the carbonate factory revisited a reexamination of sediment production functions used to model deposition on carbonate platforms there are two widely used models of sediment production on carbonate shelves 1 production varying with depth and 2 production as a function of depth and distance from the platform margin here we recalculate carbonate production on great bahama bank west and northwest of andros island using broecker and takahashi s 1966 pioneering geochemical study in the calculations residence time and depth are used to calculate yearly carbonate production based on equilibrium thermodynamics of caco3 loss from water over the banks whereas broecker and takahashi assumed a uniform depth of 4 5 m in their work here we use realistic depths and residence times interpolated over the bank our results suggest that sediment production is on the order of 4 5 kg m2 y on the bank margins and that production rapidly drops off into the platform approaching zero in the shallowest innermost parts of the platform thus for a large platform such as great bahama bank with long residence times the shallowest areas of the platform may become sinks not sources of sediment

high resolution surface and subsurface sequence stratigraphy of late middle to late ordovician late mohawkian cincinnatian foreland basin rocks kentucky and virginia the late middle to late ordovician late mohawkian to cincinnatian supersequence in kentucky and virginia was deposited in a tectonically active foreland basin during a transition from the ice free early ordovician to the glacial late ordovician this sequence was deposited in less than 12 m y is 250 500 m thick and is composed of four large third order sequences each 40 150 m thick that are regionally correlative in outcrops cores and gamma ray logs smaller scale third order parasequence sets up to 20 m thick and component parasequences 1 8 m thick make up the larger sequences and are only locally correlative subtidal dominated parasequences comprise the basal part of each sequence whereas shallower subtidal or peritidal dominated parasequences compose upper parts of sequences each large third order sequence is asymmetric and marked by lowstand transgressive and highstand systems tracts with unique lithologic and gamma ray log response characteristics ramp margin wedges rmw are poorly developed and consist of marine siltstone or grainstone packstone sheets extending into deep ramp settings these sheets have high gamma ray values in their base and lower blockier gamma ray responses in their tops transgressive systems tracts tst are thin composed of high energy locally phosphatic grainstone copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 7 1996 revised manuscript received may 27 1997 final acceptance june 16 1997 2department of geology virginia tech blacksburg virginia 24061 0420 present address department of earth atmospheric and planetary sciences massachusetts institute of technology cambridge massachusetts 02139 3department of geology virginia tech blacksburg virginia 24061 0420 this study was supported by nsf grant ear 9316057 to j f read a grant from mobil oil company and grants in aid of research from the geological society of america and appalachian basin industrial associates to mike pope we thank john haynes brian keith and rick major for thorough and insightful reviews of an earlier version of this manuscript

predicting depths of gypsum dehydration in evaporitic sedimentary basins the conversion of gypsum to anhydrite upon burial and heating is accompanied by about a 39 volume decrease a four fold increase in thermal conductivity and consumption of heat accurate back stripping of an evaporite basin and tracking its thermal evolution requires a knowledge of the depth of the gypsum anhydrite transition this depth depends on temperature fluid pressure lithostatic pressure and activity of water in the pore fluid using a finite difference heat conduction program we have determined the depths of transition for gypsum in evolving basins beneath commonly associated sediments and in several tectonic environments represented in the program by different basal heat flows and sedimentation rates the activity of water is kept at 0 93 precipitation f gypsum from seawater all physical properties are recalculated at each time step as temperature increases and porosity decreases with burial an algorithm for calculating thermal conductivities using parallel series mixtures is presented the modeling results show that overlying lithologies and the tectonic environment are two important factors gypsum converts to anhydrite at shallow depths sim400 m when overlain by poor conductors like shale or gypsum in a rift environment and at great depths hypothetically 4 km when overlain by good conductors like salt in a stable cratonic region sedimentation rate and the transient heat sink effect of the endothermic reaction have little effect on transition depth

early dolomitization and recrystallization of carbonate in an evaporite basin the middle devonian ratner laminite in southern saskatchewan canada this study provides an example of early dolomitization of basinal limestone associated with a giant evaporite deposit the middle devonian ratner carbonate is extensively to completely replaced by microcrystalline to finely crystalline dolomite in the southern and central part of southern saskatchewan in the elk point basin in the northern part however individual cored intervals of the ratner laminite are either completely dolomitized or contain no microcrystalline to finely crystalline dolomite stratigraphic petrographical and sr isotopic data suggest that the ratner dolomite has formed penecontemporaneously from dense evaporitic seawater that percolated downward the distribution of the ratner dolomite is highly localized in the northern part of the study area suggesting that the downward flow of mg 2 rich fluids was restricted by interbedded anhydrite but where the fluids penetrated the underlying carbonate deposits were dolomitized the depletion in 18 o isotope varying from 4 5 to 6 6 pdb and the coarser and inhomogeneous textures of the ratner dolomite are considered to be the result of recrystallization which is interpreted to occur at elevated temperatures related to a thermal anomaly during the late devonian and carboniferous burial

widespread prolonged late middle to late ordovician upwelling in north america a proxy record of glaciation late middle to late ordovician subtidal ramp carbonates of new mexico texas and oklahoma contain abundant spiculitic chert to 70 chert by volume and phosphate 1 5 wt indicating that these rocks formed in an extensive upwelling zone upwelling began in the late middle ordovician ca 454 ma and persisted until the end of the ordovician late ordovician cherty carbonates also occur along the u s cordilleran margin in board of organic rich graptolitic shale and chert the widespread occurrence of late ordovician cherty and phosphatic carbonates on southern and western laurentia in addition to phosphate rich cool water carbonates over much of the north american midcontinent suggests vigorous thermohaline circulation related to prolonged 10 14 ma gondwana glaciation

vims sea carousel a field instrument for studying sediment transport preliminary results from field experiments using a newly developed annular sea bed flume vims sea carousel are presented to illustrate a method of in situ determination of critical threshold shear stress necessary for sediment entrainment this flume has been deployed in the lower chesapeake bay and on the inner shelf of the middle atlantic bight we applied a series of controlled bed shear stresses by changing the stress sequentially to examine the responses of nearly undisturbed natural sediment beds an optical backscatter sensor mounted on the flume indicated the occurrences of resuspension of bed material we identified clear threshold values of bed shear stress for resuspension generally the sandy inner shelf sediment had a higher critical bed shear stress 0 22 pa for resuspension than that for silty sediments 0 1 0 19 pa data from partially cohesive sediments in the lower chesapeake bay indicate significant seasonal differences in critical bed shear stress 0 19 pa at the beginning of summer and 0 11 pa at the end of summer these differences may be attributed to bioturbation the seasonal difference in the microflora living on the grains or spatial heterogeneity of bed

ordovician red river b horizontal oil play in the southern williston basin recent application of horizontal drilling technology to the ordovician red river b zone in the southern williston basin has resulted in a successful oil play with more than 100 wells drilled in 1995 and 1996 the red river b reservoir is a dolomitized laminated carbonate with microsucrosic porosity of 8 25 and permeabilities in the range of 1 66 md it occurs within the middle of three depositional cycles a b and c that form the upper red river formation each cycle consists of a lower burrowed limestone middle laminated member and capping anhydrite or lime mudstone the b reservoir is confined to the b laminated member and consists of an upper portion characterized by better reservoir quality and a lower less permeable portion horizontal drilling has the advantage of significantly increasing well bore exposure to the upper more permeable portion well data indicate the total red river b porosity zone has remarkable extent over parts of southwestern north dakota southeastern montana and northwestern south dakota productivity from horizontal wells displays considerable variation that can be correlated with structure tectonic patterns and with reservoir petrophysical character

mechanical and chemical compaction in fine grained shallow water limestones abstract artificial compaction of in situ cores of modern sediments from differing sedimentary environments resulted in 1 reduction of sediment thickness by 50 percent and more 2 reduction of initial porosities of 65 to 75 percent to 35 to 45 percent 3 creation of megascopic textures almost identical to many ancient lime mud and wackestones 4 creation of organic wispy stylolite like layers that in nature probably would initiate dissolution and stylolite development given burial and time 5 chemical compaction evidenced by thin sections showing quartz grains piercing mollusc shells without causing fractures and sem evidence of solutional interfitting of 1 to 4 m size aragonitic carbonate grains 6 obliteration of pellets and birdseye or fenestral voids in those sediments here early cementation was lacking pellets were preserved in slightly cemented bahama bank muds compacted to half the original thickness 7 obliteration of identifiable marine grasses and vertical root tube voids 8 mashing of sediment filled circular burrows to produce ellipsoidal structures significant mechanical compaction resulted from pressures simulating less than 1 000 ft 305 m of burial increasing loads to an equivalent of more than 10 000 ft 3 400 m did not significantly increase compaction or reduce sediment core length chemical compaction pressure dissolution was detected only in sediment cores compacted to pressures greater than 10 000 ft 3 400 m of burial these short term experiments suggest that chemical compaction would begin at much shallower depths given geologic time compaction experiments that caused chemical compaction lend support to the well established hypothesis that cement required to produce a low porosity low permeability fine grained limestone is derived internally dissolution ion diffusion and reprecipitation are considered the m st likely processes for creating significant thicknesses of dense limestones in the geologic record continuation of chemical compaction after significant porosity reduction necessitates expulsion of connate fluids possibly including hydrocarbons

a far field record of the end ordovician glaciation the ellis bay formation anticosti island eastern canada abstract the end ordovician was a critical period in earth history it was a time of global climatic change with large scale continental glaciations a wealth of recent work has documented the stratigraphical records from the near field areas i e from various gondwanan continental relics where glaciation was more severely felt there is an urgent need to re examine the stratigraphic record from the far field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing duration and character at the glaciation centred around the ordovician silurian o s boundary one of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on anticosti island in eastern canada the east west trending 200 km long ellis bay hirnantian outcrop belt with superb coastal exposures at both ends of anticosti island is slightly oblique to the paleoshoreline with near shore siliciclastic dominated facies restricted to the eastern part of the island and more offshore carbonate dominated facies present along the central and western parts these mixed carbonate siliciclastic facies accumulated along a storm dominated beach to offshore profile they are organized into a stacked set of shallowing upward offshore to shoreline facies successions typically coarsening upward separated by deepening upward facies successions typically fining upward formed during intervening transgressions high frequency glacio eustatic fluctuations were the dominant control on the development of tr sequences observed in the ellis bay succession of anticosti island although tectonic subsidence had a substantial secondary effect on thickness five tr sequences are recognized allowing for precise correlation along the ellis bay outcrop belt in the eastern sections relatively thin tr sequences are locally bounded by unconformities a typical sequence begins with thin transgressive limestone overlain by a maximum flooding calcareous shale grading up into regressive storm influenced proximal to transitional zone fine sandstone grainstone and shale which are sharply overlain by shoreface to foreshore sandstone grainstone in the central and western sections the thicker tr sequences have more symmetrical transgressive regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply two discrete glacial periods are recognized during the deposition of the hirnantian ellis bay succession with the younger one recording large sea level changes 50 m in magnitude caused by the development of very large ice sheets over western gondwana abundant ice conditions on the globe during the end ordovician and the resulting high amplitude eustasy appear to have left distinctive stratigraphical imprints on mixed paleotropical siliciclastic carbonate ramps

