Sedimentology (2015) 62, 1149–1183 doi: 10.1111/sed.12182 Climatic and tectonic controls on carbonate deposition in syn-rift siliciclastic fluvial systems: A case of microbialites and associated facies in the Late Jurassic ~ CONCHA ARENAS*, LAURA PINUELA† and JOSE CARLOS GARCIA-RAMOS† *Dpto. de Ciencias de la Tierra, Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain (E-mail: [email protected]) †Museo del Jurasico de Asturias (MUJA), 33328 Colunga, Spain Associate Editor – Daniel Ariztegui ABSTRACT This work provides new insights to assess the factors controlling carbonate deposition in the siliciclastic fluvial systems of rift basins. Sedimentological and stable-isotope data of microbialites and associated carbonate facies, along with regional geological information, are shown to reveal the influence of climate and tectonics on the occurrence and attributes of carbonate deposits in these settings. The Vega Formation – a 150 m thick Lower Kim- meridgian siliciclastic fluvial sequence in Asturias Province (northern Spain) – constitutes a candidate for this approach. This unit includes varied facies (stromatolites; rudstones, packstones and wackestones containing on- coids, intraclasts, charophytes and shell bioclasts; marlstones and polygenic calcareous conglomerates) that formed in a low-gradient fluvial–lacustrine system consisting of shallow, low-sinuosity oncoid-bearing channels and pools within marshy areas, with sporadic coarse alluvial deposition. The sedimentological attributes indicate common erosion by channel overflow and rapid lateral changes of subenvironments caused by water-discharge variations. The carbonate fluvial–lacustrine system developed near uplifted marine Jurassic rocks. The occurrence of the system was conditioned by nor- mal faults (active during the deposition of the unit) that favoured: (i) springs of HCO3–Ca-rich water from a Rhaetian–Sinemurian carbonate rock aquifer; and (ii) carbonate deposition in areas partially isolated from the adjacent siliciclastic fluvial system. The microbialite d13C and d18O values support deposition in a hydrologically open system, fed by ambient-temperature meteoric water, with riparian vegetation. Three types of lamination in the stromatolites and oncoids reflect distinct morphological types of cyanobacte- rial communities. The textural pattern of lamination parallels d13C and d18O changes, suggesting short-term cycles of precipitation and temperature. A moderately to strongly contrasted seasonal and/or pluriannual precipita- tion regime is inferred from the cyclic d13C pattern of the lamination and from the discontinuous and asymmetrical growth of oncoids. Thus, the iso- topic and sedimentological attributes of the carbonate deposits were linked to short-term climate changes associated with semi-arid conditions, consis- tent with the studied climatic zone. Keywords Allocyclic factors, carbonates, fluvial–lacustrine facies model, microbialites, rift basin. © 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists 1149 1150 C. Arenas et al. INTRODUCTION 2004; Andrews & Brasier, 2005; Osacar et al., 2013). Laminated microbialites (i.e. stromatolites and The Late Jurassic record of Asturias (northern oncolites) are common structures of fluvial and Spain, Fig. 1) mostly consists of a fluvial silici- lacustrine environments in different types of clastic unit overlain by shelf lagoonal and deltaic basins throughout the geological record. Many sandstone, limestone and marl deposits, all of ancient oncoidal and associated carbonate which formed in the extensional tectonic regime deposits are reported to occur mainly in fluvial that affected the region from the beginning of the and fluvial–lacustrine systems (Leinfelder & Late Jurassic to the Early Cretaceous (Garcıa- Hartkopf-Froder,€ 1990; Zamarreno~ et al., 1997; Ramos et al., 2011). The middle and distal flu- Hernandez Gomez, 2000; Melendez & Gomez- vial deposits (mudstones and sandstones, Vega Fernandez, 2000; Arenas et al., 2007; Astibia Formation) of the siliciclastic unit (Kimmerid- et al., 2012). During the Late Jurassic–Early Cre- gian) contain several limestone beds consisting taceous these deposits were particularly abun- of oncoids, intraclasts and bioclasts, along with dant in relation to syn-rift fluvial and lacustrine calcareous conglomerate beds, that are interbed- systems (Leinfelder, 1985; Perry, 1994; Hernan- ded at different stratigraphic positions (Garcıa- dez Gomez, 2000; Melendez & Gomez-Fern an- Ramos et al., 2010a,b). These carbonate deposits dez, 2000; Shapiro et al., 2009; Bosence, 2012); are the main focus of this work. Their distinct their occurrence and evolution have been facies, their occurrence within a siliciclastic explained in terms of