In the northern portion of the Paraguay Belt, in Mato Grosso, the Cuiabá Group comprises approximately 5,300 meters of terrigenous deposits with a small volcaniclastic/chemical/organic contribution (Fig. 1). This work subdivided the Cuiabá Group into three formations: Campina de Pedra, Acorizal, and Coxipó (Tokashiki & Saes, 2008).
5.1. CAMPINA DE PEDRA FORMATION: FACIES DESCRIPTION
The Campina de Pedra Formation is exposed in the northern region of Vila Chumbo, encompassing the localities of the Campina de Pedra quilombola community and the core of the Bento Gomes Braquiantiform in the southwest of the area. The lower contact of this formation has not been directly identified, possibly occurring in a nonconformity overlying the basement of the Amazonian Craton (Fig. 1). In turn, the upper contact is characterized by an interdigitation with sandstones and conglomerates of the Acorizal Formation (Pindaival Member), which can be observed in the Serra da Descida do Buriti, located on the northwest flank of the Bento Gomes Antiform (Fig. 2). We estimate an approximate thickness of 1000 m for the entire extension of the Campina de Pedra Formation, as illustrated in Fig. 3. It comprises subunits 1 and 2 of Luz et al. (1980), Lower Unit of Alvarenga (1988), and the Campina de Pedra Formation of Tokashiki and Saes (2008).
This formation comprises a stratigraphic succession composed of five main lithofacies (Fig. 3), described from the base to the top, including massive pelites with flat and wavy lamination, with colors ranging from gray, yellow, and red (Pml) (Fig. 4A). This lithofacies underlies and interdigitates the next lithofacies: black pelites with rhythmic lamination (Pc), which alternate with frequent decimetric layers of massive sandstones or with gradational bedding (Fig. 4B).
These sandstones have grain sizes ranging from coarse to fine and are characterized by colors ranging from dark gray to black with blue quartz grains (Ac) (Beal, 2013) (Figs. 4C and 4D). B) Located approximately 5 kilometers north of Vila Chumbo and interposed between the Pc and Ac lithofacies, there is a lenticular body with a thickness of around 30 m of calcilutites and intraclastic calcarenites, laminated and with colors ranging from dark gray to black (Cc) (Fig. 4E).
At the top of the formation is observed an interdigitation between the Ac and Pc lithofacies with fine to coarse-grained sandstones, presenting a greenish-gray coloration that may be massive or laminated. Low-angle cross-stratifications are rare, and the grains are predominantly composed of red pelites, feldspar, and hyaline quartz (Al) (Fig. 4F) (Beal, 2013) (Fig. 4C).
5.2. CAMPINA DE PEDRA FORMATION: DEPOSITIONAL INTERPRETATION
The description of the lithofacies of the Campina de Pedra Formation is relevant to understanding the complex sedimentary architecture present in this geological sequence. These detailed observations are essential for future paleoenvironmental and interpretative analyses of the geological evolution of the study area similar to examples reported in several basins around the world.
The structure above and stratigraphic content led the authors to use several studies for comparisons and interpretations. Some of these studies are directly associated with the accumulation and preservation of high organic carbon content (> 2%) (e.g., Tucker (2003), indicative of deep, anoxic lacustrine environments controlled by rift tectonics, as exemplified in the lakes of East Africa (Talbot & Laerdal, 2000); (Scholz et al., 2003). On the other hand, other sedimentary models, such as those proposed by Kuchle et al. (2007) and Martins-Neto & Catuneanu (2010), describe the progradation of more proximal environments over previously established distal deposits, evident in thicker siliciclastic facies. From the bottom to the top, the basin would go through three stages of filling: The phase of rapid creation of space and little sediment supply (phase 1), the phase of balance between creation of space and sediments (phase 2), and the last as being the filling phase where the sediment load is greater than the volume of space created (phase 3).
