Late Proterozoic glaciation in central-eastern Brazil: Synthesis and model

Palaeogeography, Palaeoclimatology, Palaeoecology, 65 (1988): 1-21 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

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LATE PROTEROZOIC GLACIATION IN CENTRAL-EASTERN BRAZIL: SYNTHESIS AND MODEL JOACHIM KARFUNKEL 1 and ANDREAS HOPPE 2 lInstituto de Geoci~ncias da UFMG, C.P. 2608, 30.000 Belo Horizonte (Brazil) 2Geologisches Institut tier Universitdt, Albertstrasse 23 B, D-7800 Freiburg (F.R.G.)

(Received April 13, 1987; revised and accepted September 29, 1987)

Abstract Karfunkel, J. and Hoppe, A., 1988. Late Proterozoic glaciation in central-eastern Brazil: synthesis and model. Palaeogeogr., Palaeoclimatol., Palaeoecol., 65:1 21. In the surroundings of the Middle Proterozoic S~o Francisco Craton of central-eastern Brazil, numerous traces of an ancient glaciation can be found. Almost all criteria necessary to prove an ancient glaciation occur, including scratched and facetted pebbles, a striated surface, quarrying, outwash plains, eskers, varvites, dropstones, "d~collement" structures etc. Poorly sorted tillites with varying pebble composition are widely distributed. Continental tillites and glaciomarine sequences interfinger in the east and southeast of the craton. Traces of ice movement suggest flow directions of the ice towards the east and the southeast of the craton. At the coast, the wetbase glaciers passed over to floating shelf ice. In the northeast, glaciers also came from the Serrinha Craton in northeastern Bahia. The glaciation covered an area of at least 1300 x 700 km. The sequences containing traces of glaciation discordantly overlie Archaean to Middle Proterozoic rocks. They in turn are overlain by Late Proterozoic calcareous and psammopelitic sediments of the Bambui Group. Present data indicate that glaciation took place around 1000 Ma ago.

Introduction G l a c i a l s e d i m e n t s in c e n t r a l - e a s t e r n B r a z i l h a v e b e e n t h e focus of g e o l o g i c a l r e s e a r c h for a l o n g time. In 1870, H a r t t , a m e m b e r of t h e Agassiz expedition, already observed "glacial d r i f t " in t h e u p p e r c o u r s e of t h e A r a c u a i r i v e r in M i n a s G e r a i s . L a t e r , h o w e v e r , h i s d i s c o v e r ies w e r e r e c o g n i z e d as t a l u s f o r m a t i o n s . Disc o v e r i e s of p r e - Q u a t e r n a r y g l a c i a t i o n t r a c e s w e r e m e n t i o n e d in a n u m b e r of p a p e r s up to t h e l a t e s i x t i e s (Derby, 1888, 1905; B r a n n e r , 1919; De M o r a e s a n d G u i m a r a e s , 1930, 1931; De M o r a e s Rego, 1930; W i l l i a m s , 1930; De M o r a e s , 1937; L e o n a r d o s , 1940; M a a c k , 1957; K e g e l , 1959; M a c i e l , 1959; De A l m e i d a , 1964; De C a r v a l h o a n d Da C o s t a , 1968; I s o t t a et al., 1969). A g e d e t e r m i n a t i o n a n d c o r r e l a t i o n be0031-0182/88/$03.50

t w e e n t h e v a r i o u s o c c u r r e n c e s a l w a y s rem a i n e d u n c e r t a i n . In s p i t e of t h e s e u n c e r t a i n t i e s t h e a s s u m p t i o n of a L a t e P r e c a m b r i a n or Infracambrian glaciation took hold (Maack, 1957) a n d a t t a i n e d r e c o g n i t i o n in i n t e r n a t i o n a l l i t e r a t u r e as p a r t of t h e w o r l d w i d e ~'Infracamb r i a n g l a c i a t i o n " - m o d e l ( S c h w a r z b a c h , 1964; C a h e n , 1963; H a r l a n d , 1964a, b). T h i s a g e a s s i g n m e n t w a s r e v i s e d by Pflug a n d S c h S l l (1975). S t a r t i n g in t h e s e v e n t i e s , d e t a i l e d m a p p i n g a n d r e s e a r c h on t h e s e g l a c i a t i o n t r a c e s , n o w a c c e p t e d as b e i n g P r o t e r o z o i c , w e r e done. In eastern and southeastern Minas Gerais, most of t h i s w o r k w a s c a r r i e d o u t by c o - w o r k e r s of Pflug ( S c h m i d t , 1972; H e t t i c h , 1975, 1977; W a l d e , 1976, 1978; K a r f u n k e l a n d K a r f u n k e l , 1977) a n d by t h e U n i v e r s i t y of Belo H o r i z o n t e

© 1988 Elsevier Science Publishers B.V.

( H e t t i c h and K a r f u n k e l , 1978; K a r f u n k e l , 1978; K a r f u n k e l et al., 1983, 1984; K a r f u n k e l and Noce, 1983; Hoppe et al., 1984b). In w e s t e r n Minas Gerais and e a s t e r n Goi~s, r e s e a r c h was carried out by the U n i v e r s i t y of Brasilia (De F a r i a et al., 1970; D a r d e n n e et al., 1978a). In Bahia, glaciogenic sediments were described by SSfner (1973), Inda and B a r b o s a (1978), Misi (1979) and M o n t e s et al. (1985). A r e v i e w was given by R o c h a Campos and Hasui (1981a-d). This p a p e r summarizes the above o b s e r v a tions e n h a n c e d by new d a t a and, w h a t is new,

a t t e m p t s a synthesis of available d a t a as well as the p r e s e n t a t i o n of a L a t e P r o t e r o z o i c c o n t i n e n t a l g l a c i a t i o n model for centrale a s t e r n Brazil.

Geologic framework T h e t r a c e s of g l a c i a t i o n are located w i t h i n the A t l a n t i c Shield (Fig.l). A r c h a e a n units in this a r e a were stable by 2600 M a (De Almeida et al., 1981; De Almeida and Hasui, 1984; S c h o b b e n h a u s et al., 1984) and formed the S~o

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Francisco Craton. Mobile zones formed around the craton and were folded onto its margins during the Early to Middle Proterozoic. Many traces of glaciation are known to exist at the margins of this restabilized craton. The formations and groups from which the glacial sediments were described are shown in Fig.2. (1) The Bebedouro Formation in Bahia. This formation discordantly (Misi, 1979) or concordantly (Inda, 1979) rests on various units of the Chapada Diamantina Formation which is thought to be of Middle Proterozoic age.

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(2) The Macafibas Group in eastern Minas Gerais which rests concordantly or discordantly on the Early to Middle Proterozoic Espinha~o Supergroup (Pflug and Renger, 1973; Hettich, 1975; Karfunkel and Karfunkel, 1977; Dardenne and Walde, 1979; Hoppe et al., 1987a). (3) The S~o Jo~o del Rei Group in southern Minas Gerais resting on Archaean basement (Ebert, 1957; Karfunkel and Noce, 1983) and rocks of a greenstone belt (Hoppe et al., 1984a). (4) The Ibi~ Formation in western Minas Gerais and eastern Goi~s which rests discordantly on Archaean rocks and Proterozoic units (Dardenne et al., 1978a,b; Dardenne and Walde, 1979). The youngest underlying unit is said to be the Parano~ Group with an attributed Middle to Late Proterozoic age (Dardenne, 1982; Campos Neto, 1984). The units containing glacial sediments are overlain by rocks of the Bambui and upper Macafibas Groups (eastern Minas Gerais) or are interpreted as the basal unit of the Bambui Group (Bahia and western Minas Gerais, eastern Goi~s); in southeastern Minas Gerais overlying rocks are absent. Lateral and vertical interfingering of facies between the Macafibas sediments and basal units of the Bambul Group have been described from eastern Minas Gerais (Campbell et al., 1966; Pflug, 1967; Schmidt, 1972; SchSll, 1972; Hettich, 1977).

Bebedouro Formation

Mixtites (see Schermerhorn, 1966) of the Bebedouro Formation in Bahia (Fig.2) cover an area greater than 60,000 km 2. They intermittently crop out along N - S trending synclines (Montes et al., 1985). The Bebedouro Formation discordantly overlies the Middle Proterozoic Chapada Diamantina Group and is in turn overlain discordantly by the Salitre Formation which consists mainly of carbonates (Montes e t al., 1985). Both the Bebedouro and Salitre Formation are ascribed to either the Bambui Group (Misi, 1979) or the Una Group (Inda, 1979).

