滇东寒武系磷矿成因新见解——风暴成矿模式初探

近年来,随着风暴流和风暴岩的新理论、新概念的不断引入,在分析沉积环境、沉积成矿作用时,不断提出了许多新思路,也不断地冲击和修正以往的传统认识,作者于1986年随同成都地矿所寒武系磷矿专题组对滇东一带包括昆阳梅树村、海口、鸣矣河、晋宁王家湾、二街、八街、安宁白登、江川清水沟、宜良大滴水、寻甸先锋等矿区的磷矿进行了详细的研究,在下寒武统中谊村段及其相当的地层中发现了多层磷质风暴沉积物。本文拟概略介绍磷质风暴沉积物的标志、特征、剖面结构并初步探讨磷矿富集与风暴事件关系的沉积演化模式。     

Lithofacies, taphonomy, and brachiopod assemblages in the Silurian of western Argentina: A review of Malvinokaffric Realm communities

Distribution of faunal associations in the Silurian sequences of the Precordilleran and Northwestern Basins of Argentina is integrated with sedimentologic and taphonomic data, demonstrating that marked temporal and spatial changes in the Malvinokaffric brachiopod assemblages are related mainly to the type and intensity of the hydraulic regime of the shelf environment. The vertical arrangement of associations within each basin is controlled by local fluctuations in water depth, whereas the spatial variations are linked to minor environmental changes (substrate type and oxygen content). The basal strata of the Middle-Upper Silurian Los Espejos Formation of the Precordilleran Basin are mudstones of an open-shelf environment. These pass upward into facies of a storm-dominated shelf and shoreface. Four brachiopod assemblages have been recognized in the muddy facies: the Harringtonina Low-Diversity and Australina-Dominated associations characterize distal storm beds; the Harringtonina High-Diversity and Coelospira-Dominated communities occur in the fine-grained rocks of proximal tempestites. Storm shell beds, which dominate the middle-upper part of the sequence, are represented by allochthonous and parautochthonous coquinas. The Clarkeia Association flourished in shallow-water, high-energy, nearshore environments. Silurian rocks in the Northwestern Basin were deposited in an offshore, low-energy, restricted environment, and two main faunal assemblages have been recognized: a more distal Heterorthella Association and a relatively more proximal Ancillotoechia Association.     

四川龙门山区下泥盆统的介壳岩形成模式及其意义

四川龙门山区(前山)早泥盆世地层中,特别是在白柳坪、甘溪、谢家湾三组地层中,含有相当数量的介壳层。它包括呈两向伸展的介壳毯和三向伸展的介壳层(包括透镜体),据在地层中产出的层位,自身的特点及其与围岩的关系等方面综合分析,表明介壳毯形成于正常波基面向海一侧,介壳岩则是位于波基面以下,正常风暴浪基面附近向海一侧的部位;前者主要为簸扬作用和沉积速率快慢相间的幕式沉积过程的结果,后者则和风暴作用有关,属近积风暴岩,介壳岩的研究不仅对正确划分白柳坪、甘溪、谢家湾三组提供了充分的岩性依据,而且也为正确解释古环境及盆地沉积史提供大量可靠依据。     

Late Eocene marginal marine deposits and paleoenvironment characterisation from the Maadi Formation (Northern Eastern Egypt)

