Chronostratigraphy of the Barremian-Early Albian of the Maestrat Basin (E Iberian Peninsula): Integrating Strontium-Isotope Stra
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1 Chronostratigraphy of the Barremian-Early Albian of the Maestrat 2 Basin (E Iberian Peninsula): integrating strontium-isotope 3 stratigraphy and ammonoid biostratigraphy 4 5 Telm Bover-Arnal a,*, Josep A. Moreno-Bedmar b, Gianluca Frijia c, Enric Pascual-Cebrian d, 6 Ramon Salas a 7 8 a Departament de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, 9 Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain 10 b Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 11 Coyoacán, 04510 México D.F., Mexico 12 c Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl Liebknecht-Str. 24- 13 25, Potsdam-Golm 14476, Germany 14 d GeoScience Limited, Falmouth Business Park, Bickland Water Road, Falmouth TR11 4SZ, 15 UK 16 17 * Corresponding author. 18 E-mail address: [email protected] (T. Bover-Arnal). 19 20 Abstract. A revised chronostratigraphy of the Barremian-Early Albian sedimentary record of 21 the Maestrat Basin (E Iberian Peninsula) is provided based on a comprehensive synthesis of 22 previous biostratigrahic data, a new ammonoid finding and numerical ages derived from 23 87Sr/86Sr values measured on shells of rudists, oysters and brachiopods. The succession, which 24 comprises eight lithostratigraphic formations, is arranged into six major transgressive- 25 regressive sequences and plotted against numerical ages, geomagnetic polarity chrons, 26 ammonoid zones and the stratigraphic distribution of age-diagnostic ammonoids, orbitolinid 27 foraminifera and rudist bivalves. The oldest lithostratigraphic unit sampled, the marine 28 Artoles Formation, is Early to Late Barremian. Above, the dinosaur-bearing deposits of the 29 Morella Formation and its coastal to shallow-marine equivalent, the Cervera del Maestrat 30 Formation, are of Late Barremian age and span at least part of the Imerites giraudi ammonoid 31 zone. 87Sr/86Sr ratios from oyster shells in the upper part of the overlying marine Xert 32 Formation are consistent with a latest Barremian-earliest Aptian age, while an ammonite 33 belonging to the Late Barremian Martelites sarasini Zone was collected within the lowermost 34 part of this latter formation. The Barremian-Aptian boundary is tentatively placed close above 35 the base of the succeeding transgressive marls of the Forcall Formation by analogy with 36 nearby Tethyan basins, where major transgressive records contain latest Barremian 37 ammonoids in their basal parts. The rest of the Forcall Formation and the platform carbonates 38 of the Villarroya de los Pinares Formation are of Early Aptian age. The transition from the 39 Barremian into the Aptian occurred in the course of a wide transgression, which was 40 accompanied by the proliferation of Palorbitolina lenticularis. This transgressive event 41 drowned Late Barremian carbonate platforms (Xert Formation) throughout the basin. 42 Extensive carbonate platforms (Villarroya de los Pinares Formation) recovered coevally with 43 a post-OAE1a late Early Aptian major regression of relative sea level. The last 44 lithostratigraphic unit analyzed, the marine Benassal Formation, spans the terminal Early 45 Aptian-Late Aptian interval. Based on ammonite distributions, the lower part of the overlying 46 coastal to continental coal-bearing Escucha Formation is Early Albian in age. This improved 47 chronostratigraphic knowledge allows a more precise correlation of the sedimentary record 48 studied with other coeval successions worldwide. 49 50 Key words. Strontium-isotope stratigraphy, Geochronology, Biostratigraphy, Ammonoids, 51 Early Cretaceous, Iberian Chain, Tethys 52 53 1. Introduction 54 55 In the Maestrat Basin (Fig. 1), the boundary between the Barremian and the Aptian 56 stages has been classically placed within the Artoles Formation (boundary A1 in Fig. 2; e.g., 57 Salas 1987, Salas et al. 1995, 2001, Aurell and Vennin 2001, Liesa et al. 2006, Embry et al. 58 2010) or at the limit between the Artoles and the Morella/Cervera del Maestrat formations 59 (boundary A2 in Fig. 2; e.g., Gàmez et al. 2003, Salas et al. 2005, Moreno-Bedmar et al. 60 2009, 2010, Bover-Arnal et al. 2009, 2010). The stratigraphic calibration of this boundary was 61 mainly based on charophyte, ostracod and/or benthic foraminifera biostratigraphic data (e.g., 62 Canérot et al. 1982, Salas et al. 1995) and geomagnetic polarity (e.g., Salas et al. 2005). 