calcium isotopes as a record of the marine calcium cycle versus carbonate diagenesis during the late ediacaran abstract calcium isotope ratios in ancient carbonate rocks can provide insight into the global marine calcium cycle as well as local conditions during carbonate mineral precipitation and diagenesis we compare two extraction techniques for the separation of calcium from other ions before 44ca analysis using an automated ion chromatograph and using manual gravity columns the two techniques produce the same 44ca within error 2 we present 31 44ca analyses of carbonate rocks from the nama group namibia which record a negative shift in 44ca of 0 35 between 550 and 547 ma from 1 25 to 1 60 followed by persistently low 44ca 1 48 0 06 between 547 and 539 ma very low 44ca

evolution of oceanic lithosphere a driving force study of the nazca plate the role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the nazca plate results indicate that ridge push forces when distributed over that portion of the oceanic lithosphere giving rise to the force predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest in particular the component of stress normal to the ridge vanishes near the ridge for the new modeling predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on nazca intraplate stress stress states have also been calculated for a variety of slab pull models which include possible dependence on age rate and angle of subduction the predicted stresses vary substantially near the trench among these models with less variation farther from the trench combinations of ridge push and slab pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab pull over ridge push simple viscous modeling with only distributed ridge forces predicts relative motion between the nazca and south american plates that is in fairly poor agreement with the observed direction the fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the antarctic plate

a simple method of determining sand shale ratios from seismic analysis of growth faults an example from upper oligocene to lower miocene niger delta deposits a plot of fault throw vs depth is a simple geometric tool that graphically represents strata thickness variations in growth fault and growth fold settings and has previously been used to infer fault kinematics in this paper we use it as a prediction tool for lithological change employing only seismic data if growth faulting is a continuous process intervals of shale deposition are recorded by unthickened units while intervals of sand fill in topographic lows the throw vs depth plot easily allows depiction of unthickened and thickened sedimentary intervals from even rough seismic records and can therefore be used to predict sand shale ratios the method is here applied to a growth fault in the niger delta that affects oligocene to lower miocene deltaic deposits most shale intervals are identified and the sand shale ratios are predicted we suggest that the method can be a valuable tool in oil exploration

demise of ediacaran dolomitic seas marks widespread biomineralization on the siberian platform the trigger for biomineralization of metazoans in the terminal ediacaran ca 550 ma has been suggested to be the rise of oxygenation or an increase in seawater ca concentration but geochemical and fossil data have not been fully integrated to demonstrate cause and effect here we combine the record of macrofossils with early marine carbonate cement distribution within a relative depth framework for terminal ediacaran to cambrian successions on the eastern siberian platform russia to interrogate the evolution of seawater chemistry and biotic response prior to ca 545 ma the presence of early marine ferroan dolomite cement suggests dominantly ferruginous anoxic aragonite dolomite seas with a very shallow oxic chemocline that supported mainly soft bodied macrobiota after ca 545 ma marine cements changed to aragonite and or high mg calcite and this coincides with the appearance of widespread aragonite and high mg calcite skeletal metazoans suggesting a profound change in seawater chemistry to aragonite seas with a deeper chemocline by early cambrian stage 3 the first marine low mg calcite cements appear coincident with the first low mg calcite metazoan skeletons suggesting a further shift to calcite seas we suggest that this evolution of seawater chemistry was caused by enhanced continental denudation that increased the input of ca into oceans so progressively lowering mg ca which combined with more widespread oxic conditions facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy

dolomitization history and porosity evolution of a giant deeply buried ediacaran gas field sichuan basin china abstract the deeply buried 7 km upper ediacaran sinian dengying formation ca 551 1 541 ma in the sichuan basin china is the largest precambrian dolostone gas reservoir worldwide gas exploration from the dengying formation however is hampered by a limited understanding of its complex dolomitization history and porosity evolution new petrological geochemical and petrophysical analyses were performed and are discussed here to develop a better understanding of the formation s complexities microbialite dolo grainstone and crystalline dolostone lithologies from the platform margin have high primary porosities relative to dolo mudstone and less common microbialite lithologies from the low energy platform interior spatially variable primary porosity was subsequently overprinted by meteoric dissolution and dolomitization often inducing secondary porosity the various forms of dolomitization sabkha reflux and burial dolomitization and cementation increased the resistivity of these rocks to chemical and physical compaction and porosity destruction in the deep burial domain medium to coarse crystalline dolomite cements and saddle dolomite precipitated at fluid temperatures of 120 160 c and 160 220 c respectively the significance of deep burial dissolution is critically discussed and constrained by i corrosion of late diagenetic minerals ii pores cross cutting or forming around stylolites and iii the occurrence of solid bitumen in the center of secondary pores vuggy inter and intracrystalline pores developed during a late and deep burial stage and comprise ca 20 of the overall dengying reservoir porosity mechanisms which induced corrosion at this burial stage include hydrothermal pulses and thermochemical sulfate reduction similar to many carbonate reservoirs in the phanerozoic this study documents that the platform margin has better porosities and higher gas production compared to the platform interior due to its specific carbonate rock properties and diagenesis data shown here document the complex multiphase dolomitization history and high potential for gas production from deeply buried precambrian dolostone reservoirs

quantifying the distribution and transport of pelagic sediments on young abyssal hills the post depositional transport of pelagic sediments on rough seafloor is approximated as a nonlinear diffusive process the topographic slope distribution of the sediment water interface depends primarily on the apparent sediment diffusivity the average sediment load l and the root mean square height of the abyssal hill basement h these parameters have been determined for the arsrp corridor on the west flank of the mid atlantic ridge by fitting theoretical models to the slope distribution observed from center beam hydrosweep data the average value of the apparent diffusivity is h varies between 188 and 285 m with a mean of 230 7 m l is estimated to be as thin as 10 m on 5 ma crust near the ridge crest thickening to values greater than 50 m at 10 15 ma and then decreasing to less than 30 m on 25 ma crust in the western portion of the corridor the decrease in sediment load with age may be associated with a shallower carbonate compensation depth prior to the late miocene

