Jurassic Guyots on the Southern Iberian Continental Margin: a Model of Isolated Carbonate Platforms on Volcanic Submarine Edifices J
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Jurassic guyots on the Southern Iberian Continental Margin: a model of isolated carbonate platforms on volcanic submarine edifices J. A. Vera*1 J. M. Molina2 P. Montero3 and F. Bea3 1Departamento de EstratigrafõÂa y PalaeontologõÂa, Universidad de Granada, 18071 Granada, Spain; 2Departamento de GeologõÂa, Universidad de JaeÂn, 23071 JaeÂn, Spain; 3Departamento de MineralogõÂa y PetrologõÂa, Universidad de Granada, 18071 Granada, Spain ABSTRACT During the middle Jurassic on the Southern Iberian Continental megasequence (Bajocian±Bathonian) with hummocky cross- Margin (at the westernmost end of the northern Tethys) isolated stratification strata below oolitic shoals, shows that very carbonate platforms developed over volcanic edifices, forming shallow isolated carbonate platforms developed above the guyots. The volcanic edifices were composed of K-rich pillow- volcanic edifices, with similar facies to those recognized in lavas and pyroclastic rocks with a radiometric age & 170 Myr. other guyots, but with a different age and geodynamic context. Such phenomena have not been described until now in this continental margin nor in other passive continental margins of Terra Nova, 9, 163±166, 1997 Alpine domains. The presence of a shallowing-upward part of the tilted fault blocks (Vera et same biozone, to groups of superim- Introduction al., 1988). posed flows some hundreds of metres The Betic Cordillera (Southern Spain) Jurassic submarine volcanic rocks thick with interbedded sedimentary rocks is the westernmost European Alpine (JSVR) have been studied by various from different stages. In the time inter- chain. The External Zones of this Cor- authors (Portugal et al., 1995, and pre- val and areas studied, the total thick- dillera (Prebetic and Subbetic) are com- vious works cited therein) who, in their ness of the volcanic rocks is over 300 m posed mainly of sedimentary rocks, more recent papers, consider them (Fig. 1d). The area chosen for this study deposited from the Triassic to the early transitional-alkaline, originating in an (Alicu n de Ortega) has excellent JSVR Miocene in the Southern Iberian Con- extensional process of crustal thinning. outcrops, where surface alteration of tinental Margin. The Subbetic corre- The main aim of this work is to the volcanic rocks is low. We have sponds to the part of this margin in describe shallow marine materials over analys ed two sets of samples (Al and which pelagic facies from the early the Jurassic volcanic edifices of this V in Fig. 1d), attempting to establish Pliensbachian dominated, when a rift- margin. Our data indicate that the edi- radiometric dating with Rb/Sr in each, ing phase began, with the initial devel- fices were guyots. We use the term guyot the first in the lower lava flows (samples opment of a passive continental margin in the sense of the Glossary of Geology V-1 to V-12) and the second in the that later evolved into an active margin (Bates and Jackson, 1987) as `a type of higher ones (samples AL-1 to Al-12). and terminated in a continental colli- seamount that has a platform top' and sion with the Internal Zones, originat- is therefore not necessarily located on Geochemical results and ing the Cordillera during the Burdiga- oceanic crust. The main characteristic interpretation lian (Garcõ a-Herna ndez et al., 1980; of the guyots, their flat-topped features Vera, 1988). with summits hundreds of metres below The composition of the AL samples are In the palinspastic reconstruction of water-level, has been interpreted as the ultrapotassic while the second set of the Southern Iberian Margin during result of erosional truncation in volcanic samples (V-1 to V-12) have shoshonitic the middle and late Jurassic, four pa- islands, although some authors have affinities (Vera et al., unpubl. data). To laeogeographical domains can be dif- suggested a primary constructional ori- discuss a petrogenetic model for these ferentiated in the Subbetic, that were gin (Bonatti and Tazieff, 1970). rocks is beyond the scope of this paper, elongated parallel to the margin itself but their chemistry clearly points to (Fig. 1). Jurassic volcanism, mainly a specific geodynamic environment. Volcanism in the Subbetic represented by pillow-lavas interca- Shreyer et al. (1987) proposed that lated between pelagic sediments, ap- similar rocks formed as a consequence Geological and palaeogeographical peared only in the Median Subbetic of KMg-rich fluids released during the settings (Fig. 1). In the Subbetic pelagic sedi- transformation of the assemblage, K- ments of the middle and late Jurassic The JSVR in the Median Subbetic feldspar-biotite, in the presence of ex- have been described sedimentary