Johannes H. Schroeder

The origin of Jurassic reefs: Current research developments and results

SummaryIn 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.

Submarine and vadose cements in Pleistocene Bermuda reef rock

Abstract Well-rounded pebbles and cobbles of reef rock, which in bio- and lithofacies closely resembles the Recent algal cup reefs, occur in the basal conglomerate of the Pleistocene Devonshire Formation cropping out along Grape Bay on the south shore of Bermuda. The framebuilders of this rock include coralline red algae, vermetid gastropods, and barnacles. Study by petrographic microscope, electron microprobe, and SEM revealed aragonite-needle and spherulitic cements, high-Mg calcite micrite and palisade cements including calcified algal filaments. Two or three generations of these may occur in a given pore. By analogy with the Recent cup-reef material, on the basis of their composition (high strontium content of aragonite, high magnesium content of calcite) and of circumstantial evidence, these are considered submarine cements. In addition, a vadose blocky low-Mg calcite cement occurs, which never precedes any, but frequently succeedes one, two, or three of the above cements. Its fabrics vary: it may exhibit an increase in crystal size toward the center or not, it may or may not be bounded by the meniscus outline characteristic of the vadose diagenetic environment. The limitation of cement precipitation by a meniscus gives rise to a large diversity of forms determined by respective combinations of solution supply, permeability of the micro-environment, and substrate morphology. To illustrate the diversity, the meniscus cement is shown in relation to sediment particles, aragonite-needle cement, and calcified filaments. From the blocky low-Mg cement, selective neomorphism extends into some aragonitic gastropod shells; it is controlled, in part, by original shell structure. The resulting low-Mg calcite mosaic is characterized by relic structures and plane crystalline boundaries. Aragonite needles between blocky calcite cement and neomorphic calcite were not affected by the process of mineralogic alteration. This study shows that cements recognized earlier in Recent Bermuda cup reefs can be traced into the past and that additions to the diagenetic sequence can be distinguished. Knowledge of these features and their sequences should help decipher the diagenetic history of other fossil reefs.

Steuerungsprozesse der Entwicklung von Oberjura-Spongiolithen Süddeutschlands: Kenntnisstand, Probleme und Perspektiven

