depositional facies of a mud shoreface in suriname south america a mud analogue to sandy shallow marine deposits abstract immense volumes of argillaceous muds are being deposited along the world s longest continuous mud coastline which faces the open equatorial atlantic not only are these muds deposited in an environment normally associated with sand deposits but just as in sandstones sedimentary structures within these muds record the dynamic processes that deposited them this study investigates a 100 km long portion of the suriname coast utilizing cores bathymetric surveys and field observations from nearshore and innershelf environments using terminology borrowed from sand coastlines the innershelf nearshore of suriname is divided into transition zone shoreface foreshore and backshore the transition zone consists of a mix of offshore sands and shoreface muds shoreface and foreshore are subdivided into bank and interbank zones these zones are transitory because mud banks which make up the bank zones migrate westwardly mud banks are up to 5 m high 50 to 60 km long 10 to 20 km wide and oriented at oblique angles to the coastline these banks generally consist of fluid muds which are so fluid that they interact with surface waves causing them to be altered and damped banks are attached to the shoreline and commonly produce progradation of the coastline substrate within the interbank zone is more consolidat d than within the mud banks because of lack of wave dampening wave energies are relatively high within the interbank zone producing erosion of the shoreface and shoreline sand beaches are generally formed within the erosional interbank shoreline the vertical sedimentary sequence within the shoreface muds is a record of bank migration and other sedimentary processes present wave and current energies are recorded by scour and fill features discontinuity features micro cross laminations lenticular laminations and parallel to subparallel laminations laminations appear to be created by boundary layer shearing within fluid muds which are above wave base and made mobile by wave generated shore parallel currents large scale evidence for migration of the mud banks is westward dipping acoustical reflectors within the banks these reflectors which are over a kilometer apart mark the boundaries of annual migration events biogenic structures are generally absent within the fluid bank muds but are present within the more consolid ted interbank zone and associated with discontinuity features which are buried interbank surfaces migration and vertical stacking of mud banks are recorded by vertical sequences of massive and laminated muds which were deposited by mud banks and discontinuity features which were formed by interbank zones the coastal deposits of suriname are a mud analogue to nearshore and innershelf sand deposits transition zones of both mud and sand systems reflect a transition from offshore to shoreface processes and sediments both mud and sand shoreface foreshore systems reflect sediment input and wave current and tidal processes that act on them mud banks of suriname resemble linear sand ridges on the continental shelf of the eastern u s a in shape oblique orientation to the coastline and orientation with respect to dominant direction of transport processes the sequence of massive and laminated muds with discontinuity features resembles laminated to burrowed sequences found in ancient and recent nearshore and shallow marine sands

late paleozoic glacial episodes in gondwanaland reflected in transgressive regressive depositional sequences in euramerica the late paleozoic glaciation of gondwanaland comprised two short episodes in the famennian i and visean ii confined to brazil and adjacent northwest africa and a long episode that started in the namurian iiia of eastern australia and bolivia argentina expanded to cover much of gondwanaland in the stephanian asselian iiib and collapsed in the early sakmarian iiic dropstones in eastern australia indicate that small ice centers lingered to the kazanian across the belt of low latitudes north of gondwanaland short ranging fossils in widespread shallow marine and paralic deposits indicate synchronous deposition of transgressive regressive sequences in different parts of euramerica these sequences correlate with glacial events in gondwanaland at three levels a four major regressions in euramerica in the famennian 1 visean 2 namurian 3 stephanian 4 and the tastubian transgression that preceded the sterlitamakian regression 5 also recorded in gondwanaland correlate with glacial episodes i ii and iiia iiib and iiic b the time interval of cyclothemic deposition in euramerica brigantian or latest visean to sterlitamakian correlates with that of glacial episode iii and c the dominant period of the euramerican cyclothems as estimated from the middle and late pennsylvanian deposits of the mid continent of north america and of the thickest known gondwanaland glacigenic sediment the earliest permian lyons group of western australia is 0 4 ma equivalent in turn to the long orbital eccentricity period of the quaternary ice age and the dominant period of fluctuation of the late miocene antarctic ice cap the three levels of correlations confirm wanless and shepard9s 1936 hypothesis that the late paleozoic cyclothems are controlled largely by sea level fluctuations related to the gondwanaland glaciation

