deep structure of northern mississippi embayment in september 1980 the u s geological survey conducted a seismic refraction investigation of the northern mississippi embayment during the investigation 34 shots from nine shotpoints were recorded along a series of profiles the profiles were parallel to and across an inferred precambrian rift zone which is outlined by a series of magnetic anomalies and covers an area at least 200 km 125 mi long and 70 km 45 mi wide along the northeast southwest trending axis of the rift a 0 7 to 1 1 km 2 297 to 3 609 ft thick 1 8 km sec layer representing the unconsolidated tertiary and cretaceous sediments overlies a 2 km 6 562 ft thick 5 95 km sec layer representing a paleozoic carbonate sequence beneath the carbonates is a 3 km 9 843 ft thick low velocity layer which probably consists of late precambrian clastics the presence of this low velocity layer is indicated on the seismic profiles by an abrupt cutoff of the 5 95 km sec arrivals at about 20 km 12 mi from the shotpoints this cutoff is followed by a 30 to 40 km 20 to 25 mi long zone of no arrivals beyond which are seen delayed arrivals from a 6 2 km sec crystalline basement the 11 km 6 8 mi thick 6 2 km sec basement overlies a lower crus composed of a 6 6 km sec layer whose thickness decreases to the northeast owing to its replacement by an anomalous high velocity 7 3 km sec basal layer the base of the crust ranges from a 39 km 24 mi depth at the southern end of the profile to a 46 km 29 mi depth at the northern end where the 7 3 km sec layer is 20 km 12 mi thick below the base of the crust moho the upper mantle has a velocity of 8 0 km sec the velocity structure beneath the west flank of the embayment is simpler than that of the axis 5 95 6 2 6 6 7 3 and 8 0 km sec the low velocity layer is absent and the anomalous 7 3 km sec layer is thinner than along the axial profile the profiles perpendicular to the rift show that the low velocity layer is restricted to the axial zone and that the anomalous 7 3 km sec basal crustal layer reaches maximum thickness there the tectonic model proposed to explain the origin of this embayment velocity structure includes a late precambrian mantle plume that intruded the lower crust of the northern embayment causing uplift and or crustal stretching and subsequent rifting of the axial area this was followed by erosion subsidence and subsequent deposition of sediments in the resulting trough

distal orogenic effects on peripheral bulge sedimentation middle and upper ordovician of the nashville dome abstract a major switch in depositional style in the ordovician carbonates of the nashville dome corresponds closely with the onset of the late middle ordovician taconic orogeny this time marks a shift from tropical type to temperate type carbonates the initiation of widespread major phosphate deposition the introduction of large amounts of terrigenous silt and clay the occurrence of widespread seismically induced soft sediment deformation and a change from a low energy flat topped carbonate shelf to a high energy doubly dipping carbonate ramp soft sediment deformation and the introduction of siliciclastics are direct effects of the taconic orogeny the switch from tropical type to temperate type carbonates the initiation of phosphate deposition and the switch in carbonate ramp are lar ely oceanographic effects triggered by the orogeny in particular phosphate deposition and the switch to temperate type limestones appears to have been driven by upwelling along the eastern side of the nashville dome within the newly deepened taconic foreland basin a fourfold decrease in the rate of relative sea level rise occurred on the nashville dome nearly 3 m y following the onset of thrusting and foreland basin initiation subsidence rates were constant before and after this decrease and no evidence of a change in subsidence rates is seen to coincide with the onset of thrusting the slowing of subsidence may reflect viscoelastic uplift of the nashville dome but the abrupt change from one constant subsidence rate to another is not predicted by existing foreland basin models

a mid ordovician temperate carbonate shelf the black river and trenton limestone groups of southern ontario canada abstract mid ordovician limestones of southern ontario are usually considered to be tropical shelf limestones and interpreted using facies models derived from recent tropical carbonate environments nevertheless depositional rates grain types faunas erosion surfaces mineralogy and geochemistry are more compatible with a temperate or even cold shelf environment in view of the low latitudes 20 s indicated by paleomagnetism this suggests deposition in relatively cold seas deposition below a tropical thermocline or that the paleomagnetic data are wrong similar features in other caradocian limestones of eastern north america show that these inferred cooler conditions were widespread if deposition occurred below a tropical thermocline most shallow water caradocian limestones must be re interpreted as deeper water deposits more likely in view of the evidence for temperate conditions in shallow water is that the whole eastern seaboard of north america was affected by cold currents arising from southern polar ice caps and thus was a temperate shelf environment

