the devonian lacustrine sediments of orkney scotland implications for climate cyclicity basin structure and maturation history the orcadian basin of northern scotland largely of middle devonian age is a major lacustrine rift basin with the widely correlateable achanarras sandwick lake bed extending for at least 800 km along the rift above this lake bed there are about 540 m of cyclic lacustrine sediments with lake sedimentation terminated by alluvial fan progadation the upper stromness flagstone formation and the rousay flagstone formation making up this lacustrine sequence are given revised and precise definitions within four reference sections from the orkney islands the sequence consists of about 45 first order cycles averaging about 12 m in thickness which resulted from long term climatic fluctuations in rainfall with a cycle time scale of about 25 000 years longer climatic fluctuations of about 100000 and 400000 years are also present these periods correspond to the precession and two eccentricity orbital cycles detailed knowledge of the stratigraphy constrains the structural and maturation history of the basin the basin formed in a series of extensional half graben and the orkney area suffered later inversion the lake sediments first reached the oil window at relatively shallow depths c 2000 m probably in the early carboniferous implying very high geothermal gradients during the early part of the basin history as corroborated by the presence of devonian volcanics in the region in the orkney area uplift probably started in the late carboniferous

lacustrine deposits in the karheen formation fortify links between alaska s alexander terrane and the old red sandstone continent in the late silurian early devonian the karheen formation of southeastern alaska alexander terrane is an 1800 m thick redbed molasse sequence that formed during the late silurian early devonian in the wake of the klakas orogeny previous research interpreted thin platy limestone in the karheen formation as marine in origin we describe new evidence including a lack of marine fossils which indicates that the rhythmically laminated limestone formed in deep stratified lakes intercalated siliciclastic lake margin facies record cycles of lake expansion and contraction comparative analysis with coeval lake orcadie deposits in scotland shows that the karheen formation is strikingly similar in age thickness lithology paleontologic characteristics and tectonic setting to the old red sandstone together with other published geologic data from southeastern alaska the karheen s lacustrine deposits strengthen paleogeographic connections between the alexander terrane and the n atlantic caledonide region in the mid paleozoic

the genesis and recognition of anastomosed fluvial deposits data from the st mary river formation southwestern alberta canada abstract data from modern fluvial systems show that the term anastomosed is best reserved for suspended load systems with multiple channels and very low slope that flow in regions where the hydrograph has a seasonal peak the interchannel areas are topographic lows relative to the channel hanks and are places where crevasse splay marsh and lacustrine sedimentation occurs channel geometry is intrinsically a function of the rate of sediment partitioning the coarsest sediment is deposited in the channels on the levees and in proximal splay complexes whereas the silts and clays are deposited on the rest of the floodplain extrinsic controls on the formation of these systems are geological and climatic they include a source of suspended sediment and a highly seasonal water budget preservat on of anastomosed fluvial deposits requires a mechanism for maintaining very low depositional slopes over geologic time scales data from the st mary river formation in southwestern alberta show that the stratigraphic equivalents of these rivers are ribbon sandstone bodies encased in thin sandstones shales and siltstones the total distribution of facies rather than proof of simultaneous flow in multiple channels can be used to interpret correctly a deposit as anastomosed fluvial although anastomosed fluvial deposits form in a variety of tectonic settings they are apparently most common in foreland basins because these basins provide suitable source rocks and thrusting as a mechanism that can maintain the low depositional slopes over millions of years the mutual interaction among source rock climate and subsidence all of which play critical roles in forming and preserving these distinctive fluvial deposits allows inferences to be drawn regarding paleoclimate and paleoslope that may help constrain models of basin evolution

