marine oxygenation lithistid sponges and the early history of paleozoic skeletal reefs abstract microbial carbonates were major components of early paleozoic reefs until coral stromatoporoid bryozoan reefs appeared in the mid ordovician microbial reefs were augmented by archaeocyath sponges for 15 myr in the early cambrian by lithistid sponges for the remaining 25 myr of the cambrian and then by lithistid calathiid and pulchrilaminid sponges for the first 25 myr of the ordovician the factors responsible for mid late cambrian microbial lithistid sponge reef dominance remain unclear although oxygen increase appears to have significantly contributed to the early cambrian explosion of marine animal life it was followed by a prolonged period dominated by greenhouse conditions as sea level rose and co2 increased the mid late cambrian was unusually warm and these elevated temperatures can be expected to have lowered oxygen solubility and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia greenhouse conditions would also have stimulated carbonate platform development locally further limiting shallow water circulation low marine oxygenation has been linked to episodic extinctions of phytoplankton trilobites and other metazoans during the mid late cambrian we propose that this tendency to dysoxia hypoxia in shallow marine environments also limited many metazoan reef builders in contrast during the mid late cambrian the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs these conditions ameliorated during the ordovician as temperature decline promoted ocean ventilation the prolonged time gap occupied by low diversity reefs between the cambrian explosion and the great ordovician biodiversification event reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments

structure and composition of organic reefs and carbonate mud mounds concepts and categories defined here as essentially in place calcareous deposits created by sessile organisms organic reefs are diverse and complex structures with a long geological history their classification has been the subject of fierce debate often characterized by reliance on subjective features such as wave resistance and qualitative attempts to discriminate between first and second class reefs in contrast emphasis is here placed on the objective characteristic of the type of sedimentary support which largely determines the sedimentary composition of the deposit constructional and depositional processes result in three principal sedimentary components matrix m essentially in place skeletons s and cavity cement c whose proportions can be represented on msc triangular plots separately or together these components also provide the structural support for the reef on these compositional and structural bases three main categories of organic reef are recognized 1 matrix supported reefs agglutinated microbial reefs cluster reefs segment reefs 2 skeleton supported reefs frame reefs 3 cement supported reefs cement reefs agglutinated microbial reefs possess laminated clotted or aphanitic fabrics created by microbial trapping of particulate sediment in place skeletons and large primary cavities are rare early cementation may provide added support topographic relief is limited by the need for currents to provide sediment to accreting surfaces cluster reefs skeletal reefs in which essentially in place skeletons are adjacent but not in contact resulting in matrix support characterized by relatively high matrix skeleton ratios and low volumes of extra skeletal early cement sediment trapping is an important corollary of skeletal growth and cluster reef organisms are tolerant of loose sediment absence of framework limits the topographic relief that cluster reefs can attain relative to spatial extent and may permit bedding to develop within the reef close cluster reefs have skeletons up to 1 unit distance apart spaced cluster reefs have skeletons more than 1 and up to 2 unit distances apart with increasing separation of skeletons they grade to level bottom communities segment reefs matrix supported reefs in which skeletons are adjacent and may be in contact but are mostly disarticulated and mainly parauthochtonous matrix abundance is high and early cement relatively low moderate relief can develop in response to intense on reef sediment production frame reefs skeletal reefs in which essentially in place skeletons including calcified microbes are in contact characterized by relatively high skeleton matrix ratio skeletal support enables them to raise themselves above the substrate independently of cementation and particulate sedimentation simultaneously by creating partly open shelter cavities skeletal support may facilitate early cementation both relief and early lithification promote marginal talus formation skeletal shape and orientation distinguish conical stick like dendritic domical and laminar frames each of these may be open or filled open frame reefs

influence of dissolved oxygen on secular patterns of marine microbial carbonate abundance during the past 490 myr abstract shallow marine benthic microbial carbonate sediments declined episodically during the phanerozoic they were relatively abundant during the cambrian mid ordovician late devonian mississippian mid permian mid late triassic and to a lesser degree in the late jurassic early cretaceous they were least abundant when invertebrate especially reef diversity was relatively high in the mid ordovician late devonian early mid permian late triassic mid late jurassic and early cretaceous to present day these intervals of microbial carbonate abundance and invertebrate diversity generally alternate transitionally with one another but lows and periods of overlap also occur lows when benthic microbial carbonate abundance and invertebrate diversity both declined are apparent during the pennsylvanian and early mid jurassic overlaps when microbial carbonates and invertebrates were both relatively common occur in the early cambrian and late jurassic overall these patterns broadly support suggestions that microbial carbonate abundance declined as calcified invertebrates increased to further explore these relationships we compared microbial carbonate abundance and invertebrate diversity with estimates of changes in sea surface temperature and marine dissolved oxygen for the past 490 myr this analysis suggests that invertebrate diversity varied directly with oxygen availability invertebrate diversity was low when dissolved oxygen was low in the early ordovician late devonian early triassic and early mid jurassic it appears that by reducing invertebrate diversity low oxygen levels favored microbial carbonates at the same time low oxygen levels likely stimulated anaerobic metabolisms favoring carbonate precipitation this study suggests that dissolved oxygen was a major control on secular patterns of marine microbial carbonate abundance during the phanerozoic together with marine temperature and carbonate saturation state

geochemical characteristics of water and sediment from the dal lake kashmir himalaya constraints on weathering and anthropogenic activity two hundred and forty water samples in four seasons and seventeen sediment samples have been analyzed to monitor the natural and anthropogenic influences on the water and sediment chemistry of the dal lake kashmir himalaya the scatter diagrams ca mg total cations tz ca mg hco3 ca mg hco3 so4 na k tz ca mg na k and the geological map of the study area suggest predominance of carbonate and silicate weathering lower ph and high total dissolved solids electrical conductivity and text no text 3 values in the gagribal basin and in some patches of other basins reflect anthropogenic inputs in the form of sewage from surrounding population houseboats hotels etc the dal lake is characterized by high chemical index of alteration cia 87 95 reflecting extreme weathering of the catchment area relative to the average carbonates the lakebed sediments are enriched in al ti zn cu and co and depleted in ni and mn compared to the post archean shale the sediments have higher al zn and cu contents and lower ni and co there are distinct positive anomalies of al mn zn and cu and negative anomalies of ni and pb with respect to the upper continental crust geoaccumulation index igeo and the us environmental protection agency sediment quality standards indicate that the gagribal basin and some patches of the nagin basin are polluted with respect to zn cu and pb these data suggest that the dal lake is characterized by differential natural and anthropogenic influences

molecular records of microbialites following the end permian mass extinction in chongyang hubei province south china abstract by using gas chromatograph gc and gc mass spectrometry a series of biomarkers were identified in the aliphatic fractions of extracts from microbialites following the end permian faunal mass extinction in chongyang section hubei province south china the dominance of lower molecular weight n alkanes in the samples from the section suggests that algae and bacteria were the dominant contributors to the organic fraction preserved in this marine section with bacteria particularly enhanced within the stromatolite the end permian biotic crisis coincided with a short term suppression of autotrophs indicated by the ratio of pristane pr and phytane ph to c 17 and c 18 n alkanes and proliferation of heterotrophs shown by the ratio of branched to normal c 17 alkanes the biotic crisis also may have been associated with a transient oxygenation event in the shallow marine indicated by elevated pr ph ratios and lowered c 27 18 h 22 29 30 trisnorneohopane ts to c 27 17 h 22 29 30 trisnorhopane tm and gammacerane to c 31 homohopane c 31 hp ratios subsequently three short term episodes of intensified anoxia or enhanced salinity at chongyang are recorded by decreases in pr ph ratios in conjunction with lithological boundaries in the section

early ordovician reef construction in anhui province south china a geobiological transition from microbial to metazoan dominant reefs the lower ordovician records a transition from microbial to metazoan dominant reefs immediately prior to the great ordovician biodiversification event the hunghuayuan formation late tremadocian early floian of the jianxin section in anhui province south china includes well preserved reefs which consist of a combination of microbial boundstones and microbial lithistid sponge receptaculitid boundstones the microbial boundstones are characteristically made up of nodular or columnar stromatolites produced by an irregular alternation of girvanella rich layers and thick micrite layers in which the filamentous calcimicrobe girvanella and bioclasts occur sporadically microbial biofilms including the filamentous microbes may have trapped and bound lime mud and or precipitated micrite during their growth and metabolic activities these activities contemporaneously contributed to substrate stabilisation encrustation and the construction of microbialite frameworks stromatolites in contrast the microbial lithistid sponge receptaculitid boundstones are characterised by more micritic constituents than skeletal reef building constituents thin and or domal crusts produced by peloid and dense micrite together with various proportions of girvanella occur upon bioclastic sediments and reef building skeletal organisms these micritic crusts with girvanella therefore played roles as binders and stabilisers and as consolidators by encrusting reef building skeletal organisms in the boundstones it is remarkable that the degradation of lithistid sponges due to microbial decay produced micrite in the sponge bodies as a result lithistid sponges played an apparently inconspicuous but potentially significant role as micrite producers as well as automicrite framework constructors these lower ordovician reefs in which microbialites e g stromatolites dominate but coexist with metazoans e g sponges were widespread at the time they provide excellent examples for understanding reefal palaeoecology during the transition from microbe dominant reefs to reefs enriched in metazoans

