FACIES 29 41-54 PI. 9-10 12 Figs. -- ERLANGEN 1993 Microbial Carbonate Crusts - a Key to the Environmental Analysis of Fossil Spongiolites ? Helmut Keupp, Berlin, Angela Jenisch, Hamburg, Regina Herrmann, Fritz Neuweiler and Joachim Reitner, Berlin DFG-Schwerpunkl KEYWORDS: MICROBIALITES - SPONGIOLITES - JURASSIC - CRET ACEOUS BIOO~NE SEDIMENTATION SUMMARY Morphological and geochemical comparisons between modern cryptic microbialites from Lizard Island/Great Barrier Reef and fossil counterparts in the Upper Jurassic (Southern Germany, Dobrogea/Romania) and late Lower Cretaceous (Aptian/ Albian from Cantabria/Spain) spongiolitic environments show that there are common factors controlling the crust formations mostly independ- ent of light despite of diverging (paleo-) oceanographic positions as well as relationships of competitors. Factors such as increased alkalinity ,oligotrophy, and reduced allo- chthonous deposition are of major importance. Thrombo- litic microbialites are interpreted as biologically induced and therefore calcified in isotopic equilibrium with the surrounding sea water. Corresponding with shallowing upward cycles, microbial mats which produce stromatolitic peloidal crusts become more important. Different biomarkers are introduced for the first time extracted and analyzed from spongiolitic limes tones ofLower Kimmeridgian age from Southern Germany. 1 INTRODUCTION Throughout the earth's history , spongiotitic time- stones (sensu GEYER, 1962) are characterized by two features, the more or less rich occurrence of siliceous sponges (hexactinellids and lithistid demosponges) pre- served partly or entirely and presumed microbialitic carbonates (="microbialites" sensu BURNE & MOORE 1987). This facies is often dominated by partly thrombolitic, partly peloidal stromatolitic crusts. KEupp et al. (1990: 155) pointed out concerning the synopsis of actual often contradictory discussed knowledges of processes con- trolling the formation ofLate Jurassic spongiolites: "Studies on the formation of siliceous sponge buildups is, to a great extent, an interdiscipli- nary approach and hence must be undertaken by carbonate sedimentologisls, biochemists and palaeontologists. Clarifi- cation of existing hypothetical ..... Rlft-Evolutlon und Krelde-Sedlmenl.tlon models for the fossilization of siliceous sponges and elucida- tion of carbonate crust formation play a key role in this approach." We take up this proposal with an attempt to explain essenti~1 controlling factors of the fossil spongiolitic facies on the base of comparisons between morphological, geochemical, biogeochemical, and biological data of both modern carbonate crusts which are built under more or less known cryptic conditions, and their similar Jurassic and Cre- taceous counterparts. This contribution compares Recent and fossil carbonate crusts to point out corresponding and diverg- ing environmental factors, respectively. REITNER (1993) describes and interprets modern cryptic microbialites which were collected from coral reef caves; they appear to be very similar to both Recent deeper water crusts described by BRACHERT & DULLO (1991) and some fossil crusts from different spongiolitic limestones. In conclusion of the above-me~tioned descriptions of modern cryptic microbialitic crusts lrom Lizard Island, Great Barrier Reef, the following factors controlling their growth ' can be deduced: 1. The modern crusts are restricted to cryptic, dysphotic to aphotic environments controlled by the competition with coralline red algae. 2. The crusts form only on places where the detrital supply is reduced, or where only few detrital components are able to remain on the bottom due to exposed or steep slope position. The establishment of the very slowly growing carbonate crusts seems not be possible on places characterized by higher sedimentation rates due to rapid burial effects. Addresses: Prof. Dr. H. Keupp, R. Herrmann, F. Neuweiler, Doz. Dr. J. Reitner, Institut für Paläontologie, Freie Universität Berlin, Malteserstr. 74-100, Haus D, D-12249 Berlin; Fax: Germany-030-77622070; Dr. A. Jenisch, Institut für Biogeochemie und Meereschemie, Universität Hamburg, Bundesstraße 55, D-20146 Hamburg 42 .i c: o .s::. i= Global Sea I.JMII SpnIadlng 01 Spange facie8 (PoneoI & Val 1991 ) (Geyer & GwInner 1986) 3. The deseribed modem ernsts from St. Croix and Lizard Island are restrieted to islands on whieh erystalline rocks are exposed. Therefore, influenees of weathering and, possibly, also of freshwater which inereases the alkalinity of the sea water, seem to play an important role in ernst formation. 4. The very slow carbonate aeeumulation, whieh is alterating with both leaehing aetivities including inernstations by Fe/ Mn-baeteria and boring aetivities of various organisms, results from different processes: * Trapping of sediment particles by mierobial films. * Mierobialite formation oceurs only uneommonly by caleification of microbes under direct metabolie eontrol. Therefore, the stable isotopes (Ö13C) of the mierobialites whieh are in equilibrium with the surrounding sea water, do not contain an appreciable amount of metabolie 13C (cf. WEFER & BERGER 1991). Metabolie processes, neighter mi- erobial photosynthesis nor respiration seem to play an im- portant role in carbonate eementation. * "Biologieally-indueed" (LOWENSTAM 1981) mieritie eementation of organic sheets, partly under anaerobie eon- ditions. * Inerustation by foraminifers, eoralline sponges, ser- pulids, bryozoans, braehiopods, and other organisms. 