鄂尔多斯盆地西缘砾质冲积扇沉积学特征

晚三叠世鄂尔多斯西缘冲积扇沉积主要由碎屑流、颗粒流、片流及河道砾岩等组成。文中讨论了各类砾岩的沉积学特征,同时指出,不同成因的砾岩,其粒度分布曲线形态及某些粒度参数亦不同.据此本文提出了根据-系列新的粒度分布曲线形态和粒度参数Mdf,Rgs和Mdf/Rgs等甄别冲积扇中不同类型砾岩的设想。     

Hybrid event bed character and distribution linked to turbidite system sub‐environments: The North Apennine Gottero Sandstone (north‐west Italy)

This study documents the character and occurrence of hybrid event beds (HEBs) deposited across a range of deep‐water sub‐environments in the Cretaceous–Palaeocene Gottero system, north‐west Italy. Detailed fieldwork (>5200 m of sedimentary logs) has shown that hybrid event beds are most abundant in the distal confined basin‐plain domain (>31% of total thickness). In more proximal sectors, hybrid event beds occur within outer‐fan and mid‐fan lobes (up to 15% of total thickness), whereas they are not observed in the inner‐fan channelized area. Six hybrid event bed types (HEB‐1 to HEB‐6) were differentiated mainly on basis of the texture of their muddier and chaotic central division (H3). The confined basin‐plain sector is dominated by thick (maximum 9·57 m; average 2·15 m) and tabular hybrid event beds (HEB‐1 to HEB‐4). Their H3 division can include very large substrate slabs, evidence of extensive auto‐injection and clast break‐up, and abundant mudstone clasts set in a sandy matrix (dispersed clay ca 20%). These beds are thought to have been generated by highly energetic flows capable of delaminating the sea floor locally, and carrying large rip‐up clasts for relatively short distances before arresting. The unconfined lobes of the mid‐fan sector are dominated by thinner (average 0·38 m) hybrid event beds (HEB‐5 and HEB‐6). Their H3 divisions are characterized by floating mudstone clasts and clay‐enriched matrices (dispersed clay >25%) with hydraulically fractionated components (mica, organic matter and clay flocs). These hybrid event beds are thought to have been deposited by less energetic flows that underwent early turbulence damping following incorporation of mud at proximal locations and by segregation during transport. Although there is a tendency to look to external factors to account for hybrid event bed development, systems like the Gottero imply that intrabasinal factors can also be important; specifically, the type of substrate available (muddy or sandy) and where and how erosion is achieved across the system producing specific hybrid event bed expressions and facies tracts.     

Flat-pebble conglomerate: its multiple origins and relationship to metre-scale depositional cycles

Carbonate flat‐pebble conglomerate is an important component of Precambrian to lower Palaeozoic strata, but its origins remain enigmatic. The Upper Cambrian to Lower Ordovician strata of the Snowy Range Formation in northern Wyoming and southern Montana contain abundant flat‐pebble conglomerate beds in shallow‐water cyclic and non‐cyclic strata. Several origins of flat‐pebble conglomerate are inferred for these strata. In one case, all stages of development of flat‐pebble conglomerate are captured within storm‐dominated shoreface deposits of hummocky cross‐stratified (HCS) fine carbonate grainstone. A variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat‐pebble conglomerate. These features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits. Failure was induced by either storm or seismic waves, and many beds failed along discrete slide scar surfaces. Centimetre‐scale laminae within thick amalgamated HCS beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds. In some cases, buckled masses accelerated downslope until they exceeded their internal friction, completely disarticulated into clasts and transformed into a mass flow of individual cm‐ to dm‐scale clasts. This transition was accompanied by the addition of sand‐sized echinoderm‐rich debris from local sources, which slightly lowered friction by reducing clast–clast interactions. The resulting dominantly horizontal clast orientations suggest transport by dense, viscous flow dominated by laminar shear. These flows generally came to rest on the lower shoreface, although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds. The clasts in these beds show no evidence of extensive reworking (i.e. not well rounded) or condensation (i.e. no rinds or coatings). A second type of flat‐pebble conglomerate bed occurs at the top of upward‐coarsening, metre‐scale cycles. The flat‐pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or, in some cases, may represent transgressive lags. The clasts are well rounded, display borings and have iron‐rich coatings. The matrix to these beds locally includes glauconite. These beds were considerably reworked and represent condensed deposits. Thrombolites occur above the flat‐pebble beds and record microbial growth before initial transgression at the cycle boundaries. A third type of flat‐pebble conglomerate bed occurs within unusual metre‐scale, shale‐dominated, asymmetric, subaqueous cycles in Shoshone Canyon, Wyoming. Flat‐pebble beds in these cycles consist solely of clasts of carbonate nodules identical to those that are in situ within underlying shale beds. These deeper water cycles can be interpreted as either upward‐shoaling or ‐deepening cycles. The flat‐pebble conglomerate beds record winnowing and reworking of shale and carbonate nodules to lags, during either lowstand or the first stages of transgression.     

