Shell Beds Research Articles

Early Cambrian (Stage 4) brachiopods from the Shipai Formation in the Three Gorges area of South China

AbstractDiverse and abundant fossil taxa have been described in the lower Cambrian Shipai Formation in the Three Gorges area of Hubei Province, South China, but the taxonomy and diversity of the co-occurring brachiopod fauna are still far from clear. Here we describe the brachiopod fauna recovered from the Shipai Formation in the Three Gorges area of South China, including representatives of the subphylum Linguliformea: linguloids (Lingulellotreta ergalievi,Eoobolus malongensis, and Neobolidae gen. indet. sp. indet.), and an acrotretoid (Linnarssonia sapushanensis); and representatives from the subphylum Rhynchonelliformea: the calcareous-shelled Kutorginates (Kutorgina sinensis,Kutorginasp., andNisusia liantuoensis). This brachiopod assemblage and the first occurrence ofLinnarssonia sapushanensisshell beds permit correlation of the Shipai Formation in the Three Gorges area of Hubei Province with the Stage 4 Wulongqing Formation in the Wuding area of eastern Yunnan. This correlation is further strengthened by the first appearance datum (FAD) of the rhynchonelliform brachiopodNisusiain the upper silty mudstone of both the Shipai and Wulongqing formations. The new well-preserved material, derived from siliciclastic rocks, also gives critical new insights into the fine shell structure ofL.sapushanensis. Microstructural studies on micromorphic acrotretoids (likeLinnarssonia) have previously been restricted to fossils that were acid-etched from limestones. This is the first study to carry out detailed comparative ultrastructural studies on acrotretoid shells preserved in siliciclastic rocks. This work reveals a hollow tube and solid column microstructure in the acrotretoid shells from the Shipai Formation, which is likely to be equivalent of traditional column and central canal observed in shells dissolved from limestones.

Lake-stream transition zones support hotspots of freshwater ecosystem services: Evidence from a 35-year study on unionid mussels

Securing adequate supply of high-quality water is of increasing global importance and relies in large part on ecosystem services provided by freshwater biota. Unionid mussels are important keystone species and habitat engineers that shape freshwater ecosystems through water filtration, nutrient cycling and provision of habitats; their rapid global declines result in dramatic losses of ecosystem functions. Maintenance and enhancement of the services they provide depend on the identification of their crucial habitats. Following theoretical assumptions, this study analyses the importance of lake-stream transition zones for unionid mussels, based on data collected in 1984 and 2019 from an undisturbed stream flowing through five consecutive lakes. Mussel distribution matched the distribution of host fish and was strongly influenced by lakes: densities were highest near lake outlets, reaching 290 ind. m−2 (14.7 kg m−2) in 2019, and declined with downstream distance following a negative power function. This pattern was spatially consistent and sustained over time. All six unionid species native to north-central Europe were present, but common species (Anodonta anatina, Unio pictorum, U. tumidus) contributed about 80% of individuals and were responsible for most of the ecosystem services provided by unionid mussels. Estimated 1.9 × 106 mussel individuals inhabiting 3.2 km of stream length filtered a water volume equivalent to the total stream discharge approximately 2.5 times daily. Aggregations of spent shells, up to 17 kg m−2, accumulated downstream of lakes, forming extensive shell and mussel beds, providing habitats and contributing shell hash that improved stream-bed conditions. Globally invasive Dreissena polymorpha was present at low densities and did not spread or increase in abundance, indicating a long-term biotic resistance of the natural native community. Our study underscores the importance of undisturbed lake outlets, longitudinal connectivity of riverine ecosystems, and of common mussel species in maintaining freshwater ecosystem functionality and provision of vital services.

Stratigraphy and palaeontology of the lower Miocene Kiel Sand Member (Berchem Formation) in temporary exposures in Antwerp (northern Belgium)

The stratigraphic position of the lower Miocene Kiel Sand Member of the Berchem Formation in the Antwerp area (northern Belgium) is not well constrained and its depositional environments are poorly known. Due to a spatial limited decalcification front, the Kiel Sand Member is completely decalcified in southern Antwerp and gradually becomes fossiliferous to the north-east of the city. The stratigraphy and palaeontology of the fossiliferous sediments in three temporary exposures are presented. The dinoflagellate cyst analysis of fossiliferous horizons shows the relative progress of a transgression in the southern North Sea Basin during the early–middle Burdigalian, that probably initiated in the late Aquitanian. The Kiel Sand Member contains an important mollusc fauna, with several species reported for the first time from this member. The taphonomy and fauna of the shell beds indicate a shallow marine, high energetic depositional environment, strongly influenced by storms, currents, waves and a rather low sedimentation rate. The climate was warm-temperate to subtropical. In all studied sections, the Kiel Sand Member could be clearly distinguished from the Antwerpen Sand Member: similarities and differences are discussed. Moving to the north of Antwerp, the erosive base of the Antwerpen Sand Member cuts deeper into the Kiel Sand Member. The Early Miocene Unconformity (EMU) is suggested at this contact.

