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Bulletin de la Societe Geologique de France; May 2002; v. 173; no. 3; p. 219-227; DOI: 10.2113/173.3.219
© 2002 Societe Geologique de France
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Stratigraphic and structural framework of the Neoproterozoic Paracuellos Group, Iberian Chains, NE Spain

J. Javier Álvaro1 and Marie-Madeleine Blanc-Valleron2

1 UPRESA 8014 CNRS, Cité Scientifique SN5, Université de Lille I, 59655-Villeneuve d’Ascq, France. Fax : 03 20 43 69 00, e-mail : Jose-Javier.Alva-ro{at}univ-lille1.fr
2 CNRS-FR 2400, Laboratoire de Géologie, Muséum National d’Histoire Naturelle, 43, rue Buffon, 75005-Paris, France. Fax : 01 40 79 57 39, e-mail : valleron{at}mnhn.fr

Abstract

The Neoproterozoic Paracuellos Group of the Iberian Chains constitutes the core of two disconnected faulted blocks, named the Paracuellos and Codos antiforms. Precise lithostratigraphic correlations between both areas are not possible due to the structural complexity and because marker beds do not persist laterally. This paper presents a crustal cross-section of the Neoproterozoic axial core (the Paracuellos antiform) based on surface geology, boreholes and seismic reflection profiles. Seismic reflection data reveal that the basement was directly involved by a major Hercynian structure, named here the Paracuellos fault, which splits longitudinally the Paracuellos axial core. In seismic profiles this fault occurs as a northeasterly-dipping reflector (60–70° steep), evidencing a bivergent geometry of the lateral crustal elements.

The sedimentary evolution of the Neoproterozoic Iberian platform ranges from transgressive, non-cyclic, offshore to hemipelagic, black and green shales (Sestrica Formation) to progradational trends recording shoaling during episodes of rapid sediment influx (Saviñán Formation), presumably in response to a low standing sea-level. The siliciclastic succession is punctuated in the inner platform by deposition of phosphatic limestones (Codos Bed), representing a major shoaling event and demarcating a sharp regional change of sedimentation separating two similar siliciclastic tendencies. A diagenetically induced bedded chert (Frasno Bed) occurs in the outer platform, and is interpreted as being the product of at least two silicification episodes. Both the Codos and Frasno Beds are overlain by the Aluenda Formation, which exhibits nearshore to offshore features. An important sedimentary discontinuity appears across the Neoproterozoic-Cambrian transition. The Cambrian(?) Bámbola Formation is paraconformable with the Paracuellos Group displaying a gradual transition in inner platform areas, whereas an erosive unconformity occurs in outer areas. The horizon of the Neoproterozoic-Cambrian boundary is not identified in the Iberian Chains, where neither Cadomian deformation nor discordances are recognisable.

Key Words: Stratigraphy • Crustal cross-section • Neoproterozoic • Iberian Chains • Spain






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