Interaction of an antecedent fluvial system with early normal fault growth: Implications for syn‐rift stratigraphy,western Corinth rift (Greece)

Continental ‘overfilled’ conditions during rift initiation are conventionally explained as due to low creation of accommodation compared with sediment supply. Alternatively, sediment supply can be relatively high from the onset of rifting due to an antecedent drainage system. The alluvial Lower Group of the western Plio–Pleistocene Corinth rift is used to investigate the interaction of fluvial sedimentation with early rifting. This rift was obliquely superimposed on the Hellenide mountain belt from which it inherited a significant palaeorelief. Detailed sedimentary logging and mapping of the well‐exposed syn‐rift succession document the facies distributions, palaeocurrents and stratigraphic architecture. Magnetostratigraphy and biostratigraphy are used to date and correlate the alluvial succession across and between fault blocks. From 3·2 to 1·8 Ma, a transverse low sinuosity braided river system flowed north/north‐east to east across east–west‐striking active fault blocks (4 to 7 km in width). Deposits evolved downstream from coarse alluvial conglomerates to fine‐grained lacustrine deposits over 15 to 30 km. The length scale of facies belts is much greater than, and thus not directly controlled by, the width of the fault blocks. At its termination, the distributive river system built small, stacked deltas into a shallow lake margin. The presence of a major antecedent drainage system is supported by: (i) a single major sediment entry point; (ii) persistence of a main channel belt axis; (iii) downstream fining at the scale of the rift basin. The zones of maximum subsidence on individual faults are aligned with the persistent fluvial axis, suggesting that sediment supply influenced normal fault growth. Instead of low accommodation rate during the early rift phase, this study proposes that facies progradation can be controlled by continuous and high sediment supply from antecedent rivers.     

Stream-dominated alluvial fan and lacustrine depositional systems in Cenozoic strike-slip basins, Denali fault system, Yukon Territory, Canada

Ancient stream-dominated (‘wet’) alluvial fan deposits have received far less attention in the literature than their arid/semi-arid counterparts. The Cenozoic basin fills along the Denali fault system of the northwestern Canadian Cordillera provide excellent examples of stream-dominated alluvial fan deposits because they developed during the Eocene-Oligocene temperate climatic regime in an active strike-slip orogen. The Amphitheatre Formation filled several strike-slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike-slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream-dominated alluvial-fan systems. Proximal fan deposits in the Bates Lake Basin are characterized by disorganized, clast-supported, boulder conglomerate and minor matrix(mud)-supported conglomerate. Proximal facies are located along the faulted basin margins in areas where C:S = 80 to 100 and where the average MPS ranges from 30 to 60 cm. Proximal fan deposits grade into middle fan, channelized, well organized cobble conglomerates that form upward fining sequences, with an average thickness of 7 m. Middle fan deposits grade basinward into well-sorted, laterally continuous beds of normally graded sandstone interbedded with trough cross-stratified sandstone. These distal fan deposits are characteristic of areas where C:S = 20 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe deposits consist of lacustrine and axial fluvial facies. Palaeogeographic reconstruction of the Bates Lake Basin indicates that alluvial-fan sedimentation was concentrated in three parts of the basin. The largest alluvial-fan system abutted the strike-slip Duke River fault, and prograded westward across the axis of the basin. Two smaller, coarser grained fans prograded syntaxially northward from the normal-faulted southern basin margin. Facies analysis of the Burwash Basin indicates a similar transition from proximal to distal, stream-dominated alluvial fan environments, but with several key differences. Middle-fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m thick composed of fine-grained lithofacies and coal in the lower part, trough cross-stratified sandstone in the middle, and conglomerate in the upper part of the sequence. Upward-coarsening sequences, 90–140 m thick, also are common in the fan fringe lacustrine deposits. These sequences coarsen upward from mudstone, through fine grained, ripple-laminated sandstone, to coarse grained trough cross-stratified sandstone. The upward-coarsening sequences are basinwide, facies independent, and probably represent progradation of stream-dominated alluvial-fan depositional systems. Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream-dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal development in the intervening middle and distal fan areas was suppressed by the high frequency of unconfined flow events and lateral channel mobility.     

