Shell Beds Research Articles

Facies and fauna of the Pennsylvanian Buckhorn Asphalt Quarry deposit: a review and new data on an important Palaeozoic fossil Lagerstätte with aragonite preservation

The Pennsylvanian Buckhorn Asphalt Quarry contains the best-preserved Palaeozoic mollusc fauna in the world. Early impregnation of mixed siliciclastic–carbonate rocks (mudstones, pack to grainstones, shell beds, and conglomerates) with hydrocarbons prevented aragonite destruction (“Impregnation Fossil Lagerstatte”). The exceptional preservation comprises shell microstructures, microornaments and early ontogenetic shells. Most gastropods had planktotrophic larval development indicating a high primary production although the remains of phytoplankton are very rare in this and other Late Palaeozoic deposits. Deposition occurred close to a shallow-water coastal area. Mass flow processes (density currents) triggered by storms were involved in the transport mechanisms of some units. Shells of benthic molluscs yield the most diverse known Palaeozoic microboring assemblage, indicating at least partly euphotic conditions. The invertebrate fauna comprises about 160 species and is dominated by molluscs, which is unusual for a Palaeozoic deposit, suggesting that aragonite dissolution produces a major bias in the fossil record. However, most mollusc genera in the Buckhorn deposit are also known from other Pennsylvanian occurrences as recrystallised shells. This shows that preservation bias via preferential aragonite dissolution may be overestimated.

Aboriginal Kitchen Middens and Marine Shell Beds.

Aboriginal Kitchen Middens and Marine Shell Beds.

Experimental Study on Bed Load Transport of Shell fragment-Mixed Sand under Waves

Beneficial use of shell for constitutive materials of beach is proposed instead of its disposal. Since the shape of shell fragment is oblate, the knowledge on littoral drift of sand with spherical shape could not be applied directly for understanding of its behavior under waves. In this study, critical shields number and fall velocity of shell fragment are evaluated changing oblate shapes of the fragments, and the characteristics of bed load transport of shell and shell-mixed sand beds are examined by a movable-bed experiment. From the experiments, the effects of oblateness on the increase of resistance to wave and current are elucidated. It is found that the interaction of sand and shell on their movements has an effect on the bed - load rate changing the height of sand ripples.

Bioerosive structures formed by Miocene rock-boring bivalves in Hokkaido, Japan

Fossilized rock-boring bivalves and associated borings in the Miocene Sankebetsu Formation in northern Hokkaido, Japan, are useful for reconstructing paleoenvironmental changes. The sediments are typical of shallow-marine facies in Hokkaido, and are rich in rock-boring bivalve traces. The Miocene host formation (Sankebetsu Formation, 23–19 Ma) unconformably overlies an earlier Eocene formation (Sakasagawa Formation). The inferred paleoenvironmental changes associated with deposition of the Miocene Sankebetsu Formation at the study locality started with development of a wave-cut bench of stiff mud or soft rock that was extensively bored by the rock-boring bivalve Platyodon nipponica to form unnamed ichnofossils. The bench was subsequently covered by sediments associated with a transgression and were gradually lithified. A later regression lead to the development of new bench of hard rock that was bored by the rock-boring bivalve Penitella kotakae to form the boring ichnofossil identified as Gastrochaenolites turbinatus. Finally, the new bench was covered by sediments during a major transgression, forming a shoreface environment characterized by storm-generated shell beds. The analysis of bioerosion on marine rocky surfaces is a useful tool for interpreting geological episodes such as relative oscillations of sea level, erosion/sedimentation rates, and the intensity of physical disturbance.

The role of the Mediterranean region in the development of sedimentary geology: a historical overview

AbstractThe Mediterranean region, source of so much knowledge in the world, is the site of major advances in sedimentary geology. In addition to its economic and cultural richness, the geological and geographic diversity of the region, plus its active geological processes, have long stimulated indigenous scholars, along with attracting talented outsiders such as Steno, Lyell, Walther, Kuenen and Bagnold. Since classical Hellenic times, debates about the origin of fossils and the changing positions of sea‐level served as catalysts for studies of sediments, sedimentary rocks and ancient life. The presence of geologically young and easily interpreted marine shell beds in many Mediterranean coastal areas adjacent to their modern analogues was a particular stimulus for progress in sedimentary geology, for example, the very advanced stratigraphic ideas of Leonardo da Vinci, expressed solely in his unpublished notebooks. Impeding progress was the geological complexity of facies, faunas and structure in the circum‐Mediterranean Alpine belts. Once the secrets of these were unlocked, however, the Mediterranean region became the source of major discoveries about syn‐sedimentary tectonics, carbonate platforms, pelagic and anoxic sediments, turbidites, evaporites, aeolian processes, cyclostratigraphy, magnetic stratigraphy and impact events. In many of these Mediterranean discoveries, the critical element is the occurrence of extensive Mesozoic‐Cenozoic pelagic successions whose precise age dating was made possible by pioneering biostratgraphic studies using microfossils.

