Unusual intraclastic limestones in Lower Triassic carbonates and their bearing on the aftermath of the end-Permian mass extinction

Flat pebble conglomerates were a common carbonate facies in Cambrian to Early Ordovician open marine settings, but they become extremely rare in these environments after this time. However, the Early Triassic witnessed an anachronistic reappearance of flat pebbles, together with other intraclast types, in a range of carbonate depositional settings. In south China, flat pebble conglomerates are encountered in storm-dominated, platform carbonates to deep basinal settings, while prefossilized bivalve intraclasts and flat pebbles are common in mid-ramp facies of northern Italy. The emplacement mechanisms of the intraclast-bearing beds appear to have been diverse and to have included basinal turbidity flows and storm-generated hyperconcentrated flows: true storm beds, deposited under combined flow conditions, are rare. The cause of the widespread early lithification implied by the Early Triassic intraclasts appears to have been twofold: suppression of bioturbation, allowing the preservation of thin beds, and rapid submarine lithification. Both features appear to be a response to the widespread development of benthic dysoxia/anoxia during and following the end-Permian mass extinction. This event appears to have temporarily recreated the conditions that pertained in Cambro-Ordovician shelf seas. Flat pebble conglomerates may, therefore, constitute a proxy indicator of stressed environmental conditions associated with global anoxic/dysoxic events.     

鲁南新元古界石旺庄组风暴岩特征及其意义

鲁南地区新元古界石旺庄组为一套以白云岩为主的地层,其中发育多层竹叶状砾屑灰岩,同时还发育丘状层理、粒序层理等沉积构造以及波状、不规则状、含V字形、U字形槽模等底面构造所代表的冲刷面,表现为较典型的风暴岩特征。研究表明,石旺庄组竹叶状砾屑灰岩形成于缓坡型台地的浅缓坡环境之中,为揭示鲁南新元古界岩相古地理特征提供了重要依据。竹叶状砾屑灰岩所代表的风暴岩在前寒武纪碳酸盐岩地层、尤其是潮下缓坡相碳酸盐岩沉积中较为普遍,对该类型沉积及其共生的沉积构造进行系统描述将有助于前寒武纪古环境的恢复。     

Mass flow sedimentology within the HYC Zn–Pb–Ag deposit,Northern Territory,Australia: evidence for syn-sedimentary ore genesis

Laterally continuous mass-flow deposits are an important feature of the HYC stratiform sediment-hosted Zn–Pb–Ag deposit, which reveal more about the HYC mineralising system than has been previously recognised. Mass flow deposits are interbedded with sheet-like mineralised lenses in a carbonaceous dolomitic siltstone host rock. Sedimentological processes of mass-flow deposit emplacement are proposed that constrain stratiform mineralisation to the top metre of the sediment pile, based on mass-flow geometry and detailed clast petrology. Four distinct sedimentary facies are identified within the mass-flow units: framework-supported polymictic boulder breccia; matrix-supported pebble breccia; and gravel-rich and sand-rich graded turbidite beds. The boulder breccias are weakly reverse graded and show rapid lateral transition into the other facies, all of which are distal manifestations of the same sedimentary events. The flow geometry and relationships between these facies are interpreted to reflect mass-flow initiation as clast-rich debris flows, with transformation via the elutriation of fines into a subsequent turbulent flow from which the turbidite and matrix-supported breccia facies were deposited. All the mass-flow facies contain clasts of the common and minor components of the in-situ laminated base-metal mineralised siltstone. Texturally these are identical to their in-situ counterparts, and are clearly distinct from other sulphidic clasts that are of unequivocal replacement origin. In the boulder breccias, intraclasts may be the dominant clast type and the matrix may contain abundant fine-grained sphalerite and pyrite. Dark coloured sphaleritic and pyritic breccia matrices are distinct from pale carbonate-siliclastic matrices, are associated with high abundance of sulphidic clasts, and systematically occupy the lower part of breccia units. Consequently, clasts that resemble in-situ ore facies are confirmed as genuine intraclasts that were incorporated into erosive mass flows prior to complete consolidation. Disaggregation and assimilation of sulphidic sediment in the flow contributed to the sulphide component of the dark breccia matrices. The presence of laminated sulphidic intraclasts in the mass-flow facies constrains mineralisation at HYC to the uppermost part of the seafloor sediment pile, where this material was susceptible to erosion by incoming clast-rich mass flows.Editorial handling: N. White     

