coarse grained sediment gravity flow facies in a large supraglacial lake near williams lake in the central interior of british columbia the fraser river exposes long sections of late pleistocene glaciolacustrine sediments selectively preserved within a bedrock trough the dominant facies types are thick normally graded gravels and sands that occupy steeply dipping multistorey channels up to 300 m wide and several tens of metres deep channels appear to have been simultaneously cut and filled by high density turbidity currents in a glacial lake floored by stagnant ice fining upward sediment gravity flow sequences up to 50 m thick may be the product of quasi continuous surging turbidity flows triggered by catastrophic meltwater discharges into the trough or retrogressive failure of ice cored sediments large scale post depositional deformation structures such as synclinal folds normal faults sedimentary dyke swarms and dewatering structures record gravitational foundering of sediment and pore water expulsion caused by the melt of underlying glacier ice melting of buried ice masses along the floor of the trough appears to have controlled the flow paths of turbidity currents by producing sub basins within the overlying sediment pile an idealized model of supraglacial lacustrine sedimentation is developed that may be applicable to other glaciated areas with similar bedrock topography

ice lubricated gravity spreading of the olympus mons aureole deposits abstract gravity sliding and spreading at low strain rates can account for the general morphology and structure of the aureoles and basal scarp of olympus mons detachment sliding could have occurred around the volcano if either pore fluid pressures were exceptionally high greater than 90 or the rocks had very low resistance to shear about 1 105 pa or 1 bar because of the vast areal extent and probable shallow depth of the detachment zone development of ubiquitous high pore fluid pressures beneath aureole forming material was unlikely however a zone of sufficiently weak material consisting of about 10 interstitial or interbedded ice could have been present if so a simple rheologic model for the aureole deposits can be applied that consists of a thin ductile layer overlain by a thicker brittle layer according to this model extensional deformation would have occurred near the shield and compressional deformation in its distal parts proximal grabens and distal corrugations on aureole surfaces support this model a submarine slide at kitimat arm british columbia is a valid qualitative analogy for the observed features and inferred emplacement style of the aureole deposits ground ice processes have been considered the cause of many geologic features on mars a 3 average concentration of ground ice in the regolith is predicted by theoretical models for the ice budget and cryosphere ice may have been deposited in higher concentrations below the aureole forming material the source of the ice could have been juvenile water circulated hydrothermally by olympus mons volcanism the basal scarp of olympus mons apparently demarcates the transition between the upper stable part of the shield and its lower part that decoupled and formed the aureole deposits this transition may reflect a change in the bulk shear strength of the shield caused either by a radial dependence in the abundance of ice or fluid in the shield materials or by the concentration of intrusive dikes within the volcano other martian volcanoes exhibit virtually no evidence of similar large scale gravity spreading and basal scarps perhaps such evidence if it existed has been buried by lava flows or perhaps the smaller size of other volcanoes did not permit the development of these features

catastrophic mass failure of a neoproterozoic glacially influenced continental margin the great breccia port askaig formation scotland abstract the great breccia is a thick over 50 m diamictite that occurs close to the base of the neoproterozoic port askaig formation in scotland and contains matrix supported clasts up to 100 m across in a poorly sorted dolomitic matrix detailed logging and mapping of cliff and shoreline exposures through the great breccia on the garvellach islands show that it is not a simple megabreccia but consists of three sedimentary units including a megabreccia unit 1 diamictites interbedded with dolomitic conglomerate sandstone and mudstone unit 2 and a diamictite with metre sized dolomite clasts unit 3 the sedimentological characteristics of the great breccia indicate that it formed by a variety of subaqueous sediment gravity flow processes including matrix rich debris flows and turbidites clasts up to 100 m across contained within the megabreccia unit record catastrophic collapse and failure of local calcareous siliciclastic deposits other diamictite units were deposited by debris flows conglomerates were deposited from high concentration turbulent flows and sandstones and fine grained facies also exhibit characteristics typical of turbidites continued downslope remobilization of sediment in the basin following deposition of the great breccia is recorded by overlying deformed diamictites detrital dolomites and siltstones of the disrupted beds the great breccia and associated facies within the lowermost part of the port askaig formation are similar to those found in carbonate megabreccia successions that record catastrophic mass failure of carbonate platforms failure events in the port askaig formation most likely relate to localized faulting and subsidence of the dalradian basin although the port askaig formation is considered a neoproterozoic glacial succession there is little direct evidence for glacial influence on sedimentation in the great breccia making it difficult to fully assess paleoclimatic conditions

sedimentology of a marine intermontane pleistocene gilbert type fan delta complex in the crati basin calabria southern italy the crati basin is a pliocene holocene extensional basin filled by the progradation of different types of marine fan delta systems coarse grained gilbert type fan deltas developed during the pleistocene they represent the sedimentary response to a strong differential uplift involving the basin margins and the basin itself the differential uplift was responsible for the fragmentation of the basin into several sub basins into which these fan deltas prograded the protection and lateral confinement by structural highs steep coastlines and the absence of strong tidal action in the adjacent ionian sea allowed the regular progradation of these fan deltas in the restricted gulfs and narrow embayments of the crati basin for the classical gilbert type fan deltas in lacustrine settings homopycnal inflow favours a rapid mixing of water masses and deposition of sediment close to the river mouth in the case of the example described here the density contrast between the sea water and the inflowing river water caused the separation of the muddy fraction from the coarse sandy and conglomeratic part of the sediment this allowed the development of steep mud poor coarse grained delta foresets slope instability features slump scars conglomeratic flow slides are fairly scarce in the proximal parts of the san lorenzo del vallo system towards the north where protection from the ionian sea was less they increase in importance

interlaminated ice proximal glacimarine sediments in muir inlet alaska muir inlet in glacier bay alaska is a glacial fjord receiving a tremendous volume of sediment annually the rate of sediment accumulation is greatest proximal to muir glacier about 9 m yr 1 and decreases away from the glacier the primary sediment sources are meltwater streams discharging at subglacial and ice marginal positions to form overflows interflows and underflows continuous turbidity currents overflows and interflows interact with diurnal tidal currents and their volume and sediment concentration varies diurnally and annually with meltwater discharge these effects produce cyclic deposits of a thin fine grained sand or silt lamina that grades normally to a thicker poorly to very poorly sorted mud lamina this lamina couplet is termed a cyclopel underflows are suggested to occur in this glacimarine environment because of conditions unique to subglacial fluvial systems underflow deposits occur only in proximal positions 0 5 km from glacier face and are coarse grained reverse to normal graded and exhibit an increase in sorting and sand content up layer ice rafted debris identified as particles 177 m is ubiquitous though low 5 by weight and occurs as isolated particles frozen pellets or as lenses that in cores may have a lamina appearance proximally ice rafted debris is difficult to identify because proximal sediment is often as coarse grained deposited sediment may be reworked by tidal currents and sediment gravity flows depositional processes operating in muir inlet produce interlaminated sand silt clay that characterizes sediment proximal to a glacier and fines seaward to mud sediment is classified into one of three sediment types 1 1 type i sediment is very fine grained mean 8 65 7 17 o low in sand 0 1 11 2 and very poorly to poorly sorted it is the dominant sediment type in muir inlet and is transported by plumes and deposited by suspension settling 2 2 type ii sediment is fine to coarse grained mean 6 70 3 12 o low to high in sand 5 1 86 6 and very poorly to moderately sorted it represents reworked sediment proximal plume deposits or coarse grained laminae of cyclopels 3 3 type iii sediment is coarse grained mean 3 89 2 38 o high in sand 58 0 100 0 and poorly to well sorted it is deposited by sediment gravity flows or underflows

a depositional record of deglaciation in a paleofjord late carboniferous pennsylvanian of san juan province argentina the role of catastrophic sedimentation the combination of high sediment supply rates and ample accommodation within fjords provides high resolution records of deglaciation ancient fjord fills provide the potential for three dimensional views of the evolution of depositional environments in response to changing sediment supply and base level through the deglacial process we describe the fill jejenes formation of a well exposed late carboniferous pennsylvanian glacial valley and its tributaries these deposits record the progressive marine flooding and infilling of topography by glacially derived sediments the geometry of the valley and its tributaries is controlled by the underlying lower paleozoic lithologies a deep valley with steep sides exists where the bedrock is massive limestones and a broader shallower valley exists where the bedrock is generally a fine grained olistostrome the valleys are locally floored with diamictites including both in situ tillites and remobilized diamictites in the trunk valley these are locally overlain by a small shallow water delta the major part of the valley fill consists of a 150 m thick mudstone dominated succession probably generated by plumes of glacial outwash containing numerous dropstones that decrease in abundance down the fjord the mudstones contain numerous thin sandstone and conglomerate turbidites that were supplied laterally via subaqueous gravel fans feeding in from the tributary valleys each of which has a distinctive clast suite related to the local subcrop the entire succession is overlain by 300 m of sandy turbidites of which the upper part includes large mass transport complexes intercalated within the succession in the trunk valley are structureless graded silty mudstones lacking dropstones but with abundant large wood fragments close to the steep western margin of the trunk valley each of these massive mudstones is underlain by a slump or debris flow locally containing meter scale blocks of ordovician limestone from the valley side we interpret these as a consequence of rockfalls from the steeper valley sides triggering debris flows on the subaqueous fjord slopes we suggest that large solitary waves were generated as the rockfalls entered the water traveling along the fjord and stripping vegetation from the shoreline large amounts of mud and silt were thrown into suspension during these events and subsequently settled from suspension to form the structureless graded beds these deposits and other mass flow deposits within the succession emphasize the potential importance of catastrophic sedimentation within deglacial successions