petrophysical characteristics and facies of carbonate reservoirs the red river formation ordovician williston basin similarly shaped capillary pressure curves classified according to pore throat size sorting the maximum threshold entry radius mter and percent recovery efficiency re delineate petrophysical facies capillary pressure curves of carbonates that have a well sorted ws pore throat size distribution are characterized by mter occurring at less than 20 mercury imbibition and horizontal to subhorizontal plateaued injection curves resulting from a unimodal pore throat size distribution capillary pressure curves of carbonates that have a moderately sorted ms pore throat size distribution are generally sinusoidal in shape and have mter between 10 and 40 cumulative pore volume imbibition capillary pressure curves of carbonates that have a poorly sorted ps pore throat size distribution are generally oblique or diagonal and have no plateau and poorly defined mter the concept of petrophysical facies is applied to the red river formation ordovician of the williston basin capillary pressure curves are used to determine the spatial distribution of petrophysical characteristics within the red river carbonates curve types are spatially clustered and subdivide the formation into petrophysical facies which are laterally and vertically continuous capillary pressure curves of dolostones that have high porosity and good reservoir potential are characterized by ws pore throat size distributions mter within the range from 0 448 to 8 55 m and greater than or equal to 30 re optimum reservoir potential occurs in dolostones that have greater than 70 dolomite ws pore throat size distribution greater than or equal to 13 5 apparent porosity ap more than 49 total pore system saturation by 500 psia and more than 87 total pore system saturation by 1000 psia during mercury porosimetry 0 1 to 0 65 m average pore throat radius apr 0 18 to 1 m median pore throat radius mpr 0 6 to 3 6 m mter and 46 to 63 re a positive correlation occurs between ap and re where as porosity increases to 21 the re increases to 64 pore diameters that have optimum reservoir potential are not necessarily the largest found in dolostones moderate size pore diameters occurring in intercrystalline pore systems produce the highest porosity and re the ap is also a function of the pore throat radius highest porosity occurs in dolostones that have relatively small pore throat radii as the apr increases from 1 to 3 2 m the ap slightly decreases dolostones that have apr larger than 3 2 m have no ap lillian hess tanguay is an assistant professor of geology and director of the graduate program in environmental studies at the c w post campus of long island university liu her previous position at liu was the assistant dean of the college of liberal arts and sciences she received her ph d in earth and environmental sciences from the graduate school of the city university of new york in 1993 prior to her doctoral studies she was an exploration geologist at husky oil company in denver colorado gerald m friedman is distinguished professor of geology at the city university of new york serving at brooklyn college and the graduate school and university center his previous position was at rensselaer polytechnic institute where he is now emeritus professor at the research center of amoco in tulsa oklahoma he was supervisor of sedimentary geology within the last 30 years 10 500 petroleum geologists in industry have taken his short courses in carbonate geology and sedimentology he is an honorary member of aapg sepm and the international association of sedimentologists aapg has honored him with the distinguished educator award and the sidney powers memorial medal

the ordovician sebree trough an oceanic passage to the midcontinent united states the sebree trough is a relatively narrow shale filled sedimentary feature extending for several hundred kilometers across the middle and late ordovician carbonate platform of the midcontinent united states the dark graptolitic shales within the trough stand in contrast to the coeval bryozoan brachiopod echinoderm rich limestones on the flanking platforms we infer from regional stratal patterns thickness and facies trends and temporal relations established by biostratigraphy and k bentonite stratigraphy that the sebree trough initially began to develop during late turinian to early chatfieldian time mohawkian series as a linear bathymetric depression situated over the failed late precambrian early cambrian reelfoot rift rising sea level and positioning of a subtropical convergence zone along the southern margin of laurentia caused the rift depression to descend into cool oxygen poor phosphate rich oceanic waters that entered the southern reaches of the rift from the iapetus ocean the trough apparently formed in a system of epicontinental estuarine circulation marked by a density stratified water column trough formation was accompanied by cessation of carbonate sedimentation deposition of graptolitic shales development of hardground omission surfaces substrate erosion and local phosphogenesis the carbonate platforms on either side of the trough are dominated by bryozoan brachiopod echinoderm grainstones and packstones that were deposited in zones of mixing where cool nutrient rich waters encountered warmer shelf waters concurrently lime mudstone and wackestone were deposited shoreward northern illinois wisconsin iowa minnesota michigan in warmer more tropical shallow seas coeval upward growth of the flanking carbonate platforms sustained and enhanced development of the trough shale facies five widespread diachronous late mohawkian and cincinnatian omission surfaces are present in the carbonate facies of the midcontinent these surfaces include sub deicke k bentonite ds1 top of black river limestone ds2 base and top of the guttenberg limestone member of the decorah formation ds3 and ds4 and top of the trenton limestone ds5 some of the surfaces correspond to previously described depositional sequence boundaries all five surfaces which embody deepening phases on top of highstand systems tracts converge in the sebree trough indicating that the trough was a long lived feature and was the source of eutrophic waters that episodically spread across the adjacent platforms terminating carbonate production late turinian and early chatfieldian incipient drowning episodes were followed by a final drowning event that began in the sebree trough during the late chatfieldian climacograptus spiniferus zone and reached southernmost minnesota and other regions far within the platform interior by richmondian time amorphognathus ordovicicus zone

controls on microbial activity and tidal flat evolution in shark bay western australia microbial deposits at shark bay constitute a diverse living microbial carbonate system developed in a semi arid highly evaporative marine setting three tidal flats located in different embayments within the world heritage area were investigated in order to compare microbial deposits and their holocene evolution the stressing conditions in the intertidal subtidal environment have produced a microbial ecosystem that is trapping binding and biologically inducing caco3 precipitation producing laminated stromatolites tufted smooth and colloform non laminated thrombolitic forms pustular and cryptomicrobial non laminated forms microbial pavement a general shallowing upwards sedimentary cycle was recognized and correlated with holocene sea level variations where microbial deposits constitute the younger 2360 years bp and shallower sedimentary veneer in addition sediments have been documented with evidence of exposure during the holocene from 1040 to 940 14c years bp when sea level was apparently lower than present filamentous bacteria constitute the dominant group in the blister tufted and smooth mat types and coccus bacteria dominate the pustular colloform and microbial pavement deposit types in the subtidal environment within colloform and pavement structures microbial communities coexist with organisms such as bivalves serpulids diatoms green algae acetabularia crustaceans foraminifera and micro gastropods which are responsible for exoskeleton supply and extensive bioturbation the internal fabric of the microbial deposits is laminated sub laminar scalloped irregular or clotted depending on the amount of fine grained carbonate and the natural ability of microbial communities to trap and bind particles or induce carbonate precipitation nilemah tidal flat contains the thickest 1 3 m and best developed microbial sedimentary system its deposition pre dated the rocky point and garden point tidal flats with the most positive isotope values for 13c and 18o reflecting strong microbial activity in a highly evaporative environment there is an evolutionary series preserved within the tidal flats reflecting relative ages and degree of salinity elevation

lateral continuity of facies and parasequences in middle cambrian platform carbonates carrara formation southeastern california u s a abstract lateral and vertical changes within a 40 m thick interval of peritidal carbonates middle cambrian jangle limestone member carrara formation se california produce a mosaic pattern of laterally continuous and discontinuous facies shallowing upward cycles 2 20 m thick parasequences and surfaces bounding parasequences this mosaic pattern was traced bed by bed across 1 8 km of continuously exposed outcrop and then correlated to sections in nearby ranges within parasequences contacts between successive subtidal facies tend to be sharp but may pass laterally into gradational contacts most subtidal facies extend laterally for less than 300 m passing gradationally over tens of centimeters to meters into other laterally adjacent subtidal facies lateral facies contacts between subtidal nodular ribbon rock and intertidal fenestral limestone are gradual over a few meters whereas contacts between vertically juxtaposed facies generally are sharp parasequence boundaries generally are sharp and locally show several tens of centimeters of erosional relief over lateral distances of several meters most parasequences and their bounding surfaces extend for the 1 8 km of outcrop studied and correlate to nearby ranges with varying degrees of confidence several parasequences however do not extend laterally for more than a few hundred meters and are not distinguishable in a vertical profile from those that extend laterally for tens of kilometers the stratigraphic relationships at eagle mountain southeastern california support a model in which the lateral continuity of parasequence surfaces is principally determined by accommodation variations and the lateral continuity of facies is most strongly influenced by the intrinsic dynamics of spatially and temporally heterogeneous sediment production the presence of parasequence boundaries of highly variable lateral extent suggests that the magnitude of accommodation variations was comparatively small consistent with the independent evidence for passive margin subsidence and transitional greenhouse conditions for the middle cambrian

settling velocities of fine suspended particles in the changjiang estuary china abstract point sampled suspended sediment concentration profiles grain size largely c of sediment in suspension the best fit equation for the field settling velocity of fine particles in the changjiang estuary can be expressed by the power law ws 2 37 c 0 84 r2

use of high resolution ichnological and stable isotope data for assessing completeness of a k p boundary section agost spain high resolution ichnological analysis has been performed at the agost section betic cordillera southeast spain in order to assess the completeness of the k p boundary transition below the biostratigraphic resolution intrasubzone high resolution planktonic foraminifera biozonation revealed that the first appearance of the palaeogene index species does not coincide with the k p boundary but is located a few centimetres above and calibrated 65 7 6 7 ky later the k p boundary to establish the continuity of this first interval of the lowermost danian hedbergella holmdelensis subzone and the existence of any hiatus below biostratigraphic resolution trace fossil analysis was revealed to be a useful tool planolites thalassinoides and alcyonidiopsis structures belonging to the upper tiers colonizing at or immediately after the k p boundary event were studied focusing on the isotopic composition of the filling materials and the genesis of thalassinoides rapid firmground generation and thalassinoides tracemaker colonization is mainly attributed to cessation of precipitation and or deposition of calcium carbonate but an incipient lithification is not disregarded the d 13 c data obtained from trace fossils fit well with those corresponding to a the k p boundary layer from 1 00x to 1 30x b the stratigraphic interval between 2 5 and 5 0 cm above the k p boundary between 0 32x and 0 47x and c the interval between 7 0 and 11 0 cm above the k p boundary between 0 69x and 0 79x revealing continuity in the colonization from the k p boundary event to the dark clay deposition above this macrobenthic colonization occurred continuously during the dark layer deposition from the k p boundary layer including the first centimetres evidencing completeness and the absence of hiatuses below biostratigraphic resolution intrasubzone at the agost section d 2005 elsevier b v all rights reserved

