@article{ChristMaerzKutscheraetal.2017, author = {Christ, Nicolas and Maerz, Sven and Kutschera, Edgar and Kwiecien, Ola and Mutti, Maria}, title = {Palaeoenvironmental and diagenetic reconstruction of a closed-lacustrine carbonate system}, series = {Sedimentology : the journal of the International Association of Sedimentologists}, volume = {65}, journal = {Sedimentology : the journal of the International Association of Sedimentologists}, number = {1}, publisher = {Wiley}, address = {Hoboken}, issn = {0037-0746}, doi = {10.1111/sed.12401}, pages = {235 -- 262}, year = {2017}, abstract = {Chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events, on their repetition and prediction through time. Diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction. Here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the Miocene Ries Crater Lake (Germany). This area is critical because it represents the transition from shoreline to proximal domains of a hydrologically closed system, affected by recurrent emergent events, representing the boundaries of successive sedimentary cycles. Chemostratigraphy targets shifts related to subaerial exposure and/or climatic fluctuations. Methods combine facies changes with C-13-O-18 chemostratigraphy from matrix carbonates across five closely spaced, temporally equivalent stratigraphic sections. Isotope composition of ostracod shells, gastropods and cements is provided for comparison. Cathodoluminescence and back-scatter electron microscopy were performed to discriminate primary (syn-)depositional, from secondary diagenetic features. Meteoric diagenesis is expressed by substantial early dissolution and dark blue luminescent sparry cements carrying negative C-13 and O-18. Sedimentary cycles are not correlated by isotope chemostratigraphy. Both matrix C-13 and O-18 range from ca -75 to +40 parts per thousand and show clear positive covariance (R=097) whose nature differs from that of previous basin-oriented studies on the lake: negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis, while positive values probably represent primary saline lake water chemistry. Noisy geochemical curves relate to heterogeneities in (primary) porosity, resulting in selective carbonate diagenesis. This study exemplifies that ancient lacustrine carbonates, despite extensive meteoric weathering, are able to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation to those primary conditions. Also, it emphasizes the limitation of chemostratigraphy in fossil carbonates, and specifically in settings that are sensitive for the preservation of primary environmental signals, such as lake margins prone to meteoric diagenesis.}, language = {en} } @article{FoellmiHofmannChiaradiaetal.2015, author = {F{\"o}llmi, Karl B. and Hofmann, Helene and Chiaradia, Massimo and de Kaenel, Eric and Frijia, Gianluca and Parente, Mariano}, title = {Miocene phosphate-rich sediments in Salento (southern Italy)}, series = {Sedimentary geology : international journal of applied and regional sedimentology}, volume = {327}, journal = {Sedimentary geology : international journal of applied and regional sedimentology}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0037-0738}, doi = {10.1016/j.sedgeo.2015.07.009}, pages = {55 -- 71}, year = {2015}, abstract = {The upper Middle to lower Upper Miocene (Serravallian to Tortonian) sedimentary succession in Salento (southern Italy) includes glauconite- and phosphate-rich deposits, which are associated with pelagic micrite. In Baia del Ciolo and Marittima (southern Salento), the succession is composed of shallow-water platform carbonates of Late Oligocene age (Chattian; Porto Badisco Formation), which are overlain by a 20- to 30-cm-thick level of glauconite-rich micrite with abundant reworked particles and fossils of the underlying Porto Badisco Formation. This interval is in turn covered by an up to 15 cm thick phosphatic crust ("Livello ad Aturia"), which itself is overlain either by a hemipelagic chalk-like carbonate of Middle to Late Miocene age ("Pietra Leccese"; Marittima) or directly by a micrite of Late Miocene age (Messinian; Novaglie Formation; Baia del Ciolo), which shallows upwards into a shallow-water platform carbonate. A large hiatus is present in this succession, which likely includes the Lower and lower Middle Miocene. In the region of Lecce, two discrete levels enriched in glauconite and phosphate-each associated with a major discontinuity-occur within the Pietra Leccese. The strontium-isotope ages derived on phosphate nodules and phosphatized and non-phosphatized fossils and calcareous nannofossil ages indicate a time interval of phosphogenesis between 13.5 and 7.5 Ma, with two clusters at 12 and 10.5 Ma. The glauconite and phosphate-rich sediments resulted from a current-dominated regime, which was characterized by low overall sedimentation rates, erosion and sediment reworking, and authigenesis. This regime was likely related to a generally westward-directed bottom current, which was forced to upwell once it arrived at the western border of the eastern Mediterranean basin. The timing of the principal phosphogenic phases can only partly be correlated to those of other occurrences in this part of the Mediterranean (Malta, Gozo, southern Sicily, Matese, Latium-Abruzzi platform). The partial diachrony in phosphogenesis may express the effect of lateral switching in and/or focusing of upwelling zones. (C) 2015 Elsevier B.V. All rights reserved.