earthquake induced sediment failures on a 0 25 slope klamath river delta california on november 8 1980 a major earthquake magnitude 6 5 to 7 2 occurred 60 km off the coast of northern california a survey of the area using high resolution seismic reflection and side scan sonar equipment revealed the presence of extensive sediment failure and flows in a zone about 1 km wide and 20 km long that trends parallel to the shelf on the very gently sloping c and a prominent 1 to 2 m high seaward facing toe scarp on the outer margin of the terrace this toe scarp is sinuous appears to be nearly continuous for a distance of 20 km and closely parallels the 60 m isobath evidence indicates that the toe scarp is the terminus of lateral spreads and lobes of sediment flows side scan sonar records show evidence of gas vents and small 10 3 0 5 m high pressure ridges formed seaward of and parallel to the sediment flow scarp indicators of liquefaction sand boils and collapse craters are present on the sediment flow terrace extensive sediment failure occurred on a sea floor slope of less than 0 25 and we can unequivocally pinpoint the cause as an earthquake of known location magnitude and time

marine tailings disposal simulation the disposal of mine tailings in deep water body requires comprehensive assessment of the potential environmental impacts and estimates of the effective storage capacity of the bottom bathymetry for accommodating the anticipated volume of disposed tailings over the life of a mine a mathematical simulation model was developed for this purpose to predict the distribution and long term deposition patterns of tailings in a fjord the model describes the mechanics of the tailings slurry flow along the bottom of the receiving water body and the settling of suspended particles as well as the redistribution of the deposited sediments due to slumping of unstable slopes specific processes simulated in the model also include entrainment at the top of the density current entrainment associated with internal hydraulic jumps in areas of changing bottom slopes hindered settling and coagulation of suspended particles and sediment deposition and bathymetric changes resulting from the deposition of large quantities of tailings long term tailings deposition patterns are predicted by analyzing the flow of the tailings slurry in a series of time increments from a few months to a year the bathymetry is assumed to remain unchanged within each time increment and the model first simulates the steady state flow field for the tailings slurry along the bottom and the resulting deposition rates for several sizes of suspended particles then the estimated tailings deposition rates are combined with slope stability considerations to predict the new bathymetry of the receiving water body data from several operating marine tailings disposal systems were reviewed to assess the applicability of specific relationships used in the model the model was calibrated using data from one of the most extensively studied marine tailings disposal systems the island copper mine in rupert inlet of vancouver island british columbia comparison of simulated tailings deposition patterns in rupert inlet with data from field observations and measurements suggests that calibration of the model is satisfactory and provides confidence in the capability of the model to predict deposition patterns and suspended sediment concentrations for proposed mines

geometry of submarine slides in the bowland basin dinantian and their relation to debris flows sedimentary slide deformation structures in the lower carboniferous dinantian sediments of the bowland basin show a spoon shaped morphology with extension in the head region and compression in the toe the particular structures developed depend on the rheological properties of the sediment involved slides in soft sediment have deformation distributed throughout the slide mass with folding being the dominant deformation style slides within more lithified sediment have deformation concentrated along slip planes both types of deformation may be present within a particular slide depending on diagenetic factors and the depth to which slide planes cut down disaggregation of slides to yield material for sediment gravity flows is explained by processes operating within the slide unit

slumping and sliding in miocene and recent developing arc basins onshore and offshore sumba indonesia abstract neogene slidemasses in eastern sumba are discussed and compared to neogene and quaternary analogues in seismic profiles from the offshore lombok and savu basins the onshore examples were deposited in deep marine base of slope environments within the reach of large amounts of clastics derived from a volcanic arc i e in a setting comparable to that of the recent slides onshore three types of slides are distinguished ranging from mud clast rich debris flows type 1 their possible seismic origin is discussed elongate lenticular slumps type 2 to over 100 m thick slide masses of intimately mixed and folded strata together with rafts of less deformed sediments type 3 the offshore analogues may be twice as thick both in the field and in the seismic profiles such slides are found at various superimposed levels five superimposed units can be distinguished in a seismic profile in the savu basin near the westernmost extension of the offshore sumba ridge a giant blocky slide occurs all slide masses are located on slopes with increasing gradients due to differential vertical movements tectonically induced oversteepening is therefore considered a main cause of failure in outcrops the upper surfaces of the slides are almost flat this contrasts with a hummocky topography seen in some of the offshore examples

the igneous petrology and magmatic evolution of the midcontinent rift system klewin k w and shirey s b 1992 the igneous petrology and magmatic evolution of the midcontinent rift system in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 33 40 the 1100 ma midcontinent rift system of north america is one of the world s great igneous provinces containing over one million km3 of mafic volcanic and plutonic rocks volcanic rocks range from picrite to rhyolite although tholeiitic basalt predominates plutonic rocks range from peridotite to granite with plagioclase rich gabbros and troctolites being most abundant the igneous history of the rift can be divided into early and late stages flows of the early stage form a trend having low alumina high magnesia picrite as the primitive endmember whereas flows from the late stage form a trend having high alumina olivine tholeiite as the primitive endmember the early basalts have enriched nd isotopic compositions and were derived largely from old heterogeneous subcontinental lithosphere the late basalts have chondritic to depleted nd isotopic compositions and were derived from large degree partial melts of a mantle plume that had mixed with isotopically depleted asthenospheric mantle apparently melting began initially in the subcontinental lithosphere as it was stretched subsequently a lower mantle plume which had entrained asthenospheric mantle during its ascent rose beneath the rift and melted at large degrees to yield the voluminous later basalts

the tsunami earthquake of 13 april 1923 in northern kamchatka seismological and hydrodynamic investigations we present a seismological and hydrodynamic investigation of the earthquake of 13 april 1923 at ust kamchatsk northern kamchatka which generated a more powerful and damaging tsunami than the larger event of 03 february 1923 thus qualifying as a so called tsunami earthquake on the basis of modern relocations we suggest that it took place outside the fault area of the mainshock across the oblique pacific north america plate boundary a model confirmed by a limited dataset of mantle waves which also confirms the slow nature of the source characteristic of tsunami earthquakes however numerical simulations for a number of legitimate seismic models fail to reproduce the sharply peaked distribution of tsunami wave amplitudes reported in the literature by contrast we can reproduce the distribution of reported wave amplitudes using an underwater landslide as a source of the tsunami itself triggered by the earthquake inside the kamchatskiy bight

geochemistry and paleomagnetism of keweenawan basalt in the subsurface of nebraska abstract basalts of keweenawan age mesoproterozoic extend from the lake superior region southwestward into the subsurface of nebraska and kansas one of the best examples of these subsurface basalts occurs in the williams 1 radenslaben well in saunders county nebraska the well is located on the flank of a linear magnetic high within the confines of the midcontinent rift system a core from the well preserves 186 continuous feet 56 7 m of keweenawan basalt five lava flows are present in the core the flows have a coarse ophitic texture and consist of labradorite augite fe ti oxides olivine pseudomorphs chlorite the primary mineralogy is well preserved in the top three flows a through c which represent almost 75 of the total core interval sporadic small patches of low grade secondary minerals are concentrated in the upper portions flow tops of flows a through c but decrease to minimal amounts in the massive flow interiors in contrast the bottom two flows d and e and an alteration zone on the top of flow b show the effects of incipient burial metamorphism and contain chlorite albitized plagioclase prehnite epidote and k feldspar megacrysts a paleomagnetic reversal is recorded in the middle of flow c the rock below the reversal has reversed polarity and the rock above normal polarity there is no evidence that any part of the core has been faulted the original stratigraphic sequence is preserved the age of the reverse normal polarity in these rocks is not constrained but is similar in sequence to the main keweenawan reversal recorded at 1097 ma the geochemical data suggest however that the polarity reversal may be later and record an event that is not presently recognized in the exposed flows of the lake superior region the overall tholeiitic character of these basalts is demonstrated by their primary mineralogy cipw normative chemistry and immobile trace element concentrations trace element abundances in flows a b and c are primary and have not been modified by the burial metamorphism when normalized to chondrites the trace elements form in a tight coherent array however flows d and e clearly show the chemical effects of the burial metamorphism and some of the trace elements for example ba rb sr na and to a lesser extent k have been mobilized the radenslaben basalts have geochemical similarities to e morb but with an additional lree and lile enrichment component they are similar in trace element and nd isotopic characteristics to the main later sequence of keweenawan flood basalts exposed at lake superior in containing relatively depleted lree and lile enrichment vectors and initial positive nd values

petrology of the keweenawan mamainse point lavas ontario petrogenesis and continental rift evolution the mamainse point formation forms a 5250 m thick sequence of flood basalts ranging from high mgo picrites and basalts to basaltic andesites that formed in the keweenawan 1100 ma midcontinent rift of north america this section is the most continuous and complete sequence of keweenawan lavas and records almost the entire igneous history of the rift the section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics a variety of petrogenetic processes including partial melting of different sources at decreasing depth periodically replenished tapped and fractionated magma chambers crustal contamination and polybaric fractionation were responsible for the evolution of the magmas the dominant petrogenetic process changes with stratigraphy which suggests that the physical evolution of the rift controlled which process operated at any time the initial fracturing of the crust presumably allowed high mgo liquids generated at depth to be erupted the early stage of rifting was apparently slow and allowed the formation of long lived magma chambers in which complex processes such replenishment mixing and assimilation contamination could occur the late stage of rifting however was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted these conclusions from the chemostratigraphy agree with the physical stratigraphy which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation

lithofacies and seismic reflection interpretation of temperate glacimarine sedimentation in tarr inlet glacier bay alaska abstract high resolution seismic reflection profiles of sediment fill within tarr inlet of glacier bay alaska show seismic facies changes with increasing distance from the glacial termini five types of seismic facies are recognized from analysis of huntec and minisparker records and seven lithofacies are determined from detailed sedimentologic study of gravity vibro and box cores and bottom grab samples lithofacies and seismic facies associations and fjordfloor morphology allow us to divide the fjord into three sedimentary environments ice proximal iceberg zone and ice distal the ice proximal environment characterized by a morainal bank depositional system can be subdivided into bank back bank core and bank front subenvironments each of which is characterized by a different depositional subsystem a bank back subsystem shows chaotic seismic facies with a mounded surface which we infer consists mainly of unsorted diamicton and poorly sorted coarse grained sediments a bank core depositional subsystem is a mixture of diamicton rubble gravel sand and mud seismic reflection records of this subsystem are characterized by chaotic seismic facies with abundant hyperbolic diffractions and a hummocky surface a bank front depositional subsystem consists of mainly stratified and massive sand and is characterized by internal hummocky facies on seismic reflection records with significant surface relief and sediment gravity flow channels the depositional system formed in the iceberg zone environment consists of rhythmically laminated mud interbedded with thin beds of weakly stratified diamicton and stratified or massive sand and silt on seismic reflection profiles this depositional system is characterized by discontinuously stratified facies with multiple channels on the surface in the proximal zone and a single channel on the largely flat sediment surface in the distal zone the depositional system formed in the ice distal environment consists of interbedded homogeneous or laminated mud and massive or stratified sand and coarse silt this depositional system shows continuously stratified seismic facies with smooth and flat surfaces on minisparker records and continuously stratified seismic facies which are interlayered with thin weakly stratified facies on huntec records

deciphering the sedimentological imprint of paleoseismic events an example from the aptian codo formation northern brazil abstract a stratigraphic horizon bearing several styles of soft sediment structures occurs sandwiched within entirely undisturbed deposits in the lacustrine codo formation aptian located in the eastern part of the grajau basin northern brazil these deposits show a variety of sedimentary features distinguished according to their deformation style as follows from bottom to top a zone 1 consisting of widespread molar tooth structures i e fine grained spar filled cracks that are interconnected with small scale faults fissures and stylolites inclined at a high angle to bedding b zone 2 represented by complex convolute folds with lateral gradation of smooth synclines and anticlines into isoclinic folds and internal layers that retain the normal bedding features recumbent folds nappe folds with associated thrust faults pseudonodules and mound and sag structures c zone 3 consisting of normal faults and fissures that are vertical to near vertical show ragged morphology with small delicate peaks taper both downward and upward after a few centimeters and are associated with large up to 2 5 m long collapsed boulders and d zone 4 characterized by shales with irregular convolute folds these deformation zones are interpreted to reflect sediment rheology and alternating periods of sediment accumulation and deformation under the influence of synsedimentary seismic events as suggested mostly by 1 occurrence between undeformed deposits 2 lateral correlation for several kilometers of distance 3 similarity with other features formed in areas affected by seismic activity 4 presence of structures attesting to sediment deformation in an unconsolidated to semi consolidated state and 5 complex association of brittle and ductile deformation structures hence zone 1 records small scale cracking and faulting of early cemented cohesive deposits zone 2 is dominated by ductile deformation structures formed on water saturated unconsolidated sediments probably as a reflex of compressional stress zone 3 records a change in tectonic style to dominantly distension as evidenced by the horizon with normal faults finally zone 4 shows structures attributed to ductile deformation of water saturated muds which was probably formed during the same episode of deformation responsible for the faults and fissures in zone 3 this succession of deformation events appears to be consistent with transtensional and compressional shear stresses during the sedimentary evolution of the grajau basin

early cambrian teichichnus dominated ichnofabrics and palaeoenvironmental analysis of the caerfai group southwest wales uk abstract the early cambrian caerfai group of pembrokeshire in southwest wales yields an intensely bioturbated teichichnus dominated ichnofabric and provides evidence for the early evolution of infaunal organisms three units of the caerfai group comprise the focus of this study in sections exposed at caerfai bay in the st non s sandstone few primary sedimentary structures are observed bedding being predominantly massive with subordinate parallel and current ripple cross lamination the topmost st non s sandstone contains common soft sediment deformation phenomena and collectively deposition is believed to have taken place in the proximal subaqueous reaches of a steep fronted delta system teichichnus is the dominant trace with subordinate planolites palaeophycus and possible rhizocorallium the overlying caerfai bay shales were deposited from mass flows with evidence of downslope creep suggesting deposition on a steep delta front sandstone debrites are common as are tuffaceous beds with evidence of reworking by bottom currents sandstone and tuff event beds contain common skolithos and arenicolites with less frequent planolites interbedded mudstones contain infrequent teichichnus the caerfai bay shales coarsen and thicken upwards into the caerbwdy sandstone debrite sandstone beds dominate and bed thickening upsection indicates progradation of the delta system the dominant trace fossil within the unit is teichichnus which is constrained to the top few centimetres of individual sandstone beds less common are planolites and palaeophycus the size and abundance of teichichnus in the caerfai group are similar to or exceed those in contemporaneous sections in avalonia and baltica local environmental factors were probably the primary control on trace fossil distribution substrate and salinity are proposed as the main influences on ichnodiversity and size in the caerfai group teichichnus is most abundant in the st non s sandstone where the proximal reaches of the delta experienced reduced salinity and possibly higher oxygen levels burrow diameter progressively decreases upsection and bioturbation becomes less pervasive in subsequent units of the caerfai group this indicates that by the cambrian stage 2 significant advances into reduced salinity environments had occurred along with the dispersal of the teichichnus producer throughout the avalonia baltica seaway correlation within avalonia indicates that a transgression in the cambrian stage 2 initiated the deposition of the caerfai group marking the onset of the welsh basin

glaciogenic coarse clastics in a permo carboniferous bedrock trough in india a sedimentary model abstract physical aspects of sedimentation particularly of coarse clastics within a bedrock trough in the basal 28 5 m of the permo carboniferous talchir formation in west bokaro india have been addressed herein the trough fill facies assemblage bearing abundant signs of glacial activity was eventually buried under wave agitated open nearshore and shelf sediments analysis of several sedimentary characteristics and their interrelationship help to recognize various types of glacial sediments and their frequent resedimentation through various mass flow processes it brings to light plausible transportational and depositional mechanism facies patterns in relation to the various morphometric parts of the trough reveal a changing pattern of sedimentation whereas available palaeocurrent data indicate palaeoflows converging from the flanks and turning axis parallel on the floor of the trough trough filling through fan progradation initially from single but subsequently from multipoint sources failed to impart systematic changes in bed thickness clast size and composition in the sequence this continued till the major coastal onlapping attributed to rapid deglaciation a model with predictive generalization has been proposed for coarse clastic sedimentation in fjord like coastal glacier fed troughs on the basis of observations made herein

proglacial sediment instability features on the scotian slope at 63 w abstract a small area of the scotian slope has been investigated in detail as a type example of a high latitude continental slope adjacent to a major ice outlet the geologic framework has been defined by airgun seismic profiles which show that late tertiary shales have irregular dips but overlie undisturbed strata these shales form a zone of diapirs at around 1200 m water depth which have ponded the prograding quaternary sediments upslope high resolution sparker profiles seamarc i mid range sidescan imagery and piston cores have been used to investigate the late quaternary sedimentary sequence a prominent gullied horizon about 40 m subbottom that is overlain by acoustically unstratified sediments sands is correlated with the occurrence of an early middle wisconsinan grounded icesheet at the top of the continental slope it is overlain by a thick sequence of proglacial muds and a thin veneer of holocene sediments much of the late wisconsinan sequence has been dissected by a series of closely spaced gullies both sidescan and subbottom profiler data show that slumping is common in the late wisconsinan sequence as a result of high sedimentation rates and local gully erosion piston cores provide evidence for debris flows and turbidity currents depositional sediment mounds in the diapir zone have accumulated sediment at three times the regional sedimentation rate suggesting that large amounts of fine sediment reached the slope in suspension and that much of the proglacial sediment by passed the continental slope

chemostratigraphy of the neoproterozoic alona bay lavas ontario a basal sequence of flood basalt lavas associated with the neoproterozoic midcontinent rift system crops out in alona bay along the southeastern shore of lake superior in ontario the alona bay lava succession is about 1200 m thick and lies just north of the well studied contemporaneous mamainse point formation detailed chemostratigraphy of the alona bay lavas suggests they are grossly correlative with the basal portion of the mamainse point formation for instance like the basal part of mamainse point formation the alona bay section contains numerous high mgo lavas and can be subdivided into 4 5 groups with distinct chemical characteristics chemical variations within the alona bay groups are largely the result of fractional crystallization likely at moderate pressures one small group of alona bay lavas also carries the compositional imprint of crustal contamination the remaining inter group chemical distinctions at alona bay are the consequence of temporal changes in partial melting and source ch

