ABSTRACT
The
site aims at protecting a classical exposure of varvite
from the Itararé Subgroup (Permo-Carboniferous), near Itu, in central-eastern
State of São Paulo.
The
Itu varvite is made up of rhythmites consisting of a lower, coarser,
light-colored bed/lamina of fine sandstone/siltstone overlain by a thin, dark
lamina of siltstone/argillite. Contact between pairs as well as between light
and dark layers of the same pair is sharp. Thickness of the light beds/laminae
vary vertically, but that of the dark laminae is constant.
Sedimentological,
palynological and paleomagnetic evidence indicate probable seasonal (annual)
control on the deposition of the lithologic pairs of the varvite, as in Pleistocene varve clays. About 300 pairs are present in the Itu
quarry.
Typical
sedimentary structures of the varvite are beautifully exposed in the quarry, as
well as abundant ichnofossils assigned to aquatic, benthonic invertebrates.
Though relatively rare, dropstones of varied composition and size (up to 2
meters in diameter) and mounds of glaciogenic debris liberated from icebergs are
found dispersed in the rock.
Probable
depositional environment of the varvite was a proglacial water body or lake
partially or temporarily in contact with the glacier margin. The light,
sandstone/siltstone beds/laminae were deposited by dense flows/turbidity
currents during summer, followed by the settling of muds from suspension during
winter, when the lake was frozen over.
The
Itu quarry, the best exposure of glacial rhythmite known in the Paraná Basin,
represents a classic geological monument related to late Paleozoic glaciation.
The monument is well preserved within the Parque do Varvito (Varvite Park).
INTRODUCTION
The
present geological site is being proposed to protect the magnificent exposure of
rhythmites of the Itararé Subgroup (Permo-Carboniferous, Paraná Basin) known
in the literature as varvite, that crops out in the historical Itu quarry,
within the urban area of Itu, State of São Paulo.
Although
the terms varvic and varvoid have been used for laminated rocks of the Itararé
Subgroup, occurrences of regular rhythmites similar to the Itu varvite are
relatively rare in the Paraná Basin (Rocha-Campos and Sundaram, 1981;
Rocha-Campos et al., 1981; Santos et al., 1996). The Itu quarry constitutes one
of the most extensive, thickest and best preserved exposures of varvite in the
Paraná Basin, and it is undoubtely the widest known nationally and
internationally. Besides.documenting one of the typical sedimentary environments
of the Gondwana glaciation, the quarry presents is also important historically
linked to the settlement of western São Paulo State. Flagstone from the Itu
quarry has been utilized as a building stone since at least the foundation of
the town of Itu, some 300 years ago. Location of the quarry, near São Paulo,
made it the focus of great scientific and educational attention for high school
and college students and the general public as well as a reputable place for
visits by national and international scientists interested in the geology of
Gondwana and late Paleozoic glacial deposits of the Paraná Basin. It is
undoubtly a classical visiting site for all field trips devoted to the Gondwanic sedimentary sequence of the northern
Paraná Basin (Rocha-Campos et al., 1972; 1988).
LOCATION
The
quarry is situated on the premises of the Parque do Varvito (Varvite Park), in
the urban area of Itu, at Avenida do Varvito (Varvite Avenue), unnumbered, about
96 km northwest of São Paulo. The park may be reached from km 27 of the Rodovia
do Açucar (Açucar Highway), (SP-79), by travelling northeastward along the
Avenida do Varvito (Fig.1).
HISTORY
The well know flagstone of the Itu quarry has been used for paving buildings and
sidewalks of the main streets of Itu since at least the beginning of the 18th century. José Bonifácio de Andrada e Silva (the “Father of the
Independence of Brazil”) and his brother, Martim Francisco Ribeiro de Andrada
referred to a rock corresponding to the varvite in their report: “Viagem
Mineralógica na Província de São Paulo” (Mineralogical Voyage in the
Province of São Paulo), published in 1820 (Mendes and Rocha-Campos, 1963).
Other explorers and naturalists who travelled in the Itu area in the end of 19th
century and beginning of 20th century also made reference to this rock
(Oliveira, 1887; Gonzaga de Campos, 1888; Florence, 1907; apud Mendes, 1944).
The notable Itu painter Miguelzinho Dutra left a visual record of the
“Pedreira de Itu” (Itu quarry) in a significant watercolor painted in 1841.
The Itu varvite seems to have been deposited in an environment predominantly
dominated by density underflows/turbidity currents associated with subsidiary
inter and overflows, produced by melting water moving down marginal deltas
during summer. The summer sandy-silty layers were capped by a winter
silty-clayey laminae deposited from suspension when the water body was covered
over by ice. Though the dark laminae are uniform in thickness throughout the
quarry, the presence of clayey partitions suggests that less intense underflows
or turbidity currents may have formed during winter.
