Oncoid growth and distribution controlled by sea-level fluctuations and climate (Late Oxfordian,Swiss Jura Mountains)

Abundant lagoonal oncoids occur in the Late Oxfordian Hauptmumienbank Member of the Swiss Jura Mountains. Four oncoid types are observed in the studied sections and classified according to the oncoid surface morphology, the structure and composition of the cortex, and the texture and fauna of the encasing sediment. Micrite-dominated oncoids (types 1 and 2) have a smooth surface. Type 1 has a rather homogeneous cortex and occurs in moderate-energy environments. Type 2 presents continuous or discontinuous micritic laminae. It is associated with a low-diversity fauna and occurs in high-energy facies. Bacinella and Lithocodium oncoids (types 3 and 4) display a lobate surface. They are dominated by microencrusters (Bacinella irregularis and Lithocodium aggregatum) and are found in low-energy facies. The stratigraphic and spatial distribution of these oncoid types shows a correlation with the sequence-stratigraphic evolution of the studied interval, and thus with relative sea-level fluctuations. It can be shown that these sea-level fluctuations were controlled by orbital cycles with 100- and 20-kyr periodicities. At the scale of 100- and 20-kyr sequences, types 1 and 2 oncoids are preferentially found around sequence boundaries and in transgressive deposits, while types 3 and 4 oncoids are preferentially found around maximum floodings and in highstand deposits. This implies that changes of water energy and water depth were direct controlling factors. Discrepancies in oncoid distribution point to additional controlling factors. Platform morphology defines the distribution and type of the lagoon where the oncoids flourished. A low accumulation rate is required for oncoid growth. Additionally, humidity changes in the hinterland act on the terrigenous influx, which modifies water transparency and trophic level and thus plays a role in the biotic composition and diversity in the oncoid cortex.     

Triassic trace fossils from lacustrine shoreline deposits of the Passaic formation,douglassville, pennsylvania

Lake‐margin deposits of the Late Triassic Passaic Formation, Douglassville, Pennsylvania, have yielded a moderate variety of trace fossils. The greatest diversity and abundance of trace fossils occurs on the sole of a thin gray claystone overlain and underlain by gray siltstones. Specimens of Cochlichnus anguineus, Helminthoidichnites tenuis, Helminthopsis hieroglyphica, Treptichnus pollardi, and paired trails reflect simple, unspecialized, horizontal grazing as well as feeding traces under very shallow water lacustrine conditions. The lack of commonly associated Scoyenia burrows in these deposits may possibly be related to the degree of maturation of the organic debris available. The traces, as well as a lack of meniscate burrows, compare favorably to the Mermia ichnofacies, except Passaic deposits exhibit evidence of subaerial exposure. This unit most likely records a lacustrine expansion whereby grazing trails were emplaced under fully subaqueous conditions. Subsequent shallowing and desiccation, followed by sediment influx during rainstorms, favored preservation of these traces.

In contrast, the Scoyenia ichnofacies consists of feeding burrows of Scoyenia gracilis and Spongeliomorpha milfordensis within reddish brown siltstones and mud‐stones (redbed sequence). The Scoyenia ichnofacies records limited exploitation, by opportunistic infaunal deposit feeders, of lake‐margin nutrients carried in during occasional rainstorms that punctuated otherwise extended periods of aridity.     

The Ichnofossils of the Triassic Hope Bay Formation,Trinity Peninsula Group,Antarctic Peninsula

The Triassic Hope Bay Formation (Trinity Peninsula Group) includes a diverse ichnocoenosis in the Puerto Moro succession (Hope Bay, Antarctic Peninsula). The Hope Bay Formation is a thick turbidite succession with a minimum vertical exposure of 533 meters along the Hope Bay coast. The rocks are locally affected by contact metamorphism related to later arc magmatism. The ichnofossils are found mainly in thick- and thin-bedded sandstone-mudstone facies composed of a monotonous repetition of sandstone-mudstone cycles. The sandstones are usually medium grained, massive or parallel laminated; the mudstones are massive and rarely laminated. In the fine-grained rocks, mainly the mudstones, there are distinct densities of bioturbation, and at least six patterns were observed. The following ichnogenera were recognized: Arenicolites Salter 1857, Lophoctenium Richter 1850, Taenidium Heer 1877, Palaeophycus Hall 1847, Phycosiphon von Fischer-Ooster 1858 and Rhizocorallium Zenker 1836. All appear to be feeding-traces. The trace fossil assemblages occur mainly in black mudstones rich in organic material that suggest a low oxygen environment. The stratigraphic interval in which they occur is interpreted as progradational supra-fan lobes with channel fill and levee deposits. The thin-bedded turbidite and mudstone lithofacies, where the ichnofossils are abundant, is interpreted as a distal fan turbidite or levee deposit related to a long-term channel fill. This study is the first significant report of trace fossils in the Hope Bay Formation.     

