PD Dr. Markus Wilmsen
e-mail: m.wilmsen@mail.uni-wuerzburg.de
phone: ++49/+931/31-2510
fax: ++49/+931/31-2504
List of publications.
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PD Dr. Markus Wilmsen e-mail: m.wilmsen@mail.uni-wuerzburg.de phone: ++49/+931/31-2510 fax: ++49/+931/31-2504 List of publications. |
During 1994 to 1996, I was carrying out a regional study on the Mid - Cretaceous succession of the North Cantabrian Basin in northern Spain (WILMSEN 1997; see the abstracts of this monograph in
German,
English, and
Spanish
versions).
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Biosedimentology of Mesozoic marine carbonate depositional systems - Case studies from Mesozoic successions in Morocco, Iran, Spain and northern Germany -
Marine carbonate depositional systems can be regarded as ‘living' systems because most of their sediments are derived from, and processed by, biotic activity. As such living systems, they are strongly dependent on physico-chemical (environmental) parameters, evolutionary change, and geodynamic processes. Biosedimentology aims towards integrated analyses of these ‘open' carbonate systems which are characterized by diverse biotic communities as well as complex interactions and feedback mechanisms of energy and particle fluxes. It deals with all processes and products of biogenic sediment production, accumulation and dispersal, and with interactions between organisms and their sedimentary ecosystems. In order to decipher some of the dominant controlling factors on carbonate sedimentation during the Jurassic and Cretaceous periods, four biosedimentological case studies are presented. The investigations of case study A are focussed on an Early Jurassic rift basin developing in tropical palaeolatitudes (central High Atlas of Rich, Morocco ). A large-scale carbonate platform system of Middle-Late Jurassic age in east-central Iran is the focus of case study B , developing at the tropical-subtropical northern margin of the Middle Eastern Tethys. A small Cretaceous (Aptian-Cenomanian) intra-shelf basin (Cantabria, northern Spain) is investigated in case study C , situated on the subtropical, passive north Iberian continental margin. The case studies A-C all dealt with ‘benthic' carbonate systems, i.e., systems the sediments of which were predominantly produced by benthic biotic activity. Case study D , in contrast, is focussed on a Cenomanian (Cretaceous) pelagic carbonate system of northern Germany . In this system, deposition of fine planktic carbonate particles took place in a temperate, epicontinental shelf-sea. The multi-facetted analyses of this study combined an integrated approach of classical microfacies analysis combined with high-resolution bio-, event, and cyclostratigraphy, biofacies analysis, sequence stratigraphy, and stable isotope geochemistry. The investigations confirmed the importance of geodynamic processes for marine carbonate systems. Due to the breakup of Pangaea, numerous rift basins developed during the Triassic-Jurassic, eventually evolving into new ocean basins. A concomitant 1 st -order sea-level rise during the Jurassic to early Late Cretaceous (Turonian) supported the development of extensive epi- and peri-continental shelf seas. An elevated CO2 content in the atmosphere due to high rates of sea-floor spreading and the emplacement of numerous large igneous provinces (LIPs) caused a generally warm and equable climate. The sea-water chemistry (Mg/Ca and Sr/Ca ratios) was likewise affected by changing rates of sea-floor spreading and transgressions. These geodynamic processes facilitated the deposition and preservation of Jurassic-Cretaceous marine carbonate successions, and strongly influenced the evolution of Mesozoic carbonate-producing communities. However, the geodynamic factors also caused repeated evolutionary “back-steps” due to severe mass extinction events, especially affecting the marine communities. The end-Triassic, Pliensbachian-Toarcian, and end-Cenomanian mass extinctions were presumably all caused by environmental perturbations (climatic warming, sea water eutrophication, anoxia) in response to peaking sea-floor spreading rates and LIP emplacement. The investigation also stressed the importance of regional tectonic events for the development of carbonate systems. In case study B, the tilting of a large fault block during the late Middle Jurassic provided the nucleus for the development of a large-scale, rimmed barrier platform at the (sub-) tropical northern Tethyan margin. Case study A provided the possibility of studying the development of a carbonate system in a canon of regional tectonics and evolutionary change during the Early Jurassic (i.e., the interplay between marine opening of a rift basin and the increasing complexity of marine communities after the end-Triassic mass extinction). Physico-chemical parameters of water masses such as water temperature and water energy, salinity, oxygenation, and nutrient levels strongly governed the development of the investigated carbonate systems on a regional scale. Biofacies, sedimentological and geochemical proxies were used to infer the properties of those (palaeo-) water masses and the state of the contemporaneous ocean-atmosphere system. Proxy-based palaeoceanographic reconstructions have a great potential to explain the distribution of biofacies and their temporal and spatial changes. For example, trade wind-driven water mass circulation patterns are held responsible for the development of special biofacies pattern in case studies A and B. Downwelling of oceanic, presumably well oxygenated and oligotrophic surface waters starting at the transition from the Sinemurian to the Pliensbachian terminated the Late Sinemurian deep water heterozoan communities of the central High Atlas basin