mississippian stratigraphy and tectonics of east central nevada post antler orogenesis high resolution stratigraphic studies in the diamond mountains are revealing details of tectonic deformation and basin filling that post date the emplacement of the antler allochthon and predate the sonoma orogeny antler foreland basin strata were deformed uplifted and eroded in the middle mississippian and a late mississippian successor basin was established that persisted into the pennsylvanian the antler peripheral foreland basin of eastern nevada formed in the early mississippian as a result of obduction of an accretionary prism onto the western edge of the north american craton this basin filled with submarine fan sediments shed from the west that range in age from late osagean to middle meramecian or slightly younger in the diamond mountains these strata are deformed into open folds and are truncated by a regional unconformity the unconformity documents an uplift and erosion event in the late meramecian that the authors have called the diamond mountain phase of deformation oldest strata above the unconformity are early chesterian renewed subsidence in chesterian time resulted in formation of a wide and shallow successor basin bounded on the west by the relict antler highland and on the east by a siliciclastic shelf and the craton margin this successor basin was filled with recycledmore antler foreland sediments and intrabasinal shelf carbonates reworked antler clastics were distributed throughout the basin by a system of mainly south flowing fluvial and deltaic distributaries this mixed clastic carbonate basin persisted well into the pennsylvanian less

astronomical frequencies for pre quaternary palaeoclimate studies the sensitivity of the earth s orbital frequencies involved in the astronomical theory of palaeoclimates to the earth moon distance and consequently to the length of the day and to the dynamical ellipticity of the earth has been investigated for the last billion years the shortening of the earth moon distance and of the length of the day back in time induces a shortening of the fundamental astronomical periods for the obliquity and the precession a shortening which is larger for the longer periods these periods of precession and obliquity influenced by the changing earth moon distance length of the day and dynamical ellipticity of the earth have been recomputed for the last 440 million years

traces in the dark sedimentary processes and facies gradients in the upper shale member of the upper devonian lower mississippian bakken formation williston basin north dakota u s a abstract black organic rich rocks of the upper shale member of the upper devonian lower mississippian bakken formation a world class petroleum source rock in the williston basin of the united states and canada contain a diverse suite of mudstone lithofacies that were deposited in distinct facies belts the succession consists of three discrete facies associations fas these comprise 1 siliceous mudstones 2 quartz and carbonate bearing laminated mudstones and 3 macrofossil debris bearing massive mudstones these fas were deposited in three facies belts that reflect proximal to distal relationships in this mudstone system the macrofossil debris bearing massive mudstones fa 3 occur in the proximal facies belt and contain erosion surfaces some with overlying conodont and phosphate lithoclast lag deposits mudstones with abundant millimeter scale siltstone laminae showing irregular lateral thickness changes and shell debris in the medial facies belt quartz and carbonate bearing laminated mudstones dominate exhibiting sub millimeter thick siltstone layers with variable lateral thicknesses and localized mudstone ripples in the distal siliceous mudstone facies belt radiolarites radiolarian bearing mudstones and quartz and carbonate bearing laminated mudstones dominate overall total organic carbon toc contents range between about 3 and 10 wt with a general proximal to distal decrease in toc content abundant evidence of bioturbation exists in all fas and the lithological and toc variations are paralleled by changes in burrowing style and trace fossil abundance while two horizontal traces and two types of fecal strings are recognized in the proximal facies belt only a single horizontal trace fossil and one type of fecal string characterize mudstones in the distal facies belt radiolarites intercalated into the most distal mudstones are devoid of traces and fecal strings bedload transport processes likely caused by storm induced turbidity currents were active across all facies belts suspended sediment settling from near the ocean surface however most likely played a role in the deposition of some of the mudstones and was probably responsible for deposition of the radiolarites the distribution pattern of high toc sediments in proximal and lower toc deposits in some distal facies is interpreted as a function of higher accumulation rates during radiolarian depositional events leading to a decrease in suspension derived organic carbon in radiolarite laminae the presence of burrows in all fas and nearly all facies in the upper bakken shale member indicates that dysoxic conditions prevailed during its deposition this study shows that in intracratonic high toc mudstone successions such as the upper bakken shale member bed load processes most likely dominated sedimentation and conditions promoted a thriving infaunal benthic community as such deposition of the upper bakken shale member through dynamic processes in an overall dysoxic environment represents an alternative to conventional anoxic depositional models for world class source rocks

sequence stratigraphy of the red river formation williston basin usa stratigraphic signature of the ordovician katian greenhouse to icehouse transition abstract the upper katian upper ordovician red river formation is a carbonate evaporite supersequence that formed in the epicratonic williston basin of western north america this study focuses on the upper part of the formation in the southern us part of the basin the studied succession is made up of three transgressive regressive sequences rr1 rr2 and rr3 each 0 5 1 m y in duration and composed of relatively few meter scale high frequency cycles parasequences sequence boundaries are characterized by intraclastic breccia laminated dolomite successions that have been truncated and capped by evaporite deposits subaqueous anhydrites within each sequence represent evaporite lowstand systems tracts lsts that formed during relative sea level lows that lead to the precipitation of gypsum later recrystallized into anhydrite in a quiet salina type environment transgressive tst and early highstand systems tracts hst are composed of subtidal facies of predominantly skeletal mudstone to wackestone packstone with abundant burrow mottling late hsts are predominantly composed of peritidal porous laminated dolomite the study illustrates how the sequence stratigraphic framework of carbonate evaporite deposits from the williston basin relates to the late ordovician glacio eustasy transitional climates and low basin subsidence in an arid setting

ichnological analysis of the bidart and sopelana cretaceous paleogene k pg boundary sections basque basin w pyrenees refining eco sedimentary environment abstract ichnological analysis of two cretaceous paleogene boundary sections at bidart se france and sopelana n spain has been conducted in order to refining eco sedimentary environment and to a make comparison with previous interpretations based on microfossils in both sections trace fossil assemblage is low diverse consisting of chondrites planolites thalassinoides trichichnus zoophycos and phycosiphon ascribed to the zoophycos ichnofacies however with distinct differences in the bidart section early danian dark filled trace fossil assemblage is more abundant in large thalassinoides zoophycos and larger chondrites and less abundant in trichichnus and small chondrites in comparison to the sopelana section sopelana is thus interpreted as a more offshore deeper section than bidart although both were located in the upper bathyal zone of the basin oxygenation below the sediment water interface was enough to favour a generalized total bioturbation of sediments the presence of chemichnia and trace fossils which tracemakers sequestered sediments from the sea floor in deep burrows zoophycos suggests at least periodical food deficiency the distribution and content of trace fossils across the k pg boundary do not change significantly revealing minor incidence of the k pg boundary event on the macrobenthic environment the k pg boundary transition including the boundary layer is nearly totally bioturbated hence a redistribution of microbiotic and abiotic components by tracemakers is possible

depositional facies and platform architecture of microbialite dominated carbonate reservoirs ediacaran cambrian ara group sultanate of oman intrasalt carbonates of the ediacaran cambrian ara group constitute a significant reservoir element of the intrasalt stringer play in oman in which dolomitic carbonates are encased in salt at depths of 3 to 7 km 1 9 to 4 3 mi these reservoir carbonates have significant microbial influences although ara group reservoirs are mostly latest precambrian the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded in lacustrine settings where calcified invertebrates are not a significant source of carbonate or after periods of mass extinction before faunal recovery a broad range of carbonate facies provides the context in which to understand the origin of the microbialite dominated reservoirs developed across both ramp and rimmed shelf profiles major facies associations include carbonate evaporite transition zone deep ramp and slope subtidal microbialites clastic textured carbonates and restricted peritidal carbonates microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties and that help in predicting the location of reservoir fairways microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities 50 shallow subtidal pustular laminites with cm scale variability of lamina morphology deeper subtidal crinkly laminites that show mm scale variability of lamina morphology and intertidal tufted laminates that show mm to cm scale tufted textures other reservoir facies are more conventional grainy carbonates including ripple cross stratified grainstone packstone hummocky cross stratified grainstone packstone flat pebble conglomerate ooid and intraclast grainstone packstone and cloudina grainstone packstone these facies are almost invariably dolomitized and all have moderate to excellent reservoir quality these facies comprise carbonate platforms broken up during salt tectonics that range up to 160 m 525 ft in thickness and extended laterally prior to halokinesis for tens to over 50 km 31 mi the distribution of reservoir facies follows sequence stratigraphic predictions with microbialites occurring in every accommodation profile late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build ups whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate

characteristics distribution and morphogenesis of subtidal microbial systems in shark bay australia abstract the distribution nature and extent of microbial deposits in hamelin pool shark bay have been investigated and mapped with emphasis on the occurrence external morphologies internal fabrics constructional mechanisms microbial communities growth rates and sediment associations in the intertidal and previously little researched subtidal zone detailed georeferenced substrate mapping revealed extensive subtidal microbial deposits occupying approximately 300 km 2 of the total holocene 1400 km 2 area of hamelin pool the microbial pavement covers 227 km 2 of the subtidal substrate that together with columnar structures reveals a subtidal microbial habitat which occupies an area 10 times larger than the area of the intertidal deposits microbial carbonate is composed of aragonite 80 98 that reveals high positive values of 13 c 4 46 to 5 88 and 18 o 3 06 to 3 88 as a characteristic of the highly evaporative environment with extensive microbial activity oldest dated heads are 1915 and 1680 14 c years bp and the overall system was deposited in two stages the first between 2000 and 1200 and the last from 900 years bp to the present slow growth rates vary from less than 0 1 mm year to 0 5 mm year different internal fabrics were constructed according to their position in relation to the littoral zone by distinct microbial communities and lateral fabric relations have been established evidence of shallowing upward fabric sequences of microbial origin reflects relative falling sea levels during the late holocene and is likely useful in ancient environmental interpretation a sequence of events and mechanisms are described emphasizing differences between the stromatolitic thrombolitic and cryptomicrobial deposits in shark bay the new substrate map and depositional history for this distinctive and peculiar microbial habitat establish the significance of subtidal structures and emphasize the geoscientific importance of hamelin pool especially with respect to early life studies and ancient analogues for understanding microbial activity deposit characteristics fenestral fabrics and distribution

permian wolfcampian admire c new exploratory potential in the northern denver basin significant frontier exploratory potential exists in the lower wolfcampian admire c unit a regionally continuous porous dolomite interval occurring at depths of 7800 8500 ft 2379 2592 m in western nebraska united states based on core mud log thin section and sem scanning electron microscope image analysis as well as both regional and detailed local mapping the admire c has been found to be the most prospective of several reservoir zones in this region based on 1 consistency and occurrence of oil shows 2 superior reservoir quality and distribution and 3 established patterns of production continuity of the reservoir directly reflects transgressive regressive cycles responsible for carbonate deposition in a shallow intermittently restricted basin alliance basin affected by glacio eustatic sea level changes highest reservoir quality is concentrated in higher energy packstones and rare grainstones that exhibit a combination of intercrystalline moldic and vuggy porosity the admire c unit has low to moderate permeabilities 0 5 30 md that restrict short term flow capacity a number of wells however have shown recoveries in the 75 000 130 000 bbl range at low and decreasing rates of annual decline suggesting that they are in contact with a larger more continuous reservoir than was previously thought source beds include black shales within the pennsylvanian and wolfcampian sections little free water exists in the admire c zone suggesting mobile water may have been displaced by hydrocarbon migration a number of intriguing similarities between the admire c and the ordovician red river b reservoir in the williston basin imply that exploration strategies successful in the latter might be applied to the admire

