calcareous duricrust al qasim province saudi arabia occurrence and origin in al qasim province saudi arabia calcareous pedogenic duricrust overlies the upper permian carbonates and clays of the khuff formation and ranges in thickness from 60 cm to 2 m it consists of white sandy duricrust capped by brown nodular porous duricrust petrographic investigation led to recognition of three lithofacies that show spatial distribution 1 structureless sandy calcareous mudstone 2 brecciated nodular calcareous mudstone and 3 pisolitic calcareous mudstone the diagenetic changes in duricrust are aggrading neomorphism and formation of calcite rims around quartz grains microprobe chemical analysis was performed on the diagenetically altered material the development of the duricrust is controlled by four factors the climate changes the type of underlying rocks source of carbonate and biological effects the repeated changes from semi arid climate with alternating wet and dry seasons are the main controlling factor in generation of duricrust development during humid periods descending water carries the dissolved carbonates downward as bicarbonates whereas dry and interpluvial period the carbonate rich solutions are carried upwards by capillary or evaporation the claystone and sandstone interbeds possibly affect the development of calcareous duricrust more than the sandstones and dolostones because the former have the ability to attach calcium carbonate to form the duricrust the development of calcareous duricrust is mainly attributed to the pleistocene and early holocene alternating humid and arid periods that prevailed over the arabian peninsula during this time

the concentration of uranium in freshwater vadose and phreatic cements in a holocene ooid cay a method of identifying ancient water tables abstract the concentration of uranium has been measured using the fission track technique in vadose and freshwater phreatic cements from south joulters cay in the bahamas these analyses reveal dramatic differences in the u concentration of vadose 0 28 0 11 ppm and freshwater phreatic 1 80 75 ppm cements this distinctive change in concentration is proposed to result from 1 variations in the co2 of the waters of the phreatic and vadose environments and 2 selective exclusion of u during the precipitation of calcite the pco2 is important because it controls the ph and hence the concentration of co32 ions in the pore fluids as well as the amount of dissolution of the u rich aragonitic precursors for example an increas in pco2 will lower the ph and decrease the concentration of co32 present in the porewaters as the u present as the uranyl ion uo22 is mainly complexed with co32 a decrease in the concentration of co32 will not only alter the speciation of u but also increase the uo2 co3 22 co32 ratio of the fluid and hence the concentration of u in the precipitated carbonate cements the second mechanism for explaining the increase of u in phreatic cements relies on the low distribution coefficient for the incorporation of u into low mg calcite lmc the precipitation of lmc therefore leads to an increase in the concentration of u in the residual solution as fluids do not remain in the vadose zone but rather accumulate in the freshwater phreatic portion of the lens phreatic waters become relatively enriched in u compared to vadose fluids the clear difference in the u concentrations of cements in these two zones may enable the u concentration of cements to be used as an indicator of paleo water tables in ancient rocks even where other geochemical and petrographic evidences are equivocal

dendritic calcite crystals formed by calcification of algal filaments in a vadose environment abstract uncalcified and calcified algae grow on and color walls of cavities that occur in karst breccia which fills pockets in the tertiary bluff formation of grand cayman island the calcified algae are important because the calcification has been achieved through the formation of dendritic calcite crystals although common in metals such branching crystals have not been widely recognized in naturally occurring calcite the dendritic calcite crystals are exactly like most dendritic crystals in that they have a primary needle or stem with secondary and tertiary branches most dendritic calcite crystals have an open treelike form and are easy to recognize as calcification proceeds the dendritic crystals become progressively filled in and are increasingly difficult to recognize as dendritic crystals filling of the dendritic crystals can result in two dimensional sheets or three dimensional columns assuming the three dimensional columnar dendritic crystals developed planar and or curving smooth surfaces then lacking other evidence they would be indistinguishable from normal columnar calcite crystals and their dendritic origin would be hidden variables thought to control dendrite crystal formation include a creation of localized highly supersaturated solutions leading to rapid rates of nucleation and crystal growth and b impurities in the form of organic molecules and other substances algae are excellent candidates for controlling a via their metabolism and contributing to b via their decay combining these ideas with the results of this study and with the known common occurrence of algae in many speleothemic environments provides an impetus for a reexamination of the origin of subaerially formed calcite crystals

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

lateral and vertical changes of deformation style in the osen roa thrust sheet oslo region abstract changes in deformation style and amounts of shortening in the osen roa thrust sheet of the oslo region occur vertically and laterally approaching the thrust front in the south deformation in the cambromiddle ordovician sequence passes laterally from closely spaced imbricates in the north 50 60 shortening through triangle pop up and imbricate zones toward the south 20 37 shortening to widely spaced zones of deformation up to 20 shortening approaching the thrust front changes in deformation style are attributed to changing boundary conditions across the klekken thrust declining end of orogenic forces and an increase in thickness of competent units in the ordovician rocks to the south vertical changes in deformation style are attributed to the increasing percentage of competent units upward in the cambro silurian sedimentary rocks in the north the accompanying decrease in shortening upwards requires a structurally necessary upper detachment horizon to separate folded late middle ordovician silurian sediments from imbricated early cambro middle ordovician sediments below while southward in the oslo area the upper detachment needs to be placed between silurian and cambro ordovician units finally in eiker with less than 20 shortening the whole cambrosilurian sequence appears to have deformed as a single unit in the northern oslo region the upper detachment probably has a backthrust sense of motion above an imbricate stack passive roof duplex further south the upper detachment is probably directed toward the foreland

reconstruction of continental margin architecture deformed by the contraction of the lagonegro basin southern apennines italy thrust propagation through previously rifted continental margins may result in fold and thrust belts whose structure is strongly controlled by the inherited basin architecture as it occurs in southern italy the lagonegro units of the southern apennines comprise a deformed pelagic basin succession showing variable stratigraphic characteristics mainly lateral variations in both facies and thickness interpreted to be due to a complex basin topography related to a triassic rifting event in contrast to previous studies cross section balancing and restoration indicate that the lagonegro units exposed in the high agri valley area suffered relatively limited internal shortening 8 km i e 35 early deformation of these rocks later incorporated into a large displacement thrust sheet was dominated by folding around present day roughly north south trending axes the attainment of a regional decollement level was favoured by an early mild inversion of the basin producing a roughly similar structural elevation of both hanging wall and footwall successions to mesozoic faults most of the contractional deformation was accommodated by buckling of the mesozoic syn rift strata between synsedimentary faults which represented major mechanical interfaces early strain localization in the lagonegro basin ahead of the active thrust front was most probably mechanically controlled by a faulted crustal segment which originally lay within the continental margin between two massive carbonate platforms

the effects of pre existing normal faults on thrust ramp development an example from the northern apennines italy the umbria marche foreland fold and thrust belt in the northern apennines of italy provides excellent evidence to test the hypothesis of synsedimentary structural control on thrust ramp development this orogenic belt consists of platform and pelagic carbonates late triassic to miocene in age whose deposition was controlled by significant synsedimentary extension normal faulting mainly active from jurassic through late cretaceous paleogene time resulted in significant lateral thickness variability within the related stratigraphic sequences by late miocene time the sedimentary cover was detached from the underlying basement and was deformed by east verging folds and west dipping thrusts two restored balanced cross sections through the southernmost part of the belt show a coincidence between the early synsedimentary normal faults and the late thrust fault ramps these evidences suggest that synsedimentary tectonic structures such as faults and the related lithological lateral changes can be regarded as mechanically important controlling factors in the process of thrust ramp development during positive tectonic inversion processes

kalahari valley calcretes their nature origins and environmental significance abstract calcretes that form in non pedogenic settings have been widely reported in the geomorphological and geological literature yet they are still poorly understood in comparison with pedogenic varieties this may be because there are assorted types of non pedogenic calcretes often loosely referred to as groundwater calcretes but encompassing groundwater phreatic open valley or confined channel calcrete types forming within vadose and phreatic environments in different geomorphological settings relatively few studies have described the detailed petrological characteristics of such calcretes leading wright and tucker calcretes international association of sedimentologists reprint series 1991 vol 2 blackwell scientific publications oxford p 10 to suggest that much more work is needed to define the groundwater calcrete facies and to devise criteria for its recognition so that the various non pedogenic calcretes can be identified within the quaternary as well as further back in the geological record most descriptions of groundwater calcretes are highly generalistic and do not take into account the variability that may occur as a result of calcretes forming in different positions within a landscape and affected by separate controls thus the range of groundwater calcretes that exists now needs to be studied individually to identify patterns within the distinct types as a step towards expanding our knowledge of the variety of non pedogenic calcretes this paper describes the petrology micromorphology and mode of development of one of these types of calcrete namely that which forms within valley locations but not in definite fluvial channels late quaternary to holocene valley calcretes are described from various dry valleys mekgacha across the kalahari region of central botswana with the majority of samples collected from trunk and tributary valleys of the okwa samples have been analysed in thin section and under scanning electron microscope in order to determine the carbonate matrix type and calcrete micromorphology in general most samples consist of grains of quartzose kalahari sand cemented by fine crystalline often glaebular grain coating and pore filling micrite cement types are fairly consistent and not as micromorphologically variable as has been noted for other kalahari calcretes biological inputs are prevalent in many samples and include networks of calcified rootlets these characteristics reflect the fact that the calcretes formed in a relatively near surface environment with relatively high rates of evaporation