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

comparing the smooth parabolic shapes of interfluves in continental slopes to predictions of diffusion transport models abstract computer models of stratigraphic development at continental margins often include elements in which sediment topography evolves according to a diffusion equation representing effects of down slope sediment transport however there has been little effort to verify the model for example by showing whether deposit geometries are as expected from it or by considering whether transport is consistent with the model s assumptions the issue is important for understanding transport erosion and deposition in continental slopes more generally and the extent to which such processes are recorded in seabed morphology examples of slope topography illustrating diffusive like characteristics are examined here in the uppermost slope of the usa central atlantic margin sandy areas between canyon heads are smoothly convex upwards and almost parabolic a morphology similar to subaerial soil mantled hillslopes known to evolve according to a diffusion equation other data of interfluves in muddy sediment from both atlantic and pacific usa margins also show this smooth convex upwards form in subaerial hill crests the upwards convex parabolic shape arises where steady erosion by streams along channels provide constant flux boundary conditions i e the parabola is the steady state solution to the diffusion equation although submarine processes will likely be different the similar morphology of submarine interfluves prompts the question as to whether erosion along channels and movements of sediment between channels can also sometimes occur to cause a diffusive like evolution of topography the discussion therefore focuses on submarine processes affecting erosion and deposition in particular to address whether the morphology results from true diffusion which requires that sediment moves down slope with a transport flux proportional to local bed gradient or is fortuitous sediment transport is instead unrelated to local bed gradient and the morphology arises from other effects on accumulation rates

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

mechanisms of crustal deformation thirty sixthwilliam smith lecture the energy source for crustal deformation is isotopic heating and secular cooling of the mantle in a true solid heat would be lost to the surface by conduction however solid state creep processes allow the earth s solid mantle to exhibit a fluid behaviour thus thermal convection can convert heating into directed motion variations in temperature lead to variations in densitythrough thermal expansion and contraction although the resultant body forces are vertical horizontal variations in temperature lead to horizontal body forces at an ocean ridge the variations in the thickness of the lithosphere lead to the elevation of the ridge the result is a substantial horizontal ridge push force this force along with the trench pull force on the descending lithosphere is the primary force driving plate tectonics lithospheric thinning must occur beneath continental rifts and plateau uplifts i suggest that lithospheric thinning is caused by the diapiric penetration of hot asthenospheric rock to the crust mantle boundary this penetration may also cause lithospheric stoping and plateau uplifts lithospheric thinning leads to large tensional stresses above the zone of thinning these forces are generally much larger than the compressional forces generated by crustal thinning the forces generated by lithospheric thinning are quite large and may be responsible for the propagation of continental rifts in island arcs and andean type orogenic belts both lithospheric thinning and crustal thickening result in large tensional forces probably the most complex zone of crustal deformation is the continental collision variations in types of continental collisions are discussed

diffusion transport model for pelagic sediments on the mid atlantic ridge the diffusion model is potentially useful for quantifying the effect of downslope gravity transport on sedimentation rate variations which are commonly found between deep sea drilling project and ocean drilling program sites if appropriate the model and stratigraphy could be used for example to constrain the amount and timing of fault block rotation deep tow profiler records from the french american mid ocean undersea study area of the mid atlantic ridge are examined to determine whether they are consistent with a diffusion model sedimentary contacts with basement show that a variety of processes may be operating some of which are inconsistent with the diffusion model for example there are moats around some contacts which are typical of scouring or nondeposition due to high current velocities more than half of the contacts however show onlapping which is qualitatively consistent with the diffusion model sediments generally fill low areas of the topography and have smooth surfaces as expected from a diffusion model where the fluxes of sediment entering a basin are constant the diffusion model predicts that the sediment surface should evolve to a parabola the steady state solution some 20 curved surfaces in the profiler data were digitized and least squares parabolas fitted to them with rms errors of less than 1 m the slopes of the model parabolas provide values for the ratio of sediment flux to diffusivity at the edges of the basins steep surfaces are produced by low diffusivity or high fluxes this ratio is combined with estimates of the fluxes to determine the apparent sediment diffusivity kapp for eacri basin flux is estimated by assuming the abyssal hill topography acts as a simple sediment trap so that the lateral flux equals the width of a basin s pelagic catchment area times the area s mean sedimentation rate s using this method median kapp is 0 04 0 11 m2 yr 1 assuming s 10 30 m m y 1 variations in kapp and the assumptions behind the diffusion model are discussed and alternative causes of sediment surface tilts are considered in particular differential compaction which is modelled using a simple porosity depth function

using t z plots as a graphical method to infer lithological variations from growth strata the t z plot method consists of plotting the throw of sedimentary horizons across a growth fault versus their depth in the hanging wall this method has been initially developed for the analysis of growth fault kinematics from seismic data a brief analytical examination of such plots shows that they can also provide valuable information about the evolution of fault topography when growth is a continuous process stages of topography creation fault scarp and filling of the space available in the hanging wall are related to non dynamic draping mud prone pelagic settling and dynamic sand prone dynamically deposited sedimentation respectively in this case the t z plot analysis becomes a powerful tool to predict major lithological variations on seismic profiles in faulted settings q 2004 elsevier ltd all rights reserved

do geochemical estimates of sediment focusing pass the sediment test in the equatorial pacific 1 the paleoceanographic recording fidelity of pelagic sediments is limited by chemical diagenesis and physical mixing bioturbation and horizontal sediment transport diagenesis and bioturbation are relatively well studied but the effects of physical sedimentation have been largely ignored modeling u series isotopes e g 230th can potentially quantify horizontal sediment movement but model horizontal sediment focusing often equals or exceeds the vertical particle rain we find no evidence of this level of sediment focusing in the equatorial pacific from geophysical data or sediment core comparisons the overestimate of sediment focusing by 230th is probably caused by poor model assumptions that sediment does not fractionate does not sort according to size during transport and that 230th cannot leak from slowly accumulating sediments both assumptions are weak u series methods do hold promise to quantify sediment focusing if properly calibrated with calibration the trade offs between seeking high sedimentation rates for better time resolution and the blurring by horizontal sediment focusing can be better assessed

