late archean quetico accretionary complex superior province canada the 1200 km long quetico subprovince consists of monotonous metagraywacke with derived migmatite and granite in thrust and or transcurrent fault contact with the adjacent wabigoon and wawa metavolcanic subprovinces imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10 100 km wide stratigraphically north facing pile thermal relaxation in the 20 m y following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low pressure metamorphism

compositional variations in calciturbidites due to sea level fluctuations late quaternary bahamas piston cores in tongue of the ocean bahamas revealed an alternating sequence of periplatform ooze and bankderived turbidites of the past 5 glacial and interglacial periods by using the point count method we have analyzed the composition of the turbidite sediment variations in the bank derived fraction are clearly linked to the glacial interglacial cycles nonskeletal components pellets ooids grapestones are most abundant in interglacial turbidites while skeletal components calcareous algae peneroplids and reef builders dominate in glacial turbidites we attribute this pattern to the exposure and flooding of the surrounding carbonate platforms during late quaternary sea level cycles because nonskeletal sediment is produced in the interior of the platforms its growth and subsequent export to the flanks are at a maximum when the banks are flooded i e during interglacial periods during glacials skeletal grains dominate because the banks are exposed and carbonate production is limited to a narrow belt of skeletal sands and fringing reefs the resulting compositional signal appeared to have a good correlation with aragonite variations in the periplatform ooze between the turbidites further compositional variation parallels the change in turbidite frequency described earlier highstand bundling frequency and composition of turbidites are both unaffected by diagenesis and therefore provide an excellent monitor of sealevel fluctuations

facies changes in the triassic jurassic boundary interval in an intraplatform basin succession at csővar transdanubian range hungary a continuous pelagic marine triassic jurassic boundary section is exposed in newly excavated trenches in the surroundings of cso var ne of budapest hungary in the late triassic this area was located close to the offshore margin of the dachstein carbonate platform system that was segmented by intraplatform basins based on detailed facies analysis of the rhaetian hettangian platform foreslope basin succession a long term second order and superimposed shorter term third and fourth order changes of the relative sea level could be revealed after a period of highstand platform progradation in the late norian large amounts of larger plant fragments and sporomorphs of continental plants mark a significant sea level drop in the early rhaetian presumably exposing large parts of the platform a renewed transgression led to the formation of smaller build ups fringing the higher parts of the previous foreslope crinoid meadows may have occupied the slopes a potential source area of the bioclastic carbonate turbidites rising relative sea level that followed the marked early rhaetian lowstand is also reflected in the general facies trend from lithoclastic debris flows and proximal to the very distal turbidites and radiolarian basin facies up to the earliest hettangian meter scale high frequency probably fourth order deepening upward cycles probably indicating sea level oscillation could also be recognized within this interval at the base of the last rhaetian cycle about 10 m below the assumed tr j boundary the amount of bioclasts including conodonts drastically decreases biotic decline in the radiolarian basin facies characterizing the topmost part of this cycle a significant negative shift in both the d 13 ccarb and d 13 corg and a marked decrease in the organic material toc was recognized that can be interpreted to reflect reduced productivity in connection with the triassic jurassic boundary event it is overlain by a lithoclastic horizon then by laminites where the last conodonts were found the next significant facies change in the early hettangian is marked by appearance of redeposited oncoid grapestone beds suggesting survival of shallow marine carbonate factories in the neighborhood of the basin and the end of the rhatian to earliest hettangian sequence third order the recurrence of basin facies above this interval indicates a new short term third order transgression and continuation of the long term second order deepening trend in the early hettangian d 2004 elsevier b v all rights reserved

pelagic sedimentation on an inactive gullied slope northwest providence channel bahamas abstract the southern flank of little bahama bank is a deep water carbonate slope into which a series of closely spaced canyons 10 200 m deep have been cut it is similar in morphology to many other slopes in the bahamas and these slopes are generally considered to be created by the erosion of downslope sediment gravity flows however little evidence for downslope sediment movement or associated erosion were found in the holocene or latest pleistocene sediments of the slope reported on here the sediments from water depths below 260 m consist of a uniform pelagic drape covering both the gullies and inter gully ridge tops high resolution seismic reflectors are not truncated along individual gully walls and the fine scale stratigraphy of the sediments recorded in glacial interglacial changes in the aragonite calcite ratio of the sediments is preserved in sediments from gullies and ridge tops it appears that for the last two to three glacial cycles there has been only minor downslope transport of sediment despite the gullied morphology erosion is not presently occurring on the slope

stratigraphic response of a carbonate platform to relative sea level changes broken ridge southeast indian ocean in contrast to terrigenous margins the stratigraphic response of carbonate platforms to relative sea level changes is influenced by carbonate productivity dissolution and diagenesis using seismic reflection and ocean drilling program odp drilling data from broken ridge we assess the importance of these processes in controlling the stratigraphic response of a carbonate platform to relative sea level changes prior to middle eocene rifting broken ridge and kerguelen heard plateau constituted a large carbonate platform odp drilling data from the kerguelen heard plateau leg 120 and broken ridge leg 121 reveal that the carbonate platform had two major episodes of tectonic uplift the early maastrichtian uplift was confined to the southern section of the kerguelen eard plateau sites 747 748 and gently tilted the platform toward the north the middle eocene rift induced uplift affected both broken ridge and the northern section of the kerguelen heard plateau sites 752 755 watergun seismic reflection and drilling data indicate the following stratigraphic response of the platform to relative sea level changes 1 prograding clinoforms consisting of carbonate sediments were deposited during a long term relative sea level rise submergence of the uplifted parts of the platform southern kerguelen heard plateau increased the area available for productivity allowing the carbonate sediment to prograde northward into the basin during the rise 2 onlapping sequences consisting of carbonate detritus were deposited in the basin during the middle eocene relative sea level fall the carbonate strata on broken ridge were uplifted to at least wavebase and eroded the detritus was transported northward toward the basin and onlaps the preexisting strata the approx mate volume balance between the sediment eroded from the crest of broken ridge and the onlapping sediments deposited along the northern slope suggests that mechanical erosion and transportation of carbonate sediment into the deep sea were important processes controlling the stratigraphy at broken ridge during the relative sea level fall we propose that the stratagraphic response of broken ridge to the middle eocene relative sea level fall is consistent with stratigraphic predictions based on terrigenous margins however during a relative sea level rise biogenic carbonate production may allow carbonate sediment to prograde basinward despite the fact that water depths are increasing stratigraphic models based on terrigenous margins do not account for the increase in sediment supply along carbonate margins during a relative sea level rise and thus require minor modification in order to predict the development of the prograding clinoforms