Depositional controls on lacustrine coquinas from an early cretaceous rift lake: Morro do Chaves Formation, Northeast Brazil

The Morro do Chaves Formation is a hybrid sedimentary succession composed of shell beds interbedded with mudstones and sandstones deposited during the rifting process of the Gondwana in a complex lake rift system, forming one of the thickest coquina deposits around the world. Despite being deeply investigated over the last decades in outcrops, the entire stratigraphic organization is not well understood yet, mainly because of the absence of continuous well core data, in addition to challenging geometries and facies distribution of the shell beds. This study focus on the characterization of facies and facies associations, and the development of a conceptual depositional model for the Morro do Chaves Formation, based on the analysis of four continuous well cores and the stratigraphic correlation among them. The described carbonate and carbonate-hybrid rocks are composed mainly of bivalves that form flat shell banks developed during dry periods with low tectonic activity. The siliciclastic rocks (mudstones, sandstones and hybrid rocks) occur interbedded with the carbonate rocks and indicates periods of active tectonic and/or climatic wet periods. The deposits are arranged in a hierarchical organization of 6 m-scale T-R cycles showing aggradation and progradation patterns. The system is controlled mainly by allocyclic (e.g., tectonics and climate) and the autocyclic (e.g., waves and currents) processes. The maximum regressive surface record an important change in the environment with a gradual increase of terrigenous input into the basin until the fast creation of accommodation space for the deposit of extensive shales that drowned and mark the end of the shell bank system. This study on facies, geometries and stratigraphic surfaces adds new and useful knowledge for helping in the understanding of complex coquina reservoirs and is applicable in the exploration of new areas with correlate lacustrine systems.

Influence of Crepidula fornicata on suspended particle dynamics in coastal systems: a mesocosm experimental study

In coastal environments, strong feedback can exist between benthic fauna and sediment dynamics. Benthic populations can modify sediment dynamics through biofiltration and bioturbation, while hydro- and sediment dynamics directly determine local environmental conditions for benthic organisms. However, these complex feedbacks are difficult to study in situ. Here, we carried out mesocosm experiments to characterize the effects of a benthic species (the gastropod Crepidula fornicata) on mud dynamics under hydrodynamic conditions representative of their natural habitat. Different experimental tests related to the inclusion (or not) of dead or live crepidula reveal that biofiltration can increase particle settling up to 40%. Flocculation, which is strengthened by organic matter enrichment in shell beds, also substantially increases particle settling. Thus, both processes enhance sedimentation above live crepidula habitats. Furthermore, these experiments highlight serious technical, physical and biological challenges that have to be tackled for properly investigating bio-physical interactions.

New filamentous bivalve rich event beds in the Eocene deposits from Gafsa Basin: sedimentology, sequential analysis and environmental significance in Tethyan platform

Detailed analysis of Middle Eocene deposits in the Tamerza region, located in western part of Gafsa Basin, shows the presence of several metric corrugated levels. A sedimentological study was applied to these levels showing their geodynamic characteristics, significance in energy, residence time on the sea-floor, and sequential analysis. To study these levels, detailed field work was combined with stratigraphic and sedimentological analyses, as well as systematic sampling of these levels. This facies represents shallow subtidal deposits of a moderate-energy environment. Detailed analysis of undulated facies, under petrographic microscope, reveals the stacking of micrometric sequences (400–500 μm thick) marked by an abrupt change from packstone and, sometimes, grainstone to mudstone textures. Filamentous bivalves occur in massively bedded limestones, which consist of wackestones-packstones containing planktonic foraminiferas. The filaments are smaller (200 μm in average length), isolated, and show varied orientations. In the upper part, the lamination of filament, evolution of facies texture has been a key feature in interpreting them as turbidities. The differences between the three subfacies (F1, F2 and F3) are related to the higher energy. Slope deposits include a wide variety of processes requiring detailed study to set up mechanisms involved and their geological significance. Gravitational flows seem for most, resulting from the differences in density between the fluid/sediment and the ambient fluid. Shell beds have been interpreted as hiatal shell concentrations, as transgressive system track deposits associated to special currents, or as primary biogenic concentrations, among other origins. These bio-events are the expression of communications between different local basins and can be used to establish correlations in North Africa and in other continents during the Middle Eocene interval.