accretionary tectonics of the north american cordillera continental geology stands on the threshold of a change that is likely to be as fundamental as plate tectonic theory was for marine geology ongoing seismic reflection investigations into the deep crustal structure of north america are verifying that orogenic zones are underlain by low angle faults of regional extent brown et al 1981 the growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture bounded fragments particularly within the north american cordillera such interdisciplinary studies are leading to a consensus that the cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in mesozoic and early cenozoic time such crustal growth is referred to as accretionary tectonics in this paper we review some of the important concepts in accretionary tectonics discuss the nature of the materials accreted between central alaska and southern california in jurassic and cretaceous time and consider the general relations between cordilleran accretion and the movement of lithospheric plates the concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest with the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen along with mechanisms for crustal attrition accretion mechanisms include the growth of imbricated sedimentary prisms along inner trench walls slicing off of submarine topographic irregularities within subducting plates and collision of continents and volcanic arcs by ocean basin closure tectonic attrition mechanisms include rifting transform faulting and strike slip or underthrust removal of inner trench wall materials coincident with or in place of accretionary prism growth growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments an important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins since attrition by nature leaves little material evidence of having operated one of the major problems confronting cordilleran geologists lies in the recognition of such attrition within the ancient record particularly when interspersed with accretionary events the spectrum of accretion and attrition mechanisms viewed at cm yr 1 plate transport rates over time scales of 100 m y leads one to suspect a highly mobile history for continental margin orogens the serial arrangement of subducting transform and rifting links along the modern cordillera plate juncture system and both serial and parallel arrangements in the western pacific systems show the complex interplay of such mechanisms through space similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the cordillera which presently resemble a collage of crustal fragments davis et al 1978 recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and earth history the first problem to be considered is the recognition of native north american crust from exotic fragments that have been accreted to its edge

island arc carbonates from the silurian heceta formation of southeastern alaska alexander terrane abstract the heceta formation of early to late silurian age forms part of a well preserved paleozoic island arc sequence that is exposed within the alexander terrane of southeastern alaska limestones and limestone breccias predominate within the formation and are interbedded with polymictic conglomerates and volcaniclastic sandstones these deposits conformably overlie the lower ordovician lower silurian descon formation which is characterized by volcaniclastic wackes and mudstones that accumulated as turbidites in a deep water setting interbedded with the turbidite deposits are basalt andesite pillow flows and volcanic breccias that formed during incipient stages of island arc construction the limestones within the heceta formation record the earliest occurrence of widespread carbonate deposition within the arc although previously described as having only a shallow marine origin the limestones can be differentiated into shallow water platform platform margin and slope facies shallow water deposits comprise a diversity of skeletal grains that are preserved as micritized fragments associated with cyanobacterial encrustations they are also characterized by low diversity brachiopod communities amphipora and oncolites that have a predominance of dasycladacean alga nuclei these units reflect restricted quiet areas of deposition that were affected by intermittent turbulence on the platform stromatolitic buildups and stromatoporoid reefs constructed at the pla form margin generally are not preserved in situ they occur predominantly as slumps that initiated carbonate debris flows and deformed the underlying slope deposits during periodic collapse of the platform margin graded skeletal grainstones containing shelly debris of shallow water origin and finely laminated lime mudstones represent turbidites that were affected by synsedimentary deformation in a slope environment as indicated by the convoluted bedding slump folds microfaults and truncation surfaces rapid facies changes shallowing and deepening upwards sequences and thick polymictic conglomerates record phases of tectonism within the arc that periodically disrupted carbonate deposition the predominance of cyanobacteria in the limestones suggests that these organisms were largely responsible for stabilizing the substratum by constructing stromatolitic buildups and encrusting skeletal debris during early stages in carbonate platform development and following periods of tectonic disturbance within the arc

nearshore carbonate deposits in lake tanganyika an exceptionally wide variety of carbonate fades dominated by high magnesian calcite occurs along the littoral and shallow sublittoral zones 50 m of lake tanganyika in central africa these facies include exposed and submerged calcite cemented ridges of nearshore terrigenous sand ooid sand shoals and lithified oolite ridges chara meadows of bioturbated calcareous silts gastropod shell blankets and related coquinas and extensive thrombolitic microbial reefs though texturally and compositionally similar to many modern and ancient shallow water marine facies these deposits record carbonate deposition and cementation in a large tropical deep water lake of tectonic origin lithofacies along lake tanganyika represent the broadest spectrum of carbonate deposits yet reported from any modern lake and serve as important analogues for lacustrine carbonate sequences in the stratigraphic record

devonian lake margin environments and processes se shetland scotland the devonian lake or lakes of se shetland were characterized by 4 environments 1 a marginal environment dominated by the high energies of the surf zone and just lakeward of the breaker zone 2 an additional marginal environment consisting of small shallow oxidizing lakes on a low gradient coastal flood plain over which waters from the main body of the lake fluctuated 3 an intermediate environment characterized by continuous to intermittent wave agitation in water depths of probably 5 10m the marginal environment was influenced by both wave and current processes a friction dominated outflow produced sedimentary structures that indicate a south easterly flow and depositional slope outflow competed with shoaling waves which advanced from e to w onto the lake shoreline where outflow was weakest wave processes dominated longshore drift was an additional component in the movement of the nearshore waters the devonian lake or lakes of se shetland possessed rather shallow shoaling margins

