Reinhold R. Leinfelder
Upper Jurassic reef types and controlling factors -A preliminary report.
1-45
Christoph G. Koban
Faziesanalyse und Genese der quartären Sauerwasserkalke von Stuttgart, Baden-Württemberg.
47-118
Reinhold R. Leinfelder
A sequence stratigraphic approach to the Upper Jurassic mixed carbonate - siliciclastic succession of the central Lusitanian Basin, Portugal.
119-140
Günter Schweigert
Subboreale Faunenelemente (Ammonoidea) im oberen Weißjura (Oberkimmeridgium) der Schwäbischen Alb.
141-155
Institut für Geologie und Paläontologie Universität Stuttgart, 1993
Reinhold R. Leinfelder
Abstract
Reefs occurred widespread during the Late Jurassic, particularly along the northern Tethyan shelf and the marginal basins of the young North Atlantic Ocean. They thrived in a variety of settings such as on intrabasinal tectonic and halokinetic uplifts, within lagoons or within siliciclastic fan deltas. Most frequently they grew in homoclinal to steepened ramp settings, where they occupied a wide bathymetric field from the innermost, partly even hypohaline, part down to outer ramp settings. Compositionally they comprise the end members 'coral facies', 'siliceous sponge facies' and 'microbial facies', but transitions and successions are frequent. Microbial crusts are important not only in the microbial facies where they build thrombolitic reefs up to 30 metres thick but also within the siliceous sponge and coral facies where they occur at variable quantities and are largely responsible for constructing a positive relief. Reef facies without crusts is mostly biostromal. Siliceous sponge facies is frequently developed as sponge - microbial crust - mudmounds which occur in a belt from Romania down to Portugal.
Important factors determining the occurrence, composition and fabric of reefs are bathymetry, background sedimentation rate and oxygen fluctuations. Bathymetric interpretation based on sequential analysis of shallowing upward successions and on comparative semiquantitative palaeoecology shows that coral facies, mixed coral - sponge facies and siliceous sponge facies follow each other along a deepening gradient, although the zones overlap broadly. Decrease and cessation of background sedimentation increased diversity and favoured the growth of microbial crusts. An increasing rate of oxygen/nutrient fluctuations excluded reef macrofauna and eventually led to thrombolitic reefs. These were more frequent in deeper settings but occurred over a wide bathymetric range.
The success and broad compositional range of Upper Jurassic reefs is largely related to the high global sea level of this time. It provided wide epicontinental seas as well as the deeper shelf settings suitable for the expanse of sponge reefs. The much larger expansion of Upper Jurassic coral and sponge reefs relative to their Middle Jurassic counterparts is largely due to the generally rising Jurassic sea and only to a small proportion to evolutionary diversification of reef biota. The high sea level of the Late Jurassic also resulted in climatic buffering, giving rise to a general 'greenhouse'-type climate. This lowered atmospheric and oceanic circulation and resulted in the occurrence of dysaerobic bottom water in the deeper shelf, where thrombolitic reefs could thrive. Widespread occurrence of black shales and dysaerobic facies suggests that Upper Jurassic seas were widely stratified. The weak oceanic circulation was particularly driven by evaporization. It is suggested that weak but widespread upwelling along the northern Tethyan shelf was responsible for the occurrence of oxygen-controlled thrombolites on the shelf. Such oxygen-controlled reefs occurred even shallower when during times of intra-Upper Jurassic transgressions climate was aditionally buffered, leading to a rise of the dysaerobic zone.
Comparison of occurrence pattern of reefs, condensed marker horizons and clay-rich intervals across the northern Tethyan shelf indicates that modifications of sequence stratigraphic concepts have to be made for ramp systems and 'greenhouse' times. The occurrence of microbial reefs and the lateral expanse of deeper water reef facies is considered to be of special significance for recognizing sea level rises, whereas due to climatic feedbacks and tectonic effects clay-rich intervals are not diagnostic for sequence stratigraphic interpretation. A model is suggested where climatic and oceanic feedback during sea level rise as well as lowstand condensation is considered. The causes for intra-Upper Jurassic sea level changes appear to be mostly of tectonic origin, although, for higher order oscillations, the observed climatic feedbacks point to an autocyclic component related to the carbon cycle.
Christoph G. Koban
Summary
The travertines of Stuttgart accumulated during warmer stages in the Pleistocene and Holocene. They are obviously related to still occurring mineral waters rich in carbon dioxide and calcium carbonate, which ascend from deep-seated faults within the valley of the River Neckar. The travertine deposits rest on different fluvial terraces of the River Neckar (fluvial gravel and flood plain deposits). Different carbonates developed on these substrata. It must be pointed out that the Stuttgart "Travertines" do not only consist of travertines in the strict sence. Based on a new genetic classification for non-marine carbonates (Koban & Schweigert 1993), it is possible to distinguish lacustrine limestones, calcareous tufas and calcareous sinters besides travertines. Therefore the popular name "Sauerwasserkalk" (acid-water limestone) is more appropriate for these formations.
Based on microfacies analysis, several facies types are separated, which are combined to the following facies associations: A subaqueous slope/shallow pool association is dominated by laminated microbial mats, built by Schizothrix and peloidal facies types. Cyanobacterial shrub-layers, gas-bubble-layers and laminar microbial mats occur within pools during quiet conditions. On the other hand, intraclasts, coated grains and siliciclastic micrites arise during and after times of desiccation and allochthonous sediment input. Lake to swamp associations are characterized by micritic limestones with gastropods, ostracods, characean stems and gyrogonites, while smaller bogs are typified by calcareous tufas. Within gullies, intraclasts and coated grains are common besides calcareous tufas built by moss.