The Bebedouro Formation is between 1 and 16 m (Montes et al., 1985) or up to 70 m thick (Rocha Campos and Hasui, 1981d). Greater thicknesses are often found in erosion channels which cut into the underlying Chapada Diamantina Group. The frequency of gravelsize clasts increases upward within the predominantly silty formation, thus giving the rock a tillite-like appearance (Montes et al., 1985). The diameters of the clasts within the mixtites and siltstones vary from 1 to 50 cm. The clasts mostly consist of granite-, gneiss-, and quartzite fragments. Schist-, phyllite-, pegmatite- and carbonate-clasts occur less frequently, basic igneous rocks are very rare. The clasts are angular to subrounded (Fig.3), some are facetted and slightly scratched. Slightly scratched and polished bedrock surfaces have also been described (Rocha Campos and Hasui, 1981d; Montes et al., 1985). SSfner (1973) described large clasts occurring in varve-like sediments at the base of the formation. Near Raqado at Jacobina, also at the base of the formation, Montes et al. (1985) discovered gneiss dropstones larger than 50 cm in diameter in thinly laminated silty argillites.

Chattermark trails on the surface of garnet (Montes et al., 1985) also support the assumption of a glaciation (Ellis, 1962; Folk, 1975; Gravenor, 1982). SSfner (1973) already assumed an eastern source area for the Bebedouro Formation. Statistics show a decrease in clast diameters from northeast to southwest, thus indicating a source area near the Serra de Jacobina (Montes et al., 1985).

Macadbas Group The name Macafibas was introduced by De Moraes and Guimaraes (1930) to denote mixtites with facetted clasts at the margins of the southern Serra do Espinhaqo. They interpreted the mixtites as tillites and correlated them with conglomerates of the Espinha~o Zone near Diamantina, naming the latter Sopa conglomerates. Pflug (1965, 1967) demonstrated that the Sopa conglomerates belong to the Early to Middle Proterozoic Minas Supergroup (now known as Espinhaqo Supergroup) and show no traces of glaciation (Pflug and SchSll, 1975). Moreover, the Espinha~o Supergroup is

Fig.3. Typical mixtite from the Bebedouro Formation. BR 242, 34 km west of the bifurcation to Len~ois (Foto by A. H.).

often discordantly overlain by the Macaflbas Group (Pflug, 1965; Isotta et al., 1969; Gorlt, 1972; Pflug and Renger, 1973; Hettich, 1975; Viveiros and Walde, 1976; Karfunkel and Karfunkel, 1977; Walde et al., 1978). Mixtites are widely distributed within the Macafibas Group (Fig.4). They have been found near Salinas in the north (north of Fig.5; Cobra, 1969; Karfunkel and Karfunkel, 1976; Karfunkel, 1978; Viveiros et al., 1979) and as far as the Serra do Cip6 in the south (Belezkij, 1959; Von Sperber, 1977; Dossin, 1983; Dossin and Dardenne, 1984). In the area of Itacambira-Terra Branca, Karfunkel and Karfunkel (1977) divided the M a c a f b a s Group into (1) a pre-tillitic Califorme Formation (named after a place 30 km northeast of Terra Branca) consisting of quartzites and quartzitic conglomerates between 0 and 200 m thickness which are overlain by (2) tillites, 30-350 m thick, and associated rocks of the Terra Branca Formation, those in turn being overlain by (3) the Carbonita Formation (Fig.4). Within the Terra Branca Formation, Karfunkel and Karfunkel (1977) illustrated a lateral facies change beginning with terrestrial tillites (Jequitai facies) in the west, followed by glaciomarine (Caqaratiba facies; named after a village 15 km northeast of Terra Branca) and finally marine sediments (Turmalina facies) in the east. In the adjoining southern area of Carbonita, Itamarandiba and Couto Magalh~es de Minas, the M a c a f b a s Group reaches almost 1000 m of thickness (Hettich, 1975). Glaciation traces, above all mixtites, can be found there within Unit B, which reaches thicknesses of 200m (Fig.4). The thickness of the tillites decreases towards the west to 2-50m in the Serra do Cabral area (Walde, 1976). The mixtites (Fig.5) contain clasts of gneiss, granite, quartzite, quartz, metapelite, carbonate and rarely of basic volcanic rock. Clast diameters may reach 1.5 m. In a few cases, carbonates underly the glaciogenic sediments. Sch611 (1976) and Sch611 and Foga~a (1980) described such stromatolite-bearing carbonates from Mendanha and Conselheiro Mata.

Mixtites up to 60 ~ in thickness crop out at the Parafina dam. A decrease of sedimentclasts can be observed there from base to top, while the frequency of crystalline clasts increases towards the top, thus indicating a progressive erosion in the source area of the mixtites (Karfunkel et al., 1984). Besides typical mixtites with a wide range in composition, clast- and grain-size, and various degrees of roundness and sphericity, a large number of other indications for a glaciation period have been identified. Scratched, grooved and polished bedrock surfaces underlying mixtites can be found 10km southeast of Jequitai. The lineations "vary from very fine scratches that have produced a polished effect on the pavement to grooves up to 20 cm wide and 5 cm deep"; most are V-shaped, some are U-shaped grooves (Isotta et al., 1969). Such rock surfaces are also known from the eastern border of the Serra do Cabral, south of Buen6polis (Walde, 1976) and from the vicinity of the Parafina dam where in addition crescentic rock fractures have been observed (Karfunkel et al., 1984). Some mixtite clasts are scratched and facetted (De Moraes and Guimaraes, 1930, 1931; Isotta et al., 1969; Walde, 1976; Hettich and Karfunkel, 1978; Walde et al., 1978; Karfunkel et al., 1984; cf. Fig.6). North of Jequitai and at the Parafina dam, lenticular sand bodies (quartzites) interpreted as eskers are intercalated in mixtites (Hettich, 1977; Hettich and Karfunkel, 1978; Karfunkel et al., 1984). North of Jequitai, they are crossbedded and show an extension of 5 x 20 m in a road cut. Gravenor and Monteiro (1983) interpreted these quartzites as "rafts of bedrock which were sheared up into the glacier", the orientation of quartzite blocks and shear-zones indicating an ice-transport towards the north and northeast. In the area of the P a r a 6 n a dam, however, two such sandbodies with diameters of 2 x 7 m are linked by a thin sand layer (quartzites, Fig.7), thus excluding the interpretation as rafting of bedrock or as a boulder within a tillite. Lenticular rhythmites containing clasts up

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Fig.5. Typical mixtites of the Maca~bas Group from the vicinity of the Para~na dam with carbonate, gneiss, quartzite and quartz rock fragments (Foto by J. K.).

to 5 cm in diameter are intercalated in mixtites at the Parat]na dam (Fig.8). The rhythmites are interpreted as varvites, the clasts as dropstones (Karfunkel et al., 1984). Varvites have also been described from Joaquim Fellcio (Hettich, 1977; Hettich and Karfunkel, 1978). Intercalations of sandy material interpreted as outwash sediments (Karfunkel and Karfunkel, 1976, 1977; Karfunkel et al., 1984) are widely distributed within the mixtites. Such sand layers have been described from mixtites near Jequitai (Isotta et al., 1969) and from the Bocaifiva area (Schmidt, 1972) but were not further explained. Within the sequences of the Terra Branca region sandy intercalations are frequent; they have thicknesses from several centimeters to some tens of meters and an areal distribution between some square meters and some tens of square kilometers. Restricted occurrence and lack of different grainsize fractions characterize the intercalations (Karfunkel and Karfunkel, 1977). Pebbles are frequent and better rounded than those within the mixtites.

The data available permit a reconstruction of the ice movement as being from west to east. This is indicated by W - E directed scratches and grooves on rocks underlying the mixtites as well as by crescentic rock fractures. F u r t h e r evidence is given by lateral facies alteration. Unlayered mixtites in the west pass over to layered mixtites and finally to marine sediments containing dropstones further to the east. The ancient shoreline ran east of Gr~o Mogol (north of Fig.4) to the south across Terra Branca, the Parafina dam and the Serra do Cip6 (Fig.4).

S~o Jo~o del Rei Group In 1940, Leonardos already interpreted a mixtite 5 km north of Carandal as a tillite, though not discussing criteria for glaciogenic origin. Ebert (1958) described the same rock as a pebbly greywacke without excluding a glaciogenic origin, and in 1966, Guimaraes interpreted it as a volcanic breccia. During detailed mapping of the C a r a n d a i -

Fig.6. Striated and facetted glacial pebbles from a mixtite 4 km northeast of Jequital (Macafibas Group; Foto by A. H.).

Fig.7. Lenticular sand-bodies in mixtites underlying quartzites (Macafibas Group) in the vicinity of the Parafina clam. The sand lenses are linked by a t h i n sand layer and are interpreted as eskers (Foto by J. K.).