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

ca isotope study of ordovician dolomite limestone and anhydrite in the williston basin implications for subsurface dolomitization and local ca cycling abstract a difference of 0 61 is reported between dolomite 1 66 and its precursor limestone 1 05 in the yeoman formation red river equivalent of the williston basin southeastern saskatchewan the significance of the large difference found and the preference for light isotope enrichment in the dolomite is evaluated with assistance from 87 sr 86 sr ratios and dolomitization models previously proposed for the studied rocks in particular i explore the possibility that 44 ca values in dolomite reflect the 44 ca values of the dolomitizing fluids without any correction for mineral fluid fractionation this hypothesis is based on recent studies showing negligible isotopic fractionation between carbonates and ca 2 bearing waters at very low precipitation rates fantle and depaolo 2007 jacobson and holmden 2008 if correct calcium isotopes hold promise as a tool for discriminating among numerous hydrological models of dolomite formation as a case in point the 44 ca value of the yeoman dolomite 1 66 is too low to reflect ca derived from the following sources 1 the original limestone 1 05 2 overlying beds of anhydrite 1 28 or 3 evaporated seawater 0 25 paleozoic carbonates ranging as low as 1 7 appear to be the lightest source of ca in the basin succession suggesting that the mg containing dolomitizing fluid was a connate water with a previous history of water rock interactions with paleozoic carbonates possibly involving earlier episodes of dolomitization and light ca release into migrating basinal fluids the relatively high seawater 44 ca values inferred from measurements of yeoman limestone and lake alma anhydrite suggest that seawater in the williston basin 450 ma was 0 22 0 46 higher in 44 ca than the contemporaneous ocean based on globally distributed brachiopods farkas et al 2007 this finding suggests that 44 ca values preserved in the deposits of epeiric seas may bias the 44 ca reconstruction of the ocean secular record if local ca cycling effects are not taken into account equations describing the isotope balance of ca in an epeiric sea indicate that seawater 44 ca values may be higher lower or equal to the 44 ca value of the ocean depending on the sizes of local scale ca deposition and weathering fluxes in relation to seawater ca exchange fluxes

new instrumentation for the investigation of sediment suspension processes in the shallow marine environment abstract an instrumentation system has been developed that continuously measures flow velocity water depth and suspended sediment concentration profiles in the nearshore zone the system consists of four components 1 an array of electronic sensors including an electromagnetic current meter a surface piercing resistance wave gauge and a five element array of optical back scatterance sensors 2 a signal processing electronics unit 3 a guyed mast support structure and 4 a data acquisition system the small size of the sensors and their response characteristics make the array suitable for monitoring sediment transport to within a few centimeters of the bed under moderately large breaking waves 2 m results from an experiment on an exposed ocean beach on the coast of washington u s a indicate that sand suspension events of long duration 15 40 sec relative to the period of incident waves contribute a significant quantity of sand to the total littoral drift work is now in progress to modify the system for self contained operation on the continental shelf field investigations with the modified system will focus on the acquisition of a data set to test the validity of the suspended sediment distribution equation

geochemical analysis and familial association of red river and winnipeg reservoired oils of the williston basin canada abstract light oils reservoired in the lower ordovician winnipeg formation williston basin have a unique geochemical signature separating them from previously recognized oil families most importantly they are geochemically distinct from the stratigraphically adjacent upper ordovician red river formation oils winnipeg oils are characterized in the gasoline fraction by very high paraffin indices 4 16 and variations in c 7 parameters the saturate fraction is distinguished by a high abundance of c 20 n alkanes low carbon preference index and low amounts of pristane and phytane sterane biomarkers show a predominance of c 27 c 28 c 29 suggesting an algal source different from that contributing to red river oils in addition the terpane biomarkers of winnipeg oils show a high abundance of rearranged hopanes including an unknown c 30 compound labelled uc30 and 17 h c 30 diahopanes c 30 moreover these oils have unambiguous amounts of 18 h 30 norneohopanes c 29 ts which are in low abundance in red river formation oils geochemical analysis of lower ordovician winnipeg formation reservoired oils from the williston basin suggests that an additional hydrocarbon source not yet defined may exist

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

fe oxide spherules infilling thalassinoides burrows at the cretaceous paleogene k p boundary evidence of a near contemporaneous macrobenthic colonization during the k p event a detailed ichnological analysis focused on thalassinoides structures recorded just below the cretaceous paleogene k p boundary layer of the agost section southeast spain aimed to evaluate macrobenthic colonization during the k p boundary event stereomicroscopy and field emission scanning electron microscopy analyses conducted on material filling in thalassinoides trace fossils reveal the presence of dark infill material that includes spherule like elements similar to those previously differentiated in the k p boundary layer energy dispersive x ray spectrometry indicates a ferruginous composition goethite spherules this represents the first record of spherules related to the k p boundary event within trace fossil infilling material the passive filling of this material into the thalassinoides bioturbation allows interpretation of the gravitational deposition just after thalassinoides trace maker colonization the absence of erosive sedimentary structures or other material deposited between the thalassinoides trace maker colonization and the spherule deposition is interpreted as evidence of rapid colonization during the k p boundary event nearly contemporary with the spherule layer deposition

rost and beast devices for in situ measurement of particle settling velocity abstract the principles and design of two devices are described that measure the particle settling velocity in situ the remote optical settling tube rost designed by a group at osu has been successfully deployed at the hebble site the benthic autonomous settling tube beast has recently been operated at the same site

petroleum generation and migration in the cambro ordovician laurentian margin succession of nw scotland fluid inclusion and organic biomarker data show that there was formerly a hydrocarbon system in the cambro ordovician laurentian margin rocks of nw scotland oil fluid inclusions occur in stylolitized eriboll formation sandstone in k feldspar cements with an 40 ar 39 ar age of 415 5 5 5 8 ma 2 analytical precision full external precision organic extracts from durness group black limestones yield biomarker ratios characteristic of high thermal maturity organic maturation to yield oil probably occurred during orogenic deformation along the moine thrust zone the recognition of a hydrocarbon system in scotland adds to a huge hydrocarbon province in laurentian rocks including north america and greenland supplementary material technical data for ar ar dating are available at https doi org 10 6084 m9 figshare c 3879616

dolomite reservoirs porosity evolution and reservoir characteristics systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbon producing dolomite reservoirs occurs in 1 peritidal dominated carbonate 2 subtidal carbonate associated with evaporitic tidal flat lagoon 3 subtidal carbonate associated with basinal evaporite and 4 nonevaporitic carbonate sequence associated with topographic high unconformity platform margin buildup or fault fracture reservoir characteristics vary greatly from one dolomite type to another depending upon the original sediment fabric the mechanism by which dolomite was formed and the extent to which early formed dolomite was modified by postdolomitization diagenetic processes e g karstification fracturing and burial corrosion this paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types

the hydrological setting occurrence and significance of gypsum in late quaternary salt lakes in south australia well exposed often laminated gypsum sequences occur in many quarternary salt lakes in southern south australia the gypsum in the salt lakes is classified by increasing grain size into gypsite gypsarenite and selenite the salt lakes are classified by age and hydrological setting into coastal salinas which are holocene sea water fed groundwater lakes and continental playas which are late pleistocene endorheic basins a study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes as the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non salina gypsum deposits

field and modelling studies of cambrian carbonate cycles virginia appalachians reply abstract field studies coupled with computer modelling of carbonate cycles in the elbrook conococheague formations middle to upper cambrian in the appalachians have helped define the mechanisms of formation the sequence is up to 1 6 km thick and composed of cyclic peritidal carbonates these formed on an aggraded rimmed shelf on a mature passive continental margin the cycles 1 7 m thick have lower parts of subtidal intertidal carbonate and caps of dolomitic laminite containing minor quartz arenite shale and breccia sedimentation rates for these peritidal carbonate environments exceeded long term subsidence of the platform 0 01 to 0 1 m 1 000 yr thus the shelf stayed near to sea level i e was aggraded milankovitch 20 000 to 100 000 yr low amplitude sea level fluctuations few meters superimposed on 3rd order sea level fluctuation of 1 to 3 m y duration and less than 20 m amplitude caused periodic submergence of the gently sloping platform less than 0 02 m km slopes resulting in deposition of over 400 peritidal cycles in about 26 m y cycles that formed during 3rd order sea level rise are thick and limestone rich whereas those formed during 3rd order fall are thin quartzose and highly dolomitized the modelling suggests that fewer and thinner cycles formed during the long term falls compared to the rises reflecting decreased accommodation cycle frequency decreases into areas of lower subsidence rate open marine cycles typical of the outer shelf and cycles formed during 3rd order sea level rise contain digitate bioherms grainstone and ribbon carbonates in lower parts they reflect high subsidence rate and an open marine setting inner shelf cycles and cycles that formed during 3rd order sea level fall commonly are thrombolitic or have thin subtidal parts grainstone conglomerate or stromatolite and reflec the low subsidence rates interior areas of quaternary carbonate shelves are characterized by incipiently drowned facies and pinnacle reefs and are punctuated by soil caliche horizons and karst surface that reflect 100 m glacio eustatic sea level fluctuations in contrast many ancient shelves like the cambro ordovician shelf of the appalachians were flat topped and dominated by cyclic peritidal sequences lacking evidence of high amplitude sea level fluctuations such aggraded shelves may represent the typical state of mature carbonate shelves that formed in response to low amplitude milankovitch sea level fluctuations as opposed to the large scale fluctuations that typified the quaternary

visual identification and quantification of milankovitch climate cycles in outcrop an example from the upper ordovician kope formation northern kentucky

pool characterization of ordovician midale field implication for red river play in northern williston basin southeastern saskatchewan canada the upper ordovician midale field is located in the northern williston basin in southeast saskatchewan canada hydrocarbons are hosted mainly in the dolomite reservoirs with burrowed textures in the upper yeoman formation these reservoirs are characterized by intercrystalline porosity in the dolomitized matrix with variable amounts of vugs and fractures and can be divided into four zones reservoir zones 1 and 2 typically 6 10 m 20 33 ft thick in total are situated in the upper part of the traps and commonly bear oil although the underlying zones 3 and 4 are thicker they commonly contain only water because they are located below the spillpoint of the hydrocarbon traps the seismic reflection of the red river reservoirs in the midale field is characterized by a weak to medium amplitude trough immediately above the positive reflection of the winnipeg shale where all four zones are present an additional peak occurs on the seismic profile above the original reservoir reflection this additional peak however disappears where reservoir zones 3 and 4 pinch out where there is an increase in the thickness of reservoir zones 1 and 2 or amalgamation of zone 1 with zone 2 the red river reservoirs are characterized by high amplitude and high frequency reflections on seismic profiles the ordovician oil pools in the midale area are associated with low relief anticline structures these low relief structures are interpreted as the compactional drape of red river strata over local precambrian basement highs the source of hydrocarbons in the red river reservoirs is ordovician kukersites a wide range of api fractions for the oils from the midale pools suggests a mixing of low maturity oils sourced from local kukersite beds and high maturity oils that migrated over a long distance from the south the hydrocarbon production from red river midale pools is characterized by the fast rise of water cut and high water output which can be attributed to the small pool size and the fracture systems connecting oil and water zones

