Benjamin Constant Formation yielded a characteristic north Gondwana assemblage of late Arenig to early. Llanvirn age. Overlying beds are of Silurian age.
ORDOVICIAN CHITINOZOA AND BIOSTRATIGRAPHY OF BRAZIL YNGVE GRAHN Stocholm University, Department of Goology and Geochemistry, S-10691 Stockholm, Sweden. Present address : Swedish Museum of Natural History, Department of Paleozoology, Box 50007, S-10405 STOCKttOLM,Sweden.
ABSTRACT
Marine sedimentary rocks of Ordovician and supposed Ordovician age have been investigated for chitinozoans in five Paleozoic basins of Brazil, viz. Solimfes, Amazonas, Parnafba, Jatobfi, and Paranti basins. The lithology is predominantly sandstones with subordinate siltstones and shales. In the Solimfes Basin, the basal Paleozoic Benjamin Constant Formation yielded a characteristic north Gondwana assemblage of late Arenig to early Llanvirn age. Overlying beds are of Silurian age. Within or in connection to the Parnafba and Paranfi basins continental sedimentation occurred in restricted basins with molasses and acid to intermediate igneous rocks of supposed Early - Middle Ordovician age. A more widespread transgression started probably in late Caradoc and gave rise to the first Paleozoic marine sedimentation of the Amazonas (Autfis-Mirim Fro), Parnafba (lpu Fro), possibly Jatobfi (Tacaratu Fro), and Paranfi (Rio Ivai Fro) basins. A diagnostic Ashgill (Rawtheyan) chitinozoan assemblage is present in the upper part of the AutEs-Mirim Fm. The Ordovician-Silurian boundary lies approximately at the boundary between the Autfis-Mirim Fm and the overlying Nhamundfi Fm. No contemporary chitinozoan faunas are known elsewhere in Brazil. CHITINOZOMRES El' BIOSTRATIGRAPHIEDE L'ORDOVICIENDU BILESIL.
RI~UMI~
Une recherche de chitinozoaires a 6t6 effectu6e dans les s6diments marins ordoviciens (ou d'$ge Ordovicien suppos6) de cinq bassins pal6ozoiques du Br6sil (bassins de Solim6es, de l'Amazone, de Parnafba, de Jatobti et de Paran~i). Des gr~s avec quelques intercalations silteuses ou argileuses constituent le facies dominant. Dans le bassin de Solimfes, un assemblage de chitinozoaires caract6ristiques de l'Arenig superieur - Llanvirn inf6rieur nord gondwanien a 6t6 reconnu darts la Formation du Benjamin Constant qui repr6sente les premiers d6p6ts pal6ozoiques de la r6gion. Ces terrains sont directement surmont6s par le Silurien. De petits bassins a s6dimentation continentale, avec molasses et roches volcaniques acides et interm6diaires, d'fige Ordo'Acien inf6rieur a moyen suppos6, existent/i proximlt6, ou en relation avec les bassins de Parnafba et de Paranfi. Dens les bassins de l'Amazone (Formation d'Autfis-Mirim) et de Parnafba (Formation d'Ipu) et eventueUement dans les bassins de Jatob~i (Formation de Tacaratu) et du Paranfi (Formation de Rio Ivaf) la transgression pal6ozo~que se d6veloppe probablement d~s le Caradoc sup6rieur. La partie sup6rieure de la Formation de Auttis-Mirim ayant livr6 un assemblage de chitinozoaires de l'Ashgill (Rawtheyen), la limite Ordovicien-Silurien se placerait approximativement entre cette formation et l'unit6 susjacente • la Formation de Nhamundfi. Les chitinozoaires de l'Ordovicien sup6rieur restent inconnus dans les autres r6gions du Br6sil.
KEY-WORDS : PALYNOLOGY, CHITINOZOA, ORDOVICIAN, BRAZIL. MOTS-CLt~S : PALYNOLOGIE, CHITINOZOAIRES, ORDOVICIEN, BRt~SIL.
Manuscrit d6pos6 le 01.07.1991 Manuscrit accept6 d6fintitivement le 09.12.1991
~Geobios, 1992"] n ° 25, fasc. 6 [ ,._pp.703-723 ~)
704 Late Ordovician (late Richmondian) age. The fossils of this sequence were described by Derby (1878) and Clarke (1899), who assigned the Trombetas Group to the Silurian. Lange (1967, 1972) and Caputo (1984a, 1984b) remarked that Late Ordovician rocks could be present in the Amazonas Basin, but their opinions were based solely on extrapolations. It was not until recently that Quadros (1986) demonstrated the presence of Ordovician rocks in the Solimfes Basin with the help of acritarchs. Besides acritarchs and chitinozoans, only ichnofossils are known from the Ordovician of Brazil.