Sedimentary facies, depositional environments and palaeogeographic evolution of the Neogene Denizli Basin, SW Anatolia, Turkey

The Denizli Basin (southwestern Anatolia, Turkey) contains a record of environmental changes dating since the Early Miocene. Detailed facies analysis of the Neogene formations in this half-graben enables us to document successive depositional regimes and palaeogeographic settings. Sedimentation commenced in the Early Miocene with the deposition of alluvial-fan and fluvial facies (K?z?lburun Formation). At this stage, alluvial fans sourced from elevated areas to the south prograded towards the basin centre. The Middle Miocene time saw the establishment of marginal lacustrine and wetland environments followed by the development of a shallow lake (Sazak Formation). The uppermost part of this unit consists of evaporitic saline lake and saline mudflat facies that grade upward into brackish lacustrine deposits of Late Miocene-Pliocene age (Kolankaya Formation). The lake became shallower at the end of the Pliocene time, as is indicated by expansion shoreface/foreshore facies. In the Early Quaternary, the Denizli Basin was transformed into a graben by the activation of ESE-trending normal faults. Alluvial fans were active at the basin margins, whereas a meandering river system occupied the basin central part.Oxygen isotope data from carbonates in the successive formations show an alternation of wetter climatic periods, when fresh water settings predominated, and very arid periods, when the basin hosted brackish to hypersaline lakes. The Neogene sedimentation was controlled by an active, ESE-trending major normal fault along the basin's southern margin and by climatically induced lake-level changes. The deposition was more or less continuous from the Early Miocene to Late Pliocene time, with local unconformities developed only in the uppermost part of the basin-fill succession. The unconformable base of the overlying Quaternary deposits reflects the basin's transformation from a half-graben into a graben system.     

Tectono-sequence stratigraphy and U–Pb zircon ages of the Rincón Blanco Depocenter,northern Cuyo Rift,Argentina

Extensional processes that followed the Gondwanan Orogeny rise to the development of a series of rift basins along the continental margin over older accreted Eopaleozoic terranes. Stratigraphic, structural, paleontological, and isotopic studies are presented in this work in order to constrain the ages of the sedimentary infilling and to analyze the tectosedimentary evolution of one of the Cuyo basin depocenters, known as Rincón Blanco. This asymmetrical half-graben was filled by continental sediments under a strong tectonic control. The infilling was strongly controlled by tectonics which in term produced distinctive features along the whole sedimentary sequence. Using a combination of lithological and structural data the infilling was subdivided into packages of genetically linked units bounded by regional extended surfaces. Several tuffs and acid volcanic rocks have been collected across the whole section of the Rincon Blanco sub-basin for SHRIMP and LA-MC-ICPMS U–Pb zircon dating. The ages obtained range from 246.4 ± 1.1 Ma to 230.3 ± 1.5 Ma which is the time elapsed for the deposition of three tectono-sequence units separated by regional unconformities and mainly constrained to the Middle Triassic. They are interpreted as a result of a reactivation of the extensional system that has evolved along strike as segments of faults that linked together and/or as laterally propagating faults. Regional correlation with coeval rift basins permits to establish north-south propagation in the extensional regime along the western margin of SW Gondwana. This trending started in the lowermost Triassic and extended until the latest Triassic. Two of them were precisely correlated with Cerro Puntudo and Cacheuta half-graben systems. The new data indicate that the three sequences were mostly deposited during the Middle Triassic (246 to 230 Ma), with no evidence of sedimentation during Norian and Rhaetian, which is in conflict with some previous biostratigraphic studies.     