Trace fossils of the Jurassic,blue Lias,Lyme Regis,Southern England

The Lower Jurassic, Blue Lias Formation at Lyme Regis, Dorset, England is rhythmically bedded with symmetrical rhythms consisting of laminated black shale followed by dark then pale marls (sometimes cemented into pale limestones) and back to laminated black shale. Its trace fossil fauna includes nine ichno‐genera and represents a shallow shelf paleoenviron‐ment. The ichnofauna occurs in three assemblages named after consistently occurring ichnogenera. A Chondrites assemblage (diversity = 1 ichnogenus) and a Chondrites‐Arenicolites assemblage (diversity = 2–3 ichnogenera) are typical of dark marls but may penetrate tops of laminated black shales. A thalassi‐noides‐Chondrites‐Arenicolites assemblage (diversity up to 7 ichnogenera) is typical of pale marls or limestones. Blue Lias limestones are diagenetic: dark laminated limestones and pale bioturbated limestones are cemented equivalents of laminated black shales and pale marls, respectively. However, pale burrow fills in darker sediments and vice versa confirm the rhythms are primary. Rhythms represent redox cycles, fluctuating from anoxic laminated black shales lacking mac‐robenthos or infauna (bioturbation index = 0) to oxic, bioturbated, pale marls or limestones with large mac‐robenthos such as Gryphaea or Plagiostoma and diverse infauna (bioturbation index = 5).     

黔西北桐梓水坝塘下奥陶统桐梓组碳酸盐岩微相

黔西北桐梓水坝塘剖面下奥陶统特马豆克阶桐梓组沉积时位于上扬子区陆表海极浅水区,抑或部分时段存在盐度异常的云坪,与下伏寒武系第四统娄山关群和上覆下奥陶统红花园组均呈整合接触。桐梓组岩性多为灰色、深灰色灰岩,后生动物大化石和陆源碎屑岩含量显然偏低。常见内碎屑滩相堆积,偶见含砾屑滩,砾屑边缘弱磨圆,属非强水流的近源搬运颗粒类型;少许鲕粒灰岩层。微生物岩颗粒丰富并作为碳酸盐岩的主要供物来源,部分微生物群落形成的细纹层粘结岩属小型叠层石礁相单元,表明在贵阳乌当和桐梓红花园剖面的叠层石可北延超过100km到达水坝塘地区,构建了狭长的叠层石礁相带。黔西北桐梓组部分碳酸盐岩层中出现单一而广适性的钙质微生物造礁群落,与特马豆克阶全球生物礁演化发展阶段相吻合,在奥陶纪生物大辐射的初期,生物礁具有地理分布局限、低生物多样性以及钙质微生物占主导地位的特征。     

河南泌阳凹陷古近纪核桃园组遗迹化石组合及其环境意义——兼论深水湖泊遗迹相特征

南襄盆地泌阳凹陷古近纪核桃园组湖泊沉积中发育大量遗迹化石,含5个遗迹组合,形成于从滨湖到深湖的不同沉积环境中。Taenidiumserpentinum遗迹组合反映稍有覆水并周期性暴露的低能滨湖环境;Skolithos linearis遗迹组合代表湖泊中上临滨环境;Planolites montanus遗迹组合与低能浅湖环境有关;Skolithos verticalis遗迹组合形成于浅湖的远源风暴沉积中;Semirotundichnus dongyingensis遗迹组合发育在贫氧的半深湖-深湖沉积环境中。     

Short-term colonization trace-fossil assemblages in a carbonate lacustrineKonservat-Lagerstätte (Las Hoyas fossil site, Lower Cretaceous, Cuenca, Central Spain)

