Stephen Kershaw

A ?microbialite carbonate crust at the Permian–Triassic boundary in South China, and its palaeoenvironmental significance

A 1 m thick carbonate crust, layered and commonly domal, caps crinoidal limestones on reef complexes of the top Permian Changxing Formation in the Huaying Mountains, eastern Sichuan, China. The crusts stratigraphic level lies at a sharp change in facies, and is overlain by poorly fossiliferous laminated micrites and shales of the Lower Triassic Feixianguan Formation. The crust therefore appears to coincide with the end-Permian extinction event, although the dating of the strata is currently imprecise. The crust is composed mostly of digitate carbonate, locally thrombolitic, with remnant lobate fabric, and resembles microbialites, but is mostly recrystallised and a microbial origin is unconfirmed. It is enclosed in micrite with pyrite crystals, ostracode and other shell debris. The crust is absent from interreef areas. Previous interpretations of karstification and calcrete formation are not upheld, and the facies were deposited under water. Overlying sediments are low energy, with abundant ferroan calcite and pyrite, reflecting anoxia associated with rising sea level. Various interpretations of the crust are possible: (1) it was organic, and microbia grew preferentially on topographic highs of reef tops; (2) microbia were a disaster biota in the absence of grazers; (3) microbia took advantage of favourable conditions for calcification, associated with a rapidly rising environmental-CO2 content during the Late Permian; (4) the crust was an inorganic precipitate associated with CO2-rich water. Microbial crusts are rare after the Cambrian, but whether this crust is organic or not, its presence in these strata reinforces the view that oceanic-atmospheric conditions in the Permian–Triassic boundary interval were unusual.

Classification and geological significance of biostromes.

SummaryBiostrome and bioherm were described as terms byCumings (1932), and bioherm has become synonymous with reef because of the discrete mound or lens shape in vertical section. The phrase “reefs and biostromes” is common in the literature and emphasises that biostromes are normally regarded explicitly as not reefal structures, because of the lack of topographic relief and common absence of a framework. However, the position adopted here is that bioherm and biostrome are most usefully applied to simply describe the outline shape of an organic accumulation, and not to denote any particular inherent internal structural organisation. Furthermore, the view here is that biostromes are most usefully considered as single organic layers (i.e. beds). Observations of biostromes of numerous ages and settings indicate that a considerable variety of internal structure exists within the outline which defines biostrome. Often, the structure comprises frame-works and dense clusters of in-place organisms and is just as much “reefal” as similar constructions with a biohermal shape. In other cases biostromes consist of beds of skeletal debris consistent with the concept of biostrome used by. many workers. These differences demonstrate that classification of biostromes is needed in order to allow comprehensive palaeoenvironmental analysis, and highlight the long-standing problem of using ‘reef’ to describe organic buildups. For biostromes,autobiostrome, autoparabiostrome, andparabiostrome are introduced to describe a continuum from structures where the constructing organisms are mostly in place (autobiostromes), to mostly debris of the structure (parabiostromes), with autoparabiostrome as intermediate.Allobiostrome, describes biostromes formed of material derived from allochthonous sources, for example skeletal plankton sedimented onto the sea bed. Most biostromes are of calcareous construction and their composition is most adequately described by existing limestone classification terminology.Other descriptive terms include: a) for biostrome geometry—ribbon and sheet; b) for internal layering—internally unbedded andinternally bedded, because some biostromes show lateral facies changes resulting in single layers becoming internally divided by bedding; c) for internal packing variation of constructors, using terminlogy introduced by R. Riding—dense (where constructors are closer together than one unit distance), andsparse (where constuctors are more distantly spaced). Biostromes are further categorised to account for thickness variations. Adaptation of terminology used for bed thickness descriptions is applied; standard bed thickness categories are not appropriate to biostromes, which are often thicker than 1m. Instead:Very thin biostromes-up to 0.1m,thin biostromes-0.1–0.5m,medium biostromes-0.5–2.0m,thick biostromes-2.0–5.0m,very thick biostromes->5.0m. Autobiostromes which form significant features in sedimentary successions may be regarded as the peak of in place benthic organic skeleta buildup (=reefsensu lato) development, and their recognition is required to permit full palaeoenvironmental analysis of facies containing them. Particular emphasis may be placed on their role in identifying sea level change in shallow water carbonate sequences, and thence their utility in models to explain such change.