strains developed in the hangingwalls of thrusts due to their slip propagation rate a dislocation model reply abstract fault planes propagate radially from a point source and this failure propagation process is very similar to the movement of a dislocation through a crystal lattice an elastic strain represented by an extra half plane is necessarily imparted to the lattice at an edge dislocation and this is equivalent to the ductile bead which accompanies the propagating tip of a thrust fault this ductile bead migrates with the fault tip and imparts a characteristic internal strain to the thrust sheet a two dimensional model is presented to illustrate the inter relationship between fault plane slip fault tip propagation and internal strain in multilayered sequences internal strain is usually represented by asymmetric folds verging in the thrust transport direction a simple technique the displacement distance plot utilizes the fact that displacement dies out towards the fault tip this plot can be used to quantify the relative stretch measured parallel to the fault movement which is dependent on the slip propagation rate and it may be used to define exactly the position of the fault tip on a cross section examples of fold thrust structures from devon england and pembrokeshire wales are used to illustrate the technique

vertical strain variations in the osen roa thrust sheet north western oslo fjord norway abstract vertical variations in deformation style and shortening within the osen roa thrust sheet are examined on the north western side of the oslo fjord using thrust distribution diagrams and graphs of throw against height in the stratigraphy particular formations are identified in which thrusts are either created or destroyed many thrusts are killed by a combination of 1 dissipation of slip by splaying of thrusts in shales and 2 the presence of a thick competent unit that requires a thrust to attain a specific displacement value in order to propagate through the unit in one jerk the upper didymograptus shales and ampyx limestone make an effective combination that kills thrusts with less than 50 m of throw within the thrust sheet deformation styles change vertically from imbricate slices pop up and triangle zones with tip line folds in the cambro ordovician shales and limestones to buckle folds in the silurian limestones sandstones and shales this change is accompanied by a decrease in shortening from 34 in the former to 17 in the latter hence one or several higher bedding parallel detachment horizons need to be invoked to separate areas of varying deformation character

origin of a thrust related fold geometric vs kinematic tests abstract geometric tests to determine the origin of fault related folds are common but as is typical in structural geology more than one fold origin may yield the final natural geometry thus the results of geometric tests are usually non unique in contrast kinematic tests of origin which employ both geometry and data about deformation commonly yield more constrained if not unique results unfortunately the necessary data collection requires much more work than for a geometric test in this study the thrust related barclay anticline is analyzed both geometrically and kinematically to determine which test is more effective geometric tests using angular relationships indicate three possible origins fault bend fault arrest and break thrust for the kinematic test predicted deformations for interlayer and flexural slip flexural flow simple shear and bending strains are compared to micro and mesostructural distributions solution strain and finite strain from the anticline strain measurements indicate that microscale deformation is uniformly distributed through the structure and is lithification dominated the microscale deformation does not match kinematic predictions and did not accommodate fold formation fold growth was achieved primarily through layer parallel slip restricted mostly to the forelimb and absent in the hinge which eliminates fault bend and fault arrest origins that require material transport from forelimb to backlimb the barclay anticline is therefore interpreted to be a break thrust structure interestingly a suite of contraction faults in the forelimb and hinge indicates material transport from forelimb to backlimb such transport has been discounted for break thrust folds the most important result of this study is that a kinematic test was a necessary step for distinguishing fold origin geometric testing alone was insufficient given that the barclay anticline has geometric characteristics typical of many thrust related folds kinematic testing appears necessary to determine their structural origin

fold generated imbricates examples from the caledonides of southern norway abstract the cambro silurian sequence of the oslo region contains fold trains and closely spaced imbricate thrust faults the imbricates could be interpreted as having ramped off the sole detachment and generated fault propagation folds several imbricate thrusts however appear to die out both downwards and upwards into folds this suggests that the thrusts were generated within the cambro silurian sequence and propagated down to the sole thrust rather than propagating upwards from the sole thrust the mechanical stratigraphy is apparently responsible for this style where during folding relatively brittle limestone members faulted in the highly strained fold forelimbs prior to through going faulting developing in the shales understanding the origin of such imbricates is important when choosing a method of section restoration

quaternary aminostratigraphy of eolianite on lord howe island southwest pacific ocean abstract amino acid racemization aar dating of the eolianite on lord howe island is used to correlate several disparate successions and provides a geochronological framework that ranges from holocene to middle pleistocene time the reliability of the aar data is assessed by analysing multiple samples from individual lithostratigraphic units checking the stratigraphic order of the d l ratios and the consistency of the relative extents of racemization for a suite of seven amino acids three aminozones are defined on the basis of the extent of racemization of amino acids in land snails placostylus bivaricosus and whole rock eolianite samples aminozone a includes placostylus from modern soil horizons e g mean d l leucine ratio of 0 03 0 01 and whole rock samples from unconsolidated lagoonal and beach deposits 0 10 0 01 0 07 0 03 aminozone b includes placostylus 0 45 0 03 and whole rock samples from beach 0 48 0 01 and dune 0 45 0 02 0 30 0 02 units of the neds beach formation deposited during ois 5 the oldest aminozone c comprises placostylus recovered from paleosols 0 76 0 02 and whole rock eolianite samples 0 62 0 00 from the searles point formation which indicate the formation was likely deposited over several oxygen isotope stages ois during and prior to ois 7 these data support independent lithostratigraphic interpretations and are in broad agreement with u th ages of speleothems from the searles point formation and corals from the neds beach formation and with several tl ages of dune units in both formations the aar data reveal that eolianite deposition extends over a significantly longer time interval than previously appreciated and indicate that the deposition of the large dune units is linked to periods of relatively high sea level

early carbonate diagenetic fabrics in the rhizosphere of late pleistocene aeolian sediments analysis of a range of aeolianite deposits from the western mediterranean roquetas in southern spain and campo de tiro in mallorca and the bahamas crab cay on san salvador island has shown that biota can be important in the diagenesis of deposits in the vadose zone and this may aid in the interpretation of palaeoenvironments particularly where pedogenic calcretes are absent this paper looks at how the growth of roots into dune sediments and the associated profusion of micro fauna and micro flora can have on a local scale a profound affect on the early diagenetic and sediment fabric these fabrics are affected at three levels 1 at the site of the former root 2 directly surrounding the root the peritrophic zone and 3 from the peritrophic zone to a few tens of centimetres from the former root

the role of skeletal porosity in aragonite neomorphism strombus and montastrea from the pleistocene key largo limestone florida abstract the neomorphism of aragonite to calcite involves complex textural and chemical alterations that are in part controlled by the degree of communication between diagenetic and ambient formation fluids two taxa montastrea annularis and strombus gigas from the vadose zone of the pleistocene key largo formation in southern florida were examined in order to evaluate the effect of initial skeletal porosity on system closure during neomorphism fabrics of replacement calcite in both taxa are similar consisting of blocky crystals largely restricted to areas of former skeletal aragonite oriented aragonite and organic inclusions result in the preservation of relic fabrics in both taxa the chemical signatures of coralline and molluscan aragonite and calcite are strongly dependent on initial skeletal porosity compared to original aragonite montastrea calcite is enriched in magnesium 1 151 64 ppm versus 2 114 619 ppm and depleted in strontium 7 397 376 ppm versus 2 510 43 ppm these data in conjunction with large calcite oxygen depletions 3 8 0 2 versus 5 7 0 3 and variable calcite carbon depletions 0 6 0 6 versus 1 3 to 7 3 demonstrate significant exchange between neomorphic front and formational fluids in contrast strombus calcite composition is largely inherited from original aragonite and is only slightly enriched in strontium and magnesium 2 299 316 ppm versus 2 519 759 ppm and 96 23 ppm versus 173 79 ppm while carbon exhibits a small depletion 5 3 0 5 versus 4 4 0 8 similarity of str mbus aragonite and calcite compositions reflects high rock water ratios in relatively closed diagenetic systems calcite oxygen depletion 0 5 0 5 versus 3 6 0 4 observed in strombus however requires some exchange between formation and neomorphic front fluids differences in the thickness of replacement calcite rims between taxa reflect variable skeletal porosities and rates of migration of neomorphic fronts neomorphism was six times faster by volume in porous montastrea than in dense strombus skeletal structures thus skeletal porosity influences degrees of system openness during aragonite neomorphism