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

early devonian conodonts from prince of wales island southeastern alaska early devonian conodonts from 11 localities on prince of wales island and adjacent small islands southeastern alaska include pandorinellina optima moskalenko pandorinellina exigua philipi klapper pandorinellina exigua exigua philip pandorinellina boucoti klapper pelekysgnathus serratus jentzsch pelekysgnathus cf p klamathensis savage pelekysgnathus n sp eognathodus sulcatus sulcatus philip eognathodus sulcatus kindlei lane and ormiston eognathodus sulcatus n ssp a eognathodus sulcatus n ssp b ozarkodina eberleini savage kimognathus alexeii mashkova kimognathus n sp a and kimognathus n sp b the occurrences indicate that lower devonian deposits are widespread on prince of wales island on wadleigh island zlichovian and probably older deposits underlie eifelian beds of the wadleigh limestone previously thought to include limestone only of middle and late devonian age the early devonian faunas of this part of southeastern alaska contain species characteristic of cordilleran north america there are strong affinities with nevada eastern australia and central asia but lesser affinities with central and eastern europe affinities with eastern north america and western europe are relatively weak regional variations in conodont color alteration indices suggest that the southern half of prince of wales island has a different post devonian thermal history from the northern half

nd and sr isotopic characterization of the wrangellia terrane and implications for crustal growth of the canadian cordillera nd and sr isotopic data are reported from samples of most of the major lithologic units of the wrangellia terrane exposed on vancouver island and the queen charlotte islands initial epsilon nd values range from 1 0 to 7 3 and initial 87 sr 86 sr ratios range from 0 70323 to 0 70481 these ratios are similar to those of modern island arcs and demonstrate the unevolved juvenile nature of this terrane these data indicate that wrangellia probably resided in an intra oceanic environment until its accretion to north america nd crustal residence ages for clastic sedimentary rocks of wrangellia mean t cr 0 77 pm 0 33 ga t dm 0 57 pm 0 35 ga are much closer to their stratigraphic ages than is the case for the ages of many other phanerozoic sedimentary rocks thus providing strong evidence for phanerozoic crustal addition to north america early proterozoic crustal components cannot comprise more than about 6 of wrangellia therefore the terrane consists of 94 new mantle d

a review of molecular organic proxies for examining modern and ancient lacustrine environments molecular organic geochemical proxies are increasingly being utilized to reconstruct past environmental conditions as new tools continue to be discovered and developed to date organic geochemical proxies have been developed mainly for use in marine systems and are widely used in paleoceanography in contrast organic proxies have been less commonly used on lacustrine sedimentary records one reason for this is that the wide range in the physical and chemical properties of lakes complicates application of some organic geochemical proxies in lacustrine settings furthermore in comparison to marine studies presently only a small number of studies have conducted or are currently conducting fundamental research aimed at developing organic geochemical proxies for use in lacustrine settings despite this an increasing number of paleo limnological studies are currently applying organic geochemical techniques to examine present and past environmental conditions in this manuscript we review the use of a number of commonly utilized organic geochemical and isotopic proxies and discuss their potential for environmental reconstruction in quaternary lacustrine deposits

palaeoenvironmental significance of lacustrine stromatolite forms from the middle old red sandstone of the orcadian basin the form of microbialite accumulations is largely the product of environmental processes and microbial activity recent work has largely concentrated on the identification and classification of microbialites with little attention being paid to their environmental significance this study describes the environmental distribution of the varied stromatolite forms recorded from the middle old red sandstone sequences of the orcadian basin comparisons are made with triassic examples from east greenland and modern microbialite accumulations the middle old red sandstone of northern scotland was deposited in a predominantly lacustrine setting stromatolites are recorded from both steep basin margin coincident settings and lower gradient settings where the lake margin was distant from the basin margin in the latter case stromatolite development is largely restricted to transgressive lacustrine sequences during the deposition of which reduced rates of sedimentation resulted from the migration of sediment input points towards the basin margin stromatolite sheets domal mounds aligned mounds and associated runnels sand cored stromatolite mounds and reefal stromatolite accumulations have been identified representing the transition from more sheltered to more exposed environments in basin margin coincident settings stromatolite accumulation is restricted to areas of low sedimentation where microbialites coat boulders and pebbles a model for the palaeoenvironmental distribution of the stromatolite forms described is proposed and is shown to be applicable to similar examples from the triassic of east greenland it is suggested that this model may be more widely applicable to stromatolitic accumulations in similar lacustrine settings through large portions of the phanerozoic

