porosity reduction and burial history of siliceous rocks from the monterey and sisquoc formations point pedernales area california porosity grain density and composition were determined for different rock types from a 1 2 km thick neogene section of the monterey and sisquoc formations in the point pedernales area california porosity decreases and dry bulk density increases with burial depth in these predominantly siliceous rocks porosity reduction is controlled largely by initial composition and silica diagenesis there is a positive correlation between porosity and biogenic silica diatom content of diatomaceous shales the porous incompressible diatom frustule maintains a high porosity relative to detrital organic and calcareous components of the rock porosity reduction by physical compaction from increasing overburden pressure is much slower at burial depths greater than 0 5 km solution of the porous diatom frustule and precipitation of cryptocrystalline opal ct resulted in a highly variable porosity reduction porosity reduction during the opal a to opal ct transformation depends on the amount composition and texture of the nonsilica grains a large porosity reduction occurred locally in nodules and beds of pore filling dolomite and silica the porosity depth relation of siliceous sediments and rocks from this and previous studies is used with biostratigraphic age data and estimated paleobathymetry to reconstruct the burial history of the point pedernales area the sedimentation rate corrected for compaction quadrupled during deposition of the sisquoc formation using the constant water depth proposed for the monterey formation and assuming that basin filling occurred during rapid deposition of the sisquoc formation results in a tectonic subsidence curve similar to that observed for cooling oceanic lithosphere upwelling of asthenosphere near the surface would have produced a significantly higher paleogeothermal gradient and the opal a to opal ct transformation is predicted to have occurred 3 m y earlier than if a constant presentday geothermal gradient is assumed basin subsidence by faulting or flexure without upwelling of asthenosphere does not significantly alter the thermal history

easter microplate evolution geochemical evidence confirms that there is current active spreading on both the east and west rift zones of the easter microplate and northwards propagation of the east rift with the active tip at 25 s the east rift is influenced by lateral flow of plume derived material from a hotspot located beneath either sala y gomez or easter island spreading rate variation does not seem to be influential in controlling the composition of erupted basalts

ridge segmentation faulting and crustal thickness in the atlantic ocean the mid atlantic ridge mar between the kane and atlantis fracture zones consists of segments 20 85 km in length1 bull s eye patterns in the mantle bouguer gravity anomaly field2 centred on several segments associated with narrow rift valleys3 have been interpreted as centres of strong mantle upwelling and thick crust here i present a map of normal faults inferred from 5 104 km2of sea beam bathymetry4 along the mar between 28 n and the atlantis fracture zone faults are mapped both on and off ridge using a criterion that distinguishes them from volcanic topography abyssal hill lineations seem to form at the rift valley walls through the growth of the normal faults towards ridge segment ends these faults are more widely spaced with larger throws than those at segment centres and often define the boundaries of the gravity bull s eyes these two different faulting styles which probably reflect changes in lithospheric strength are preserved into the rift mountains large throw faults are highly correlated with the mantle bouguer highs suggesting that amagmatic extension on the large faults contributes to the crustal thinning inferred towards the segment ends

morphological development of basin scale silica diagenetic fronts revealed with 2d seismic reflection data offshore sakhalin russian far east two dimensional 2d seismic reflection data reveal two silica diagenetic boundaries located in the north sakhalin basin offshore sakhalin island these boundaries form distinctive high amplitude seismic reflections that predominantly cross cut miocene stratigraphy the boundaries represent the opal a to opal ct cristobalite and tridymite transition and the subsequent deeper opal ct to quartz transition and cover an area of c 107 000 km 2 the reflections represent diagenetic reaction fronts that advance through silica rich host sediments during burial the oldest strata within which the diagenetic fronts have been identified are of miocene age and the fronts commonly show a parallel relationship to a late miocene unconformity these diagenetic fronts develop a variety of morphological forms in the north sakhalin basin these front geometries are probably the result of the interference of the diagenetic fronts with inclined and folded stratigraphy together with lateral changes in the composition and therefore depth of transformation of the sediment we define criteria for the recognition of these 2d cross sectional morphological elements and by so doing provide the basis for the objective description and categorization of diagenetic front geometry

clay organic association as a control on hydrocarbon generation in shale abstract programmed pyrolysis experiments investigating the influence of the mineral matrix on hydrocarbon generation from organic compounds have shown inconclusive results in past studies this study tests the hypothesis that the inhibitory vs catalytic influence of clay minerals is dependent on the physical association of the mineral matrix with organic compounds by comparing natural samples from two shale formations with different mineral organic associations samples from the miocene monterey formation have a greater area of surface contact between organic matter and mineral surfaces when compared to the more particulate texture and discrete nature of the organic matter in the samples from the permian stuart range formation comparison of kerogen extracts to the whole rock containing mineral and organic matter samples during programmed pyrolysis shows higher hydrocarbon generation from isolated kerogen compared to the whole rock sample it was extracted from in both the monterey and the stuart range formation samples the difference between extracted kerogen and whole rock samples is greater and more variable 56 210 mghc gtoc in the stuart range than the monterey formation 85 142 mghc gtoc implying greater hydrocarbon retention by the free mineral surfaces the temperature of maximum hydrocarbon generation rock eval t max is greater in whole rock than kerogen isolates 20 c mean in the monterey formation samples but shows negligible difference 2 c mean in the samples from the stuart range formation standard kerogen typing using rock eval pyrolysis data places the same kerogen as type ii to type iii on a modified van krevelen diagram depending whether it is isolated or pyrolysed with its mineral host this study identifies that not only the bulk mineralogy and kerogen type but also the degree of surface contact of the phases influence hydrocarbon generation