migration of seismic activity along intraplate seismic belts in the japanese islands abstract migration of seismic activity after eight large earthquakes which occurred in and near japan is investigated it is found that migration velocities are in most cases in the range between several km yr and 100 km yr however in some cases seismic activity in the regions more than a few hundred kilometers distant from the focal regions of great earthquakes are induced just after the earthquake occurrence in these cases apparent migration velocities are more than 100 km day although large earthquakes above m 6 seldom or never occur in almost all cases seismic activity migrates along intraplate seismotectonic belts containing the focal region of the large earthquakes which have triggered the migrational activity it is interesting to note in relation to the prediction of large intraplate earthquakes that the activity of small earthquakes of about m 4 are sometimes observed in the whole of the seismic belts in a relatively short period preceding the commencement of migration of larger earthquakes in those belts when a gap remains behind the migration front it is likely to be filled later a seismic gap concept seems to be most effectively applicable to these gaps as far as intraplate seismic activity is concerned although the physical mechanism of migration has not been found as yet the phenomena are sometimes useful for forecasting future large earthquakes and help us to understand intraplate seismotectonics

neogene tephra correlations in eastern idaho and wyoming implications for yellowstone hotspot related volcanism and tectonic activity the explosive rhyolitic eruptions that define the track of the snake river plain yellowstone volcanism have produced a large volume of tephra found in late miocene and younger basin fill sediments throughout the western united states here we use 40ar 39ar isotopic dating paleomagnetic analysis major and trace element geochemistry and standard optical techniques to establish regional tephra correlations we focus on tephra deposits in three neogene basins in spatially separated areas grand valley in eastern idaho jackson hole in northwestern wyoming and the granite mountains area in central wyoming these basins have experienced relatively continuous deposition from the late miocene to the holocene we found tephra layers that directly tie the stratigraphy between all three basins using these correlations we found that basins experienced discrete pulses of extension separated by long periods of relative quiescence the dates of which are staggered between basins an early pulse of extension occurred in grand valley and jackson hole between 10 3 ma and 16 33 ma with a second pulse initiating between 4 54 ma and 2 09 ma the granite mountains basin experienced a single pulse of extension sometime between 11 14 ma and 6 75 ma these pulses of accelerated extension along with evidence of similar pulses in other basins present a pattern of west to east migration that we suggest is related to the yellowstone hotspot the later pulse of activity in grand valley and jackson hole corresponds to the migration of the north american plate over the tail of the yellowstone hotspot we speculate that the earliest pulse in each basin is related to the more rapid movement of the sublithospheric hotspot head as it spreads out from its earliest known location where the columbia river plateau flood basalt province initiated in southeastern oregon to its outermost edge under central wyoming our results are consistent with this model of a plume head though not unique to it the results of our study also indicate that previous suggestions that the rate of snake river plain explosive volcanism has decreased by a factor of 2 or 3 since emplacement of the middle miocene trapper creek tuffs likely underestimate post trapper creek eruption rates we have discovered a large number of previously unidentified post middle miocene major eruptive events both as ash flow tuffs and as vitric air fall tuffs recalculation to include these newly discovered events results in a rate of major eruptions that is fairly uniform until ca 4 54 ma however there is a substantial gap in major silicic eruptions in the interval between 4 54 ma and 2 09 ma which we call the post heise eruptive gap with the exception of this gap the rate of major eruptions on the snake river plain has been roughly constant since inception of the eastern snake river plain yellowstone volcanic track between 16 ma and 17 ma

kinematics and timing of three superposed extensional systems east central idaho evidence for an eocene tectonic transition cenozoic crustal extension in east central idaho began about 50 ma and continues at present three distinct episodes characterize one of the longest intervals of cenozoic extension yet documented in the north america cordillera crosscutting relationships between ne striking normal faults and volcanic rocks regional dike trends and slickenline data indicate nw se extension during peak eocene volcanism about 49 48 ma episode 1 ne striking normal faults with at most a few kilometers of offset formed in an intraarc setting during rapid ne subduction of oceanic plates under the pacific northwest north to nnw striking and west dipping normal faults with offsets up to 10 15 km formed during a younger middle eocene to oligocene basin forming event episode 2 this newly documented episode was the most important extensional event in east central idaho and began during the waning phases of challis volcanism wsw ene to sw ne extension during episode 2 was nearly perpendicular to the extension direction during episode 1 and perpendicular to the grain of the idaho montana fold and thrust belt the flip in extension direction between episode 1 and episode 2 is tightly constrained by 40ar 39ar age determinations to have taken place at the end of eocene challis magmatism about 46 48 ma i infer that plate boundary forces controlled the geometry of normal faults and dikes during episode 1 whereas internal stresses within previously thickened crust drove major sw to wsw directed extension during episode 2 a drop in convergence rates between the north american and farallon plates between 59 ma and 42 ma stock and molnar 1988 may coincide with the onset of gravitational spreading during episode 2 and may also explain the abrupt end of eocene magmatism in the pacific northwest miocene and younger sw dipping basin and range faults episode 3 extended the region in a ne sw direction although faults formed during episode 2 and episode 3 are not parallel slickenlines indicate only small changes in slip vector trends suggesting little rotation of the extension direction in east central idaho since 46 ma