The Hunghae Formation, SE Korea: Miocene debris aprons in a back-arc intraslope basin

ABSTRACT During early to middle Miocene times a sudden opening of the Ulleung (Tsushima) back-arc basin in the East Sea (Sea of Japan) led to the development of intraslope basins along the rifted southwestern margin (southeast Korea). Abrupt subsidence resulted in the deposition of the 200 m thick Hunghae Formation (middle Miocene), a sand/mudstone sequence that can be divided into five facies. Facies I (sand and mudstone couplet) and II (coarse sand) are turbiditic in origin, as evidenced by massive, graded, crudely-layered and parallel-laminated sand beds. Facies III (homogeneous mudstone) is characterized by various lignite and plant fragments, clastic and biogenic grains that are randomly oriented, suggestive of hemipelagic deposition. Facies IV (chaotic deposit) is characterized by the disruption of beds, the presence of isolated siltstone blocks (or balls) and large clasts in the muddy matrix, indicative of retrogressive rockfall and slide/slump. Facies V (conglomerate) is of debris flow origin, as evidenced by clast- and matrix-supported features, floating large clasts and absence of traction structures. Individual facies are organized into two types of facies association: (1) homogeneous mudstone (facies III) associated randomly with the rest (facies I, II, IV and V), indicative of hemipelagic and episodic sediment-gravity flow processes, respectively; (2) conglomerate (facies V), coarse sand (facies II) and sand/mudstone couplet (facies I), representing the flow transformation from debris flow to high- and low-concentration turbidity currents. These facies associations are similar in many respects to modern and ancient debris (or slope) aprons found elsewhere. Numerous isolated slide/slump blocks, wedged conglomerates with armoured mudstone balls, discontinuous lignite-containing sand/mudstone beds, chaotic structure and growth faults suggest that the deposition occurred on a steep slope (intraslope basin) off coalescing fan-deltas, mainly by unchannellized sediment-gravity flows. Ancient deposits with irregular facies sequences can be viewed as debris-apron systems, which provide alternatives to submarine-fan models in many clastic basins with a line rather than point source.     

Limestone pseudoconglomerates in the Late Cambrian Gushan and Chaomidian Formations (Shandong Province, China): soft-sediment deformation induced by storm-wave loading

This paper focuses on the formative processes of limestone pseudoconglomerates in the Gushan and Chaomidian Formations (Late Cambrian) of the North China Platform, Shandong Province, China. The Gushan and Chaomidian Formations consist mainly of limestone and shale (marlstone) interlayers, wackestone to packstone, grainstone and microbialite as well as numerous limestone conglomerates. Seventy‐three beds of limestone pseudoconglomerate in the Gushan and Chaomidian Formations were analysed based on clast and matrix compositions, internal fabric, sedimentary structures and bed geometry. These pseudoconglomerates are characterized by oligomictic to polymictic limestone clasts of various shapes (i.e. flat to undulatory disc, blade and sheet), marlstone and/or grainstone matrix and various internal fabrics (i.e. intact, thrusted, edgewise and disorganized), as well as transitional boundaries. Limestone pseudoconglomerates formed as a result of soft‐sediment deformation of carbonate and argillaceous interlayers at a shallow burial depth. Differential early cementation of carbonate and argillaceous sediments provided the requisite conditions for the formation of pseudoconglomerates. Initial deformation (i.e. burial fragmentation, liquefaction and injection) and subsequent mobilization and disruption of fragmented clasts are two important processes for the formation of pseudoconglomerates. Burial fragmentation resulted from mechanical rupture of cohesive carbonate mud, whereas subsequent mobilization of fragmented clasts was due to the injection of fluid materials (liquefied carbonate sand and water‐saturated argillaceous mud) under increased stress. Storm‐wave loading was the most probable deformation mechanism, as an external triggering force. Subsequent re‐orientation and rounding of clasts were probably prolonged under normal compactional stress. Eventually, disrupted clasts, along with matrix materials, were transformed into pseudoconglomerates by progressive lithification. Soft‐sediment deformation is prevalent in alternate layers of limestone and mud(marl)stone and/or grainstone, regardless of their depositional environments.     