Summary Rhythmically laminated fine-grained lacustrine carbonatcs of the Cretaceous La Huérguina Limestone Formation at Las Hoyas fossil site, central Spain, contain not only a spectacular body fossil fauna, but also vertebrate and invertebrate trace fossils. The study of the Las Hoyas invertebrate ichnofauna provides valuable taphonomic, paleoecological and paleoenvironmental information and represents one of the first attempts to study in detail a lacustrine trace-fossil assemblage in carbonate rocks. The Las Hoyas ichnofauna includesCruziana problematica, Helminthoidichnites tenuis, Lockeia isp.,Palaeophycus tubularis, andTreptichnus pollardi. This assemblage is characterized by low ichnodiversity and small burrow size, and it is dominated by surface trails and extremely shallow burrows produced mostly by detritus feeders. The Las Hoyas ichnofauna indicates low to moderately low energy, permanently submerged, shallow-lacustrine areas. The ichnofauna is suggestive of environmental stress, most likely oxygen-depleted conditions in interstitial waters. Occurrence of traces in event beds indicates opportunistic colonization by epifauna and very shallow infauna during brief periods of improved oxygenation related to the passage of density underflows and dilute turbidity currents. The Las Hoyas ichnofauna resembles other shallow lacustrine trace-fossil assemblages of the archetypalMermia ichnofacies and differs significantly from the archetypalScoyenia ichnofacies. It is therefore best regarded as a proximal, depauperate example of theMermia ichnofacies. The Las Hoyas assemblage is similar to the slightly older lacustrine Montsec ichnofauna, but it is remarkably different from marine ichnocoenoses of other Mesozoickonservat-lagerst?tten in lithographic limestones such as those of Cerin, Solnhofen and Nusplingen.     

Microbial encrustations from the Middle Oxfordian-earliest Kimmeridgian lithofacies in the Prebetic Zone (Betic Cordillera, southern Spain): characterization, distribution and controlling factors

Two main types of microbial encrustation were identified in Middle Oxfordian to lowermost Kimmeridgian deposits in the Prebetic Zone (southern Spain), showing existing relationships between skeletal content, fabric and morphology of these organosedimentary structures. Laminated planar and concentric encrustations relate to peloidal fabrics (mainly constituted of microbes = microbial laminated fabrics s. str. and microbial oncoids s. str.), as well as to dense microbial fabrics periodically colonized by encrusting foraminifera (microbial laminated fabrics with nubeculariids and microbial oncoids with nubeculariids). Sedimentation rates, substrate stability and grain size, as well as illumination, influenced microbial growth pattern as major controlling factors in low-energy conditions, and forced palaeogeographic and stratigraphic patterns of distribution. Significant encrustation was identified in terrigenous-poor lithofacies from the middle (Transversarium-Bifurcatus zones) to the outer (Transversarium-Bimammatum zones) shelf in the Prebetic Zone. Rare-to-absent encrustation characterized terrigenous-rich deposits (Bimammatum and Planula zones) in the area.     

Plio-Pleistocene facies environments from the KBS Member, Koobi Fora Formation: implications for climate controls on the development of lake-margin hominin habitats in the northeast Turkana Basin (northwest Kenya)

Climate change is hypothesized as a cause of major events of Plio-Pleistocene East African hominin evolution, but the vertically discontinuous and laterally confined nature of the relevant geological records has led to difficulties with assessing probable links between the two. High-resolution sedimentary sequences from lacustrine settings can provide comprehensive data of environmental changes and detailed correlations with well-established orbital and marine records of climate. Hominin-bearing deposits from Koobi Fora Ridge localities in the northeast Turkana Basin of Kenya are an archive of Plio-Pleistocene lake-margin sedimentation though significant developmental junctures of northern African climates, East African environments, and hominin evolution. This study examines alluvial channel and floodplain, nearshore lacustrine, and offshore lacustrine facies environments for the approximately 136-m-thick KBS Member (Koobi Fora Formation) exposed at the Koobi Fora Ridge. Aspects of the facies environments record information on the changing hydrosedimentary dynamics of the lake margin and give insights into potential climatic controls. Seasonal/yearly climate changes are represented by the varve-like laminations in offshore mudstones and the slickensides, dish-shaped fractures, and other paleosol features overprinted on floodplain strata. Vertical shifts between facies environments, however, are interpreted to indicate lake-level fluctuations deriving from longer-term, dry-wet periods in monsoonal rainfall. Recurrence periods for the inferred lake-level changes range from about 10,000 to 50,000 years, and several are consistent with the average estimated timescales of orbital precession ( approximately 20,000 years) and obliquity ( approximately 40,000 years). KBS Member facies environments from the Koobi Fora Ridge document the development of lake-margin hominin habitats in the northeast Turkana Basin. Environmental changes in these habitats may be a result of monsoonal rainfall variations that derive from orbital insolation and/or glacial forcing.     