Fossil decapods from the Upper Quaternary in Shinjima Island in Kagoshima, Kyushu, Japan, and description of a new species of ghost shrimp (Axiidea, Eucalliacidae).

Five species of decapod crustaceans, including Calliax nishiki sp. nov. (Axiidea, Eucalliacidae) and Carcinoplax longimana (de Haan), are described from the upper part of the Moeshima Silt Bed (latest Pleistocene to Holocene: ca 13,000 to 8,000 cal BP). Ten decapods, including Laticallichirus grandis (Karasawa Goda) and Ebalia tuberculosa (A. Milne-Edwards) are described from the Moeshima Shell Bed (Holocene: ca 8,000 to 2,000 cal BP). Calliax nishiki is abundant and Carcinoplax longimana is common in the decapod assemblage of Moeshima Silt Bed. The new species of Calliax seems to have dispersed around the deep marine setting with a reducing environment. The decapod assemblage of the Moeshima Shell Bed is characterized by a predominance of L. grandis. The present record suggests that L. grandis is abundant under sandy and gravelly bottoms mixed with shells of the lower sublittoral to subtidal zones during the Quaternary.

First report of acrotretoid brachiopod shell beds in the lower Cambrian (Stage 4) Guanshan Biota of eastern Yunnan, South China

AbstractBrachiopod shell accumulations are abundant and diverse in the lower Cambrian strata of Yunnan Province, South China, but most commonly they are composed of linguloid and acrotheloid brachiopods. Here, we describe the first record of shell beds with high-density accumulations of microscopic acrotretoid brachiopods (usually <2 mm in width) in the muddy deposits of the Wulongqing Formation (Guanshan Biota, Cambrian Stage 4) in the Wuding area of Yunnan Province. The acrotretoid shell beds from the Wulongqing Formation vary from thin mm-thick pavements to more well-developed beds, several centimeters thick. The occurrence of remarkably rich acrotretoid shell beds indicates that microscopic lingulates began to exert an important role in hardening and paving the soft-substrate seafloor during the early Cambrian evolution of Phanerozoic “mixgrounds.” The new Guanshan material is referred to a new species, Linnarssonia sapushanensis n. sp., which differs from other species of Linnarssonia mainly in having a well-developed internal pedicle tube, as well as a relatively longer dorsal median septum. The occurrence of Linnarssonia sapushanensis n. sp. in the Wulongqing Formation in eastern Yunnan extend the oldest record of the genus on the Yangtze Platform of South China back to at least Cambrian Stage 4.UUID: http://zoobank.org/3e0c3878-6ce2-4eed-87bf-e39647c310c4

Relationship of sediment influx to ostracode populations on the variably deforested Luiche and Mahale platform coasts of Lake Tanganyika, Tanzania

We investigated the hypothesis that ostracode populations are sensitive to variations in sediment discharge related to watershed differences at Lake Tanganyika. Many ostracode species are documented to be sensitive to high turbidity which increases during runoff from strong rainfall events and changes in ostracode populations can impact organisms higher in the food web. We compared surficial death assemblages of ostracodes around two sites with extensive shell beds, the highly deforested Luiche Platform area, heavily populated since the 20th Century, and the less deforested Mahale Platform area, with recent population increases, to understand the impacts of increasing sedimentation within and between sites. The study found that whereas the Mahale Platform area is less disturbed, the faunal differences between sites is not large. The data showed high species richness and abundance of ostracodes at 10–20 m depth at both sites, within shell bed zones, with no statistical difference in raw species richness between the sites. The Mahale Mountain area ostracode populations were however statistically more diverse (Shannon H) than at Luiche. We conducted a multi-response permutation procedure and found that the sites are statistically different, but with very small differences in species richness and ostracode abundance. Non-metric multidimensional scaling ordination of species assemblages shows that most samples overlap on both major axes. This study synthesized with ongoing research about shell bed communities in less disturbed regions of the lake will give insights into how conservation projects can be optimized to preserve the lake’s benthic ecosystem from the effects of deforestation.

