reservoir implications of modern karst topography tropical karst landscapes exhibit a predominance of positive relief features relative to the negative relief features i e sink holes dolines etc of temperate karst areas and yet paradoxically have never been observed in the subsurface on seismic sections to understand why this is the case topographic profiles were made over the positive karst relief features of china java and jamaica and were used to construct synthetic seismic sections the results indicate that in most instances the magnitude of the topographic relief is sufficient to be seen on seismic sections a possibility of misinterpreting karst highs as reefs seems clear but more important in explaining the paradox is the fact that the magnitude of modern karst relief expression is largely attributable to uplift over an extended period the probability of preservation of these positive relief solution residuals is dependent on the karstification process being aborted by the deposition of overlying sediment an event not likely to occur in an uplift area uplift of well lithified carbonate minimally results in an areally extensive runoff surface that controls the development of positive karst landforms uplift also commonly induces faulting and fracturing that further control the distribution of karst land forms hydrocarbon production from the resulting karst is geared to matrix porosity the higher the matrix porosity the greater the rate of deliverability of matrix oil to produced fracture and karst conduit oil in contrast the lowering of the base level of erosion due to a fall in sea level is generally less than that induced by uplift and consequently minimizes positive karst relief development relative to that of uplift subaerial exposure induced by passive sea level fall also minimizes the potential for the related occurrence of fracturing and faulting which of itself mitigates against the general development of positive karst relief the subaerial consequences of passive sea level fall are commonly preserved in the geologic record and result in karst reservoirs dominated by extensive moldic and vuggy porosity interconnected by solution channels these reservoirs lack the production problems commonly attending fractured karst reservoirs with little matrix porosity

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

diagenesis of an upper triassic reef complex wilde kirche northern calcareous alps austria the wilde kirche reef complex early late rhaetian grew as an isolated carbonate structure within the shallow kossen basin at the triassic jurassic boundary a single brief c 10 50 ka period of subaerial exposure occurred the preserved karst profile 70 m thick displays a vadose zone enhanced dissolution at a possible palaeo watertable 5 15 m below the exposure surface and a freshwater phreatic zone karst porosity was predominantly biomouldic primary cavities and biomoulds were enlarged and interconnected in the freshwater phreatic zone cavity networks developed preferentially in patch reef facies resubmergence of the reef complex allowed minor modification of the palaeokarst surface by sea floor dissolution and fe mn crust deposition on a sediment starved passive margin fibrous calcite fc radiaxial fibrous calcite rfc and fascicular optic calcite foc cements preserved as low mg calcite lmc are abundant in primary and karst dissolution cavities fc cement is restricted to primary porosity particularly as a synsedimentary cement at the windward reef margin fc rfc and foc contain microdolomite inclusions and show patchy non bright cathodoluminescence 18o values of non luminescent portions interpreted as near original are 1 16 to 1 82 0 close to the inferred 18o of calcite precipitated from late triassic sea water 13c values are constant 3 to 2 2 0 these observations suggest fc rfc and foc were originally marine high mg calcite hmc precipitates and that the bulk of porosity occlusion occurred not in the karst environment but in the marine environment during and after marine transgression the hmc to lmc transition may have occurred in contact with meteoric water only in the case of fc cement the most altered brightly luminescent portions of rfc foc cements yield 18o 2 44 to 5 8 0 suggesting hmc to lmc alteration at up to 34 c in the shallow burial environment at depths of 180 250 m abundant equant cements with 18o 4 1 to 7 1 0 show crisp uniform or zoned dull luminescence they are interpreted as unaltered cements precipitated at 33 36 c at 200 290 m burial depth from marine derived fluids under a slightly enhanced geothermal gradient fluids carrying the equant cements may have induced the hmc to lmc transition in the fibrous cements

sedimentation by river induced turbidity currents field measurements and interpretation turbidity currents initiated from spring runoffs of an influent river were observed in the upper region of a reservoir in hokkaido japan by measuring water temperature velocity and suspended sediment concentration their profiles offer some physical parameters for the sedimentary conditions assuming the turbidity currents to be quasi uniform the bottom sediment deposited by the turbidity currents was then collected by a portable core sampler the bottom sediment consists of more than 90 silt and clay and thus offers a hydraulically smooth bed for shear flow a plane bed as a bed configuration was formed on the reservoir bed probably because of the low shear velocity and small grain size of sediment using a graphic method with log normal probability paper the bottom sediment is divided into several overlapping log normal subpopulations grain size analysis indicates that the bottom sediment may be regarded as cohesionless criteria for complete deposition of transported grains can then be incorporated into the extended shields diagram giving the minimum shear stress to erode bottom sediment applying the new diagram to the grain size distribution of the bottom sediment it is suggested that each of the log normal subpopulations was deposited in each of four different modes of deposition i e traction saltation or intermittent suspension suspension and suspension under equilibrium the last mode may be observed under a sedimentary condition where upward flux of suspended sediment by eddy diffusion is almost equal to its depositional flux due to gravity the mean and critical grain sizes for bottom sediment and each of the corresponding subpopulations decrease consistently with an increase of fd2 log10re fd is the densimetric froude number and re is the flow reynolds number correlates inversely with shear velocity which bears a linear relationship to mean velocity these results lead to the conclusion that relatively fine suspended sediment is deposited as a result of decreasing bottom friction with a relative decrease of turbulent energy

paleocave carbonate reservoirs origins burial depth modifications spatial complexity and reservoir implications paleocave systems form an important class of carbonate reservoirs that are products of near surface karst processes and later burial compaction and diagenesis features and origins of fractures breccias and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale pore networks and spatial complexities spatial complexities in paleocave reservoirs result from near surface and burial processes near surface processes include dissolutional excavation clastic sedimentation chemical precipitation and localized fracturing brecciation and collapse of cave walls and ceilings burial processes begin as cave systems subside into the subsurface remaining cave passages commonly collapse and early formed breccia clasts are rebrecciated differential compaction of strata around and over collapsed passages produces fractures crackle breccias and mosaic breccias near surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed paleocave systems the coalescing of passages in a cave system into larger connected porosity zones results from a combination of multiple cave forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured this combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across commonly forming large exploration targets final size pore network types and spatial complexities of coalesced collapsed paleocave systems are products of their evolution from near surface development through burial into the deeper subsurface the coalesced collapsed paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved

facies characteristics of a proglacial turbiditic sand lobe at svalbard abstract a shallow marine turbiditic lobe inferred to be associated with an ice contact delta was deposited during the last glaciation weichselian at svalbard the norwegian arctic the postglacially raised sandy lobe is exposed in a 200 m long river section and exhibits six units that reflect an early phase of glaciomarine mud deposition followed by three episodes dominated by deposition from sandy turbidity currents the lobe is characterized by a complex pattern of turbidite filled erosional features from thin scour fills reflecting a highly mobile flow pattern to stacked chute fill deposits showing confined flow the sandy turbidites show rhythmic deposition of massive to graded laminae deposited from suspension alternating with faint laminae or ripple cross lamination deposited from tractional transport semi continuous turbidity currents are thought to have been generated by retrogressive slumping of ice proximal well sorted sandy deposits on the delta front and slope the lateral and vertical changes of lithofacies within the sandy lobe are mainly attributed to normal mechanisms of turbiditic lobe growth with only minor effect from synsedimentary glacier terminus movement and minor changes in the meltwater outflow position this detailed mapping of facies changes allows a fairly accurate reconstruction of the axis of the former discharge outlet

hyperpycnal plume formation from riverine outflows with small sediment concentrations a series of laboratory experiments has been conducted in order to elucidate the sediment induced mixing processes accompanying riverine outflows specifically the discharge of a warm fresh particle laden fluid over a relatively dense cool brine in a parameter regime analogous to recently acquired field measurements hypopycnal surface plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm fresh sediment laden plume and particle settling within it convection was robust in the presence or absence of intense turbulence at sediment concentrations as low as 1 kg m 3 and took the form of millimetre scale sediment laden fingers descending from the base of the surface plume a consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal bottom riding flow the experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient the resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond

petrological and geochemical constraints on diagenesis and deep burial dissolution of the ordovician carbonate reservoirs in the tazhong area tarim basin nw china abstract deeply buried 4500 7000 m ordovician carbonate reservoirs in the tazhong area tarim basin nw china show obvious heterogeneity with porosity from null in limestones and sweet dolostones to 27 8 in sour dolostones from which economically important oils sour gas and condensates are currently being produced petrographic features c o sr isotopes were determined and fluid inclusions were analyzed on diagenetic calcite dolomite and barite from ordovician reservoirs to understand controls on the porosity distribution ordovician carbonate reservoirs in the tazhong area are controlled mainly by initial sedimentary environments and eo genetic and near surface diagenetic processes however vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction filling and cementation in some locations or wells especially zg5 zg7 oilfield nearby zg5 fault burial diagenesis e g thermochemical sulfate reduction tsr probably played an important role in quality improvement towards high quality reservoirs c2 calcite and dolomite cements and barite have fluid inclusions homogenization temperatures ths from 86 to 113 c from 96 to 128 c and from 128 to 151 c respectively we observed petrographically corroded edges of these high temperature minerals with oil inclusions indicating the dissolution must have occurred under deep burial conditions the occurrence of tsr within ordovician carbonate reservoirs is supported by c3 calcite replacement of barite and the association of sulfur species including pyrite anhydrite or barite and elemental sulfur with hydrocarbon and 12c rich as low as 7 2 v pdb c3 calcite with elevated ths 135 153 c the tsr may have induced burial dissolution of dolomite and thus probably improved porosity of the sour dolostones reservoirs at least in some locations in contrast no significant burial dissolution occurred in limestone reservoirs and non tsr dolostone reservoirs the deeply buried sour dolostone reservoirs may therefore be potential exploration targets in tarim basin or elsewhere in the world

morphology and stable and radiogenic isotope composition of pedogenic carbonates in late quaternary relict soils new mexico u s a an integrated record of pedogenic overprinting abstract this petrographic and stable 13c and 18o and radiogenic 14c isotope study of pedogenic carbonates from late quaternary soils in the rio grande rift region of new mexico documents that carbonate formation in semiarid to arid soils aridisols occurs principally by overprinting and can result in a complex petrographic and isotopic record with increasing duration of exposure 7 to 900 ky of the four studied profiles pedogenic carbonates exhibit 1 an increase in the degree of intraprofile 13c and 18o heterogeneity 2 increased development of down profile 13c and 18o trends 3 an increase in the degree of intermorphologic and intramorphologic stable isotope variation commonly in samples distributed laterally within narrow depth intervals and 4 development of covariation between 13c and 18o values radiocarbon ages of the carbonates permit assessment of the degree of isotope heterogeneity in contemporaneous carbonates and recognition of temporal isotopic trends the range of exposure times recorded by the studied soils are analogous to those associated with the formation of fossil soils this makes the studied soils ideal for evaluating the origin of stable isotope heterogeneity in paleosol carbonates and the implications of those heterogeneities for the use of stable isotopes as paleoenvironmental proxies specific implications of this study include the following 1 pedogenic carbonates that form by overprinting are a cumulative record of the soil conditions present during their accumulation thus isotope analysis of overprinted carbonates can result in time averaging of changing climatic paleoenvironmental and paleoecologic conditions 2 carbonates may record only a fraction of the duration of development of their host profile 3 there is a high level of uncertainty associated with using the sequence of morphologic development of carbonate as a function of time or maturity in paleosols 4 an exponential decrease in carbonate 13c values with depth in a paleosol does not imply that the carbonates are contemporaneous or accumulated under invariant soil conditions 5 shifts in soil and paleoclimatic conditions may be recorded by the 13c and 18o values of carbonates distributed within narrow depth intervals of a given soil in addition to those distributed down profile or even from within individual carbonate samples

twentieth century floods recorded in the deep mediterranean sediments flood generated turbidity currents represent an important process of marine sedimentation however no deposit related to this process has yet been described at sea turbiditic sequences cored at 2000 m water depth in the mediterranean show tractive structures and superposition of reverse and normal grading they are related to floods triggered during the twentieth century deposits from flood generated turbidity currents have a great impact on the interpretation of deep ocean paleoclimate records obtained near continental margins by linking deep marine sedimentary records to continental climatic changes through flood frequency and magnitude implications of this study should help redefine the reservoir character of oil fields in fine grained turbidites

supersequence and composite sequence carbonate platform growth permian and triassic outcrop data of the arabian platform and neo tethys abstract permian and triassic carbonate platforms of the arabian peninsula gondwana and seamounts of the neo tethys hawasina and batain basins are characterized by distinctive supersequences second order duration 5 20 million years my and composite sequences third order duration 0 5 5 my the presented sequence stratigraphic framework will be compared with existing sea level curves to discuss the validity of different regional oscillations during the dispersal of pangea the carbonate succession of the haushi and akhdar groups of the arabian platform is composed of four permian p1 p4 and four triassic supersequences tr1 tr4 isolated platforms of the hawasina and batain basins comprise two permian supersequences and one triassic supersequence in contrast to the continuous development of the arabian shield carbonate platform growth of the seamounts was restricted to the guadalupian lopingian and to the middle upper triassic and ceased after drowning events composite sequences exhibit a well developed stacking pattern during the guadalupian lopingian saiq formation lowstand systems tracts lsts occur during the cisuralian gharif formation haushi group and triassic mahil formation akhdar group open marine depositional environments prevail during transgressive systems tracts tsts with diverse biota including rugose and scleractinian corals chaetetids bryozoans and crinoids highstand system tracts hsts exhibit a twofold pattern during the transgressive phase of supersequences composite sequence highstands are dominated by reef or level bottom communities with corals cyclic platform deposits or monotonous mud and wackestone accumulated during the turnaround or late second order highstand of a supersequence correlation of maximum flooding surfaces with published data suggests that supersequences p1 p2 and tr4 can be traced across the arabian platform into the neo tethys basins while supersequences p3 p4 and tr1 tr3 resulted from local tectonic events at the margin of the arabian platform hulw half graben the presented sea level curve corresponds therefore to the tethyan sea level curve during the cisuralian and guadalupian but differs significantly during the lopingian as a result of the dispersal of pangea the middle and upper triassic sea level curve from oman is again in good correlation with published data the permian and triassic sequence architecture on the arabian plate and adjacent neo tethys was predominantly triggered by the global warming after the permian carboniferous glaciation the initial rifting of neo tethys and subordinately by eustatic sea level changes

depositional cycles composite sea level changes cycle stacking patterns and the hierarchy of stratigraphic forcing examples from alpine triassic platform carbonates carbonate platform deposits record a complex interplay of numerous geodynamic variables of which eustasy subsidence and sediment accumulation are prime factors in determining both the kilometer scale depositional sequence scale and meter scale depositional cycle scale stratigraphic packaging in this study we looked particularly at composite eustasy that is superimposed sea level fluctuations with different frequencies defined as orders and different amplitudes and the role it plays in the linkage between meter scale cyclic stratigraphy and kilometer scale sequence stratigraphy specifically we have investigated the relationship between low frequency third order 1 10 m y period depositional cycles and their component high frequency fourth 0 1 1 m y and fifth 0 01 0 1 m y order cycles through detailed stratigraphic analyses of alpine triassic platforms complemented by computer modeling on the basis of our results we suggest that in general there exists a hierarchy of stratigraphic forcing driven by composite eustasy that results in organized stacking patterns thickness subfacies character early diagenetic attributes of high frequency typically fourth and fifth order shallow water carbonate cycles dictated by low frequency third order relative sea level effects we suggest that systematic vertical changes in stacking patterns of high frequency cycles across a larger depositional sequence are due to systematic and predictable differences in depositional space available during the rising and falling stages of a relative third order sea level change we also suggest that these systematic variations in cycle stacking patterns will exist regardless of the mechanism responsible for generating the high frequency cycles be it an autocyclic or allocyclic mechanism this approach has major implications for the use of high frequency fourth and fifth order cycle characteristics to identify third order cycles in outcrops and cores of shallow water carbonates where stratigraphic control may be less than desirable this would constitute a valuable bridge between cyclic stratigraphy at the meter scale and sequence stratigraphy at the seismic scale we present two examples from triassic buildups of the alps the ladinian latemar buildup and the norian dolomia principale where a systematic succession of high frequency cycle stacking patterns and early diagenetic features exists within an overriding third order cycle sequence reflecting the interplay of short term high frequency fourth fifth order eustasy and long term low frequency third order eustasy in accordance with the hierarchy of stratigraphic forcing central to the interpretation of these examples is the demonstration that true eustatic rhythms are recorded in the high frequency cyclicity as verified by time series analyses and the use of fischer plots these examples can be modeled by computer under conditions of lag depth constrained sedimentation uniform subsidence and composite eustasy we also present two examples one from the alpine triassic the norian lofer cyclothems and one from the pleistocene of south florida that lack both a systematic succession of high frequency cycle stacking patterns and identifiable composite rhythms in the stratigraphic record despite the existence of composite high frequency eustasy in these examples we call upon 1 local tectonic forcing in the form of short term deviations in subsidence via faulting lofer example and 2 large differences in the relative amplitudes of different orders of high frequency sea level oscillations pleistocene example to explain the lack of composite rhythms and we present computer simulations to illustrate the concepts an understanding of composite relative sea level changes and the potential for a hierarchy of stratigraphic forcing provides the link between cyclostratigraphy and sequence stratigraphy and also has important implications regarding the stratigraphy of early diagenesis