2 CARBONATE CRUSTS OF LA TE JURASSIC SPONGIOLITES Many former studies ofLate Jurassie spongiolites, par- tieularly from Southem Germany, deal with the detailed description and classifieation ofmierobial ernsts (e.g. FRITZ lMnId climale Fig. 1. Upper Jurassic Spongiolitic second order cyclicityofSouthemGer- many. The spreading of spongiolitic facies is cor- related with the transgres- sive systems tracts (LEIN- fFLDER eta!. at DFG-round table, Neustadt 1992). ~ -, ._- ._- ._- -.,,' ._- 1958, HUMMEL 1960, HILLER 1964, WAGENPLAST 1972, Nrrzo- POULOS 1974,GWINNER 1976, FLüGEL & SrnGER 1981,GAIl1..ARD 1983, LANG 1989, KOTT 1989). These authors agree that "microbial" - or "algal" -ernsts prineipally are of non-skeletal type eontaining various amounts of enernsting organisms. The so ealled "filamentous algal ernsts" sensu FLüGEL & STEIGER (1981) represent misinterpreted struetureless mieritie mierobialites including bundles of dermal spieules oflithistide demosponges (pI. 9/2). In Southern Germany, the Late Jurassie sponge facies is subjeet to a more or less distinet eyclie development (KEupp et al. 1990): A) The development of two larger seale (eaeh 4-5 million I years) shallowing upward eyles during Oxfordian and Kimmeridgian time (Fig. 1), -and possibly a third one in the Lower Tithonian- apparently eontradiets the global trans- gressive sea level trend 01 AlL et al. 1977). However, the progressive shallowing ean be explained by overeompensa- tion by relatively high carbonate produetions of both the small subsidenee rate of the platform at the passive margin of the Tethyan Oeean and the falling sea level. The first eycle starts with smaller spongiolitie bodies of Middle Oxfordian age (pI. 9/1) which are embedded in marly lime- stones. The facies is eharaeterized by both a dominanee of hexaetinellid sponges including Tremadictyon radicatum (QuENSTEDT) the only deseribed Upper Jurassic species whieh eould anehor itself in soft sediment (MÜllER 1991) and a small amount of - mostly aphanostromatie - carbonate ernsts. In the earliest Upper Oxfordian spongiolites, the bioherms become larger and the amount of carbonate ernsts including gj o ~ ..8~ _.- ..c: '" (.) 0 Oe.. - CI) ';1--0 43 Fig. 2. Main factors controlling the small scale spongiolitie eycIes whieh ean be developed in the dimension of eaeh Upper Jurassie spongiolitie bank. VI detritical bedded facies V Crust-dominated Facies IV Sponge-Crust-Facies with thrombolites and peloidal stromatolites m Sponge-thrombolite-Facies n Growth of siliceous sponges and mummy preservation oc8~ _____ _ Ia Consolidation I Omission, partly reworking steadly more stromatolitic peloidal structures increases (cf. LANG 1989). Above the platynota-marls of earliest Kim- meridgian age the second cycle starts: most of the buildups are now dominated by Iithistid demosponges. All siliceous sponges reported from these buildups need solid substrates for settling. In the stratigraphical lower spongiolites only poor carbonate crusts exist. Above the middle part ofLower Kimmeridgian strata (Malm gamma 2), thrombolitic crusts become important (PI. 9/4). In the Middl, and Late Kimmeridgian, thrombolitic and also stromatolitic crusts often dominate the biohermal and biostromal spongiolites (cf. KOTI 1989, MEYER 1975, SCHORR & KOCH 1985). The associated sediments contain still more high energy struc- tures (resedimentation, grainstones, and ooids (cf. MEYER 1975, WIRSING & KOCH 1986, POMONI-PAPAIOANNOU et a1 . 1989). B) Small scale cycles are reported from mounds and thick- bedded autochthonous limestones ("TreuchtIinger Marmor") by SCHORR & KOCH (1985), MEDER (1989), LANG (1989), KOTI (1989), REHFELD-KIEFER (1992) , and others. These cycles are interpreted by LANG (1989) and KOTI (1989) 10 be orbitally forced (Milancovitch-cycles). The following gen- eralized sequence can be deduced (Fig. 2): Following an omission phase connected with a consoli- dation of the bottom, siliceous sponges grow probably very slowly under oligotrophie conditions. Several modern Fig. 3. Two generations of dense (=aphanie) peloidal mierobialitie erusts have grown under dysphotie eonditions inside a spongoeoel of a toppled silieeous sponge. Their growth eonditions have been interrupted by temporary infilling with detrieal sediment (floatstones); Oxfordian, Dobrogea/Romania hexactinellids and lithistid demospongiae are extreme K- strategists: therr optimal adaption to poor living conditions enabled their survivaI, reproduction and competition with other organisms in a dynamic equilibrium near the minimal capacity . Therefore, they propagate mostly by sexual repro- ductions and are probably temporarely Iiving without any active particle feeding, but together with endosymbiontic chemoautotrophic bacteria. Corresponding with the repro- ductive fitnesss of K-strategists, we can. find