INTRODUCTION There are several reports on the occurrence of Ordovician rocks in Brazil (for a summary, see Acefiolaza & Baldis 1987). However, many of these supposed Ordovician sequences are probably Proterozoic in age (Schobbenhaus et aL 1984). The first mention of possible Ordovician rocks was by Schuchert (1906), who based on the investigations by Ulrich in North America, considered the Trombetas Group (Grahn & Paris 1992) in the Amazonas Basin to be of 65 °
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Brdsll et la localization des forages profonds d~ultds, ayant traversd des terrains ordoviaens (ou supposds ordoviciens).
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The absence of other macrofossils, as well as the lack of outcrops with Ordovician marine rocks in the Paleozoic basins with the possible exception of the Parnm'ba and Jatobfi basins, have hindered the recognition of the Ordovician System for many decades in Brazil. The lithologies are mosdy fluviatile/deltaic sandstones, and it is only in the subsurface of the Solim6es Basin that more extensive fos~iliferous shale heds occur. This paper is the first to describe Ordovician Chitinozoa and their biostratigraphy in Brazil. It summarizes the results obtained from the investigation of five Paleozoic basins, viz. Solimfes, Amazonas, Parnat'ba, Jatobfi and Paranfi basins (Fig. 1). For a general presentation of the basins references are made to Melo (1988). The stratigraphy of the Ordovician or supposed Ordovician formations in these basins will be discussed below.
OUTLINE OF THE ORDOVICIAN STRATIGRAPHY OF BRAZIL The Ordovician was a time of tectonism and differentiation of the South American platform. In Brazil reactivation of weak zones and resulting trends in the basement created huge fiat-lying intracratonic basins in the northern part of the Brazilian platform (Caputo 1984a), whereas a marginal basin tilted to the west were formed in the southern part, in the region now occupied by the Paran~i Basin (Fulfaro et al. 1982). Except for the SolimSes Basin, the Lower and Middle Ordovician of Brazil, when present, is supposed to have developed as continental sedimentation (molasse) mixed with acidic to intermediate magmatism (Schobbenhaus et al. 1984). This is considered to have taken place in small isolated
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basins situated within or adjacent to the future Parnafba and Paran,'i basins. However, there is no fossil evidence for the age of the supposed continental sedimentation, and the radiometric datings are controversial. An isolated case of Early Ordovician marine sedimentation occurred in the SolimSes Basin (Jandiatuba Sub-basin), which was connected to the Andean Bek in the west and therefore developed differently from other Early Paleozoic basins in Brazil during the Ordovician. A more widespread marine sedimentation started probably in the late Caradoc, and the resulting rocks are mainly sandstones with subordinate siltstones and shales.
SOLIMOES BASIN The only known unit of Ordovician age from the Solim5es Basin is the Benjamin Constant Formation. It was earlier correlated with the Trombetas Group in the Amazonas Basin (Caputo et aL 1971, 197~ ; Carneiro & Schneider 1976 & Esteves 1982), of which the Solim6es Basin was previously regarded as the western part (Upper Amazonas Basin). However, through the works of Caputo (1984a, 1985) and Silva (1987, 1988) it became evident that the Amazonas and Solimfes basins have evolved independently of each other during pre-Carboniferous time. The Benajmi- Constant Formation was named by Silva (1987, 1988) for an erosional wedge, up to 103 m thick, of shales and sandstones that overlies the basement in the Jandiatuba Sub-basin of the Solim6es Basin (Fig. 1). No outcrops are known, and the formation has therefore been studied for chitinozoans in three wells (Figs. 1-2)~ The type section is between 1075-1178 m in well
706 Figure 3 - Lithology and sampling levels in the investigated wells from Amazonas Basin. LtOo/o#, et prdl~varaents dana las putts dtudids du Bassin da l'Amazone.