Palaeozoic stromatoporoid taphonomy: ecologic and environmental significance

Abstract Epibenthic sessile marine organisms, such as the aspiculate stromatoporoids, show a wide range of growth forms in different settings across Palaeozoic carbonate banks and reefs, making them useful taphonomic tools in palaeoenvironment analysis. Field observations of Ordovician through Late Devonian stromatoporoid-bearing biostromal and biohermal successions on different palaeocontinents have revealed three main post-mortem tapho-signatures that affect palaeoecologic analysis: (1) variability of physical breakage and bioerosion prior to burial; (2) loss of morphologic and taxonomic information due to post-burial diagenesis; and (3) minor localized, reorientation during karstification. Physical fragmentation, abrasion, and lesser bioerosion and dissolution represent the main processes responsible for post-mortem destruction. Hydrodynamic behaviour of stromatoporoid clasts was similar to other carbonate skeletal clasts in their rounding and sorting characteristics. Although bioerosion and bioturbation did not play an important role in Palaeozoic stromatoporoid taphofacies characterization, a marked increase in bioerosion of stromatoporoids is evident, beginning in certain Silurian (Wenlock) morphotypes and in some Devonian morphotypes. Stromatoporoids offer varying profiles to shelf currents and seasonal storm surges, and apparently have different skeletal strength (durability) depending on skeletal architecture. Cyst-dominated skeleton architecture appears to have been fairly resistant to fragmentation. Latilaminae, a common skeletal feature in different families of calcified sponges, represent inherent zones of weakness that appear to have enhanced equidimensional (breakage away from latilaminae) to oblate (breakage along latilaminar surfaces) fragmentation. Construction of lamina-pillar architecture, amongst the most common skeletal attributes of Siluro–Devonian stromatoporoids, gives the visual impression of varying robustness, but this is supported by few data. Stromatoporoid response to increasing hydrodynamic energies varies greatly from no movement or simple overturning of skeletons to severe fragmentation and size sorting. Although such variation broadly reflects palaeogeographic setting and thus periodic disturbance by tropical storms, familial and generic skeletal traits greatly influenced taphofacies characterization. These differences in modularity and ontogeny in the stromatoporoid groups has hampered development of generalized taphofacies models, as devised for shell-rich deposits. Taphofacies are not comparable between modern coralgal reefs, especially the highly specialized acroporid facies, and Palaeozoic stromatoporoid-bearing reefs for the following reasons: significant differences exist in diversity of observable growth strategies (nature of attachment of skeletons to substrate and variety of morphotypes); probable significant differences in life modes between these groups (feeding strategies and positions and areal extent of clonal tissue within the skeletons, and their subsequent susceptibility to infestation by microborers); and, Palaeozoic macrobioeroders were fewer than modern representatives and apparently less effective in stromatoporoid taphofacies characterization than modern reef macrobioeroders are for coral taphofacies development.

Geochemical and facies evidence for palaeoenvironmental change during the Late Ordovician Hirnantian glaciation in South Sichuan Province, China

Abstract Late Ordovician shales at Kuanyinchiao, southern Sichuan Province, China, contain a muddy micritic limestone (the Kuanyinchiao Formation) 0.4 m thick, representing the Hirnantian glaciation, and therefore contain the two extinction events which together comprise the Late Ordovician mass extinction. Wufeng Formation shales underlie, and Lungmachi Formation shales overlie, the limestone, and the Ordovician–Silurian boundary is c. 0.6 m above the base of the Lungmachi Formation. Geochemical analysis of closely spaced samples aids assessment of tectonic and palaeoceanographic change influencing sedimentary facies throughout the interval, but also highlights the value and dangers of geochemical applications in palaeoenvironment study. The immobile elements Hf, Sc, Ta and Th show a consistent asymmetric pattern of abundance; they declined gradually in the upper Wufeng Formation to lower concentrations in calcareous sediments of the Kuanyinchiao Formation because of suppressed erosion and lower sedimentation rate during glaciation. The decline may represent gradual cooling and reduction in erosion rate from source, before preserved sedimentary evidence of the glaciation itself. A sharp return to shale (Lungmachi Formation), and Hf, Sc, Ta and Th concentrations to pre-glaciation levels, suggest rapid glacial collapse and renewed erosion, probably of the nearby Dianqian Land. These immobile elements co-vary with most REE (especially Ce and La), demonstrating that these were also source-controlled, and unaffected by contemporary environmental, or diagenetic, processes. However, partial co-variance of Eu and U with immobiles suggests a combined control of Eu and U by both source variations and redox changes. Anomalies in Ce, Eu and U are often used to identify shifts in oxidation state; at Kuanyinchiao, anomalies which suggest anoxia in the Wufeng Formation are contradicted by benthic faunas which prove the seafloor was oxygenated then, and indicate complex control of those elements. Ba, As and Sb trends depart markedly from immobiles, and show no consistent trends; they are interpreted as modified by diagenesis. Overall, the immobile element suite suggests that the site received clastic supply from local uplift throughout the interval, which was interrupted by the glacial phase.

Factors controlling the growth of stromatoporoid biostromes in the Ludlow of Gotland, Sweden

Abstract Stromatoporoid biostromes and stromatoporoid-algal-coral bioherms in the Silurian of Gotland display differing gross morphology and internal construction which suggests a difference in factors controlling their formation and development. Lower Wenlock bioherms of the Visby and Hogklint Formations, biotically zoned, grew during a relative sea level fall, on substrates of limited lateral extent, and sediment accumulated around and on the reefs as they grew. In contrast, the growth and morphology of middle Ludlow Hemse Group biostromes was determined by low levels of clastic supply, laterally extensive suitable substrate, relatively stable sea level and a largely flat sea bed in shallow water conditions. These are interpreted to result in dense accumulation of skeletal biota in biostrome form. In detail, the biostromes contain low- and high-profile stromatoporoid growth forms and exhibit differences from upper Ludlow reef stromatoporoids, which contain the same stromatoporoid species suite but as low-profile forms only. Controls on stromatoporoid growth form differences are unclear but may include substrate consistency, environmental energy and closer adaptation to environment by the same species between middle and upper Ludlow times. The abundance of biostromes on Gotland may be due to climate; drier episodes result in lower clastic supply, and if sea level was also stable, then biostromes could result. Hemse Group biostromes in one locality occur during dry phases of a two-state dry/wet model proposed for the Silurian, and demonstrate the potential of global controls on these reefs.

Spatial competition among clonal organisms in extant and selected paleozoic reef communities

SummaryOccurrences of densely packed benthic organisms in extant reefs are of two types: 1) live-live interactions, where two living organisms interact, and 2) live-dead associations, where only one is alive and uses the other as a substrate. The latter are common in reef deposits due to biostratinomic feedback, i.e. dense skeletal accumulations provide hard substrates for clonal recruitment, thus facilitating greater frequency of live-dead encounters than in lower biomass level-bottom communities dominated by solitary organisms. Differentiating between these two types in ancient reefs is difficult, often impossible.Most live-live interactions among clones in extant reef communities involve competition for space. Clonal spatial competition is divisible into four types: 1) direct-aggressive: encrusting overgrowth; 2) indirect-passive: depriving neighbors of resources, chiefly sunlight, by growth above them; 3) stand-off: avoidance of competition by organisms adopting positions that avoid or minimize direct polyp/zooid contact; and 4) overwhelming: one clone/ species volumetrically or numerically overwhelms the other, meeting minimal resistance. Despite class-order level differences in taxa, our results indicate that extant analogs, based on the arrangement and distortion of skeletons, are valuable for recognizing live-live interactions in Silurian and Carboniferous reefs and interpreting the types of spatial competition represented.Comparison of overhead (plan) views of live-live coral competition in Polynesian reefs with vertical sections of Silurian and Carboniferous sponge-dominated reefs and biostromes suggests that direct-aggressive competition is more common among extant than among Paleozoic reef-builders. Stand-offs showing clone margin distortion and overwhelming with minor skeletal distortion are most common in our fossil examples and probably relate to the dominance of these reefs by sponges. Success by extant sponges in spatial competition is largely due to allelochemical deterrence which may explain the predominance of stand-off and overwhelming confrontations in fossil sponges rather than tentacle-mesentery based direct aggression among extant corals and bryozoans.