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

detailed geomorphology and neotectonics of the endeavour segment juan de fuca ridge new results from seabeam swath mapping a 36 km wide corridor of seabeam bathymetry was collected on the juan de fuca ridge north of 48 00 n in may 1983 these data merged with previous seabeam bathymetric compilations for the cobb offset as well as with sea marc i and sea marc ii side scan sonar seismic reflection magnetometer and deep tow photography data have been used to identify morphological variations along the spreading center of the endeavour segment of the juan de fuca ridge between the cobb offset and sovanco fracture zone the endeavour segment consists of two ridge sections separated by the 13 km wide endeavour offset which formed when spreading jumped from middle valley to west valley 200 000 yr ago the locus of spreading occurs along the entire endeavour segment as a narrow 1 2 km wide shallow 10 30 m deep inner rift which is superimposed upon larger structures that range from broad 5 10 km wide deep as much as 3 000 m median valleys to narrow 5 km wide shallow 2 100 m volcanic ridges an abrupt discontinuity in the tectonic fabric occurs at about 48 05 n south of this latitude spreading occurs within an axial high endeavour ridge which gradually deepens to the south south endeavour valley as it approaches the zone of overlap with the propagating rift at the cobb offset north of 48 05 n the axis is characterized by broad deep median valleys north endeavour valley and west valley these large scale morphological variations reflect a complex interplay of the spreading center with seamount volcanism waning magma supply at the distal ends of ridge axes thermal contrasts across ridge offsets propagating rifts and an unstable triple junction the pronounced axial deep of west valley is interpreted as reflecting the youth of the structure 200 000 yr and may be primarily due to collapse with little extension and magmatism the rift axis jump to west valley fortuitously isolated endeavour seamount the youngest edifice in the heck seamount chain from the pacific plate and trapped it between a pair of overlapping spreading centers at the newly formed endeavour offset development of a remarkably simple overlapping conjugate rift pair at endeavour offset has resulted in spite of the complex and highly variable crustal thickness magma supply and pre existing structural grain rifting in south west valley has caused pre existing topography generated at the endeavour ridge to be destroyed by subsidence and burial thereby creating the apparent discontinuity in morphology north of 48 05 n

effects of biogenic silica on acoustic and physical properties of clay rich marine sediments the physical properties of marine sediments are influenced by compaction and diagenesis during burial changes in mineralogy chemistry density porosity and microfabric all affect a sediment s acoustic and electrical properties sediments from the japan trench illustrate the dependence of physical properties on biogenic silica content increased opal a content is correlated with increased porosity and decreased grain density and compressional velocity variations with depth in opal a concentration are therefore reflected in highly variable and at times inverse velocity depth gradients the diagenetic conversion of opal a to opal ct and finally to quartz was investigated at a site in the san miguel gap california distinct changes in microfabric particularly in the porosity distribution accompany the diagenetic reactions and contribute to a sharp velocity discontinuity at the depth of the opal a to opal ct conversion evaluation of this reaction at several sites indicates a systematic dependence on temperature and age in clay rich and moderately siliceous sediments in ocean margin regions sediments are buried rapidly and opal a may be converted to opal ct in less than 10 m y temperatures of conversion range from 30 degrees to 50 degrees c much longer times 40 m y are required to complete the conversion in open ocean deposits which are exposed to temper tures less than 15 degrees c in the absence of silica diagenesis velocity depth gradients of most clay rich and moderately siliceous sediments fall in the narrow range of 0 15 to 0 25 km s km which brackets the gradient 0 18 km s km determined for a type pelagic clay section relationships such as these can be useful in unraveling the history of a sediment sequence including the evolution with time of reservoir properties and seismic signatures

magnetic and tectonic studies of the dueling propagating spreading centers at 20 40 s on the east pacific rise evidence for crustal rotations we present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20 40 s on the east pacific rise a majority of the data used were collected during a cruise aboard the r v moana wave during which continuous seamarc ii coverage was obtained these data were combined with additional data to produce an anomaly map which extends to anomaly 2 aged crust a three dimensional inversion in the presence of bathymetry was carried out for the area the resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity the results indicate consistent asymmetric spreading faster to the east discontinuous high magnetizations in the discordant zone associated with the discontinuity and southward migration of the feature at a rate of 90 100 mm yr between jaramillo and brunhes time 0 95 to 0 73 ma with slowing during the brunhes to less than 10 mm yr there is also an overlapping jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the nazca to the pacific plate in addition areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector results from this second model support the presence of highly rotated pre brunhes nazca crust within brunhes pacific crust which has been deformed by bookshelf faulting this indicates at least two episodes of crustal transfer from the nazca plate to the pacific plate the discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the nazca plate between the discontinuity and the easter microplate to the south the detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