extreme ocean anoxia during the late cryogenian recorded in reefal carbonates of southern australia abstract the neoproterozoic was a time of great change in the earth s surface and marine environments including extensive climate variability the widespread oxygenation of the oceans and the accompanying rise of animal life however the timing of ocean oxygenation remains uncertain particularly in regard to cryogenian seas which were disrupted by large periods of global glaciation interglacial cryogenian reef complexes in the northern adelaide fold belt of australia contain abundant primary marine dolomite cements these cements have well preserved textural and growth zonation indicating they preserve their original marine chemistry and can be used as geochemical proxies for late cryogenian paleooceanography analysis of marine cements from peritidal nearshore facies shallow platformal facies and deep framework facies of the reef complexes reveals significant geochemical gradients with paleo depth while nearshore cements have low fe contents and commonly contain iron oxide inclusions the shallow and deep cements have very high fe concentrations chalcophile elements cu cd pb zn etc are most abundant in the nearshore cements whereas rare earth elements are found in highest concentrations in the deeper facies cements rare earth element profiles are unusual with shallow and deep facies having convex profiles with negligible ce ce anomalies and positive eu eu anomalies being constrained by sedimentology this carbonate geochemistry provides a window into interglacial cryogenian ocean chemistry and structure the marine cements reveal pronounced chemical stratification in this late cryogenian ocean a thin veneer of oxic surface waters existed at the ocean surface in peritidal facies with increasingly anoxic and fe rich seawater at depth the distribution of strongly chalcophile elements like cd and cu across the chemocline suggests that although ferruginous deeper anoxic waters probably contained some dissolved sulphide these conditions describe a ferro sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late precambrian a return to archean like ocean conditions at this time suggests large scale disruption of the ocean system during the neoproterozoic

testing the preservation potential of early diagenetic dolomites as geochemical archives early marine diagenetic dolomite is a rather thermodynamically stable carbonate phase and has potential to act as an archive of marine porewater properties however the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity here the archive potential of early marine dolomites exposed to extreme post depositional processes is tested using various types of analyses including petrography fluid inclusion data stable 13c and 18o isotopes 87sr 86sr ratios and u pb age dating of various dolomite phases in this example a triassic carbonate platform was dissected and overprinted diagenetic temperatures of 50 to 430 c in a strike slip zone in southern spain eight episodes of dolomitization a dolostone cataclasite and late stage meteoric vadose cementation were recognized the following processes were found to be diagenetically relevant i protolith deposition and fabric preservation and marine dolomitization of precursor aragonite and calcite during the middle late triassic ii intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a proto atlantic opening stress regime t ca 250 c during the early middle jurassic iii dolomite cement precipitation during early alpine tectonism rapid burial to ca 15 km and high grade anchizone overprint during alpine tectonic evolution in the early eocene to early miocene iv brecciation of dolostones to cataclasite during the onset of the carboneras fault zone activity during the middle miocene and v late stage regression and subsequent meteoric overprint data shown here document that under favourable conditions early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years evidence for this preservation includes preserved late triassic seawater 13cdic values and primary fluid inclusion data data also indicate that oversimplified statements based on bulk data from other petrographically complex dolomite archives must be considered with caution

transient shallow ocean oxidation associated with the late ediacaran nama skeletal fauna evidence from iodine contents of the lower nama group southern namibia abstract the terminal ediacaran interval from 550 ma to 541 ma witnessed the first appearance of skeletal metazoans and complex ecosystems in earth history this biotic innovation event is thought to be related to increase of oxygen in earth s surface environments but many studies suggested that pervasive oceanic anoxia continued through the late ediacaran and early cambrian to further evaluate the redox conditions of the terminal ediacaran interval and their potential relationship with biotic changes we analyzed the iodine contents in the lower nama group 550 547 ma from the driedoornvlagte and zebra river sections southern namibia the i ca mg values fluctuate from 0 08 to 6 20 mol mol with an average of 0 94 mol mol in the driedoornvlagte section and 0 70 mol mol in the zebra river section high i ca mg values 2 6 mol mol occur immediately above the boundary between the lower and upper omkyk members with a peak up to 6 20 mol mol indicative of well oxygenated surface waters comparable with those of the modern marine environments abundant skeletal fossils including cloudina namacalathus namapoikia and trace fossils are observed only in the intervals after the high i ca mg peak the results are consistent with the interpretation that the nama skeletal communities grew in oxic o2 20 70 m and dysoxic o2 10 m waters above the chemocline the i ca mg ratios show significant tempo spatial variations which is also consistent with previous studies that suggested redox stratified terminal ediacaran sedimentary basins and highly heterogeneous oceanic redox conditions

pelagic sedimentation on rough seafloor topography 2 inversion results from the north atlantic acoustic reverberation corridor seafloor slope distributions are sensitive to basement topography the surface shape of sediment distributions and the average thickness of sediment within a region by comparing slope distributions computed from bathymetric data to distributions from realistic models of sedimented topography we determine how characteristic values of average sediment thickness l rms basement variability h and apparent diffusivity vary throughout the office of naval research acoustic reverberation corridor located on the western flank of the mid atlantic ridge north of the kane fracture zone 25 5 27 25 n 44 5 49 w values of l from basin centered subregions show thinner sediments near the axis thickening westward with maximum values found on 10 20 ma seafloor lower values of l are found for older late oligocene early miocene seafloor values of l on young seafloor suggest a sediment accumulation rate of 6 5 2 3 m m y since 10 ma values of h computed from hill centered regions range from 217 to 306 m with an average of 267 27 m hills are generally larger to the north of a low running through the study area values of range from 0 12 to 0 77 m2 yr with an average of 0 32 0 19 m2 yr lower values of kbetter describe the surface shapes of individual high standing ponds while higher values are needed to model the surface shapes of thick deposits in regional lows this difference in with geological setting is probably due to variations in sediment transport mechanisms

