Facies analysis of the Lower-Middle Toarcian in the External Subbetic (provinces of Murcia and Granada, Southern Spain): palaeoenvironmental conditions

Toarcian sedimentary rocks are well recorded in the Subbetic (Betic External Zones, Southern Spain) as part of the Zegrí Formation (upper Pliensbachian-Aalenian). These rocks were deposited in the South Iberian Palaeomargin in the Western Tethys. We study the lower-middle Toarcian facies in two sections in the External Subbetic and their palaeoenvironmental interpretation. The lower-middle Toarcian have more than 160 m in thickness, the maximum for this time in all the Betic External Zones. Five types of lithofacies are differentiated: 1) Grey-yellow marl-marly limestone rhythmite and limestones (lower part of the Polymorphum Zone); 2) dark marls (upper part of the Polymorphum Zone and lower part of the Serpentinum Zone); 3) thin bedded grey-yellow limestones, locally with chert and abundant slumps (upper part of the Serpentinum Zone); 4) grey marls and marly limestones (Bifrons and Gradata Zones); and 5) yellow-brown laminated calcisiltites and calcarenites, intercalated in facies 3 and 4. Facies 1 to 4 are interpreted as hemipelagites, deposited by the slow accumulation, on a quiet sea floor of biogenic and very fine terrigenous particles. Facies 2 was deposited in rather depleted oxygen conditions with slightly dysoxic bottom waters but discarding completely anoxic conditions. The Toarcian Oceanic Anoxic Event (T-OAE) is recorded in this facies 2 by some increase of total organic carbon (maximum of 1.05 wt.%) and redox sensitive elements, the decrease of CaCO₃, and the negative excursion of δ¹³C observed at the base of Serpentinum Zone. Facies 5 are mainly peloidal grainstone with bioclasts (brachiopods, bivalves, and echinoderms), ooids and allochthonous shallow water foraminifera, and packstone-wackestone of bioclasts (mainly radiolarians) and peloids. This facies 5 with parallel lamination and locally with normal grading, low angle, wavy, and hummocky cross stratification is interpreted as tempestites related with tropical cyclones, and/or internalites. The influence of adjacent emerged lands and carbonate platforms, differential subsidence by local tectonics, sediment winnowing by currents, sedimentation rates, bioturbation, and diagenesis, may have had more importance in the distribution of the facies types than depth. The evolution during the lower-middle Toarcian was mainly controlled by tectonics after the Pliensbachian break-up of the Lower Jurassic platform, together with a relative sea-level change. Also the beginning of basaltic submarine volcanism to the South in some Median Subbetic areas had influence. The diversified physiography related to synsedimentary tectonic activity and oceanic circulation patterns, determined different intensities of winnowing and oxygenation on the sea-floor. The T-OAE is recorded in the base of Serpentinum Zone. The general re-oxygenation after the T-OAE could be favoured by changes in oceanic currents and by the tempestite/internalite inputs during the upper part of Serpentinum and Bifrons zones.