INSUGEO   12554
INSTITUTO SUPERIOR DE CORRELACION GEOLOGICA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Refinning the chronology of the Seilacherian "Cruziana Stratigraphy": Additional data from an outstanding outcrop in the Ordovician of
Autor/es:
ACEÑOLAZA, G.F.
Lugar:
Kiel, Alemania
Reunión:
Simposio; 20th Colloquium on Latin American Earth Sciences; 2007
Institución organizadora:
Christian Albrechts University - Kiel & DFG
Resumen:
Epicratonic marine sandstones characterize thick sequences on the Lower Paleozoic Gondwanan realm, a facies type that are the least suitable for preserving body fossils (Fortey and Seilacher, 1997). At the same time, this type of lithology (clastic rocks, and particularly sandstones) is the best for preserving trace fossils. Even though the last are mostly regarded as poor biostratigraphic indicators - as they were produced by facies restricted benthic organisms (Magwood and Pemberton, 1990) - some elements shall be highligthed by their unique paleontological value. In this context, the stratigraphical distribution of traces assigned to Cruziana provided a clear and reliable scheme for the dating of large and thick successions of monotonous unfossiliferous strata in northern and western Gondwana. The morphological changes reflected by Cruziana between the Lower Cambrian to the upper Devonian and Lower Carboniferous is known as “Cruziana stratigraphy” (Seilacher, 1970; 1994). This scheme has proved to be a fast dating field method in several basins around Africa and South America (Seilacher, 1990; 1992; 1993; El-Khayal and Romano, 1998; Aceñolaza and Aceñolaza, 2002; Aceñolaza and Milana, 2005). For the South American Cambrian – Ordovician successions, the record of C. semiplicata is restricted to the Furongian and Tremadocian, whilst C. furcifera occurs in the Tremadocian and Arenigian, but not in the Furongian (Crimes, 1970; Seilacher, 1994). Up in the strata, C. rugosa is restricted to the uppermost Tremadocian and Arenig strata (Aceñolaza and Aceñolaza, 2002) (Fig. 2). Even though this so called “ichnostratigraphic paradigm” has been challenged with the decription of elements belonging to the “Cruziana rugosa” group in the Lower Cambrian of Alberta (Magwood and Pemberton, 1990), recent revision of additional material from Canada revealed the ratification of the stratigraphical significance of Cruziana, and clear morphological differences in the Canadian material, with the establishment of Cruziana pectinata (Seilacher, 1994) Trilobite burrows allow a higher level of distinction than other invertebrate trace fossils because of his paleobiologic and biostratinomic signatures (Seilacher, 1994). In this sense, the Cruziana stratigraphy must be considered as a reliable fast field method for dating non fossiliferous strata in the Early Paleozoic Gondwanan seas. Lower Paleozoic Cruziana is mostly regarded as a trilobite crawling exite-produced trace (see Aceñolaza nd Milana, 2005). Due to its striking morphology has provoked many remarkable papers focusing different aspects related to its production, preservation and variability (e.g. Seilacher and Crimes, 1969; Crimes, 1970; 1975; Seilacher, 1970; Crimes and Marcos, 1976; Bergström, 1976; 1979; 1981; Pickerill and Fillion, 1983; Fillion and Pickerill, 1990). Outstanding Cruziana pavements have been recently localized in the Lower Ordovician strata of the Sierra de Zenta of northwest Argentina (Aceñolaza and Milana, 2005). Traces are mostly assigned to the “rugosa group” of Seilacher (1970), and were found in the upper sector of the Santa Victoria Group close to the Abra Blanca locality of the Sierra de Zenta (figures 1, 2, 3B-I). Traces are preserved as positive hyporelief produced by casting on sand of previously excavated layers and as rare negative epireliefs in the upper surface of strata. The preservation of very fine morphological details, as claw and exite marks, are a result of the interaction of the particular paleoenvironmental conditions of sequences (e.g. environmental energy, substrate coherence, water content of sediments) and the ethology of the producer organism.