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

time specific black mudstones and global hyperwarming on the cambrian ordovician slope and shelf of the laurentia palaeocontinent abstract the early paleozoic featured nine intervals of strong expansion of an upper slope dysoxic anoxic d a water mass with eustatic rise or epeirogenic transgression strong expansion of this d a water mass led to deposition of time specific macroscale alternations of dark grey black mudstone within oxic green to red mudstone on the middle lower slope this d a facies even onlapped warm carbonate and cool water siliciclastic shelves as in the mesozoic d a muds were deposited in shallow water perhaps tens of metres deep with sea level rise these nine d a macroscale alternations correspond to intervals of global hyperwarming times of very intense greenhouse conditions that resulted from a feedback initiated by higher insolation and heat storage as shallow seas onlap tropical palaeocontinents warm epeiric seas heated the ocean and thermal expansion accelerated eustatic rise ever more extensive epeiric seas heightened oceanic and global temperature as heat storage capacity increased deep ocean circulation intensity fell below that of a greenhouse interval and lead to d a deposition low on the slope and on the platforms to provide the signature of global hyperwarming global hyperwarming differs from a hothouse interval as it does not require co 2 input from large igneous provinces to produce high temperatures and never shows deep sea anoxia late ordovician and late devonian black mudstones that cover much of laurentia record epeirogenic transgressions that led to global hyperwarming and suggest that cold water upwelling or plant terrestrialisation had nothing to do with epeiric sea anoxia global hyperwarming reduced oxygen solubility in these seas and erosion of orogens produced muddy water that limited light penetration and promoted shallow water anoxia the global hyperwarming hypothesis means that relative eustatic and epeirogenic sea levels complement the effect of global pco 2 on climate and sea level must also be regarded as a primary driver of phanerozoic climate

carbonate shelf edge off southern australia a prograding open platform margin the southern continental margin of australia is an extensive shelf that has been a site of cool water carbonate deposition since eocene time the platform has no rim and is swept by high energy waves and swells throughout the year the shelf is deep 40 to 100 m and typified by bryozoan rich sediments the shelf margin is a gentle incline that becomes progressively steeper seaward except where it laps down onto offshore terraces the edge of the eucla platform in the great australian bight is used to illustrate that the margin is a series of extensive prograding clinoforms progradation is interpreted to be the result of off shelf sediment transport and in place carbonate production by actively growing deep water bryozoa and sponges this area is a potential model for ancient high energy platform margins during geologic periods when large skeletal reef building metazoans were scarce

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

sequence stratigraphy of a carbonate clastic ramp the cincinnatian series upper ordovician in its type area the cincinnatian series consists of five depositional sequences each is composed of a thin transgressive systems tract and a much thicker shallowing upward highstand systems tract whereas the highstand systems tracts are relatively poor in micrite but rich in terrigenous shale the transgressive systems tracts are composed of cleaner carbonate facies ranging from coarse grained calcarenites to micrite rich wackestones and micstones this alternation of carbonate and terrigenousfacies types resulted from the trapping of clastic sediment near its presumed source in the appalachians during a relative sea level rise and it may be a feature common to sequences in other mixed carbonate clastic settings these depositional sequences were produced by eustatic fluctuations as shown by the limited extent to which the sequences can be traced into more proximal portions of the appalachian basin the recognition of depositional sequences clarifies the regional paleogeography and simplifies regional lithostratigraphic nomenclature within current biostratigraphic resolution sequences in the cincinnatian series appear to have chronostratigraphic significance and thereby offer an additional means of intrabasinal correlation in the upper ordovician

chemical composition stratigraphy and depositional environments of the black river group middle ordovician southwestern ohio the chemical composition and stratigraphy of the black river group in southwestern ohio were studied chemical analyses were done on two cores of the black river from adams and brown counties ohio these studies show that substantial reserves of high carbonate rock 96 to 98 caco3 plus mgco3 are present in the black river at depths of less than 800 ft in proximity to cincinnati and the ohio river stratigraphic studies show that the black river group has eight marker beds in its middle and upper portions and three distinct lithologic units in its lower portion these marker beds and units are present throughout southwestern ohio the black river group correlates well with the high bridge group of kentucky depositional environments of the black river are similar to those of the high bridge and to present day tidal flats in the bahamas