geochemical characteristics of biogenic gases in china abstract a number of contributions have demonstrated that the stable carbon and hydrogen isotopic compositions together with molecular compositions have a great significance on determination for gas origin and gas generation pathways of biogenic gases together with some data from published work a total of 143 gas samples were compiled from different biogenic gas producing locations in china to investigate the gas origin and gas generation pathways by the molecular composition stable carbon and hydrogen isotopes the results demonstrate that biogenic gases in china are dominated by methane with low level of c2 3 generally less than 0 5 and low 13cch4 values generally 55 the relatively high level of c2 3 up to 11 79 in the biogenic gases from the baise and songliao basins indicates the possible mixture of low mature thermogenic gases biogenic gases from the jiangzhe coastal area including shanghai zhejiang and jiangsu provinces and yingqiong basin have dch4 values greater than 190 indicating marine salt water depositional environments however all other gases have dch4 values less than 190 possibly indicating a terrestrial freshwater or freshwater to slightly saline water depositional environment the methanogenic pathway of biogenic gases in china is dominated by biogenic carbonate reduction though a number of these biogenic gases were generated in a terrestrial freshwater or freshwater to slightly saline water depositional environments compared to the bacterial carbonate reduction methanognic pathway in the luliang basin gas formation mechanism in the baoshan basin was more toward bacterial methyl type fermentation likely due to the relatively higher geothermal gradient in the baoshan basin

δ13c and δ18o values of triassic brachiopods and carbonate rocks as proxies for coeval seawater and palaeotemperature abstract a dataset of 160 isotope 13c and 18o values from anisian ladinian carnian and rhaetian articulate brachiopod shells complemented by 158 carbon and oxygen isotope values from whole rock carbonates define the first continuous stable isotope baseline trends for the triassic seawater the carbon isotope data suggest the existence of several short term high amplitude excursions in the early triassic followed by a predominance of values around 0 5 1 during the middle triassic a rise to 3 5 during the carnian plateau at this level during the late carnian to early norian and a 1 5 decline in the middle norian to values around 2 during the late norian rhaetian interval the causation scenarios for these rapid oscillations are at present equivocal but may in part reflect a biological instability of the carbon cycle following the recovery from the end permian extinction event and or an input of mantle derived co2 from enhanced volcanic activity the 18o values from well preserved brachiopods from the tethyan realm range from 3 9 to 0 6 v pdb these values require open marine triassic seawater 18o values close to 0 v smow for the calculated temperatures to be within the range of tolerance of the coexisting reef building corals a 2 18o increase within the uppermost cordevolian and early julian suggests either a distinct temperature decline in the southern alps during that time interval an increase in seawater salinity or their combination oxygen isotope values for the muschelkalk brachiopods range between 6 2 and 2 0 and likely reflect a strong influx of meteoric waters into the germanic basin that shifted the oxygen isotopes to more negative values

an ice contact rhythmite turbidite succession deposited during the november 1996 catastrophic outburst flood jokulhlaup skeiðararjokull iceland abstract this paper presents new evidence of coarse grained deposition from a turbulent suspension within a low energy slack water location during a recent well documented volcanically related high magnitude glacier outburst flood or jokulhlaup skei ararjokull iceland this study uses established spatial and temporal constraints on processes observed during the november 1996 jokulhlaup to interpret resultant flood sediments coarse grained sediments were deposited within flood slack water conditions from suspension load by repeated turbulent flow pulses with durations of seconds minutes depositional processes are thought to be analogous to turbidity currents a minimum thickness of 15 m of rhythmites record suspended sediment dynamics at a conduit mouth during the late rising and early falling stages of the jokulhlaup the morphology and sedimentology of deposits in a large ice walled chamber are consistent with previous models proposed for bar deposition during cataclysmic floods flood flows within subglacial conduits and ice walled open channels were sufficiently powerful to carry material up to boulder size in suspension macro turbulent flows carried huge quantities of suspended sediment to high levels within the main outlet channel high frequency short duration flow pulses entering a complex ice walled channel geometry generated highly unsteady and non uniform flows capable of both erosional and depositional work this study lends support to the deposition of multiple rhythmites per flood under main flow slack water conditions identification of high magnitude jokulhlaups in the ice marginal sedimentary record will depend on the identification and correct interpretation of feeder channel and slack water turbidite sediments the results described here may also have application to rhythmic deposits formed by large turbulent flows in other environmental settings

ptb mass extinction and earliest triassic recovery overlooked new evidence for a marine origin of lower triassic mixed carbonate siliciclastic sediments rogenstein member germany abstract the central european permo triassic basin was land locked with occasional connections to the north pangaea shelf and tethys a thick sequence of siliciclastic fluvial and floodplain sediments separates the late permian marine carbonate evaporite cycles of the zechstein basin from marine middle triassic marls and carbonates of the germanic basin within this latest permian middle triassic siliciclastic interval the location of the permian triassic boundary ptb is difficult to recognize the first limestones after the ptb are induan mixed carbonate siliciclastic oolites and microbialites of the bernburg and calvorde formations untere buntsandstein group consensus emerged after a long lasting controversy that the carbonates of the so called rogensteine are the relics of an alkaline playa lake the idea of the marine genesis was finally rejected because of the obvious lack of calcified metazoans the existing depositional playa lake model will be tested using previously unconsidered data such as currently discussed early triassic recovery scenarios of marine benthic communities after the permian triassic mass extinction global patterns of early triassic carbonate production permian triassic tectono sedimentary evolution of northern pangaea and stable oxygen and carbon isotope data the rogenstein is interpreted as marine carbonate showing the characteristic features of marine carbonates of the early triassic recovery period calcified invertebrates are rare in many marine settings during the aftermath of the permian triassic mass extinction thin shells of the oolites resemble opportunistic bivalves flourishing in marine early triassic oceans the oolites and microbialites are the product of abiotic and biotically induced carbonate precipitation following the permian triassic mass extinction while biomineralisation is insignificant rogenstein microbialites look like marine early triassic microbial reefs the extraordinary size of the rogenstein ooids resembles marine neoproterozoic ooids the term anachronistic has been used in the literature to describe the similarities in the fabric and composition of early triassic and early palaeozoic neoproterozoic marine carbonates stable carbon isotopes of least altered rogenstein ooids and stromatolites exhibit trends similar to the composition of triassic seawater the presence of flat pebble conglomerates and sediment structures similar to herring bone cross stratification confirm the marine genesis i propose that cool seawater transgressed during a global rise of late permian early triassic base level from the barents shelf southward along reactivated extensional graben structures and flooded the central european permo triassic basin there the extremely hot and arid climate of the megacontinent pangaea caused increased evaporation of seawater and subsequent supersaturation with respect to calcium carbonate insignificant biomineralisation and high water energy in the shallow sea give rise to largely abiotic and biotically induced carbonate precipitation which resembles lake sediments at first sight benthic tropical carbonate production started with the next transgression during the rot formation leading to the sedimentation of marls and limestones during the spathian late olenekian

fossil contourites a critical review abstract despite three decades of study there is still great controversy over the recognition and interpretation of fossil contourites exposed in ancient series on land in order to best examine this problem we briefly review the evidence from modern systems including the many examples of cenozoic contourites that have been recovered from dsdp odp drilling on major drifts in the present day oceans the range of contourite facies described from both deep water 2000 m and mid water 300 2000 m drifts are mostly fine grained bioturbated and homogeneous often with a distinct bedding cyclicity and with some coarser grained sandy contourites developed under higher energy bottom currents there are also a number of current controlled sediment bodies that have formed in outer shelf upper slope settings 50 300 m under the influence of counter currents underflows and major surface currents these are not considered contourites sensu stricto but may be mistaken as such in ancient examples the most commonly described fossil contourites in the literature have been interpreted by the authors concerned as bottom current reworked turbidites however a critical review suggests that these are the facies most subject to misinterpretation and many of the sediments claimed as fossil contourites are almost certainly fine grained turbidites whereas others were more likely formed under outer shelf upper slope current systems there remain very few ancient examples that are more closely comparable to modern contourites these include the cretaceous talme yafe formation in israel the ordovician jiuxi drift in china and parts of the paleogene lefkara formation cyprus and the neogene misaki formation in japan we present a set of possible criteria for the recognition of fossil contourites and bottom current reworked turbidites