rare earth major and trace element composition of monterey and dsdp chert and associated host sediment assessing the influence of chemical fractionation during diagenesis abstract chert and associated host sediments from monterey formation and deep sea drilling project dsdp sequences were analyzed in order to assess chemical behavior during diagenesis of biogenic sediments the primary compositional contrast between chert and host sediment is a greater absolute sio2 concentration in chert often with final sio2 98 wt this contrast in sio2 and si al potentially reflects precursor sediment heterogeneity diagenetic chemical fractionation or both sio2 concentrations and si al ratios in chert are far greater than in modern siliceous oozes however and often exceed values in acid cleaned diatom tests compositional contrasts between chert and host sediment are also orders of magnitude greater than between multiple samples of the host sediment calculations based on the initial composition of adjacent host observed porosity reductions from host to chert and a postulated influx of pure sio2 construct a chert composition which is essentially identical to observed sio2 values in chert thus precursor heterogeneity does not seem to be the dominant factor influencing the current chert composition for the key elements of interest in order to assess the extent of chemical fractionation during diagenesis we approximate the precursor composition by analyzing host sediments adjacent to the chert the sio2 concentration contrast seems caused by biogenic sio2 dissolution and transport from the local adjacent host sediment and subsequent sio2 reprecipitation in the chert along with sio2 other elements are often added with respect to al to monterey and dsdp chert during silicification although absolute concentrations decrease the two monterey quartz chert nodules investigated in contrast to the opal ct and quartz chert lenses formed primarily by extreme removal of carbonate and phosphate thereby increasing relative sio2 concentrations dsdp chert formed by both carbonate phosphate dissolution and sio2 addition from the host manganese is fractionated during chert formation resulting in mno al 2 o 3 ratios that no longer record the depositional signal of the precursor sediment ree data indicate only subtle diagenetic fractionation across the rare earth series ce ce values do not change significantly during diagenesis of either monterey or dsdp chert eu eu decreases slightly during formation of dsdp chert la n yb n is affected only minimally as well during formation of one monterey opal ct chert lens ree al ratios show subtle distribution changes at gd and to a lesser extent near nd and ho ree compositional contrasts between diagenetic states of siliceous sediment and chert are of a vastly smaller scale than has been noted between different depositional environments of marine sediment indicating that the paleoenvironmental ree signature is not obscured by diagenetic overprinting

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

fossilisation of a silica diagenesis reaction front on the mid norwegian margin the diagenetic transformation of biogenic silica from opal a to opal ct was recognised on seismic reflection data over an area of 78 103 km2 on the mid norwegian margin the opal a ct diagenetic boundary appears as a positive high amplitude reflection that generally cross cuts the hosting stratigraphy we demonstrate that it is not a sea bottom simulating reflection bsr and also that is not in thermal equilibrium with the present day isotherms we present arguments that three styles of deformation associated with the opal a ct reflection polygonal faulting regional anticlines and synclines and differential compaction folding indicate that the silica diagenesis reaction front is fossilised at a regional scale isochore maps demonstrate the degree of conformity between the opal a ct reflection and three seismic horizons of late miocene to early pliocene age that potentially represent the paleo seabed when fossilisation of the reaction front took place the seismic interpretational criteria for recognition of a fossilised diagenetic front are evaluated and the results of our study are integrated with previous studies from other basins of the ne atlantic in order to determine if the arrest of silica diagenesis was diachronous along this continental margin

phosphogenesis and organic carbon preservation in the miocene monterey formation at naples beach california the monterey hypothesis revisited the middle part of the miocene monterey formation at naples beach west of santa barbara california is predominantly composed of organic rich mudstone interstratified with phosphatic laminae minor lithologies include volcanic ash dolomite porcelanite and chert and condensed phosphatic beds sediments dated as 14 3 13 5 ma have average total organic carbon toc values around 8 5 wt and organic carbon oc accumulation rates are around 565 mg cm2 k y sediments dated as 13 5 13 ma are characterized by average toc values of 12 6 wt and oc accumulation rates of around 1130 mg cm2 k y the interval between 13 and 10 6 ma is marked by condensation average toc values are around 8 6 wt and oc accumulation rates diminished to around 55 mg cm2 k y the last interval studied is dated as 10 6 9 4 ma and average toc values are around 6 wt whereas oc accumulation rates rose again to 320 mg cm2 k y the presence of erosional surfaces angular unconformities and reworked clasts and nodules suggests that bottom current activity and gravity flow deposition have been instrumental in sediment accumulation the phosphatic laminae were precipitated at a very early stage of diagenesis during periods of nonsedimentation they formed less permeable sedimentary lids and may as such have contributed to enhanced oc preservation between 13 and 10 6 ma the thus formed phosphatic laminae were frequently subjected to subsequent sediment winnowing and reworking resulting in the formation of condensed phosphatic beds calculated p c molar ratios suggest that 1 the measured section is highly enriched in phosphorus p relative to oc 2 regeneration of organic p from organic matter decomposition was negligible and 3 the source of p was external likely upwelled bottom water rich in inorganic p in spite of good preservation conditions and correspondingly high toc contents the overall oc accumulation rates are moderate in comparison to those of actual high productivity areas which is mainly due to the episodic character of depositional processes and the intervening long periods of nondeposition and sediment reworking they preclude this section and by extrapolation the monterey formation in general from being an important oc sink during the middle miocene alternatively large oc sinks were probably created on the continent lignite deposits and in sedimentary depocenters which received increasing amounts of detrital sediments due to a combination of climate change spreading of grasslands and the increasing importance of mountain chains such as the himalaya the associated high nutrient fluxes may have been involved in the backstepping and drowning of carbonate platforms and in the generation of widespread phosphat rich deposits during the late early and early middle miocene