Concurrent Aspergillosis and Ascites with High Mortality in a Farm of Growing Broiler Chickens

Two flocks of broiler chickens aged 15 to 30 days presented respiratory signs such as dyspnea and up to 25% mortality. These were the only two flocks in the farm where a bed of sunflower shells was used instead of the rice-hull bedding used in other flocks. At necropsy, severe ascites, right heart hypertrophy, pulmonary congestion, and extensive multifocal granulomatous pneumonia were recorded. Histopathologic examination revealed chronic multifocal mycotic granulomatous pneumonia. Aspergillus fumigatus was identified by microbiologic study from pulmonary specimens. After disinfecting the floor and changing the bedding, no clinical signs were recorded in the farm. Severe chronic granulomatous pneumonia caused by A. fumigatus in the chickens of the present study may have caused hypoxia, leading to pulmonary hypertension, heart failure, and ascites.

Particle-size distribution of inferred tsunami deposits in Sur Lagoon, Sultanate of Oman

The sedimentary characteristics of shell beds within an interpreted tsunami deposit from Sur Lagoon, Oman were examined using shell taphonomy and high-resolution particle-size analysis. The tsunami bed was deposited by the 28 November, 1945 tsunami generated by the Makran subduction zone. Q-mode cluster analysis of particle-size data was evaluated as a means of discriminating individual tsunamite shell layers from lagoonal intertidal deposits. Results showed that the tsunami shell bed was more poorly sorted, and heterogeneous (in both the digested and undigested samples) than the background lagoonal sediments. The tsunami bed thickness correlated generally with the thickness of the shell-bed, however, cluster analysis extended the tsunami unit several centimeters above or below the shell bed in some cores. The particle-size analysis also showed subtle textural trends in the tsunami unit, suggesting that the tsunami bed was deposited in several distinctive phases during tsunami incursion into Sur Lagoon. The findings indicate that cluster analysis of particle-size data can be used to identify tsunami beds in intertidal environments and holds potential for identifying paleotsunami deposits in sediments from embayed intertidal–subtidal siliciclastic systems where obvious sedimentary structures may be absent.

Mineralogical control of rare earth elements in acid sulfate soils

Major, trace and rare earth element concentrations were measured in porewater, surface water and sediments at an acid sulfate soil site. The concentrations of La and Ce in porewater are up to 1–3 ppm. There is a strong correlation between REE concentration and acidity, except that the maximum concentrations were consistently found below the horizon of maximum acidity, associated with an increase in pH (to ca. 4) and change in mineralogy from jarosite-dominated to goethite-dominated mottles. Jarosite replacement by goethite is as expected with the rise in pH, which in turn is due to the occurrence of a fossil shell bed just below. The rare earth element patterns in the porewaters are enriched in the MREE with respect to Post-Archaean Australian Shale (PAAS). Measurements and calculations show that this is in accord with experiments on low-degree partial dissolution of jarosite, even when the jarosite itself is highly enriched in LREE. There is a clear fractionation in the patterns between the clay-rich soil matrix, which is slightly depleted in the LREE when normalized to PAAS (La/Yb PAAS ∼0.5), and the secondary mineral phase jarosite, which is enriched in the LREE (La/Yb PAAS = 15–50). The REE pattern in the porewater changes with the transition from jarosite- to goethite-rich mottles, becoming relatively more enriched in the LREE compared to the HREE, which is consistent with the incongruent dissolution of jarosite to form goethite and the release of greater amounts of jarosite REE to solution, including proportionately more of the jarosite-compatible LREE. Maximum surface water REE concentrations in acidic water were 100–200 ppb La and Ce. REE patterns in surface water were very similar to the porewater transition zone, enriched in the MREE, but asymmetric, relatively enriched in the LREE compared to the HREE.