did anoxia terminate ediacaran benthic communities evidence from early diagenesis abstract the ediacaran oceanic redox landscape was heterogeneous where many basins had a shallow and highly dynamic chemocline above anoxic ferruginous or euxinic or low oxygen manganous waters seawater mmg ca ratio was also high promoting early diagenetic dolomitisation how the benthos responded to these conditions is fundamental to understanding their ecological dynamics here we utilise redox sensitive elements in early marine carbonate cements to investigate possible water column redox controls on the distribution and growth of the oldest metazoan communities skeletal communities in the zaris sub basin of the nama group namibia 550 547 ma grew in shallow waters where fine grained carbonate sediment often shows evidence of early dolomitisation mid ramp cloudina reefs are composed of open highly porous structures that formed multiple successive assemblages each assemblage is terminated by thin reef cements show a paragenetic sequence from synsedimentary to early marine cement and final burial which we infer were precipitated under dynamic redox conditions first acicular pseudomorphed aragonite cement formed under oxic conditions low fe and mn next the presence of iron rich dolomicrite sediment often associated with a recrystallised ferroan dolomite crust suggests that originally aragonitic or calcitic sediment and a high mg precursor cement were preferentially dolomitised dolomitisation may have been enhanced either via upwelling of deeper water anoxic ferruginous seawater or by later fluid remobilisation from adjacent shales a following mn rich calcite cement is inferred to be early marine due to its inclusion rich fibrous form and well preserved cl zonation the final blocky cement precipitated under oxic conditions probably during shallow burial the cements likely record a general shallow to deeper water transect from oxic shallow waters to low oxygen manganous waters then to oxic shallow burial conditions we hypothesize that cloudina reef communities were terminated by episodic sediment incursions during short term transgressive cycles possibly accompanied by upwelled anoxic ferruginous and dolomitising waters although the timing for this is poorly constrained more generally such incursions may have terminated ediacaran benthic communities that grew close to the chemocline

characterising the extent of volcanism at the galapagos spreading centre using deep tow sediment profiler records abstract volcanism at the galapagos spreading centre consists of sheet flows and other distributed extrusives the surficial volcanics in the axial region should therefore be generally young over a finite area and those on the ridge flanks should be younger on average than would be predicted from simple seafloor spreading a method for determining an upper bound on the mean age anomaly of the flanks which is interpreted as equivalent to the mean extent of volcanism is demonstrated using sediment thicknesses digitised from deep tow profiler records of the galapagos spreading centre as found previously the sediments thicken linearly with distance from the ridge reflecting simple seafloor spreading up to 1 ma deviations from the linear trend are due to sediment redistribution short wavelength fluctuations and the age distribution of extrusives in particular due to large volcanic flows revealed by regions of uniform thickness the full width of the extrusion zone is obtained by regressing sediment thickness on across axis distance for both flanks and observing the separation of the two regression lines where they cross zero thickness since the method is quantitative and provides an average value for the length of the profile it is more representative than estimating the width from morphological observations of the spreading axis due to scattering from the basement which obscures sediment in depressions sediment thickness is generally under estimated and the measured axial width is an upper bound 10 12 km for the galapagos spreading centre the mean sediment thickness bias is calculated from high resolution topography data of lava flows and is 1 5 m suggesting that the true mean width of the extrusion zone is 8 10 km using this corrected value the surface of the crust is younger than predicted by seafloor spreading by 120 150 kyr on average this large value implies that the flanks of the galapagos spreading centre are covered with a series of large overlapping volcanic flows

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

cavity filling dolomite speleothems and submarine cements in the ediacaran dengying microbialites south china responses to high frequency sea level fluctuations in an aragonite dolomite sea speleothems mostly composed of calcium carbonate are widely present in modern karst originated caves but have rarely been reported in palaeokarst systems this study presents a novel type of dolomite speleothem and subsequent submarine dolomite cement which are widely present in the upper ediacaran dengying formation in the upper yangtze area these precipitated materials occur in the cavity system that cuts across several peritidal cycles the interconnected cavity networks with irregular shapes embayed walls internal breccias on cavity floor and their preferential development in the shallower cycle tops for example tepee deformed beds suggest that they were initially generated by subaerial dissolution as the earliest infills the hemispherical protrusions icicle like pendants and ground up columns show similar morphological features and occurrence patterns to the cave popcorn stalactites and stalagmites respectively thus these earliest infills are speleothems resulting from associated meteoric precipitation during subaerial exposure the isopachous growth pattern of subsequent more extensive fibrous dolomite cements points to a submarine diagenetic environment in which they were precipitated microscopically the micritic to micro crystalline dolomite acicular dolomite in speleothems and the subsequent fibrous dolomite share similar crystal fabrics to metastable precursors for example mg calcite meanwhile the carbon oxygen isotope compositions of the speleothem and fibrous dolomite although partly altered by burial diagenesis share a large overlap with host rock and coeval marine carbonates all over the yangtze platform for these reasons these speleothems and fibrous cements are considered to have been initially precipitated as metastable carbonate precursors in meteoric and submarine environments respectively and stabilized during submarine mimetic dolomitization the cyclic occurrence of cavity systems filled with speleothems and submarine cements reflects periodic subaerial exposure and marine flooding of broad tidal flat in the upper yangtze area driven by high frequency sea level fluctuations furthermore the neoproterozoic seawater chemistry that favoured early dolomitization of carbonate precursor mineralogies was an advantage for the preservation of fabrics from metastable precursor minerals