centrifuge modelling of fold thrust structures in a tripartite stratigraphic succession abstract analog models measuring 127 76 mm in plan were deformed at 2500 4000 g in a centrifuge scaled stratigraphic sequences were constructed of anisotropic multilayers with individual layers of plasticine and silicone putty as thin as 40 m the plasticine silicone putty multilayers are analogs for interbedded competent carbonates and clastics and incompetent pelites given the model ratios of acceleration 2500 g length 5 10 6 specific gravity 0 6 time 10 10 modelling of fold thrust tectonics emphasizes the influence of stratigraphic succession on structural evolution the models are constructed with a tripartite stratigraphic succession comprising basal and upper well laminated and incompetent units and a middle somewhat more isotropic and competent unit the models deform by three mechanisms layer parallel shortening folding and thrust faulting they reproduce a number of fold thrust relationships that have been observed in nature folds are typically periclinal in en echelon arrays folds and thrusts are arcuate in plan reflecting differential shortening fold attitudes grade from upright at high levels to overturned at deeper levels within a structural panel reflecting drag against the basal decollement fold axial surfaces and thrust faults are listric while competent units may be offset by localized displacement on thrust faults the discrete faults may die out both upwards and downwards into regions of ductile strain in less competent units thrust faults appear to follow staircase trajectories through the strata transecting incompetent units at shallow angles to bedding and competent units at steeper angles however the apparent staircase pattern results from propagation of a fault along a relatively straight trajectory through previously folded strata foreland verging thrusts are more common than back thrusts the latter have steeper dips the models suggest a mechanism of thrust ramp nucleation following detachment folding long wavelength buckling of a competent unit can initiate localized strain folding and layer parallel shear in an underlying incompetent unit beneath the anticlines of the competent unit thrust faults propagate up section from these high strain zones through the foreland dipping limbs of buckle folds in the competent unit this mechanism may explain the commonly observed spatial periodicity of thrust ramps the model results bear similarities to natural fold thrust belts in which the stratigraphic succession consists of three mechanical units for example the asiak foreland and bear creek hills fold thrust belts of the slave province northwest territories canada

formation of holocene limestone sequences by progradation cementation and erosion two examples from the bahamas abstract holocene limestones on north bimini and joulter cays bahamas reveal sedimentary structures that are directly comparable to structures in the modern beach environment a complete sequence comprises shallow subtidal intertidal and supratidal deposits holocene and equivalent modern sedimentary features are found on more or less the same topographic levels freshwater cementation phreatic and vadose takes place after accretion and progradation of the shore subsequent erosion exposes the consolidated sediments secondary cementation in the intertidal zone leads to beachrock formation continued erosion may break off blocks of beachrock and cliffs which will be incorporated into the basal part of the sequence during a new phase of progradation a freshwater cemented sequence going from shallow subtidal to supratidal and eolian deposits does not therefore necessarily imply sea level changes but can be created by a cycle of progradation cementation and erosion

three dimensional geometry and kinematics of experimental piggyback thrusting the three dimensional geometry and kinematics of piggyback stacks of imbricate thrust sheets are illustrated and discussed using a single model shortened in a squeeze box strike parallel geometric elements simulated include lateral ramps eyed sheath folds splays and thrust thrust interference fine details of these structures were exposed by eroding a shortened wedge of sand using a newly developed vacuum eroding technique a kinematic analysis of the model shows a stepwise increase in imbricate thrust spacing and or a decrease in rate of nucleation of imbricate thrusts in the direction of thrust transport despite the steady forward advance of a rear wall the piggyback wedge accreted episodically recording different strain domains in longitudinal cross sections strain partitioning in single layers by bed length balancing showed an increase in layer shortening with volume loss and a corresponding decrease in imbricate thrusting and ramp folding with depth

influence of deflection on a fold to fault progression field evidence from near marietta south carolina abstract four periods of deformation d1 d4 are recognized in the lion park road borrow pit near marietta south carolina although each period is characterized by distinct structures d3 produced two structural styles d3a d3b resulting from layer parallel shortening d3a is characterized by detachment folding at the tip of an underlying thrust d3b is a fold to fault progression that was localized by east dipping quartz filled gash fractures the fold to fault progression demonstrates the influence of a mechanical anisotropy on ramp development the early stages of d3b were formed by deflection of northwest directed layer parallel shortening and active down section propagation of folds and thrusts following connection with a splay of basal detachment later d3b stages resulted from up section movement that produced kink folding and a throughgoing thrust this up section movement deformed and modified the geometries of older down section structures detailed mesoscopic field observations integrated with a combination of current thrust fault models are used to interpret the d3b fold to fault progression

alternative model of thrust fault propagation a widely accepted explanation for the geometry of thrust faults is that initial failures occur on deeply buried planes of weak rock and that thrust faults propagate toward the surface along a staircase trajectory we propose an alternative model that applies gretener s beam failure mechanism to a multilayered sequence invoking compatibility conditions which demand that a thrust propagate both upsection and downsection we suggest that ramps form first at shallow levels and are subsequently connected by flat faults this hypothesis also explains the formation of many minor structures associated with thrusts such as backthrusts wedge structures pop ups and duplexes and provides a unified conceptual framework in which to evaluate field observations

characteristics evolution and distribution of quaternary channel calcretes southern jordan cemented and calcretized conglomerates of quaternary age are found within ancient river channels exposed at various heights between 1 and 125 m above the current bed of the wadi dana in southern jordan in cross section the calcrete deposits are typically lensoid in shape and are found in lling palaeochannels cut into the bedrock the fossilized channel sediments preserve evidence of past river conditions sediment loads source areas phases of river down cutting and sediment accretion as well as post depositional alteration processes samples have been analysed in thin section and under the scanning electron microscope in order to determine the calcrete micromorphology cement types and crystal sizes the results show that the river deposits have undergone signi cant displacement and replacement of the detrital grains by caco3 and this has resulted in the cementation in places of over 4 m of river material the cements display an alpha type fabric with no evidence of biogenic activity the nature of the cement fabric within and between deposits differs as a function of two key variables i the speci c location within the channel and ii height above the modern wadi oor firstly in many of the deposits cement crystal size varies with sparite dominating at depth within impermeable palaeowadi channels whereas on the anks of the channels and near the top of the channels micrite is the main type of cement these variations are thought to be a result of differences in the length of residence time of calcium carbonate rich uids in different parts of the channel and the spatially varying in uence of evaporation secondly distinct changes in cement fabric can be seen in channels of different ages with a reduction in the amount of micrite and an increase in the amount of secondary recrystallized pore lling spar over time copyright 2004 john wiley sons ltd

kinetic control of morphology composition and mineralogy of abiotic sedimentary carbonates abstract conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid my ca ratios and crystal morphology crystal composition and carbonate mineralogy resulting in the formation of meteoric equant low magnesium calcite and marine acicular high magnesium calcite and aragonite problematically calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals and aragonite may coexist with calcite in either environment commonly however a systematic relation exists between crystal morphology composition mineralogy and rates of reactant supply to growing crystal surfaces for example equant rather than acicular crystals of calcite form in modern deep and or cold marine meteoric phreatic and deep burial settings where the degree of carbonate saturation and or rates of fluid flow are low in areas of higher saturation and or fluid flow such as in warm shallow marine and meteoric vadose environments acicular calcite may predominate this relation is also seen in systems in which aragonite and calcite form in intimate association aragonite precipitation is favored when rates of reactant supply are high calcite forms when rates are low such relations suggest that crystal morphology composition and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants principally carbonate ions at growth sites precipitating phases are the ones which can best accommodate such excess reactants ambient mg ca ratios only indirectly control the nature of inorganically precipitated carbonate phases

fault propagation folds above thrusts with constant dip abstract previous models of fault propagation folds have been extended to include the case where folding initiates above a thrust ramp of constant dip the model may be useful in analyses of fold thrust structures in thin and thick skinned compressional terrains it may be most applicable to fault propagation folds that develop as a fault propagates from a thick massive brittle unit into a thinly layered anisotropic unit

