The biostratigraphy and formational relationships of the upper aeronian and lower telychian (llandovery,silurian) formations of western mid-wales

The stratigraphical relationships of the upper Aeronian-lower Telychian formations in the Aberystwyth area are examined in the light of a new biostratigraphical subdivision of this interval based on graptolites, in which part of the Monograptus turriculatus Biozone is divided into six subzones and the Stimulograptus halli Biozone is resurrected. In the Aberystwyth area (B.G.S. 1:50 000 Sheet 163 (1984)) there is shown to be greater diachroneity of formational —boundaries than previously recognized. Notably, the Cwmsymlog Formation facies persisted in the northeast of the area far longer than anywhere else, due to the persistence of a bathymetric high into the lower Telychian. Also, the basal beds of the Aberystwyth Grits in the west of the area are shown to be of the same age as the uppermost beds of the Devil's Bridge Formation in the east. Thus for a short period the area was subject to turbiditic incursions from two sources. The degree of diachroneity of the base of the Aberystwyth Grits Formation as a whole is less than previously thought — the oldest beds are approximately one subzone older in the south than in the north. The core of the in the clinal pericline responsible for the crescentic outcrop pattern of the Aberystwyth Grits is demonstrated to lie in the vicinity of Llanrhystud, rather than at Aberarth as previously suggested. Four new graptolite species are described and one new genus is erected.     

Upper Llandoverian Grogal Sandstones and Aberystwyth Grits in the New Quay area,Central Wales: A possible upwards transition from contourites to turbidites

The lower boundaries of the Aberystwyth Grits Formation and the underlying Grogal Sandstones Formation (new name) in the New Quay area are defined. The Grogal Sandstones Formation is described, and interpreted as a contourite deposit on a continental rise; we believe these are the first rocks in the British Isles to be identified as contourites. The transition within a few tens of metres from Grogal Sandstones Formation contourite facies, displaying bimodal current directions, to Aberystwyth Grits Formation turbidite facies, with unimodal current directions, can be seen well in two places on the coast southwest of New Quay. Regionally, an upward transition from finegrained contourites to turbidites of varying facies can be recognised. Our results are compatible with Ziegler's interpretation of early Upper Llandoverian palaeogeography.     

The world's longest graptolite?

An incomplete specimen of Stimulograptus halli 1.45 m long is recorded from the Aberystwyth Grits Formation north of Clarach, western mid‐Wales. This is the longest graptoloid graptolite known. Assuming a growth rate similar to that of modern Rhabdopleura it lived for at least 25 years. Copyright © 2001 John Wiley & Sons, Ltd.     

Sedimentation,cleavage and tectonic thinning in the aberystwyth grits between clarach and wallog,cardigan bay

In recent years the folding and cleavage seen in the Aberystwyth Grits has been ascribed to downslope movement in soft unlithified sediment, not to tectonic forces. A detailed examination of folded beds in a small part of the outcrop was undertaken to test this theory. It is concluded that the folding and cleavage was initiated by movement from a rising tectonic ridge, but that the sediments may not have been completely lithified at the time of folding.     

Facies and depositional processes in an Upper Jurassic to Lower Cretaceous pelagic sedimentary sequence, Antarctica

The Nordenskjöld Formation (?Oxfordian-Berriasian age) is exposed on the east coast of the Antarctic Peninsula, where it consists of interbedded ash layers and biosiliceous mudstones which accumulated under anaerobic to dysaerobic bottom waters. The mudstones were deposited by pelagic settling and the ash layers by pelagic settling from suspension or as fallout from subaerial eruption columns. The lower part of the succession accumulated in a basinal setting under anaerobic bottom waters and is characterized by parallel bedding. Mudstones deposited in this setting preserve abundant zooplankton faecel pellets. Compaction of these pellets has given rise to a bedding parallel fissility. The upper part of the succession accumulated under dysaerobic bottom waters in a slope setting. The sequence is wavy bedded and contains abundant evidence of post-depositional sediment instability and resedimentation, much of which was caused by tectonic activity. Discrete slide masses are absent from the slope sequence and it appears that slope processes were dominated by creep. Examination of the mudstones shows that as levels of dissolved oxygen in bottom waters increase, pelleted mudstones give way to structureless mudstones before visible bioturbation is noted.     

