Manfred Krautter

The origin of Jurassic reefs: Current research developments and results

SummaryIn order to elucidate the control of local, regional and global factors on occurrence, distribution and character of Jurassic reefs, reefal settings of Mid and Late Jurassic age from southwestern Germany, Iberia and Romania were compared in terms of their sedimentological (including diagenetic), palaeoecological, architectural, stratigraphic and sequential aspects. Upper Jurassic reefs of southern Germany are dominated by siliceous sponge—microbial crust automicritic to allomicritic mounds. During the Oxfordian these form small to large buildups, whereas during the Kimmeridgian they more frequently are but marginal parts of large grain-dominated massive buildups. Diagenesis of sponge facies is largely governed by the original composition and fabric of sediments. The latest Kimmeridgian and Tithonian spongiolite development is locally accompanied by coral facies, forming large reefs on spongiolitic topographic elevations or, more frequently, small meadows and patch reefs within bioclastic to oolitic shoal and apron sediments. New biostratigraphic results indicate a narrower time gap between Swabian and Franconian coral development than previously thought. Palynostratigraphy and mineralostratigraphy partly allow good stratigraphic resolution also in spongiolitic buildups, and even in dolomitised massive limestones.Spongiolite development of the Bajocian and Oxfordian of eastern Spain shares many similarities. They are both dominated by extensive biostromal development which is related to hardground formation during flooding events. The Upper Jurassic siliceous sponge facies from Portugal is more localised, though more differentiated, comprising biostromal, mudmound and sponge-thrombolite as well as frequent mixed coral-sponge facies. The Iberian Upper Jurassic coral facies includes a great variety of coral reef and platform types, a pattern which together with the analysis of coral associations reflects the great variability of reefal environments. Microbial reefs ranging from coralrich to siliceous sponge-bearing to pure thrombolites frequently developed at different water depths. Reef corals even thrived within terrigeneous settings.In eastern Romania, small coral reefs of various types as well as larger siliceous sponge-microbial crust mounds grew contemporaneously during the Oxfordian, occupying different bathymetric positions on a homoclinal ramp.Application of sequence stratigraphic concepts demonstrates that onset or, in other cases, maximum development of reef growth is related to sea level rise (transgressions and early highstand) which caused a reduction in allochthonous sedimentation. The connection of reef development with low background sedimentation is corroborated by the richness of reefs in encrusting organisms, borers and microbial crusts. Microbial crusts and other automicrites can largely contribute to the formation of reef rock during allosedimentary hiatuses. However, many reefs could cope with variable, though reduced, rates of background sedimentation. This is reflected by differences in faunal diversities and the partial dominance of morphologically adapted forms. Besides corals, some sponges and associated brachiopods show distinct morphologies reflecting sedimentation rate and substrate consistency. Bathymetry is another important factor in the determination of reefal composition. Not only a generally deeper position of siliceous sponge facies relative to coral facies, but also further bathymetric differentiation within both facies groups is reflected by changes in the composition, diversity and, partly, morphology of sponges, corals, cementing bivalves and microencrusters.Criteria such as authigenic glauconite, dysaerobic epibentic bivalves,Chondrites burrows or framboidal pyrite in the surrounding sediments of many Upper Jurassic thrombolitic buildups suggest that oxygen depletion excluded higher reefal metazoans in many of these reefs. Their position within shallowing-upwards successions and associated fauna from aerated settings show that thrombolitic reefs occurred over a broad bathymetric area, from moderately shallow to deep water. Increases in the alkalinity of sea water possibly enhanced calcification.Reefs were much more common during the Late Jurassic than during the older parts of this period. Particularly the differences between the Mid and Late Jurassic frequencies of reefs can be largely explained by a wider availability of suitable reef habitats provided by the general sea level rise, rather than by an evolutionary radiation of reef biota. The scarcity of siliceous sponge reefs on the tectonically more active southern Tethyan margin as well as in the Lusitanian Basin of west-central Portugal reflects the scarcity of suitable mid to outer ramp niches. Coral reefs occurred in a larger variety of structural settings.Upper Jurassic coral reefs partly grew in high latitudinal areas suggesting an equilibrated climate. This appears to be an effect of the buffering capacity of high sea level. These feedback effects of high sea level also may have reduced oceanic circulation particularly during flooding events of third and higher order, which gave rise to the development of black shales and dysaerobic thrombolite reefs. Hence, the interplay of local, regional and global factors caused Jurassic reefs to be more differentiated than modern ones, including near-actualistic coral reefs as well as non-actualistic sponge and microbial reefs.

