吕梁－陕北地区马家沟组中小层序特征及其成因

吕梁－陕北地区马家沟组碳酸盐岩－蒸发岩向上变浅层序由一系列小层序垂向叠置而成。本文主要通过小层序特征及其分布规律的研究，认为小层序是由叠加在三级海平面变化旋回上的频繁小级别海平面升降旋圆形成的，这种小级别海平面旋回与地球轨道旋回引起的米兰科维奇旋回有关。     

Porosity distribution in cyclic dolomites of the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin,China

Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China. In this study, the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity. The Lower Qiulitag Group includes ten lithofacies, which were deposited in intermediate to shallow subtidal, restricted shallow subtidal, intertidal, and supratidal environments on a carbonate ramp system. These lithofacies are vertically stacked into repeated shallowing-upward, meter-scale cycles which are further grouped into six third-order depositional sequences (Sq1 to Sq6). There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks, including interparticle, intraparticle, and fenestral pores of primary origin, inter crystal, and vuggy pores of late diagenetic modification. The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity. In contrast, the high-energy ooidal grainstones generally have very low porosity. In this case, the microbialite-based peritidal cycles and peritidal cycle-dominated highstand (or regressive) successions have relatively high volumes of pore spaces, although highly fluctuating (or vertical inhomogeneous). Accordingly, the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity. This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.     

The St George Group (Lower Ordovician) of western Newfoundland: tidal flat island model for carbonate sedimentation in shallow epeiric seas

The St George Group consists of peritidal carbonate rocks deposited on the continental shelf of North America bordering the ancient Iapetus Ocean. These Lower Ordovician rocks are similar to other lower Palaeozoic limestones and dolostones that accumulated in epeiric seas and veneer cratonic areas worldwide. A wide variety of facies in the St George is grouped into seven lithotopes, interpreted to represent supratidal, intertidal and shallow, high- and low-energy subtidal environments. Rapid lateral facies changes can be observed in some field exposures, and demonstrated by correlation of closely spaced sections. The stratigraphic array of these lithotopes, although too irregular to be simplified into shallowing-upward cycles, suggests that they were deposited as small tidal flat islands and banks. Shallow subtidal areas around islands generated sediment and permitted tidal exchange. Tidal flat islands were somewhat variable in character at any one time, and evolved with changing regional hydrographic conditions. The St George rocks suggest an alternative theory of carbonate sedimentation in large, shallow epeiric seas, namely as small islands and banks built by processes that operated in a tidal regime. Furthermore, this island model provides a framework for a mechanism of cyclic carbonate sedimentation, by which small-scale, peritidal cycles represent tidal flat islands that accreted vertically and migrated laterally as local sediment supply from neighbouring subtidal areas waxed and waned during relatively constant subsidence.     

Depositional facies,environments and sequence stratigraphic interpretation of the Middle Triassic–Lower Cretaceous (pre-Late Albian) succession in Arif El-Naga anticline,northeast Sinai,Egypt

The Middle Triassic–Lower Cretaceous (pre-Late Albian) succession of Arif El-Naga anticline comprises various distinctive facies and environments that are connected with eustatic relative sea-level changes, local/regional tectonism, variable sediment influx and base-level changes. It displays six unconformity-bounded depositional sequences. The Triassic deposits are divided into a lower clastic facies (early Middle Triassic sequence) and an upper carbonate unit (late Middle- and latest Middle/early Late Triassic sequences). The early Middle Triassic sequence consists of sandstone with shale/mudstone interbeds that formed under variable regimes, ranging from braided fluvial, lower shoreface to beach foreshore. The marine part of this sequence marks retrogradational and progradational parasequences of transgressive- and highstand systems tract deposits respectively. Deposition has taken place under warm semi-arid climate and a steady supply of clastics. The late Middle- and latest Middle/early Late Triassic sequences are carbonate facies developed on an extensive shallow marine shelf under dry-warm climate. The late Middle Triassic sequence includes retrogradational shallow subtidal oyster rudstone and progradational lower intertidal lime-mudstone parasequences that define the transgressive- and highstand systems tracts respectively. It terminates with upper intertidal oncolitic packstone with bored upper surface. The next latest Middle/early Late Triassic sequence is marked by lime-mudstone, packstone/grainstone and algal stromatolitic bindstone with minor shale/mudstone. These lower intertidal/shallow subtidal deposits of a transgressive-systems tract are followed upward by progradational highstand lower intertidal lime-mudstone deposits. The overlying Jurassic deposits encompass two different sequences. The Lower Jurassic sequence is made up of intercalating lower intertidal lime-mudstone and wave-dominated beach foreshore sandstone which formed during a short period of rising sea-level with a relative increase in clastic supply. The Middle-Upper Jurassic sequence is represented by cycles of cross-bedded sandstone topped with thin mudstone that accumulated by northerly flowing braided-streams accompanying regional uplift of the Arabo–Nubian shield. It is succeeded by another regressive fluvial sequence of Early Cretaceous age due to a major eustatic sea-level fall. The Lower Cretaceous sequence is dominated by sandy braided-river deposits with minor overbank fines and basal debris flow conglomerate.     