The Rio Dell Formation: a Plio-Pleistocene basin slope deposit in Northern California

The Rio Dell Formation (Pleistocene and Pliocene), exposed south of Eureka, California, is a prograded sequence of basinal turbidites overlain by basin slope and shelf deposits. The slope deposits studied in the Centerville Beach section accumulated in a steadily shallowing environment delineated by analysis of palaeobathymetrically significant benthonic foraminiferal biofacies in turn suggesting deposition at depths of 1000–100 m. Lower slope deposits interfinger with basinal turbidites derived from the Eel River delta to the north. Slumped blocks of silty mudstone, and associated silt and mud beds, are common. The middle slope deposits are mudstones; coarser sediments bypassed this zone. Mudstones and muddy siltstones alternate on the upper slope. Shallow depressions, probably slump scars, that have been rapidly filled by upper slope sediment are common. The transition to shelf deposits is marked by an increase in sediment grain size, in the degree of oxidation, and in the abundance of megafossils. High percentages of benthonic foraminifera displaced from shelf depths indicate that resedimentation processes are most important on the upper slope.     

A diverse ichnofauna from silurian flysch of the aberystwyth grits formation,Wales

The most extensive ichnofauna yet recorded from a deep water Lower Palaeozoic sequence occurs within the distal turbidites of the Lower Silurian Aberystwyth Grits Formation of Central Wales. The strata contain an abundant assemblage comprising 25 ichnogenera: Asteriacites, Bergaueria, Chondrites, Cochlichnus, Cosmorhaphe, Glockerichnus, Gordia, Helicolithus, Helminthopsis, Helminthoida, Hormosiroidea, Lorenzinia, Megagrapton, Monomorphichnus, Neonereites, Nereites, Palaeophycus, Paleodictyon, Planolites, Protopaleodictyon, Spirorhaphe, Spirophycus, Squamodictyon, Subphyllochorda, Taphrhelminthopsis; 36 ichnospecies are described, three of which (Asteriacites aberensis, Helminthopsis regularis, Cosmorhaphe elongata) are new. The inorganic sedimentary structures and trace fossils of some 418 sandstone beds were examined in detail; 16 per cent of the beds commence with Divisions A or B and 84 per cent with Division C of the turbidite sequence. This indicates a relatively distal environment, mainly receiving low velocity turbidity currents, and favouring trace fossil preservation. The most common traces were Helminthopsis, Paleodictyon, and Squamodictyon which were found on 46 per cent, 34 per cent, and 19 per cent of the beds examined. Data from this, and other recently described sequences, confirms that there was a gradual increase in trace fossil diversity in the deep oceans throughout the Lower Palaeozoic, in contrast to the situation in shallow water shelf seas where a peak was reached as early as the Lower Cambrian.     

Beds comprising debrite sandwiched within co-genetic turbidite: origin and widespread occurrence in distal depositional environments

Co‐genetic debrite–turbidite beds occur in a variety of modern and ancient turbidite systems. Their basic character is distinctive. An ungraded muddy sandstone interval is encased within mud‐poor graded sandstone, siltstone and mudstone. The muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite. The mud‐poor sandstone, siltstone and mudstone show features indicating progressive layer‐by‐layer deposition and are thus called a turbidite. Palaeocurrent indicators, ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event. Detailed field observations are presented for co‐genetic debrite–turbidite beds in three widespread sequences of variable age: the Miocene Marnoso Arenacea Formation in the Italian Apennines; the Silurian Aberystwyth Grits in Wales; and Quaternary deposits of the Agadir Basin, offshore Morocco. Deposition of these sequences occurred in similar unchannellized basin‐plain settings. Co‐genetic debrite–turbidite beds were deposited from longitudinally segregated flow events, comprising both debris flow and forerunning turbidity current. It is most likely that the debris flow was generated by relatively shallow (few tens of centimetres) erosion of mud‐rich sea‐floor sediment. Changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition. The association with distal settings results from the ubiquitous presence of muddy deposits in such locations, which may be eroded and disaggregated to form a cohesive debris flow. Debrite intervals may be extensive (> 26 × 10 km in the Marnoso Arenacea Formation) and are not restricted to basin margins. Such long debris flow run‐out on low‐gradient sea floor (< 0·1°) may simply be due to low yield strength (? 50 Pa) of the debris–water mixture. This study emphasizes that multiple flow types, and transformations between flow types, can occur within the distal parts of submarine flow events.     