geochemical mapping of carbonate sediments in the makgadikgadi basin botswana using moderate resolution remote sensing data this paper maps the carbonate geochemistry of the makgadikgadi pans region of northern botswana from moderate resolution 500 m pixels remotely sensed data to assess the impact of various geomorphological processes on surficial carbonate distribution previous palaeo environmental studies have demonstrated that the pans have experienced several highstands during the quaternary forming calcretes around shoreline embayments the pans are also a significant regional source of dust and some workers have suggested that surficial carbonate distributions may be controlled in part by wind regime field studies of carbonate deposits in the region have also highlighted the importance of fluvial and groundwater processes in calcrete formation however due to the large area involved and problems of accessibility the carbonate distribution across the entire makgadikgadi basin remains poorly understood the modis instrument permits mapping of carbonate distribution over large areas comparison with estimates from landsat thematic mapper data show reasonable agreement and there is good agreement with estimates from laboratory analysis of field samples the results suggest that palaeo lake highstands reconstructed here using the srtm 3 arc second digital elevation model have left behind surficial carbonate deposits which can be mapped by the modis instrument copyright c 2006 john wiley sons ltd

late quaternary vadose carbonate diagenesis in coastal and desert dune and beach sands is there a palaeoclimatic signal a study of the variation in cement amounts in aeolianite and sandy beach calcarenites that range in age from holocene to last interglacial is used to assess whether a palaeoclimatic control on vadose diagenesis can be identi ed examples are taken from modern day arid to subhumid settings and represent a geographical distribution ranging from libya and oman to the mediterranean the caribbean and mexico the results indicate that a palaeoclimatic signal can be identi ed in some deposits however in other sediments there is substantial variability in mean cement abundance within deposits as well as between these sand bodies moreover many of the better cemented sediments are located in areas where there is accelerated diagenesis such as in the sea spray zone close to the groundwater table or near to a palaeosurface rather than in climatically wet regions the inference is that palaeoclimatic interpretations are substantially complicated by other factors that affect diagenetic processes and change therefore caution is needed when studying the role of climate in vadose diagenesis in the light of the effects of other intrinsic and extrinsic controls this paper does not aim to provide a de nitive comparison of sites from different climatic zones the approach taken here is a to see if there is an overall palaeoclimatic signal in the samples studied b to use examples to illustrate how explanations controls other than those relating to climate can account for the variabilities observed and c if there is a climatic effect to see if it is the main over riding control on vadose diagenesis the conclusions drawn from this research highlight that it is possible to misinterpret evidence if the study is approached with preconceived notions of simplistic relationships between diagenesis and climate copyright 2004 john wiley sons ltd

the kinematics of break thrust folds abstract the kinematics of asymmetric open to close often overturned thrust related folds are commonly explained with fault propagation or detachment fold models field observations of thrust related folds exposed in tennessee virginia wyoming and montana that exhibit this geometry indicate that models of fault propagation and detachment folds do not adequately describe the kinematics of these structures existing models of fault propagation and detachment folding employ migrating kink shaped hinges and relate fold geometry entirely to fault geometry slip and to the thickness of the basal detachment layers the models exclude the relation between fold shape and amplitude and they do not predict a correlation between competent layer thickness and fold wavelength as expected from buckling theory thrust related folds examined in this study exhibit unique distributions of fabrics in the hinges and forelimbs which demonstrate a lack of hinge migration furthermore the ratios of dominant member thickness to fold wavelenght agree with those expected for folds controlled by a single layer and they are within the expected range for folds with multiple evenly spaced harmonically deforming layers our analyses suggest these folds evolved from an initial brief stage of sinusoidal buckling to a later stage of fixed hinge kinking and thrusting thrust related folds which exhibit a geometry and mesofabric distribution suggesting this type of evolution are defined as break thrust folds

influence of detachment layer thickness on style of thin skinned shortening abstract there are several parameters which may determine whether detachment folds or thrust faults develop in thin skinned compressional systems a fundamental but little publicised parameter is apparent from consideration of detachment fold growth as a detachment fold amplifies its core must be filled by redistribution of ductile material if the mobile material were insufficient to supply the volume required by the fold core through redistribution fold growth would be inhibited and eventually faulting would occur the volume of a detachment fold core is compared here with the volume of available underlying mobile material for various amounts of shortening and sizes of fold the occurrence of detachment folds versus thrusts in several compressional belts is successfully predicted by this relationship

the importance of sea spray to the cation budget of a coastal hawaiian soil a strontium isotope approach soil nutrients such as ca mg and k are traditionally thought to be derived primarily from rock weathering here we show that sea spray is a significant source of nutrient elements to modern and buried soils developed on 30 000 year old pahala ash deposits 50 m from the coast at south point hawaii the soil profiles evolved in a semi arid climate and have always been above sea level and the water table rhizoliths fossilized root traces and horizontal laminated carbonate sheets found in buried soils are composed of high mg calcite up to 14 mol mgco3 differences in strontium isotopic composition between marine aerosols 87sr 86sr 0 7092 and tephra parent material 0 7035 allow quantification of cation sources to the labile soil reservoir and to pedogenic carbonate mixing equations indicate that 50 80 of labile soil sr and approximately half of carbonate sr was derived from marine sources using the sr isotopic signatures and sr ca ratios of seawater and tephra as end members we determined that up to 2 3 of the ca in the labile reservoir and up to 1 3 of ca in the carbonates has a marine origin carbonate 87sr 86sr ratios are fairly constant with depth but labile 87sr 86sr ratios indicate decreasing sea spray aerosol influence with depth this trend could be due either to sequestering of aerosol derived sr in the upper part of the profile or to lower aerosol input in the past due to lower sea level the unusual occurrence of high mg pedogenic calcite probably results from high labile mg ca ratios during earlier stages of weathering coupled with rapid calcite precipitation during soil pore water evaporation

rates of vadose diagenesis in quaternary dune and shallow marine calcarenites warrnambool victoria australia abstract progressively younger cycles of quaternary dune limestones and shallow marine calcarenites infill a coastal embayment near warrnambool in southeastern australia the formations have been dated using u th and c 14 methods or have had their ages estimated by comparison with adjacent deposits they provide a well dated framework for the quantification of diagenetic rates in the fresh water vadose zone the sequence as a whole shows progressive increase in diagenetic alteration with age related to increasing duration of subaerial exposure diagenesis includes alteration of high mg calcite to low mg calcite dissolution of aragonite and precipitation of increasing amounts of void filling low mg calcite spar these changes are reflected in the progression of bulk rock isotope values from the youngest to the oldest strata the rocks at warrnambool pass through 3 main stages of diagenetic evolution the end of stage 1 is marked by the disappearance of high mg calcite this occurs in just less than 90 000 years the end of stage 2 occurs between 400 000 and 700 000 years and is marked by the disappearance of aragonite the end of stage 3 and the beginning of stage 4 is marked by the complete infilling of all pore space by low mg calcite spar this is not generally seen in this sequence but is restricted to specific diagenetic sites such as immediately below paleosol horizons

multistory duplexes with forward dipping roofs north central brooks range alaska the endicott mountains allochthon has been thrust far northward over the north slope parautochthon in the northern brooks range progressively younger units are exposed northward within the allochthon to the south the incompetent hunt fork shale has thickened internally by asymmetric folds and thrust faults northward the competent kanayut conglomerate forms a duplex between a floor thrust in hunt fork and a roof thrust in the kayak shale to the north the competent lisburne group forms a duplex between a floor thrust in kayak and a roof thrust in the siksikpuk formation both duplexes formed from north vergent detachment folds whose steep limbs were later truncated by south dipping thrust faults that only locally breach immediately overlying roof thrusts within the parautochthon the kayak lisburne and siksikpuk equivalent echooka formation form a duplex identical to that in the allochthon this duplex is succeeded abruptly northward by detachment folds in lisburne these folds are parasitic to an anticlinorium interpreted to reflect a fault bend folded horse in north slope basement with a roof thrust in kayak and a floor thrust at depth these structures constitute two northward tapered internally deformed wedges that are juxtaposed at the base of the allochthon within each wedge competent units have been shortened independently between detachments located mainly in incompetent units the basal detachment of each wedge cuts upsection forward northward to define a wedge geometry within which units dip regionally forward these dips reflect forward decrease in internal structural thickening by forward vergent folds and hindward dipping thrust faults

distribution of fault slip in outcrop scale fault related folds appalachian mountains abstract current kinematic models of ramp related folds predict a direct relationship between ramp angle and fold shape and imply specific sequences of deformation analyses of outcrop scale structures in the valley and ridge province of the appalachians reveal configurations that depart from model predictions the models fail to account for the presence of footwall synclines and are inconsistent with measured displacement distributions on some natural faults observations support the interpretation that faults can grow by propagation both up and down dip from a nucleation point fault propagation in either direction may result in the formation of folds primarily on the side of the fault that is displaced in the direction of fault propagation