paleogeographic significance of a permian sedimentary megamictite in the central belt of the northern sierra nevada the central belt of the northern sierra nevada at oroville lake california includes large outcrops of permian limestone as clasts surrounded by siliceous diamictite limestone ranges from pebbles to megaboulders as much as 100 m in size and five clasts contain wolf campian to leonardian corals and fusulinids the diamictite which encloses limestone clasts has a pebbly mud texture and a chaotic fabric paleozoic megafossils occur in the mud matrix of the diamictite and mixing of limestone clasts and diamictite was a permian event the diamictite and enclosed limestone megaboulders collectively termed a megamictite probably represent a base of slope sedimentary deposit limestone boulders and megaboulders are tabular stratigraphic fragments which detached from lithified carbonate escarpments and moved downslope by sliding along their flat surfaces lithologies of limestone clasts indicate derivation from a shallow carbonate platform corals and fusulinids within limestone clasts include endemic taxa found in the mccloud limestone of the eastern klamath mountains and they represent a permian biogeographic province distinct from those of western terranes of the klamath mountains and the north american craton a large carbonate platform in the eastern klamath area is proposed as the source of limestone clasts in the central belt at oroville lake

metallogeny of the midcontinent rift system of north america abstract the 1 1 ga midcontinent rift system of north america is one of the world s major continental rifts and hosts a variety of mineral deposits the rocks and mineral deposits of this 2000 km long rift are exposed only in the lake superior region in the lake superior region the rift cuts across precambrian basement terranes ranging in age from 1850 ma to more than 3500 ma where exposed the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks beneath the center of lake superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles this region hosts two major classes of mineral deposits magmatic and hydrothermal all important mineral production in this region has come from hydrothermal deposits rift related hydrothermal deposits include four main types 1 native copper deposits in basalts and interflow sediments 2 sediment hosted copper sulfide and native copper 3 copper sulfide veins and lodes hosted by rift related volcanic and sedimentary rocks and 4 polymetallic five element veins in the surrounding archean country rocks the scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals where hydrothermal sulfides occur i e shale hosted copper sulfides the source of sulfur was local sedimentary rocks magmatic deposits have locally supported exploration and minor production but most are subeconomic presently these deposits occur in intrusions exposed near the margins of the rift and include cu ni pge and ti fe v in the duluth complex u ree nb in small carbonatites and breccia pipes resulting from local hydrothermal activity around small felsic intrusions mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization most of the sulfide deposits in intrusions however contain sulfur derived from country rocks the interaction between magma and country rocks was important in generation of the magmatic cu ni sulfide deposits a mantle plume origin has been proposed for the formation of the midcontinent rift more than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines these brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume hydrothermal deposits were emplaced for at least 30 40 m y after rift magmatism and extension ceased this time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift or may be due to the timing of post rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift

stratigraphy and geochemistry in the proterozoic mafic volcanic rocks of the nagu korpo area sw finland the volcanic rocks of the nagu korpo area in sw finland form a thin 1000 m formation that is folded into narrow synforms separated by migmatite filled antiforms in nagu the volcanic formation comprises three units a lower unit of subvolcanic banded gabbro sills a middle unit of amphibolitic volcanic rocks and an upper unit consisting of a thin ultramafic volcanic rock layer the volcanic formation is interlayered within a thicker sequence of metasedimentary gneisses geochemically the metavolcanic rocks are tholeiitic and plots based on ti y zr and other trace elements show that they resemble recent within plate lavas morb and mantle normalised trace element plots show patterns strongly differing from those of recent arc lavas indicating a tectonic environment of plate boundary related initial rifting the upper ultramafic lavas are enriched in lree relative to the lavas of the much thicker amphibolitic middle unit below a model for the initial formation of the volcanic rocks of the svecofennian fold belt must acknowledge both the episode of initial rifting and the later evolution of an island arc like environment

reexposure and advection of 14c depleted organic carbon from old deposits at the upper continental slope represents an additional source of 14 c depleted organic carbon supplied to the ocean in parallel with the weathering of fossil organic carbon delivered by rivers from land to understand the dynamics and implications of this reexposure at the shelf edge a biogeochemical study was carried out in the gulf of lions mediterranean sea where erosive processes driven by shelf dense water cascading are currently shaping the seafloor at the canyon heads mooring lines equipped with sediment traps and current meters were deployed during the cascading season in the southwestern canyon heads whereas sediment cores were collected along the sediment dispersal system from the prodelta regions down to the canyon heads evidence from grain size x radiographs and 210 pb activity indicate the presence in the upper slope of a shelly coarse surface stratum overlying a consolidated deposit this erosive discontinuity was interpreted as being a result of dense water cascading that is able to generate sufficient shear stress at the canyon heads to mobilize the coarse surface layer eroding the basal strata as a result a pool of aged organic carbon d 14 c 944 5 24 7 mean age 23 650 3 321 ybp outcrops at the modern seafloor and is reexposed to the contemporary carbon cycle this basal deposit was found to have relatively high terrigenous organic carbon lignin 1 48 0 14 mg 100 mg oc suggesting that this material was deposited during the last low sea level stand a few sediment trap samples showed anomalously depleted radiocarbon concentrations d 14 c 704 4 62 5 relative to inner shelf d 14 c 293 4 134 0 mid shelf d 14 c 366 6 51 1 and outer shelf d 14 c 384 47 8 surface sediments therefore although the major source of particulate material during the cascading season is resuspended shelf deposits there is evidence that this aged pool of organic carbon can be eroded and laterally advected downslope

morphology volcanism and mass wasting in crater lake oregon crater lake was surveyed nearly to its shoreline by high resolution multibeam echo sounding in order to define its geologic history and provide an accurate base map for research and monitoring surveys the bathymetry and acoustic backscatter reveal the character of landforms and lead to a chronology for the concurrent filling of the lake and volcanism within the ca 7700 calibrated yr b p caldera the andesitic wizard island and central platform volcanoes are composed of sequences of lava deltas that record former lake levels and demonstrate simultaneous activity at the two vents wizard island eruptions ceased when the lake was 80 m lower than at present lava streams from prominent channels on the surface of the central platform descended to feed extensive subaqueous flow fields on the caldera floor the wizard island and central platform volcanoes andesitic merriam cone and a newly discovered probable lava flow on the eastern floor of the lake apparently date from within a few hundred years of caldera collapse whereas a small rhyodacite dome was emplaced on the flank of wizard island at ca 4800 cal yr b p bedrock outcrops on the submerged caldera walls are shown in detail and in some cases can be correlated with exposed geologic units of mount mazama fragmental debris making up the walls elsewhere consists of narrow talus cones forming a dendritic pattern that leads to fewer wider ridges downslope hummocky topography and scattered blocks up to 280 m long below many of the embayments in the caldera wall mark debris avalanche deposits that probably formed in single events and commonly are affected by secondary failures the flat floored deep basins contain relatively fine grained sediment transported from the debris aprons by sheet flow turbidity currents crater lake apparently filled rapidly ca 400 750 yr until reaching a permeable layer above glaciated lava identified by the new survey in the northeast caldera wall at 1845 m elevation thereafter a gradual climatically modulated rise in lake level to the present 1883 m produced a series of beaches culminating in a modern wave cut platform commonly 40 m wide where suitable material is present the new survey reveals landforms that result from intermediate composition volcanism in rising water delineates mass wasting and sediment transport into a restricted basin and yields a more accurate postcaldera history leading to improved assessment of volcanic hazards

sedimentary features of seismoturbidites triggered by the 1983 and older historical earthquakes in the eastern margin of the japan sea abstract both mass failure deposits and turbidites are widely distributed in the epicentral area of the 1983 earthquake mw 7 9 in the eastern margin of the japan sea this basin is semi enclosed with no active conduits for the entrance of coarse detrital sediments seismic reflection surveys and sediment cores reveal that mass failure deposits which are distributed on the continental slopes merge into turbidites in the basin plain hence the mass wasting process is a major transportation process in this region one gravity core gh93 816 was recovered from the southeastern margin of the japan basin close to the epicentral area of the 1983 japan sea earthquake sedimentological results show that five turbidite beds are intercalated in the top 68 cm of the column occurrence of the b tm tephra at the depth of 68 71 cm indicates that these turbidite beds were developed during the last 1000 years the high 137cs concentration in the top 20 cm of the core implies that the uppermost turbidite bed was deposited after 1954 based on these age limits for the top and the bottom of the studied interval we estimate the recurrence intervals of turbidites as 160 330 years we suggest that these turbidites were generated by submarine slides which were triggered by the 1983 and older historical earthquakes in this region these seismoturbidites are characterized by amalgamated beds irregular structure sequences grain size breaks fluctuations abrupt changes in composition within bed and variable composition among beds from these features we infer that deposition resulted from a series of turbidity currents associated with multiple or line sources for example multiple shallow slides that were regionally initiated by the same earthquake the sedimentary features described herein may be useful as criteria for identifying modern seismoturbidites in other settings as well as seismoturbidites in the geologic record however further study is needed for the distinction of this type of turbidites from other types e g tempestites reflected turbidites

crustal contamination identified in keweenawan osler group tholeiites ontario a trace element perspective in the lake superior area voluminous middle proterozoic tholeiitic basalt with minor rhyolite and interflow sediments occupy the midcontinent rift and overlie archean and older proterozoic basement the 3000 m thick section of gently dipping 10 sse basalts on the black bay peninsula is divided into three suites based on the relationship between stratigraphy petrography and geochemical variations e g mg number a1 2 o 3 la sm and gd yb the lower suite consists of augite olivine porphyritic lavas with high mg number sio 2 tio 2 and gd yb but low la sm the overlying central suite flows are either aphyric or plagioclase porphyritic and have lower mg number sio 2 la sm and gd yb decline upward through the central suite the upper suite lavas are either aphyric or plagioclase porphyritic and element ratios plot on slightly different trends compared to the central suite the lower suite has a narrow compositional range the average of the four most primitive lavas is used