diagenetic evolution of the eocene ramp carbonates a paradigm from the nile valley egypt petrographical and geochemical attributes abstract carbonate rocks of east nile valley in egypt have been investigated as a paradigm for revealing the diagenetic evolution of the eocene ramp carbonates the integration of petrographical elemental geochemical and stable isotopic data revealed that the studied carbonates have been influenced by a sequence of diagenetic processes which are included in three diagenetic episodes i e eogenesis mesogenesis and telogenesis and are related to four diagenetic environments i e marine phreatic meteoric phreatic burial and uplifting environments the paragenetic sequence of the studied carbonates commences with the eodiagenetic episode which includes the marine and meteoric phreatic diagenetic environments the marine phreatic diagenetic environment comprises micritization glauconitization pyritization marine cementation fibrous calcite and syntaxial overgrowth rim cements early dolomitization formation of fine crystalline matrix replacive dolomites and early stages of mechanical compaction before significant burial the meteoric phreatic diagenetic environment includes the neomorphism dissolution meteoric cementation earlier phases of sparry calcite cements and precipitation of megaquartz in the void spaces of the peritidal carbonates during the mesogenesis episode the studied carbonates were exposed to burial diagenesis the shallow burial diagenesis resulted in the creation of point tangenetial contacts between the allochemical constituents and the precipitation of a medium coarse crystalline dolomite cement filling the dissolved fossils and dissolution pores the deep burial diagenetic processes embody the chemical compaction condensed fitted interpeneteration and pressure dissolution fabrics fracturing and burial cementation the telodiagenesis represents the last episode in the diagenetic history of the studied carbonates it witnessed the fracturing meteoric vadose cementation and hematitization processes

deciphering the impact of sea level changes and tectonic movement on erosional sequence boundaries in carbonate successions a case study from tertiary strata on grand cayman and cayman brac british west indies abstract the stratigraphic architecture of carbonate successions that develop on geographically isolated islands reflects the balance between sea level highstands lowstands and tectonic activity this is readily apparent in the tertiary carbonate sequences on the cayman islands that are formed of the bluff group which includes the brac formation lower oligocene cayman formation middle miocene and pedro castle formation middle pliocene these strata are overlain by the pleistocene ironshore formation the unconformities that define the boundaries between these formations are variable because some developed during one cycle of past erosion others developed through two or more past erosional cycles and some are still developing today some unconformities like the one at the top of the cayman formation are geographically variable because they underwent different developmental histories in different areas the challenge with architectural complex successions like those on the cayman islands is that of deciphering the impact of sea level changes as opposed to tectonic influences during sea level lowstands the older carbonate successions were exposed on land and modified by surface and subsurface karst development at the same time marine erosional processes affected the coastal areas surface karst modification which commonly produced rugged topographies with erosional relief at least 62 m was controlled largely by rainfall runoff and stratal dip weathering on grand cayman at the end of the miocene for example produced a dish shaped topography with elevated peripheral rims in contrast uplift of the east end of cayman brac between the late pliocene 3 6 ma and 400 ka elevated the basal part of the cayman formation 33 m above sea level subsequent karst development which is still ongoing today removed most of the cayman formation on the eastern part of the island and produced peripheral rims that are higher than those on grand cayman during some lowstands like that between the late pliocene and 400 ka coeval coastal erosion led to the development of coastal benches that cut into the older carbonate strata the combination of karst development in the islands interiors and coastal erosion produced complex rugged topographies that strongly influenced patterns of deposition during the following highstand the complex stratigraphic architecture of the carbonate successions on the cayman islands reflects the variable impact of tectonics karst development and coastal erosion that was associated with each lowstand highstand cycle

the impact of milankovitch scale climatic forcing on sediment supply abstract stratigraphy is the main technique to predict the occurrence of potential reservoirs and seals in the oil industry stratigraphic tools used are stratigraphic observations from drilled wells rock samples and wireline logs and seismic interpretation nowadays computer modelling allows scenario forecasting of the sequence to be expected application of these tools indicates that the key input parameters are initial basin topography subsidence eustasy and sediment supply the first three parameters are usually well constrained or can readily be deduced from well and seismic data however the fourth sediment supply is loosely defined and because of that it is used to calibrate the model in the present article the controls on sediment supply are reviewed and the impact of in particular milankovitch scale climatic forcing on sediment supply is tested on the neogene of the niger drainage and delta system sediment supply is controlled by a complex interplay of geographic and climatic factors such as 1 relief or mountain height 2 the size and temporal storage capacity of the drainage area 3 overall climate 4 amount and seasonality of precipitation and 5 vegetation cover studies in geomorphology have indicated that although complex computer models can be made to predict erosional product for the present a large scale sediment supply prediction should be based on the combination of palaeogeographical palaeoclimatological and erosion potential maps more detailed sediment supply predictions should include milankovitch scale variations the climatic impact of these variations is mainly felt when we are in a transition between two climates then the area will experience regular climatic changes from e g wet rainforest to dry monsoonal climate or low to high sediment supply respectively this concept was tested in the forward modelling package stratagem on the neogene of the niger delta four scenarios were evaluated using various combinations of large and milankovitch scale sediment supply curves combined with global or milankovitch scale eustatic sea level curves the results confirm the impact of both large scale and milankovitch scale climatic forcing on sediment supply however in an icehouse world the various scenarios also suggest that the effect of eustatic sea level fluctuations overshadows the impact of milankovitch scale sediment supply by contrast in a greenhouse world the impact of milankovitch scale sediment supply is expected to be more significant in the absence of significant sea level fluctuations

discontinuity surfaces on a shallow marine carbonate platform berriasian valanginian france and switzerland discontinuities in sedimentation are commonly expressed as surfaces in outcrop sections and are due to rapid and substantial environmental changes on shallow marine carbonate platforms most such surfaces represent hiatuses below biostratigraphic resolution and detailed analysis is necessary to identify and evaluate the environmental change involved surfaces in nine sections of the lower cretaceous of the french and swiss jura platform are characterized on the basis of eight universally applicable criteria geometry lateral extent morphology biological activity mineralization facies contrast diagenetic contrast and biostratigraphy nine different surface types are distinguished by their common features and environment of formation all of them are related to environmental changes in the form of subaqueous erosion subaerial exposure subaqueous omission or changes in texture and facies the distribution of surface types in the studied sections shows that condensation and exposure related surfaces tend to occur repetitively in certain intervals calibrated by biostratigraphy these surface zones can be correlated across the platform from proximal to distal positions in comparison with the global sequence stratigraphic framework hardenbol et al 1997 most exposure zones correlate with third order sequence boundaries condensation zones fall in between in the studied sections third order eustatic sea level drops appear to be represented rather by zones of small scale discontinuities than by widespread and well marked single sequence boundaries this is explained by the superposition of high frequency low amplitude sea level fluctuations on a larger scale sea level trend under greenhouse conditions the lateral extent of the surface zones varies through time and indicates important changes in platform morphology changes in local subsidence rate indicated by variable thicknesses of the deposits in comparison with second and third order sea level trends suggest an evolution of the french jura platform from a ramp type morphology in the late middle berriasian to a flat topped platform in the late berriasian the early valanginian again is characterized by increased differential subsidence and well marked platform morphology this study demonstrates that 1 small scale and short lived discontinuities can reflect large scale variations of relative sea level 2 on shallow platforms characterized by small topographic variations and lateral facies changes third order sequence stratigraphic surfaces are not necessarily expressed by one widespread single surface but by zones of surfaces indicating repeated environmental changes surface zones can serve as an additional tool for correlation and interpretation of platform evolution and 3 small scale discontinuities form an integral part of the stratigraphic record and should receive the same attention as the sedimentary deposits they delimit