Two beds containing relatively thick and highly concentrated oyster shell banks occurring in the densely fossiliferous Wadi Al Abraq outcrop (Upper Eocene Maadi Formation, Cairo-Sukhna Road, Egypt). Such shell beds are almost monospecific. The upper level consists mainly of Ostrea clotbeyi Bellardi, topped by a considerable concentration of Carolia placunoides Cantraine, while in the lower level occurs Ostrea multicostata Deshayes. In both beds, the upper contact with the overlying marl bed (about 2 m thick) is sharp and sometimes erosive. Similarly, the contact with the underlying shale (1.5 m thick) is sharp and erosional. The shell beds are composed mainly of loosely packed oyster shells and their fragments (usually with a high proportion of disarticulated specimens) set in the sandy limestone matrix. The collected oysters show bioerosion and skeletobiont assemblages. Bioerosion trace fossils comprise ten ichnospecies while skeletozoans comprise five taxa. In general, shells of O. clotbeyi show a higher incidence of bioerosion than O. multicostata. The shells' chaotic orientation and their moderate fragmentation indicate that the shell bed formation was associated with high energy events. On the other hand, the low frequency of articulated specimens suggests that the shell beds are parautochthonous oyster banks' remnants. The taphonomic features of the studied assemblage indicate deposition in a shallow-water, wave-dominated environment.     

Lower Devonian tempestites in western Yangtze,South China: insight from Zoophycos ichnofabrics

A Lower Devonian (Emsian) tempestite in the western Yangtze plate, China, was studied based on ichnological and stratigraphical features. The results indicated that the local tempestite can be grouped into clastic constituent types, which include bottom erosion structures, graded beds, swaley cross‐stratification (SCS), parallel lamination, bioturbation and the new composition Zoophycos ichnofabrics. Five storm sequences and three Zoophycos ichnofabrics (Zoophycos–Chondrites ichnofabric, Zoophycos–Chondrites–Thalassinoides ichnofabric and Zoophycos–Thalassinoides–Palaeophycus ichnofabric) were discerned. It is demonstrated that the Zoophycos‐producers from the complex types of Zoophycos in storm sequences were opportunistic organisms (r‐strategists). This study reveals storm‐generated physical and biogenic structures and textures and provides a good ichnological method to identify storm deposits. Copyright © 2013 John Wiley & Sons, Ltd.     

Integrated ichnological and sedimentological studies of the Parahio Formation (Cambrian) of the Zanskar region (Zanskar-Spiti Basin), Northwest Himalaya

Lower-Middle Cambrian (Lungwangmiaoan-Hsuchuangian stage) Parahio Formation (Kunzam La Formation) exposed at Purni Village, Niri-Tsarap Chu Valley of Zanskar region of Tethyan Himalaya has yielded a high diversity and abundant ichnofossils with myriapod trackways. It includes Cruziana isp., Phycodes palmatum, Diplocraterion isp., Diplichnites isp., Dimorphichnus isp., Tapherhelminthopsis cf. circularis, Teichichnus isp., Monomorphicnus isp., Lockeia isp., Skolithos isp., Planolites isp., Palaeophycus isp., Isopodichnus isp., and arthropod appendage marks. Integration of ichnological and sedimentological studies for measured part of the Parahio Formation (Cambrian) reveals that alternating energy conditions due to recurring storm events were superimposed on low energy fair-weather sediments. The post-depositional Arenicolites ichnofacies, preserved within the high-energy storm beds of shoreface deposits, commonly overlies the fair-weather assemblage of pre-depositional Cruziana ichnofacies of relatively more offshore deposits, which contains more diverse and varied behavioral signatures of various deposit feeders. The traces of the Cruziana ichnofacies are abruptly replaced by traces of Arenicolites ichnofacies and show reiteration throughout the measured part of the Parahio Formation. The ichnofabric indices in these deposits range from ii3–ii5. No discrete sets of tiers are observed; perhaps there were two coeval communities. Ichnologically, an idealized wave-dominated progradational cycle in the Parahio Formation is characterized, from base to top, by a Cruziana ichnofacies (lower-upper offshore to offshore transition) and storm related Arenicolites ichnofacies (shoreface). Each progradational cycle reflects a progressive increase in sand content, degree of oxygenation, hydrodynamic energy and dearth of food. These environmental factors controlled the vertical distribution of trace fossils in Parahio Formation. The occurrence of Cruziana ichnoassociation much below the Middle Cambrian trilobite horizon in the Parahio Formation restricts the age of this part of the formation to the pre-Middle Cambrian, probably series 2 of Cambrian system.     