2 5 b y of punctuated earth history as recorded in a single rock deciphering the absolute chronology of ancient crustal rocks is often problematical because of isotopic reequilibration and or mixing during multiple geologic events ion probe u pb age data from texturally distinct zircon crystal domains in a single sample of high grade gneiss from the nimrod group in antarctica reflect successive resorption and crystallization under disequilibrium conditions our data indicate that archean crust of the east antarctic shield initially formed from juvenile magmas ca 3000 ma extends to the central transantarctic mountains successive zircon growth ca 2955 1720 and 530 ma records a sequence of distinct widely spaced high temperature metamorphic and or anatectic events related to archean paleoproterozoic and eocambrian orogenesis

sea level highstand and lowstand shedding related to shelf margin aggradation and emersion upper eocene oligocene of maiella carbonate platform italy abstract sequence stratigraphic analysis of the upper eocene lower oligocene of the maiella carbonate platform allows the timing of shelf aggradation and erosion to be linked to shedding onto the slope and relative sea level stands the shelf was eroded during lowstands and the lowstand slope sediments are thin channelized lithic breccias with clasts eroded from an emerged reef tract highstand shedding of skeletal sands from the inner shelf resulted in thick coarsening and thickening upward successions of turbidites

simultaneous pure and simple shear the unifying deformation matrix abstract any simultaneous combination of finite simple shear and finite pure shear is a linear transformation which can be expressed as a single transformation matrix for two dimensions the matrix is upper triangular with an off diagonal term called the effective shear strain is a simple function of the pure and simple shear components for three dimensions a simultaneous combination of thrusting in the x direction thrusting in the y direction and a wrench in the x direction in addition to 3 orthogonal components of coaxial strain can also be represented by a 3 3 upper triangular matrix here three off diagonal terms xy x y and y z occur xy is a simple function of the horizontal coaxial strain values and thrusting in the x direction yz depends on the coaxial strain components in the y and z directions and the thrusting in the y direction while xz is related to all six strain components the matrix also allows for volume change either homogeneously or preferentially in a single direction a method of decomposing the deformation matrix into a series of incremental deformation matrices where each incremental deformation records the same kinematic vorticity number as the finite deformation is shown the orientation and magnitude of the finite strain ellipsoid ellipse is easily and accurately found at any increment during the deformation

assessment of the relationship between emplacement of the algoman plutons and regional deformation in the rainy lake region ontario the algoman suite of granites and granodiorites located within the rainy lake region of the superior province intruded the wabigoon quetico subprovince boundary about 2686 ma deformation occurred during the late archean collision between the wabigoon and wawa volcanic arcs the algoman plutons are the youngest rocks in the region their apparent lack of macroscopic deformation fabrics has led previous researchers to use their age as a constraint for the cessation of regional deformation however contrasting competence between the relatively rigid granites and surrounding volcanic and metasedimentary rocks suggests that obvious deformation fabrics may not be observed in the plutons even if they were present during deformation a field anisotropy of magnetic susceptibility ams gravity inversion model and microstructural study were undertaken on two neighboring algoman plutons to determine their emplacement history relative to the regional deformation there is a steep macroscopic foliation in the adjacent rocks and in parts of the plutons the magnetic foliation is very strong and roughly coincident with the field foliations the gravity inversion models indicate that the plutons are relatively shallow 1 km and 4 5 km at their deepest points and their shapes are consistent with emplacement during dextral transpression quartz microstructures such as undulose extinction and subgrain formation indicate that there was some solid state deformation accommodated by dislocation creep mechanisms the field ams gravity and microstructural evidence indicate that the plutons were most likely emplaced syntectonically thus the pluton age does not provide a conclusive constraint to the cessation of regional deformation

decompression reactions and p t conditions in high grade rocks northern labrador p t t paths from individual samples and implications for early proterozoic tectonic evolution

late proterozoic to early palaeozoic tectonism in dronning maud land antarctica supercontinental fragmentation and amalgamation the mid proterozoic early palaeozoic tectonic development of western dronning maud land is examined in the light of recent proposals that suggest geological continuity between laurentia and gondwana during the middle proterozoic the sweat hypothesis gondwana amalgamation from the remaining constituents followed fragmentation and rifting of laurentia from this supercontinent culminating in a late cambrian orogeny at c 500 ma the mid proterozoic early palaeozoic tectonic development of western dronning maud land was significantly different to that observed in the transantarctic mountains and the c 500 ma event in particular is characterized by 1 a regional isotopic resetting event at c 470 ma 2 scarce coeval magmatism comprising only one major intrusion of alkaline affinity and volumetrically small s type granitic sheets derived from local anatexis of gneisses 3 structural deformation considerably less intense than during the preceding kibaran 1100 ma orogeny comprising relatively open to gentle folding 4 reactivation of thrusting initiated during the kibaran orogeny with associated metasomatism it is concluded that the effects of the late cambrian c 500 ma event are consistent with crustal thickening through large scale underplating rather than subduction related processes as seen in the ross orogeny of the transantarctic mountains it is also concluded that gondwana amalgamation occurred along suture lines not currently exposed in outcrop in dronning maud land these data neither contradict nor support the sweat hypothesis as currently argued

controls on grain size patterns in periplatform carbonates marginal setting versus glacio eustacy the importance of grain size as a fundamental property controlling other physically derived properties is well known recent grain size studies on periplatform sediments of the caribbean and western north atlantic have shown that grain size variability is coupled with changes in carbonate platform productivity and export in response to pleistocene holocene glacio eustacy these findings showed that periplatform sediments are dominated by fine grained material during sea level highstands and coarser material during sea level lowstands this paper addresses the question of grain size patterns in periplatform settings and their relationship to glacio eustacy furthermore it considers whether these patterns are applicable to different types of margin settings accretionary erosional here existing data pleistocene holocene from the accretionary western leeward margin of great bahama bank odp leg 166 and new data from the erosional eastern windward margin odp leg 101 are used the results show that the grain size patterns of both margins are controlled by late neogene sea level fluctuations the accretionary leeward margin sediments indicate the same grain size pattern as previously described in the literature however the erosional windward margin shows a reverse pattern with coarser sediments in interglacials and finer deposits in glacials this proposes that the leeward margin grain size pattern is controlled by the productivity export mode of the platform this is indicated by the platform top derived fine muds transported off bank to form a thick sediment wedge on the low angled slope during interglacials in contrast on the windward margin the sediment flux pattern is controlled by the impact of mass transport processes this is indicated by the increased occurrence of turbidites during interglacials that might be more frequent at this time because the steep angled erosional slope is less apt for the deposition of excess fine material this material is eroded and or bypasses the slopes and is redeposited in the periplatform basin setting

glacial interglacial differences in circulation and carbon cycling within the upper western north atlantic we investigated glacial interglacial changes in the circulation and carbon cycling in the western north atlantic subtropical gyre using hydrographic data and downcore records of the stable isotopic compositions of individual shells of bahamian benthic foraminifera potential temperature salinity depth relations show that modern thermocline 200 1000 m and deep 1000 2000 m waters in the providence channels bahamas originate in the sargasso sea and are typical of the subtropical gyre gradients in the stable isotopic compositions of late holocene planulina and cibicidoides species from the bank margins 400 to 1500 m depth reflect temperature nutrient and isotopic gradients of modern subtropical gyre waters the difference between the 18o of glacial maximum and late holocene foraminifera is 2 1 for the upper 900 m of the water column and 1 6 for deeper waters indicating that these waters were 4 c and 2 c cooler respectively during glacial time the glacial temperature gradient dt dz was similar to today while the base of the thermocline was 100 m shallower these results differ significantly from our earlier results from multiple shell 18o analyses which implied upper thermocline waters were only 1 c cooler and dt dz was greater during the glacial maximum the difference occurs because bioturbation adversely affects multiple shell analyses of glacial aged samples from shallow water depths at all depths above 1500 m foraminiferal 13c are greater during the glacial maximum than the late holocene by at least 0 1 to 0 2 as much as 0 6 in the lower thermocline indicating that nutrient concentrations throughout the thermocline were reduced and there was no oxygen minimum zone during the glacial maximum this suggests greater more uniform ventilation of the thermocline results of single and multiple shell 13c analyses of glacial age foraminifera compare favorably because samples most affected by mixing correspond to water depths where the glacial interglacial change of 13c was small cooler upper ocean waters during the glacial maximum reflect cooler temperatures at the ocean surface where isopycnal surfaces outcrop including large areas of the subtropical ocean a shallower thermocline base is consistent with southward migration of the northern edge of the subtropical gyre or increased mode water production enhanced thermocline ventilation is consistent with more vigorous winds and all isopycnal surfaces outcropping in the area of ekman downwelling