middle triassic orbital signature recorded in the shallow marine latemar carbonate buildup dolomites italy a new time frequency analysis of sea level controlled carbonate platform cycles in the middle triassic latemar massif dolomites italy reveals a strong depositional signature with characteristics of dominant forcing by climatic precession modes corresponding to long and short precession components at 1 21 7 k y and 1 17 6 k y underwent amplitude modulations matching earth9s orbital eccentricity with major frequency components at 1 400 k y 1 125 k y and 1 98 k y obliquity appears as a minor component at 1 35 4 k y the latemar signature thus constitutes the oldest pristine milankovitch signature yet observed in the geologic record its fidelity rivals that of the pliocene pleistocene record originally used to confirm the theory of orbitally forced climates this evidence deepens a widely noted disagreement between radiometric and cyclostratigraphic time scales for the latemar buildup the latemar cycles indicate that orbitally forced sea level oscillations were operative in the ice free middle triassic hothouse world

astronomically induced paleoclimatic oscillations reflected in pliocene turbidite deposits on corfu greece implications for the interpretation of higher order cyclicity in ancient turbidite systems the growth pattern of fan lobes in the early pliocene corfu turbidite system greece demonstrates that regional climatic fluctuations in the land locked mediterranean completely concealed global glacioeustatic effects the temporal evolution of the fan lobes studied closely matches the astronomical precession cycle in this time interval 23 ka strongly suggesting that regular waxing and waning of the sediment supply to the delta fed turbidite system were directly driven by changes of precipitation and continental runoff the common lack of high resolution time control in ancient turbidite systems may lead to mistaken interpretations of higher order cyclicity in ancient turbidite sequences produced by precession induced variations of sediment supply at low paleolatitudes

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

facies architecture of an isolated carbonate platform tracing the cycles of the latemar middle triassic northern italy the 720 m thick succession of the middle triassic latemar massif dolomites italy was used to reconstruct the lagoonal facies architecture of a small atoll like carbonate platform facies analysis of the lagoonal sediments yields a bathymetric interpretation of the lateral facies variations which reflect a syndepositional palaeorelief based on tracing of lagoonal flooding surfaces the metre scale shallowing upward cycles are interpreted to be of allocyclic origin short term sea level changes led to subaerial exposure of wide parts of the marginal zone resulting in the development of a tepee belt of varying width occasional emergence of the entire lagoon produced lagoon wide decimetre thick red exposure horizons the supratidal tepee belt in the backreef area represented the zone of maximum elevation which circumscribed the sub to peritidal lagoonal interior during most of the platform s development this tepee rim the subtidal reef and a sub to peritidal transition zone in between stabilized the platform margin the asymmetric width of facies belts within individual metre scale cycles was caused by redistribution processes that reflect palaeowinds and storm paths from the present day south and west the overall succession shows stratigraphic changes on a scale of tens of metres from a basal subtidal unit overlain by three tepee rich intervals separated by tepee poor units composed of subtidal to peritidal facies this stacking pattern reflects two third order sequences during the late anisian to early middle ladinian

the interpretation of cyclic successions of the middle and upper triassic of the northern and southern alps abstract theories regarding the formation of sedimentary cycles in the 3rd 4th and 5th order bands are reviewed with reference to the middle and upper triassic of the northern calcareous alps nca and southern alps milankovitch autocyclic and tectonic theories are discussed together with an evaluation of concepts of chaotic sedimentation and a case example from the nca concerning eustasy 3rd 4th and 5th order sea level fluctuations were probably a low amplitude low rate phenomenon caused by fluctuations in the volume of mountain glaciers and ocean water during the triassic the mid and late triassic was a non glacial interval in which polar regions may have been ice free so glacio eustasy can not be expected eustatic sea level variations in the 3rd 4th and 5th order bands seem to have left no useful imprint on cyclic successions in the region whatever record there may be is inextricably mixed with two other signals tectonic activity and autocycles the review shows how sedimentation in the triassic of the area was strongly influenced by tectonic activity this is as true for the middle and late triassic of the nca as it is for the southern alps tectonic activity may be responsible for large scale cyclicity 4th to 3rd order scale although seismogenic structures have yet to be identified and described in carbonate successions of the alps candidates do exist slumped and microfaulted layers in laminated sediments of the seefeld basin upper triassic nca have been described as the products of fault movements the sedimentary record from the nca and southern alps also leaves little doubt that autocyclic processes were important in all environments except perhaps the deep sediment starved basins most small scale platform cycles 5th order scale in the region can be related to autocyclic processes and in shallow basinal successions to events such as storms previous workers have not been consistent in their interpretation of cyclic successions in the area applying diverse theories to similar successions so far the steinplatte hochkonig platform with attached kossen basin is the only example interpreted with reference to tectonics and autocyclicity eustasy was probably not the most important factor in cycle generation in the triassic of the nca and southern alps such an approach could prove useful in future studies

sediment laden gravity currents with reversing buoyancy there are many natural occurrences of sediment laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid although the bulk density of the current is greater such currents are driven by the excess density of suspended particles however after sufficient particles have sedimented the current will become buoyant cease its lateral motion and ascend to form a plume examples of such currents include brackish underflows in deltas turbidity currents and pyroclastic flows experimental studies are described which show that due to sedimentation sediment laden gravity currents decelerate more rapidly than saline currents of the same density there is little difference in the experiments between a sediment laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift off a marked deceleration is then observed and a plume is generated with lift off occurring along the length of the current the resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank the deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift off distance and has a flatter profile than that from a simple sediment current a theoretical model is presented which is based on the two layer shallow water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current the model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift off distance reasonably well these processes have implications for the behaviour of turbidity currents the interpretation of turbidites mixing processes in the oceans and the lift off of pyroclastic flows

packstones and floatstones ambiguous textures and origins in need of critical appraisal abstract the depositional textural classification of allochthonous limestones by dunham 1962 and its modifications by embry iii and klovan 1971 are widely used by carbonate petrologists during petrographic field studies and core examinations the origin and textures of packstones and floatstones used in this classification scheme need re appraisal towards their unified and unequivocal usage by carbonate petrologists in academia and the petroleum industry such usage will help avoiding inadequate interpretations of the depositional sequence stratigraphic or reservoir quality distribution models based on examination of jurassic cretaceous and holocene carbonate deposits from united arab emirates and oman it is suggested here that packstones should not be used to interpret energy of depositional environments because the majority are originally deposited as grainstones i e free of mud matrix thus sand and mud sized particles are expected to be hydraulically separated i e not deposited simultaneously therefore packstones might be originally grainstones that have incorporated variable amounts of mud matrix by various post depositional processes primarily bioturbation and or compaction deformation of peloids known as pseudomatrix dickinson 1970 we further observed that the 2 mm allochems in floatstones float in grainstones and packstones and not in mud wackestone matrix

evolution and distribution of porosity associated with subaerial exposure in upper paleozoic platform limestones west texas middle pennsylvanian lower permian limestones in the subsurface of west texas were studied to determine how subaerial exposure and freshwater diagenesis karstification affected porosity distribution in meter scale cycles approximately 87 depositional cycles are present in the gross reservoir interval depths of 2600 3000 m and each cycle is interpreted to represent a glacio eustatic sea level fluctuation using recent radiometric age dating average cycle duration is estimated at 160 000 yr per cycle reservoir grade porosity 4 occurs in 5 25 of the gross reservoir section porosity is stratified occurring in 1 6 m thick intervals in the upper part of cycles that were subaerially exposed however many cycles that were subaerially exposed now lack porosity diagenesis and porosity development have distinct patterns related to duration of subaerial exposure four stages of porosity development are identified 1 very brief or no subaerial exposure estimated at less than 5000 yr caused little or no diagenetic change 2 brief to moderate subaerial exposure estimated at 5000 50 000 yr resulted in most primary pores being filled with calcite cement and dissolution creating fine matrix pores molds and intercrystalline pores 3 moderately long subaerial exposure estimated at 50 000 130 000 yr resulted in cements filling primary pores and some fine secondary pores and dissolution creating small conduit pores vugs fractures fissures 4 prolonged subaerial exposure estimated at greater than 130 000 yr resulted in most primary and secondary matrix pores being filled with calcite cement but dissolution enlarged conduit pores vugs fractures fissures present subsurface porosity in this field preferentially occurs in thick grainstones phylloid algal boundstones and a few wackestone packstones in cycles subjected to brief subaerial exposure stages 2 and 3 matrix porosity molds intercrystalline pores is dominant because most conduit pores formed during prolonged subaerial exposure were filled with either shale during subsequent transgressions or burial cements derived from pressure solution associated with the shales the distribution of porosity in the southwest andrews area indicates that duration of subaerial exposure and supply of clastics are major factors determining ultimate porosity in limestones subjected to subaerial exposure and karstification