Shelf construction in a foreland basin: storm beds, shelf sandbodies, and shelf-slope depositional sequences in the Upper Cretaceous Mesaverde Group, Book Cliffs, Utah

The Upper Cretaceous (Campanian) Kenilworth Member of the Blackhawk Formation (Mesaverde Group) is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of North America. Analysis of this section at a combination of small (sedimentological) and large (stratigraphical) scales reveals the dynamics of progradation of a shelf-slope sequence into a subsiding foreland basin. Four major lithofacies are present in the upper Mancos and Kenilworth beds of the Book Cliffs. A lag sandstone and channel-fill shale lithofacies constitutes the thin, basal, transgressive sequence, which rests on a marine erosion surface. It was deposited in an outer shelf environment. Shale, interbedded sandstone and shale, and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave-dominated delta and its flanking strand plains prograded seaward. Analysis of grain size and primary structures in Kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies. The fine- to very fine-grained graded strata of the interbedded facies are tempestites, deposited out of suspension by alongshelf storm flows (geostrophic flows). There is no need to call on cross-shelf turbidity currents (density underflows) to explain their presence. Very fine- to fine-grained hummocky strata are likewise suspension deposits created by waning storm flows, but were deposited under conditions of more intense wave agitation on the middle shoreface. Cross-strata sets in this region are bed-load deposits that accumulated on the upper shore-face, in the surf zone. Lag strata are multi-event, bed-load deposits that are the product of prolonged storm winnowing. They occur on transgressive surfaces. While the graded beds are tempestites in the strict sense, all four classes of strata are storm deposits. The distribution of strata types and their palaeocurrent orientations suggests a model of the Kenilworth transport system driven by downwelling coastal storm flows, and probably by a northeasterly alongshore pressure gradient. The stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth, but also because of progressive sorting, which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments. The Kenilworth Member and an isolated outlier, the Hatch Mesa lentil, constitute a delta-prodelta shelf depositional system. Their rhythmically bedded, lenticular, sandstone and shale successions are a prodelta shelf facies, and may be prodelta plume deposits. Major Upper Cretaceous sandstone tongues in the Book Cliffs are underlain by erosional surfaces like that beneath the Blackhawk Formation, which extend for many tens of kilometres into the Mancos shale. These surfaces are the boundaries of Upper Cretaceous depositional sequences. The sequences are large-scale genetic stratigraphic units. They result from the arranging of facies into depositional systems; the depositional systems are in turn stacked in repeating arrays, which constitute the depositional sequences. The anatomy of these foreland basin sequences differs     

Sequence stratigraphy of the Mancos Shale,lower Tres Hermanos Formation,and coeval middle Cenomanian to middle Turonian strata,southern New Mexico,USA

Sequence stratigraphic analysis of four widely spaced outcrops of middle Cenomanian to middle Turonian strata deposited in the Western Interior foreland basin in southern New Mexico, USA, defines ten sequence boundaries in a marine shale‐rich interval ca 200 m thick. The majority of sequence boundaries are based on basinward shifts in lithofacies characterized by either a non‐Waltherian contact between distal‐bar or lower shoreface sandstone and underlying lower offshore shale, or an erosional contact between distal‐bar or lower shoreface sandstone and underlying upper offshore shale. The sequence boundaries commonly correlate basinward to packages of storm‐deposited sandstone and to beds of sandy grainstone composed of winnowed inoceramid shell fragments. In several cases, however, the sequence boundaries pass basinward into presumably conformable successions of lower offshore shale. Maximum flooding surfaces within the sequences are represented by one or more beds of locally phosphatized globiginerid wackestone and packstone or exist within a conformable succession of lower offshore shale. Following initial south/south‐westward transgression into the study area, the regional trend of palaeoeshorelines was north‐west to south‐east, although isopach data indicate that lobes of sandstone periodically spread south‐eastward across the study area. The ten sequences in the study area are arranged into a third‐order composite megasequence that is characterized by overall upward‐deepening followed by upward‐shallowing of sequences. The composite megasequence is similar but not identical to the previously established T‐1 transgression and R‐1 regression in New Mexico. Based on radioisotopic dates of bentonites, the average frequency of the sequences within the study area was ca 327 kyr, which is consistent with fourth‐order cycles of ca 400 kyr interpreted in coeval marine strata elsewhere in the world.     

