossil-bearing. The coal layers are massive, black, from 3 up to almost 7 m in thickness, with normal to gradational base contacts to the clayey sediments, and with normal to erosive contact to the above sediments. Between these two coal layers is always found red to grey clayey sediments. This sequence could be interpreted both as a crevasse splay environment or as restricted lake sediments. These layers dip from 50 to 80 degrees near the eastern border, with decreasing dip through the middle of the basin, indicating tectonic impact over this portion of this basin. Above them, it is found sandy to sand clayey sediments, with sigmoid format and stratification structures, close to the braided river. This unit is interpreted as a register of opening basins, with erosive flows, migrating to a river lacustrine environment and buried by debris flows. The top unit is a massive red to brown coarse-grained sediment, with a clayey matrix, with clasts of itabirite, phyllite, dolomite, and canga fragments, roughly rounded. Tectonic occurrences and unstable steep natural slopes over the basin area are considered the triggers for the sediment origin. Interpreted as debris flow sediments, its source is the Proterozoic soil and rocks. The units are affected by normal faults, with 1 to 3m long, punctually with a horst and graben aspect, being these faults present in the top units with smaller displacement. The eastern portion of the basin is affected by a single great thrust fault overlapping part of the sedimentary units with the Paleoproterozoic rocks, buried by the debris flow influx. The basin starts as a river-dominated process, with local lake occurrences. Its volume increases controlled by tectonic activities, that appropriate over the local plane structures of the karstic basement, associated with a strike-slip flower major structure of Gandarela Syncline. The sedimentary influx decreases as the basin is filled upwards and the tectonic activity reduces. More exposure to the sedimentary units was a great addition to the study of the Gongo Soco Basin, as strategic contacts could be accessed, and the coal layers were unveiled, greatly enhancing the knowledge of the basin using basic field investigations. The authors thanks VALE S.A. for the data supply.

Palavras Chave

Gandarela Syncline; Cenozoic; Neotectonics