Filament lumachelle on top of Middle Jurassic oolite limestones: event deposits marking the drowning of a Tethysian carbonate platform (Subbetic, southern Spain)

Bioclastic accumulations often occur on top of Tethysian carbonate platforms and crinoids are a common constituent of these bioclastic deposits on Lower and Middle Liassic carbonate platforms. In contrast, the relevant literature contains few examples in which the main constituent of the lumachels is thin-shelled bivalves (filaments). This paper presents a study of a filament lumachelle cropping out on top of a Middle Jurassic carbonate platform. The carbonate platform is represented by the Jabalcuz Formation, found in one of the northernmost Subbetic units (south of Jaen city). The lumachelle marks the demise of the carbonate platform and has special features that characterize the drowning phase. This process has been related with the syn-rift extensional tectonics associated to the opening of the Tethys westwards. Stratigraphically, the lumachelle occurs on top of shallow-water oolite limestones (Middle Jurassic) and is overlain by radiolarian-rich pelagic and resedimented deposits (Latest Callovian–Oxfordian). It occurs as a body (about 1.5 km wide and up to 8 m thick) made up entirely of densely packed thin bivalve shells. A remarkable feature of the bivalve shell beds is stromatolite-like crumpled lamination at the outcrop. The observations made at the outcrop scale, by microscope under transmitted light, and by cathodoluminiscence favor a diagenetic origin for this striking structure. Other hypotheses, such as its possible relation with seismicity, cannot be confirmed. The filaments would have filled one of the former basins that originated in relation with syn-rift fault-block tectonics leading to the demise of the carbonate platform. Tectonics was one of the main factors setting in motion a carbonate productivity crisis and the inhibition of a diverse benthic community. Once production failed in the carbonate factory, storms and probably hurricanes as well, swept shell deposits from the shallowest areas of the shallow-water carbonate platform and accumulated them in a coevally formed small half-graben basin. At least three main depositional stages can be differentiated in the fill of this half-graben basin, which was a sediment trap for the accumulation and preservation of the tiny bivalve shells against ebb surges. The massive accumulation of valves, the shortage of micrite around the filaments, outcrop morphology and facies relationships, along with regional geology, are among the arguments supporting this interpretation.

Taphonomic windows and molluscan preservation

Recent studies on silicified fossil biotas have suggested that substantial skewing of the molluscan record resulted from early aragonite dissolution in mid-outer carbonate ramp settings. If those rare skeletal lagerstätten are representative, then the quality and completeness of the molluscan record are thrown into doubt. Yet database studies suggest that the bivalve fossil record is actually relatively complete. If so, then biodiversity must be captured by other processes that preserved shells vulnerable to early dissolution, and which operated on a relatively high frequency, i.e., less than the species duration for bivalves. Storm beds, shell plasters and submarine hardgrounds are identified as fossil deposits that can preserve the labile aragonitic component of the fauna and thus represent potential taphonomic windows. Many storm event beds include rich accumulations of shelly benthos. Differences between storm bed faunas and those of the background facies could reflect transportation effects. However, some storm bed assemblages are rich in originally aragonitic infaunal bivalves that are not represented in background facies or more proximal shelf equivalents, and here rapid burial and removal of organic matter by winnowing may be the keys to aragonite shell preservation. Despite Palaeozoic to Cenozoic changes in the thickness and frequency of shell beds that reflect the predominant bioclast producers, shallow infaunas are commonly concentrated together with epifauna in such deposits. Some low energy, organic-rich mud-dominated settings are associated with preservation of aragonitic molluscs. Infaunal bivalves are a prominent component of shell plasters or pavements in such settings, linked to episodic bottom water anoxia. Decaying algal blooms drew the redox boundary up above the sediment–water interface, and brought populations of infaunal bivalves to the surface where they died. Isolated from the oxic taphonomically active zone, the shells were not dissolved and were buried as thin shell layers. In similar settings, aragonitic shells were preserved as moulds through early pyritisation, or even through preservation of original shell aragonite. In oxic environments, bioturbational reworking of surface sediment destroyed moulds of aragonitic shells after early dissolution. In some hardgrounds, these delicate moulds were preserved due to synsedimentary cementation, probably using carbonate released by aragonite dissolution. The examples included here come from both intervals of “calcite” and “aragonite” seas, and it is not possible to assess whether the saturation state (with respect to aragonite) of the ambient sea water played a role in the selective removal of aragonitic shells. While taphonomic windows may have captured the diversity of individual groups, it is clear from quantitative data involving skeletal lagerstätten that the scale of loss from early aragonite dissolution has drastically altered the trophic composition of some fossil assemblages commonly used as the basis for reconstructions of past communities.