ZusammenfassungDie jurassischen Spongiolithe der epi- bis perikontinentalen Randentwicklung der Tehys erreichen im Malm ihre maximale Entfaltung. Sie können in der Evolution von Riffassoziationen während der Erdgeschichte als ein historisch einmaliges Phänomen gesehen werden. Der Beitrag diskutiert den derzeitigen, zum Teil kontrovers beurteilten Kenntinisstand über Genese und Rahmenbedingen dieser autochthonen Kieselschwamm-Gesteine Süddeutschlands und seiner Nachbargebiete. Im Hinblick auf das Teilprojekt ‘Riffevolution’ des DFG-Schwerpunktprogrammes ‘Biogene Sedimentation’ beleuchtet er insbesondere folgende genetischen und prozeßorientierten Aspekte: 1.Stratigraphische Rahmenbedingungen (Bio-, Litho- und Mineralostratigraphie).2.Die geotektonischen und paläogeographischen Rahmenbedingungen auf dem breiten passiven nördlichen Tethys-Schelf werden anhand der Faktoren Schelfbreite, Schelfstabilität, Landferne und terrigener Sedimenteintrag sowie Meeresspiegel erörtert.3.Für die aus der Sedimentologie ableitbaren klimatischen Fluktuationen liegt noch keine widerspruchsfreie Bestätigung durch die Organismenverteilung vor.4.Die klassische Deutung der sich in der stratigraphischen Abfolge verändernden Wuchsformen biostromaler und biohermaler Spongiolithe als Stillwasserbildungen des tieferen Schelfs, die sich unter anderem auf die (wechselnden) Mikrofazies-Merkmale stützt, wirdneben paläogeographischen und sedimentologischen Argumenten auch durch die sedimentbindende Bedeutung biogener Karbonatkrusten in Frage gestellt.5.Aspekte sedimentologischer Rahmenbedingungen der oberjurassichen Spongiolithe konzentrieren sich im wesentlichen auf die Fragen nach gesicherten bathymetrischen Hinweisen, sedimentären Zyklen, dem möglichen Zusammenhang mit re- und transgressiven Entwicklungen und auf biogene Interaktionen als mutmaßliche Initiatoren der Kieselschwamm-Fossilisation einerseits und der Boundstone-Bildung andererseits.6.Biologische Rahmenbedingungen lassen sich aus den in der Zeit wechselnden benthonischen Organismen-Assoziationen und aus der wechselnden Substratbindung der Organismen ableiten. Neue phylogenetische Errungenschaften innerhalb der dominanten Kieselschwämme sind als Auslöser der Buildups derzeit nicht erkennbar.7.Für die Analyse der steuernden Parameter der Schwammfazies (Paläogeographie, Bathymetrie, Sedimentationsraten, Nährstoffangebot) ist der Vergleich der mit der Schwammfazies vertikal und lateral assoziierten Korallenfazies von grundsätzlicher Bedeutung.8.Die Bedeutung diagenetischer Prozesse für die Interpretation der Spongiolithe wird anhand von früher Lithifizierung, Lösung und epigenetischer Dolomitisierung bzw. Dedolomitisierung diskutiert.SummaryThe sponigolitic facies reaches its widest distribution along the epicontinental to pericontinental margin of the Northern Tethys during the Upper Jurassic. These buildups formed by siliceous sponges represent a singular event within the evolution of reef communities during earth’s history. In this paper the present knowledge of causal and environmental aspects of the spongiolithic limestones in Southern Germany and adjacent areas is discussed. With regard to the research project ‘Evolution of Reefs’ (a major part of the Schwerpunktprogramm ‘Biogenic Sediments’ established by the Deutsche Froschungsgemeinschaft in 1990) the following genetically and process orientated aspects are of interest: 1)The stratigraphic classification using biostratigraphic, lithostratigraphic and mineralostratigraphic methods.2)The paleogeography, oceanography and geotectonics along the broad passive northern shelf of the Tethys are discussed with regard to width and stability of the shelf, distance to the land mass and terrigenous influence and sea level fluctuations.3)Certain sedimentological features seem to be indicative of climatic fluctuations, but this is not always proved by the distribution patterns of organisms.4)The classical interpretation that spongiolites formed below the wave base gets oppositon by the significance of biogenic carbonate crusts as well as sedimentological and paleogeographic arguments.5)Important sedimentological factors of the Upper Jurassic spongiolites are paleobathymetry, sedimentary cycles, possible control of spongiolite growth by sea level fluctuations, as well as biogenic interactions which probably initiated the fossilization of siliceous sponges as well as the formation of spongiolitic buildups.6)Paleoecological conditions can be reconstructed by the community patterns of benthonic organisms and substrate control patterns, both changing through time. Phylogenetic innovations of related sponge groups (hexactinellids, lithistide demospongeans) most probably are not responsible for the initial growth of buildups.7)Comparison of the spongiolites with the laterally and vertically associated coral facies is of major importance for the interpretation of controlling parameters (bathymetry, sedimentation rates, nutrient and nutrient availability) of the sponge facies.8)The genetic significance of diagenetic processes in spongiolites is discussed with regard to early lithification, dissolution, epigenetic dolomitization and dedolomitization.