carbonate deposition in an ephemeral siliciclastic alluvial system jurassic shuttle meadow formation newark supergroup hartford basin usa abstract nonmarine carbonate accumulation from a sedimentologic perspective has multiple possible origins clastic load springs groundwater discharge soil development biochemical seasonal changes cave speleothems and eolian influx many of these carbonates contain diagnostic textures from which origin is easy to ascertain however differentiating between clastic and spring groundwater carbonates especially in depositional systems dominated by siliciclastics is not always simple here criteria for recognition are developed subsurface hydrology and tectonic origin of the depositional basin source rocks surrounding the basin sedimentologic packaging and extent of the carbonates in question as well as their sedimentary structures and fossil content and stable isotopic analyses of the carbonates can all aid in determining carbonate origin and correctly assessing sedimentary and basin evolution the present study focuses on a nonmarine carbonate deposit found within the jurassic shuttle meadow formation of the hartford basin one of many rift basins which formed as a result of the opening of the north atlantic ocean during the late triassic to early jurassic the upper 50 m of the formation at plainville connecticut is interpreted as continental in origin and is composed of five facies red ripple cross laminated muddy siltstones to sandstones fr sr massive red siliciclastic mudstones fm buff fine to medium grained horizontally laminated sandstones sh red to white disrupted calcareous shales fl3 and rare micritic carbonates pm facies fr sr sh and fm are interpreted as an alluvial plain system in an ephemeral setting with bedload sheetflooding as the dominant depositional mechanism similar to the channel country drainage system of central australia in addition preservation of lateral accretion sandstone sheets accretionary benches indicate that the shuttle meadow alluvial system was rapidly accreting the anomalous existence of carbonate beds in facies fl3 and pm in the smf bedload dominated siliciclastic sequence is attributed mainly to the unique nature of the clastic sediment load in the basin no direct evidence for a spring groundwater input is apparent the shallow carbonate lake margin or a pond with vegetation and periodic exposure is envisioned as the depositional environment the intraclastic texture of the micrites with rare primary relict lamination indicates early diagenetic alteration from root penetration desiccation cracks and burrowing

major controls on sedimentation during the evolution of a continental basin pliocene pleistocene of the guadix basin betic cordillera southern spain abstract sequence stratigraphy based on climatic tectonic and base level parameters can be used to understand carbonate sedimentation in continental basins the uppermost continental fill of the guadix basin betic cordillera containing both siliciclastics and carbonates is investigated here in its central sector a thick succession of fluvio lacustrine sediments appear hosting several important pliocene and pleistocene macrovertebrate sites fonelas project the need to characterize the stratigraphic and sedimentologic context of these important paleontologic sites has lead to litho magneto and biostratigraphic studies these data together with the sedimentologic analysis of the pliocene and pleistocene siliciclastic and carbonate successions establish a sedimentary model for the fluvio lacustrine sedimentation of the two last stages of sedimentation in the guadix basin units v and vi unit v comprises mostly fluvial siliciclastic sediments with less abundant carbonate beds interpreted as floodplain lakes or ponds the latter unit vi is dominated by vertically stacked carbonate palustrine successions using two pre existent continental stratigraphic models the influence of climate tectonism and stratigraphic base level during the last 3 5 ma on the sedimentary evolution of the fluvio lacustrine system in the guadix basin especially the carbonate sedimentation patterns is outlined

coastal fluvial palaeoenvironments and plant palaeoecology of the lower devonian emsian gaspe bay quebec canada