It is important to emphasize that the distinction between lacustrine deposits, shallow marine, and epicontinental sediments in the Precambrian remains a controversial topic, with divergent opinions from some researchers (Hallam, 1981; Miall, 2010; Schieber, 1988). In this sense, other relevant studies, such as those conducted by Catuneanu et al. (2005) and Aunaud & Eyles (2006), were used in this work for comparison purposes, aiming the application of evolutionary models for Precambrian basins and providing a more informed approach to the evolution of rift systems. Based on the available data and correlations with information from other regions of the world, as well as on the interpretations of Freitas (2003), Tokashiki & Saes (2008), and Saes et al. (2008), it is possible to suggest the main depositional processes for the unit under study.
The basal pelitic facies are linked to periods of low sedimentation rate, decantation of suspended material (Agua Vermelha Member), and outcropping near the homonymous community. Moving up the stratigraphy, the carbonaceous sandstone facies with gradational bedding reach carbonaceous pelites that become common (Chumbo Member), interpreted as originating in hyperpycnal flows associated with floods in relatively deep waters (Tokashiki & Saes, 2008). The stratification of this lake is due to high subsidence rates that favor the deposition and preservation of black shales and represent the central, anoxic parts of the lake (Saes et al., 2008). The black limestones (Cc) associated with carbonaceous sediments were interpreted as the result of periods of climatic aridity and increased salinity of the water body (Freitas, 2003).
Finally, how the non-carbonaceous facies, which occur at the top of the stratigraphic sequence (Topo da Serra Member), are stratigraphically organized suggests a possible progradation, indicating increased sedimentation from the edge of the water body towards the center of the basin (Fig. 5). The interpretations presented here, although in agreement with previous studies (Freitas, 2003; Tokashiki & Saes, 2008; Saes et al., 2008), highlight the approach of this research that considers the Campina de Pedra Formation exclusively deposited in a rift environment, with sedimentary variations that range from shallow to deep lacustrine or marine environments, and again shallow due to the progradation of proximal facies over distal ones (filling stage in the model by Martins-Neto & Catuneanu, 2010).
5.3. ACORIZAL FORMATION: FACIES DESCRIPTION
The Acorizal Formation is exposed on both the Bento Gomes Antiform’s flanks and to the northeast, in the core of the Acorizal near the IX ECB (Engineering and Construction Battalion) quarry and Pindaival antiforms (Fig. 6). This unit comprises eight main lithofacies, which individually or in association constitute the three members of the formation (Pindaival, Engenho, and Cangas). Represented by the strata of subunits 3 and 4 of Luz et al. (1980), proximal facies of the Middle Turbidite Glaciogenetic Unit of Alvarenga (1988) and the lower and middle part of the Acorizal Formation of Tokashiki & Saes (2008). It rests on the Campina de Pedra Formation with a conformal contact. The Pindaival Member, named after the outcrops near the village of the same name, located approximately 35 km northeast of the city of Acorizal, is essentially composed of polymictic conglomerates (Cp), sandstones (Al) and massive and laminated pelites (Pml), organized in successive finning-upward cycles with thicknesses of up to 20 m (Fig. 7).
The conglomerates are clast-supported, massive, greenish-gray, with clasts ranging from 2 to 50 cm, and include granite, gneiss, feldspar, chert, and black and brown claystone. Sedimentary pebbles are more common at the base, while igneous and metamorphic pebbles predominate at the top of the unit (Figs. 8A and 8B). Sandstones (Al) are coarse to fine-grained, massive, or have plane-parallel lamination or wave marks (hummocks), with greenish/reddish gray colors and layers up to 10 m thick. The grain composition is equally distributed between hyaline quartz, lithic fragments, and feldspars. Pelitic rocks (Pml) are silt-clayey, massive or finely laminated, with green and red dropstones, and occur mainly near the member base. These facies are well exposed in the IX BEC quarry, north of the village of Campina de Pedra, and in the Serra da Descida do Buriti, where they come into interdigitated contact for about 300 m with the Campina de Pedra Formation. The estimated thickness for the Pindaival Member in the studied region is about 900 m (Beal, 2013). The Engenho Member is named after the village located 12 km north of the city of Acorizal, on the banks of the Cuiabá River, and was chosen by Almeida F. F. (1964) as the type area for the Engenho Formation/Jangada Group. It consists of massive purple-gray diamictites (T) with a sandy-clayey matrix containing pebbles, blocks, and boulders of gneisses, granites, and, subordinately, quartzites and milky quartz, poorly sorted. Rare lenses of intercalated sandstones are also observed (Fig. 7). Approximately 60% of the clasts are faceted and have drag striations (Fig. 8C). The thickness of this unit was estimated at 400 m in places where primary sedimentary structures can rarely be observed, mainly in the Jangada/Acorizal region, but reduced to about a dozen meters on the northwest flank of the Bento Gomes Antiform.