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

dissecting calathium microbial frameworks the significance of calathids for the middle ordovician reefs in the tarim basin northwestern china abstract as an obconical macrofossil with porous double wall calathium was commonly present in reefs of early to early middle ordovician age the calathium bearing reefs thrived globally during the early ordovician but this ecosystem collapsed in middle ordovician a rare case of calathium microbial reefs was found from the middle part of the yijianfang formation darriwilian late middle ordovician of the bachu area located in the northwestern margin of the tarim basin northwestern china surrounded by bioclastic grainstones patch reefs here are well developed and vary in size with 1 4 m in thickness and 2 18 m in diameter three facies types are distinguished within the reef limestones 1 calathium microbial framestone 2 echinoderm calathium bafflestone and 3 bryozoan microbial bindstone as a dominant type the calathium microbial framestone shows a three dimensional skeletal framework that is mainly constructed by calathium and stabilized by microbialites although most specimens are toppled calathium displays well developed lateral outgrowths which connected individuals of the same species morphological characters of calathium in thin sections show that calathids are hypercalcified sponges rather than receptaculitid algae unlike the early ordovician lithistid sponge calathium reefs the tarim reefs studied herein contain very few lithistid sponges instead bryozoans are fairly common and act as the most important non microbial encrusters attaching to the walls of calathium noticeably the co occurrence of pelmatozoans and calathium is similar to those reported from late ordovician calathids echinoderm reef communities from tennessee the presence of calathium microbial reefs in tarim indicates that locally calathids were major reef builders at least before the latest middle ordovician

the ecology of lower cambrian buildups from zuune arts mongolia implications for early metazoan reef evolution reefal buildups in western mongolia of mid early cambrian late atdabanian age flourished during a period when shelf seas were globally widespread the succession at zuune arts records the transition from shallow marine siliciclastic sediments bayan gol formation to shallow marine but still clastic influenced carbonates salaany gol formation the salaany gol formation is interpreted as having been deposited as a series of shallowing upwards cycles on a shallow gently inclined shelf in a rapidly subsiding epicontinental sea cycles commenced with the growth of subtidal metazoan calcimicrobe aggregative communities on an open shelf the resultant buildups were commonly engulfed by extensive massive microbial stromatolites when they grew in agitated intertidal conditions latterly they were smothered by ooid shoals in response to rapid sea level rise four types of reefal buildup are distinguished 1 green coloured calcimicrobe tarthinia epiphyton gordonophyton and renalcis boundstones 2 red or green coloured cambrocyathellus tarthinia epiphyton bafflestones 3 red coloured okulitchicyathus bindstones and 4 red coloured radiocyath archaeocyath cribricyath bioherms each is interpreted as having grown at increasing depths and possibly sedimentation rates the buildups are commonly enclosed within graded and planar bedded bioclastic grainstones and packstones and are best developed towards the top of the formation when sea level was high thickets of large solitary archaeocyaths are also inferred in the deeper interbiohermal areas these buildups had synoptic relief and constructed porous structures with growth framework cavities housing diverse coelobiontic communities extensive synsedimentary cements are present including pseudomorphed aragonitic fans and possible pseudomorphed aragontic botryoids these early reefs thus have geological fabrics similar to later phanerozoic representatives it is proposed however that this ecosystem was largely composed of generalist and opportunistic filter and suspension feeders which were dependent upon a far higher input of nutrients than modern day reefal developments bacteria were probably the main primary producers from both planktic and benthic cyanobacterial communities the diversity of each buildup assemblage appears to be controlled by primary cavity size the richest fauna belonging to the highly tiered radiocyath dominated community inferred to have lived in the deepest waters the communities at zuune arts can be compared with other buildups from the early cambrian and with lower ordovician receptaculid calcimicrobe solitary sponge bioherms known from the usa and siberia

tightening the siberian connection to western laurentia newly available geological and geophysical data tighten the proterozoic connection between the rifted margins of the northern siberian craton and western laurentia and permit a siberia laurentia australia troika with northern australia connected to the southern margin of the siberian craton the continental assembly is linked by a prominent 2 0 1 8 ga collisional belt and the ca 1 3 1 0 ga grenville orogen the reconstruction also aligns a 1 5 1 45 ga dike sill swarm that extends from the wyoming province through the belt purcell basin into the northern siberian craton separation of australia and siberia may have occurred by the early neoproterozoic but separation of siberia and laurentia may not have been completed until the early cambrian continental extension began along the zigzag siberia laurentia rift zone in the early mesoproterozoic rift related igneous and sedimentary assemblages dated at ca 1 5 1 38 and 1 2 to 1 0 ga correlate across the reconstructed siberia laurentia rift zone renewed rifting in the neoproterozoic and vendian culminated with seafloor spreading and thermal subsidence of the conjugate rift shoulders correlative archeocyathan reefs endemic olenellid trilobite fauna and exchange of detritus between the cratons imply that the rift may have remained relatively narrow until the atdabanian stage black sulfidic shales buried the archeocyathan reefs on the rapidly subsiding rift margins during the botomian sinsk event

the middle late cambrian reef transition and related geological events a review and new view abstract the reefs of the middle late cambrian cambrian epoch 3 furongian have long been simply considered as microbialites that flourished in the aftermath of the archaeocyath extinction thorough review of the reefs in these periods shows however that the cambrian epoch 3 and the furongian actually yield different types of reefs the cambrian epoch 3 reefs are dominated by thrombolites and dendrolites largely constructed by the calcified microbes epiphyton and renalcis on the other hand the furongian reefs consist mainly of maze like maceriate reefs and columnar stromatolites the maceriate reefs most likely formed by siliceous sponges and calcified microbes including girvanella and tarthinia whereas the columnar stromatolites were mainly constructed by girvanella tarthinia and minor siliceous sponges other microbial reefs e g non columnar stromatolites persisted during the cambrian epoch 3 and the furongian mainly as small patch reefs or reefal crusts lithistid sponge microbial reefs initially formed in the cambrian epoch 3 and occurred throughout the furongian but occupied only a minor portion during these periods several geological events occurred across the boundary between the cambrian epoch 3 and the furongian including positive carbon and sulfur isotope excursions a eustatic sea level drop major faunal turnover of trilobites and diversification of new organisms the coincidence of these events and the transition in reefal fabrics suggest that reef ecosystems were significantly influenced by at least some of these events among the events it is likely that sea level change would have affected reefs although further studies are required in order to test whether and how the other events influenced the reef transition epiphyton and renalcis and thrombolites and dendrolites formed by them declined greatly most likely due to sea level fall and thus decreased in shallow marine habitat at the end of cambrian epoch 3 the cambrian epoch 3 thrombolites and dendrolites were gradually replaced by a sponge microbial association of the maze like maceriate reefs and the columnar stromatolites in the furongian coincident with a sea level rise

microbial metazoan reefs of the terminal proterozoic nama group c 550 543 ma namibia thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal proterozoic nama group c 550 543 ma of central and southern namibia the reefs form integral parts of several carbonate platforms within the nama group including the kuibis platform of the northern nama basin zaris subbasin and huns platform witputs subbasin of the southern nama basin the reefs are composed of both thrombolites and stromatolites that form laterally continuous biostromes isolated patch reefs and isolated pinnacle reefs ranging in scale from a metre to several kilometres in width in the majority of cases the reefs occur stratigraphically as an integral facies within the transgressive systems tracts of sequences making up the kuibis and huns platforms this suggests that a regime of increasing accommodation was required to form well developed reefs though reefs also occur sporadically in highstand systems tract settings within a given transgressive systems tract a regime of increasing accommodation through time favours the transition from sheet like biostromal geometries to more isolated patch and pinnacle biohermal geometries similarly increasing accommodation in space such as a transect down depositional dip shows a similar transition from more sheet like geometries in updip positions to more isolated geometries in downdip positions reefal facies consist of thrombolitic domes columns and mounds with well developed internal clotted textures in addition to stromatolitic domes columns and mounds with crudely to moderately well developed internal lamination stromatolites are better developed in conditions of relatively low accommodation and updip locations under conditions of higher current velocities and greater sediment influx thrombolites are better developed in conditions of relatively high accommodation and low sediment influx both types of microbialites are intimately associated with the first calcifying metazoan organisms which may have attached themselves to the sediment surface or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats the appearance of thrombolitic textures during terminal proterozoic time is consistent with colonization of cyanobacterial mats by higher algae and metazoans which would have been an important process in generating clotted textures fabrics in the nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine carbonate interpreted as former aragonite this was followed by emplacement of geopetal micrite fills and precipitation of dolomite as an isopachous rim cement followed by occlusion of remaining porosity by blocky calcite spar