内蒙古西胡里吐盆地下白垩统大磨拐河组沉积相及砂岩型铀矿成矿作用

西胡里吐盆地是一个早白垩世断陷盆地，盆地盖层由下白垩统大磨拐河组构成。受同沉积断裂作用的影响，盆地的古地理格局总体为南陡北缓，南部主要发育冲积扇相、扇三角洲相，盆地北部以河流相、三角洲相为主，仅上部层位发育冲积扇相，而盆地中部的广大区域则以巨厚的湖泊相沉积占优势。大磨拐河组可进一步划分为三个岩性段。第一段形成于盆地断陷发育的初始阶段，以冲积扇、河流、三角洲发育为特征；第二段形成于湖泊迅速扩张至最大时期，沉积了一套巨厚的湖相泥岩，构成盆地盖层的主体，在晚期有河流、三角洲等粗碎屑沉积；第三段以冲积扇的极大发育为特征，代表盆地进入萎缩阶段。三个段总体表现为一段、三段岩性粗、煤层少且薄，二段岩性细、泥岩厚、煤层发育多。从第一段到第三段，基本构成了盆地一次完整的沉积充填历史。主要含矿层位大磨拐河组的沉积特点不仅控制着盆地内铀矿化的空间展布，而且也控制着本区的铀矿化类型。“粗～细～粗”的地层结构特点决定了本区的铀成矿作用主要发生于湖泊最大扩张的上部地层，即第二段上部及第三段的粗碎屑沉积物中，主要矿化层位为该套地层内的冲积扇、辫状河与三角洲相沉积，含矿岩石主要为渗透性好、富含有机质的砂砾岩、砂岩；同时由于上部含矿层位具有“砂多泥少”的特点，决定了本区铀矿化类型主要为具垂直分带的潜水氧化带型，仅局部存在潜水转层间氧化带型。     

Tectono‐sedimentary evolution of the northernmost margin of the NE German Basin between uppermost Carboniferous and Late Permian (Rotliegend)

The tectono‐sedimentary evolution of the Rotliegend deposits of the northernmost margin of NE German Basin (NEGB) has been analysed on the basis of detailed sedimentary logs of 300 m of core material together with the re‐evaluation of 600 km of seismic lines. Three distinct phases were recognized. During the initial Phase I, basin geometry was largely controlled by normal faulting related to deep‐seated ductile shearing leading to a strong asymmetric shape, with a steep fault‐controlled eastern margin and a gently, dipping western margin. The results of forward modelling along a cross‐section fit the basin geometry in width and depth and reveal a footwall uplift of c. 1000 m. Adjacent to the steep faults, local sedimentation of Lithofacies Type I was confined to non‐cohesive debris flow‐dominated alluvial fans, whereas the gently dipping western margin was dominated by alluvial‐cone sedimentation. During the post‐extensional period (Phase II), cooling of the lithosphere generated additional accommodation space. The sediments of Lithofacies Type II, comprising mainly clast‐supported conglomerates, are interpreted as braided ephemeral stream flow‐surge deposits. Tectonic quiescence and an increase in flood events resulting from wetter climate led to progradation of this facies over the entire region. At the end of this period, the accommodation space was almost completely filled resulting in a level topography. Phase III was controlled by the thermal‐induced subsidence of the southerly located NEGB in post‐Illawarra times. The formerly isolated region tilted towards the SW, thus forming the northern margin of the NEGB during uppermost Havel and Elbe Subgroup times. The sediments of Lithofacies Type III were divided into a marginal sandstone‐dominated environment and a finer‐grained facies towards the SW. The former consists of poorly‐sorted coarse‐grained sandstones of a proximal and medial ephemeral stream floodplain facies. The latter comprise mud flat fines and fine‐grained distal ephemeral stream deposits. The end of the tectono‐sedimentary evolution is marked by the basinwide Zechstein transgression. Copyright © 2001 John Wiley & Sons, Ltd.     