MASS-MORTALITY LAYERS OF FOSSIL STICKLEBACK FISH: CATASTROPHIC KILLS OF POLYMORPHIC SCHOOLS

Stickleback fish (Gasterosteus doryssus) from late middle-Miocene lacustrine deposits of the Truckee Formation in west-central Nevada are abundant and well preserved. They occasionally occur in unusually high densities on single annual laminations (varves) in “mass-mortality layers.” We demonstrate that stickleback mass-mortality layers consist of members of schools and may be used as population samples. Comparison of stickleback mass-mortality samples to time-averaged samples, which accumulated over hundreds or thousands of years in the same deposit, indicates that, for some purposes, the time-averaged samples are acceptable surrogates for instantaneous population samples from the G. doryssus lineage. Mass-mortality and time-averaged samples are similar for variation of pelvic structure and dorsal-spine number, but associations between states of different characters may be weaker in mass-mortality samples than in time-averaged samples. Thus, character variances in time-averaged samples of G. doryssus are comparable to those of living populations, but character correlations are suspect. Character variances and correlations in time-averaged fossil samples must be evaluated on a case-by-case basis and interpreted with caution.     

Lacustrine sedimentation in the diapir-controlled Miocene Bicorb Basin, eastern Spain

The Miocene Bicorb Basin is a small elongated basin developed by normal faulting of a thick Jurassic–Cretaceous carbonate succession and subsequent diapirism of Upper Triassic mudstones and evaporites. The basin fill comprises a sequence over 650 m thick formed by two units. The lower, alluvial unit consists of a complex alternation of conglomerates, sandstones, mudstones, and minor lacustrine limestones. The upper unit comprises marginal alluvial and lacustrine deposits in which seven lacustrine facies associations have been distinguished. The inner lacustrine deposits comprise mudstones and carbonates with minor evaporitic deposits. At the northeast and southwest basin edges, alluvial inputs led to the development of delta and fan-delta environments where sandstone and conglomerate deposits dominate. The latter stages of the lake evolution are represented by an expansive thick limestone sequence which close to the NW fault-bounded margin pass laterally to breccias. The hierarchical arrangement of the upper unit shows five different orders of sequences. The first-order sequence defines a major vertical trend of lake expansion whereas three second-order sequences are linked to major flooding-expansion episodes. All these sequences are related to the tectonic evolution of the diapir. Metre-scale, third- and fourth-order sequences and fifth-order sequences, which consist of finely laminated rhythmite deposits probably record climatically forced processes. The diapir–graben system evolution and the climatic variations have exerted major controls on the sequential arrangement and evolution of the lacustrine system. Both factors have also strongly influenced the changes in the gastropod assemblages.     

河南登封寒武系第三统张夏组核形石与遗迹化石的耦合变化

河南登封关口剖面寒武系第三统张夏组下部碳酸盐岩中发育了大量的微生物成因的核形石和后生动物遗迹化石。在野外和显微镜下对核形石和遗迹化石进行观察,并统计它们在地层中所占比例,表明核形石和遗迹化石存在着耦合关系。下部地层以发育凝块石和形状不规则、纹层不连续、代表一种低能弱搅动水体的Ⅰ型核形石为主,不含遗迹化石,表明此时微生物在海洋生态系统中占主导地位。中部地层则以发育浑圆形、纹层连续的Ⅱ型核形石和遗迹化石Planolites为特征;而且随水体能量的增强,Planolites丰度逐渐升高,核形石丰度逐渐降低;二者的丰度变化说明后生动物的存在对核形石的数量有一定影响,但未破坏核形石的生长条件。上部地层发育大量Thalassinoides和生物扰动构造,缺乏核形石;后生动物对沉积基底进行反复扰动,彻底破坏了原始层理以及微生物造岩的环境,核形石消失。可见,在张夏组沉积时期,微生物与后生动物以及环境之间存在着特殊的相互作用关系。     

