Abstract The earliest named stromatolite Cryptozoon Hall, 1884 (Late Cambrian, ca. 490 Ma, eastern New York State), was recently re-interpreted as interlayered microbial mat and non-spiculate (keratosan) sponge deposit. This “classic stromatolite” has come to be central to a fundamental debate concerning the significance or even existence of non-spiculate sponges in carbonate rocks of the Neoproterozoic (Tonian) onward. We determine herein that Cryptozoon has three types of primary carbonate layers: clotted-pelletoidal micrite with microbial filaments, clotted-pelletoidal micrite with vesicular structure, and dense microcrystalline laminae. Using contextual fabric analysis, elemental mapping, cathodoluminescence microscopy, fluid inclusions, electron backscatter diffraction, U–Pb carbonate dating, and regional burial history, the sponge interpretation is rebutted. We conclude that suspect fabric elements are secondary in nature and best explained as products of deep burial alteration. Incipient carbonate metamorphism is early Carboniferous in age (Mississippian, terminal Acadian orogeny). Key petrographic observations include heterogenous recrystallization (aggraded Ostwald ripening) associated with interfingering reaction fronts typical for partially miscible fluids, a granoblastic calcite texture exhibiting preferred crystallographic orientation, and subsequent authigenic white mica (deepest burial; late Carboniferous and Permian Appalachian orogeny). Topotype Cryptozoon is a sub-greenschist metacarbonate stromatolite. The published Tonian to Phanerozoic record of non-spiculate sponges needs to be reassessed.
Kershaw, Stephen 1980 10 15: Cavities and cryptic faunas beneath non-reef stromatoporoids. Lethaia, Vol. 13, pp. 327–338. Oslo. ISSN 0024–1161. Stromatoporoids from level-bottom shales and argillaceous limestones in the Silurian of Gotland, Sweden, form substrates for a variety of encrusting and boring organisms. Overturned stromatoporoids have encrusters and borers on both upper and lower surfaces, while coenostea preserved in situ have encrusters on both surfaces but borings on upper surfaces only. This suggests that cavities, now infilled, existed below coenostea. The stromatoporoids are isolated and not part of a reef framework where growth alone could have created overhangs and cavities. The scouring activity of currents removing sediment from around and beneath the edges of coenostea, and small current-controlled movements leaving stromatoporoids imperfectly settled on the uneven substrate or partly overlying skeletal debris, are invoked to explain the presence of cavities and hence encrusters on stromatoporoid lower surfaces. Both processes probably operated on many specimens. The lower surfaces of these stromatoporoids also show basal concavities which range from shallow to deep and reflect the topography of the substrate and the success of stromatoporoids growing on positive features from which they could shed sediment easily. Overturned stromatoporoids and coenostea with deep, encrusted, basal concavities, point to violent environmental events, such as storms, more powerful than currents producing scour and small movements of coenostea.
'Anachronistic facies' and 'disaster forms' are interpretive terms applied from the early 1990s to sedimentary deposits and biotas in the aftermath of mass extinctions; both terms have been used especially for the deposits formed directly after the end-Permian mass extinction. Microbial carbonates (disaster forms) are abundant in the earliest Triassic and often considered as a return to environmental conditions typical of Neoproterozoic to Cambro-Ordovician times. However, this view does not take into account: (i) the growing evidence that microbialites are stimulated by bicarbonate-supersaturated waters irrespective of mass extinction; (ii) the potential oceanic and climatic effects of the Siberian Traps volcanics; and (iii) the unique global plate-tectonic setting of Pangaea at that time. The configuration of land masses led to near-isolation of Tethys from Panthalassa, with modelled slow circulation and accumulation of anoxic deep water in Tethys. Evidence of catastrophic overturn of the Tethys Ocean reflects instability, possibly driven by climate changes, which released anoxic bicarbonate-rich waters to the surface. Items (ii) and (iii) are features of the Permian–Triassic boundary transition and are not parallels of earlier episodes of Earth history. Taking the argument wider, not all mass extinctions are followed by widespread anachronistic facies and disaster biotas. Therefore, it may be argued that application of anachronism and disaster biota concepts is an oversimplification of mass extinction processes in general, and the Permian–Triassic boundary extinction in particular. Continued use of these terms generates a narrowed view of processes and hinders development of comprehensive interpretations of changes of facies and biotas in mass extinction research.
Skeletal and non-skeletal components of marine sedimentary rocks have been analyzed for the purpose of reconstructing the rare earth element (REE) and yttrium (Y) compositions of paleo-seawater, but skeletal carbonates frequently have proven to be unreliable recorders of seawater chemistry. Here, we present a systematic multi-technique assessment of rare earth and other trace elements in ooid sands from the modern Great Bahama Bank (GBB–marine) and Great Salt Lake (GSL–continental) based on strong-acid (hydrofluoric and nitric) and weak-acid (acetic) digestions, as well as laser ablation (LA) of ooid cortices and nuclei. The results show that Bahamian ooid cortices possess shale-normalized REE + Y features nearly identical to those of shallow seawater, including limited contamination from siliciclastic REEs. An admixture of even 0.2% of detritally sourced material can modify the primary marine REE + Y patterns by, for example, increasing the light REE (LREE) content. Mean values of LA data for Bahamian ooid cortices exhibit similar REE + Y signatures to those produced by acetic acid digestion, but LA data are generally noisier, primarily as a result of low REE concentrations and the small volume of carbonate ablated in analysis. Screening out samples with ΣREE <0.9 ppm reveals more uniform, seawater-like REE + Y patterns in individual ooid cortices as well as high, uniform REE distribution coefficients (116 ± 21, from La to Lu) with respect to ambient seawater. Ooid laminae show no distinct alternation between oxic and anoxic pore fluid characteristics, suggesting that growing ooids primarily formed under oxic conditions in carbonate shoal settings, even when buried during their resting phase. Unlike Bahamian ooids, shale-normalized REE + Y patterns of GSL ooids digested in weak acid show slightly depleted LREE compositions that may deviate from ambient fluids owing to release of REEs from an included clay fraction based on comparison between digestion and LA methods. Multiple ablation spots within the same cortex allowed direct comparison of more and less contaminated portions of a single ooid. Nearly uniform positive Ce anomalies may be related to strongly alkaline water conditions. Individual ablation spots that are partially overprinted by siliciclastic-derived REEs contain variable Zr contents (from 0.096 to ~4 ppm) and flattened shale-normalized LREE patterns. The least-contaminated GSL ooid cortex yields an LREE-depleted REE + Y distribution. Normalization of the REE + Y distributions of least-contaminated GSL ooids using reliable GBB ooid distributions (i.e., with ΣREE > 0.9 ppm) returns a flat pattern, suggesting similar degrees of LREE depletion controlled by carbonate complexation under similar aqueous alkalinity conditions in Great Salt Lake and Bahamian waters. In summary, ooids can be a reliable proxy for REE + Y characteristics of ambient surficial waters when adopting suitable analytical methods, including laser ablation, that allow the identification and isolation of a contamination signal from siliciclastic detritus.
Abstract Sponges of the Lower Greensand Group (LGS) are well preserved, and occur in sediments of a sandy matrix. Abundant in the Faringdon Sponge Gravel Member (FSG), these sponges, mostly Calcareans are found in Oxfordshire, with notable preservation at Little Coxwell quarries. Classical researchers described sponges and spicules from the LGS, including Lhuyd (considered to have been the first to publish illustrations of LGS sponges), Sharpe, Sowerby and Parkinson. In addition to the FSG, the Folkestone, Hythe, and Atherfield Clay formations within the LGS also contain sponge remains, including spicules as well as whole sponge fossils. These sponges include mostly samples from traditional sponge class Calcarea and a taxon of Hexactinellida; Altogether, the sponge assemblage developed in warm seas of the Lower Cretaceous, and display diverse shapes of sponge bodies and robust spicules. This study provides descriptions of common species following updated Porifera classification and recent sponge taxonomy research, illustrated with specimens from the Natural History Museum, London (NHM) and British Geological Survey (BGS) collections. The following taxa are recorded and described: 1) Calcareans: Barroisia anastomosans (Parkinson, 1811), Barroisia clavata (Keeping, 1883), Barroisia irregularis (Hinde, 1884), Dehukia crassa (de Fromentel, 1861), [Elasmoierea] faringdonensis (Mantell, 1854), [Elasmoierea] mantelli (Hinde, 1884), Peronidella gillieroni (Loriol, 1869), Peronidella prolifera (Hinde, 1884), Peronidella ramose (Roemer, 1839), Oculospongia dilatate (Roemer, 1864), Tremospongia pulvinaria (Goldfuss, 1826), Raphidonema contortum (Hinde, 1884), Raphidonema porcatum (Sharpe, 1854), Raphidonema farringdonensis (Sharpe, 1854), Raphidonema macropora (Sharpe, 1854), Raphidonema pustulatum Hinde, 1884, Endostoma foraminosa (Goldfuss, 1829); 2) Hexactinellids: Lonsda contortuplicata Lonsdale, 1849.
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