The interdigitation of the lithofacies of both units marks the contact between the Pindaival and Engenho members. This interdigitation forms a succession of strata where metric layers of purplish or pinkish gray diamictites (T) rest in abrupt, erosive contact on asymmetric, finning-upward, and coarsening-upward cycles of the Cp, Al, Pml facies or sandstones, rhythmites with dropstones (Rd) (Fig. 7). Centimetric layers of banded iron formations are frequent, intercalating with the pelitic facies and marking the middle part of the cycles, which have a thickness of around 15 m. This association, informally called Transition Layers (Figs. 8d and 8E), outcrops on the edges of the Pindaival and Acorizal Antiform and the northwest flank of the Bento Gomes Antiform, with an estimated thickness of around 400 m. It is also important to emphasize that with each new layer that cuts the top of the diamictite, centimeter-wide lenses of litho-feldspathic sandstones occur, which soon become polymictic conglomerates (Beal, 2013).
The Cangas Member of the Acorizal Formation occurs in a narrow strip, approximately 10 km wide, on the southeast flank of the Bento Gomes Antiform, extending from Poconé to Nossa Senhora do Livramento, passing through the village of the same name (Tokashiki & Saes, 2008). This member is characterized by the intercalation of massive diamictites (Dml), sandy-pelitic rhythmites (Rd) with dropstones (< 25 m) (Fig. 8F), massive polymictic conglomerates (Cp), massive and laminated pelites (Pml), banded iron formations (BIFs) and levels enriched in magnetite (magnetitites), which confer to the unit an important regional magnetic anomaly detected by Dall'oglio et al. (2008). The Cangas Member presents thicknesses of up to 1000 m. However, this thickness may be overestimated due to the intense deformation in recumbent folds that affect the entire package. The upper contact of this member is generally tectonic but presumably abrupt since no interfingering is observed between the Engenho and Cangas Members with the overlying units (Fig. 7).
5.4. ACORIZAL FORMATION: DEPOSITIONAL INTERPRETATION
The stratigraphic organization of the Acorizal Formation and its interrelations with the underlying unit and internal gradations were approached based on studies in several similar sedimentary basins worldwide. Among these studies, those that stand out are those that focused on rift basins with high-frequency cycles (Martins-Neto & Catuneanu, 2010), analyses of sedimentary cyclicity in rift basins after the tectonic pulse caused by the movement of rift-edge faults (Blair & Bilodeau, 1988), studies for field recognition of subglacial deposits (Dreimanis & Schlüchter, 1985), as well as tunnel mouth deposits or glacial deposits remobilized by subglacial meltwater (eskers), according to the model proposed by Powell & Molnia (1989). In addition, the interdigitation of facies resulting from deposition during glacial advance and retreat (LØnne, 1995) was also considered, as was the influence of glacial events in the distal parts of the basin, where the main deposition process involves the variation in the flow of meltwater currents that form layers of debris associated with glacial masses floating ice (Visser, Self-destructive collapse of the Permo-Carboniferous marine ice sheet in the Karoo Basin: evidence from the southern Karoo, 1991). This multidisciplinary approach allowed a better understanding of the Acorizal Formation not observed in the literature that addressed this stratigraphic unit and provided a more in-depth analysis of the stratigraphic and sedimentary relationships within the context of rift basins, contributing to the advancement of knowledge in this area of research. In this sense, the Acorizal Formation comprises basal facies that interdigitate with the underlying units, forming a coarsening-upward stacking. Freitas (2003) associated it with suspension processes and high-density subaqueous flows, resulting in high-frequency cycles and characterizing what is known as lacustrine fan-deltas (Freitas, 2003; Saes et al., 2008). Additionally, we interpret the facies as being similar to those described by Martins-Neto & Catuneanu (2010), with depositional pulses creating a coarsening-upward sequence with internal finning-upward cycles, where the basal facies are generally of the lacustrine environment (Campina de Pedra Formation) passing to deltaic and finally fluvial (Fig. 9). In the case of the Cuiabá Group, the upper transition of environment did not occur to fluvial deposits due to the already known and accepted glacial influence (Engenho Formation, Almeida (1964); Subunit 4, Luz et al. (1980); proximal facies of the Middle Turbidite Glaciogenetic Unit of Alvarenga (1988).