upper permian lacustrine organic facies evolution southern junggar basin nw china abstract the jingjingzigou lucaogou and hongyanchi formations of the southern junggar basin together comprise over 1000 m of organic rich lacustrine facies they record an overall evolution from relatively shallow evaporative lakes to freshwater lakes associated with fluvial systems the jingjingzigou formation is characterized by low toc and hi and biomarker features such as abundant carotane consistent with a specialized saline or hypersaline biota biomarker distributions in jingjingzigou formation extracts most closely resemble oils from the giant karamay oilfield the lucaogou formation represents one of the richest and thickest lacustrine source rock intervals in the world yet it contradicts conventional lacustrine source rock models in at least two important aspects first deposition occurred at middle paleolatitudes 39 43 n rather than in the tropics second limited nutrient supply in a drainage basin dominated by intermediate volcanic rocks appears to have caused low to moderate primary productivities stable salinity stratification and low inorganic sedimentation rates in a deep lake nonetheless resulted in deposits with up to 20 toc and hi near 800 the overlying hongyanchi formation has 1 5 toc but low hi and was deposited in freshwater oxic to sub oxic lakes

models of miocene marginal lacustrine sedimentation in response to varied depositional regimes and source areas in the madrid basin central spain abstract a succession of lake complexes formed through miocene times in the madrid basin central spain the complex formed during the aragonian stage provides an outstanding opportunity to evaluate the role of source areas and depositional regimes in the construction of coeval marginal lacustrine deposits sedimentary facies of the aragonian lake indicate a shallow lake that was fringed to the nw and s by arkosic alluvium to the n by litharenitic alluvial fans and to the ne and e by alluvial fans distally dominated by large floodplains marginal lacustrine deposits in the arkosic realm mostly consist of mg smectites and dolomite sequences that laterally interfinger with sepiolite beds which were deposited in small discontinuous ponds in the distal alluvial fan areas occasionally micaceous rich sandy streams entered the lake forming sheets at their entry points in contrast the northern margin of the lake was formed by muddy flats which were influenced by fluctuation in the water table in this setting illitic mudstones are commonly interbedded with biotite rich flood sands showing a sheet like geometry meandering channels occasionally cut into the mud flats facies assemblages on this side of the lake were mainly derived from low grade metamorphic areas to the northeast the marginal lacustrine facies consist of silty clays carbonate paleosols and paludal to lacustrine limestones these facies reflect an interfingering between the lake and a wide stream floodplain system derived from a carbonate source area the mosaic of marginal lacustrine facies in the madrid basin during the aragonian is discussed in the light of lithological and geochemical factors related to variations in source areas

synoptic reconstruction of a major ancient lake system eocene green river formation western united states numerous 40ar 39ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the greater green river piceance creek and uinta basins of wyoming colorado and utah constrain 8 m y of the eocene epoch multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance 40ar loss and 39ar recoil when considered in conjunction with existing radioisotopic ages and lithostratigraphy biostratigraphy and magnetostratigraphy these new age determinations facilitate temporal correlation of linked eocene lake basins in the laramide rocky mountain region at a significantly increased level of precision to compare our results to the geomagnetic polarity time scale and the regional volcanic record the ages of eocene magnetic anomalies c24 through c20 were recalibrated using seven 40ar 39ar ages overall the ages obtained for this study are consistent with the isochroneity of north american land mammal ages throughout the study area and provide precise radioisotopic constraints on several important biostratigraphic boundaries applying these new ages average sediment accumulation rates in the greater green river basin wyoming were approximately three times faster at the center of the basin versus its ramp like northern margin during deposition of the underfilled wilkins peak member in contrast sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled tipton and laney members sediment accumulation patterns thus reflect basin center focused accumulation rates when the basin was underfilled and supply limited accumulation when the basin was balanced filled to overfilled sediment accumulation in the uinta basin at indian canyon utah was relatively constant at 150 mm k y during deposition of over 5 m y of both evaporative and fluctuating profundal facies which likely reflects the basin margin position of the measured section the most rapid sediment accumulation for the entire system 1 m k y occurred between 49 0 and 47 5 ma when volcaniclastic materials from the absaroka and or challis volcanic fields entered the green river formation lakes from the north our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the green river formation basins coupled with previously published paleocurrent observations our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes the overall character of eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding laramide landscape freshwater overfilled lakes were initially dominant 53 5 52 0 ma possibly related to high erosion rates of remnant cretaceous strata on adjacent uplifts expansion of balanced fill lakes first occurred in all green river formation basins at 52 0 51 3 ma and again between 49 6 and 48 5 ma evaporative underfilled lakes occurred in various basins between 51 3 and 45 1 ma coincident with the end of the early eocene climatic optima and subsequent onset of global cooling defined from marine record however evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate evaporative terminal sinks were initially located in the greater green river and piceance creek basins 51 3 48 9 ma then gradually migrated southward to the uinta basin 47 1 45 2 ma this history is likely related to progressive southward construction of the absaroka volcanic province which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient the greater green river and piceance creek basins were eventually filled from north to south with absarokaderived detritus at sedimentation rates 1 2 orders of magnitude greater than the underlying lake deposits