structural models of faulted detachment folds faulted detachment folds represent a common structural style in several hydrocarbon producing fold and thrust belts including the zagros idaho wyoming utah carpathian and albanide belts and several deep water fold belts including the mississippi fan and per dido belts these structures form in units with high competency contrasts and display a transition in deformational behavior from detachment folding to progressive fault propagation with increasing shortening the structures first form by symmetric or asymmetric detachment folding with the fold wavelength controlled by the thickness of the dominant units volumetric constraints require the movement of material from the synclines to the anticlines contin ued shortening results in tighter folds with limb rotation by hinge migration limb segment rotation and internal shear rotation of limb segments between locked hinges results in thrust faults which propagate through deformation zones on the frontlimb and possibly the backlimb the faults may terminate both up and down section within brittle or ductile deformation zones eventually a through going fault connects one of the major faults with the basal detach ment variations in the structural geometry are related to the rela tive thickness and competency of the units and the mechanisms of deformation because faulted detachment folds superficially resem ble fault propagation folds their geometry can be misinterpreted in areas of poor data quality characteristic features for distinguish ing these structures include 1 smaller variations in anticlinal length during fold evolution 2 a more open and rounded fold geometry 3 complex and nonuniform variations in fault slip 4 a significant variation in deformational behavior among different units and 5 fold fault relationships indicating transition from folding to faulting such as footwall synclines and decapitated fold geometries

the character and genesis of calcrete in late quaternary alluvial deposits gujarat western india and its bearing on the interpretation of ancient climates late quaternary deposits in mainland gujarat contain sediments deposited in subhumid and semi arid climates the 30 35 m succession shows the presence of vertisols at the base and a red bed horizon ferric calcisol when pedogenic in origin that roughly bisects the succession a widespread development of calcretes is observed throughout the succession the various varieties of calcrete include pedogenic calcrete groundwater calcrete calcrete conglomerate transported calcrete and rhizogenic calcrete pedogenic calcrete nodules associated with vertisols and the red soil show marked differences in morphology dimensions and the distribution of microscopic features these differences arise due to contrasting climate controlled physicochemical environs under which they formed pedogenic calcrete nodules associated with vertisols acquire large 5 10 cm dimensions and are characterised by either showing the presence of a nucleus of soil matter or showing a dense micritic groundmass cut by thick sparitic veins in contrast calcrete nodules that formed in the red soil are 3 cm in size and do not exhibit dense networks of sparitic veins calcretes associated with vertisols and the ferric calcisol also exhibit differences in the morphology of rhizoliths these differences also show up in the distribution of microfabrics of calcretes grain coats are present in rhizogenic and pedogenic calcretes but absent in groundwater calcretes and less profuse in hydromorphic soil calcretes clotted micrite is present in all types except groundwater calcretes sparitic veins however are observed in each type but are relatively less developed in groundwater calcretes a similar distribution of displacive and replacive textures is also seen although some grains in groundwater calcretes showed signs of corrosion the vertisol associated calcretes represent a subhumid 500 700 mm climate whereas the red soil calcretes suggest a semi arid 100 500 mm climate calcretes from the vertisol association show a range in 13c composition constrained between 9 and 5 whereas the red soil calcretes exhibit the whole spectrum of values from 9 to 1 based on mineralogical associations calcretes usually taken to reflect semi arid episodes in the earth s history may be classified further calcretes when associated with sepiolite palygorskite suggest an arid climate mean annual rainfall 50 100 mm when associated with smectite haematite and the absence of hydromorphism a semi arid climate mean annual rainfall 100 500 mm and when found in smectitic vertisols subhumid climates mean annual rainfall 500 700 mm

early diagenesis atherton formation quaternary a guide for understanding early cement distribution and grain modifications in nonmarine deposits abstract the atherton formation represents an extensive sheet of outwash and related materials of quaternary age these sand q77f8r15 and gravel sediments are locally lithified and present an excellent opportunity for evaluating the effects of early diagenesis on a terrigenous deposit the principal cement present is calcium carbonate 98 occurring as euhedral to anhedral grain contact pore lining and porefilling varieties limonite and gypsum are present as rare cements cement distribution was controlled at various levels by 1 stratification character 2 grain size and sorting 3 framework grain composition and 4 proximity to near surface processes initial cement development and subsequent early growth were primarily parallel to laminae within beds this precipitation site preference is related to initially higher relative permeabilities estimated at 2 to 2 5 x parallel versus normal to stratification when associated with trough cross strata cements preferentially are located in the lower portion of troughs or directly below trough set boun aries there is a slight preference for early formed cements to be present along the axis instead of the flanks of a trough about 50 of pebble gravel and sandy pebble gravel units contain locally cemented zones while 17 of finer grain size strata are locally lithified over 95 of locally cemented units are within 7 m of the present ground surface suggestive of a causal relation to near surface perhaps soil related processes the sources for the various constituents composing cements are related to the dissolution of previously cemented layers and framework components such as carbonate rock fragments calcareous pelitic rock fragments and iron bearing silicate grains the examination of oversized voids elongate pores and intraparticle porosity suggests that at least locally multiple episodes of cementation followed by dissolution have taken place reactions between carbonate cement and limestone rock fragments have destroyed a high percentage of detrital carbonate grain boundaries such reactions combined with early dissolution can result in an information loss in excess of 50 of the limestone rock fragment component in a sandstone a combination of organic acids and co2 mixed with meteori waters is likely responsible for the dissolution and transportation of cement constituents

the geographic distribution of sr isotopes from surface waters and soil extracts over the island of bornholm denmark a base for provenance studies in archaeology and agriculture abstract in this paper we report the sr isotope signatures and sr al and na concentrations of 30 surface waters lakes ponds and rivers creeks and 19 soil sample extracts from the island of bornholm denmark and present a categorized 87 sr 86 sr value distribution map that may serve as a base for provenance studies including archaeological migration and authenticity proof for particular food products the sr isotopic compositions of surface waters range from 87 sr 86 sr 0 7097 0 7281 average 0 7175 0 0049 1 whereas 0 1 m hno 3 0 05 m hno 3 and 0 01 m cacl 2 soil extracts range from 87 sr 86 sr 0 7095 0 7197 and define somewhat lower but statistically indistinguishable averages of 0 7125 0 003 1s these compositions are lower than the values expected from the precambrian granitoid basement 87 sr 86 sr 0 758 0 944 and from the overlying mainly clastic paleozoic sediments combined sr isotope composition vs sr na and al concentration relationships of soil extracts imply that lowering of the isotopic composition of leachable sr on bornholm results as a consequence of significant admixture to this fraction of sr deposited as marine salts aerosols and that rainwater only has a minor influence on the sr budget of the surface waters positively correlated al na and 1 sr vs 87 sr 86 sr relationships in soil extracts and surface waters indicate that the surface run off on bornholm is characterized by two predominant sources namely marine aerosols sea salts with high sr and low 87 sr 86 sr values and a source with lower sr delivering radiogenic sr to the surface waters which we equate with sr leached from the products of mineral weathering soils a feasibility study for using sr isotopic compositions of surface waters and soil extracts as a proxy for bioavailable sr signatures was performed with a few samples collected in the vicinity of the eleventh century ad ndr grodbygard cemetery site in sw bornholm from where sr isotope compositions of modern fauna samples and tooth enamel of humans buried in the cemetery have been reported waters and soil extracts studied herein from around this site range from 87 sr 86 sr 0 7104 0 7166 and correspond to sr compositions extracted from snail shells in this area which span a range of 87 sr 86 sr 0 7095 0 7160 some human tooth enamel is characterized by more radiogenic values 87 sr 86 sr up to 0 718 which points to a possible provenance of these humans from the granite gneiss terrain in the north of the island and or to immigration of these humans in their childhood from other places for example from mainland sweden to bornholm if the total compositional range of 87 sr 86 sr 0 709 0 718 n 44 recorded in human enamel from the ndr grodbygard site is considered representative for the variation of bioavailable sr on bornholm then our soil leachate and surface water data entirely covers this range we therefore propose that the combination of sr isotope analyses of surface waters and soil leachates are an easy fast and relatively cost efficient way to characterize a local bioavailable 87 sr 86 sr signature and consequently propose that the overall average of 87 sr 86 sr 0 7153 0 0048 1 n 50 can be taken as a band for bioavailable sr fractions suitable to discriminate between local and non local signatures in provenance studies in the field of archaeology and for food and plant authenticity control in agricultural applications

