Ediacaran/Early Cambrian Serra da Saudade Formation, Bambuí Group: the sedimentary record of a foreland basin in Southeastern Brazil

Moreira, Débora Silvano; Uhlein, Alexandre; Uhlein, Gabriel Jubé; Sial, Alcides Nóbrega; Koester, Edinei

doi:10.1590/2317-4889202120210029

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Brazilian Journal of Geology

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ARTICLE • Braz. J. Geol. 51 (03) • 2021 • https://doi.org/10.1590/2317-4889202120210029 copy

Ediacaran/Early Cambrian Serra da Saudade Formation, Bambuí Group: the sedimentary record of a foreland basin in Southeastern Brazil

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

The Serra da Saudade Formation, Ediacaran/Cambrian Bambuí Group, cropping out along the ridge of the same name in Central-Western Minas Gerais, Brazil, contains important deposits of glauconitic siltstones associated with phosphatic rhythmite as well as other siliciclastic, volcaniclastic and carbonate lithofacies. Lithofacies indicate deposition by proximal and distal storm waves on a continental marine shelf. The basal gray siltstone and part of the green siltstone represent a transgressive systems tract. The glauconitic siltstone indicates sea level rise, with the maximum flooding surface located approximately at the interval of highest K₂O content. Biogenic carbonate and an upward increase in sandstone beds are interpreted as a highstand systems tract. Gradual lateral facies changes reflect the transition from foredeep (to the west) to forebulge (to the east) deposition in a foreland basin as the final stage of a shallowing-upward second-order sequence. Black organic-rich limestone has δ¹³C values between -8.78 and 9.79‰, negative δ¹⁸O values from -11.50 to -9.35‰, and ⁸⁷Sr/⁸⁶Sr around 0.7075. This isotopic signature, similar to that of other Bambuí carbonates, indicates a probably-restricted depositional environment with organic sedimentation, methanogenesis and high evaporation rates from the Ediacaran through the early Cambrian period.

KEYWORDS:
Glauconitic siltstone; phosphorite, foredeep/forebulge transition; C-O-Sr isotopic geochemistry

Type	Lithofacies	Description	Interpretated Depositional Environment
Siliciclastic facies	Green to yellowish sandstone	Lithofacies predominant in the east, in the region of Cedro do Abaeté (*Lima et al. 2007*Lima 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. 2007*Lima 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. 2007*Lima 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.
Volcaniclastic facies	Volcaniclastic rocks	Whitish to greenish, very friable, containing vesicular cavities and a porous matrix.

Sample		SR-02	SR-06	SR-07	SR-12
Height (m)	Unit	1003	1013	1014	1032
SiO₂	%	4.69	4.07	2.15	21.84
Al₂O₃	%	0.14	0.42	0.14	4.51
Fe₂O₃	%	0.31	0.44	0.23	1.66
CaO	%	50.07	51.9	53.4	36.24
MgO	%	0.39	0.34	0.53	1.22
MnO	%	< 0.01	< 0.01	0.01	0,03
TiO₂	%	< 0.01	0.03	0.01	0.26
P₂O₅	%	0.20	0.38	0.17	0.23
Na₂O	%	0.02	0.04	0.04	0.08
K₂O	%	0.06	0.14	0.07	1.51
LOI %	%	41.66	41.37	42.63	30.24
Ba	ppm	94.00	165.00	119.00	253.00
Ca	ppm	35.78	37.09	38.16	25.90
Co	ppm	0.60	1.30	0.50	5.20
Cs	ppm	0.07	0.21	0.11	1.62
Cu	ppm	28.00	35.00	45.00	31.00
Ga	ppm	0.20	0.60	0.20	6.00
Hf	ppm	0.19	0.45	< 0.05	1.85
Nb	ppm	1.72	0.54	0.07	5.10
Ni	ppm	11.00	11.00	7.00	20.00
Rb	ppm	< 0.2	3.10	< 0.2	50.40
Sn	ppm	1.30	< 0.3	< 0.3	0.70
Sr	ppm	2,155.00	3,357.00	2,051.00	2,340.00
Ta	ppm	0.06	< 0.05	< 0.05	0.23
Th	ppm	0.60	0.30	< 0.1	3.60
U	ppm	0.78	1.15	1.02	1.71
V	ppm	5.00	27.00	5.00	57.00
W	ppm	< 0.1	< 0.1	< 0.1	0.20
Zn	ppm	22.00	< 5	< 5	< 5
Zr	ppm	17.00	24.00	< 10	67.00
La	ppm	3.8	11.6	2.1	23.9
Ce	ppm	3.8	8	3.3	34.9
Pr	ppm	0.4	0.77	0.29	4.23
Nd	ppm	2	3.4	1.5	16.4
Sm	ppm	0.4	0.8	0.2	3.3
Eu	ppm	0.08	0.16	0.08	0.61
Gd	ppm	0.58	1.09	0.3	2.84
Tb	ppm	0.06	0.14	< 0.05	0.35
Dy	ppm	0.58	1.12	0.36	2.33
Y	ppm	4.4	7.63	2.64	12.37
Ho	ppm	0.11	0.19	0.06	0.47
Er	ppm	0.28	0.63	0.21	1.29
Tm	ppm	< 0.05	0.06	< 0.05	0.17
Yb	ppm	0.2	0.4	0.1	1.1
Lu	ppm	< 0.05	0.06	< 0.05	0.16
La/La*		3.72	4.61	3.04	1.33
Ce/Ce*		1.37	1.33	1.70	0.92
Eu/Eu*		0.92	0.85	2.27	0.85

Sample	SR-02	SR-06	SR-07	SR-12
Height (m)	1003	1013	1014	1032
δ¹³C	-8.75	9.26	9.79	9.22
δ¹⁸O	-11.5	-10.24	-9.94	-9.35

Sample	SR-02	SR-06	SR-12
Height (m)	1003	1013	1032
⁸⁷Sr/⁸⁶Sr	0.7076	0.7074	0.7077
SE (%)	0.0027	0.0030	0.0005

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[1] *Corresponding author.