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

anomalous compaction due to silica diagenesis textural and mineralogical evidence from hemipelagic deep sea sediments of the japan sea abstract although anomalous compaction dramatic physical property changes is widely recognised from scientific drilling of sections of marine biosiliceous sediments that have undergone silica diagenesis the precise mechanisms governing vertically abrupt compaction are poorly understood to better understand relationships between silica diagenesis and anomalous compaction the microfabric and composition of hemipelagic sediments at the ocean drilling program sites 794 and 795 in the sea of japan were analysed in detail textural and mineralogical examination of core samples from these stations shows that dissolution of opal a and precipitation of opal ct are the two major controls on anomalous compaction other observed components of the diagenetic history such as structural ordering of diagenetic opal and precipitation of authigenic phases clay minerals and pyrite do not strongly affect physical properties above the opal a to opal ct transition zone anomalous compaction interval opal a deposits impart significant porosity to the sediments because of this high porosity and the relative incompressibility of siliceous tests normal mechanical compaction of biosiliceous units is retarded and mainly postdates silica diagenesis across the transition zone a sharp reduction in opal a content under dissolution from 35 to 12 in site 794 and from 45 to 7 in site 795 leads to a significant reduction in sediment framework stability which makes it vulnerable to sudden collapse abrupt reduction in intergranular and intragranular porosity and increasing bulk density and a decrease in pore water content although later opal ct precipitation does influence the petrophysical response mineralogical analyses suggest a lesser role for opal ct precipitation on host porosity than precursor opal a dissolution this is attributed to the inhibiting effects of high abundance of clay sized components that have restricted significant precipitation of opal ct

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

porosity reduction during diagenesis of diatomaceous rocks with lithification and burial diatomaceous rock transforms from opal a to opal ct and then quartz this study documents two different patterns of porosity reduction and their associated mineralogical and density changes using samples from the monterey formation california samples that have one set of properties are called group 1 and those showing other trends are assigned to group 2

mineralization of organogenic ammonium in the monterey formation santa maria and san joaquin basins california usa abstract inorganic fixed ammonium nh4 contents as high as 0 28 wt were measured in organicrich quartz grade siliceous rocks of the miocene monterey formation from the santa maria and san joaquin basins california the greatest amount of fixed nh4 was found in rocks associated with hydrocarbons in the point arguello and lost hills oil fields where the nh 4 nh 4 k molar ratio of bulk samples ranges from 0 17 0 35 the formation of nh4 illite is suggested by the parallel increase in the percent of illite in the mixed layered illite smectite i s and in the nh 4 nh 4 k molar ratio of the clay sized fraction with increasing burial depth mineralization of nh4 appears to be promoted by the coincident timing of the smectite to illite clay mineral transformation and the release of nh4 during catagenesis nh4 feldspar may form at shallow burial depths in rocks from the point arguello field that contain a greater amount of detrital k feldspar and in which the i s contains only 10 20 illite quartzgrade siliceous monterey rocks from coastal outcrops in the lions head area lack significant amounts of hydrocarbons and have nh 4 nh 4 k molar ratios of 0 14 0 21 rocks from the lions head area show a strong positive correlation between diagenetic illite and fixed nh4 contents with nh4 constituting 18 21 mol of the fixed interlayer cations in the i s low grade opal a and opal ct siliceous miocene rocks from coastal outcrops in the pt pedemales area have low fixed nh4 contents 0 01 wt because these rocks contain minor amounts of diagenetic k bearing minerals i s contains 10 illite and they lack significant amounts of generated or migrated hydrocarbons the increase in the organic c n ratio with burial depth most likely reflects the preferential release of n compared to c and suggests that nh4 release is greatest during shallow burial bacterial degradation and deep burial catagenesis oil generation the results of this study support the suggestion of williams et al 1989 that high fixed nh4 contents may provide a long term geologic record of low temperature

measurement of diagenetic compaction strain from quantitative analysis of fault plane dip we developed a new technique for quantifying the amount of porosity loss during diagenetic transformation of opal a to opal ct the technique is based on 3d seismic data from offshore norway where the widely developed biosiliceous sediments of the brygge formation are deformed by a polygonal fault system evidence is shown from two study areas that the dip of the polygonal fault planes reduces abruptly across the opal a to opal ct diagenetic boundary suggesting that the fault planes were passively rotated into shallower dips due to diagenesis the reduction of fault plane dip was used to quantify the vertical compaction strain due to diagenesis using an independent assessment of the magnitude of the porosity loss we are able to validate the method based on fault plane dips and also to evaluate whether the porosity loss results in an exclusively vertical strain additionally the impact of silica diagenesis on the shear strength of the sediments and fault growth is investigated we present evidence suggesting that deviations from classical mohr coulomb behaviour may be expected during the combined processes of diagenesis and porosity collapse and may indeed be sufficient to promote continued fault growth

