An enigmatic kilometer-scale concentration of small mytilids (Late Miocene, Guadalquivir Basin, S Spain).

Upper Miocene heterozoan carbonates crop out extensively in a NE-SW-trending belt (42 km long and 1.5-8 km wide) along the so-called El Alcor topographic high, from Carmona to Dos Hermanas (Seville, S Spain). These carbonates formed at the southern active margin of the Guadalquivir Basin, the foreland basin of the Betic Cordillera. They change to marls basinward (NE) and to sands landward (SE and SW). Therefore, carbonate production was constrained to a limited area in an otherwise siliciclastic shelf. The carbonates (up to 40 m thick) overlie a gradually coarsening-upward succession of marls followed by silts and sandstones. The carbonate sequence can be divided into three subunits corresponding, frombottom to top, to lowstand, transgressive, and highstand system tract deposits. The lower subunit, exhibiting extensive trough cross-bedding, is interpreted as a shallow-water bar deposit. The intermediate subunit onlaps underlying sediments and was deposited in deeper, lowturbulence conditions. The upper subunit deposits accumulated in a well-oxygenated outer platform based on benthic foraminiferal assemblages. The presence of hummocky and swaley cross-stratification in these latter deposits suggests that theywere affected by storms. Pervasive fluid-escape structures are also observed throughout the carbonates. The three subunits consist of bioclastic packstones to rudstonesmade up of abundant fragments of smallmytilids. Isotopic data from serpulid polychaete Ditrupa tubes show 13C-depleted values (up to −16.1¿), whereas δ18O yields normal marine values. Additional isotopic data on shells of scallops, oysters, and small mussels, as well as bulk sediment, show diagenetic alterations. Based on actualistic examples of massive concentrations of mussels, the nearly monospecific composition of the El Alcor deposits, together with negative δ13C values of Ditrupa tubes, indicates that cold seeps presumably promoted carbonate formation. However, the absence of typical features of cold-seep deposits, such as authigenic carbonatesmediated by anaerobic bacterial activity and the typical chemosynthetic shelly organisms, makes the large carbonate body of El Alcor an unusual cold-seep deposit

An enigmatic kilometer-scale concentration of small mytilids (Late Miocene, Guadalquivir Basin, S Spain)

UpperMiocene heterozoan carbonates crop out extensively in a NE–SW-trending belt (42 km long and 1.5–8 km wide) along the so-called El Alcor topographic high, fromCarmona to Dos Hermanas (Seville, S Spain). These carbonates formed at the southern active margin of the Guadalquivir Basin, the foreland basin of the Betic Cordillera. They change to marls basinward (NE) and to sands landward (SE and SW). Therefore, carbonate production was constrained to a limited area in an otherwise siliciclastic shelf. The carbonates (up to 40 m thick) overlie a gradually coarsening-upward succession of marls followed by silts and sandstones. The carbonate sequence can be divided into three subunits corresponding, frombottom to top, to lowstand, transgressive, and highstand system tract deposits. The lower subunit, exhibiting extensive trough cross-bedding, is interpreted as a shallow-water bar deposit. The intermediate subunit onlaps underlying sediments and was deposited in deeper, lowturbulence conditions. The upper subunit deposits accumulated in a well-oxygenated outer platform based on benthic foraminiferal assemblages. The presence of hummocky and swaley cross-stratification in these latter deposits suggests that theywere affected by storms. Pervasive fluid-escape structures are also observed throughout the carbonates. The three subunits consist of bioclastic packstones to rudstonesmade up of abundant fragments of smallmytilids. Isotopic data from serpulid polychaete Ditrupa tubes show 13C-depleted values (up to −16.1‰), whereas δ18O yields normal marine values. Additional isotopic data on shells of scallops, oysters, and small mussels, as well as bulk sediment, show diagenetic alterations. Based on actualistic examples of massive concentrations of mussels, the nearly monospecific composition of the El Alcor deposits, together with negative δ13C values of Ditrupa tubes, indicates that cold seeps presumably promoted carbonate formation. However, the absence of typical features of cold-seep deposits, such as authigenic carbonatesmediated by anaerobic bacterial activity and the typical chemosynthetic shelly organisms, makes the large carbonate body of El Alcor an unusual cold-seep deposit

An enigmatic kilometer-scale concentration of small mytilids (Late Miocene, Guadalquivir Basin, S Spain).