Sedimentation and palaeoenvironments of Late Cretaceous crater-lake deposits in Bushmanland, South Africa

A large diameter borehole core from an epiclastic kimberlite remnant on the farm Stompoor in the Prieska district, Cape Province, contains a continuous 76 m section of fossiliferous sediments interpreted as having accumulated within a crater-lake during the Late Cretaceous. Three distinct facies associations reflect depositional processes that prevailed in offshore areas of the original lake. Facies Association A: matrix-supported pebble conglomerates comprising a chaotic assemblage of pyroclastic, basement and country rocks set in a fine-grained matrix. Flat, non-erosional basal surfaces with ‘frozen’ rip-up clasts, the protrusion of matrix-supported clasts above the upper surfaces and a direct relationship between maximum clast size and bed thickness suggest deposition from debris flows that originated subaerially on pyroclastic talus cones surrounding the crater. Facies Association B: alternating thin beds of matrix-supported granule conglomerate, structureless fine-grained sandstone and parallel laminated mudrock. Small fining-upward sequences within these beds are comparable to turbidite Bouma Tade, Tde. Numerous partings display petrified fish and frog skeletons, as well as bivalve, gastropod and ostracode shells, leaf impressions, insect wings and a possible bird bone. These beds were deposited by thin debris-flows and turbidity underflows interspersed with periods of ‘pelagic’ sedimentation. Facies Association C: microlaminated mudstone beds containing scattered ‘dropstone lapilli’. The lamination is imparted by alternating Ca-rich/Ca-poor layers which may reflect climatic seasonality. They are interpreted as the result of seasonally influenced suspension settling through a thermally stratified water column. Short-term periodicities in conglomerate bed thicknesses are interpreted as the result of successive block caving of a slump scar giving rise to several debris flows from the same source area. Seismic shock from nearby volcanism may have simultaneously triggered slumps on both subaerial and subaqueous slopes. Dropstone lapilli in Type C beds and the preponderance of load casting in Type B beds support this interpretation. An estimate of the time span involved in accumulating 76 m of crater lake sediments based on the possible seasonal imprint of Type C beds gives a figure of some 220,000 yr.     

Carbonate ramp-to-deeper shale shelf transitions of an Upper Cambrian intrashelf basin, Nolichucky Formation, Southwest Virginia Appalachians

The Nolichucky Formation (0–300 m thick) formed on the Cambrian pericratonic shelf in a shallow intrashelf basin bordered along strike and toward the regional shelf edge by shallow water carbonates and by nearshore clastics toward the craton. Lateral facies changes from shallow basinal rocks to peritidal carbonates suggest that the intrashelf basin was bordered by a gently sloping carbonate ramp. Peritidal facies of the regional shelf are cyclic, upward-shallowing stromatolitic carbonates. These grade toward the intrashelf basin into shallow ramp, cross-bedded, ooid and oncolitic, intraclast grain-stones that pass downslope into deeper ramp, subwave base, ribbon carbonates and thin limestone conglomerate. Ribbon limestones are layers and lenses of trilobite packstone, parallel and wave-ripple-laminated, quartzose calcisiltite, and lime mudstone arranged in storm-generated, fining upward sequences (1–5 cm thick) that may be burrowed. Shallow basin facies are storm generated, upward coarsening and upward fining sequences of green, calcareous shale with open marine biota; parallel to hummocky laminated calcareous siltstone; and intraformational flat pebble conglomerate. There are also rare debris-flow paraconglomerate (10–60 cm thick) and shaly packstone/wackestone with trace fossils, glauconite horizons and erosional surfaces/hardgrounds. A 15-m thick tongue of cyclic carbonates within the shale package contains subtidal digitate algal bioherms which developed during a period of shoaling in the basin. Understanding the Nolichucky facies within a ramp to intrashelf basin model provides a framework for understanding similar facies which are widely distributed in the Lower Palaeozoic elsewhere. The study demonstrates the widespread effects of storm processes on pericratonic shelf sedimentation. Finally, recognition of shallow basins located on pericratonic shelves is important because such basins influence the distribution of facies and reservoir rocks, whose trends may be unrelated to regional shelf-edge trends.     