Trace fossil evidence from the Adigrat Sandstone for an Ordovician glaciation in Eritrea, NE Africa

Trace fossils are described here from the Adigrat Sandstone formation of hitherto uncertain Palaeozoic-Mesozoic age in south-central Eritrea. The formation is subdivided into a lower unit, the Adi MaEkheno Member, and an upper informal unit, Member 2. The formation was deposited on the locally mudcracked top of the glacigenic Edaga Arbi Beds, suggesting that these two rock units were formed in a very short time interval. The Adi MaEkheno Member and the lower part of Member 2 contain trace fossils Arthrophycus alleghaniensis (Harlan), Arthrophycus ?brongniartii (Harlan), Didymaulichnus lyelli (Rouault), Palaeophycus tubularis Hall, Taenidium isp., thin winding ridges, winding ridges and furrows, simple cylinders, and ‘stellate’ forms. A. alleghaniensis is distinctively of Ordovician–Silurian (?Early Devonian) age. The trace fossil association belongs to the Cruziana ichnofacies that indicates a shallow marine environment between the normal and storm wave bases. The trace fossil data and stratigraphic relationships indicate that the Adigrat Sandstone formation and the Edaga Arbi Beds in Eritrea are Ordovician–Silurian in age. The Edaga Arbi Beds are correlated with other Upper Ordovician (Hirnantian) glacial units in northern Africa and the Arabian Peninsula, lending these beds the status of a marker unit in the Lower Palaeozoic stratigraphy of the Horn of Africa. The Jurassic “Adigrat Sandstone” in central-west and eastern Ethiopia cannot be correlated with the Adigrat Sandstone formation in its type area and in Eritrea.     

An example of a highly bioturbated,storm‐influenced shoreface deposit: Upper Jurassic Ula Formation,Norwegian North Sea

Integrated ichnological and sedimentological analyses of core samples from the Upper Jurassic Ula Formation in the Norwegian Central Graben were undertaken to quantify the influence of storm waves on sedimentation. Two main facies associations (offshore and shoreface) that form a progradational coarsening upward succession are recognizable within the cores. The offshore deposits are characterized by massive to finely laminated mudstones and fine‐grained sandstones, within a moderately to highly bioturbated complex. The trace fossil assemblage is dominated by deposit‐feeding structures (for example, Planolites, Phycosiphon and Rosselia) and constitutes an expression of the proximal Zoophycos to distal Cruziana ichnofacies. The absence of grazing behaviours and dominance of deposit‐feeding ichnofossils is a reflection of the increased wave energies present (i.e. storm‐generated currents) within an offshore setting. The shoreface succession is represented by highly bioturbated fine‐grained to medium‐grained sandstones, with intervals of planar and trough cross‐bedding, thin pebble lags and bivalve‐rich shell layers. The ichnofossil assemblage, forming part of the Skolithos ichnofacies, is dominated by higher energy Ophiomorpha nodosa ichnofossils and lower energy Ophiomorpha irregulaire and Siphonichnus ichnofossils. The presence of sporadic wave‐generated sedimentary structures and variability in ichnofossil diversity and abundance attests to the influence of storm‐generated currents during deposition. As a whole, the Ula Formation strongly reflects the influence of storm deposits on sediment deposition; consequently, storm‐influenced shoreface most accurately describes these depositional environments.     