Crassostrea patagonica (d'Orbigny, 1842) shell concentrations from the late Miocene of Río Negro province, NE Patagonia, Argentina

The Río Negro Formation (late Miocene-early Pliocene) mainly consists of continental deposits, but it contains a middle member of marine origin. It represents a transgressive-regressive sequence that can be seen at several outcrops along the N Patagonian coast. The taphonomical approach to the El Espigón marine deposits permits the identification of four main layers containing different kinds of skeletal accumulation, which mainly consists of oyster shells [Crassostrea patagonica (d'Orbigny, 1842)]. These concentrations display three different morphologies (pouches, pavements and bouquets) with a different taphonomic signature. These deposits were formed in shallow marine environments influenced by wave activity that produced valve concentrations of different entities. They contain several shell beds that represent event, composite, hiatal to lag skeletal concentrations. Trace of bioturbation in the sediment (Thalassinoides, Teichichnus) and bioerosion on the shells (Entobia, Gastrochaenolites, Caulostrepsis), and encrusters (cirripeds, bryozoans), are also abundant in the outcroup and constitute common components of these Miocene materials. Layers 1 and 2 of the sequence were deposited in shoreface(foreshore environments at the beginning of a highstand systems tract, while layers 3 and 4 were deposited at the end, or at the beginning of a forced regression, in foreshore environments, A final erosional episode cut the top of layer 4, which truncated the abundant bioturbation developed there

The utility of single foraminifera amino acid racemization analysis for the relative dating of Quaternary beach barriers and identification of reworked sediment

Abstract The identification and correlation of a sequence of Pleistocene beach barriers located within the northern Coorong Coastal Plain, South Australia, is made difficult by long-term subsidence and extensive reworking. Associated shell beds, to all but the last interglacial barrier, are subsurface and not readily accessible. The whole-rock amino acid racemization method was found to be unsuitable for distinguishing age differences of the individual barriers due to the low calcium carbonate content of the sediments, concerns for the leaching or contamination as a result of the comminuted nature of the shelly component, and the potentially high content of reworked sediment. These difficulties prompted an investigation into the applicability of AAR analysis of single foraminifera tests to ascertain whether the relative age of beach barriers could be determined and whether sediment contribution from antecedent barriers could be identified. Foraminifera tests of multiple genera (including Cibicides corticatus and Rosalina bradyi) from numerous modern depositional environments were analyzed to establish aminozones of aspartic acid and glutamic acid D/L values representative of modern/Holocene foraminifera and to assess natural variability of Holocene D/L values between depositional settings. Tests of Lamellodiscorbis dimidiatus, common in the Pleistocene barriers, were analyzed from MIS 5 and MIS 7 sediments and aminozones identified, allowing correlation of non-contiguous barrier features and the identification of reworked foraminifera. Rejection criteria removed, on average, between 25 and 30% of analytical results. The Holocene aminozone for every genus analyzed falls within the same general constraint: D/L values less than 0.40 for aspartic acid and less than 0.20 for glutamic acid. The D/L values of approximately 15% of foraminifera from the modern sediments exceed the Holocene aminozone constraint and the tests are considered reworked. The range of D/L values representative of each interglacial aminozone decreases with age, reflecting the slowing rate of amino acid racemization with time. Pleistocene sediments are found to have a higher percentage of reworked tests. Thermoluminescence dating of the Pleistocene successions was used to evaluate the AAR results. This work illustrates that foraminifera tests are surprisingly robust and can survive potentially multiple phases of deposition and reworking.

The Hettangian–Sinemurian (Lower Jurassic) strata of Redcar, Cleveland Basin, NE England: facies and palaeoecology

The foreshore at Redcar hosts the oldest Jurassic succession exposed on the Yorkshire-Cleveland Coast but has received little attention since the late nineteenth century. Temporary removal of beach sands by winter storms early in 2018 allowed for a sedimentological and palaeontological study of a nearly 60 m-thick foreshore section. The rocks are latest Hettangian to Early Sinemurian in age (Early Jurassic) and comprise five coarsening-upward cycles (parasequences) that grade from mudstones through siltstones into Gryphaea -rich shell beds. Ammonites are reasonably common along with a diverse benthos that includes abundant bivalves (e.g. Gryphaea, Cardinia, Luciniola, Plagiostoma and Oxytoma ) and rarer serpulids, gastropods, foraminifera and solitary corals. In the upper part of the section thicker-shelled taxa are commonly bored by cirripedes and bryozoans. Despite relatively high benthic diversity and intense bioturbation, pyrite framboids are common with size distributions suggesting deposition under a moderately oxygen-restricted water column. The Redcar succession post-dates the end-Triassic mass extinction by as much as 4 Ma and the diverse marine assemblages indicate that recovery was substantially complete by this time.