Alluvial Fan-lacustrine Sedimentation and its Tectonic Implications in the Cretaceous Athgarh Gondwana Basin, Orissa, India

The Athgarh Formation is the northernmost extension of the east coast Upper Gondwana sediments of Peninsular India. The formation of the present area is a clastic succession of 700 m thick and was built against an upland scarp along the north and northwestern boundary of the basin marked by an E-W-ENE-WSW boundary fault. A regular variation in the dominant facies types and association of lithofacies from the basin margin to the basin centre reveals deposition of the succession in an alluvial fan environment with the development of proximal, mid and distal fan subenvironments with the distal part of the fan merging into a lake. Several fans coalesced along the basin margin, forming a southeasterly sloping, broad and extensive alluvial plain terminating to a lake in the centre of the basin. Aggradation of fans along the subsiding margin of the basin resulted in the Athgarh succession showing remarkable lateral facies change in the down-dip direction. The proximal fan conglomerates pass into the sandstone-dominated mid-fan deposits, which, in turn, grade into the cyclic sequences of sandstone-mudstone of the distal fan origin. Further downslope, thick sequence of lacustrine shales occur. The faulted boundary condition of the basin and a thick pile of lacustrine sediments at the centre of the basin suggest that tectonism both in the source area and depositional site has played an important role throughout the deposition of the Athgarh succession of the present area. The vertical succession fines upward with the coarse proximal deposits at the base and fine distal deposits at the top, suggesting deposition of the succession during progressive reduction of the source area relief after a single rapid uplift related to a boundary fault movement.The NW-SE trending fault defining the Son-Mahanadi basin of Lower Gondwana sediments are shear zones of great antiquity and these were rejuvenated under neo-tensional stress during Lower Gondwana sedimentation. The E-W-ENE-WSW trending fault of the Athgarh basin, on the other hand, define tensional rupture of much younger date. In the Early Cretaceous period, there was a reversal of palaeoslope in the Athgarh basin (southward slope) with respect to the Son-Mahanadi basin (northward slope). During the phase drifting of the Indian continent and with the evolution of Indian Ocean in the Early Cretaceous period, the tectonic events in the plate interior was manifested by formation of new grabens like the Athgarh graben.     

Late Miocene nonmarine sedimentation and formation of magnesites in the Acıgöl Basin, southwestern Anatolia, Turkey

The Late Miocene lacustrine Acıgöl Basin, SW Turkey, formed as an orogen-top, extensional half-graben, with the subaqueous accommodation controlled by the lake level and the bulk accommodation provided by active subsidence along a WSW-trending normal fault at the basin's southern margin. The basin-fill sedimentary succession consists of terminal alluvial-fan facies overlain by ephemeral lake-margin facies and perennial lake facies, with widespread fluvial facies at the top. The distal alluvial-fan facies include massive to stratified sandstones and massive mudstones with intervening nodular dolostones and incipient pedogenic horizons. The lake-margin facies are micritic magnesites passing laterally into peloidal, irregularly laminated magnesites towards the palaeolake margin and overlain by marlstones and dolostones, all with abundant evidence of episodic subaerial exposure (desiccation cracks, pedogenic features, and tepee structures). The perennial lake facies are micritic magnesites passing upwards into clayey dolostones and dolomitic or clayey marlstones. The fluvial facies capping the succession include planar cross-stratified conglomerates (channel-fill deposits), planar parallel-stratified, planar cross-stratified and rippled cross-laminated sandstones (crevasse-fill and crevasse splay deposits), and assemblages of mudstones intercalated with thin sandstone beds (overbank floodplain deposits).The sedimentological, mineralogical and geochemical data reveal large variations in the basin's hydrological regime, including short-term oscillations and bulk rise of the lake level, periodical changes in the Mg/Ca ratio and terrigenous mud supply, and a negative covariance of δ¹⁸O and δ¹³C fluctuations. The composition of terrigenous sediment and the chemistry of water supplied to the lake were controlled by the weathering, chemical leaching and erosion of the ultramafic–dolomitic bedrock in the catchment area. The bedrock yielded Mg-rich carbonate solutions that caused the deposition of Mg-carbonates in the lake.Despite short-term lake-level fluctuations, the lake's net water budget remained positive. It is suggested that the region's present-day climate and Mg-rich alkaline lakes can serve as an analogue for the climatic and hydrological conditions in the Late Miocene Acıgöl Basin.     

