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Neoproterozoic (Vendian) ichnofossils from Lower Alcudian strata in central Spain

Published online by Cambridge University Press:  01 May 2009

Gonzalo Vidal ,
Sören Jensen  and
Teodoro Palacios
Gonzalo Vidal
Affiliation:
Uppsala University, Institute of Earth Sciences, Norbyvägen 22, S-752 36 Uppsala, Sweden
Sören Jensen
Affiliation:
Uppsala University, Institute of Earth Sciences, Norbyvägen 22, S-752 36 Uppsala, Sweden
Teodoro Palacios
Affiliation:
Universidad de Extremadura, Facultad de Ciencias, Area de Paleontologia, 06071 Badajoz, Spain
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Abstract

Simple trace fossils are reported from three localities in central Spain within a monotonous succession of shale and greywacke and classical turbidites attributed to the Neoproterozoic (Riphean) Lower Alcudian megaunit. The Lower Alcudian strata are believed to have a complex tectonic history including deformation during the Pan-African Orogeny. Chronostratigraphic control is not available for this structurally complex succession and fossil evidence is sparse. Here we report on a low-diversity ichnofossil association including the ichnospecies (isp.) Gordia marina Emmons, 1844, Gordia isp., G. aff. arcuata Książkiewicz, 1977. The stratigraphically inferred Proterozoic age of the rocks, added to the present ichnofossil association, suggests that colonization of deeper waters started before the early Cambrian. We further consider that the plausible oxygen requirements of marine invertebrate(s) which produced the present ichnofossil taxa may well have exceeded the oxygen levels in a largely dysaerobic environment. A circulation model proposed by one of us (Palacios, 1989), implying the presence of extensive upwelling and descending oxygen-rich waters, helps to explain the continuous colonization of deep-water settings in Lower and Upper Alcudian strata. We argue that the ichnofaunal record of strata older than 650 Ma is both rare and inconclusive. Moreover, the present ichnofauna, being similar to ichnofaunal associations in environmentally comparable Upper Alcudian strata, is largely documented from Neoproterozoic (Vendian) and early Palaeozoic associations elsewhere. We conclude that the new ichnofossil evidence is consistent with a Vendian or younger age for the Lower Alcudian megaunit. This view is also consistent with a recently published maximum deposition age of 565 Ma inferred from U–Pb datings of detrital zircons from the supposedly much older (Middle Riphean) Tentudia Group or ‘Serie Negra’, that appears roughly time-equivalent with Lower Alcudian strata. It thus may appear that basin formation and flysch deposition in central Iberia encompass a minor segment of Neoproterozoic time, being probably limited to the Vendian. By default, formerly postulated Pan-African deformation of Neoproterozoic Lower Alcudian strata appears implausible. We further conclude that the complex pre-Phanerozoic structural history of Alcudian strata is probably restricted to Cadomian and younger deformation. Former structural models need substantial revision.

Type
Articles
Information
Geological Magazine , Volume 131 , Issue 2 , March 1994 , pp. 169 - 179
Copyright
Copyright © Cambridge University Press 1994

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