Shallowing-upward cycles in pelagic troughs (Upper Jurassic, Subbetic, Southern Spain)
Abstract Abundant calcareous tempestites with chert and hummocky cross-stratification (HCS) occur in Upper Jurassic successions (Milanos Formation) of the Subbetic (Betic Cordillera, Southern Spain). These calcareous tempestite beds are calcisiltites and very fine calcarenites with peloids and bioclasts, and have thicknesses between 25 and 75 cm. Beds show symmetrical wave ripple lamination near the top, and are usually capped with wave ripples. According to palaeogeographic reconstructions, these strata were deposited in a subsiding trough in the pelagic domain of the Southern Iberian Continental Margin, which was the northwestern-most border of the Tethys. Calcisiltite and calcarenite beds deposited in this pelagic trough form small-scale to large-scale shallowing-upward cycles. Shallowing-upward cycles with calcareous tempestites in the lower part and oolitic limestone beds in the upper part, have also been recognized in some sectors of the Subbetic pelagic basin. The large-scale cycle occurs in the upper part of the Jarropa Radiolarite Formation, which directly underlies upper Oxfordian, Kimmeridgian and Tithonian strata of the Milanos Formation. Two types of large-scale shallowing-upward cycles have been recognized, according to the presence or absence of thick units of oolitic limestone with large-scale cross-stratification. The water depth during deposition ranged from below, but near, storm wave base at the base of the large-scale cycles to very shallow carbonate platform at the top.
Contrast comparison of differential diagenetic pathways of Lower Tithonian carbonate materials from the Betic Cordillera (S. Spain): Evidence for physico-chemical paleo-seawater properties
Abstract The reconstruction of paleoenvironmental conditions is based on the study of ancient rock materials. For this purpose, skeletal materials are often assumed to have higher preservation potential, sometimes favored over the use of matrix micrite. However, factors such as ecology, depositional setting, mineralogy and diagenetic processes may obscure original geochemical signals, and care must be taken when reading such an intricate record. This work provides a thorough comparison between geochemical signatures of different carbonate materials, interpreted in the light of their paleoenvironmental significance. Burial and diagenetic processes are taken into account, contextualized by ammonite and belemnite ecology. Two sections deposited in epioceanic Tethyan areas and corresponding to the same Lower Tithonian ammonite biozone are under scope: a condensed pelagic and cephalopod-rich limestone (the Alamedilla sector) intercalated in thick mafic rocks piles, and a typical Ammonitico Rosso facies (the Cardador section). In contrast to depositional and taphonomic histories typical for a Tethyan top-swell site (i.e., rather calcareous Ammonitico Rosso), events of carbonate-mud deposition, rapid burial and early lithification, synsedimentary sliding and over imposition of firmground horizons characterize pelagic cephalopod-rich limestones. Differential taphonomy of carbonate skeletals agrees with differential preservation, favoring inner-cast preservation in Ammonitico Rosso and shell preservation (neomorphic calcite) in pelagic cephalopod-rich limestones. Stable carbon and oxygen isotope data, together with trace element data (Sr, Mg, Fe, Mn) are presented for a wide range of carbonate materials: matrix micrite, neomorphic ammonite shells, belemnite rostra and carbonate cements. Optical inspection (including cathodoluminescence imaging) and geochemical data suggest a fair preservation of the above mentioned materials, except for diagenetic marine burial carbonate cements. Carbon isotope values fall within the typical Tethyan Late Jurassic range (ca. 2‰), except data from belemnite rostra which are depleted around − 1‰, which is interpreted as resulting from metabolic fractionation. Oxygen isotope values are more variable, but the clustering of different carbonate materials within a narrow range of values between − 1 and 1‰ is consistent with conservative diagenesis. Warmer burial fluids explain lower values in later diagenetic carbonate cements. No evidence of pervasive diagenetic imprint is revealed by elemental composition. The increased abundance of Mg, Fe, and especially Mn (up to 1500 ppm), is recognized as hydrothermal contamination. Proximity to hydrothermal sources (the Mid-Subbetic Volcanic Ridge) accords with higher Mn concentration at the Alamedilla site. Interestingly, the timing of the mineralogical stabilization of each carbonate material reveals a potential record of a post-sedimentary geochemical event. The obtained information demonstrates the complexity of ancient carbonate records, and encourages future research focused on the preservation potential of geochemical proxies throughout time. The studied case hopes to be a representative example to approach the interpretation of potential past and present rare analogs.
The distribution of 54-Mn in the enamel and dentine of the rat incisor.
Abstract The distribution of 54 Mn was studied in the lower incisors of rats. Autoradiograms show 54 Mn to be present in the enamel and dentine 5, 10 and 15 days after its administration. In the dentine, the radioisotope is first localized in a zone near to and encircling the pulp and later becomes more diffuse. Pre-existing as well as the newly formed enamel incorporates the isotope. In both enamel and dentine 54 Mn appears to be principally associated with the inorganic portion.