Eãúy Silurian glaciations in Brazil

Palaeogeography, Palaeoclimatology. Palacoecology. 99 (1992): 9-15
Elsevier Science Publishers B.V., Amsterdam
9
Eãúy Silurian glaciations in Brazil
Yngve Grahn^-' and Mário V. Caputo''
^Petróleo
" Slockholm Universily, Department of Ceology and Geochemistry, S-I069I Stockholm, Sweden
Brasileiro S.A. (Petrohrás). Northern Exploration District, Rodovia Arthur Bernardes 5511. Belém.
66000. Brazil
Para
(Received February 11. 1992; rcvised and acccplcd June 5, 1992)
ABSTRACT
Grahn. Y . and Caputo. M . V . . 1992. Early Silurian glaciations in Brazil. Palaeogeogr.. Palacoclimatol. Palaeoecol.. 99: 9-15.
The opening ol' the Brazilian Paleozoic basins during the Late Ordovician was followed by a tectonic stabilization in the
Silurian. A t the time of the Early Silurian glaciations. dcposition of continental sandslones prevailed in the basins. Offshore
environments were prcscnt in the Amazonas Basin and, to a sraaller extent. in the P a r a n á Basin. The ice shects advanced over
fluviatile and littoral sediments. except for the south margin o f the Amazonas Basin. where the glacial diamictites immedialely
overlie marine fácies. The ice produced much meltwater. even when advancing. and this was probably climatically controlled.
The ice movements were from the northwest at the northern margin of the Amazonas Basin. and towards the northeast at the
southern margin. In the Parnaiba Basin and its outliers, the J a t o b á Basin and Cariri Valley, the ice came from southeast. and
probably also from east. Further to the south. in the P a r a n á Basin, it came from the north and northeast.
Four glaciations have been dated to the Early Silurian. The oldest is dated as latest Ashgill and or carliest Llandovery in
the P a r a n á Basin. as evidenced from tillites in the upper Rio Ivai and lower Vila Maria Formalions (including the former l a p ó
Formation). It is contemporaneous with a glaciation in the Cape Basin. South Africa. The next glaciation is dated as early
middle Llandovcry (gregarius graptolite Zonc) and is present in the Amazonas Basin and lhe Andean Belt, and probably also
in the Parnaíba Basin and its outliers. The two youngest tillites were widespread in the northern Brazilian basins (Amazonas,
Parnaíba and Ja to b á basins and Cariri Valley), as well as in the Andean Belt, and are dated as early late Llandovery (possibly
starting already in the late middle Llandovery) and latest Llandovery to earliest Wenlock. The latter tillitc is the most widespread
one. Assuming these glaciations are related to polar positions, the apparent migration of the South Pole ice cap must have
been quite rapid, from central N o r t h Africa in late Ashgill towards central Brazil in the middle-late Llandovery. The glaciation
in the P a r a n á Basin took place up to 45° from the late Ashgillian South Pole. The cool climate continued during the Silurian,
but no new glaciations are known bcfore latest Devonian.
Introduction
Mountain Group in the Cape Basin, South Africa,
by Rogers (1902, 1904) and Rust (1973, 1981). In
Late Ordovician and Early Silurian glacial the Andean Belt from northern Argentina across
events have been extensively described in the litera- Bolivia to Peru (Berry and Boucot, 1972) Silurian
ture. Late Ordovician glaciations are well known glaciations (Crowell, 1978; 1981) are dated as
from central North Africa (cf. Beuf et ai., 1971) middle-late Llandovery on the basis of graptolites
and Arábia (cf. McClure, 1978), and Early Silurian (Berry and Boucot, 1972; Laubacher et al., 1982),
Traces o f Silurian glaciations in Brazil have been
glaciations have been reported from the Table
known since the late 1940s (e.g., Maack, 1947).
Correspondence lo: Y . Grahn. c/o F. Paris. Université dc Glaciations of this age are described from the
Rennes, Institute de Géologie, Laboratoire de Palcontologie et Amazonas, Parnaíba, Jatobá, and Paraná basins
Stratigraphie, Ave. du General Leclere, 35042 Rennes Ccdcx, (Fig. 1). Traces of glaciations have also been
France.
reported from the Cariri Valley (Fig. 1), east of
' Present address: Swedish Muscum of Natural History. Departthe Parnaiba Basin (Caputo, 1984). The age of
ment of Palaeozoology, Box 50007, S-10405 Stockholm,
these glaciations was previously believed to be
Sweden.