The sequence of facies described is in agreement with models of sedimentary basins where, after the period of maximum subsidence caused by the tectonic pulse originated by the movement of the rift edge faults, the stratigraphic sequence shows the advancement of the proximal facies over the distal ones (Blair & Bilodeau, 1988). In this case, the rapid subsidence is represented by the Campina de Pedra Formation, previously described, and the prograding facies are those present in the Pindaival Member. The diamictite facies (T) of the Engenho Member, interpreted here as lodgement tillite (Almeida, 1964 and 1965; Hennies, 1966; Luz et al., 1980), represent the maximum glacial stage of a sedimentary basin when subglacial deposits prograde over sediments from environments deposited in marine/lacustrine/lagoon environments (Martini & Brookfield, 1995; Assine, 1996; Brookfield & Martini, 1999) (Fig. 9). The distal facies, represented by the Cangas Member, with sedimentary features suggestive of depositional processes controlled by meltwater currents and their variations in flow, form debris deposits associated with icebergs (analogous to the Port Askaig Formation in Scotland (Arnaud & Eyles, 2006) and the Rapitan Group in the MacKenzie Mountains in Alaska (Narbone & Aitken, 1995) according to Tokashiki & Saes 2008). The contact between the Pindaival and Engenho members consists of the interdigitation of the respective facies (Fig. 5), interpreted as glacial sediments that were resedimented by subglacial meltwater or as subglacial tunnel mouth deposits, following the models proposed by Powell & Molnia (1989). These facies association results from deposition during repeated episodes of glacial advance and retreat, forming deposits of glacial contact deltas and subaqueous fans, similar to the models proposed by Lønne (1995) (Fig. 9).
5.5. COXIPÓ FORMATION: DESCRIPTION OF FACIES
The name Coxipó Formation is borrowed from the work of Guimarães & Almeida (1972), considered by these authors as superimposed on the undivided Cuiabá Group and accumulated in a periglacial environment. It consists of a thick (~ 2,200 m) sedimentary succession laid in abrupt contact with the Engenho diamictites and marked by significant facies variation (Fig. 10). The best exposures are distributed in the NE (Manso and Planalto da Serra) and SE (Cuiabá and Barão de Melgaço) regions of the studied area after the closure of the ample regional folds (Fig. 1). It corresponds to subunits 5 to 8 of Luz et al. (1980), the Middle Turbidite Glaciogenetic and Carbonate Unit of Alvarenga (1988), and the Coxipó Formation of Tokashiki & Saes (2008), in the present study, including the top unit of the Acorizal Formation of these latter authors (Pai Joaquim Member). In this context, three main facies associations are identified: Pai Joaquim, Marzagão, and Guia members and a unit informally named Mata-Mata (Fig. 6).