63 However, and besides the above-mentioned chronostratigraphic discrepancy between studies, 64 Canérot et al. (1982) and López Llorens (2007) already noted that the stratigraphic position of 65 the Barremian-Aptian boundary in the Maestrat Basin was not successfully established yet. 66 Thus, while depicting the Barremian/Aptian boundary at the limit between the Morella (or 67 Cervera del Maestrat) and Xert formations, Canerot et al. (1982, Fig. 6.1, p. 277), in their 68 descriptions of lithostratigraphic units, by contrast, give a terminal Barremian or earliest 69 Aptian age for the Morella Formation (p. 285) and a terminal Barremian to earliest Aptian 70 time span for its coastal to marine equivalent the Cervera del Maestrat Formation (p. 284). On 71 the other hand, López Llorens (2007) found an Argvethites sp. (genus determination modified 72 in Garcia et al. 2014), a Late Barremian ammonite belonging to the Imerites giraudi Zone, 73 within the marine-influenced deposits of the uppermost part of the Morella Formation, thus 74 ruling out the Early Aptian age classically assumed for this lithostratigraphic unit. 75 Later on, Moreno-Bedmar and Garcia (2011) put forward the hypothesis that the 76 Barremian-Aptian boundary was located at the lowermost part of the marls of the Forcall 77 Formation (boundary B in Fig. 2). This supposition was founded on the recognition of the 78 Deshayesites oglanlensis ammonoid Zone and the Subzone Deshayesites luppovi at the lower 79 part of the marls of the Forcall Formation. Moreno-Bedmar and Garcia (2011) also noted that 80 the Organyà Basin in north-eastern Spain and the Provençal Platform in south-eastern France 81 recorded a major transgressive event starting in the latest Barremian that would then be 82 analogous to the deposition of the hemipelagic marls of the Forcall Formation in the Maestrat 83 Basin (E Spain). Since then, Garcia et al. (2014) and Villanueva-Amadoz et al. (2014) have 84 attempted to test this hypothesis by reviewing the literature and providing new data on the 85 Barremian-Aptian ammonite biostratigraphy of the Maestrat Basin and by studying the 86 palynological content of the Morella Formation. Even though neither of these two works is 87 conclusive, they lend support to the Moreno-Bedmar and Garcia (2011) hypothesis. Garcia et 88 al. (2014) identified the species Deshayesites antiquus Bogdanova and Deshayesites sp. cf. 89 oglanlensis Bogdanova in the lower, non-basal part of the marls of the Forcall Formation. 90 These species are characteristic of the lower part of the Deshayesites oglanlensis Zone, which 91 is the first Aptian ammonoid Zone (Fig. 3; Reboulet et al. 2011, 2014). Villanueva-Amadoz et 92 al. (2014) record the dinoflagellate cysts Subtilisphaera terrula, Florentinia mantelli and 93 Oligosphaeridium abaculum, which indicate a Barremian age, from the base of the Morella 94 Formation. Villanueva-Amadoz et al. (2014) also recognize the pollen type Stellatopollis sp. 95 in the upper part of the Morella Formation and indicate that possibly this formation may be as 96 old as Late Barremian. 97 Using strontium-isotope stratigraphy and new ammonite biostratigraphic data the 98 present study conclusively locates the Barremian-Aptian boundary, while also calibrating the 99 age of the Barremian-Early Albian lithostratigraphic units of the Maestrat Basin. Strontium- 100 isotope stratigraphy is today a well-established, proven and widely adopted 101 chemostratigraphic method, which allows derivation of numerical ages from known past 102 changes in the 87Sr/86Sr ratio of seawater (e.g., Steuber 1999, 2001, 2003a, b, McArthur et al. 103 2001, 2012, McArthur and Howarth 2004, Steuber et al. 2005, Frijia and Parente 2008, Bodin 104 et al. 2009, Burla et al. 2009, Boix et al. 2011, Huck et al. 2011, Steuber and Schlüter 2012, 105 Wagreich et al. 2012, Williamson et al. 2012, Jaramillo-Vogel et al. 2013, Bonilla-Rodríguez 106 et al. 2014, Pascual-Cebrian 2014, Frijia et al. 2015). The resulting numerical ages derived 107 from 87Sr/86Sr values obtained from brachiopod, rudist and oyster shells collected in selected 108 stratigraphic intervals of the Barremian-Early Albian sedimentary succession of the Maestrat 109 Basin are plotted against lithostratigraphic units, major transgressive-regressive sequences of 110 relative sea level recorded in the basin, ammonoid zones, geomagnetic polarity chrons, and 111 ammonite, orbitolinid and rudist occurrences (Fig. 3). The results are complemented with 112 numerical ages derived from 87Sr/86Sr ratios measured in rudist shells from the western 113 Maestrat Basin by Pascual-Cebrian (2014). 114 Therefore, besides constraining the stratigraphic position of the Barremian-Aptian 115 boundary in the Maestrat Basin, the resulting chronostratigraphic framework (Fig. 3) allows 116 us: i) to establish the age of the dinosaur and other vertebrate records found in the Morella 117 and Xert formations (e.g., Yagüe et al. 2003, Canudo et al. 2008a, b, Jorquera-Grau et al. 118 2009, Pérez-García et al. 2009, 2014, Gasulla et al. 2011a, 2011b, 2012); ii) to date the major 119 Barremian-Early