related to oxygen minimum edge effects. These ‘blue water masses' also supported the development of photozoan equilibrium communities of the Saharan Platform fringing the southern basin margin throughout the Pliensbachian. In the Late Jurassic of east-central Iran , the existence of biotic communities of surprisingly low species richness and the striking absence of coral reefs in the otherwise favourable setting of an ocean-facing rimmed platform were possibly related to coastal upwelling of nutrient-rich deep waters. This upwelling system was probably triggered by an offshore blowing north-eastern trade wind system. In several cases, nutrient levels had a strong impact on the type of benthic carbonates and carbonate productivity. Under elevated nutrient levels, ‘temperate' carbonate sediments (heterozoan carbonates) were deposited in warm water settings. Such systems are generally prone to drowning due to their reduced accumulation rates and two examples of drowning related to environmental stress as a result of marine eutrophication are presented (case study C). The highest rates of carbonate production were reached by an oligotrophic, tropical photozoan system from the Pliensbachian of case study A (>350m/my). However, this highly productive system was also affected adversely by environmental perturbations, and drowned in response to the oceanic anoxic event (OAE) during the Early Toarcian. An evolutionary event strongly affecting the global carbonate budget was the advent of pelagic carbonate producers during the Mesozoic and the resultant shift of parts of marine carbonate production and deposition into the pelagic realm. Planktic carbonate productivity during Early Jurassic was still strongly limited. The bulk of the carbonate mud deposited as ‘pelagic' carbonate in the central High Atlas basin represents peri-platform mud (case study A). Inactivity of the lagoonal carbonate factory due to short-term emersion was immediately reflected by carbonate-poor basinal sedimentation, and suggests a tight coupling of the neritic and pelagic system of the central High Atlas. In contrast to the relatively low rates of pelagic carbonate production during the (Early) Jurassic, marine carbonate production and sedimentation was significantly transferred into the pelagic realm during the (Late) Cretaceous. The maximum capacity of planktic carbonate production during the Cenomanian was affected by nutrient levels of the (eu-)photic zone and degree of water column mixing. A coast-to offshore-directed decrease of nutrient availability and increased water column stratification controlled the community structure of primary producers (eutrophic calcareous cyst-producing dinoflagellates versus oligotrophic coccolithophorids; case study D). In this case, the eutrophic system shows higher accumulation rates which are, however, an order of magnitude lower than in benthic carbonates. When the different carbonate production rates (high for calcisphere and low for coccolith carbonates) are translated into organic carbon flux to the sea-floor, a coupling of densities of benthic invertebrate communities to palaeoproductivity of the overlying water masses can be demonstrated. Most carbonate systems and their biofacies show a strong dependency on water depth because physico-chemical factors that affect the physiology and ecology of marine organisms are usually strongly correlated with depth. Thus, biofacies changes show a strong connection to the architecture of depositional sequences and hence sequence stratigraphy is an important element of biosedimentology. Third-order depositional sequences are the dominant motif of the investigated succession. High-frequency sea-level cycles of 4 th - to 6 th -order affected mainly benthic shallow water carbonate systems with platform geometries where even subordinate sea-level falls resulted in subaerial exposure of large parts of the productive area (e.g., case study A). The highest diversity faunal associations of are commonly coupled to early transgressive phases of 3 rd -order and high-frequency cycles. Even during the Jurassic-Cretaceous greenhouse, temperature changes may have caused the sea-level variations, as is indicated by biofacies changes at sequence boundaries (case study C) and oxygen stable isotopes across high-frequency sea-level cycles (case study D). In the absence of major continental ice sheets during this time, the climatic changes may have been modulated by changes in humidity and the global groundwater reservoir into sea-level variations. The broad biosedimentological approach applied in this integrated analysis proved to be most successful in the reconstruction of highly complex carbonate depositional systems such as ramps, platforms, and epicontinental shelves. Biosedimentological Case Studies 1. Lias of the central High Atlas, Morocco 2. Mid-/Late Jurassic of east-central Iran 3. Cretaceous (Aptian to Cenomanian) of Cantabria, northern Spain |
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Nautiloids are the least studied Cretaceous
cephalopod group and the compiled literature stands in remarkable contrast to that of ammonites and belemnites (e.g., taking the standard reference data base "GeoRef" and the search term "Cretaceous" combined with "Nautiloidea", "Belemnoidea" or "Ammonoidea", you'll end up with a hit ratio of 1 - 4.4 - 35!). Furthermore, systematic studies are suffering from taxonomic inconsistencies and knowledge is distributed among many different papers, some very important of which are, unfortunately, in Russian (e.g., SHIMANSKY 1975). The last comprehensive accounts on post-Triassic nautiloids were presented by KUMMEL (1956) and the late Jost WIEDMANN (1960, with a bias to Cretaceous nautiloids). |
Parkeria
sphaerica CARTER, 1877, from norther Cantabria.