Spatial variations in the porosity development of carbonate sediments and rocks

SummaryDiagenetic sequences of many stages are know from various carbonate rocks, but the diagenetic spatial variations within a given facies, outcrop, hand specimen or thin section are only gradually rediscovered and taken into account in porosity studies. The variations occur in various carbonate facies such as reefs, beachrocks, shallow-water deposits, hardgrounds, and concretions. Multiple, parallel diagenetic pathways, branching in time, are recognized in such cases.Diagenetic spatial variations are caused by individual characteristics of particles and cements (mineral and chemical compositions, ultra- and microstructure, surface properties); collective characteristics of sediments and sedimentary rocks (component mix due to ecological and transport selection, fabrics as determined by primary sedimentary and skeletal structures); synsedimentary to early diagenetic processes (internal sedimentation, bioturbation, bioerosion); early to late diagenetic processes (cementation, dissolution, fracturing, decomposition of organic matter); hydrology (wave and tide ranges, sea and ground water levels, mixing zones), as well as substrate mobility and solubility. Many of the causal factors are determined, directly or indirectly, by biologic, meteorologic, or oceanographic factors. Causal factors may combine and thus, as a given diagenetic stage, provide for considerable diagenetic complexity.In the course of diagenesis, i.e. in time, the spatial variations increase: The spatial heterogeneity of one stage provides the scene for cementation, dissolution and/or alteration to act in their respective ‘local’ or ‘micro-local’ environments, and thus to increase the heterogeneity for the next scene or diagenetic stage. Superposition of respective variations gradually increases the diagenetic diversity. Porosity development therefore progresses not only in time, but also in space, and thus is four-dimensional.The study of diagenetic spatial variations and the respective detailed analyses lead to a comprehensive understanding of porosity development and of the highly variable and complex diagenetic history of carbonate rocks. The complexity provides problems for the application of diagenetic information in cement stratigraphy and porosity assessment. Cement-stratigraphic correlation, when taking into account spatial variations, uses selected pore types and pore fillings in a defined space/time framework. Examining pores and pore fillings of different diagenetic stages, the respective paleo-hydrologic gradients, the sources of solutes, and thus the hydrologic history of depositional and diagenetic environments can be established. Porosity assessment, when coping with diagenetic complexities, either is based on quantification of diagenetic pathways/products encountered, or on selection of indicative pores or pore fillings. Differentiation according to diagenetic stages permits assessing the porosities at successive stages.Realization of spatial variations in porosity development spoils fast and simple correlations, assessments, or predictions. The synthesis of the detailed analytical information, however, provides a route toward application of diagenetic complexities.ZusammenfassungDiagenetische Abfolgen mit vielen Stadien kennt man von verschiedenen Karbonatgesteinen, aber diagenetische räumliche Variationen innerhalb einer Fazies, eines Aufschlusses, Handstückes oder Dünnschliffes werden erst all-mählich wiederentdeckt und in Porositätsuntersuchungen miteinbezogen. Solche Variationen kommen in verschiedenen Karbonatfazies vor, zum Beispiel in Riffen, Strandfels, Flachwasserkalken, Hartgründen und Konkretionen. Vielfältige parallele Bahnen der Diagenese, die sich mit der Zeit immer wieder gabeln, sind in solchen Fällen zu erkennen.Zu den Ursachen solcher Unterschiede gehören individuelle Merkrmale von Partikeln und Zementen (Mineral-und chemische Zusammensetzung, Ultra- und Mikrostruktur, Oberflächenbeschaffenheit), kollektive Merkmale von Sedimenten und Sedimentgesteinen (Komponentenmischung aufgrund ökologischer oder physikalischer Selektion; Gefüge resultierend aus primären Sedimentstrukturen oder von biogener Konstruktion), synsedimentäre bis frühdiagenetische Prozesse (Internsedimentation, Bioturbation, Bioerosion), früh- bis spätdiagenetische Prozesse (Zementation, Lösung, Bruch und Zersetzung organischen Materials), weiter hydrologische Verhältnisse (Wellen- und Gezeitenhöhe, Meeres- und Grundwasserspiegelniveaus, Mischungszonen), und schließlich Mobilität und Löslichkeit des Liegenden. Viele dieser Ursachen wirken gleichzeitig, entweder zusammen oder auch gegeneinander; sie werden zum Teil selbst von biologischen, klimatologischen und ozeanographischen Faktoren bestimmt.Im Laufe der Diagenese, d.h. mit der Zeit, vergrößern sich die räumlichen Unterschiede: Die Heterogenität eines Stadiums bildet das Szenarium für den Ablauf von Zementation, Lösung oder Umwandlung im folgenden Stadium; diese Prozesse haben jeweils “lokal” oder “mikrolokal” variable Effekte, wodurch die Heterogenität wieder gesteigert wird, und die Ausgangslage für das nächste diagenetische Stadium komplexer ist als die des vorherigen. Durch die allmähliche Überlagerung der jeweiligen Variationen aufeinanderfolgender Stadien erhöht sich die räumliche diagenetische Vielfältigkeit des Gesteins. Die Porosität Entwickelt sich also nicht nur mit der Zeit, sondern wird auch ständig räumlich differenzierter; die Entwicklung ist somit vier-dimensional.Dieser Ansatz und die dazugehörige detaillierte Analyse bringen ein umfassendes Verständnis der jeweiligen Porositätsentwicklung. Die Vielfalt verursacht Probleme für die Anwendung vor allem bei der Zement-Stratigraphie und bei der Porositätsabschätzung. Zement-stratigraphische Korrelation unter Berücksichtigung der räumlichen Komplikationen konzentriert sich auf ausgewählte diagenetische Produkte, wie bestimmte Zemente oder Poren, die raum-zeitlich definiert sind; Berücksichtigung verschiedener Produkte eines Stadiums bietet Überprüfungsmöglichkeiten. Auf diese Weise sind regional Analysen möglich und hydrologische Gradienten und Herkunft von Lösungen zu bestimmen. Bezieht man entsprechende Untersuchungen auf verschiedene diagenetische, Stadien, läßt sich so die hydrologische Geschichte der Ablagerungs-und diagenetischen Bereiche eines Gesteins rekonstruieren.Bei der Porositätsabschätzung und-extrapolation kann man quantifizieren, d.h. die Proportionen der jeweiligen diagnetischen Produkte ermitteln; dabei bringen jedoch auch die Zahl der Kategorien, der Maßstab der Untersuchungen wie auch der Arbeitsaufwand eigene Probleme mit sich. Vielversprechend ist auch hier die gezielte Auswahl von bestimmten Poren oder Porenfüllungen, wobei Verteilung, Häufigkeit, Größe, Zeitpunkt des Auftretens und nachfolgende Geschichte Auswahlkriterien sind. Für beide Aufgabenbereiche gilt, daß die schnelle und simple Korrelation oder Abschätzung ohne vorhergehende Analyse der räumlichen Beziehungen und nachfolgenden Synthese nicht sehr sinnvoll ist.