origins of gypsum in deep carbonate reservoirs implications for hydrocarbon exploration and production gypsum can form penecontemporaneously in carbonate sediments and it may be altered to anhydrite during burial some workers have suggested that gypsum dehydration is complete at burial depths of 3500 ft 1067 m however gypsum has been documented at depths as great as 13 100 ft 3993 m two examples of deeply buried gypsum illustrate contrasting origins gypsum in the permian san andres formation in the permian basin of west texas at a depth of 6000 ft 1829 m occurs as anhydrite nodules with an outer rind of gypsum suggesting that anhydrite was rehydrated to gypsum by interaction with interstitial brines in contrast gypsum in the carbonate beds of the cretaceous ferry lake anhydrite of the mississippi interior salt basin at 13 100 ft 3993 m occurs as gypsum nodules with a rind of anhydrite suggesting that the gypsum is primary and in the process of dehydration to anhydrite these contrasting origins are likely a function of evaporite and carbonate stratigraphy in the permian basin a thick section of carbonate rocks is overlain by a relatively thin section of evaporites in the mississippi interior salt basin carbonates are interbedded with multiple evaporite beds which inhibit circulation of interstitial brines the burial temperature pressure and salinity of these formations suggest that gypsum could be preserved as primary or secondary gypsum the presence of gypsum in reservoir rocks can affect porosity calculations especially calculations using neutron porosity logs which measure bound water of hydration in gypsum as porosity identification of gypsum at the depth of 13 100 ft 3993 m indicates that log estimates of porosity in some deep carbonate reservoirs containing gypsum could have significant porosity and water saturation measurement errors

measurements of high concentration suspended sediments using the optical backscatterance sensor abstract an optical backscatterance sensor obs was used to measure extremely high concentration suspended sediments on the amazon continental shelf as part of the amasseds project a multidisciplinary amazon shelf sediment study using a pumping system for obtaining water samples to calibrate the sensor in situ the obs output was observed to increase approximately linearly for concentrations 36 g l the obs response was nearly continuous from 0 to 320 g l making it possible to obtain continuous suspended sediment concentration profiles through fluid muds

laboratory apparatus for calibrating optical suspended solids sensors abstract an apparatus for testing and calibrating miniature to medium sized optical suspended solids sensors is described stable suspensions of sedimentary particles ranging in size from 10 6 to 4 10 3 m can be produced in water with concentrations exceeding 100 g 1 1 flow velocities in the test volume can be varied continuously from 0 1 to 1 5 m s 1 and samples of suspensions can be withdrawn directly with a peristaltic pump for gravimetric and particle size analyses the apparatus has been used to evaluate the effects of particle size flow speed and air bubbles on the performance of optical backscatterance sensors and transmissometers

notes on the performance of an optical backscatter sensor for cohesive sediments abstract a comprehensive test on the performance of an optical backscatter sensor obs for cohesive sediments reveals that the obs responds quite differently for the three basic cohesive sediments kaolinite illite and montmorillonite its operational ranges from 0 to 0 5 g l for kaolinite from 0 to 0 8 g l for illite and from 0 to 1 2 g l for montmorillionite are significantly lower than that for silts or sands up to 20 g l it also responds differently with variation in salinity the color of water the ambient light and the scanning rate do not affect the performance this study demonstrates that calibration of an obs should be conducted in situ to obtain accuracy

a new instrument for measuring settling velocities in situ abstract an in situ settling cylinder for determining particle settling velocities has been deployed during a field experiment in san pablo bay california the instrument is a spring loaded cylinder which traps approximately 31 of water and suspended sediment between two end plates mounted between the end plates is a vertical array of five miniature nephelometers positioned 4 20 cm below the top end plate which continuously monitor 5 hz sampling rate suspended sediment concentration at each level as particles settle the resulting settling curve can be divided into components with characteristic settling velocities and a settling velocity distribution obtained for the suspension the results are conservative estimates of settling velocities for the suspension in situ estimates of particle settling velocity are higher than stokes settling velocities calculated for the disaggregated grain size distribution of a suspended sediment sample indicating that the sediment in suspension is in the form of aggregates the aggregates have settling velocities as high as those of sand size quartz spheres

inssev an instrument to measure the size and settling velocity of flocs in situ abstract an instrument has been developed to observe the settling of individual flocs in turbid water in order to measure size and settling velocity spectra of estuarine cohesive suspended sediments inssev in situ settling velocity instrument is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column the settling flocs are viewed using a miniature video system subsequent analysis of video tapes can provide direct measurements of size and settling velocity of individual flocs down to 20 m from this information floc density can be estimated the main feature of the instrument is its ability to video flocs in situ irrespective of the concentration in the estuary with as little disturbance to their hydrodynamic environment as possible preliminary field testing in the tamar estuary has given useful results in flow velocities up to 0 4 m s 1 and in concentrations up to 200 mg 1 1 the video recordings show large numbers of low density flocs with multiple structures linked by fine organic fibres

interpretation of hydrocarbon shows using light c1 c5 hydrocarbon gases from mud log data chromatograph analysis of hydrocarbon gases as reported on mud logs is the first indication of a reservoir s fluid characteristics ratio calculations of these data can enhance the interpretational value of this information giving the well site geologist an immediate indication of zones of interest that need further testing it was found that for an effective interpretation of mud log gas data the results must be plotted on a depth log which can be easily integrated with other data thus a new method of interpreting ratio calculations of c1 c5 gas data is proposed this method can be used at the well site during drilling to give an initial evaluation of the reservoir fluid type and in post well evaluation to enhance wireline and mud log data

sequence stratigraphy of a late devonian ramp situated reef system in the western canada sedimentary basin dynamic responses to sea level change and regressive reef development the alexandra formation located in the northwest territories of canada is formed of a late devonian frasnian reef system that developed on a gently sloping epicontinental ramp in the western canada sedimentary basin high resolution sequence stratigraphic analysis of its deposits delineates two reef complexes that are separated by a type i sequence boundary the second reef complex developed on the outer ramp basinward of the first after sea level fell 17 m stratigraphic complexity of the second reef complex was a result of its initiation during forced regression and its development through an entire cycle of sea level rise followed by sea level fall its highstand systems tract was not characterized by high rates of carbonate production or sediment shedding rather these features took place as sea level fell after its highstand systems tract the sequence stratigraphic framework of this regressive reef system highlights a number of depositional parameters that differ from high relief shelf situated reef systems with steep narrow margins these have implications for understanding the controls on the development of ramp situated reef systems and the nature of reef systems with gently sloping profiles this study demonstrates that the development of stromatoporoid reef systems may be far more complex than generally realized and that high resolution sequence stratigraphy may provide the tools for better understanding of complex often enigmatic aspects of these systems

the strontium isotopic composition of ordovician and silurian brachiopods and conodonts relationships to geological events and implications for coeval seawater utilized for strontium isotope measurements the results document a gradual decrease in 87 sr 86 sr from 0 7091 to 0 7087 from tremadoc to llandeilo a sharp decline to 0 7078 during the late llandeilo early caradoc little change during caradoc and the ashgil and a steady rise to 0 7087 through the silurian these long term 10 7 yr variations with magnitudes in the range of 10 23 are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering the gradual decrease in 87 sr 86 sr ratio during the early ordovician may record the reduction in uplift and weathering rates due to waning of the pan african orogenies the rapid decline near the llandeilo caradoc boundary suggests a strong hydrothermal flux likely due to increased sea floor spreading and a possible superplume event the latter may have caused the prominent transgressive phase the largest in the phanerozoic which would have muted continental flux input although the caradoc was the main interval for the taconic orogeny its impact on the sr continental flux may have been delayed until the early llandovery this effect complemented by reworking of glacial deposits near the ordovician silurian boundary and enhanced by phases of the silurian salinic orogeny may have combined to give the progressive increase in the strontium isotope ratio through the silurian the scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes copyright 1998 elsevier science ltd

cherty carbonate facies of the montoya group southern new mexico and western texas and its regional correlatives a record of late ordovician paleoceanography on southern laurentia abstract the upper ordovician montoya group in southern new mexico and westernmost texas records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently nearly entirely dolomitized the medial unit of the montoya group the aleman formation is unique because it contains abundant chert 10 70 by volume the chert occurs as 1 thin continuous beds of sponge spicules within mudstone or calcisiltite 2 discontinuous lenses or nodules within skeletal wackestones and packstones or 3 as a replacement of skeletal grains and burrows coeval skeletal grainstones and muddy peritidal facies contain little chert phosphate up to 5 wt occurs within the underlying upham formation and the aleman formation as replacement of fossils and peloids the abundance of chert and phosphate in these subtidal facies indicates they formed within a region of strong upwelling regional correlation with upper ordovician cherty units along the periphery of southern laurentia and other low latitude continents suggests that upwelling was widespread and long lived during the late ordovician the upwelling is interpreted to record vigorous oceanic circulation produced by the onset of glaciation on gondwana during this period late ordovician relative sea level curves around the periphery of laurentia indicate correlative third order 1 3 my duration fluctuations that may provide a means for high resolution global correlations however the mechanism s that produced these long term fluctuations are unclear

eustatic control on early dolomitization of cyclic peritidal carbonates evidence from the early ordovician upper knox group appalachians the early ordovician upper knox group is characterized by stacked meter scale peritidal cycles that repeat at high frequencies 10 4 to 10 5 yr stacking patterns stratigraphic trends in lithofacies and cycle thickness of meter scale cycles define five depositional sequences that in conjunction with fischer plots delineate five long term relative sea level fluctuations during upper knox deposition intrabasinal and interbasinal correlation of upper knox fischer plots suggests that the third order sea level events were eustatic meter scale peritidal cycles likely formed in response to high frequency fourth and fifth order eustatic sea level fluctuations superimposed on these third order sea level events upper knox cyclic carbonates are extensively dolomitized as much as 85 of all dolomite is stratiform and consists of early dolomite exhibiting minor to extensive modification by burial dolomite synsedimentary dolomitization likely occurred in modified sea water during tidal flat progradation governed by high frequency sea level events this is suggested by the common association of dolomite with mud cracked laminites and silicified evaporite nodules the systematic decrease in dolomite abundance below laminite cycle caps and the presence of dolomite clasts in regoliths veneering high frequency cycle tops or in transgressive limestones of the overlying cycle dolomite distribution within depositional sequences shows a strong relationship to third and fourth order eustatic sea level events indicating that long term eustasy also strongly controlled early dolomitization of upper knox carbonates mass balance calculations show that the proposed sabkha model of dolomitization in concert with composite eustasy could generate stratiform dolomite of considerable vertical and lateral extent in peritidal cyclic carbonates this reflects the duration of progradation and supratidal exposure 10 4 to 10 5 yr available for dolomitization to proceed and the broad zone of active dolomitization that would develop during continued progradation throughout each cycle period