The environmental factors limiting the distribution of shallow-water terebratulid brachiopods

AbstractThe Cenozoic genusTerebratulaseems to be an exception to the post-Permian trend in brachiopod retreat to offshore habitats, because it was species rich and numerically abundant in warm-temperate shallow-water environments in the Mediterranean and the Paratethys realms. This was so despite the general dominance of bivalves and the pervasive bioturbation and predation pressure during the Neogene.Terebratula, however, went extinct in the Calabrian (Pleistocene). The optimal environmental conditions forTerebratuladuring its prime are poorly known. The Águilas Basin (SE Spain) is an ideal study area to investigate the habitat ofTerebratula, because shell beds of this brachiopod occur there cyclically in early Pliocene deposits. We evaluate the paleoecological boundary conditions controlling the distribution ofTerebratulaby estimating its environmental tolerances using benthic and planktic foraminiferal and nannoplanktic assemblages and oxygen isotopes of the secondary layer brachiopod calcite. Our results suggest thatTerebratulain the Águilas Basin favored oligotrophic to mesotrophic, well-oxygenated environments at water depths of 60–90 m. Planktic foraminiferal assemblages and oxygen isotopes point to sea-surface temperatures between ~16°C and 22°C, and bottom-water temperatures between 17°C and 24°C. The analyzed proxies indicate thatTerebratulatolerated local variations in water depth, bottom temperature, oxygenation, productivity, and organic enrichment.Terebratulawas probably excluded by grazing pressure from well-lit environments and preferentially occupied sediment-starved, current-swept upper offshore habitats where coralline red algae were absent. Narrow temperature ranges ofTerebratulaspecies might have been a disadvantage during the high-amplitude seawater temperature fluctuations that started about 1 Ma, when the genus went extinct.

Amusium cristatum shell beds in the lower Pliocene deposits of Almería-Níjar Basin (SE Spain)

Densely packed monospecific-paucispecific Amusium cristatum-dominated shell beds are signifi cant features in the lower Pliocene deposits of the Almería-Níjar Basin (SE Spain), embedded in distal prodelta fi ne-grained sand and silt. The shells in these concentrations are exceptionally well preserved: articulation is relatively high, fragmentation is low, abrasion is almost lacking, and many shells preserve the original colour pattern. These taphonomic signatures suggest in situ deposition forming autochthonous fossil assemblages, i.e. biological concentrations. Benthic foraminiferal assemblages found in the prodelta deposits indicate that they formed in mesotrophic mid-shelf settings.The A. cristatum accumulations show similarities withrecent concentrations of A. balloti from the Shark Bay (NW Australia). The latter species form dense populations in restricted positions of the bay from 10 to 75 m depth. The lowermost depth range of A. balloti is similar to the palaeobathymetry inferred for the A. cristatum beds. Furthermore, the shell beds studied occur in the centre of a relatively enclosed palaeobay. Apart from these coincidences, the proximate ecological causes accounting for the recent concentrations of A. balloti are still obscure. Thus, no straightforward actualistic comparison explains the biological origin of the A. cristatum shell beds. Based on benthic foraminiferal assemblages in the prodelta deposits, A. cristatum concentrations are most likely related to intermittent nutrient and food-particle supply from several deltas feeding the palaeobay. The advance of the deltas to the centre of the basin guaranteed rapid burial and the excellent preservation of the shells.