SITE
DESCRIPTION
The site corresponds to an inactive rhythmite quarry locate within the Parque do
Varvito (Fig. 2). The total area of the park is 44,346 m2. The
varvite crops out on several vertical, plane rock faces that correspond to the
former areas of extraction (Fig.3). These are clearly controlled by a system of
mainly orthogona1 vertical fracture oriented N18oE, N36oE,
N91o-101oE and N141o-151oE. Main
exposures lie on the southern side of the quarry. Other outcrops may be seen in
the northwest corner of the park that seem to correspond to the oldest quarry
faces.
Figure
2 - General view of the park showing main exposures of varvite.
Maximum thickness of varvite exposed in the quarry is about 15 m. On the various
faces, a continuous series of about 260 lithological pairs or couplets has
been identified and measured (M. Ernesto, personal communication, 1990).
Thickness of pairs diminishes upward, from around 50 cm near the quarry floor
to about 1.5 cm in the upper, weathered part of the quarry (Fig. 4). Thinner
or thicker pairs occur sporadically however throughout the section. The thinning-upward trend of pairs is accompanied by a fining-upward change in
grain size, particularly visible in the light, thicker beds/laminae, that
change from fine sandstone at the base to more silty at the top. Still thinner
pairs still may be observed in the upper part of the section cropping out
along the street beside the park toward Itu. Rocha-Campos et al. (1972)
estimated a total of around 300 pairs of lithologies. Thining of pairs is
mainly due to variation in thickness of the light beds/laminae, the dark
beds/laminae maintaining an average thickness of around 5 mm.
Sedimentary structures are abundant and diversified in the varvite (Fig.5). Rocha- Campos
and Sundaram (1981) pointed out that the separation between each pair is
sharp. Sharp contact is also noted between lower, light bed/lamina and upper,
dark lamina of each pair. In the former, thin partings of dark
siltstone/argillite may occur while partings of siltstone may be seen within
the dark laminae. In addition, other sedimentary structures found in the
bright, lower beds/laminae are ripple-drift, micro-cross lamination, draped
laminae of siltstone over ripple marks, and multiple, normally graded laminae.
Clasts varying in size from mm to dm, of varied composition, mainly granite
and quartzite, are intercalated in the rhythmites, deforming the beds above
and below them (Fig.6). During construction of the park several metric-size
boulders exhumed from the weathered varvite were found. They are identified
and preserved on several points of the park.
The Itu varvite is notable for its abundance of trails made on bedding planes by
aquatic, benthonic invertebrates (Fig. 7). Two ichnogenera have been
identified: Isopodichnus and Diplichnites
(Fernandes et al., 1987) identified, possibly representing different types
of impressions of the same animal. Elliptical resting marks have also been
found. Ichnofossils are better preserved on top of dark laminae. Besides
these, other fossils present in the varvite include a diversified palynoflora
studied by several authors (e.g.: Daemon and Quadros, 1970; Kemp, 1975; Dino
et al., 1987). The biochronological interpretation of the assemblages is,
however, contradictory, some results being taken as indicative of a late
Carboniferous age, while other seem to indicate an early Permian age.
Stratigraphic evidences, however, suggest that the varvite and associated
rocks from the Itu area correspond to the younger intervals of the Itararé
Subgroup.
PROTECTIVE
MEASURES
The
former varvite quarry of Itu is presently duly protected by several measures
undertaken by municipal and state governments, as follows:
a) registration of part of the quarry area in 1974 by the Secretaria de
Estado da Cultura de São Paulo (São Paulo State Secretariat of Culture), under
a proposal from the Conselho Estadual de Defesa do Patrimônio Histórico, Artístico,
Arqueológico e Turístico do Estado de São Paulo (Council for the Protection
of the Historical, Artistic, Archeological and Touristic Patrimony of the State
of São Paulo (CONDEPHAAT);
b) expropriation of the quarry and surrounding area totaling 44,346 m2,
including the area formerly registered by the Municipality of Itu, in 1993;
c) construction of the Parque do Varvito (Varvite Park) in the above area by
the Municipality of Itu. The park, inaugurated in 1995, is the second of its
kind installed in Brazil aimed at integrating the protection of an important
geological monument represented by the varvite quarry with the planned, rational
use of the park for leisure, cultural activities and environmental education.
The park is equipped with adequate infrastructure to meet its multiple uses. It
is intensely visited by the local population and visitors from other places in São
Paulo and Brazil, including students of all levels, as well as researchers from
Brazil and abroad. It is often included in the itinerary of geological field
trips associated with scientific meetings (Rocha-Campos et al., 1972, 1988) or
organized by ecotourism companies in the State of São Paulo.
ACKNOWLEDGEMENTS.
I thank Paulo R. dos Santos, José R. Canuto, Ivo Trodstorf Jr. and Alexandre
Tomio for their help during field work in Salto and Itu. I am also grateful to
Paulo R. dos Santos for his comments to the text and Thomas R. Fairchild for the
review of the English version. This
paper is a contribution to the Projeto Temático: “Controles tectônico, climático
e paleogeográfico das características, gênese e preservação dos depósitos
glaciais pré-cenozóicos do Brasil”, supported by FAPESP (Grant
91/0546-2).
REFERENCES
