WILSON CYCLE IN THE IBERIAN VARISCIDES: GEODYNAMICAL IMPRINTS ON PALAEOICHNOCOENOSES DISTRIBUTION (A SYNTHESIS OF KNOWN DATA)

by C. Neto de Carvalho

Palaeoichnologic studies of any sedimentary basin show a need to relate conditioning elements of biologic communities installation and development with the stages of basin tectonostratigraphic evolution. Iberian Massif constitutes the most southern and vast tectonic element of Armorican plate, a continental block that was separated from Gondwana supercontinent in Lower Palaeozoic. During this time, Armorica was divided from Laurussia by Atlantic proto-ocean, from Baltica by Tornquist sea and from Gondwana by Rheic. Subsequent close of these oceans in the end of Palaeozoic and continental elements regrupment resulted in a multiphase orogeny (Pereira et al., 1998).

Iberian Massif is formed by four tectonostratigraphic terranes (Ribeiro et al., 1987 in Ribeiro et al., 1990), which have contrasting histories in stratigraphy, structure and metamorphism. These incorporate an autochthonous zone (Iberian Terrane), composed by Cadomian cycle’s amalgamated elements which passed to have a co-evolution in Variscan times. Structural and geophysics data present by Dias (1998) point in sense of an heterogeneous precambrian basement, formed by unites with different reologic behaviour separated by two main anisotropy planes corresponding to orogenic suture zones. In this way, it will be expect an heterogeneous reply of Iberian Terrane in distensive periods associated with Wilson Cycle initial stages (op. cit.).

The beginning of palaeozoic Wilson Cycle in this particular peri-gondwanic palaeogeographical area is registed in Lower Palaeozoic unites exposed in all tectonic elements of Iberian Terrane. Precocious expressions took place in early Ordovician, yet in transition between Cadomian and Variscan cycles, with a prolonged intracontinental rifting phase in transtensive regime marked by Centro-Iberian aulacogene development (Romão et al., 1992). This process occurs after abortion and migration to N of rifting  phenomenon, from Ossa-Morena Zone (O.M.Z.) to Centro Iberian Zone (C.I.Z.). During Variscan cycle, this rifting phase had been continuous between Cambrian and Silurian. Indeed, oceanization process originates Rheic opening, probably in Ordovician (Dias & Ribeiro, 1995). However, during Sardic deformation phase, which marked O.M.Z. and C. I. Z., cambrian aulacogene suffers a tectonic inversion for lateral transpression (Tomar-Córdoba megashear zone reactivation) in Cambrian-Ordovician transition, taking to emersion of ante-ordovician formations, as well as their deformation, intensified to S of C.I.Z., generating folds with sub-vertical axial planes (Pereira et al., 1998). Nevertheless, in O.M.Z.  Sardic deformation had only basculated pre-ordovician sequences. Ordovician sedimentation shows cohesive in total area of C.I.Z. presenting, however, completely distinct compositions in O.M.Z., which reveals an anisotropic reaction of precambrian basement, as a consequence of meridional suture kinematics (Dias, 1998).

Basal conglomerates outcropping in C.I.Z. have been dated from Lower Ordovician (Tremadoc or Arenig), based on lithostratigraphic framing, since palaeontologic evidences are scarce (monospecif ichnofabrics of Skolithos isp.) and have no stratigraphic importance (Sequeira, 1993). These conglomerate unites, discordantly placed (first-order angular unconformity) above Dúrico-Beirão Supergroup sequences, show sedimentation processes and depositional architecture controlled by inherited topographic irregularities (tectonic reliefs), characterised by immature and coarse deposits of gravitic nature laterally passing littoral facies, typical of coalescent alluvial fans-fan deltas. In some places (i.e., Quinta da Ventosa Formation), deep submarine fans was generated in active fault escarpments (Conde, 1966; McDougall et al., 1987; Sequeira, 1993). In continental facies there are Skolithos piperocks in channalized sandstones, revealing a rare ecological distribution to Skolithos ichnofacies (Droser, 1991).

Above breccio-conglomerates sequences deposited Armorican Quartzite Formation (A.Q.F.), in Lower-to-Middle Ordovician (see below). This unit is composed by detritical, argillaceous-to-siliciclastics sequences, corresponding to a foreshore-inner offshore, storm-influenced environmental deposition (Guitars-Marco et al., 1990). Climax ichnodiversity in Ordovician is registed in this formation (Romano, 1991), which had preliminarily accounted a presence of 18 ichnogenera (work in progress). With relation to upper limit of    A.Q.F., presence of Didymograptus (D.) murchisoni biozone elements in the base of the above Breijo Fundeiro Formation (Cooper, 1980), statistic domination verified for Eodalmanitina destombesi nava and Selenopeltis macrophtalma near lower boundary of this formation in Mação region point out to a lower Llanvirn age extent (A. Marques Guedes, pers. commun.). However, age range for A.Q.F. upper limit to N, in Buçaco region, doesn’t exceed the top of Arenig (Romano et al., 1986) and lower limit of Valongo Formation blackshales located northernmost, which lies above A.Q.F., dates from upper Arenig (Expansograptus hirundo biozone, Couto et al., 1997). In what is concerned about ichnostratigraphy, A.Q.F. can be dated with the association of Cruziana rugosa group, C. rouaulti and C. imbricata (cf. Seilacher, 1994). These ichnoassociation occurrence constitutes an evidence of the precision that ichnostratigraphy can attain in datation of “azoic” formations. For the presented data, it seems to exist sufficiently biostratigraphic evidences that indicates a N-to-S diachronism (cf. Romano et al., 1986) revealed by A.Q.F. upper boundary (Neto de Carvalho et al., 1999). Transgressive global character of Lower Ordovician formations, with general direction NE-SW (Hamman et al., 1982), relates with geodynamic regime valid in that time, of distensive nature, which is associated with installation of a epicontinental sea above a sardic palaeotopography substantially razed (Ribeiro et al., 1990), nevertheless more irregular and prominent to S (McDougall et al., 1987).

