Geobios 37 (2004) 127–133 www.elsevier.com/locate/geobio
Precambrian-Cambrian ichnofossils, an enigmatic “annelid tube” and microbial activity in the Puncoviscana Formation (La Higuera; Tucumán Province, NW Argentina) Icnofósiles Precambrico-Cámbricos, un enigmático “tubo de anélido” y actividad microbiana en la Formación Puncoviscana (La Higuera, Tucumán, Argentina) Guillermo F. Aceñolaza INSUGEO, CONICET, Universidad Nacional de Tucumán. Miguel Lillo 205, 4000 Tucumán, Argentina Received 25 October 2002; accepted 10 March 2003
Abstract The Puncoviscana Formation (s.l.) comprises a few thousand meters of mainly low-grade metamorphosed sedimentary rocks. These are dominated by mudstones, slates, and sandstones cropping out extensively in the Eastern Cordillera of NW Argentina. The formation is known by its ichnofossils, that include a variety of ichnotaxa such as Asaphoidichnus, Cochlichnus, Didymaulichnus, Dimorphichnus, Diplichnites, Glockerichnus, Helminthoidichnites, Helminthopsis, Monomorphichnus, Nereites, Oldhamia, Planolites, Protichnites, Protopaleodictyon, Protovirgularia and Treptichnus, among the most remarkable ones. The record of recurrent wrinkle structures and palimpsest ripples suggest the presence of microbial matgrounds and biofilms in different localities of NW Argentina. These have been regarded in the literature as important elements of paleoenvironments during the Vendian-Tommotian transition. Three associations have been recognized within the unit. These are the Beltanelliformis, Nereites and Oldhamia, associations, characterized by a distinct spatial distribution. They appear in NW Argentina as elongated bands (NE-SW general direction), representing different temporal steps in the Precambrian-Cambrian transition of the region. We describe an ichnoassociation represented by Helminthopsis isp., Treptichnus cf. aequalternus Schlirf, Monomorphichnus isp., comparable to an enigmatic “annelid tube” regarded as Sphenotallus? sp., and several undifferentiated microbial structures for one locality of “Beltanelliformis association” at La Higuera outcrops (Province of Tucumán). The early-referred association is included in highly folded heterolithic facies, characterized by a alternate succession of greenish-grayish fine/mid sandstones and slates. All new data on the biostratigraphy of the Precambrian-Cambrian transition in NW Argentina, contribute to a better understanding of sequences, and to the evolution of the Early Paleozoic basins of the South American margin. © 2004 Elsevier SAS. All rights reserved. Resumen Bajo la denominación de Formación Puncoviscana (s.l.) se agrupan algunos miles de metros de rocas sedimentarias arenoso-pelíticas ligeramente metamorfizadas de amplio desarrollo en la Cordillera Oriental Argentina. Dentro de esta unidad también se incluyen raras manifestaciones de rocas carbonáticas, conglomerádicas y vulcanitas. El registro icnológico de la Formación Puncoviscana (s.l.) ha generado una relativa interpretación de su cronoestratigrafía, habiéndose registrado una buena variedad de icnotaxones. Entre otros se mencionan Asaphoidichnus, Cochlichnus, Didymaulichnus, Dimorphichnus, Diplichnites, Glockerichnus, Helminthoidichnites, Helminthopsis, Monomorphichnus, Nereites, Oldhamia, Planolites, Protichnites, Protopaleodictyon, Protovirgularia y Treptichnus. Asimismo, el registro E-mail address:
[email protected] (G.F. Aceñolaza).
