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Jurassic Guyots on the Southern Iberian Continental Margin: a Model of Isolated Carbonate Platforms on Volcanic Submarine Edifices J

Jurassic Guyots on the Southern Iberian Continental Margin: a Model of Isolated Carbonate Platforms on Volcanic Submarine Edifices J

Jurassic on the Southern Iberian : 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 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 - 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 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- 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 . 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- , 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 , 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. 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 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

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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 and departure from the reference isochron displayed by both 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 , 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 , 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-

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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- karstification features at the top (Fig. form over volcanic edifices correspond 3). The karstified surface shows cavities to , more precisely to guyots. (sinkholes) that locally are some tens of They were formed by interbed volcanic metres long and several metres in depth. and sedimentary rocks, suggesting they The ages of the bottom and the top of were built up by layer on layer of sub- this shallowing-upward megasequence, marine volcanic ejecta, and are flat- based in ammonites and regional cor- topped mainly due to their form of relations, would be 170 Myr and 157 construction rather than erosion. Fig. 2 Plot of the two sets of samples in the Myr, respectively. Radiometric ages re- Guyots have been described for diagram 87Sr/86Sr vs. 87Rb/86Sr and refer- cently obtained by Portugal et al. (1995) other periods and tectonic contexts, ence isochron (170 Myr). In all the diagrams, in some JSVR range between 169 4 but this is the first time they have been crosses are AL samples (ultrapotassic + rocks) and black dots are V samples. and 155 + 3 Myr, in accordance with reported in an Alpine continental margin the afore mentioned Rb/Sr data. in an extensional phase. Gischler (1996) recognized sedimentary environments that compare favour ably to those on Discussion and conclusions recent guyots from the early Carboni- rence of HCS and symmetrical wave From our detailed palaeogeographical ferous from the Rhenish Trough of the ripples indicate that their deposition reconstruction of the Subbetic basin we Hercynian orogen in Central Europe. occurred above the storm wave base. conclude that the submarine volcanic Certain classic works (Bonatti and The existence of thickening-upward cy- edifices of the Median Subbetic and the Tazieff, 1970; Tazieff, 1972) discussed cles (Fig. 3) involving these storm layers very shallow sediments (shoaling car- some examples of guyots, presently can be interpreted as shallowing-up- bonate) at their tops pass laterally into emerged, in the Afar Rift (Ethiopia). ward cycles. pelagic sediments of the same age. One of those described by these authors There are no shallow carbonates of this is the Asmara guyot, a truncated cone FA3 Represented in the higher inter- age unrelated to volcanic highs in the 300±400 m high, & 2.3 km in diameter bedding and, mainly, in the post volca- Median Subbetic. The very shallow at its base and about 1 km at its summit, nic carbonate rocks, this is composed of dimensions that are similar to those of peloidal and oolitic limestones with the Subbetic guyots. facies and microfacies from the Camar- After the first description by Hess ena Fm., with a late Bathonian age at (1946), flat-topped seamounts, or the top (Molina, 1987). They have very guyots, mainly from the Pacific , characterist ic microfacies, dominating were analysed in several papers. Since the oolitic grainstones, with oncoids, then it has been shown that many of the aggregate grains, foraminifera (Nauti- large guyots in the Western Pacific loculina, Protopeneroplis, Trocholina, region represent former islands and Pfenderina, Valvulina), calcareous al- of Cretaceous age. Shiba (1993) gae (mainly Dasycladaceae), crinoids and DomõÂ nguez et al. (1995), for in- and solitary corals. The ooids are stance, analysed a guyot (Daiichi-Ka- clearly of bahamian type with well- shima) near the Japan trench, with developed tangential and radial struc- Aptian reefal complexes that developed tures: they are unbroken and do not on a Barremian submarine . show sign of redeposition. This oolitic Van Waasbergen and Winterer (1993) grainstone is a typical deposit of a very studied 21 guyots in the Western Paci- shallow environ- fic, finding, over the volcanic relief, ment. The peloidal limestones (wack- thick shallow-water limestones ranging estone) appear related to the oolitic from Barremian to Albian, and karstic limestones forming shoaling cycles, surfaces linked to subaerial exposure. with karstic surfaces indicating emer- Winterer et al. (1993) presented an gence episodes. overview of the geology and geophysics A clear shallowing-upward megase- of the Cretaceous guyots in the North- quence can be recognized in sediments west Pacific, in which Aptian±Albian located on the volcanic edifices (Fig. 3). shallow-water limestones with rudists This megasequence is composed in the Fig. 3 Sedimentary and stratigraphic fea- have thicknesses as much as 800 m. lower part by FA1, deposited below the tures of the shallowing-upward megase- Karstic topography developed over storm wave base, in the median part by quence deposited on the volcanic edifices these limestones and was later covered FA2 deposited over the storm wave forming guyots (Southern Iberian Con- by Turonian pelagic sediments. In all base, and in the upper part by FA3. tinental Palaeomargin). cases the depth of the marine bottom

