High-resolution correlation of coastal cliff sections in the Lagos- Portimao Formation (Lower - Middle Miocene, central Algarve, Portugal)

This paper describes a high-resolution stratigraphic correlation scheme for the early to middle Miocene Lagos-Portimão Formation of central Algarve, southern Portugal. The Lagos Portimão-Formation of central Algarve is a 60 m thick package of horizontally bedded siliciclastics and carbonates. The bryozoan and mollusc dominated biofacies is typical of a shallow marine, warm-temperate climatic environment. We define four stratigraphic marker beds based on biofacies, lithology, and gamma-ray signatures. Marker bed 1 is a reddish shell bed composed predominantly of bivalve shells in various stages of fragmentation. Marker bed 2 is a fossiliferous sandstone / sandy rudstone characterized by bryozoan masses. Marker bed 3 is also a fossiliferous sandstone with abundant larger foraminifers and foliate bryozoans. Marker bed 4 is composed of three distinct layers; two fossiliferous sandstones with an intercalated shell bed. The upper sandstone unit displays thickets of the bryozoan Celleporaria palmate associated with the coral Culizia parasitica. This stratigraphic framework allows to correlate isolated outcrops within the stratigraphic context of the Lagos-Portimão Formation and to establish high resolution chronostratigraphic Sr-isotopic dating

Deep-water Stromatolites and Frutexites Maslov from the Early and Middle Jurassic of S-Germany and Austria

Despite extensive discussions during the last 20 years stromatolites are still used by many geologists as unequivocal indicators of very shallow-water conditions. We investigated four stratigraphic units from the Lower and Middle Jurassic of southern Germany (Posidonien-Schiefer, Amaltheen-Ton) and of the Northern Calcareous Alps (Adneter Kalk, Klauskalk), which were formerly interpreted as shallow marine sediments by some authors due to the occurrence of stromatolites. Our interpretations of the macro-, micro- and ultrafacies of these sediments are not compatible with shallow-water settings. We therefore propose a deep-marine, aphotic origin of these stromatolites. Former interpretations of the Posidonien-Schiefer as a shallow-water deposit are mainly based on the occurrence of stromatolites. We favour the model of a temporarily stagnant, deep, aphotic basin for these planktonrich sediments. Particles resembling ooids, but lying within mudstones cannot be taken as evidence for shallow agitated water. They either formed within the mud or are allochthonous. The deep-water setting of the red limestone of the Alpine Early and Middle Jurassic is indicated by a lack of platform-typical components like coated grains and phototrophic benthos and by shells of plankton and nekton forming a major part of the sediment. Stromatolites occur on the steep slope of a drowned Rhaetian reef with an estimated relief of 50–100 m and immediately below and within radiolarian limestones, deposited below the aragonite compensation depth (ACD). The aphotic stromatolites show some morphological differences to their shallow water counterparts. In all of our sections they occurred during intervals of reduced sedimentation. They form only thin horizons and probably grew very slowly. Mineralizations by Fe−Mn oxides and phosphate are very common. The presence of a microbial film is evident from binding of sedimentary particles, but the nature of the microbes is not known. Growth habits within the very distinct environments of red limestone and black shales show some common features, but also clear differences. The microproblematicum Frutexites Maslov is a very common component in deep-water stromatolites, but may also itself form small crusts or dendrolites. It occurs in two different forms. Opaque, slender forms with indistinct outlines probably grew within the weakly lithified sediment. Thicker, transparent forms with well defined outlines are found in cavities and probably also grew on the seafloor. Well preserved specimens display an internal fabric of radially arranged fibres of Fe−Mn oxides and calcite. It is suggested that calcite or aragonite were one original mineralogy ofFrutexites, which was later replaced by Fe−Mn oxides or phosphate. It is not certain whether Frutexites is an organic, biomineralized structure or an inorganic mineralization, but the variable mineralogy and growth forms in different environments point to an organic origin. But even if organic, the occurrence in cryptic habitats and negative phototactic growth-directions make it clear that Frutexites was not phototrophi

Shallow burial diagenesis of skeletal carbonates: selective loss of aragonite shell material (Miocene to Recent, Queensland Plateau and Queensland Trough, NE Australia) - implications for shallow cool-water carbonates