Taxonomy, facies relationships and palaeobiology of bakevelliid bivalves from the Lower Cretaceous of west-central Argentina

Cretaceous bakevelliids in Argentina are restricted to the Lower Cretaceous. They are recorded in shallow marine deposits in two Mesozoic basins located in the Andes foothills in Patagonia. In the Austral Basin (44°–55°S) there is only one genus, Gervillella, represented by a single specimen from the Barremian. In the Neuquén Basin (30°–40°S) two genera, Gervillaria and Gervillella, occur, ranging from the Berriasian to the Lower Barremian. In the Pilmatué Member of the Agrio Formation two species are identified: Gervillaria alatior (Imlay) and Gervillella aviculoides (J. Sowerby). The former is also recorded in Mexico while the latter is more widespread, occurring in Europe, Asia, and Africa. In the Bajada del Agrio section they come from eight tabular shell beds located near and on top of shallowing-upwards cycles. They were abundant only in four of these levels. These shell beds are interpreted as having been deposited in the mid-ramp (middle of cycles), and upper mid-ramp (top of cycles). A functional analysis of shells revealed two life habits: epibyssate and endobyssate.     

Non-uniform across-shelf variations in thickness, grain size, and frequency of turbidites in a transgressive outer-shelf, the Middle Pleistocene Kakinokidai Formation, Boso Peninsula, Japan

Across-shelf variations in thickness, grain size, and frequency of sandstone beds in a transgressive outer-shelf succession were investigated from the Middle Pleistocene (ca. 0.7 Ma) Kakinokidai Formation on the Boso Peninsula, Japan. The transgressive deposits are generally muddy and contain slumps and slump scars. The intercalated sandstone beds are interpreted to have been formed from turbidity currents as a response to erosion and resuspension of sandridge-complex deposits in the southwestern upslope area during storm events. Mapping of volcanic ash beds and a transgressive surface in the base of the formation permits detailed bed-by-bed correlation of the outer-shelf sandstone beds. Although, overall, thickness, grain size, and frequency of sandstone beds decrease in the downslope direction, some sandstone beds locally thin out and coarsen in association with slump scars in the surrounding muddy deposits. These sandstone beds subsequently thicken and fine, and finally thin out in the farther downslope area. In addition to the local thinning of sandstone beds, the frequency of sandstone beds first decreases and then increases in the farther offshore direction. From this evidence, we concluded that these non-uniform patterns of across-outer-shelf variations in thickness, grain size, and frequency of sandstone beds were caused by the local increases in flow speeds and subsequent expansion and reduced speeds of turbidity currents, along with a local increase in the seafloor gradient that was induced by the development of slump scars in the transgressive outer-shelf floor. These physiographic features in the outer shelf are interpreted not to have permitted monotonous downslope thinning and fining of sandstone beds, compared with the bed-shape models of depletive turbidity currents and with the proximality trend of shelf sandstones from modern and ancient highstand-stage shelf systems.     

Coupled ‘storm‐flood’ depositional model: Application to the Miocene–Modern Baram Delta Province,north‐west Borneo