Submarine fan and slope-apron deposition in a Cretaceous Forearc Basin: The Gürsökü Formation (Kavak–Samsun, N. Turkey)

The Late Cretaceous Gürsökü Formation represents the proximal fill of the Sinop–Samsun Forearc Basin that was probably initiated by extension during the Early Cretaceous. The succession records sedimentation in two contrasting depositional systems: a slope-apron flanking a faulted basin margin and coarse-grained submarine fans. The slope-apron deposits consist of thinly bedded turbiditic sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides. The submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non-cohesive debris flows. The occurrence of both slope apron and submarine fan depositional systems in the Gürsökü Formation may indicates that the region was a tectonically active basin margin during the Late Cretaceous.     

Sand intraclasts within a diamicton mélange,southern Niagara Peninsula,Ontario, Canada

Sand intraclasts found within diamicton units along the north shore of Lake Erie in the Mohawk Bay area of the Niagara Peninsula would appear to be part of a ‘block-in-matrix’ mélange. The intraclasts are undeformed and many exhibit primary bedding structures. Numerous intraclasts have been rotated and/or tilted and are, in general, subrounded in outline. Examination of the surrounding diamicton reveals that the diamicton clast fabrics exhibit a wide scatter and are not characteristic of any known till clast fabric. Around each intraclast exists an aureole of brecciated diamicton. Other evidence in the form of macro- and microshear structures, and banding within the diamicton indicate that the diamicton has been subject to high strain. Interpretation of the sand intraclasts seems to be intrinsically linked to the origin of the diamicton and together linked to the origin of the mélange. Various hypotheses are suggested separately for the sand intraclasts, diamicton and mélange. A subglacial deformable bed hypothesis is advanced as the most acceptable explanation for the complete sediment sequence in which diamicton and frozen sand intraclasts, the latter mobilised from the substrate, are moved as a mélange below an active fast-moving ice mass. Several implications from this study emerge with regard to glacial sedimentology and stratigraphic interpretations.     

Glacial influence on Neoproterozoic sedimentation: the Smalfjord Formation, northern Norway

ABSTRACT There is much debate regarding the intensity and geographic extent of glaciation during the Neoproterozoic, particularly in response to recent geochemical work suggesting that the Neoproterozoic earth was at times ice covered from equator to poles (the ‘Snowball Earth’ hypothesis). A detailed sedimentological analysis of the Neoproterozoic Smalfjord Formation of northern Norway was conducted in order to determine the extent and intensity of glacial influence on sedimentation. In the Tarmfjorden area, the Smalfjord Formation consists of a stacked succession of diamictites interbedded with fine‐grained laminated mudstones containing rare outsized clasts. Diamictites and interbedded mudstones are interpreted as the product of subaqueous mass flows generated along the basin margin. In the Varangerfjorden area, chaotically interbedded diamictites, conglomerates and sandstones are overlain by a thick succession of stacked sandstone beds; onediamictite unit at Bigganjargga overlies a striated pavement. The Varangerfjorden outcrops appear to record deposition on a subaqueous debris apron. Although diamictites contain rare striated and faceted clasts, suggesting a glacial sediment source, their origin as subaqueous mass flows prevents the interpretation of ice mass form or distribution. Rare lonestones may be associated with floating ice in the basin, which may be of glacial or seasonal origin. Glacial ice may have contributed poorly sorted glacial debris to the basin margin, either directly or through fluvioglacial systems, but there is no evidence of direct deposition by ice at Varangerfjorden or Tarmfjorden. The overall fining‐upward trend identified in the Smalfjord Formation and overlying Nyborg Formation is consistent with depositional models of rift basin settings. This fining‐upward trend, the predominance of mass flow facies including breccias associated with scarps and the evidence for extensional tectonic activity in the region suggest that tectonic activity may have played an important role in the development of this Neoproterozoic succession. The Smalfjord Formation at Tarmfjorden and Varangerfjorden does not exhibit sedimentological characteristics consistent with severe glacial conditions suggested by the snowball Earth hypothesis.     