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Paper 129 Disc Jurassic guyots±isolated carbonate platforms . J.A. Vera et al. Terra Nova, Vol 9, No. 4, 163±166 ...... around the guyots is 3±4 km, more or nental Margin. The pelagic deposition Molina, J. M., 1987. Ana lisis de Facies del less the same as the height of the volca- depth would have been a few hundred Mesozoico en el Subbe tico Externo nic edifices. These values are not the metres in this extensional phase of the (provincia de Co rdoba y Sur de Jae n). same as in the present example, due to margin during the middle Jurassic. The Unpubl. doctoral dissertation, University of Granada, 518pp. the very different geodynamic context, origin of volcanic edifices with a similar Molina, J. M., Ruiz-Ortiz, P. A. and Vera, J. but the criterion of considering that the height to this depth allowed the pri- A., 1997. Calcareous tempestites in depth of the adjacent pelagic deposits is mary constructional origin of guyots pelagic facies (Jurassic, Betic Cordilleras, related to the height of the volcanic (with platform carbonate sedimenta- Southern Spain). . Geol., 109, edifice is applicable. tion) over the volcanic edifices, as long 95±109. The estimate of the depositional as pelagic sedimentation continued in Portugal, M., Morata, D., Puga, E., depth of the pelagic sediments around adjacent areas. Demant, A. and Aguirre, L., 1995. these guyots is more problematic be- Evolucio n geoquõ mica y temporal del cause we have no definitive criteria. magmatismo ba sico mesozoico en las Acknowledgements Zonas Externas de las Cordilleras Be ticas. However, it can be reasonably assumed Est. Geol., 51, 109±118. The authors thank Dr J. Sandoval for the that the depth would be similar to the Schreyer, W., Massonne, H. J. and Chopin, classification of the ammonites and E. height of the volcanic edifice plus the C., 1987. Continental crust subducted to Adams and C. Laurin for improving the palaeo depth estimate of the carbonate depths near 100 km: implications for English. This study forms part of the results deposition on the guyot. Assuming that and fluid genesis in collision obtained under Research Project PB-93± the marine bottom is flat, that the vol- zones. In: Magmatic Processes: 1150, financed by the DGICYT. canic edifice developed over pelagic Physicochemical principles (B. O. Mysen, sediments, and that in the adjacent ed.). Spec. Publ. Geochem. Soc., 1, 155±163. sectors of each edifice sedimentation References Shiba, M., 1993. Middle Cretaceous continued to a similar depth, the values Carbonate Bank on the Daiichi-Kashima Seamount at the Junction of the Japan of the would be the sum of Bates, R. L. and Jackson, J. A., 1987. Glossary of Geology (3rd edn). American and Izu-Bonin Trenches. In: Cretaceous the depth of the shallow deposits and Geological Institute, Alexandria, Carbonate Platforms (J. A. Simo, R. W. the height of the volcanic edifice. If the Virginia, 788pp. Scott and J. P. Masse, eds). Mem. Am. subsidence in the volcanic edifice were Bonatti, E. and Tazieff, H., 1970. Exposed Ass. Petrol. Geol., 56, 465±471. more than in adjacent sectors as a con- guyot from the Afar Rift, Ethiopia. Tazieff, H., 1972. About deep- sequence of loading (plus compacta- Science, 168, 1087±1089. volcanism. Geol. Rundsch. 