silurian llandovery wenlock conodonts from the base of the heceta limestone southeastern alaska the base of the heceta limestone exposed on cap island in southeastern alaska appears to lie close to the top of the celloni zone the succeeding 8 m of strata lies within the amorphognathoides zone and part of the ranuliformis zone this cap island faunal sequence consists of 12 previously described and largely cosmopolitan taxa and nine new possibly more provincial taxa it is particularly characterized by the distinctive new taxa tuxekania barbarajeanae neospathognathodus chapini tokeenia furcata and aspelundia capensis the section is of late llandovery to early wenlock silurian age

distinctive characteristics of a streamflow dominated alluvial fan deposit sanghori area kyongsang basin early cretaceous southeastern korea abstract in the northwestern part of the non marine kyongsang basin early cretaceous conglomeratic deposits occur at intervals along the northern basin margin and grade basinward into interbedded sandstone and mudstone one of these conglomeratic deposits referred to as the sanghori deposit is 150 m thick and characterized by amalgamated sheets of conglomerate and sandstone facies assemblages 1 and 2 alternating with interbedded sandstone and purple siltstone facies assemblage 3 facies assemblage 1 comprises clast supported disorganized and organized sheet conglomerates interbedded with massive to stratified sandstones this assemblage is interpreted as the product of proximal gravel bed stream deposition facies assemblage 2 consists of trough and planar cross stratified and horizontally stratified sandstones it represents sandy braided stream deposits in which sand bars and dunes were common sedimentary facies and architecture of the coarse grained facies assemblages suggest that the gravel bed and sandy braided streams were wide and shallow and rapidly aggraded during flash floods facies assemblage 3 consists of interbedded purple siltstone and tabular sandstone which show evidence of weak pedogenic modification and bioturbation these characteristics are interpreted as the result of floodplain processes sediments were supplied from the north and dispersed radially forming a streamflow dominated alluvial fan system the sanghori alluvial system was most probably fed from a large drainage basin resulting in the predominance of streamflow deposits and rounded clasts the alternation of the gravel bed stream and floodplain deposits was due to autocyclic switching of active fan sectors under the conditions of relatively slow and steady basin subsidence

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

geological evolution of the offshore tunisia gabes basin pelagian domain since the cretaceous constraints from subsidence curves from hydrocarbon wells data abstract the comparison of subsidence curves obtained from deep wells in sedimentary basins represents a powerful tool to identify major changes in subsidence distribution both in time and space the construction of subsidence curves from 19 wells covering a time interval from the cretaceous to the present in the gabes basin pelagian domain of the eastern tunisian platform provided the opportunity to reconstruct the subsidence patterns across the basin changes in the subsidence rates in different wells allowed the identification of six time intervals stages stage boundaries are defined by major changes in subsidence trends in the 19 wells each stage is characterized by changes in the subsidence pattern of groups of wells likely reflecting episodes of re organization of the distribution of domains with different trends increase or decrease of subsidence comparisons among the subsidence curves highlighted for each stage parts of the basins with higher or lower subsidence permitting to identify highs and lows and to reconstruct their significance in the different stages identified the basin scale changes in the distribution of subsidence rates reflect regional events of basin re organization potentially controlled by syndepositional faults or for some stages by halokinesis due to the presence of salt deposits in the southern part of the study area the comparison of the subsidence curves permitted to identify domains characterized in different time intervals by different patterns of subsidence rates providing important elements for the characterization of the different stages in the stratigraphic and tectonic evolution of the basin