华北中元古代陆表海氧化还原条件

文石海底沉淀是地球早期大气高CO2浓度、海洋贫氧条件下,CaCO3过饱和而直接沉淀于海底所形成的自生碳酸盐,可反映古海洋贫氧状态.对华北中元古界雾迷山组(ca 1.50～1.45 Ga)碳酸盐岩的研究发现:潮下带下部黑色纹层石由针状文石假晶等厚层与微生物席层交互堆叠而成;潮下带中部凝块石白云岩(A单元)主要由针状文石假...     

重庆彭水地区震旦系灯影组沉积地质新认识

重庆彭水太原乡老厂坪背斜核部的灯影组白云岩自下而上可划分为两段,下段"富藻",上段"富硅";沉积环境为浅水台地背景下的潮坪-局限台地,可据沉积物差异、沉积构造特征划分出潟湖、潮下、潮间及潮上四个微相类型。通过区域对比,建立了上扬子地区震旦系灯影组的沉积模式。纵向上,沉积相序表明其沉积时期海平面经历了两次升降变化,海侵十分短暂,海退为主体,海平面总体呈下降趋势;横向上,由北西至南东,水体逐渐加深,沉积环境经历了由潮坪-潟湖到局限台地-开阔台地,再到台地边缘以及陆棚的过渡。     

Foraminiferal biostratigraphy and sequence stratigraphy of peritidal carbonates at the Triassic–Jurassic boundary (Karaburun Peninsula,Western Turkey)

Continuous shallow marine carbonates spanning the Triassic–Jurassic boundary are exposed in the Karaburun Peninsula, Western Turkey. The studied section (Tahtaiskele section) consists of Upper Triassic cyclic shallow marine carbonates intercalated with clastics overlain by Lower Liassic carbonates. Based on the microfacies stacking patterns, three main types of shallowing-upward cycles have been recognized. Cycles are mostly composed of subtidal facies at the bottom, intertidal/supratidal facies and/or subaerial exposure structures at the top. The duration of the cycles suggests that cycles were driven by the precessional Milankovitch rhytmicity. In the sequence stratigraphic frame of the Tahtaiskele section 4 sequence boundaries were detected and globally correlated. The first sequence boundary is located at the Alaunian–Sevatian boundary nearly coinciding with the first appearance of Triasina hantkeni. The second falls in the Rhaetian corresponding to a major sea level fall which led to the invasion of forced regressive siliciclastic deposits over the peritidal carbonates. The third occurs close to the T/J boundary and the fourth lies slightly above the base of the Jurassic. In the studied section, extinction, survival and recovery intervals have been recognized based on the stratigraphic occurrence patterns of benthic foraminifera and algae. Foraminifers became nearly totally extinct in the inner carbonate shelves at the Triassic–Jurassic boundary and an interval of approximately 0.5 my passed before the begining of the recovery of Jurassic foraminifera.     

Allogenic and autogenic processes combined in the formation of shallow-water carbonate sequences (Middle Berriasian,Swiss and French Jura Mountains)

