Siliciclastic facies |
Green to yellowish sandstone |
Lithofacies predominant in the east, in the region of Cedro do Abaeté (Lima et al. 2007Lima O.N.B., Uhlein A., Britto W. 2007. Estratigrafia do Grupo Bambuí na Serra da Saudade e geologia do depósito fosfático de Cedro do Abaeté, Minas Gerais. Revista Brasileira de Geociências, 37(4 Suppl.):204-215.). Green to yellowish (when weathered) beds of 5 cm to 2 m-thick, with a fine to medium-grained sandy base and a silt-mud top. Planar, wavy, and truncated lamination. Locally, hummocky cross-stratification with low angle truncated surfaces and wavelengths between 1 to 2 m. |
Deposited by the action of proximal storm waves on a marine shelf. |
Siltstone |
Gray to beige, massive or laminated, coarse- to fine-grained siltstone locally interbedded with calciferous siltstone. |
Fine-grained sediment deposited by weak traction currents or settling in a marine shelf environment. |
Mud-silt-sand rhythmite |
Laminated rhythmites with varying proportions of mud, silt, and fine sand grains. May be intercalated with beds of fine-grained sandstone. It typically weathers to a reddish brown due to the presence of iron oxides and hydroxides. Load casts and flame structures are common. |
Sand-silt-clayey sediment transported and deposited by weak or distal storm waves in marine environment. |
Green and glauconitic siltstones |
Massive or laminated, medium to coarse grained green siltstone, occurring locally with truncated laminations. It shows grains of glauconite, subangular to subrounded grains of quartz and K-feldspar, clay minerals, detritic micas, opaques, and iron hydroxides. When glauconite content is greater than 40%, as predominant in the west, in the region of Matutina and Funchal, it is classified as glauconitic siltstone. |
Deposited by the action of distal storm waves on a marine shelf. |
Phosphatic facies |
Phosphatic rhythmite |
Phosphatic sand-silt-clay rhythmites showing dark gray or brown laminae (with manganese oxides) interbedded with light gray laminae. The light gray laminae are composed of phosphate intraclasts in a microcrystalline grained matrix rich in fluorapatite, colophane, quartz, feldspar, muscovite, chlorite, and calcite (Lima et al. 2007Lima O.N.B., Uhlein A., Britto W. 2007. Estratigrafia do Grupo Bambuí na Serra da Saudade e geologia do depósito fosfático de Cedro do Abaeté, Minas Gerais. Revista Brasileira de Geociências, 37(4 Suppl.):204-215.). |
The genesis of the phosphorite likely occurred due to marine transgression and upwelling on the marine shelf. A later regression reworked the primary phosphorite by waves and storm currents on the continental shelf (Lima et al. 2007Lima O.N.B., Uhlein A., Britto W. 2007. Estratigrafia do Grupo Bambuí na Serra da Saudade e geologia do depósito fosfático de Cedro do Abaeté, Minas Gerais. Revista Brasileira de Geociências, 37(4 Suppl.):204-215.). |
Carbonate facies |
Reworked carbonate |
Dark gray and fine-grained grainstone and intraformational breccia (rudstone), with angular to subangular intraclasts. |
Formed by the action of waves and currents on a shallow continental shelf. |
Microbial laminite |
Dark gray microbial laminite, occasionally phosphatic, showing typical crenulated and irregular laminations. |
Biogenic carbonate formed by cyanobacteria and other microbial lineages in a shallow marine environment. |
Black organic-matter-rich micritic limestone |
Black organic-matter-rich micritic limestone and light gray ooid grainstone highly fractured. |
Related to high organic matter burial in an inner to outer shelf environmental. |
Volcaniclastic facies |
Volcaniclastic rocks |
Clay-rich, whitish to greenish, 0.5 m-thick layer composed of angular quartz, muscovite, illite, kaolinite, euhedral and rounded zircon grains and a few glauconite aggregates. |
Sedimentation of volcanic ashes and resedimented volcanic material in marine shelf. |
Whitish to greenish, very friable, containing vesicular cavities and a porous matrix. |