reconstruction of palaeo depositional environment in north eastern arabian sea a sediment gravity core recovered from the north eastern arabian sea was investigated for sediment grain size total organic carbon toc and selected trace metals to understand the changes in the depositional environment over time the core exhibited three distinct zones of sediment the lower zone i middle zone ii and upper zone iii representing the varying conditions of sediment deposition the lower zone is dominated by the silt sized fraction with low organic carbon that revealed a shallow depositional environment and led to oxidation of organic matter while the low metal concentration in this zone was either due to the low intensity of monsoon or the dilution by biogenic components and aeolian sediment influx on other hand the middle zone represented a transition phase where metal along with clay and organic carbon concentration started increasing due to strengthening monsoon intensity further the upper zone that represented the holocene sediments pointed to the increase in concentration of metals al fe co and cu that indicated an increase in the intensity of the south west monsoon and led to large fluvial inputs toc also increased towards the surface indicating an increase in productivity that was controlled by south west monsoon

early recovery of sponge framework reefs after cambrian archaeocyath extinction zhangxia formation early cambrian series 3 shandong north china abstract reefs dominated by the anthaspidellid sponge rankenella zhangxianensis the calcimicrobe epiphyton and the stem group cnidarian cambroctoconus orientalis together with encrusting microstromatolites occur early in the middle cambrian series 3 late stage 5 of shandong eastern china in the zhangxia formation these in situ components created a tight framework with centimeter scale growth cavities mainly filled by fine grained matrix among them r zhangxianensis and c orientalis mutually attached and locally formed metazoan dominated frameworks these metazoan microbial reefs form thin lenses 505 ma these rankenella reefs indicate recovery of framework building metazoans within 5 million years of the archaeocyath reef decline in structure they resemble archaeocyath reefs as well as early ordovician lithistid sponge microbial reefs having conjoined conical macroskeletons thickly veneered by calcimicrobes and microbial carbonate with largely matrix filled intervening cavities in combination with other sponge microbial reefs reported from australia iran korea and the usa they demonstrate that an anthaspidellid sponge microbial reef consortium was widespread throughout the mid late cambrian

incipiently drowned facies within a cyclic peritidal ramp sequence early ordovician chepultepec interval virginia appalachians the chepultepec interval 145 to 260 m thick in virginia although dominated by peritidal carbonates contains thick subtidal sequences that formed during repeated incipient drowning of the early ordovician ramp cyclic peritidal sequences make up the lower member up to 150 m thick and upper member up to 85 m thick subtidal limestone and bioherms make up the middle member up to 110 m thick and pass landward into cyclic peritidal facies average long term subsidence rates were up to 4 cm 1 000 yr mature passive margin rates and ramp gradients commonly were extremely low 2 cm km computer modeling helped define mechanisms involved in formation of the stratigraphic sequence the peritidal sequences are upward shallowing cycles average period 140 000 to 200 000 yr of pellet and skeletal limestone pellet grainstone thrombolites intraclast grainstone and laminite caps these resulted from periodic sea level oscillations of several metres amplitude and possible slow long teim third order sea level fall superimposed on long term subsidence tidal flats were shifted far westward after each sea level rise but had sufficient time to prograde back across the ramp during sea level fall the middle number contains repeated sequences of subtidal skeletal limestone and pellet limestone with storm beds and burrowed units formed below fair weather wave base local rippled pellet limestone and thrombolite bioherms subwave base to wave agitated shoals that may have erosional tops overlain by thin grainstone sheets tidal flat facies are absent these sequences formed by periodic sea level rise more than 12 to 17 m amplitude which caused incipient drowning of the platform and migration of tidal flats several hundred kilometres back onto the platform these flats were unable to prograde onto the outer ramp during regression because of insufficient time and because seaward retreat of the shoreline outpaced progradation of tidal flats this left them abandoned on the inner ramp similar sequences reflecting incipient drowning are widespread in the geological record

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

lithistid sponge microbial reefs nevada usa filling the late cambrian reef gap abstract cambrian ordovician sponge microbial mounds in the great basin of the western usa reveal reef structure and composition immediately prior to the great ordovician biodiversification event gobe here we describe lithistid sponge microbial reefs from the upper cambrian furongian stage 10 strata of the arrow canyon range nevada the reefs are mound like structures up to 1 to 2 m high and a few meters wide that consist of an unidentified thin walled bowl shaped anthaspidellid sponge columnar microstromatolite fabric and the calcified microbe angusticellularia the reefs formed in low energy subtidal environments in which lime mud filled spongocoels and inter reef spaces around undisturbed in place thin walled sponges the reefs colonized stable substrates provided by oolitic and bioclastic grainstone shoals the mutually attached lithistid sponges form the main framework of the reefs these thin walled and bowl shaped lithistids most likely were adapted to low energy environments spaces beneath the overhanging sponge walls were filled by microbial carbonates these include pendent micro dendritic angusticellularia attached to dermal sponge surfaces and upward growing masses of microstromatolites after death the lithistid spongocoels were mainly filled by micritic sediment that hosted soft bodied burrowing organisms and keratose like sponges these lithistid sponge microbial reefs together with an earlier example of late cambrian paibian dendrolite lithistid reefs in the same area characterize skeletal microbial reefs immediately prior to the gobe

structure and diversity of oldest sponge microbe reefs lower cambrian aldan river siberia the oldest sponge reef is a small early cambrian bioherm at the base of the tommotian stage 535 540 ma in southeast siberia the mainly archaeocyath construction may be a response to turbid conditions cambrocyathellus bowls fused to create a rigid cavernous frame colonized by cryptic archaeolynthus and calcified microbes renalcis in addition to these constructors and binders other reef guilds are present bafflers other archaeocyaths spiculate sponges and hyoliths and dwellers hyoliths mollusks and many others this is the oldest known reef possessing an open skeletal frame structure built by animals and a mixed animal autotroph composition it provides a blueprint for younger phanerozoic reefs

p t evolution of abukuma metamorphic rocks in north east japan metamorphic evidence for oceanic crust obduction the abukuma metamorphic terrane of northeast japan is well known for the type terrains of andalusite sillimanite progressive metamorphism of miyashiro s classic studies 1958 1961 later however relict kyanite was found in the high grade pelitic gneisses kano kuroda 1968 and there have been arguments about whether the abukuma metamorphism was plurifacial or polymetamorphic earlier kyanite sillimanite type and later andalusitesiuimanite type since 1984 we have been studying the geology and petrology of the abukuma metamorphic rocks to reveal their p t t paths and the tectonic evolution of the terrane the purpose of this paper is to outline our new insight into the p t t paths of rocks and their correlation with deformation events of the terrane detailed geology mineralogy and petrology will be presented elsewhere in this paper mineral abbreviations of kretz 1983 are used to represent compositions and reactions of minerals geological and petrological setting the abukuma metamorphic terrane is one of the major geological units of north east japan which is bounded from south west japan by the tanakura tectonic line fig 1 the abukuma metamorphic rocks which are extensively intruded by the cretaceous plutonic rocks shibata uchiumi 1983 shibata tanaka 1987 are lithologically divided into two the gosaisyo series to the north east and the takanuki series to the south west fig 1 the gosaisyo series is composed mainly of metabasites and metacherts with subordinate amounts of meta ultrabasites metapelites and limestones having affinities with oceanic crust original structures and textures however are poorly preserved owing to intense deformation hiroi et al 1987 reported

the pre cretaceous deep seated tectonics of the abukuma massif and its place in the structural framework of japan abstract the structures and microstructures of the takanuki and hitachi areas in the abukuma massif northeast japan are described in the takanuki area the basic gosaisho series thrusts the pelitic takanuki ones in a hp metamorphic context the nappe structure is afterwards refolded by a migmatitic dome the samegawa dome in a ht metamorphic context microtectonic analysis shows that the nappe was transported from south to north along the stretching lineation geometric features suggest that the samegawa dome was emplaced by diapirism the role of the thrust surface as an instable interface promoting the doming is emphasized the hitachi metamorphic rocks composed of basic schist limestone and sandstone shist thrust the pelitic rocks of the western hitachi gneisses as for the takanuki area the thrusting occurred in ductile synmetamorphic conditions with a north or northeastward displacement owing to lithologic petrologic structural similitudes the nappe of the hitachi metamorphic rocks and that of the gosaisho series are unified into a unique nappe with a northward motion the emplacement occurred between late permian and late cretaceous likely in late jurassic the allochthonous units of the abukuma massif are correlated with the green schist nappe described in southwest japan since they are surrounded by the same zones namely the tanba zone and the kurosegawa kitakami one moreover both in southwest and northeast japan the emplacement of the green schist nappes is due to a shear deformation inducing rotational structures along the stretching lineation indicating the same sense of transport that is eastward in southwest japan and northward in northeast japan owing to the late bending of the japanese islands the late jurassic nappe structure is obliquely overprinted by a ht metamorphism ryoke in southwest japan abukuma in northeast japan and afterwards cut by late faults as the median tectonic line or the tanakura fault giving rise to the present complexity