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

evidence for an earthquake triggered basin collapse in saguenay fjord canada abstract on february 5 1663 perhaps the largest earthquake ever witnessed in eastern north america struck the saguenay fjord basin with such force that landslides and submarine slides were triggered over a wide region over 3 km 3 of clayey holocene sediments collapsed from the margins of the fjord resulting in amalgamated debris flow and slide deposits that covered much of the basin floor these deposits reached thicknesses of up to 100 m and extended over an area of 100 km 2 the ambient seafloor was compressed under the impact and load of these debris masses and or liquefied under the cyclic stress of the seismic wave a 0 2 km 3 landslide also occurred at this time blocking the saguenay river and possibly facilitating an extra large spring freshet discharge biotracers planktonic and benthic foraminifera arcellaceans pollen and wood in conjuction with mass physical properties of sediment cores and high resolution seismic and sidescan profiler data have been used to identify local and distal sediment sources including their original water depth of deposition and their subsequent mode of mass gravitational transport the landslide material was eroded by the river and super elevated sediment concentrations are considered responsible for a long lasting 28 days turbidity current that entered the saguenay fjord depositing a single 0 3 km 3 turbidite the 2 to 10 m thick turbidite is considered to have been self igniting having eroded into the surface of the collapsed basin debris paleo hydraulic calculations suggest that this turbid flow averaged 0 45 m s and reached a flow thickness of 30 m

mass transport deposits and gas hydrate occurrences in the ulleung basin east sea part 1 mapping sedimentation patterns using seismic coherency abstract seismic coherency measures such as similarity and dip of maximum similarity were used to characterize mass transport deposits mtds in the ulleung basin east sea offshore korea using 2 d and 3 d seismic data several slope failure masses have been identified near drill site ubgh1 4 the mtds have a distinct seismic character and exhibit physical properties similar to gas hydrate bearing sediment elevated electrical resistivity and p wave velocity sediments recovered from within the mtds show a reworked nature with chaotic assemblage of mud clasts additionally the reflection at the base of mtds is polarity reversed relative to the seafloor similarly to the bottom simulating reflector commonly used to infer the presence of gas hydrates the mtds further show regional seismic blanking absence of internal reflectivity which is yet another signature often attributed to gas hydrate bearing sediments at the drill site ubgh1 4 no gas hydrate was recovered in sediment cores from inside a prominent mtd unit instead pore filling gas hydrate was recovered only within thin turbidite sand layers near the base of the gas hydrate stability zone with the analysis of seismic attributes the seismic character of the prominent mtd unit 3 was investigated the base of the mtd unit exhibits deep grooves interpreted as gliding tracks from either outrunner blocks or large clasts that were dragged along the paleo seafloor similar seismic features were identified on the seafloor although the length of the gliding tracks on the seafloor is much shorter a few hundred meters to 1 km compared to over 10 km long tracks at the base of the mtd the seismic coherency attributes allowed to estimate the volume of the failed sediment as well as the direction of the flow of sediment tracking the mtd and extrapolating its spatial extent from the 3 d seismic volume to adjacent 2 d seismic profiles a possible source region of this mass failure was defined 50 km upslope of site ubgh1 4

sediment transport processes on an active submarine fault escarpment gulf of corinth greece abstract sediment transport processes on the escarpment of a listric fault in the tectonically active and terrigenous sediment dominated corinth graben in greece are studied using 3 5 khz and airgun profiles and sidescan sonar images the whole of the slope area is affected by gravitational mass movements slumps debris mud flows and liquified and turbidity flows in canyon fan systems large volumes of sediment are transported through slumping debris mud and turbidity flows into the basin here they build coalescing fans which consist of intercalations of debris flow deposits and turbidites in delta front slope fan apron systems sediments are transported radially downslope through slumping debris mud flows liquified flows and high density turbidity flows into the basin where they build coalescing fan aprons these consist of massive homogenized deposits fluxoturbidites in shelf slope systems retrogressive rotational slumping is the dominant instability feature however due to the sharp increase in the slope gradient caused by the slumping debris and turbidity flows are triggered the major factors responsible for triggering gravitational mass movements in the region are earthquakes high sedimentation rates off the river mouths and heavy rainfalls and high seas these mass movements appear to occur at least once every 2 yrs as indicated by the failure of submarine cables

early ordovician continental break up in variscan europe ndsr isotope and trace element evidence from bimodal igneous associations of the southern massif central france abstract the high grade marvejols group and the low grade albigeois cevennes sedimentary sequence contain bimodal igneous rocks of early ordovician age which are representative of a widespread thermal event in the european variscides comparison of their nd sr isotope and trace element characteristics provides additional evidence for their origin in an ensialic extensional setting as an alternative to the back arc model proposed by previous authors we propose that these associations record a continental break up episode unrelated to contemporaneous subduction in this model the widespread early paleozoic bimodal magmatism marks the birth of a mid european oceanic arm we propose that the break up was controlled by both transtensional processes and mantle plume activity high grade associations such as the marvejols group and low grade bimodal associations such as the albigeois cevennes might be regarded as representing opposite rifted passive margins the northern marvejols type margin was involved in a late ordovician silurian subduction like process with hp ht metamorphism in contrast the southern albigeois cevennes margin remained largely unaffected possibly as part of the overriding plate of the subduction zone pseudo calc alkaline signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break up episodes in this case negative nb anomalies are produced by the addition of crustal components enriched in th and lree relative to nb this inferred alternative origin of nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data

experimental diatom dissolution and the quantification of microfossil preservation in sediments abstract four laboratory experiments on fresh modern diatoms collected from lakes in the northern great plains of north america were carried out to assess the effects of dissolution on diatom abundance and composition marked differences in mean dissolution susceptibility exist between species despite sometimes significant intra specific variation between heterovalves twenty four taxa were ranked according to susceptibility to dissolution using an exponential decay model of valve abundance this dissolution ranking was used to derive two weighted indices of sample preservation a third index f was based on a simple binary classification of valve morphology into dissolved and pristine categories as distinguished by light microscopy lm when compared against rank indices and a measure of species diversity this diatom dissolution index was found to be the best predictor of the progress of dissolution as estimated by total valve abundance or biogenic silica bisi loss strong empirical relationships between f index values and diatom abundance r2 0 84 n 32 and bisi r2 0 89 n 32 were developed and applied to a diatom sequence from a short core from devils lake north dakota and compared to diatom inferred and observed salinity at this site the f index is a simple effective diagnostic tool to assess important aspects of diatom preservation the index can provide insight into si cycling and record changes in conditions pertinent to diatom dissolution and has a role in validation of transfer functions or other inferences derived from compositional data

eocene seismicity in the pyrenees from megaturbidites of the south pyrenean basin spain abstract the triggering mechanism for emplacement of nine calcareous megaturbidites mt in the clastic turbidite eocene basin of the pyrenees is examined the thickness of each mt is 10 to 200 m and their volume may reach 200 km 3 their mobilization is thought to result from seismic activity related to thrust faults along the northern border of the basin we attempt to define the seismic characteristics for a volume of 100 km 3 the dimensions of the slope failures are estimated to be 100 5 10 0 1 0 2 km and related to a fault rupture length of about 50 100 km and liquefaction of basal clayey layers given these two constraints superficial earthquakes of m 7 would be required to initiate landslides

morphology of a submarine slide kitimat arm british columbia a digitally acquired scale corrected side scan sonar survey yielded high resolution imagery of a submarine landslide in british columbia the landslide in a fjord head setting at kitimat was last active in 1975 and created a wide area of deformed sea floor the sediment failure involved shallow rotational movements on the slopes of a fjord head delta marginal tearing translational sliding compressional folding and block gliding of fjord bottom marine clays the slide is shallow and elongate and appears to have been produced by failure in mobile low strength sediments

glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on quaternary examples abstract three variables control the type of glacial debris source and where and when debris is introduced to an ocean 1 type of glacial ice source ice sheet or valley glacier 2 condition of a grounded glacier melting freezing or frozen base and cold or warm internal ice and 3 type of glacier front ice shelf or tidewater using these variables in different combinations eight glacimarine regimes are postulated to exist by integrating information from holocene sediment and pleistocene glacimarine sequences with the eight inductive regimes lithofacies relationships are predicted and modelled the models show a complexity of glacimarine lithofacies the importance of recognizing this complexity lies in realistic interpretations of ancient sequences ice shelf sediment may be limited in the geologic record because of tectonic and climatic constraints required for ice shelf formation alternatively in some tidewater glacier regimes larger volumes of glacimarine sediment are produced than in ice shelf regimes ice proximal sediment is distinctive especially near subglacial meltwater stream effluxes ice shelves and tidewater glaciers may produce similar ice proximal lithofacies e g morainal bank associations and laminites in certain regimes however details of lithofacies may aid in distinguishing ice shelf from tidewater regimes ice shelf and tidewater iceberg zone sediment can be distinguished from each other if basal and high level englacial debris can be distinguished ice shelf zone sediment out from the grounding line has a basal debris source high level englacial debris is deposited in more distal ice shelf zones resulting in sediment by pass in corresponding positions of a tidewater glacier regime iceberg rafted debris comprises mixed basal and high level englacial debris deeper water zones receive large volumes of sediment when a melting freezing base grounding line is at the edge of a continental shelf when the grounding line retreats sediment supply may be drastically reduced because of a back sloping continental shelf and trapping by morainal banks cool water bioclastic carbonates may accumulate once a grounding line has retreated although ice shelf iceberg zones are more extensive than those in tidewater regimes ice shelf icebergs have low debris content and resulting ice rafted debris may be scarce the smallest volume of sediment is produced by a frozen base ice sheet ending as a tidewater front and the largest volume by a melting freezing base valley tidewater glacier glacimarine sediment has a relatively high preservation potential near continental interiors or margins however at continental margins mass flow olistostrome and tectonic melange deposits may be preserved with glacimarine diamictites these deposits may be difficult to distinguish from each other especially if metamorphosed such as at a convergent margin the models presented do not fully represent the glacimarine environment because many other variables are omitted however the principal concepts of generating glacimarine lithofacies associations are considered continued research will verify if each regime exists and will provide data on those regimes that exist today to compare and modify these models

models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences abstract this paper argues that glaciomarine environments can be regarded as special glacially influenced types of continental margin environments e g continental shelf slope rise and basin plain knowledge of the stratigraphic architecture and typical sedimentary sequences of non glacial margins is becoming well known but remains limited for those that have been glacially modified the principal influences on sedimentation in these environments relate to the glacial sediment input controlled by relief of basin margin glacier thermal regime and ice flow dynamics and depositional environments influence of traction currents substrate relief and proximity to nearby ice margins typical ranges of sedimentation rates can be established for glacially influenced continental margin environments and these may provide a framework for ancient sequences starvation of sediment supply to glacially influenced continental margins is common the nature of sub ice shelf sedimentation a model that has been applied to many ancient glacial sequences is critically reviewed the significance of such sedimentation in the rock record has probably been exaggerated because of oversimplistic interpretations of diamictite sequences existing process models of glaciomarine sedimentation derived from study of modern environments are sometimes difficult to employ in investigation of ancient sedimentary sequences because simple lithofacies criteria and typical vertical profiles are not available to aid in interpretation in addition compositional data emphasized by many workers for distinguishing glaciomarine from continental glacial diamict ite s frequently fingerprint sediment source and not mode of deposition the importance of facies analysis methods for isolating depositional environments is illustrated by three examples of ancient glaciomarine sequences these are the early proterozoic gowganda formation 2 3 ga of northern ontario canada the late proterozoic port askaig formation 670 ma of scotland and ireland and the late cenozoic yakataga formation 20 ma to recent of the gulf of alaska these examples illustrate the significance of detailed genetic studies of ancient glacial rocks in the interpretation of palaeogeographic and tectonic settings diamictite units in ancient glaciomarine sequences cannot be easily interpreted in terms of climatic or ice advance retreat cycles because of the varied controls on diamict accumulation and diamictite preservation in marine basins

geometry and genesis of glacigenic debris flows on the north sea fan tobi imagery and deep tow boomer evidence abstract acoustic data have been used to study the upper sequence of glacigenic debris flows gdfs deposited during the last glacial maximum on the north sea fan the presumed gdf release area at the shelf break 450 m water depth exhibits a sequence up to 90 m thick thinning to less than 45 m basinward at approx 750 800 m depth on a low slope 0 6 0 8 a seismically featureless sequence at the shelf break changes distinctly the first kilometers basinward showing a significant occurrence of features interpreted as infilled transport chutes the straight or sinuous chutes feed into well defined relief building gdf lobes 10 40 km from the shelf break a lack of failure scarps at the shelf break suggests that the gdfs have been sourced by a relatively continuous release of material flow lines concentrated along the chutes and center of the gdfs suggest a laminar movement pattern in the gdf core supported by passive flanks based on the flow morphology high pore pressure and low sediment strength within a central core of the flow is assumed to have contributed to the mobility of the gdfs dissipation of pore pressure along the margins of the flow may have caused the margins freeze while the sediment in the core remained mobile

particulate silica and si recycling in the surface waters of the eastern equatorial pacific the distributions of biogenic and lithogenic silica concentrations and net silica production rates in the upper 120 m of the eastern equatorial pacific eep were examined in december 2004 on two transects situated at 110 w 4 n to 3 s and along the equator 110 w to 140 w lithogenic silica lsio2 was generally 10 nmol si l 1 with maximum concentrations reaching 25 nmol l 1 in surface waters these low concentrations confirm low atmospheric inputs of particulate si consistent with reported low inputs of wind borne material in the eep in spite of active upwelling of silicic acid rich waters the biogenic silica bsio2 concentrations were generally low falling between 100 and 180 nmol si l 1 in the upper 50 m and decreasing to less than 50 nmol si l 1 below not vert similar90 m estimates of net bsio2 production rates revealed that the rate of production exceeded that of dissolution in the upper euphotic layer 0 40 m along 110 w with net production extending somewhat deeper 60 100 m to the west along the equator net production rates in the surface layer were low ranging between 5 and 40 nmol si l 1 d 1 consistent with previous observations that diatoms are small contributors to autotrophic biomass in the eep net silica dissolution predominated in the lower euphotic layer 40 120 m indicating active si recycling which diminished the strength of the silica pump in this region

an assessment of the age and tectonic setting of volcanics near the base of the windermere supergroup in northeastern washington implications for latest proterozoic earliest cambrian continental separation mafic metavolcanics of the huckleberry formation in northeastern washington occur near the base of the windermere supergroup a sequence of immature clastic rocks thought to have been deposited dur

magma genesis in a late proterozoic proto oceanic rift ree and other trace element data from the keweenawan mamainse point formation ontario canada abstract the keweenawan mamainse point formation of ontario consists of a sequence of tholeiitic lavas and transitional basalt dykes with trace element characteristics similar to other proto oceanic rift sequences stratigraphic variation of geochemistry allows the subdivision of the sequence into five series each of which has suffered shallow level fractional crystallization differences in incompatible element ratios and ree patterns however can only be explained by variations in the melting process ree modelling suggests that parental magmas may have been derived from a heterogeneous mantle with flat chondritic and lree depleted sources however the systematic evolution of ree patterns and other trace elements are better explained by dynamic partial melting within two subsequent rising mantle diapirs

eocene paleoseismic record of the green river formation fossil basin wyoming u s a implications of synsedimentary deformation structures in lacustrine carbonate mudstones abstract laterally extensive intervals containing sedimentary deformation features are identified in outcrops of lacustrine deposits of the eocene 52 51 ma green river formation in fossil basin southwestern wyoming fossil basin is the smallest of the paleo lakes in the green river system and is located in the sevier fold and thrust belt which was tectonically active during deposition deformation structures include convolute lamination load structures hybrid brittle ductile features sedimentary dikes microfaults fluid escape features such as cusps and pipes oil shale breccias and mass transport deposits in most cases these structures are hosted by finely laminated variably organic rich carbonate mudstones that accumulated in a low energy profundal lacustrine environment deformed intervals range from a few centimeters to 5 m in thickness and are bound above and below by undeformed strata of the same facies which demonstrates the early post depositional timing of deformation the distribution of these features indicates a primary control exerted by sediment rheology resulting from variation in grain size clay and organic content and degree of lithification which in turn were governed by the depositional environment and early diagenesis based on their morphological attributes lateral extent and recurrence at different stratigraphic levels along with their sedimentary environment and the tectonic setting of the basin the deformation structures are interpreted to be seismites i e the result of stresses induced by syndepositional earthquakes and other potential trigger agents are rejected moreover stratigraphic relationships indicate that these small scale deformation features as well as the mass transport deposits formed on a virtually flat lake floor ruling out instability caused by the gravitational regime syndepositional deformation in fossil basin points to tectonic activity along the nearby absaroka and or hogsback thrusts and related fault systems seismites point to possible tectonic forcing as a mechanism for changes in bathymetry and sedimentation in the green river formation and consequently demonstrate their potential for elucidating the tectonic history of other lacustrine and marine basins

mafic rocks in a deep crustal segment of the variscides the gory sowie sw poland evidence for crustal contamination in an extensional setting coronitic metagabbronorites so called hyperites and metabasites occur within gneisses migmatites and minor granulites in the high grade metamorphic gory sowie block in the sudetes sw poland incompatible trace element and end data combined with field and petrographic evidence suggest that three groups may be distinguished 1 enriched amphibolites zr nb ca 8 end500 1 2 5 2 depleted amphibolites zr nb 30 end500 5 6 and 3 metagabbronorites zr nb 13 19 end500 scattered between 5 3 and 1 these characteristics favour an extensional within plate setting consistent with their field occurrence as small bodies scattered within metasedimentary and felsic metaigneous gneisses in that setting enriched mantle sources were partially melted to produce a range of basic magmas showing affinities with continental tholeiites within plate type basalts presumably in a more advanced stage of rifting more depleted varieties transitional to n morb were emplaced these mafic magmas were variably contaminated during their ascent in the crust the metagabbronorites with their well preserved igneous textures were previously interpreted as having been intruded after the main deformation and after the peak of amphibolite facies metamorphism some authors suggesting a relation to the circum gory sowie ophiolites however the geochemical characteristics and p t estimates suggest that the metagabbronorites are unrelated to the ophiolitic gabbros found in the neighbourhood of the gory sowie block and in contrast to them have experienced a complex polybaric p t path