Fig.8. Dropstone within varvite (Macafbas Group) near the Parahna dam (Foto by J. K.).

S~o Jo~o del Rei area (Karfunkel and Noce, 1983) these occurrences were reinterpreted (Karfunkel et al., 1983; Hoppe et al., 1984b). The S~o Jo~o del Rei Group can now be divided (like the Macafibas Group) into a lower unit (Tiradentes Formation) with shallow water conglomerates, sands and siltites (850m of thickness), a middle unit with glaciogenic sediments (70 m of thickness) and a marine, upper psammopelitic unit with intercalations of limestone (Rio Elvas Formation, > 1000 m of thickness; Fig.9). The middle unit is called the Carandai Formation and can be divided into sub-units: mixtites up to 70 m of thickness form the lower part; in the upper part laminated sediments are predominant. At the type locality, the Carandai Formation consists of mixtites up to 50 m thick containing clasts up to 1.5 m in diameter. The majority of the clasts is granitic and gneissic, some components are quartzites, schists and phyllites. Less abundant are carbonates, basic volcanic rocks and pegmatites. Some clasts show flat-iron forms.

At S~o Jo~o del Rei (Vila de Santa Cruz), the mixtite is separated from the underlying Tiradentes Formation by an unconformity. It is a step-like unconformity (Fig.10) strongly resembling the quarrying out of a glacier after it has passed a riegel. Similar erosion forms have been described by Walde et al. (1978) from the M a c a f b a s Group near the Serra do Cabral. At Vila de Santa Cruz, pelitic intercalations underlying the mixtites are deformed (Fig.ll). This deformation is attributed to the push of the glacier causing deformation of the pelitic rocks due to their high plasticity (cf. Boulton, 1986); these structures can be interpreted as d6collement structures showing the direction of ice transport. Dropstones can be observed at Vila de Santa Cruz and at the national road BR 383 north of S~o Jo~o del Rei at the bifurcation to Agua Santa. Well-layered pelites with a dropstonerich horizon (1.5 m of thickness) can be traced laterally over a distance of 15 m (Fig.12). Most dropstones are granite and gneiss fragments, some of them with their long axis almost

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vertical relative to bedding (Hoppe et al., 1984b; Karfunkel et al., 1983). In southeastern Minas Gerais, the d6collement structures as well as a lateral facies alteration from glacioterrestrial sediments in the north to glaciomarine sediments in the south indicate ice movement from north to south. Ibid Formation

The mixtites in western Minas Gerais/eastern Goihs State are part of the sedimentary sequence of the Brasilia Fold Belt (De Almeida, 1968). They occur discontinuously along an arcuate belt of 850km in length (Fig.13). A large number of local names and different stratigraphic assignments impeded a synopsis of these occurrences for a long time. The glaciogenic rocks at the western edge of the Silo Francisco Craton have been reinvestigated and reinterpreted by Dardenne et al. (1978a). These authors considered them as the

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basal sequence of the mainly carbonaceous Bambul Group, compared them with the glaciogenic rocks from Jequital in eastern Minas Gerais and defined them as Jequitai/Ibih Formation. As the city of Jequital is situated in eastern Minas Gerais (within the area of the Macafibas Group) we here prefer the name "Ibih Formation" in order to avoid confusion.

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The mixtites discordantly overlie Archaean and Proterozoic rocks (Arai, Canastra and Arax~ Groups). The quartzitic Parano~ Group containing dolomite lenses with stromatolites is thought to be the youngest unit underlying the glaciogenic rocks. The Parano~ Group is assigned a Middle to Late Proterozoic age (Dardenne et al., 1978a, b; Dardenne and Walde, 1979; Dardenne, 1982; Campos Neto, 1984). Most of the Ibi~ Formation is composed of mixtites, meta-siltites and argillites; sandstones occur in subordinate quantities. The

thicknesses vary from some meters up to 150 m at Cristalina (De Carvalho and Da Costa, 1968; Barbosa et al., 1970; De Faria et al., 1970; Dardenne et al., 1978a; Rocha Campos and Hasui, 1981c; De Faria, 1985). The rock fragments within the mixtites reach diameters up to 2 m and are irregularly distributed within the matrix. Grades of roundness vary from well rounded to angular. Most of the mixtite clasts are quartzites and quartz stones; granite, gneiss, limestone, dolomite, chert, schist, phyllite and hematite-phyllite-stones are subordinate. Volcanic rocks are rare. The matrix of the mixtites is clayey, silty, sandy or locally arkosic; the feldspars are almost fresh (Dardenne et al., 1978a). Striated and facetted glacial pebbles have been described from Cristalina (De Carvalho and Da Costa, 1968; De Faria, 1985) and Nova Roma (Dardenne et al., 1978a); gouges on mixtite clasts in the area of Lagamar have been found by Dardenne et al. (1978a). Intercalations of rhythmites (varvites) containing oversized clasts (dropstones?) occur locally (De Carvalho and Da Costa, 1968; De Faria, 1985).

Fig.12. Dropstones (granites) in pelitic rocks of the S~o Jo~o del Rei Group west of Aguas Santas (Foto by A. H.).

12

lil Fig.13. The Ibi~ Formation (in black) in western Minas Gerais/eastern Goi~s State (modified after Schobbenhaus et al. 1984); older areas are unmarked, younger ones are marked with lines. Stratigraphic columns after Dardenne et al. (1978a).

Age The glaciation of central-eastern Brazil is younger than the Early to Middle Proterozoic stabilization of the S~o Francisco Craton and older than the overlying Bambul Group, or more or less contemporaneous with the basal sequences of the Bambui Group. To obtain an age determination, stromatolites from the Bambui Group have been investigated and compared with the Russian stromatolite stratigraphy. Thus, Cloud and Dardenne (1973) assigned Conophyton metula-bioherms in the area of Vazante and near Lagamar (Dar-

denne and Campos Neto, 1976) to the Bambui Group and gave the stromatolites an age between 1350 and 950 Ma. Recently, however, it has been doubted that the stromatolites from Vazante belong to the Bambui Group (Dardenne and Walde, 1979; Parenti Couto and Bez, 1981). Dardenne and Walde (1979) consider a parallelization with occurrences of Conophyton that have been found in dolomites of the Maca6bas Group at Mendanha and at Conselheiro Mata (SchSll, 1976; SchSll and Fogaqa, 1980; see Fig.4) and which were estimated to be between 1600 and 850 Ma old (SchSll and Fogaqa, 1980). The stromatolites described by SchSll (1976) are from beneath the tillites. According to Campos Neto (1984), the stromatolites from Vazante are part of the upper Parano~ Group which is thought to be of Middle Riphean age. Stromatolites of the Gymnosolenida Supergroup at Sete Lagoas (Fig.4) are undoubtedly part of the Bambui Group; they have been dated by Marchese (1974) at 950-650 Ma. Nevertheless, doubts concerning the reliability of stromatolite stratigraphy must be permitted when considering the fact that only stromatolite texture is used for dating without knowing the microfossils proper (in the majority of cases). Additionally, secondary changes of different degree during diagenesis are to be expected (cf. Hofmann and Schopf, 1983). Conophyton for example is already known from rocks more than 1 billion years older (Late Archaean and Early Proterozoic) so that we are tempted to agree with Walter (1983, p. 213): "much remains to be learned before questions such as those relating to their [the stromatolite's] potential biostratigrahic usefulness (if any) can be fruitfully examined". Besides paleontologic methods, attempts to obtain reliable ages for the Late Proterozoic rocks of central-eastern Brazil have been made by using radiometry. But Rb/Sr-, K/Ar- and Pb/Pb-methods applied for this purpose are complicated by metamorphic overprinting of the area in the latest Precambrian (Brasiliano Cycle). In addition, deep tropical weathering restrains the recovering of fresh rock samples