FRESHWATER ALGAE FROM THE UPPER TRIASSIC CUYANA BASIN OF ARGENTINA: PALAEOENVIRONMENTAL IMPLICATIONS

Abstract: Diverse freshwater aquatic palynomorphs are present among rich terrestrial palynofloras from the upper part of the Potrerillos and Cacheuta formations outcropping in the Cuyana Basin at the southern extremity of the Precordillera of Argentina. The Potrerillos/Cacheuta sequence includes fluvial, deltaic and lacustrine facies deposited in the early Late Triassic fault‐bounded syn‐rift Cacheuta hemigraben. The phytoplankton described and illustrated consist of representatives of colonial chlorococcalean algae belonging to the Hydrodictyaceae (Plaesiodictyon) and Botryococcaceae (Botryococcus), a diverse group of Zygnemataceae zygospores, a freshwater dinoflagellate cyst (Bosedinia) and acritarchs (sphaeromorphs). Zygnematacean zygospores are represented by species of Gelasinicysta?, Lecaniella, Mougeotia, Ovoidites, Peltacystia and Schizocystia. Of these, Gelasinicysta? cuyanensis sp. nov. is newly described. Quantitative analysis of the palynofloras permits interpretation of changes in the local vegetation and phytoplankton communities controlled by changes in environmental setting. Two algal associations are recognized as belonging to different stages in the evolution of the basin. Diverse zygnematacean assemblages along with hydrodictyacean, botryococcacean and leiosphere/sphaeromorph algae are common in fluvial–deltaic environments of the upper part of the Potrerillos Formation and the lowermost part of the Cacheuta Formation. Higher in the sections (in the Cacheuta Formation) Botryococcus dominates, in association with amorphous kerogen, representing deposition in a relatively deep, quiet‐water, lacustrine environment where anoxic conditions prevailed. In the Cacheuta hemigraben the lacustrine shales have average TOC values of 4 per cent (locally reaching 20 per cent), with some terrigenous components (Type II/III kerogen), but dominant amorphous, algal‐like, organic matter (Type I/II kerogen). Oils derived from these source rocks are predominantly waxy.     

Palaeoenvironmental significance of cool‐water microbialites in the Darriwilian (Middle Ordovician) of Sweden

Well‐developed oncoids and centimetre‐sized stromatolites are reported for the first time from the Darriwilian (Middle Ordovician) cool‐water ‘orthoceratite limestone’ at Kinnekulle, Västergötland, Sweden. The characteristics and stratigraphical distribution of these microbialites show an apparent relationship to fluctuations in relative sea level. The most abundant and well‐developed oncoids occur in stratigraphical intervals that are characterized by notable sea‐level lowstands. Stromatolites, which share many compositional characteristics with the oncoids, are essentially confined to a single bed associated with an especially prominent lowstand. Stromatolite‐like lamination also occurs in the uppermost part of the studied succession, but this feature may be of abiogenic origin. The microbialites appear to be originally calcareous, but synsedimentary iron‐ and/or phosphate‐enriched laminae are conspicuous, and secondary substitution by coarse calcite and barite is common. Iron staining is most prominent in poorly preserved specimens. Diagenesis has occluded the identity of the producers of these microbialites, but characteristics of associated endolithic borings suggest that they were formed in photic waters. The laminated fabrics of the documented microbialites record a depositional environment sensitive to high‐frequency environmental change. Most significantly, the microbialites have provided important information about the depositional environment of their enigmatic host limestone, and the collective observations challenge the notion that the studied strata were deposited in a deep shelf to basinal environment – rather, it appears that they are to a large extent, shallow‐water deposits, formed in waters only a few tens of metres deep.     

THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FORM MODERN FRESHWATER SETTINGS1

Some modern laminated find calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolites growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventually become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a propertuy they share with cyanobacterial stromatolite builders.     