A Pliocene mega-tsunami deposit and associated features in the Ranquil Formation,southern Chile

An exceptionally large tsunami affected the coastline of southern Chile during the Pliocene. Its backflow eroded coarse beach and coastal dune sediments and redistributed them over the continental shelf and slope. Sandstone dykes and sills injected from the base of the resulting hyperconcentrated flow into underlying cohesive muds, assisted in plucking up large blocks of the latter and incorporating them into the flow. Locally, the rip-up intraclasts were fragmented further by smaller-scale injections to form a distinct breccia of angular to rounded mudstone clasts within a medium to coarse sandstone matrix. Sandstone sills in places mimic normal sedimentary beds, complete with structures resembling inverse gradation, planar laminae, as well as ripple and trough cross-lamination. These were probably formed by internal sediment flow and shear stress as the semi-liquefied sand was forcefully injected into cracks. In borehole cores, such sills can easily be misinterpreted as normal sedimentary beds, which can have important implications for hydrocarbon exploration.     

A Cretaceous carbonate delta drift in the Montagna della Maiella,Italy

The Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is compared to newly discovered contourite drifts in the Maldives. Like the drift deposits in the Maldives, the Orfento Formation fills a channel and builds a Miocene delta‐shaped and mounded sedimentary body in the basin that is similar in size to the approximately 350 km² large coarse‐grained bioclastic Miocene delta drifts in the Maldives. The composition of the bioclastic wedge of the Orfento Formation is also exclusively bioclastic debris sourced from the shallow‐water areas and reworked clasts of the Orfento Formation itself. In the near mud‐free succession, age‐diagnostic fossils are sparse. The depositional textures vary from wackestone to float‐rudstone and breccia/conglomerates, but rocks with grainstone and rudstone textures are the most common facies. In the channel, lensoid convex‐upward breccias, cross‐cutting channelized beds and thick grainstone lobes with abundant scours indicate alternating erosion and deposition from a high‐energy current. In the basin, the mounded sedimentary body contains lobes with a divergent progradational geometry. The lobes are built by decametre thick composite megabeds consisting of sigmoidal clinoforms that typically have a channelized topset, a grainy foreset and a fine‐grained bottomset with abundant irregular angular clasts. Up to 30 m thick channels filled with intraformational breccias and coarse grainstones pinch out downslope between the megabeds. In the distal portion of the wedge, stacked grainstone beds with foresets and reworked intraclasts document continuous sediment reworking and migration. The bioclastic wedge of the Orfento Formation has been variously interpreted as a succession of sea‐level controlled slope deposits, a shoaling shoreface complex, or a carbonate tidal delta. Current‐controlled delta drifts in the Maldives, however, offer a new interpretation because of their similarity in architecture and composition. These similarities include: (i) a feeder channel opening into the basin; (ii) an excavation moat at the exit of the channel; (iii) an overall mounded geometry with an apex that is in shallower water depth than the source channel; (iv) progradation of stacked lobes; (v) channels that pinch out in a basinward direction; and (vi) smaller channelized intervals that are arranged in a radial pattern. As a result, the Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is here interpreted as a carbonate delta drift.     

An iron-bearing wave-dominated siliciclastic shelf: Facies analysis and palaeogeographic implications (Silurian-Lower Devonian Sierra Grande Formation,Southern Argentina)

