Upper Neoproterozoic-Lower Cambrian sedimentary successions in the Central Iberian Zone (Spain): sequence stratigraphy, petrology and chemostratigraphy. Implications for other European zones

Abstract

The Upper Neoproterozoic±Lower Cambrian sedimentary succession in the central areas of the Central Iberian Zone has been subdivided into 12 mostly siliciclastic lithostratigraphic units, ranging in thickness between 1800 and 3900 m. The lithology and facies of each unit are described and the facies associations are interpreted. The facies resulted mainly from turbidity currents and debris flows and, to a lesser extent, from submarine slides and traction flows. The facies associations suggest that sedimentation took place in slope and base-of-slope environments. Two depositional sequences are recognized, separated by a type-1 unconformity. The lower sequence is of Late Neoproterozoic age (units I±IV) and exhibits lowstand, transgressive, and highstand systems tracts. Most of the upper sequence is probably of Early Cambrian age (units V±XII). It begins at the base of unit V and possibly ends with the Tamames Limestone Formation. The upper sequence records a lowstand systems tract and minor-order sea-level oscillations. In the Cambrian units there are higher amounts of feldspar and smaller quantities of intrabasinal clasts than in the Neoproterozoic units. The modal data plot close to the Q±L and Qm±Lt sides of Q±F±L and Qm±F±Lt triangular diagrams, suggesting a provenance from a recycled orogen evolving into a provenance from a craton interior towards the top of the succession. The chemical results, based mainly on Al 2 O 3 , TiO 2 , Zr, and Nb abundances in shales from all the units, strongly suggest a gradual compositional change within this sedimentary succession. Together with the petrological data, the chemical results do not reveal any obvious coeval volcanic contribution to the sed-iments. On the basis of the chemical data, a comparison is made with other European zones containing detrital sediments composed of reworked crustal components.

A reactive assimilation model for regional-scale cordierite-bearing granitoids: geochemical evidence from the Late Variscan granites of the Central Iberian Zone, Spain

“…The consequence was severe chemical alteration of the siliciclastic material, resulting in intense leaching of Ca and Na as well as the redistribution of some trace elements (principally Y and the rare earth elements). These chemical effects are found in detrital rocks underlying, and especially overlying, this regional-scale unconformity, and sequence stratigraphy predicts the presence of other such unconformities, notably at the lower boundary of the Upper Neoproterozoic sequence, with similar extreme compositional characteristics (Ugidos et al 1997a(Ugidos et al , b, 2003aValladares et al 1998Valladares et al , 2000. The important implication for the purposes of this present study is that the sedimentary succession that hosts the granites and was subject to Variscan high T/low P regional metamorphism contains at least one, and probably multiple, horizons that have suffered extreme chemical alteration.…”

“…The most conspicuous characteristics of these siliciclastic rocks are their petrological and geochemical homogeneity, together with their chemical maturity over an extensive region of more than 30,000 km 2 in western Spain. Furthermore, some chemical concentrations such as TiO 2 and Zr, together with some elemental ratios (e.g., Rb/Zr<0·65, Ti/Nb>400) and relatively high Nd values in the sequences of the Upper Neoproterozoic ( Nd > 4), are clearly distinct from those of the Lower Cambrian (Rb/Zr>0·7, Ti/Nb<400, Nd < 4), reflecting different sources for the siliciclastic materials of both sequences (Ugidos et al 1997a(Ugidos et al , b, 2003Valladares et al 1998Valladares et al , 2000.…”

“…143 Nd/ 44 Nd ratios were measured in multidynamic mode and corrected for mass fractionation using an exponential law and 146 Nd/ 144 Nd=0·7219. Repeat analyses of the internal laboratory standard (JM) gave 143 Nd/ Table 1 Granite emplacement levels in relation stratigraphic units derived from sequence stratigraphy analysis in the Central Iberian Zone W and S of Salamanca (Valladares et al 2000). The stratigraphic levels of some specific sample locations are also indicated.…”

“…The Upper Neoproterozoic-Lower Cambrian boundary is an unconformity corresponding to a eustatic fall in sea level (Valladares et al 2000(Valladares et al , 2006 that led to the subaerial exposure of the uppermost Neoproterozoic. The consequence was severe chemical alteration of the siliciclastic material, resulting in intense leaching of Ca and Na as well as the redistribution of some trace elements (principally Y and the rare earth elements).…”

LP-HT anatectic processes and lithological heterogeneity in the Mindelo Migmatite Complex (NW Portugal)

“…The SchistGreywacke Complex is a thick turbidite sequence aged Ediacaran to Cambrian that occupies the major part of the Central Iberian Zone. Two main units have been recognized: a lower unit aged Ediacaran and an upper unit aged Early Cambrian (Valladares et al, 2000;Ugidos et al, 2010;Pereira et al, 2012). In Portugal these two units were nominated Beiras Group (BG) and Douro Group (DG) respectively (Sousa, 1984) (Fig.…”