Late Paleozoic glaciotectonic structures in northern Paraná Basin, Brazil

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Sedimentary Geology 130 (2000) 131–143

Late Paleozoic glaciotectonic structures in northern Parana´ Basin, Brazil A.C. Rocha-Campos Ł , Jose´ R. Canuto, Paulo R. dos Santos Instituto de Geocieˆncias, Universidade de Sa˜o Paulo, Rua do Lago 562, CEP 05508-900, Sa˜o Paulo, Brazil Received 18 March 1999; accepted 24 September 1999

Abstract Fluviatile–deltaic sediments containing thin coal layers, and glaciofluvial sediments of the Itarare´ Subgroup (late Paleozoic) cropping out near Cerquilho, northern Parana´ Basin, exhibit post-depositional deformation such as recumbent and drag folds, shear fractures, faults and shear lamination. The deformation occurs in an interval of 3–4 m thick and is confined between horizontal strata. The deformed strata are directly overlain along a horizontal erosional=tectonic contact surface by a silty–sandy, massive, 1 m thick, clast-poor tillite. Deformation shows a consistent geometry indicating a predominant vergence toward the SSW having been developed under a horizontal stress oriented from NNE to SSW. The deformation is interpreted to be of glaciotectonic origin associated with an ice readvance in the area. The glacier moved toward the SSW on relatively soft, deformable sediments, depositing the subglacial till on top of them. The tillite shows parallel, upglacier-dipping shear fractures at its base and wedge-like intrusions into the underlying sandstone, probably formed during its subglacial deposition. The tillite is overlain by cyclic (braided stream) fluviatile, sandstone beds. The top of these shows deformations (small recumbent folds and reverse faults) below another clast-poor, silty–sandy tillite. This could represent another glacial readvance with subglacial deposition of till. Occurrence of glaciotectonic deformation overlain by subglacially deposited tillites in the Cerquilho area indicates glacier readvances and permanence of the glacial influence in the upper part of the Itarare´ Subgroup. Absence of incorporated material from the underlying deformed sediments in the tillite and the style of the deformations suggest an unfrozen, water-saturated, mostly unconsolidated substratum. These conditions may have generated instability and fast ice flow of the glacier. They also suggest that the sediment may have most likely failed in this manner under rapidly applied stresses associated with a surging behavior of the retreating glacier.  2000 Elsevier Science B.V. All rights reserved. Keywords: glaciotectonic deformation; late Paleozoic; Parana´ Basin; Brazil

1. Introduction Descriptions of glaciotectonic features in prePleistocene deposits are rare in the literature. The only possible examples known to us are those of Ł Corresponding

author. Fax: C11 818 4129; E-mail: [email protected]

Visser (1990, 1991), who described small folds associated with a glacially furrowed soft-sediment pavement, and brecciated and deformed sedimentary substrate below diamictite in the late Paleozoic Dwyka Formation, and Biju-Duval et al. (1974), who recorded normal faults dipping downglacier which displace Precambrian and Ordovician glacigenic sediments and=or bedrock in the Central Sahara.