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

upper ordovician mohawkian carbon isotope δ13c stratigraphy in eastern and central north america regional expression of a perturbation of the global carbon cycle abstract the guttenberg carbon isotope excursion gice documented from eastern north america demonstrates the effects that regional geochemically distinct water masses upwelling and ocean circulation have on the carbon isotope record from carbonate platforms late turinian chatfieldian carbonates from oklahoma kentucky virginia and west virginia record a positive carbon isotope excursion 3 0 the gice excursion the gice excursion has relations to established biostratigraphy beginning in the north american midcontinent phragmodus undatus conodont zone and continuing through the plectodina tenuis zone sequence and event stratigraphy previously established models for positive carbon isotope shifts on carbonate platforms have been tested during the gice excursion where geochemically distinct water masses are defined for the upper ordovician a major eustatic sea level rise before the gice promoted a greater exchange of open ocean waters onto the carbonate platform of laurentia however restricted or sluggish circulation and exchange between water masses within the epeiric seas and the adjacent iapetus ocean were still apparent local variations documented in the gice excursion are directly related to upwelling of nutrient rich isotopically light waters increased primary productivity and the subsequent organic carbon production and burial

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

correlation of the ordovician deicke and millbrig k bentonites between the mississippi valley and the southern appalachians 1 two widespread rocklandian age middle ordovician k bentonite beds the deicke and millbrig can be correlated from southern minnesota and northwestern iowa through eastern missouri across kentucky and central tennessee to the valley and ridge of the southern appalachian mountains they are equivalent to beds previously called t 3 or pencil cave and t 4 or mud cave metabentonites and thus constitute ordovician chronostratigraphic marker horizons throughout much of the eastern mid continent previous correlations of these beds in the mississippi valley by chemical fingerprinting is extended toward the southern appalachian basin by tracing in surface exposures and on wireline logs both beds thicken toward the southeast indicating a volcanic source area that was probably ast of south carolina the interval between the k bentonite beds also thickens toward the southeast owing to increased rates of carbonate deposition between the two volcanic episodes the areal extent of known correlatives of the deicke and millbrig is minimally estimated to be 600 000 sq km and at least 1122 cu km of pre compaction deicke k bentonite accumulated as ash in eastern north america the volume of both beds suggests they rank among the largest air fall ash deposits documented in the stratigraphic record

mechanism of subsidence of ancient cratonic rift basins cratonic basins commonly occur over ancient rift zones these inactive rift basins are recognizable by a positive linear bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys many of these basins undergo discrete periods of increased subsidence rates or reactivations long after the mass excess responsible for the linear gravity high was emplaced furthermore the reactivation of many cratonic basins occurs simultaneously with large scale compressional tectonics it is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess the subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear maxwell viscoelastic rheology solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time the subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved since ancient rift mass excesses may be uncompensated over long geologic time intervals the early thermal and structural evolution of rifts may not significantly influence later basin subsidence the models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material allowing the uncompensated basin to settle toward an isostatic compensation depth faster than normal since viscosity is a strong function of temperature reactivation by a world wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent an increase of 15 in the geothermal gradient for example from 16 5 18 9 k km will cause about a 5 increase in subsidence this increase in heat flow however seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100 increase in subsidence might occur since the rheology of the lithosphere is nonlinear effective viscosity is also a strong nonlinear function of stress the presence of a regional compressive stress during periods of tectonism of 1 1 108 pa about 2 8 of the buckling strength of the lithosphere produces a short period of reactivated subsidence 105 yr during the reactivated subsidence the newly imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation this suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere this may be accomplished by an intermittent application of the regional stress over time

correlation of middle ordovician k bentonites based on chemical fingerprinting six separate k bentonite beds in middle ordovician rocks of southwestern ohio and northern kentucky have been analyzed for three major and 22 trace elements and the data used as a basis for stratigraphie correlation samples from eight cores distributed over a distance of 140 km were analyzed for dy eu mn na k sm la as ga lu sb fe yb cr tb se rb co ce ta th hf cs zn and zr by instrumental neutron activation analysis and x ray fluorescence and the results subjected to stepwise discriminant analysis within layer and between layer variations in composition were assessed and a ranking of elements established on the basis of their ability to characterize individual layers eighteen elements are sufficient to establish 100 classification in a group matrix of the six beds with the principle assignment contributions coming from th ta rb cr fe ga yb and k to test the stratigraphie usefulness of chemical fingerprinting over more extended distances seven samples of k bentonites e