Particularly the genesis of the travertine facies types is characterized by the activity of microbial communities. To a large extent, peloidal layers of bacterial origin occur. Layers of bush-like arranged calcite crystals (shrubs) formed by the cyanophycea Dichothrix are also important. Associated layers of regular open-space structures are interpreted as in situ calcified oxygen gas bubbles (a product of microbial photosynthesis) at the water/microbial mat-interface. All these structures are typical also for travertines known from other locations worldwide. They are useful for their recognition and allow to differentiate from other non-marine carbonates. The Stuttgart "Travertines" are normally horizontally bedded or dip slightly in different directions. This is the result of the given morphology and changes in drainage patterns. Depressions were caused by subrosion and partly filled by terrigenous input. Additionally subrosion induced gravitational tilting of travertine beds after dissolution of Triassic evaporites. Due to desiccation some microbial layers are transformed to tepee-structures.
Stable isotope-, cathodoluminescence- and EDX-investigations show differences between well laminated and poorly layered travertines. Hence for microbial mats an origin within highly mineralized waters is proved, while the poorly layered ones are a result of surface water mixing and terrestrial input.
The general succession of the Stuttgart "Travertines" mainly consists of calcareous tufas developed upon fluvial deposits, which are followed by travertines of different facies types. Their development is occasionally interrupted by terrestrial sediments. In the upper part lacustrine limestones occur. The uppermost sediments are often poorly consolidated calcareous tufas overlaid by loess. The deposition plane was characterized by a gentle slope towards the River Neckar. On this the mineral waters flowed in thin sheets or formed shallow pools, which controled the different development of the limestones. The accumulation continued a long time as a result of continuous subsidence
Reinhold R. Leinfelder, Stuttgart
Abstract
The Upper Jurassic of the Lusitanian Basin, west-central Portugal, represents a marine to terrestrial syn-rift succession. The major depositional systems are determined by general basin setting and synsedimentary tectonic activity, controlling amount and direction of terrigeneous input as well as formation and location of shallow-water platforms. They comprise long-lived systems such as an east-derived coarse siliciclastic fan delta, a largely coeval, northwest-derived fine-grained terrigeneous prograding slope system, and isolated shallow-water carbonate platforms on structural or halokinetic uplifts.
Sea level changes of third order only modify these structurally controlled depositional systems. During sea level rise coral biostromes and bioherms as well as oxygen-controlled thrombolites occurred within siliciclastic fan and slope settings. In terrestrial settings characterized by red bed development shallow-water ramp carbonates occurred as thin intercalations during transgressive phases and early highstand. Third-order sea-level drops terminated such short-lived carbonate development causing subaerial unconformities and karstification. They also resulted in phases of black pebble formation, subaerial erosion, and shallow karst horizons on the long-lived, structurally controlled carbonate platforms on intrabasinal highs. Due to a generally high terrigeneous influx into the basin, third-order sequences are not identifiable at all sites due to frequent overcompensation of accomodation rate. As a result of this, progradation frequently occurred even during sea level rises. Particularly basinal settings show such overcompensation effects and, hence, poor sequential resolution. On the other hand, sequences tend to amalgamate on shallow uplifts due to low subsidence rates. Moderately deep settings show the best sequential resolution. By combining the local sequences to a composite pattern, 11 depositional third-order sequences can be identified for the Arruda Subbbasin, which represents the central part of the Lusitanian Basin.
Although the biostratigraphic framework is not very dense, the Lusitanian Basin third-order sequences can be perfectly matched with the sequences of Ponsot & Vail (1991a,b). Since this is also true for many other European regions such as southeastern Portugal, France or southern Germany, the Ponsot & Vail sequences seem to reflect at least western/ central Europe-wide sea level changes. However, in the Lusitanian Basin, third-order sequence boundaries frequently coincide with tectonic activity, causing angular disconformities or collapse structures. It is therefore assumed that the Upper Jurassic sequences of Ponsot & Vail (1991a,b) reflect regional tectonic rift activity rather than glacioeustatic cycles.
Günter Schweigert, Stuttgart
Abstract
The subboreal ammonite species Aulaco-stephanus jasonoides (Pavlow), Propectinatites websteri Cope, and Eosphinctoceras magnum Mesezhnikov are reported from the the Swabian Upper Jurassic for the first time. The latter is an index species of the lowermost Volgian stage of Russia and allows to correlate the finding level with the subboreal realm.
Aulacostephanus eudoxus (d'Orbigny) and A. pinguis Durand still occur in basal parts of the Beckeri Zone of Swabia together with Tolvericeras sevogodense (Contini & Hantzpergue), T. atavum (Schneid), and Aulacostephanus contejeani (Thurmann) indicating the French contejeani horizon of the Eudoxus Zone. Stratigraphic hints for an Upper Kimmeridgian age of the "Liegende Bankkalke" Formation are given by specimens of Aulacostephanus jasonoides (Pavlow) and Gravesia lafauriana Hantzpergue. Propectinatites websteri Cope coming from a coralliferous limestone within the "Zementmergel" Formation proves paleobiogeographic connections to Southern England and therefore puts the finding level into the Autissiodorensis Zone of the Upper Kimmeridgian.