Abstract

The Fonseca Basin, located at the Quadrilátero Ferrífero region (State of Minas Gerais, Southeastern Brazil), is a classical example of Brazilian Tertiary sediments. It has called the attention of several researchers since the second half of the 19^th century for its deposits of "canga", lignite and fossiliferous sediments. The main lithotypes consist of sandy and clayey sediments of the Fonseca Formation, probably dated as Eocene. The ironstone pebble conglomerates ("canga") of the Chapada de Canga Formation cover them. The Fonseca Formation is a very fossiliferous deposit. A great amount of leaves and branches of Angiospermae, mainly from the families Melastomatacea and Mimosacea, have been recovered. The most spectacular fossil ever recovered in the Fonseca Formation belongs to the Family Bombacacea - an almost complete flower with impressions of the petals and the male reproductive organ. It represents the first record of a flower in the Cenozoic of Brazil.

Introduction

The small sedimentary basin of Fonseca is located at the Quadrilátero Ferrífero region, state of Minas Gerais, and represents a classical example of Tertiary sediments in Brazil. It is recognized as an important Brazilian paleontological site. Since the first studies of Gorceix (1876, 1884), several researchers have admitted the importance of this basin for the knowledge of the Paleogene flora of Brazil.

The geological and paleontological interest in the Fonseca Basin has economical and scientific reasons due to the "canga" and lignite deposits, which were occasionally explored. The lignite deposits preserve an unusual richness of fossil plants, an evidence of luxurious vegetation during its deposition (Gorceix, 1884; Lima & Salard-Cheboldaeff, 1981). Besides, Tertiary floras of tropical regions in the South Hemisphere are especially important to the study of Angiospermae dispersion and its migration to the northern region (Duarte, 1956).

Location

The Fonseca Basin is located at the eastern boundary of the Quadrilátero Ferrífero, in the central-eastern part of the State of Minas Gerais, near to the Fonseca Village, Alvinópolis city. It is situated at the eastern foothills of Serra do Caraça (Fig. 1; Photo 1). The Fonseca Basin, as defined by Sant’Anna & Schorscher (1997), is placed at 20º10’S and 43º20’W.

The Fonseca Basin is 107 km far from Belo Horizonte and one can reach the basin taking the non-paved road MG-326, from the city of Santa Bárbara to Catas Altas and Santa Rita Durão. The Fonseca Village is located about 15 km to southeast of Catas Altas and 12 km to east of Santa Rita Durão. The movement in the area is made using non-paved secondary roads. The main roads in the area go from Fonseca to Catas Altas and from this city to Santa Rita Durão, in the northern and western boundaries.

Historical Overview

The Fonseca Basin has called the attention of several researchers since the second half of 19^th century, for its deposits of "canga", lignite and fossiliferous sediments. Gorceix (1876) was the first researcher to study the basin, describing the "canga", the sedimentary deposits and some of the fossils. He also discussed the genesis, the probable source areas, and the age of the sediments, which he considered as Quaternary age due to the similarities between the fossil leaves and the extant regional vegetation. After, Gorceix (1884) discussed the lithology and the fossiliferous contents of the fresh water Tertiary basins of Gandarela and Fonseca, assigning to the last one a lacustrine origin and upper Miocene or Pliocene age.

Photo 1: General view of the regional morphology in the area of the Fonseca Basin. Notice, at the background, the Serra do Caraça and, at the foreground, the Chapada de Canga plateau.

Detailed works about the fossils of the Fonseca Basin were only performed latter on, when Berry (1935) described fifteen new species of fossil plants. Other fossil plants were characterized by Dolianiti (1949, 1950), Curvello (1955), Duarte (1956, 1958, 1974) e Sommer & Lima (1967). Even though fossil plants are notably more abundant, Gorceix (1976) quoted the occurrence of a fossil fish and Costa-Lima (1944) refers to the presence of fossil insects of the Order Isoptera. The study of the palynomorph contents was started a lot later. Based on the analysis of the pollen contents, Lima & Salard-Cheboldaeff (1981) suggested an Eocene age for the sediments of the Fonseca Basin.