zoophycos distribution and sequence stratigraphy examples from the jurassic and cretaceous deposits of southeastern france the trace fossil zoophycos is characteristic in certain jurassic and cretaceous basinal deposits of southeastern france these deposits are cyclic marl limestone alternations in which depositional sequences can easily be recognized two kinds of sequence have been studied the marl limestone couplet and the third order sequence detailed observations show that zoophycos has a particular position in the sequences in the marl limestone couplet the trace is concentrated towards the upper part of the calcareous beds the burrow is also present in the lower part of the beds and at the top of the underlying marls but is less abundant the same kind of burrow distribution occurs in the third order sequences zoophycos is concentrated in the upper part of the calcareous bundles which are interpreted as lowstand systems tracts while it is totally lacking in the superimposed more marly sequence which probably corresponds to the lower part of the transgressive systems tracts the burrow may be present throughout the highstand systems tracts which overlie the maximum flooding surfaces but is less abundant the particular distribution of zoophycos is related to the fluctuations of certain environmental factors the most important of which are the carbonate productivity the sedimentation rate and the substrate consistency zoophycos can be proposed as a useful marker of sequence boundaries and of the transgressive surfaces in distal deposits where other signals for sequence analysis are often lacking

cambrian ordovician magmatism in the thiel mountains transantarctic mountains and implications for the beardmore orogeny new field and laboratory studies result in a redefinition of the three main lithostratigraphic units of the thiel mountains these are 1 the thiel mountains porphyry a massive hypersthene bearing monzonite faulted against 2 the mount walcott formation a sequence of shallow water volcaniclastic sedimentary rocks and dacitic tuffs or flows and 3 the reed ridge granites coarse grained biotite granite granodiorite stocks that cut the porphyry genetic relations between the porphyry and both the sedimentary rocks and the granite are proposed evidence for a phanerozoic age indicated by the presence of fossils in the sediments is reinforced by rb sr whole rock dating that has conclusively established a late cambrian or early ordovician age 502 5 ma for the entire sequence the stratigraphic and tectonic consequences refute all evidence for magmatism in the transantarctic mountains associated with the precambrian beardmore orogeny the age and status of which are now in doubt

high pressure metamorphism in mafic schist from northern vermont the only confirmed occurrence of glaucophane and omphacite in the appalachian mountains of new england is in mafic schist from tillotson peak 44 48 72 33 north central vermont mineral assemblages observed all with epidote sphene magnetite phengitic muscovite carbonate calcite or dolomite apatite sulfide are 1 actinolite glaucophane chlorite garnet quartz 2 actinolite glaucophane chlorite albite garnet 3 actinolite glaucophane omphacite garnet 4 glaucophane chlorite garnet quartz 5 glaucophane omphacite quartz 6 albite glaucophane chlorite quartz and 7 albite paragonite chlorite quartz electron microprobe investigations show that plagioclase chlorite sphene white mica carbonate and magnetite grains are relatively homogeneous however amphibole pyroxene epidote and garnet grains are zoned indicating incomplete equilibration and changing physical conditions during metamorphism glaucophane in assemblage 6 has the general formula rim composition na 1 9 m4 ca 0 1 mg 1 8fe 1 2 2 al 1 0 vi fe 0 3 3 si 7 9 al 0 1 iv o 22 oh 2 glaucophane in assemblages 1 to 5 is somewhat poorer in the glaucophane endmember and richer in actinolite ca rich amphibole grains are zoned extensively with barroisite cores and actinolite rims omphacite grains in assemblages 3 and 5 have the general formula rim composition na 0 5 ca 0 5 mg 0 4 fe 0 1 2 al 0 4 vi fe 0 1 3 si 2o 6 in spite of the significant zoning the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition correlation of mineral compositions and assemblages with mineral stability data suggests high pressure facies series metamorphism at about 9 2 kb and 450 100 c with p h2o p total it is proposed that the metamorphism occurred during the ordovician

palaeomagnetic and rock magnetic study of a deeply exposed continental section in the charnockite belt of southern india implications to crustal magnetisation and palaeoproterozoic continental nucleii abstract this study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south india metamorphosed to granulite facies close to the archaean proterozoic boundary 2600 ma at former depths of 20 35 km the metamorphism is characterised by extensive development of charnockite produced by flushing of co2 rich fluids through older crust rock magnetic studies thermomagnetic high and low temperature susceptibility hysteresis and irm show that very low ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types it has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes magnetic susceptibility shows little increase with temperature falling to zero at 550 c implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust induced magnetisations are able to explain 50 of observed magnetic anomalies of deep origin intensities of magnetisation and qn values are high especially in charnockite where metasomatism has resulted in new magnetite formation and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation the magnitude of anisotropy of magnetic susceptibility ams is also increased by charnockite formation as are degrees of lineation and foliation however directions of ams in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism no appreciable phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by a1 d i 280 8 a2 d i 319 11 and a3 d i 1 10 this swathe is interpreted as an uplift related cooling acquisition following charnockite formation at 2600 ma and a remanence age of 2600 2400 ma is implied by the absence of most of the swathe in early palaeoproterozoic and younger dyke swarms cutting the region a second group of component directions b mean d i 238 87 is similar to magnetisations found in dykes linked to an age of 2370 ma and interpreted as somewhat younger than the a swathe we argue that the entire magnetic record was acquired before final basement uplift prior to 2000 1800 ma collective late archaean early proterozoic palaeopoles from india define a low to high latitude movement of the shield at 2600 2300 ma which is replicated in contemporaneous results from the other ancient gondwana nucleii of southern africa and australia the cratonic nucleii of these three shields have the oldest laterally extensive supracrustal covers and comprise the core of the protocontinent of ur the 2900 2200 ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities but not configurations were similar within the palaeozoic supercontinent of gondwana