Debris flow deposits of early miocene age,deadman stream,Marlborough, New Zealand

Detailed analysis is presented of a conformable succession of Early Miocene conglomerates and sandstones lying between massive marine mudstones. The coarse sediments reflect deposition by a spectrum of subaqueous debris-flow mechanisms during an early pulse of tectonism that ultimately resulted in Plio-Pleistocene eversion of the Kaikoura Mountains.Sparse pebbly mudstones and rare sandy conglomerates show disoriented clasts and reflect high-viscosity flows and slurry-creep flow mechanisms. Other deposits have little mud matrix, hence appear to reflect low-viscosity flow processes. The largest clasts in these have a preferred planar orientation, probably reflecting dispersive grain pressure, and a preferred long-axis orientation parallel to flow direction. Common sorted sandstones and some conglomeratic sandstones show diffuse parallel lamination; with rare exceptions neither grading nor traction structures are present. Other conglomeratic sandstones show trough cross-bedding which we attribute to entrained bedload movement during intersurge episodes of debris flow.Microfossil data from the mudstones indicate sedimentation in an environment of outer neritic to upper bathyal aspect. Most detritus is abraded, suggesting derivation from terrestrial or inner neritic sources, but angular calcilutitic clasts and irregular sandstone and mudstone clasts and rafts were probably derived from submarine erosion between the emergent source area and the site of accumulation. Deposits generally appear to infill broad shallow channels. Paleocurrent and fabric analysis indicate a markedly uniform flow direction throughout succession, and suggest that the locus of channeling remained relatively fixed in space throughout accumulation of hundreds of metres of superimposed, commonly amalgamated debris-flow deposits. Although lateral control away from the measured sequence is limited, we infer that the locus of deposition lay shoreward of any submarine canyon or fan.     

Glacially-influenced deep-marine sedimentation of the Late Precambrian Gaskiers Formation, Newfoundland, Canada

The Late Proterozoic Conception Group, exposed on the Avalon Peninsula in Newfoundland, Canada, is a 4 km thick turbidite succession containing a conformable 300 m thick sequence of diamictites (the Gaskiers Formation) near the base. Massive and crudely-stratified diamictites form beds up to 25 m thick which have a tabular geometry with slightly erosive basal contacts and are interbedded with mudstones and fine-grained, thin-bedded turbidites. These diamictites are interpreted as submarine debris flow deposits. Disrupted diamictites form strongly deformed units that contain large, complexly folded rafts of mudstone and turbidite facies. These diamictite units are interpreted as submarine slumps. Diamictites contain glacially-striated and faceted clasts; clasts and matrix are predominantly of volcanic provenance. One outcrop shows interbedded volcanic agglomerate and diamictite, and volcanic bombs can also be identified. The interbedding of diamictites with turbidites and the stratigraphic context provided by the thick sequences of turbidites below (Mall Bay Formation) and above (Drook Formation) indicate a deep marine slope setting of diamictite deposition. Diamictite facies record remobilization and downslope transfer of large volumes of unstable volcanic and glacial debris initially deposited in a shallower water marginal marine zone. The regional tectonic framework suggests the Conception Group accumulated in a deep, southward-opening ensialic rift basin with active but waning volcanic centres to the north. The Gaskiers Formation may be representative of other Late Precambrian glacially-influenced diamictite sequences that were deposited around the North Atlantic region and in Europe. These deep marine diamictite sequences characterized by debris flows, turbidites, and slump deposits, can be contrasted with more extensive shallow marine shelf diamictite sequences found in association with dolomites and tidally influenced shallow water facies in other basinal settings.     