Facies pattern of the middle Permian Barren Measures Formation, Jharia basin, India: The sedimentary response to basin tectonics

In the Lower Gondwana succession of the Jharia basin of eastern India, the Barren Measures Formation is characterized by the cyclic disposition of fine-grained lacustrine deposits and relatively coarse-grained fluvial deposits. The cyclic variation in the rate of coarse clastic input is attributed to the sedimentary response to basin tectonics. The sandstone-shale alternations of the Barren Measures succession can be correlated with the tectonic cyclothems developed on the hangingwall dip-slope and adjoining trough in a continental half-graben setting. Enhancement of the gradient of the hangingwall dip-slope during reactivation of the basin margin faults led to progradation of the existing fluvial system towards the half-graben trough and deposition of the coarser clastics on the fine-grained lacustrine deposits of the trough. Peneplanation of the hangingwall slope and slow increase in the lake level caused lacustrine transgression and retrogration of the fluvial system on the hangingwall block. The fluvial sediments were onlapped by the fine-grained lacustrine deposits. Episodic rejuvenation of the basin margin faults thus caused development of tectonic cyclothem on the hangingwall block. The paleocurrent pattern indicates that a persistent northward paleoslope was maintained during Barren Measures sedimentation. The inferred depositional settings were much more extensive than the present limit of the outcrop. The faults, presently defining the northern limit of the Barren Measures Formation, were possibly emplaced after Barren Measures sedimentation. The final movement along these fault planes caused preservation of the downthrown hangingwall block and the Barren Measures sediments on the footwall block were eroded during subsequent denudation. The Southern Boundary Fault came into existence after the deposition of the Barren Measures sediments.     

Synrift stratigraphic geometry in a transfer zone coarse-grained delta complex, Miocene Pohang Basin, SE Korea

The Gojusan fan delta is a coarse‐grained delta complex on the western margin of the Miocene Pohang Basin, SE Korea. The deposits consist of five facies associations (FA): alluvial‐fan conglomerates (FA I), shallow‐marine mouth‐bar sandstones (FA II), fluvial and mouth‐bar conglomerates (FA III), Gilbert‐type foreset conglomerates (FA IV) and hemipelagic mudstones (FA V). Different facies associations characterize the northern, central and southern parts of the delta complex. To the north, FA IV is laterally juxtaposed with FA I or the basement, with scarp‐derived breccias along the contact. Centrally, and to the south, FA I is laterally juxtaposed with FA II, with an abrupt facies change and local inliers of basement rocks along the contact. Early alluvial fan and mouth‐bar deposits are overlain by the topset (FA III) and truncated by the foreset (FA IV) of a Gilbert‐type delta in the central part of the fan delta complex, whereas FA II passes transitionally upward into FA III in the south, with the latter extending basinward into a gently inclined shoal water delta front. Gilbert‐type and shoal‐water geometries are therefore developed in the same delta complex. The composite delta geometry is interpreted as reflecting (1) its development near an extensional transfer zone where the hangingwall relief was variable over short distances along strike, and (2) the operation of intrabasinal faults. This interpretation contrasts with previous studies that viewed the delta complex as having formed along a pull‐apart basin margin.     