accommodation and supply a dual control on stratigraphic sequences abstract it is widely accepted that both eustatic and tectonically controlled regional changes of sea level have contributed to the record of stratigraphic sequences i suggest that environmental change be added as a third autonomous control sedimentologic principles clearly indicate that sequences and their systems tracts are controlled by the interplay of two rates the rate of change in accommodation space available for sedimentation and the rate of sediment supply sea level has direct control on accommodation but its influence on sediment supply is remote and easily overshadowed by environmental factors for instance the record of the most recent sea level rise is a transgressive systems tract where supply is low it is a prograding highstand systems tract in deltas where the supply is high examples of sequence boundaries generated by changes in sediment supply include tectonically driven shifts in sediment input into basins changes in ocean currents pulsating supply from failure of submarine slopes and drowning of carbonate platforms by environmental stress furthermore the stratigraphic sequences in fluviatile continental basins are physically removed from sea level induced changes in accommodation and must have formed by changes in the rate and pattern of supply subaerial exposure of marine sediments at the sequence boundary is a most important criterion for recognizing sea level cycles as opposed to supply cycles other criteria include downstepping of shelf breaks and characteristic patterns in the spacing of time lines within sequences some third order cycles ca 0 5 3 ma duration meet these criteria others do not cycle stacking patterns and the shifting facies belts on cratons indicate that many second and third order cycles lack pronounced exposure unconformities and represent gradual changes superimposed on more rapid shorter oscillations seismic data yield poor images of these gradational changes because they lack resolution seismic models of outcrops show that near the limits of resolution the seismic tool tends to portray rapid changes in facies and bed thickness i e lateral variations in supply as unconformities seismic unconformities do not necessarily correspond to unconformities in outcrop the succession of stratigraphic sequences is best considered a composite record of sea level and environmental change separating the various controls requires carefully planned experiments work on the oxygen isotope curve another proxy for sea level has set an example in this respect

lowstand carbonates highstand sandstones the sedimentary facies sediment dynamics and sequence architecture of modern high energy shelves in the mid and high latitudes are largely governed by wave abrasion processes cool water carbonates may form there if the influx and or net accretion of siliciclastics is kept at a minimum little dilution of the carbonate produced in situ is generally promoted by a wide epicontinental shelf subdued topography of the adjacent mainland the predominance of limestone outcrops and an arid climate the aforementioned requirements are rarely met and thus will automatically lead to the formation of mixed siliciclastic cool water carbonates such an example is found in the early to mid miocene lagos portimao formation algarve s portugal which formed on a narrow high energy shelf of the atlantic ocean that was bounded by a mountain range the sediments of the formation consist of fossiliferous sandstone fs shell beds and rhodolith blankets along strike the stratification of the formation is monotonous for tens of kilometres and well exposed in coastal cliffs whereas no outcrops of dip sections exist the bulk skeletal composition of the sediments is typical for the warm temperate climatic zone various endo and epibenthic bivalves bryozoans coralline algae echinoderms gastropods and large foraminifers heterostegina in some very rare beds a few isolated not framework forming specimens of zooxanthellate corals porites tarbellastrea indicate temporally elevated surface water temperatures close to the lower threshold of the coral reef ecosystem in sandstones the fauna is well preserved and burrowing bivalves are commonly found in life position in limestone beds the state of preservation of the grains ranges from intact to disintegrated and abraded specimens we infer an accumulation of the shell beds through winnowing of fine materials siliciclastic sand and carbonate mud at wave abrasion depth and concentration of calcareous skeletons associated with the subsequent attraction of new epibiota in a complex shell bed the vertical alternation of fossiliferous sandstone and shell beds and in phase variations of the photo index photic biota vs bryozoans and bryozoan index bivalves vs bryozoans is envisaged to document variations of water depth and sea level sandstone units built up when wave abrasion depth wad rose above the sea floor during tst and early hst whereas the shell beds formed during lst when the wad for sand intersected with the sea floor clastic sediments were probably brought on the outer shelf during early transgression and by longshore currents sea level signatures inferred in the mixed siliciclastic cool water carbonate shelf setting of s portugal therefore significantly deviate from conventional concepts of carbonate sequence stratigraphy which were developed for flat topped platforms successful interpretations of ancient mixed sequences must therefore take into consideration the processes of production concentration and accretion of the carbonate sediments

polyphase late paleozoic tectonothermal evolution of the southwestern meguma terrane nova scotia evidence from 40ar 39ar mineral ages 40ar 39ar incremental release ages of hornblende muscovite and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late paleozoic tectonothermal evolution for the southwestern meguma terrane regional d1 folding with cleavage formation under greenschist lower amphibolite facies m1 metamorphic conditions occurred at ca 400 410 ma and was followed by emplacement of a series of granitic stocks ranging in age between ca 375 and 315 ma these were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade these are locally superposed on m1 regional metamorphic assemblages and result in a complex isograd pattern 40ar 39ar mineral ages suggest episodes of contact metamorphism occurred at 1 360 375 ma possibly related to emplacement of the south mountain batholith or temporal equivalents 2 350 356 ma around the port mouton pluton and northeastern shelburne pluton 3 ca 315 325 ma near the wedgeport pluton and in

interpreting compositional variations of calciturbidites in relation to platform stratigraphy an example from the paleogene of se spain abstract in the sierra de aitana betic cordilleras southern spain a sequence of massive platform limestones outcrops study of these limestones indicates a late eocene age and has resulted in distinguishing four lithofacies associations several features suggest tilting uplift and partial exposure of this carbonate platform at the end of the eocene in a more basinward section this time interval is represented by calciturbidites the composition of which has been analysed microscopically the upper eocene turbidites are mainly composed of loose skeletal grains derived from the platform interior around the eocene oligocene boundary the composition changes abruptly and the lower oligocene turbidites mainly consist of platform derived limestone clasts and skeletal debris from the platform margin this compositional change is accompanied by a large decrease in volume of turbidite input it is concluded that both composition and amount of calciturbidites reflect the history of the adjacent carbonate platform this case study therefore provides new evidence for the mechanism of highstand shedding in carbonate depositional systems

great slave lake shear zone canadian shield reconstructed vertical profile of a crustal scale fault zone abstract the great slave lake shear zone gslsz of the northwestern canadian shield a 25 km wide early proterozoic transcurrent dextral mylonite zone is a type example of a crustal scale fault zone profile with time 1 1 the metamorphic grade decreased from granulite to greenschist facies 2 2 the locus of high strain narrowed and jumped laterally abandoning relatively older mylonites progressively younger mylonites represent both cooler temperatures and lower pressures thus the sequence of narrowing mylonite belts represents a series of progressively shallower seated sections through the gslsz 3 3 deformation evolved through the brittle ductile transition initially producing non dilational penetrative breccias followed first by discrete non dilational faulting and subsequently by dilational quartz stockworks developing crustal scale shear zones grow to an optimum width dictated by the requirement of minimum geologically reasonable strain rates for given conditions of wall rock rheology and displacement rate independent of variations in relative plate motion mature shear zones change width and migrate laterally as a function of inferred thermally activated rheological changes within the wall rocks

eustatic control of calciclastic turbidites abstract global changes in sea level can be recognized in seismic sections by establishing the relationship between sea level changes and the timing of potential deep sea carbonate reservoir facies of turbidity current and debris flow origin their occurrence may be predicted prior to drilling compilation of published data suggests that throughout the phanerozoic most calciclastic turbidites tend to correspond to lowering episodes of sea level calciclastic turbidites therefore should occur primarily in association with major uncorformities caused by lowstands of sea level the data also suggest a possible lowstand of sea level during the late cretaceous campanian maastrichtian