0037-0738/00/$ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 1 1 0 - 4

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As utilized here, the term glaciotectonic refers to structures and landforms created by glacially induced deformation of pre-existing substratum, either bedrock or sediment (Aber, 1982), of variable scale. This strict definition excludes other structures such as post-depositional folds and faults resulting from collapse due to melting of adjacent lateral or underlying buried ice (Eyles, 1977; Hart and Boulton, 1991). There are in our view two main reasons for the scarcity of information on pre-Pleistocene glaciotectonic structures. Firstly, as pointed out by Banham (1988), glaciotectonic structures may be geometrically similar to non-glacial tectonic features, as well as to those originated by soft-sediment deformation. Secondly, glaciotectonic features have been typically described from the glacial terrestrial environment, where their preservation potential in the ancient record is, therefore, probably low (Eyles, 1993). It must be recognized, however, that deformation of sediments by overriding, grounded glacial ice can also occur subaquatically. Interpretation of a general glacial influenced environment of deposition for the Itarare´ Subgroup (Middle Carboniferous–Early Permian) in central-eastern Sa˜o Paulo State is supported by the occurrence of a roche moutonne´e, striated surfaces, diamictites with faceted and striated clasts, intra- and intertillite striated boulder pavements and regular rhythmites with dropstones (Rocha-Campos, 1967; Frakes and Crowell, 1969; Rocha-Campos and Santos, 1981; Gravenor and Rocha-Campos, 1983; Santos et al., 1992, 1996). According to the latest available paleogeographic reconstruction (Santos et al., 1996), glaciers of the Kaokoveld lobe (Frakes and Crowell, 1972) that reached the margins and entered into the northern Parana´ Basin in the late Paleozoic, flowing in general from SE to NW, were extensions of the Windhoeck Ice Sheet (Santos et al., 1996) centered in southern Africa. Multiple advances of the lobe are recorded by thin, subglacially deposited tillites identified at different stratigraphic levels along the Itarare´ Subgroup section (Gravenor and Rocha-Campos, 1983; Canuto, 1985; Santos et al., 1996). At each retreat glaciers left a carpet of glaciogenic sediments mostly reworked by meltwater and=or mass-gravity processes on the land and in the sea (Frakes and Crowell, 1972; Gravenor and Rocha-Campos, 1983;

Santos et al., 1996). A temperate thermal setting for the glaciers is suggested by sedimentary facies, palynological content and paleolatitudinal data (Santos et al., 1996). As they advanced, glaciers moved on crystalline and meta-sedimentary Precambrian, and sedimentary Paleozoic basement rocks, as well as on their own deposits (Santos et al., 1996). Along the eastern outcrop belt of the Parana´ Basin glacigenic beds are separated from each other by locally thick fluvial–deltaic and=or marine strata showing little or no glacial imprint (‘interglacial’ phases). Facies studies show that most parts of the Itarare´ Subgroup succession were preserved in the marine environment of the subsiding Parana´ Basin (Gravenor and Rocha-Campos, 1983; Eyles et al., 1993; Santos et al., 1996). Only in the northern part of the basin remnants of the terrestrial glacial facies seem to be more widespread (Rocha-Campos and Santos, 1981; Gravenor and Rocha-Campos, 1983; Santos et al., 1996). In the Parana´ Basin of Brazil structures (mostly folds) affecting Itarare´ Subgroup rhythmites and diamictites were measured by Martin (1961) and used by him to deduce the sense of ice movement, have been reinterpreted as due to post-depositional slumping or resedimentation of the glacigenic sediments through mass-gravity flow processes (Rocha-Campos, 1963, 1967). Coincidently, orientation of these gravity-driven features is also indicative of the paleoslope dip along which the glaciers moved toward the Parana´ Basin. Results are, therefore, roughly consistent with glacial striae on the basement (RochaCampos, 1967). Here we describe meso-scale (dm to m) structures affecting sediments of the Itarare´ Subgroup in northern Parana´ Basin, Brazil, which we interpret as of glaciotectonic origin. The features were first identified during geological mapping in the Cerquilho area, central-eastern Sa˜o Paulo State, by RochaCampos et al. (1981). They were subsequently briefly referred to and interpreted as either glaciotectonic (Nagali and Consoni, 1984; Rocha-Campos et al., 1986; Martini and Rocha-Campos, 1991) or of non-glacial origin (Fulfaro et al., 1984, 1991). Additional field work including extensive cleaning of deeply weathered outcrops allowed detailed investigation of the sedimentary and tectonic features and collecting of additional tectonic data that helped to

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settle the existing controversy on the origin of the structures. Besides its paleoclimatic significance, identification of the glacial origin of the structures has important implications for the late Paleozoic stratigraphy and glacial history of the Itarare´ Subgroup in the Parana´ Basin. It also allows some inferences on the possible dynamics and basal stress conditions of the late Paleozoic glaciers.