tectonic subsidence analysis of the illinois basin tectonic subsidence curves calculated from three wells indicate that the illinois basin formed through initial fault controlled mechanical subsidence and subsequent thermal subsidence thermal subsidence involved an early stage of thermal linear 520 460 ma and subsequent thermal asymptotic subsidence 460 350 ma this thermal subsidence was in response to mantle intrusion which formed a three arm rift system a second subsidence episode of unknown origin was documented from middle mississippian through early permian time subsidence mechanisms controlled sedimentation and subsidence rate trends and also sedimentation patterns for each of the four distinct tectonic subsidence phases tectonic subsidence curves were extended across the sauk tippecanoe ordovician silurian and tippecanoe kaskaskia unconformities and fit a post rift thermal contraction model the sauk tippecanoe ordovician silurian and possibly the kaskaskia absaroka unconformities formed because rates of eustatic sea level fall exceede

disentangling hominin and carnivore activities near a spring at flk north olduvai gorge tanzania flk north is one of the densest concentrations of fossils found in olduvai gorge a recent taphonomic re evaluation of the collection excavated by leakey at the site suggests that it was a palimpsest in which most of the animals were accumulated and modified by carnivores the lithic tools therefore seem to have an independent depositional history from most of the fauna the present study based on new excavations expands the evidence supporting this interpretation and demonstrates a thicker deposit than was reported by leakey including new archaeological levels it also shows that in the few instances where hominins butchered carcasses meat not marrow was their main target this argues against passive scavenging hypotheses which emphasize the dietary role of marrow and instead underscores the importance of meat in the diet of early pleistocene hominins

models for circulation patterns in epicontinental seas applied to paleozoic facies of north america craton circulation in epicontinental seas develops in response to thermohaline wind driven and tidal motions in various climatic geographic and bathymetric settings mass balance relations in a seaway are established by the relative rates of outflow inflow freshwater and ocean water and evaporation which constrain two contrasting styles of epicontinental circulation quasietuarine circulation qec is established where rates of freshwater influx exceeds evaporation surface outflow from a seaway with qec typically is balanced by inflow of oceanic water at depth a density stratification can be established below the limits of vertical mixing a bottom layer of oxygen poor or anoxic water may develop in a seaway with or without an entrance sill if the bottom waters are derived from the oxygen minimum zone of the open ocean or if organic productivity exceeds the limits of aerobic respiration upwelling may be promoted in specific epicontinental settings with qec antiestuarine circulation aec is developed in arid climates where evaporation exceeds freshwater influx evaporative concentration in the surface layer causes oxygenated water to sink bottom outflow of denser more saline water is balanced by surface inflow from the open ocean a sill or barrier may inhibit bottom circulation in seas with aec permitting development of a basal brine layer stagnation in the brine layer may lead to oxygen depletion seas with aec commonly are characterized by extensive carbonate evaporite facies geographic and stratigraphic syntheses of lithofacies and biofacies provide the empirical basis for interpretations of seaway dynamics in ancient epicontinental settings examples from the upper ordovician maquoketa formation and upper devonian lime creek sweetland creek formations of cratonic north america are interpreted in the context of stratified qec patterns in each case oxygen poor to anoxic waters appear to have impinged along the basin slope below the margin of an oxygenated carbonate shelf upwelling along the shelf margin in the maquoketa example helps explain the distribution of phosphatic sediments a stratified aec pattern is interpreted for the gower formation silurian of eastern iowa a barrier to bottom circulation at the mouth of an embayment in the epicontinental sea resulted in a basal layer of hypersaline water devoid of shelly benthos surface water salinities increased progresively shoreward within the embayment but benthic organisms flourished where carbonate mounds built upward into the oxygenated layer

isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans implications for secular curves isotopic analysis of conodonts and their host limestones sampled between two regionally extensive altered volcanic ash layers in eastern laurentia shows that a 454 ma epeiric sea maintained large lateral differences in nd and c isotope compositions this is consistent with inferred temperature salinity defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments because the majority of old marine fossils and sedimentary rocks are known from epeiric seas some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses rather than global scale changes in the state of the earth ocean system