a model for atmospheric co 2 over phanerozoic time a new model has been constructed for calculating the level of atmospheric co sub 2 over phanerozoic time which is much simpler mathematically than the blag model but more complex geologically and biologically results suggest that there has been a notable pattern of varying atmospheric co sub 2 level over the past 570 my with high levels during the mesozoic and early paleozoic and low levels during the permo carboniferous and late cenozoic sensitivity analysis shows that within reasonable limits of the principal governing parameters this qualitative trend of varying co sub 2 is relatively insensitive to the actual values chosen for these parameters causes of the co sub 2 variation are multiple and no single geological or biological process can be called upon to explain all co sub 2 variation for all time the calculated trend of co sub 2 over time agrees well with independent estimations of paleoclimates thus the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study

microcodium revisited root calcification products of terrestrial plants on carbonate rich substrates abstract microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols in the cretaceous and tertiary continental and marine successions of the peri tethyan realm the main controversy about the microcodium structures is their origin and possible relation with calcified plant roots microcodium and rhizogenic root formed microfabrics were studied in calcrete profiles within the paleocene shallow marine carbonate succession in southwestern slovenia the prominent laminar calcrete horizons contain abundant calcite aggregates 150 m to 1 cm in size with perfectly preserved structural details of plant root tissues morphology and structure of these aggregates indicate that they formed through biologically controlled precipitation of calcium carbonate within the root cortical cells the morphologies intermediate between the typical microcodium aggregates composed of a single layer of individual elongate pyramidal or prismatic crystals of calcite measuring 100 500 m in length and 20 70 m in width and calcified roots with multilayer arrangement of isodiametric cells were observed and this supports previous rhizogenic interpretations of microcodium structures intermediate forms show that the typical microcodium aggregates formed through morphological transformation of the root tissue by growth of the calcite within the cortical cells which distorted the cell shape calcification of roots and the creation of microcodium structures can be explained as an effective nutrient acquiring mechanism used by certain types of terrestrial plants inhabiting nutrient poor calcareous substrates the widespread occurrence of microcodium in almost unaltered shallow marine limestones indicates that its formation took place during early stages of paleosol development probably reflecting specific types of vascular plants of a pioneer community that were able to colonize carbonate substrates during the early phases of subaerial exposure

hyperpycnites deposited 700 km away from river mouths in the central japan sea abstract relatively little is known about the characteristics of deposits and potential runout distances of hyperpycnal currents this study describes and discusses evidence of sediment deposition from sustained long duration quasi steady turbidity currents in the distal part of the toyama deep sea channel which extends ca 700 km from river mouths in the central japan sea the study is based on gravity cores and airgun seismic reflection profiles obtained from the channel distal reaches the silty turbidite beds of the channel s terminal fan show rhythmic layering that indicates sustained turbidity currents with distinct flow strength fluctuations some of the rhythmite beds show a fining upward internal trend net flow strength waning whereas others show an upward coarsening net flow strength waxing followed by fining seismic reflection profiles from the channel levees show large bedforms of climbing dune type attributed to the spillover of thick sustained turbidity currents the deposition of the terminal fan rhythmites and accretionary levee bedforms is attributed to turbidity currents generated by hyperpycnal river effluent a quantitative assessment of sediment concentrations in the coastal rivers indicates that their effluents could become hyperpycnal nearly every year or during every major seasonal flood the density of a river generated underflow would increase by the entrainment of saline water and seafloor sediment on the steep slope of the toyama bay resulting in robust long runout turbidity currents the estimated flow velocities of these currents were around 0 3 m s and their recurrence period for the last 1000 years was of the order of 70 years the estimated duration of hyperpycnal flows required for the deposition of rhythmite beds 700 km away from the river mouth is of the order of several days to 3 4 weeks the study provides new insights into the recognition and classification of hyperpycnites in distal zones of deep marine turbiditic systems

pangaean climates megamonsoons of the megacontinent the climate for a highly idealized geographic representation of the megacontinent of pangaea corresponding to the period 250 200 m y ago is explored using a low resolution atmospheric general circulation model that is coupled to a static mixed layer ocean of constant depth with a provision for sea ice formation and decay soil moisture is included as a predicted variable on the continent because there are large uncertainties about key factors that might influence the pangaean climate sensitivity experiments are used to examine the climate for two different mean elevations 0 and 1000 m two different snowcover prescriptions and two different levels of implied greenhouse heating and solar luminosity modern co2 concentration and modern solar luminosity fivefold increase in modern co2 concentration and a 1 decrease in modern solar luminosity in all of the experiments the large bipolar land masses of laurasia and gondwanaland joined near the equator in the west and separated by the tethys sea in the east exhibit extreme continentality with hot summers cold winters and large scale summer and winter monsoon circulations year round or seasonal aridity is simulated for all but the eastern coastal regions the tropical western coast and the regions poleward of 40 latitude the simulated climate of the idealized pangaean continent has many features that correspond qualitatively with geologic evidence for tropical warmth and aridity in the late permian and triassic however the model results serve mainly to indicate the importance of various climate forming factors geography topography heating detailed comparisons will require new experiments with better models and more accurate geologic boundary conditions

biotic carbonate precipitation inhibited during the early triassic at the rim of the arabian platform oman abstract sedimentological palaeontological and geochemical data of a 152 m thick composite section of the saiq formation were used to describe the facies associations and the nature of carbonate precipitation during the late permian early triassic at the eastern rim of the arabian platform sultanate of oman saih hatat wadi aday changhsingian late permian platform carbonates dominated by bryozoans brachiopods and crinoids are truncated by a mineralized discontinuity surface the disappearance of permian calcified metazoans a negative 13ccarb excursion and a sharp facies contrast collectively suggest that the permian triassic boundary lies in this interval unfossiliferous siliciclastics with nodules of iron and manganese minerals on top of the unconformity were probably deposited during a phase when sea water was undersaturated with respect to calcium carbonate after a gap late dienerian carbonate deposition started with abiotically precipitated lime mudstone and biotically induced microbialites having the lowest observed 13ccarb values facies association a further up 1 2 m of bioclastic wacke and grainstones with a positive 13ccarb excursion indicate a short lived interval of biotically controlled carbonate precipitation facies association b the overlying sequence of siliciclastics laminated or bioturbated dolomitized mudstone microbialites dolomitic siltstone and black calcite which is characterized by a 2nd negative 13ccarb shift marks the return to biotically induced carbonate precipitation facies associations c d the recrystallized black calcite at the top of the sequence is capped by a thick palaeosol and overlain by late early to middle triassic cycles consisting of biotically precipitated carbonate the facies development of the rim of the arabian platform differs from that of the interior in having a pronounced discontinuity at the permian triassic boundary and an early triassic bioclastic carbonate unit sandwiched between abiotically precipitated carbonates the observed lateral changes in carbonate precipitation across the arabian platform could have been caused by changes in oceanic circulation such as episodic upwelling of deep water

economic phosphorite from the ediacaran doushantuo formation south china and the neoproterozoic cambrian phosphogenic event abstract phosphorite of the ediacaran doushantuo formation accumulated in the central guizhou province during the neoproterozoic cambrian phosphogenic event producing the first true phosphorite giant in earth history deposition is interpreted to have occurred on a wave dominated epeiric platform along the northern margin of the qianzhong uplift lithofacies stacking patterns and two paleokarst surfaces record phosphogenesis through at least two sea level cycles as in younger phanerozoic epeiric sea phosphorites the delivery of upwelled phosphorus is interpreted to have stimulated phosphogenesis in a range of shallow water environments ten phosphatic and associated lithofacies are recognized in the doushantuo formation wavy and horizontal laminated pristine facies were the locus of phosphogenesis and reflect the authigenic precipitation of francolite in accumulating fine sediment granular cross stratified phosphorite is interpreted to have been produced through storm wave winnowing of pristine facies and the subsequent transport redeposition and concentration of phosphatic peloids into economic beds beds with the highest concentration of phosphorus formed in shallow wave agitated environments around paleo topographic highs paragenetic analysis suggests that secondary meteoric processes further enriched the phosphorus content 35 wt of these granular beds during sea level lowstands such aerially extensive upwelling related phosphorite contrasts paleoproterozoic and neoproterozoic phosphatic deposits throughout much the proterozoic francolite precipitation was restricted to shallow water photosynthetic oxygen oases where redox sensitive phosphogenic processes were focused in the sediment the doushantuo formation is interpreted to record the oxygenation of the water column and concomitant expansion of phosphogenic environments during the neoproterozoic oxygenation event this progressive ventilation of the deep ocean forever changed the biogeochemical cycling of phosphorus in the marine realm and set the stage for the deposition of phosphorite giants that punctuate the phanerozoic stratigraphic record