Upper Miocene heterozoan carbonates crop out extensively in a NE-SW-trending belt (42 km long and 1.5-8 km wide) along the so-called El Alcor topographic high, from Carmona to Dos Hermanas (Seville, S Spain). These carbonates formed at the southern active margin of the Guadalquivir Basin, the foreland basin of the Betic Cordillera. They change to marls basinward (NE) and to sands landward (SE and SW). Therefore, carbonate production was constrained to a limited area in an otherwise siliciclastic shelf. The carbonates (up to 40 m thick) overlie a gradually coarsening-upward succession of marls followed by silts and sandstones. The carbonate sequence can be divided into three subunits corresponding, frombottom to top, to lowstand, transgressive, and highstand system tract deposits. The lower subunit, exhibiting extensive trough cross-bedding, is interpreted as a shallow-water bar deposit. The intermediate subunit onlaps underlying sediments and was deposited in deeper, lowturbulence conditions. The upper subunit deposits accumulated in a well-oxygenated outer platform based on benthic foraminiferal assemblages. The presence of hummocky and swaley cross-stratification in these latter deposits suggests that theywere affected by storms. Pervasive fluid-escape structures are also observed throughout the carbonates. The three subunits consist of bioclastic packstones to rudstonesmade up of abundant fragments of smallmytilids. Isotopic data from serpulid polychaete Ditrupa tubes show 13C-depleted values (up to −16.1¿), whereas δ18O yields normal marine values. Additional isotopic data on shells of scallops, oysters, and small mussels, as well as bulk sediment, show diagenetic alterations. Based on actualistic examples of massive concentrations of mussels, the nearly monospecific composition of the El Alcor deposits, together with negative δ13C values of Ditrupa tubes, indicates that cold seeps presumably promoted carbonate formation. However, the absence of typical features of cold-seep deposits, such as authigenic carbonatesmediated by anaerobic bacterial activity and the typical chemosynthetic shelly organisms, makes the large carbonate body of El Alcor an unusual cold-seep deposit

Formation and structures of horizontal submarine fluid conduit and venting systems associated with marine seeps

Methane-rich water moves through conduits beneath the seafloor whose surfaces are formed through precipitation reactions. To understand how such submarine fluid conduit and venting systems form and grow, we develop a detailed mathematical model for this reaction-advection system and we quantify the evolution of an ensemble of similar filaments. We show that this growth can be described by a superposition of advection and dispersion. We analyze analog laboratory experiments of chemical-garden type to study the growth of a single filament undergoing a precipitation reaction with the surrounding environment. We apply these findings to geological fluid conduit and venting systems, showing that their irregular trajectories can lead to very effective spreading within the surrounding seabed, thus enhancing contact and exchanges of chemicals between the conduit and external fluids. We discuss how this methane venting leads to the formation of marine authigenic carbonate rocks, and for confirmation, we analyze two field samples from the Gulf of Cadiz for composition and mineralogy of the precipitates. We note the implications of this work for hydrate melting and methane escape from the seabed.L. A. M. Rocha gratefully acknowledges funding from the Fundação para a Ciência e Tecnologia (FCT), Portugal (grant SFRH/BD/130401/2017). C. Pimentel acknowledges funding from Juan de la Cierva-Formación (grant FJC2018-035820-I) from the Spanish Ministry of Science. J. H. E. Cartwright and C. Ignacio Sainz-Díaz acknowledge the financial support of the Spanish MINCINN projects FIS2016-77692-C2-2-P and PCIN-2017-098. I. Sánchez-Almazo thanks the crew of the Cornide de Saavedra vessel, as well as the members of the Tasyo project, funded by the Spanish Marine Science and Technology Program, for allowing her to participate in the 2000–2001 Anastasya cruises. The authors acknowledge the contribution of the COST Action chemobrionics, CA17120

Supporting Information for Formation and structures of horizontal submarine fluid conduit and venting systems associated with marine seeps

Supporting Information for Formation and structures of horizontal submarine fluid conduit and venting systems associated with marine seeps. Geochemistry, Geophysics, Geosystems.The supporting information includes additional information about the analytical techniques used in this work, as well as additional figures and a movie, complementary to the information and figures in the main article. Text S1. Analytical Techniques; Figures S1 to S6; Captions for movie S1 (X-ray tomography images complementary to image S6).Peer reviewe

Extension in the Western Mediterranean

43 pages, 31 figuresThe Miocene is an essential period in the configuration of the present-day relief of the Betic Cordillera and the South Iberian continental margin, which determined the structure and evolution of the Neogene sedimentary basins (Fig. 3.1). The crustal thinning processes that occurred during the early and middle Miocene, after the main metamorphic events, generated major low-angle normal faults that separate the main metamorphic complexes. Although a wide variety of tectonic models have been proposed for this setting, most of them are related to delamination or to subduction with associated roll-back. During the late Miocene, the relatively flat and low relief of the continental crust facilitated the accumulation of sedimentary deposits, which are interlayered with volcanic rocks in the eastern Betic Cordillera and Alborán Sea. The continuous Eurasian-African convergence finally produced regional uplift since the late Miocene and the development of large late regional E-W to NE-SW folds, which determine the main relief