Stratigraphic architecture of the fluvio-lacustrine deposits, Bathan Formation, Al-Rehaili area, North Jeddah, Saudi Arabia

A siliciclastic-dominated succession (～11 m thick) underlying Harrat Rahat, belonging to the Miocene–Pliocene Bathan Formation is recently exposed at Al-Rehaili area, North Jeddah, Saudi Arabia. It covers a wide spectrum of grain sizes varying from clay-rich mudstones to cobble grade conglomerate and consists of a variety of facies vary from fluvial to marginal and open lacustrine deposited in a half-graben basin formed along the eastern margin of the extensional Red Sea Basin. Field-based sedimentologic investigation enables to identify ten facies grouped into three facies associations (A–C). The depositional history is subdivided into two stages. The first stage represents deposition in gravel to sand-dominated fluvial system sourced from a southern source and grade northward into lacustrine delta and open lacustrine setting. The second stage on the other hand includes deposition of fluvial channels running in E–W direction with attached bank sand bar. Sequence stratigraphic interpretations of the lacustrine deposits enable to identify three unconformity-bounded sequences (SQ_1–3). The basal sequence is incomplete, consisting of three aggradationally to progradationally stacked delta plain and delta front parasequences. The second sequence is sharply and erosively overlying a red paleosol bed that defines the upper boundary of the first sequence. It includes two system tracts; upward-fining and deepening lacustrine offshore mudstones of the transgressive system tracts unconformably overlain by red paleosol of the regressive systems tracts. The top of this sequence is delineated at the sharp transgressive surface of erosion at the base of delta mouth bar deposits of sequence 3. Changes in the accommodation and sedimentation rates by basin subsidence under the influence of tectonics and sediment compaction and loading as well as climatic oscillation between semi-arid to arid conditions were the major controls on the fluvio-lacustrine sedimentation and their facies distribution. Tectonic reorganization of the drainage system resulted in the formation of E–W flowing fluvial streams in the second stage.     

Cerdanya basin geometry and its implication on the Neocene evolution of the Eastern Pyrenees

Cerdanya is a Neogene half-graben basin that lies over the Palaeozoic rocks of the Eastern Pyrenean Axial Zone. Subvertical fault scarps bound the half-graben at the southern and southeastern margins, but Neogene sediments may directly overlap Palaeozoic rocks in the northern margin.Vertical electrical soundings show that the Miocene sequence reaches a maximum thickness of over 700 m and that fine-grained lacustrine deposits from the central part of the basin grade to coarser fluvial deposits toward the margins.Macro- and micro-structural data analyzed in Cerdanya and other related Neogene basins, such as Rosselló, Confient, Capcir and Seu d'Urgell, suggest that they formed as a consequence of transtensile motion along an old northeastern-trending fault. The stress situation favoured sinistral movements along east-trending faults.     

柴北缘牦牛山地区牦牛山组沉积相组合特征

柴达木盆地北缘牦牛山地区出露的牦牛山组是一套由冲积扇和扇三角洲相共同构成的陆相沉积组合,冲积扇相砾岩-粗砂岩组合主要分布于研究区SE侧,扇三角洲相砂岩-泥岩组合主要分布于研究区NW侧。古水流分析表明牦牛山组沉积物主要来自其SE侧古隆起,但后期扇三角洲相包含少量来自NW和NE向的沉积物。该套沉积组合序列特征与区域上分布在牦牛山西侧同时期形成的湖泊相、滨浅海相沉积共同表明,柴达木盆地北缘在晚志留-早泥盆世时期存在一NW向倾斜的古斜坡,且晚期北侧发生抬升。砾岩和砂岩碎屑组成与区域岩石组合类型对比表明,牦牛山组沉积碎屑物主要来自于滩间山群。沉积组合序列特征、碎屑组成和区域构造背景综合研究表明,牦牛山组可能为柴达木板块向北俯冲过程中形成的局部断陷盆地的充填物。     

Sedimentary record of Triassic intraplate extension in North China: evidence from the nonmarine NW Ordos Basin, Helan Shan and Zhuozi Shan