2. Location and stratigraphy Features described crop out on two parallel, roughly N–S-oriented cuts of a secondary road some 1.5 km northeast from Cerquilho (Fig. 1). Sediments involved belong to the upper part of the Itarare´ Subgroup succession in the area and are situated less than 20 m below the contact with the overlying post-glacial Tatuı´ Formation (Rocha-Campos et al., 1981). Rocks exposed on the cut are deeply weathered and a detailed recognition of lithologies and features is therefore difficult. Fig. 2 shows the lithologic succession cropping out along the eastern road cut. The lower part of 80o

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60o

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the section below the deformed beds is formed by a series of horizontal fining-upward cycles, one to several meters thick, assigned to a fluviatile origin, resting on top of a badly exposed silty–clayey diamictite. Cycles start with fine to medium sandstone with rare, lenticular intercalation of feldspathic, planar cross-bedded conglomerate that might represent crevasse splay deposits, followed by faintly bedded, sandy siltstones and thin (cm) coal or coaly siltstone layers interpreted as deposited in an alluvial plain. Rootlets below some of the coal layers point to an autochthonous origin for these associated with the formation of thin paleosol horizons on the top of the alluvial plain silt layers. Exposed thickness of the deformed zone is 3–4 m, but its base was not seen (Figs. 2 and 3). The stratigraphic succession in this part of the section is highly disturbed by the deformation. In view of this, only for descriptive purpose, three sets of associated lithologies (a, b and c) bounded by tectonic and=or erosional surfaces are recognized in the deformed zone (Figs. 3 and 4). Set a (units 1–3) includes intercalated, decimetric, horizontal and folded beds of massive, fine sandstone (unit 1) and planar laminated or bedded sandy silt-

40o

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BRAZIL 7

-42

SP

20o

PARANÁ BASIN CHACOPARANÁ BASIN

40

Cerquilho

Cerquilho

o

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00

1000m

Fig. 1. Location of the glaciotectonic features in the Parana´ Basin near Cerquilho. Blank: Itarare´ Subgroup (Permo–Carboniferous); fine dots: Tatuı´ Formation (Permian); v’s: diabase (Cretaceous); coarse dots: Cenozoic (from Rocha-Campos et al., 1981).

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DEFORMATIONS

20

15

FIG.3 SETC

10

SETSAandB

COAL

ING FIN RD WA UP

55

0m

C L.

G

N

O

D

N

UD

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Fig. 2. Generalized stratigraphic log of the road cut near Cerquilho.

stone, with rare thin coal layers (unit 2), in lateral contact with silty–clayey beds containing irregular, discontinuous, centimetric thick coal or coaly layers (unit 3). A single irregular mass of conglomeratic sandstone some 50 cm long is included in the siltstone (unit 2). These beds overly the lower fluviatile beds containing thin coal layers in Fig. 2. They are interpreted as mostly of lacustrine and alluvial plain facies. Units 4–7 make up set b, a succession of (from bottom to top) decimeter-thick beds of fine sandstone with irregular lenses of clayey siltstone (unit 4), medium cross-bedded sandstone (unit 5), irregular lenses of feldspathic, conglomeratic sandstone (unit 6), and fine to medium sandstone with intercalated thin, siltstone layers (unit 7). Contacts between beds are sharp, sometimes erosional and irregular, involving loading or interpenetration of lithologies (Fig. 3). Units of this set thin out toward the SSW and are inclined toward the NNE, apparently lapping onto folded, coaly silty–clayey beds (unit 3). Geometry and facies of this set contrast clearly with that of beds below. Sedimentary structures indicate subaquatic deposition by traction currents under a relatively rapidly changing flow regime. Their interpretation as probably distally proglacial or glaciofluvial deposits is best understood in the context of a glacial origin adopted for the diamictites and associated beds in the upper part of the section (see discussion above). Units 8, 9 and 10 make up set c. The first is a bed of fine sandstone similar to unit 7. It thickens toward the SSW and is thrust against unit 9. Units 9 and 10 are in lateral contact with a large granite boulder (diameter of about 1.5 m), respectively on the north and south sides of the clast. The conglomeratic sandstone and laminated siltstone of unit 10 thin out toward the south away from the boulder and is truncated by an upper sandstone unit (unit 12). Units 9 and 10 seem to represent slices of sediments that were tectonically separated from beds stratigraphically below and above them. Set c is separated through an irregular, horizontal, erosional=tectonic surface from the overlying tabular bed of silty–sandy, massive diamictite (unit 11), which is in its turn overlain also along an irregular, erosional surface by a medium–fine, sorted sandstone bed (unit 12). The diamictite, about 1.5