quantitative controls on the regional geometries and heterogeneities of the rayda to shu aiba formations northern oman using forward stratigraphic modelling abstract the complex geometry of carbonate systems is influenced by a multitude of physical as well as biological processes the lower cretaceous carbonates of northern oman are characterised by a variability of regional scale geometries with expected vertical and lateral facies variations the main environmental and tectonic controls acting on the depositional processes of the lower cretaceous ramps and platforms through space and geological time in 4 dimensions are only partially understood in this study we use a 4d dionisosflow forward stratigraphic modelling fsm approach to explore the role of i eustasy ii subsidence iii initial paleobathymetry and iv wave energy to generate carbonate stacking patterns and heterogeneities carbonate production was maintained constant through deposition multi disciplinary and multi scale datasets were used i e seismic well and field data to constrain the fsm input parameters and sensitivity analysis was carried out to validate or refute some depositional model hypotheses results show that basement topography and eustasy have the greatest influence on the progradational geometries and the lateral continuity of clinoform architectures during the tithonian to valanginian second order super sequence in the valanginian to aptian super sequence subsidence was the primary control for the observed aggradational stacking pattern lateral and vertical stacking of carbonate lithologies textures as well as facies are thus apprehended through this fsm approach leading to a better assessment of petroleum systems elements as reservoir seal and trap

eve of biomineralization controls on skeletal mineralogy carbonate mineralogies have oscillated between aragonite and calcite seas through geological time proposed to be due mainly to secular variation in the magnesium calcium ratio driven by changing rates of ocean crust production a quantitative compilation of inorganic and biominerals from the onset of biomineralization late ediacaran middle ordovician reveals a correspondence between seawater chemistry and the first adopted mineralogy of skeletal clades ediacaran tommotian skeletons and inorganic precipitates were composed exclusively of aragonite or high mg calcite but these were replaced by low mg calcite mineralogies during the early atdabanian implying the onset of a calcite sea this transition is empirically constrained by fluid inclusion data late atbadanian botoman inorganic precipitates returned to aragonite with high mg calcite echinoderms and solitary tabulaconids and massive aragonitic tabulaconids originating during this interval middle cambrian ordovician inorganic precipitates were low mg calcite and the ordovician radiation in skeletal expression was due mostly to groups with low mg calcite mineralogies these short lived transitions can be most parsimoniously explained by minor oscillations of mmg ca around 2 during this period possibly combined with the progressive onset of greenhouse conditions during the mid late cambrian

geochemistry of synsedimentary cements in early cambrian reefs abstract synsedimentary fibrous calcite cements in reefs representing different periods of early cambrian time from eastern siberia southern australia and eastern canada were analyzed to determine whether they retained information about the chemical and isotopic compositions of the seawater from which they precipitated although petrographically similar the sr o and c isotopic compositions as well as the sr and mg contents of the cements from the three areas are different such differences can be attributed to varying degrees of diagenesis of the cements that correlate with the tectonic history of each area cements from the siberian craton are the least altered cements from near the appalachian orogen are partially altered and cements from the flinders fold and thrust belt are extensively altered strontium contents of the fibrous cements generally increase with mg contents although the correlation among these two elements is more variable than that reported for devonian or holocene cements those cements with sr mg ratios near 0 04 which include clear fibrous calcite have the lowest 87 sr 86 sr ratios and have been least altered strontium isotopic ratios in the least altered cements are lower than whole rock ratios and probably approach the original values of early cambrian seawater 18o values in the cements are variable because of exchange with diagenetic fluids whereas 13c values exhibit local variations that do not match temporal curves constructed from whole rock data for the same regions thus most of the original chemical and isotopic compositions of these cements have been variably changed through diagenesis although limited information about the composition of early cambrian seawater such as sr and c isotopic compositions have been retained in some samples

neoproterozoic aragonite dolomite seas widespread marine dolomite precipitation in cryogenian reef complexes the extraordinary abundance of dolomite in the proterozoic challenges our understanding of precambrian marine environments here we show that synsedimentary marine dolomite precipitation was pervasive within cryogenian reef complexes from the adelaide fold belt south australia although these reefs are composed of dolomite textural evidence indicates an originally aragonitic mineralogy for depositional components in common with many other neoproterozoic carbonates allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts indicating synsedimentary dolomitization in the reefs we describe several new forms of fibrous marine dolomite cement from the reefs that have a length slow optical character these fascicular slow radial slow and rhombic dolomite cements have finely preserved cathodoluminescent growth zones and optical characteristics that indicate they originally precipitated as dolomite rather than replacing calcite or aragonite cements abundant early marine dolomite precipitation implies a radically different seawater chemistry for the cryogenian perhaps these aragonite dolomite seas are associated with extreme neoproterozoic glacial events and or ocean anoxia

petrological and geochemical characteristics of the botryoidal dolomite of dengying formation in the yangtze craton south china constraints on terminal ediacaran dolomite seas abstract recent studies suggest that the massive precambrian dolomite successions were likely the products of specific seawater conditions known as dolomite seas however whether these so called dolomite seas existed in the terminal ediacaran ocean has not yet been constrained the widespread botryoidal dolomite with well preserved sedimentary fabrics from the upper ediacaran dengying formation ca 551 to 545 ma south china can be potentially an ideal target to constrain this issue results showed that within the botryoidal dolomite fibrous dolomite cements are the dominant constituent part and either grew upward from microbialite layers or occurred in sheet cracks within the microbialites implying a syn sedimentary diagenesis two different types of fibrous cements are newly redefined fascicular and radial slow dolomite well preserved chemical and cathodoluminescent zonations and length slow optical characters are direct evidence for syn sedimentary primary dolomite precipitation different with the previous alternative interpretations of mimetic dolomitization of aragonite or high mg calcite precursors in contrast microbialite matrix consists of finely crystalline dolomicrite and preserved microbial fabrics reveling an early mimetic dolomitization of aragonitic precursors the dolomite precipitation and mimetic dolomitization likely occurred in coeval seawater or seawater type fluids as indicated by comparable 13c and 87sr 86sr data with global changes in particular the presence of primary dolomite cement precipitates within botryoidal dolomite can reflect that the specific dolomite seas associated with anoxic or even euxinic shallow water conditions inferred by redox sensitive trace elements and rees and increased microbial activity also prevailed in the terminal ediacaran ocean however the occurrence of dolomite seas was not one occasional event but common even if not persistent in the early to terminal neoproterozoic oceans this study helps to improve the understanding of dolomite problem and ubiquitous distribution of precambrian dolomite worldwide