The Helan Shan and Zhuozi Shan of the NW Ordos basin, China, contain thick (up to 4 km) sequences of nonmarine Triassic strata. These rocks represent a major intraplate sedimentary basin, the paleogeography, tectonic setting and provenance of which are poorly understood and controversial. Studies of the sedimentary geology of the basin, supported by new palinspastic reconstruction of younger deformation, demonstrate that the basin filled from three sides by fluvial, lacustrine-deltaic and alluvial fan depositional systems. The basin forms a westward-thickening wedge that reaches its maximum thickness along the western margin of the Helan Shan and thins to a relatively constant 600–800 m east of the Zhuozi Shan. The stratigraphy of the basin is strongly asymmetric; alluvial fan strata are restricted to the extreme western margin of the basin and interfinger with axial fluvial deposits low in the section and deep lacustrine facies high in the section. Much of the eastern part of the basin is dominated by west-flowing meandering river and deltaic systems. Large structures of Triassic age have not been identified in the Helan Shan or Zhuozi Shan, but small Triassic normal faults have been documented in the western and central Helan Shan. These characteristics most strongly support an extensional origin for the Triassic basin in NW Ordos. The basin is interpreted to have been a north-trending half graben, bound along its western margin by an east-dipping normal fault, presently concealed beneath Quaternary cover west of the Helan Shan. The eastern margin, now found in the Zhuozi Shan, has simple ramp-margin geometry. Driving mechanisms for this extension are not obvious due to limited documentation of Triassic structure throughout the region, but probably relate to far-field stresses from the Qinling or Jinsha active margins interacting with the stable Ordos block.     

Alluvial fan and lacustrine sediments from the Stephanian A and B (La Magdalena, Ciñera—Matallana and Sabero) coalfields, northern Spain

Vertical sequence analysis within 1500-2500 m thick coarse-grained coalfield successions allows six sedimentary associations to be distinguished. These are interpreted in terms of depositional environments on, or related to alluvial fans which fringed a fault bounded source region. (i) Topographic valley and fanhead canyon fills: occurring at the bases of the coalfield successions and comprising sporadically reddened, scree, conglomeratic thinning and fining upward sequences, and fine-grained coal-bearing sediments. (ii) Alluvial fan channels: conglomerate and sandstone filled. (iii) Mid-fan conglomeratic and sandstone lobes: laterally extensive, thickly bedded (1-25 m) and varying from structureless coarse conglomerates and pebbly sandstones, to stratified fine conglomerates and cross-bedded sandstones. (iv) Interlobe and interchannel: siltstones, fine-grained sheet sandstones, abundant floras, thin coals and upright trees. (v) Distal fan: 10 cm-1.5 m thick sheet sandstones which preserve numerous upright trees, separated by silt-stones and mudstones with abundant floras, and coal seams. The sheet sandstones and normally arranged in sequences of beds which become thicker and coarser or thinner and finer upwards. These trends also occur in combination. (vi) Lacustrine: coals, limestones, and fine-grained, low-energy, regressive, coarsening upward sequences. Proximal fan sediments are only preserved in certain basal deposits of these coalfields. The majority of the successions comprise mid and distal alluvial fan and lacustrine sediments. Mid-fan depositional processes consisted of debris flows and turbulent streamflows, whilst sheetfloods dominated active distal areas. A tropical and seasonal climate allowed vegetation to colonize abandoned fan surfaces and perhaps resulted in localized diagenetic reddening. Worked coals, from 10s cm-20 m thick, occur in the distal fan and lacustrine environments. These alluvial fan deposits infill‘California-like’basins developed and preserved along major structural zones. In many of their characteristics, in particular the occurrence of thinning and fining, and thickening and coarsening upward sequences and megasequences, these sediments have similarities to documented ancient submarine fan deposits.     