in Jurassic spongiolites mostly single sponge individuals originated by sexual reproduction and grew due to settlement of a single larva, while asexual budding only very rarely OCCUfS. • Peloidal crust, first generation • Peloidal crust, aecond generation o Siliceous spange ~Floatstone with litho- and bioelasts m Sparitic cement 44 -lern 1 ;0P~ Peloidal erust _ Dense peloidal erust ~Ml@ Silieeous sponge ~~ Lithoelasts, bioelasts, tubero i ds ~ sparitie eernent Fig. 4. The primary position of peloidal microbialites from Dobro- gea (Oxfordian) below apreexistent lithistid siliceous sponge let presume dysphotic growth conditions (cf. PI. 9/5). The second phase is eharaeterized by sponge-earbonate ernst assemblages. The mostly aphanostromatie, often thrombolitie ernsts eontain different enernsting organisms as weIl as biodetrital eomponents (pI. 9/3). These ernsts resemble a hardground eharaeterized by eoneomitant in- ernstation anqboring aetivities. The upper part of a spongiolitie body is often eharaeterized by a deereasing amount of benthie organisms and an inereasing importanee of a more or less pure mierobialitie facies including both thrombolitie and stromatolitie ernsts. 2.1 Morphology of Upper Jurassic Microbialites All the modified types of autochthonous carbonate produets and their numerous transitional morphotypes eonnected with the Late Jurassie siliceous sponge facies, instruetively figuredin GAILLARD (1983: p. 133, fig.57), ean beredueed to 3 basie types: 1. Mieritie to peloidal mummies preserving the mor- phology and internal struetures of silieeous sponges. Their formation took plaee under anoxie eonditions (see ehapter 2.2) . 2. Thrombolitie and dendritie ernsts rieh in both detrital eomponents and enernsting organisms, partieularly foraminifers (including the enigmatie organism-eommunity Tubiphytes morronensis), polyehaete worms (serpulids and Terebella), eyclostomate bryozoans, eoralline sponges (Neuropora, minehineIlid Calcarea). In general, no distinet internal laminations oeeur, but we often find brownish eoloured iron-rieh films penetrating the ernsts irregularly. Their ealeifieation is eharaeterized by a slow growth rate. It seems to be restrieted to the sediment/surfaee boundary. Some of these thrombolitie mierobialites are eharaeterized by repeated alternations of earbonate aeeumulation and rem oval by leaehing (?) and boring organisms (e.g. Aka: R EITNER & KEupp 1991, lithophagous peleeypods). GYGI (1992: Fig. 18) reported very similar eolumnar mierobialites from Late Jurassie eoral bioherms, whieh permit to draw a parallel between modem eoral facies oceurenees as de- seribed by REITNER and ZANKL (this volume) and theJurassie sponge faeies . 3. Stromatolitie laminar to domal peloidal erusts be- eome more and more important within the higher parts of shallowing upward eycles ofLate Jurassie spongiolites (pI. 9/5-7). On polished slabs these appear milky. Exeept numer- Lipidic Acids ~OOH ~ro~ i- and ai- Fig. 5. Generalized drawing of spe- cific components of biomarkers ex- tracted and analyzed from diffemt Kimmeridgian spongiolitic limestones of Southem Germany and their affin- ity to discrete organisms. Eubacteria Cyanobacteria Archebacteria Eucaryotic organisms Higher Plants Hydrocarbons Hopanoids Hydrocarbons (n-CI7 ) branched Hydrocarbons Squalane Lycopane Steroids Hydrocarbons long-chain Lipidic Acids Di-acids I ~ ~ J.. 1 1 1 ~(CH2) .. ............... ~ (CHz) ................. COOH HOOC~ (CHz) ................. COOH T HYDROCARBONS I branched e17 2 3 Calcified siliceous sponge mununy lbrombolitic crust Fig. 6. Section of gas chromatographie speetra of hydrocarbons showing the aleanes Cl? and ClS eharaeterizing partieularly their origin from eyanobacterians. The eorresponding graphs of differ- ent analyzed spongiolitic faeies elements (1. ealeified lithistid sponge from Middle Kirnmeridgian, SW Treuehtlingenl Franconia; 2. thrombolitie crusts from Lower Kimmeridgian, Core B 110 near Gei'slingenlSwabia; 3. detritic sediment from a Late Kim- meridgian bioherrn near Gosbach/Swabia) prove the homogenous distribution of organie material and does not allow to classify the biomarkers with a distinct facies element. ous peloids, which are presumed to be of in situ origin, only a small content of biodetritus can be observed. Encrnsting and boring organisms can be found only rarely. Each lamina is characterized by a dense micritic coating at its surface and a more or less loose arrangement of peloids below it varying in size. The laUer are fixed by (micro-)sparry calcite ce- ments. The lamination is normally not disturbed or re- worked. Therefore, we assume that the mat calcification is related to self burial processes including the autochthonous formation of peloids below the micobial mat comunity analogous to the Recent anaerobic origin of peloids as described by REITNER (this volume). Apart from the sponge mummies, the other basic types ofUpper Jurassic microbialites, particularly the thrombolites, resemble morphologically the aphotic crnsts of Lizard Is- land and StCroix, respectively. In addition to this morpho- logical congruence, the following further common features can be observed: 45 * The crnsts favour obviously' exposed positions. Due to this observation, photic growth conditions have been postu- lated (cf. WaGENPLAST 1972: 30; GAIIl.ARD 1983: p.I26). However, the exposed positions are not only characterized by optimal light conditions, but also by absence or at least a reduced rate of sediment deposition. * All the desribed types of microbialites inside the spongocoel of toppled sponges as weIl as below large platy sponges (pigs.3-4, PI. 915; GAILLARD, 1983: Fig. 12, p. 125) can be found. Their growth seems tQ proceed under at least dysphotic conditions. Therefore, we can presume a facultative aphotic growth potential of the described mirobialites. With the intention to find hints of the possible primary crnsts producers which are not documented by skeletal or filamental relics, we used biogeochemical and stahle iso- topic methods, in addition to the description of thin sections. 2.2 Biogeochemistry Spongiolitic sampies of different localities and different stratigraphical positions of the S wabian and the Franconian HYDROCARBONS Calcified siliceous sponge mwnmy Ph Pr Ly 1 thrombolitic crust 33 29 sediment Fig. 7. Complete gas chromatographie spectra ofhydrocarbons of the same sampIes as represented in Fig. 6. The pristane (Pr) and phytane (Ph) relationships of sponge mummy give hirlts for anoxie eonditions, while the chlorophyll ofboth other facies types (mierobialite erust and detrital sediment) seem to be altered under more oxie conditions. The biomarkers squalane (Sq) and lycopane (Ly) are derived from anoxic arehebaeterians (arrows). 46 Stromatolitic peloidal crust Upper K1mmeridgian. Gosbach f L, i .. 27 aß 30 aß 29 aß 31 ,., aß aß 32 31 ,... aß ~2 aß 33 Hopanoic Acids Hopanes ~~~'V"'<'\ .. h\. Fig. 8. Mass spectrometric analyses of hopanoids from an LateKimmeridgian stromatolithic peloidal crust of the Gosbach bioherm (Swabia). The hopanoic acids have been analyzed from the fraction of acids, the hopanes from the fraction of hydrocar- bons. All the different series of hopanoic acids and of hopanes derived from the same bacteriohopantetrol during diagenetic proc- esses (oxidation/hydrogenation). Alb have been analyzed by methods of gas chromatography and mass spectrometry. In spite of some methodical prob- lems during the sampie extraction (particularly for sugar analysis) caused by the little C content of the pure lime- stones, characteristic biomarkga'have been found (Fig. 5). Unfortunately, the spectrum of biomarkers within the different types of sampies analyzed from the spongiolitic facies (sponge-mummies, thrombolitic carbonate ernsts, dedrital sediment), particularly of the hydrocarbons, seems to be more or less identical (Figs. 6-7). Therefore, we suggest that the distribution ofbiomarkers may be rather the result of detrital mechanisms than a original in situ signal. Due to this homogenization, differentiated interpretations seem to be questionable because we cannot establish spe- cific markers to distinct facies elements. Moreover. the long chain alcanes n-C25 to n-C33 which can be traced back probably to eucaryotic bigher plants, support the existence of detritic C influx. org. Being aware of the fact that a correlation of the biomarkers with a distinct sediment feature might not be reliable, as de- scribed above, the following interpretations seem to be possible: The different pentacyclic hydrocarbons (hopanes: Fig. 8), diagenetically derived from the Bacteriohopantetrol, mark the influence of non-differentiable eubacterians, wh ich were active during or after deposition of organic mate- rial, while the n-alkanes C I7 and CI8 (Fig. 6), occurring in most of the sampies, are quite typical for cyanobacterians. The hydrocarbons pristane and phytane can be presumed to have originated from chlorophyll (phytol), while pristane derives under oxic conditions and phytane under anoxic conditions. These isoprenoids give hints for photosyn- thetic activities, but we cannot determine their origin from neither cyanobacterians nor detritalland plants. On the other hand, different phytane/pristane-relationships prove probably anoxic conditions of the sponge mum- mies and more oxic conditions for both the thrombolitic crusts and the sediment (Fig. 7). Also, the hydrocarbons squalane and lycopane seem to prove anoxic archebacterian activities within crusts and sediment. In conclusion, there is evidence of activity of different anaerobic and aerobic bacterians including cyanobacterians (cf. LANG 1989). However, their participation in carbonate precipitation, particularly in the formation of thrombolitic and laminated crusts, can not be judged on account of the results of the used biochemical methods unfortunately. 2.3 Stable Isotopes Measurements of stable isotopes (18C and l3C) have been performed on different spongiolitic facies of a Kimmeridgian core drilled near Geislingen/Swabian Alb (borehole 120) and of an outcrop of Oxfordian spongiolitic 13 f ......... I .......... li .. ... I ......... .. j.... f: C i ............ j ............ + ............ ~ ........... " .········· ··i······ ···t I, ............ " ......... ~it.:.:.!".·:.: ... ~.·.:. : . •. · .... 