degassing water around air bubbles entrapped in the vadose zone as a mechanism of carbonate precipitation a hypothesis abstract irreversible water degassing connected with periodic entrapment of pore air and formation of bubbles after the wetting of unsaturated sediments followed by the subsequent release of air from the bubbles after the sediments dry is proposed as a possible mechanism for the precipitation of calcium carbonate in the vadose zone the process involves an initial stage during rainy periods when pore air may be entrapped due to preferential water flow in large pores equilibration of the water air system causes pressure in the entrapped air bubbles to decrease to a value close to the sum of atmospheric and capillary pressure 40 95 of atmospheric pressure this pressure decrease leads to degassing of pore water adjacent to the bubbles bubble expansion and a shift in the equilibrium of all aqueous reactions involving dissolved gases in particular the degassing must accelerate carbonate precipitation a second stage develops when entrapped air is released during lowering of the groundwater table and or drying of sediment release of entrapped air that has been subjected to a pressure lower than atmospheric should draw in a portion of free pore air towards the water air interface that free pore air has a lower gas concentration compared to that in the released air which causes fast mixing between the two in addition transfer of a portion of the degassed water towards the groundwater occurs in combination these processes cause the degassing and corresponding geochemical processes in the zone of air entrapment and release to be irreversible the proposed degassing mechanism of carbonate precipitation may take place in the wetting front zone that passes through unsaturated sediments from the land surface downwards after rainy periods this mechanism may also occur at the upper boundary of the fluctuating capillary fringe of phreatic aquifers including perched groundwater periodically formed within unsaturated sediments that lie upon more clayey less permeable layers for a sandy capillary fringe the minimum rate of carbonate precipitation by this mechanism is estimated in a wide range of 120 to 175 200 mg m 2 yr 1 at these rates within a 1 m zone of capillary fringe fluctuations sand might become cemented until pourability is lost for from 800 to 1 170 000 years

late pleistocene mixing zone dolomitization southeastern barbados west indies field geochemical and petrographic data for late pleistocene dolomites from southeastern barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters the dolomite is localized in packstones and wackestones from the algalamphistegina fore reef calcarenite facies stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization carbon isotopes in pure dolomite phases average about 15 0 pdb this light carbon is attributed to the influence of soil gas co2 and precludes substantial mixing with seawater a narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint dolomitization likely occurred with as little as a five per cent admixture of seawater strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy the dolomite is characterized by low sodium values low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization a sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix limpid euhedral dolomite cements precipitated in primary intra and interparticle porosity subsequent to replacement dolomitization as waters became progressively less saline dolomite cements alternated with thin bands of syntaxial calcite cement the final diagenetic phase precipitated was a blocky calcite spar cement representing diagenesis in a fresh water lens this sequence of diagenetic features arose as the result of a single fall in eustatic sea level following deposition a stratigraphic eustatic diagenetic model constrains both the timing and rate of dolomitization in southeastern barbados dolomitization initiated as sea level began to fall immediately following the oxygen isotope stage 7 3 high stand some 216 000 yr bp due to the rapidity of late pleistocene glacio eustasy dolomitization locally complete is constrained to have occurred within about 5000 yr

the hydrogeochemistry of early meteoric diagenesis in a holocene deposit of biogenic carbonates abstract this study presents the results of a quantitative hydrogeochemical investigation of early meteoric diagenesis in a deposit of biogenic mixed mineralic holocene carbonate sediments at ocean bight in the southeastern bahamas ground water samples were collected from a network of 27 wells arranged into 4 transects across the 1 5 km2 strand plain deposit four wells provided multi depth samples at 0 5 m intervals thus allowing for the construction of vertical chemical profiles analyses of the water consisted of temperature ph alkalinity ci ca2 mg2 sr2 so42 and 13c data derived from these analyses were used in a series of equilibri m and mass balance equations to calculate mineral saturation levels and magnitudes and rates of mineral transformation reactions interpretation of measured and calculated parameters provided a detailed picture of patterns and processes of meteoric diagenesis at ocean bight four features of meteoric diagenetic processes operating at ocean bight are distinct from processes documented at previously studied non biogenic and aragonite dominated holocene systems 1 lighter cementation with respect to magnitudes of calcite precipitation 2 diminished magnitudes of aragonite dissolution 3 elevated rates of mineral stabilization and 4 lower efficiencies of transformation reactions another distinction of this study which is not thought to be attributed solely to the presence of mixed biogenic carbonates is the isolation of transformation reactions to the vadose and upper phreatic zone this result contradicts the popularly held belief that metastable carbonate deposits undergo rapid equilibration throughout the freshwater phreatic zone this distribution of diagenetic reactions along with the analytical results characterizes a chemical environment dominated by carbonate mineral transformation reactions linked mainly to the oxidation of organic matter and not only to the relative solubility of the different carbonate minerals this finding may prove important in attempts to explain the preservation of metastable minerals in ancient geologic sequences despite repeated exposure to freshwater phreatic zones this finding also questions the assumption that the extent of paleo phreatic lenses is equal to the observed extent of freshwater phreatic alteration

review of major shale dominated detachment and thrust characteristics in the diagenetic zone part i meso and macro scopic scale there is considerable variability in the thickness structure and macroscopic deformation style within shale prone major thrust zones which also impacts the structural style in the overlying and sometimes the underlying section the properties and percentages of clay minerals and percentages of organic material quartz and calcite affect fault zone weakness and behaviour the distribution of competent units within a shale prone zone can also significantly affect fault zone architecture even over short strike distance of 10s to 100s of metres other influences include high pore fluid pressures and associated veins deformation of the fault zone may disrupt the original principal displacement zones and force them to relocate including switching lithologies calcite veins can become concentrated locally in the fault zone and become a focus of fault zone weakness we still need many more descriptions of shale prone fault zones to understand the potential variations in thrust sheet structural style caused by differences in shale detachment characteristics the variations in structural style in the hanging walls of shale detachments are broad and range between the sledge runner style in many appalachian examples and the extensive closely spaced imbricate style of the osen roa detachment southern norway such contrasts are related to several factors including thrust sheet mechanical stratigraphy thickness of the overburden lateral continuity of the detachment detachment mechanical properties composition pore fluid pressures presence absence of higher level detachments and strain rate in many structural locations and at many scales shale prone sequences will pile up to produce over thickened masses cleavage duplexes s c fabrics duplexes imbricates pseudo duplexes and vein formation can all contribute to the thickening at nested scales a mixture of brittle and solution diffusion processes overall cause the affected unit to exhibit flow like characteristics while the broad principles are now well established considerable work remains to be done understanding the full range of structural geometries associated with shale detachments what factors regarding the nature of the detachments affect overlying structural styles rates of deformation and the controls on deformation mechanisms within the thrust zones

the role of fungal biomineralization in the formation of early carboniferous soil fabrics paleosols in the lower carboniferous limestones of south wales commonly contain needle fibre calcite which is an unusual form of calcite recently shown to form by the calcification of fungal hyphae in present day soils the needle fibre calcite occurs in two associations in the paleosols as coatings on sediment grains and as rhizocretions the former can be compared with the microbial grain coatings of quaternary calcretes the latter represent the sites of fungal coats on roots and are interpreted as probable ectomycorrhizae a symbiotic fungal sheath root association these findings suggest that biomineralization was important in the formation of soil fabrics during the carboniferous as it is in present day soils

thrusting in a folded regime fold accommodation faults in the ruhr basin germany abstract the late carboniferous ruhr basin in germany has been described as an authochtonous foreland basin of the variscan orogen the structure of the basin is fold dominated with fold accommodation faults as secondary elements vast and excellently documented 3d mining exposures provide extensive data on the geometry of thrusts and the relationship between thrusting and folding stratigraphic throw is the determining factor in the geometry of thrusts and displacement and shortening are functions of throw material behaviour and bedding dip built up on a basic geometric model a complex system of different fault types can be described linked regularly to each other and developed during the folding process

fabric selective diagenesis in the late pleistocene miami limestone abstract the miami limestone is a late pleistocene ooid sand body which has undergone 130 000 years of meteoric diagenesis the resultant limestone has a generally high but variable 15 60 porosity which is indirectly related to the amount of primary aragonite remaining in the limestone 0 45 the predominant textural theme in porosity development is the role of the primary sediment fabric which controls early cementation and porosity development on the scale of individual grains and sedimentary structures and is still influential during late development of channel and cavernous porosity the porosity development within the miami limestone reflects a change from an early system wherein fabric selective diagenesis occurred primarily as a function of the ability of the sediment to retain pore water to a system wherein the cemented sediments channelize water flow and effectively isolate portions of metastable aragonitic limestone from meteoric waters

fold thrust structures where have all the buckles gone special publication title folding and fracturing of rocks 50 years of research since the seminal text book of j g ramsay we dedicate the paper to the memory of martin casey 1948 2008 who did much through good humored argument to ensure that buckling ideas were not lost to what he called the ramping club the thrust belt community the fold thrust research group has been funded by interoil oilsearch and santos we thank paul griffiths and anonymous referee for comments together with hermann lebit for scientific editing the views expressed here of course remain those of the authors

the mechanics and geological implications of dust transport and deposition in deserts with particular reference to loess formation and dune sand diagenesis in the northern negev israel

