viscosity and finite strength of the mantle as determined from water and ice loads summary some recent examples of transient earth loads lake bonneville utah glacier bay alaska northeast greenland indicate that both the viscosity and finite strength of the mantle are lower than is commonly presumed a time constant 1 e of 4000 years is estimated for lake bonneville and of 1000 years for northeast greenland a strain rate of 10 14 is typical these figures imply viscosities in a homogeneous half space ranging from 1020 to 1021 poises an upper limit of finite strength is set by lake bonneville at a few times 106 dyn cm2 if mountain ranges like the sierra nevada or himalaya are regarded as dynamically supported rather than static systems this low value is not incompatible with other geologic observations

the grace satellite gravity and geoid fields in analysing large scale cratonic or intracratonic basins the recently released gravity potential field development derived from the gravity recovery and climate experiment satellite allows an unprecedented opportunity to use the gravity field to make global comparisons of structures of geological interest the spatial resolution of the gravity field is sufficiently good to map large scale or intracratonic and cratonic basins as the areal extent of these basins is 0 5 x 10 6 km 2 and greater we present the gravity anomaly bouguer geoid and terrain corrected geoid fields for a selection of nine large scale basins and show that the satellite derived field can be used to successfully identify distinctive structures of these basins e g extinct rifts underlying the basins and generally the isostatic state the studied basins are the eastern barents sea west siberian tarim congo michigan amazon solimoes parnaiba and parana basins we complete the mapping of the gravity field with a description of the basins in terms of areal extension and depth sedimentary age and presence and age of volcanism interpretation of the satellite gravity anomalies and considerations regarding the crustal thickness as known from seismic investigations allows us to conclude that for the greater part of the basins there is evidence for high density material in the lower crust and or upper mantle this density anomaly is at least partly compensating for the low density sedimentary infill instead of the crustal thinning mechanism for our selection of basins crustal thickness variations and moho topography cannot be considered as mechanisms of compensation of the sedimentary loading which is a clear difference to well defined rift basins

lower crustal erosion induced by mantle diapiric upwelling constraints from sedimentary basin formation followed by voluminous basalt volcanism in northwest kyushu japan abstract a sequence of geological events beginning with basement subsidence to form a shallow water sedimentary basin and subsequent voluminous basalt volcanism and uplift of land has been observed in the back arc region of northwestern kyushu japan the basin consists of a succession of marine and non marine sediments with a total thickness of 1000 1500 m which range in age from 43 to 10 ma the basalt volcanism commenced at 10 ma and continued until 1 ma uplift started at around 30 ma and continued after the cessation of the basalt volcanism these geological phenomena may be explained by the convective coupling between the ductile lower crust and upper mantle induced by mantle diapiric upwelling thus surface subsidence leading to sedimentary basin formation is attributed to lower crustal erosion by mantle diapiric upwelling for an earth model with lower crustal and upper mantle viscosities of 10 19 10 20 pa s 5 km of lower crust can be eroded 10 20 myr after the start of convective coupling between the ductile lower crust and upper mantle compatible with the period estimated by observations in this process the melt due to adiabatic mantle diapiric upwelling accumulates beneath the lower crust the accumulation of low density melt in the space originally occupied by mantle material causes crustal uplift when the stress state became extensional as inferred from the extension of okinawa trough during the middle to late miocene 1 the melt filling the eroded lower crustal area may have reached the surface leading to voluminous basalt volcanism

tsunami history of an oregon coastal lake reveals a 4600 yr record of great earthquakes on the cascadia subduction zone bradley lake on the southern oregon coastal plain records local tsunamis and seismic shaking on the cascadia subduction zone over the last 7000 yr thirteen marine incursions delivered landward thinning sheets of sand to the lake from nearshore beach and dune environments to the west following each incursion a slug of marine water near the bottom of the freshwater lake instigated a few year to several decade period of a brackish 4 salinity lake four additional disturbances without marine incursions destabilized sideslopes and bottom sediment producing a suspension deposit that blanketed the lake bottom considering the magnitude and duration of the disturbances necessary to produce bradley lake s marine incursions a local tsunami generated by a great earthquake on the cascadia subduction zone is the only accountable mechanism extreme ocean levels must have been at least 5 8 m above sea level and the cumulative duration of each marine incursion must have been at least 10 min disturbances without marine incursions require seismic shaking as well over the 4600 yr period when bradley lake was an optimum tsunami recorder tsunamis from cascadia plate boundary earthquakes came in clusters between 4600 and 2800 cal yr b p tsunamis occurred at the average frequency of 3 4 every 1000 yr then starting 2800 cal yr b p there was a 930 1260 yr interval with no tsunamis that gap was followed by a 1000 yr period with 4 tsunamis in the last millennium a 670 750 yr gap preceded the a d 1700 earthquake and tsunami the a d 1700 earthquake may be the fi rst of a new cluster of plate boundary earthquakes and accompanying tsunamis local tsunamis entered bradley lake an average of every 390 yr whereas the portion of the cascadia plate boundary that underlies bradley lake ruptured in a great earthquake less frequently about once every 500 yr therefore the entire length of the subduction zone does not rupture in every earthquake and bradley lake has recorded earthquakes caused by rupture along the entire length of the cascadia plate boundary as well as earthquakes caused by rupture of shorter segments of the boundary the tsunami record from bradley lake indicates that at times most recently 1700 yr b p overlapping or adjoining segments of the cascadia plate boundary ruptured within decades of each other

lake deposits of moderate salinity as sensitive indicators of lake level fluctuations example from the upper rotliegend saline lake middle late permian northeast germany abstract the rotliegend saline lake periodically covered wide areas of the southern permian basin in northwest europe during the permian the sedimentology mineralogy and geochemistry of lake deposits were studied to document very high frequency lake level fluctuations and to evaluate their triggers increased precipitation and marine ingressions into the basin resulted in lake extension increased run off is documented by intercalated fluvial deposits in low salinity deposits of the lake because lake deposits reflect mainly deposition in relatively wet climate phases they are not correlatable to halite deposits in other basin areas decreased precipitation is followed by shrinkage of the lake desiccation at its margins and higher lake salinity due to concentration of the brine carbonate and anhydrite contents of lake deposits increased considerably before the areas fell dry but halite is not preserved in the study area falling lake level is also reflected by the occurrence of wave ripples reflecting decreased water depth and finally by desiccation and formation of evaporite crusts not only the mineralogical content but also the colour of lake claystones changes with varying salinity anhydrite and carbonate free red claystones were replaced by violet to green and grey anhydritic and calcareous claystone or marls with increasing salinity the amount of boron adsorbed on illite also corresponds to changes in salinity and can therefore be used as a palaeosalinity indicator rare earth element concentrations within lake deposits do not reflect variations in salinity but different palaeogeographical settings marine ingressions into the lake are neither reflected by the amount of boron adsorbed onto illite nor by rare earth element contents of the lake deposits highly saline deposits are partly characterised by intensive deformation which can be interpreted as seismites or dissolution breccias

exceptionally preserved microbially induced sedimentary structures from the ediacaran post glacial successions in the kimberley region northwestern australia abstract a set of exceptionally preserved microbially induced sedimentary structures miss is described from the post glacial yurabi formation of late ediacaran age in the kimberley region northwestern australia these miss comprise oriented grains bamboo leaf like cracks polygonal cracks worm like cracks sinuous cracks gas domes pyrite concentrated laminae and micrite laminae most of the yurabi miss are morphologically comparable with their ancient and modern counterparts micrometre or nannometre scale bacteria or biofilm like inorganic objects are detected from most miss and are interpreted as bacterial bodies or remains that have been replaced with inorganic minerals upon fossilization oriented grains indirectly support microbial control on their accumulation both pyrite conformable laminae and micrite laminae suggest degradation and decay of original microbial mats upon diagenesis sinuous cracks were likely generated in a soft sediment deformation process whereas other sand cracks bamboo leaf like cracks worm like cracks polygonal cracks could have formed from sediment surface layer microbial organics shrinkage upon desiccation gas domes may have formed as the result of anoxic redox reactions and being filled with gases and sediments transported from below two proxies bedding plane coverage percentage and accumulative thickness of single miss bearing beds are developed to semi quantitatively evaluate miss abundance on bedding plane and vertical outcrop respectively the yurabi miss have substratum preference to fine sandstone facies and intertidal settings the yurabi formation yields the most abundant and diverse post glacial miss assemblages during the late ediacaran worldwide both kimberley and global fossil records indicate that miss may have proliferated in shallow marine ecosystems soon after the retreat of ediacaran gaskiers glaciation during late neoproterozoic

phase change and thermal subsidence of the williston basin summary a mechanism of subsidence for the williston basin is proposed and tested the mechanism combines cooling and thermal contraction of the lithosphere with a phase transformation in the lower crust the cooling and the phase boundary movement following a sudden change in the thermal boundary condition at the base of the lithosphere are calculated isostatic adjustments and sea level variations are included to determine subsidence and sediment accumulation at the basin s centre which are compared with the data the subsidence record could not be explained by temperature boundary conditions the calculations show that 1 the heat flow boundary condition explains the long duration more than 350 myrs of subsidence of the williston basin and 2 the delay of the phase change subsidence about 40myrs explains an acceleration of subsidence in the early stages of the basin s evolution when isostatic adjustments and the effect of sea level variations are included the calculated sediment accumulation history fits well the sediment record of the williston basin

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

surface and subsurface sedimentary structures produced by salt crusts the growth and subsequent dissolution of salts on or within sediment may alter sedimentary structures and textures to such an extent that it is difficult to identify the depositional origin of that sediment and as a result the sediment may be misinterpreted to help to overcome such problems with investigating ancient successions results are presented from a comprehensive study of the morphology and fabrics of three large areas of modern salt flats in se arabia the sabkhat matti inland region and the at taf coastal region both in the emirate of abu dhabi and the umm as samim region in oman these salt flats are affected by tidal marine alluvial and aeolian depositional processes and include both clastic and carbonate dominated surficial sediments the efflorescent and precipitated salt crusts in these areas can be grouped into two main types thick crusts with high relief 10 cm and a polygonal or blocky morphology or thin crusts with low relief 10 cm and a polygonal or blister like appearance the thin crusts may assume the surface morphology of underlying features such as ripples or biogenic mats a variety of small scale textures were observed pustular growths hair like spikes and irregular wrinkles evolution of these crusts over time results in a variety of distinctive sedimentary fabrics produced by salt growth sediment deformation salt solution sediment collapse sediment aggradation and compound mixtures of these processes salt crust processes produce features that may be confused with aeolian adhesion structures an example from the lower triassic ormskirk sandstone formation of the irish sea basin demonstrates how this knowledge of modern environments improves the interpretation of the rock record a distinctive wavy laminated facies in this formation had previously been interpreted as the product of fluvial sheetfloods modified by soft sediment deformation and bioturbation close inspection of laminations seen in core reveals many of the same sedimentary fabrics seen in se arabia associated with salt crusts this facies is the product of salt growth on aeolian sediment and is not of fluvial origin

sediment loading on the western platform of the new zealand continent implications for the strength of a continental margin abstract mechanical properties of the new zealand lithosphere are determined by forward modelling the gravity and isostatic response of a plio pleistocene sediment loaded shelf margin the giant foreset formation on the western platform of new zealand is a 2 km thick 250 km long and 100 km wide sediment package both the downward flexed pliocene miocene boundary beneath the package and a 35 50 mgal peak to trough gravity anomaly over the sediment package are best matched with a flexural rigidity in the region of 5 6 10 22 1 5 10 23 n m effective elastic thickness t e 20 28 km the giant foresets are a particularly important load to model because unlike most sediment loaded passive margins there is a time gap of about 70 80 ma between rifting and subsequent deposition of the sedimentary load furthermore the width of the giant foreset load is sufficiently small to afford a reasonable resolution of t e

caldera formation by magma withdrawal from a reservoir beneath a volcanic edifice caldera formation has been explained by magma withdrawal from a crustal reservoir but little is known about the conditions that lead to the critical reservoir pressure for collapse during an eruption the reservoir pressure is constrained to lie within a finite range it cannot exceed the threshold value for eruption and cannot decrease below another threshold value such that feeder dykes get shut by the confining pressure which stops the eruption for caldera collapse to occur the critical reservoir pressure for roof failure must therefore be within this operating range we use an analytical elastic model to evaluate the changes of reservoir pressure that are required for failure of roof rocks above the reservoir with and without a volcanic edifice at earth s surface with no edifice at earth s surface faulting in the roof region can only occur in the initial phase of reservoir inflation and affects a very small part of the focal area such conditions do not allow caldera collapse with a volcanic edifice large tensile stresses develop in the roof region whose magnitude increase as the reservoir deflates during an eruption the edifice size must exceed a threshold value for failure of the roof region before the end of eruption the largest tensile stresses are reached at earth s surface indicating that faulting starts there failure affects an area whose horizontal dimensions depend on edifice and chamber dimensions for small and deep reservoirs failure conditions cannot be achieved even if the edifice is very large quantitative predictions are consistent with observations on a number of volcanoes

glacial isostatic adjustment i the forward problem summary the isostatic adjustment of a radially stratified visco elastic spheroid is treated using space time green functions for the associated surface mass load boundary value problem these impulse response functions are convolved with a heaviside function to give the time dependent deformation of the planet which would be produced by a unit point mass brought up from infinity at t 0 and allowed to remain on the surface the resulting heaviside green functions can be employed to simulate all of the important signatures of glacial isostatic adjustment given a space time dependent surface mass load consisting of ice sheet ablation histories and a model of the simultaneous filling of the ocean basins these source terms are simply convolved with the heaviside green function appropriate to a specific response signature a realistic model of the spatial distribution of the main late pleistocene ice loads and of their temporal disintegration is constructed the response of two visco elastic earth models is computed and compared to a global set of relaxation data relative sea level curves on the basis of this initial comparison of theory and observation the possibility that the lower mantle has a viscosity which is significantly in excess of the viscosity of the upper mantle is excluded in addition clear evidence of the presence of the lithosphere has been found in relaxation data from sites which were near the edge of the laurentide ice sheet such data should therefore prove useful as a basis for analysis of lateral variations in lithosphere thickness further extensions of the calculation are suggested

investigation of the near infrared spectral character of putative martian chloride deposits 1 putative chloride salt deposits observed throughout the southern highlands of mars by the compact reconnaissance imaging spectrometer for mars crism display featureless red slopes in near infrared ratio spectra it is hypothesized that the admixture of anhydrous chlorides or unoxidized sulfides with silicate rocks or minerals could imitate this spectral behavior three different sets of spectra were collected 1 simple mixtures of halite with labradorite and flood basalt at multiple concentrations 2 halite crusts formed on both labradorite and flood basalt and 3 simple mixtures of 25 and 50 wt acid washed pyrite with labradorite and flood basalt in all three instances multiple grain sizes were used to evaluate the effect of particle size on spectral results spectra of the mixtures and crusts were divided by pure labradorite and flood basalt spectra of the same grain size to produce ratio spectra comparable to the crism ratio spectra our study rules out pyrite as a possible component of these deposits whereas flood basalt mixtures with halite reproduced the observed red slope under some conditions this allows us to place some broad constraints on halite proportions and the effective grain sizes of these deposits

structural sequences and styles of subsidence in the michigan basin subsidence in the michigan basin produced 5 km of sedimentation over a period of more than 200 m y during paleozoic time utilizing well log correlations and constrained by compaction corrections and estimates of paleobathymetry we recognize four different styles of subsidence in the basin trough shaped regional tilting narrow basin centered and broad basin centered subsidence began as a trough shaped northerly extension of the illinois basin during late cambrian to early ordovician time this was followed by narrow basin centered subsidence in early to middle ordovician time basin centered subsidence ceased for 30 m y during long wavelength 1000 km eastward tilting in middle to late ordovician time a pattern incompatible with thermal contraction subsidence models basin centered subsidence resumed in silurian time but with a broader distribution a second episode of narrow basin centered subsidence occurred in latest silurian through middle devonian time and was replaced by broad basin centered subsidence at the end of middle devonian time the geometry of upper devonian and younger paleozoic deposits suggests another eastward tilting event but results remain inconclusive due to erosion of strata and uncertainties in their paleobathymetry in addition to these subsidence patterns two distinct unconformity styles are present basin wide and marginal erosion there is no evidence for significant basin centered unconformities as predicted by purely thermal mechanisms a history of episodic subsidence reactivations is interpreted as the result of a stress induced crustal weakening mechanism for the narrow basin centered subsidence whereas broad basin centered subsidence is interpreted as thermal contraction related to lower crustal attenuation during the narrow subsidence episodes recently proposed dynamic topography related to initiation of ordovician subduction provides a driving mechanism for long wavelength eastward tilting together with a temporal correlation to appalachian tectonism these mechanisms provide a plate tectonic framework for the history of the michigan basin

identifying goethite and hematite from rock magnetic measurements of soils and sediments we describe a sequence of experiments involving the acquisition of isothermal remanent magnetization irm in high 2 t fields followed by orthogonal demagnetization and the cooling and heating of remanent magnetization mr over the temperature range 196 c to 680 c the main aim of these experiments is to improve the basis for identifying the imperfect antiferromagnetic minerals contributing to the magnetic properties of natural soil and sediment samples which often contain complex mineral mixtures the procedures developed are first applied to a series of ore and synthetic samples of hematite and goethite each characterized by x ray diffraction xrd spectra and second to controlled mixtures of the two minerals the results confirm the mineral diagnostic nature of the experiments used they are subsequently used to identify goethite and hematite contributions to the rock magnetic properties of a variety of soils and recent sediments goethite can be unambiguously identified often accompanied by hematite in the majority of the samples studied despite indications from the synthetic mixtures that the ratio of goethite to hematite may have to exceed an order of magnitude before the goethite signatures are readily recognizable the rock magnetic tests are a much more sensitive basis for recognizing the presence of goethite in natural samples than are xrd analyses the results suggest that goethite s contribution to the rock magnetic properties of soil and sediment samples may often have been underestimated

dust sources in arid and semiarid china and southern mongolia impacts of geomorphological setting and surface materials abstract analysis of the geomorphological settings and surface materials in arid and semiarid china and southern mongolia revealed that the dominant dust sources are gobi deserts and alluvial fans adjacent to gobi deserts in the piedmonts of the kunlun qilian helan and gobi altai mountains these areas have persistent supplies of dust sized particles deposited by intermittent floods from the mountains and fine materials are also generated by weathering and aeolian processes acting on the coarse fractions of gobi surfaces clear spatial differences in mineral types and contents and in the salt contents of surface materials also strongly affected the intensity of dust emissions inconsistencies in monitoring simulation and calculation of dust emissions in previous studies of northern china and mongolia may have been caused by insufficiently detailed considerations of variations in the geomorphological setting where the geomorphological settings in these areas are not considered in sufficient detail deviations arise in the estimation and prediction of dust using current models although the present study did not quantify the persistence and replenishment of dust sources in the study area our conclusions about the importance of geomorphology can improve future studies of dust sources and controlling factors improve assessments of the effects of climate change on dust emissions and facilitate the interpretation of paleoclimate records

effects of the quaternary sea level change on the subsidence of a sedimentary basin a case study of the osaka bay sedimentary basin japan abstract surface load redistribution related to the quaternary ice age may play an important role on the formation of quaternary sedimentary basin the basin is in fact formed by tectonic processes but the quaternary sea level change may effect the subsequent evolution of the basin in particular the subsidence history thus the time dependent surface load and its concomitant sediment loading produce a maximum crustal subsidence of about 100 m if the following conditions are satisfied 1 maxwell relaxation time of relatively thin lithosphere is less than the period of the glacial and interglacial cycle of 10 5 yr and 2 sediments are sufficiently supplied into the deformed areas by the water loading crustal subsidence of 100 m corresponds to about 20 for the depression of osaka bay basin during the past 1 2 myr these two conditions seem to be satisfied in active island arcs with relatively weak lithosphere and osaka bay sedimentary basin formed in the quaternary is a suitable place to test this mechanism and or to examine its effect on the crustal subsidence related to the sedimentary basin

synsedimentary dissolution pipes and the isolation of ancient bacteria and cellulose coarse clear halite from synsedimentary dissolution pipes in the permian salado formation in southeastern new mexico usa yielded viable halotolerant bacteria and well preserved cellulose believed to be permian in age here we show that geologic and hydrologic conditions have isolated these rocks since permian time pipes were dissolved most likely along cracks created by thermal contraction or desiccation and the boundaries of salt polygons saucers on exposed salado formation salt pan surfaces down to the level of the water brine table macropores developed at the brine level in some horizons as the water level rose coarse halite cemented the open space fluid inclusions millimeter scale trapped bacteria which were probably in a spore state as well as cellulose inclusion water from pipes in some cycles may have isotopic values reflecting permian meteoric water while other cycles may show evaporated permian seawater exposure surfaces are more prominent in the upper part of salado formation depositional cycles due to basin desiccation as indicated by cracks and dish shaped laminae clays were concentrated on many exposure surfaces by floods wind and dissolution dissolution pipes formed from these surfaces and a synsedimentary age is confirmed by undisturbed overlying halite beds salado formation halite has very low permeability 10 22 m 2 effectively preventing significant fluid flow and passage of fluids through the formation either to recrystallize salt or to introduce modern bacteria stratigraphic relationships halite textures fluid inclusion chemistry and hydraulic properties are all consistent with a synsedimentary origin of the dissolution pipes their crystals and the bacteria and cellulose recovered from the salado formation

crustal structure and the mechanical properties of extended continental lithosphere in the valencia trough western mediterranean seismic refraction and reflection gravity and well data have been used to constrain the structure and evolution of the valencia trough a young extensional basin in the western mediterranean seismic refraction data reveal the present day crustal structure while backstripping of seismic reflection and well data reveal the structure at the time of rifting by comparing the seismically constrained crustal structure to that inferred from backstripping we have been able to evaluate whether the moho is in the expected position given the tectonic subsidence uplift and sediment loading history of the trough the best overall fit between the predicted and seismically constrained crustal structure in the trough is for a model in which the extended continental crust and lithosphere has acquired little or no strength following rifting a low strength is in accord with the observed free air gravity anomaly as well as results from other types of rift basins such as the north sea and the continental margin off the east coast of the usa there are discrepancies between the observed and predicted moho depths beneath the flanks of the valencia trough but they are attributed to either the removal of crustal material during or following rifting iberian peninsula or to orogenic loading of previously extended continental crust mallorca the largest discrepancy up to 10 km correlates with the ebro delta which has prograded into the trough by more than 60 km since the pliocene the discrepancy cannot be explained by the same large scale processes that modified the moho beneath the trough flanks and requires that the lithosphere beneath the ebro delta region is associated with a high 60 t c

fayetteville green lake new york u s a viii mass balance for 137cs in water varved and non varved sediments abstract a mass balance for 137 cs was done in fayetteville green lake 43 of total 137 cs input was found in shallow water 0 18 m sediments 16 was found in deep water varved sediments 9 was found in the water column and 32 remained to be carried away by lake outflow shallow water sedimentary 137 cs must be resuspended and or dissolved to maintain current levels of 137 cs in the water column and current fluxes of 137 cs to deep water varved sediments 137 cs activities in individual varve year samples from 1946 to 1976 followed closely the pattern of published bomb fallout records but the varves contained only 28 34 of cumulative unit area fallout for this area in order to preserve the yearly record of 137 cs sedimentation the pore water diffusion coefficient for 137 cs must be 11 cm 2 s 1 bioturbation and physical mixing of these sediments does not occur some of the cores of shallow water sediments accumulated 137 cs in amounts greater than was supplied by fallout to the lake surface area this may be due to 1 horizontal transport of surface water 137 cs to shallow water sediments for direct absorption 2 bioconcentration by benthic algae and mosses or 3 localized funnelling of sediments

thermal subsidence and generation of hydrocarbons in michigan basin temperature histories for selected stratigraphic horizons in the michigan basin are computed from three dimensional continually filled subsidence models mechanical evolution of the michigan basin is modeled as flexure of the lithosphere caused by thermal contraction results are compatible with the subsidence record of the sediments and free air gravity anomalies paleotemperature is determined from excess temperature due to the thermal anomaly plus burial temperature predicted from subsidence curves for an equilibrium temperature gradient of 22 c km 1 2 f 100 ft surface temperature of 20 c 68 f and an equilibrium surface heat flow of 1 1 hfu excess temperature paleotemperature and surface heat flow do not exceed 15 c 27 f 110 76 c 230 f and 2 5 hfu respectively these estimates are consistent with upper limits set by paleomagnetic studies the low value for excess temperature is caused by concentration of the thermal anomaly below 15 km 9 mi in agreement with gravity results the great depth of the thermal anomaly can explain the lack of evidence for an initial heating event prior to subsidence once the thermal history of the sediments is specified the oil potential of the basin can be determined from laboratory derived kinetic equations for degradation of kerogen to petroleum for the michigan basin predicted temperature conditions are sufficient for source rock 25 conversion of type i and type ii kerogen only in ordovician and older rocks in the southern peninsula of michigan by this model petroleum found in rocks younger than ordovician would have had to migrate upward from the older rocks geochemical studies of dundee devonian and trenton ordovician crude oils in michigan are compatible with this interpretation niagaran silurian crude oils appear to come from a different source than the dundee and trenton oils if their source is silurian rocks on the flanks of the basin either the scheme used to calculate maturity is not applicable to these oils or the temperature of the source rocks was significantly higher in the past than today more likely the model underestimates the depth of burial of silurian source rocks near the center of the basin especially in local regions of downfaulting

late permian non marine marine transitional profiles in the central southern permian basin northern germany the transition from rotliegend to zechstein within the southern permian basin is one from continental desert to a marine environment during the upper rotliegend ii a huge playa lake existed there this lake was temporarily influenced by precursors of the zechstein transgression therefore the mega playa evolved into a sabkha system one of these early marine ingressions is known from an outcrop in schleswig holstein laminated silt and claystones deposited within a standing water body are intercalated in siltstones of a salt flat environment the lake sediments are characterised by high frequency cyclicity shown by the sedimentary record and also by palaeontological data the section contains fresh water as well as brackish marine and marine fauna climatically forced cycles interact with marine incursions after the zechstein transgression had flooded the basin completely sedimentation was controlled by sea level fluctuations two sections in the southern north sea and in schleswig holstein are presented in this paper cyclicities with different frequencies controlled the sedimentation of the kupferschiefer t1 and the werra carbonate ca1 sediments of the north sea sequence were deposited within a shallow bay at the margin of an elevation therefore the high frequency cyclicity became obvious within the sedimentary patterns and in the faunal content

evolution of the lithosphere beneath the michigan basin using depths to formation tops from 480 wells changes in shape of the michigan basin during the phanerozoic are determined flexural rigidity of the lithosphere is then estimated for several time intervals by assuming that changes in shape were caused by deflection of an elastic plate subjected to an axisymmetric load it is found that the flexural rigidity generally increased over time from less than 10 22 n m early in the basin s history to more than 10 23 n m late in the basin s development this increase in rigidity is attributed to cooling and thickening of the elastic portion of the lithosphere as the basin was subsiding low flexural rigidity early in the basin s history indicates that the lithosphere was probably rejuvenated heated thinned and possibly uplifted prior to basin subsidence the location of the load remained approximately fixed over time the rejuvenation process is investigated using a one dimensional finite difference thermal model in which a 142 km plate is temporarily heated from below taking the depth to the 450 c isotherm to be the effective elastic thickness of the lithosphere rigidity predicted by the thermal model is consistent with the observed increase in rigidity over time furthermore phase changes produced by heating during thermal rejuvenation combined with the predicted thermal contraction following rejuvenation explain the two episodes of subsidence observed in the michigan basin we conclude that the lithosphere in this area was rejuvenated approximately 500 m y ago as the lithosphere cooled it thickened it also contracted and subsided temperatures near the surface of the crust apparently were not raised sufficiently by lithospheric rejuvenation to alter the keweenawan and older radiometric and paleomagnetic dates of basement rocks

coeval versus reciprocal mixed carbonate siliciclastic deposition middle devonian baltic basin eastern europe implications from the regional tectonic development the middle devonian narva succession in the baltic basin represents a significant turnaround in the history of the basin the detailed study of core and outcrop sections and the three dimensional correlations across the baltic basin reveal a carbonate dominated mixed retrogressive succession overlain by a siliciclastic dominated progradational succession the palaeogeographic reconstructions show how the shallow tide influenced basin expanded from south west to north east and later during the transgression also to the north south and east the transgressive portion of the basin fill is dominated by carbonate rich sabkha and supratidal to intertidal deposits on the basin margins and subtidal carbonates in the basin centre siliciclastic material was derived by tidal currents and storm waves from the south west through a tidal inlet and flood tidal delta complex this initial transgressive phase is characterized by the lack of subsidence or even episodic uplifts in the northern north western part of the basin margin shown by convergence of timelines and the thin 30 m transgressive succession in contrast on the southern margin the facies associations stack vertically into a 70 to 80 m thick succession indicating significantly higher subsidence rates the upper part of the transgressive phase indicates subsidence across the whole basin the upper progradational portion of the basin fill is dominated by coarse siliciclastic tide influenced deltaic deposits that rapidly prograded from north west to south east this detailed study on the narva succession shows that siliciclastic and carbonate deposition was coeval and that mixing occurred at different temporal and spatial scales the mixing was controlled by grain size volume and location of siliciclastic input rather than relative sea level changes as suggested in widely used reciprocal mixing models it is suggested that the forebulge of the scandinavian caledonian fold and thrust belt migrated to the north western margin of the baltic basin during the earliest eifelian as indicated by the lack of subsidence and probable uplift in the northern north western margin during the early transgressive phase the forebulge migration ceased although the forebulge had already started to subside during the later stages of the transgressive phase the deltaic progradation is interpreted to be associated with the orogenic collapse and uplift in the scandinavian caledonides that caused the erosion of the foreland basin fill and the coarse sediment transport into the baltic basin

quantification of carbonate ramp sedimentation and progradation rates for the late holocene abu dhabi shoreline abstract we utilize new radiocarbon dates and detailed sedimentological logging of the abu dhabi sabkha to produce the first well constrained internally consistent rates for late holocene carbonate ramp sediment accumulation and progradation for the southern shore of the persian gulf these data are applied to test the hypothesis that in low angle carbonate ramp systems ramp geometry is the primary control on sedimentation and progradation rates and therefore estimates acquired from other depositional environments cannot be applied to low angle carbonate ramp systems average calculated progradation rates of 0 75 m yr are higher than progradation rates proposed for other late holocene and recent carbonate settings a relationship that is explained by the lack of accommodation space in the low angle laterally extensive ramp system in the sedimentary record such relatively high progradation rates would produce laterally extensive lithologically uniform units with little or no evidence of progradational features and would appear to be largely contemporaneous across their extent the total average sediment accumulation rate of 0 029 cm yr is lower than those proposed from most other late holocene to recent shallow water carbonate depositional environments again a relationship interpreted to result from the limited accommodation space of low angle carbonate ramp systems calculated sediment accumulation rates show little variation over the past 1 400 years yet the calculated progradation rates exhibit a sharp decline from a maximum of 1 03 m yr before 677 946 cal bp to 0 70 m yr afterwards this is interpreted to reflect a rise in relative sea level or distal steepening of the ramp system applying the progradation rates proposed in this paper the lagoon system in the study area would be completely infilled in less than 4 000 years calculated sediment accumulation rates for syndepositional and early postdepositional interstitial displacive evaporite formation are higher than those for carbonate and microbial mat intervals continued precipitation of evaporite minerals during burial would result in a further increase in their relative contribution to the sediment pile conversely compaction particularly of microbial mat intervals will result in a decrease in apparent sediment accumulation rates for non evaporite intervals these processes need to be considered when modeling the basin evolution of mixed evaporite carbonate sequences in the stratigraphic record during calculation of sedimentation rates it is important to consider both a correction for compaction and a correction for evaporite formation

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

synsedimentary brecciation in the eifelian middle devonian baltic basin sudden catastrophe or diagenetic collapse the eifelian sedimentary record in the epeiric baltic basin contains an extensive brecciated interval known as the narva breccia during recent decades the origin of this breccia has been attributed to 1 palaeoseismicity 2 extraterrestrial impact event or 3 diagenetic collapse our studies show the presence of two different breccia types that form 11 stratigraphic horizons and contain several laterally and stratigraphically unrelated breccias the occurrence of type 1 and type 2 breccias together with carbonate sabkha and carbonate inter to supratidal tidal flat deposits on the basin margins indicates synsedimentary brecciation formed in a sabkha environment type 2 breccia could also originate from wave cycling loading

consortium for continental reflection profiling michigan surveys reprocessing and results reprocessing of consortium for continental reflection profiling data from central michigan with emphasis on deconvolution and velocity filtering before stack and more detailed velocity analysis has resulted in greatly improved imaging of a previously reported keweenawan rift buried beneath the paleozoic of the michigan basin the seismic stratigraphy of the sub paleozoic units can now be related to the basin filling middle keweenawan volcanic sequence and the upper keweenawan clastic assemblage in greater detail important structural and stratigraphic features can be seen for the first time for example reprocessing has imaged an unconformity within the upper keweenawan it is possible some of these newly imaged features are associated with a late keweenawan compressive episode furthermore reprocessing also reveals previously unrecognized sub keweenawan structures including a deeply penetrating reflection which may represent a major crustal fault and possible weak reflections from midcrustal 25 km and moho 43 km depths

some consequences of volcanic edifice destruction for eruption conditions abstract destruction of a volcanic edifice by landslides or phreatic explosions unloads the upper crust induced changes of stress field around and of magmatic pressure within a magma reservoir are investigated with an analytical model for the deformation of a liquid filled cavity within an elastic half space unloading affects the reservoir pressure and hence the net result depends on how the liquid filled reservoir responds to a change of remote stress magma compressibility is taken into account and may dampen changes of internal pressure in small volatile rich reservoirs the main consequence of edifice destruction is a decrease of magmatic pressure and stresses on the reservoir walls in some cases this may be responsible for dyke closure at the reservoir walls which stops magma withdrawal and may prevent eruption if an eruption does occur edifice destruction affects the volume of magma erupted depending on edifice size magma reservoir size and depth the erupted volume may be smaller or larger than that which would be erupted with no damage to the edifice these results suggest that major phreatic explosions may prevent magmatic eruptions

mid proterozoic dolomitic varves and microcycles from the mcarthur basin northern australia abstract the middle proterozoic 1 75 ga wollogorang formation of the unmetamorphosed intracratonic mcarthur basin of northern australia contains a dolomitic black shale facies that has distinctive millimetre and sub millimetre scale bedding laminations and microcycles of alternating organic rich mudstone and organic poor dolostone the microcycles are between 1 and 20 mm thick and their internal laminations are arranged rhythmically into a lower subdivision of mainly dolostone with subordinate mudstone laminae and an upper subdivision of mainly mudstone with thin dolostone laminae the microcycles vary systematically in both thickness and character the thinnest laminations visible around 0 2 mm thick are interpreted as chemical varves the microcycles represent bundles of varves deposited over longer periods over these longer periods each microcycle shows a gradual upward decrease in the amount of dolomite precipitated and or preserved this is related in part to original climatic controls in the environment of deposition and also to a secondary but pre lithification process of dissolution similar to that invoked by trendall 1972 1973 for the formation of mesobands in the bif s of the hamersley basin western australia simple statistical calculations of varve thickness variations on about 100 microcycles from a 1 5 m long core indicate the presence of 7 to 11 year and 300 year periods these periods are comparable with those published by williams 1981 for glaciogenic varves from late proterozoic 680 ma rocks in south australia he relates the former to sunspot cycles and the later to longer term climate periods as evident from tree ring studies although nowhere near as conspicuous as the sunspot cycles described by williams 1981 or the double sunspot cycles of trendall 1972 from the latest and earliest proterozoic respectively these microcycles from the middle proterozoic may be due to similar sunspot related processes and therefore add support to their suggestions for long term stability of solar processes

a model of thick plate deformation and its application to the isostatic movements due to surface subsurface and internal loadings summary a model of thick plate deformation is presented without any restriction on stress or strain conditions in the plate an analytical solution is given for the vertical displacement of a thick elastic plate the present method is exact and applicable to 3 d plate deformation problems associated with surface subsurface and internal loadings the conventional thin plate theory which ignores the variation of vertical strain of a plate may not be appropriate to be used as a basic model to study deformation problems of the continental lithosphere since it is shown that the actual surface isostatic adjustment due to surface loading could reach an amount twice as much as predicted by thin plate theory even on a relatively thin plate with 30 km thickness and young s modulus e lo00 kbar and poisson s ratio y 0 25 the isostatic adjustment due to surface load depends on the stress condition at the lower boundary of the elastic lithosphere if there is an active mantle upwelling or downwelling force acting on the boundary then the isostatic adjustment depending on the magnitude of the mantle force is either greatly hindered or amplified to a degree that is beyond the normal prediction of isostatic movements in addition the viscous effect of mantle material on the isostatic movements is shown to be negligible on geological time scales if the mantle viscosity is below ld4poise it is also shown that the airy isostasy of the total sediment load in a surface depression with small wavelength could be achieved only if a considerable period of geological time is allowed for the stress relaxation of viscoelastic or viscous material in the lithosphere the rheological property of the continental lithosphere has a significant effect on basin subsidence caused by sediment loading alone while pure elastic lithosphere restricts surface basin development because of the strong basement support throughout the evolution history

relaxation of continental lithosphere an explanation for late cretaceous reactivation of the sabine uplift of louisiana texas with few unconstrained assumptions a simple quantitative model for flexural isostasy between two crustal blocks of different thickness that subsequently undergoes partial relaxation of accumulated stress can explain reactivation of the sabine uplift in the mid cretaceous thin salt over the sabine uplift indicates that it was a positive area in the middle jurassic but began to subside during the late jurassic during the late jurassic and early cretaceous the sabine uplift had no topographic expression and was the center of a large flat bottomed basin covering most of east texas and north louisiana reactivation of the sabine uplift in the mid cretaceous caused a minimum of 150 m of uplift and extensive erosion of lower cretaceous rocks during the late cretaceous and tertiary the sabine uplift resumed subsiding but at a slower rate than the adjacent basins since the mid cretaceous the sabine uplift has risen more than 1 km relative to the east texas basin we suggest that this structural history is consistent with the following simple quantitative model for flexural isostasy during formation of the gulf of mexico in the late triassic early jurassic by rifting and extension of the lithosphere brittle deformation of the crust created small scale wavelength of tens of kilometers lateral variations in crustal thickness initially the newly formed margin was in point wise airy isostatic equilibrium thus synrift and or early postrift subsidence was characterized by a series of rift valleys or half grabens separated by uplifts however as the margin cooled and contracted the lithosphere became stronger and subsequent loading was regionally compensated thus postrift subsidence was characterized by a broad regional downwarp if at some later point in time the rigid portion of the lithosphere was weakened or relaxed then lateral density variations would have been recompensated at shorter wavelengths thus areas of little or no crustal extension would have been uplifted and areas of high extension would have experienced additional subsidence gravity and total tectonic subsidence interpretations agree that the sabine uplift region is underlain by thicker crust than the adjacent east texas and north louisiana basins possible mechanisms for relaxation of continental lithosphere are thermal rejuvenation horizontal in plane stress viscous relaxation and extensional faulting

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

new insights into the basement structure of the west siberian basin from forward and inverse modeling of grace satellite gravity data 1 the oil and gas rich west siberian basin is underlain by a layer of flood basalts of late permian triassic age that are coeval with the siberian traps the extent and thickness of the basalts are unknown but knowing their thickness is important for discussions on the end permian mass extinction because basalt volume constrains estimates of emitted volatiles we have used grace satellite and terrestrial gravity data to study the structure of the crust and basalt distribution published seismic sections are used to constrain the sediment isopachs and to estimate a depth density function we use published models of crustal thickness and basement depth to reduce the observed gravity field to the basement level the resulting three dimensional density model gives information on density anomalies in the lower crust and upper mantle and on the basalt thickness we identify several rift graben structures that are presumably filled with basalt the lower crust below the west siberian basin shows considerable density variations and these variations allow the region to be divided into four major blocks the eastern part of the basin toward the siberian platform shows an arch shaped density increase in the lower crust that is accompanied by a linear high density anomaly at shallower depths our work demonstrates the way in which the grace gravity field can be applied to map geological structures like buried rifts and large basins the same techniques can be used for other large remote basins such as those in cratonic south america

characteristics of sedimentary varve chronologies a review abstract we constructed a varve data base vdb to study the worldwide distribution of varved sediment records and the fidelity of their varve chronologies clusters of lakes with varved sediments exist in central north america in the canadian arctic and in northern and central europe among the 108 sites included in the vdb varved records are typically 200 500 cm long and cover a period of 1000 2000 years their varve chronologies are often based on counting of annual layers from fresh sediment surfaces and photographs or from epoxy embedded sediment blocks and thin sections the vdb indicates that chronological errors associated with counting sedimentary varves fall generally between 1 and 3 but only 57 of published records are providing quantitative error estimations for their chronologies with a careful documentation of varves and by applying radiometric dating methods to surface sediments as well as using historical events as time markers there is the potential of having centennial long varved records with a very precise age depth control however it is unrealistic to expect errors significantly below 1 from several millennia long sections we found no indication that varve chronologies would have been substantially more accurate and precise in some parts of the world than in others or statistically dependent on varve thickness or temporal extension of a varved sequence according to the vdb close to 90 of the published varve chronologies have been cross checked with some independent dating methods

constraints on the structure of brooks range and colville basin northern alaska from flexure and gravity analysis tectonic thickening of the crust within the adjacent brooks range fold thrust mountain belt has been suggested as a probable cause for rapid subsidence of the colville basin in the early cretaceous a simple mechanical model in which the response of the lithosphere to loading was approximated by a semi infinite broken elastic plate overlying a weak fluid was used to analyze deflection of the basement beneath the colville basin comparison between the calculated and observed depth to basement suggests that the emplacement of thrust sheets onto the lithosphere during mountain building cannot produce a load sufficient to create the observed basin geometry and observed topography regardless of the position of the slab end or flexural rigidity assumed for the lithosphere an additional subsurface load capable of producing a deflection of the foreland basin roughly equal to the depression caused by the topographic load of the brooks range is required bouguer gravity data from the region are characterized by an asymmetric gravity low which spans the brooks range and colville basin and a gravity high to the south gravity modeling is consistent with a subsurface load with a mass of approximately 2 5 1013 g cm located beneath the southern brooks and the yukon koyukuk province to the south the origin of the subsurface load is interpreted as obduction of crustal blocks onto an old rifted continental margin during attempted continental a type subduction the colville basin was also influenced by the contemporaneous development of the northern arctic continental margin loading by subsurface and topographic loads to the south and loading on the continental margin to the north resulted in the formation of the barrow arch just offshore of northern alaska a best fitting value for mechanical thickness of the lithosphere beneath the colville basin is approximately 65 km

lithosphere structure beneath the phanerozoic intracratonic basins of north america four intracratonic basins of north america the hudson bay michigan illinois and williston basins have similar ages and are close to one another yet they exhibit different subsidence histories characterised by different time scales and sediment thicknesses they can be explained by local lithosphere thinning and by the cooling of the induced thermal anomaly within the framework of 1d thermal models for vertical heat transport each basin requires a different lithosphere thickness or a different boundary condition at the base of the lithosphere heat flow and seismic studies show that beneath the north american craton the lithosphere is too thick for the assumption of purely vertical heat transfer to be valid thermal models are developed to account for finite thermal anomaly width and for two types of basal boundary conditions fixed temperature or fixed heat flux different subsidence histories are explained by deep lithospheric anomalies of different sizes the stability of thick continental roots requires the mantle part of the lithosphere to be compositionally buoyant with respect to normal convecting mantle localised lithospheric thinning due for example to plume penetration results in the emplacement of compositionally denser mantle into the lithosphere this represents a load which drives permanent flexure the cooling time and the characteristics of flexure allow constraints on the dimensions of these deep lithospheric anomalies there are no solutions for lithosphere thicknesses less than 170 km the williston and illinois basins are associated with wide v200 km and thin anomalies v100 km whereas the michigan and hudson bay are located on top of narrow v100 km and tall v200 km anomalies fl 2000 elsevier science b v all rights reserved

plate tectonics and basin subsidence history tectonic setting exerts fi rst order control on basin formation as refl ected in basin subsidence history while our approach ignores the effects of fl exural loading and eustatic sea level change consistency of backstripped subsidence histories i e with local loading effects of sediment removed suggests consistent tectonic driving mechanisms in each tectonic setting with the possible exception of forearc basins based on published subsidence curves and open fi le stratigraphic data we show the subsidence characteristics of passive margins strike slip basins intracontinental basins foreland basins and forearc basins passive margin subsidence is characterized by two stages rapid initial synrift subsidence and slow post rift thermal subsidence with increasing subsidence rates toward the adjacent ocean basin subsidence of intracontinental basins is similar in magnitude to that seen in passive margin settings but the former is generally slower longer lived and lacks initial subsidence long lived subsidence for many intracontinental basins is consistent with cooling following thermal perturbation of thick lithosphere found beneath old parts of continents basins associated with strike slip faults are usually short lived with very rapid subsidence changes in local stress regimes as strike slip faults evolve and migrate over time coupled with three dimensional heat loss in these small basins likely explain this subsidence pattern foreland basin subsidence rates refl ect the fl exural response to episodic thrust loading resultant subsidence curves are punctuated by convex up accelerating segments forearc basins have the least consistent subsidence patterns subsidence histories of these basins are complex and may refl ect multiple driving mechanisms of subsidence in forearc settings second order deviations in subsidence suggest reactivation or superimposed tectonic events in many basin settings the effects of eustatic sea level change may also explain some deviations in curves for many of these settings subsidence histories are suffi ciently distinctive to be used to help determine tectonic setting of ancient basin deposits

annually laminated lake sediments and the study of quaternary environmental changes a review abstract annually laminated lake sediments non glacial varves have in recent years been discovered in increasing abundance especially since the development of in situ freezer samplers so far the majority of examples have been found in three main areas eastern north america northern and central europe and east and central africa a much more widespread distribution seems highly probable however the formation of annual laminations in freshwater lakes is governed by a number of factors the most important of which appears to be morphometry seasonality of sediment supply which may be controlled by physical biological or even cultural processes in the lake or in its catchment is also clearly significant four main types of laminations here termed ferrogenic calcareous biogenic and clastic have so far been recorded although it is quite clear that others exist special field and laboratory techniques are needed in order that the particular properties of varved sediments may be fully exploited annually laminated sediments have so far been applied to the investigation of a number of aspects of the quaternary these include the study of a geochronology b climatic change c vegetation history especially the role of fire in forest ecosystems d the calculation of rates of sediment influx to lakes and of rates of erosion from their catchments and e the monitoring of processes in the modern environment such as heavy metal pollution eutrophication and acid rain annually laminated lake sediments combine an internal independent chronology with the preservation of discrete visible increments of seasonal and annual sedimentation given this unique property and the likelihood that many more examples will be discovered the prospects for further applications of studies of these sediments to the investigation of the quaternary seem very good indeed

a 3 d backstripping method and its application to the eromanga basin in central and eastern australia summary a general 3 d backstripping method based on a precise model of plate deformation is developed for analysis of the tectonic history of sedimentary basins and it has been applied to the eromanga basin in central and eastern australia the 3 d method is more appropriate when dealing with the subsidence history of basins since it takes into account both the flexural effect of surface load and the stress relaxation in the lithosphere due to viscous creep of mantle material furthermore this method is more accurate than conventional thin plate models it is shown that the 3 d model provides a different yet simple and clear evolving picture of the areal extent of the tectonic subsidence of basins the 3 d analysis results show that the formation of the eromanga basin is characterized by a gradual increase in the extent of sedimentation with localized subsidence from the late triassic to the early cretaceous and a large extension of sedimentation with rapid subsidence during the early to middle cretaceous followed by local and gradual subsidence after the middle cretaceous

phase changes and thermal subsidence in intracontinental sedimentary basins summary a model of tectonic subsidence is developed to explain the late acceleration of subsidence observed in some intracratonic sedimentary basins the proposed mechanism combines the effect of thermal contraction of an initially hot lithosphere with the effect of a subcrustal phase transformation that moves under changing pressure and temperature conditions the subsidence following a sudden change in temperature at the base of the lithosphere is calculated the calculations show that 1 phase changes if present and activated contribute substantially to the subsidence of sedimentary basins 2 because the effect of phase change is delayed subsidence accelerates after a time on the order of 20 myr and 3 the duration of the subsidence is on the order of 100 to 150 myr during the late stages of subsidence the phase change is the dominating mechanism an application to the michigan basin is presented the calculated sediment accumulation history fits the record well when the effect of sea level changes is included in the model

oil generation in the michigan basin a biological marker and carbon isotope approach abstract palaeozoic strata in the michigan basin produced crude oils which despite their rather small volume and economic value are interesting because of the considerable age of the basin and because of the opportunity to study generation and migration of oils within a relatively simple but ancient geological setting based on n alkane profiles biological marker distributions and carbon isotope ratios the oils belong to three main families of different genetic origin and a few less important mixed types silurian oils from salina and niagara limestones have broad n alkane distributions and abundant isoprenoid hydrocarbons a strong phytane over pristane predominance and the lack of diasteranes indicate a carbonate source for these oils chemical maturity parameters show that they are more mature than the oils from the other main families from which they are also clearly distinguished by carbon isotope ratios of hydrocarbon fractions and single n alkanes oils found in the calcareous ordovician trenton formation contain n alkane cyclohexylalkane and alkyl phenanthrene distributions typical of immature oils many oils from the devonian dundee reservoirs are very similar in overall composition to the trenton oils devonian traverse oils are considered to be mainly from a devonian source with some contribution of ordovician type oil and are of intermediate maturity based on calculations using kinetic parameters of biological marker reactions considerably deeper subsidence of the devonian source rock in the past is implied

annually laminated lake sediments and environmental changes in bashang plateau north china abstract angulinuo lake is the biggest lake on the bashang plateau north china and is 47 6 km2 in area and 2 6 m in depth a core from the inner part of angulinuo lake was sliced and the sediment was observed by scanning electronic microscope sem annual laminations characterized by variable color and grain size were found and interpreted as recording the cyclic deposition of lacustrine clay and aeolian dust the results of chemical analysis of coarse grains conducted by sem edax and grain size analysis of modern aeolian dust in the ice on angulinuo lake support an aeolian origin for the light coarse layers image analysis technique was used to calculate the size and number of coarse grains in each layer the coarse grains were fractionated into four classes 42 m 14 42 m 14 4 2 m and 1 4 4 2 m in general the abundance of the four classes shows similar temporal variation patterns around angulinuo lake the winter monsoon is strong and transports aeolian dust into the lake when the winter monsoon is strong the size and amount of coarse grains are expected to increase we infer that the winter monsoon was weaker during 8430 5440 year bp and was unstable in the later part of this period from 5440 year bp the winter monsoon became stronger and then weaker from 3250 to 2490 year bp during 2490 1170 year bp the winter monsoon was slightly stronger but since 1170 year bp it has become weaker again the changes of the winter monsoon intensity recorded in the annual laminations in angulinuo lake sediments correspond well to environmental changes in north china and to changes in sea level during the same period periods of weaker winter monsoon correspond to times of higher sea levels while the periods of stronger winter monsoon correspond to the neoglaciation stage in china and the periods of lower sea levels

distribution of palaeosols and deposits in the temporal evolution of a semiarid fluvial distributary system bauru group upper cretaceous se brazil abstract the stratigraphic and sedimentological knowledge of the bauru group upper cretaceous se brazil is still generally insufficient and controversial a sedimentological and palaeopedological study allowed to interpret the south eastern portion of the bauru group according to the model of a fluvial distributary system this work has two objectives 1 to include palaeosols in the interpretation of a fluvial distributary system and 2 to give detailed information on the sedimentological and stratigraphic features of the se portion of the bauru group in order to support biostratigraphical taphonomic and palaeoecological studies in the south eastern portion of the bauru group three genetic stratigraphic units were described and interpreted here informally called lower intermediate and upper units the lower unit is constituted of muddy sandstone salt flat deposits and sandstone sheet deltas deposits and is interpreted as a basinal part of a fluvial distributary system the intermediate unit is formed of very fine to fine grained sandstone filled ribbon channel and sandy sheet shaped beds suggesting a distal or medial portion of a fluvial distributary system the upper unit does not match with the present models of the fluvial distributary system because mostly constituted of moderately developed well drained medium to fine grained sandstone palaeosols which testify pauses of sedimentation to the order of 104 years preserved features of sedimentary structures suggest that the parent material was formed by occasional catastrophic unconfined flows this unit may represent the most distal portion of a fluvial distributary system generated by retrogradation of the alluvial system due to aridification of the climate the upper unit may be interpreted also as proximal portion of fluvial distributary system if considering the coarser grained and the well drained palaeosols however the absence of channel deposits makes this interpretation unconvincing

thermal history of michigan basin the elevated organic maturity observed in shallowly buried units from the michigan basin implies that higher temperatures and thicker overburdens once existed in the basin evidence from sediment accumulation rates regional dips and maturity of pennsylvanian age coals suggests that up to 1 000 m 3 280 ft of sediment were removed by erosion prior to the late jurassic when the basin became stable geothermal gradients during paleozoic basin subsidence probably ranged from 35 to 45 c km 19 to 25 f 1 000 ft in contrast to the average present value of 25 c km 14 f 1 000 ft depths to the top of the oil window ranged from 1 900 to 2 300 m 6 230 to 7 540 ft during the paleozoic post pennsylvanian erosional uplift and further thermal m turation of the basin have combined to raise the top of the oil window to its present level of 500 m 1 640 ft

temperature seasonality control on modern halite layers in the dead sea in situ observations layered halite sequences are found in deep basins throughout the geological record however analogs for such sequences are commonly studied in shallow environments here we studied active precipitation of halite layers from the only modern analog for deep halite precipitating basins the hypersaline dead sea in situ observations in the dead sea link seasonal thermohaline stratification halite saturation and the characteristics of the actively forming halite layers the spatiotemporal evolution of halite precipitation in the dead sea was characterized by means of monthly observations of 1 lake thermohaline stratification temperature salinity and density 2 degree of halite saturation and 3 textural evolution of the active halite deposits we present the observed relationships between the textural characteristics of layered halite deposits i e grain size consolidation and roughness and the degree of saturation which in turn reflects the limnology and hydroclimatology the lake floor is divided into two principal environments a deep hypolimnetic lake floor and a shallow epilimnetic lake floor in the deeper hypolimnetic lake floor halite continuously precipitates with seasonal variations 1 during summer consolidated coarse halite crystals form rough surfaces under slight supersaturation 2 during winter unconsolidated fine halite crystals form smooth lake floor deposits under high supersaturation these observations support interpretations of the seasonal alternation of halite crystallization mechanisms the shallow epilimnetic lake floor is highly influenced by the seasonal temperature variations and by intensive summer dissolution of part of the previous year s halite deposit which results in thin sequences with annual unconformities this emphasizes the control of temperature seasonality on the characteristics of the precipitated halite layers in addition precipitation of halite on the hypolimnetic floor at the expense of the dissolution of the epilimnetic floor results in lateral focusing and thickening of halite deposits in the deeper part of the basin and thinning of the deposits in shallow marginal basins

gravity anomalies and flexure of the lithosphere at the middle amazon basin brazil the middle amazon basin is a large paleozoic sedimentary basin on the amazonian craton in south america it contains up to 7 km of mainly shallow water sediments a chain of bouguer gravity highs of approximately 40 to 90 mgals transects the basin roughly coincident with the axis of maximum thickness of sediment the gravity highs are flanked on either side by gravity lows of approximately 40 mgals the observed gravity anomalies can be explained by a steeply sided zone of high density in the lower crust varying in width from 100 to 200 km wide within this region the continental crust has been intruded replaced by more dense material to more than half its original thickness of 45 50 km the much wider sedimentary basin results from regional compensation of the subsurface load and the subsequent load of accumulated sediments by flexure of the lithosphere the observed geometry of the basin is consistent with an elastic lithosphere model with a mechanical thickness of 15 20 kin although this value is lower than expected for a stable cratonic region of early proterozoic age it is within the accepted range of effective elastic thicknesses for the earth rapid subsidence during the late paleozoic may be evidence of a second tectonic event or lithospheric relaxation which could lower the effective mechanical thickness of the lithosphere the high density zone in the lower crust as delineated by gravity and flexural modeling has a complex sinuous geometry which is narrow and south of the axis of maximum sediment thickness on the east and west margins and wide and offset to the north in the center of the basin the linear trough geometry of the basin itself is a result of smoothing by regional compensation of the load in the lower crust

annual laminations in the sediments of loe pool cornwall annually laminated freshwater lacustrine sediments have been recorded at several sites in central europe1 5 north america6 13 and fennoscandia14 24 the presence of laminations may reflect either 1 regular changes within the lake ecosystem itself or 2 variation in the intensity of erosion and transport of material from the catchment particularly where instability in the lake watershed system has occurred as a result of human activities22 the principal cause of lamination is therefore seasonal variation of environmental conditions particularly climate lakes with laminated sediments tend to be physically deep exhibit a strong seasonal stratification and be situated in areas of continental climate we describe here what we believe to be the first reported instance of a long sequence of laminated lake sediments from great britain unlike most of the previous examples these have been formed in a shallow polymictic lake in an oceanic climate

thermal contraction and flexure of intracratonal basins a three dimensional study of the michigan basin summary cooling of continental lithosphere by thermal conduction to the surface following a heating event imposes a load on the lithosphere and causes subsidence as the basement rocks contract accumulation of sediments in the resulting depression forms a sedimentary basin studies of the geometry of sedimentary basins with horizontal dimensions of a few hundred kilometres suggests the lithosphere responds to loads by regional flexure of a strong elastic or viscoelastic upper lithosphere two and three dimensional models for flexure of the lithosphere owing to thermal contraction are applied to the michigan basin 2 d flexural models are compatible with previously estimated rheological parameters for an elastic lithosphere an effective flexural rigidity of about 5 1029 dyne cm can explain observed gravity anomalies and structural contours across the michigan basin a viscoelastic lithosphere requires an effective flexural rigidity of about 1032 dyne cm for a viscosity of 1025 poise for a 3 d elastic or viscoelastic lithosphere effective flexural rigidity must be approximately an order of magnitude smaller 2 1028 and 1031 dyne cm respectively larger values leave an excessive remaining load at the centre of the basin which is not indicated by observed gravity anomalies the decrease in effective flexural rigidity is attributed to greater difficulty in equi dimensional bending of a 3 d plate the magnitude and spatial distribution of the computed thermal contraction load are consistent with the presumed mechanism as the thermal anomaly of the initial heating event was present mainly at great depths in the lithosphere the lack of geological evidence for an initial heating event prior to subsidence is not a fatal objection to the thermal contraction mechanism of subsidence a driving load presumably from subsurface processes including crustal stretching and emplacement of dense rocks into the crust is necessary to explain the net subsidence of the basin from the time before the heating event until the lithosphere beneath the basin cooled without this driving load the thermal expansion would produce uplift but the surface would subside only back to sea level after the lithosphere cooled theoretical gravity results indicate that the driving load is centred at a depth of approximately 15 km deviations in subsidence curves from exponentials associated with thermal contraction can be explained by changes in sediment supply spatial variations in sediment load caused by regional facies changes produce migration of the centre of maximum deposition water and basement depths are determined for sequential time intervals during basin development the predicted depositional environments are consistent with lithofacies maps of the middle devonian

taphonomic controls on animal tracks at saline alkaline lake bogoria kenya rift valley impact of salt efflorescence and clay mineralogy abstract the morphologies of subaerial vertebrate tracks and invertebrate traces in surficial sediments can be used in paleoenvironmental reconstructions by providing details about the initial substrate conditions e g water content and any changes experienced during trace preservation a qualitative field study was undertaken to identify the factors affecting track taphonomy in saline lake margin environments supported by experiments to isolate selected factors that modify track morphology and substrate characteristics flamingo tracks ubiquitous around saline alkaline lake bogoria in the kenya rift were the focus of field study although other vertebrate tracks and surficial invertebrate trails and tunnels were also considered a complex and interacting set of environmental factors were identified such as the effect of lake level fluctuations on substrate grain size and capillary evaporation several factors affect the early taphonomic modification destruction stabilization and or cementation of tracks and their associated substrates the laboratory experiments tested the effects of pore water salinity clay mineralogy e g swelling or non swelling clays drying method e g solar radiation or wind and wetting and drying cycles on tracks these data were supported by statistical analyses of impression depth and planimetric area the results show that track morphology in smectitic substrates is altered rapidly by wetting and drying particularly if the substrates contain saline pore waters but morphology is less affected in non swelling clays the degree of morphologic alteration and or resistance to wetting and drying varies with the rate of substrate drying and the type of the resulting salt efflorescence e g interstitial vs surficial crust these short term process related taphonomic effects help to explain the morphologic variants of tracks in lake marginal settings and provide insights into the preserved distribution patterns of subaerial ichnofossil assemblages

fayetteville green lake new york u s a abstract a varve chronology extending back to 1900 with dated stratigraphic markers has been established for deep water sediment in the northern part neck of meromictic fayetteville green lake and correlated to a similar chronology established earlier in the deeper southern end of the lake main basin precise estimates of net annual varve sedimentation rates are given for each year between 1938 and 1977 for the neck these rates are 27 higher than those previously reported in the main basin and range between 454 and 1090 g m 2 yr 1 varve sedimentation rates have significantly increased throughout the lake during the last 30 years with no changes in the relative ratio of varve sedimentation rates in the neck and the main basin although there is considerable year to year variation in this ratio higher varve sedimentation rates occur in shallower portions of the permanently stratified water probably because resuspension is unlikely in this low energy environment because of basin morphometry and meromixis sediment focusing creates higher varve sedimentation rates in relatively shallow water near the upper limit of the permanently stratified water rather than in the deepest water

climatic and geochemical implications of archean pedogenic gypsum in the moodies group 3 2 ga barberton greenstone belt south africa abstract lithic sandstones of braided fluvial to supratidal facies in the paleoarchean moodies group 3 22 ga barberton greenstone belt south africa include several regionally traceable units with common to abundant in places rock forming nodular concretions of megaquartz pseudomorphs after gypsum barite and calcite concretionary accumulations are stratiform and commonly associated with aqueously reworked fine grained tuffaceous sediment of originally rhyodacitic composition and can grow to fist sized agglomerates in crusts tens of m in lateral extent weathering of tuffaceous material and feldspar delivered alkali cations such as ca ba and k while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a co2 rich atmosphere sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic s or so2 concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone dominated by seasonal fluctuations of the groundwater level under evaporitic conditions the concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols their formation and composition constrain the local occurrence of sulfate in the archean atmo and hydrosphere their interaction with the emerging biosphere archean weathering regime local climate and vadose zone hydrodynamics

mesoproterozoic erg and sand sheet system architecture and controlling factors galho do miguel formation se brazil abstract aeolian systems of the precambrian time were established in different palaeoclimatic and palaeogeographic conditions than the present different morphologies of dunes draas as linear oblique and star forms are widespread in the recent aeolian systems however the ancient aeolian deposits are rarely interpreted as these types of bedforms and usually referred to as transverse dunes the nature and complexity of bedform morphology in precambrian aeolian systems as well as the factors that allow the construction accumulation and preservation of these environments are still not well defined extensive exposures of galho do miguel formation in southeastern brazil represent a mesoproterozoic erg succession and exhibit a distinct facies architecture which contributed to the understanding of these deposits three architectural elements were identified i tangential cross stratified sandstone with superimposed trough cross bedded sandstone ii planar laminated sandstone and iii extensive flat bedded sandstone dominated by wrinkly laminae these elements are organized into three succession groups representative of simple transverse dune oblique compound draa and sand sheet systems the simple to compound dune draa system corresponds to the most central parts of the erg associated with a higher sediment availability and lower water table level conversely the sand sheet system occurs under low sediment availability and higher water table level in the marginal portions of the erg precambrian climatic oscillations may have changed the aeolian system morphology allowing the alternation of simple and complex aeolian dunes resulting in high frequency changes in sand sheet system and the remobilization of previously accumulated sediments over large geographic areas

the polygonal surface structures in the dalangtan playa qaidam basin nw china controlling factors for their formation and implications for analogous martian landforms