Comparative palaeolimnology of the Middle Devonian Orcadian Basin
The stratigraphy of the Middle Devonian Orcadian Basin is summarized and a regional review of all related structural and sedimentological data suggests that the basin area formed by the reactivation of old Caledonian thrusts as normal faults bounding extensional basin areas, within an overall regional sinistral strike slip tectonic regime. The depositional environment of Orcadian Basin fish beds is reassessed on the basis of preservational predictions for fish and the delicate sedimentary hicrolamination with the result that the deep stratified lake origin is upheld. The major Achanarras lacustrine transgression event is investigated and the sedimentary and palaeontological data collected allow the development of palaeogeographic and palaeoecologic schemes for this spectacular phase of lake extension. Two fish bed horizons from the Upper Caithness Flagstone Group are described and relationships between high salinity levels and mass fish mortalities are shown by fish distribution, carbonate isotope geochemistry and penecontemporaneous chert development. Patterns of fish preservation are related to taphonomic variables in turn reflecting changing levels of hydrodynamic energy during fish bed formation. Using a variety of bulk and molecular organic goechemical analyses, the primary organic production is shown to have comprised predominantly algae, cyanobacteria and a varied microbial assemblage including a halophilic component. The principal organic carbon cycles are suggested and the critical lack of lake sediment bioturbators within such schemes is notable. Aspects of carbonate chemistry, C/S ratios, and sedimentary features are used to suggest palaeosalinity levels consistent with a brackish-saline lake system. A model of brine evolution via a zonal precipitation sequence of insoluble carbonates is used to postulate the presence of evaporites in the Orcadian Basin centre during low lake stands. The oxygen and carbon isotape compositions of coexisting calcite and dolomite are used to suggest that dolomite formed principally as a primary or early diagenetic mineral within Orcadian lakes. Lake depth/time curves are generated from facies associations and inferred sedimentation rates from average varve thicknesses, within Upper Flagstone Group cyclic lacustrine sequences. Frequency analyses of these time curves demonstrates the importance of 20 Kyr and 90 Kyr periodicities in cycle development, in turn related to long term Milankovitch climatic control. A synthesis of the various aspects of the Orcadian Basin discussed is presented, and comprises a summary description of the physical environment and of the biota together with a discussion of community palaeoecology and probable functional adaptations within early gnathostome lineages. Three examples, of progressively younger lacustrine systems, the Upper Devonian Escuminac Formation, the Early Mesozoic Newark Supergroup and the Eocene Green River Formation are investigated as comparative examples with emphasis on the content and structure of biotic communities represented and the ontogeny of lake environments. A final synthesis is presented which attempts to identify the aspects most important in the evolution of lacustrine ecosystems through the Phanerozoic; lacustrine cycling, related to the Milankovitch theory of extrinsic climatic forcing by orbital parameters, is considered to have been the single most important physical influence, after the formation of a continental area fit to drain water into, in driving the evolutionary patterns seen in lacustrine biotic communities.