The Sierra Grande Formation (Silurian-Early Devonian) consists of quartz arenites associated with clast supported conglomerates, mudstones, shales and ironstones. Eight sedimentary facies are recognized: cross-stratified and massive sandstone, plane bedded sandstone, ripple laminated sandstone, interstratified sandstone and mudstone, laminated mudstone and shale, oolitic ironstone, massive conglomerate and sheet conglomerate lags. These facies are interpreted as shallow marine deposits, ranging from foreshore to inner platform environments. Facies associations, based on vertical relationships among lithofacies, suggest several depositional zones: (a) beach to upper shoreface, with abundant plane bedded and massive bioturbated sandstones; (b) upper shoreface to breaker zone, characterized by multistorey cross-stratified and massive sandstone bodies interpreted as subtidal longshore-flow induced sand bars; (c) subtidal, nearshore tidal sand bars, consisting of upward fining sandstone sequences; (d) lower shoreface zone, dominated by ripple laminated sandstone, associated with cross-stratified and horizontal laminated sandstone, formed by translatory and oscillatory flows; and (e) transitional nearshore-offshore and inner platform zones, with heterolithic and pelitic successions, and oolitic ironstone horizons. Tidal currents, fair weather waves and storm events interacted during the deposition of the Sierra Grande Formation. However, the relevant features of the siliciclastics suggest that fair weather and storm waves were the most important mechanisms in sediment accumulation. The Silurian-Lower Devonian platform was part of a continental interior sag located between southern South America and southern Africa. The Sierra Grande Formation was deposited during a second order sea level rise, in which a shallow epeiric sea flooded a deeply weathered low relief continent.     

Mid-Cretaceous basin margin carbonates, east-central Mexico

Isolated, high relief carbonate platforms developed in the intracratonic basin of east-central Mexico during Albian-Cenomanian time. Relief on the platforms was of the order of 1000 m and slopes were as steep as 20–43°. Basin-margin debris aprons adjacent to the platforms comprise the Tamabra Formation. In the Sierra Madre Oriental, at the eastern margin of the Valles-San Luis Potosi Platform, an exceptionally thick (1380m) progradational basin to platform sequence of the Tamabra Formation can be divided into six lithological units. Basinal carbonate deposition that preceded deposition of the Tamabra Formation was emphatically punctuated by an allochthonous reef block 1 km long by 0·5 km wide with a stratigraphic thickness of 95 m. It is encased in Tamabra Formation unit A, approximately 360 m of peloidal-skeletal wackestone and lithoclastic-skeletal packstone that includes some graded beds. Unit B is 73 m of massive dolomite with sparse skeletal fragments and intraclasts. Unit C, 114m thick, consists of structureless skeletal wackestone passing upward into graded skeletal packstone. Interlaminated lime mudstone and fine grained bioclastic packstone with prominent horizontal burrows are interspersed near the top. Unit D is 126 m of breccia with finely interbedded skeletal grainstone and burrowed or laminated mudstone. The breccias contain a spectrum of platform-derived lithoclasts and basinal intraclasts, up to 10 m in size. The breccias are typically grain supported (rudstone) with a matrix of lightly to completely dolomitized mudstone or skeletal debris. Beds are up to several metres thick. Unit E is 206 m of massive, sucrosic dolomite that replaced breccias. Unit F is approximately 500 m of thick bedded to massive skeletal packstone with abundant rudists and a few mudstone intraclasts. Metre scale laminated lime mudstone beds are interspersed. The section is capped by El Abra Formation platform margin limestone, consisting of massive beds of caprinid packstone and grainstone with many whole valves. Depositional processes within this sequence shift from basinal pelagic or peri-platform sedimentation to distal, platform-derived, muddy turbidity currents with a large slump block (Unit A); through more proximal (coarser and cleaner) turbidity currents (Unit B?, C); to debris flows incorporating platform margin and slope debris (Units D, E). Finally, a talus of coarse, reef-derived bioclasts (Unit F) accumulated as the platform margin prograded over the slope sequence. Interspersed basinal deposits evolved gradually from largely pelagic to include influxes of dilute turbidity currents. Units containing turbidites with platform-derived bioclasts reflect flooding of the adjacent platform. Breccia blocks and lithoclasts were probably generated by erosion and collapse of the platform during lowstands. Laminated, black, pelagic carbonates, locally cherty, are interbedded with both breccias and turbidites. At least those interbedded with turbidites may have been deposited within an expanded mid-water oxygen minimum zone during relative highstands of sea level. They are in part coeval with mid-Cretaceous black shales of the Atlantic Ocean.     

