glenelg river complex western margin of the lachlan fold belt or extension of the delamerian orogen into western victoria abstract low grade regionally metamorphosed slates phyllites and metagreywackes and higher grade low pressure high temperature metamorphic rocks in the glenelg river complex are structurally lithologically and sedimentologically similar to cambrian sequences in the south australian kanmantoo group peak metamorphic conditions 650 c 3 4 kbar occurred during the d2 deformation and coincided with synkinematic intrusion of cambro ordovician i and s type granites and pegmatite d2 folds are non cylindrical on all scales verge westwards towards the australian craton and include tight to isoclinal nw trending structures which have been variably rotated into parallelism with the d2 extension direction these folds and their accompanying high grade metamorphism post date low grade regional metamorphism and are inferred to have developed during west vergent thrust faulting and crustal thickening in common with other parts of the delamerian fold belt to the east the glenelg river complex is faulted against cambrian slates and an unconformably overlying sequence of late silurian earliest devonian siliciclastic sediments grampians group which have been intruded by 400 ma i type granites this fault marks the eastern limit of delamerian granite magmatism in victoria and possibly represents the boundary between the lachlan and delamerian fold belts an analogous crustal boundary lanterman fault zone identified in antarctica northern victoria land provides an important constraint on plate tectonic reconstructions of the southwest pacific sector of gondwana

evidence of a tectonically driven sequence succession in the middle ordovician taconic foredeep the late mohawkian through early cincinnatian ordovician trenton group carbonates and coeval siliciclastic succession of the central mohawk valley new york state were deposited in a foreland basin during the taconic orogeny development of this facies mosaic records changes in stacking patterns that took place during episodic basin deepening and simultaneous carbonate ramp shallowing along a ramp to basin transect located on the distal slope of the foreland basin differential rates of subsidence across the region produced accommodation rates that increased by an order of magnitude from west to east high rates of siliciclastic sediment supply and even higher subsidence rates resulted in a thick deep water succession in the eastern part of the study region the coeval trenton group succession in the west formed under conditions of moderate sediment supply and lower subsidence rate resulting in a net shallowing upward succession these relationships suggest that active margin tectonism was the proximate cause of the mohawk valley facies architecture

ordovician k bentonites issues in interpreting and correlating ancient tephras abstract prominent upper ordovician k bentonites include the deicke and millbrig beds in the late ordovician of eastern north america and the kinnekulle k bentonite in the uppermost sandbian of baltoscandia these beds are thought to represent some of the largest ash fall deposits known in the phanerozoic record this report presents a comprehensive study of biotite compositions from the three beds covering a far more extensive geographic range than has previously been described these data show that the kinnekulle and millbrig are multiple event ash beds some parts of which are indistinguishable from one another tectonomagmatic discrimination diagrams combined with mg number data indicate that the deicke millbrig kinnekulle sequence represents the progressive transformation from calc alkaline to peraluminous magmatic sources consistent with a model of progressively evolving magmatism during the closure of the iapetus ocean it is concluded that the millbrig and kinnekulle beds are coeval and represent primarily simultaneous episodes of explosive volcanism although it cannot be excluded that some portions of both beds were produced by the same event

δ13c stratigraphy of a turinian chatfieldian upper ordovician foreland succession ottawa embayment central canada resolving local and inter regional isotope excursions in a tectonically active basin abstract positive 13ccarb excursions are correlated through an upper turinian to lower chatfieldian carbonate platform succession along the axis of the ottawa embayment and into outliers of the northern ottawa bonnechere graben in central canada successive turinian excursions e1 and e2 are lithostratigraphically constrained by erosional surfaces and hosted within the watertown and overlying l orignal formations respectively the latter coeval with the selby formation in the adjacent northern appalachian basin the excursions coincide with periods of regional transgression but geographic patterns of 13c depletion versus enrichment coincide with structurally defined areas of stratigraphically condensed and preferentially thickened formation successions respectively differential subsidence is interpreted to have created bathymetric variation resulting in intrabasinal restriction of seawater exchange between these areas with preferential corg recycling with stratigraphic condensation by early chatfieldian time segmentation of the once regional carbonate platform l orignal formation produced a regional mosaic of low energy muddy carbonate banks rockland formation and a deeper water platform lower hull formation settings subject to fluctuating high to low energy current flow excursion e3 occurs in both successions but 13c enrichment is associated only with the bank top muddy facies this may identify preferential photosynthetic drawdown of 12c across the bank tops due to limited seawater exchange across the bank deeper platform boundaries excursions e1 to e3 and a younger excursion e4 in the hull formation are correlated with varying confidence with excursions across southern laurentia excursion e3 being the local expression of the guttenberg 13c excursion our study supports local modulation of regional if not global 13c excursions arising from structurally controlled changes in oceanography and productivity