12

Setc

11 BB

Fig.5

CC 88

Setb

11

0.5

Fig.4

DD AA

55

22

99

66 77

10

44

111 33

Seta

11 22

0m

Fig. 3. Diagram of the structures. Numbers 1–12 correspond to different lithologies; A, B, C, D indicate specific structures; a, b, c indicate sets of lithologies. Obliquely hatched: covered. Boxes show approximate positions of Figs. 4 and 5 (see text for description).

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SSW

NNE

135

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Fig. 4. (A) Shear surfaces, truncated fold and other deformation associated with the granite boulder. (B) Photointerpretation to enhance features. Numbers correspond to different lithologies (see text for description). Oblique hatched: covered.

m thick, contains rare, chaotically dispersed clasts, predominantly of quartzite and granite, mostly centimetric, a single one metric in diameter.

The overlying sandstone comprises two superposed fining-upward cycles, several meters thick each, which grade from medium–fine sandstone to

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a thin siltstone at the top (Fig. 2). The sandstone varies from massive to cross-laminated and contains locally medium-sized trough cross-bedding, whereas the siltstone is faintly laminated. These beds are thought to have been formed in a braided-channel, distal proglacial environment. The top of the upper sandstone shows small recumbent folds and reverse faults below the sharp contact with an uppermost silty–sandy massive, clast-poor, 2 m thick diamictite (Fig. 2).

3. Structures The most prominent structure identified in the deformed zone (Fig. 3) is a long, horizontal to slightly inclined and undulating shear surface, which seems to cut across the whole outcrop. At the southern sector of the exposure, the shear surface separates sets a and c; to the right of the boulder it ascends reaching and then possibly following the contact between set c and the upper sandstone bed (unit 12). Toward the north, the shear surface seems to follow the top of the conglomeratic lens, and then cuts across unit 7 (sandstone) of set b, reaching the contact between this and the overlying diamictite (unit 11). The general shape of the shear surface is, therefore, amply concave upward. Besides this, other slightly inclined, possibly older shear fractures were identified cutting across units 7 and 8. Beds of set a form an isoclinal, recumbent fold truncated by the shear surface. It forms a well developed, cylindrical, recumbent drag fold with a subhorizontal axis, showing the synclinal axis extruded by two fractures (Figs. 3 and 4). The truncated, inverted limb of this structure dips toward the SSW. Tectonic movement from NNE to SSW is also indicated by tenuous polishing and striae trending in average NNE–SSW, and secondary shearing on the long shear surface. Other minor structures are small faults in the more competent sandstone beds and crumpling of the siltstone near the axis of the recumbent fold. Beds of set b have apparently been thrust toward the SSW, which resulted in deformation or upward ‘squeezing’ of the more ductile, coaly, silty–clayey bed, forming a sort of truncated, asymmetrical fold, also with vergence toward the SSW. Evidence of shear may also be seen in units 7 and