Middle Jurassic (Bathonian) encrusted oncoids from the Polish Jura,southern Poland

Oncoids from two localities (Ogrodzieniec and Blanowice) of the Polish Jura, southern Poland, have been investigated with respect to their genesis and paleoecology. These oncoids occur within Middle Jurassic (Bathonian) deposits. Those from Ogrodzieniec are large, elliptical, and embedded within a presumably condensed carbonate bed. Those from Blanowice, on the contrary, are significantly smaller, irregular to box-like in shape, and occur within the ore-bearing clays. The oncoids from both localities consist of a distinct carbonate core and laminated cortex that is significantly thicker and better preserved in the Ogrodzieniec oncoids. SEM and optical microscopic investigation of the oncoid cortices revealed the presence of carbonate and silicate layers with web-like structures similar to those occurring in recent cyanobacterial microbialites. Thus, the oncoid cortices investigated may have formed in a photic zone environment with the aid of coccoid and filamentous cyanobacteria. Oxic conditions prevailed during oncoid cortex formation within the siliciclastic setting, which is manifested by low total organic carbon content, high pristane/phytane (Pr/Ph) ratio, and significant predomination of the C₃₁ homohopanes. On the cortices’ surfaces, as well as between particular laminae, various encrusting organisms have been found. The encrusters, dominated by serpulids and bryozoans, are cryptic species that inhabited the undersides and recesses of the oncoids. Their presence on both the upper and lower surfaces of the oncoids indicates that the oncoids were episodically overturned on the seafloor. The much better developed cortex lamination and much higher diversity and abundance of encrusters in the Ogrodzieniec oncoids may point to better trophic conditions prevailing in a shallower marine environment characterized by transparent waters, as opposed to a deeper siliciclastic environment with less transparent waters and probably worse trophic conditions prevailing during formation of the Blanowice oncoids.     

The Permian Moradi Formation of northern Niger: Paleosol morphology,petrography and mineralogy

Three basic paleosol morphologies, named Type A, Type B and Type C, are described from the middle–upper Permian strata of the Moradi Formation, Tim Mersoi Basin, northern Niger. The Moradi Formation is a typical alluvial redbed succession dominated by red mudrocks with fine to coarse-grained pebbly channel sandstones and matrix-breccias. Type A paleosols are hosted by well-sorted fine to medium grained trough cross bedded and massive sandstones and preserve abundant vertical to horizontal micritic and microspar calcite tubules, interpreted as rhizoliths. Lateral variability of rhizoliths in Type A paleosols, and their close association with fluvial channel-fill sediments suggests they are the roots of grove stands of phreatophytic vegetation that grew within unstable anabranching stream systems. Type B paleosols are hosted by mudrocks and preserve well-developed ped structure, abundant micritic calcite nodules and vertically-stacked micritic calcite nodular bodies, as well as rare calcite with satin-spar texture interpreted as a pseudomorphic replacement of pedogenic gypsum. The morphology of Type B paleosols suggests they were formed in well-drained floodplain deposits on stable landforms. Type C paleosols are similar to Type B but preserve pedogenic structures indicative of soil volume expansion and contraction, as well as more abundant Stage II pedogenic carbonate nodules. The morphology of Type C paleosols suggests that they developed periodically rather than seasonally in poorly-drained deposits that nevertheless occupied a relatively stable part of the landscape such as the plains flanking ephemeral lakes or sabkhas.X-ray diffraction analysis of the < 2 μm fraction from the Moradi Formation strata indicates that paleosol phyllosilicates are composed of illite, smectite, and occasionally kaolinite and talc. Illite is likely a detrital mineral, whereas smectite and kaolinite are likely pedogenic weathering products. The presence of talc in the Moradi Formation paleosols is unusual. It is limited to paleosol horizons that also preserve evidence for pedogenic gypsum accumulation and is therefore most likely related to a pedogenic weathering process. It is possible that this talc is a relatively low-temperature (~ 50–100 °C) diagenetic alteration product of pedogenic Mg–phyllosilicates such as sepiolite.The range of morphologies, petrographic textures and mineralogy of the paleosol profiles indicates semi-arid to hyper-arid climatic setting. This paleoclimatic reconstruction is in agreement with Middle and Late Permian conceptual paleoclimate models and quantitative general circulation models. Nevertheless, and in spite of an arid climate, Moradi paleosols and their host strata also indicate a relatively shallow groundwater table. Importantly, this shallow groundwater resource undoubtedly helped to support the moderately diverse fossil vertebrate assemblage and large-stature macrophytes preserved in the Moradi Formation.     