Maxwell (1972) defined the Fonseca Formation for the area, describing the type section near the Fonseca Village. According to this author, the unit comprises an 86m thick succession of fluviolacustrine sandy-clayey sediments, covered by ironstone pebble conglomerates ("canga"). The "canga" was originally included in this unit because it was interpreted as a remnant sedimentation of the basin border. Maxwell (1972) also presented the first geologic map of the Fonseca Basin, in 1:100.000 scale, showing the Fonseca deposits covering an area of 35 km². In recent stratigrafic review of the Fonseca Basin, Sant’Anna (1994) and Sant’Anna & Schorscher (1997) recognized two stratigraphic units in the basin: Fonseca Formation and Chapada do Canga Formation.

Site Description

Geology

The Fonseca Basin is developed over Archean rocks of the Quadrilátero Ferrífero ("Iron Quadrangle"). These rocks belongs to the TTG Association (Tonalite-Throndjemite-Granodiorite) of the Regional Complex of Granitic (sensu lato) Rocks (Schorscher, 1992) and to the Rio das Velhas Supergroup (Schorscher, 1978) – Fig. 1. Proterozoic metasedimentary rocks composed by quartzites and itabirites of the Minas Supergroup and quartzites of the Espinhaço Supergroup (Dorr, 1969; Inda et al., 1984) constitute the highlands that limit the area (Schorscher, 1980; Luchesi, 1991; Davies, 1993).

The most important lithologic unit of the Fonseca Basin, as a Brazilian paleontological site, is Fonseca Formation, recently redefined by Sant’Anna & Schorscher (1997). This unit comprises sandy, sandy-clayey (sometimes fossiliferous and papiraceous) and clayey sediments. The most representative columnar section of the Fonseca Formation is presented in figure 2.

The deposits of the Fonseca Formation represent a Tertiary meandering fluvial system. It was developed over the regional Precambrian basement during an interval of relative tectonic quietude, probably under humid climatic conditions (Sant’Anna & Schorscher, 1997). The sandy, sandy-clayey and clayey sediments were deposited in this sedimentary environment, showing a fining-upward pattern.

Sometimes, the clayey-sandy sediments are fossiliferous and papiraceous. They were produced in oxbow lakes, where the low rate of sedimentation was responsible for the preservation of the macrofossils. Organic-rich laminated mudstones and sandstones presenting fossil remnants (fragments of branches and leaves) are typical deposits of the Fonseca Formation. Lamination is the predominant sedimentary structure in this unit, being well developed in the sandy-clayey and clayey sediments. Slump bedding (atectonic convolute bedding) and bioturbation are locally preserved in the mudstones. Sandstones show low-angle tabular cross-stratification at the basal portion of the available exposures, and they are massive at the middle part of the available exposures. There are not outcrops showing the basal portion of the Fonseca Formation and its maximum thickness that can be observed at the available exposures is equal to 20 m.

The main source areas of the sediments of the Fonseca Formation were the Archean units of the Quadrilátero Ferrífero. The granitic rocks of the TTG Association were the main contributor to the quartzose composition of the sandstones and to the kaolinitic (detrital kaolinites) of the mudstones. The mafic and ultramafic rocks of the Rio das Velhas Supergroup have also contributed to the sediment composition.

The diagenesis of the Fonseca Formation led to the compactation of the sediments, particularly the paper shales, and to the generation of authigenic kaolinites with a face-to-face texture. Besides the erosion of the deposits of the Fonseca Formation and the chemical decomposition of the pre-existing minerals, modern-weathering processes is promoting the generation of kaolinites with a wormlike texture.

Based on paleobotanical studies, Sommer & Lima (1967) considered the Fonseca Formation as Miocene. Based on more recent palynological studies, Lima & Salard-Cheboldaeff (1981) dated this unit as Eocene.

The deposits of the Fonseca Formation are preserved in a small graben with approximately 2.2 km². The graben is inserted on basement rocks and is limited by NE and NW oriented, post-sedimentation normal faults (Sant’Anna et al., 1997).

Ironstone conglomeratic deposits recover the sediments of the Fonseca Formation and the Precambrian basement rocks. Gorceix (1876) originally named these deposits as "canga". They occur as metric-thick tabular bodies constituted by brown to black colored, oligomictic orthoconglomerates composed by sub-rounded to angular pebbles and cobbles, also presenting a ferruginous clayey matrix. The composition of the gravels is quartz, quartzite and itabirite. These deposits are related to important regional plateaus.