both tectonic and climatic sequence and their close association with normal factors. Tectonics primarily control the origin faults in the underlying marine Jurassic and evolution of such continental basins, the sequence provide an excellent scenario for dis- extent and lithology of catchment areas and the cussion of the combined influence of tectonics preservation potential of deposits through subsi- and climate on the fluvial environment. In addi- dence (Miall, 1996). Most fluvial and fluvial– tion, well-preserved laminated structures in the lacustrine carbonate systems are principally oncoids provide an opportunity to examine the linked to water supplies from carbonate-rock effects of environmental factors on microbial catchments and aquifers, which are directly deposits through textural and stable-isotope related to climate and bedrock structure analyses. ~ (Ordonez & Garcıa del Cura, 1983; Evans, 1999; Based on the sedimentological and stable-iso- Dunagan & Turner, 2004; Arenas-Abad et al., tope analyses of the carbonate deposits in the 2010). Vega Formation, complemented with regional Microbial lamination records a complex inter- stratigraphic data, the purpose of this work was: action of physical, chemical and biological (i) to characterize the different carbonate facies parameters (Monty, 1976; Golubic, 1991; Merz- and depositional environments and propose a Preiß & Riding, 1999; Riding, 2000; Noffke & sedimentary facies model that explains their dis- Awramik, 2013) that greatly depend on climate tribution and evolution through space and time; and hydrology. The analysis of lamination in (ii) to infer the climatic conditions and their ancient microbialites has been the focus of many imprint in the microbial laminated deposits; and ~ sedimentological works (Zamarreno et al., 1997; (iii) to discern the influence of climate and tec- Seong-Joo et al., 2000; Suarez-Gonzalez et al., tonics on the occurrence and evolution of the 2014). However, the interpretation of the envi- carbonate deposits. The results provide new ronmental and temporal significance of lamina- insights that may help to interpret other exam- tion from thickness and textural features is not ples in which carbonate deposits develop in always straightforward and unequivocal expla- association with siliciclastic fluvial systems in nations are uncommon (Seong-Joo et al., 2000; rift basins. Storrie-Lombardi & Awramik, 2006; Petryshyn et al., 2012). The stable-isotope composition d13 d18 ( Cand O) reveals that lamination in micro- GEOLOGICAL SETTING – SEDIMENTOLOGY bialites can record short-term climatic and AND PALAEOGEOGRAPHY OF THE hydrological changes on different time scales JURASSIC IN THE STUDIED AREA (i.e. seasonal, interannual and decadal); there- fore, laminated microbialites are considered The most spectacular and best-preserved Juras- high-resolution records of palaeoenvironmental sic outcrops in Asturias (northern Spain) extend conditions (Chafetz et al., 1991; Woo et al., almost continuously along a narrow section of © 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 1149–1183 Carbonate deposition in a syn-rift fluvial system 1151 A B Fig. 1. Location (A) and geological map of the area (B), with the main studied sections: 1 – La Griega–Lastres; 2 – Vega; 3 – Abeu; 4 – La Fumarea; 5 – Fuente del Gato. coast (ca 60 km long) between the Cabo Torres, originated on a low and irregular coast, rich in in Gijon, and Arra beach, in Ribadesella (Garcıa- carbonate muds and evaporites (coastal sabkha). Ramos et al., 2011; Fig. 1). The Jurassic outcrops This succession also includes some stratiform are part of the so-called Gijon-Villaviciosa Basin calcareous breccias of metre-scale thicknesses. (Ramırez del Pozo, 1969), whose western end is Their origin is related to dissolution of gypsum bounded by the Verina~ fault, located 2 km west layers intercalated among highly fractured lime- of Gijon (Lepvrier & Martınez Garcıa, 1990). The stones (collapse breccias; Valenzuela et al., basin extends eastward to the Ribadesella fault, 1986; Garcıa-Ramos et al., 2006). coinciding with Arra beach (Alonso et al., During the Late Sinemurian, a gradual relative 2009). The Jurassic to Cenozoic rocks in eastern rise in the sea-level caused a great part of the Asturias (Fig. 1B) overlie diverse Variscan units region to be submerged, at times to depths (Precambrian to Carboniferous of the Cantabrian greater than 100 m. The Rodiles Formation rep- Zone) and Permian–Triassic units. Further west resents the resulting marine deposits. This for- and south, outside of Fig. 1B, mostly Variscan mation has two distinct parts (Fig. 2A): the pre-Devonian rocks crop out (Western Asturian– lower part is made up of nodular limestones Leonese