fluid flow due to the advance of basin scale silica reaction zones the conversion of biogenic silica opal a to opal ct cristobalite and tridymite in biosiliceous sediment causes increased rates of water expulsion because of the reduction in sediment porosity and dehydration of the amorphous opal a phase this release of water occurs over large tracts of sedimentary basins during sediment burial within discrete diagenetic reaction zones analysis of two dimensional and three dimensional seismic data sets from basins in the northern hemisphere provides geophysical evidence for a variety of fluid conduits and roughly circular erosional depressions at the contemporaneous seabed we interpret these features as indicative of water expulsion and focused fluid flow emanating from opal a to opal ct reaction zones at burial depths within the range 200 800 m the rate at which water is expelled depends upon the degree of porosity reduction and the weight fraction of bound water at the reaction zone as well as the rate of advance of the reaction zone where the reaction is actively taking place within homogeneous biosiliceous sediment the rate of water expulsion is independent of the reaction rate this is because water is released across the entire reaction zone therefore slow reaction rates are compensated for by expulsion of water across wider reaction zones we calculate the rate and volume of water expulsion for the faeroe shetland basin where the sediment immediately below the reaction zone contains on average 30 opal ct by weight the estimated volumetric rate of water expulsion per unit surface area at the present day is 6 m3 my 1 per square meter which is greater than the vertical flux of water at the same depth from compaction of the deeper basin fill the average volumetric rate of water expulsion is 120 km3 my 1 across the whole basin biogenic silica is particularly rich in neogene successions in high latitude and equatorial regions and where silica reaction zones are identified they should be factored into sediment compaction and fluid flow histories

ultrasonic velocities of opaline rocks undergoing silica diagenesis experimental results of laboratory measurements at ultrasonic frequencies on dry opaline rocks are documented from the monterey formation a siliceous unit in the san joaquin basin in these samples the silica is primarily biogenic opal a in origin but progressively alters to opal ct and then quartz with depth the diagenetic transitions are accompanied by a marked porosity reduction compressional and shear wave velocities were measured on core plugs at effective reservoir pressure experimental results on samples from the opal a opal ct transition and the opal ct quartz transition show very little pressure dependence of elasticwave velocities when their elastic moduli are normalized by those of the solid phase the moduli porosity trend is very close to that of chalk and in the lower porosity domain of some clean sandstones

thin section analysis of lithified paleosols from dagshai formation of the himalayan foreland identification of paleopedogenic features and diagenetic overprinting and implications for paleoenvironmental reconstruction abstract in the present study we report micromorphology of diagenetically altered paleosols 31 ma old from dagshai formation himalayan foreland the fluvial sequence exposed along the koshaliya river nw himalaya contains four types of paleosols with decreasing abundance of well developed paleosols from basal to upper part of the dagshai formation burial diagenesis at 7 5 km depth caused compaction 54 78 current thickness compared to pre burial thickness and cementation of the paleosols accompanied by internal reorganization of groundmass within peds staining of ped walls with iron oxide plugging of voids disruption of textural pedofeatures fracturing of large mineral grains and nodules coarsening of pedogenic calcite crystals redoximorphic features and mineralization of root channels despite diagenetic alteration evidence of paleopedogenic processes is still well preserved in these fossil soils in the form of microstructures b fabrics pedogenic calcite bioturbation and textural pedofeatures thin section analysis helped to distinguish pedogenic and diagenetic features of lithified paleosols and to infer the paleoenvironment of the dagshai paleosols the paleopedological characteristics of the fossil soils suggest humid to sub humid conditions during their formation in early oligocene the paleoclimate inferred here is consistent with prevalence of tropical paleovegetation cf ficus l reported from dagshai sediments

density and mineralogy variations as a function of porosity in miocene monterey formation oil and gas reservoirs in california the miocene monterey formation long known as the critical source rock in california also includes significant fractured chert and porous diatomite reservoirs what is not widely recognized is that there are high matrix porosity reservoirs within the opal ct and quartz phase rocks using density porosity and mineralogy data we have identified two distinct groups of monterey formation reservoirs group 1 porosity changes gradually during silica diagenesis porosities of 55 70 exist not only in opal a dominated samples but also in samples that have undergone the transition from opal a to opal ct in group 2 porosity decreases abruptly at the transition and rocks below the transition are tight the main mineralogic difference between groups 1 and 2 is a higher clay content in group 1 resulting in different silica diagenesis pathways for the two groups this led us to expect all san joaquin basin samples to fall into group 1 and all coastal california samples to fall into group 2 because of the different depositional histories of the two areas although our prediction holds in most instances we discovered that some san joaquin basin samples exhibit group 2 characteristics therefore it is important to use petrophysical seismic and or geological information to determine if a monterey formation reservoir is likely to be a fracture dominated type group 2 or if there might be a matrix production component group 1 once one has made this diagnosis the characteristic density porosity relationships of each group allow one to easily calculate an accurate matrix porosity