pore size and pore throat types in a heterogeneous dolostone reservoir devonian grosmont formation western canada sedimentary basin the devonian grosmont formation in northeastern alberta canada is a giant heavy oil reservoir the main reservoir rocks are dolomitized and karstified platform and ramp carbonates and the best reservoir facies occur in the upper grosmont ugm units 3 and 2 in these units reservoir properties are highly heterogeneous hand specimen thin section uv and sem petrography as well as grading scales mercury capillary pressure curve analysis and statistics have been used to characterize reservoir heterogeneity our investigation led to a new pore size classification for carbonate reservoirs this new classification has four pore sizes microporosity pore diameters 256 mm a combination of microscopic observations and capillary pressure curve characteristics led to the recognition of four pore throat texture types on the microporosity scale and to five types on the mesoporosity scale microporosity pore types include 1 intracrystal dissolution porosity 2 pervasive intercrystal and intracrystal dissolution porosity 3 intergranular and or intercrystal porosity in grainstones and 4 primary or solution microporosity in mud matrix only in limestones mesoporosity pore types include 1 intercrystal porosity 2 solution enhanced intercrystal porosity 3 oversized porosity 4 intragranular solution porosity and 5 intergranular solution porosity some of these types are homogeneous e g non fabric selective dissolution porosity and inte crystal primary porosity whereas others are heterogeneous generally hydrocarbon recovery efficiency is good in the homogeneous pore throat types but poor in the heterogeneous types reservoir heterogeneity has further been characterized by comparison and statistical analysis of plug and full diameter core fdc data that indicate 1 plug porosities on average are higher than fdc porosities 2 plug permeabilities on average are higher than fdc permeabilities 3 plug porosity and permeability show relatively good linear relationships in certain core intervals but fdc data do not and 4 generally the more homogeneous reservoir rocks have relatively high matrix porosity heterogeneity is common in grosmont dolostone reservoir rocks and affects overall reservoir quality at the megascopic scale the reservoir properties can be classified as homogeneous dual porosity and multilayered the dual porosity model has three subcategories fractured channeled and pressure solution derived

evolution of diagenetic system and its controls on the reservoir quality of pre salt dolostone the case of the lower ordovician majiagou formation in the central ordos basin china abstract the evolution of diagenetic system and its impact on the carbonate reservoir quality are of paramount importance new investigation of the lower ordovician majiagou formation central ordos basin china has revealed that the diagenesis and quality of pre salt dolostone reservoirs were controlled by the evolution of diagenetic system three different reservoir types grain crystalline and algal dolostone and five kinds of reservoir space intergranular pore intragranular pore intercrystalline pore dissolution pore and vug in karst system and parallel dissolution pore of laminar algae are identified in the majiagou formation pre salt dolostone of the central ordos basin based on the petrographic characteristic the diagenetic process of pre salt dolostone can be divided into four stages the penecontemporaneous diagenesis the early diagenesis the epidiagenesis and the mesodiagenesis late diagenesis petrographic and geochemical analyses of the studied dolostone reservoir indicated that the diagenetic system was open during the penecontemporaneous diagenesis and became closed during the early diagenesis open and closed diagenetic system existed in the west and east of the central ordos basin during the epidiagenesis respectively after that the western and eastern pre salt dolostone reservoirs both evolved into a closed diagenetic system during the mesodiagenesis late diagenesis subaerial meteoric dissolution in an open diagenetic system secondary pore formation and subsequent pore preservation in a closed diagenetic system are the critical factors in the improvement of pre salt dolostone reservoir quality although two cycles from open to closed diagenetic system developed in the western dolostone reservoir the pore space there was filled with overlying weathering mud in the second open diagenetic system during the epidiagenesis which reduced the reservoir quality the eastern dolostone reservoir experienced just one cycle from open to closed diagenetic system the reservoir quality was improved and then preserved thus only a good match among the diagenetic systems during different diagenetic stages will be good for the reservoir formation and preservation therefore the potential high quality reservoirs in the pre salt dolostones mainly located in the eastern part of the central ordos basin

kalkowsky s stromatolites revisited lower triassic buntsandstein harz mountains germany abstract the term stromatolite was proposed by kalkowsky 1908 z dt geol ges 60 68 125 for structures he discovered in the carbonates of the lower triassic buntsandstein of the harz mountains in central germany we have re examined these localities to clarify kalkowsky s descriptions and to deduce the environmental conditions of stromatolitic growth the lower buntsandstein was deposited in a closed lacustrine basin the central european permo triassic basin the depositional environment was a playa lake with variable salinity in the centre of the basin oolite beds are intercalated in the mainly siliciclastic succession which is composed of 20 fining upward cycles thought to have been produced by climatic variations stromatolites occur almost exclusively on the surface of oolite beds stromatolitic structures consist of millimetre thick laminated crusts and centimetre high branching columnar forms smaller and bigger 2 m high domes are composed of secondary order columns and intercalated crusts occasionally upper surfaces of the domes are pitted due to syndepositional dissolution despite of the overall intense sparitization two groups of stromatolitic microstructures can be distinguished spongy fenestrate and fan like fabrics the spongy fenestrate fabric is similar to that one caused by the recent cyanobacteria phormidium calothrix and dichothrix and the fan like fabric to rivularia laminated crusts called stromatoid by kalkowsky are interpreted as being of cement origin and formed by quick precipitation the change from columnar to domical forms may reflect changes of hydrodynamic conditions clear photoautotrophism suggests that cyanobacteria or eukaryotes were the dominant organisms involved in the formation of kalkowsky s stromatolites influx of ooids or siliciclastic mud interrupted or terminated the stromatolitic growth the morphology growth pattern and fabrics of stromatolites were most likely controlled by ecological factors which all seem to be related to the general hydro climatic evolution of the lower buntsandstein

relationship between karstification and burial dolomitization in permian platform carbonates lower khuff oman large breccia fabrics associated with karst constitute an important structure in massive limestone successions the dimensions and shapes of breccia structures are controlled by the initial fracture pattern of the limestone and preferential pathways of the karstifying fluids but subsequently breccia fabrics can also govern the migration of later fluids therefore breccias are highly relevant features to capture for reservoir characterisation outcrop analogues for lower khuff units in the middle east present in the central oman mountains reveal brecciated fabrics up to 10 s of meters in diameter these brecciated units are closely associated with dolomite bodies of late diagenetic origin based on an integrated set of data the breccias are interpreted as collapsed karst cavities either formed by meteoric or hypogenic fluids the exact origin of the fluids could not be constrained due to an overprint by later dolomitizing fluids based on the composition of the clasts and matrix in the breccias two dolomitization events are interpreted to have affected the succession one prior to early diagenetic ed dolomite and one after brecciation late diagenetic dt2 dolomite dolomite of shallow burial origin ed dolomite was only observed as clasts within breccia and is much more frequent than late diagenetic medium to deep burial dolomite clasts thus the timing of the brecciation and collapse is assumed to postdate shallow burial early diagenetic dolomitization late diagenetic replacive dolomite dt2 dolomite forms 90 of the matrix in the breccia fabrics with the exception of a small area that was not affected by dolomitization but is rarely present as clasts stable isotope measurements 18o 2 5 to 6 vpdb and 13c 2 9 to 4 8 vpdb suggest a burial origin for the late diagenetic dolomite potentially with the participation of hydrothermal fluids the dolomitized matrix indicates a migration of late dolomitizing fluids subsequent to or postdating the collapse of the karstic cavities thus early karstification processes seem to have played a big role in controlling subsequent loci of late dolomitization in the oman mountains and potentially in other similar settings elsewhere