13 in many places, and rocks suitable for radiometric dating (magmatic rocks) are often lacking. Therefore, mostly sediments of the Bambui Group have been dated during the last years using the Rb/Sr-method (whole rock and fine fractions). The data obtained range between 890 and 450 Ma (Amaral and Kawashita, 1967; Bonhomme, 1976; Parenti Couto et al., 1981; Clauer, 1982), without excluding the possibility that primary ages have been altered by later metamorphic overprint (Bonhomme et al., 1982). Besang et al. (1977) dated granites from Monte Carmelo-Ibi~ (Fig.13). Intrusive contacts of these granites show that they are partly younger than the Ibi~ Formation. A whole-rock isochrone of 733 Ma (6 whole-rock analyses, STSr/S6Sri=0.7066) has been interpreted as the minimum age of the Ibi~ Formation. Macedo and Bonhomme (1981) have tried to determine the age of the Bebedouro Formation in Bahia more precisely. They dated the early diagenesis of the Bebedouro Formation at Olho d'Agua at 960__+31 Ma (STSr/S6Sri=0.723; fine fractions). A later diagenesis has been dated by K/Ar and Rb/Sr at 896 + 30 Ma (STSr/S6Sri=0.731). Samples from the Bebedouro Formation of Tabua (Fig.3) gave an age of 812_+22Ma (STSr/S6Sri=0.713) which has also been assigned to a late diagenesis (Bonhomme et al., 1982). Further samples from Caatinga de Moura (Macedo and Bonhomme, 1984) gave Rb/Sr-isochrons of 932__+30 Ma (STSr/S6Sri=0.7397) and 911+27 Ma (STSr/S6Sri = 0.7362), also assigned to a late diagenesis. K/Ar-fine fraction ages (6 samples) were between 901 + 21 and 876 + 20 Ma. Field observations and available data point to a Late Proterozoic age of the glaciation: the glaciogenic units postdate the Middle Proterozoic stabilization of the S~o Francisco Craton, estimated to have taken place around 1000 Ma or earlier (cf. De Almeida and Hasui, 1984; Macedo and Bonhomme, 1984; Schobbenhaus et al., 1984), and they are older than 950 Ma as metamorphic overprinting may have obscured the primary age. At the moment, the most

reasonable age for the glaciogenic rocks in the surroundings of the Late Proterozoic S~o Francisco Craton seems to be approximately 1000 Ma. This would correspond to the oldest period of Late Proterozoic glaciations, the Infracambrian II (cf. Windley, 1984, p. 124). Further Precambrian glaciogenic rocks are known from other areas in Brazil as well (cf. Rocha Campos and Hasui, 1981a, b). The Salobo Formation of eastern Brazil (southern Bahia State) has been interpreted as part of the molasse basin of the Aracuai Fold Belt (Rocha Campos and Hasui, 1981a) and is therefore probably younger than the glaciogenic rocks described above, the Aracual Fold Belt being contemporaneous with the Macafibas Group (De Almeida, 1977). From the western parts of Mato Grosso State, glaciogenic rocks have been described within the J a n g a d a Group and the Puga Formation (Rocha Campos and Hasui, 1981b). The Jacadigo Group in the area of Corumb~, noted for rich deposits of banded iron-manganese formation contains mixtites and dropstones (Walde, 1981); the available age determinations vary between 950 and 850 Ma (cf. Hoppe et al., 1987b; Walde, 1987). The Jacadigo Group is overlain by the Corumb~ Group which yielded the first Late Precambrian megafossils of South America (Hahn et al., 1982; Walde et al., 1982; Walde, 1987) attesting to an Ediacarian/Vendian age (Cloud and Glaessner, 1982; Sokolov and Fedonkin, 1984) of this group. Discussion

Since the term "tillite" for fossil moraines was introduced by Penck in 1906 it has been applied in different ways. Some authors restricted the term to consolidated deposits of terrestrial glaciers (Pettijohn, 1957; Flint, 1961, 1971); others used it for terrestrial and marine glaciogenic sediments (Crowell, 1964; Harland, 1964a). In this paper the term "tillite" is applied to consolidated terrestrial and/or marine sediments with a high proportion of glaciogenic components. Different environments are separated by terms like glacioterres-

14 trial, glaciomarine, glaciolacustrine, glaciofluvial. Glaciogenic and non-glaciogenic r o c k s m a y be v e r y similar in a p p e a r a n c e . This led Scherm e r h o r n (1966) to propose the n e u t r a l t e r m " m i x t i t e " for coarse-grained, poorly sorted sediments. The analysis of evidence for preP l e i s t o c e n e g l a c i a t i o n is often complicated by v e r y limited o u t c r o p conditions. T h e r e f o r e , m a n y c r i t e r i a h a v e been discussed which should be fulfilled before a glacial period can be p o s t u l a t e d with good c o n s c i e n c e (cf. Charlesworth, 1957; Flint, 1961, 1971; K r i n s l e y and T a k a h a s h i , 1962; Cailleux, 1964; C h u m a k o v , 1964, 1973; Crowell, 1964, 1983; Ovenshine, 1964, 1970; Schwarzbach, 1964, 1974; H a r l a n d et al., 1966; Lindsey, 1966; S c h e r m e r h o r n , 1966, 1974; L a n d i m and Frakes, 1968; F r a k e s and Crowell, 1969; K r 6 n e r and R a n k a m a , 1973; Hettich, 1975; Walde, 1976; K a r f u n k e l and K a r f u n k e l , 1977; Frakes, 1979; B o u l t o n and Deynoux, 1981; G r a v e n o r et al., 1984). To us it seems i m p o r t a n t to d e m o n s t r a t e m a n y of those c r i t e r i a w i t h i n a closely definable geological interval. The evidence of a c o n t i n e n t a l g l a c i a t i o n r e q u i r e s in addition a wide areal d i s t r i b u t i o n of g l a c i a t i o n t r a c e s as well as lateral and v e r t i c a l i n t e r f i n g e r i n g of different facies types (cf. H a r l a n d et al.,

1966). These c r i t e r i a are fulfilled in the case of the o c c u r r e n c e s described above (see also Table I). V o l c a n i c r o c k s are not associated with the glaciogenic rocks. T h e greenschists included in the Macaflbas G r o u p overlie the glaciogenic u n i t (Fig.4, u n i t E). T h e lack of volcanic r o c k s corresponds to the o b s e r v a t i o n of Schermerh o r n (1983), who found a general s c a r c i t y of volcanism along with extensive c a r b o n a t e deposition (indicating w a r m depositional environment) d u r i n g the L a t e Proterozoic. Scherm e r h o r n (1983) tried to solve this p a r a d o x (evidence for widespread glaciations and extensive c a r b o n a t e platforms) by suggesting an a t m o s p h e r i c CO 2 depletion due to the s c a r c i t y of volcanism which could h a v e led to a t h i n n e r a t m o s p h e r e with a steeper t e m p e r a t u r e gradient and a lower snowline. T h e wide areal d i s t r i b u t i o n of glaciogenic r o c k s in centrale a s t e r n Brazil and the lateral interfingering of facies from g l a c i o t e r r e s t r i a l facies in the c e n t e r of the c r a t o n to g l a c i o m a r i n e r o c k s at its margins as well as the flow of ice t o w a r d s the sea precludes the i n t e r p r e t a t i o n of a g l a c i a t i o n affecting only m o u n t a i n ranges, as suggested by S c h e r m e r h o r n (1983). Our k n o w l e d g e of this e n o r m o u s a r e a is still f r a g m e n t a r y . However, the glaciogenic sedi-

TABLE I Criteria for a glaciation in Late Proterozoic sequences of central-eastern Brazil (cf. Fig.I) proved until today. See text for further explanation

Mixtites Striated and grooved bedrock with crescentic fractures Glacial pebbles, striated facetted fiat iron forms Plucking/quarrying Outwash plains Eskers Varvites Dropstones D6collement structures Chattermark trails on garnet

Bebedouro Formation

Macaflbas Group

S~o Jo~o del Rei Group

Ibi~ Formation

x ?x

x x x x x x x x x x x

x

X

x x x x

x x x

?x x x

x x x

x x

?x x

15 ments in the surroundings of the Late Proterozoic S~o Francisco Craton show very good correspondence in their stratigraphic settings and their lithological successions. We therefore believe that the mixtites of central-eastern Brazil must be interpreted as tillites and that these mixtites have a key position concerning the stratigraphic correlation of Late Proterozoic sediments in central-eastern Brazil. Conclusions and model