magnesian calcite and chamositic ooids forming shoals peripheral to late ordovician ashgill muddy siliciclastic shores southern ontario in southern ontario ooids are associated with two distinct facies associations in the queenston formation the final stage of late ordovician ashgill taconic basin fill one facies consists of thin ooid and bioclastic grainstones interbedded with mudrock and lies near the base of the formation and in southwestern ontario also forms a local nw thickening wedge near the middle of the formation ooids have radial fibrous and radial concentric fabrics type a with chamosite illite and fe oxide laths at intercrystalline sites vertical lithologic and ooid abundance patterns indicate that thresholds to carbonate production were sensitive to changes in terrigenous sediment supply sea level circulation accommodation space and tectonism ooids in the second facies association are admixed with abraded fragments of open marine biota or occur burrow fills within a 30 cm thick interval of mudrock near the top of the preserved queenston succession a few metres below the ordovician silurian unconformity ooids have radial concentric and crosscutting patchy microcrystalline fabrics type b this unit may represent a transgressive or stillstand deposit modified by bioturbation the extent of preserved fabric suggests that both ooid types were originally magnesian calcite but type a ooids underwent greater burial alteration this is shown by crystalline mosaics that cross cut relict primary fabrics 13c values 1 82 to 0 67 and 18o values 4 46 to 10 57 more negative than marine calcite of similar age mn and fe concentrations more elevated above expected marine values and a luminescence similar to that of intergranular cements burial meteoric diagenesis was likely promoted by excellent permeability of the host sand we interpret authigenic chamosite and fe oxide to reflect diagenesis of iron bearing and clay detritus trapped during ooid growth type b ooids suffered less alteration 13c 1 1 to 6 64 and 18o 3 04 to 4 81 values overlap the expected marine range including 13c enrichment that occurs within the hirnantian latest ordovician excursion although mn and fe values are still higher than those of modern calcitic ooids negligible luminescence suggests that recrystallization occurred in the presence of marine derived pore fluids further burial alteration was inhibited due to low permeability of the host mud type a ooid facies in the queenston formation forms an ancient analogue for lesser known quaternary ooid shoals peripheral to tropical deltaic systems the facies of type b ooids while more enigmatic may preserve a geochemical herald of latest ordovician climate change the presence of minor chamosite in type a ooids defines a possible distal facies of the well known oolitic ironstones of similar age in the mid continental usa

phase relationships in buchan facies series pelitic assemblages calculations with application to andalusite staurolite parageneses in the mount lofty ranges south australia low pressure medium to high temperature buchan type regional metamorphism of pelitic rocks in the mount lofty ranges south australia is defined by the development of biotite staurolite andalusite fibrolite prismatic sillimanite and migmatite zones k feldspar makes its first appearance in the prismatic sillimanite zone and here we restrict our discussion to lower grade assemblages containing prograde muscovite concentrating particularly on well developed andalusitestaurolite parageneses in general the spatial distribution and mineral chemical variation of these assemblages accord with the predictions of petrogenetic grids and p t and t xfe pseudo sections constructed from the internally consistent data set of holland and powell 1990 in the system kfmash assuming a h2o 1 although analysed white mica compositions are systematically more aluminous than predicted importantly the stability ranges of most critical assemblages predicted by these grids and pseudo sections coincide closely with p t estimates calculated using the data set of holland and powell 1990 from the mount lofty ranges assemblages with the exception of mn in garnet and zn in one staurolite cordierite muscovite assemblage non kfmash components do not significantly appear to have affected the stability ranges of the observed assemblages an apparent local reversal in isograd zonation in which andalusite first appears down grade of staurolite suggests a metamorphic field gradient concave towards the temperature axis and together with evidence for essentially isobaric heating of individual rocks is consistent with the exposures representing an oblique profile through a terrain in which heat was dissipated from intrusive bodies at discrete structural levels