the magnetochronology of barstovian mammals in southwestern montana and implications for the initiation of neogene crustal extension in the northern rocky mountains magnetostratigraphic and biostratigraphic data have been used to construct a chronologic framework for the deposition of the hepburn9s mesa formation in the yellow stone valley of southwestern montana deposition spanned an interval from 16 8 ma to at least 14 3 ma and possibly to 13 8 ma the magnetic data help to constrain the barstovian land mammal age in the northern rocky mountains as well as two interval zones defined within it the major faunal boundary ages determined here are indistin guishable from ages for these same boundaries at the barstovian type locality in southern california consequently observed differences in generic and species composition between these regions are likely to reflect biogeographic and ecological variability rather than temporal separation when used in conjunction with biostratigraphic data from jackson hole wyoming these new dates improve the time constraints that can be placed on the duration 1 m y of the mid tertiary unconformity in the northern rockies and on the inception at 17 ma of the interval of crustal extension that led to extensive intermontane basin development during the neogene

preserved pedogenic mineral magnetic signature pedogenesis and paleoclimate change pennsylvanian roca shale virgilian asselian central kansas usa abstract this study focuses on the field characteristics and mineral magnetism of four stacked paleosols in the upper pennsylvanian virgilian asselian roca shale exposed near manhattan kansas usa the roca shale paleosols pi at base p4 at top include a protosol p1 argillic calcisol p2 calcic argillisol p3 and calcic vertisol p4 soil climatic changes during the development of each soil profile are suggested by cross cutting relationships and superposition observations including 1 clasts of a lower vertic part are incorporated in an upper calcic horizon and 2 carbonate glaebules have argillans covered by hematitic ferrans decreasing carbonate content more strongly developed of vertic properties and argillans and more mature profiles in successive paleosols are consistent with an interpretation of a longer term soil climate shift from more arid to more seasonal or humid during development of the four paleosols several observations are consistent with a preserved pedogenic mineral magnetic signature in particular 1 each type of soil horizon b bt bk be and c within and among paleosols contains consistent magnetic susceptibility natural remanent magnetization nrm anhysteretic remanent magnetization arm and isothermal remanent magnetization irm signatures different from other horizons 2 similar redox trends are indicated by magnetic mineralogy and uranium concentrations 3 observed magnetic character and uranium concentrations are compatible with those expected in gleyed and oxidized intervals as defined by field observations 4 pedogenic carbonate glaebules and adjacent pedogenically altered siliciclastic strata preserve the same trends and 5 paleosols contain single domain sd magnetite and some maghemite similar to many modern soils comparison with other paleozoic vertisols indicates that the roca shale paleosols are characterized by more complex pedogenic and mineral magnetic attributes this variability in the roca shale paleosols is probably related to glacial interglacial climatic variability during pedogenesis the results of this study show that coupled field observation and mineral magnetic characterization of other pre quaternary paleosols potentially will yield significant paleoclimatic and pedogenic information

diversity of magma types in a lower paleozoic island arc marginal basin system eastern klamath mountains california u s a abstract a trace element investigation has been conducted on ordovician to devonian volcanics of the eastern klamath mountains spatially associated to the siluro ordovician trinity peridotite and gabbros all the studied volcanics low k tholeiites and calc alkaline volcanics present strong island arc geochemical features low tio 2 contents negative nb zr and ti anomalies mild lree enrichment and depletion some volcanics from lovers leap and copley exhibit strong immature island arc geochemical affinities very depleted lree patterns and very low nb contents suggesting that they were formed by high degrees of partial melting 20 of a residual lithosphere diminution of the partial melting degree and addition of sediments to the source will explain the occurrence of less depleted or slightly lree enriched volcanics the grey rocks basaltic pillow lavas thrust onto the trinity peridotitic sheet probably represent the extrusive member of the ophiolitic sequence their strong island arc geochemical affinities suggest that the ophiolite was formed during the first stages of a back arc basin development differentiation took place in the arc by melting of the deeper basaltic layers leading to the formation of abundant rhyolitic and trondhjemitic magmatism representing the first stages of continental accretion