The Pai Joaquim Member, a basal unit, has its type area located in the homonymous village north of the community of Nossa Senhora da Guia and outcrops from the northeastern part of Cuiabá to the vicinity of Acorizal (Fig. 1). This unit is composed of orthoquartzitic conglomerates (Cm) with a sandy matrix, massive or laminated quartzarenites (Aml) with a yellowish-white coloration, and massive or laminated pelites (Pml) with a greenish-gray coloration (Fig. 10). The facies are organized in finning-upward cycles, with an average thickness of 10 m at the base, without pelites, reaching more than 50 m at the top, where the Aml and Pml facies predominate. At the base of the cycles, the conglomerates or sandstones rest on cut-and-fill structures and syn-sedimentary deformation structures (Figs. 11A and 11B). The approximate thickness for the Joaquim Parent Member is 1200 m.
The Marzagão Member (redefinition of the Marzagão Formation/Jangada Group proposed by Almeida in 1964) is named after the village located approximately 30 km north of the Rio Manso Hydroelectric Power Plant. This member rests concordantly on the pelites at the top of the Pai Joaquim Member and is composed mainly of massive and, more rarely, laminated diamictites (Dml) of greenish-gray to pink coloration with a sandy-pelitic matrix (> 70%), containing blocks and boulders (> 10 m) of silicified sandstones, greenish-gray pelites and subordinate granites/gneisses (Fig. 10). Intercalated with the diamictites are common metric layers of medium/coarse, massive, and laminated quartz arenites (Aml) of light gray color and thick layers of laminated pelites (Pml), pink, dark gray to green (Fig. 11C). A layer of purple diamictite (Dml) marks the top of the Marzagão Member, in contact with the overlying unit. The approximate thickness of the Marzagão Member is approximately 1000 m.
The Guia Member rests in transitional contact over the diamictites of the Marzagão Member and outcrops discontinuously, from the quarry 30 km north of Cuiabá (Fig. 12) to the vicinity of Planalto da Serra. This transition is marked by a metric layer of gray dolomitic limestones (Ca), a decametric layer of massive and laminated pink pelites (Pml), and a sequence of light gray marl (Ma), light to dark gray calcitic limestone (Ca), and black marl (Ma) (~ 8m). This member is primarily characterized by the presence of thick lenses of carbonate rocks (Ca) (Fig. 10), including calcilutites, calcarenites (oolitic, peloidal, and intraclastic), and calcirudites (breccias) (Fig. 11D) in light to dark gray, with planar-parallel and wavy lamination (Fig. 11E), and metric layers of dolomitic limestone (Ca) (Saes et al., 2008).
The carbonate rocks are intercalated or covered by calcitic marls and siliciclastic sediments: pink and green sandstones rhythmites in layers 4 to 10 m thick (R), thin, massive, and laminated greenish-gray and yellowish sandstones (Aml) and massive and laminated greenish-gray, yellow, red and pink pelites (Pml). The total estimated thickness of the Guia Member is 300 m near the district of the same name and 180 m in the Planalto da Serra region. In the southeastern portion of the studied area (Barão de Melgaço/São Vicente), the alignment of mountain ranges with a general direction of N50°E is notable, in which thick layers of fine to medium-grained yellowish-white quartzarenites (Aml) outcrop, with wave-truncated laminations (hummockys), intercalated with massive, greenish pelites (Pml) and, more rarely, metric layers of gray-green, massive or laminated diamictites (Dml) (Fig. 6). This set, still little studied, is informally called the Mata-Mata Unit.
5.6. COXIPÓ FORMATION: DEPOSITIONAL INTERPRETATION
The lithofaciological organization of the Coxipó Formation can be compared to studies in different geographic areas. Particular emphasis is given to the studies of El-Ghali (2005) on deposits that originated in proglacial meltwater fans, where high-energy currents from the meltwater flow were responsible for the deposition. Boulton et al. (1985) Boulton (1986 and 1990) and Martini (1990) also contributed models of proglacial fans, common on glacial margins in terrestrial and subaqueous environments, characterized by high sedimentation rates, evidenced by syn-depositional fluidization structures. In turn, Visser et al. (1987) and Visser (1997) addressed the retreat of the ice cover, which causes retrogradational stacking and base-level variation, resulting in the deposition of pelitic sediments from plumes of fine suspended material, either in interchannel areas or during times of reduced melt flow. Rust & Romanelli (1975), in a study in the Ottawa region, Canada, pointed out that the deposits present similarities with environments with fluvial characteristics but were recognized as having originated in a subaqueous environment, more precisely as subaqueous proglacial outwash fans. During post-glaciation, that is, the removal of the ice cover, several factors begin to influence deposition, as detailed by Vesely & Assine (2004) in their analysis of the Permocarboniferous section of the Paraná Basin, where remobilized and resedimented deposits resulting from gravitational flows or turbidity currents were identified, possibly associated with the uplift of the basin edges due to the adjustment of load after the removal of the ice cover (glacio-isostatic rebound).