1,. . ·, aa . i - ... :L\························t 2 I ......................... ~ ........... + ....... . . . ( ., ........... ; ...... : t----+--_-+~--t----2-1-' ---11----.. -+1- & 180 Core GeIaIIngen 120 (KInvneridg1an aponglolltes) x MIcrobIaI crusts • Sediment C Dobrogea / Romania: MIcrobIaI CI'U8t8 of Oxfordlan aponglolltes Fig. 9. Uncorrected values of Ö l3e and Ö 180 measurements of primary calcitic Late Jurassic microbialites and of detrital sedi- ment: Kimmeridgian of GeislingenlSwabia (core B 120) and Oxfordian of DobrogeaIRomania. I f I \ \ \ \ \ ---_ ... , 4 \ \ \ IX: Middle Jurassic algal limes tones from Scotland (ANOREWS 1986) ./ 2 VII 47 Fig. 10. The comparative overview of isotope ratios from different modem and fossil primary ca1citic rnirobialites (Up- per Jurassic to Recent) shows the follow- ing pattern (values uncorrected): Group 1: Distribution of stahle isotopes in modem cavity-dwelling marine micro- bialites: I: Mg-calcite of non-lithified mud, Be- +6180 lize (MAclNTYRE 1984) 11: Mg-calcite of micobialite, Belize (MAclNTYRE 1984) I1I: Mg-calcite cements (microbialite, J amaica: MAcINTYRE. & MARSHALL 1988, LANO & GOREAU 1970) IV: Microbialite, Belize Reef (JAMES & GINS BURG 1979) L: Cryptic microbialites from Lizard Is- land (REITNER 1993) Group 2: Crnsts of the Red Sea and of fresh water environments: V: Deeper water calcitic ernsts calci- fied in equilibrium with Red Sea water (STOPFERS & BOTZ 1990) VI: The high positive a 180 values of this field result from aragonitic crust mineralogy (STOPFERS & BOTZ 1990). VIII: Fresh water lake (Sleaford Mere, Fellmongery) caIcitic microbialites (BURNI! & MOORE 1987) Group 3: Deep water ernsts from the Red Sea which consist of diagenetic dolomite and siderite. The light a 13C values are probably linked with anaerobic sulfate reduction within the sediment: VII: Carbonatecrust "groupill" (S'1'OffBRS & BOTZ 1990) Group 4: Microbialites with portions of photosynthetic fractionation pattern char- acterized by partly light a 13C values: X: Middle Jurassie algallimestones from Loch Bay/Scotland (ANOREWS 1986). Group 5: Various fossil microbialites without recognizable photosynthetic organisms. This group i/; depleted in 180, a typical pattern for meteoric diagenetic influences (also seen in group 4). The a 13C patterns fit more ore less with the' modem ones of group 1: J: Upper Jurassic microbialites from core B 120, Geislingen/Swabia (cf. Text-Fig. 9) K: Albian microbialites from Northem Spain (NElJWEILER, this volume) T: Carnian microbialites from Cassian Cipit boulders (Northern Italy) XI: Late Jurassie microbialites from Southern Germany (WIRSING 1988). limestones of Dobrogea/Romania: thrombolites. calcified lithisthid demospongians. sediment surrounding the microbialites, and sediment inside an ammonite test. They all show a narrow range of Öl3C and a spatial variation conceming Ö 180 (Fig. 9). Our data from the Kimmeridgian of Geislingen (Fig. 10: J). particularly of the Ö13C. corre- spond with the published data by WIRSING (1988). The primary calcite precipitation seems to have happened more or less in equilibrium with the sea water analogous to modem microbialites from Lizard Island. We do not have evidence for a considerable fractionation in connection with the lithification of the entire sediment. However, "biological calcification is essentially a disequilibrium process, due to its useofC02 asan importantraw material ... " (McCONNAUGHEY 1989). Typically. carbonate precipitation produced by pho- tosynthetic activities is much faster than anorganic calcifi- cation from the same water. Therefore. we should expect a lighter 13C composition due to leinetie isotope effects. A substantial equilibration occurs when the calcifieation is very slow, or initiated through extraeellular catalysis ofC02 reaetions (McCONNAUGHEY 1989, WEFER & BERGER 1991). The isotopic data of Upper Jurassic spongiolites prove 48 Alkalinity (Eutrophication) Ught Alloc:hthonous deposition Fig. 11. Main factors controlling the Late Jurassie spongiolitic facies in comparison with both the coral facies and the pure microbialite facies at this period. similar cementation processes within a closed pore water system as described by NEUWEILER (1993) from the Albian mounds. Both early lithification of microbial crusts and the dominating micritic sediment characterized by only low permeability are favourable for the preservation of almost primary isotopic sea water composition, also during burial cementation (compare the inverted "J" model of LoHMANN (1988) and discussion by NEUWEILER (this volume)). Ac- cording to investigations of sampies of spongiolitic Late Jurassic limestones from both cores and outcrops, later exchanges of carbon isotopes between pore and/or clevage water masses by meteoric influences are also excluded by WIRSING (1988:281). 