a proposed integrated multi signature model for peritidal cycles in carbonates abstract a new integrated multi signature model for meter scale peritidal carbonate cycles presented here is relevant to the interpretation of these cycles in the stratigraphic record meter scale shallowing upward micro facies cycles are the most common type of peritidal parasequence peritidal parasequences are a fundamental component of carbonate sequence stratigraphic models however the existence of meter scale cycles in the geological record has been questioned owing to the failure to prove statistically the existence of these cycles using data on micro facies and stacking pattern taken from previous studies nevertheless the shallowing upwards micro facies interpretive template remains widely used to identify these cycles and parasequences we propose that the sedimentary signatures of meter scale peritidal cycles vary systematically during the various phases of an underlying 3rd order relative sea level cycle although during specific phases of a 3rd order relative sea level cycle meter scale peritidal cycles may be characterized by shallowing upward units e g early transgressive systems tract during other periods the stratigraphic signal of cyclicity may be more cryptic and complex causing them to be overlooked e g late transgressive systems tract and highstand systems tract we also propose that there is a half cycle phase shift in periods of deposition and the formation of drowning surfaces relative to 4th to 5th order relative sea level cycles over the course of a 3rd order relative sea level cycle these points have ramifications for defining parasequences and using stacking patterns to determine peritidal cycle periodicity specifically parasequences defined using flooding surfaces identify sedimentary units that inconsistently partition 4th to 5th order relative sea level cycles at different stages of a 3rd order relative sea level cycle we propose testable new physical stratigraphic signatures for identifying meter scale cycles that have consistent genetic significance each cycle recording a relative sea level cycle characterized by a 4th to 5th order relative sea level rise and fall meter scale peritidal cycles are recorded by an ordered pattern of systematically varying micro facies signatures related to their position within a 3rd order relative sea level cycle the integrated multi signature model of meter scale peritidal carbonate cycles we propose here may assist in explaining the apparently contradictory evidence both for and against their existence in the geological record as a result of the detailed consideration in this paper of the nature of parasequences and their variations with 3rd order relative sea level changes a redefinition of parasequence is required and one is presented here

eustatic and tectonic controls on deposition of hybrid siliciclastic carbonate basinal cycles discussion with examples changes in sea level represent one of the dominant controls on basinal sedimentation adjacent to mixed carbonate siliciclastic sediment source areas sedimentary responses to sea level change of the siliciclastic and carbonate components of these hybrid systems commonly result in deposition of alternating siliciclastic and shelf derived carbonate basinal deposits such deposition is particularly pronounced adjacent to carbonate platforms where alternate immersion exposure of the flat platform top results in carbonate highstand deposition and siliciclastic lowstand deposition in contrast small to moderate amplitude sea level drops in distally steepened ramp settings may expose only the inner part of the gently sloping shelf resulting in lowstand deposition of mixed sil ciclastic allodapic footnote 3 carbonate sediments along active margins tectonic uplift and subsidence of sediment source areas act as additional major controls on deposition of siliciclastic carbonate basinal cycles two of the most important tectonic effects are 1 uplift of siliciclastic sediment source areas resulting in increased erosion and swamping of carbonate shelf sources and 2 exposure or immersion of shelf carbonate sediment source areas due to vertical tectonic movements most published seismic stratigraphic studies of sea level control on deep sea sedimentation focus on lowstand deposition of siliciclastic sediment and commonly neglect the contribution of allodapic carbonate sediment deposited during sea level highstands however the examples discussed in this paper indicate that highstand carbonate deposits may make up a significant proportion of many hybrid carbonate siliciclastic basinal sequences

comparative evolution of material flux through two dsdp successions from central atlantic and gulf of mexico preliminary results of a new approach to sedimentary dynamics in pelagic environments abstract a method for gaining knowledge of sedimentary fluxes is applied to early cretaceous sequences containing high frequency sedimentary cycles the cycles are used to determine the relative fluctuations of 1 the sedimentation rate and 2 carbonate and silicate terrigenous fluxes expressed as linear values proportional to the real fluxes visual and automatic image analysis of black and white core photographs have been used to calculate the masses of different materials deposited in equal time intervals and to determine the distribution of the material within lithological units of cores according to a scale of grey intensities characters of these distributions allow definition of a flux variability index assumed to be negatively correlated to ocean continent distance related itself to sea level at dsdp sites 534 western atlantic off florida and 535 gulf of mexico the valanginian hauterivian interval has been chosen for determining the relative variations of carbonate and silicate terrigenous linear fluxes their variability and their distribution within lithological cycles these data provide information about the relative importance of carbonate sources lands platforms and plankton and their evolution and of other forcings upon planktonic productivity such as supply of nutrients from the continents and upwelling by comparing the data in the two sites it is possible to detect common climate and eustasy and local factors tectonics and upwelling which have controlled the material fluxes compared with site 534 site 535 was supplied by more numerous active and close carbonate sources and less active but closer terrigenous sources at the two sites carbonate flux was controlled by common factors sea level being the most prominent one terrigenous flux was submitted to more local controls this new method for investigating pelagic deposits should be tested on other deep sea sedimentary sequences

magnetic fabric and pluton emplacement in a transpressive shear zone system the itaporanga porphyritic granitic pluton northeast brazil the itaporanga granitic pluton intruded into the lithological contact separating neoproterozoic pelitic metasediments from meso to paleoproterozoic rocks that constitute the cachoeirinha belt of northeastern brazil an aureole of pelitic gneisses containing sillimanite and andalusite indicates that emplacement occurred at 12 15 km depth the objective of this work is a detailed study of its magmatic structures mainly by means of anisotropy of magnetic susceptibility ams regularly spaced sampling 61 stations yielded a magnetic fabric map which shows two domains 1 a western lobe which has lineations close to vertical and e trending foliations that merge southwestward as the transcurrent igarassi shear zone is approached isz and 2 an eastern lobe showing se trending lineations associated with inward dipping magmatic foliation situated in the center of the lobe the magnetic fabric is mostly paramagnetic in origin i e related to crystallographic preferred orientation of biotite and hornblende a low susceptibility magnitude 0 4 10 3 si typically of the ilmenite series granites is found in porphyritic granites diorites or their hybrid products hysteresis data indicate that the ferrimagnetic contribution to the bulk susceptibility does not reach 30 the magma was emplaced into a chamber developed by local wnw ese extension induced by an overall n s shortening melt ascent may have been facilitated by the pre existence of a steep fault zone bounding crustal discontinuities the magma must have ponded along a mechanical barrier formed by the contact between phyllitic metapelites and its gneissic basement which controlled the final emplacement structures of the pluton

argon retentivity of hornblendes a field experiment in a slowly cooled metamorphic terrane hornblende from samples of amphibolite and granitic gneiss collected within a single outcrop in the central adirondacks yield significantly different sup 40 ar sup 39 ark sub k dates of 948 plus minus 5 and 907 plus minus 5 ma assuming that this terrane cooled slowly following high grade metamorphism and that the samples have experienced the same thermal history the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples the fe fe mg mn of the hornblende samples are 0 8 and 0 6 the higher ratio corresponding to the younger sup 40 ar sup 39 ar sub k date transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution but contain zones of fibrous phyllosilicates approximately 0 1 to 2 mu m wide parallelling 100 and 110 these alteration zones probably formed during post metamorphic cooling as a result of the migration of fluids through the hornblendes and are obvious pathways for argon escape from hornblende as these features are more abundant in the hornblende sample with the younger sup 40 ar sup 39 ar date and higher fe fe mg mn they may influence the argon blocking temperature by effectively partitioning the hornblende grains intomore diffusion domains of varying size biotite from the granitic gneiss yields an sup 40 ar sup 39 ar sub k date of 853 plus minus 2 ma with a mildly discordant step heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite plagioclase from the amphibolite yields a sup 40 ar sup 39 ar sub k integrated date of 734 plus minus 3 ma less