sedimentary characteristics and origin of dolomitic ooids of the terminal ediacaran dengying formation at yulin chongqing south china abstract the ediacaran was a crucial transition period in the evolutionary history of the earth ediacaran ooids in the dengying formation from the yulin section in southwestern china are important archives of paleoenvironmental information although these ooids have been totally dolomitized the cortical fabric characteristics are locally preserved well indicating that these ooids were mimetically replaced the common occurrence of concentric radial fabrics and low level of sr concentrations 60 3 8 8 ppm in the studied ooid cortex is more likely suggestive of a primary low mg calcite mineralogy and that a calcite sea might exist during deposition of the second member of the dengying formation in situ geochemical data for the ooid cortices obtained by laser ablation inductively coupled plasma mass spectrometry show that these ooids have middle rare earth element enrichment prn smn 0 7 0 1 and smn ybn 1 5 0 4 and high fe contents 8200 1800 ppm this suggests ferruginous pore waters which were related to the selective or preferential absorption and release of rees by fe mn oxyhydroxides

morphotectonic and morphometric analysis of the nazca plate and the adjacent offshore peruvian continental slope implications for submarine landscape evolution we use new swath bathymetry data acquired during the rv sonne cruise geopeco and complement them with swath data from adjacent regions to analyse the morphotectonics of the peruvian convergent margin the nazca plate is not covered with sediments and therefore has a rough surface along the entire peruvian trench the styles of roughness differ significantly along the margin with linear morphological features trending in various directions most of them oblique to the trench and roughness magnitudes of a few to several hundred meters the lower slope is locally very rough and at the verge of failure throughout the entire peruvian margin as a result of subduction erosion causing the lower slope to over steepen using curvature attributes to quantitatively examine the morphology in the yaquina and mendana areas revealed that the latter shows a larger local roughness both seaward and landward of the trench however the amplitude of morphological roughness is larger in the yaquina area we identified a 125 km2 large slump on the lima middle slope morphometric dating suggests an age of 74500 years within 35 to 40 error estimated incision rates on the upper slope are between 0 1 and 0 3 mm per year suggesting that landscape evolution on the peruvian submarine continental slope is similarly slow than that in the atacama desert

trends in oolite dolomitization across the neoproterozoic cambrian boundary a case study from death valley california abstract changes in dolomite fabric within the neoproterozoic cambrian stratigraphic succession in death valley california preserve evidence for a first order biogeochemical change in early diagenetic environments through the neoproterozoic cambrian transition oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time two key generalizations can be made 1 late neoproterozoic dolomitization in death valley occurred very early in the paragenetic sequence was volumetrically significant and preserved fine detail of the original aragonite or calcite precursor and 2 early cambrian dolomitization in death valley occurred late in the paragenetic sequence with burial was commonly incomplete and obliterated details of the calcite or aragonite precursor early diagenetic dolomitization of any kind was absent the dolomitization fabrics in death valley parallel the presumed pattern of seawater mg ca with respect to mimetic dolomitization in the paleozoic low mg ca little no mimetic dolomitization and cenozoic higher mg ca more common mimetic dolomitization however the volume of neoproterozoic mimetic dolomitization is immense compared to the cenozoic examples we speculate that a fundamental shift in the early diagenetic environment occurred in late neoproterozoic time perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment capping microbial mats an oscillatory trend in the fabric and volume of dolomite through the phanerozoic has been suggested but the termination of volumetrically significant mimetic dolomitization across the neoproterozoic cambrian transition a critical time in earth history for many reasons dwarfs the oscillatory trends noted in the phanerozoic

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

high resolution record of tectonic and sedimentary processes in growth strata growth strata are used to determine the kinematics of synsedimentary structures such as faults classical methods of analysis such as thickness versus throw plot consider that the available space created by fault slip in the hanging wall of faults is instantaneously filled up by sediments this has lead many previous works to identify a cyclic activity for growth faults here we perform a careful analysis of the variation of strata thicknesses on each side of a very well documented normal growth fault in the niger delta we show that these thickness variations are induced by the alternation of sedimentary processes during continuous fault slip suspended load processes induce either uniform or slightly variable thickness of a large majority of mudstone layers bedload processes result in a preferential thickening of sand layers in the hanging wall these high quality data thus provide strong grounds for doubting the polycyclic growth diagnosed for some faults at the scale of sedimentary cycles and supports the notion that fault displacement rates can be very well behaved our study emphasizes the important conclusion that stable fault growth and related displacement rates can appear to be punctuated when viewed at the scale of sedimentary cycles it follows that care should be taken when attempting to derive displacement rates on temporal scales equivalent to those of alternating sedimentological cycles