2.4 Environmental Iitterpretation The comparison of described Jurassic microbialit fea- tures and growth conditions of similar modem cryptic, mainly cavity dwelling carbonate crusts allow to claim the foUowing processes controlling the Jurassic sponge facies: A. Paleogeographic configuration and its consequences for water chemistry and nutriation: During the Kimmeridgian, the time of the maximal distribution of spongiolitic fades, the position of the pelagic, weakly inclined ramp along the wide passive northem shelf of the Tethys Ocean, far from slope conditioned upwelling systems, produces principally oligotrophie circumstances for spongiolite formation (KEupp et al. 1990). Thesupposedcarbonateprecipitationoftheslowlygrowing aphanostromatic microbialites, which is only biogenetically induced by organic macromolecules (proteins, polysaccharids) and/or biofilms, is in equilibrium with the surrounding sea water, whereas the process could only be induced provided that the alkalinity of the water masses during Upper Jurassie increased. This may be induced (1) by the epicontinental carbonate platform model after KEMPE (1990), which obtained the alkalinity from a water exchange with an an aerobic basin connected with the open- ing of the Atlantic Ocean, (2) by freshwater influences (cf. NeUWElLER, 1993). How- ever, the second modelis improbable because of the large distribution of the spongiolitic facies, its position far from the coast, and its association with a normal marine fauna. B. Global sea level development and its a) sedimentologic consequences b) climatic consequences The successive expansion of the spongiolitic facies in Southem Germany follows exactly the transgressive sys- tems tracts of the global sea level chart (PONSOT & V AlL P I a t e 9 Microbialites of various Late Jurassie spongiolites Fig. 1. Polished seetion through a small spongiolitic buildup (45 x 30 cm) from Late Ox,fordian marls of Streitbergl Northem Frankenalb (coll. T. Steiger, Munich). Microbialites are restricted ,to 'formation of calcareous sponge mummies and few thrombolitic crusts only. Scale bar = 10 cm. Fig. 2. Lithistid sponge surrounded with a thrombolitic microbialite crust incorporating Bundles of dermal scleres. Similar structures were formerly described as "filamentous algal crusts" (FLüGEL & STEIGER 1981) or "Algal Problematicum I...ADWEIN 1976" (BRACHERT 1986). Middle Kimmeridgian, (Treuchtlinger Marmor), Treuchtlingen /Southem Frankenalb. Scale bar = 1 mm. Fig. 3. Mummy of a hexactinellid sponge encrusted with aphanic thrombolite which includes tubes of polychaete worms (arrows) and biodetritus. Late Oxfordian, Urspring/Northem Frankenalb. Scale bar = 1 mm. Fig. 4. Dendroid thrombolites growing on a lithistid sponge. Middle Kimmeridgian (Malm Delta), Kaider/Northem Frankenalb); scale bar = 1 mm. Fig. 5. Densely laminated stromatolitic hemispheroid constructed of peloids which are presumed to be of in situ origin. It grew below a overtumed platy lithistid sponge under dysphotic conditions upon a thrombolitic aggregate rich in Tubiphytes morronensis (arrow). Biostromal spongiolite of Middle Kimmeridgian age. Quarry of the Treuchtlinger Marmorwerke south-west ofTreuchtlingen/ Southem Frankenalb. Scale bar = 1 mm. Figs. 6.-7. Peloidal stromatdlitic microbialites from Oxfordian of Dobrogea/Romania. The various density of lamina- tions result from different growth rates of the microbial mats. Scale bar = 1 mm. Plate 9 49 50 1991) as pointed out by LEINFELDER, KRAUITER & WERNER (1992; Fig. 1). The obviously redueed allochthonous sedi- mentation, whieh is pobably the main faetor to establish both the slowly growing siliceous sponges and the aphano- stromatie thrombolites, may be eaused by the reeeding shoreline. The sponge facies progrades during the Upper Jurassie from deeper slope positions (ef. GYGI & PERSOZ 1986, 1987) to the shallowing platform sueeessively with eaeh transgressive thrust and reaehes its maximum regional distribution during the Middle and Late Kimmeridgian (GWINNER 1976). ent orbital-foreed facies sequenees indieate a similar mode of ehanging in climate eonditions. C. Bathymetry, partieularly its eonsequenees of light pen- etration: The modem microbialitie erusts as described by REITNER (1993) and ZANKL (1993) seem to be restrieted to eavities and other dysphotie habitats owing to the eompetition with the mueh more rapidly growing eoralline red algae. How- ever, photie influenees were without remarkable effeets on erust formation during the Jurassie below the maximum depth of hermatypic eorals (about 50-60 m), due to the still missing eorallinaeeans. Based on the faet that both facies, the spongiolites and the eoral reefs, claim - exeept the light - nearly the same environmental eonditions (oligotrophy, mostly solid substrates, normal marine salinity), we suppose that the sponge facies was generally situated deeper than the eoral facies analogous to the Upper Jurassie sections of Portugal deseribed by LEINFELDER (1992). Therefore, the transition from sponge to eoral faeies in Southem Germany is deseribed in different vertieal shallowing upward sections only (BAUSCH 1963, ALDINGER 1968, GWINNER 1976, MEYER 1977,1978; MEYER & SCHMIDT-KALER 1983). However we Aeeording to LEINFELDER et al. (1992), the climatic eonsequenees of the sea level development may be respon- sible for the 4-5 million year sequenees. Humid climate and redueed water eireulation during the high stands possibly produee an eutrophication of the surfaee water and a de- ereasing ventilation of the bottom water. Subsequently, the oligotrophie benthic fauna disappear, and the more or less pure mierobialitie facies eould be explained. It is dominated by stromatolitie peloidal struetures, which are presumed to have grown more rapidly without remarkable partieipation of other benthie organisms situated predominantly at the upper part of the eycle. The obviously small-sealed, eongru- Plate 10 Figs. 1 -3. Fig. 1. Fig.2. Fig.3. Figs.4.