palaeolimnology of palaeozoic lakes focussing on a single lake cycle in the middle devonian of the orcadian basin scotland there are few palaeolimnological studies of palaeozoic lake deposits using methodologies developed for the study of quaternary lakes new research described here indicates the benefits of applying multidisciplinary high resolution sampling and analysis on the middle devonian lake deposits of the orcadian basin in order to understand lake palaeoecology and response to water chemistry change and possible climate change in a continuously sampled 4 m thick lake cycle in two correlated boreholes in caithness there are two distinct features there is a broad trend in 13c of sedimentary organic matter from high values in the shallow water deposits to lower values in the deeper water deposits attributed to changing contributions from terrestrial plant material 13corg c 28 and algal material 13corg c 33 this variation is consistent with palynological data and is best explained by proximal distal variation as the lake filled a smaller but well correlated up section decreasing 13corg trend occurs over approximately 1 m starting at the cycle centre where the lake was at its deepest since the same feature is seen in two boreholes we suggest that local effects of migrated bitumen diagenesis or facies changes are unlikely to be the cause and that changes in the carbon cycle of the lake are responsible explanations for this trend may relate to the source of carbon that algae utilised as the lake changed from a fresher to a more saline environment and to changes in the carbon isotope composition of the lake water during periods of enhanced productivity

late paleozoic tectonic amalgamation of northwestern china sedimentary record of the northern tarim northwestern turpan and southern junggar basins sedimentary rocks contained in basins adjacent to the tian shan provide a long and complex record of the late paleozoic continental amalgamation of northwestern china complementing that provided by rocks preserved within the range this record which comprises dramatic changes in sedimentary facies sediment dispersal patterns sandstone provenance and basin subsidence rates broadly supports previous interpretations of a two part evolution of the tian shan late devonian to early carboniferous collision of the tarim continental block with the central tian shan followed by collision of this combined block with island arcs in the north tian shan and bogda shan in late carboniferous early permian times the first collision resulted in widespread angular unconformities within the tarim basin continued convergence following the collision created a long lived flexural foredeep along the northern margin of the tarim block which received at least 2000 m of lower carboniferous through lower permian fluvial and marine sediment derived from the interior of tarim subsequent early permian continental extension of the northern tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks sandstone within this interval was derived from the paleo tian shan and is composed predominantly of lithic volcanic grains similar to the rhyolite in contrast to the tarim basin calc alkaline volcanic rocks and volcanogenic sedimentary rocks dominated carboniferous and permian sedimentation in the northern turpan and northwestern junggar basins volcanic arcs remained active in the north tian shan and bogda shan through the early late carboniferous depositing a kilometers thick interval of deep marine sediment gravity flows in the northwestern junggar basin major arc magmatism ceased in the late carboniferous in response to closure of the oceanic basin between the combined tarim central tian shan block and the north tian shan bogda shan arcs upper carboniferous through lower permian rocks in the northwestern junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth on the northern side of the volcanic arcs culminating in a 1000 m thick marine regressive sequence middle to upper permian sandstones were derived from the uplifted paleo tian shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range late permian subsidence of the junggar basin accommodated 5 km of nonmarine sediments however the cause of this subsidence and its relationship to regional tectonic events remain controversial

lead contents of s type granites and their petrogenetic significance an evaluation of pb and ba contents in s type granites can provide important information on the processes of crustal partial melting primary low t s type granites which form mainly by fluid absent muscovite melting may acquire a significant enrichment in pb when compared to higher t s type granites for a given ba content we consider the following factors are responsible for this enrichment muscovite is a major carrier of pb in amphibolite facies metapelites and thus large quantities of pb can be liberated upon its breakdown the typical restite assemblage of qz bt sil pl grt kfsp that forms during low t fluid absent muscovite melting can take up only minor amounts of this pb this is because the crystal melt pb distribution coefficients for these restite minerals are low to very low only k feldspar is moderately compatible for pb with a crystal melt distribution coefficient of 3 but its modal content in restites is usually low at the same time the restite assemblage will retain much ba owing to the very high ba uptake in both biotite and k feldspar which is an order of magnitude higher than for pb thus during a low t anatectic event involving a low degree of crustal melting pb as an incompatible element can become strongly enriched in the partial melt relative to ba and also relative to source rock values in the case of higher t anatexis and larger partial melt amounts the pb becomes less enriched and the ba less depleted or even enriched relative to source rock values during fractional crystallization of a s type granite magma ba behaves strongly compatibly and pb weakly compatibly the concentrations of both elements decrease along the liquid line of decent owing to this sympathetic fractionation behavior the primary source related pb ba fingerprint with weak or strong pb enrichment remains in evolved s type granites this facilitates a distinction between primary low t s type granites which are related to muscovite melting and secondary low t s type granites that evolve through fractional crystallization from a higher t parental magma we show in this paper that a simple logarithmic pb versus ba diagram can be a valuable aid for interpreting the petrogenesis of s type granite suites