© 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.geobios.2003.03.008
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repetitivo de estructuras ondulosas y del tipo « kynneia » sugieren la presencia de matas microbianas y biofilmes en diferentes localidades de la Cordillera Oriental Argentina. Tres asociaciones han sido reconocidas en la unidad: Beltanelliformis, Nereites y Oldhamia, que se diferencian en su distribución tanto espacial como temporal de sus diferentes elementos. En esta oportunidad se da a conocer una icnoasociación conformada por Helminthopsis isp., Treptichnus cf. aequalternus Schlirf y Monomorphichnus isp., junto al primer registro de un « tubo de anélido » comparable con Sphenothallus en la unidad, así como algunas estructuras microbianas indiferenciadas para los afloramientos de La Higuera, provincia de Tucumán (NW Argentina). Toda nueva información que aporte al conocimiento bioestratigráfico de la transición Precámbrico-Cámbrico en el NW de Argentina aporta elementos que contribuyen a clarificar diferentes aspectos de las secuencias Eopaleozoicas del margen Andino Sudamericano. © 2004 Elsevier SAS. All rights reserved. Keywords: Ichnofossils; Annelid tube; Microbial activity; Precambrian-Cambrian; Argentina Palabras clave: Icnofósiles; Tubo de anélido; Actividad microbiana; Precámbrico-Cámbrico; Argentina
1. Introduction Trace fossils frequently appear below the first record of Cambrian shelly fauna in Northwestern Argentina, providing important evidence for the evolution and early behavior of metazoans in the region. The early referred data has been the key to get closer to the understanding of the Precambrian – Cambrian transition in this part of Gondwana’s margin, taking into account the little knowledge on fossil organisms during that interval of time. Previous research in the region showed the ichnological diversity of the Precambrian-Cambrian Puncoviscana Formation (s.l.), as well as the very few references to probable soft-bodied elements described in the succession (Aceñolaza and Durand, 1973, 1984, 1986; Aceñolaza et al., 1976; Aceñolaza, 1978; Durand and Aceñolaza, 1990; Durand et al., 1993; Buatois et al., 2000). These early studies have allowed a better understanding of the chronostratigraphy of the metasedimentary sequence, helping on the comparison with other Precambrian/Cambrian sequences in the world. Promising results are expected, taking in to account the little knowledge on this matter in this region (Conway Morris, 1990). In this contribution, an interesting ichnoassociation supporting an early Cambrian age for the strata is presented. The first “annelid tube”, and few structures regarded as microbial mats from the locality of La Higuera, (Tucumán Province) are described. Environmental considerations are also given.
2. The Puncoviscana Formation: geological background The Neoproterozoic to early Cambrian strata in northwestern Argentina consists mainly of over 2000 meters of highly folded, slightly metamorphosed slates and schists (Aceñolaza and Miller, 1982; Aceñolaza and Durand, 1986; Jezek, 1990; Durand and Aceñolaza, 1990; Aceñolaza et al., 1999). The Puncoviscana Formation (s.l.) conforms the basement rocks for the stratigraphic sequence of the Eastern Cordillera
of Argentina and Bolivia. These are characterized by a highly folded very low metamorphic sediments grading into schists, few turbidites, pelagic clays, limestones and rare volcanic rocks in the central part of the basin. Outcrops are widely distributed in the northern provinces of Jujuy, Salta, Tucumán, Catamarca and La Rioja, aligned on an elongated strip of over 800 km long and 300 km wide. This configuration reflects the basin morphology for the Puncoviscana Formation, that was limited by the Rio de la Plata Craton crystalline rocks to the east (south branch of the Brasilian Craton – sensu De Almeida et al., 1976), and from the Arequipa Massif to the west. Northwards, the Puncoviscana Basin was connected to other Paraguayan and Brazilian basins within the South American platform. Siliciclastic and carbonate sequences of a Neoproterozoic – Lower Cambrian age have been described in the upper Paraguay River, with tillites of the Jangada and Puga Formations. These are overlain by marine limestones of the Arará and Itapucumí formations and the Corumbá Group (Brasil Lemos and Pinto Oliveira, 1983, with references). The southern extension of the basin is not well known, but it is assumed to be connected to the northern Patagonian El Jaguelito Formation (González et. al., 2002).