Litho- and biofacies analysis of postglacial marine mud using CT-Scanning

Marine silty clay deposited during the Late-Wisconsinian postglacial marine transgression of eastern Québec (Goldthwait Sea) is ubiquitous in the sedimentary column of intertidal zones of the St-Lawrence Estuary. This mud is very compact and limits the penetration of organisms composing the modern Macoma balthica community. In order to describe the characteristics of intertidal sediments containing Goldthwait Sea mud, axial tomography (CT-Scan) is used. CT-Scan is a non-destructive method that can be used to describe sediment characteristics (grain size, mineralogy, primary and secondary sedimentary structures, fabric, shape and roundness, bedding contact), and to obtain high resolution, 3D representations of structures within sediment cores. Based on differences in the densities of analysed materials, the different lithologies, lithofacies, and organisms within the core can be discriminated, and a quantification of the volume occupied by the different components of the material can be made. Here, CT-Scan images provide information on the distribution, orientation and interweaving of thanatocœnosis shell beds that alternate with massive or faintly laminated postglacial marine mud beds, as well as on ichnofacies characteristics. In addition, we show 3D images of bioturbation structures within the recent sediment layer, which is distinguished from the underlying Goldthwait Sea mud. When coupled with conventional sedimentary (grain size statistics) and radiochronological ( 14C) analyses, these data provide information which is valuable for identifying depositional processes within sedimentary environments.

Generation of brachiopod‐dominated shell beds in the Miocene rocks of Carriacou, Lesser Antilles

AbstractBrachiopod‐dominated shell concentrations occur within the turbiditic rocks of the Miocene Grand Bay Formation on the Antillean island of Carriacou. The shells, mainly Tichosina, form dispersed and loose accumulations. Exceptional conditions on the deep‐water flanks of submarine volcanic edifices encouraged the recruitment of large populations of Tichosina and the subsequent entrainment of the shells in gravity‐driven deposits. The shells are a rare example, in the Caribbean Basin, of transported Archaic‐type shell beds constructed by a taxonomic group more typical of the Palaeozoic fauna. Copyright © 2008 John Wiley & Sons, Ltd.

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 argillaceous lime mud. Among these, undoubted reefs were constructed from a framework of digitate Rankenella hamdii with thick Girvanella encrustations. These encrustations locally developed as subvertical columnar ministromatolites, which could also merge laterally to form more extensive masses. Subsequent pervasive cementation generated isopachous rinds that preserved the reef framework intact. Coeval and younger Cambrian anthaspidellid–calcimicrobial reefs are known from California–Nevada and Texas, USA. These were heralds of the Early Ordovician resurgence of metazoan reefs.

Macroevolution of ecosystem engineering, niche construction and diversity

Organisms influence their environments through activities that range from bioturbation to modification of redox gradients and construction of structures. Some of these activities modify the selective regime of the builder (niche construction) and some influence the ecological success of other species (ecosystem engineering) as well as their evolutionary prospects. In this article, I argue that these processes produce effects that persist over geological time, modulating macroevolutionary patterns and diversity. Examples include greater sediment bioturbation and increased thickness and persistence of shell beds. The impact of these processes has been increasing over time, with recent communities encompassing greater ecosystem engineering than those of the early Phanaerozoic. Thus, positive feedback through environmentally mediated selection seems to have increasingly enhanced biodiversity through the Phanaerozoic.

Origin and composition of organic matter in tidal flat sediments from the German Wadden Sea

Two sediment cores of up to 550 cm length from an intertidal flat of the German Wadden Sea near the island of Spiekeroog were investigated for the quantity and composition of fossil organic matter (OM). The lowermost parts of the cores are dominated by grey mud of a salt marsh facies containing mainly terrestrial OM estimated to account for 60–75% of the total OM, based on δ 13C values and the ratio of short to long chain n-alkanols. The terrigenous origin of the dominant fraction is indicated, among others, by high proportions of C 29 sterols and long chain n-alkanes typical of plant waxes. Coarse shell beds overlying the grey mud at 2–2.5 m depth represent a flooding and erosion event possibly related to heavy storm floods in the Middle Ages. Within the intertidal sand-dominated sediments in the upper parts of the cores total organic carbon (TOC) contents are generally low, ranging from 0.1% to 0.5%, and correlate well with the amount of mud fraction ( r 2 0.90). At the surface, marine OM has not undergone intense diagenetic alteration and so is the dominant fraction. Eroded peat particles are common throughout most of the sequence and values of the Phragmites peat indicator (PPI) > 5 indicate an origin from reed peat due to a high relative abundance of the n-C 24 alkane. Changes in the composition of microbial communities over the depth interval investigated are documented by varying compositions of unsaturated fatty acids with 16 and 18 carbons. Eicosapentaenoic acid (EPA) was detected along the entire cores and indicates the presence of EPA-producing bacterial strains.

