Paleoecology ...Of ... Reefs from the Late Jurassic
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Project: Le “Vergleichende Palokologie und Faziesentwicklung oberjurassischer Riffstrukturen des westlichen Tethys-Nordrandes * und des Lusitanischen Beckens (Zentraiportugal)/E, Project Leader: R. Leinfelder (Stuttgart) . Paleoecology, Growth Parameters and Dynamics of Coral, Sponge and Microbolite Reefs from the Late Jurassic Reinhold R. Werner, Martin Nose, Dieter U. Schmid, Laternser, Martin Takacs & Dorothea Area of Study: Portugal, Spain, Southern Germany, France, Poland Environment: Shallow to deep carbonate platforms Stratigraphy: Late Jurassic Organisms: Corals, siliceous sponges, microbes, microen- crusters Depositional Setting: Brackish-lagoonal to deep ramp set- tings Constructive Processes: Frame-building, baffling and binding (depending on reef type and type of reef- building organisms) Destructive Processes: Borings by bivalves and sponges; wave action Preservation: Mostly well preserved Research Topic: Comparative facies analysis and paleo- ecology of Upper Jurassic reefs, reef organisms and communities Spongiolitic facies Fig. I: Distribution of Upper Jurassic reefs studied in detail. Abstract framework and pronounced relief whenever microbolite crusts provided stabilization. Reefs in steepened slope set- Reefs from the Late Jurassic comprise various types of tings are generally rich in microbolites because of bypass coral reefs, siliceous sponge reefs and microbolite reefs. possibilities for allochthonous sediment. Reef rimmed shal- Upper Jurassic corals had a higher ratio of heterotrophic low-water platforms did occur but only developed on pre- versus autotrophic energy uptake than modern ones, which existing uplifts. Upper Jurassic sponge-microbolite mud explains their frequent occurrence in terrigenous settings. mounds grew in subhorizontal mid to outer ramp settings Coral communities changed along a bathymetric gradient and reflect a delicate equilibrium of massive and peloidal but sedimentation exerted a stronger control on diversities microbolite precipitation and accumulation of allochthonous than bathymetry. One coral community was adapted to mud and fine allochems, determined by the distance to brackish waters. shallow-water carbonate factories. Disturbances in this Reefal siliceous sponge biostromes and sponge microbo- equilibrium lead to the development of sponge biostromes lite mud mounds generally occur below the coral facies, and or the disappearance of sponge facies. hexactinellid-dominated sponge communities generally oc- The growth of Upper Jurassic reefs was largely restricted cur below a zone of mixed “lithistid’‘-hexactinellid growth. to, or strongly facilitated by, rising global or regional This distribution mirrors differences in nutrient conditions, sealevel, reducing sediment influx occurring during 4th or !jth with coral facies related to stable, moderately oligotrophic to order transgressive pulses within the window of 3rd order mesotrophic conditions whereas siliceous sponges could sealevel rise. Consequently, transgressive/early highstand tolerate fluctuating levels and hence may range from ex- shallow-water reefs are rich in microbolite crusts and highly tremely oligotrophic to strongly mesotrophic settings. This is diverse, whereas the rare late highstand/lowstand coral due to the fact that hexactinellid sponges can largely live on reefs are of low diversity and have little framework pre- osmotrophy and “lithistid” sponges develop deposits of liv- served. In deeper waters, the frequency of sponge-micro- ing organic matter by hosting a huge mass of bacteria. bolite mounds is correlated with the sealevel development. Microbolite crusts demand strongly reduced sedimenta- Together with basin configuration climatic and oceano- tion and are important framework contributors in many coral graphic response to sealevel rise account for a predisposi- and sponge reefs. Eutrophication or oxygen depletion may tion for eutrophication and oxygen depletion particularly exclude reef fauna, giving rise to pure microbolite reefs. around the lberian Peninsula, giving rise to the occasional Most Upper Jurassic reefs developed in ramp settings. occurrence of pure microbolite reefs or repetitive succes- High-energy reefs contain little preserved framework, sions of coral-microbolite to pure microbolite reefs at fairly whereas low-energy reefs may have excellently preserved shallow water depths. The origin of Upper Jurassic reefs can only be unraveled by taking the paleogeographical, structural and sequence J., NEUWEILER, F. 8 GUNKEL, F. (eds., 1996): stratigraphic framework into account. On the other hand, Global and Regional Controls on Biogenic Sedimentation. the lateral and bathymetric distribution patterns of reef types I. Reef Evolution. Research Reports. - from the Late Jurassic provide valuable tools for the better Gcttinger Arb. Geol. Palaont., Sb2, 227-248, Gottingen Paleoecology of Late Jurassic reefs dome shaped to conical growth bands Fig. 2: The influence of sedimentation rate and water depth on the morphology MicrosoIenof a agariciformis (atier NOSE 1995) understanding of shelf dynamics and climate of this epoch more thorough discussion of selected aspects. Additional at a regional and even global scale. papers and monographs are in preparation. 