Günter Schweigert

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.

Microbial origin of travertine fabrics—two examples from Southern Germany (Pleistocene stuttgart travertines and miocene riedöschingen Travertine)

SummaryIn Southern Germany, two examples of travertines of different age and depositional morphology were examined in detail. Travertines are laminated carbonate rocks formed by precipitation from mineral and/or thermal waters. They include characteristic facies types, such as bushy layers (‘shrubs’) referred to calcification of branching microbes (‘Dichothrix’-morphotype), laminar microbial mats, peloidal layers, and gas bubble layers formed within the sediment. In travertines, microbial activity is the most important factor for carbonate precipitation.Tufas differ from travertines by their abundance of molds of higher plants (leaves, reed, moss, green algae). They may be associated with travertines, but do not exhibit strict travertine facies types. Tufas are common in normal fresh water environments. Contrary to travertines and tufas, calcareous sinters usually occur in restricted areas like spring fissures, caves, or in pores, where microbial activity is not totally absent, but not of paramount importance for precipitation.Pedogenetic processes, which can alter travertine deposits, are responsible for large-scale features such as tepee-structures, and some intraclastic layers, and microscopic structures like endolithic borings andMicrocodium. Travertines may also grade into lacustrine limestones with Characeae, ostracods, and aquatic gastropods.

MIDDLE JURASSIC BELEMNITES AND AMMONITES (CEPHALOPODA) FROM TELMA-DAREH, NORTHERN IRAN

Ammonites and belemnites from a Middle Jurassic section at Telma-Dareh (Alborz Mountains, northern Iran) are described. The lithology of the studied section consists of an alternation of marls and limestones attributed to the Dalichai Fm., overlying the top of the Shemshak Fm. (sandstones) and underlying the Lar Fm. (limestones). Fossils are rather abundant but concentrated in ten scattered levels, ranging from the Aalenian (Scissum-Murchisonae zones) up to the Bajocian. Ammonites ( Tmetoceras scissum , Leioceras cf. comptum , Ludwigia cf. murchisonae , Onychoceras ? sp., and Leptosphinctes sp.), belemnites ( Brevibelus breviformis , Holcobelus xa0cf.xa0 munieri xa0) and bivalves are the most abundant and well-preserved fossils; accessory elements are scarce brachiopods and gastropods. The palaeobiogeographic affinities of the belemnite fauna is Subboreal-Submediterranean, whereas the ammonites have Submediterranean-Tethyan affinities. The Aalenian age of the lower part of the Dalichai Fm. at Telma-Dareh differs from the Bajocian age attributed by other authors to this formation in other localities.

NEW EFFORTS FOR A REVISION AND CORRELATION OF THE AMMONITE FAUNA OF THE NEUBURG FORMATION (TITHONIAN, SW GERMANY)

Ammonites collected bed-by-bed by the late K.W. Barthel allow a high biostratigraphic resolution of the succession of the Neuburg Formation (Unterhausen Member) into 2 regional Biochronozones (Ciliata Zone, Palmatus Zone) and 5 ammonite faunal horizons. The new finds are important for a revision of the taxa described in the monographs of Th. Schneid. The fauna consists of a Submediterranean stock of perisphinctids, together with some Tethyan and Subboreal faunal elements. Especially the co-occurring Tethyan immigrants Volanoceras schwertschlageri , Virgatosimoceras rothpletzi , and Richterella cf. richteri point to a correlation of basal parts of the Neuburg Formation with the Fallauxi Zone. In the younger faunal horizons the Tethyan influx strongly decreases, so that a direct correlation with the Tethyan standard zonation is impossible. Nevertheless, the ammonites of the younger horizons allow a correlation with the Subboreal zonal scheme.

The hansrieberi biohorizon (Aalenian; Opalinum Zone) in the Opalinuston Formation of Donzdorf-Grünbach (Eastern Swabian Alb, Germany)

Abstract The lithostratigraphy and ammonite fauna of the Early Aalenian Opalinuston Formation (Zillhausen Member) at Donzdorf-Grünbach is described. A new chronospecies of Leioceras, L. hansrieberi, is erected. It characterizes the hansrieberi biohorizon of the Opalinum Subzone (Opalinum Zone). The macroconchs of L. hansrieberi represent the largest specimens hitherto described within the genus Leioceras. In addition, the associated macrofauna is described which allows to draw some conclusions on the palaeoecology of this locality.

Additional Tithonian and Berriasian ammonites from the Vaca Muerta Formation in Pampa Tril, Neuquén Basin, Argentina

The ammonite fauna of the Tithonian–Berriasian of the Vaca Muerta Formation in Pampa Tril has been recently described in detail. New important specimens and additional information are presented in this paper. The phyletic evolution of Choicensisphinctes, passing from C. platyconus to C. erinoides is confirmed, as well as the sexual dimorphic correspondence of this latter with C. mendozanus. A microconch of the genus Krantziceras is described for the first time. New specimens of Substeueroceras koeneni identical to the paralectotype, along with material already described from the koeneni Hz. (Koeneni Zone), point to the fixation of this horizon as the type horizon of the species. New specimens of Subthurmannia boissieri from the Damesi Zone match clearly the range of variation of this species in Spain, thus providing an element for time-correlation with the Tethyan standard scale. Additional material from the internispinosum alpha Hz. confirms the origin of W. internispinosum from C. proximus by the inception of an evolutionary innovation in the juvenile ontogeny.