科研之友
微信
新浪微博
Facebook
分享链接
摘要
Early marine seafloor lithification of carbonate sediments leads to the formation of hardgrounds and is known from rocks as old as the Proterozoic. Hardground surfaces, however, are more commonly recorded in the Phanerozoic sedimentological archive. While ecological studies of hardground biota abound, the environmental and physico-chemical parameters leading to the development of seafloor lithification remain, in many cases, poorly understood and documented. This paper reviews published evidence on the petrography, mineralogy and geochemistry of Phanerozoic carbonate hardgrounds within a process-oriented context of their environmental controls. Hardgrounds typically develop in warm and shallow, agitated tropical waters that are saturated with respect to CaCO3, but are also reported from hemi-pelagic to bathyal realms and from cool-water, temperate settings. A range of early marine cement types are documented from present-day hardgrounds whereas (preserved) cement fabrics and related mineralogies are less diverse in early Phanerozoic hardgrounds. Carbonate hardgrounds are widespread during calcite sea periods (e.g., Ordovician and Cretaceous), as opposed to some (preservation bias?) ancient aragonite seas (e.g., Permian and Triassic). Obviously, the relation between seawater chemistry and hardground abundance and attributes is not an easy one. Here, the concept of aragonite versus calcite seas serves as a first-order template for the sake of a structured discussion whereas modem aragonite mode oceans document a significant diversity in spatial and bathymetric seawater properties. This spatio-temporal complexity is perhaps reflected in the scarce record of calcite sea hardgrounds from the Devonian or Paleogene rock record contrasted by the abundance in aragonite Holocene and Recent seas. Holocene hardgrounds allow for the direct assessment of rates of, and reasons for, early subaquatic lithification and in most cases escaped subsequent non-marine diagenetic overprint. Conversely, studies of ancient hardgrounds are often biased by early biological and mechanical erosion, the degree of diagenetic overprint, and tectonic to orogenic processes. The formation, distribution and physical properties of hardgrounds depend on the global climatic context (greenhouse/icehouse modes), on sea-level changes, on ocean stratification, on the spatial extent of epicontinental seas and carbonate depositional environments, and on the spatially different mineralogies and rates of carbonate production. The fabrics and mineralogies of early hardgrounds cement seem to substantially hinge on variations in atmospheric CO2. Thus, the application of lessons from modern to ancient hardground case examples is not straightforward. A holistic model explaining the full variability of controls on submarine hardground formation and their cement petrography throughout the Phanerozoic is as yet lacking. A more critical view on secular changes in hardground formation patterns is clearly needed. Future work should rely on the combination of sedimentological, stratigraphic, palaeoecological, petrographic, and geochemical approaches. Specific emphasis should be given to time intervals (Triassic, Carboniferous, Cambrian etc.) where the bulk of reported case studies is at best scarce.
- 出版日期2015-12