3. Locality, age and fossil association An ichnofossiliferous assemblage associated to an “annelid tube” is described from the Precambrian/Cambrian transition in the early Paleozoic Central Andean Basin. Outcrops of the Puncoviscana Formation are exposed along the road that cuts through the Morro de la Higuera Hill, close to La Higuera locality (Trancas Department, NW Tucumán Province; Fig. 1). A highly folded heterolithic facies sequence represented by sligthly metamorphosed greenish-grayish fine/mid sandstones and shales characterize the outcrops at La Higuera, Tucumán Province, Northwestern Argentina. All fossiliferous material comes from the same unit, within a sector of 3.2 Km along the road (Fig. 2). Aceñolaza and Durand (1987) and Durand and Aceñolaza (1990) chronologically analyzed the ichnofauna of the
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Trancas 65º10´
Partial schematic section at La Higuera, near Choromoro
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Fig. 1. Locality map of the outcrops near La Higuera, with regional distribution of the Puncoviscana Formation (Tucumán Province, NW Argentina). Schematic section showing stratigraphy. Fossiliferous levels (modified from Durand et al., 1993) indicated by asterisks. Fig. 1. Mapa de localización de los afloramientos cercanos a la localidad de La Higuera, y distribución regional de la Formación Puncoviscana (Provincia de Tucumán, NO de Argentina). Perfil estratigráfico esquemático con ubicación de los niveles fosilíferos (modificado de Durand et al., 1993).
metasedimentary basement of NW Argentina, discriminating an early ichnoassociation of Nereites (Praecambridium, Vendia, Nereites saltensis, and Nereites isp.) followed by a late Oldhamia association, characterized by a number of traces referred to early arthropod activities such as Oldhamia radiata, O. flabellata, O. antiqua, Protichnites isp. and Monomorphichnus isp. Recently Aceñolaza and Alonso (2000) and Aceñolaza and Aceñolaza (2001) refer to the previously mentioned distribution within the same basin, discussing three associations, whose chronological and spatial distribution is being matter of debate (Beltanelliformis, Nereites and Oldhamia associations). Aceñolaza and Aceñolaza, (2001) suggested that the corresponding Neoproterozoic-Lower Cambrian transition should be placed within the Oldhamia ichnoassociation. Among associated trace fossils recovered from the strata cropping out near Choromoro, the presence of several undifferentiated limb marks and trace fossils as Monomorphichnus restricts the material to the lowermost Cambrian (Crimes, 1987, 1992; Crimes and Anderson, 1985).
Fig. 2. Outcrops of the Puncoviscana Formation at La Higuera, Tucumán Province, NW Argentina. Metasedimentary facies with fossiliferous levels near the Choromoro River. Fig. 2. Afloramientos de la Formación Puncoviscana en La Higuera, Provincia de Tucumán, NW de Argentina. Facies metasedimentarias con niveles fosilíferos a orillas del Río Choromoro.