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

variability in early vadose carbonate diagenesis in sandstones abstract early carbonate diagenesis in calcareous and siliciclastic sandstones in the vadose environment is reviewed and shown to be more extensive than generally recognized and more importantly highly variable in character both spatially and temporally spatial variability is classed into three hierarchical scales macro regional meso sediment body and micro individual and groups of pores dependent and independent variables relating to the diagenesis are shown to vary according to scale at the macro scale the independent variables are tectonic activity climate sea level and lithofacies at the meso scale they include location of the sediment body relative to the watertable and sea level and exposure to sea spray plant cover water balance relationships texture and allochem content micro scale independent variables are the chemistry amounts and mobility of the pore waters and individual particle microstructures and mineralogies dependent variables at one scale are shown to exist as independent variables at the next more detailed scale the infinitely complex combinations of variables operating over different time scales explain the observed and characteristic variability in diagenetic processes and products within the vadose zone

the structure and diagenesis of rhizoliths from cayman brac british west indies abstract rhizoliths from the pleistocene ironshore formation are formed of a brown micrite core surrounded by sediment that is well cemented with 1 calcite rhomb chains 2 isolated calcite rhombs 3 calcified spherical bodies 4 types i and ii calcified epilithic and chasmolithic filaments and 5 grain coating calcite needle mats the friable host sediment has significantly less cement and lacks the grain coating calcite needle mats and type ii calcified filaments of the rhizoliths the sediment however contains calcite needles and a cryptocrystalline cement which do not occur in the rhizolith the contrast in the amount and type of cement between the rhizoliths and the encasing sediment points to the significant role that plants can play in carbonate diagenesis not only do the plant roots act as conduits for water of varying composition they also provide sites at which algae fungi and bacteria thrive these microorganisms serve to promote diagenesis in the rhizoliths the occurrence of the sediments in a coastal setting means that they are subjected to many different types of water ranging from freshwater to saltwater such variability along with the associated changes in the water chemistry may be responsible for the considerable variation in the cement associated with the rhizoliths

fault related folding in sandbox analogue models of thrust wedges abstract the nucleation and growth of thrust fault related folds has been simulated using high resolution sandbox coulomb wedge type analogue models thrust related anticlines grew progressively from initial layerparallel shortening to decollement folding thrust tip folding and eventually thrust ramp folding thrust ramps nucleated in the central sector of the overturned forelimb of decollement folds as a consequence of limb thinning thrust tip folding occurred during outward migration of the ramp tips the fault propagation rate was greater at the lower ramp tip which quickly joined the basal thrust flat while folding continued beyond the upper tip the development of the analogue model thrust fault related folds by progressive changes in kinematics from decollement folding to thrust tip folding and finally thrust ramp folding compares well with analyses of naturally occurring thrust fault related folds

mechanical and fracture stratigraphy using examples from core studies this article shows that separate identification of mechanical stratigraphy and fracture stratigraphy leads to a clearer understanding of fracture patterns and more accurate prediction of fracture attributes away from the wellbore mechanical stratigraphy subdivides stratified rock into discrete mechanical units defined by properties such as tensile strength elastic stiffness brittleness and fracture mechanics properties fracture stratigraphy subdivides rock into fracture units according to extent intensity or some other observed fracture attribute mechanical stratigraphy is the by product of depositional composition and structure and chemical and mechanical changes superimposed on rock composition texture and interfaces after deposition fracture stratigraphy reflects a specific loading history and mechanical stratigraphy during failure because mechanical property changes reflect diagenesis and fractures evolve with loading history mechanical stratigraphy and fracture stratigraphy need not coincide in subsurface studies current mechanical stratigraphy is generally measurable but because of inherent limitations of sampling fracture stratigraphy is commonly incompletely known to accurately predict fractures in diagenetically and structurally complex settings we need to use evidence of loading and mechanical property history as well as current mechanical states

rifting in heterogeneous lithosphere inferences from numerical modeling of the northern north sea and the oslo graben oslograbenevolvedinthecoldandstableprecambrian lithosphere of fennoscandia whereas the northern northsearifttookbirthinfreshlyreworkedcaledonian lithosphere huge volumes of magmatic rocks characterize the relatively narrow oslo graben in contrast little magmatism is documented for the wide northern north sea rift differences in timing between both rifts are inferred but still debated we present numerical thermomechanical models along a lithospheric e w section that involves both the oslo graben and the northern north sea area because the modeled section crosses the boundary between caledonian and proterozoic provinces thermal and compositional heterogeneities are considered as is suggested by various geophysical data sets we also consider lithospheric thickness heterogeneities in the precambrian lithosphere modeling results suggest that the northern north sea was on top of weak lithosphere very sensitive to far field stresses consequently we suggest that rifting in the northern north sea began as early as regional extension was effective i e late carboniferous early permian and does not postdate the oslo graben as it is commonly assumed rifting in the strong precambrian lithosphere is unexpected modeling results suggest thatapre existinglithosphericthicknesscontrastwithin the fennoscandian lithosphere favored rifting in the oslo graben index terms 8109 tectonophysics continental tectonics extensional 0905 8159 tectonophysics evolution of the earth rheology crust and lithosphere 8020 structural geology mechanics 9335 information related to geographic region europe 9614 information related to geologic time paleozoic keywords numerical modeling rifting rheology lithosphere oslo graben north sea citation pascal c and s a p l cloetingh rifting in heterogeneous lithosphere inferences from numerical modeling

deepwater fold and thrust belt classification tectonics structure and hydrocarbon prospectivity a review abstract deepwater fold and thrust belts dwftbs are classified into near field stress driven type 1 systems confined to the sedimentary section and type 2 systems deformed by either far field stresses alone or mixed near and far field stresses dwftbs can occur at all stages of the wilson cycle up to early stage continent continent collision type 1 systems have either weak shale or salt detachments they occur predominantly on passive margins but can also be found in convergent related areas such as the mediterranean and n borneo examples include the niger and nile deltas the west coast of africa and the gulf of mexico type 2 systems are subdivided on a tectonic setting basis into continent convergence zones and active margin dwftbs continent convergence zones cover dwftbs developed during continent arc or continent continent collision and those in a deepwater intracontinental setting e g w sulawesi makassar straits active margins include accretionary prisms and transform margins the greatest variability in dwftb structural style occurs between salt and shale detachments and not between tectonic settings changes in fold amplitude and wavelength appear to be more related to thickness of the sedimentary section than to dwftb type in comparison with shale salt detachment dwftbs display a lower critical wedge taper more detachment folds long and episodic duration of deformation and more variation in vergence structures unique to salt include canopies and nappes accretionary prisms also standout from other dwftbs due to their relatively long continuous duration rapid offshore propagation of the thrust front and large amount of shortening in terms of petroleum systems many similar issues affect all dwftbs these include the oceanward decrease in heat flow offshore increase in age of mature source rock and causes of trap failure e g leaky oblique and frontal thrust faults breach of top seal by fluid pipes one major difference between type 1 and type 2 systems is reservoir rock high quality continent derived quartz rich sandstones are generally prevalent in type 1 systems more diagenetically reactive minerals derived from igneous and ophiolitic sources are commonly present in type 2 systems or many are simply poor in well developed turbidite sandstone units however some type 2 systems particularly those adjacent to active orogenic belts are partially sourced by high quality continent derived sandstones e g nw borneo s caspian sea columbus basin in some cases very high rates of deposition in accretionary prisms adjacent to orogenic belts coupled with uplift due to collision results in accretionary prism related fold belts that pass laterally from sub aerial to deepwater conditions e g s caspian sea indo burma ranges the six major hydrocarbon producing regions of dwftbs worldwide gulf of mexico niger delta nw borneo brazil west africa s caspian sea stand out as differing from most other dwftbs in certain fundamental ways particularly the very large volume of sediment deposited in the basins and or the great thickness and extent of salt or overpressured shale sdetachments

geochemistry of meteoric calcite cements in some pleistocene limestones in this study the stable isotope and trace element geochemistries of meteoric cements in pleistocene limestones from enewetak atoll western pacific ocean cat island bahamas and yucatan were characterized to help interpret similar cements in ancient rocks meteoric calcite cements have a narrow range of 18o values and a broad range of 13c values in each geographical province these pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location enewetak calcite cements have a mean 18o composition of 6 5 0 pdb and 13c values ranging from 9 6 to 0 4 0 pdb sparry calcite cements from cat island have a mean 18o composition of 4 1 0 and 13c values ranging from 6 3 to 1 1 0 sparry cements from yucatan have a mean 18o composition of 5 7 0 and 13c values of 8 0 to 2 7 0 the mean 18o values of these pleistocene meteoric calcite cements vary by 2 4 0 due to climatic variations not related directly to latitude the 13c compositions of meteoric cements are distinctly lower than those of the depositional sediments variations in 13c are not simply a function of distance below an exposure surface meteoric phreatic cements often have 13c compositions of less than 4 0 0 which suggests that soil derived co2 and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements concentrations of strontium and magnesium are quite variable within and between the three geographical provinces mean strontium concentrations for sparry calcite cements are for enewetak atoll 620 ppm 510 ppm for cat island 1200 ppm 980 ppm and for yucatan 700 ppm 390 ppm equant cements intraskeletal cements and bahamian cements have higher mean strontium concentrations than other cements equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements magnesium concentrations in pleistocene meteoric cements are similar in samples from enewetak atoll mean 1 00 mol mgco3 0 60 mol mgco3 and cat island mean 0 84 mol mgco3 0 52mol mgco3 but yucatan samples have higher magnesium concentrations mean 2 20 mol mgco3 0 84mol mgco3 higher magnesium concentrations in some yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water

milankovitch band cyclicity in bedded halite deposits contemporaneous with late ordovician early silurian glaciation canning basin western australia abstract the mallowa salt of the early palaeozoic carribuddy group canning basin western australia is a halite mudstone anhydrite dolomite evaporite sequence of late ordovician to early silurian age deposited in barred marginal marine to ephemeral saltpan and saline mudflat environments bhp utah minerals potash exploration well gingerah hill no 1 obtained drill core of a 477 m stratigraphic interval of the mallowa salt which permitted the study of geochemical variations in a thick halite sequence contemporaneous with late ordovician early silurian glaciation on other continents a strip of uniform width was continuously ground from the core and the powdered rock bagged at 1 m intervals for geochemical assay the resulting geochemical stratigraphic series display conspicuous cyclicity several orders of cycles ranging from 3 m to100 m stratigraphic thickness are discernible in the plots of minor constituents br mgo and k2o in extracted soluble salts and also in the plots of na2o and total salts content fast fourier transform fft of the br mgo and k2o series gives principal spectral peaks in the 0 00 0 10 cycles m frequency band at 250 113 35 4 22 1 and 19 7 m as well as peaks at 65 m and between 2 8 and 13 5 m fft of the na2o and total salts series shows dominant spectral peaks at 220 259 and 109 114 m if the strongest spectral peak at 113 m is taken to represent the relatively stable eccentricity period of 100 ka and a constant net rate of accretion assumed for long sections the other main periods in the mallowa salt as exemplified by the br spectrum would be31 3 3 0 19 6 1 1 and17 4 1 1 ka these figures are consistent with predicted late ordovician early silurian 440 ma periods for obliquity 30 5 ka and precession 19 3 and 16 4 ka based on evolutionary change in the earth moon system additional spectral peaks identified with implied periods of approximately 206 233 57 5 8 12 and 2 5 3 5 ka also may be of palaeoclimatic relevance the relative amplitudes and structure of spectral peaks in the 0 00 0 10 cycles m frequency band support the identification of climatic oscillations forced by orbital cycles the data indicate a precession eccentricity dominated pattern which accords with the know low palaeolatitude

quaternary calcarenite stratigraphy on lord howe island southwestern pacific ocean and the record of coastal carbonate deposition abstract lord howe island is a small mid ocean volcanic and carbonate island in the southwestern pacific ocean skeletal carbonate eolianite and beach calcarenite on the island are divisible into two formations based on lithostratigraphy the searles point formation comprises eolianite units bounded by clay rich paleosols pore filling sparite and microsparite are the dominant cements in these eolianite units and recrystallised grains are common outcrops exhibit karst features such as dolines caves and subaerially exposed relict speleothems the neds beach formation overlies the searles point formation and consists of dune and beach units bounded by weakly developed fossil soil horizons these younger deposits are characterised by grain contact and meniscus cements with patchy pore filling micrite and mirosparite the calcarenite comprises several disparate successions that contain a record of up to 7 discrete phases of deposition a chronology is constructed based on u th ages of speleothems and corals tl ages of dune and paleosols ams 14 c and amino acid racemization aar dating of land snails and aar whole rock dating of eolianite these data indicate dune units and paleosols of the searles point formation were emplaced during oxygen isotope stage ois 7 and earlier in the middle pleistocene beach units of the neds beach formation were deposited during ois 5e while dune units were deposited during two major phases the first coeval with or shortly after the beach units the second later during ois 5 e g ois 5a when the older dune and beach units were buried large scale exposures and morphostratigraphical features indicate much of the carbonate was emplaced as transverse and climbing dunes with the sediment source located seaward of and several metres below the present shoreline the lateral extent and thickness of the eolianite deposits contrast markedly with the relatively small modern dunes these features indicate that a slight fall 2 10 m in sea level may be required to mobilise relatively large volumes of sediment onto the island the stratigraphy of the calcarenite combined with the shallow depth of the platform surrounding the island 30 50 m present water depth and the geochronological data suggest that cycles of carbonate deposition on the island are linked to interglacial and interstadial periods of high or falling sea level

thrust breakthrough of asymmetric anticlines observational constraints from surveys in the brooks range alaska abstract to gain insights into the processes governing the thrust truncation of anticlines we conducted a field study of the thrust truncated folds in the remote brooks range of northern alaska where there is a transition in fold style from symmetric detachment folds to thrust truncated asymmetric folds in order to document the detailed geometry of the km scale folds exposed in cliff forming largely inaccessible outcrops a new surveying technique was developed that combines data from a theodolite and laser range finder the field observations survey profiles and cross section reconstructions indicate that late stage thrust breakthrough of the anticlines within the mechanically competent lisburne group carbonates accommodated continued shortening when other mechanisms became unfeasible including fold tightening forelimb rotation and parasitic folding in the anticline forelimbs these results provide constraints on the processes that govern the transition from buckle folding to thrust truncation in fold and thrust belts worldwide

geometry of thin skinned tectonic systems in relation to detachment layer thickness in sedimentary basins characterizing features of both extensional and compressional thin skinned systems may be controlled by the thickness of the underlying mobile material within extensional fault arrays detachment layer thickness in conjunction with detachment dip controls variations in fault spacing and polarity because rigid fault blocks are only free to rotate as long as there is enough underlying mobile material to redistribute between the fault block keels different styles of extensional fault array range from single polarity arrays with closely spaced faults thin detachment to arrays where the fault blocks rotate until the fault blocks separate from one another thick detachment departure from this relationship depends on the amount of internal deformation that can be sustained by the domino fault blocks within contractional zones detachment folds occur where the amount of mobile material can fill the evolving fold core via redistribution thrusts occur where there is insufficient mobile material to sustain detachment fold growth the variations in extensional and contractional style that are described here in relation to mobile layer thickness can be combined to give an idealized model of regional variation in structural style in relation to regional variation in stratigraphy applying this model to a salt basin an allochthon in the area of thickest salt may be characterized by updip raft tectonics and downdip detachment fold belts this style would be laterally transitional toward areas of thinner or lower net salt to updip domino fault arrays and downdip thrust systems

deformation pattern and evolution of the structures in the penganga group the pranhita godavari valley india probable effects of grenvillian movement on a mesoproterozoic basin abstract recent studies reveal imprints of tectonic deformation in the mesoproterozoic rocks of the pranhita godavari p g valley india outcrop scale structures that record a progressive sequence of deformation can thus be used to clarify the kinematic evolution of the penganga group a mesoproterozoic succession in the northern and central parts along the sw marginal proterozoic belt of the pg valley overprinting relationships among the structures in the northern part indicate three distinct structural stages 1 layer parallel shortening 2 thrusting and folding in combination and 3 extensional joint and fracture opening the former two stages are explained in the framework of a progressive deformation model the extensional structures were produced during exhumation similar structures are also developed in the central part of the sw belt of the pg valley though the two areas slightly differ in the overall pattern of deformation the geometry of the progressive deformation structures from both regions of the belt suggests that they were produced under a regime of ne sw ene wsw directed regional compression the deformation in the proterozoic basin is possibly related to the grenvillian orogeny recorded in the eastern ghats mobile belt and may represent a small scale meso neoproterozoic convergence event