upper devonian frasnian stromatactis bearing mud mounds western alberta canada reef framework dominated by peloidal microcrystalline calcite

a middle ordovician darriwilian calathium reef complex on the carbonate ramp of the northwestern tarim block northwest china a sedimentological approach this study was supported by national natural science foundation of china 41072002 xdb10010503 and 41521061

upper silurian lithistid sponge reefs on somerset island arctic canada the upper ludlow douro formation contains the first reported silurian sponge reefs these relatively small 5 35 m diameter mound shaped structures contain on average 35 lithistid demosponges reefs are surrounded by irregular haloes of crinoid debris abundance and diversity of all fossil groups decreases away from the reefs each reef is underlain by a lens of crinoid wackestone to grainstone rich in crinoid holdfasts trepostomate bryozoans solenoporacean algae and rhynchonellid brachiopods are locally common the bulk of each reef consists of lime mudstone with abundant lithistid sponges this is capped by a thin layer of wackestone with abundant tabulate and rugose corals and fewer lithistid sponges calcareous algae trepostomate bryozoans and stromatoporoids this zonation in which a sponge colonization community was replaced by a coral diversification community is similar to that reported from some middle ordovician upper jurassic and holocene sponge reefs the douro sponge reefs were relatively low structures with about 3 m maximum topographic relief they grew on a broad carbonate platform probably in warm tranquil turbid waters of normal or near normal marine salinity periodic influxes of terrigenous mud adversely affected reef size and caused biotic changes some of the reef lime mud was derived from non reef sources but significant quantities were also produced on the reefs reefs underwent synsedimentary lithification bioerosion and minor storm erosion fabrics and compositions of sparry calcite in cavities record three generations of meteoric cementation originally siliceous spicules of the lithistid sponges were dissolved and the moulds later filled with sparry calcite early dissolution of siliceous spicules is common in reef environments and may have caused fossil sponges to be under represented in ancient reefs

biostratigraphy of the vendian cambrian sukharikha river section northwestern siberian platform the sukharikha river section contains more than 800 m of fossiliferous vendian and lower cambrian carbonate rock deposited in ramp shelf and slope environments a diverse fauna of small shelly fossils calcibionts brachiopods trilobites and archaeocyaths has allowed us to develop a multi taxa biostratigraphic framework for this section a dearth of distinctive fossils low in the sukharikha formation prevents us from determining the position of the vendian cambrian boundary abundant small shelly fossils and archaeocyaths in the uppermost sukharikha formation and low in the krasnoporog formation provide ample biostratigraphic control near the base of the tommotian stage but the nemakit daldynian tommotian boundary as defined at ulakhan sulugur on the aldan river is temporally ambiguous for this reason there is no precise definition of this boundary in the sukharikha river section we have provisionally placed the base of the tommotian stage at the first occurrence of nochoroicyathus sunnaginicus

middle late cambrian rankenella girvanella reefs of the mila formation northern iran following the collapse of the early cambrian archaeocyathan calcimicrobial reef consortium the middle late cambrian furongian was an interval dominated by purely microbial dendrolite and stromatolite reefs however among these latter a few exceptional occurrences of metazoan reefs are known one such reef complex occurs in the late middle early late cambrian furongian portion of the mila formation of northern iran in the otherwise low energy interval of this formation the anthaspidellid demosponge rankenella hamdii sp nov is associated with encrusting girvanella eocrinoid plates rhynchonelliformean brachiopod valves and subordinate hyoliths and trilobites in tempestite shell beds these beds underwent synsedimentary cementation on the seafloor to form hardgrounds in the succeeding higher energy interval a complex of metre scale bioherms and or taphoherms incorporates toppled or transported rankenella hamdii in association with brachiopods echinoderm plates trilobites and some red arg

filamentous microfossils in the early proterozoic transvaal supergroup their morphology significance and paleoenvironmental setting abstract well preserved filamentous microfossils siphonophycus transvaalensis n sp are described here from the carbonate campbellrand subgroup to iron formation kuruman iron formation transition of the transvaal supergroup south africa estimated to be 2 5 2 3 ga years old the microfossils occur in petrographic thin sections of a core sample of carbonate chert they are preserved by permineralization in both chert and in sparry ferroan dolomite stratigraphically the fossiliferous core sample occurs as part of an upward transition from a stromatolitic dolomite and limestone sequence campbellrand to the overlying iron rich sediments of the kuruman griquatown iron formations the average 13 c value of the kerogen in the sample is about 36 9 the microfossils are filamentous unbranched tubular to somewhat flattened 15 25 m in diameter and a few to many hundred microns in length they exhibit a coriaceous finely granular external surface texture resulting from the presence of adhering randomly distributed fine mineralic carbonate needles in salient morphological characteristics they are comparable to the tubular originally polysaccharide encompassing sheaths of extant oscillatoriacean cyanobacteria in comparison with previously described precambrian microfossils these fossil filaments are unusual because of their preservation in sparry carbonate in addition to chert their relatively large diameter and their coating by adhering precipitated carbonate needles they appear to be among the oldest assured microfossils now known from proterozoic age sediments the microfossils are interwoven occurring in subparallel aggregates that form a stromatolitic mat like fabric they are considered to be of an endogenetic in situ benthic origin occurring at the proximal margin of a deep shelf environment at the front distal margin of the campbellrand carbonate platform the water depth for this environment at the break in slope between deep shelf and euxinic basin is estimated to have been of the order of 40 45 m paleomagnetic data support our interpretation that the micro organisms inhabited a warm water marine environment probably at low latitudes

the fabrics and origins of peloids immediately after the end permian extinction guizhou province south china abstract peloids are major contributors to modern and ancient limestones although their origins are not yet comprehensively understood upper permian to lower triassic carbonate successions are well preserved at the bangeng area in guizhou province south china where microbialites thrombolites coccoidal microbes and peloids are preserved together these peloids are classified into three types peloid a b c that typically occur together along with coccoidal microbes and restricted bioclasts in a variety of microenvironments peloid a is most common and exhibits close relationship with coccoidal microbes peloid a1 has spherical grain and a size range typical for coccoidal microbes 10 60 m in diameter whereas peloid a2 is marked by larger spherical and subrounded grains with diffuse margins ranging from 70 to 200 m in diameter peloid a1 is in part produced by the complete micritic filling in coccoidal microbes by their metabolic activities whereas peloid a2 is related to the calcification of a colony of coccoidal microbes by their metabolic activities or simply to the aggregation of individual peloids and coccoidal microbes peloids of this type might also originate from other carbonate precipitation processes peloid b is derived merely by micrite filling in ostracodes and gastropods skeletons whereas peloid c is of micritized bioclast origin these peloidal origins do not explain those of all environments and ages peloids are polygenetic in origin irrespective of the presence of coccoidal microbes however it is emphasized that the peloids formed after the end permian extinction are dominated by microbes related ones those kinds of peloids might have occurred repeatedly and predominantly especially in severely deteriorated environments such as those after the end permian mass extinction