Sediment production and accumulation on shallow carbonate platforms are controlled by allogenic, externally controlled processes (such as sea level, climate, and/or platform-wide subsidence patterns) as well as by autogenic factors that are inherent to the sedimentary system (such as lateral migration of sediment bodies). The challenge is to determine how and in which proportion these processes interacted to create the observed sedimentary record. Here, a case study of Middle Berriasian, shallow-marine carbonates of the Swiss and French Jura Mountains is presented. Based on vertical facies evolution and bedding surfaces, different orders of depositional sequences (elementary, small-scale, medium-scale) have been identified in the studied sections. The hierarchical stacking pattern of these sequences and the time span represented by the investigated interval imply that eustatic sea-level fluctuations in the Milankovitch frequency band were an important controlling factor. The small-scale and medium-scale sequences relate to the 100 and 400-kyr orbital eccentricity cycles, respectively. The elementary sequences are attributed to the 20-kyr precession cycle. Differential subsidence additionally produced accommodation changes. The present study focuses on one specific small-scale sequence situated at the base of the transgressive systems tract of large-scale sequence Be4, which is identified also in other European basins. This small-scale sequence has been logged in detail at eight different outcrops in the Jura Mountains. Detailed facies analysis reveals that different depositional environments (tidal flats, internal lagoons, open lagoons, carbonate sand shoals) were juxtaposed and evolved through time, often shifting position on the platform. The boundaries of the small-scale (100-kyr) sequence can be followed over the entire study area and thus must have formed through predominantly allogenic processes (eustatic sea-level fall, the effect of which was locally modified by differential subsidence). In two sections, five well-developed elementary sequences constitute the small-scale sequence. In the other sections, the identification of elementary sequences often is difficult because sedimentation was dominated by autogenic processes that overruled the influence of sea-level fluctuations. In low-energy, tidal-flat and internal-lagoonal settings, orbitally induced sea-level changes were recorded more faithfully, while high-energy shoals were mainly submitted to autogenic processes and the allogenic signal is masked. Consequently, the studied Jura platform experienced a combination of auto- and allogenic processes, which created a complex facies mosaic and a complex stacking of depositional sequences. Nevertheless, the 100-kyr orbital signal was strong enough to create correlatable sequence boundaries. Within a 100-kyr sequence, however, the unambiguous definition of sequences related to the 20-kyr orbital cycle is often difficult and the prediction of their lateral or vertical facies evolution impossible.     

Milankovitch cyclicity in Purbeck peritidal limestones of the Prebetic (Berriasian, southern Spain)

Peritidal carbonate rocks (Purbeck facies) of the uppermost Portlandian to Berriasian in the type section of the Sierra del Pozo Formation in the Prebetic Zone, southern Spain, are divisible into 141 shallowing upward cycles averaging 2 m in thickness. The subtidal facies in these cycles consist of micritic or marly limestones with dasycladacean algae and lituolids; the intertidal facies are micritic limestones containing birdseyes and miliolids; the supratidal facies comprise laminated algal limestones, the tops of which display desiccation cracks and rhizocretions, or more locally palaeosols, calcretes, or palaeokarst surfaces. A statistical study, using power spectra of the Fast Fourier Transform, demonstrates that the periodicity of these cycles is in the Milankovitch frequency band. Most sedimentary cycles correspond to the obliquity cycles; eccentricity and precession cycles have also been recognized. Using a Fischer plot, third-order tectono-eustatic cycles are recognized, which can be correlated with the eustatic curve of the Exxon chart. The shallowing upward sequences are characterized by a distinctive pattern of geochemical parameters. Carbon and oxygen isotopic (δ¹³C and δ¹⁸O) variations, calcium and magnesium carbonate contents and the abundance of organic matter and trace elements (Mn and Sr) all have predictable patterns of distribution within the sequences. The Sr content of the subtidal facies is relatively high whereas the δ¹³C and δ¹⁸O ratios are quite low; in the intertidal facies the Sr and Mn levels fall concomitantly with a rise in δ¹³C and δ¹⁸O. The highest δ¹³C and δ¹⁸O values occur in the lower part of the supratidal facies, whereas in the upper part of δ values and Sr contents drop sharply. Cyclic variations in evaporation and in meteoric water influence, determined from oxygen isotopic composition, reveal that the cyclicity of the beds containing the most limestones (supratidal) and those with the most marls (subtidal) is related to climatic changes. The coldest periods are those represented by supratidal deposits, when the sea level was at its lowest. During the warmest periods, when the overall sea level was higher, subtidal deposits accumulated in the region. A genetic model is proposed, according to which the asymmetrical sedimentary cycles occur in response to glacio-eustatic changes with a periodicity similar to that of Plio-Pleistocene sea-level variations, but with a much lower range due to the smaller extent of polar ice caps during the Early Cretaceous. The glacio-eustatic changes involved a rapid sea-level rise and a slow sea-level fall.     