0031-0182/92 S05.00
;c 1992 Elsevier Science Publishers B.V. A l i rights reserved
10
Fig. 1. Map showing the Brazilian Paleozoic basins discussed in lhe text. The ice movements are indicated by arrows. Outcrops
shallow borings with tillites are marked with triangles and subsurface (decp borings) tillite occurrences with dots.
Early Silurian in the Paraná Basin, and Late Brazil (Fúlfaro et al., 1982). The foramtion of the
Ordovician in the other basins (Caputo and Cro- basins in the late Ordovician was followed by
well, 1985). New biostratigraphic investigations tectonic stabilization during the Silurian (Grahn,
from Brazil, based on graptolites (Jaeger, 1976) in press). A t the time of the Early Silurian glaciaand chitinozoans (Grahn and Paris, 1992), have tions, deposition o f fluviatile (unidirectional and
made it possible to date lhe glaciations more high-angle cross-bedding) and littoral (thick vertireliably. For a historical review and a general cal burrows or Skolilhos in fine- to medium-grained
description of the Brazilian Paleozoic basins see quartzites) sands prevailed in the basins. Glacial
channels in the Amazonas Basin are represented
Melo (1988).
by quartzitic subwackes with slumping and contorted structures (Carozzi et al., 1973). Ofl"shore
Geologic setting and lithology
environments were present in the Amazonas Basin
The marine Paleozoic sedimenlation recorded and, to a smaller extent, in the Paraná Basin. Early
nowadays in the Amazonas, Parnaiba and Jatobá Silurian glacial and periglacial beds correspond to
basins (Fig. 1) took place in huge intracratonic the Nhamundá/lower Pitinga formations in the
basins (Caputo, 1984; Grahn, in press). In the Amazonas Basin, Ipu/Tianguá formations in the
Early Paleozoic the future Paraná Basin (Fig. 1) Parnaíba Basin, Ipu Formation in the Cariri
formed a marginal basin tilted to the west in south Valley, Tacaratu Forination in the Jatobá Basin,
11
and the Rio Ivai/Vila Maria Formations (including sideritic interbeds and nodules occur throughout
the former lapó Formation) in the Paraná Basin the unit (Caputo, 1984). A transition zone is
(Figs. 1 and 2). The Rio Ivai Formation probably present in the top of the N h a m u n d á sandstones.
This zone has chamositic wackes and oolitic haemalso includes late Ordovician glaciogenic beds.
The N h a m u n d á Formation consists of quartzites atite with phosphate and late diagenetic siderite
(dominant), quartzitic subwackes, quartzitic cement. Alternating beds are of massive siderite.
wackes, chamositic wackes and diamictites (Car- The transition beds indicate low sediment supply
ozzi et al., 1973). It can be divided into three units. from the continent, a slow rate of sedimentation,
The lower unit is made up of angular, fine- and with predominantly chemical deposition, and
medium-grained, kaolinitic and highly cross- destruction of the faunal remains in a reducing to
bedded sandstone beds. This part is not exposedoxidizing environment (Carozzi et al., 1973).