During Lower Ordovician times, O.M.Z: attend to a progressive fill of the cambrian aulacogene, with deposition of thick sedimentary series with turbiditic features. Nereites ichnofacies, where it can be found a Pascichnia-type diversified ichnofauna, is ubiquitous (Nery Delgado, 1910) except for an area (Mourão) where was found in proximal turbidite-like sedimentary layers an ichnoassociation (Cruziana cf. furcifera, Daedalus halli and ?Arthrophycus isp.) typical of C.I.Z. (A.Q.F.) coastal facies (Perdigão, 1964). Pelagic facies nature, with frequent euxinic periods, predominated in this terrane until the end of Silurian.

Between Middle Ordovician and Silurian, cadomian substract stretchment extends in C.I.Z., inducing abundantly bimodal magmatism (e.g., Buçaco) and a subsidence regime continuity, both characteristic of intraplate environments. During Middle Ordovician, there was a deposition of  pelitic sequences, in progressively more distal offshore environments, in frequent anoxic regime. Cruziana ichnofacies is ubiquitous, characterised by low ichnodiversity, dominated by Tomaculum (Romano, 1991) structures and with sporadic occurrences of horizontal burrows (Planolites isp.). In North of C.I.Z., sedimentation is homogeneous until Upper Ordovician (i.e., Valongo Formation), while in Mação region, the southwesternmost area, records of sudden and frequent variations of pelagic to coastal detritical facies (e.g., Monte da Sombadeira Formation), whenever exist slight sea-level eustatic variations (Pereira et al., 1998). Along Caradoc, there are siliciclastic lithofacies progradation in a large scale regressive event, which is related with a glaciation that reached Iberian Terrane in the end of Ordovician. Glaciogenic deposits known (e.g., Casal Carvalhal Formation), formed by silto-pelitic sequences with dropstones (Young, 1988), suggest a Armorica relative proximity to Gondwana continent, in that time positioned at high latitudes in South hemisphere (Bonhommet & Perroud, 1986 in Ribeiro et al., 1990) and where occurs strong evidences for the presence of extensive glaciar cap at North African region.

During Silurian/Early Devonian, oceanization process reaches to its climax. Pelitic and euxinic sedimentation of deep pelagic nature attains almost total area of Iberian Terrane. Graptolitic faunas dominate orictocoenoses. Nevertheless, in some areas like East Trás-os-Montes, occur siliciclastic episodes with Cruziana, which lake conclusive palaeoichnological studies. This kind of facies, with unusual characteristics for Iberian Terrane during this period, seems to point for a palaeogeographic conditioning caused by activation of septentrional large shear zone (Dias, 1998). Armorican plate reaches in that period of time to the highest velocity of drift with relation to Gondwana, passing from polar palaeolatitudes to a position near Equator at Devonian.

Beginning of oblique collision between Laurussia and Gondwana blocks (in the middle of which armorican plate would be catched), ophiolitic sheet obduction and thrust mantles installation in the North-western Iberia are contemporary of diachronically deposition, from NE and E, of flysch sequences (Lower Devonian-Lower Carboniferous). Diachronism is opposed to vergence direction of Variscan first-phase structures, which possibly indicates subduction orientation. In these early stages, there are intense type-I magmatism (Ribeiro et al., 1990). Tectonic activity began with Pulo do Lobo e Beja-Acebuches unites obduction above meridional border of Iberian Terrane, with the same polarity as yet active subduction. This process was responsible for the O.M.Z. arrangement, between allochthonous South Portuguese Terrane and C.I.Z., both in contact, according with a tectonic flake geometry. Allochthonous accretionary oceanic elements at NW of Iberian Terrane can be genetically correlated with Pulo do Lobo ophiolite (Ribeiro et al., 1990).

During Lower-to-Middle Devonian, oceanic close in S originated accretion of continental terrane, with deformation and uplift of Variscan Chain. South Portuguese Terrane is filled by thick flysch series, typical of this particular orogenic stage at foreland basins, in which their most distal portions Nereites ichnofacies is characteristic and where adopted behaviour by benthonic palaeocommunities is very similar to those produced structures in O.M.Z. Lower Ordovician, at identical sedimentary regime (see Nery Delgado, 1910; Perdigão, 1961).

Oblique collision of armorican and avalonian continental plates and structures moultage around Cantabrian indentator, which acted like a promontory during Lower Carboniferous, are responsible for Iberian Terrane structuration, with generation of Ibero-Armorican Arc (Dias & Ribeiro, 1995). Reactivation of cadomian lithosferic criptosuture, generated during the end of Proterozoic deu to O.M.Z. exotic terrane accretion to Iberian Terrane and reutilized in Lower Cambrian intracontinental rifting process, is given by the late stages of Variscan orogeny like shear zone with antagonistic movements, Porto-Tomar and Tomar-Córdoba, inside of Blastomilonitic Belt (Pereira et al., 1998).

Iberian palaeozoic Wilson Cycle epilogue occurs in Upper Carboniferous, with tectonic inversion and total emersion of Iberian Massif. The last sediments to deposit are dated from tardi-Carboniferous and suit with lacustrine facies of intramontane basin features (Dúrico-Beirão Trough, Buçaco and Santa Susana Carboniferous Basins).

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