The majority of investigators studying the tillites of central-eastern Brazil were engaged in the genetic interpretation of the different rock types and only a few of them worked on lateral and vertical interfingering of facies and reconstruction of paleo-environments (e.g. Hettich, 1975; Walde, 1976; Karfunkel and Karfunkel, 1977; Karfunkel et al., 1984; Montes et al., 1985). Based on the different characteristics of the tillites, four different depositional environments can be reconstructed: (1) A glacioterrestrial environment, characterized by tillites (sensu strictu) without bedding and heterogeneous granularity, as well as by associated features like striated and grooved bedrock surfaces, crescentic rock fractures and gouges. (2) A glaciolacustrine environment with varvites of restricted areal distribution within the tillites and the occasional occurrence of dropstones. (3) A glaciofluvial environment represented by quartzites and metasiltites associated with the tillites. Eskers in the surroundings of the Paraflna dam and near Jequital and outwash sediments in the region of Terra Branca and near the Paraflna dam (Macaflbas Group) belong to this facies. (4) A glaciomarine environment with laminated sediments of wide areal distribution occasionally containing dropstones. Within the Macaflbas Group, a subdivision can be made into (1) a proximal facies with tillites of carbonate-free detritus and (2) a distal facies

with laminated tillites and some carbonate (Karfunkel and Karfunkel, 1977). Precise fixation of the facies boundaries is complicated by limited outcrop conditions, but within the Macaflbas Group (Fig.4) a relatively precise delimitation is possible. There, the direction of ice transport can be deduced from W - E directed, striated and grooved bedrock, from crescentic rock fractures pointing to the east, and from a lateral interfingering of facies changing from glacioterrestrial rocks in the west to glaciomarine sediments in the east. The boundary between glacioterrestrial and glaciomarine facies is in some places marked by subaquatic slumps. The coastline passed east of Itacambira and the Paraflna dam to the Serra do Cip6 in the south (cfi Fig.4), running more or less parallel to the eastern border of the Silo Francisco Craton sensu De Almeida (1977). In addition, a division of the Macaflbas Group into three approximately N-S extending zones is possible (Fig.14). They are from west to east (1) glaciers in contact with the continent (terrestrial zone), (2) an area with moraines deposited within a marine environment (grounded shelf-ice zone) and (3) an area where ice masses floated on the shelf sea (floating shelf-ice zone; cf. Carey and Ahmad, 1961; Reading and Walker, 1966; Powell, 1984; Edwards, 1986). For the S~o Francisco Supergroup (Macaflbas and Bambui Groups) at the eastern margin of the Late Proterozoic Silo Francisco Craton the following evolution stages can be separated (Fig.15): (A) Preglacial stage: in small epicontinental basins carbonates were deposited. Contemporaneously, at the border of the craton, sand, silt and conglomerates were deposited in a shallow sea; in some places carbonates formed here as well. (B) Glacial stage: the craton was uplifted. The climatic conditions changed. The continental highlands were deeply eroded during a continental glaciation and moraines as well as glaciomarine sediments were deposited at the border of the craton.

16

W

E

:'"~

basal till

W

'-

/ ~ I " .. " ~

englacial .~_L_=_

~

./\e+ +. ~1~ :

,

;'eetl'

"=,'= , ~

subaqueous sediment gravity fl°w (slumping)

melt water

I

°u'ra+ terrestrial zone

]grounded shelf-ice

I floating shelf-ice

zone

t_

+ J.,, . . . . .

zone

Fig.14. Situation at the eastern edge of the continent during the glaciation period (schematically). Above: the ice-covered craton, below: depositional model (wet base glacier) for a regressive stage.

W

E

--=__ +

,

+

•

+ +

"

, ~

,

+ + +.

"sheet"

,

~

,

+

,,

: : " . . . . . ~

...

~

,

*

*

~

•

.

.

I I

i

÷

t ~

+ "¢C:/~---~-~ +

t

~

+

) I

I

I r

~-,.--~=._r~/, ÷

4-

~

+

.

,

"

.

V

•

~

.o~'-~.-:-

~..?. :

i

_

- = ~ ~ - - ~ .

*

t

_

y'+

L , A f ~ : L . ' : , " . \ \ ~ .

+

v " •

A'~--"~-:.÷

*

-

:A:&:&:

°

.

•

"

v - . _ - . - . - _ .. • ". ~ . -V

-vv

-

-,

. v

:--:A. - A

J K / A H 198S ~

÷

Fig.15. Paleogeographic evolution (schematically, not to scale) of the eastern parts of the Silo Francisco Craton during the Late Proterozoic (Macafibas and Bambul Groups). See text for further explanation.

17

(C) Further development (traces of a postglacial isostatic uplift have not been observed yet): Epicontinental basins subsided and thick layers of silts and carbonates filled a basin of wide areal extension in the area of the former higher continental area, while at the border of the craton mainly psammopelitic sediments were deposited. These psammopelitic sediments contain a few volcanic rocks (Macaflbas greenschists) and, at the top of the sequence, even less carbonates (S~o Jo~o del Rei Group). In southeastern Minas Gerais (S~o Jo~o del Rei Group, Fig.9), the Carandai tillite can be divided into a basal terrestrial part and a glaciomarine part at the top of the sequence. This change in facies was probably caused by a retreat of the ice and a transgression of the sea, a situation corresponding to Walther's law (1894), which says t h a t a vertical change in facies always goes along with a lateral one as well. The parallelisation of the different glaciogenic units (Macaflbas and S~o Jo~o del Rei Groups, Bebedouro and Ibi~ Formations, Fig.l) seems to be guaranteed by the equal stratigraphic positions of these units. All units overlie Archaean or Early to Middle Proterozoic rocks and underlie the Late Proterozoic Bambui Group or are regarded as the basal part of the Bambui Group. Directions of ice transport cannot be reconstructed for the western part of the area affected by continental glaciation (Ibi~ Formation) at the moment. In Bahia, directions of ice movement from northeast to southwest have been suggested. Thus, at least two different continental highlands are likely to have existed: the Late Proterozoic S~o Francisco Craton and the Serrinha Craton (De Almeida and Hasui, 1984) in northeastern Bahia State stabilized during the Early Proterozoic. In general, the following conclusions can be drawn: The glaciogenic units of central-eastern Brazil are underlain by Middle Proterozoic rocks and are overlain by the Late Proterozoic Bambui Group or are part of the basal sequences of the Bambui Group.

The indications described suggest a continental glaciation which affected an area of at least 1300 km N-S and 700 km W - E extension. Available age data point to a glacial period around 1000 Ma. Four different glacial environments can be distinguished: glacioterrestrial, glaciolacustrine, glaciofluvial, glaciomarine. The glaciogenic rocks described here are all part of one glacial period.

Acknowledgements We t h a n k Dr. Barbara Lott (Hawaii) for various contributions and Angelika Werchau (Freiburg) for correcting the English text. Prof. Dr. R. Pflug (Freiburg) critically read the manuscript. Financial support of the Conselho Nacional de Pesquisa, the Conselho de Pesquisa and the Instituto de Geoci~ncias da Universidade Federal de Minas Gerais for J. K. and of the Deutsche Forschungsgemeinschaft and the Deutscher Akademischer Austauschdienst for A.H. is gratefully acknowledged. References Amaral, G. and Kawashita, K., 1967. Idade do Grupo Bambui. Bol. Paranense Geoci~nc., 26: 214-217. Barbosa, O., Braun, O. P. G., Dyer, R. C. and Da Cunha, C. A. B. R., 1970. Geologia da regi~o Triangulo Mineiro. Bol. Serv. Fom. Prod. Min., 136, 140 pp. Belezkij, V., 1959. S6bre uma occorencia singular de platina e geologia da parte central da Serra do Cip6. Bol. Div. Fom. Prod. Min., Dep. Nac. Prod. Min., 106: 7-79. Besang, C., Eberle, W. G., Lahner, L., Lenz, H., Mollat, H., Mfiller, P. and Paulsen, S., 1977. Radiometrische Altersbestimmungen an Gesteinen aus Minas Gerais und Espirito Santo/Brasilien. Geol. Jahrb., B 24: 149-179. Bonhomme, M.G., 1976. Mineralogie des fractions et datations rubidium strontium dans le Groupe Bambul, M. G., Br~sil. Rev. Bras. Geoci~nc., 6(4): 211 222. Bonhomme, M. G., Cordani, U. G., Kawashita, K., Macedo, M. H. F. and Thomaz Filho, A., 1982. Radiochronological age and correlation of Proterozoic sediments in Brazil. Precambrian Res., 18: 103-118. Boulton, G.S., 1986. Push-moraines and glacier-contact fans in marine and terrestrial environments. Sedimentology, 33: 677-698. Boulton, G.S. and Deynoux, M., 1981. Sedimentation i n glacial environments and the identification of tills and tillites in ancient sedimentary sequences. Precambrian Res., 15: 397-422. Branner, J. C., 1919. Outlines of the geology of Brazil to