Texture and structure of resedimented conglomerates: examples from Książ Formation (Famennian—Tournaisian), southwestern Poland

The Famennian-Tournaisian conglomerates and sandstones of the Ksiaz Formation are interpreted as marine resedimented deposits. Matrix- and clast-supported conglomerate beds are equally common, and two textural sequences (motifs) have been recognized: (I) matrix-rich conglomerate → pebbly sandstone → sandstone, and (II) clast-supported conglomerate → sandstone. Variation in clast type partly controls motif type, and therefore, to some extent, matrix percentage in the conglomerates generally. Grading is extremely common in both clast- and matrix-supported conglomerates: inverse (19%), inverse-to-normal (14%) and normal (26%). The studied succession, itself part of a 4 km thick, fan delta, basin-fill sequence, is organized into large (110–150 m) and small-scale (5–30 m) sequences, both of which show (1) upward coarsening and thickening, (2) upward trend of sandstones and pebbly sandstone → matrix-rich conglomerates → clast-supported conglomerates and (3) a less clear upward tendency of massive and normally graded beds → inversely graded beds. Variation in matrix percentage in beds is therefore also partly controlled by fan processes, during the progradation of fan bodies and lobes. It is predicted that individual resedimented conglomerate beds or motifs show general downfan trends in thickness, texture and structure opposite to those evident in the vertical sequences.     

An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

The feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south-west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (R_w), maximum projection sphericity (δ_p), oblate-prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δ_p and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade-shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and R_w is observed, hence the abundance of disc-shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.     

Lithofacies and origin of late Quaternary mass transport deposits in submarine canyons, central Scotian Slope, Canada

Mass transport deposits, up to 3·9 m thick, have been identified from piston cores collected from canyon floors and inter-canyon ridges on the central Scotian Slope. These deposits are characterized by four distinct mass-transport facies – folded mud, dipping stratified mud, various types of mud-clast conglomerate, and diamicton. Commonly, the folded and stratified mud facies are overlain by mud-clast conglomerate, followed by diamicton and then by turbidity current deposits of well-sorted sand. Stratified and folded mud facies were sourced from canyon walls. Overconsolidation in clasts in some mud-clast conglomerates indicates that the source sediment was buried 12–33 m, much deeper than the present cored depth, implying a source in canyon heads and canyon walls. The known stratigraphic framework for the region and new radiocarbon dating suggests that there were four or five episodes of sediment failure within the past 17 ka, most of which are found in more than one canyon system. The most likely mechanism for triggering occasional, synchronous failures in separate canyons is seismic ground shaking. The facies sequence is interpreted as resulting from local slides being overlain by mud-clast conglomerate deposits derived from failures farther upslope and finally by coarser-grained deposits resulting from retrogressive failure re-mobilizing upper slope sediments to form debrisflows and turbidity currents.     

The ‘old red sandstone’ of county mayo,Northwest Ireland

The distribution and stratigraphic relations of the Old Red Sandstone of the Clew Bay area. County Mayo are described. They comprise two groups both dominated by conglomerates, one Lower Devonian and one Middle Devonian, which have differing clast contents and which are separated by faults. The sediments are described in terms of five facies which are all interpreted as alluvial fan deposits. Provenance data from palaeocurrents and clast composition suggest transport predominantly to the west-northwest but one formation shows an opposite sense of transport. The rocks are folded about northeast-southwest axial traces and are affected by numerous faults. Structures are more complex than those of the Lower Carboniferous rocks which unconformably overlie the Devonian. A similar tectonosedimentary setting to the Old Red Sandstone of parts of the Scottish Caledonides is indicated.     