4. Newly described material Collected specimens are deposited in the Invertebrate Paleontological Collection of the Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán (Argentina), with a designated prefix PIL (Paleontología Invertebrados Lillo). Class and Order UNCERTAIN Family UNCERTAIN Sphenothallus? sp. Fig. 3(1–3) Material: one relatively well-preserved sample with mould. Description: large tube of smooth external surface (length = 12.6 cm; anterior aperture = 1.2 cm). Tube widens posteriorly before closing. The widening reaches up to 1.4 cm an is located in the mid-posterior sector of the tube. Average width = 0.9 cm. No annulations or preserved internal soft tissues were observed. Few folds can be seen in the wide sector of the tube. Whether this faint folds are due to decay processes or have a different origin is not clear. Irregularities in the margins of the tube, as well as the previously
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Fig. 3. 1–3. Sphenothallus? sp. (PIL 14. 545). 2. Mould of sample 1 (PIL 14.546). 3. Close-up of same sample (PIL 14.546). 4. Helminthopsis isp. winding trace, slightly meandering from the fossiliferous locality (PIL 14.549). 5. Wrinkle structure interpreted as produced by microbial activity La Higuera. 6. Monomorphichnus isp., from Puncoviscana Formation at La Higuera (PIL 14.548). 7. Treptichnus cf. aequalternus Schlirf (PIL 14.547). 8. Slab with different apendicular scratch marks, including Monomorphichnus isp (PIL 14.548). 9. Structure interpreted as produced by microbial activity. 10. Same as Figure 9. 11, 12. Sample with mould of polygonal structures of suspected microbial origin. Scale bar in all figures is 1 cm. Fig. 3. 1–3. Sphenothallus? sp. (PIL 14. 545). 2. Molde de ejemplar 1 (PIL 14.546). 3. Acercamiento del anterior (PIL 14.546). 4. Helminthopsis isp. Traza curvada, ligeramente meandriforme (PIL 14.549). 5. Estructura tipo « wrinkle », interpreada como producto de la actividad microbiana en la secuencia aflorante en La Higuera. 6. Monomorphichnus isp., de las pizarras de la Formación Puncoviscana en La Higuera (PIL 14.548). 7. Treptichnus cf. aequalternus Schlirf (PIL 14.547). 8. Pieza con diferentes marcas apendiculares, incluyendo Monomorphichnus isp (PIL 14.548). 9, 10. Estructuras microbianas frecuentes en la misma localidad. 11, 12. Estructuras poligonales genéticamente vinculadas a tapetes microbianos. La barra de escala es de 1 cm en todas las figuras.
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mentioned folds suggest that wall may have suffered some post-mortem deformation. Discussion: the high relief displayed by the sample supports the idea of a semi-rigid tube. Material is probably flattened. Metamorphism suffered by the host rock did not allowed preservation of shelly material. No element allows to determine the original composition, either phosphatic or calcareous. The tube morphology and preservation type resembles those of ?Sphenothallus from the Cuyahoga Formation (Mason and Yochelson, 1985: Fig. 4(1–4)). The lack of annulations on the surface does not support the idea of a correspondence with the enigmatic genus Byronia Matthew. Sphenothallus is a frequent fossil in the Ordovician and Silurian series of North America and Europe (Mason and Yochelson, 1985, with references). The genus has been interpreted as an annelid type of organism with unknown affinities. The analyzed material from La Higuera locality is attributed with some doubts to this genus, on the basis of its general morphology. Fedonkin (pers. comm.) highlighted the high relief of the fossil, supporting the idea that it is different from Selkirkia. The widening of the tube in its mid-posterior sector and the smooth surface of the shell characterize this sample. No internal soft tissues are preserved. The widening of the anterior aperture seems to have been related to the position of the proboscis or other soft tissues. Sphenothallus? sp. from La Higuera locality differs from any previously described material recovered from the Puncoviscana Formation by its unique preservation, and distinctive morphology of its tube. The general aspect of the material resembles Selkirkia Walcott, but differs from Silkirkia by its sclerotized tube preserved with a remarkable relief. Selkirkia is a common fossil in North American Lower and Middle Cambrian localities (Resser and Howell, 1938; Conway Morris, 1977; Conway Morris and Robison, 1986). Whereas these groups evolved to modern phogonophorans with usually stiff chitinous tubes is a matter of discussion (Sokolov, 1972; Fedonkin, 1990; Ruppert and Barnes, 1993). Phogonophoran walls display a full laminar structure capable of recording wrinkle structures and folds as exemplified by Sphenothallus (Urbanek, 1979). 4.1. Trace Fossils Ichnogenus Monomorphichnus Crimes, 1970 Type ichnospecies: Monomorphichnus bilinearis Crimes, 1970 Monomorphichnus isp. Fig. 3(6, 8) Material: two rock slabs with undifferentiated apendicular trace fossils including Monomorphichnus isp. (PIL 14.548) Description: deep straight to slightly comma-like scratches grouped in a series of up to 15 parallel to subparallel individuals. Principal deeper and secondary shal-
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lower scratches can be recognized. Length varies from 0.2 to 0.7 mm. Discussion: Monomorphichnus is one of the many controversial ichnogenera that have generated many discussions in the literature. It has been recorded from the earliest Cambrian Treptichnus pedum zone to Triassic, in shallow marine and estuarine environments. Fillion and Pickerill (1990) and Jensen (1997) summarize the present status of the ichnogenus. The material from La Higuera is associated to several undifferentiated arthropod apendicular trace fossils. Ichnogenus Treptichnus Miller, 1889 Type ichnospecies: Treptichnus bifurcus 1889 emend. Schlirf, 2000.