The laminated sediments of Loch Ness, Scotland: Preliminary report on the construction of a chronology of sedimentation and its potential use in assessing Holocene climatic variability

Two sediment cores, ca. 6 m long, have been recovered from the north basin of Loch Ness, Scotland. Each consists of ca. 4.5 m of laminated gyttja, terminating in a basal grey, unlaminated clay. A suite of three ¹⁴C AMS dates have been obtained, and place the base of the gyttja at ca. 9500 yr B.P. Investigations into the structure, composition and formation of the laminae are in progress. Analysis by Backscatter Scanning Electron Microscopy (BSEM) has demonstrated that they consist of couplets comprising dark, clay-rich sediments thought to be deposited from spring to autumn, and pale, silt-rich layers believed to represent sedimentation through winter. The laminae are thus thought to record incidence and intensity of streamflow into the Loch over the past nine millennia, and thus illustrate environmental change over the region for most of the Holocene. X-ray densitometry has been utilised in order to count the laminations and test the hypothesis that they are varves. It has, however, proved difficult to obtain a continuous sequence of countable laminations, although it has been possible to construct a fragmented, floating chronology which indicates that the hypothesis may be correct. Examination of lamination thickness reveals that although the average rate of sedimentation throughout the time periods studied seems to have remained fairly constant, significant variations have occurred. As Loch Ness is located on the northwest oceanic fringe of Europe, any climatic signal which the laminations contain will be closely related to even larger scale events over and within the North Atlantic Ocean, which is a major controller of global climate. Preliminary statistical investigation of sequences of laminae is being carried out in order to search for periodicity of sedimentation which may then be related to appropriate climatic indices.     

Dynamics of the lacustrine fauna from the Early Cretaceous Yixian Formation,China: implications of volcanic and climatic factors

Pan, Y., Sha, J., Fürsich, F.T., Wang, Y., Zhang, X. & Yao, X. 2011: Dynamics of the lacustrine fauna from the Early Cretaceous Yixian Formation, China: implications of volcanic and climatic factors. Lethaia, Vol. 45, pp. 299–314. The taphonomy and palaeoecology of the famous Lower Cretaceous Jehol biota of northeastern China are two of its least resolved aspects. The biota occurs in lacustrine sediments characterized by abundant volcanic ash layers. The general view is that these tuff layers correlate strongly with vertebrate mass mortality events. However, though aquatic invertebrates also suffered mass mortality, in the majority of cases individuals tend to occur on bedding planes of finely laminated sediments, suggesting that each mass mortality event is not related to volcanic activity. Based on data collected in the course of two excavations at Zhangjiagou and Erdaougou, the role of volcanic activity and other factors that could have controlled the dynamics of the fauna were investigated. Cluster analyses of fossil assemblages from both localities show similar results, and eight fossil communities are recognized. In the lacustrine Yixian Formation, frequent and often severe volcanic activity represented by the abundant tuff layers influenced the water quality, causing repeated collapse of the aquatic ecosystem. Bedding planes with remains of the eight different communities were analysed, each recording the community dynamics of a shallow eutrophic lake system that was most probably controlled by fluctuations of oxygen level related to climate. A mortality model, in which oxygen‐level fluctuations play the decisive role, is proposed to explain the existence and distribution of the fossil communities, as well as the unfossiliferous layers. □China, Jehol biota, lacustrine community, mass mortality, palaeoclimate, volcanic activity.     

松辽盆地三肇地区泉头组三段上部介形类化石的指相意义

松辽盆地三肇地区白垩系下统泉头组三段上部(杨三组—杨一组)以大套的紫红、红色砂、泥岩沉积为主,在紫红色泥岩中含有较丰富、完整的介形类及其它门类化石。以往认为,这套以红色为基调的地层属河流相沉积。对其中介形类等化石的古生态、埋藏特征及丰度变化的研究表明,该地层的沉积环境应为浅水湖泊环境,而非河流等水上环境。湖相沉积自杨三组就已存在,向上分布范围不断扩大,反映了湖泊水域面积呈现逐渐扩大的趋势。