depositional framework and controls on mixed carbonate siliciclastic gravity flows pennsylvanian permian shelf to basin transect south western great basin usa mixed carbonate siliciclastic sediment gravity flow deposits of late pennsylvanian to early permian age are exposed in the death valley owens valley region of east central california the mexican spring unit constitutes the upper part of the keeler canyon formation and is characterized by turbidites debris flow deposits and megabreccias all of mixed carbonate siliciclastic composition the mixed composition of the keeler canyon formation provides an opportunity to link facies architecture to controls on depositional system development depositional relationships indicate that the deposits represent a non channellized base of slope carbonate apron system with inner outer and basinal facies associations these gravity flow deposits are characterized by repeated stacked small scale 100 m coarsening and thickening upward cycles contemporaneous outer shelf and upper slope deposits of the tippipah limestone are exposed at syncline ridge on the nevada test site the deposits consist of carbonate buildups directly overlain by cross bedded quartz rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf detritus was transported to the west down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the keeler canyon formation this style of deposition differs from point sourced siliciclastic submarine fan depositional systems however the keeler canyon system has lithofacies similar to some sandy siliciclastic turbidite systems such as the delta fed submarine ramp facies model which is a line sourced shelf fed system that is not supply limited the mixed clastic apron systems of the keeler canyon formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply relative sea level changes and or tectonic events interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system relative sea level changes serve two important functions 1 they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf and 2 shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles therefore an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level which is a characteristic of late palaeozoic sea level history

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

early cambrian reefs reef complexes and associated lithofacies of the shackleton limestone transantarctic mountains the shackleton limestone formed a carbonate platform that bordered part of the greater antarctic craton in middle and late early cambrian time in the holyoake range of the central transantarctic mountains this unit records deposition on a stable shelf on which flourished ecological reefs composed of microorganisms and archaeocyathans burrow mottled lime mudstone wackestone and packstone with patch reefs represent accumulation in shelf areas of relatively low to moderate energy thick ooidal grainstone units reflect deposition in higher energy shoals and as sand sheets that were associated with extensive reef complexes the framework of these reefs was principally the product of micro organisms by inference mostly cyanobacteria archaeocyathans constitute as much as 30 of some reefs but commonly they form less than 10 and are absent from some on the basis of microbial composition three reef types are recognized the first type is a renalcis boundstone that lacks archaeocyathans within these abundant upward directed thalii of renalcis formed a framework that trapped fine grained sediment the second type which forms the core of some larger reefs is composed of stromatactis bearing microbial boundstone the third yet most common reef type is variable in composition it is characterized by the presence of abundant epiphyton but may include archaeocyathans and the microbial microfossils girvanella and renalcis as well as cryptomicrobial clotted micrite in this type of reef frame building organisms typically constructed highly porous structures that had small interparticle and fenestral pores and large growth framework cavities as well as rare metre sized caverns within these spaces epiphyton and less commonly renalcis encrusted framework elements fine grained sediments accumulated and pervasive sea floor cements were precipitated boundstone fabrics in the shackleton limestone are highly complex with fabrics analogous to younger more metazoan rich reefs as well as deep water stromatactis bearing mud mounds the epiphyton girvanella archaeocyathan frameworks and stromatactis bearing boundstones both of which seemingly first appeared in the middle early cambrian are regarded as the precursors in structure composition and preferred hydrologic setting of the more extensive reefs and complex framework styles of later phanerozoic time

rapidity of freshwater calcite cementation implications for carbonate diagenesis and sequence stratigraphy abstract in less than 10 years subaerially exposed holocene oolitic sand on eleuthera island bahamas was completely case hardened by freshwater vadose calcite cementation this phenomenon demonstrates the rapidity of freshwater carbonate diagenesis and explains in part why carbonate platforms preferentially shed unconsolidated sediment during highstands and not during lowstands when they are subaerially exposed it further proves contrary to what the exxon conceptual sequence stratigraphic models imply that carbonate deposits do in fact respond differently to subaerial exposure than siliciclastic deposits case hardening of unconsolidated carbonate sediment in response to subaerial exposure and early near surface freshwater diagenesis helps explain why shallow water platform sediments are preferentially shed into adjacent basins during highstands of sea level and not during lowstands during lowstands rapid subaerial lithification of carbonate sediments retards their reworking by wind or water once lithified these deposits are further stabilized by vegetation and karstification inhibiting their transport as unconsolidated sediment into adjacent deeper water basins observations from eleuthera island also document the reciprocal processes associated with freshwater carbonate diagenesis calcium carbonate released by partial dissolution of aragonitic ooids was locally reprecipitated as pore filling low magnesian calcite cement this reciprocity is confirmed by the confinement of freshwater dissolution and cementation to the upper 10 20 cm of the subaerially exposed body of oolitic sand below this case hardened surface oolitic sands remained unconsolidated only very minor amounts of calcite cement were required for effective casae hardening

denudation rates thermal evolution and preservation of blueschist terrains the erosion of low temperature high pressure blueschist facies metamorphic rocks presents a number of problems if their rate of erosion is not sufficiently rapid then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages the problem has been investigated using a one dimensional two layer model to mimic an accretionary prism overlying oceanic lithosphere the results suggest that if no erosion takes place the maximum time that blueschist conditions may exist is about 26 m y if levels of radioactive self heating are high and approximately 100 m y if levels are very low if erosion takes place the maximum temperature reached by an element of rock is at a higher level in the earth s crust than its initial depth of burial rocks may re equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved this could result in the geotherm