Facies architecture of an isolated carbonate platform: tracing the cycles of the Latemàr (Middle Triassic, northern Italy)

The 720-m-thick succession of the Middle Triassic Latemàr Massif (Dolomites, Italy) was used to reconstruct the lagoonal facies architecture of a small atoll-like carbonate platform. Facies analysis of the lagoonal sediments yields a bathymetric interpretation of the lateral facies variations, which reflect a syndepositional palaeorelief. Based on tracing of lagoonal flooding surfaces, the metre-scale shallowing-upward cycles are interpreted to be of allocyclic origin. Short-term sea-level changes led to subaerial exposure of wide parts of the marginal zone, resulting in the development of a tepee belt of varying width. Occasional emergence of the entire lagoon produced lagoon-wide decimetre-thick red exposure horizons. The supratidal tepee belt in the backreef area represented the zone of maximum elevation, which circumscribed the sub- to peritidal lagoonal interior during most of the platform's development. This tepee rim, the subtidal reef and a sub- to peritidal transition zone in between stabilized the platform margin. The asymmetric width of facies belts within individual metre-scale cycles was caused by redistribution processes that reflect palaeowinds and storm paths from the present-day south and west. The overall succession shows stratigraphic changes on a scale of tens of metres from a basal subtidal unit, overlain by three tepee-rich intervals, separated by tepee-poor units composed of subtidal to peritidal facies. This stacking pattern reflects two third-order sequences during the late Anisian to early middle Ladinian.     

Depositional environments of Upper Miocene (Messinian) evaporite deposits of the Sicilian Basin*

In Sicily, Messinian evaporitic sedimentary deposits are developed under a wide variety of hypersaline conditions and in environments ranging from continental margin (subaerial), to basin-margin supratidal, to intertidal, to subtidal and out into the hypersaline basin proper. The actual water depth at the time of deposition is indeterminate; however, relative terms such as ‘wave base’ and ‘photic zone’ are utilized. The inter-fingering relationships of specific evaporitic facies having clear and recognizable physical characteristics are presented. These include sub-aerial deposits of nodular calcium sulphate formed displacively within clastic sediments; gypsiferous rudites, arenites and arenitic marls, all of which are reworked sediments and are mixed in varying degrees with other clastic materials (subaerial, supratidal, and intertidal to deep basinal deposits). Laminated calcium sulphate alternating with very thin carbonate interlaminae and having two different aspects; one being even and continuous and the other of a wavy, irregular appearance (subtidal, intertidal, and supratidal deposits). Nodular calcium sulphate beds, usually associated with wavy, irregular laminated beds (supratidal, sabkha deposits); very coarsely crystalline gypsum beds (selenite), associated with more even, laminated beds (subaqueous, intertidal to subtidal deposits); wavy anastomozing gypsum beds, composed of very fine, often broken crystals (subaqueous, current-swept deposits); halite having hopper and chevron structures (supratidal to intertidal); and halite, potash salts, etc. having continuous laminated structure (subaqueous, possibly basinal). Evidence for diagenetic changes is observed in the calcium sulphate deposits which apparently formed by tectonic stress and also by migrating hypersaline waters. These observations suggest that the common, massive form of alabastrine gypsum (or anhydrite, in the subsurface) may not always be ascribed to original depositional features, to syndiagenesis or to early diagenesis but may be the result of late diagenesis.     

陆表海台地沉积充填模式及内克拉通碳酸盐岩勘探新启示

内克拉通陆表海盆地是古老碳酸盐建造形成的重要场所。除其顶部风化壳岩溶储层外,广泛分布的内幕非岩溶改造的白云岩勘探长期未受到足够的重视。近年来,随着深层、超深层勘探技术的进步,内幕陆表海白云岩陆续获得油气勘探新发现,但其预测难度远比镶边台地中的礁滩"移动靶"更大。为更好地揭示这类未经岩溶改造的白云岩储层的宏观发育分布规律,本文研究了鄂尔多斯盆地奥陶系马家沟期和四川盆地雷口坡期陆表海台地的高精度岩相古地理和沉积充填模式。结果表明:陆表海台地一般形成于温室期低纬度干燥气候下的稳定内克拉通盆地,与盐湖伴生的台地潮坪广泛分布是其标志性的沉积特征,微环境可进一步划分为潮上带泥云坪、上潮间带云坪、上潮间带微生物席、下潮间带席状潮缘滩、上潮下带灰云质澙湖、下潮下带膏云质澙湖;虽然内克拉通陆表海台地是一个相对静态的沉积环境,其沉积建造并非平板一块,海平面的升降变化往往会在相对平坦的陆表海内引起大幅度的相带迁移,造成潮上带、潮间带、潮下带频繁交互;相带频繁往复迁移过程中,潮间带的晶粒/颗粒/微生物白云岩受到盐湖、古隆起或潮坪岛的制约,形成大面积分布的席状富孔白云岩透镜体;陆表海沉积层序结构为典型的向上变浅序列,主要发育潮坪进积楔和潮坪岛拼嵌两种沉积充填模式,这两种模式形成的席状白云岩透镜体在层序结构和厚度都能很好地对比,通常延伸都在100km以上,甚至可跨越整个陆表海台地,是一个能够媲美台缘礁滩相的油气勘探新领域。     