A stable isotope study of the Mercia Mudstones (Keuper Marl) and associated sulphate horizons in the English Midlands

ABSTRACT Widespread evaporitic sulphate horizons occur in the frequently dolomitic Mercia Mudstones (Upper Triassic) of the English Midlands. The mudstones were deposited on an extensive peneplain which had areas of upstanding minor relief (Charnwood Massif, Derbyshire—Pennines, London Platform). Horst and graben structures formed during the early and middle Triassic, controlled late Triassic sedimentation rates and peneplain slope directions. A stable isotope study (S,C,O) of the sulphates and dolomites of the Trent Formation indicates that during the deposition of the lower Fauld Member grabens such as the Need wood Basin in Staffordshire and areas marginal to a graben such as Newark in Nottinghamshire had a marine brine regime with minor continental input. Using constraints applied by sulphate concentrations of modern waters in arid environments, the ratio of volume of marine input to volume of continental input lay between 3:1 and 4:1. The horst area (East Leake, Nottinghamshire) was strongly influenced by continental brines which derived sulphate from the exposed Carboniferous Hathern Anhydrite Series. A predominantly continental brine regime existed across the whole area during deposition of the overlying Hawton Member. Periodic marine influxes gave rise to thin sulphate horizons whilst continental run-off was occasionally sufficient to form temporary lakes. The Blue Anchor Formation (Rhaetian) was deposited in a lacustrine environment with waters of mixed marine and continental origin. Salinities were lower than those of the preceding Trent Formation brines.     

Ashgill trilobite faunas from the Llŷn Peninsula,North Wales,U.K.

The Crugan Mudstone Formation is formalized and the Dwyfor Mudstone Formation proposed for restricted Ashgill outcrops respectively near Llanbedrog and Llanystwmdwy in the Llŷn Peninsula. These mudstones contain brachiopods of the Foliomena fauna and a total of 26 determined genera of trilobites, one, Pseudosphaeroxochus represented by a new species, P. seabornei sp. nov. Elements of the trilobite fauna are used to demonstrate a Rawtheyan age for the mudstones. The generic assemblages both of brachiopods and of trilobites present in the mudstones indicate relatively deep water environments of deposition, that of the Dwyfor Mudstones probably being the deeper. The trilobites belong to what is here described as the Nankinolithus-Opsimasaphus association.     