was phanerozoic reef history controlled by the distribution of non enzymatically secreted reef carbonates microbial carbonate and biologically induced cement throughout most of the phanerozoic reef rigidity resulted as much or more from early lithification by microbial carbonates and biologically induced cements non enzymatic carbonates than from biological encrustation of or by large enzymatically secreted metazoan skeletons reef framework is divided into four categories 1 skeletal metazoan 2 non skeletal microbialite stromatolite and thrombolite 3 calcimicrobe and 4 biocementstone in which small or delicate organisms serve as scaffolds for rigid cement crusts the last three categories are dominated by non enzymatic carbonates skeletal framework and non skeletal microbialite framework were the most abundant framework types through the phanerozoic the composition and abundance of skeletal framework was controlled largely by mass extinction events but most reefs consisted of both microbialite and skeletal organisms in a mutually beneficial relationship microbialite framework was abundant throughout the palaeozoic and early mesozoic but declined after the jurassic calcimicrobe framework was important during the cambrian early ordovician and devonian and biocementstone framework was important from the late mississippian to the late triassic the phanerozoic history of reefs does not correlate well with the stratigraphic distribution of large skeletal reef builders or with a variety of physicochemical parameters including sea level history wilson cycle or global climate cycles because non enzymatic carbonates result from induction by non obligate calcifiers and not enzymatic precipitation by obligate calcifiers the distribution of these carbonates was controlled to a larger extent by temporal changes in physicochemical parameters affecting the saturation state of sea water with respect to carbonate minerals changes in pco ca mg ratios cation concentrations and temperature may have affected the abundance of non enzymatic carbonates and hence reefs independently from the effects of these same parameters and mass extinction events on skeletal reef biota the decline in abundance of reefal microbialite and absence of calcimicrobe and biocementstone reef framework after the jurassic may be a result of relatively low saturation states of sea water owing to increased removal and sequestration of finite marine carbonate resources by calcareous plankton since the jurassic reef history is difficult to correlate with temporal changes in specific global parameters because these parameters affect skeletal biota and biologically induced carbonate precipitation independently hence reef history was regulated not just by skeletal reef biota but by parameters governing non enzymatic carbonates

comments on the paper the pre cretaceous deep seated tectonics of the abukuma massif and its place in the structural framework of japan by m faure f lalevee y gusokujima j t iiyama and j p cadet commentaire sur l article de m faure f lalevee y gusokujima j t iiyama et j p cadet paru dans earth planet sci letters nld 1986 77 384 398

an upper ordovician sponge bearing micritic limestone and implication for early palaeozoic carbonate successions abstract a potentially new type of non reef sponge bearing micritic limestone is reported from the upper ordovician xiazhen formation of south eastern china the sponges are preserved as incomplete skeletons that consist of curved bifurcated and trifurcated spicules embedded in dark micrite and can only be recognized under a petrographic microscope the characteristics of the spicule networks suggest that the sponges are probably belonging to demosponges however based on the absence of features such as desma zygome a distinct dermal layer and a canal system they are not considered to be lithistids the majority of the sponges are found in a lime mudstone facies together with some micritic portions of wackestone to grainstone facies comprising approximately 13 of the 50 m thick micritic limestone successions it is interpreted that the non lithistid demosponges flourished on soft substrates over shallow marine carbonate platform such sponge bearing carbonates are similar to spiculites and spongolites in terms of being a major constituent of the sedimentary rocks and their potential contribution as sediment producers but affinity and modes of preservation of the xiazhen sponges are significantly different to those of the spiculites and spongolites in light of the present finding it is suggested that non lithistid demosponges may have been more widespread in early palaeozoic non reef carbonates than has previously been recognized thus indicating the critical need for further detailed studies if we are to understand their distributions and sedimentological contributions

distinguishing coral reef facies from coral bearing open platform facies examples from ordovician ordos basin northwest china abstract fossil coral communities are often simply considered as coral reefs in fact coral bearing communities are not necessarily coral reefs the key criterion is whether the corals are in situ and if they are abundant enough to form topographic relief two outcropped profiles in the western margin of the ordos basin northwestern china drill cores of the lianglitag formation from 4 wells in the tarim basin and 500 large thin sections were examined in this study to reveal the difference between coral reefs and coral bearing carbonate open platform facies in upper ordivician carbonate succession the second layer of the shijiezigou section is massive in internal structure mound like in shape 5 0 m in height and 33 6 m in width and is interpreted as a patch reef in an open platform this layer has a variety of corals that are preserved in situ reef dwelling organisms include dasycladaceans crinoids and gastropods other layers of the section are bedded limestones many of which bear corals and crinoids while others bear dasycladaceans or only the dasycladacea genus vermiporella unlike in the reef the corals in the open platform facies are sparse and generally occur as fragments most of which are not preserved in situ according to the dominant species four communities are distinguished from the shijiezigou section namely the vermiporella community coral community crinoid coral community and dasycladacean crinoid community each of which adapted to some special environmental conditions the patch reef of the shijiezigou section contain abundant and diverse tabulate corals which form bafflestones while stromatoporoids are the primary builders in the platform margin reefs of well tz 822 and the lijiapo section the common biotic components of the open platform of the shijiezigou section and well tz 822 are crinoids and dasycladaceans the restricted platform of the shijiezigou section is dominated by the dasycladacean vermiporella while the organisms of the restricted platform facies in well tz 35 are mainly composed of cyanobacteria ostracods and tetradium like organisms

crouching shells hidden sponges unusual late ordovician cavities containing sponges abstract marine cavities harbouring cryptic organisms have been ubiquitous throughout the phanerozoic however our knowledge of early cryptic communities is as yet insufficient and how metazoans began to utilize such habitats remains unknown in this study we document demosponge remains within intraskeletal cavities embedded in the micritic succession of a shallow carbonate platform in the upper ordovician katian xiazhen formation of south china molluscs gastropods bivalves and nautiloids and corals the solitary rugosan tryplasma and colonial agetolitids within the succession commonly contain patches of spicular demosponge remains 11 n 45 415 mainly occupying intraskeletal spaces with areas of 1 30 mm2 in thin section sponge occurrence varies according to sedimentary facies within lime mudstone facies sponges commonly occur both inside and outside intraskeletal cavities suggesting that sponges would have inhabited and become preserved within any available space in this environment in contrast when other sessile organisms co occur in wackestone to packstone facies there are fewer sponge occurrences both inside and outside cavities possibly due to competition in open habitats and or their poorer preservation in such environments overall this result suggests that sponges would have exploited cryptic habitats by normal expansion of the open surface biota in addition compared with coeval reef and hardground crypts the xiazhen intraskeletal cryptic biota is monotonous in composition suggesting decoupled occupation of cryptic habitats in different environments

untangling intricate microbial sponge frameworks the contributions of sponges to early ordovician reefs abstract the early ordovician witnessed an increased influence of skeletal organisms on the construction of microbial dominant reefs which preceded the middle to late ordovician expansion of skeletal dominant reefs skeletal frameworks of early ordovician reefs built by stromatoporoids bryozoans pelmatozoans and calathiids have recently been reported in this study we document the occurrence of localized archaeoscyphia frameworks and additional unidentified non anthaspidellid spiculate sponges that played diverse roles in the construction of microbial siliceous sponge reefs preserved in the early ordovician dumugol formation korea which have not been previously described in coeval reefs elsewhere two types of boundstone textures are recognized microbial dominated boundstone and sponge microbial boundstone microbialites were primarily responsible for reef construction making up about one third of reefs both as framework builders and as encrusters of other reef components the archaeoscyphia and spiculate sponges contributed in various ways to reef development archaeoscyphia constituting about one fourth of the dumugol reefs mainly constructed sponge microbial boundstone together with encrusting microbialites clusters of archaeoscyphia individuals sporadically formed centimeter sized sponge framestone 3 of reefs with primary cryptic spaces spiculate sponges 4 of reefs a previously unknown component of early ordovician reefs also played diverse roles such as encrusting other constituents dwelling in cryptic spaces stabilizing reef flank sediments and even building frameworks such framework building siliceous sponges together with previously reported skeletal frameworks in other early ordovician reefs reflect an early phase of the transition from microbial to skeletal dominated reefs in the early paleozoic

the late early cambrian microbial reefs immediately after the demise of archaeocyathan reefs hunan province south china abstract the archaeocyath bearing reefs that first appeared in south china in the atdabanian were last seen in the lower toyonian e g in the tianheban and jindingshan formations the upper toyonian lower amgan qingxudong formation of hunan province examined in this study includes large sized reefs formed solely by calcimicrobes of epiphyton kordephyton girvanella hedstroemia and renalcis during the middle and upper cambrian small scale stromatolite reefs were formed the qingxudong reefs are therefore interpreted to be the most well developed microbial reefs of the post archaeocyath bearing reefs in the late early cambrian the transition from archaeocyath bearing to purely microbial reefs was caused by a decline in the diversity of metazoans and was accompanied by a reduction in the abundance of reefs possibly related in part to late early cambrian regression the late early cambrian reefs of south china contain a well preserved record of reef succession and provide important insights into the factors that led to the demise of archaeocyath bearing reefs and the subsequent long lasting hindrances to the development of skeletal dominated reefs during the middle and late cambrian