Analyzing the facies stacking in the study area and correlating it with models presented by authors in other regions, the contact between the Coxipó Formation and the underlying formation is discordant. When reviewing the studies presented over time, it is possible to correlate them. For example, Alvarenga C. S. (1985) interpreted the Pai Joaquim Member as feeder fans of coarse turbidite facies, while Alvarenga (1988) described it as submarine turbidities fan deposits of channels and interchannels.
In this work, we interpret the Pai Joaquim Member as the result of high-energy currents in subaqueous proglacial outwash fan systems, whose rapid glacier retreat results in retrogradational stacking, reworking previously deposited sediments. This glacial retrogradation is responsible for a high sedimentation rate, presenting syn-depositional structures of fluidization (El-Ghali, 2005; Boulton, 1986, 1990; Martini, 1990; Visser et al., 1987; Visser, 1997) (Fig. 13). The configuration of the cycles of this member bears similarities to those described as liquefied flow (high-density turbidite currents) and low-density turbidite currents by Lowe (1982).
The Marzagão and Mata-Mata Unit Members are in contact with the Pai Joaquim Member. Alvarenga and Saes (1992) related the Marzagão Member to environments in the upper part of the slope (proximal facies), where diamictites were found intercalated with conglomerates and sandstones. Alvarenga & Trompette (1993) associated these facies with a deep marine environment dominated by gravitational flows without glacial influence, which deposited the laminated diamictites. Our interpretation of the Marzagão Member differs from previous studies. The end of high-energy currents promotes the decantation of plumes and the eventual fall of iceberg clasts, a process called "rain out," becoming more common, giving rise to massive to stratified diamictites. The Marzagão Member, in part, is composed of reworked deposits of the Pai Joaquim Member, evidenced by the frequent presence of olistrostromes of pelites and sandstones in the diamictites. In this case, we correlate the interpretation presented by Vesely & Assine (2004) in the Parana Basin, where retraction in the outwash fans is covered by distinct facies, representing the retreat of the glacier and the deposition of sediments by debris flows and turbidity currents, with remobilization and sedimentation processes being standard.
The intercalation of facies in the Marzagão Member (Fig. 10) records the slow deposition of fine material in suspension on the fringes and distal portions of the lobes. The removal of the ice cap generates crustal adjustment (glacial-isostatic rebound), remobilizing deposited material and contributing material carried by an iceberg, responsible for the presence of fine-grained facies with dropstones, which are intercalated in a rhythmic-sandy-pelitic manner. Regarding the Guia Member, Alvarenga (1988) correlates it with the Araras Formation, forming a single carbonate platform called the Medium Carbonate Unit. Here, the Guia Member and the Mata-Mata Unit constitute deposits with structures, composition, and maturity indicative of environments influenced by storm waves and currents, with the scarcity of terrigenous supply. This suggests accumulation in a shallow marine siliciclastic/carbonate depositional context on a tectonically stable platform with low intensity of current flow, without glacial influence, in a semiarid climate.
A fundamental point to understand the exclusion of the diamictites of the Marzagão Member from glaciation is the approach adopted in this work. The research seeks to revisit the stratigraphic data and reevaluate sedimentary patterns. Although previous works have sometimes considered the diamictites as subglacial, new evidence and methodologies allow for a new interpretation and the correct stacking of the formations and rock sequences, considering the tectonic contexts at each evolutionary stage. Observations of interbedded carbonate rocks suggest the influence of sedimentary processes other than glacial ones.