安徽宿松坐山中二叠统栖霞组碳酸盐岩微相和黏土矿物特征及其对沉积相和古气候的指示

安徽宿松坐山剖面中二叠世地层发育,栖霞组、孤峰组和武穴组连续出露,为下扬子地区中二叠统的典型剖面之一。其中栖霞组主要为碳酸盐岩,其次为硅质岩和碎屑岩。栖霞组碳酸盐岩中共识别出9种微相类型,分别可以与Flügel整理的7种标准微相对比。栖霞组主要为斜坡沉积,其次为台地沉积,而斜坡相又可以进一步分为上斜坡相和下斜坡相。其中,上斜坡相主要出现在臭灰岩段的中部,该层位以产砾屑灰岩为特征,微相类型以MF3为主。其次出现在本部灰岩段的中偏上部,该处微相类型为MF3、MF8和MF9。此外,顶部灰岩上部发育灰岩砾石的层位中的微相类型主要为MF3和MF9,也属于上斜坡相。下斜坡相主要发育于上、下硅质岩段和本部灰岩段的顶部,主要特征为发育薄层硅质岩或长条状硅质结核,微相类型主要为MF4和MF5。台地相主要出现在本部灰岩中下部,其次出现在臭灰岩段的下部和上部及顶部灰岩段的下部,且其微相类型以MF1为主,其次为MF2、MF6和MF7,表明台地相整体属于开阔海台地。微相分析表明栖霞组整体形成于一个海侵过程中,期间经历了4次较明显的海退。黏土矿物分析显示栖霞组碳酸盐岩的黏土矿物以伊利石为主(平均含量为61.97%),高岭石次之(平均含量为27.25%),伊蒙混层(平均含量为9.43%)和绿泥石(平均含量为1.36%)较少。高岭石/(伊利石+绿泥石)的比值与伊利石的相对含量变化共同表明栖霞期由早期的相对干冷变为晚期的相对暖湿,期间发生了3次较为明显的气候变化周期。同时,栖霞期气候整体变暖湿、海平面整体上升的趋势与晚古生代冰川(LPIA)消融的时间对应,则暗示古气候变化导致的冰川消融事件控制了栖霞期的整体海平面变化。而沉积相变化所反映的栖霞期4次相对海平面变化与3次明显的气候波动能够较好的吻合,则显示出气候对海平面变化和沉积记录的控制作用。表现在气候变暖湿导致冰体融化,海平面上升,风化指数(WI)升高,主要发育碳酸盐斜坡相。气候变干冷、冰体增大导致海平面和风化指数(WI)的下降,以碳酸盐台地沉积为主。     

The role of slump scars in slope channel initiation: A case study from the Miocene Jatiluhur Formation in the Bogor Trough,West Java

This paper investigates slope channel initiation by seabed irregularities that were initially formed by slump scars in the lower to middle Jatiluhur Formation, part of the middle- to late Miocene successions in the Bogor Trough, West Java. This Miocene succession is up to 1000 m thick in the study area, and is interpreted as a prograding slope–shelf system that formed during a period of falling- and lowstand stages in relative sea level. The lower part of the formation is a siltstone-dominated siliciclastic succession, containing slump deposits, slump-scar-fill deposits, and minor channel-fill deposits, which formed in slope and shelf-margin environments. In contrast, the middle part, which gradationally overlies the lower part, is characterized by shallow-marine carbonates.The slump-scars-fill deposits have an overall lenticular geometry, and are 140–480 m wide and 0.4–1.6 m thick. Some have distinct erosional bases, which cut into the underlying siltstones, in association with medium- to coarse-grained sandstones with lateral-accretion surfaces and tractional structures common in channel-fill deposits. The incident link of slump-scar-fill deposits and channel-fill deposits in the prograding slope–shelf succession of the lower to middle Jatiluhur Formation suggests that some slump scars formed incipient seabed irregularities that may have played an important role in the development of slope channels. The present study provides one example of the various potential mechanisms that can result in channel formation in a slope setting.     

川东南—黔西北桐湾Ⅲ幕岩溶古地貌恢复

通过对川东南—黔西北灯影组露头、钻井岩芯的详细观察分析,认为灯影组中上部发育一系列的古岩溶标志,包括岩溶角砾岩、充填的溶洞溶缝、含铁质古风化壳等。在结合前人对上扬子桐湾运动研究的基础上,认为川东南—黔西北大部分地区由Ⅲ幕和Ⅱ幕叠加形成了寒武系与震旦系之间的平行不整合面。采用"印模法"恢复桐湾Ⅲ幕岩溶古地貌,"印模"为下寒武统"牛蹄塘组+明心寺组+金顶山组";"印模"如果包括麦地坪组,"印模"底界将具明显穿时性。根据"印模"地层分布及古地貌指示,研究区岩溶古地貌主体呈南北向展布,具有东高西低的特征,东部发育岩溶高地,西侧为岩溶洼地,中部则发育岩溶斜坡。岩溶高地和岩溶斜坡可以进一步划分为残丘、平台以及上斜坡、下斜坡等次级岩溶古地貌单元。     