The Miocene to Modern Baram Delta Province is a highly efficient source to sink system that has accumulated 9 to 12 km of coastal–deltaic to shelf sediments over the past 15 Myr. Facies analysis based on ca 1 km of total vertical outcrop stratigraphy, combined with subsurface geology and sedimentary processes in the present‐day Baram Delta Province, suggests a ‘storm‐flood’ depositional model comprising two distinct periods: (i) fair‐weather periods are dominated by alongshore sediment reworking and coastal sand accumulation; and (ii) monsoon‐driven storm periods are characterized by increased wave‐energy and offshore‐directed downwelling storm flow that occur simultaneously with peak fluvial discharge caused by storm precipitation (‘storm‐floods’). The modern equivalent environment has the following characteristics: (i) humid‐tropical monsoonal climate; (ii) narrow (ca <100 km) and steep (ca 1°), densely vegetated, coastal plain; (iii) deep tropical weathering of a mudstone‐dominated hinterland; (iv) multiple independent, small to moderate‐sized (10² to 10⁵ km²) drainage basins; (v) predominance of river‐mouth bypassing; and (vi) supply‐dominated shelf. The ancient, proximal part of this system (the onshore Belait Formation) is dominated by strongly cyclical sandier‐upward successions (metre to decametre‐scale) comprising (from bottom to top): (i) finely laminated mudstone with millimetre‐scale silty laminae; (ii) heterolithic sandstone–mudstone alternations (centimetre to metre‐scale); and (iii) sharp‐based, swaley cross‐stratified sandstone beds and bedsets (metre to decimetre‐scale). Gutter casts (decimetre to metre‐scale) are widespread, they are filled with swaley cross‐stratified sandstone and their long axes are oriented perpendicular to the palaeo‐shoreline. The gutter casts and other associated waning‐flow event beds suggest that erosion and deposition was controlled by high‐energy, offshore‐directed, oscillatory‐dominated, sediment‐laden combined flows within a shoreface to delta front setting. The presence of multiple river mouths and exceptionally high rates of accommodation creation (characteristic of the Neogene to Recent Baram Delta Province; up to 3000 m Ma⁻¹), in a ‘storm‐flood’‐dominated environment, resulted in a highly efficient and effective offshore‐directed sediment transport system.     

唐古拉山中侏罗统缅甸光牡蛎(Liostrea birmanica REED)壳层的事件沉积成因及其环境意义

唐古拉山地区中侏罗统产出丰富的牡蛎类化石,特别是缅甸光牡蛎Liostrea bir—manica Reed分布更为广泛,而且多以介壳层形式产出。在唐古拉山的冬曲,窝布茸曲及青藏公路108道班等地剖面上,栖息于软基底的表生底栖双壳类Liostrea birmanica形成近原地埋葬的介壳层堆积;内生的Ceratomya动物群则多散布于沉积物中,形成原地埋葬的化石群．它们和以丛藻迹Chondrites为主体的遗迹化石群依次重复出现,构成一个和风暴沉积事件相关的底栖动物群替代序列。Liostrea birmanica介壳层是风暴高峰期的侵蚀和再沉积所形成的,风暴期以后水下沉积间断所形成的轻度固化的软基底则迅速地被潜穴生物占领(丛藻迹Chondrites ich—nosp．和根珊瑚迹Rhizocorallium ichnoSp．);而以深挖穴滤食类型为主的Ceralomya组合则是好天气时的原地埋葬。在大多数情况下,它们指示着介于正常浪基面以下,风暴浪基面以上的低沉积速率浅水碳酸岩台地沉积环境。     

Organic fraction of the total carbon burial flux deduced from carbon isotopes across the Permo-Triassic boundary at Meishan,Zhejiang Province,China

By combining the carbon cycle model with the records of carbonate and organic (kerogen) carbon isotope, this paper presents the calculation of the fraction of organic carbon burial (f _org) of beds 23–40 at the global boundary stratotype section and point (GSSP) of the Permian-Triassic boundary at Meishan, Zhejiang Province. The resulting calculation produces two episodes of f _org maxima observed to occur at beds 23–24 and 27–29, which respectively corresponds to the two episodic anoxic events indicated by the flourish of green sulfur bacteria. Two episodic f _org minima occurred at beds 25–26 and 32–34, generally coincident with the flourish of cyanobacteria (bed 26 and upper part of beds 29 to 34) as shown by the high value of 2-melthyhopnoanes. It appears that the f _org is related to the redox conditions, with greater f _org values observed under the reductive condition. The relationship between f _org and the total organic carbon (TOC) content was complex. The f _org value was low at some beds with a high TOC content (such as bed 26), while high observed at some beds with a low TOC content (e.g. bed 27). This association infers the important contribution of primary productivity to the TOC content. The original organic burial could be thus calculated through the configuration of the function of the primary productivity and f _org, which can be used to correct the residual TOC measured today. This investigation indicates that compiling the organic-inorganic carbon isotopes with the carbon cycle model favors to understand the fraction of organic carbon burial, providing information for the reconstruction of the coupling among biota, environments and organic burial. Journal of China University of Geosciences, 2007, 32(6): 767–773 [译自: 地球科学—中国地质大学学报]     