non antipodal directions in magnetostratigraphy an overprint bias s u m m a r y paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal as one would expect for an axial geocentric dipole model specifically in magnetostratigraphic studies of tertiary sediments which include large data sets of paleomagnetic directions of both polarities it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions we investigate the conventional explanation for such asymmetry which involves the presence of a persistent partially unremoved present day field magnetization in the studied rocks both plate motion history and inclination shallowing which we evaluate in detail can play an important role in the paleomagnetic record our analysis shows that alternatively it is plausible that the inclination anomaly is due to a contribution of an axial octupole in accordance with observations of the recent 0 5 ma geomagnetic field recorded in lava flows

u pb dates of paleosols constraints on late paleozoic cycle durations and boundary ages u pb ages of paleosol samples from cyclothemic sections in the sacramento mountains of new mexico and the permian basin of west texas bracket the pennsylvanian permian and carboniferous permian boundaries linear interpolation and extrapolation to boundaries are accomplished by using cyclothems as periodic subdivisions the carboniferous permian boundary is 301 2 ma 2 the pennsylvanian permian boundary is 302 2 4 2 and the missourian virgilian boundary is 307 3 ma 2 these ages are within the uncertainties of recent estimates for the boundaries but are significantly more precise the average cycle duration for this upper pennsylvanian and lower permian interval is 143 64 ka 2 within the uncertainty of the 100 k y cycle that dominated pleistocene sea level history this new estimate which is much shorter than those published previously suggests that the pennsylvanian and early permian cyclothems may have been driven in a manner similar to that which caused glacio eustatic cycles of the pleistocene

late paleozoic depositional sequences are synchronous and worldwide more than fifty transgressive regressive depositional sequences are present in carboniferous and permian shallow marine successions on stable cratonic shelves worldwide these were synchronous depositional events resulting from eustatic sea level changes that generally ranged from 100 to 200 m each transgressive regressive sequence is correlatable using current fossil knowledge they average about 2 m y and range from 1 2 to 4 m y in length the presence within these strata of numerous synchronous unconformities of considerable duration and worldwide extent suggests that the fossil record is very incomplete and that we are studying a punctuated fossil record and not a punctuated evolution based on a highly irregular mutation rate these late paleozoic transgressive regressive depositional sequences facilitate correlations because depositional histories of a rock succession can support interpretations of faunal assemblages and faunal similarities in evaluating age relationships

characteristic space time patterns in seismic activity in the northwest chubu district of honshu island japan and the 1984 nagano ken seibu earthquake abstract the spatial pattern of the epicentral distribution in the northwest chubu district of honshu island japan changed drastically after the occurrence of the 1948 fukui earthquake m 7 1 the change can be explained by invoking that the wnw ese trending seismic belt which contains the focal region of the fukui earthquake was activated and that the activity along another seismic belt trending ne sw which had been conspicuous before the fukui earthquake disappeared a notable feature is that large earthquakes occurred successively along the newly activated seismic belt migrating to the east the 1984 nagano ken seibu earthquake m 6 8 occurred on the extension of that migrational activity the focal region of the nagano ken seibu earthquake is situated at a distinct inflection point in the seismic zone which was activated after the fukui earthquake the spatial pattern of the seismic zone is very similar to the distribution of quaternary volcanoes in that region and the same pattern is also recognized in the bouguer anomaly these features suggest that the seismic belt is a weak zone in the crust corresponding to the boundary of the tectonic blocks the focal region of the nagano ken seibu earthquake had been very quiet over a period of 10 years with the seismic activity subsequently resuming several years before the nagano ken seibu earthquake presenting two active centers in the seismic gap near one of which a large destructive earthquake had occurred in 1906 it seems that the occurrence of the nagano ken seibu earthquake could have been predicted if the characteristics of the seismic activity after the fukui earthquake and the tectonic features in the region had been correctly interpreted