diachronous tectonic collapse of the ordovician continental margin eastern canada comparison between the quebec reentrant and st lawrence promontory the upper ordovician trenton group of southern quebec represents the last taconian foreland basin carbonate unit in the quebec reentrant prior to final collapse of laurentia s continental margin and its burial under synorogenic flysch the trenton group either conformably or unconformably overlies the black river group and is in turn conformably overlain by the utica shales the tripartite trenton carbonate unit records progressive deepening 1 very shallow to shallow subtidal 2 shallow to deep carbonate ramp and 3 shallow to deep outer shelf regional facies distribution lithotectonic elements and thickness variations indicate that the trenton shelf was dissected by extensional faults delineating blocks subsiding at various rates this scenario compares favourably with taconian foreland basin development in the middle ordovician table head group at the st lawrence promontory newfoundland a similar stratigraphic succession and tectono sedimentary history occurring 10 15 ma earlier at the s

oroclines in the tasmanides abstract oroclines are features of orogenic belts that exhibit curvature in plan on a map scale with the additional requirement that this curvature resulted from relative rotation of the limbs around a vertical or near vertical axis much debate has centred on contrasts between thin skinned and thick skinned systems and on the question of whether the structural fabric was imposed prior to or synchronous with the development of curvature leading to a variety of inconsistent terminology a simple hierarchy in which all map scale curved orogenic features which pass a test for rotation are termed oroclines as distinct from primary arcs and in which these are then subdivided by timing into progressive synchronous fabric and curvature and secondary pre existing fabric is then rotated and by mechanism into thin or thick skinned is applied to a suite of curved orogenic features in the tasmanides of eastern australia this analysis includes the first formal application of the orocline test palaeomagnetic or palaeocurrent directions plotted against structural strike to the tasmanides the neoproterozoic to early mesozoic tasmanides comprise four orogens each of which contains curved orogenic features which are highlighted by extensive aeromagnetic datasets the delamerian orogen developed on the gondwana margin exhibits two curved structures in the neoproterozoic to cambrian adelaide fold belt termed the fleurieu and nackara arcs previous structural interpretations have indicated that the nackara arc is non rotational and hence not oroclinal re examination of existing palaeomagnetic data including a positive palaeomagnetic orocline test shows the nackara arc to in fact be a hinge of the adelaide orocline curvature in the adelaide orocline was dominantly progressive and thin skinned but may have been influenced by secondary rotation of the pre existing partly underlying curnamona craton mega kinks in a buried delamerian arc volcanic chain revealed in aeromagnetic images follow the curvature of the nackara arc suggesting an accompanying thick skinned secondary oroclinal deformation further from the continental margin orogenic curvature in the palaeozoic lachlan orogen has only been widely recognised since the advent of tilt filtered aeromagnetic images and a model of oroclinal folding with two hinges riverina and tambo and strike slip offset during the silurian bindian orogeny has recently been proposed to explain perplexing repetitions of sequence and inversions of structural vergence and directions of sediment provenance palaeomagnetic data indicates rotation of elements within the proposed orocline and a preliminary study indicates a positive palaeomagnetic orocline test for the riverina hinge palaeocurrent data around the tambo hinge provide a positive if scattered orocline test an independently conceived numeric model of the roll back response to congestion of a subduction zone has generated an oroclinal structure which shares many features with the proposed lachlan orocline and deep crustal ambient noise seismic data confirms that the major curved and strike slip features persist throughout the crust the lachlan orocline appears to be secondary and thick skinned orogenic curvature in the largely covered thomson orogen occurs along its southern border with the lachlan orogen the putative olepoloko orocline and around its geometrically complex boundary with the mossman orogen in northern queensland the charters towers orocline rotation of the olepoloko structure cannot be confirmed and competing models for its formation leave open the question of whether this is a primary arc a thin skinned progressive orocline or a thick skinned secondary orocline palaeomagnetic data from the charters towers orocline are limited to two poles but a change in declination between the silurian pole and the devonian pole matches the apparent rotation of the orocline which appears to have been thick skinned and secondary in contrast to the other orogens consensus that at least part of the curvature in the palaeozoic to triassic new england orogen is oroclinal has been broad although not universal there remains substantial debate over the number of hinges from two to four and mechanism palaeomagnetic poles have previously been cited as evidence of rotation of blocks within the orocline but this paper presents the first formal palaeomagnetic orocline test which is positive for the manning and texas hinges a palaeocurrent orocline test of the manning hinge in younger rocks than the palaeomagnetic sample is negative constraining rotation in the southern manning hinge to the carboniferous before 322 ma while rotation in the northern texas hinge appears to be latest carboniferous or permian the existence of a hastings hinge is questionable but if real its rotation also appears to be younger than that of the manning hinge thick skinned secondary rotation of the hinges of the new england orocline appears to have been diachronous