61, 470±480. tion), the possible error would be in Dominguez, S., Lallemand, S. E., Van Waasbergen, R. J. and Winterer, E. L., excess. Taking into account the real Malavieille, J. and Pouclet, A., 1995. 1993. Summit Geomorphology of values of the maximum thickness of Nouvelle interpre tation structurale du Western Pacific Guyots. In: The Mesozoic the volcanic rocks (hundreds of metres), mont sous-marine Daiichi Kashima Pacific: Geology, Tectonics, and (Fosse du Japon) et essai de Volcanism. Geophys. Monogr., 77, 355± we can estimate that the pelagic sedi- reconstruction ge odynamique. C. R. 366. ments surrounding the guyots were at Acad. Sci. Paris, 320(IIa), 403±409. Vera, J. A., 1988. Evolucio n de los sistemas least 200±500 m in depth. Garcõ a-Herna ndez, M., Lo pez-Garrido, A. de depo sito en el margen ibe rico de las The geodynamic context in which C., Martõ n-Algarra, A., Molina, J. M., Cordilleras Be ticas. Rev. Soc. Geol. these guyots appear is a passive con- Ruiz-Ortiz, P. A. and Vera, J. A., 1989. EspanÄa 1, 373±391. tinental margin in its initial rifting Las discontinuidades mayores del Vera, J. A., Ruiz-Ortiz, P. A., Garcõ a- phase. Garcõ a-Herna ndez et al. (1989) Jura sico de las Zonas Externas de las Herna ndez, M. and Molina, J. M., 1988. considered that the prerifting phase Cordilleras Be ticas: ana lisis e Paleokarst and related sediments in the continued until 190 Ma, and that at this interpretacio n de los ciclos sedimentarios. Jurassic of Subbetic Zone, Southern  Spain. In: Paleokarst (N.P. James and J. time the generalized break of the car- Cuad. Geol. Iberica, 13, 35±52. Garcõ a-Herna ndez, M., Lo pez-Garrido, A. Choquette, eds), pp. 364±384. Springer, bonate platform occurred, as well as the C., Rivas, P., Sanz de Galdeano, C. and New York. start of pelagic sedimentation in the Vera, J. A., 1980. Mesozoic Winterer, E. L., Natland, J. H., Van Subbetic. The synrifting phase lasted paleogeographic evolution of the External Waasbergen, R. J., eds., 1993. Cretaceous between 190 and 157 Myr (the interval Zones of the Betic Cordillera. Geol. Mijnb. Guyots in the Northwest Pacific: an in which the materials studied here 59, 155±168. overview of The Geology and appeared) and the postrifting phase Gischler, E., 1996. Late Devonian-Early Geophysics. In: The Mesozoic Pacific: (or the beginning of the expansion) Carboniferous deep-water coral Geology, Tectonics, Volcanism. Geophys. started at 157 Ma. assemblages and sedimentation on a Monogr., 77, 307±334. From geochemical data and palaeo- Devonian seamount: Iberg Reef, Harz Mts., Germany. Palaeogeogr., geographical evolution reconstruc- Palaeoclimatol. Palaeoecol., 123, 297±322. tions, we can conclude finally that the Hess, H. H., 1946. Drowned ancient islands volcanism occurred in the initial rifting of the Pacific Basin. Am. J. Sci., 244, 722± Received 28 February 1997; revised version stages of the Southern Iberian Conti- 791. accepted 19 September 1997

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