conditions for the ignition of catastrophically erosive turbidity currents abstract the basic impediment to a clear understanding of eroding and depositing turbidity currents has been the lack of a proper formulation of bed sediment entrainment this problem is addressed herein a detailed analysis indicates that the bagnold criterion is necessary but insufficient for self sustaining turbidity currents the analysis reveals two possible equilibrium states a relatively low velocity ignitive state and a high velocity dense catastrophic state a stability analysis indicates that the ignitive state is unstable flows below it invariably die out and flows above it ignite i e accelerate and entrain sediment to the catastrophic state which is stable the ignitive state thus defines the criterion for a self sustaining turbidity current estimates of the catastrophic state suggest that it is highly erosive and competent to scour out submarine canyons

three dimensional architecture of a coalesced collapsed paleocave system in the lower ordovician ellenburger group central texas the three dimensional interwell scale architecture of a lower ordovician ellenburger coalesced collapsed paleocave system was constructed through the integration of ground penetrating radar gpr shallow core and outcrop data the data were collected near marble falls in central texas over an area 800 1000 m 2600 3300 ft that could cover several oil well locations 160 ac 0 65 km2 typical of a region such as west texas integration of core based facies descriptions with gpr reflection response identified several paleocave facies that can be recognized and mapped with gpr data alone 1 continuous reflections image the undisturbed strata 2 relatively continuous reflections over tens of meters characterized by faults and folds image the disturbed strata and 3 chaotic reflections having little to no perceptible continuity image heterogeneous cave related brecciated facies recognized in core that cannot be individually resolved with the gpr data these latter facies include the highly disturbed strata coarse clast chaotic breccia fine clast chaotic breccia and sediment fill the three dimensional architecture of the coalesced collapsed paleocave system based on core and gpr data indicates that there are trends of brecciated bodies that are as much as 350 m 1100 ft wide greater than 1000 m 3300 ft long and tens of meters high these brecciated bodies are coalesced collapsed paleocaves between the brecciated bodies are areas of disturbed and undisturbed host rock that are jointly as much as 200 m 660 ft wide as a cave system is buried many structural features form by mechanical compaction these features include folds sags and faults the folds and sags measure from a few meters to several hundred meters wide the collapse related faults are numerous and can have several meters of displacement most are normal faults but reverse faults also occur

quantification of macroscopic subaerial exposure features in carbonate rocks abstract the macroscopic features that characterize subaerial exposure surfaces in carbonates are well known but their significance has not been quantitatively evaluated this study presents such an analysis in the lower oligocene suwannee limestone of west central florida eleven cores were point counted on a foot by foot basis for the abundance of caliche rhizoliths karst breccia open vugs infiltrated sediment fractures and pedogenic alteration these features occur at and below intraformational exposure surfaces which represent hiatuses estimated at 104 to 105 years and an uppermost sequence bounding unconformity representing 0 5 myr as revealed by sr isotope data statistical analyses of the point count data reveal only a few significant relationships 1 the hierarchy of exposure surfaces and by inference duration of exposure is differentiated only at a marginally significant level by sediment filled vugs preferentially associated with the sequence boundary duration of exposure did not have a significant impact on the relative abundance of all other features 2 proximity 5 ft to both types of surfaces fractures open vugs and infiltrated sediment show no proximal or distal preference for either type of surface 3 depositional texture has no statistically significant affect on the presence or abundance of the exposure features with the exception that rhizoliths and open vugs are preferentially more abundant in packstones relative to grainstones this is interpreted to be the result of a soil moisture effect factor analysis defines four factors that explain 46 to 52 of the total variance in the abundance data relative to the sequence boundary and the intraformational surfaces respectively the loading of each exposure feature on each factor is the same with respect to both types of surfaces which is further evidence that the abundance of exposure features is independent of duration of exposure factor 1 is interpreted to be the amplitude of base level changes and controls the abundance of karst breccia factor 2 is interpreted to be abundance of vegetation and relates to the abundance of rhizoliths and fractures factor 3 is interpreted to be a combination of soil zone pco2 and the availability of water and affects the abundance of pedogenic overprinting caliche and open vugs factor 4 is stratigraphic proximity to the sequence boundary which controls the presence of sediment filled voids the amount of uncorrelated unique variance associated with infiltrated sediments pedogenic overprinting caliche and open vugs is large 60 meaning that feature abundance is also influenced by other unidentified site specific factors these results demonstrate that quantifying the abundance of macroscopic subaerial exposure features in limestones has the potential to yield more insight into the significance of those features than a mere qualitative assessment this is particularly true when assessing the potential role of the many variables that can affect the development of these features

initiation of turbidity currents outcrop evidence for eocene hyperpycnal flow turbidites abstract the most widely used although not as widely documented explanation for initiation of turbidites is the conversion of catastrophic sediment failures into turbidites such instantaneous sediment failures generate short lived surge type flows that deposit turbidite beds described as bouma sequences in this communication we present data from the eocene central basin of spitsbergen that shows turbidite systems that are volumetrically dominated by thick ungraded and laminated sandy turbidite beds deposited by gradual aggradation from sustained flows sustained flows are long lived and more or less continuous they can be generated by a variety of mechanisms e g 1 instability during volcanic eruptions and the consequent remobilisation of unconsolidated material 2 seismically triggered subaerial sliding within the drainage 3 storm surges 4 retrogressive slope failure 5 breaching and 6 hyperpycnal flows our database shows that in the eocene central basin the sustained flows were generated by hyperpycnal flows i e by direct river effluent the hyperpycnal flow turbidites have been recognized on the basis of 1 physical connection between fluvial and turbidite channels at the shelf edge 2 abundance of thick turbidite sandstone beds 3 sand prone nature of the turbidite systems 4 downslope changes of individual thick sandy turbidite beds and their collapsed pinch out segments 5 great abundance of continental material leaves coal fragments in turbidite beds 6 low abundance of associated slumped or debris flow beds and 7 the occurrence of turbidites in systematically accreted shelf margins these features strongly suggest that hyperpycnal flow generated by direct river effluent deposited significant number of turbidite beds in the documented slope and basin floor turbidite successions we also argue that the hyperpycnal flows are preferentially fed into the deepwater slopes beyond the shelf edge during the falling stage and lowstand of relative sea level the turbidite beds were studied in the context of seismic scale 1 15 km dip oriented outcrops by measuring detailed vertical sections following individual beds and packages of beds downslope for distances over 5 km lateral mapping and helicopter taken photomosaics

predicting the migration rates of subaqueous dunes in this paper we develop a simple physically based method for predicting what fraction of sediment moving over the crests of dunes will bypass their lee faces the bypass fraction is found by calculating a characteristic excursion length for every grain size making up a particular train of dunes all particles with excursion lengths greater than the downstream span of the lee face of the average dune are assumed not to contribute to dune propagation bypass fractions based on distributions of excursion lengths account for the discrepancies between total sediment discharge and dune migration rate measured by stein 1965 and guy et al 1966 in laboratory flumes as well as by us in the north loup river of nebraska calculations and these data agree over sediment transporting conditions associated with the entire stability field for dunes composed of medium sand the two laboratory studies show that commonly 30 60 of all sand moving over fully developed dunes is not deposited on slip faces measurements from the north loup river reveal that of all sediment moving over the dune crests there roughly 45 is not being captured on lee faces even though 99 of sediment is transported within 2 cm of the bed the method developed herein successfully estimates measured values for the bypass fraction from 0 to 80 our analysis indicates that the division between grains that are deposited versus those that are bypassing falls within the range of sand sizes making up the suspended load at dune crests

origin and significance of isotope shifts in pennsylvanian carbonates asturias nw spain abstract the primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water an inadequate understanding of this variability as well as different diagenetic environments severely limit the interpretation of the stable isotope record of shoalwater carbonates in order to investigate this primary and diagenetic variability along a bashkirian moscovian platform to basin transect 13c and 18o analyses have been performed on more than 1000 matrix micrite carbonate cement and brachiopod shell samples in isotope analysis these different carbonate materials tend to complement each other inasmuch as they have different advantages and shortcomings the resulting data reveal spatial trends in 13c and 18o signatures from platform top lower values to basin higher values in the case of 13c from pristine brachiopods this trend can be explained by the long residence time aging of platform top water masses in the case of brachiopod 18o this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break micrite and marine cement isotopic values from the platform interior were reset lowered during pervasive early meteoric diagenesis in contrast micrite and marine cement isotopic values from the outer platform slope and basin show higher values close to the assumed pennsylvanian seawater isotopic composition this implies that isotopic data from shoalwater carbonates including pristine brachiopod shells might not necessarily reflect paleoceanographic trends of the open ocean water masses because of changes in coastal water mass isotope signature and interaction with early meteoric fluids