Sandy high-energy flood sedimentation — Some criteria for recognition,with an example from the devonian of S.W. England

Many modern sand deposits resulting from episodic floods in semi-arid zones are dominated by parallel laminations, but parallel-laminated sands are not a major feature in perennial streams. Single flood events may deposit over 1.5 m of parallel-laminated sand. Hence, ancient parallel-laminated, sand-dominated alluvial deposits may be the product of ephemeral flows.The Trentishoe Formation (Hangman Sandstone Group — Middle Devonian) of North Devon consists, in part, of laterally extensive beds of parallel-laminated, fine-grained red sandstones that build multistorey sand bodies. Individual beds are little more than 1 m thick, but may extend laterally for hundreds of metres. A vertical sequence of erosion surface, sparse silt clasts, parallel laminations, silt drape, is often found, suggesting high flow stage deposition of plane-bedded sand, followed by a very rapid waning of flow with silt fallout in the final stages of the flood. This sequence bears marked similarity to certain modern ephemeral flow sands.     

Sedimentology and stratigraphy of a transgressive, muddy gravel beach: Waterside Beach, Bay of Fundy, Canada

Sediments exposed at low tide on the transgressive, hypertidal (>6 m tidal range) Waterside Beach, New Brunswick, Canada permit the scrutiny of sedimentary structures and textures that develop at water depths equivalent to the upper and lower shoreface. Waterside Beach sediments are grouped into eleven sedimentologically distinct deposits that represent three depositional environments: (1) sandy foreshore and shoreface; (2) tidal‐creek braid‐plain and delta; and, (3) wave‐formed gravel and sand bars, and associated deposits. The sandy foreshore and shoreface depositional environment encompasses the backshore; moderately dipping beachface; and a shallowly seaward‐dipping terrace of sandy middle and lower intertidal, and muddy sub‐tidal sediments. Intertidal sediments reworked and deposited by tidal creeks comprise the tidal‐creek braid plain and delta. Wave‐formed sand and gravel bars and associated deposits include: sediment sourced from low‐amplitude, unstable sand bars; gravel deposited from large (up to 5·5 m high, 800 m long), landward‐migrating gravel bars; and zones of mud deposition developed on the landward side of the gravel bars. The relationship between the gravel bars and mud deposits, and between mud‐laden sea water and beach gravels provides mechanisms for the deposition of mud beds, and muddy clast‐ and matrix‐supported conglomerates in ancient conglomeratic successions. Idealized sections are presented as analogues for ancient conglomerates deposited in transgressive systems. Where tidal creeks do not influence sedimentation on the beach, the preserved sequence consists of a gravel lag overlain by increasingly finer‐grained shoreface sediments. Conversely, where tidal creeks debouch onto the beach, erosion of the underlying salt marsh results in deposition of a thicker, more complex beach succession. The thickness of this package is controlled by tidal range, sedimentation rate, and rate of transgression. The tidal‐creek influenced succession comprises repeated sequences of: a thin mud bed overlain by muddy conglomerate, sandy conglomerate, a coarse lag, and capped by trough cross‐bedded sand and gravel.     

Debris flow (olistostromes) and slumping on a distal passive continental margin: the Palombini limestone–shale sequence of the northern Apennines

An olistostrome accumulation up to 530 m thick occurs in the Casanova area of the northern Apennines. It lies within or above the calciturbiditic Palombini limestone-shale sequence, and is part of the allochthonous Vara Complex—sediments originally deposited on oceanic crust. The olistostromes are poorly sorted, monomict, matrix-supported, submarine debris flow deposits with rigid plugs. They have a compactional foliation and a compaction-modified, planar clast fabric created during flow. Although diachronous in the main part of the area, the olistostromes have a vertical gradational contact with the overlying slumped Palombini, in which recumbent asymmetric fold hinges and trains and slump boudins are present. Many criteria indicate soft-sediment deformation. Fold asymmetry indicates a uniformly SW-dipping palaeoslope. The textural gradation from slumps to olistostrome beds, plus slump folds and boudins as olistostrome clasts show that the olistostromes are dismembered slumps. In vertical sections, variations in limestone petrography, volume percentage and size of clasts confirm that the olistostromes were derived from the Palombini as a series of bed-by-bed slumps keeping pace with sedimentation of the Palombini. From olistostrome clast sizes and bed thicknesses, a depositional slope of ～ 4° is estimated. The olistostromes are not precursor sediments shed from advancing nappes, as in the Bracco ridge model of some authors; rather, they were formed at the foot of a distal, block-faulted passive continental margin, long before nappe emplacement.     