fea- crops out WSW±ENE in a strip 5±10 cess water to phengite + quartz, which tures against the attribution of great km wide and 250 km long. Basalts with occurs when crustal rocks of suitable depth in the deposition (Molina et al., pillow-lava flows dominate and are in- composition are subducted to a depth 1997) admitting the occurrence of local terbedded with pyroclastic rocks. The of 50±70 km. The reaction of these and temporal emersions of the higher thickness of the volcanic rocks and the fluids with the surrounding mantle first time interval represented by them produces metasomatism and then, when change locally from flows a few metres the geodynamic regime allows, partial *Correspondence: E-mail: [email protected] thick in sedimentary rocks from the melting and segregation of K-rich #1997 Blackwell Science Ltd 163 Ahed Fig marker Bhed Table marker Paper 129 Disc Ched Ref end Dhed Ref start Ref marker Jurassic guyots±isolated carbonate platforms . J.A. Vera et al. Terra Nova, Vol 9, No. 4, 163±166 ............................................................................................................................................................................................................................................. melts. In our case, this idea also finds support from strontium isotope data. On the 87Sr/86Sr vs. 87Rb/86Sr dia- gram, rocks from the V-1 to V-12 set fit the 170 Myr old reference isochron reasonably well, but not the AL set (Fig. 2), which has higher 87Sr/86Sr values. The different dispersion and departure from the reference isochron displayed by both rock series cannot be attributed to post consolidation phe- a nomena, since both rock types are equally affected by them, but alterna- tively they most likely reflect primary differences. The fact that ultrapotassic lavas also have a high proportion of radiogenic strontium could indicate a higher contribution of crustal compo- nent in the form of KMg fluids. Based b on their age relationships, chemistry and isotope geology, we therefore sug- gest that the JSVR originated during the beginning of an extensional regime c in a zone with a thick continental crust, probably overthickenned during the Hercynian convergence. Shallowing-upward megasequence related to volcanic rocks Three types of facies associations (FA) can be differentiated in the sedimentary rocks interbedded between the volcanic rocks and those located on the volcanic edifices. FA1 This is very frequent in the lower sedimentary intercalations. It com- prises pelagic marl/limestone rhythmite with ammonites (Toxamblyites and Skirroceras genera) belonging to the higher part of the early Bajocian. Mud- stone and bioclastic (mainly `filaments') and peloidal wackestone-packstone dominate. In some beds slumps, intra- formational breccias and soft deforma- tion structures occur. They are depos- ited clearly below the storm wave base, with episodes of tectonic instability. FA2 This appears mainly in the med- d ian sedimentary intercalations and is composed of marl/limestone rhythmite Fig. 1 (a) Palaeogeographical reconstruction of the Western Mediterranean area during with some strata of calcisiltites with the Middle Jurassic (after Garcõ a-Herna ndez et al., 1989, modified), with indication of hummocky cross-stratification (HCS) the location of the palinspastic reconstruction cross-section in (b) and (c). Key: 1, and symmetrical wave ripples. Parallel Prebetic; 2±5, Subbetic (2, Intermediate Domain; 3, External Subbetic; 4, Median laminations, marked by alternative pe- Subbetic; 5, Internal Subbetic). 6, Area with minimum thickness of continental crust, in loidal and bioclastic laminae, are also which oceanic crust appeared during the Late Jurassic. (b)±(c), Palinspastic reconstruc- frequent. Nodular and bedded chert tions of the Southern Iberian Continental Margin (b), Middle Toarcian; (c), Upper frequently appear in these calcisiltite Bajocian-Bathonian); (d), Stratigraphic section in the guyot of the Alicu n de Ortega beds, just as described Molina et al. area. (1997) in analogous facies. The occur- 164 #1997 Blackwell Science Ltd Paper 129 Disc Terra Nova, Vol 9, No. 4, 163±166 Jurassic guyots±isolated carbonate platforms . J.A. Vera et al. ............................................................................................................................................................................................................................................. There are several shallowing-upward sediments were deposited in isolated cycles 2±5 m thick, with peloidal wack- platforms located over the volcanic edi- estone in the lower part changing gra- fices, several kilometres wide, that were dually in the upper part to oolitic grain- later covered by younger pelagic sedi- stone, showing shoaling cycles with ments. These isolated carbonate plat-