黔南罗甸沫阳中二叠世茅口组风暴岩沉积特征及地质意义

黔南罗甸沫阳剖面中二叠世茅口组以海相碳酸盐为主,下部地层中发育风暴岩,露头特征明显,主要以介壳灰岩为识别标志。风暴沉积构造包括底面侵蚀构造、截切构造、波痕层理、递变层理、块状层理等。风暴沉积主要包括一个底面构造A和B、C、D、E等四个沉积单元:A.侵蚀底面,代表风暴流对海底沉积物的作用;B.介壳灰岩层,代表风暴浪、风暴涡流沉积;C.粒序层,代表风暴衰减期重力分异沉积或风暴浊流沉积;D.块状层,代表风暴快速悬浮沉积;E.波痕层理段,代表风暴衰减后期沉积。它们共组成4种风暴沉积序列,分别代表不同深度的风暴沉积,反映了不同的风暴沉积作用和风暴流的类型。沫阳地区风暴沉积的沉积特征表明,该区风暴沉积主要发育于风暴浪基面之上的内陆棚沉积环境,风暴强度大,为近源风暴岩。仅个别风暴沉积发育于风暴浪基面之下的外陆棚沉积环境,为风暴引起的浊流沉积,为远源风暴岩。风暴岩类型的正确识别,对确定该区沉积相与深入认识扬子碳酸盐台地南部边缘沉积演化提供参考依据。     

元古界会理群凤山营组风暴流沉积特征

四川省会理县洪川桥地区,是会理群凤山营组广泛分布地区之一,出露面积达百余平方公里。在洪川桥凤山营组剖面的上部(洪剖119层—108层),发育一套约845m厚的风暴流沉积。现将其特征及形成机制作一初步分析,以供参考。一、岩性特征风暴流沉积物,主要为灰—深灰色粉砂质粉屑灰岩和部分泥质灰岩。有时两者呈薄的互层出现。一般层厚几毫米到几厘米,沿走向在数十厘米或几米内尖灭,形成一个个薄的透镜     

Fluvial, tidal and storm sedimentation in the Chilhowee Group (Lower Cambrian), northeastern Tennessee, U.S.A.

The Lower Cambrian Chilhowee Group of northeastern Tennessee consists of the Unicoi, Hampton and Erwin Formations, and is divided into four facies. The conglomerate facies occurs only within the lower 200 m of measured section (the Unicoi Formation) and consists of fine-grained to pebbly quartz wacke with rare thin beds of laminated siltstone. Low-angle to horizontally laminated, fine-grained sandstone with laminae and lenses of granules and pebbles represents upper flow-regime, overbank deposition within a braided stream system that was close to a coastline. Medium-scale, planar-tabular cross-stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels, as flood stage waned and larger bedforms ceased to migrate. Large-scale, planar-tabular cross-stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage. The sandstone facies occurs throughout the Chilhowee Group, and is therefore interbedded with all other facies. It consists of mainly medium- to very coarse-grained, subarkosic to arkosic arenite. Thinly interbedded, laminated siltstone and sandstone, which may exhibit wavy or lenticular bedding, represents deposition during slack water periods between ebb and flood tides. Large-scale planar-tabular and trough cross-stratification reflects deposition within the deepest areas of subtidal channels, whereas medium-scale cross-stratification represents deposition in shallower water on shoals separating channels. Fining- and thinning-upward sequences most likely resulted from the longshore migration of channels and shoals. The hummocky facies occurs only in the Erwin Formation and consists of horizontally laminated to hummocky stratified, fine-grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone. It represents deposition between storm and fairweather wave-base by combined-flow storm currents. The quartz arenite facies is characterized by an absence of fine-grained units and lithologically consists of a super-mature, medium- to coarse-grained quartz arenite. Large-scale planar-tabular cross-stratification and abundant low-angle cross-stratification with rare symmetrical ripples (lower quartz arenite facies) occurs interbedded with the braided fluvial conglomerate facies, and was deposited within either a ridge-and-runnel system or a system of nearshore bars. Large-scale, planar-tabular cross-stratification (upper quartz arenite facies), which forms the top of two 40 m-thick coarsening-upward sequences of the type: hummocky faciessandstone faciesquartz arenite facies, probably represents deposition on sand ridges that formed on a sand-starved shelf as transgression caused the detachment and reworking of shoreface channel-shoal couplets. Palaeocurrent data for the Chilhowee Group are unimodal but widely dispersed from 0° to 180°, and exhibit a minor mode to the west. The data are interpreted to reflect the influence of longshore, tidal and storm currents. The ichnofossil assemblage changes upsection from one characterized only by Paleophycus to a Skolithos ichnofacies and finally to a Cruziana ichnofacies. The facies sequence, biogenic and palaeocurrent data reflect the interaction through time of (I) non-marine and marine processes; and (2) transgression coupled with shoreline progradation. The Chilhowee Group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents, and finally to a storm shelf environment that periodically shoaled upward.     