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708
AMAZONAS BASIN No marine sedimentation is known from the Amazonas Basin since the closure of the Proterozoic basin during the Riphean up to the be#nning of the Late Ordovician transgression, nor any continental sedimentation similar to that of the Parnafba and Paranfi basins. The first sedimentary rocks of Paleozoic age are represented by the Autfis-Mirim Formation. This formation was named by Caputo et al. (1971, 1972) for a sandy sequence in the basal Trombetas Group of the Amazonas Basin (Fig. 1 herein ; Grahn & Paris, 1992). The formation is only known from subsurface and has been studied in two wells (Figs. 1,3). The type section is in well 1-AM-I-AM (Autfis-Mirim n* 1, State of Amazonas) between 1943-2235 m (see AM-1-AZ, Lange 1967). The Urubu and Cajari members as defined by Lange (1967) are now considered as parts of the overlying Nhamundfi and Pitinga formations, and are no longer in use. The basal part of the Autfis-Mirim Formation yields greenishbrown sandstones which become white upwards. Beds with greenish-grey siltsones and dark-grey shales are common. Crossbedding and ichnofossils (e.g. Skolithos) are present, and Caputo (1984a) interpreted the AutfisMirim Formation to have been deposited in a continental-marine-continental cycle. The maximum thickness of the Autfis-Mirim Formation is about 350 m (Caputo 1984a). Except for the ichnofossils mentioned above no other fossils were previously recorded from this formation. However, a chitinozoan assemblage ( 844 /zm; maximum width of vesicle (coeff. 0.7) 63-67/~m ; width of aperture not observed.
Amazonas Basin, Autfis-Mirim Fm, 1-AM-I-AM : 2091 m. Rhabdochitina gracilis is a long-ranging Ordovician species known from Arenig to Ashgill. The type specimen was described from the late Arenig part of the Holen Limestone at Fj~icka, Dalarna, Sweden (Eisenack 1962). Rhabdochitina gracilis is a common species in the Ordovician of Sweden (Eisenack 1968a ; Grahn 1978, 1980, 1981a, 1981b, 1982) and Estonia (Eisenack 1968a ; Grahn 1984). In Estonia occurs a complex of curved specimens in the middle AshgiU (NSlvak, personal communication 1987), and this seems to be a primary feature. Rauscher (1968) reported it from the Arenig of Montagne-Noire, south France, and the species is also known from the Llanvirn of Normandie, western France (Rauscher 1973 ; Rauscher & Doubinger 1967a, 1967b). Paris (1981) described Rhabdochitina gracilis from the lower part of the Pissot and Andouill6 Fins with the same age as those from Normandie. Elaouad-Debbaj (1984) reported it from the Ktaoua Fm of late Caradocearly Ashgill age. Occurrence
Genus Tanuchitina JANSONIUS, 1964.
Species in the Ordovician of Brazil Tanuchitina anticostiensis (Achab 1977a) and Tanuchitina hyalophrys (Eisenack 1959).
TANUCHITINA ANTICOSTIENSIS (ACHAB, 1977) : Fig. 9, 5. 1977a - Cyathochitin.a anticostiensis n. sp. : Achab, p. 422, pl. 4, figs. 1-2,5. 197719 - Tanuchitina anficostiensis (ACHAB) : Achab, p. 12, pl. 1, fig. 5. 1985 - Tanuchitina anticostiensis (ACHAB) : Molyneux & Paris, pl. 6, fig, 7. v1989 - Tanuchitina anticostiensis (ACHAB) : Grahn & Nchr-Hansen, fig. 3 H. Discussion - This very characteristic species has a long,
elongated subcylindrical vesicle. The flanks are slightly convex and taper about .2/3 of the total length oralwards into a cylindrical neck. Flexure indistinct and shoulder absent. The aperture is straight. Inside the basal edge occurs an inverted conical carina (ca. 25/zm long). Base convex. Vesicle wall perfectly smooth. Dimensions (7 specimens) - Length of vesicle 470-485 /zm ; maximum width of vesicle (coeff. 0.8) 75-80/zm ; width of aperture (coeff. 0.8) 45-52/zm. Remarks - The Brazilian specimens are much smaller than those described from Anticosti Island, Canada (Achab 1977a, 1977b).
- Amazonas Basin, Aut~s-Mirim Fm, 1-AM-I-AM : 2091 m Tanuchitina anticostiensis was originally described from well New Associated Consolidated Paper Anticosti no. 1 (823 m) in the Vaur6al Formstion (Zone of Dicellograptus complanatus), Anticosti Island, Canada (Achab 19T/a, 197719). Molyneux & Paris (1985) illustrated it from the Ashgill of Libya, and Grahn & Nchr-Hansen (1989) from the Ashgill of north Greenland. Paris (1988) reported the species unillustrated from the Ashgill of NE Libya. Occurrence
TANUCHITINA HYALOPHRYS (EISENACK, 1959) : Fig. 9, 6-7.
1959 - Cyathochitina hyalophrys n. sp. : Eisenack, p. 11-12, pl. 2, figs. 6-7, pl. 3, fig. 15.
Type species - Tanuchitina ontwiensis JANSONIUS, 1964. Remarks - Paris (1981, p. 213) modified the original
definition by Jansonins (1964, p. 910). He pointed out two characteristics in addition to those mentioned by Jansonius : the angle between the basal margin and the base, which is 90 degrees or less for Tanuchitina, and the maximum width of the vesicle just above the basal
nlargixl.