examination of downward hydrocarbon charge within the bakken three forks petroleum system williston basin north america abstract the bakken and three forks formations of the williston basin have been targeted by over 10 000 unconventional style wells horizontal drilling coupled with hydraulic fracturing exploration and developmental drilling initially focused on the middle bakken but later expanded into the underlying upper three forks located directly below the bakken and eventually the middle and lower three forks which are positioned 10s to 100 feet below the bakken hydrocarbons oil and associated gas within both formations are believed to be sourced from the upper and lower bakken shale members which are both highly organic rich 10 toc thermally mature petroleum source beds however minimal literature exists examining the relationship between hydrocarbons generated by the lower bakken shale and expelled downward into the various reservoirs of the three forks core plug oil and water saturation data from 65 partial to complete three forks cores were utilized to map out fluid saturation trends within the upper middle and lower three forks over 500 core chip analyses from 52 cores were used to spatially delineate the present day average hydrogen index thermal maturity of the lower bakken shale while a lower shale isopach map was created from wireline logs of 240 wells lastly the geochemical and standard core plug data from four complete bakken three forks cores were used to calculate and compare the volumetric amount of oil generated and expelled from the lower bakken shale with the original oil in place ooip of the entire three forks section the results show a strong spatial relationship between increased thickness thermally maturity of the lower bakken shale and increased core plug oil saturations across the upper to middle three forks members and a similar but less pronounced relationship with the lower three forks the calculated lower bakken shale expelled oil volumes ranged from 13 to 51 million barrels of oil mmbo per section 640 acres which overall equaled to exceeded the ooip of the entire three forks section 9 25 mmbo section these spatial and volumetric relationships indicate that local hydrocarbon generation and expulsion from the lower bakken shale is the primary control for hydrocarbon charge within the underlying three forks formation where in some locations hydrocarbons have been pushed downwards 200 ft understanding downward hydrocarbon charge has implications in exploring for and developing hydrocarbon resources within the bakken three forks as well as numerous other unconventional petroleum systems

three dimensional structural model of composite dolomite bodies in folded area upper jurassic of the etoile massif southeastern france the three dimensional 3 d geometry of fractures and fault related dolomite is difficult to access with classical subsurface prospection tools therefore we have investigated an outcrop to improve the subsurface prediction for complex dolomite bodies this outcrop is located in the etoile massif southeastern france within a fault bend anticline the sedimentary units are of upper triassic to lower barremian age the fold results from the pyreneo provencal shortening during the late cretaceous to the eocene the anticline hosts three types of dolomite bodies 1a massive dolomite of middle to late oxfordian age 1b syndepositional stratabound dolomite of tithonian age and 2 isolated dolomite bodies associated with fractures and faults large scale geometries of fault related dolomite bodies have been modeled in 3 d the 3 d geometries of these bodies show diapir finger and wall like structures these bodies are located close to the main thrusts in strata of middle oxfordian to early barremian age and are linked to the compressive fold bending phase during the late cretaceous fault related dolomitization occurred because of magnesium removal from the hydraulic brecciation and the pressure solution of type 1 dolomite with overpressured fluids these fluids flushed upward along the main thrust and laterally by following the reservoir property contrasts in the host rocks fault related dolomite bodies are either spread far apart from faults in grainy limestones with good initial reservoir properties or are restricted to fault vicinity in muddy limestones with poor initial reservoir properties the study of the structural and stratigraphic framework was essential in the understanding of the dolomitization process

origin and geometry of red river dolomite reservoirs western williston basin remarkably uniform distribution of limestone laminated dolomite and anhydrite as determined from compensated neutron density logs suggests that the entire ordovician red river formation of the central williston basin was deposited in subtidal brining upward sequences study of cores and thin sections verifies this interpretation and reveals that the red river b and c zones consist of from bottom to top 1 highly burrowed locally dolomitized fossiliferous wackestones and packstones 2 laminated to evenly bedded unfossiliferous mudstones dolomitized in many wells and 3 bedded anhydrite no evidence of subaerial exposure was observed in these rocks units dolomitization in the red river c zone is highly localized an empirical study of dolomite distribution using data from well logs reveals the presence of dozens of pods of dolomite immediately beneath the c anhydrite the pods are up to 200 ft 60 m thick and 1 mi 1 6 km in diameter and consist of concentered lenses of 1 tight locally anhydritic cryptocrystalline dolomite up to 40 ft 12 2 m thick and 3 300 ft 1 000 m in diameter 2 fine to medium grained porous dolomite that forms the reservoirs and 3 still farther from the cryptocrystalline dolomite relatively tight partly dolomitized limestones these dolomite pods formed by the subtidal downward migration of magnesium rich brines the brines formed during or immediately after precipitation of the subtidal anhydrite probably originally gypsum brine migration was localized by holes in the anhydrite and or upwelling compaction water from the sediments beneath the anhydrite the holes probably formed by fracturing minor faulting or expulsion of water during compaction where the magnesium rich brines first entered the carbonate sediments they formed cryptocrystalline dolomite but as they migrated away from the holes and lost their potency they produced porous dolomite the nature of which was partly controlled by the host sediment and still farther away partly dolomitized limestones because certain layers of the c zone carbonates were quite permeable to the dolomitizing brines highly irregular reservoirs characterized by interfingering limestones and dolomites formed several oil fields in the western williston basin could have been found through application of this model and others could be more efficiently developed

late ordovician early silurian ashgillian llandovery sea level curve derived from conodont community analysis anticosti island quebec abstract earlier workers have established sea level curves for the ashgillian llandovery sequence on anticosti island quebec canada based on sedimentology and on brachiopod and reef communities the complete anticosti sequence has been collected for conodonts to provide a database of over 77 600 conodonts derived from 272 samples through the 800 1100 m sequence this paper uses a statistical analysis of the database to establish the pattern of conodont communities and their interpreted water depths this allows the establishment of a eustatic sea level curve for the sequence this curve is in general agreement with those of the earlier workers but is more detailed and better constrained stratigraphically given the large sampling program the curve reflects more oscillating sea level changes for the whole sequence than those constructed earlier and different water depths for certain intervals from those established earlier the curve is valuable given that the anticosti sequence is one of the most complete in the world for this interval and its divergence from global eustatic curves for this interval is noted

ichnological analysis of the cretaceous palaeogene boundary interval at the caravaca section se spain abstract ichnological analysis of the cretaceous palaeogene k pg boundary at caravaca betic cordillera southeast spain reveals significant data for the interpretation of the k pg boundary event the uppermost maastrichtian marls below the cretaceous palaeogene k pg boundary at caravaca contains the dark coloured trace fossils chondrites targionii planolites thalassinoides zoophycos and alcyonidiopsis the composition of trace fossil associations in the upper maastrichtian and lower danian marls is generally the same yet much more poorly preserved in the latter showing in general no ichnological evidence of a severe macroinfaunal crisis at the k pg boundary the dark coloured trace fossils penetrate from two bioturbated horizons in the dark boundary layer separated by two horizons with primary lamination the ichnofauna rapidly recovered after the k pg event following deposition of the lower parallel laminated bed 14 mm thick that occurs in the dark boundary layer above the k pg boundary zoophycos is the deepest trace fossil penetrating as far as 90 cm below the k pg boundary probably in stiff or partly firm substrate which was dewatered to some extent during the slow accumulation of the danian dark boundary layer the firmness probably prevented the chondrites trace maker from deeper burrowing the maastrichtian sediments may be contaminated with danian microfossils due to bioturbation

sequence architecture of the bighorn dolomite wyoming usa transition to the late ordovician icehouse abstract although major late ordovician glaciation was confined to the terminal ordovician hirnantian stage lasting less than 2 million years recent isotopic evidence suggests a prolonged interval of slow growth in polar ice caps tropical carbonate platforms from passive margins and epicontinental seas offer the opportunity to monitor changes in eustatic sea level changes that accompanied this transition from a greenhouse towards an icehouse climate the late ordovician bighorn dolomite of northern wyoming and adjacent states preserves such a transition the bighorn dolomite was deposited on a northeastward facing subequatorial platform and it consists of three third order depositional sequences each is bounded by a regional erosional surface commonly with erosional truncation visible at outcrop scale and commonly mantled by a distinctive brecciated horizon the bighorn dolomite is dominated by highly bioturbated and fossiliferous open marine shallow subtidal facies but portions also contain dolomitic bioturbated restricted marine shallow subtidal and laminated peritidal facies as well as thin bedded argillaceous and fossiliferous deep subtidal limestone the three depositional sequences preserve a transition in their internal cyclicity that is consistent with computer modeling of the stratigraphic signal of increasing period and amplitude of eustatic sea level changes the lowermost sequence which spans the lander and steamboat point members consists of homogeneous acyclic shallow subtidal facies with a few meter scale peritidal parasequences near its top the middle sequence which corresponds to the leigh member contains acyclic to weakly cyclic restricted shallow subtidal and peritidal facies the uppermost sequence consisting of the horseshoe mountain member is dominated by five decameter scale parasequences composed of open marine shallow subtidal restricted shallow subtidal and peritidal facies overall the bighorn dolomite is interpreted to represent deposition during a shift from low amplitude a few meters short period 20 kyr cyclicity to moderate amplitude 10 m long period 100 kyr cyclicity indicating a shift from greenhouse conditions towards icehouse conditions a shift that agrees with recent isotopic evidence for changing late ordovician climate

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

ordovician metre scale cycles implications for climate and eustatic fluctuations in the central appalachians during a global greenhouse non glacial to glacial transition abstract metre scale shallowing upward cycles in ordovician carbonates of the central appalachian basin record climate and eustatic fluctuations during a transition from early ordovician global greenhouse to late ordovician icehouse conditions peritidal facies and shale abundance suggest a long term trend in this area from semi arid early ordovician to more humid middle to early late ordovician conditions with a return to semi arid conditions during the late ordovician the climatic fluctuations were most likely produced by tectonics uplift and erosion related to taconic orogenesis plate motion of north america and the areal extent of water covering the shelf peritidal cyclic facies indicate that high frequency relative sea level fluctuations were of small amplitude 20 m relative sea level fluctuations that decreased into the later ordovician if these sea level changes are eustatic then the increased amplitude may mark the early initiation of continental glaciation on gondwana in the late middle ordovician followed by waning of ice sheets prior to the latest ordovician glaciation

polar front shift and atmospheric co2 during the glacial maximum of the early paleozoic icehouse our new data address the paradox of late ordovician glaciation under supposedly high pco2 8 to 22 pal preindustrial atmospheric level the paleobiogeographical distribution of chitinozoan mixed layer marine zooplankton biotopes for the hirnantian glacial maximum 440 ma are reconstructed and compared to those from the sandbian 460 ma they demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the polar front through time from 55 70 s to 40 s these changes are comparable to those during pleistocene interglacial glacial cycles in comparison with the pleistocene we hypothesize a significant decline in mean global temperature from the sandbian to hirnantian proportional with a fall in pco2 from a modeled sandbian level of 8 pal to 5 pal during the hirnantian our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end ordovician mass extinction