Sediment supply versus storm winnowing in the development of muddy and shelly interbeds from the Upper Ordovician of the Cincinnati region, USAThis article is one of a series of papers published in this Special Issue on the themeThe dynamic reef and shelly communities of the Paleozoic. This Special is in honour of our colleague and friend Paul Copper.

Shell-bed development can be a product of complex sedimentological and biological factors. The Upper Ordovician sediments near Cincinnati, Ohio constitute a succession of thinly interbedded shelly carbonates and mudrocks. Despite years of study, the development of Cincinnatian shell beds and metre-scale cycles has, until recently, been attributed solely to storm reworking. This “storm-winnowing model” treats shells as passive sedimentary clasts, ignoring other factors of shell-bed development. A recently proposed alternative is Brett and Algeo’s idea that these shell beds grew during long periods of normally low sedimentation, while most mud accumulated during brief periods of high sedimentation. Under this “episodic starvation model,” any storms would winnow pre-existing muds and shell beds alike. We tested both models in the Edenian–Maysvillian (early to mid Katian) strata of the Cincinnati region by compiling observations on their petrologic, taphonomic, and paleoecologic characteristics. The storm-winnowing model does not explain several observed features that the episodic starvation model does, including (i) storm-related sedimentary structures in mudrocks and limestones; (ii) lack of a sufficiently fossiliferous precursor deposit to winnow; (iii) deep-water faunas in grainstones; (iv) mixed taphonomic conditions of shell-bed fossils; (v) ubiquitous discontinuity surfaces; (vi) carbonate concretion horizons; (vii) unwinnowed shell beds; and (viii) micrite in packstones. Episodic starvation is a superior explanation because it explains all of these features and allows for the complex interplay of other environmental and biological factors that contribute to shell-bed growth. It may also be applicable to other deposits, previously interpreted as tempestites.

Water mass conditions during the past 11,000 years in the innermost part of the Kagoshima Bay, South Kyushu, Japan : evidence from the fossil shell assemblages, oxygen and carbon isotopic compositions

Quantitative analysis of fossil shell assemblage was carried out in Shinjima Island located in the innermost part of the Kagoshima Bay, South Kyushu, to confirm the change of mollusks’ habitat conditions. In addition, stable carbon and oxygen isotopic ratios of carbonate fossil shells were also measured to clarify the water-mass conditions of the Kagoshima Bay during the past 11,000 years. Both the fossil shell community and carbon isotopic ratios clearly indicate that the water-masses in the innermost part of the Kagoshima Bay were influenced strongly by the pelagic water, which replaced the fresh water mass initially present in the paleo-caldera lake. Thus, the pelagic water mass increased progressively, and became the maximum during the period between 6,000-2,300 yrs BP when the shell bed (Moeshima Shell Bed) was formed. The analysis of fossil shell assemblage and shell size distribution, however, indicates that the Neopycnodonte cochlear, the dominant species in the Moeshima Shell Bed, did not reproduce in the bay, but supplied as the planktonic larvae from the pelagic water mass. After that, it started that the influence of pelagic water-mass became to be weak at 2,300 yrs BP, when the supply of the larvae was stopped. As a result, the dominant pelagic species disappeared from the innermost part of the Kagoshima Bay, and consequently the shell community was replaced by the assemblage comprised of the inner bay species. Such a change in mollusks community appears to be depended on the time-series variation of the Kuroshio Current influence.