}, language = {en} } @phdthesis{Wichura2011, author = {Wichura, Henry}, title = {Topographic evolution of the East African Plateau : a combined study on lava-flow modeling and paleo-topography}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-52363}, school = {Universit{\"a}t Potsdam}, year = {2011}, abstract = {The East African Plateau provides a spectacular example of geodynamic plateau uplift, active continental rifting, and associated climatic forcing. It is an integral part of the East African Rift System and has an average elevation of approximately 1,000 m. Its location coincides with a negative Bouguer gravity anomaly with a semi-circular shape, closely related to a mantle plume, which influences the Cenozoic crustal development since its impingement in Eocene-Oligocene time. The uplift of the East African Plateau, preceding volcanism, and rifting formed an important orographic barrier and tectonically controlled environment, which is profoundly influenced by climate driven processes. Its location within the equatorial realm supports recently proposed hypotheses, that topographic changes in this region must be considered as the dominant forcing factor influencing atmospheric circulation patterns and rainfall distribution. The uplift of this region has therefore often been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While the far-reaching influence of the plateau uplift is widely accepted, the timing and the magnitude of the uplift are ambiguous and are still subject to ongoing discussion. This dilemma stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. In order to quantify the amount of plateau uplift and to find evidence for the existence of significant relief along the East African Plateau prior to rifting, I analyzed and modeled one of the longest terrestrial lava flows; the 300-km-long Yatta phonolite flow in Kenya. This lava flow is 13.5 Ma old and originated in the region that now corresponds to the eastern rift shoulders. The phonolitic flow utilized an old riverbed that once drained the eastern flank of the plateau. Due to differential erosion this lava flow now forms a positive relief above the parallel-flowing Athi River, which is mimicking the course of the paleo-river. My approach is a lava-flow modeling, based on an improved composition and temperature dependent method to parameterize the flow of an arbitrary lava in a rectangular-shaped channel. The essential growth pattern is described by a one-dimensional model, in which Newtonian rheological flow advance is governed by the development of viscosity and/or velocity in the internal parts of the lava-flow front. Comparing assessments of different magma compositions reveal that length-dominated, channelized lava flows are characterized by high effusion rates, rapid emplacement under approximately isothermal conditions, and laminar flow. By integrating the Yatta lava flow dimensions and the covered paleo-topography (slope angle) into the model, I was able to determine the pre-rift topography of the East African Plateau. The modeling results yield a pre-rift slope of at least 0.2°, suggesting that the lava flow must have originated at a minimum elevation of 1,400 m. Hence, high topography in the region of the present-day Kenya Rift must have existed by at least 13.5 Ma. This inferred mid-Miocene uplift coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa. Accordingly, the combination of my results regarding the Yatta lava flow emplacement history, its location, and its morphologic character, validates it as a suitable "paleo-tiltmeter" and has thus to be considered as an important topographic and volcanic feature for the topographic evolution in East Africa.}, language = {en} } @phdthesis{MarcanoRomero2008, author = {Marcano Romero, Gabriela Helena}, title = {Investigations on sedimentology and early diagenesis in shallow-water warm-temperate to tropical miocene carbonates : a case study from Northern Sardinia, Italy}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-29207}, school = {Universit{\"a}t Potsdam}, year = {2008}, abstract = {This study investigated the warm-temperate to tropical shallow-water Miocene carbonates of the Perfugas basin (Anglona area), northern Sardinia, Italy (Central Mediterranean). The aim of this study was to identify and document the existence and significance of early diagenesis in this carbonate system, especially the diagenetic history, which reflects the diagenetic potential in terms of skeletal mineralogy. The motivation behind the present study was to investigate the role that early cementation has over facies stabilization linked to differences in biotic associations in shallow-water settings. Principal to this was to unravel the amount, kind and distribution of early cements in this type of carbonates, in order to complement previous studies, and hence acquire a more global perspective on