estimates of large magnitude late cambrian earthquakes from seismogenic soft sediment deformation structures central rocky mountains a variety of unusual early post depositional deformation structures exist in grainstone and flat pebble conglomerate beds of upper cambrian strata western colorado including slide scarps thrusted beds irregular blocks and internally deformed beds thrusted beds up to tens of centimetres thick record thrust movement of a part of a bed onto itself along a moderate to steeply inclined 15 to 40 ramp locally producing hanging wall lenses with fault bend geometries thrust plane orientations are widely distributed and in some cases nearly oppositely oriented in close proximity indicating that they did not form as failures acted upon by gravity forces irregular bedded to internally deformed blocks are isolated on generally flat upper bedding surfaces these features represent parts of beds that detached moved up onto and some distances across the laterally adjacent undisturbed bed surfaces deformation of thin intervals of mud on the ocean floor by moving blocks rules out the possibility of storm induced deformation because the mud was not eroded by high shear stresses that would accompany the extremely large forces required to produce and move the blocks finally internally deformed beds are characterized by large blocks fitted fabrics of highly irregular fragments and contorted lamination which represent heterogeneous deformation such as brecciation and liquefaction the deformation structures were produced by earthquakes linked to the reactivation of mesoproterozoic crustal scale shear zones in the central rockies during the late cambrian analysis of the deformation structures indicates very large body forces and calculated earthquake generated ground motion velocities of ca 1 6 m sec 1 these correspond to moment magnitudes of ca 7 0 or more and a mercalli intensity of x these are the only known magnitude estimates of phanerozoic other than quaternary large intensity earthquakes for the rocky mountain region and they are as large as or larger than previous estimates of proterozoic earthquakes along these major shear zones of the central rockies

a tectonically triggered late holocene seismite in the southern amazonian lowlands brazil abstract definitive criteria for identifying the triggers of soft sediment deformation structures ssdss from sedimentary deposits are still under construction in this work we aimed to contribute to advancing this issue describing the sedimentological characteristics of an interval of ssdss formed in a region of the amazonian lowlands having numerous morphostructural evidences of neotectonic reactivation the deformed interval is 1 5 m thick and consists of load and flame structures interbedded with massive beds sandwiched between well stratified deposits of a fluvial sandy channel succession with a radiocarbon age of 1301 1382 cal yr bp the ssdss are intercepted by numerous small scale normal graded faults the most interesting is that these faults display segmented nw se trending planes that propagate upward after horizontal displacements of a few millimeters processes intrinsic to channel dynamics could not justify this fault morphology which is related to recurrent short lived seismic perturbations with intervening sedimentation following a main tremor a relation to fault reactivation is proposed with basis on the fault trend compatible with the regional structural pattern and the numerous morphostructural anomalies verified in the study area including trellis and sub trellis drainage rectangular and to a less extent annular channels anomalously enlarged channels orthogonal channels and valleys channel captures sudden changes in channel morphology moderate to high basin asymmetry factor af between 10 and 27 moderate to high transverse topography symmetry factor t 0 3 0 5 and anomalous longitudinal river profiles with concavity values 11 to 30 indicative of rivers in process of adjustments the morphology of the fault graded beds described herein might constitute a feature of relevance for identifying seismites in other fluvial deposits

evidence of intensified biogenic silica recycling in the black sea after 1970 abstract the black sea has been subject to increased levels of nitrogen and phosphorus loading and a decrease in silicate input after around 1970 changes in phytoplankton community composition from diatoms to non diatom groups have been attributed to the decrease in silicate however a discrepancy between the decreasing silicate input and the increasing silicate pool in the deep sea reported elsewhere implies that another silicate source exists which challenges the current paradigm of widespread silicate limitation in this study we investigate changes in the dissolution state of siliceous protists over the last 140 years and show that siliceous protists became significantly more dissolved after the late 1960s indicating a reduction of the silicate pool preserved in the deep sea sediment we hypothesize that the decline in the dissolution state is caused by increased recycling of biogenic silica in the water column due to an increased annual production driven by nitrogen enrichment

un courant d eau continu peut il generer des turbidites resultats d experimentations analogiques abstract the arenite pelite or limestone marl alternations of turbiditic sequences are today considered as resulting from quasi instantaneous turbidite flows separated by long relatively calm periods i e background sedimentation during which marls or pelites are deposited this is the classical interpretation of the turbidity concept considering the frequent incoherency of the sequences as well as the presence of silts in the sometimes normally graded pelites we propose an alternative explanation for the origin of turbiditic deposits which emphasizies the continuous nature of turbidity flows rather than alternating mechanisms we therefore carried out experiments involving the reproduction of turbidites in a tank for the first time the turbiditic alternation of arenitic and pelitic facies has been formed from a continuous unidirectional current rather than from the intermittent currents usually used in previous experiments the arenite pelite alternations were produced by introducing a given quantity of sediment into the continuous flow at various stages the basin i e the tank occasionally undergoes turbiditic pulsations whereas the supplying current remains completely continuous the turbidity of the water current increases rapidly after sediment addition and decreases very slowly until all the sediment is deposited the procedure was repeated several times at lengthening intervals and with various quantities of sediment arenites and pelites were deposited in spite of the constancy of the current this situation corresponds well to that in a river in which the sediment load variations are controlled by variations in floods which subsequently enter the basin these experiments therefore demonstrate that turbidites can form under fluviatile type conditions the facies discontinuities within turbidites need not reflect drastic systematic variations in turbid flows but could result from turbidity variations within a continuous turbid flow because rivers are the main carriers of sediments from the continents to the basins they could be the cause of the turbidite formation such conditions could explain many sequential aberrations within turbidites such as intrasequence erosion pelitic laminae within ripple foresets and the high proportion of silts in the pelites between the turbiditic sequences these pelites should not be considered as background sedimentation but as the product of a genuine turbid flow such as that which produced the coarse deposits this model leads to significant modification of the sequence interpretation of turbiditic basins which will be much more linked to drainage systems rather than to a particular geodynamic context

delta progradation driven by an advancing sediment source coupled theory and experiment describing the evolution of elongated deltas 1 most of the models of alluvial fan and delta morphodynamics to date have used one of two assumptions concerning the upstream boundary condition 1 a fixed sediment feed point at the vertex or 2 a feed point corresponding to a bedrock alluvial transition that e g migrates upstream under conditions of constant base level here however we present both experimental and numerical results pertaining to a new configuration i e one in which the sediment feed point migrates downstream the research is motivated by the operation of the tailings pond of a mine in which a delta forms the mine operators must frequently move the outfalls of their slurry pipelines downstream to avoid their burial under sediment as the delta aggrades this downstream migration results in a characteristic delta configuration under the right conditions as the tailings delta progrades downstream it does not have time to fill all the available space in the lateral direction thus leaving unfilled open water on either side the result is the formation of an elongated delta here we use both experiments and numerical modeling to characterize the control of the feed point migration rate on the lateral extent of the prograding deltas we use these results to obtain a partial explanation of natural elongated deltas in the case of such deltas the downstream progradation of natural levees provides an analog to the downstream migration of the outfall of a slurry pipeline

anatomy of the kitimat fiord system british columbia abstract the geomorphic complexity of the kitimat fiord system on the active margin of british columbia canada is analysed from several perspectives sub glacial landforms and sediments show that grounded ice exiting the fiord system at the last glacial maximum streamed down moresby trough towards the queen charlotte trough mouth fan after brief halts on the inner shelf grounded ice margins cleared the fiord threshold perhaps by c 15 5 ka cal yrs bp and certainly before 13 ka cal yrs bp just outside the fiords meltwater plumes deposited stratified glaciomarine sediments interbedded with submarine slides inside the fiords thick glaciomarine sediments were deposited and large transverse moraines formed during temporary halts in retreat several glacial outburst floods eroded the kitkiata moraine and deposited distinctive mud deposits postglacial sedimentation on fiord floors has been spatially variable drifts of mud 90 m thick corresponding with areas of low current velocity alternate with areas of non deposition and erosion corresponding with areas of high velocity the fiord system hosts more than a hundred morphologically diverse fan deltas that can be classified in the prior and bornhold 1989 1990 system submarine mass transport was most frequent immediately following ice retreat 15 5 11 5 ka cal yrs bp the largest event 1 2 km 3 involved failure of glaciomarine sediment on a submarine moraine at squally channel and consequent movement of material into the adjacent deep basin this event occurred post 13 ka cal yrs bp in the postglacial phase mass transport continued on a lesser scale up to the present day most intensively in kitimat arm from the perspective of glacial landforms postglacial sedimentation and mass transport this pacific active margin fiord system has some parallels with fiord systems on canada s east coast passive margin and with norwegian fiords but the intensive development of holocene fan deltas is strongly distinctive