the influence of shale depositional fabric on the kinetics of hydrocarbon generation through control of mineral surface contact area on clay catalysis abstract accurately assessing the temperature and hence the depth and timing of hydrocarbon generation is a critical step in the characterization of a petroleum system clay catalysis is a potentially significant modifier of hydrocarbon generation temperature but experimental studies of clay catalysis show inconsistent or contradictory results this study tests the hypothesis that source rock fabric itself is an influence on clay mineral catalysis as it controls the extent to which organic matter and clay minerals are physically associated two endmember clay organic fabrics distinguish the source rocks studied 1 a particulate fabric where organic matter is present as discrete 5 m particles and 2 a nanocomposite fabric in which amorphous organic matter is associated with clay mineral surfaces at sub micron scale high resolution electron imaging and bulk geochemical characterisation confirm that samples of the miocene monterey formation california are representative of the nanocomposite source rock endmember whereas samples from the permian stuart range formation south australia represent the particulate source rock endmember kinetic experiments are performed on paired whole rock and kerogen isolate samples from these two formations using open system non isothermal pyrolysis at three different heating rates 0 7 2 and 5 k min to determine the effects of the different shale fabrics on hydrocarbon generation kinetics extrapolation to a modelled geological heating rate shows a 20 c reduction in the onset temperature of hydrocarbon generation in monterey formation whole rock samples relative to paired kerogen isolates this result is consistent with the monterey formations s nanocomposite fabric where clay catalysis can proceed because reactive clay minerals are intimately associated with organic matter by contrast there is no significant difference in the modelled hydrocarbon generation temperature of paired whole rock and kerogen isolates from the stuart range formation this is consistent with its particulate fabric where relatively large discrete organic particles have limited contact with the mineral matrix and the clay minerals are mainly diagenetic and physically segregated within pores while heating rate may have a control on mineral matrix effects this result shows that the extent to which organic matter and clay minerals are physically associated could have a significant effect on the timing of hydrocarbon generation and is a function of the depositional environment and detrital vs diagenetic origin of clay minerals in source rocks

structure of a silica diagenetic transformation zone the gjallar ridge offshore norway three dimensional seismic data from the gjallar ridge were used together with x ray diffraction data scientific boreholes and wireline logs to analyse the seismic structure of a silica diagenetic transformation zone the following features were identified i an interval some 150 to 300 m thick that contains anomalously high amplitude reflections ii a strong reflection event at the top of this interval which cross cuts stratigraphy interpreted as the transformation boundary between opal a rich and opal ct rich sediment iii amplitude variations on stratigraphic reflections within the interval attributed to variations in the proportions of opal a and opal ct and iv a second deeper cross cutting reflection event within the interval interpreted as the transformation boundary between opal ct rich and quartz rich sediment the base of the interval containing the anomalously high amplitude reflections is interpreted as a stratigraphic reflection demarcating the base of the silica rich strata on a stratigraphic reflection within the interval of high amplitudes roughly circular regions of anomalously high amplitude with diameters of 0 8 to 2 5 km are separated by lower amplitude regions this pattern is similar to the cells previously identified at the opal a to opal ct transformation boundary and probably results from more complete transformation of opal a to opal ct all of these observations provide the first recognition from seismic data that silica diagenetic transformations are not always narrow boundaries represented by single cross cutting seismic reflection events as implied previously but can be heterogeneous and hundreds of metres in thickness as observed at outcrop

geometry of inverted faults and related folds in the monterey formation implications for the structural evolution of the southern santa maria basin california abstract a wide variety of mesoscopic structures observed in the monterey formation of coastal california reveal contrasting styles of deformation among mechanical units and provide a relative chronology of neogene deformation for the southern santa maria basin these structures which include ptygmatically folded veins folded beds of chert inverted normal faults fault propagation folds and axial planar breccia zones document an early extensional phase in the middle late miocene followed by two distinct episodes of contraction between the pliocene and the present miocene normal faults in interbedded carbonates and mudstones were inverted resulting in geometries that include normal faults truncated by bedding plane detachments low angle thrusts and thrust duplexes and normal faults reactivated in reverse fault block geometry drag folds and culmination folds are characteristic features that help identify inverted structures normal fault inversion coincides with the development of early chert folds and related structures higher in the stratigraphie section representing a regional phase of layerparallel contraction the second phase of regional contraction resulted in the development of a fold and thrust belt which in the monterey formation is manifested by detachment and fault related folds aligned parallel to regional fold axes effects of silica diagenesis contribute to the development of mesoscale structures throughout the deformation history of the monterey formation

silica diagenesis porosity evolution and mechanical behavior in siliceous mudstones mowry shale cretaceous rocky mountains u s a quartz in mudstones of the mowry shale cretaceous rocky mountain region takes several forms extrabasinal detrital silt some with transported overgrowths replacement of skeletal debris minor overgrowth on detrital quartz and replaced radiolaria pore filling in the intragranular pores of allochems and authigenic microquartz that is dispersed through the clay size matrix the matrix dispersed microquartz has cl character similar to the overgrowths with which it is intergrown supporting the interpretation that it is authigenic and pore filling radiolaria are widely observed in these mudrocks but are poorly preserved by partial mineral replacement quartz dolomite calcite pyrite we interpret dissolved radiolaria and possibly now cryptic diatoms as the likely source of silica for the overgrowths and microquartz through a reaction pathway that entailed the formation of opal ct lepispheres that subsequently recrystallized to microquartz a newly devised variation of the igv intergranular volume concept a parameter igvm is applied for partitioning the causes of porosity decline in mudstones all of the samples examined contain authigenic microquartz which averages around 40 of the rock volume the large volume of microquartz together with the volumes of surviving intergranular pores and pore filling solid hydrocarbon constrain the porosity to around 50 at the time of initial lepisphere formation despite the exceptional abundance of cement overall porosity decline is compaction dominated the presence of significant volumes of cement has implications for mechanical rock properties in the mowry shale which is expected to manifest brittle behavior where cements are most prominently developed