广西晚二叠世合山组沉积相分析

晚二叠世合山组总体上是在低能的浅水局限台地中沉积下来的，由于海平面下降，局限台地露出水面，发生泥炭沼泽化，堆积泥炭．根据沉积学特征、生物组合类型及岩石组合类型，在合山组中识别出４种微相类型：单粒度基底型、双粒度单模式型、双粒度双模式型、过渡类型和７种沉积相：滨岸平原沼泽－泥炭沼泽相、潮上沼泽－泥炭沼泽相、潮上相、潮间－潮上沼泽－泥炭沼泽相、潮间相、浅水潮下相、较深水潮下相．     

Sedimentary environments and facies of the subtropical mgeni estuary,Southeast Africa

The Mgeni Estuary is situated on the subtropical, mainly microtidal Natal coast. Modern sedimentary environments in the estuary comprise two groups. Barrier-associated environments include inlet channel, inlet beachface, tidal delta, washover fans, transverse intertidal bars and aeolian dunes. Estuarine environments include subtidal channels, interidal bars, back-barrier lagoon, tidal creek, tidal creek side-attached bars, creek mouth bar, mangrove fringe and supratidal mudflats. Each sedimentary facies is described in terms of grain-size, sedimentary structures, and sedimentary processes. The distinctive flora and fauna play an important role in facies recognition. Vertical sequences produced by infilling of the estuary and subsequent coastal erosion are discussed. The facies are considered sufficiently distinct to warrant recognition in the geological record.     

History of the Middle Berriasian transgression on the Jura carbonate platform: revealed by high-resolution sequence- and cyclostratigraphy (Switzerland and France)

The Middle Berriasian deposits of the Jura platform in Switzerland and France have already been well studied in terms of high-resolution sequence stratigraphy and different orders of depositional sequences (large-, medium-, and small-scale) have been defined. The hierarchical stacking pattern of the sequences and the time span represented by the investigated interval imply that sea-level fluctuations in the Milankovitch frequency band as well as differential subsidence caused the observed changes of accommodation on the Jura platform. The present study focuses on three small-scale sequences within the transgressive interval of a large-scale sequence. The initial flooding of the platform is marked by a facies change from supra- and intertidal (Goldberg Formation) to shallow-marine subtidal deposits (Pierre Chatel Formation). Detailed logging and facies analysis of 11 sections allow recognizing small environmental changes that define elementary sequences within the well-established small-scale sequences and distinguishing between autocyclic and allocyclic processes in sequence formation. It is concluded that the small-scale sequences correspond to the 100-ka orbital eccentricity cycle, while allocyclic elementary sequences formed in tune with the 20-ka precession cycle. Based on the correlation of elementary and small-scale sequences it can be shown that the Jura platform has been flooded stepwise by repeated transgressive pulses. Differential subsidence and pre-existing platform morphology further controlled sediment accumulation and distribution during the transgression. The combination of high-resolution sequence stratigraphy and cyclostratigraphy then enables the reconstruction of hypothetical palaeogeographic maps in time increments of a few ten thousand years.     