but is present in the subsurface throughout the Corresponding beds in the Parnaiba Basin and
basin. The middle unit yields light grey, hummocky the Cariri Valley make up the Ipu Formation. This
cross-stratified, and ripple cross-laminated sands comprises pebbly sandstones, conglomerates, sandwith Skolithos and minor grey carbonaceous shale stones and diamictites. The pebbly sandstone is
interbeds. In the upper unit the beds are silty with massive or cross-bedded, white to cream coloured,
increasingly argillaceous sandstone and medium- friable to well-cemented and contains scattered
to fine-grained, highly cross-bedded sandstone quartz pebbles up to 5 cm in diameter. Conglomerinterlayers. Ichnofossils are common (Caputo, ates are recurrent and mainly constituted of white
1984), The N h a m u n d á Formation crops out from to light grey quartz, quartzite boulders and pebbles
the Rio Negro to the Rio Pitinga in the northwest with a sandy and clayey matrix. The Ipu Formapart of the basin. The best exposures are along tion pinches out from northeast to the west, where
the Rio Urubu (Swan, 1957). The N h a m u n d á interfingering shale beds of the Tianguá Formation
sandstones interfinger with, and are also overlain are present (Caputo, 1984; Grahn, 1991). The
by, shales of the lower Pitinga Formation that Tianguá shales consist of dark grey, bioturbated
constitutes a shaly lateral fácies of the sandstonesshales with mudstone and siltstone intercalations
(Grahn and Paris, 1992). The Pitinga Formation and light grey sandstones in the middle part
consists of green-grey to dark grey, micaceous, (Caputo, 1984). East of the Parnaiba Basin a series
thinly laminated, soft, carbonaceous, pyritic, partly of outliers occurs. In the largest one, the Jatobá
sideritic shale with interbedded shaly siltstone. Basin (Fig. 1), glacial and periglacial deposits have
Haematite interbeds are present at the base, andbeen found in the Tacaratu Formation. This for-
r
A,
j'^'^!.
^
-^^P
Fig.
2. Corrclation diagram for the Ashgill- Wenlock in lhe Brazilian basins discussed. Dashcd lings indicate diamiclitc layers.
12
malion can bc partly correlated with the Serra dal weathering. Since the fissures are filled wit
Grande Group in the Parnaíba Basin and is com- siltstones from overlying beds, these must ha
posed of conglomerates, sandstones and kaolin been in situ when lhe ice transgression reach
lenses (Caputo, 1984).
them (Grahn and Melo, 1990). These layers a
In the Paraná Basin the Ordovician-Silurian also present on the southern margin along the R
transition probably occurs within the upper Rio Xingu (i.e., Altamira and Belo Monte). They are
Ivai and lower Vila Maria Formations (Fig. 2). generally 1-2 m thick, but may reach 10 m (a
The former yields sandstones with rarc shale inter- Rio Trombetas). These beds are of the same
calations, basal conglomerates and diamictites as the tillites in the N h a m u n d á Formation and a
(Grahn, in press). The sandy conglomeratic and connected with the glaciations (for the dating, s
diamictitic lower part of the Vila Maria Formation below). Locally, a deglaciation is indicated in the
is overlain by greenish-grey to brown calcareous top o f the N h a m u n d á Formation by fluviatile
shales, above which occur calcareous sandstonessandstones.
and siltstones.
In the Parnaíba Basin tillites are known from
the Serra Vermelha and Serra da Capivara, St
of
Piauí (Fig. 1). In the former locality tillites up
Glacial and periglacial sedimentary structures
to 6 m thick occur, with diamictites containing
In the Amazonas Basin three diamictitic hori- quartz, quartzite, fine-grained sandstone and cry
zons have been described from the upper Nha- talline rock clasts within a massive sandy an
mundá Formation. The N h a m u n d á Formation is clayey matrix. A l the latter locality the tillites are
widely distributed in the subsurface, and diamic- up to 20 m thick and yield fine- to coarse-grained
tites have been reported from four wells (Rodrigues massive to strongly cross-bedded, poorly sorte
et al., 1971) situatcd at lhe margins of the basin argillaceous and micaceous, white lo light gre
diamictites. Kegel (1953) found some faceted pe
(i.e. 2 - S L - l - A M , 2 - M N - l - A M , 2-PE-2-AM, and
bles in the tillites, but did not observe any striate
2-AI-2-AM; Fig. 1). The diamictites in the Amazonas Basin form distinct lobes, and were therefore ones. Diamictites have also been found in we
interpreted as true tillites (Carozzi et al., 1973). 1-CI-I-MA (Fig. 1).
