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The Neoproterozoic-Cambrian Transition in Central Spain with a Discussion of Terminal Ediacaran Stratigraphy

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The Neoproterozoic-Cambrian Transition in Central Spain with a Discussion of Terminal Ediacaran Stratigraphy The Neoproterozoic-Cambrian Transition in Central Spain with a Discussion of Terminal Ediacaran Stratigraphy S. Jensen & T. Palacios Area de Paleontologia, Facultad de Ciencias, Universidad de Extremadura, E-06071 Badajoz, Spain Email: [email protected] Introduction A thick (>2 km) succession of terminal Neoproterozoic (Ediacaran) - Cambrian siliciclastics and minor carbonates crop out over a wide area of central Spain. The importance of this area to studies of the Ediacaran - Cambrian transition was first documented by Brasier et al.(1979) and has been further corroborated by new palaeontological finds (Palacios 1989; Vidal et al.,1994a; Vidal et al.1999; Palacios, et al., 2002). The outcrops cover a wide area, involve modest structural complications, and their investigation remains at a preliminary stage. Here we summarize the current knowledge and attempt to put the succession into a broader context of Ediacaran stratigraphy. The succession represents a general shallowing upward from turbiditic to shallow- water and near-shore deposits. Deposition took place on an immature passive margin, in part in grabens or semi-grabens, and records the terminal stages of the Cadomian Orogeny (Vidal, et al., 1999; Palacios, et al, .2002). The lithostratigraphical nomenclature has for long been in a state of flux; here we use the scheme of Vidal et al.(1994a) and Palacios & Vidal (1996). The basal Domo Extremeño Group consists of a thick succession of immature, turbiditic sandstones. Simple trace fossils occur low in the Domo Extremeño Group (Vidal, et al., 1994b) suggesting that these strata are no older than about 560 Ma. The only other palaeontological data consist of acritarchs such as Palaeogomphosphaera (Palacios, 1989) and, towards the top, more diverse trace fossils. Of particular interest is the occurrence of trace fossils showing angular junctions and traces preserved on lower bedding planes as rows of discontinuous segments or pods (Palacios, 1989; Vidal, et al., 1994a). These appear to represent surface expressions of three-dimensional burrow systems comparable to Treptichnus. In the area of the Ibor Anticline the Domo Extremeño Group is succeeded by the Ibor Group with a thin basal unit, which includes platform carbonates. Further to the east, in the Valdelacasa Anticline, the lower part of the Rio Huso Group largely consists of mudstones with scattered dolomitic sandstones and olistostromic levels in places developed as a megabreccia (Fuentes Olistostrome). Finds of the skeletal fossil Cloudina, including Cloudina hartmannae, in storm influenced platform carbonates of the Ibor Group (Palacios 1989, Vidal et al.1994a) suggest a latest Ediacaran age. Garcia Hidalgo (1993) reported trace fossils, including Hormosiroidea, at a stratigraphical position immediately above Cloudina-bearing units in the Ibor Anticline. In more eastern areas Cloudina is found also in olistostromic units. Higher in the succession are mudstones containing carbonaceous discoidal fossils that have been compared to Chuaria and Beltanelliformis (Beltanelloides), as well as Phanerozoic- aspect trace fossils (Brasier, et al., 1979; Vidal, et al., 1994a; Gamez, Vintaned and Linan ,1996). The middle part of the Ibor and Rio Huso Groups consists of black micro-laminated shales that in places are phosphatic, and which has yielded Sabellidites (Vidal, et al., 1994a) in the Ibor Anticline. Both groups are terminated by green shales, sandstones and minor limestones. The higher strata of the Ibor and Rio Huso groups contain early Cambrian skeletal fossils, including trilobites and the mollusc Anabarella, as well as age-diagnostic acritarchs (Vidal, et al., 1999, Palacios, et al., 2002). Notable is also the presence of abundant carbonaceous filaments. Discussion The current palaeontological data suggest that strata of the Domo Extremeño and Ibor/Rio Huso groups encompass the Ediacaran to Cambrian transition. The occurrence of Cloudina suggests that, as a starting point, the central Iberian succession should be compared with other Cloudina bearing regions. A terminal Ediacaran age is currently accepted for Cloudina in Namibia, South China, Oman, western USA and probably Brazil. Recent radiometric dates in Namibia and Oman confirm that Cloudina ranges close to the currently accepted age for base of the Cambrian. It has been suggested that a Nama Assemblage contains the youngest of three Ediacaran Assemblages (Gehling and Narbonne, 2002; Waggoner, 2003), characterized by the mineralized and agglutinated taxa Cloudina, Namacalathus and Archaeichnium and a low diversity of Ediacara- type fossils including Swartpuntia and Ernietta. Geographic and palaeoenvironmental factors must also be considered in addition to the still relatively scant information (cf. Waggoner, 2003; Grazhdankin, 2004), but this scheme nevertheless forms