Porosity development and distribution in the Rüdersdorfer Schaumkalk (Middle Triassic) of the Gas Store Berlin, Germany

SummaryGas is stored in the Triassic Buntsandstein units above the salt-pillow structure of Spandau. Brines obtained collaterally when bringing the gas back to the surface are reinjected into the oomoldic limestone of the Lower Muschelkalk. Cores of the limestone were investigated with respect to microfacies, diagnetic development, porosity, and permeability to provide the input for modeling reservoir configuration and capacity of this unit. In the porous portions of this unit, six microfacies types were distinguished by varying proportions of ooids, peloids and biogenic (skeletal) particles. Microfacies types do not correlate with porosity and permeability. The rocks were altered by varying successions of dissolution and cementation stages; four diagenetic grades were defined on the basis of development of moldic pores and their fillings. The diagenetic grades correlate with porosity and permeability showing that kind and intensity of diagenetic alterations largely overprinted initial control of microfacies on fluid flow.

Ecology, sedimentology, and diagenesis of recent and fossil reefs

ZusammenfassungWährend des VIII. Internationalen Sedimentologen-Kongresses in Heidelberg (1971) wurde ein Symposium zum obigen Thema veranstaltet; eine Auswahl der Beiträge ist hier zusammengestellt. In dieser Einführung werden anhand der verschiedenen Ansätze der folgenden Untersuchungen Trends und Aufgaben der Rifforschung aufgezeigt.AbstractAt the VIII. International Sedimentological Congress held in Heidelberg in 1971, a symposium with the above title was convened; a selection of the papers presented is assembled in this issue. In the introduction the approaches of the following studies are reviewed to show some trends and problems in reef research.RésuméDurant le VIIIe congrès international de sédimentologie, à Heidelberg, un symposium sur le thème susdit fut organisé; une sélection des articles, est ici rassemblée. Dans cette introduction, les approches des études suivantes sont examinées afin de montrer les tendences et les problèmes dans les recherches concernant les récifs.Краткое содержаниеВ этой статье приведе ны результаты послед них исследований по рифо образованию и о значении рифов в ис тории Земли, обсуждав шиеся в 1971 г в Гейдельберге на 8-? конгрессе седимен тологов, и указываетс я на целый ряд проблем, ожидающих своего раз решения.