a comparative analysis of pre silurian crustal building blocks of the northern and the southern appalachian orogen the new york promontory serves as the divide between the northern and southern segments of the appalachian orogen antiquated subdivisions distinct for each segment implied that they had lithotectonic histories that were independent of each other using new lithotectonic subdivisions we compare first order features of the pre silurian orogenic building blocks in order to test the validity of the implication of independent lithotectonic histories for the two segments three lithotectonic divisions termed here the laurentian iapetan and the peri gondwanan realms characterize the entire orogen the laurentian realm composed of native north american rocks is remarkably uniform for the length of the orogen it records the multistage neoproterozoic early paleozoic rift drift history of the appalachian passive margin formation of a taconic seaway and the ultimate demise of both in the middle ordovician the iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the iapetus ocean between laurentia and gondwana in the northern segment the realm is divisible on the basis of stratigraphy and faunal provinciality into peri laurentian and peri gondwanan tracts that were amalgamated in the late ordovician south of new york stratigraphic and faunal controls decrease markedly rock associations are not inconsistent with those of the northern appalachians although second order differences exist exposed exotic crustal blocks of the peri gondwanan realm include ganderia avalonia and meguma in the north and carolinia in the south carolinia most closely resembles ganderia both in early evolution and late ordovician silurian docking to laurentia our comparison indicates that to a first order the pre silurian appalachian orogen developed uniformly starting with complex rifting and a subsequent drift phase to form the appalachian margin followed by the consolidation of iapetan components and ending with accretion of the peri gonwanan ganderia and carolinia this deduction implies that any first order differences between northern and southern segments post date late ordovician consolidation of a large portion of the orogen

taconian orogeny in the new england appalachians collision between laurentia and the shelburne falls arc tectonic models of the ordovician taconian orogeny in western new england usually invoke a collision between the laurentian margin and a magmatic arc identified as the bronson hill arc however in central massachusetts and southern new hampshire rocks in the bronson hill arc are 454 to 442 ma and therefore younger than much of the taconian deformation and metamorphism in western new england and eastern new york which began by 470 ma u pb and single grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc the shelburne falls arc that formed west of the bronson hill arc at 485 to 470 ma the shelburne falls arc formed above an east dipping subduction zone by the early ordovician the taconian orogeny was the result of the collision between laurentia and the shelburne falls arc beginning ca 475 to 470 ma the younger bronson hill arc formed above a west dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the taconian orogeny the taconian orogeny ended when plate convergence between laurentia and iapetus was accommodated by the newly developed west dipping subduction zone instead of by crustal shortening in the taconian thrust belt the tectonic history of the new england appalachians is inconsistent with a middle ordovician collision between laurentia and the proto andean margin of gondwana

surface and deep ocean circulation in the subpolar north atlantic during the mid pleistocene revolution 1 we investigated surface and deep ocean variability in the subpolar north atlantic from 1000 to 500 thousand years ago ka based on two ocean drilling program odp sites feni drift site 980 55 29 n 14 42 w and bjorn drift site 984 61 25 n 24 04 w benthic foraminiferal stable isotope data planktic foraminiferal faunas ice rafted debris data and faunally based sea surface temperature estimates help test the hypothesis that oceanographic changes in the north atlantic region were associated with the onset of the 100 kyr world during the mid pleistocene revolution based on percentage of neogloboquadrina pachyderma s records from both sites surface waters during interglacials and glacials were cooler in the mid pleistocene than during marine isotope stages mis 5 and 6 in particular interglaciations at bjorn drift site 984 were significantly cooler faunal evidence suggests that the interglacial arctic front shifted from a position between the two sites to a position northwest of bjorn drift site 984 after ca 610 ka as during the late pleistocene we find faunal evidence for lagging surface warmth at most of the glacial initiations during the mid pleistocene each initiation is associated with high benthic 13c values that are maintained into the succeeding glaciation which we term lagging nadw production these findings indicate that lagging warmth and lagging nadw production are robust features of the regional climate system that persist in the middle to late pleistocene