Glacial meltwater impounding: Evidence from the late Quaternary glaciogenic sediments in the Sangla valley, district Kinnaur, Himachal Pradesh, India

Sangla valley is situated at an altitude of ~ 3500 m above mean sea level and lies in the Kinnaur district of Himachal Pradesh. It is fed by river Baspa, a tributary of river Sutlej, that entrenches through the Quaternary glaciogenic deposits before emerging out of the valley and joining the river Sutlej at Karcham. The unstratified to stratified glaciogenic deposits consist of large boulders to fine silt and are classified into four major depositional facies on the basis of sedimentary texture and depositional environment. The facies — basal conglomerates, debris flow, water/sheet flow and laminites — represents the change in the environment of deposition from glaciofluvial to lacustrine and also the extent of the glacier to the valley floor during late Quaternary.     

Facies,environments and sedimentary cycles in the Middle Eocene,Bracklesham Formation of the Hampshire Basin: evidence for global sea-level changes?

The Bracklesham Formation is of Middle Eocene age and occurs throughout the Hampshire Basin of southern England. The basin is elongated east-west and filled with Lower Tertiary sediments. Its southern margin is marked by either large, northward-facing monoclines, or faults, both of which underwent differential movement, with uplift of the southern side throughout the Middle Eocene. The Bracklesham Formation, which is up to 240 m thick, shows pronounced lateral facies changes with dominantly marine sediments in the east passing to alluvial sediments in the west. Four principal sedimentary environments: marine, lagoonal, estuarine and alluvial are distinguished. Marine sediments comprise six facies including offshore silty clays and glauconitic silty sands, beach and aeolian dune sands, and flint conglomerates formed on pebble beaches. Offshore sediments predominate in the eastern part of the basin, as far west as Alum Bay, where they are replaced by nearshore sediments. Lagoonal sediments comprise four facies and formed in back-barrier lagoons, coastal marshes and, on occasions, were deposited over much of the basin during periods of low salinity and restricted tidal motion. Five estuarine facies represent tidal channels, channel mouth-bars and abandoned channels. These sediments suggest that much of the Bracklesham Formation was deposited under micro- to meso-tidal conditions. Alluvial sediments dominate the formation to the west of Alum Bay. They comprise coarse to fine sands deposited on the point-bars of meandering rivers, interbedded with thick sequences of laminated interchannel mudstones, deposited in marshes, swamps and lakes. Extensive layers of ball clay were periodically deposited in a lake occupying much of the alluvial basin. In alluvial areas, fault movement exposed Mesozoic rocks along the southern margin of the basin, the erosion of which generated fault-scarp alluvial fan gravels. Locally, pisolitic limestone formed in pools fed by springs emerging at the faulted Chalk-Tertiary contact. In marine areas, flint pebbles were eroded from coastal exposures of chalk and accumulated on pebble beaches and in estuaries. From other evidence it is suggested that older Tertiary sediments were also reworked. The Bracklesham Formation is strongly cyclic and was deposited during five marine transgressions, the effects of which can be recognized throughout the basin in both marine and alluvial areas. Each of the five transgressive cycles is a few tens of metres thick and contains little evidence of intervening major regression. The cycles are thought to represent small-scale eustatic sea-level rises (‘paracycles’) superimposed upon a major transgressive ‘cycle’ that began at the base of the Bracklesham Formation, following a major regression, and was terminated, at the top of the Barton Formation by another major regression. This major cycle can be recognized world-wide and may reflect a period of rapid northward extension of the mid-Atlantic ridge.     

Sedimentology of a lacustrine fan-delta system, Miocene Horse Camp Formation, Nevada, USA

A 1600-m-thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east-central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan-delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan-delta facies associations include: ungraded, matrix-and clast-supported conglomerate; normally graded, matrix- and clast-supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan-delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan-delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan-delta environment by debris flows, high- and low-density turbidity currents, and suspension fallout. The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High-energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore-directed currents produced uncommonly large (typically 1–2 m thick) trough cross-stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high-energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout. A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan-delta, subaqueous fan-delta and offshore lacustrine deposition. This model suggests that high-energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan-delta environment and promoting development of a well-defined nearshore lacustrine environment. Low-energy nearshore currents induced by shallow water along the