the ichnogenus cruziana in the upper ordovician

SEGUNDA REUNION ARGENTINA DE ICNOLOGIA, SAN JUAN, 9-17 DE OCTUBRE, 1995.

THE ICHNOGENUS CRUZIANA IN THE UPPER ORDOVICIAN SASSITO FORMATION, CENTRAL PRECORDILLERA OF SAN JUAN, ARGENTINA. Silvio H. Peralta1, Oliver Lenhert2 y Martin Keller3. 1

)

2, 3

Instituto de Geología - F.C.E.F.N. - U.N. SAN JUAN, CONICET. ) Institut für Geologie der Universität Erlangen-Nürnberg

INTRODUCTION. The occurrence of ichnogenus Cruziana, which characterize the Cruziana Ichnofacies sensu Seilacher (1967), is reported from the Upper Ordovician (Late Caradoc-Early Ashgill?) Sassito Formation (Astini & Cañas, 1995). This unit is 24,5 m thick, and is composed in the lower part mainly of black shales, 5,5 m thick, with a basal thin horizon of chert and limestone conglomerate and coarse calcilithites which cover unconformably the San Juan Formation, and carbonate and mixed deposits, 19 m thick, which predominate upward sequence (Astini & Cañas, 1995). At the top, this unit underlies uncoformably the Silurian siliclastic deposits of the Tambolar Formation. The reported trace fossils has been found by O. Lenhert, at the basal part of the Sassito Formation just above of the basal conglomerate, in a dark carbonate level. In spite Ordovician rocks are widely distributed all over Precordillera, it is noteworthy that this is the first record of Cruziana ichnogenus for these rocks in Precordillera, although it has been previously recorded for the Middle-Upper Silurian Los Espejos Formacion, in the Central Precordillera of San Juan Province (Peralta & Aceñolaza, 1985; Benedetto & Herrera, 1986), and in allochthonous carbonate blocks of Cambrian age, in the Jáchal area (Astini et al., 2000). Out of Precordillera, Cruziana is a common trace fossil in Ordovician rocks from the North Western of Argentina, mainly in the Eastern Cordillera, where occurs in Lower to Middle Ordovician siliciclastic units, and in the Tandilia and Famatina System (Aceñolaza, 1978). The Cruziana ichnogenus has been, previously recorded in the Central Precordillera of San Juan Precordillera, by Aceñolaza & Peralta (1986) and Benedetto & Herrera (1988) in the Upper Silurian Los Espejos Formation, and later by Astini et al. (2000) in carbonate olistolith of Middle Cambrian age, in the Los Sombreros Formation, at Western Precordillera.

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OUTLINED FEATURES OF THE ICHNOGENUS CRUZIANA FROM THE UPPER ORDOVICIAN SASSITO FORMATION. In this report we follow the criteria supported by Osgood (1970), Rodwansky & Roniewics (1972), Crimes (1975) and, as in Argentina, by Aceñolaza & Durand (1978), to recognize Rusophycus as a resting trail produced by artropods, as trilobitemorphs and Cruziana as crawling trace produced as result of arthropods displacement. The Cruziana trace from the Upper Ordovician Sassito Formation, is a bilobate traces characterized by wide lobes covered by coarse, rounded, bifid scratch marks, and preserved as positive hyporeliefs. Length 13 cm, and width is 7 cm. It is composed by two parallel and symmetrical lobes 3 cm wide each, which are separated by a central furrow near 1 cm wide. Lateral scratches out from the central furrow. V-angle is 160ª to 180ª. V-marking are typically transverse. The median depression is relatively shallow, irregular and slightly undulated. Exopodite marking and lateral ridges are typically absent. Material herein assessed as Cruziana omanica group-related, is relatively scarce. In spite is shows close similarities in scratch configuration and size range with some rusophyciform traces, it well may be interpreted as cruzianiform trace. In agree with such features, the Cruziana trace from the Sassito Formation, resemble C. omanica as representative from the C. omanica group (Seilacher, 1992; Cruziana petraea group of Seilacher, 1970). However, the last is believed a stratigraphycally useful ichnospecies, probably restricted to the Upper Cambrian-Tremadoc (Seilacher, 1970, 1990, 1992).

Figure 2: PI-UNSJ Cruziana sp., from the basal level of the Sassito Formation, at Río Sassito section. Coin is about 2,3 cm diameter.

REFERENCES. Aceñolaza, F.G., 1978. El Paleozoico inferior de Argentina según sus trazas fósiles. Ameghiniana, 15(12):15-64. Buenos Aires. Aceñolaza, F.G. and Durand, F., 1978. Trazas de trilobites en los estratos del Ordovícico basal de la Puna Argentina. Acta Geológica Lilloana, v. 15, Nº 1, p. 1-12. San Miguel de Tucumán.

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Aceñolaza, F.G. and Peralta, S.H., 1985. Interpretación del comportamiento de la cuenca silúrica en función de sus icnofacies en el área de Talacasto, Precordillera de San Juan. 1ras. Jornadas Sobre Geología de Precordillera, Asociación Geológica Argentina, Serie “A”, Monografías y Reuniones, Nº 2, Actas, p. 151-156. Astini, R., Mángano, M.G., and Thomas, W. A., 2000. El icnogénero Cruziana en el Cámbrico temprano de la Precordillera argentina: el registro más antiguo de Sudamérica. Ameghiniana, 55(1-2):111-120. Buenos Aires. Benedetto, J.L. and Herrera, Z.A., 1988. Hallazgo del icnogénero Cruziana en la Formación Los Espejos (Silúrico), provincia de San Juan. Ameghiniana, 25(1): 17-22. Buenos Aires. Crimes, P. 1975. The production and preservation of trilobite resting and furrow traces. Lethaia, 8(1): 3548. Lehnert, O., 1995. Ordovizische Conodonten aus der Präkordillere Westargentiniens: Ihre Bedeutung für Stratigraphie und Paläogeographie. Erlanger Geologische Abhandlungen, 125: 1-193. Erlangen. Osgood, R.G., 1970. Trace fossils of the Cincinnati area. Paleontology American, 6(4): 281-244. Rodwansky and Roniewicz, 1972. A long trilobite trackway, Cruziana semiplicata Salter, from the Upper Cambrian of the Holy Cross Mountains. Act. Geol. Polonica, 22: 439-447. Seilacher, A., 1970. Cruziana stratigraphy of “non-fossiliferous” Paleozoic sandstones. In: Crimes, T.P., and Harper, J.C. (eds.), Trace Fossils. Geological Journal Special Issue, 3:447-476. Seilacher, A., 1990. Paleozoic trace fossils. In: Said, R. (ed.), The Geology of Egypt. A.A. Balkema, Rotterdam Brookfield, pp. 649-670. Seilacher, A., 1992. Un updated Cruziana stratigraphy of Gondwanan Paleozoic sandstones. In: Salem, M.J. (ed.), The Geology of Libia, 5. Elsevier, Amsterdam and New York,pp. 1565-1580. Seilacher, A., 1967. Bathymetry of trace fossils. Marine Geology, 5: 413-428.

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