Miller,
Treptichnus cf. aequalternus Schlirf Fig. 3(7) Material: one sample with a well-preserved trace (PIL 14.547). Description: horizontal to sub-horizontal regularly alternating trace of elliptical burrows (probably flattened). Very shallow burrow (diameter = 4-5 mm). Burrows raise 0.4 to 0.2 mm above the surface of rock sample. No internal structures or wall lining visible. Discussion: the material from La Higuera locality has been compared to Treptichnus aequalternus due to the regularity of the projections on either side of the main trace. The scarce development of the trace does not allow any full confident assignment of the same. Treptichnus has been interpreted as a feeding structure or a farming trace (Schlirf, 2000) possibly produced by a vermiform organism (see Buatois and Mángano, 1993 for summary). Ichnogenus Helminthopsis Heer, 1877 Type ichnospecies: Helminthopsis magna Heer, 1877 by subsequent designation (Ulrich, 1904), emend. Fillion and Pickerill, 1990. Helminthopsis isp. Fig. 3(4) Material: one sample with one preserved trace (PIL 14.549). Description: slightly widening-meandering horizontal, subcylindrical trail (4 mm wide length up to 21 cm). Width of trace constant along the trace. No burrow fill or lining of trace preserved. Remarks: a similar material has been described previously from different localities of the Puncoviscana Formation (Aceñolaza and Durand, 1986). The material from La Higuera displays a pattern of loosely developed meanders, not regularly traced that do not allow a clear ichnotaxonomic differentiation. Helminthopsis is regarded as a produced by polychaete annelids or priapulids (Fillion and Pickerill, 1990).
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5. Final remarks and conclusions The Precambrian - Cambrian transition has long been regarded as included in the strata of the Puncoviscana Formation (Durand, 1993; Durand and Aceñolaza, 1990 with references). No stratigraphical characterization of the unit has been consensuated due to the highly folded and tectonically disturbed sequences. Until now, the only means to get closer to a better understanding of the unit is by biostratigraphic information. The presence of the first “annelid tube” assigned to Sphenothallus? sp. and associated trace fossils (Helminthopsis isp., Treptichnus cf. aequalternus Schlirf and Monomorphichnus isp.) adds important elements within the so called Beltanelliformis association of NW Argentina, and are new important pieces of information useful for the understanding the Precambrian-Cambrian transition in NW Argentina. The ichnofossiliferous record of the Puncoviscana Formation supports a wide spectrum of organisms living in these seas during the Neoproterozoic-Cambrian transition. Associated wrinkle structures and distinctive surface textures that appear in the slates of the Puncoviscana Formation are referred to the development of microbial mats, reflecting the importance of the microbial sediment binding already in Precambrian-Cambrian times (Fig. 3(5, 9, 11–13)). Further work must be done to get a better definition of the biostratigraphy and stratigraphy of the unit. The outcrops at La Higuera (Tucumán) seem to the particularly important for the resolution of this question.
Acknowledgements The early version of this paper benefited greatly from the critical remarks made by the referees, Drs. Zhu Maoyan and Eladio Liñán as well as by the precise comments of Dr. Michael Fedonkin on the “tube worm”. I also thank D. Ruiz Holgado for the drawing of figures. Special thank to the Fundación Antorchas (Buenos Aires, Argentina) for the financial support.
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