shell structure and distribution of cloudina a potential index fossil for the terminal proterozoic cloudina bearing biosparites and biomicrites in the lower part of the nama group namibia contain a wide morphological diversity of shell fragments that can all be attributed to the two named species c hartmannae and c riemkeae the curved to sinuous tubular shells of cloudina were multi layered each shell layer was 8 to 50 micrometers thick and in the form of a slightly flaring tube with one end open and the other closed growth appears to have been periodic with successive shell layers forming within older layers each added layer was slightly elevated from the previous layer at the proximal end and was asymmetrically placed within the older layer so that only a portion of the new shell layer was fused to the previous layer this type of growth left a relatively large unminerialized area between the shell layers which was often partially or fully occluded by early marine cements the thin shell layers exhibit both plastic and brittle deformation and were likely formed of a rigid caco3 impregnated organic rich material often the shell layers are preferentially dolomitized suggesting an original mineralogy of high magnesian calcite both species in the nama group formed thickets or perhaps bioherms and this sedentary and gregarious habit suggests that cloudina was probably a filter feeding metazoan of at least a cnidarian grade of organization the unusual shell structure of cloudina gives rise to a characteristic suite of taphonomic and diagenetic features that can be used to identify cloudina bearing deposits within the nama group and in other terminal proterozoic deposits around the world species of cloudina occur in limestones from brazil spain china and oman in sequences consistent with a latest proterozoic age assignment in addition supposed lower cambrian pre trilobitic shelly fossils from northwest mexico and the white inyo mountains in california and nevada including sinotubulites nevadatubulus and wyattia are all either closely related to or con generic with cloudina hence it is probable that these outcrops are latest proterozoic in age and that cloudina or cloudina like organisms were widely distributed at that time it is possible moreover to suggest that metazoan biomineralization occurred on a global scale by the latest proterozoic at the same time that evidence for complex multicellularity and locomotion in animals appears in siliciclastic ediacaran rocks in the form of body and trace fossils

storm influenced siliciclastic and carbonate ramp deposits the lower ordovician dumugol formation south korea the dumugol formation lower ordovician in the southern part of the baegunsan syncline south korea contains mixed siliciclastic and carbonate ramp deposits the ramp sediments were frequently influenced by storm events resulting in tempestites of sandstone mudstone couplets bioclastic grainstones to packstones flat pebble conglomerates a skeletal lag layer and laminated calcisiltites all tempestites are characterized by an erosive to sharp base poor grading and a transitional upper boundary the difference in lithology of tempestites appears to have been controlled by the nature of substrates and by proximality for example laminated calcisiltites have developed on the shallow carbonate ramp flat pebble conglomerates are closely associated with nodular limestones on shallow and deep ramps and thin skeletal lag layers from fossiliferous argillaceous sediments formed in a basinal setting the stratigraphic succession of the dumugol formation represents an initial transgression followed by a regression the vertical facies change records the transition from a shallow siliciclastic ramp to a deep carbonate ramp to a basin shallowing to a deep carbonate ramp and to a shallow carbonate ramp storm effects are mostly well preserved in shallow to deep ramp deposits

sequence stratigraphy of the taebaek group cambrian ordovician mideast korea abstract this study focuses on the sequence stratigraphic implications of the taebaek group cambrian ordovician a mixed carbonate siliciclastic sequence ca 1200 m thick in the mideastern part of the korean peninsula the group contains 11 lithologic units formations that can be partitioned into four packages supersequences i iv each package comprises alternation of siliciclastic and carbonate successions corresponding to the second order sequence supersequence formed in a period of 10 100 m y supersequence i jangsan myeonsan myobong and daegi fms represents the initial inundation and subsequent drowning during the late early to middle middle cambrian it experienced the drowning between the middle and late middle cambrian forming type 3 sequence boundary supersequence ii sesong and hwajeol fms was formed by prolonged marine flooding during the late middle and late cambrian it was terminated by a large supply of siliciclastics due to the rejuvenation of the drainage basin and the subsequent shallowing at the cambrian ordovician boundary type 2 sequence boundary supersequence iii dongjeom dumugol and makgol fms comprises the lowstand coarse grained siliciclastics transgressive shale by marine flooding and shallow platform carbonates by highstand progradation in the early ordovician following the large scale eustatic fall of sea level in the earliest middle ordovician supersequence iv jigunsan and duwibong fms formed on the subaerial exposure surface type 1 sequence boundary by resumed marine flooding in the middle ordovician this study provides basis for basin scale correlation with the coeval sequences in northeastern china

a model for the origin and petrogenesis of the red stromatactis limestone of paleozoic carbonate mounds abstract from study of the devonian recifs rouges of belgium and the silurian mounds of the quebec appalachians we propose that deposition of the red stromatactis limestone facies of cambrian devonian carbonate mounds was controlled by sponges and that the red color and stromatactis result from early diagenesis within a few meters below the substrate water interface in the deep marine environment the common presence of sponge body fossils as well as various stages of sponge preservation ranging from easily delineated to indiscernible bodies in the belgian recifs rouges support the conclusion that a large part of the red finely crystalline limestone originated from early cementation of sponge communities or spicule rich organic mats derived from degradation of sponge communities the role of bacteria and other microbes although widely advocated as the main primary builders of carbonate mounds is difficult to assess microbial communities may have contributed to mound accretion as primary builders and or mud producers but their presence cannot be documented we suggest that they may have acted as agents for concurrent sponge putrefaction and early cementation biodiagene is during the sulfate reduction phase under alkaline ph and anoxic conditions we interpret stromatactis as a spar body that resulted from early marine cementation of a cavity network created by excavation of uncemented material in partly indurated decaying sponges and spicule rich organic mats derived from degradation of sponge communities through circulation of interstitial water in the uppermost few meters of the sediment this cavity network was connected to sea floor and flushed by oxic waters that changed the diagenetic environment from anoxic to oxic converting amorphous iron sulfide to hematite and giving the facies its red color recognition of the dominant role of sponge communities in the construction of red stromatactis limestone mounds of the cambrian devonian time interval has important implications for the geological record this community was the main deep water mound building community during this time but is difficult to recognize because of its variable but often poor preservation

ordovician reef systems and settings in south china before the late ordovician mass extinction abstract ordovician reefs of the south china block occur chiefly in three stratigraphic units 1 the middle tremadoc fenhsiang formation and the upper part of the lunshan formation 2 the late tremadoc hunghuayuan formation and 3 the middle ashgill xiazhen sanjushan and daduhe formations the region therefore records part of the early palaeozoic reef expansion episode and permits a broader assessment of change in ordovician reef facies during middle tremadoc time lithistid sponge bryozoan calathium calcimicrobially dominated patch reefs of the fenhsiang formation occur in the high energy belt of the central yangtze platform columnar nonskeletal stromatolites of the lunshan formation coeval with fenhsiang formation occur on the southeastern margin of the platform where low diversity reef attached organisms and the generally fine grained character of the sediment are consistent with a depth below normal wave base calathium and lithistid sponges were the principal reef builders of late tremadoc reefs and were widespread in the areas of yichang hubei province and dongzhi anhui province from the platform centre to its margin settings bryozoan reef builders occur only in the platform centre with a high diversity of reef dwellers such as brachiopods trilobites and nautiloids however during the same time at the platform margins microbes played an important part in reef building together with calathium and lithistid sponges the middle late tremadoc lithistid sponge bryozoan calathium microbial community was replaced by a middle ashgill coral stromatoporoid community and shows that the succession of community replacement during the ordovician took a relatively long time restricted by the black shales of the main part of the yangtze region middle ashgill reef complexes can be found only on the northeast platform of cathaysian land between the yushan jiangxi and changshan zhejiang regions carbonate mudmounds are present on the western margin of the yangtze platform on the northeast platform of cathaysian land patch reefs some higher relief with talus and biostromes of the xiazhen formation consist of high diversity biotas of corals stromatoporoids calcimicrobes brachiopods and gastropods the sanjushan formation is age equivalent to the xiazhen formation and contains carbonate mudmounds that are composed of abundant calcareous algae and calcimicrobia uplift forced a northward extension of cathaysian land and caused a regional relative sea level fall eliminating this reef complex prior to the first extinction event of the late ordovician the daduhe formation carbonate mudmounds occur in the nearshore belt of kangdian land western margin of the yangtze platform and are paraconformably overlain by the hirnantian nancheng formation in total the ordovician reefs of south china show a range of habitats and controls on growth and demise

trace element distributions in aquatic sediments of danang hoian area vietnam distribution of the trace elements cr cu ni pb and zn in surficial sediments of the river sea environment in danang hoian area vietnam was investigated to examine the degree of metal pollution caused by anthropogenic activities point sources from domestic and industrial wastes are identified as dominant contributors of trace element accumulation surficial sediments of hoian river show extremely high total concentrations of cu average concentration 295 g g ni ac 112 g g pb ac 396 g g and zn ac 429 g g that exceed assigned safety levels er m similarly the sediments of han river show high pb ac 188 g g and zn ac 282 g g contents in marine sediments of thanhbinh beach pb is also enriched 138 g g above guideline levels in contrast the sediments of the cude river are dominated by trace element concentrations close to background values