In burial environments, carbonate sediments undergo mineralogical stabilization and increasing lithification with depth. As yet, however, little knowledge exists with respect to the corresponding effects on fossil preservation and taphonomic modification of the original sediment composition. Countings of particles (>63 μm) in Miocene to Recent periplatform sediments (ODP Leg 133, NE Australia) exhibit a clear trend of reduction of skeletal aragonite downcore. Low- and high-Mg-calcite grains occur in a continuous order of magnitude over the studied interval (<600 m sub-bottom depth). Original microtextures are retained in high-Mg-calcite biota, although converted to low-Mg-calcite. Thus, the conversion to low-Mg-calcite appears to occur without introducing a significant quantitative bias. Aragonite skeletons (pelagic gastropods), however, exhibit a successive exposure of deeper crystal layers and a chalky preservation with burial depth, which we interpret to result from dissolution. Hints for the originally more numerous existence of aragonite biota exist in soft sediments and chalks by the presence of internal moulds, shells replaced by microspar, and mouldic porosity in early cemented hardgrounds. In deep sections barren of aragonite, the number of casts and replaced shells remains unchanged and is insignificant as compared to aragonitic biota probably originally present within the sediment (<2% in ooze/chalk vs. 30–50% of grains in modern periplatform sediments). Therefore, palaeontological information must be significantly biased through selective removal of aragonite. Rates of preservation and destruction depend on external factors during sediment accretion (sedimentation rates, clay content, total organic carbon content) and rates of fluid flow within the sediment. These observations are relevant with respect to the diagenetic potential and patterns of fossil preservation in little cemented calcitic cool-water carbonates, because they may originally have contained more aragonite biota as important constituents of an ecosystem than is commonly suspected, and calcite/aragonite ratios in ancient carbonate sediments may not necessarily reflect original input signals (climate or sea level)

Vermetid reefs and their use as palaeobathymetric markers: new insights from the Late Miocene of the Mediterranean (Southern Italy, Crete)

Vermetids are a poorly known family of reef-building gastropods characterized by an extremely irregular growthof the shell. Vermetid reefs are known from the Badenian (Middle Miocene) to present-day and their mainimportance is related to their use as sea-level indicators, strictly associated to the intertidal or immediate subtidalzone. During their evolution, two different vermetid genera have been recognized as reef-builders: Petaloconchuswas the major component of vermetid reefs until the Holocene when, for still unknown reasons, it was almostcompletely replaced by Dendropoma. In the Mediterranean Basin, Petaloconchus reefs have been frequently foundin close association to Late Miocene coral bioconstructions. New finding of Petaloconchus reefs within the slopefacies of the Salento Messinian reef complex (southern Italy), together with new data from late Tortonian\u2013earlyMessinian reefs of Crete (Greece) provide further information on the palaeoecology of these peculiarbioconstructions. Stratigraphical and sedimentological contexts of Salento and Crete vermetid reefs have beendescribed, together with an account of their size, shape and inner structure. Biotic (vermetids, serpulids,bryozoans and coralline algae) and abiotic components (earlymarine cements and intra-reef sediment) have beenidentified by microfacies analysis, and their percentage-abundance has been reported. These data allow thedescription of vermetid reefs from two different depositional settings: along the shelf edge and, for the first time,within the coral reef slope. Shelf-edge and slope Petaloconchus reefs show the same inner structure andcomposition, but they differ by their shape, by the density of the framework, and by the relative proportion of theassociated reef-building organisms (serpulids, bryozoans and coralline algae). Additional information on theirpalaeobathymetric setting have been acquired from the presence of other reef-building biota (Porites corals andHalimeda green algae)which occur in close proximity to slope vermetid reefs, together with observations on someanatomical feature of Petaloconchus shells. Results clearly indicate for Petaloconchus reefs a palaeobathymetricrange spanning from the upper subtidal zone down to 30\u201350 m of depth. The present study confirms vermetidreefs as important tools for the reconstructions of ancient sea-level, but great attention has to be paid to theidentification of the main reef-building vermetid. Actually, while a great number of present-day Dendropomareefs can be associated to the mean sea-level, Late Miocene vermetid reefs from Salento and Crete indicate forPetaloconchus bioconstructions a wider palaeobathymetric range