mesoarchean and paleoproterozoic history of the nimrod complex central transantarctic mountains antarctica stratigraphic revisions and relation to the mawson continent in east gondwana abstract high grade metamorphic and igneous rocks originally mapped as the nimrod group represent the only known crystalline basement of the east antarctic shield exposed in the transantarctic mountains shrimp u pb age data from zircon show that this assemblage preserves multiple geologic events spanning 2 5 b y of archean to early paleozoic history culminating in thermomechanical reworking and active margin magmatism during the ross orogeny new age data from igneous and metamorphic rocks as well as from detrital zircons in metasedimentary quartzites refine the mesoarchean to paleoproterozoic history and indicate the need to abandon the stratigraphic term nimrod group and its five formations in its place we redefine only the igneous and high grade gneissic parts of the assemblage as the nimrod complex and all other metasedimentary rocks exposed in the miller and geologists ranges are included in a newly defined argosy schist within the nimrod complex the oldest layered gneisses represent magmatic mesoarchean crust formed between about 3150 and 3050 ma correspondence of u pb zircon crystallization ages with whole rock sm nd model ages indicates formation from juvenile mantle melts magmatism at ca 3100 ma was followed closely by high temperature metamorphism recorded by zircon crystallization at 2955 2900 ma likely due to high advective heat transfer and or thermal insulation of newly stabilized crust metaigneous units with ages of ca 2500 ma indicate a late neoarchean period of anatexis and or magmatism although the geologic context for these events is uncertain gneissic eclogitic and metaigneous rocks record an important period of deep crustal metamorphism thickening and magmatism between 1730 and 1700 ma referred to as the nimrod orogeny that may have played an early role in assembly of east gondwana cratons a new lithodemic unit argosy schist consists of interlayered mica schist quartzite amphibolite and calc silicate schist quartzites from the schist unit have two distinctive detrital zircon provenance characteristics one type has populations that correlate with dated gneissic and metaigneous units of the nimrod complex indicating a proximal provenance whereas a second type has detrital zircon ages resembling those from neoproterozoic beardmore group sandstones indicating a common broader east antarctic shield provenance their maximum depositional ages range widely from about 2000 to 900 ma leaving unresolved their age and geologic relationship to either the nimrod complex or beardmore group yet they comprise a metasedimentary assemblage that is distinctive from the nimrod complex recent findings of mesoarchean gneisses in the gawler craton geochronology from terre adelie and our new age data from the nimrod complex thus highlight the importance of ca 3 1 2 5 and 1 7 ga events in formation of the composite east antarctic shield adjacent to the modern transantarctic mountains correlative crustal units may extend to the southern prince charles mountains and shackleton range collectively this extensive crustal province referred to as the mawson continent represents an elongate belt of primary mesoarchean crust that experienced major reworking at ca 2 5 and 1 7 ga the latter period includes both high and low p t petrologic signatures providing evidence of crustal thickening and magmatism that may signify collisional processes related to paleoproterozoic cratonal amalgamation during formation of the nuna supercontinent

40 ar 39 ar mineral age constraints on the paleozoic tectonothermal evolution of high grade basement rocks within the ross orogen central transantarctic mountains the precambrian to early paleozoic orogenic history of the central transantarctic mountains is obscured by uncertainties in the regional extent relative intensities and tectonic significance of three or more events the nimrod beardmore and ross orogenies significant problems exist in correlation of structural styles structural geometry and lithotectonic units across major tectonic boundaries as well as in conflicting geochronological results in an effort to resolve these disparities 40 ar 39 ar ages were determined for hornblende and muscovite from several lithotectonic subunits of the nimrod group exposed in the miller range including amphibolitic gneiss ca 1 7 ga orthogneiss and metasedimentary mica schist these rocks display ductile deformation fabrics that formed prior to intrusion of ca 500 ma post kinematic granitic plutons hornblende concentrates from eight gneiss samples in the miller range display internally discordant 40 ar 39 ar apparent age spectra due to va

eclogite facies metamorphism preserved in tectonic blocks from a lower crustal shear zone central transantarctic mountains antarctica in the geologists and miller ranges of the central transantarctic mountains mafic and ultramafic tectonic blocks occur in an early cambrian amphibolite to granulite facies lower crustal transpressional shear zone mafic blocks consist of garnet clinopyroxene symplectite hornblende plagioclase quartz ilmenite orthopyroxene biotite mineral compositions in these blocks generally reflect equilibration under upper amphibolite to lower granulite facies conditions t 700 c p 8 12 kbar these conditions are the same as those indicated by peak syn kinematic assemblages in the enclosing shear zone tectonites mineral and textural evidence for an earlier higher pressure p 12 25 kbar eclogite facies metamorphism includes 1 plagioclase reaction rims around anhedral garnet 2 complex clinopyroxene intergrowths consisting of augite plagioclase quartz hornblende orthopyroxene interpreted as exsolved na rich clinopyroxene omphacite and 3 unusual magnesian staurolite atomic mg mg fe 0 58 inclusions in pyrope almandine garnet we suggest that the relict eclogite facies metamorphism records an earlier period of crustal thickening that occurred prior to formation of the transpressional shear zone perhaps as an early stage in ross orogenesis

depositional bias and environmental change important factors in sequence stratigraphy abstract differences among depositional systems here called depositional bias strongly influence sequence patterns siliciclastics and shallow water carbonates for instance shed most of their sediment during opposite phases of a sea level cycle lowstand shedding and highstand shedding respectively furthermore the two systems generate their own system specific relief on the sea floor disperse their sediment load along different avenues and differ in the way they are deactivated reefs and carbonate platforms can be drowned whereas siliciclastic deposition can be shut off and renewed at any depth as a consequence of these differences pronounced unconformities drowning unconformities develop where carbonate platforms are terminated and buried by siliciclastics the siliciclastic to carbonate transition tends to be more gradual drowned platforms and drowning unconformities appeared world wide in great abundance in the miocene cretaceous valanginian turonian jurassic toarcian and devonian frasnian famennian examples of drowning unconformities interpreted as sequence boundaries include those of the early cretaceous platforms off new jersey and off morocco the mid cretaceous unconformity in the gulf of mexico and miocene unconformities on top of reefs in the far east the eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms either the rates of rise are an order of magnitude lower than the growth potential of platforms or cycle amplitudes are too small or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone the critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems thus drowning unconformities demonstrate the importance of environmental change as a control on sequence development other examples of dominantly environment controlled sequences include slope deposits shaped by shifting currents e g florida and blake plateau and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland e g gulf of mexico environmental change must be considered as a third independent factor that competes with eustatic and tectonically driven regional changes of sea level for control of sequences the definition of sequences and sequence boundaries should be broad enough to include the possibility of non sea level controls

anatomy of a modern open ocean carbonate slope northern little bahama bank the open ocean carbonate slope north of little bahama bank consists of a relatively steep 4 upper slope between water depths of 200 and 900 m and a more gentle 1 2 lower slope between depths of 900 and 1300 m the upper slope is dissected by numerous small submarine canyons 50 150 m in relief that act as a line source for the downslope transport of coarse grained carbonate debris the lower slope is devoid of any well defined canyons but does contain numerous small 1 5 m hummocks of uncertain origin and numerous larger 5 40 m patchily distributed ahermatypic coral mounds sediments along the upper slope have prograded seaward during the cenozoic as a slope front fill seismic facies of fine grained peri platform ooze surface sediments show lateral gradation of both grain size and carbonate mineralogy with the fine fraction derived largely from the adjacent shallow water platform near surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well lithified hardgrounds to patchily cemented nodular ooze to unlithified peri platform ooze controlled by lateral variations in diagenetic potential and or winnowing by bottom currents submarine cementation stabilizes the upper part of the slope allowing upbuilding of the platform margin and controls the distribution of submarine slides as well as the headward extent of submarine canyons where unlithified sediments are heavily bioturbated and are locally undergoing dolomitization upper slope sediments are also conditioned eustatically resulting in vertical cyclic sequences of diagenetically unstable aragonite and magnesian calcite rich and stable calcite rich carbonates that may explain the well bedded nature of ancient peri platform ooze sequences lower slope sediments have prograded seaward during the cenozoic as a chaotic fill seismic facies of coarse grained carbonate turbidites and debris flow deposits with subordinate amounts of peri platform ooze coarse clasts are internally derived from fine grained upper slope sediments via incipient cementation submarine sliding and the generation of sediment gravity flows gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge shaped apron of debris parallel to the adjacent shelf edge consisting of a complex spatial arrangement of localized turbidites and debris flow deposits a proximal apron facies of thick mud supported debris flow deposits plus thick coarse grained ta turbidites grades seaward into a distal apron facies of thinner grain supported debris flow deposits and thinner finer grained ta b turbidites with increasing proportions of peri platform ooze both the geomorphology and sedimentary facies relationships of the carbonate apron north of little bahama bank differ significantly from the classic submarine fan model as such a carbonate apron model offers an alternative to the fan model for palaeoenvironmental analysis of ancient open ocean carbonate slope sequences