on the periodicity and magnitude of late carboniferous glacio eustatie sea level changes data from sedimentary cycles in basins from northern england the appalachians and midcontinent us afford an insight into periodicities and controls of late carboniferous glacio eustatic sea level changes lithofacies with identifiable palaeobathymetry are used to assess the magnitude of sea level change in the pennine basin of northern england the method yields a minimum value of 42m walsh power spectral analysis is employed to resolve periodicities of about 120ka within the pennine basin of northern england 212 140 and 93 ka within the midcontinent us succession and 109 84 67 63 ka within the appalachian basin within the pennine basin periodicities and magnitudes are found to be independent of their position within the basin thus any tectonism can be assumed to have had a negligible effect on the transgressions a simplified modelled climatic curve is derived from milankovitch parameters and pleistocene 18 o data this is used in a basin fill model that simulates the thermally subsiding pennine basin the resulting distribution of marine deposits is seen to depend upon both the steady regional subsidence and the magnitude and extent of glacio eustatic transgressions

sea level curve for pennsylvanian eustatic marine transgressive regressive depositional cycles along midcontinent outcrop belt north america at least 55 cycles of marine inundation and withdrawal are recognized in the mid desmoinesian to mid virgilian midcontinent outcrop sequence in north america they range from widespread major cycles classic cyclothems with deep water facies extending across the northern shelf through intermediate cycles persisting as marine horizons across the shelf to minor cycles developed on the lower shelf or as parts of major cycles biostratigraphic differentiation of the cycles should establish interbasinal correlation on a scale fine enough to allow evaluation of differential tectonics and sedimentation sequential groupings of cycles are more irregular than proposed megacyclothems or mesothems but they may be obscured by the distinctness of the major cyclothems estimates of cycle periods range from about 40 to 120 x 10 sup 3 yr for the minor cycles up to about 235 to 400 x 10 sup 3 yr for the major cyclothems the range for all cycles corresponds well to the range of periods of earth s orbital parameters that constitute the milankovitch insolation theory for the pleistocene ice ages and it further supports gondwanan glacial control for the pennsylvanian cycles even though the dominant period of the major pennsylvanian cyclothems is up to four times longer than the dominant 100 000 yr period inmore the pleistocene the shapes of both curves display rapid marine transgression rapid melting of ice caps and slow interrupted regression slow buildup of ice caps which suggest similar linkages between the climatic effects of the orbital parameters and ice cap formation and melting at the two different scales widely separated in time 28 references 3 figures less

estimating carboniferous sea level fluctuations from gondwanan ice extent the magnitude of carboniferous sea level fluctuations is not well known but the estimates are important for constraining cyclothem models in this paper we apply area volume relations developed for quaternary ice sheets to estimate the glacio eustatic component of sea level fluctuations in the carboniferous utilizing the best estimate of gondwanan ice area at the time of its greatest extent we arrive at a figure of 60 15 m for the isostasy corrected glacio eustatic component this value is nearer the lower limit of sea level estimates for cyclothem fluctuations from other sources

from cyclothems to sequences the record of eustasy and climate on an icehouse epeiric platform pennsylvanian permian north american midcontinent the stratigraphic succession of the north american midcontinent contains a high resolution 100 000 year record of sea level and climate change spanning the pennsylvanian permian boundary outcrop based sequence stratigraphic analysis on the upper wabaunsee admire and lower council grove groups in kansas and nebraska reveals a hierarchy of fifty one fifth order meter scale cycles grouped into five fourth order composite sequences fifth order cycles are bounded by subaerial unconformities in nearshore settings and correlative conformities in offshore settings and are therefore very thin depositional sequences lowstand systems tracts observed only in nearshore deposits are expressed as sandy incised valley fills in these meter scale cycles transgressive systems tracts which are dominated by carbonate deposition include coastal evaporites indicating arid climatic conditions highstand systems tracts which are dominated by siliciclastic deposition include thin but persistent coals in muddy coastal successions indicating humid climatic conditions paleosols at sequence boundaries evolve from vertic to calcic indicating climate change from relatively humid to relatively arid conditions during subaerial exposure this indicates that relatively arid climate coincided with eustatic lows while relatively humid climate coincided with eustatic highs on this low latitude platform composite sequences are bounded by subaerial angular unconformities and display a transgressive regressive stacking pattern of meter scale cycles the high resolution stratigraphic record of the midcontinent helps constrain the rate and magnitude of environmental change in ancient icehouse conditions and can serve as a baseline of environmental change for comparison with other coeval successions