origin of cratonic basins tectonic subsidence curves show that the illinois michigan and williston basins formed by initial fault controlled mechanical subsidence during rifting and by subsequent thermal subsidence thermal subsidence began around 525 ma in the illinois basin 520 460 ma in the michigan basin and 530 500 ma in the williston basin in the illinois basin a second subsidence episode middle mississippian through early permian was caused by flexural foreland subsidence in response to the alleghanian hercynian orogeny resurgent permian rifting in the illinois basin is inferred because of intrusion of well dated permian alnoites such intrusive rocks are normally associated with rifting processes the process of formation of these cratonic basins remains controversial past workers have suggested mantle phase changes at the base of the crust mechanical subsidence in response to isostatically uncompensated excess mass following igneous intrusions intrusion of mantle plumes into the crust or regional thermal metamorphic events as causes of basin initiation cratonic basins of north america europe africa and south america share common ages of formation around 550 to 500 ma histories of sediment accumulation temporal volume changes of sediment fills and common dates of interregional unconformities their common date of formation suggests initiation of cratonic basins in response to breakup of a late precambrian super continent this supercontinent acted as a heat lens that caused partial melting of the lower crust and upper mantle followed by emplacement of anorogenic granites during extensional tectonics in response to supercontinent breakup intrusion of anorogenic granites and other partially melted intrusive rocks weakened continental lithosphere thus providing a zone of localized regional stretching and permitting formation of cratonic basins almost simultaneously over sites of intrusion of these anorogenic granites and other partially melted intrusive rocks

tectonic development of the new madrid rift complex mississippi embayment north america abstract geological and geophysical studies of the new madrid seismic zone have revealed a buried late precambrian rift beneath the upper mississippi embayment area the rift has influenced the tectonics and geologic history of the area since late precambrian time and is presently associated with the contemporary earthquake activity of the new madrid seismic zone the rift formed during late precambrian to earliest cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions the configuration of the buried rift is interpreted from gravity magnetic seismic refraction seismic reflection and stratigraphic studies the increased mass of the crust in the rift zone which is reflected by regional positive gravity anomalies over the upper mississippi embayment area has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex the correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures the slippage is due to reactivation of the structure by the contemporary nearly e w regional compressive stress which is the result of plate motions

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

high resolution sequence stratigraphy of the upper ordovician montoya group southern new mexico and western texas outcrop analog of an unconventional chert and carbonate reservoir the upper ordovician montoya group crops out in southern new mexico and westernmost texas and records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently almost entirely dolomitized the montoya group is a third order composite sequence composed of six regionally correlative shallowing upward third order depositional sequences m0 m5 sequence m0 has sandstone at its base that is overlain by skeletal packstone grainstone sequence m0 occurs only locally and was likely deposited in a topographic low formed during regional development of the unconformity following el paso group deposition sequence m1 marking the initial widespread transgression over the ellenburger unconformity consists of sandstone updip that passes downramp into skeletal packstone the highstand systems tract hst of m1 consists of a prograding skeletal grainstone that was subaerially exposed upramp sequence m2 which contains the second order maximum flooding surface has abundant subtidal cherty carbonate at its base which shallows upward into a widespread prograding coral packstone grainstone in the hst sequence m3 also contains abundant downramp chert that passes upramp into an aggrading crinoidal shoal and farther upramp into peritidal mudstone sequence m4 records an extensive basinward shift in facies as peritidal burrowed and cryptalgalaminated mudstone prograded over subtidal carbonate sequence m5 is only locally developed downramp and consists of crinoidal grainstone with abundant evidence of subaerial exposure a regional unconformity separates the montoya group from the silurian fusselman dolostone or younger units parasequences meter scale cycles recording low to moderate amplitude relative sea level fluctuations are ubiquituous features at individual outcrops but are difficult to correlate regionally the abundance of syn or early depositional chert in the subtidal facies indicates that the montoya group was deposited within a region of strong regional upwelling along southern laurentia this early formed chert was the reservoir facies in a successful upper ordovician gas play in ward and reeves counties texas

