The origin of Jurassic reefs: Current research developments and results |
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Authors: | Prof Dr Reinhold R Leinfelder Dr Manfred Krautter Dipl-Geols Ralf Laternser Martin Nose Dieter U Schmid Günter Schweigert Dr Winfried Werner Prof Dr Helmut Keupp Dipl-Geols Hartmut Brugger Regina Herrmann Dr Ursula Rehfeld-Kiefer Prof Dr Johannes H Schroeder Dipl-Geol Carsten Reinhold Profs Dres Roman Koch Arnold Zeiss Prof Dr Volker Schweizer Dipl-Geols Heinrich Christmann Götz Menges Prof Dr Hanspeter Luterbacher Reinhold R Leinfelder |
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Institution: | 1. Institut für Geologie und Pal?ontologie der Universit?t, Herdweg 51, D-70174, Stuttgart 2. Bayerische Staatssammlung für Pal?ontologie und historische Geologie, Richard-Wagner-Str. 10, D-80333, München 3. Institut für Pal?ontologie der Freien Universit?t, Malteserstr. 74-100, D, D-12249, Berlin 4. Institut für Geologie und Pal?ontologie der Technischen Universit?t, Ernst-Reuter-Platz 1, D-10587, Berlin 5. Institut für Pal?ontologie der Universit?t, Loewenichstr. 28, D-91054, Erlangen 6. Geologisch-Pal?ontologisches Institut der Universit?t, Im Neuenheimer Feld 234, D-69120, Heidelberg 7. Institut und Museum für Geologie und Pal?ontologie der Universit?t, Sigwartstr. 10, D-72076, Tübingen
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Abstract: | Summary In order to elucidate the control of local, regional and global factors on occurrence, distribution and character of Jurassic
reefs, reefal settings of Mid and Late Jurassic age from southwestern Germany, Iberia and Romania were compared in terms of
their sedimentological (including diagenetic), palaeoecological, architectural, stratigraphic and sequential aspects. Upper
Jurassic reefs of southern Germany are dominated by siliceous sponge—microbial crust automicritic to allomicritic mounds.
During the Oxfordian these form small to large buildups, whereas during the Kimmeridgian they more frequently are but marginal
parts of large grain-dominated massive buildups. Diagenesis of sponge facies is largely governed by the original composition
and fabric of sediments. The latest Kimmeridgian and Tithonian spongiolite development is locally accompanied by coral facies,
forming large reefs on spongiolitic topographic elevations or, more frequently, small meadows and patch reefs within bioclastic
to oolitic shoal and apron sediments. New biostratigraphic results indicate a narrower time gap between Swabian and Franconian
coral development than previously thought. Palynostratigraphy and mineralostratigraphy partly allow good stratigraphic resolution
also in spongiolitic buildups, and even in dolomitised massive limestones.
Spongiolite development of the Bajocian and Oxfordian of eastern Spain shares many similarities. They are both dominated by
extensive biostromal development which is related to hardground formation during flooding events. The Upper Jurassic siliceous
sponge facies from Portugal is more localised, though more differentiated, comprising biostromal, mudmound and sponge-thrombolite
as well as frequent mixed coral-sponge facies. The Iberian Upper Jurassic coral facies includes a great variety of coral reef
and platform types, a pattern which together with the analysis of coral associations reflects the great variability of reefal
environments. Microbial reefs ranging from coralrich to siliceous sponge-bearing to pure thrombolites frequently developed
at different water depths. Reef corals even thrived within terrigeneous settings.
In eastern Romania, small coral reefs of various types as well as larger siliceous sponge-microbial crust mounds grew contemporaneously
during the Oxfordian, occupying different bathymetric positions on a homoclinal ramp.
Application of sequence stratigraphic concepts demonstrates that onset or, in other cases, maximum development of reef growth
is related to sea level rise (transgressions and early highstand) which caused a reduction in allochthonous sedimentation.
The connection of reef development with low background sedimentation is corroborated by the richness of reefs in encrusting
organisms, borers and microbial crusts. Microbial crusts and other automicrites can largely contribute to the formation of
reef rock during allosedimentary hiatuses. However, many reefs could cope with variable, though reduced, rates of background
sedimentation. This is reflected by differences in faunal diversities and the partial dominance of morphologically adapted
forms. Besides corals, some sponges and associated brachiopods show distinct morphologies reflecting sedimentation rate and
substrate consistency. Bathymetry is another important factor in the determination of reefal composition. Not only a generally
deeper position of siliceous sponge facies relative to coral facies, but also further bathymetric differentiation within both
facies groups is reflected by changes in the composition, diversity and, partly, morphology of sponges, corals, cementing
bivalves and microencrusters.
Criteria such as authigenic glauconite, dysaerobic epibentic bivalves,Chondrites burrows or framboidal pyrite in the surrounding sediments of many Upper Jurassic thrombolitic buildups suggest that oxygen
depletion excluded higher reefal metazoans in many of these reefs. Their position within shallowing-upwards successions and
associated fauna from aerated settings show that thrombolitic reefs occurred over a broad bathymetric area, from moderately
shallow to deep water. Increases in the alkalinity of sea water possibly enhanced calcification.
Reefs were much more common during the Late Jurassic than during the older parts of this period. Particularly the differences
between the Mid and Late Jurassic frequencies of reefs can be largely explained by a wider availability of suitable reef habitats
provided by the general sea level rise, rather than by an evolutionary radiation of reef biota. The scarcity of siliceous
sponge reefs on the tectonically more active southern Tethyan margin as well as in the Lusitanian Basin of west-central Portugal
reflects the scarcity of suitable mid to outer ramp niches. Coral reefs occurred in a larger variety of structural settings.
Upper Jurassic coral reefs partly grew in high latitudinal areas suggesting an equilibrated climate. This appears to be an
effect of the buffering capacity of high sea level. These feedback effects of high sea level also may have reduced oceanic
circulation particularly during flooding events of third and higher order, which gave rise to the development of black shales
and dysaerobic thrombolite reefs. Hence, the interplay of local, regional and global factors caused Jurassic reefs to be more
differentiated than modern ones, including near-actualistic coral reefs as well as non-actualistic sponge and microbial reefs. |
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Keywords: | Coral Reefs Siliceous Sponge Reefs Microbial Reefs Massive Limestones Sedimentology Automicrites Microbial Crusts Palaeoecology Diagenesis Sequence Stratigraphy Ammonite Stratigraphy Palynostratigraphy Mineralostratigraphy Sedimentation Rate Bathymetry Oxygenation Evolution Shelf Style Palaeogeography Palaeoclimate Palaeocirculation Southern Germany Portugal Spain Romania Middle Jurassic Upper Jurassic |
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