INVESTIGADORES
MILANA Juan Pablo
artículos
Título:
Remarkable Cruziana beds in the Lower Ordovician of the Cordillera Oriental, NW Argentina
Autor/es:
ACEÑOLAZA, G. AND MILANA, J.P
Revista:
AMEGHINIANA
Editorial:
ASOCIACION PALEONTOLOGICA ARGENTINA
Referencias:
Lugar: Buenos aires; Año: 2005 vol. 42 p. 3 - 3
ISSN:
0002-7014
Resumen:
Upper Cambrian - Lower Ordovician strata of the South American Central Andean Basin contain abundant and well-preserved trace fossil assemblagesthat have been mostly studied after the mid 1960´s (Borrello, 1966; Ramos, 1973; Aceñolaza, 1978; Alonso and Marquillas, 1981; Toro et al., 1988, 1990; Mángano et al., 2001; Aceñolaza and Aceñolaza, 2002 with references). Early reports of these fossils were provided by d´Orbigny (1842), who described Bolivian material from the surroundings of Zudañez (Cochabamba and Chuquisaca departments) and introduced Cruziana, an ichnogenus that nowadays is one of the most intriguing trace fossils. Lower Paleozoic Cruziana are mostly regarded as trilobite crawling exite-produced traces, and due to their striking morphology have provoked many remarkable papers focusing on different aspects of their 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). Extraordinary well-preserved Cruziana pavements have been recently located in Lower Ordovician strata of the Sierra de Zenta in Jujuy province (northwest Argentina). The traces are assigned to the “rugosa group” of Seilacher (1970) and come from the upper part of the Santa Victoria Group, close to the Abra Blanca locality (figures 1, 2,3 B-I). The objective of this paper is to present this outstanding new ichnofossiliferous locality, advancing some results of an interdisciplinary project that is being carried out in the Sierra de Zenta of northwest Argentina. The ichnogenus Cruziana d`Orbigny, 1842 Cruziana is mostly represented by a bilobate and elongated furrow covered by transverse or herringbone-shaped ridges. Traces may be marginated by lateral outer zones with or without ridges (Häntzschel, 1975; Pickerill et al., 1984; Fillion and Pickerill, 1990). The ichnogenus ranges from the Lower Cambrian to the Triassic, and has been reported from a varied spectrum of paleoenvironments, ranging from typical shallow water marine to freshwater settings (Crimes, 1987; Bromley andsgaard, 1979; Seilacher, 1985). Seilacher (1970) grouped different ichnospecies of Cruziana by means of their morphological and biostratigraphical setting. The “rugosa group” displays a prevalence of long ploughings over resting tracks, and includes C. rugosa d´Orbigny, C. furcifera d´Orbigny, C. goldfussi (Rouault) and C. barriosi Baldwin. Cruziana rugosa d´Orbigny stands out as its most remarkable element, being characterized by strong corrugations across the lobes and numerous sharp scratches indicating up to 12 subequal claws for each endopodite (figure 2) (Seilacher, 1970). The “rugosa group” is widely distributed in the perigondwanan realm: Bolivia, Argentina, Newfoundland, Portugal, Spain, France, Wales, Turkey, Jordan, Libya, Iraq, Saudi Arabia, Afghanistan, and China (Seilacher, 1970, 1992; El-Khayal and Romano, 1988; Seilacher et al., 2002; Aceñolaza and Aceñolaza, 2002). The group has repeatedly been the subject of Lower Paleozoic literature, and there is still considerable debate regarding the distinction of its different ichnospecies (Fillion and Pickerill, 1990). Although some authors have supported an ichnosubspecific assignment (Seilacher, 1996), we prefer to provisionally retain the individual ichnospecies, fol-lowing the insightful discussions by Pickerill et al. (1984), Durand (1985) and Fillion and Pickerill (1990). A detailed systematic analysis of over 1.000 well preserved samples is being carried out within the current project. Most Lower Paleozoic Cruziana are regarded as trilobite traces, although some may have been produced by different types of arthropods, some brachiopods, aglaspidis or even vertebrates (Seilacher, 1970; Bromley and Asgaard, 1979; Fisher, 1978; Shone, 1979; Fillion and Pickerill, 1990). Asaphacean trilobites have been suggested by several authors as possible producers of the “rugosa” group (Bergström, 1973; 1976; Mángano et al., 2001; Aceñolaza, 2003), but unfortunately, to date no trilobites have been found associated with the trace fossils in the new locality. Sedimentology and the Cruziana association of Zenta Ordovician strata in the Sierra de Zenta display a thick sequence where sheet-like and ribbon-like sand bodies alternate with muddy intervals. Most sand bodies are almost completely altered by bioturbation. Trace fossils are better preserved in the less bioturbated mudstone intervals. The latter are characterized by a dominantly heterolithic sequence, in which silty massive packages alternate with sandy beds where most Cruziana pavements are found. Sandy intervals are, with the exception of a faint stratification, depleted of almost all primary depositional structure due to bioturbation. Sand bodies show a coarsening-up grain size tendency and fragmented shell debris lags are frequent in these units. In spite of the turbidite-like appearance, all the sequence has been interpreted as deposited within an outer shoreface environment with frequent shoaling bars, becoming shallower towards the top. In addition, there is a remarkable absence of important unconformities in the sequence. Strata display a thick shoreface succession, suggesting a quite stable tectono-eustatic framework for Tremadocian-Arenig times. Traces represent mostly shallow crawling open furrows, but frequent deep steeply walled specimens were observed, some of them regarded as true burrows in a same manner that mentioned