a useful starting point for discussion. At present, comparison of the central Iberian succession to the Nama Assemblage is based on the occurrence of Cloudina hartmannae. The Cloudina-bearing intervals in central Spain are currently under investigation in the search for a higher diversity of skeletal fossils. Nonmineralized Ediacara-type fossils are so far unknown from central Spain, with Beltanelliformis the only possible exception. An interesting similarity between Namibia and central Spain is, however, the occurrence of three-dimensional burrow systems, which consist of numerous interconnected parts and, therefore, can be considered moderately complex traces. Trace fossils in the Urusis Formation of Namibia have been identified as Treptichnus sp. (Jensen, et al., 2000). This occurrence is constrained between 548 and 545 Ma and widely accepted to represent a pre-Cambrian age. The presence of treptichnid-type trace fossils may, therefore, be added as a further characteristic of the Nama assemblage. The stratigraphic context of the central Iberian Cloudina is consistent with a latest Ediacaran age, and provides support for a latest Ediacaran trace fossil zone containing the first treptichnid- type trace fossils (Jensen, 2003), occurring in close association to Cloudina. The Ediacaran-Cambrian boundary is in many areas of the world located in close stratigraphic proximity to a major unconformity of possible eustatic origin (e.g., Pyle, et al., 2004). The Ediacaran-Cambrian boundary as defined on trace fossils is located at or somewhat below this unconformity. The Proterozoic-Cambrian boundary in Spain is widely represented by a period of non-deposition and erosion (e.g. Valladeres, et al., 2002). Valladeres, et al. (2000) identified a level correlative to the Fuentes olistostrome to represent a regional sequence boundary. However, Vidal, et al (1994a) interpreted this interval to be largely explicable by instability in the carbonate platform. An important task for further studies is renewed examination of the evidence for and against a break in sedimentation at this level. References Brasier, M.D., Perejón, A. and San Jose, M.A., 1979. Discovery of an important fossiliferous Precambrian-Cambrian sequence in Spain. Estudios Geologicos 35: 379-383. Gámez Vintaned, J.A. and Liñan, E., 1996. Significant ichnological data during the Neoproterozoic - early Cambrian transition in Iberia. II Field Conference of the Cambrian stage subdivision working groups, Field trip guide and Abstracts: 101-102. García Hidalgo, J.F., 1993. Las pistas fosiles del Alcudiense superior en el anticlinal de Ibor. Consideraciones cronoestratigráficas. Geogaceta, 13: 33-35. Gehling, J.G. and Narbonne, G.M., 2002. Zonation of the terminal Proterozoic (Ediacaran). Geological Society of Australia Abstracts 68: 63-64. Grazhdankin, D., 2004. Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution. Paleobiology, 30: 203-221. Jensen, S., 2003. The Proterozoic and earliest Cambrian trace fossil record: patterns, problems and perspectives. Integrative and Comparative Biology, 43: 219-228. Jensen, S., Saylor, B.Z., Gehling, J.G. and Germs, G.J.B., 2000. Complex trace fossils from the terminal Proterozoic of Namibia, Geology 28: 143-146. Palacios, T. 1989. Microfosiles de pared organica del Proterozoico superior (Région central de la Península Ibérica). Memorias del Museo Paleontologico del la Universidad de Zaragoza, 3(2): 1-91. Palacios, T. and Vidal, G., 1996. El Neoproterozoic Superio - Cámbrico inferior del centro de España. XII Jornadas de Palentología, Comunicaciones: 170-179. Palacios, T., Eguiluz, L. and Marti, M., 2002. New paleontological and structural data from the Upper Vendian - Lower Cambrian of Valdelacasa Anticline (Central Spain). VIII Conference of the Cambrian Stage Subdivision working group, Programme and Abstracts: 31. Pyle, L.J., Narbonne, G.M., James, N.P., Dalrymple, R.W. and Kaufman, A.J., 2004. Integrated Ediacaran chronostratigraphy, Wernecke Mountains, northwestern Canada. Precambrian Research, 132: 1-27. Valladeres, M.I., Barba, P. and Ugidos, J.M., Colmenero, J.R. and Armenteros, I., 2000. Upper Neoproterozoic - Lower Cambrian sedimentary successions in the Central Iberian Zone (Spain): sequence stratigraphy, petrology and chemostratigraphy. Implications for other European zones. International Journal of Earth Sciences, 89: 2-20. Valladeres, M.I., Barba, P. and Ugidos, J.M. 2002. Precambrian. In Gibbons, W. & Moreno, M.J., (eds.). The Geology of Spain: 8-16. Vidal, G., Palacios, T., Diez Balda, M.A., Gámez Vintaned, J.A. and Grant, S.W.F., 1994a. Neoproterozoic - early Cambrian geology and palaeontology of Iberia. Geological Magazine, 131: 729-765. Vidal, G., Jensen, S. and Palacios, T., 1994b. Neoproterozoic (Vendian) ichnofossils from the Lower Alcudian strata in central Spain. Geological Magazine, 131: 169-179. Vidal, G., Palacios, T., Moczydlowska, M. and Gubanov, A.P., 1999. Age constraint from small shelly fossils on the early Cambrian terminal Cadomian Phase in Iberia. GFF 121: 137-143. Waggoner, B. 2003. The Ediacaran biota in space and time. 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