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8. These include anastomosing joints and interlaminated, sheared fine sandstone lenses and sandstone in the zone close to the boulder (Fig. 3, A, and Fig. 5). Extensional deformation is also evidenced by step-like normal faults in unit 1 (Fig. 3). The geometry of the structures which affect the sediments on the northeast and southwest sides of the boulder is highly suggestive of its behavior as an obstacle to sediment displacement during deformation. A less compressible zone of relatively greater shear strength was thus created. Compressive forces accumulating against the northeastern face of the boulder resulted in deformation and detachment of a slice of sediment from the base of the diamictite (unit 9). This was overthrust by a wedge of highly sheared sandstone (unit 8; Fig. 3, A). Southwest of the boulder, deformation (folding) of the conglomeratic sandstone and siltstone (unit 10), adjacent to the face of the clast (Fig. 3, D) could have been caused by the same, essentially horizontal, compressive regime (see discussion below.) Although megascopically massive, polished samples of the diamictite show a mixed texture of deformed sandy and silty laminae. Deformations include stretched isoclinal folds and fragmented sandy laminae (sand boudins?) forming pods crossed by fractures and sand dykes (Fig. 3). Other features found in the upper diamictite are less clearly related to the deformation process. Examples of these are two parallel, oblique, wedge-like bodies of diamictite (unit 11) that seem to be intruded into the underlying sandstone (unit 8; Fig. 3, B), and parallel, oblique, decimetric fractures in the basal part of the diamictite (Fig. 3, C). The former may have resulted from pressing of the diamicton into fractures formed in the underlying sandstone during deformation. Alternatively, they may have been formed by shearing of the sand and its injection into the overlying diamicton. The interpretation of these features in the context of the glacial history of the area is further discussed below. The parallel, inclined features, on the other hand, seem to correspond to complementary (shear) fractures associated with the main shear surface. It seems, therefore, that structures indicative both of compressive and extensive stress regimes may be recognized in the Cerquilho outcrop. Fig. 6 synthesizes the measurements of the main structural

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Fig. 5. (A) Anastomosing shear fractures in unit 8 (see text for description). (B) Photointerpretation to enhance features.

features that show orientation. It includes, for comparison, the orientation of a slightly asymmetrical anticlinal, cylindrical fold cropping out on the west-

ern road cut (not illustrated). Both the axis and the asymmetry of the fold indicate a vergence toward the south associated with a horizontal thrust from north

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3

1

Fig. 6. Principal stress axes derived from structural data at Cerquilho (Angelier’s method; Schmidt-Lambert, lower hemisphere stereogram). Blank squares: poles (7 measurements); black squares: axis of drag fold (east cut); blank triangles: poles (5 measurements); black triangle: axis of anticlinal fold (west cut); contours: shear planes (5 measurements).

to south, coherently with the movement deduced for the structures on the eastern cut (Fig. 6).

4. Origin of the structures The intraformational nature of the deformations which are confined between horizontal strata of the Itarare´ Subgroup rules out the possibility of their origin due to post-Paleozoic tectonism. The geometry and deduced structural regime of the Cerquilho deformations also exclude sliding=slumping or gravitational instabilities associated with stagnant ice as a cause. For the same reasons as cited above, though some of the features described (e.g. the folds in the coaly beds; unit 3; Fig. 3) clearly indicate a ductile behavior of the less competent sediments during deformation, assignment of all the structures described solely

to soft-sediment deformation (mud diapirism) as the origin, as suggested by Fulfaro et al. (1984, 1991), seems highly improbable. No evidence of vertical stresses were found in the outcrop either. Characteristics of particular importance indicative of the probably glaciotectonic nature of the deformations are: the overall consistent geometry and orientation of the structures (fold planes and hinges, drag folds and shear planes); the dominant vergence toward the southeast and the progressive change in structural styles and of intensity of deformation (Banham, 1988). In analogy with Pleistocene occurrences, examples of brittle and ductile deformation are found in Cerquilho. Evidence for both compressive and extensional, essentially horizontal, stresses was also noticed. In the Cerquilho outcrop, the deformed zone is immediately overlain by a tabular bed of silty–sandy, massive diamictite. Another diamictite bed of similar