Discovery of a “Living Dinosaur”: Globally unique modern hexactinellid sponge reefs off British Columbia, Canada

SummaryGlobally unique hexactinellid sponge reefs occur on the continental shelf off British Columbia, Canada. They cover about 425 km2 of seafloor on the continental shelf off British Columbia (Canada) in water depths between 165 and 240 metres and occur on a low-angle deep shelf, iceberg scoured seafloor, characterized by very low sedimentation rates and very stable environmental conditions. The sponge bioherms are up to 19 metres high with steep flanks, whereas the biostromes are 2–10 metres thick and many kilometres wide. They all consist of dense populations of only seven hexactinellid species. Three of them, all hexactinosan species (Aphrocallistes vastus, Heterochone calyx, Farrea occa) are the main frambuilders, composing a true rigid framework of sponge skeletons encased in a organic rich matrix of modern clay baffled by the sponges. Growth rates of hexactinosan sponges range in the order of 0–7 centimetres per year. The base of the oldest sponge reefs date from approximately 9000 years b.p.Different invertebrate and fish faunas occupy the reefs than occur on adjacent seafloor areas and some species appear to use the sponge reef complex structures as refugia where they can hide.Sidescan sonar data and direct observation by manned submersible clearly show that large areas of sponge reefs have been heavily damaged by seafloor trawling in the past decade.These unique extant siliceous sponge reefs can be used as a modern analogue for a better understanding and interpretation of fossil siliceous sponge reefs, known from many ages and many locations world wide.

RECENT HEXACTINOSIDAN SPONGE REEFS (SILICATE MOUNDS) OFF BRITISH COLUMBIA, CANADA: FRAME-BUILDING PROCESSES

Abstract Hexactinosidan sponges are important reef-building organisms in Earth history as they are able to create a three-dimensional reef framework and thereby form topographic relief comparable to that produced by scleractinian corals. Study of modern hexactinosidan sponge skeletons from water depths of 165–240 m on the continental shelf off British Columbia, Canada, demonstrate the hitherto undescribed frame-building process that leads to the formation of large and so far unique siliceous sponge reefs in this area. The fundamentals of the frame-building process are based on the production of siliceous envelopes around spicules of dead hexactinosidan sponges. In addition to the development of a three-dimensional reef framework, mound growth is supported by the current baffling effect of the sponges. Fine-grained siliciclastic suspended sediment is trapped and deposited within the gaps in the sponge skeletons and in voids in the reef surface preventing the framework from collapsing as the reef grows. Analogous but tropical examples from the Lower Jurassic of Portugal show that the frame-building potential of hexactinosidan and other siliceous sponges has existed, substantially unchanged, for more than 180 million years. In contrast to well-known fossil mud mounds of various geologic ages, in which the in situ precipitation of automicrite via microbial processes plays a major role, the matrix of the hexactinosidan sponge mounds of British Columbia consists exclusively of baffled fine-grained siliciclastics; automicrite is absent. Existing mud mound classification schemes do not encompass these depositional characteristics, therefore this new type of mound is consequently here classified as a silicate mound.

Sponge reefs in the Queen Charlotte Basin, Canada: controls on distribution, growth and development

Sponge reefs in the Queen Charlotte Basin exist at 165–240 m depth within tidally influenced shelf troughs subject to near bottom current velocities of 25–50 cm s−1 where nutrient supply from coastal runoff is augmented by wind-induced upwelling of nutrient rich water from the adjacent continental slope. Large reef mounds to 21 m in elevation affect tidally driven bottom currents by deflecting water flows through extensive reef complexes that are up to 300 km2 in area. Three hexactinellid species construct reefs by building a siliceous skeletal framework through several frame-building processes. These sponge reefs exist in waters with 90 to 150 µM dissolved oxygen, a temperature range of 5.9 to 7.3°C and salinity of 33.2 to 33.9 ‰. Relatively high nutrient levels occur at the reef sites, including silica, which in bottom waters are typically >40 µM and may be up to 80 µM. A high dissolved silica level is potentially an important control on occurrence of these and other dense siliceous sponge populations. The sponge reefs are mainly confined to seafloor areas where exposed iceberg plough marks are common. Sediment accumulation rates are negligible on the relict, glacial surface where the reefs grow, and trapping of flocculated suspended particulate matter by hexactinosidan or framework skeleton hexactinellid sponges accounts for a large proportion of the reef matrix. Suspended sediment concentration is reduced within the nepheloid layer over reef sites suggesting efficient particle trapping by the sponges. The reef matrix sediments are enriched in organic carbon, nitrogen and carbonate, relative to surrounding and underlying sediments. The sponges baffle and trap suspended sediments from water masses, which in one trough have a residence time of approximately 6 days, ensuring a close association of the sponges with the bottom waters. The location of the reef complexes at the heads of canyons provide a means of regionally funnelling particulate material that sponges can trap to enrich their environment with organic carbon and biogenic silica. Like deepsea coral reefs, the sponge reefs are a remote and poorly known ecosystem that can present logistical challenges and survey costs. Also like deep-sea coral reefs, many of the hexactinosidan sponge reefs have been damaged or destroyed by the groundfish trawl fishery.