1 Introduction 2 Paleobiology of Upper Jurassic Reef Organisms Reefs grew widespread during the Late Jurassic. Most Paleobiology of organisms in general can be deduced were situated along the margins of the Tethys and its shelf from the fossilized remains, which are the product of ge- seas but also developed in some parts of the North Atlantic netically fixed constructional characteristics specific for a rifl systems and its adjacent epicontinental seas (central certain organism (intrinsic factors, such as given micro- Portugal, northern Germany, southern England) as well as structure and other general bauplan elements) and a flexi- in the Pacific realm (e.g. southern Argentina, Japan). Along ble reaction or adaptation to environmental conditions or the Tethys they were particularly frequent at its northern their change (extrinsic factors). The latter might be unrav- marginal seas. We analyzed, in a comparative approach, eled by well fixed or flexible morphology and growth rates, the belt-like reef region at the tectonically stable northern or isotopic signatures. margin of the Tethys (focusing on S-Germany, N-Switzer- land, NE-France, NE-Spain and S-Portugal) as well as the 2.1 Corals from the Late Jurassic reefs from the strongly differentiated and tectonically active Atlantic rift (Lusitanian Basin, west-central Portugal) and The ecological demands of hermatypic scleractinian cor- adjacent marginal seas (SW-France). Despite all differ- als from the Late Jurassic are similar to those of modern ences in the settings, a common feature of most regions is zooxanthellate scleractinians, although some significant the development of an Upper Jurassic, large-scale shal- differences exist. In the Upper Jurassic, the associated fa- lowing-upward succession, into which reefs are more or ties and fauna (e.g. oolites, green algae) as well as the less frequently intercalated at different positions. paleogeographical occurrence point to shallow, warm-water Reefs comprise very variable types and communi- environments in which hermatypic scleractinians grew and ties of coral reefs, siliceous sponge reefs and microbolite formed reefs. Nevertheless, the low latitude reef belt was reefs. These reefs were most widespread during the Ox- probably wider than today, since remarkable shallow-water fordian and, in some basins, during the Kimmeridgian, coral reefs were also described from the Neuqukn Basin in whereas they grew only locally during the Tithonian. Our Argentina (50“ paleolatitude) 1991), Northern main scope was to identify the factors responsible for the Germany (39’ paleolatitude) 1993a) and the Ox- occurrence and differences of Late Jurassic reefs, and to fordian of England (40’ paleolatitude) 1984). This indi- discuss their mutual interdependence and interaction in a cates a more equilibrated greenhouse-type climate during geological and paleogeographical framework. Our interpre- the Upper Jurassic (LEINFELDER 1993a, 1994a, LEINFELDER tation is based on qualitative and semi-quantitative paleo- et al. 1993a). ecological fauna1 analysis of reefal and non-reefal commu- In contrast to modern corals Upper Jurassic scleractini- nities (constructional morphology, taxonomy and population ans frequently occur in siliciclastic environments. Beside dynamics of reef organisms), on the sedimentology of reefs forms adapted to elevated sedimentation already by their (microfacies, petrography, geochemistry), as well as on phaceloid growth (e.g. Calamophylliopsis) highly morpho- their regional geological framework (biostratigraphy, se- variable taxa are able to cope quence stratigraphy, field mapping, incorporation of pre- with elevated sedimentation rates (Fig. 2). The latter exhibit existing data on paleogeography and basin tectonics). a change to “pseudobranching” morphotypes with non- This paper summarizes results of a six-year research enveloping growth bands indicative of elevated sediment project on reefs from the Late Jurassic which was per- input (cf. JAMES & BOURQUE 1992, NOSE 1995) and a strong formed by the Stuttgart reef group in the course of the Pri- increase in growth rate (up to 10 mm/yr). The growth rates ority Program “Global and Regional Controls on Biogenic of Upper Jurassic scleractinians from the investigated Sedimentation - Reef-Evolution”