Microbial crusts of the late jurassic: Composition,palaeoecological significance and importance in reef construction |
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Authors: | Prof Dr Reinhold R Leinfelder Dipl-Geol Martin Nose Dipl-Geol Dieter U Schmid Dr Winfried Werner |
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Institution: | 1. Institut für Geologie und Pal?ontologie, Universit?t, Herdweg 51, D-70174, Stuttgart, Germany 2. Bayerische Staatssammlung für Pal?ontologie und historische Geologie, Richard-Wagner-Str. 10, D-80333, München
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Abstract: | Summary 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 thrombolities 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, waterenergy, 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 inLithocodium could be identified as the foraminiferBullopora aff.laevis, possibly living as a parasite or symbiont in theLithocodium algal tissue.Lithocodium andBacinella 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. |
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Keywords: | upper jurassic microbial crusts reefs palaeoecology algae foraminifera microproblematica bathymetry sedimentation rate oxygen sea level fluctuations climate upper jurassic |
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