Ichnology and Depositional Environment of the Cambrian Nagaur Sandstone (Nagaur Group) Along the Dulmera Section,Bikaner-Nagaur Basin,Rajasthan

Abstract: The Lower Cambrian Nagaur Sandstone (Marwar Supergroup) has yielded trace fossils Treptichnus, Cruziana, serially repeated Rusophycus, Diplichnites, Monomorphichnus, Bergaueria, arthropod swimming traces and leap frogging marks of Cruziana ichnofacies. This ichnofossils assemblage is dominantly preserved in fine to medium grained red sandstone beds of the Mohra Member (Nagaur Sandstone). The presence of graded rip-up clasts, current ripples, dune cross-stratification with mud drapes and tidal bundles indicates a subtidal paleoenvironment for the deposition of Nagaur Sandstone Formation, which is corroborated by the Cruziana ichnofacies trace fossil assemblage.     

四川盆地及其周缘地区寒武系洗象池群颗粒滩特征及分布

通过对野外露头剖面、钻井以及1︰200000区域地质调查等资料的综合研究,识别出四川盆地及其周缘地区寒武系洗象池群颗粒滩的岩石类型包括颗粒云岩、颗粒灰岩、细—中晶白云岩,颗粒类型主要为砂屑,次为鲕粒、砾屑以及少量生屑。发育潟湖—台内滩—潟湖、潟湖—台内滩—台坪、潮坪—潮缘滩—潮坪这3种向上变粗的沉积序列。单个旋回中的颗粒滩的厚度一般小于2.5m,垂向上多表现为频繁叠置的小规模薄型滩体,且横向规模小、可对比性差。平面上,颗粒滩主要分布在古隆起区和水下相对高地,整体为东北向分布。海平面升降和沉积能量的高低控制着颗粒滩发育规模,而构造条件决定了颗粒滩的横向连续性和平面分布。     

Mega-debris flow deposits from the Oligo-Miocene Pindos foreland basin, western mainland Greece: implications for transport mechanisms in ancient deep marine basins

The turbidite dominated, Oligo-Miocene Pindos foreland basin of western mainland Greece contains two thick (60–72 m), matrix supported conglomerates. The conglomerates are ungraded and contain three clast types: (1) polymict, rounded, extrabasinal clasts (long axes 3–50 cm); (2) tightly folded, intrabasinal clasts (long axes 1–10 m); and (3) tabular, largely undeformed, intrabasinal blocks (long axes 18–300 m). Clasts are isolated within a slit dominated matrix. These chaotic, matrix supported conglomerates are interpreted as mega-debris flow deposits. During transport, extrabasinal clasts were supported by a combination of matrix cohesion and clast dispersive pressure, folded intrabasinal clasts were supported by a combination of buoyancy (Archimedes principle) and clast dispersive pressure. The large tabular clasts were transported by gravity sliding/gliding within the flow on films at high pore fluid pressure. These different clast support mechanisms were active simultaneously within the Pindos mega-debris flow deposits. As a result, the deposits have no systemic vertical stratigraphy, in contrast to many described large scale mass flow deposits. The mega-debris flow deposits are significantly thicker than most described ancient siliciclastic debris flow deposits and provide an ancient analogue for the thick Recent siliciclastic debris flow deposits on continental margins.     

Storm-influenced siliciclastic and carbonate ramp deposits, the Lower Ordovician Dumugol Formation, South Korea

The Dumugol Formation (Lower Ordovician) in the southern part of the Baegunsan syncline, South Korea, contains mixed siliciclastic and carbonate ramp deposits. The ramp sediments were frequently influenced by storm events resulting in tempestites of sandstone-mudstone couplets, bioclastic grainstones to packstones, flat-pebble conglomerates, a skeletal lag layer and laminated calcisiltites. All tempestites are characterized by an erosive to sharp base, poor grading and a transitional upper boundary. The difference in lithology of tempestites appears to have been controlled by the nature of substrates and by proximality. For example, laminated calcisiltites have developed on the shallow carbonate ramp, flat-pebble conglomerates are closely associated with nodular limestones on shallow and deep ramps, and thin skeletal lag layers from fossiliferous argillaceous sediments formed in a basinal setting. The stratigraphic succession of the Dumugol Formation represents an initial transgression followed by a regression. The vertical facies change records the transition from a shallow siliciclastic ramp to a deep carbonate ramp, to a basin, shallowing to a deep carbonate ramp, and to a shallow carbonate ramp. Storm effects are mostly well preserved in shallow to deep ramp deposits.     