a deep root for the cambrian explosion implications of new bio and chemostratigraphy from the siberian platform much uncertainty remains as to the temporal relationship between the ediacaran and cambrian biotas yet this is critical to our understanding of the rise of metazoans here we present new high resolution carbon isotope chemostratigraphy and biostratigraphy for a terminal ediacaran to cambrian succession on the eastern siberian platform russia which shows the presence of a succession of diverse fossil assemblages before the start of the basal cambrian negative carbon isotope excursion bace soft bodied ediacaran biota beltanelliformis occur before the start of the late ediacaran positive carbon isotope plateau epip a mixed ediacaran and cambrian skeletal biota cloudina anabarites cambrotubulus appears within the epip and diverse cambrian type small shelly fossils including protohertzina and other protocondonts halkieriids chancelloriids hyoliths hyolithelminthes and the burrowing trace fossil diplocraterion appear at the beginning of the bace these integrated data show that taxa attributed to so called ediacaran and earliest cambrian skeletal biotas in fact overlap without notable biotic turnover and thus refute the presence of a large isotope excursion coincident with mass extinction of all ediacaran biota we propose a new biozone the cloudina namacalathus sinotubulites assemblage zone to precede the known small shelly fossil ssf zones these observations raise doubts as to whether there is any true separation between the ediacaran and cambrian skeletal biotas and suggest that there is a deep root for the cambrian explosion of metazoans

the tonian beck spring dolomite marine dolomitization in a shallow anoxic sea abstract the reason for the abundance of dolomite lithologies in earth s early geological record compared to modern environments remains contentious this study provides new insight into this precambrian dolomite problem by revisiting one of the most controversial dolomite localities the beck spring dolomite of death valley usa consistent with some previous studies petrographic evidence indicates that although the beck spring dolomite now consists almost entirely of dolomite it was originally precipitated largely as aragonite and high mg calcite depositional constituents microbialites and ooids were likely originally aragonitic and early marine length fast cements now dolomite are suggested to have precipitated as high mg calcite then replaced syntaxially by dolomite based on petrographic and geochemical evidence we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit and for the most part involved syntaxial and mimetic replacement further a length slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement this is indicated by the length slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation this new evidence for tonian marine dolomite precipitation reinforces the idea of precambrian marine environmental conditions including the chemical composition of seawater promoting dolomite formation at this time the trace metal geochemical composition of well preserved marine components especially dolomite marine cements reveals information about redox conditions in this tonian shallow seawater in terms of rare earth element geochemistry the beck spring dolomite has no significant ce anomaly and a ubiquitous positive eu anomaly consistent with widespread oceanic anoxia during deposition furthermore the relatively low levels of iron and chalcophile elements co cu pb and zn in marine components compared to other neoproterozoic carbonates suggest euxinic conditions both anoxic and sulphidic prevailed during deposition of the beck spring dolomite

creep in pelagic sediments and potential for morphologic dating of marine fault scarps cores from the flanks of the galapagos spreading centre show that the top 25 cm of pelagic sediments is highly bioturbated kenyan and turcotte 1985 have proposed that bioturbation may cause down slope creep of the sediment surface and bioturbation may therefore smooth sediment topography in a similar way to hillslope creep on land assuming this produces movement consistent with the diffusion transport model sediment topography across scarp crests digitised from deep tow profiler records is modelled with a simple analytic solution to the diffusion equation and with a mean diffusion constant k of 0 007 m yr this small value suggests creep rates are modest for example 2 cm yr for a 30 slope and is an upper bound since sediment avalanching and bottom currents probably also remove sediment from scarp crests although requiring further work these results suggest that morphologic dating methods may be applicable to marine fault scarps and could be useful in relatively sediment free areas such as mid ocean ridges where it is commonly not possible to use datable offset seismic horizons

precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites dolomites are more common in precambrian than in phanerozoic rocks and from a petrographic isotopic study of the middle to upper proterozoic beck spring dolomite of eastern california it is suggested that precambrian dolomites are different from those of the phanerozoic petrographically all fabric details of precambrian dolomites studied are preserved just as if they were limestones and isotopically they show the same trend as limestones increasingly negative i8 o and to a lesser extent increasingly light i8 c from depositional components through early fibrous dolomite cements to later dolomite spar cements the data suggest that in precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis and this implies that precambrian seawater was different from that of the phanerozoic

effect of far field slope on morphologic dating of scarplike landforms the principal finding of this paper is that the far field slope has a first order effect on model age determinations of scarplike landforms in weakly consolidated terrains observationally this can be demonstrated in two ways using the lake bonneville and lahontan shoreline scarps as separate and combined data sets use of the reduced scarp slope tan theta sub s minus b where theta sub s is the maximum scarp angle and b is the far field or fan slope instead of tan theta sub s alone as the measure of scarp slope measurably reduces separation between the two data sets induced by different average fan slopes for the two data sets and significantly reduces scatter in the slope offset plot for both the separate and combined data sets theoretically the argument can be put even more strongly at least within the range of linear and nonlinear diffusion models that the authors consider here together with a mathematical transformation of the empirical approach of r c bucknam and r e anderson when one correctly takes into account the far field slope one will basically get the same age determination no matter which of these models one uses conversely without accounting properly for the effect of far field slope one is virtuallymore guaranteed to get an erroneous age determination no matter which model is used less

investigating carbonate platform types multiple controls and a continuum of geometries abstract current classifications of carbonate platforms use depositional gradient as the main criterion for separating systems into two end member types ramps and flat topped platforms ftps however many examples do not conform to this simple classification to investigate why this is and to better understand probable controls on platform development we have used a series of 2d numerical forward model runs to investigate how sediment production diffusional sediment transport and other controls such as tectonic subsidence antecedent topography and relative sea level oscillation interact to determine platform geometry modeling results reaffirm that rates of down dip sediment transport relative to rates of autochthonous production are a critical factor in maintaining a ramp profile in stable cratonic settings under a constant rate of relative sea level rise type of carbonate production versus water depth curve for example euphotic versus oligophotic is not a significant control in our model cases both euphotic and oligophotic production profiles produce ftps when diffusion coefficients are low relative to production rates and ramps when diffusion coefficients are relatively high these results suggest a continuum of platform types ranging from transport dominated low gradient systems at one end of the spectrum to in situ accumulation dominated systems at the other a system may be transport dominated because high energy processes are able to break down and transport even bound sediment or because carbonate factories produce only sediment that is easily transportable under even low energy conditions time evolution is also probably important initially low gradient systems will in the absence of sufficiently high sediment transport rates tend to evolve towards high gradient flat topped steep margined platforms many observed or inferred platform geometries are therefore likely to be transient forms and this could complicate interpretation investigating how basin bathymetry and style of subsidence control platform geometry suggests that in transport dominated systems strata simply drape the underlying topography and that pre existing breaks of slope and differential fault subsidence are a stronger control on platform geometry in in situ accumulation dominated systems rotational subsidence tends to create transport dominated systems during rotation as the topographic gradient increases and transport rate increases and outpaces in situ production rate relative sea level oscillations tend to move the locus of sediment production laterally along any slope present on the platform distributing the sediment accumulation across the whole width of the platform suppressing progradation and steepening and so favoring development of low gradient systems based on all these results we suggest that a simple cutoff classification into ramp and flat topped platform types can still be useful in some circumstances but a more meaningful approach may be to describe and predict platform strata in terms of a multiple dimension platform parameter space containing a continuum of geometries controlled by sediment production sediment diffusion coefficient antecedent topography differential subsidence effects relative sea level oscillations and perhaps other as yet unappreciated controls