18 accompany the geologic map of Brazil. Bull. Geol. Soc. Am., 30: 189-338. Cahen, L., 1963. Glaciation anciAnnes et derive des continents. Ann. Soc. GAol. Belg., 86: 19-83. Cailleux, A., 1964. Petrographische Eigenschaften der GerSlle und SandkSrner als Klimazeugen. Geol. Rundsch., 54(1): 5-15. Campbell, D. F., Fonseca, J. M., Leite, L. C. and Mansares, V.E., 1966. Correlaqfio estratigrAfica preliminar das principais unidades da Bacia do Alto Silo Francisco. An. 20 Congr. Bras. Geol., 1: 94-95. Campos Neto, M. da C., 1984. Litoestratigrafia, relaqSes estratigrAficas e evoluqfio paleogeogrAfica dos Grupos Canastra e ParanoA (regifio de Vazante-Lagamar, M.G.). Rev. Bras. GeociAnc., 14(2): 81-91. Carey, W. and Ahmad, N., 1961. Glacial marine sedimentation. In G. O. Raasch (Editor), Geology of the Arctic, 2. Univ. Toronto Press, Toronto, pp. 865-894. Charlesworth, J.K., 1957. The Quaternary Era, with Special Reference to its Glaciation. Arnold, London, 1700 pp. Chumakov, N. M., 1964. Pr/ikambrische Tillit-/ihnliche Gesteine der Sowjetunion. Geol. Rundsch., 54: 83-102. Chumakov, N. M., 1973. Continental ice-sheets and Precambrian shields: discussion. Bull. Geol. Soc. Am., 84: 1839-1640. Clauer, N., 1982. The rubidium-strontium method applied to sediments: certitudes and uncertainties. In: G. S. Odin (Editor), Numerical Dating in Stratigraphy. Wiley, Chichester, pp. 245-276. Cloud, P. and Dardenne, M., 1973. Proterozoic age of the Bambui Group in Brazil. Bull. Geol. Soc. Am., 84: 1673-1676. Cloud, P. and Glaessner, M.F., 1982. The Ediacarian period and system: metazoa inherit the earth. Science, 217(4562): 783-792. Cobra, R. Q., 1969. Laminag~o de fluxo e cisalhamento na Serra do Espinhago, Minas Gerais. Ser. Espec. Div. Geol. Serv., SUDENE, 11, 38 pp. Crowell, J. C., 1964. Climatic significance of sedimentary deposits containing dispersed megaclasts. In: A. E. M. Nairn (Editor), Problems in Paleoclimatology. Interscience, New York, 86-99. Crowell, J. C., 1983. The recognition of ancient glaciations. Mem. Geol. Soc. Am., 161: 289-297. Dardenne, M., 1978. Les mineralisations des plomb, zinc, fluor du Proterozoique SupArieur dans le BrAsil central. Avec une mise au point sur la chronologie du PrAcambrien brAsilienne. Thesis. Univ. Paris VI, Paris, 251 pp. Dardenne, M., 1982. Chrono-stratigraphie et metallogenie du PrAcambrien dans le BrAsil central. Actas 5 Congr. Latinoam., 3: 65-77. Dardenne, M. and Campos Neto, M. da C., 1976. Geologia da regifio de Lagamar (Minas Gerais). In: Resumos 29 Congr. Bras. Geol., pp. 17-17. Dardenne, M. A. and Walde, D. H. G., 1979. A estratigrafia dos Grupos Bambul e Macafxbas no Brasil Central. Bol. Soc. Bras. Geol., Ndcleo Minas Gerais, 1: 43-53. Dardenne, M., Faria, A., Magalhaes, L.F. and Soares, L. A., 1978a. O tilito da base do Grupo Bambui na borda

ocidental do Craton S~o Francisco. Bol. Inf. Soc. Bras. Geol., Nfacleo Centro-Oeste, 7-8: 85-97. Dardenne, M., Magalhaes, L. F. and Soares. L.A., 1978b. Geologia do Grupo Bambui no vale do Rio ParanA (GoiAs). An. 30 Congr. Bras. Geol., 2: 611-621. De Almeida, F. F. M., 1964. G l a c i a ~ o eocambriana em Mato Grosso. Not. PrA1. Div. Geol. Min., 117, 11 pp. De Almeida, F. F. M., 1968. Evolu¢~o tectAnica do centrooeste brasileiro no ProterAzoico Superior. An. Acad. Bras. Ci~nc., 40: 285-295. De Almeida, F. F. M., 1977. O craton do S~o Francisco. Rev. Bras. Geoci~nc., 7: 349-364. De Almeida, F. F. M. and Hasui, Y. (Editors), 1984. O PrACambriano do Brasil. Blficher, Sao Paulo, 378 pp. De Almeida, F. F. M., Hasui, Y., De Brito Neves, B. B. and Fuck, R.A., 1981. Brazilian structural provinces: an introduction. Earth Sci. Rev., 17: 1-29. De Carvalho, R. T. and Da Costa, L. A. M., 1968. Indicios de glaciaq~o em Cristalina, GoiAs. Miner. Met., 48: 73-75. De Faria, A., 1985. Geologia do domo de Cristalina, GoiAs. Rev. Bras. Geoci~nc., 15: 231-240. De Faria, A., De Almeida, J. A. P., Veloso, J. A. V. and Dardenne, M., 1970. Excurs~o no. 6: Cristalina e Paracatfl. In: 24 Congr. Bras. Geol. Rot. Exc., Ndcleo Centro-Oeste, 2: 97-114. De Moraes Rego, L.F., 1930. Glaciaq~o eopaleozAica no centro do Brasil. An. Acad. Bras. Ci~nc., 2: 109-112. De Moraes, L. J., 1937. Geologia general. In: Geologia econAmica do norte de Minas Gerais. Bol. Serv. Fom. Prod. Min., 19: 1-103. De Moraes, L. J. and Guimaraes, D., 1930. Geologia da regi~o diamantifera do norte de Minas Gerais. An. Acad. Bras. Ci~nc., 2: 153-186. De Moraes, L. J. and Guimaraes, D., 1931. The diamond bearing region of northern Minas Gerais. Econ. Geol., 26: 502-530. Derby, O. A., 1888. t~ber die Spuren einer carbonen Vereisung in Siidamerika. Neues Jahrb. Miner. Geol. Palaeontol., 1888: 172-176. Derby, O. A., 1905. The geology of the diamond and carbonado washings of Bahia, Brazil. Econ. Geol., 1: 134-142. Dossin, I., 1983. Geologia, g~nese e controle de depAsitos de mangan~s associados ao Grupo Maca/tbas na borda ocidental da Serra do CipA-MG (area do Inhame). Thesis, Dep. Geoci~nc., Brasilia, 127 pp. Dossin, I. A. and Dardenne, M. A., 1984. Geologia da borda ocidental da Serra do CipA, Minas Gerais (area do Inhame). An. 33 Congr. Bras. Geol., 7: 3104-3117. Ebert, H., 1957. Beitrag zur Gliederung des Pr/ikambriums in Minas Gerais. Geol. Rundsch., 45(3): 471-521. Ebert, H., 1958. Discordancias PrA-Cambrianas em Carandai, Minas Gerais. Bol. Div. Geol. Min., Dep. Nac. Prod. Min., 183, 48 pp. Edwards, M., 1986. Glacial environments. In: H. G. Reading (Editor), Sedimentary Environments and Facies. Blackwell, Oxford, 2nd ed., pp. 445-470. Ellis, C. W., 1962. Marine sedimentary environments in the vicinity of the Norwalk Islands, Connecticut. Bull. State