Tempestita del Holoceno Medio en la isla San José, Área de Conservación Guanacaste, Costa Rica

Shell beds on coastal zones are common and can be generated by phenomena such as storms (hurricanes), tsunamis or anthropic activity. The shell, sandstone and clasts were deposited at the San Jose Island, and they are interpreted as originated by storms occurring 4360+/-30 BP (before present). This age corresponds with the last part of the Northgrippian (Middle Holocene). They constitute tempestite deposits, where Pinctada mazatlanica is the dominant species (75%), with disarticulated valves and broken borders, and which are closely packed, predominantly concave-up. There are also completed and fragmented shells of other bivalves genera such as Megapitaria sp., Cardita sp., Trachycardium sp., Anadara sp. and Spondylus sp.; as well as reworked gastropod fragments (parts of the columella and the spiral), encrusted and perforated by bryozoans, sponges, polichaetes, serpulids, and some barnacles, which constituted secondary hard substrates. Among them Mallea sp., Conus sp., Strombus sp., Strombus gracilior, Strombus granulatus, Siphonaria sp., Fissurella sp., Murex sp. with sizes These deposits can be correlated with climatic events in the Northern hemisphere, which produced an increase in storm activity and aridity in tropical regions as well as changes in global atmospheric circulation. From a hydrodynamic point of view, this deposit corresponds to several tempestite events, with maximum heightwaves of 7.6 m that corresponds to category 5 cyclonic storm.

A taphofacies interpretation of shell concentrations and their relationship with petrophysics: A case study of Barremian-Aptian coquinas in the Itapema Formation, Santos Basin-Brazil

The Itapema Formation in the Santos Basin is described as a carbonate coquina interval, a type of sedimentary rock composed mainly by shells and their fragments. These rocks have different textural characteristics related to depositional and diagenetic processes, which influence the pore system and consequently the permeability. Understanding how the different facies are distributed and their relationship with petrophysics is essential to preview the quality of a reservoir. In this study, we classify the shell beds of the Itapema Formation through taphonomic characteristics, applying the concept of taphofacies, where taphonomic patterns such as orientation of shells, degree of packing, fragmentation of shells, abrasion, rounding, and sorting (type of grains) are used to differentiate shell concentrations. Six taphofacies (Tf-1 to Tf-6) were described in a core interval of 12 m, which corresponds to the Itapema Formation. Taphofacies Tf-1 and Tf-2 are well-sorted and unsorted grainstones/rudstones with parallel-oriented and densely packed shells with a braded valves. Taphofacies Tf-3 and Tf-4 are well-sorted and unsorted grainstones/rudstones with oblique oriented shells, generally well-preserved but often with high shell dissolution. Taphofacies Tf-5 and Tf-6 are well-sorted and unsorted grainstones/rudstones with randomly oriented shells and densely packed deposits. In general, all the well-sorted taphofacies are composed mainly of shells and their fragments, do not show mud matrix or even shell fragments smaller than 0.2 mm, the size of valves varies from 0.5 to 5 mm and shell fragments are the smallest components. The unsorted taphofacies contain grains smaller than 0.2 mm, most of them peloids, and very small shell fragments. Through the analysis of thin sections, x-ray tomography (CT), high resolution tomography (H-CT) and Nuclear Magnetic Resonance (NMR) it was possible to quantify the porosity in the described taphofacies and classify them according to Choquette and Pray (1970) and Ahr (2008). In well-sorted taphofacies primary pores are preserved; the total porosity varies from 9% to 21% in thin sections, and mainly consists of interparticle, intraparticle, and moldic pores and vugs. The control of porosity varies from depositional to hybrid 1, according to Ahr (2008), and pores are generally connected. In unsorted taphofacies, preserved primary porosity is uncommon and most pores are moldic and vugs. The total porosity in this facies varies from 2 to 15%, although almost all values are between 10% and 15%. The control of porosity varies from hybrid 1 to diagenetic, based on Ahr (2008), and the connectivity varies. Despite these textural differences between well-sorted and unsorted taphofacies, in general, the porosity is good and varies between 10% and 25%. The permeability in well-sorted taphofacies varies from 0.3 to 8 D, the high values being associated with a connected porous system, whereas in unsorted taphofacies the permeability varies from 0.01 to 0.4 D due to the occurrence of isolated vugs and moldic pores. The well-sorted coquinas were interpreted as deposits influenced by storm-induced currents and waves where the energy was sufficient to winnow the finer grains and preserve the primary pores. The taphonomic characteristics indicate tempestites to shoreface deposits, whereas in unsorted taphofacies the energy was not sufficient to remove finer grains generally associated with distal tempestites and backshore deposits. • Shell concentration (coquinas) pore types and petrophysics. • Taphofacies interpretation and their relationship with porosity and permeability. • Porosity control in carbonate coquina deposits. • Permeability distribution in carbonate coquinas reservoir.