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

possible significance of early paleozoic fluctuations in bottom current intensity northwest iapetus ocean sedimentologic and geochemical characteristics of red and green deep water mudstone exposed in the central appalachian orogen define climatically induced fluctuations in bottom current intensity along the northwest flank of the iapetus ocean in early and middle ordovician time red mudstone accumulated under the influence of moderate to vigorous bottom current velocities in oxygenated bottom water produced during climatically cool periods interbedded green mudstone accumulated at greater sedimentation rates probably from turbidity currents under the influence of reduced thermohaline circulation during global warming periods the close association of green mudstone and carbonate turbidites of early ordovician late tremadocian to early arenigian age suggests that a major warming phase occurred at this time increasing temperatures reduced bottom current velocities and resulted in increased production of carbonate sediment and organic carbon on the carbonate platform of eastern north america much of the excess carbonate sediment and organic carbon was transported into deep water by turbidity currents although conclusive evidence is lacking this eustatic event may reflect a climatic warming phase that followed the postulated glacio eustatic black mountain event subsequent middle ordovician fluctuations in bottom current intensity recorded by thin red green mudstone couplets probably reflect periodic growth and shrinkage of an ice cap rather than major glacial episodes

deposition of a muddy sediment drift in the southern straits of florida during the late quaternary abstract bottom currents associated with the florida current in large part control deposition of muddy biogenic sediments in the constricted deep sea passage of the southern straits of florida the unit is similar to muddy contourite drifts based on sediment structures changing thickness of biostratigraphic zones and distribution of biogenic sands in twenty piston cores current activity affected deposition more during the last glacial period than during the present and penultimate interglacial periods of the last 127 000 years based on the presence of an erosional regional unconformity that ended in the upper part of the glacial interval and on the change in pattern of silt and clay accumulation rates in the glacial interval relative to interglacial intervals

genesis of limestone megabreccias and their significance in carbonate sequence stratigraphic models a review a re appraisal of the factors controlling the deposition of carbonate megabreccias and their sequence stratigraphic significance is presented based on an examination of the mechanisms for instigating carbonate seafloor slope instability and a review of the depositional settings of these distinctive rock deposits limestone megabreccias have classically been interpreted as formed by the catastrophic collapse of high angle metastable oversteepened carbonate platform margins however a review of megabreccia occurrence shows that these deposits formed on a broad range of carbonate slope angles and that metastable oversteepened slopes are not necessary for their genesis a re evaluation constructed from first principles of mechanisms promoting submarine gravitational instability indicates that pore water overpressure of confined aquifer horizons beneath the seafloor rather than slope oversteepening is the critical control on megabreccia deposition catastrophic release of overpressure may initiate gravitational instability of previously kinematically stable slopes the relative importance of endogenic processes intrinsically linked to the depositional system and exogenic processes operating independently in generating megabreccias is assessed two particularly important mechanisms in generating megabreccias are endogenic processes causing overpressure at discrete hydrologically confined horizons beneath the seafloor during relative sea level falls and increases in stress as pore fluid drains from the sediment when the platform top becomes subaerially exposed during relative lowstands of sea level the increased likelihood of megabreccia genesis during relative sea level falls is supported by an empirical trend identified from a review of the stratigraphic distribution of documented megabreccias exogenic causes of gravitational instability with inherently random periodicity especially seismicity may account for the small number of documented megabreccias that do not conform to this underlying trend the proposed endogenic pattern favouring megabreccia deposition during relative sea level falls is important in a sequence stratigraphic context megabreccia deposits may build volumetrically significant toe of slope wedges and aprons during relative lowstands of sea level partially compensating for the decrease loss in the supply of fresh granular sediment as the platform top carbonate factory contracts lowstand wedges and aprons in carbonate systems therefore generally differ in composition from equivalent highstand deposits in being composed not of calciturbidite deposits but of megabreccias cannibalized from the upper slope

40 ar 39 ar mineral age record of early caledonian tectonothermal activity in the baltoscandian miogeocline central scandinavia in central portions of the scandinavian caledonides greenschist facies volcanosedimentary successions within the koeli nappe complex have been thrust several hundred kilometers eastward onto the baltoscandian platform these were derived from eugeoclinal terranes situated outboard west of the baltica continent during the early paleozoic the koeli nappe complex is tectonically underlain by higher grade units within the seve nappe complex these are composed of amphibolite and granulite facies rocks and locally contain eclogites the seve nappes tectonically separate koeli units from structurally lower allochthons derived from more inboard environments along the baltoscandian miogeocline previous mineral isotopic age determinations from seve and koeli units have been in the 430 to 390 ma range and have been interpreted to presumably date cooling following scandian middle silurian to early devonian metamorphism however incremental release sup 40 ar sup 39 ar dates recorded by minerals within some of the koeli and seve nappes exposed in jaemtland sweden taennforsen and are districts provide evidence of earlier tectonothermal activity hornblendes from the seve and koeli nappe complexes display variably discordant age spectra as a result of low temperature experimental evolution of loosely bound extraneous argon components however in most analyses plateau ages of 510 to 475 ma koeli and 465 to 455more ma seve are defined in contrast muscovite and biotite from all tectonic units record scandian cooling ages between 245 and 410 ma the older events recorded by hornblende within these seve and koeli units are evidence of early caledonian tectonothermal activity and subsequent diachronous cooling during the early middle ordovician less

search for eustacy record in deep tethyan deposits through the study of sedimentary flux variations application to the upper tithonian lower aptian series at dsdp site 534 central atlantic abstract a new method is proposed for determining the relative variations of sedimentary flux in alternating pelagic successions with a first application to 51 cores of the upper tithonian lower aptian interval at dsdp site 534 the bulk of carbonate in the sediments is deduced from the grey intensity analysis of black and white core photographs the average rate of sedimentation in cores is proportional to the number of marl limestone cycles over 5 cm thick counter within each of these intervals this relationship allows the expression of average fluxes based on the carbonate and terrigenous fractions present in the successive cores at the scale of the tithonian aptian succession the average carbonate and terrigenous fluxes of cores vary in parallel or opposite way at the scale of cores the fluxes are represented by the carbonate and terrigenous distributions within the color spectrum of corresponding sediments they vary in a subparallel way the sorting of the carbonate distribution is used to calculate an index of carbonate flux variability vfc for each core the relationships of this index with the sediimentation rate and the strontium content of deposits indicate a correlation between vfc and the relative sea level fluctuations the low sea levels deduced from this relation correspond to an increase of the carbonate flux at site 534 the comporison between the vfc and haq et al 1987 curves identifies periods where the carbonate flux was anomalously high upper tithonian berriasian or low lower aptian this method improved by using image analysis by computer will be tested on other ancient and recent series deposited at variable distances from continental margins it needs also additional geochemical and mineralogical data