nature and regional significance of unconformities associated with the middle ordovician hagan k bentonite complex in the north american midcontinent stratal patterns of the middle ordovician hagan k bentonite complex and associated rocks show that the black river trenton unconformity in the north american midcontinent formed through the complex interplay of eustasy sediment accumulation rates siliciclastic influx bathymetry seawater chemistry and perhaps local tectonic uplift the unconformity is diachronous and is an amalgamated surface that resulted from local late turinian lowstand exposure followed by regional early chatfieldian transgressive drowning and sediment starvation the duration of the unconformity is greatest in southern wisconsin northern illinois and northern indiana where the deicke and millbrig k bentonite beds converge at the unconformity on the basis of published isotopic ages for the deicke and millbrig beds it is possible that in these regions erosion and nondeposition spanned a period of as much as 3 2 m y two broad coeval depositional settings are recognized within the north american midcontinent during early chatfieldian time 1 an inner shelf subtidal facies of fossiliferous shale spechts ferry shale member and ion shale member of the decorah formation and argillaceous lime mudstone and skeletal wackestone guttenberg and kings lake limestone members extended from the canadian shield and transcontinental arch southeastward through minnesota wisconsin iowa and missouri 2 a seaward relatively deep subtidal sediment starved middle shelf extended eastward from the mississippi valley region to the taconian foreland basins in the central and southern appalachians and southward through the pericratonic arkoma and black warrior basins in the inner shelf region the black river trenton unconformity is a composite of at least two prominent hardground omission surfaces one at the top of the castlewood and carimona limestone members and the other at the top of the guttenberg and kings lake limestone members both merging to a single surface in the middle shelf region the inner and middle shelves redeveloped later in approximately the same regions during devonian and mississippian time

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

chemical correlation of k bentonite beds in the middle ordovician decorah subgroup upper mississippi valley strata of the champlainian middle ordovician decorah subgroup in the upper mississippi valley region have been correlated on the basis of the chemical composition of k bentonite beds in widely distributed outcrops and cores the four principal k bentonite beds in the decorah the deicke millbrig elkport and dickeyville can be differentiated by their unique chemical fingerprints which were established using a linear discriminant function analysis the elements that served as the best discriminators of differences between beds were in order of atomic number na sc ti zr sm eu tb dy yb lu hf ta and th although no one element serves to delineate a k bentonite bed completely from others a combination of elements can do so the chemical signatures of the deicke and millbrig k bentonite beds the two thickest and most widespread k bentonites in the mississippi valley were recognized in outcrop and subsurface from southern minnesota to southeastern missouri a distance of about 900 km the elkport and the dickeyville k bentonites were chemically identified in a limited area in northern illinois southwestern wisconsin and northern iowa the decorah consists of widespread lithologic units that are approximately parallel to k bentonite beds in some areas but in other areas lateral gradation of lithologies as shown by k bentonite correlations indicates contemporaneity of decorah lithofacies

biotic response to a middle ordovician paleoceanographic event in eastern north america global paleontological events such as mass extinctions have been the focus of attention in recent years yet regional biotic events account for a greater proportion of extinction in the fossil record and should receive greater attention if we are to fully understand extinction processes over the phanerozoic we use a sequence stratigraphic framework to document a major extinction and migration episode that occurred in the middle and late ordovician of north america this event is characterized in eastern north america by coordinated changes in faunal assemblages carbonate lithologies siliciclastic influx and phosphatization we ascribe these changes to a regional paleoceanographic event brought on by thrust driven flexural subsidence and sea level fluctuations that lowered water temperature and increased turbidity and nutrient input

characteristics of weathering zones of granitic rocks in malaysia for geotechnical engineering design tropical climatic conditions cause severe weathering to rock masses resulting in thick weathering profiles consisting of different weathering zones with complex characteristics feasible geotechnical engineering design requires a detailed understanding of the characteristics of each weathering zone a typical weathering profile is usually characterized by field scale observation and geological study this paper presents the characteristics of weathering zones of various granitic rocks in the south of malaysia this region has a tropical climatic condition the study was carried out in four active quarries located in johor province malaysia and forty sections were investigated in the study areas a typical granitic weathering profile was characterized based on the geological and structural parameters i e joint characteristics corestone occurrence rock soil ratio mass homogeneity color of rock and discoloration at joints surfaces the profile includes common zones ranging from fresh to residual soil however two subzones are introduced based on corestone boulder occurrence in the highly weathered and completely weathered zones this study based on detailed field observations at the mentioned quarries followed by analysis presents an amalgamated picture of weathering zones in granitic profiles of a tropical region like malaysia moreover it is believed that the characterization of the typical weathering profile allows further classification of generally complex weathered rock masses for a specific geotechnical engineering design