Análise estrutural e caracterização faciológica da Formação Gongo Soco, porção central do Sinclinal do Gandarela - Quadrilátero Ferrífero - MG

Abstract

The Gongo Soco Formation represents the sedimentary fill of a Cenozoic intramontane basin located in the northeastern portion of the Quadrilátero Ferrífero Mineral District, southeastern Brazil, overlying Paleoproterozoic units of the Minas Supergroup. The basin records the development of a braided fluvial fan system, with its stratigraphic architecture and facies distribution strongly controlled by inherited basement structures. The integration of stratigraphic, sedimentological, structural, and geophysical data reveals a complex infill history shaped by episodic tectonic activity and paleoclimatic variations. Facies analysis based on outcrops, core logging descriptions, and petrography allowed the identification of twelve lithofacies grouped into four main units. The basal and upper portions are composed of debris-flow and braided fluvial deposits, characteristic of high-energy alluvial fan environments. The intermediate unit (~15 m thick) includes lenticular beds of finely laminated organic-rich mudstones and lignite, indicating sedimentation in low-energy palustrine to lacustrine settings under anoxic conditions. The presence of ferricrete horizons at multiple stratigraphic levels indicates episodes of subaerial exposure and lateritization under fluctuating paleoclimatic regimes. Structural mapping and morphotectonic analysis reveal that the Gongo Soco Basin is bounded by fault systems trending NW–SE, NE–SW, and ENE–WSW, which reactivated pre-existing discontinuities. Paleostress inversion based on shear fracture and fault-slip data using the WinTensor® software identified two main tectonic phases: D₁, an initial transtensional regime (σ₁ vertical, σ₃ NW–SW), responsible for synsedimentary normal faulting and subsidence forming NW–SE-trending horst-and-graben structures; and D₂, a subsequent transpressional phase (σ₁ ~E–W), which generated reverse, strike-slip, and oblique-slip faults affecting both the basement and the Cenozoic cover. A third deformational phase (D₃), associated with compressional stress relaxation, is suggested by the structural record but is not evident in the paleostress analysis. Electrical resistivity profiles and borehole data corroborate a subsurface horst-and-graben geometry trending NW–SE, with vertical offsets of up to 90 m. The sedimentary architecture and structural framework are consistent with a rhombohedral pull-apart basin formed along a NE–SW-trending dextral strike-slip corridor. Stratigraphic correlation with dated organic-rich horizons in adjacent basins (Fazenda Gandarela, Fonseca, and Bandeira) suggests that sedimentation in began in the Late Eocene and possibly extended into the Miocene. The Gongo Soco Basin thus represents a significant example of continental sedimentation controlled by brittle tectonics in the southern border of the São Francisco Craton, providing valuable insights into the tectono-sedimentary evolution of the Southeastern Brazilian Cenozoic Rift System.