水城矿区龙潭组浅绿色泥岩的特征及成因

水城矿区龙潭组二段下部赋存的两层浅绿色泥岩（粘土化凝灰岩）,特征显著,分布稳定。通过对其进行宏观观察、镜下鉴定、化学分析,差热分析、X 射线衍射分析等综合研究,认为它主要由火山物质在沼泽-近海湖泊里沉积而成,可以作为矿区和区域煤层对比的可靠标志层。     

Trench-slope channels from the New Zealand Jurassic: the Otekura Formation, Sandy Bay, South Otago

The Otekura Formation (Early Jurassic, Pseudaucella zone) at Sandy Bay comprises part of a 10+ km thick, regressive, forearc shelf and slope sequence, the Hokonui facies belt of the Rangitata Geosyncline. The Otekura Formation is dominantly fine grained, being mostly mudstone, silty mudstone and siltstone. The sediments are volcanogenic throughout. The upper 150 m of the formation contains two 20 m thick, channelized bodies of medium-thick bedded sandy flysch, each associated with thin bedded muddy flysch interpreted as overbank turbidites. Directional indicators within the channel sequence indicate emplacement from the south-southwest. In contrast, rare turbidites that occur below the channel sequence, within the background mudstone sediment, were emplaced from the east, i.e. at right angles to the channelized flows. The immediately overlying Omaru Formation contains more abundant macrofossils, intraclastic conglomerates, and appreciable amounts of traction-emplaced cross-bedded sand. Bioturbated calcareous siltstones with an in situ molluscan fauna follow (Boatlanding Formation), and are of shelf origin. The Omaru Formation is therefore interpreted as a shelf-slope break deposit, and the Otekura Formation as an upper slope facies. Reconnaissance studies indicate that the Otekura Formation is underlain by several kilometres of dominantly fine grained, deep water slope sediments, containing occasional sand and conglomerate filled channels similar to those here described in detail from the Otekura Formation. Such channels are inferred to form when a mass-transported sand, derived from failure higher on the slope, ploughs erosively into the sea floor. After their incision, the channels served for a short time as conduits for downslope transport of sediment, the redeposited deposits of which are found filling each channel. Both channel fills at Sandy Bay are capped by thin-bedded turbidites inferred to have overspilled from similar channels nearby on the slope.     

First record of an arthropod from the Passaic Formation (Late Triassic), near Milford, New Jersey

Mudstones of the Triassic Passaic Formation, near Milford, New Jersey, have yielded the first evidence of an arthropod impression in that formation. Associated trace fossils include Helminthoidichnites, Lockeia, Scoyenia, Spongeliomorpha, and the reptile footprint Rhynchosauroides, representing the Scoyenia ichnofacies. Associated sedimentary structures include desiccation cracks and raindrop impressions. The Passaic sediments were deposited under shallow water lacustrine shoreline conditions subject to periodic subaerial exposure.     

The geometry and organization of fluviodeltaic channel sandstones in the Jurassic Saltwick Formation, Yorkshire, England

Within the Middle Jurassic Saltwick Formation of Yorkshire, England, 65% of the fluvial channel sandstones (n= 22) have width to thickness ratios in the range 17–30. More than 90% lie in the range 17–67 and are classified as narrow sheet channel sandstones. A broad sheet channel sandstone (c. 15 m thick, 3 km wide, width to thickness ratio of 200) occurs at the base of the formation in the proximal part of the studied depositional system. The thicknesses of the channel sandstones are in the range 4 m to more than 22 m, with 90% in the range 4–12 m. The widths of the channel sandstone sections are in the range 100–800 m, with 80% between 100 and 400 m. In plan view the channel sandstones display relatively constant thicknesses across the main part (80%) of the channel body. Channel sandstones are usually multistorey. The storeys are separated by major scour surfaces, with depths of scour ranging from 2 m to 8–10 m. The individual storeys are 4–13 m thick, and transverse and oblique storey sections have lateral extents of 60–480 m. Vertical stacking of storeys is accompanied by a decrease in the width to thickness ratio of the channel sandstones, whereas lateral stacking of storeys is accompanied by an increase. Mudstones and siltstones within channel sandstone bodies occur at the top of storeys and were in most cases deposited during channel abandonment. The Saltwick Formation was deposited during increasing accommodation forming a transgressive systems tract. The channel sandstone to interchannel sediment ratio is highest in the lower parts of the transgressive systems tract. The quantitative geometry and architecture data from the Saltwick Formation may be useful for describing and modelling humid delta plain hydrocarbon reservoirs of transgressive systems tracts.     