Comparative analysis of marine paleogene sections and biota from West Siberia and the Arctic Region

The analysis of the main biospheric events that took place in West Siberia and the Arctic region during the Early Paleogene revealed the paleogeographic and paleobiogeographic unity of marine sedimentation basins and close biogeographic relations between their separate parts. Most biotic and abiotic events of the first half of the Paleogene in the Arctic region and West Siberia were synchronous, unidirectional, and interrelated. Shelf settings, sedimentation breaks, and microfaunal assemblages characteristic of these basins during the Paleogene are compared. The comparative analysis primarily concerned events of the Paleocene-Eocene thermal maximum (PETM) and beds with Azolla (aquatic fern). The formation of the Eocene Azolla Beds in the Arctic region and West Siberia was asynchronous, although it proceeded in line with a common scenario related to the development of a system of estuarine-type currents in a sea basin partly isolated from the World Ocean.     

High‐frequency cycles of brachiopod shell beds on subaqueous delta‐scale clinoforms (early Pliocene,south‐east Spain)

During the early Pliocene, subaqueous delta‐scale clinoforms developed in the Águilas Basin, in a mixed temperate carbonate–siliciclastic system. The facies distribution is consistent with the infralittoral prograding wedge model. Stacking patterns and bounding surfaces indicate that the clinoforms formed during the highstand and falling sea‐level stages of a high rank cycle. Twenty‐two prograding clinothems were recognized over a distance of ≥1 km. Biostratigraphic data indicate a time span shorter than 700 kyr for the whole unit (MPl3 biozone of the Mediterranean Pliocene). Cyclic skeletal concentrations and occasional biostromes of suspension feeders (terebratulid brachiopods, modiolid bivalves and adeoniform bryozoan colonies), slightly evolved glauconite and occasional Glossifungites ichnofacies formed on the clinoforms during high‐frequency pulses of relative sea‐level rise. During such stages, increased accommodation space in the topsets of the clinoforms caused a strong reduction of terrigenous input into the foresets and bottomsets. This provided favourable conditions for the development of these suspension feeder palaeocommunities. During stillstand stages, however, reduced accommodation space in the topsets eventually resumed progradation in the foresets. There, the abundance of Ditrupa tubes indicates frequent siltation events that extirpated the terebratulid populations and other epifaunal suspension feeders in the foreset and bottomset subenvironments. The occurrence of shell beds on the clinoforms suggests that this case study represents lower progradation rates than standard examples where shell beds bound the clinobedded units at their base and top only. Importantly, the distributions of biofacies and ichnoassemblage associations contribute significantly to the understanding of the effects of relative sea‐level fluctuations on the evolution of subaqueous delta‐scale clinoform systems.