Along with the tillites, sideritic, hematitic and
Part of the Ipu Formation consists of glaciogen
chamositic shale beds and subaerial deposits occurdeposits reworked by periglacial outwash slream
(Caputo, 1984). The youngest tillite (10-15 m but fan delta front environments were present
thick) displays a clayey and silty matrix with the central part of the basin (Caputo, 1984). I
dispersed sand-sized grains, granules and pebblesthe Jatobá Basin the glacial deposits of lhe Taca
with an average diameter of around 3 cm atu Formation display quartzitic and felspar peb
(Rodrigues et al., 1971; Carozzi et al., 1973). The bles in an argillaceous matrix (Mabesoone, 1977
matrix is mainly of chlorite, which predominates and in the Cariri Valley a tillite outcrops conlain
over illite. The diamictites at the northern margin dispersed rounded and angular pebbles and co
mainly originate from sedimentary rocks and, at bles from igneous and metamorphic rocks of th
the southern margin, from sedimentary, metamor- basement. these are supported by a sandy a
phic and igneous rocks. In the outcrop belt diamic- clayey groundmass. Diamictites occur in the tran
tites are present at Rio Carabinani, a tributary of ition between the Rio Ivai and Vila Maria Forma
the Rio Negro (Fig. 1), in the State of Amazonas tions of the Paraná Basin (Faria, 1982; Zalán et a
(Caputo and Saad, 1974). In shoreface and offshore 1987). These are reddish in outcrop and grey
positions shale beds are lateral equivalents of the the subsurface. The clasts are angular and sub
tillites.
gular, up to 100 cm in diameter, and supported b
A t the northern margin of the basin, on the Rio a sandy-silty- clayey matrix. The pebbles consi
Trombetas (i.e.. Cachoeira Viramundo), basal dia- mostly of granite, gneiss, rhyolite and quartzite
mictites or breccias are found to include blocks The thickness of the diamictite layers varie
and pebbles from the basement that show sphaeroi-between ca. 16 m in outcrops and almost 70 m
13
the subsurface. In the Vila Maria outcrop belt Age of the glaciations
from Rio Araguaia to Rio Caiapó, State of Goiás,
diamictites have been observed at Tres Barras The oldest Silurian glaciation in Brazil is ind
Farm and Aldeia Creek, Bom Jardim County, and rectly dated by the overlying beds in Vila Mar
Formation. In Goiás State these consist of shal
in the Vila Maria locality south of Arenópolis.
Rio Ivai (lapó) diamictites crop out along the road marine, perhaps lagoonal, shales with shelly fos
between Pirai and Ventania in the State of Paraná. Gray et al. dated the beds as early LlanDiamictites have also been reported from wells dovery, and Grahn as early-middle Llandovery. In South Africa diamictites occur in th
-RI-l-PR,- M O - l - P R and-JT-l-PR (Fig.
upper part of the Pakhuis Formation within t
Table Mountain Group. The overlying Cedarbe
Early Silurian ice movements in Brazil
shales have been dated as early Llandovery
Berry and Boucot This circumstance delimThe major glacial lobes in the Amazonas Basin its the diamictites in the Cape Ranges to la
advanced basinwards from northwest along the Ashgill and/or earliest Llandovery, so they a
northern shore. During the latest Llandovery one most probably contemporary with the diamictit
of the lobes shifted to a more westward direction in the Paraná Basin, as suggested by Caputo
Lima This conclusion is further strength(cf. Carozzi et al., There the glaciofluvial
fácies grades basinwards into littoral beach and ened by paleogeographic reconstructions sugg
then into shoreface and offshore conditions (Car- ing a short distance between the Paraná and C
basins during the Silurian (Smith et al., and
ozzi et al., The glacial lobes at the south
margin advanced from the southwest towards others). No fossiliferous Silurian beds young
northeast and, in contrast to the ones at the than middle Llandovery are so far known fro
northern margin, overlap marine fácies. A t the the Paraná Basin (Grahn,
I
I
I
southern margin there is a very broad zone of
The dating of the next glaciation is problemat
littoral fácies with eroded glacial materiais from It records an advance of continental ice during
the southwest. Basement clasts have been trans-early middle Llandovery (M. gregarius graptoli
ported basinwards for more than km from the
Zone). Fóssil evidence is present only at the no
present surface position of the basement, on a ern margin of the Amazonas Basin, from be
substrate dipping about (Caputo, The
immediately overlying the lowest diamictite lay
glacial lobes are always surrounded by fluvial at Rio Trombetas (e.g., Monogniptus cf. gregar
deposits grading into littoral and offshore condi- Conochitina cf. iklaensis, and others). There is