On the frequency distribution of turbidite thickness

The frequency distribution of turbidite thickness records information on flow hydrodynamics, initial sediment volumes and source migration and is an important component of petroleum reservoir models. However, the nature of this thickness distribution is currently uncertain, with log‐normal or negative‐exponential frequency distributions and power‐law cumulative frequency distributions having been proposed by different authors. A detailed analysis of the Miocene Marnoso Arenacea Formation of the Italian Apennines shows that turbidite bed thickness and sand‐interval thickness within each bed have a frequency distribution comprising the sum of a series of log‐normal frequency distributions. These strata were deposited predominantly in a basin‐plain setting, and bed amalgamation is relatively rare. Beds or sand intervals truncated by erosion were excluded from this analysis. Each log‐normal frequency distribution characterizes bed or sand‐interval thickness for a given basal grain‐size or basal Bouma division. Measurements from the Silurian Aberystwyth Grits in Wales, the Cretaceous Great Valley Sequence in California and the Permian Karoo Basin in South Africa show that this conclusion holds for sequences of disparate age and variable location. The median thickness of these log‐normal distributions is positively correlated with basal grain‐size. The power‐law exponent relating the basal grain‐size and median thickness is different for turbidites with a basal A or B division and those with only C, D and E divisions. These two types of turbidite have been termed ‘thin bedded’ and ‘thick bedded’ by previous workers. A change in the power‐law exponent is proposed to be related to: (i) a transition from viscous to inertial settling of sediment grains; and (ii) hindered settling at high sediment concentrations. The bimodal thickness distribution of ‘thin‐bedded’ and ‘thick‐bedded’ turbidites noted by previous workers is explained as the result of a change in the power‐law exponent. This analysis supports the view that A and B divisions were deposited from high‐concentration flow components and that distinct grain‐size modes undergo different depositional processes. Summation of log‐normal frequency distributions for thin‐ and thick‐bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power‐law trend. Thus, the occurrence of both log‐normal and segmented power‐law frequency distributions can be explained in a holistic fashion. Power‐law frequency distributions of turbidite thickness have previously been linked to power‐law distributions of earthquake magnitude or volumes of submarine slope failure. The log‐normal distribution for a given grain‐size class observed in this study suggests an alternative view, that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters, in addition to the settling velocity of the grain‐sizes present.     

羌塘盆地第一口油气科学钻探井油气地质成果及勘探意义

羌塘盆地位于青藏高原腹地,是我国陆上面积最大、勘探程度最低的油气勘探新区,长期以来,由于缺乏油气科学钻探井的验证,对盆地油气地质条件认识不清。最近,中国地质调查局成都地质调查中心在北羌塘中北部的半岛湖地区组织实施了第一口深达4696.18米的油气科学钻探井—羌科1井,本文对该科探井的油气地质成果进行了报道。钻遇了从上侏罗统至上三叠统的连续地层,其中下-中侏罗统雀莫错组与下伏上三叠统那底岗日组地层为整合接触,沉积环境为三角洲-局限台地环境。羌科1井在雀莫错组中首次钻遇了烃源岩,并发现13层气测异常,其中,3层为重要含气层,此外还识别出两套重要的区域性盖层和一套直接封盖层。这些成果表明半岛湖地区油气保存条件较好,是盆地油气勘探的有利地区。