19 Geol. Nat. Hist. Surv. Conn., 94:26 27 (cited in Montes et al., 1985). Flint, R. F., 1961. Geological evidence of cold climates. In: A. E. M. Nairn (Editor), Descriptive Paleoclimatology. Interscience, New York, N.Y., pp. 140-155. Flint, R. F., 1971. Glacial and Quaternary Geology. Wiley, New York, N.Y., 892 pp. Folk, R. L., 1975. Glacial deposits identified by chattermark trails in detrital garnet. Geology, 3: 473-475. Frakes, L. A., 1979. Climates Throughout Geologic Time. Elsevier, Amsterdam, 310 pp. Frakes, L. A. and Crowell, C., 1969. Late Paleozoic glaciation: I, South America. Bull. Geol. Soc. Am., 80: 1007-1042. Gorlt, G., 1972. Fazieswechsel und Metamorphose in der westlichen Serra Negra (Espinhaqo-Zone, Minas Gerais, Brasilien). Geol. Rundsch., 61: 166-201. Gravenor, C. P., 1982. Chattermarked garnets in Pleistocene glacial sediments. Bull. Geol. Soc. Am., 93: 751-758. Gravenor, C. P. and Monteiro, R. L. B. P., 1983. Ice-thrust features in the Proterozoic Macaflbas Group, Jequitai area, Minas Gerais, Brazil. J. Geol., 91: 113-116. Gravenor, C. P., Von Brunn, V. and Dreimanis, A., 1984. Nature and classification of waterlain glaciogenic sediments exemplified by Pleistocene, Late Paleozoic and Late Precambrian deposits. E a r t h Sci. Rev., 20: 105-166. Guimaraes, D., 1966. As formaq5es Carandai-Prados. In: Os metamorfitos denominados "Micaxistos SeridS". Bol. Div. Geol. Min., 237: 20-31. Hahn, G., Hahn, R., Leonardos, O.H., Pflug, H.D. and Walde, D. H. G., 1982. KSrpertich erhaltene ScyphozoenReste aus dem Jungpr~ikambrium Brasiliens. Geol. Palaeontol., 16: 1-18. Harland, W. B., 1964a. Evidence of Late Pre-Cambrian glaciation and its significance. In: A . E . M . Nairn (Editor), Problems in Paleoclimatology. Interscience, New York, N.Y., pp. 119-149. Harland, W. B., 1964b. Critical evidence for a great infraCambrian glaciation. Geol. Rundsch., 54: 45-61. Harland, W. B., Herod, K. N. and Krinsley, D. H., 1966. The definition and identification of tills and tillites. E a r t h Sci. Rev., 2: 225-256. Hartt, C. F., 1870. Geology and Physical Geography of Brazil. Fields and Osgood, Boston, 620 pp. (1975 reprint: Krieger, Huntington, N.Y.). Hasui, Y. and De Almeida, F. F. M., 1985. The Central Brazil Shield reviewed. Episodes, 8: 29-37. Hettich, M., 1975. Zur Genese des Macadbas und Geologie des Gebietes nSrdlich der Serra Negra (Minas Gerais, Brasilien). Geol. Jahrb., B 14: 47-85. Hettich, M., 1977. A g l a c i a ~ o ProterozSica no centronorte de Minas Gerais. Rev. Bras. Geoci~nc., 7:87 101. Hettich, M. and Karfunkel, J., 1978. Um esker, um varvito e seixos estriados no Grupo Macaflbas norte de Minas Gerais. Rev. Esc. Minas Ouro Preto, 34: 5-8. Hofmann, H. J. and Schopf, J. W., 1983. Early Proterozoic microfossils. In: J. W. Schopf (Editor), E a r t h ' s Earliest Biosphere. Princeton Univ. Press, Princeton, N.J., pp. 321-360. Hoppe, A., Karfunkel, J. and Noce, C. M., 1984a. Rochas

komatilticas no centro-sul do Estado de Minas Gerais. In: Resumos 33th Congr. Bras. Geol., pp. 147-147. Hoppe, A., Karfunkel, J. and Noce, C. M., 1984b. Evidencias da glacia9~o Proteroz6ica no centro-sul do Estado de Minas Gerais. Cifinc. Terra, 9: 16-16. Hoppe, A., Lammerer, B., Miiller, G., Pflug, R. and WeberDieffenbach, K., 1987a. Aspectos da geologia e petrologia da Serra do Espinha~o-sul e entre o Quadrilgltero Ferrifero e do cinturao m6vel Brasiliano no sul do Espirito Santo, Brasil. In: H. Miller (Editor), Investigaclones recientes en Latinoam6rica: Geologia. Dtsch. Forschungsgem., VCH, Weinheim, pp. 85 97. Hoppe, A., Schobbenhaus, C. and Walde, D. H. G., 1987b. Precambrian iron formation in Brazil. In: P. W. U. Appel and G. LaBerge (Editors), Precambrian Iron Formations. Teophrastus, Athens, pp. 347 390. Inda, H. A. V. (Editor), 1979. Geologia e recursos minerais do Estado da Bahia. Text. Bas., Secr. Min. Energ., CPM, 12: Gov. Est. Bahia, Salvador, 165 pp. Inda, H. A. V. and Barbosa, J. H., 1978. Texto explicativo para o mapa geol6gico do Estado da Bahia 1:1.000.000. Secret. Min. Energ., CPM, Gov. Est. Bahia, Salvador, 137 pp. Isotta, C. A. L., Rocha-Campos, A. C. and Yoshida, R., 1969. Striated pavement of the upper Precambrian glaciation in Brazil. Nature, 222:466 468. Karfunkel, B. a n d Karfunkel, J., 1976. Estudos petrofaciol6gicos n a por~fio mediana da Serra do Espinha~oMG. An. 19 Congr. Bras. Geol., 2: 179-188. Karfunkel, B. and Karfunkel, J., 1977. Fazielle Entwicklung der mittleren Espinhaqo-Zone mit besonderer Beriicksichtigung des Tillit-Problems (Minas Gerais, Brasilien). Geol. Jahrb., B 24: 3-91. Karfunkel, J., 1978. A geologia da regifio pegmatitica de Coronel Murta, MG. Rep. Inst. Geoci~nc. Univ. Fed. Minas Gerais, Belo Horizonte (unpublished). Karfunkel, J. and Noce, C.M., 1983. Desenvolvimento faciol6gico do Pr&Cambriano Superior da regi~o de Carandai-Sfio Jo~o del Rei, Minas Gerais. Bol. Soc. Bras. Geol., Nflcleo Minas Gerais, 3: 16-29. Karfunkel, J., Noce, C.M. and Hoppe, A., 1983. A F o r m a ~ o Carandal no centro-sul do Estado de Minas Gerais: g~nese, ambiente de deposiq~o e correla~Ses. Bol. Soc. Bras. Geol., Nficleo Minas Gerais, 3: 30-37. Karfunkel, J., Horta Moreira, P.C., Ribeiro, M.C. and Costa Franco, A.L., 1984. Aspectos gen~ticos e deposicionais do Grupo Macafibas n a regi~o da barragem do Paraflna e sua importancia n a contribuiq~o para um modelo paleogeogrhfico e geotectSnico. An. 33 Congr. Bras. Geol., 7: 3091-3103. Kegel, W., 1959. Estudos geolSgicos n a zona central da Bahia. Bol. Div. Geol. Min., 198, 35 pp. Krinsley, T. and Takahashi, T., 1962. Surface textures of sand grains - - an application of electron microscopy: glaciation. Science, 138: 1262-1264. KrSner, A. and Rankama, K., 1973. Late Precambrian glaciogenic sedimentary rocks in southern Africa: a compilation with definition and correlation. Bull. Geol. Soc. Finl., 45: 79-102. Landim, P. M. B. and Frakes, L.A., 1968. Distinction

20 between tills and other diamictons based on textural characteristics. J. Sediment. Geol., 38: 1213-1223. I.~onardos, O. H., 1940. Tilito metam6rfico do Carandai. Minas Gerais. An. Acad. Bras. Ci~nc., 12: 243-259. Lindsey, D. A., 1966. Sediment transport in a Precambrian ice-age: the Huronian Gowganda Formation. Science, 154: 1442-1443. Maack, R., 1957. Uber Vereisungsperioden und Vereisungsspuren in Brasilien. Geol. Rundsch., 45: 547-595. Macedo, M. H. F. and Bonhomme, M. G., 1981. Data95es R b - S r e K - A r das Forma~Ses Bebedouro e Caboclo na Chapada Diamantina. An. Simp. Craton Silo Francisco, CPM, Soc. Bras. Geol., N/lcleo Bahia, pp. 98-99. Macedo, M. H. F. and Bonhomme, M. G., 1984. Contribui9fio a cronoestratigrafia das Forma~Ses Caboclo, Bebedouro e Salitre n a Chapada Diamantina (BA) pelos m6todos R b - S r e K-Ar. Rev. Bras. Geoci~nc., 14: 153-163. Maciel, P., 1959. Tilito cambriano (?) no Estado-de Mato Grosso. Bol. Soc. Bras. Geol., 8: 31-39. Marchese, H. G., 1974. Estromatolitos Gymnosolenidos en el lado oriental de Minas Gerais, Brasil. Rev. Bras. Geoci~nc., 4: 257-271. Misi, A., 1979. O Grupo Bambul no Estado da Bahia. Text. Bas. Secr. Min. Energ., CPM, 1: Gov. Est. Bahia, Salvador, pp. 119-154. Montes, A. S. L., 1977. O contexto estratigr~fico e sedimentolAgico da Formagfio Bebedouro n a Bahia: um possivel portador de diamantes. Thesis. Univ. Fed. Bahia, Salvador, 100 pp. Montes, A. S. L., Gravenor, C. P. and Montes, M. L., 1985. Glacial sedimentation in the Late Precambrian Bebedouro Formation, Bahia, Brazil. Sediment. Geol., 44: 349-358. Ovenshine, A. T., 1964. Glacial interpretation of Precambrian Gowganda Formation, n o r t h shore of Lake Huron, Canada. Abstr. Spec. Pap. Geol. Soc. Am., 82: 146-146. Ovenshine, A. T., 1970. Observations of iceberg rafting in glacial bay, Alaska, and the identification of ancient icerafted deposits. Bull. Geol. Soc. Am., 81: 891-894. Parenti Couto, J.G. and Bez, L., 1981. A glaciaqfio Jequitai: um guia estratigr~fico para o Pr6-Cambriano Superior no Brasil. Rev. Bras. Geoci6nc., 11: 17-21. Parenti Couto, J. G., Cordani, U. G., Kawashita, K., Iyer, S. and Moraes, N. M. P., 1981. Considera95es s6bre a idade do Grupo Bambui corn base em an~lises isot6picas de Sr e Pb. Rev. Bras. Geoci~nc., 11: 5-16. Penck, A., 1905. Sfid-Africa und Sambesiffille. Geogr. Z., 12: 600-611. Pettijohn, F. J., 1957. Sedimentary Rocks. Harper, New York, N.Y., 718 pp. Pflug, R., 1985. A geologia da parte meridional da Serra do Espinhaqo e zonas adjacentes, Minas Gerais, Brasil. Bol. Div. Geol. Min., 226, 51 pp. Pflug, R., 1967. Die prfikambrische Miogeosynklinale der Espinhago-Kordillere, Minas Gerais, Brasilien. Geol. Rundsch., 55: 825-844. Pflug, R. and Renger, F., 1973. Estratigrafia e evoluq~o geol6gica da margem SE do Craton Sanfranciscano. An. 27 Congr. Bras. Geol., 2: 5-19.