A Cenomanian–Santonian rudist–bearing carbonate platform on the northern Arabian Plate, Turkey: facies and sequence stratigraphy

Recent studies have shown that Cenomanian–Santonian carbonate sedimentary rocks rich in rudists are widespread throughout southeastern Turkey. The Derdere and Karababa formations have been analysed in the Sabunsuyu section (Kilis Province). On a field scale, we can distinguish rudist–rich beds that rhythmically alternate with bioclastic levels composed of bivalves, gastropods and roveacrinids in these formations. Although pelagic faunal elements (predominantly planktic foraminifera and pithonellids) are documented in the lowermost part of the series, the rudist–rich facies are represented mainly in the Derdere and Karababa formations. Rudists, benthic and planktic foraminifera indicate middle–late Cenomanian and Turonian–Santonian ages for the Derdere and Karababa formations, respectively. Based on studied facies, five microfacies have been recognised and categorised in three facies groups: inner ramp, mid ramp and outer ramp. Evidence such as gradual changes in facies and absence of extensive uninterrupted barrier reefs indicate that the Derdere Formation was deposited in a ramp depositional system. The sedimentology and taphonomic signature of the rudist shell beds have been described in order to obtain a better understanding of the depositional environment and the physical processes that controlled Cenomanian–Santonian sedimentation. Monospecific tabular beds characterise mainly the upper part of the series (topmost part of Derdere Formation and Karababa Formation); more complex rudist concentrations, characterised by moderate species diversity, increase upsection. The rudist levels are associated with highstand systems tract deposits because of the suitability of trophic conditions in the rudist–dominated ramp.

PALEOENVIRONMENTAL IMPLICATIONS OF TIME-AVERAGING AND TAPHONOMIC VARIATION OF SHELL BEDS IN LAKE TANGANYIKA, AFRICA

ABSTRACTThe lake bottom along structural platforms in Lake Tanganyika, Africa, is carpeted with numerous large shell beds, known to be of late Holocene age, but of uncertain assemblage process. The shell beds may be the result of sedimentological (physical) assembly processes, or biological processes, or both. Previous work focused on the distribution of shell-rich facies, and showed time averaging of the surficial shell bioclasts over the last ∼ 1600 calendar years BP. We focus on an extensive shell deposit along a deltaic platform in Kungwe Bay, Tanzania and examine time-averaging and taphonomy of Neothauma tanganyicense shells to constrain sedimentological and biological processes forming concentrations of shells. New radiocarbon dating indicates that Neothauma shells are time-averaged over the last ∼ 3000 calendar years. Younger shells predominate shallow-water and exhibit unimodal age distributions, while shells from deeper-water exhibit a broader age distribution. Taphonomic results indicate that water depth and distance from the delta river mouth influence shell abrasion and encrustation with more encrustation developing away from sediment input points. Shells with black coatings and reddish-orange oxidation patinas suggest local burial and exposure. The age-frequency distributions of the shells suggest production rates of the shells vary over time and with water depth, tracking climatically driven lake-level changes (e.g., Little Ice Age, ∼ 100–650 BP). In addition, age-distributions suggest that (1) mixing of different populations are more prevalent along the steeper deltaic slopes, and (2) recent decreasing production rates may reflect anthropogenic land-use change and attendant sedimentation, which has implications for Neothauma itself, and for organisms that are obligate occupants of the shell beds. These results suggest both climatic and depositional processes play unique roles in the distribution and accumulation of shell beds in Lake Tanganyika, which informs interpretation of similar paleoenvironments in the geologic record.

Non-condensed shell beds in hiatal successions: instantaneous cementation associated with nutrient-rich bottom currents and high bivalve production