SEDIMENTATION IN THE RIFT ZONE OF THE JUAN DE FUCA RIDGE

The sediments of the upper Swartkops River are almost exclusively gravels and boulder beds derived from the Cretaceous Uitenhage Group and the Paleozoic Cape Supergroup rocks. Many of the cobbles and boulders are second-cycle clasts, the great majority of which are quartzitic in composition. Pebble size and shape were examined and fabric analysis was performed on samples from 22 sites in the study area. Pebble imbrication planes dip consistently upstream at angles of 20? to 50? and pebble long axes generally are aligned normal to the flow direction. Clasts in the braid-plain deposits range from a few millimeters to tens of centimeters (large boulders over a meter in diameter are not uncommon). Pebble roundness ranges from 0.2 to 0.9 (averaging 0.43) and sphericity values range from 0.3 to 0.9 (averaging 0.59). The gravel clasts are angular to well-rounded, but are predominantly subrounded. Zingg diagram plots show a majority of discoidal pebbles, but there is a diversity of shapes reflecting the complex source area from which some resedimented clasts originated.

Channel and bar morphology is complex, with gravel bars often merging laterally and longitudinally with main and secondary channels. Both channels and bars are terraced stepwise downstream and across the braid plain. Bar tops are armored by both small and large clasts, whereas channels may be lined with cobbles or boulders, but often exhibit small pebble lags. Algal mats occur as fresh curtains in all standing pools of water and dried crusty deposits on pebbly substrates in inactive channels.

Imbrication studies demonstrate conclusively that pebble imbrication is the most meaningful indicator of flow direction in a gravel deposit and is far more reliable than rare cross-bedding encountered in bar-top sands, where bedforms often migrate laterally rather than downstream. The Swartkops braid-plain gravels resemble the ancient deposits of the Ventersdorp Contact Reef, both deposits being characterized by boulder-rich gravels, poor clast sorting, resedimented pebbles from a proximal fault-bounded source, and algal mats. Although heavy minerals are lacking in the Swartkops, trapping of fines by algal filaments appears to occur during low-flow conditions.     

Shallow‐water microbialite‐volcaniclastic facies association in the Cambro‐Ordovician Mt Windsor Subprovince,Australia

The Trooper Creek Formation is a mineralised submarine volcano‐sedimentary sequence in the Cambro‐Ordovician Seventy Mile Range Group, Queensland. Most of the Trooper Creek Formation accumulated in a below‐storm‐wave‐base setting. However, microbialites and fossiliferous quartz‐hematite ± magnetite lenses provide evidence for local shoaling to above fairweather wave‐base (typically 5–15 m). The microbialites comprise biogenic (oncolites, stromatolites) and volcanogenic (pumice, shards, crystal fragments) components. Microstructural elements of the bioherms and biostromes include upwardly branching stromatolites, which suggest that photosynthetic microorganisms were important in constructing the microbialites. Because the microbialites are restricted to a thin stratigraphic interval in the Trooper Creek area, shallow‐water environments are interpreted to have been spatially and temporarily restricted. The circumstances that led to local shoaling are recorded by the enclosing volcanic and sedimentary lithofacies. The microbialites are hosted by felsic syneruptive pumiceous turbidites and water‐settled fall deposits generated by explosive eruptions. The microbialite host rocks overlie a thick association (≤?300 m) of andesitic lithofacies that includes four main facies: coherent andesite and associated autoclastic breccia and peperite; graded andesitic scoria breccia (scoriaceous sediment gravity‐flow deposits); fluidal clast‐rich andesitic breccia (water‐settled fall and sediment gravity‐flow deposits); and cross‐stratified andesitic sandstone and breccia (traction‐current deposits). The latter three facies consist of poorly vesicular blocky fragments, scoriaceous clasts (10–90%), and up to 10% fluidally shaped clasts. The fluidal clasts are interpreted as volcanic bombs. Clast shapes and textures in the andesitic volcaniclastic facies association imply that fragmentation occurred through a combination of fire fountaining and Strombolian activity, and a large proportion of the pyroclasts disintegrated due to quenching and impacts. Rapid syneruptive, near‐vent aggradation of bombs, scoria, and quench‐fragmented clasts probably led to temporary shoaling, so that subsequent felsic volcaniclastic facies and microbialites were deposited in shallow water. When subsidence outpaced aggradation, the depositional setting at Trooper Creek returned to being relatively deep marine.     