microbial laminite versus rooted and burrowed caps on peritidal cycles salinity control on parasequence development early cretaceous isolated carbonate platform croatia some parasequences in the interior of the early cretaceous isolated carbonate platform croatia which formed under a greenhouse climate lack the typical regressive tidal flat laminites instead these bioturbated emergent parasequences have burrowed and rooted upper parts capped with thin greenish paleo sols and are reminiscent of triassic lofer cycles we propose that the bioturbated cycle tops reflect coastal salinities that are normal marine brackish or metahaline whereas the transgressive rare and abundant regressive tidal flat laminites formed under hypersaline coastal waters thus these features may be used to track gross salinity changes of coastal waters on the platform through time in addi tion we suggest ways to evaluate whether the salinity changes relate to hypersaline embay ments on the platform or changes in the cli mate at the parasequence scale we propose that the bioturbated emer gent parasequences could have formed under semiarid conditions seaward of subtle shallow embayments on the platform con temporaneously in the bayheads micro bial laminites developed these would require time equivalent updip and downdip laminate capped and bioturbated emer gence capped parasequences however the para sequences could also have been con trolled by climate changes the laminite capped parasequences could have resulted from salinity increase with shallowing of the platform interior under semiarid condi tions stable climate in contrast the bio turbated emergence capped parasequences could have formed during times of more humid climate punctuating the overall semi arid climate the humid phase would have favored brackish to normal marine salinities in the coastal zone along with macrophytes and intense animal and plant bioturbation which would suppress development of lami nite caps thus these parasequences provide a means of tracking gross salinity of coastal waters and if climate induced then they have implications for fluctuating climates in greenhouse worlds

stratigraphic and geochemical investigation of kukersites petroleum source beds within the ordovician red river formation williston basin abstract the upper ordovician red river formation has been a prolific producer of oil and gas in the williston basin where it has cumulatively produced more than 750 million bbl of oil equivalent over the past half century previous studies have recognized petroleum source beds referred to as kukersites in the red river formation but have not determined their complete extent or hydrocarbon generation significance examination and analysis of 28 cores and greater than 300 wireline logs have revealed 10 distinct kukersites in the red river d zone that can be correlated individually for tens to hundreds of miles tens to hundreds of kilometers across the western quarter of north dakota although each red river kukersite is typically thin 1 2 ft 0 3 0 7 m thick they combine to reach net thicknesses of greater than 12 ft 3 7 m with average present day total organic carbon toc values of typically 3 6 wt hydrogen index hi values from kukersite samples range from primarily greater than 800 mg hydrocarbons hc g toc within the northern flank of the basin to systematically decreasing to less than 100 mg hc g toc within the basin center this systematic decrease in hi is interpreted to be a function of increased thermal maturity where hydrocarbon generation has depleted kukersite organic richness preliminary calculations of hydrocarbon volumes generated from red river kukersites based on a previously developed method that calculates the volumetric decrease in original to present day kerogen content total approximately 66 billion bbl 1 05 10 10 m 3 of oil equivalent this approximate generation total is more than enough to account for cumulative red river production and supports the idea that the red river is a self sourced petroleum system with potentially significant remaining exploration potential

reservoir performance in ordovician red river formation horse creek and south horse creek fields bowman county north dakota the contiguous horse creek and south horse creek fields produce oil from the ordovician red river formation s d zone equal to the c burrowed member these fields produce from dolomite reservoirs at depths of about 9000 ft 3000 m in the southern williston basin on the northeastern flank of the southern end of the cedar creek anticline gentle 1 degree northeast regional dip allows oil entrapment in both areas of updip porosity pinch out and small 2 km diameter low relief 30 m structural closures each field consists of two red river d oil pools a stratigraphic porosity pinch out trap drained by 15 wells in the two fields to the west and structural closures drained by up to 3 wells each to the east reservoir rocks in both types of traps are burrowed dolomitized carbonate mudstones and wackestones deposited in open to restricted shelf environments the best reservoir rocks occur where up to 25 porosity is present between completely dolomitized burrow fills reservoir quality porosity is mainly intercrystalline and vuggy in finely crystalline dolomites but even in the most porous intervals permeability only locally exceeds 30 md amounts of porosity in wells producing from the d zone can be used to estimate a well s ultimate oil recovery when integrated with data on structural position thickness of porous dolomite and the nature of the fluid saturation best indicated by bulk volume water values production in the structurally trapped d zone oil pools in each field where initial water saturation was 22 will average about 625 thousand bbl of oil well with initially negligible water but with increasing watercut through time the stratigraphically trapped oil pools in the fields where initial water saturations ranged from 32 to 66 will average 237 thousand bbl of oil well with higher initial watercuts but little increase in watercut through time

ecological zonation during the carbonate transgression of a late ordovician rocky shore northeastern manitoba hudson bay canada late ordovician ashgillian carbonates belonging to the port nelson formation transgress a resistant rocky shore near churchill manitoba on the southwest flank of the hudson bay basin in canada examples of ancient rocky shores are very rare and this locality offers the first opportunity to document extensive offshore biological zonation with spatial reference to a well defined shoreline five biofacies zones are parallel to a 350 m long shoreface carved into the precambrian churchill quartzite eroded boulders of the dark quartzite blanket the shoreface and clast size is observed to decrease dramatically up section through 4 5 m of tan colored carbonates from an onshore to offshore position the ecological zones include 1 boulder encrusting favositid corals and a related high energy fauna 2 a shell pavement dominated by the inarticulate brahciopod monomerella 3 large free standing tabulate corals including palaeophyllum calapoecia and catenipora 4 tunneled trace fossils including chondritid thalassinoid and less common vertical volcano burrows and 5 surface cruziana burrows associated with rare large isotelid asaphacean trilobites siphuncular debris from aminocerid cephalopods abundantly scattered through zones 2 3 provides data on current orientation consistent with long shore drift a wide field of ripple marks preserved near the top of the stratigraphic sequence confirms that wave trains advanced shoreward at an indirect angle of about 45 although currents deflected by natural groin fields may have swept coarse clastics more seaward at some spots than others the biological zones are remarkably linear and persistent the churchill site is a comparatively simple case of landward migration by shore parallel biofacies primarily controlled by factors related to water depth and distance from shore this interpretation does not preclude the possibility that more random facies patterns may have existed in protected embayments or along more irregularly cliffed sectors of the ancient churchill coast

a late ordovician high energy temperate water carbonate ramp southern quebec canada implications for late ordovician oceanography the trenton group late ordovician the youngest carbonate unit in the taconic foreland basin of southern quebec is a tripartite unit with a distinctive coarse grained middle part the deschambault formation lithofacies of the deschambault formation are dominated by coarse grained bioclastic intraclastic limestones finer grained lithofacies are ubiquitous but subordinate the complete spectrum of lithofacies indicates sedimentation ranging from above fairweather to below storm wave base skeletal components are indicative of the modern temperate water bryomol association non skeletal elements are represented by peloids and intraclasts accretion rates from areas of continuous sedimentation were low 14 cm 103 years from sedimentological and faunal evidence it is proposed that the late ordovician deschambault ramp was bathed by temperate waters the model compares favourably with modern cool water shelves rimming the southern edge of the australian continent palaeomagnetic data locate southern quebec in a low latitudinal setting during the late ordovician upper ordovician facies distribution in eastern canada and progressive disappearance of some faunal provinces through late ordovician time are used to conclude that the initiation of the late ordovician glaciation that covered most of gondwana was instrumental in easing northward movement of cold oceanic currents this resulted in the rapid contraction of the southern hemisphere warm water tropical belt from a 30 latitudinal wide zone in the early caradoc to a 15 zone in the late caradoc

carbon isotope composition of bioturbation infills as indication of the macrobenthic colonization timing across the cretaceous tertiary boundary agost section se spain

porosity permeability relationships in interlayered limestone dolostone reservoirs porosity and permeability data from five carbonate platform successions of different settings ages and burial depths are examined to identify overall similarities and differences in the reservoir quality of interlayered limestones and dolostones each succession consists mainly of limestone and dolostone with subordinate proportions of intermediate partly dolomitized compositions in the three deeply buried platforms the key features are that limestones have much lower average porosity than associated dolostones and that limestones and dolostones show little difference in average permeability for given porosity in contrast the shallowly buried platforms show little difference in average porosity between limestones and dolostones and also display higher average permeability for given porosity in dolostones than limestones these data suggest the following general guidelines for depositional and diagenetic controls on reservoir architecture in carbonates consisting of interlayered limestone and dolostone 1 reservoir compartmentalization by the formation of tight limestone barriers is largely a burial diagenetic process involving calcite cementation locally produced by chemical compaction 2 both the pattern of early dolomitization and the distribution of clay minerals because they influence the localization of chemical compaction are key factors that determine the distribution of tight limestone barriers separating flow units thus the pattern of eventual burial compartmentalization follows a template that is hardwired into the stratigraphic architecture by depositional mineralogy and early diagenesis 3 after burial 2 3 km 1 2 1 8 mi dolostones should not be expected to have higher permeability for given porosity than associated limestones these rules assume dolomitization to occur early relative to chemical compaction which will commonly be the case because of combined hydrologic and mass balance constraints dolomitization and concentration of clay appear linked to cycle and sequence architecture in the present examples and thus may have a useful degree of predictability at least in terms of statistical parameters such as net gross and probability of flow unit thickness distribution

suspended sediment transport in an estuarine tidal channel within san francisco bay california abstract a recently developed instrumentation system has been used to monitor simultaneously flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within san francisco bay california measurements were made every 15 min over six successive flood and ebb tidal cycles they included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed mean suspended sediment concentration profiles from seven miniature nephelometers placed within 1 m of the seabed near bottom pressure fluctuations vertical temperature gradient and bottom photographs additionally suspended sediment was sampled from four levels within 1 m of the seabed three times during the tidal cycle the instrument system was retrieved during each slack water period to exchange suspended sediment sample bags while the instrument was deployed std nephelometer measurements were made thoughout the water column and water samples were collected each 1 h and bottom sediment was sampled at the deployment site size distributions of the suspended sediment samples estimates of particle settling velocity s friction velocity u and reference concentration c a at z 20 cm were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles three suspended sediment particle conditions were evaluated 1 individual particle sizes in the 4 11 62 5 0 5 m size range with the reference concentration c a at z 20 cm c 2 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration c a at z 20 cm c f and 3 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress an analysis was also carried out on the sensitivity of the suspended sediment distribution equation to deviations in the primary variables s u and c a in addition computations of mass flux were made in order to show vertical variations in mass flux for varying flow conditions

comparison of sequences formed in marine sabkha subaerial and salina subaqueous settings modern and ancient marine evaporites occurring in modern subaqueous salina settings and subaerial sabkha settings are different subaqueous holocene evaporites occur as shoaling upward lacustrine sequences up to 10 m thick they are evaporite dominated and are composed primarily of bottom nucleated crystals that may be deposited as massive laminated or rippled units each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments in plan view they show a well developed bull s eye pattern with a sulfate center and a carbonate rim in contrast subaerial sabkha evaporites occur as part of a laterally prograding shoaling upward peritidal sequence in which the supratidal unit is usually no more than 1 m thick sabkha sequences are matrix dominated not ev porite dominated with the bulk of the sulfate phase occurring as diagenetic nodules enteroliths or diapirlike structures these sulfates were formed during syndepositional diagenesis by replacement and displacement processes the various facies of the sequence tend to accumulate in belts parallel with the shoreline relative to the sea level or the brine level sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows some of the characteristics that distinguish holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies even when gypsum has been converted to nodular anhydrite the distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates

jens munk archipelago ordovician silurian islands in the churchill area of the hudson bay lowlands northern manitoba abstract the name jens munk archipelago is proposed for a cluster of small monadnocks that rise a maximum of 45 m above present sea level on the shores of hudson bay in the churchill area of northern manitoba resistant precambrian quartzite formed shoals and islands during the late ordovician and early silurian periods outcrop control in the present tundra environment of the hudson bay lowlands is spotty at best but the quartzitic monadnocks are fringed by upper ordovician and lower silurian ramp deposits mainly carbonate in composition five isolated outliers of ordovician silurian strata are described and the location of four other potential outliers is indicated ramp dip at any one site generally ranges from 10 to 15 and the surviving ramps on opposite flanks of paleoislands suggest preservation of near synsedimentary conditions extensive conglomerates of reworked quartzite are most abundant on a northern exposure within the islands ripple marks and oriented cephalopod debris are related t

the effect of particle settling velocity on computed suspended sediment concentration profiles abstract a field study was performed at a site in san pablo bay california to evaluate how well suspended sediment distribution equations reproduce observed concentrations near bed measurements of current velocity suspended sediment concentrations and fluid and particle characteristics were made an in situ settling cylinder was used to determine settling velocity values of aggregated suspended sediments and photographs from a plankton camera provided size estimates in situ weighted average settling velocities were generally an order of magnitude greater than a weighted average for stokes calculations of settling velocity for the grain size distribution of the reference suspended sediment sample and two orders of magnitude greater than a stokes settling velocity for the median grain size of suspended sediment flocs were commonly 100 m in diameter and as large as 450 m with settling velocities as high as 0 2 cm s 1 calculated suspended sediment concentration profiles using in situ estimates of settling velocity yielded closer agreement with observed concentration profiles within 25 than using settling velocities calculated from the grain size distribution of suspended sediment samples within 40 50

middle late ordovician darriwilian sandbian decoupling of global sulfur and carbon cycles isotopic evidence from eastern and southern laurentia abstract middle late ordovician sequences from the appalachian basin and arbuckle mountain regions of north america were analyzed for carbonate associated sulfate 34 s cas and pyrite 34 s pyr paired with carbonate 13 c carb and organic matter 13 c org chemostratigraphy two major negative drops in 34 s cas 12 excursions are recognized the older decline in 34 s cas occurs within the histiodella holodentata phragmodus polonicus conodont zones and the younger drop is within the cahabagnathus sweeti amorphognathus tvaerensis baltoniodus gerdae subzone zones these overall these negative shifts in 34 s cas have an antithetical relationship with positive shifts in 34 s pyr 10 and 13 c carb 2 recorded in the same successions the older negative 34 s cas shift is coincident with the widely documented mid darriwilian 13 c excursion mdice and the younger negative 34 s cas shift is coincident with another positive 13 c carb shift in the early sandbian geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative 34 s cas trends additionally a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends while increased crustal weathering is broadly consistent with a sea level lowstand and the seawater 87 sr 86 sr isotope record of change in continental weathering in the late darriwilian stage of the ordovician geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in 34 s cas these antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates pulses of oceanic ventilation would have expanded habitable environments for marine organisms and thus is broadly consistent with major increases in biodiversification during this period of the ordovician

long lived glaciation in the late ordovician isotopic and sequence stratigraphic evidence from western laurentia the timing and causes of the transition to an icehouse climate in the late ordovician are controversial results of an integrated 13c and sequence stratigraphic analysis in nevada show that in the late ordovician chatfieldian stage mid caradoc a positive 13c excursion in the upper part of the copenhagen formation was closely followed by a regressive event evidenced within the prominent eureka quartzite the chatfieldian 13c excursion is known globally and interpreted to record enhanced organic carbon burial which lowered atmospheric p co2 to levels near the threshold for ice buildup in the ordovician greenhouse climate the subsequent regressive event in central nevada previously interpreted as part of a regional tectonic adjustment is here attributed in part to sea level drawdown from the initiation of continental glaciation on gondwana this drop in sea level which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p co2 by suppressing shelf carbonate production signals the transition to a late ordovician icehouse climate 10 m y before the widespread hirnantian glacial maximum at the end of the ordovician

an optical settling tube for the determination of particle size distributions abstract the particle size distribution the settling velocity distribution and the concentration weighted settling velocity are important parameters in the study of sediment dynamics these parameters can be determined by means of an optical settling tube in which the concentration of particles is determined by means of a beam attenuation meter and the particle size is determined by stokes settling results obtained by this method compare very well with coulter counter and gravimetric settling tube data various correction factors are discussed the instrument is designed for eventual use in situ

deposition and early alteration of evaporites summary the depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories marginal mixed shallow subaqueous and subaerial shallow and deep subaqueous these three environmental groupings hold for both marine and nonmarine settings although the details of continental evaporites may be far more complex than in most marine fed water bodies the primary evaporite morphologies from many continental playa hypersaline marine and marine marginal depositional settings are reasonably well understood because of the numerous detailed studies of recent holocene and cenozoic deposits that serve as models for sedimentary interpretation the sedimentological features that develop in deeper water settings are inferred based on examination of unaltered cenozoic deposits each environmental setting develops characteristic depositional features and patterns and one facies grades into the next because there may be significant physiochemical changes in water composition during deposition as well as sudden change s in both water depth and basinal circulation description and interpretation of evaporative rocks should not be based on mineralogy alone but on the distinct sedimentary characteristics of each part of a deposit in cases where sediments still reflect their primary mineralogy and morphology most of the environments can now be recognized however geochemical studies are commonly required to determine the source s of the original water the determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions bromine content of chloride salts and the stable isotopes of strontium sulphur carbon and oxygen only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled

sequence development on a sediment starved low accommodation epeiric carbonate ramp silurian wabash platform usa mid continent during icehouse to greenhouse transition abstract twelve cored wells through the silurian section of the wabash platform indiana u s a and 2 non cored basinal wells with cuttings were studied to define the sequence stratigraphy of a sediment starved ramp within a low latitude epeiric sea during the latest ordovician icehouse to silurian greenhouse transition the wabash platform approximately 200 000 km2 area was bounded to the north by the michigan basin to the east by the appalachian foreland basin and passed to the southwest into the vincennes basin which was open to the ocean facies developed include platform fringing buildups stromatactis wackestone core stromatoporoid skeletal wackestone caps and crinoidal rudstone packstone flank facies and non buildup crinoid grainstone packstone sheets shoals skeletal wackestone variably cherty between fair weather and storm wave base deep lagoon and deep ramp and carbonate mudstone clean to argillaceous sub storm wave base deep lagoon and deep ramp basin the eleven sequences 2 m to 30 m thick and 1 to 4 m y durations average 2 5 m y generally can be correlated to global sea level cycles the lower three sequences are disconformity bounded reflecting moderate ice sheets but the others are relatively conformable the most easily mapped regional surfaces are the transgressive surfaces because the correlative conformities are cryptic the wide well spacing prevented mapping of lowstand system tracts the transgressive systems tracts are upward deepening upward fining carbonate units that become more argillaceous and silty upward the highstand systems tracts are upward coarsening carbonate mudstone to wackestone packstone and rare grainstone barrier bank complexes and isolated buildups aggraded during deposition of the upper 4 sequences to form a discontinuous raised rim 40 m relief to the ramp while the interior of the platform remained sediment starved subsidence rates in the basin were very low 0 6 to 1 2 cm k y but the low platform accumulation rates 0 2 cm k y to 0 8 cm k y prevented the seafloor from building to sea level in all but the lower three sequences and later over barrier banks accumulation rates 2 cm k y parasequence development in high accommodation settings elsewhere in the silurian in north america records transition from moderate ice sheets to an ice free world this is weakly expressed on the wabash platform due to the dominantly deeper subtidal setting the remarkably low sedimentation rates resulted from the epeiric sea being isolated from the open ocean and they likely are typical of the large epeiric seas of the paleozoic

organic geochemistry of mid continent middle and late ordovician oils ordovician oils in mohawkian and cincinnatian reservoirs of the united states mid continent retain the biochemical imprint of middle and upper ordovician oceanic life before the evolution of land plants and most vertebrates thus these oils have some geochemical features that distinguish them from younger oils these features include 1 a predominance of n c15 n c17 and n c19 alkanes in the saturated hydrocarbon fraction 2 relatively low amounts of longer chain n alkanes 3 low amounts of chlorophyll derived isoprenoids such as pristane and phytane and 4 abundant c29 sterane relative to c27 with rearranged forms diasteranes predominant over normal steranes ordovician oils also generally contain little sulfur and have a somewhat variable light stable carbon isotopic composition with dgr13csat and dgr13caro values of 28 to 31 pmil pdb but these features are typical of many marine oils the unusual chemistry of these ordovician oils supports the interpretation of reed illich and horsfield 1986 that prokaryotic organisms provided the organic matter for most ordovician oils although their claim for gloeocapsamorpha a problematic unicellular prokaryote possibly a blue green alga or an unusually large bacterium cannot be proven from oil chemistry alone knowing that indigenous mid continent ordovician oils were derived from prokaryotic organisms may aid in future exploration for these reservoirs

stable carbon isotope record of shallow marine evaporative epicratonic basin carbonates ordovician williston basin north america secular variations in stable carbon isotope values of marine carbonates are used widely to correlate successions that lack high resolution index fossils various environmental processes however commonly may affect and alter the primary marine carbon isotope signal in shallow epicratonic basins this study focuses on the marine carbon isotope record from the carbonate evaporite succession of the upper katian upper ordovician red river formation of the shallow epicratonic williston basin usa it documents the carbon isotope signal between the two major ordovician positive shifts in 13c the early katian guttenberg and the hirnantian excursions eight 13c stages are identified based on positive excursions shifts from positive to negative values and relatively uniform 13ccarb values a correlation between carbon isotope trends and the relative sea level changes based on gross facies stacking patterns shows no clear relation based on the available biostratigraphy and 13c trends the studied williston basin curves are tied to the isotope curves from the north american midcontinent quebec anticosti island and estonia which confirm the late katian age aphelognathus divergens conodont zone of the upper red river formation the differences in the 13c overall trend and absolute values coupled with the petrographic and cathodoluminescence evidence suggest that the carbon isotope record has been affected by the syndepositional environmental processes in the shallow and periodically isolated williston basin and stabilized by later burial diagenesis under reducing conditions and the presence of isotopically more negative fluids

estuarine flocs their size settling velocity and density the size and settling velocity of flocs of suspended sediments from the chesapeake bay were measured in laboratory experiments and in the field the curves between floc diameter and settling velocity showed a non stoke s relationship that was best described by the following settling velocity cm s 1 73 floc diameter cm 0 78 furthermore the relationship between floc diameter and settling velocity indicated that the floe density decreased as the floes increased in size in addition to changing density a cylinder shape was found to be the best shape factor