ABSTRACT
The striated pavements observed in the Santa Fé de Minas region, in the northwestern of the Minas Gerais State, are not well known by the Brazilian geologists. However these structures are important in the regional geologic context of the Sanfranciscana Basin, and were fundamental to the identification of the depositional environment of the basal units of the São Francisco Craton Phanerozoic covers. The structures are limited in area, but are very well preserved and must be integrated to the geological inheritance of Brazil as one stratigraphic site.

INTRODUCTION

    The Permo-Carboniferous glaciation is preserved in many Paleozoic basins in the Gondwana continent (for example: the Itararé and Aquidauana groups in the Paraná Basin; the Dwyka Formation in the Karroo Basin, South Africa; Queensland Formation,Bowen Basin in Australia; Koel Basin in India and many other occurrences in Antarctica. Crowell & Frakes, 1971 and 1972, Frakes & Crowell 1969 and 1970, Frakes et al. 1971 and 1975). However, in the Sanfranciscana Basin this record is limited, and the preservation is just observed in paleogeographic situation associated to paleovalleys in the bed rock. The identification and confirmation of the glacigenic event in the interior of the São Francisco Craton was just possible after the mapping of the striated pavements in the Santa Fé de Minas region.
    The glacigenic sediments were attributed to the Santa Fé Group (including the Floresta and Tabuleiro formations), what is the basal unit of the basin (Campos 1992, Campos & Dardenne 1997) and the structures were initially described by Dardenne et al. (1990).

LOCATION

    The outcrops are located 5,0 kilometers west of the Santa Fé de Minas town (in the northwest of Minas Gerais State, in the Paracatu River Valley). The access to the outcrops is made by the earth road that links Santa Fé de Minas to Bonfinópolis de Minas. A good distance reference is the bridge over the Lavado River, what is 1.800 meters far from the site. The outcrops are about 20 meters from the left margin of the road (travelling from Santa Fé to Bonfinópolis). The outcrops region is located on 45^o 31' 54" W - 16^o 43' 34"S.
    The bed rock that is striated and polished is represented by Três Marias Formation arkoses, belonging to the uppermost sequence of the Bambuí Group (Neoproterozoic of the Brasilia Fold and Thrust Belt and cratonic areas).
    The age of the glacial sequence was determined by the ichnofossils present in glacio-lacustrine facies, where the trails marked in siltstone and shales were identified as isopodichnus and diplichnites. Although the referred trace fossils do not make possible the determination of a well defined age, they are commonly characteristic of the Paleozoic and allow the correlation of the glacigenic succession to the gondwanic glacial event.

Figure 1 – Localization of the study areas where the glacigenic sediments are found and where there is the presence of striated pavements.

SITE DESCRIPTION

    The site is characterized by few outcrops, where the largest is about 40 square meters, but showing excellent quality (Figure 2). The pavements are represented by shallow, fines and parallel grooves that make up sequences of fluting which can be follow for several meters. Locally the pavement is polished or with shallower grooves, getting a smooth aspect. Besides all kinds of stria, grooves and fluting, locally the surface of the pavement is undulated and abrasion marks of pebble or crescent fractures can be observed (figures 3 to 5).

Figure 2. Geologic map of the Santa Fé de Minas region (Minas Gerais state)

    Associated to parameters as petrographic analysis of arkoses, pebble composition, heavy minerals assemblage and statistic of crossed stratification, the striated pavements were important to define the direction of the ice cape flux, indicating that the transport was from northeast to southwest.
    At the place of the geological site, the pavements are associated to diamictites and dropstone bearing shales facies, both containing clasts (blocks and pebbles) of variable nature, faceted shapes and occasionally also striated. The glacigenic facies are characteristically reddish brown in color and fill valleys. The valleys are pre-existing in the basement or are resulted by the glacial geomorphology.
    The Santa Fé Group is not restricted to this region, but is also observed in many other regions of the basin (Urucuia, Canabrava and São Romão regions in the Minas Gerais State, and Posse and São Domingos regions in the Goiás State), however the pavements described here are the unique mapped until now.

Figure 3 – Striated and undulated pavement developed over arkoses of the Três Marias Formation.

Figure 4 – Striated pavement with crescentic fractures.

Figure 5 – Polished and striated pavement. General direction of the striation is N20E-S20W, and transport is towards SW.

    The importance to the preservation of this stratigraphic geological site is associated to the following factors:

• These pavements are the only occurrence of the main record that demonstrates the existence of Paleozoic glaciation in the Sanfranciscana Basin;
• They are one kind of sedimentary structure that is very difficult to be preserved, because the surface now exposed must be exactly the same where the ice caps have slipped. In the case it is some centimeters deeper, the pavements were be eroded and if it is some centimeters above the surface, the pavements were be buried by glacigenic facies;
• This kind of structure is just preserved in cohesive and well lithified rocks. Over non cohesive rocks the ice caps promote erosion and the abrasion of the bed rock can be registered and
• These structures may help further palaeogeographic investigations in an area that is poor in geological and geomorphological studies.

PROTECTION PROGRAM

    There are no protection measures. As this site is situated in the margin of a vicinal not important road, the risk what it is submitted is to be buried or fragmented by machines when the road will be enlarged or repaired. In this sense would be very interesting to the preservation of the site, to show to the local administrators the importance of the area for the geologic sciences.

Campos,J.E.G. 1992. A Glaciação Permo-Carbonífera nas regiões de Canabrava e Santa Fé de Minas – MG. Dissertação de Mestrado, Universidade de Brasília. 104 p.

Campos,J.E.G.; Dardenne,M.A. 1997. Estratigrafia e sedimentação da bacia Sanfranciscana: Uma Revisão. RBG 27(3):269-282.

Crowell,J.C.; Frakes,L.A. 1971. Late Paleozoic Glaciation: Part IV, Australia. Geological Society of America Bulletin, 82:2515-2540.

Crowell,J.C.; Frakes,L.A. 1975. The Late Paleozoic Glaciation. In; Gonduana Geology, Campbell, K.S.W. (edit) Part IV, Karroo Basin. Geological Society of America Bulletin, 83:1887-1912.

Dardenne,M.A.; Gonzaga,G.M.; Campos,J.E.G. 1990. Descoberta de pavimentos estriados de origem glacial sobre os arcóseos da Formação Três Marias na região de Santa Fé de Minas. REM-Revista da Escola de Minas,43(4):65-66.

Frakes,L.A.; Crowell,J.C. 1969. Late Paleozoic Glaciation: Part I, South America. Geological Society of America Bulletin, 80:1007-1042.

Frakes,L.A.; Crowell,J.C. 1970. Late Paleozoic Glaciation: Part II, Africa, Exclusive of the Karroo Basin. Geological Society of America Bulletin, 81:2261-2286.

Frakes,L.A.; Kemp,E.M.; Crowell,J.C. 1975. Late Paleozoic Glaciation: Part VI, Asia. Geological Society of America Bulletin, 86:454-464.

Frakes,L.A.; Matheus,J.L.; Crowell,J.C. 1971. Late Paleozoic Glaciation: Part III, Antactica. Geological Society of America Bulletin, 82:1581-1604.