stress magnitudes in the crust constraints from stress orientation and relative magnitude data the world stress map project is a global cooperative effort to compile and interpret data on the orientation and relative magnitudes of the contemporary in situ tectonic stress field in the earth s lithosphere horizontal stress orientations show regionally uniform patterns throughout many continental intraplate regions these regional intraplate stress fields are consistent over regions 1000 5000 km wide or ca 100 times the thickness of the upper brittle part of the lithosphere ca 20 km and about 10 15 times the thickness of typical continental lithosphere ca 150 200 km relative stress magnitudes or stress regimes in the lithosphere are inferred from direct in situ stress measurements and from the style of active faulting the intraplate stress field in both the oceans and continents is largely compressional with one or both of the horizontal stresses greater than the vertical stress the regionally uniform horizontal intraplate stress orientations are generally consistent with either relative or absolute plate motions indicating that plate boundary forces dominate the stress distribution within the plates since most regions of normal faulting occur in areas of high elevation the extensional stress regimes in these areas can be attributed to superimposed bouyancy forces related to crustal thickening and or lithosphere thinning stresses derived from these bouyancy forces locally exceed midplate compressional stresses evaluating the effect of viscous drag forces acting on the plates is difficult simple driving or resisting drag models with shear tractions acting parallel or antiparallel to plate motion are consistent with stress orientation data however the large lateral stress gradients across broad plates required to balance these tractions are not observed in the relative stress magnitude data current models of stresses due to whole mantle flow inferred from seismic tomography models and with the inclusion of the effect of high density slabs predict a general compressional stress state within continents but do not match the broad scale horizontal stress orientations the broad regionally uniform intraplate stress orientations are best correlated with compressional plate boundary forces and the geometry of the plate boundaries

secular variations in precambrian seawater chemistry and the timing of precambrian aragonite seas and calcite seas an extension of the seawater secular variation model that successfully predicted the observed timing of phanerozoic mgso 4 vs kcl marine evaporites and aragonite seas vs calcite seas has been applied to the prediction of the secular variations in the major ion chemistry of seawater and aragonite seas vs calcite seas during the precambrian testing of the predictions was based on those precambrian seafloor carbonate precipitates that have been interpreted by others to have formed originally as aragonite of 16 examples of precambrian seafloor aragonite 14 fall within the 6 periods of aragonite seas predicted for the late archean proterozoic by the model 1 falls on the transition between an aragonite sea and calcite sea and 1 falls in a period of calcite seas this strong correlation supports the following predictions of the model 1 precambrian seawater was a saline nacl water with ca hco 3 since at least the late archean 2 the major ion compositions of precambrian seawater chemistry and their secular variations are in the same ranges as those of phanerozoic seawater 3 the mg ca mole ratio in seawater has controlled the types of caco 3 polymorphs that have precipitated from earth9s oceans throughout the phanerozoic and most if not all of the precambrian

massive formation of early diagenetic dolomite in the ediacaran ocean constraints on the dolomite problem paleozoic and precambrian sedimentary successions frequently contain massive dolomicrite camg co3 2 units despite kinetic inhibitions to nucleation and precipitation of dolomite at earth surface temperatures 100 c thus raising doubt about the validity of these deposits as archives of earth surface environments we present a high resolution 63 my long clumped isotope temperature t 47 record of shallow marine dolomicrites from two drillcores of the ediacaran 635 to 541 ma doushantuo formation in south china our t 47 record indicates that a majority 87 of these dolostones formed at temperatures of 100 c when considering the regional thermal history modeling of the influence of solid state reordering on our t 47 record further suggests that most of the studied dolostones formed at temperatures of 60 c providing direct evidence of a low temperature origin of these dolostones furthermore calculated 18o values of diagenetic fluids rare earth element plus yttrium compositions and petrographic observations of these dolostones are consistent with an early diagenetic origin in a rock buffered environment we thus propose that a precursor precipitate from seawater was subsequently dolomitized during early diagenesis in a near surface setting to produce the large volume of dolostones in the doushantuo formation our findings suggest that the preponderance of dolomite in paleozoic and precambrian deposits likely reflects oceanic conditions specific to those eras and that dolostones can be faithful recorders of environmental conditions in the early oceans

first finds of problematic ediacaran fossil gaojiashania in siberia and its origin we describe the first occurrence of the problematic fossil gaojiashania outside china in the ediacaran yudoma group of the siberian platform in both areas gaojiashania characterizes the lower upper ediacaran strata and precedes the appearance of cloudina and other skeletal fossils which highlights its significance for the ediacaran subdivision and correlation features of this fossil such as indeterminate length the absence of a distinct growth pattern and self avoiding behaviour indicate its trace fossil origin but do not necessarily imply metazoan affinities for its producers several organisms including stem group social amoebozoans and unicellular protists may have been proterozoic trace fossil producers