precise u pb zircon dating of a post d2 meta dolerite constraints for rapid tectonic development of the southern adelaide fold belt during the cambrian numerous morb like mafic dykes plugs and sills intruded the kanmantoo group and older meta sediments of the southern adelaide fold belt from pre d1 to post d2 during the delamerian orogeny a post d2 meta dolerite yields a precise u pb zircon age of 510 2 ma this age combined with a published shrimp zircon age of 526 4 ma reveals a rapid tectonic evolution of the fold belt during the cambrian events including the thick kanmantoo group9s deposition major mafic magmatism low p high t regional metamorphism and multiple deformations all took place from about 526 to 510 ma this rapid evolution supports the suggestion that the heat source for the metamorphism originated from continental extension associated with mantle upwelling and mafic magmatism

early palaeozoic foreland thrusting and basin reactivation at the palaeo pacific margin of the southeastern australian precambrian craton a reappraisal of the structural evolution of the southern adelaide fold thrust belt abstract regional and detailed structural mapping kinematic analysis and balancing and restoration of cross sections has lead to a re interpretation of the adelaide fold belt in south australia in this belt sedimentary rocks of the late proterozoic adelaidean and early cambrian normanville and kanmantoo sequences were deposited during at least two episodes of subsidence related to major crustal attenuation contraction and crustal thickening accompanied by granitoid intrusions are related to the cambro ordovician delamerian orogeny during this event both basins were reactivated inverted and the sedimentary rocks are now incorporated in a wnw verging foreland fold and thrust belt at the margin of the proterozoic southeast australian craton maximal shortening of the orogen was around 55 within the adelaidean basin shortening is dominantly accommodated by major mylonitic shear zones and reverse faults in the cambrian kanmantoo basin several thrusts are demonstrably reactivated growth faults across which thickness changes of cambrian sedimentary rocks are revealed by balancing and restoration of cross sections owing to the steep easterly dips of these faults in the western part of the kanmantoo basin lateral shortening of up to 58 is largely accommodated by intense folding and fold axial planar flattening strain further east strain magnitudes wane and the overall shortening of around 30 is accommodated by both discrete thrust zones and regional folds the basin reactivation along the australian part of gondwana s palaeo pacific craton margin reflects the rapid transition from passive to active tectonism which possibly is a consequence of the disintegration of the formerly conjugate margins of gondwana and laurentia

the late ordovician dawson point formation timiskaming outlier ontario key to a new regional synthesis of richmondian hirnantian carbonate and siliciclastic magnafacies across the central canadian craton current paleogeographic reconstructions extend late ordovician taconic derived siliciclastics across the cen tral canadian craton prior to the terminal ordovician glacioeustatic lowstand revision of the late ordovician dawson point formation of the timiskaming outlier greatly reduces the distribution of these siliciclastics and documents a greater spread of shallow water carbonate of richmondian age as revised the dawson point formation contains two informal members a deep water graptolitic shale that grades upward into shallow water siliciclastic redbeds and an up per member of shallow water muddy crinoidal limestone with interbedded shale likely representing low energy shoals on a muddy shelf deep water shale accumulation began in the upper manitoulinensis graptolite zone following foun dering of the regional foreland carbonate platform basin development documents a northward younging 1 million years from southern ontario foreland basins in keeping with regional tectonic driven transgression along eastern north america the shale to carbonate succession of the dawson point formation correlates with the georgian bay forma tion on manitoulin island wherein the upper carbonate dominated divisions of both formations are equivalent to the siliciclastic queenston formation of southern ontario in absence of additional biostratigraphic information the upper member of the dawson point formation is likely richmondian or late ashgillian in age the revised late ordovician history of the timiskaming outlier may identify a once significant volume of shallow water carbonate across the central canadian craton with related sequestration of carbon dioxide possibly aiding global cooling erosion of the carbonate driven by developing glacioeustatic lowstand conditions was likely contemporaneous with early hirnantian peritidal de position of the uppermost queenston formation in southern ontario