Pelagic carbonate deposits formed by the thin-shelled, epifaunal, originally bimineralic bivalve Bositra buchi were geographically widely distributed in the Tethyan basins during the Middle Jurassic. Here, to evaluate conditions that allowed the formation of peculiar, metre-scale, densely-packed shell beds primarily formed by Bositra, we assess size distributions and preservation of this bivalve in thin sections at ten sites in the Pieniny Klippen Belt and Peri-Klippen units (Western Carpathians), representing a bathymetric transect from pelagic-platform tops with shell beds up to slope environments where small filaments occur with spicules and radiolarians. Although Bositra shell beds are modulated by transport and winnowing, three types of evidence indicate that they primarily reflect high bivalve productivity. First, we find that size distributions of this species form a bathymetric gradient, from the dominance of remains smaller than 0.5 mm in low- energy slope environments, to 0.5-2 mm on muddy platform edges, up to lensoid shell beds with large valves (~2-15 mm) occurring on platform tops exposed to bottom currents. Although sediment winnowing from shell beds contributed to the rarity of small-sized specimens in platform-top environments, the bathymetric shift in the shape of size distributions is not purely driven by fragmentation and by size-selective transport of small specimens into slope environments because the average valve thickness declines with depth and thick fragments do not occur in slope environments. High abundances of suspension-feeding Bositra preferentially associated with indicators of bottom currents at oligophotic or aphotic depths indicate that plankton productivity was probably sourced by nutrient-rich internal waves that intersected platform tops, leading to low juvenile mortality in Bositra populations. In contrast, populations in deeper environments with the limited input of particulate organic matter failed to achieve maturity. Second, the inner, originally nacreous shell layer of Bositra is now represented by neomorphic calcite that is luminescent, enriched in Mn and depleted in Mg, indicating that this layer was not dissolved in the taphonomic active zone. Third, fibrous- acicular low-Mg calcite cements that characteristically coat Bositra in shell beds show blotchy luminescence and highly irregular Mg distribution, indicating that they were precipitated as high-Mg calcite cements. Fibrous-acicular cements in shell beds do not coat upward- facing sides of valves covered by a first phase of micrite whereas they fully coat elevated portions of the same valves. Therefore, they were precipitated at very high rates concurrently with micrite deposition in shelters. Nutrient-rich bottom currents thus simultaneously increased (1) Bositra survivorship by enhancing primary productivity and (2) cementation rate by renewal of saturation of pore waters in the taphonomic active zone at platform tops. In spite of the association of Bositra shell beds with major hiatal surfaces, (1) the rapid precipitation of fibrous-acicular cements, (2) the rarity of iron-stained Bositra valves in shell beds, and (3) the significantly smaller concentrations of iron in shell-rich muds than in shell-poor muds indicate that shell beds do not represent long-term hiatal or lag concentrations. They rather represent composite shell beds that record high population densities of these bivalves at ecological time scales. Spatial variation in intensity of bottom currents and in sea- floor topography generated by faulted blocks resulted (1) in hiatal surfaces with mineralized hardgrounds at high-energy current-swept sites and (2) in preservation of up to 1 m-thick lensoid shell beds at sites with less intense but persistent currents. Bositra shell beds thus ultimately have patchy horizontal and stratigraphic distribution.

Time-richness and phosphatic microsteinkern accumulation in the Cincinnatian (Katian) Ordovician, USA: An example of polycyclic phosphogenic condensation

Millimeter-scale phosphatic steinkern preservation is a feature of the taxonomically enigmatic Early Cambrian “small shelly faunas”, but this style of preservation is not unique to the Cambrian; it is ubiquitous, if infrequently reported, from the Phanerozoic record. The polycyclic phosphogenic condensation (PPC) model envisions both the genesis and concentration of phosphatic microsteinkerns as natural outcomes of shell bed genesis through episodic sediment starvation. This model predicts that more reworked and condensed shell bed limestones will contain more phosphatic microsteinkerns, but that even the least reworked limestones may contain some phosphatic particles. We test this model through examination of vertical thin sections densely collected through a 10-meter interval from the classic Cincinnatian (upper Katian, middle Maysvillian North American Stage) upper Fairview Formation, Miamitown Shale, and lower Grant Lake formations at four localities near Cincinnati, Ohio. For each of approximately 50 distinguishable limestone depositional units in each locality, a 2 × 2 cm square was selected for study. Each square was assigned a textural classification (mud content of intergranular space) and a breakage rank (pristine to comminuted). Phosphatic particle distribution was quantified both by visual estimation and by particle counting, with counts ranging from none detected to over 1000 per 4 cm2. Our analyses show a strong positive relationship between phosphate content and both textural maturity and fragmentation. This positive relationship is consistent with the PPC model and confirms that textural maturity can reflect the degree of condensation as well as depth-related environmental energy. This finding suggests that shell bed processes of repeated deposition and reworking make a significant contribution to the generation and accumulation of phosphatic particles. If local-scale sedimentary processes and conditions can control this accumulation, temporal changes in phosphatic sediment deposition rates may be linked to earth changes more complexly than through changing ocean chemistry on a global scale.