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

a basaltic ferrobasaltic granulite association oonagalabi gneiss complex central australia magmatic variation in an early proterozoic rift extremely fractionated basaltic to ferrobasaltic amphibolites and granulites comprise two spatially associated mafic tholeiitic suites deformed sills within the early proterozoic oonagalabi basement gneiss complex harts range central australia the metatholeiites are characterised by high to very high feo tio2 and p2o5 contents and variable depletion in cao and al2o3 despite similar zr nb ratios the rocks from the two suites show different degrees of enrichment in lree and other immobile incompatible elements the basaltic melts which were parental to the two mafic suites were not comagmatic and the rocks cannot be related simply by fractionation of realistic assemblages of low pressure fractionating phases

morphology and origin of smaller scale mass movements on the continental slope off northern norway abstract little attention has been paid to smaller scale mass movements on continental slopes even though they occur much more frequently than their large scale counterparts swath bathymetry side scan sonar sub bottom profiler and seismic data from the continental slope offshore the lofoten islands northern norway reveal evidence of repetitive smaller scale translational sliding involving spreading and multi phase retrogression in water depths between 1100 and 2500 m three styles of failure have been identified occurring in close proximity style 1 is characterized by a 4 7 km wide and up to 100 m deep amphitheater shaped headwall a relatively deep glide plane 130 mbsf detached sediment ridges and a run out area with rafted sediment blocks style 2 consists of a staircase pattern of secondary escarpments caused by the activation of several glide planes between 30 and 110 mbsf headwalls and secondary escarpments have a height of up to 30 and 70 m respectively the run out area shows an almost complete sediment evacuation style 3 is more subtle as it is only identified on the side scan sonar data due to its higher spatial resolution this style shows different phases of on going evolution illustrating the gradual disintegration of a slab of sediments moving over a shallow glide plane at 13 mbsf zones with sediment slabs are up to several hundreds of meters wide and are sharply delineated by shear margins or escarpments the spatial variation in the failure style is inferred to have been caused by the activation of different glide planes which is probably a result of the thinning of contouritic sediments towards the south west in the north east the mounded contouritic sediments contain more potential glide planes and higher slope angles the smaller scale mass movements are suggested to have been triggered by undercutting and removal of support at the foot of the slope due to large scale mass movements that have occurred immediately south of the study area such as the traenadjupet or nyk slides the geophysical data indicate that relatively recent mass movement processes have taken place in this area through the formation of shallow cracks and graben structures by the movement of 13 m thick slabs of sediments style 3 which could have severe implications for offshore infrastructure

self reversed magnetization held by martite in basalt flows from the 1 1 billion year old keweenawan rift canada in some basalt flows of the 1 1 billion year old keweenawan rift exposed at mamainse point ontario there is a magnetic phase that holds a remanence antiparallel to the populations of magnetite and hematite that are typical of flows in the succession the paleomagnetic and geological context of this component demonstrates that it is not a chemicaloverprint whoseremanence is attributableto a subsequently reversed geomagnetic field but that the component is a self reversal of the primary magnetization here we use rock magnetic experiments and mossbauer spectroscopy to show that this phase occurs in the most oxidized flowsandisheldbya fine grainedpopulationofhematitethatacquiredits self reversedremanencethrough interactions with a phase of lower blocking temperature we propose that this self reversed hematite is a manifestation of the self reversal phenomena that has been observed to occur experimentally during the transformation of maghemite to hematite and thathas been attributed to negative exchange coupling across the crystal lattices we suggest that hematite formed in association with iron silicates in the basalt flows carries a remanence reflective of the field in which it formed but that martite hematite pseudomorphed after magnetite which formed through the progressive oxidation of magnetite to maghemite with subsequent inversion to hematite carries the self reversed remanence this study marks the second time that a naturally occurring self reversed magnetization has been attributed to this mechanism and marks the oldestreportedinstanceof anytypeof self reversedremanence themartitethatformsthrough thisprocess has the ability to retain a record of the negative exchange coupling from the time of its formation for hundreds of millions of years

high resolution bathymetry and acoustic geophysical data from santa maria di leuca cold water coral province northern ionian sea apulian continental slope abstract a total of 800 km 2 of multibeam echo sounder coverage roughly 800 km of chirp sonar data and 18 km of side scan sonar profiles 100 500 khz were acquired a few km offshore santa maria di leuca south eastern italy from 200 m to 1300 m water depth the explored area belongs to the upper slope of the gently south eastward dipping apulian continental margin northern ionian sea acoustic datasets were collected by three different oceanographic expeditions where evidence of living cold water coral cwc colonies were documented by previous surveys and samples high resolution multibeam bathymetry indicated an extensive rough seafloor with an irregular faulted upper surface to the west reflecting large scale tectonic control on the margin and a highly disrupted upper slope formed by prominent downslope mass movements to the north a broad area in the east was influenced by mass transport deposition which resulted in a very complex hummocky seafloor mainly shaped by detached block like features and failure related bedforms i e low scarps downslope lineations and compressional ridges from the shallow seismic stratigraphic data failure events appeared to be multiple and recurrent and chaotic reflectors both buried and exposed at the seafloor affected most of the investigated area drift sedimentation was also recognised along a central large ridge resulting in an interplay between contour currents and downslope turbidity currents the spatial distribution of the cwc reefs was inferred from the acoustic facies interpretation based on video images and ground truthed by sediment samples it appeared that 1 within the investigated area living coral frameworks were located along large topographic highs facing the main flow of the bottom currents where hard and firm substrata and or failure related sediment bedforms occurred 2 cwc mainly settled on clustered and isolated mound like features tens to a few hundreds of metres long and no more than 25 m high and were located between 600 and 900 m water depth within the broad area affected by downslope mass transport deposits such mound like morphologies could thus be interpreted as a result of sediment accreted by coral growth with the consequent sediment trapping on small scale positive seafloor irregularities formed by different types of pleistocene exposed mass transport deposits their burial prevented by bottom currents

dynamic process of turbidity generation triggered by the 2011 tohoku oki earthquake 1 we analyzed the distribution of turbidity in seawater near the epicenter of the 2011 tohoku earthquake mw 9 0 as well as the mineral composition and grain size distribution of suspended particles and of shallow sediments in cores from the same area one month after the earthquake in an area where large changes in bathymetry accompanied the earthquake we observed a strong turbidity anomaly above the seafloor maximum 5 mg l that greatly exceeded turbidity measured several years before the earthquake average 0 59 mg l the suspended particles were composed of material similar to the surface material of the sediment cores and we inferred that they were generated locally by a disturbance due to slope failure we estimated slope stability on the basis of the geophysical characteristics of the sediment cores as well as the acceleration of the earthquake our results showed that a submarine landslide could have been induced by the very large ground acceleration of the tohoku earthquake as high as 10 15 m s2 even if the sediment layer on the sliding surface was thin we interpreted the turbidity anomaly observed one month after the tohoku earthquake as the result of shallow submarine landsliding and stirring up of unconsolidated sediment around the epicentral area

biogenic silica dissolution in the oceans reconciling experimental and field based dissolution rates 1 on average 50 60 of diatomaceous opal produced in the euphotic zone redissolves within the upper 100 m of the water column high specific silica dissolution rates in the surface ocean contrast with order of magnitude lower values in deep sea sediments the lack of a mechanistic understanding of these large variations in dissolution kinetics is a major source of uncertainty in models of nutrient silicon cycling in the oceans here we show that the observed variability in biogenic silica reactivity is consistent with results of experimental dissolution studies differences in aluminum content and specific surface area of the diatom skeletons plus differences in temperature and degree of undersaturation all contribute to lowering silica dissolution rates in deep sea sediments relative to the euphotic zone when variations in these material and environmental properties are accounted for the predicted specific opal dissolution rate in average surface ocean water is over 2 orders of magnitude faster than the rate measured in recently deposited biosiliceous oozes of the southern ocean the predicted specific dissolution rate however is approximately 5 times higher than the value obtained from global estimates of the depth integrated dissolution rate and biogenic silica concentration in the upper water column this discrepancy likely reflects the inhibitory effect of protective organic coatings on fresh diatom shells in surface waters the experimental data further imply that aluminum incorporation in the silica matrix during biomineralization or after death of the organisms enhances the preservation efficiency of biogenic opal in marine sediments

regional and local patterns of low grade metamorphism in the north shore volcanic group minnesota usa the north shore volcanic group in northern minnesota is part of the middle proterozoic keweenawan sequence one of the largest plateau lava provinces in the world the primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment the subaerial lava flows mainly amygdaloidal olivine tholeiites and tholeiites have undergone low grade metamorphism from zeolite to lower greenschist facies on the basis of alteration phases replacing the primary magmatic minerals infilling amygdales and veins and replacing secondary minerals the following zones have been distinguished 1 thomsonite scolecite smectite 2 heulandite stilbite smectite 3 laumontitechlorite albite 4 laumontite chlorite albite prehnite pumpellyite and 5 epidote chlorite albite actinolite zone in addition to the overall zonation based on mineral parageneses zonations in the composition of the ab content of the newly formed albite replacing primary ca rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows mafic phyllosilicates in the upper part of the sequence mainly smectites and mixed layer smectite chlorites display high si and ca na k contents whereas in the lower part of the sequence the amounts of si and ca na k are markedly lower mainly chlorites and mixed layer chlorite smectites similar zonations are observed within the individual flows the albite content of the newly formed plagioclase is highest and the si and ca na k content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior the above features suggest that the overall pattern is one of burial type metamorphism associated with extension in the rift setting in detail the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts this suggests that at the first stages of alteration lowest grade the patterns of fluid flow were important effects in controlling the assemblages at greater burial depth assemblages are more homogeneous perhaps representative of a more even and pervasive flow pattern using the observed assemblages at face value to define grade and or facies different conditions would be assigned within the different morphological flow portions thus at low grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions