Nota Paleontológica

Remarkable Cruziana beds in the Lower Ordovician of the Cordillera Oriental, NW Argentina

Guillermo F. Aceñolaza ¹ and Juan Pablo Milana ²

¹ Instituto Superior de Correlación Geológica, (CONICET-UNT). Miguel Lillo 205, 4000 Tucumán. insugeo@csnat.unt.edu.ar
² Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Geología (UNSJ). Av. Ignacio de La Roza y Meglioli, 5400 San Juan. jpmilana@unsj-cuim.edu.ar

Introduction

Upper Cambrian - Lower Ordovician strata of the South American Central Andean Basin contain abundant and well-preserved trace fossil assemblages that 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).

Figure 1. Localition map of fossiliferous outcrop (asterisk)/ Mapa de ubicación de la nueva localidad (asterisco) .

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 and Asgaard, 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).

Figure 2. Isolated steeply walled sample of Cruziana rugosa d`Orbigny displaying characteristic strong corrugations across the lobes with numerous and sharp scratches (Scale 1 cm; PIL 15.099)/ Ejemplar de Cruziana rugosa d`Orbigny con sus características corrugaciones transversales y marcas lobulares (Escala 1 cm; PIL 15.099).

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, following 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 earlier for other Gondwanan localities ( e.g ., Seilacher, 1955; 1970; 1982; Baldwin, 1977; Goldring, 1985). Traces are preserved in medium to fine-grained sandstone beds up to 40 cm thick as concave epireliefs and undertracks, types of preservation that were not previously described from the Lower Paleozoic of South America. The last ones usually display washed out surfaces and generally do not show fine details as those preserved as convex hyporeliefs. Associated traces are represented by several forms of Lockeia , Skolithos , Monomorphichnus and Dimophichnus .

Brief description of the material

Part of the material described in this paper is housed in the paleontological collections of the Facultad de Ciencias Naturales e Instituto Miguel Lillo of the Universidad Nacional de Tucumán (PIL 15.099-15.101). The sample figured in 3 C, E is housed and exhibited at the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” of Buenos Aires (MACN Icn 2253).

Cruziana furcifera d´Orbigny 1842
(Cf in figures 3 D-F, I)

Description and remarks. Large-to mid- sized traces with inner lobes and lacking outer lobes. Mostly bifid scratch marks are developed regularly over lobes, reaching the median groove with an acute angle of 15º to 65º degrees. Criss-crossing is recognized close to the median groove, producing faint rhombic patterns in some material, as also described previously by Fillion and Pickerill (1990) and El-Khayal and Romano (1988). Width of traces varies from 30.3 to 70.6 mm, depth up to 60 mm, and length reaches in some specimens up to 800 mm. Minor sectors of many long specimens display a transitional morphology to C. rugosa . This is a common ichnospecie, representing 15 % of the total analyzed traces.

Cruziana goldfussi (Rouault 1850)
(Cg in figures 3 C, E, H)

Description and remarks. Mid- to small-sized Cruziana displaying fine and relatively parallel scratch marks along the lobes. Inner and outer lobes can be distinguished in most of the samples. Width of traces range between 18.4 to 75.2 mm. Outer lobes represent approximately 5-10 % of trace width. Scratches reach the median groove at angles varying from 20º to 70º. The presence of faint corrugations by sectors in some of the samples resembles the morphology of C. rugosa ; while rare faint criss-crossing of scratch marks in certain samples is reminiscent of C . furcifera . Cruziana goldfussi represents approximately 18% of the total analyzed samples.

Cruziana rugosa d´Orbigny 1842
(Figure 2; Cr in figures 3. C-H)

Description and remarks. High relief large and robust bilobed traces up to 78 mm wide. Traces display well-developed scratch marks and a moderately deep irregular median furrow (up to 15 mm deep). Large ribbon-like traces are frequent, reaching a length of 400 to 500 mm. Transverse coarse corrugations characterize this ichnospecies, developed by sets with 8 to 15 scratch marks, each one representing individual appendage impressions. As previously mentioned by several authors ( e.g ., Pickerill et al ., 1984; Fillion and Pickerill, 1990; Mángano et al ., 2001; Aceñolaza and Aceñolaza, 2002), our material transitionally passes into C . furcifera d`Orbigny, while fewer transitions to C . goldfussi Rouault were also recorded.
Cruziana rugosa d`Orbigny (1842) is the type species of the ichnogenus and represents almost a 70 % of the traces assigned to Cruziana in the outcrop.

Final comments and conclusions

The “ Cruziana stratigraphy” concept is recognized as an effective biostratigraphic method for field-work dating, specially in the non-fossiliferous shallow siliciclastic platforms of Gondwana (Crimes, 1969; 1975; Seilacher, 1970, 1992; Baldwin, 1977; Crimes and Marcos, 1976; Pickerill et al ., 1984).
In NW Argentina the association C. semiplicata / C . furcifera characterizes late Cambrian-Tremadocian beds, while elements of the “ rugosa group” ( C. rugosa , C. goldfussi and C. furcifera ) are restricted to the upper Tremadocian-Arenig as mentioned by Mángano et al . (2001) and Aceñolaza and Aceñolaza (2002). Although early papers have provided, with a limited number of samples from discontinuous sections, some data about the Cruziana stratigraphy of the Central Andean Basin ( e.g ., Mángano et al ., 2001), no integrated biostratigraphical-sedimentological analysis with abundant traces have been done in the South American Gondwanan margin. As pointed out by Seilacher (1970) and El-Khayal and Romano (1988), “ Cruziana stratigraphy” is reliable only if research is done over assemblages with numerous traces.

Figure 3. A, Outcrops with sandstone beds bearing the outstanding Cruziana pavements in Jujuy Province (NW Argentina). B, Infrequent preservation of Cruziana as negative epireliefs on the sandstone beds (hammer for scale, material in locus). C, Steeply walled C . rugosa , associated with C. goldfussi (scale 1 cm; MACN Icn 2253). D, Sandstone bed with an overcrossing association of C. rugosa and C . Furcifera (scale 5 cm, material in locus). E, Large sandstone bed with several samples of the C. rugosa group (scale 5 cm; material exposed at the Museo “Bernardino Rivadavia” Buenos Aires; MACN Icn 2253). F, C. rugosa with well developed transverse corrugations along the trace and overimposed C. furcifera (scale 5 cm; PIL 15.100). G, In situ Cruziana on a sandstone sole of the locality (scale 5 cm; material in locus). H, partially preserved C. rugosa associated with a small sample of C. goldfussi (scale 5 cm; PIL 15.101). I, Clearly developed C. Furcifera associated with Monomorphichnus multilineatus developed over a rare smooth sample of C . rugosa (scale 1 cm; material in locus)/ A, Nuevos afloramientos con Cruziana en Jujuy (NO de Argentina). B, Preservación poco frecuente del icnogénero Cruziana como epirelieves negativos (foto de campo). C, Ejemplar profundo de C. rugosa asociado a C. goldfussi (escala 1 cm; MACN Icn 2253). D, Nivel arenoso con una asociación con entrecruzamientos de C. rugosa y C. furcifera . (escala 5 cm; material in locus ). E, Base de arenisca con numerosos ejemplares asignados al grupo rugosa (escala 5 cm; material expuesto en el Museo Bernardino Rivadavia, Buenos Aires, MACN Icn 2253). F, C. rugosa asociada a C. furcifera mostrando típicas características de las icnoespecies (escala 5 cm; PIL 15.100). G, Base de arenisca con material de Cruziana in situ (escala 5 cm; material in locus ). H, C. rugosa parcialmente preservada en asociación a C . goldfussi (escala 5 cm; PIL 15.101). I, C. furcifera vinculada a Monomorphichnus multilineatus desarrollado sobre un ejemplar superficialmente liso de C . rugosa (escala 1 cm; material in locus ).

The quality and quantity of traces, and the stratigraphical continuity of the sequence of the Sierra de Zenta (including the Tremadocian/Arenig transition), will give a better sight of the biostratigraphy in the Lower Ordovician strata of Northwest Argentina. Most sequences of the Cordillera Oriental are fragmentary due to its complicated tectonic setting, thus the Lower Ordovician of Zenta represents a unique field laboratory to understand the sedimentary history of the basin. Promising results are expected from this new outstanding locality.

Aknowledgements

The manuscript was improved by the insightful review of R. Pickerill (New Brunswick) and an anonymous referee. C. Ek kindly helped with the English text. Several colleagues from the INSUGEO (Tucumán) assisted in field trips. The staff of the Museo Bernardino Rivadavia provided invaluable assistance with the sample housed in Buenos Aires. We thank E. Gómez-Hasselrot and D. Ruiz Holgado for the line drawings.

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Recibido: 29 de marzo de 2004.
Aceptado: 11 de noviembre de 2004.