华北地台晚寒武世层序地层及其与北美地台海平面变化的对比

华北地台在晚寒武世为一个较为典型的缓坡型台地。在其北部地层中,以发育风暴砾屑灰岩及生物丘灰岩闻名,包括崮山组、长山组和凤山组。在该套地层中,深缓坡相泥岩和泥灰岩与浅缓坡相颗粒灰岩和泥粒灰岩一起构成若干潮下型碳酸盐米级旋回,这些米级旋回本身即为有序的异成因岩相序列而成为基本工作单元。在长周期三级层序中。米级旋回形成有序垂直叠加形式,反映了三级层序本身是一个与三级相对海平面变化相关的、环境加深和变浅过程中所形成的沉积相序列根据这些特征,在华北地台北部晚寒武世地层中可识别出4个三级层序。从北向南,相变为一套白云岩,其中三级层序的高水位体系域则发育较多的潮坪相白云岩以及由其构成的环潮坪型米级旋回。这些层序的典型特征是：层序界面为淹没不整合面,层序的相序序列以“深缓坡相—浅缓坡相”为特征,从而构成特殊的“CS(?) HST”序列;而在华北地台南部,层序界面多为暴露间断面,层序的相序组构多以“缓坡相—潮坪相”为特征。从北向南的变化,形成了一个富有规律性的层序地层格架。从旋回到层序、从岩相序列到沉积相序列的层序地层研究表明：作为基本工作单元,米级旋回实际上是由“间断—加积作用”旋回机制所形成的岩相序列,由米级旋回的有序叠加形式所构成的三级层序则是一个沉积相序列。由各剖面点的古水深变化曲线,可以定性地综合出华北地台晚寒武世的海平面变化曲线。与北美晚寒武世海平面变化曲线相对比,既存在相似性更存在差异性,说明了长周期海平面变化既受全球性海平面变化的控制,同时也受区域因素的影响。     

Temperate carbonate sediments of Northern Spencer Gulf, South Australia: a high salinity 'foramol' province

Cool-water skeletal carbonate sediments are forming in Spencer Gulf, South Australia, an area of high salinity and moderate tidal range. Four environments can be distinguished: deeper marine areas (10–20 m); shallow subtidal platforms and banks (2–10 m); intertidal and supratidal zones; and coastal springs and lakes fed by saline continental groundwaters. The sediments are predominately bioclastic carbonate sands; muddy sediments occur in protected intertidal environments. The most common grain types are gastropods, bivalves, foraminifera, coralline algae and quartz. Indurated non-skeletal carbonate grains have not been seen. Composition of the sediment varies little between environments, but considerable textural variation results from variation in the stability of the substrate, hydrodynamic conditions, depth of water, period of tidal inundation, supply of terrigenous grains, temperature, and salinity. The Spencer Gulf data suggests that temperature, and particularly minimum temperature, controls the distribution of skeletal and non-skeletal grain associations in high-salinity environments. The textures of the sedimentary facies of Spencer Gulf closely parallel those of equivalent environments in warm-water carbonate provinces.     

Platform and off-platform carbonates of the Upper Cambrian of western Maryland, U.S.A.

Upper Cambrian carbonates in western Maryland are comprised of platform facies (Conococheague Limestone) west of South Mountain and basin facies (Frederick Limestone) east of South Mountain. Conocheague platform carbonates contain interbedded non-cyclic and cyclic facies. Non-cyclic facies consist of cross-stratified grainstones, thrombolitic bioherms, and graded, thin-bedded dolostones. These were deposited in shallow, subtidal shelf lagoons. Cyclic facies are composed of repeated sequences of cross-stratified grainstone; ribbon-rock; wavy, prism-cracked laminite; and planar laminated dolostone. The cyclic facies are shallowing-upward cycles produced by lateral progradation of tidal flats over shallow, nearshore subtidal environments. Cyclic and non-cyclic facies are interbedded in the Conococheague in a layer cake fashion, but no higher-order cyclicity can be found. The Frederick Limestone is dominated by monotonously thick sequences of graded, thin-bedded limestones, interbedded with massive peloidal grainstones and beds of breccia up to 10 m thick in the lower Frederick. The breccias contain transported megaclasts of Epiphyton-Girvanella boundstones. The basal Frederick was deposited in a slope-to-basinal setting east of a rimmed shelf. An Epiphyton-Girvanella marginal reef along the shelf edge was the source of the blocks in the breccias. The upper Frederick Limestone formed on a carbonate ramp.     