Fossil Hexactinellida: An Overview

Hexactinellida (Porifera) are known from hexactin spicules from the Late Proterozoic of Mongolia (Brasier et al., 1997) and China (Steiner et al., 1993), possibly representing the oldest lineage of animals alive on earth today. Hexactinellida flourished and radiated rapidly during the Middle Cambrian times, giving rise to many new taxa and new skeletal plans (Rigby, 1986c; Walcott, 1920). Our knowledge of Early Paleozoic Hexactinellida is largely derived from isolated spicules from sediments. Both Amphidiscophora and Hexasterophora are known since the Early Paleozoic but most Paleozoic families did not survive the Paleozoic-Mesozoic boundary. Lyssacinosida are the first Hexactinellida known since the Proterozoic. They are an ultra-conservative group and their body plan still exists in living species today. The first representatives of Hexactinosida are known since the Devonian, with maximum radiation and diversity occurring during the Mesozoic. The first lychniscosidan sponges were described from the Middle Jurassic of Europe and during the Cretaceous where they reached their maximum peak of diversity. After the Jurassic-Cretaceous there has been a gradual decline in diversity leading up to the present. The present overview discusses about 120 genera in 18 families and two orders.

Kieselschwämme aus dem unterjurassischen Misonekalk der Trento-Plattform (Südalpen): Taxonomie und phylogenetische Relevanz

The Lower Jurassic Misone Limestone of the Trento Platform (Southern Alps, Italy) contains a siliceous sponge fauna which is here described. Besides the well-known Moroccan sponge carbonates, these Lower Jurassic spongioliths from the Trento Platform are presently the second mass occurrence of siliceous sponges, which is known from the southern margin of the Tethys. They differ from each other in regard of the composition of the sponge fauna and the absence of microbial crusts in the spongioliths of the Trento Platform. There, hexactinosans and lithistid demosponges occur in equal proportions. Sphinctozoans are another very characteristic element. Because of the richness in both sphinctozoans and siliceous sponges, the Trento occurrences may be considered as a transitional fauna between the late Palaeozoic-Triassic sponge fauna dominated by sphinctozoans and the post-Liassic sponge fauna dominated by more modern groups of siliceous sponges. Two new siliceous sponge genera with their species are established:Misonia baldensis n. gen. n. sp. (Hexactinosa) andBenacia princeps n. gen. n. sp. (lithistid Demospongiae). The rarity of siliceous sponge dominated spongioliths in the Early Jurassic is due to the restricted occurrence of low energy, deeper shelf areas.KurzfassungDie Kieselschwamm-Vorkommen auf der Trento-Plattform stellen neben den marokkanischen Spongiolithen das bisher zweite bekannte Vorkommen von unterjurassischen Schwammgesteinen auf dem Südschelf der Tethys dar. Die Spongiolithe des Misonekalks unterscheiden sich von den nordafrikanischen vor allem durch die Zusammensetzung der Schwammfauna und durch die Abwesenheit von mikrobiellen Krusten. Hexactinose Schwämme und lithistide Demospongiae treten gleichermaßen auf. Daneben sind segmentierte, coralline Demospongiae ein charakteristisches Element der südalpinen Schwammfauna. Aufgrund dieser reichlich vorhandenen „Sphinctozoen“ wird eine Übergangsfauna zwischen den jungpaläozoisch-triassischen, Sphinctozoen-dominierten Schwammfaunen und den postliassischen Kieselschwamm-dominierten Schwammfaunen postuliert. Zwei neue Schwammgattungen mit den dazugehörigen Arten werden aufgestellt:Misonia baldensis n. gen. n.sp. (Hexactinosa) undBenacia princeps n.gen. n.sp. (lithistide Demospongiae). Die Seltenheit Kieselschwamm-dominierter Spongiolithe des Unterjura wird auf das noch geringe und lokal begrenzte Angebot an niederenergetischen Tiefschelf-Bereichen zurückgeführt.