tions. It would appear that the ice sheets continu- definite evidence for early middle Llandoveria
ously produced meltwater even during advances. glaciations elsewhere in Brazil. In the Andean B
This phenomenon is probably climatically con- there appear to have been contemporary gla
tions (Berry and Boucot, Laubacher et al.,
trolled, occurring where the ice lobes reached
warmer áreas (Carozzi et al., In the Parnaiba
and Jatobá basins and the Cariri valley the contiThe next advance of the continental ice occur
nental ice advanced towards the northwest, and during the early Telychian (late Llandovery),
probably also towards the west, over the basin possibly still in the latest Aeronian (middle Lla
(Mabesoone, Caputo and Lima, Furdovery). This event is well documented ali ov
ther to the south, in the Paraná basin, the ice the Amazonas Basin. Shale layers in the ba
moved from the north and northeast from the lateral to the actual tillite at the margins ha
Brazilian and Guaporé shields, as is shown from yielded a characteristic early Telychian chitinozo
the lithology of diamictite clasts (Caputo and fauna. Shelly fossils are unknown from this lev
as well as in older parts of the Nhamundá a
Crowell, This indicates that the glacial
centre was the same as for the Cape Basin in SouthPitinga formations. This glaciation is probab
also represented in the Parnaiba and Jatobá b
Africa (cf., Rust,
14
and Cariri Valley. In the former basin, for instance, (1992), the apparent migration of the geographic
two diamictite horizons are present (Lima and South Pole and related ice cap was quite rapid,
Leite, 1978), of which the lower one may be of from central North Africa in the late Hirnantian
(late Ashgill), towards South America in the Aeroearly Telychian age.
The youngest tillite is the most widespread one nian (middle Llandovery). The glaciations in the
and is dated as late Telychian to earliest Wenlock Cape and Paraná basins probably extended up to
(Fig. 2). The shelly fauna described from the Ama- 45" away from the South Pole of that time, since
zonas Basin by Derby (1878) and Clarke (1899) is the South Pole could hardly have been subjected
restricted to this age interval (Grahn and Paris, to apparent migration from North to South Africa
1992). A hiatus above the N h a m u n d á sandstonesand back to central Brazil during the early Llancomprises a gap from earliest Wenlock up to late dovery, a time interval extending only over ca. 2-3
Wenlock (Grahn and Paris, 1992; Grahn, 1991). million years. The next glaciation in South America
The tillites in the Parnaíba Basin and the outliers following the Early Silurian ones is only recorded
(Jatobá Basin and Cariri Valley) are contemporary. in the middle Famennian (Caputo and Crowell,
Beds of interfingering shales of the Tianguá For- 1985; Caputo, 1985). The climate remained cool
mation in the uppermost Ipu Formation, corre- in the Silurian and Early Devonian, although there
sponding to the tillites, are dated as late is regional sedimentological and paleontological
Llandovery (Grahn, 1991). In the Ipu sandstones evidence for a warmer climate during the Middle
the only macrofossil known is a specimen tenta- Devonian and early Late Devonian in South Amertively identified as Arthrophycus sp. (Moore, 1963). ica (Melo, 1988).
a trace fóssil considered to be Ordovician or
Sijurian (Hântzschel, 1975). Seemingly, the tiUites
Acknowledgements
in the Parnaiba Basin and the outliers are coeval
with those in the Amazonas Basin, and are thus
We are indebted lo Petrobrás-Petróleo Brasileiro
of Llandovery age. The most widespread tillites
S/A for lhe permission to publish this paper. The
present are probably of late Llandovery to earliest
following colleagues have provided us with comWenlock age.
ments on the manuscript; J.H.G. Melo (Petrobrás,
Rio de Janeiro), E . M . Meister (Petrobrás, Rio de
Concluding remarks
Janeiro), M . Lindslrõm (Slockholm Universily,
Stockholm) and J. Bergstrõm (Swedish Museum
Several glaciation centres of Early Silurian age of Natural History, Stockholm). S. Wolff (Petrodeveloped in the uplands in central Gondwana. brás, Curitiba) provided us with information about
The ice sheets advanced on three or four occasionsthe Paraná Basin. A grant from the Swedish
during the Llandovery over fluviatile and littoral
National Science Research Council ( N F R G - G U
sediments and, at the southern margin of the 8811-302) is gratefully acknowledged by Yngve
Amazonas Basin, also over marine environments. Grahn. Our sincere thanks lo ali.
These circumstances, plus the absence of tillites
outside South America (possibly with the exception
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