late middle miocene volcanism in northwest borneo southeast asia implications for tectonics paleoclimate and stratigraphic marker abstract explosive volcanic events often produce pyroclastic materials that can be recognized from the geological record these discrete pyroclastics form regional marker beds here we report the occurrence of a tephra layer interbedded within very thick coal beds near mukah sarawak borneo traceable for tens of kilometers in the mukah area of sarawak this tephra layer can be considered as regional stratigraphic marker with precise chronostratigraphic control systematic sedimentological mineralogical geochemical and zircon u pb geochronological studies have revealed a major effusive volcanic event during the latest middle miocene presumably contemporaneous and or related to a magmatic event of an earlier phase of the mt kinabalu pluton or magmatism in west sarawak or east sabah the volcanic event had promoted catastrophic flooding of coastal swamps and fall out from the ash clouds that formed a regionally monotonous tephra layer across the serravallian tortonian boundary in conjunction with the regional occurrences of trap rocks structural trends and known tectonic events we constrained the regional depositional environments and climate the tephra layer was deposited in a coastal plain swamp seasonal shallow high moderate energy fluvial channel lacustrine environmental setting wherein atmospheric fallout and eroded material from regoliths formed over older basement and volcanic rocks of the hinterland which were mixed to produce the tephra layer this tephra layer is sandwiched between the very thick coal beds a pre existent volcanic chamber that was active for a long time also experienced periodic explosive activity from probably the same magma chamber and conduit and including a major explosive activity that recycled early formed crystals and felsic magma rhyolite dacite during the major effusive event are also recognized our findings provide robust evidence for the prevalence of intensive chemical weathering under a wet humid climate and relative tectonic quiescence before the major effusive event and the existence of vast monotonously gently sloping coastal plains and luxuriant vegetation akin to the present

the komandorsky basin as a product of spreading behind a transform plate boundary abstract a long term study of the komandorsky basin in particular by the cruises of r vvulkanolog and r vdmitry mendeleev in the summer of 1988 made it possible to describe in detail the main structural features of this marginal basin several ne and n s trending spreading axes were detected in the basin in the rear of the transform sector of the aleutian arc spreading axes are shifted relative to each other along nw striking transform faults there is variation in style from north to south along the strike of the spreading axes and a less well defined symmetry compared to that typical of mid oceanic ridges the tectonic pattern and heat flow reflect several stages starting from the eocene in the history of the opening of the basin caused by the changes in the velocity and direction of the relative motion between the pacific and north american plates

late ordovician chatfieldian catastrophic volcanism and abrupt carbonate platform interior subsidence a tectonic link across a taconian foreland basin quebec embayment due to inherited crustal weakness the late ordovician trenton group of new york state and equivalent strata in south central canada peripheral to the quebec embayment form a regional carbonate platform succession within the taconian foreland basin the upper and lower group boundaries record geodynamically controlled changes in regional subsidence whereas a higher order allocyclic stratigraphy traced south along the foreland interior of the eastern united states may identify shorter term eustatic controls a tectonomagmatic event extending across the foreland basin is recorded a few million years prior to the regional collapse of the foreland platform in the quebec embayment within the lower c spiniferus graptolite zone platform margin retreat 50 km in southern quebec is contemporaneous with a shale rich limestone succession that thins 30 10 m westward over 300 km into the ottawa embayment eastern ontario west of the appalachian structural front this stratigraphic record coincides with catastrophic volcanism within the once distant volcanic arc terranes as recorded by a bentonite traced geophysically from the platform margin to the michigan basin core from the ottawa embayment demonstrates western directed transport of lithic bearing ash the lithic material representing reworked deep water sedimentary and or volcaniclastic sediment was entrained by eruption of calc alkaline felsic to basaltic andesitic and trachyandesitic magmatic sources the above geological history documents a short period of diminishing influence of subsidence moving into the platform interior an event that enhanced briefly cross platform oceanographic circulation as recorded by elevated shale abundance and change in macrofaunal assemblages short lived differential subsidence of the continental crust in this region in response to distal tectonism may have been predicated by inherited weakness of a shallowly buried intracratonic 400km precambrian fault system manifest today by the ottawa bonnechere graben the resulting thin 10 to 30 m stratigraphic record of the tectonomagmatic event is of similar thickness to high order allostratigraphic patterns within the taconic foreland interior of the eastern united states because there is a heterogeneous distribution of precambrian fault systems underlying the taconian foreland interior of eastern laurentia differential structural response of the foreland interior can be anticipated the apparent regional allostratigraphy may contain subtle facies differences along the strike of the foreland basin that reflect this tectonic component