Recognition of massive Upper Cretaceous carbonate bodies as olistoliths using rudist bivalves as internal bedding indicators (Campanian Merfeg Formation,Central Tunisia)

The Merfeg Formation (upper Campanian) of Central Tunisia crops out around the southwestern periclinal termination of Jebel el Kébar, near Sidi Bouzid. At its base is a massively bedded unit of locally dolomitized, sparsely fossiliferous micritic to microbioclastic limestone that contains several discrete, plurimetric mound-like bodies (lithosomes) of micritic limestone containing locally abundant rudists and corals. The lithosomes are separated laterally from one another by megabreccias and conglomerates containing clasts of similar lithology and are overlain, with sharp contact, by onlapping argillaceous pelagic limestones, within which are intercalated at least two more, somewhat thinner rudist/coral limestone units. This complex of facies is laterally equivalent to thicker, deep platform limestones of the Abiod Formation to the north and east, and to restricted carbonate platform facies of the Berda Formation to the south and west. The lithosomes have previously been interpreted as in situ downslope mudmounds that became capped by rudist and coral formations, cemented, and then surrounded by erosively emplaced debris flows. However, our detailed studies of rudist orientations imply variable and in some cases relatively high angles of bedding within the lithosomes with respect to the regional dip of the host strata. Such steep inclinations of internal bedding are unlikely to have been primary. Accordingly, we propose an alternative interpretation that the lithosomes were platform-derived olistoliths, emplaced along with the associated debris flow deposits. Micritic beds, neighbouring the olistoliths are of variable thickness and contain rare large inoceramids and randomly oriented rudists, as well as locally developed microbioclastic beds with planar and small-scale swaley cross stratification. These micritic and microbioclastic beds are, by contrast, interpreted as primary (i.e., non-olistostromal) slope deposits. Whether the proposed catastrophic collapses of the original platform margin were induced by sea-level fall or seismically triggered (or a combination of the two) remains uncertain.     

A terrestrial-shallow marine transition in the archean opemisca group east of Chapais,Quebec

The Stella and Hauy Formations of the Archean Opemisca Group in the eastern Chapais Syncline feature numerous fluvial to shallow marine transitions throughout the stratigraphic sequence. During the first depositional phase (Stella Formation), relatively high gradient alluvial fans prograde into a shallow marine environment. The coarse clastic sediments reflect a Scott-type depositional environment. The initial depositional environment is envisaged to be a high-energy, wave- and storm (flood) -dominated coastline bordered by coastal alluvial fans. Shallow marine deposits consist of coarse- to very coarse-grained sandstones and intraclast and pebble to cobble conglomerates. The prevalent shallow marine sediments are related to storm and/or flood deposition.Terrigenous sediments of the Donjek-type depositional environment prevail during the second depositional phase (Hauy Formation). The shallow marine sediments, although still wave- and storm (flood) -dominated, are finergrained, and when correlated with the terrigenous sediments imply a lower gradient and a maturing of the coastline. Furthermore, contemporaneous volcanism of andesitic (shoshonitic) to basaltic composition, occurs during this evolutionary stage. The sedimentary basin gradually evolved during the two depositional phases.The repetition of terrigenous and shallow marine sediments throughout the stratigraphic sequence, coarsening- and fining-sequences in the conglomerates, and the alluvial fan setting strongly suggest a fault-bounded basin margin. Tectonic uplift of the hinterland initiates alluvial fan progradation onto the shelf, whereas quiescent periods are documented by marine transgression. The clast composition of the conglomerates and the alluvial fan environment suggest local derivation of clasts.The synvolcanic Chibougamau Pluton and the Lake Dore Complex acted as a stable basement during deposition of the Opemisca Group. The small landmass served as a stable platform for shallow marine deposition, even when tectonically influenced. A late phase intraplutonic (cratonic) development of an initial backarc basin system (internal zone of the Abitibi greenstone belt) is proposed as the overall depositional setting of the Chapais basin.