characteristic patterns in the space time distribution of large shallow earthquakes in the japanese islands abstract characteristic patterns of intraplate large shallow earthquake occurrence in the japanese islands are investigated it is shown that in each district there are seismic belts or seismotectonic active zones where large shallow intraplate earthquakes frequently occur in some cases the activation of seismic belts seems to be related to the occurrence of large shallow earthquakes such as the 1891 nobi earthquake with a magnitude of 8 0 and the 1923 kanto earthquake with a magnitude of 7 9 also it is probably caused by stress increase due to interplate coupling such as in the chugoku district southwest japan before the 1946 nankaido earthquake with a magnitude of 8 1 in the kyushu district other unique tectonics seem to be involved in the intraplate seismic activity non overlapping of active areas and the concentration of earthquake occurrence within a relatively short period are usually observed for each seismotectonic zone further migration of seismic activity and synchronous activity as well as correlation of earthquake occurrences in distant parts along seismotectonic belts are recognized in most tectonic zones it is pointed out that the same concept of seismic gap of the first kind originally proposed for interplate large shallow earthquakes along the deep sea trench may also be applicable to the occurrence of intraplate large earthquakes along seismic belts these characteristic patterns are considered to give a promising clue to selecting the sites where large earthquakes are expected to occur in the near futue seismic belts or seismotectonic active zones probably play an important role in intraplate tectonics they are a kind of boundary of blocks along which change in stress field can be transferred easily though their tectonic origin may be different for each seismotectonic zone

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

walther s law climate change and upper paleozoic cyclostratigraphy in the ancestral rocky mountains geologists routinely apply walther9s law in interpreting paleoenvironments and paleogeographic scenarios from stratigraphic successions the application of walther9s law however may be problematic in strata that have been influenced by significant and geologically rapid climate change such climatic fluctuation can force depositional environments to change fundamentally rather than migrate laterally resulting in a stratigraphic succession wherein depositional facies vertically superposed rarely or never coexisted laterally upper pennsylvanian carbonate siliciclastic cycles deposited in basins of the southern ancestral rocky mountains display stratigraphic relations that cannot be explained by autogenic and or glacioeustatic mechanisms alone rather intracyclic glacial interglacial climate change was a significant influence cycles of the northern pedregosa basin for example comprise eolian marine siltstone in sharp contact with subtidal peritidal carbonate facies absence of facies mixing suggests a climatic control on silt influx cycles of the western orogrande basin comprise deltaic siliciclastic strata in sharp contact with subtidal peritidal carbonate facies absence of facies mixing truncated siliciclastic progradation and carbonate facies with emergence features similarly support a climatic influence on the siliciclastic component in both systems intracyclic glacial interglacial climatic shift reconfigured depositional environments in cases such as these application of walther9s law could produce erroneous conclusions severe glacial interglacial climate change helps explain the pervasive association of mixed carbonate siliciclastic cyclicity that typifies upper paleozoic strata in many parts of the world

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

localized rotation during paleogene extension in east central idaho paleomagnetic and geologic evidence we collected paleomagnetic data from 137 sites in middle eocene volcanic rocks of the northeastern basin and range province in order to characterize the three dimensional kinematics of continental extension in the lost river and lemhi ranges idaho and to test previous block rotation models statistically significant flattening of directions at three of 10 localities reflects either insufficient averaging of paleosecular variation or erroneous tilt corrections five of the seven remaining locality means are statistically indistinguishable from the middle eocene reference direction this indicates that vertical axis rotation did not accompany post 50 ma normal faulting discordant declinations from two localities in the northern lost river range indicate 30 40 of counterclockwise rotation of a large domain of nnw striking volcanic rocks inclination flattening at one of the two rotated localities is moderate but statistically significant f f 9 9 6 5 in total the paleomagnetic data conflict with earlier predictions of uniform clockwise rotation in response to right lateral transtension adjacent to the eastern snake river plain there is a close correspondence between counterclockwise rotated localities and domains of nnw striking volcanic rocks and between unrotated localities and domains of north striking volcanic rocks new mapping shows that counterclockwise rotated localities are all in the hanging wall of a major west dipping low angle normal fault the donkey fault named for the first time this fault loses displacement southwards was active in eocene oligocene time and is probably a tilted high angle fault the spatial distribution of rotated and unrotated localities is best explained by counterclockwise rotation of the hanging wall of the donkey fault during opening of a southward tapering half graben according to this hanging wall rotation model the footwall of the donkey fault remained fixed relative to its rotating hanging wall our proposed kinematic model is generally applicable to extending continental lithosphere and provides a mechanism for coeval crustal thinning and vertical axis rotation thus vertical axis rotation may occur in the absence of strike slip faults in extensional orogens

precursory activity in seismic belts containing the focal region of forthcoming large intraplate earthquake abstract seismic activity in the region surrounding the foci is investigated for three severe earthquakes two with a magnitude of 6 1 and one with a magnitude of 5 3 which have occurred in japan in recent years the most conspicuous feature commonly noticed is precursory activation of seismic belts which include the focal regions of main shocks the repetition of the same pattern in the space time distribution of earthquake occurrence along the seismic belt is also observed for each case the precursory activity of seismic belts terminates in rather a short period and after that the area around the focus of the forthcoming large earthquake becomes quiescent which demonstrates the appearance of the seismic gap of the second kind mogi 1979 the periods of seismic quiescence for the cases investigated in this paper are longer than those which are given by the regression relationship between earthquake magnitude and precursor time proposed for example by sekiya 1977 however our definition of anomalous seismic activity is clear and it is possible to give a physical meaning to it as an increase in the local stress field in the seismic belt we propose that a kind of coupling between intraplate tectonic blocks analogous to interplate coupling in the subduction region is responsible for the formation of the stress field relevant to these earthquakes although this is at present only one of the possible viewpoints on the formation of the focal region of large intraplate earthquakes it may be worthwhile to study various precursory phenomena in connection with this hypothesis

astronomical forcing of contrasting rainfall changes in tropical south america between 12 400 and 8800 cal yr b p abstract today precipitation over tropical south america is largely controlled by the seasonal movements of the inter tropical convergence zone itcz during the summer the itcz is shifted southward due to the warming of the continent paleoclimate data from southeastern amazonia and the central andes indicate that these two areas evolved similarly during the last 30 000 yr however between 12 400 and 8800 cal yr b p eastern amazonia received substantial moisture whereas the bolivian altiplano was arid this suggests that the itcz during summer was then farther north than it is today

refining a pedogenic carbonate co2 paleobarometer to quantify a middle miocene greenhouse spike abstract estimates of respired soil co2 concentration in paleosols are needed to calculate past atmospheric co2 levels from the difference between organic and carbonate carbon isotopic values in paleosols a new compilation of co2 concentrations late in the growing season in modern calcareous soils can be used to estimate respired soil co2 by subtracting atmospheric co2 concentrations furthermore respired co2 shows a significant relationship with depth to calcic horizon and this relationship can be used to estimate respired co2 concentrations in paleosols these values as well as estimates of 13c of atmospheric co2 from 13c of soil organic matter and of paleotemperature from bw horizon chemical composition refine the cerling co2 paleobarometer these refinements are applied to quantify the middle miocene greenhouse which has been controversial because it was not detected by marine alkenone and boron paleobarometers new estimates of middle miocene atmospheric co2 from paleosols in railroad canyon idaho reveal levels at 16 ma of 852 86 ppmv this is similar to predicted values by 2100 and 3 times pre industrial values pil 280 ppmv

periodicity of mesoscale phanerozoic sedimentary cycles and the role of milankovitch orbital modulation data on more than 200 mesoscale sedimentary cycles indicate that phanerozoic cycle periods are randomly distributed with respect to the four major milankovitch orbital parameters except late mississippian through late pennsylvanian cycles which show positive clustering about the 413 000 yr eccentricity period cycle periods calculated from average thicknesses and accumulation rates are strongly dependent on the former correlation coefficient 0 802 but relatively independent of the latter correlation coefficient 0 360 hence calculated period is largely a function of average cycle thickness and milankovitch range periods will be preferentially calculated for cycles with average thicknesses between 1 m and 20 m regardless of actual causes of cyclicity for individual cyclic sequences arguments for milankovitch control based solely on average period are unfounded comparison of short term holocene sedimentation rates and long term phanerozoic accumulation rates for cyclic peritidal and delta

high precision 40ar 39ar spectrum dating on sanidine from the donets basin ukraine evidence for correlation problems in the upper carboniferous two 40 ar 39 ar spectrum measurements on well preserved volcanic feldspar sanidine from the coal tonstein horizon 1 3 1 from the lower kashirsky group middle moscovian stage of the donets basin upper carboniferous ukraine yielded well defined plateaus with an average age of 305 5 1 5 ma the age is to be interpreted as sedimentation age of the coal tonstein and thus defines a chronostratigraphic time mark in the east european carboniferous taking the correlation of the 1 3 1 horizon and the recently published time scales for the upper carboniferous as a basis for comparison the age of 305 5 ma is about 6 ma too young this age discrepancy obviously points to inaccurate or even erroneous correlation and we therefore are compelled to suggest a reconsideration of the validity of the current biostratigraphic correlation between the carboniferous of western and east europe

seismic coupling and uncoupling at subduction zones seismic coupling has been used as a qualitative measure of the interaction between the two plates at subduction zones kanamori 1971 introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern pacific subduction zones a quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables age of the subducting lithosphere and convergence rate the largest earthquakes occur in zones with young lithosphere and fast convergence rates while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane asperities are strong regions that resist the motion between the two plates the zones with the largest earthquakes have very large asperities while the zones with smaller earthquakes have small scattered asperities this observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface while the variation in asperity size is used to establish a connection between earthquake size and tectonic stress it also implies that plate age and rate affect the asperity distribution plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory i e the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity indirect influences are many including oceanic plate topography and the amount of subducted sediments all subduction zones are apparently uncoupled below a depth of about 40 km and we propose that the basalt to eclogite phase change in the down going oceanic crust may be largely responsible this phase change should start at a depth of 30 35 km and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change

recognition of interfluve sequence boundaries integrating paleopedology and sequence stratigraphy in shallow marine and coastal plain strata sequence boundaries are easily recognized at the base of incised valley fills between valleys however it is more difficult to recognize sequence boundaries where they are expressed as interfluve paleosols we suggest that interfluve paleosols can be most reliably identified using a combination of 1 stratigraphic position 2 field observations such as thickness structure color and degree of rooting and 3 micromorphological features such as bioturbation fabric clay coatings ferruginous features and siderite and barite only micromorphology permits recognition of temporal changes in drainage surface stability and protracted pauses in sedimentation that typify these surfaces this three part approach has been successfully employed to identify interfluve sequence bounding paleosols in the cenomanian dunvegan formation in alberta and british columbia canada

paleoclimate controls on stratigraphic repetition of chemical and siliciclastic rocks climate is a primary control on sediment flux from continental sources into sedimentary systems in warm climates siliciclastic input is greatest under highly seasonal rainfall nonseasonal conditions favor formation of end member chemical rocks perennially wet climates are conducive to coal formation whereas dry climates produce carbonates and or evaporites stratigraphic repetition of siliciclastic and chemical rocks therefore appears to be related to paleoclimate cycles as well as to transgressive regressive events and tectonics

stratigraphic responses to geologic processes late pennsylvanian eustasy and tectonics in the pedregosa and orogrande basins ancestral ricky mountains the primary factors that influence stratigraphy are tectonics eustasy and sediment supply because a change in any of these factors produces a concomitant change in relative base level or accommodation space the link between a given process and its stratigraphic response is nonunique despite this ambiguity distinguishing the origins of stratigraphic signals is theoretically possible by examining the temporal and especially the spatial scales of the signal correlation is a particularly critical tool for distinguishing global eustasy from regional tectonism upper pennsylvanian strata of the pedregosa and orogrande basins southern ancestral rocky mountains were deposited during a time of major continental collision coincident with extensive continental glaciation and thus they contain a composite record of significant changes in tectonism eustasy and sediment supply high frequency stratigraphic cyclicity expressed as repetitive successions of lithofacies at the 10 m scale is prevalent in all sections and displays several features that collectively imply a primarily glacioeustatic origin 1 abrupt juxtaposition of dissimilar lithofacies signaling rates of base level change best attributed to glacioeustasy 2 apparent intrabasinal interbasinal and provisional interregional correlation of high frequency cycles across and between contrasting tectonic environments and 3 apparent cycle frequencies that bracket the 413 k y periodicity typical of orbital eccentricity the probable forcing mechanism for pennsylvanian glaciations aside from the prevalent high frequency cyclostratigraphy multiple cycle trends in facies and or thicknesses define low frequency stratigraphic patterns at the 10 2 m scale isolating and identifying the controlling factors for these patterns is not straightforward nevertheless qualitative analysis of multicyclic trends implicates distinct eustatic and tectonic processes as contributing influences the eustatic component may derive partially from low frequency glacioeustasy the tectonic component almost certainly reflects marathon ouachita collisional orogenesis