Macrobenthic community structure in a high-productivity region: Upper Campanian Mishash Formation (Israel)

A paleoecological study of molluscan fossils from the Upper Campanian Mishash Formation of southern Israel was undertaken to provide new insights into the inference of bottom-oxygen levels, food availability and bottom water energy on high-productivity seafloors. Quantitative data for molluscan analysis were collected from each of the main lithologies that comprises the high-productivity facies-mosaic of the Mishash sea, i.e., organic-rich carbonates, thin-porcelanite and -chert beds, and loosely- to densely-packed shell beds. Diagenesis has obliterated faunal remains in carbonate layers. Pollution studies in modern environments have demonstrated the close connection between stressful conditions – such as organic load, toxicity and water and sediment aeration – and benthic community structure, as expressed through species richness, evenness, taxonomic composition, body sizes, and trace fossil assemblages. These connections and behavior patterns were investigated in the varied benthic fossil assemblages of the Phosphate Member of the Mishash Formation. Bivalves and gastropods dominated the macrofauna in all rock types but in varying abundance and taxonomic compositions, changing from diverse assemblages (15–22 species per sample) to less rich ones (5–7 species) and even barren intervals. This range of abundance neatly describes the lateral mosaic and heterogeneous nature of the upwelling environment of the Upper Campanian and is comparable to the mosaic of sub-facies that characterize modern upwelling facies-belts today (e.g., in the Benguela upwelling system). The variations shown for the macrobenthic assemblages within the Mishash Formation high-productivity regime are associated with proximity to the high-productivity center or to periods of intensified upwelling. The inferred, variable oxygen levels and organic-load changes point up how dynamic these regions really are. This is expressed in community structure and composition, which vary between two extremes: The low-oxygen biofacies 1 is characterized by a smaller range of shell sizes, infaunal deposit feeders and low species richness, while the high-oxygen biofacies 3 is characterized by greater ranges in shell size, infaunal but also epifaunal suspension feeders and higher species richness. The chert and porcelanite beds exhibit a variety of sub-environments and seem to represent an intermediate facies that bridges the two extremes of biofacies 1 and 3. The above ecological criteria may provide evidence of the original carbonate environment for the thin-chert and -porcelanite layers. The chert or porcelanite samples that contain a moderately even and rich assemblage of species intermediate to the values of the two end-member facies are taken to represent a shallow-water carbonate marine environment marginal to the upwelling center, whereas layers of chert and porcelanite containing very low species richness and evenness are taken to characterize primary depositional environments proximal to the paleo-upwelling cells.

A Bathysiphon (Foraminifera) ‘shell bed’ from the Cretaceous of northern California, USA: Example of a parautochthonous macro-skeletal deposit in deep-ocean turbidites

Shell beds consisting of concentrations of minimally transported, slightly damaged skeletal remains of indigenous organisms—comparable to bedded shelly accumulations of certain shallow-marine environments—have rarely been reported from truly deep-ocean turbidites. The general expectation is that shelly accumulations, when they do occur, ought to be derived from upslope sources and many kilometers away from the site of deposition. A Cretaceous thin-bedded turbidite in the Franciscan Complex of northern California, however, hosts a concentration of large specimens of the giant foraminiferan, Bathysiphon aaltoi, reflecting localized transportation and deposition in the original life habitat. The tests were derived from a densely populated thicket of the bathysiphonid probably located only a few metres/10s-of-metres away, decimated by a turbidity current that either overflowed an active submarine fan channel or spread outward from a suprafan lobe. As such, this unusual bathysiphonid-rich deposit can be viewed as a kind of deep-ocean level bottom ‘shell bed’.

Identifying tsunami deposits using bivalve shell taphonomy

On 28 November 1945, in the Makran trench off Pakistan, a large earthquake (8.1 Mw) produced a tsunami that struck the coast of Oman and left a taphonomically distinct shell bed in Sur Lagoon. The shell bed was thick (5–25 cm) and laterally extensive, covering a >1 km2 area. The shell assemblage from the deposit contained a mean of 59% for articulated bivalves of allochthonous offshore and subtidal species (e.g., Tellina palatam ) as well as a mean of 20% for lagoonal species, indicating large-scale erosion and transport. Taphonomic traits (e.g., articulation, rounding, fragment angularity) of all bivalve material >5.6 mm were quantified for eight sample horizons, and compared with a tsunamite from Caesarea, Israel. Some of the taphonomic characteristics between the shell beds from these two different depositional settings were similar, and three tsunamigenic specific traits were identified: (1) thickness and lateral extent of the shell deposit, (2) presence of allochthonous articulated bivalves out of life position, and (3) extensive angular fragmentation. These results show that tsunamis form shell accumulations and cannot be ignored when assessing shell bed origin for the geological record. When these three traits are collectively found, a tsunamigenic origin should be considered for the shell bed.