-6. Fig.4. Fig.5. Fig.6. Figs.7.-9. Fig. 7. Fig.8. Fig.9. Comparative representation of similar mirobialites through time: Late Jurassie (left eolumn), Albian (CantabriaIN Spain, middle eolumn) and Reeent (Lizard Island, right eolumn): Mierobialites with eommon eolumar thrombolitie features exhibiting irregular to missing laminations and many included allochems: Oxfordian (Dobrogea/Romania); seale bar = 1 mm. Late Albian (buildup from La Sia, Northem Spain); seale bar = 5 mm. Modem reef eave from Lizard Island; seale bar = 1 mm. Facies eontaets of mierobialites of different ages: Middle Kimmeridgian biostromal limes tone of Petersbueh near EiehstättiSouthem Frankenalb: A badly preserved lithistid sponge (L) is overgrown by thrombolitie mierobialites including aminehinellid ealeareous sponge (M) and "Tubiphytes" (T). The remaining pore spaee of the thrombolitie erust is infilled with stromatolitie peloidal erusts (S). Seale bar = 1 mm. Dense (=aphanie) mieritie/fenestral mierobialite eontaining some specimens of the enerusting foraminifer Placopsilina (P) overgrow a lithistid sponge (L) inside a small eave. Wider spaeedl'ediment binding laminae ) originated from the main hemispheroidal mierobialite strueture (arrow) demonstrate inereasing sediment influx into the remaining spaee.Middle Albian from AndarrosaINorthem Spain; seale bar = 5 mm. Charaeteristie facies sueeession of a Recent reef eave mierobialite from Lizard Island. Its base is eharaeterized by eoralgal struetures. The scleraetinians are represented by Leptoseris types, the eoralline algae by Lithophyllum,Neogoniolithon andPeyssonnelia. The sueeession is formed by aphanic mierobialites includ- ing biodetritital material and enerusting foraminifera, serpulids, bryozoans, eoralline sponges and braehiopodes. It is interrupted by mineralized films of Fe-Mn baeterians. The remaining pore spaee is filled up with Ca- binding organie mueus (M) in whieh detritus is bound. Seale bar = 5 mm. The thrombolitie mierobialites of all the deseribed oceurenees are eharaeterized by poorly laminated struetures that eonsist of mierite associated with detrital grains and various enerusting organisms: Oxfordian mierobialite from Dobrogea/Romania enerusting a silieeous lithistid sponge (L) with enclosed serpulids (S) and foraminifers (F). The dark, limonitie laminae probably eorrespond with the mineralized films of Fe/Mn baeterians (eompare PI. 10/9). Seale bar = 1 mm Detail of the Middle Albian mierobialite of Fig.5 with altemating enerustations of Placopsilina and polyehaete worms. Seale bar = 1 mm. PiIlar-like mierobialite eovered by a film of Fe/Mn baeterians from Lizard Island. It shows the integrated eharaeteristie serpulid-foraminifer assemblages. Seale bar = 1 mm. Plate 10 51 52 do not know both synchroneous facies immediately linked in lateral positions, but only over larger distances. 3 MICROBIALITES OF ALBIAN SPONGIOLITIC MOUNDS OF TUE V ASCOCANT ABRIAN BASIN (NORTUERN SPAIN) Sedimentary, paleontological and geochemical features of Albian microbialites from northem Spain (NEUWEILER, this volume) serve as an additional example in the compara- tive study of calcareous microbialites. Despite of some individual characteristics (basin type, paleogeographic set- ting and phylogenetic trends of involved organisms), micro- and macroscopic structures of microbialites and related deposits are partly very similar to spongiolitic limestones from the Upper J urassic and cryptic microbialites of modem age. The common quality of microbialites is weIl expressed by microbialitic microstructures, stable isotope geochemistry and special chemical environment. 3.1 Microbialite microstructures Some types of microbialite microstructures described by NEUWEILER (this volume) can be transferred to their possible Jurassic counterparts in the following manner: 3.1.1 Dense (=aphanic) micritic/fenestral microbialites forming laterally linked hemispheroids and vertically stacked hemispheroids most probably correspond to thrombolitic and dendritic ernsts as described above. Variations of micro- structure is caused by different rates of sediment supply. Microbialites of this type are potentially stromatolitic, de- fined by the rate of allogenic particle flux. Calcification occurs in situ and is related to the biofilm/water interface. Dense micritic/fenestral microbialites are the main con- tributors to microbialite reef development, situated at mid to lower slope environments of marginal platform areas. Their distribution extends below the photic zone. In contrast, some other Late Albian mounds of Northem Spain seem to be formed similarly to modem environments by a coralgal frame which is penetrated and overgrown by microbialites (REITNER 1987). In situ calcified microbialites induced hardground conditions with boring activities of sponges (Aka) and lithophagous pelecypods. The principal associ- ated organisms are lithistid demosponges, coralline sponges, encrusting foraminifera, and to a minor extent polychaetes, bryozoans, terebratulid, and thecidean brachiopods. 