Microbial crusts of the Late Jurassic: composition, palaeoecological significance and importance in reef construction.-
Facies, 29, 195-230 (Spec. vol. "Microbial Carbonates "), Erlangen.
Upper Jurassic reefs contain variable amounts of calcareous microbial crusts. In examples from Portugal, Spain and southern Germany they occur within coral biostromes and bioherms, mixed coral-siliceous sponge reefs, siliceous sponge meadows and mudmounds, and build up thrombolites with or without additional reef metazoans. The crusts are of paramount importance for the establishment and development of positive buildups. Commonly, reef growth starts with crusts which develop from a narrow base and rapidly expand laterally by rising above the sea floor. Reef associations with little or no microbial crust normally did not develop distinct relief.
The basic microbial crust type is characterised by a dense to peloidal, mostly clotted, hence thrombolitic fabric which developed due to calcification triggered by microbes. Morphological evidence for this organic nature are positive relief, bridge-structures, and the shape and arrangement of peloids. The basic thrombolitic crust type is a eurytopic feature, equally occurring in settings of different bathymetry, water-energy, salinity and oxygen/nutrient concentrations. However, the crusts also comprise additional micro-encrusters of variable abundance and diversity. The concurrent occurrence of these encrusters and diversity trends allows discrimination between crusts of different environments, particularly of different water depths. Microbial crusts from non-reefal marine oncoids show both similarities and differences with reefal crusts. For some of the mostly enigmatic micro-encrusters new clues to their nature could be detected. For instance, bubble-like structures, formerly interpreted as sporangia in Lithocodium could be identified as the foraminifer Bullopora aff. laevis, possibly living as a parasite or symbiont in the Lithocodium algal tissue. Lithocodium and Bacinella are regarded as different organisms. 'Tubiphytes' morronensis clearly represents a symbiotic intergrowth between a nubeculinellid foraminifer and a microbe of unknown nature. The main prerequisite for the occurrence of microbial crusts is a cessation of background sedimentation which commonly can be tied to rises in sea level. This results in the development of crust-rich reefs. Fluctuations in oxygen and nutrient levels are indicated by dysaerobic bivalves and richness in authigenic glauconite, and led to the microbes outcompeting reefal metazoans, and to the development of thrombolites. Such thrombolites occur at very different depths which is interpreted to be related to a rise of dysaerobic waters due to climatic buffering and lowering of oceanic circulation during sea level rises. Microbial crusts in modern reefs are largely restricted to shaded, cryptic settings which contrasts with the wide distribution of crusts in Upper Jurassic reefs. Microbial crusts were increasingly replaced by coralline red algae since the Late Mesozoic, but despite their restricted modern habitat seem to still play an important, commonly overlooked role in the stabilisation of reef framework.
Last changes Nov. 04 by Reinhold Leinfelder