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Tubular carbonate concretions of up to 1 m in length and perpendicular to bedding, occur abundantly in the Upper Pliensbachian (upper Amaltheus margaritatus Zone, Gibbosus Subzone) in outcrops (Fontaneilles section) in the vicinity of Rivière-sûr-Tarn, southern France. Stable isotope analyses of these concretions show negative &delta¹³C values that decrease from the rim to the center from −18.8‰ to −25.7‰ (V-PDB), but normal marine δ¹⁸O values (−1.8‰). Carbon isotope analyses of Late Pliensbachian bulk carbonate (matrix) samples from the Fontaneilles section show clearly decreasing C-isotope values across the A. margaritatus Zone, from +1‰ to −3‰ (V-PDB). Isotope analyses of coeval belemnite rostra do not document such a negative C-isotope trend with values remaining stable around +2‰ (V-PDB). Computer tomographic (CT) scanning of the tubular concretions show multiple canals that are lined or filled entirely with pyrite. Previously, the formation of these concretions with one, two, or more central tubes, has been ascribed to the activity of an enigmatic organism, possibly with annelid or arthropod affinities, known as Tisoa siphonalis. Our results suggest tisoan structures are abiogenic. Based on our geochemical analyses and sedimentological observations we suggest that these concretions formed as a combination of the anaerobic oxidation of methane (AOM) and sulfate reduction within the sediment. Fluids rich in methane and/or hydrocarbons likely altered local bulk rock carbon isotope records, but did not affect the global carbon cycle. Interestingly, Tisoa siphonalis has been described from many locations in the Grands Causses Basin in southern France, and from northern France and Luxemburg, always occurring at the same stratigraphic level. Upper Pliensbachian authigenic carbonates thus possibly cover an area of many thousand square kilometers. Greatly reduced sedimentation rates are needed to explain the stabilization of the sulfate-methane transition zone in the sedimentary column in order for the tubular concretions to form. Late Pliensbachian cooling, reducing run-off, and/or the influx of colder water and more vigorous circulation could be responsible for a halt in sedimentation. At the same time (thermogenic) methane may have destabilized during a major phase of Late Pliensbachian sea level fall. As such Tisoa siphonalis is more than a geological curiosity, and its further study could prove pivotal in understanding Early Jurassic paleoenvironmental change

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Geological mapping, definition of facies distributions and reconstruction of platform-interior growth geometries of the Messinian Cariatiz carbonate platform (Sorbas basin, South Spain), were performed to evaluate the controlling factors in platform growth and to test a 3-D computer simulation program. For the simulation with the program REPRO, five platform-related facies were modelled: (1) the reef crest facies by the numerical solution of a Fisher equation; (2) the lagoonal facies by a function of water depth-dependent carbonate production; (3) the proximal and middle slope facies (breccia and block facies, calcarenite facies) by a subroutine simulating gravity-driven particle export from the reef crest; (4) a distal slope; and (5) a basinal facies by a pelagic rain function. Development of a fan delta conglomeratic system is simulated by using a siliciclastic point source and gravity-driven particle redistribution. A best fit between the observed platform growth geometries and modelling results is achieved by assuming that high-frequency sea-level changes superimposed onto a longer term sea-level fall controlled platform growth. For the modelling, a relative sea-level curve was reconstructed, which is based on a deep-sea benthic foraminiferal stable oxygen isotope record at ODP Site 926 with a 45 m eustatic sea-level fall, and a tectonic uplift component of 20 m. The consistency of 3-D simulation results is corroborated by the coral growth rates provided by the Fisher-equation subroutine. These rates of 2-8 mm year )1 compare well to the coral growth rates in Recent fringing reefs. We propose that during the early stage of platform evolution the high-frequency fluctuations were obliquity-modulated precessional cycles, whereas precessional cycles control later stages of platform growth. REPRO provides a separate visualization of the different facies bodies as a function of time and space, showing the intrinsic pattern of facies distribution in the platform. This is the result of a combination of platform growth and syndepositional subaerial erosion. For example, only the youngest stages of reef framework facies in the development of the Cariatiz carbonate platform are preserved.

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