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characteristics occurs in the upper part of the outcrop. Their sedimentological characteristics, as well as their stratigraphical relationships both local and within the regional geological context are consistent with their interpretation as subglacial tillites. The silt layer that partially surrounds the south side of the large granite boulder in the Cerquilho outcrop (unit 10; Fig. 3, D) is reminiscent of the sand drapes occurring over clasts that Shaw (1982) considers as diagnostic of melt-out till. Rappol (1986), however, argues that this feature may be explained by deformation, as a result of the more rigid behavior of the clast in a more ductile till matrix. A similar explanation is preferred in the present case (see below). Features around the large granite boulder and their deduced structural evolution are at first sight suggestive of models for variation in pressure and shear in the vicinity of large stones during subglacial sedimentation, as proposed by Boulton and Paul (1976) and Boulton et al. (1976, fig. 17d). It seems obvious from the geometry of the structures adjacent to the boulder that it served as an effective obstruction to movement, altering the stress field in its surroundings. The apparent absence of a higher concentration of clasts against its up-glacier face, as predicted in the above-mentioned models, is, however, more consistent with the features being generated after the subglacial deposition of the diamict. Structures similar to these are thought to develop during deformation of subglacial sediments due to the interference of a competent body (a clast for instance) with the deforming mass (Hart and Boulton, 1991, fig. 5B). Presence of a silt layer around the large boulder and deformation of the conglomeratic sandstone on its lee (down-glacier) side could then have resulted from rotation or slight displacement of the clast along the basal shear surface. Other features exhibited by the diamictites, particularly the lower one, are commonly found associated with Pleistocene subglacial (lodgement, deformation) tills (Hicock and Dreimanis, 1984; Rappol, 1986; Johnson and Hansel, 1989; Hart et al., 1990; Hart and Boulton, 1991; Hart, 1995; Pedersen, 1995; Klint and Pedersen, 1995; etc.). Lamination and associated structures found in the diamictite are closely similar to features interpreted as resulting from deformation under intense subglacial shearing (Hart and Boulton, 1991, fig. 4). Features compa-

rable to the wedge-shaped diamictite bodies found along the contact between the lower diamictite and underlying sandstone (unit 8) have been interpreted as related to the down-glacier opening of tension cracks due to glacial drag, subsequently infilled with till (Hicock and Dreimanis, 1984). Our structures, however, dip toward the north and, thus, against the deduced down-glacier orientation (see below). Wedges in Cerquilho are also comparable in both morphology and orientation to features illustrated by Johnson and Hansel (1989) and Hart (1995) and interpreted as being formed in a deforming till bed under high shear strain. The nature and distribution of structures in the deformed zone in the Cerquilho outcrop remind us of the model of constructional deformation described by Hart et al. (1990, fig. 3). In this model, the amount of deformation and percentage of fartravelled material increase upward through the deformed succession. As demonstrated by Pedersen (1995) and Klint and Pedersen (1995), however, different glaciotectonic phases may be recognized in sedimentary successions that have been subject to progressive deformation related to proglacial to subglacial regimes. Characteristic structures for each phase reflect the changing stress regime, as well as the rheological and hydrologic conditions of each bed involved. Although no attempt of glaciodynamic analysis has been made in Cerquilho, the relative age of some structures can be deduced on the basis of their apparent mutual spacial relationships. It seems also evident from the occurrence of compressive and extensional deformations that the sediments have been affected by progressive proglacial to subglacial glaciotectonism.