1970 - Cyathochitina ontariensis (JANSONIUS) : Jenkins, p. 274, pl. 50, figs. 1-9. ?1972b - Cyathochitina hyalophrys EISENACK : Eisenack, p. 119, pl. 36, fig. 3. Description - Species with a slender, elongated conical vesicle. The body is conical with straight flanks that taper into a cylindrical neck about 2/3 of the total
720 length. The aperture is straight. The basal edge is slightly rounded and the base flat. Immediately inside the basal edge is a short carina. Although this is not clear from the illustration by Eisenack (1972b, pl. 36, fig. 3), it can be seen in the Brazilian specimens. The vesicle wall is smooth.
paleoecological indications. Conochitina micracantha and Conochitina pervulgata ? dominate the Early Ordovician chitinozoan faunas of Brazil. The latter species and Conochitina havliceki ? seem to be restricted to the upper part of the Benjamin Constant Fm, while the others range through the whole formation.
Dimensions (7 specimens) - Length of vesicle 560-570 /~m ; maximum width of vesicle (coeff. 0.8) 80-85/.tin ; width of aperture (coeff. 0.8) 55-60 pro.
The Ashgill assemblage in the Aut~is-Mirim Fm contains Tanuchitina anticostiensis, which is an excellent index species for the Rawtheyan (Richmondlan early Gamachian). The Brazilian fauna recalls that in north Gondwana and, tO some extent, in Quebec, but yields also Tanuchitina hyalophrys, a species previously unrecorded outside the Midcontinent U.S.A. Contemporary chitinozoan faunas in Baltoscandia, Podolia and the Midcontinent U.S.A. differ in not containing the genus Arrnoricochitina. Tiffs probably has paleogeographic significance, and emlShasizes the influence of north Gondwana faunas in the Paleozoic basins of Brazil during the Late Ordovician transgression. Later, when the transgression reached its peak in the Llandovery, this connection became more obvious (Grahn, in press ; Grahn & Paris 1992).
Remarks - The position of the carina inside the basal edge is a diagnostic character for the genus Tanuchitina. Hence, "Cyathochifina hyalophrys" is herein transferred from Cyathochitina to Tanuchifina. Grahn & Bergstr6m (1984, p. 114-115) discussed specimens similar to Tanuchitina hyalophrys (Cyathochitina cf. hyalophrys) from the Llandeilian upper part of Lenoir Limestone in the Southern Appalachians, U.S.A. These specimens differ in having a more convex base provided with a wide basal scar. Occurrence Amazonas Basin, Aut~-Mirim Fm, 1-AM-I-AM : 2091 m. Tanuchitina hyalophrys ranges from late Caradoc to middle AshgiU. It was originally described by Eisenack (1959) from the Upper Ordovician of the Cincinnati Region. The type locality is unknown. Tanuchitina hyalophrys is a well known species in the Maysvillian of the Midcontinent U.S.A. (Miller 1976), and it was described from the early Ashgill Sylvan Shale in Oklahoma (Jenkins 1970).
DISCUSSION Ordovician Chitinozoa from Brazil have only been obtained from the SolimSes and Amazonas basins, where marine shales occur to greater extent. All Brafilian chitinozoans discussed in this study were recovered from shales. The late Arenig - early Llanvirn chitinozoam in the Benjamin Constant Formation show a striking similarity to contemporary faunas in north Gondwana, from where all specimens, except for Conochitina sp. A, have previously been described. However, the Brazilian ehitinozoans constitute mainly a simple Conochitina Lagenochitina assemblage in contrast to contemporary faunas in north Gondwana. They are in this respect more similar to those described from Spitzbergen (Bockelie 1980), Quebec (Achab 1980, 1986) and recently Queensland, Australia (Playford & Miller 1988). Early Ordovician faunas from other regions are further differentiated with additional genera such as Eremochitina, Sagenochitina, Tanuchitina, Velachitina and others. As pointed out by Playford & Miller (1988, p. 27) this could have paleobiogeographic as well as
Acknowledgements - I am indebted to Florentin Paris (Rennes,
France), Jos6 Henrique Gonqalvesde Melo (Rio de Janeiro, Brazil) and M~trioVicente Caputo (Bel6m, Brazil) for valuable comments on the manuscript and to Petrobr~is - Petroleo Brasilero S.A. for the permission to publish this paper. My work has also been supported by a grant fiom the Swedish Natural Science Research Council (NFR-G-GU8811-302).
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