non-tropical carbonate settings. The shallow-buried Sedini Limestone Unit was investigated for variations on early diagenetic features, as well as for the type of biotic association, and oxygen and carbon stable isotope stratigraphy. Results showed, that particularly at the Perfugas basin (< 15 km2), which evolves in time from a ramp into a steep-flanked platform, shallow-water facies are characterized by a "transitional" type of biotic association. The biotic assemblages change gradually over time from a heterozoan-rich into a photozoan-rich depositional system. This transition implies a change in the depositional environmental control factors such as temperature. It is considered that sedimentation took place under warm-temperate waters, which shifted to more warmer or tropical waters through time. Moreover, it was noticed that along with these changes, marine early syn-depositional cements (high-Mg calcite), with particular fabrics (e.g. fibrous), gradually contributed to the early lithification of rocks, favoring a steepening of the platform relief. The major controls for the shift of the depositional geometry was triggered by the change of the type of biotic associations (carbonate factory), related with the shift towards warmer conditions, and the development of early marine cementation. The identification of the amount and distribution of different cement phases, porosities and early diagenetic features, within facies and stratigraphy, showed that diagenesis is differential along depth, and within the depositional setting. High-Mg calcite cements (micrite, fibrous and syntaxial inclusion-rich) are early syn-depositional, facies-related (shallow-water), predominant at the platform phase, and marine in origin. Low-Mg calcite cements (bladed, syntaxial inclusionpoor and blocky) are early to late post-depositional, non-facies related (shallow- to deep-water) and shallow-burial marine in origin. However, a particular difference exists when looking at the amount and distribution of low-Mg calcite bladed cements. They become richer in shallow-water facies at the platform phase, suggesting that the enrichment of bladed cementation is linked to the appearance of metastable grains (e.g. aragonite). In both depositional profiles, the development of secondary porosity is the product of fabric-selective dissolution of grains (aragonite, high-Mg calcite) and/or cements (syntaxial inclusion-rich). However, stratigraphy and stable isotopes (oxygen and carbon), indicate that the molds found at shallower facies located beneath, and close to stratigraphic boundaries, have been produced by the infiltration of meteoric-derived water, which caused recrystallization without calcite cementation. Away from these stratigraphic locations, shallow- and deep-water facies show molds, and recrystallization, as well as low-Mg calcite cementation, interpreted as occurring during burial of these sediments by marine waters. The main cement source is suggested to be aragonite. Our results indicate that the Sedini Limestone Unit was transformed in three different diagenetic environments (marine, meteoric and shallow-burial marine); however, the degree of transformation in each diagenetic environment differs in the heterozoan-dominated ramp from the photozoan-dominated platform. It is suggested that the sediments from the ramp follow a diagenetic pathway similar to their heterozoan counterparts (i.e. lack of marine cementation, and loss of primary porosity by compaction), and the sediments from the platform follow a diagenetic pathway similar to their photozoan counterparts (i.e. marine cementation occluding primary porosity). However, in this carbonate setting, cements are Mg-calcite, no meteoric cementation was produced, and secondary porosity at shallow-water facies of the platform phase is mostly open and preserved. Despite the temporal and transitional change in biotic associations, ramp and platform facies (shallow- to deep-water facies) showed an oxygen isotope record overprinted by diagenesis. Oxygen primary marine signatures were not found. It is believed that burial diagenesis (recrystallization and low-Mg calcite cementation) was the main reason. This was unexpected at the ramp, since heterozoan-rich carbonates can hold isotope values close to primary marine signals due to their low-Mg calcite original composition. Ramp and platform facies (shallow- to deep-water facies) showed a carbon isotope record that was less affected by diagenesis. However, only at deep-water facies, did the carbon record show positive values comparable with carbon primary marine signals. The positive carbon values were noticed with major frequency at the platform deep-water facies. Moreover, these values usually showed a covariant trend with the oxygen isotope record; even that the latter did not hold positive values. The main conclusion of this work is that carbonates, deposited under warm-temperate to tropical conditions, have a unique facies, diagenesis and chemostratigraphic expression, which is different from their cool-water heterozoan or warm-water photozoan counterparts, reflecting the "transitional" nature of biotic association.}, language = {en} }