the hypothetical mesoproterozoic supercontinent columbia implications of the siberian west laurentian connection abstract the siberian west laurentian connection is a postulated reconstruction of a proterozoic craton that formed between 1800 ma and 1500 ma correlation of geological trends and of rift facies between the siberian platform and western laurentia indicates that although the two cratons underwent significant intracontinental rifting in mesoproterozoic and neoproterozoic times they did not undergo complete separation until early cambrian ca 530 ma although this interpretation does not support the hypothetical columbia supercontinent model of rogers 2000 and rogers and santosh 2002 some elements of it are compatible with that model

questioning the microbial origin of automicrite in ordovician calathid demosponge carbonate mounds calathid demosponge carbonate mounds are a feature of early to middle ordovician shallow marine carbonate depositional environments of tropical to subtropical palaeolatitudes these mounds contain an important amount of autochthonous non skeletal microcrystalline calcium carbonate automicrite conventionally considered microbial in origin here the automicrite of calathid demosponge carbonate mounds tarim basin north west china is broken down into five distinct fabrics an in situ peloidal spiculiferous fabric am 1 an in situ peloidal fabric am 2 an aphanitic microtubular fabric am 3 a minipeloidal fabric am 4 and a laminoid cerebroid fabric am 5 type am 1 occurs with am 2 being succeeded by an assemblage of am 3 and am 4 types am 4 and am 5 are separated by an erosional disconformity a good correlation of fluorescence and cathodoluminescence of automicrites indicates that induced and supported organomineralization produced automicrite probably via the permineralization of non living organic substrates adsorbing dissolved metal humate complexes using a spreadsheet with six parameters and 17 characters am 1 to am 4 turn out to be non microbial in origin instead these automicrites represent relics of calcified metazoan tissues such as siliceous sponges non spiculate sponges or the basal attachment structures of stalked invertebrates fabric am 5 is a microbial carbonate but is post mound in origin forming a drape within a reefal framework established by am 4 the five automicritic fabrics individually or as an assemblage are a common element of ordovician calathid demosponge carbonate mounds in general the reassessment of the origins of these automicritic fabrics holds consequences for understanding of the great ordovician biodiversification event in terms of community structure reef ecology and reef evolution episodically these fabrics are also present in other carbonate build ups stretching from the neoproterozoic over the entire phanerozoic eon the massive calcification of metazoan soft tissue am 1 to am 4 characterizes episodes and conditions of enhanced marine calcification and might be of value to refine secular trends of pco2 ca concentration and mg ca ratio at the scale of individual sedimentary basins

latest middle cambrian metazoan reef from northern iran middle and late cambrian reefs were built mainly by cyanobacterial communities a few reefs with a metazoan as well as an algal component however are known from this interval a middle cambrian reef formed primarily by spicular demosponges is described here from the mila formation in the elburz mountains northern iran the reef is enclosed within calcareous grainstones which contain terminal middle cambrian late mayan trilobites the mila formation reef was constructed by sponges of the family anthaspidellidae and bacterial algal sheaths and is the earliest metazoan reef to be documented from the interval after the demise of archaeocyath sponges the reefal community is typical of subsequent reefal communities of early middle ordovician age the ordovician examples differ only by the incorporation of additional metazoan elements

cambrian series 3 carbonate platform of korea dominated by microbial sponge reefs abstract metazoans have been considered as negligible components of cambrian series 3 and furongian microbial dominated reefs in contrast to their presence in earlier terreneuvian cambrian series 2 microbial archaeocyath reefs however recent discoveries of sponges in cambrian series 3 furongian reefs of australia china iran usa and korea have raised question regarding their contribution in terms of carbonate platform development which have never been assessed this study examines cambrian series 3 deposits of the daegi formation korea to elucidate this question the 100 m thick middle part of the daegi formation is dominated by boundstone facies which occupies 45 of the study interval as well as bioclastic wackestone to packstone bioclastic grainstone and ooid packstone to grainstone facies the daegi reefs are primarily thrombolitic in composition with 90 n 26 29 of the reefs containing an average of 9 sponges in aerial percentage calculated from thin sections lithistid sponges composed of peloidal fabrics some desma spicules and spicule networks commonly occupy the interstitial space in microbial clusters are encrusted by mesoclots and epiphyton and are surrounded by micrite subordinate non lithistid demosponges occur within clusters of microbial elements the middle daegi formation can be largely subdivided into shoal environment dominated by grainstone to packstone facies and shallow subtidal platform interior environment located behind shoal with wackestone to packstone facies the microbial sponge reefs mainly developed around platform interior as patch reefs the current study indicates that metazoans in the form of lithistid and non lithistid demosponges are nearly ubiquitously incorporated in daegi reefs and contributed greatly to the formation of microbial sponge reefs as well as carbonate platform during the time study of these microbial sponge reefs and their distribution within the carbonate platform may help us to understand how carbonate sedimentary environments responded to the extinction of archaeocyaths

middle cambrian siliceous sponge calcimicrobe buildups daegi formation korea metazoan buildup constituents in the aftermath of the early cambrian extinction event abstract numerous decimetre to metre scale carbonate buildups dominated by siliceous sponges and the calcimicrobe epiphyton are reported from the middle cambrian series 3 daegi formation of korea these siliceous sponge epiphyton buildups consist predominantly of grey micritic boundstones with dark clots and or white clumps the boundstones contain sponge spicule networks interpreted as the calcified remains of siliceous sponges the white clumps and dark clots in the boundstones represent variously preserved epiphyton siliceous sponges form constructional pore space and are commonly encrusted by epiphyton the sponges were probably the primary frame builders providing substrates for the attachment and subsequent growth of epiphyton epiphyton is considered to be a binder when covering the surface of siliceous sponges and a subordinate frame builder when filling depositional voids created by siliceous sponges or growing on top of other epiphyton growth bundles the siliceous sponge epiphyton buildups of the daegi formation show similarities to previously described late cambrian furongian anthaspidellid calcimicrobe buildups from iran and the usa together with recently reported examples from the zhangxia formation of eastern china the sponge epiphyton buildups from korea represent some of the oldest metazoan calcimicrobe buildups after the extinction of most archaeocyaths at the end of the early cambrian series 2 this implies that the incorporation of metazoans in middle cambrian series 3 carbonate buildups occurred much earlier than previously known the buildup forming siliceous sponges described in this study demonstrate that their role in early phanerozoic carbonate buildups has been grossly underestimated

a new middle ordovician bivalve siliceous sponge microbe reef building consortium from north china abstract a new reef building consortium from a middle ordovician succession of the western north china platform is described consisting mainly of bivalves siliceous sponges and microbial carbonates epiphyton like renalcis like and girvanella calcified microbes and microcrystalline microstromatolites in addition to minor brachiopods amsassia and rhabdotetradium the bivalves are thin walled and mostly articulated indicating in situ preservation the siliceous sponges are characterized by regularly spaced spicule networks embedded within micrite which partly grade into peloidal textures three main types of bivalve sponge associations are found 1 larger bivalves 2 13 mm encrusted by sponges 2 sponges occupying internal spaces of larger bivalves and 3 smaller 0 2 0 4 mm bivalves embedded within sponge spicule networks microbial carbonates either cover the upper surfaces of siliceous sponges and bivalves or occur independently as centimeter scale patches the reefal boundstones were constructed mainly by bivalves siliceous sponges and microbes which were subsequently encrusted and stabilized by additional sponges and microbes extensive early marine cementation forming fibrous cement helped stabilize the reef framework the co occurrence of bivalves and siliceous sponges is possibly analogous to modern day counterparts in which sponges encrust bivalves or bivalves are living within sponges suggesting a symbiotic relationship the bivalve siliceous sponge microbial reefs of this study together with other ordovician reefs represent the changeover from microbial to skeletal dominated reefs during the middle ordovician the current example may represent an ancestral association of bivalve siliceous sponge microbe reefs similar to those in the mesozoic and cenozoic thus shedding light on the roots of such associations

weathering and anthropogenic influences on the water and sediment chemistry of wular lake kashmir himalaya this paper presents a study on the wular lake which is the largest fresh water tectonic lake of kashmir valley india one hundred and ninety six 196 water samples and hundred 100 sediment samples n 296 have been collected to assess the weathering and anthropogenic impact on water and sediment chemistry of the lake the results showed a significant seasonal variability in average concentration of major ions being highest in summer and spring and lower in winter and autumn seasons the study revealed that lake water is alkaline in nature characterised by medium total dissolved solids and electrical conductivity the concentration of the major ion towards the lake central showed a decreasing trend from the shore line the order of major cations and anions was ca2 mg2 na k and hco3 so4 2 cl respectively the geochemical processes suggested that the chemical composition lake water is mostly influenced by the lithology of the basin carbonates silicates and sulphates which had played a significant role in modifying the hydrogeochemical facies in the form of ca hco3 mg hco3 and hybrid type chemical index of alteration values of wular lake sediments reflect moderate weathering of the catchment area compared to upper continental crust and the post archean shale the sediments have higher si ti mg and ca contents and lower al fe na k p zn pb ni cu content geoaccumulation index igeo and us environmental protection agency sediment quality standards indicated that there is no pollution effect of heavy metals zn mn pb ni and co the study also suggested that wular lake is characterised by both natural and anthropogenic influences