Geochemistry of post-extinction microbialites as a powerful tool to assess the oxygenation of shallow marine water in the immediate aftermath of the end-Permian mass extinction

Rapid and profound changes in earth surface environments and biota across the Permian–Triassic boundary are well known and relate to the end-Permian mass extinction event. This major crisis is demonstrated by abrupt facies change and the development of microbialite carbonates on the shallow marine shelves around Palaeo-Tethys and western Panthalassa. Microbialites have been described from a range of sites in end-Permian and basal Triassic marine sedimentary rocks, immediately following the end-Permian mass extinction. Here, we present geochemical data primarily focused on microbialites. Our geochemical analysis shows that U, V, Mo and REE (Ce anomaly) may be used as robust redox proxies so that the microbialites record the chemistry of the ancient ambient sea water. Among the three trace metals reputed to be reliable redox proxies, one (V) is correlated here with terrigenous supply, the other two elements (U and Mo) do not show any significant authigenic enrichment, thereby indicating that oxic conditions prevailed during the growth of microbialites. REE profiles show a prominent negative Ce anomaly, also showing that the shallow marine waters were oxic. Our geochemical data are consistent with the presence of some benthic organisms (ostracods, scattered microgastropods, microbrachiopods and foraminifers) in shallow marine waters that survived the mass extinction event.

Permian–Triassic boundary microbialites (PTBMs) in southwest China: implications for paleoenvironment reconstruction

Permian–Triassic boundary microbialites (PTBMs) are commonly interpreted to be a sedimentary response to upwelling of anoxic alkaline seawater and indicate a harsh marine environment in the Permian–Triassic transition. However, recent studies propose that PTBMs may instead be developed in an oxic environment, therefore necessitating the need to reassess the paleoenvironment of formation of PTBMs. This paper is an integrated study of the PTBM sequence at Yudongzi, northwest Sichuan Basin, which is one of the thickest units of PTBMs in south China. Analysis of conodont biostratigraphy, mega- to microscopic microbialite structures, stratigraphic variations in abundance and size of metazoan fossils, and total organic carbon (TOC) and total sulfur (TS) contents within the PTBM reveals the following results: (1) the microbialites occur mainly in the Hindeodus parvus Zone but may cross the Permian–Triassic boundary, and are comprised of, from bottom to top: lamellar thrombolites, dendritic thrombolites and lamellar-reticular thrombolites; (2) most metazoan fossils of the microbialite succession increase in abundance upsection, so does the sizes of bivalve and brachiopod fossils; (3) TOC and TS values of microbialites account respectively for 0.07 and 0.31 wt% on average, both of which are very low. The combination of increase in abundance and size of metazoan fossils upsection, together with the low TOC and TS contents, is evidence that the Yudongzi PTBMs developed in oxic seawater. We thus dispute the previous view, at least for the Chinese sequences, of low-oxygen seawater for microbialite growth, and question whether it is now appropriate to associate PTBMs with anoxic, harsh environments associated with the end-Permian extinction. Instead, we interpret those conditions as fully oxygenated.

Biotic structure and morphology of patch reefs from South China (Ningqiang Formation, Telychian, Llandovery, Silurian)

SummaryNingqiang Formation (late Telychian, Llandovery, Silurian), characterized by nearly 3000 m of shales in tercalated with carbonates, is situated between Ningqiang (S. Shaanxi Province) to Guangyuan (N. Sichuan Province) adjacent to the northwest margin of the Yangtze Platform. The high diversity “Xiushan Fauna”, and abundant reef development, illustrate a relatively warm and persistent shallo marine environment in these early Silurian sediments. The sequence shows reef radiation after recovery from the end Ordovician mass extinction envents. Multiple horizons of reef-building occurred within a relatively short geological interval and resulted in more than 30patch reefs up to 200 m in diameter and 1–50 m vertically, composed of abundant fossils. Reef biota include frame-building corals, stromatoporoids, bryozoans, and microbialites, and reef-associated oranisms such as crinoids, brachiopods, trilobites, gastropods, nautiloids and ostracods. Three reefrelated biotic associations are recognised: a) reefs dominated by framework with crinoids and microbia; b) reefs dominated by only crinoids and microbia; and c) crinoiddomainated facies. Seven representative reef examples illustrate different morphologies and growth styles. A high terrigenous debris input and shallow epicontinental ramp, which lacked obvious topographic variation, were major controls which resulted in rather simple reefs; sedimentation was apparently the main constraint on lateral and vertical extension of reefs, and prevented large-scale reef complexes developing.

Comment—Earliest Triassic microbialite micro- to megastructures in the Huaying area of Sichuan Province, south China: implications for the nature of oceanic conditions after the end-Permian extinction (Ezaki et al., 2003)

Abstract