Pflug, R. and SchSll, W. U., 1975. Proterozoic glaciations in eastern Brazil: a review. Geol. Rundsch., 64: 287-299. Powell, R. D., 1984. Glaciomarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on Quaternary examples. Mar. Geol., 57: 1-52. Reading, H. G. and Walker, R. G., 1966. Sedimentation of Eocambrian tillites and associated sediments in Finmark, n o r t h e r n Norway. Palaeogeogr., Palaeoclimatol., Palaeoecol., 2: 177-212. Rocha-Campos, A. C. and Hasui, Y., 1981a. Late Precambrian Salobro-Formation of Brazil. In: M . J . Hambrey and W.B. Harland (Editors), E a r t h ' s Pre-Pleistocene Glacial Record. Cambridge Univ. Press, Cambridge, pp. 928-930. Rocha-Campos, A. C. and Hasui, Y., 1981b. Late Precambrian J a n g a d a Group and Puga Formation of centralwestern Brazil. In: M . J . Hambrey and W.B. Harland (Editors), E a r t h ' s Pre-Pleistocene Glacial Record. Cambridge Univ. Press, Cambridge, pp. 916-919. Rocha-Campos, A. C. and Hasui, Y., 1981c. Proterozoic diamictites of western Minas Gerais and eastern Goi~s, central Brazil. In: M . J . Hambrey and W.B. Harland (Editors), E a r t h ' s Pre-Pleistocene Glacial Record. Cambridge Univ. Press, Cambridge, pp. 920-923. Rocha-Campos, A. C. and Hasui, Y., 1981d. The Late Precambrian Bebedouro Formation, Bahia, Brazil. In: M. J. Hambrey and W. B. Harland (Editors), E a r t h ' s PrePleistocene Geologic Record. Cambridge Univ. Press, Cambridge, pp. 924-927. Schermerhorn, L. J. G., 1966. Terminology of mixed coarsed-fine sediments. J. Sediment. Petrol., 36: 831-835. Schermerhorn, L. J. G., 1974. Late Precambrian mixtites: glacial and/or nonglacial. Am. J. Sci., 274: 673-824. Schermerhorn, L. J. G., 1983. Late Precambrian glaciation in the light of CO 2 depletion in the atmosphere. Mere. Geol. Soc. Am., 61: 309-315. Schmidt, H. L., 1972. Fazieswechsel in der S~o Francisco Serie (Bambui) bei Bocaiuva, Espinhaqo-Zone, Minas Gerais, Brasilien. Beih. Geol. Jahrb., 121: 59-94. ~ Schobbenhaus, C., Campos, D. de A., Derze, G.R. and Asmus, H. E., 1984. Geologia do Brasil. Texto explicativo do mapa geol6gico do Brasil e da /trea oce~nica adjacente . . . . . . . . . . dep6sitos minerais, escala 1:2.500.000. Dep. Nac. Prod. Min., Brasilia, 501 pp. Sch611, W. U., 1972. Der siidwestliche Randbereich der Espinha¢o-Zone, Minas Gerais, Brasilien. Geol. Rundsch., 61: 201-216. SchSll, W. U., 1976. Estromatolitos (Conophyton) em dolomitos do Grupo Maca6bas. An. 29 Congr. Bras. Geol., 2: 67-73. SchSll, W. U. and Fogaqa, C. A. C., 1980. Estromatolitos em dolomitos do Grupo Macadbas (Supergrupo S~o Francisco) n a regi~o de Conselheiro Mata, M.G. Bol. Inst. Geoci~nc. USP, 11: 40-42. Schwarzbach, M., 1964. Criteria for the recognition of ancient glaciation. In: A. E. M. Nairn (Editor), Problems in Paleoclimatology. Interscience, New York, N.Y., pp. 81-85.

21 Schwarzbach, M., 1974. Das Klima der Vorzeit. Enke, Stuttgart, 3rd ed., 380 pp. SSfner, B., 1973. Observa~Ses s6bre a estratigrafia do Pr~Cambriano da Chapada Diamantina sudeste e da ~rea contigua. An. 27 Congr. Bras. Geol., 2: 23-32. Sokolov, B. F. and Fedonkin, M. A., 1984. The Vendian as the terminal system of the Precambrian. Episodes, 7: 12-19. Viveiros, J. F. M. and Walde, D., 1976. Geologia da Serra do Cabral, Minas Gerais, Brasil. MUnster. Forsch. Geol. Pal/iontol., 38-39:15 27. Viveiros, J. F. M., S~, E. L., Vilela, O. V., Santos, O. M., Moreira, J. M. P., Neto, F.H. and Vieira, W.S., 1979. Geologia dos vales dos Rio Peixe Bravo e Alto Vac~ria, norte de Minas Gerais. Bol. Soc. Bras. Geol., Nficleo Minas Gerais , 1:75 87. Von Sperber, M., 1977. Geologie des Pr/ikambriums am Sfidwestrand der Serra do Cip6 (Serra do Espinha~o, Minas Gerais, Brasilien). Geol. Jahrb., B 24: 93-148. Walde, D. H. G., 1976. Neue Hinweise fur eine proterozoische Vereisung in Ostbrasilien. MUnster. Forsch. Geol. Pal/iontol., 38-39:47 59. Walde, D. H. G., 1978. Desenvolvimento faciol6gico do Pr~cambriano entre a Serra Mineira e a Serra do Cabral (regi~o sudoeste da Serra do Espinhaqo, Minas Gerais). An. 30 Congr. Bras. Geol., 2: 711-725. Walde, D. H. G., 1981. Die Mangan- und Eisenerzvorkommen von Urucum, Mato Grosso, Brasilien. Erste Ergeb-

nisse und weiteres Programm. Zentralbl. Geol. Pal~iontol., 1: 505-513. Walde, D. H. G., 1987. Estratigrafia y desarollo del Pr~cambrico tardio en las immediaciones de Corumb~, Brasil. In: H. Miller (Editor), Investigaciones alemanas recientes en Latinoam~rica: Geologia. Dtsch. Forschungsgem., VCH, Weinheim, pp. 98-107. Walde, D. H. G., Karfunkel, J. and Karfunkel, B., 1978. O Grupo Macafibas em Minas Gerais: estratigrafia, genese e correlaq6es. Bol. Inf. Soc. Bras. Geol., Nfcleo CentroOeste, 7-8: 98-113. Walde, D. H. G., Leonardos, O. H., Hahn, G., Hahn, R. and Pflug, H. D., 1982. The first Precambrian megafossil from South America: Corurnbella werneri. An. Acad. Bras. Ci~nc., 54:461 461. Walter, M. R., 1983. Archean stromatolites: evidence of the earth's earliest benthos. In: J.W. Schopf (Editor), E a r t h ' s Earliest Biosphere. Princeton Univ. Press, Princeton, N.J., pp. 187-213. Walther, J., 1894. Lithogenesis der Gegenwart. Beobachtungen fiber die Bildungen der Gesteine an der heutigen Erdoberfl~iche. Einleitung in die Geologie als historische Wissenschaft, 3. Fischer, Jena, pp. 535 1055. Williams, H. E., 1930. Estudos geSlogicos na Chapada Diamantina, Estado da Bahia. Bol. Serv. Geol. Min., 44, 16 pp. Windley, B. F., 1984. The Evolving Continents. Wiley, New York, N.Y., 2nd. ed., 399 pp.