3.1.2 Dense micritic/peloidal microbialites revealing low amplitude laterally linked hemispheroids or subplanar ori- entated growth forms correspond to stromatolitic laminar or domal peloidal ernsts of the Jurassic. Calcification is related to selfburial processes in conjunction with decaying organic matter. Microbialites of this type are restricted to shallow marine conditions with a bathymetric range comparable with the photic zone. They occasionally bind coarse bioclastic debris related to buildup developments in small intraplatform basins or occur as relatively pure peloidal bindstones cover- ing microbialite reefs. 3.2 Geochemistry of microbialites Geochemical results obtained from microprobe analy- ses (Mg-content) and stable isotope geochemistry (ö 13C, Ö 180 ) indicate a primary high Mg-calcite mineralogy of microbial carbonates and precipitation in isotopic equilib- rium with corresponding inorganically precipitated marine calcites. Therefore, major metabolie effects and influences of 12C enriched organie carbon derived from autotrophie microorganisms are regarded as negligable. 3.3 Environmental Interpretation Chemical considerations, i.e. the increased supply of Ca2+, HC03-, alkalimetal and sulfate ions into the marine realm are discussed as the primary control on episodes of in situ calcifying microbial mats (NEUWEILER 1993). This sup- ply is controlled by the fluviatile influx of weathering products (in connection with the terrestrical Utrillas facies), diapir derived brines and paleokarst developments. In addi- tion to increased alkalinity, possible effects of toxic [Ca2+] comprise the increased release of Ca2+ -affine organic films, favouring self burial effects of peloidal microbialites. 4 CONCLUSION Similar morphological features and geochemical data including stable isotope geochemistry prove some corre- sponding controlling mechanisms of Recent cryptic micro- bialites and fossil microbialites associated with spongiolitic limestones of Jurassie and Cretaceous age. However, his- torical facts, particularly phylogenetic reasons and different geological settings, cause essential modifications of the actualistic principle (Fig. 12). Common features: * All calcareous ernsts considered here grew under essen- tially oligotrophie conditions of normal marine salinities. * They contain corresponding sessile assemblages. * Their calcification is possibly controlled by phases of increased alkalinity. * They needed more or less weIl consolidated substrates. * Particularl y the thrombolitic arld dense micritic microbialites represent slowly growing primary Mg-calcite precipitates induced by organie macromole~ules and/or biofilms and are, therefore, independent of light and potentially aphotic to dysphotic. * Reduced allochthonous sedimentation (controlled by sea level changes or regional tectonic factors). * Peloidal calcitic grains associated frequently with the spongiolitic facies are interpreted to be mostly produced in situ under anoxie conditions by self burial processes below the surface of microbial mats. Diverging features: * Paleogeographic and tectonic positions * In modem ernsts light plays an important role, favouring the competition with corallinaceans. However, corallinaceans are lacking or of minor importance, respectively, in fossil communities of Jurassie and Lower Cretaceous age. Photic 2: buIIdupe .. lnIIIUId and CIIII1II'oIIId by mlamtl Ibl 3:nat ..... Diverging factars 1: Regional setting 2: Importance of microbialites 3: CoraIIinacean oompetition 111 Modem reef environment (Uzard lsIand) 11 Alblan mounde (N Spaln) Comma" controlling factprs A; Normal marine conditions B: Increased aIkaIinity C: Oligotrophy 0: Reduced a11oc:hthc:>ncg depoait8 E: C8Icification independent of light F: CaIcification in isotopic aqulibrUn with sea water G: Common sessile asaociations 53 Fig. 12. Comparative synopsis of factors controlling microbialite formations of t1uee selected occurences tIuough time (Late Jurassic, Albian, Recent). conditions for analogous Mesozoic microbialites, particu- larly of J urassic age, became a significant controlling factor, however, limited to water depths above 60-50 m, where hermatypic corals replaced the sponge facies. Below this critical boundary, the microbialites could grow extensively provided that the sedimentation rate was low and the car- bonate saturation sufficient due to increased alkalinity. * In contrast to the known Late Albian and particularly modem counterparts, in which the buildups are initiated by coralgal frames, the similar Upper Jurassic and Albian microbialites described by NEUWElLER (1993) construct and often initially control the buildups. ACKNOWLEDGMENTS The studies are financially supported by the DFG (project Ke 322/10). We thank P. Böttcher, Dr. D. Mehl (Berlin) and particularly Dr. R. 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