heat capacities and entropies of andalusite and sillimanite the influence of fibrolitization on the phase diagram of the al 2 sio 5 polymorphs

famennian mud mounds in the proximal fore reef slope canning basin western australia abstract famennian late devonian carbonate buildups and in particular mud mounds are poorly known in general and few have been documented in detail relatively small famennian mud mounds occur in proximal fore reef slope settings in the canning basin western australia the famennian platform margin facies passes from typical shoaling carbonate facies in the back reef through massive calcimicrobial cement rich reef margin facies to relatively steeply dipping 20 30 well bedded fore reef slope facies containing shelf derived winnowed grainy sediments and extremely coarse reef block debris isolated or coalescing mounds occur in the proximal slope immediately adjacent to and in some cases possibly grading into the margin facies mounds are elongate perpendicular to the margin and some had synoptic relief greater than 2 m mounds are lithologically variable and consist of varying proportions of micrite multiple generations of marine cement abundant rothpletzella renalcis poorly preserved sparry microbial crusts and sporadically distributed laminar stromatoporoids surrounding grainy slope facies abut and slope off of mound flanks mound facies are very similar to nearby reef margin facies with the exceptions that stromatoporoids have not been observed in margin facies and solenoporoid algae which occur in the margin have not been observed in the mounds stromatolites are conspicuously absent from both facies mound facies appear to be more closely related to frasnian and famennian calcimicrobe cement dominated reef margin facies than to famennian deep water stromatolite sponge mound facies such as those that occur elsewhere in the canning basin the observed canning famennian reef and mound frameworks were constructed by communities that appear to be very similar to earlier frasnian communities despite the frasnian famennian extinction event and provide good examples of microbial reef framework construction in a high energy setting

cavity dwelling biota in middle ordovician chazy bryozoan mounds from quebec bryozoan mounds dominated by batostoma chazyensis from the laval formation of the chazy group early middle ordovician near montreal quebec contain framework cavities that house a coelobiontic cavity dwelling biota reports of coelobionts of ordovician age are rare and this is the first report of chazy group mounds containing coelobionts algae sponges and bryozoa batostoma dominate the cavity biota and are found encrusting cavity walls and roofs the algae are represented by mammillary encrustations now micrite and solenpora sp on the roofs the sponges are not identified but occur both as isolated monaxial triaxial and six rayed spicules and as body fossils within the cavities coccoid cells of unknown affinities are also found these cavities provide the first evidence of coelobiontic biological erosion trypanites a compound morphogenus is the only macroborer and is found in cavity walls endolithic boring algae are present and indicate that as early as the middle ordovician b

cryptalgal metazoan bioherms of early ordovician age in the st george group western newfoundland bioherms are common in the st george group a sequence of shallow water carbonate rocks deposited on the western continental shelf of iapetus ocean they range from small heads and metre sized mounds to extensive banks and complexes many metres thick and hundreds of metres in lateral extent the cores of these bioherms are principally composed of thrombolites unlaminated branching columnar stromatolites structures quite distinct from laminated stromatolites which are common in intertidal beds associated with thrombolites is a diverse fauna of burrowing invertebrates trilobites nautiloids pelmatozoans brachiopods gastropods rostroconchs and archaeoscyphiid sponges on the basis of framework building components three main bioherm types are distinguished 1 thrombolite mounds 2 thrombolite lichenaria or sponge mounds and 3 thrombolite lichenaria renalcis reef complexes the framework of the last is the most complex with abundant cavities and a demonstrably uneven growth surface of thrombolites corals and free standing renalcis heads walls and roofs some cavities were active sediment conduits while others were protected their roofs draped with renalcis and their walls coated by cryptalgal laminites these bioherms possess the attributes of shallow water ecologic reefs they span a critical time gap in the development of reefs the transition period from algal dominated bioherms of the precambrian and cambrian to the metazoan dominated bioherms of the middle ordovician and remaining phanerozoic

degradative calcification of a modern siliceous sponge from the great bahama bank the bahamas a guide for interpretation of ancient sponge bearing limestones abstract organic colloids are involved in the early calcification of the modern siliceous sponge spheciospongia vesparium lamarck 1815 from the great bahama bank electron microscopy and in situ fluorescence microspectometry studies indicate that colloids attached within or onto a collagen network promote the precipitation of aragonite crystals in these sponges calcification occurs within those portions of the sponge that are buried in the sediment preferentially in regions of agglutinated sediment particles with sponge connective tissue being subjected to necrosis and significant degradation of the extracellular collagen matrix the dismantling of collagen bundles leads to collagen scaffolds which act as a sorbent and have significant adhesive effects for ions and or organic colloids bacteria and other microorganisms support tissue degradation but neither act as a substrate for aragonite precipitation or are present in significant numbers at calcification sites this process of early calcification may explain the origin of fossil calcified siliceous sponges sponge mummies and tuberoids as well as the occurrence of patchy calcified sponge materials thereby calling into question the commonly accepted idea that pelletoidal texture associated with these fossil sponges indicates that bacteria are directly responsible for the calcification

tetradiid siliceous sponge patch reefs from the xiazhen formation late katian southeast china a new late ordovician reef association decimetre scale domal to tabular patch reefs consisting of dendroid tetradiids are found in the late ordovician late katian xiazhen formation southeast china it is intriguing to observe that siliceous sponges occur together with the tetradiids in many patch reefs as this has not been previously reported the tetradiid rhabdotetradium jiangshanense are preserved in situ and the siliceous sponges where present as spicule networks calcimicrobe ortonella encrusting tetradiid tubes and burrows filled with faecal pellets also constitute as subordinate components of the patch reef the tetradiid tubes are regarded as frame builders that are commonly surrounded and encrusted by the siliceous sponges the sponges wherever observed form the bulk of the patch reef and are regarded as a primary constructor as well as binder of the patch reef burrows filled with faecal pellets often penetrate the area between tetradiids and sponge spicule networks in the boundstone and these are considered to be the loci of dweller where the sponges and tetradiids co occur the tetradiid tubes are sparsely distributed and commonly display non quadripartite fission i e bipartite or tripartite in contrast where the sponges are rare in the patch reef the tetradiid tubes are densely spaced and predominantly display quadripartite fission this indicates that the tetradiid growth was influenced by the presence of siliceous sponges this finding of the new late ordovician reef association and their mutual interplay extend our understanding of the early palaeozoic carbonate reefs it reaffirms the need for multidisciplinary studies of carbonate buildups where similar interactions with siliceous sponges might have been overlooked

paleoenvironments and growth of early neoproterozoic calcimicrobial reefs platformal little dal group northwestern canada abstract although the calcimicrobial reefs of the basinal little dal group 780 ma are well described little is known about related shallow water facies to the southeast study of these platformal carbonates shows that they represent inner mid and shallow outer ramp zones of a homoclinal storm influenced ramp seven depositional sequences are recognized and correlated with reef bearing strata in the little dal basin and in the shaler supergroup of victoria island reef distribution is distinctly depth partitioned across the little dal ramp inner and mid ramp carbonates contain meter scale mounds patch reefs and biostromes composed of fine grained stromatolites including tungussia baicalia and jurusania these reefs are strikingly similar to those reported from coeval shallow water strata in the shaler supergroup of victoria island and the bitter springs formation of australia multi storey stromatolitic biostrome bioherm complexes greater than 100 m in height seem to be restricted to the mid ramp zone of the little dal ramp and probably reflect high rates of subsidence in the mackenzie basin calcimicrobial reefs occur only in the deepest water strata of the study area associated with outer ramp sediments these reefs contain calcified cyanobacterial filaments in varying degrees of preservation and are petrographically indistinguishable from the kilometre scale basinal calcimicrobial reefs to the northwest the association of calcimicrobial reefs with deep low energy environments suggests initial establishment of reef building calcimicrobes in deep water benthic environments

preservation of siliceous sponges in the jurassic of southern england and northern france reef forming lithistid sponges in the middle jurassic bathonian of normandy france were lithified on the sea floor by a peloidal cement probably of an original low mg calcite composition precipitated under bacterial influence this early lithification permitted early dissolution of the siliceous skeleton without the collapse or distortion of the resulting moulds these moulds were encrusted and bored this pathway for the fossilization of sponges is shown by other examples of jurassic lithistids and is likely to be widely applicable throughout the fossil record particularly at times during which calcite was the primary marine precipitate