Biotically constrained palaeoenvironmental conditions of a mid-Holocene intertidal lagoon on the southern shore of the Arabian Gulf: evidence associated with a whale skeleton at Musaffah,Abu Dhabi,UAE

Whale remains (a left and right mandible, scapula, humerus and fragmentary radius and ulna as well as parts of the cranium and rostrum) belonging to a probable humpback whale (Megaptera cf. novaeangliae) were found in the well-described sabkha sequence exposed in the Musaffah Industrial Channel, Abu Dhabi, United Arab Emirates. More precisely, the whale remains were found in a series of sediments representing a range of lagoonal facies. The sediments surrounding the whale bones were age-dated at approximately 5200 ¹⁴C yrs BP and are therefore interpreted to correspond to the previously documented late Flandrian sea-level peak, preceding a fall in sea-level which culminated in the supratidal sabkha overprint of the carbonates. Associated with the whale remains is an assemblage of molluscs, foraminifera and ostracods. Together with the inferred presence of sea grass and algae, these facies are interpreted to indicate a very shallow subtidal to intertidal lagoonal environment. Cirripede remains found associated with the skeleton were identified as those of the whale barnacle Coronula diadema and hence had their origins with the whale. Significantly, the low species diversity of microfossils suggests that higher salinities existed in the mid-Holocene lagoon than are present in modern counterparts. This is here inferred to be related to the onset of continental aridity in Arabia during the mid-Holocene.     

Anatomy of a cyclically packaged Mesoproterozoic carbonate ramp in northern Canada

Carbonates in the upper member of the Mesoproterozoic Victor Bay Formation are dominated by lime mud and packaged in cycles of 20–50 m. These thicknesses exceed those of classic shallowing-upward cycles by almost a factor of 10. Stratigraphic and sedimentological evidence suggests high-amplitude, high-frequency glacio-eustatic cyclicity, and thus a cool global climate ca. 1.2 Ga.The Victor Bay ramp is one of several late Proterozoic carbonate platforms where the proportions of lime mud, carbonate grains, and microbialites are more typical of younger Phanerozoic successions which followed the global waning of stromatolites. Facies distribution in the study area is compatible with deposition on a low-energy, microtidal, distally steepened ramp. Outer-ramp facies are hemipelagic lime mudstone, shale, carbonaceous rhythmite, and debrites. Mid-ramp facies are molar-tooth limestone tempestite with microspar-intraclast lags. In a marine environment where stromatolitic and oolitic facies were otherwise rare, large stromatolitic reefs developed at the mid-ramp, coeval with inner-ramp facies of microspar grainstone, intertidal dolomitic microbial laminite, and supratidal evaporitic red shale.Deep-subtidal, outer-ramp cycles occur in the southwestern part of the study area. Black dolomitic shale at the base is overlain by ribbon, nodular, and carbonaceous carbonate facies, all of which exhibit signs of synsedimentary disruption. Cycles in the northeast are shallow-subtidal and peritidal in character. Shallow-subtidal cycles consist of basal deep-water facies, and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies. Peritidal cycles are identical to shallow-subtidal cycles except that they contain a cap of dolomitic tidal-flat microbial laminite, and rarely of red shale sabkha facies or of sandy polymictic conglomerate. A transect along the wall of a valley extending 8.5 km perpendicular to depositional strike reveals progradation of inner-ramp tidal flats over outer- and mid-ramp facies during shoaling. The maximum basinward progradation of peritidal facies coincides with a zone of slope failure that may have promoted the development of the stromatolitic reefs.The sea-level history of the Victor Bay Formation is represented by three hectometre-scale sequences. An initial flooding event resulted in deposition of the lower Victor Bay shale member. Upper-member carbonate cycles were then deposited during highstand. Mid-ramp slumping was followed by late-highstand reef development. The second sequence began with development of an inner-ramp lowstand unconformity and a thick mid-ramp lowstand wedge. A second transgression promoted a more modest phase of reef development at the mid-ramp and shallow-water deposition continued inboard. A third and final transgressive episode eventually led to flooding of the backstepping ramp.Overall consistent cycle thickness and absence of truncated cycles, as well as the high rate and amount of creation of accommodation space, suggest that the periodicity and amplitude of sea-level fluctuation were relatively uniform, and point to a eustatic rather than tectonic mechanism of relative sea-level change. High-amplitude, high-frequency eustatic sea-level change is characteristic of icehouse worlds in which short-term, large-scale sea-level fluctuations accompany rapidly changing ice volumes affected by Milankovitch orbital forcing. Packaging of cyclic Upper Victor Bay carbonates therefore supports the hypothesis of a late Mesoproterozoic glacial period, as proposed by previous workers.