5. Implications for stratigraphy Fulfaro et al. (1984, 1991) utilized the name Rio Tieteˆ Formation (Barbosa and Gomes, 1958) to designate a predominantly sandy succession of deltaic, fluviatile and marine sediments with intercalated diamictite and thin coal beds in the Cerquilho area, in erosional contact with both the underlying Itarare´ ‘Formation’ and the overlying Tatuı´ Formation. The Rio Tieteˆ Formation is considered by them as postglacial and laterally contemporaneous with the coal-

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bearing Rio Bonito Formation (Early Permian) of the southern Parana´ Basin. This concept is essentially based on the interpretation of a non-glacial origin of the local diamictites, that are thought to represent former glacigenic deposits resedimented as debris flows in a marine environment. The diapiric origin adopted by Fulfaro et al. (1984, 1991) for the deformations in Cerquilho is not supported by the essentially horizontal compressive stress regime. The glaciotectonic origin proposed here for the deformation in the Cerquilho outcrop, and the interpretation of the diamictites as documenting glacial readvances in the area, the lower one tectonically affecting the fluviatile=deltaic and proglacial deposits, do not support the above stratigraphic proposal (Martini and Rocha-Campos, 1991). Additional evidence for the recurrence of glacial conditions in the uppermost part of the Itarare´ Subgroup in the central-eastern State of Sa˜o Paulo is furnished by an intradiamictite striated boulder pavement found at Jumirim, some 8 km to the northwest of Cerquilho (Rocha-Campos et al., 1968, 1969). According to these data, coal beds of the Cerquilho area are therefore considered as ‘interglacial’ (Martini and Rocha-Campos, 1991). The stratigraphic evidence in Cerquilho and Jumirim is also suggestive of a relative fast final deglaciation in this part of the Parana´ Basin.

(Hicock and Dreimanis, 1984; Hart and Boulton, 1991). Extension of these concepts to the pre-Pleistocene glacial record obviously presents numerous difficulties, some of them already commented upon above. In spite of this, attempts have been made to systematically describe and interpret features found at the base of or affecting sediments in erosional contact below thin, massive diamictites, similar to Recent and Pleistocene ones considered indicative of subglacial processes (Santos, 1979; Canuto, 1985; Canuto et al., 1991). Some features observed in the Cerquilho outcrop allow a few inferences about the rheological and stress conditions beneath the late Paleozoic glaciers and their dynamics. Absence of evidence of subglacial ‘floes’ in the overlying tillite and the style of the deformation indicate that the substratum over which the glacier moved was unfrozen, possibly water-saturated, deformable, mostly unlithified, fine sediment (Boulton, 1979). Presence of glaciotectonic deformation in Cerquilho may be indicative of the presence of a deforming bed (Boulton and Jones, 1979; Alley et al., 1986), which may have led to instabilities in late Paleozoic glaciers such as fast ice flow or surging. Such surging can help to explain the fast deglaciation rate that apparently took place in the Cerquilho area.

6. Late Paleozoic glacier dynamics

Acknowledgements

In recent years a considerable amount of literature has been produced on the glacier dynamics, rheology and stress conditions under glaciers flowing on unlithified to variably lithified sediments (see, for instance, Boulton, 1974, 1976; Boulton et al., 1976; Boulton and Jones, 1979; Boulton and Hindmarsh, 1987; Hart et al., 1990; Hart and Boulton, 1991; Hart, 1995). In addition to the more theoretical approach, analyses of glaciotectonic and other features associated with subglacial deformation (e.g. Hicock and Dreimanis, 1984; Van der Meer, 1987; Croot, 1988; Hart and Boulton, 1991) are also available. By interpreting the effects of former glaciers upon different substrata it may be possible to draw qualitative inferences of glacier dynamics and basal stress conditions

We thank Georg Sadowski, Claudio Riccomini, Carrie J. Patterson and Marcos E. da Silva for helpful comments on an earlier draft of this paper and to Ivo Trosdtorf for his help with the preparation of the figures. Support for this research was provided by FAPESP (Proc. 91=0546-2). We are also grateful to the reviewers Jane K. Hart and A.J. van Loon for their valuable suggestions for the improvement of the text.

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