power law scaling of topographic depressions and their hydrologic connectivity topographic depressions areas of no lateral surface flow are ubiquitous characteristics of the land surface that control many ecosystem and biogeochemical processes high density of depressions increases the surface storage capacity whereas lower depression density increases runoff thus influencing soil moisture states hydrologic connectivity and the climate soil vegetation interactions with the widespread availability of high resolution lidar based digital elevation model ldem data it is now possible to identify and characterize the structure of the spatial distribution of topographic depressions for incorporation in ecohydrologic and biogeochemical studies here we use ldem data to document the prevalence and patterns of topographic depressions across five different landscapes in the united states and quantitatively characterize the probability distribution of attributes such as surface area storage volume and the distance to the nearest neighbor through the use of a depression identification algorithm we show that these probability distributions of attributes follow scaling laws indicative of a structure in which a large fraction of land surface areas can consist of high number of topographic depressions of all sizes and can account for 4 to 21 mm of depression storage this implies that the impacts of small scale topographic depressions in the landscapes on the redistribution of material fluxes evaporation and hydrologic connectivity are quite significant

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

investigation of spreading center evolution by joint inversion of seafloor magnetic anomaly and tectonic fabric data spreading center segments that have experienced a complex tectonic history including rift propagation may have a complicated signature in bathymetric and magnetic anomaly data to gain insight into the history of such regions we have developed techniques in which both the magnetic anomaly patterns and seafloor fabric trends are predicted theoretically and the combined predictions are compared numerically with the data to estimate best fitting parameters for the propagation history fitting functions are constructed to help determine which model best matches the digitized fabric and magnetic anomaly data such functions offer statistical criteria for choosing the best fit model we use this approach to resolve the propagation history of the cobb offset along the juan de fuca ridge in this example the magnetic anomaly data prove more useful in defining the geometry of the propagation events while the fabric with its greater temporal resolution is more useful for constraining the rate of propagation it thus appears that joint inversion of magnetic and seafloor fabric data can be valuable in tectonic analyses

differential compaction and subsidence in sedimentary basins due to silica diagenesis a case study the conversion of opal a to opal ct is a thermochemical diagenetic process that can cause kilometer scale differential compaction and differential subsidence within clastic sedimentary basins this is demonstrated for an opal a to opal ct conversion imaged by three dimensional seismic data on the northeastern atlantic margin this diagenetic front has a circular to polygonal ridge depression morphology the ridges have a relief of 10 50 m and separate circular to polygonal depressions that have diameters of 1 2 km some of which link up to form interconnected networks this morphology initially developed because the host sediments mantled an underlying polygonal fault system and the front tracked deformed stratigraphy ridges formed above the tips of the underlying faults and depressions formed between them ridge relief and width then progressively increased due to earlier opal a to opal ct conversion above ridges as well as conversion laterally along beds or bed sets this caused compaction and concomitant subsidence in the overburden and the development of a circular to polygonal network of overburden troughs aligned with the ridge system a positive feedback loop was established where additional overburden sediment above the growing ridges reached the depth of conversion ahead of adjacent areas driving more compaction and differential subsidence this paper provides the first insights into the large scale morphological characteristics of opal a to opal ct diagenetic fronts it demonstrates the potential for a local stratigraphic control on diagenetic front development and morphology and lastly highlights the potential utility of seismic reflection data for understanding diagenetic processes

propagating rift explanation for the tectonic evolution of the northeast pacific the pseudomovie abstract we have developed a new computer graphic technique to use in reconstructing plate tectonic evolution and to make movies illustrating this evolution using this technique we have deciphered the evolutionary history of the juan de fuca area in the northeast pacific and show that a combination of seafloor spreading and rift propagation can explain this seemingly complex area as a simple consequence of the interaction of two rigid plates

lithology dependence of porosity in slope and deep marine sediments abstract compilation of porosity data from 11 ocean drilling program legs and 53 sites show that compaction in deep marine to upper slope sediments is at least in part a function of both burial depth and lithology overall porosities range from 4 to 94 despite the broad range of porosities there is an exponential trend toward decreasing porosity with depth comparison among samples dominated 90 to 100 by each of six lithologies based on grain size and composition reveals the lithology dependence of compaction except for biogenic silt size sediments and clastic sands in the former case this may in part be due to mixing of calcareous and siliceous nannofossils clay size clastic sediments show a clear change from rapid porosity reduction in the top 172 meters porosity 82 7 e depth 430 to less pronounced loss of porosity below porosity 61 4 e depth 1671 the remainder of the lithologies silt porosity 75 5 e depth 1091 biogenic sand porosity 88 5 e depth 1338 and micrite porosity 69 5 e depth 1235 show a good to strong exponential reduction of porosity with depth these relations compare well with literature studies taking into account the less pure nature of the lithologies used in this work the silt compaction curve is unique no previous studies of porosity separated silt from other grain sizes most sediment samples are of mixed grain sizes and types the lithology based porosity depth relations were used to test four approaches to decompaction in mixed lithology sediment used in backstripping all methods generated porosity estimates that correlated strongly to the observed values with correlation coefficients ranging from 0 629 to 0 686 these methods that have been used for years are for the first time shown to be valid

small non overlapping offsets of the east pacific rise high resolution bathymetric surveys1 2 of the east pacific rise epr reveal that in addition to overlapping spreading centres oscs the crest of the epr is offset tiny amounts by smaller features recently langmuir et al have shown that despite their subtle bathymetric expression these small offsets bends and kinks of the epr collectively termed deviations from axial linearity devals represent important petrological and tectonic boundaries some devals are small non overlapping offsets snoos that offset the epr axis by up to a few hundred metres bathymetric contours near snoos are usually disturbed from their axis parallel trend in ways that we interpret as evidence for strike slip faulting and volcanism at high angles to the trend of epr axis we offer a model for the formation of snoos and their associated faults that invokes independent magma supply stochastic active spreading and limited non rigid behaviour of the neovolcanic zone