Facies analysis of the Plateau Sandstones (Eocene to early Miocene?), Bako National Park,Sarawak, Malaysia

Sandstones, located in the Kuching area, western Sarawak, are known as the ‘Plateau Sandstones’ (of possible Eocene to early Miocene age). However, based on a number of factors, including: (i) anomalous kerogene compositions; (ii) proximity of the on-lap surface; and (iii) palaeocurrent direction (generally to the NNE), it is thought that the sands exposed on the Bako Peninsula are unrelated to the Plateau Formation (located to the south of the Bako Penisula) and therefore a new name is coined; the Bako Sandstones, which form a subgroup of the Bako Sandstone Group. The Bako Sandstones form the Bako Peninsula, a flat-topped cliffed plateau which extends into the South China Sea at a latitude of 1°30′N. The plateau has a gently dipping surface, sloping northwards from a height of about 300 to 150 m. The sandstones form a succession of very thick bedded sandstones (up to 6 m thick), with lenses of conglomerates and subordinate sandy mudstones. The sandstones consist of pebbly coarse-medium grained sands, interbedded with polymictic pebble conglomerates. The sandstones are mainly lithic arenite, poorly to moderately sorted and consist of subangular to subrounded grains. Isolated pebbles are common throughout the sandstones. The most common structure in both sandstones and conglomerates is cross-bedding; planar cross-bedding and trough cross-bedding, together with thick sequences of climbing ripples. These structures suggest extensive tractional transport, forming both ripple and dune structures along the base of the channel. The geometry of the sands is either (i) lensoidal, or (ii) tabular, with the channel-fill interpreted as scour-fill channels or migrating dunes, respectively. Both types are commonly stacked vertically or amalgamate laterally to form thick interconnected units. The conglomeratic lenses, scour-fill features and rip-up shale clasts are related to higher energy erosional events, whilst the mud-draped ripples, ripple rejuvenation surfaces and two-tiered channel margins indicate a lower energy and stasis period. Slope instabilities at the channel margin are inferred from the slump structures present and shale clast slurries. The sandstones at Bako are thought to have formed within a braided channel environment (subject to exposure, from the presence of mud cracks within the formation).     

Chemical and physical responses to deformation in micaceous quartzites from the Tauern Window, Eastern Alps

Micaceous quartzites from a subvertical shear zone in the Tauern Window contain abundant quartz clasts derived from dismembered quartz‐tourmaline veins. Bulk plane strain deformation affected these rocks at amphibolite facies conditions. Shape changes suggest net shortening of the clasts by 11–64%, with a mean value of 35%. Quartz within the clasts accommodated this strain largely via dislocation creep processes. On the high‐stress flanks of the clasts, however, quartz was removed via solution mass transfer (pressure solution) processes; the resulting change in bulk composition allowed growth of porphyroblastic staurolite + chlorite ± kyanite on the clast flanks. Matrix SiO₂ contents decrease from c. 83 wt% away from the clasts to 49–58% in the selvages on the clast flanks. The chemical changes are consistent with c. 70% volume loss in the high‐stress zones. Calculated shortening values within the clast flanks are similar to the volume‐loss estimates, and are greatly in excess of the shortening values calculated from the clasts themselves. Flow laws for dislocation creep versus pressure solution imply large strain‐rate gradients and/or differential stress gradients between the matrix and the clast selvages. In a rock containing a large proportion of semirigid clasts, weakening within the clast flanks could dominate rock rheology. In our samples, however, weakening within the selvages was self limiting: (1) growth of strong staurolite porphyroblasts in the selvages protected remaining quartz from dissolution; and (2) overall flattening of the quartz clasts probably decreased the resolved shear stress on the flanks to values near those of the matrix, which would have reduced the driving force for solution‐transfer creep. Extreme chemical changes nonetheless occurred over short distances. The necessity of maintaining strain compatibility may lead to significant localized dissolution in rocks containing rheologic heterogeneities, and overall weakening of the rocks may result. Solution‐transfer creep may be a major process whereby weakening and strain localization occur during deep‐crustal metamorphism of polymineralic rocks.     

贺兰奥拉槽早古生代深水重力流体系的沉积特征和充填样式

位于鄂尔多斯西缘的贺兰构造带为一中元古代一古生代的奥拉槽。在区内的中寒武和中奥陶统中识别了一套巨厚的深水重力流沉积,其中包括下斜坡滑塌泥石流复合体、浊积扇以及碳酸盐岩斜坡扇裙等沉积类型。主要的相单元包括充填沟道或进入扇面形成的泥石流钙质角砾岩和砾岩、充填辫状水道的多层叠置的砂岩和砂砾岩、上叠扇的砂、泥岩互层以及浊积砂屑或含砾砂屑灰岩等。在中奥陶世该奥拉槽发展成一深水一半深水海槽,沿盆地西侧发育有浊积扇,而东侧仅有碳酸盐岩滑塌扇裙。它们可能是沿深水盆地两侧深大断裂产生的陡坡或水下断崖分布的,代表了早古生代贺兰奥拉槽在强烈沉陷期特定的深水盆地充填。     

重庆老龙洞二叠系-三叠系界线地层的海平面下降事件

老龙洞的二叠系-三叠系界线地层剖面许多学者做过研究,关于其海平面变化问题的争论一直没有解决。本文的研究首次在该剖面上发现了明显的侵蚀面,说明曾经有海平面下降事件。对沉积环境的解释也有新的认识。该剖面在长兴期钙质海绵灰岩之上沉积了一套开阔台地相的沉积,其上是1.4m 厚的局限台地相沉积,具有斑点状构造。此层顶部出现一个波状起伏的侵蚀面。侵蚀面上下的岩性不同。此面之上是0.8m 厚的浅水的局限台地相沉积,具有树枝状的构造和外貌。此层顶面是一个更加明显的侵蚀面,起伏高差达到0.3m 以上。侵蚀面上下的岩性截然不同,以及其上岩石层理与侵蚀面的斜交关系,以及侵蚀面的形态把侵蚀面同缝合线区分开来。这个侵蚀面代表一次显著海平面下降事件之后的一次较长时间的出露和风化剥蚀。侵蚀面之上是透镜状分布的灰色的层状的小腹足类的颗粒岩,再上是一薄层灰黄色的泥粒岩,含丰富的小双壳类。此层以上是青灰色的薄板状的泥质灰泥岩,舍丰富的0.3～3mm 大小的、内部重结晶的同生角砾。     

Redeposited conglomerates in a miocene flysch sequence at Blackmount,Western Southland,New Zealand

The Middle Miocene Monowai Formation represents a gravel delta that prograded south into a flysch basin complex developed along the Moonlight Tectonic Zone, southern New Zealand. The delta-slope environment was characterized by a conglomeratic sequence up to 500 m thick. Most of the gravel was moved downslope by mass-transport processes. A complete spectrum exists from synsedimentary slide sheets (up to 10 m thick and 100 m long) that retain pre-sliding sedimentary structures, to more mature mass-transported sediment types in which all original structures have been destroyed. The most distal deposits include ungraded homogeneous pebble conglomerates up to 3 m thick. Some of the more mature redeposited conglomerate-sand-mud units (X–Y–Z sequences) are between 2 and 10 m thick; they comprise a basal X-division of bouldery conglomerate, a middle Y-division of pebbly mudstone or pebbly sandstone, and an upper Z-division of hydroplastically folded mudstone. Though X–Y–Z sequences may have been deposited from very proximal turbidity or fluxoturbidity currents, inertia-flow emplacement seems more likely. An inertia-flow mode of emplacement also seems most probable for the other redeposited sediment types described from the Monowai Formation.