Sant’Anna & Schorscher (1997) named this ironstone conglomerates as Chapada de Canga Formation, individualizing them apart of the geological evolution of the Fonseca Basin. Maxwell (1972) has considered these deposits on the top of the Fonseca Formation. Sant’Anna & Schorscher (1997) do not agree with this interpretation due to the lithologic differences, the presence of an important hiatus and their mappeability. According to Sant’Anna & Schorscher (1997), the Chapada de Canga Formation represents an alluvial fan system laterally associated with braided alluvial plains, deposited under tectonic influence and semi-arid climatic conditions. The source areas of these deposits would correspond to the ferriferous and carbonatic rocks of the Itabira Group, Minas Supergroup. Sant’Anna et al. (1997) recognize that the age of the Chapada de Canga Formation is still uncertain, suggesting a Tertiary or a Quaternary age for this unit.

Paleontology

The Fonseca Basin is very rich in fossil plants. With few exceptions, the papers about the paleoflora of this basin were restricted to morphological description of the fossils, with special attention to the pattern of the main and secondary venules.

A large amount of Angiospermae families are recorded on the Fonseca Formation: Annonaceae, Bignoniaceae, Bombacaceae, Combretaceae, Euphorbiaceae, Lauraceae, Leguminosae, Malphighiaceae, Melastomataceae, Meliaceae, Menispermaceae, Mimosaceae, Monimiaceae, Myrsinaceae, Myrtaceae, Rutaceae, Sapindaceae, Sapotaceae, Theaceae, Tiliaceae. The photos 2 and 3 show specimens recently collected in the Fonseca Basin. Several of these families are present in the extant flora of the State of Minas Gerais, especially around the Fonseca Village. Dolianiti (1950) considers it as suggestive that the fossil forms could be precursors of the recent allies.

The families Melastomatacea and Mimosaceae are the most abundant in the basin, but the most remarkable fossil belongs to the Family Bombacaeae - a flower, considerably well preserved, showing the impression of the petals and the male reproductive organ. This flower, with 3.6 cm length and five petals, is the first record of a flower in the Cenozoic of Brazil (Duarte, 1974).

According to palynological studies of Lima & Salard-Cheboldaeff (1981), the presence of Cicatricosisporites dorogensis, Spinizonocolpites, Perisyncolporites and the absence of Verrucatosporites usmensis, Jandufouria seamrogiformis, Magnastriates and pollens of Compostae are suggestive of an Eocene age for the Fonseca Formation. Lima & Salard-Cheboldaeff (1981) noticed the absence of dinoflagelates and acritarcs, confirming the continental origin of the Fonseca Formation deposits, and proposing for this unit a lacustrine depositional environment, with swampy areas. This interpretation is in agreement with the meandering fluvial system proposal of Sant’Anna & Schosrcher (1997), where the lacustrine and swampy conditions would be related to oxbow lakes and flooded areas. The great amount of spores of pteridophytes is suggestive of a humid climate.

Protection rules

The paleontological site of Fonseca is known for a long time and still represents a very promising place for geological and paleontological researches.

Nowadays, the Fonseca Formation deposits occur in an area, which activities are oriented to industrial reforesting. The remaining outcrops are observed in riverbeds and gullies.

Accepting a recent invitation of the Environmental Department of the city of Alvinópolis (State of Minas Gerais), researchers of the Department of Geology of the Federal University of Rio de Janeiro (UFRJ) carried out a fieldwork at the Fonseca Basin, when new fossil collects were done. These collects revived the paleontological interest in the Fonseca Formation, because of the discovery of new forms of plants and palynomorphs (Wey et al., 1999). It is sure to say that there is a great amount of fossil material to be collected and studied, being very wide the horizon for geological and paleontological researches.

The Environmental Department of Alvinópolis has made plans to create an Ecological and Paleontological Park, oriented not only to the preservation and environmental education, but also for scientific research. This proposal is absolutely possible to be fulfilled. However, it is first necessary: a) a careful identification and delimitation of the areas of geological and paleontological interest; b) a controlled deforestation in order to facilitate the accesses to the selected fossiliferous sites; c) a controlled opening of trenches in the riverbeds and hillslopes, for geological research and fossil collects.

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