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

tectonics and evolution of the juan fernandez microplate at the pacific nazca antarctic triple junction in this study magnetic and bathymetric data from an r v endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the juan fernandez microplate at the pacific nazca antarctic triple junction magnetic lineations along the east ridge of the microplate fan to the south indicating that the nazca juan fernandez pole is near the northern end of this feature relative motions of the juan fernandez pacific nazca antarctic four plate system are determined using relative plate motion inversions the resulting tectonic model predicts that the northern boundary of the juan fernandez microplate is a zone of compression and that the west ridge and southwestern boundary are spreading obliquely the southeastern boundary is predicted to be obliquely divergent but present data fail to constrain its geometry we also present a schematic evolutionary model which relates the formation of the juan fernandez microplate to differential spreading rates at the triple junction and discuss related complexities the future evolution of the fourplate system depends on whether one or both ridges remain active

impact of silica diagenesis on the porosity of fine grained strata an analysis of cenozoic mudstones from the north sea silica diagenesis has the potential to drastically change the physical and fluid flow properties of its host strata and therefore plays a key role in the development of sedimentary basins the specific processes involved in silica diagenesis are however still poorly explained by existing models this knowledge gap is addressed by investigating the effect of silica diagenesis on the porosity of cenozoic mudstones of the north viking graben northern north sea through a multiple linear regression analysis first we identify and quantify the mineralogy of these rocks by scanning electron microscopy and x ray diffraction respectively mineral contents and host rock porosity data inferred from wireline data of two exploration wells are then analyzed by multiple linear regressions this robust statistical analysis reveals that biogenic opal a is a significant control and authigenic opal ct is a minor influence on the porosity of these rocks these results suggest that the initial porosity of siliceous mudstones increases with biogenic opal a production during deposition and that the porosity reduction during opal a ct transformation results from opal a dissolution these findings advance our understanding of compaction dewatering and lithification of siliceous sediments and rocks moreover this study provides a recipe for the derivation of the key controls e g composition on a rock property e g porosity that can be applied to a variety of problems in rock physics

fast rift propagation along the east pacific rise near easter island during a 1983 seabeam cruise we surveyed the easter nazca spreading center which forms the eastern boundary of the easter microplate we have analyzed all available bathymetry and magnetic anomaly data to determine the present fine scale tectonic configuration and recent tectonic evolution of this spreading center the narrowing of the axial brunhes anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm yr assuming an average spreading rate of 120 mm yr there are indications of a short episode of faster propagation 350 mm yr prior to 0 4 ma these are the fastest rates of rift propagation yet documented the tip of the propagating rift is at about 25 s 112 25 w 180 km south of the previously hypothesized position the recent spreading direction is n85 w a large well defined sheared zone to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the easter pacific spreading center 360 km to the west is being replaced by the fast propagator

chert spheroids of the monterey formation california usa early diagenetic structures of bedded siliceous deposits chert spheroids are distinctive early diagenetic features that occur in bedded siliceous deposits spanning the phanerozoic these features are distinct in structure and genesis from similar concentrically banded wood grain or onion skin chert nodules from carbonate successions in the miocene monterey formation of california usa chert spheroids are irregular concentrically banded nodules which formed by a unique version of brittle differential compaction that results from the contrasting physical properties of chert and diatomite during shortening there is brittle fracture of diatomite around and horizontally away from the convex surface of strain resistant chert nodules unlike most older siliceous deposits the monterey formation still preserves all stages of silica diagenesis thus retaining textural mineralogical and geochemical features key to unravelling the origin of chert spheroids and other enigmatic chert structures chert spheroids found in opal a diatomite form individual nodules composed of alternating bands of impure opal ct chert and pure opal ct or chalcedony with increased burial diagenesis surrounding diatomite transforms to bedded porcelanite or chert and spheroids no longer form discrete nodules yet still display characteristic concentric bands of pure and impure microcrystalline quartz and chalcedony petrographic observations show that the purer silica bands are composed of void filling cement that precipitated in curved dilational fractures and do not reflect geochemical growth banding in the manner of liesegang phenomena invoked to explain concentrically banded chert nodules in limestone chertification of bedded siliceous sediment can occur more shallowly 100 m and rapidly 1 myr than the bulk silica phase transitions forming porcelanite or siliceous shale in the monterey formation and other hemipelagic pelagic siliceous deposits early diagenesis is indicated by physical properties deformational style and oxygen isotopic composition of chert spheroids early formed cherts formed by pore filling impregnation of the purest primary diatomaceous beds along permeable fractures and in calcareous siliceous strata