computer simulation of transport and deposition of the campanian y 5 ash analyses of grain size and modal composition of the campanian tuff ash layer y 5 from 11 deep sea cores have been carried out this layer represents ash fall that has been correlated with the 38 000 y b p campanian ignimbrite thunell et al 1979 a deposit formed by the largest eruption documented in the mediterranean region during the late pleistocene barberi et al 1978 the bulk deposit is bimodal in grain size and dominated by glass shards the calculated mean grain size of the coarse mode of the individual size distributions decreases with distance from the source and progressively approaches a near constant fine mode of approximately 13 microns distal samples are unimodal in grain size these data combined with a set of vertical profiles of wind 10 year average have been used as input to a computer model that simulates fallout of tephra modelling indicates that the downwind variation of grain size of the coarse mode can be accurately reproduced with transport of ash between 5 and 35 km the observed fine mode of the deposit cannot however be generated by transport of ash as individual particles at these elevations such transport would result in deposition of virtually all of the fine ash beyond the studied area deposition of fine ash within the studied distance of 1600 km from source can only occur by fallout as particle aggregates from a high eruption plume or as individual particles from co ignimbrite ash clouds with a maximum elevation of 3 km the large volume of ash in the fine mode 70 wt and the irregularity in azimuth of low level winds argue against major low level transport of co ignimbrite ash rather the ash may have been derived from both a plinian eruption column and high altitude clouds of co ignimbrite ash with settling of fine ash as particle aggregates

constraints on the accumulation of organic matter in upper ordovician lower silurian black shales from the lower yangtze region south china abstract the late ordovician to early silurian o s was an important period in which many significant geological events occurred during the o s transition organic rich black shales were widely deposited and became important unconventional energy sources paleoproductivity detrital input and redox conditions are the three major factors controlling organic matter om accumulation although the effect of each factor is relatively clear the complex influences of these factors remain poorly understood specifically the evolution of these factors and the corresponding influence on the accumulation of om during the o s transition has not yet been systematically investigated this paper investigates the constraints on om accumulation in o s transition black shales from the lower yangtze region based on graptolite fossil identification zircon u pb dating and geochemical analyses of a well sequenced drilling profile the results show that i the sy 1 core sequence was well constrained from graptolite zones wf2 447 62 ma to lm5 441 57 ma across the o s boundary with decreasing sedimentation rates srs in the upper katian wf2 to wf3 and increasing srs in the lower rhuddanian ii complementary volcanic inputs katian and rhuddanian and upwelling events hirnantian exerted strong control over paleoproductivity iii orogenic processes in south china controlled detrital inputs and redox conditions iv om accumulation in the lower yangtze area across the o s transition occurred in three stages in stage 1 wf2 wf3 strong volcanism boosted productivity whereas om preservation was not favored in the oxic environment in stage 2 wf4 lm3 upwelling and volcanism resulted in high productivity and provided the optimal configurations for om enrichment in stage 3 lm4 and later rare om was preserved due to terrestrial dilution and unfavorable preservation conditions eventually wf4 lm3 became the prominent om rich biozones due to the effective coordination of the controlling factors namely upwelling volcanism and redox conditions

oblique reflection of turbidity currents turbidity currents meeting obstacles for example the margins of a confined basin are subject to reflection the consequent change in flow direction is expressed in the sequence of depositional structures of the resulting bed of sediment putative examples of orthogonal reflection have been described based on 180 opposed current directions we present field evidence for the more general case of oblique reflection of turbidites and we report the results of flume experiments indicating a mechanism involving generation of internal solitary waves at an oblique ramp these propagate normal to the ramp regardless of the angle of incidence flow directions in reflected turbidites may indicate the orientation of reflecting surfaces such as basin margin slopes and thus may be of considerable help in paleogeographic and tectonic reconstructions

petrology and potential tectonic significance of a k bentonite in a taconian shale basin eastern ontario canada northern appalachians a 6 cm thick k bentonite herein defined as the russell bed occurs in an upper ordovician deep basin shale succession in eastern ontario canada forming part of the distal taconic foreland in eastern north america the bed lies within the pygmaeus graptolite biozone which is about 451 to 452 ma in age although some bentonites are reported from this interval in eastern north america we are reporting the first set of compositional data for a bentonite of this age gamma log correlation identifies a potential minimum distribution area of 5 km 2 for the k bentonite covering part of southern quebec new york state and eastern ontario the deposit coincides with the first influx of distal turbdites into this shale basin associated with taconic flysch and simultaneous abrupt ventilation of the once anoxic deep water basin which had formed initially after foundering of the upper ordovician carbonate platform concurrent intrabasinal extinction of several graptolite species suggests that change in sedimentation palaeoceanography and volcanism were linked to a regional external process compositionally the bentonite is distinct from the older ordovician platform deposits in eastern north america the deposit contains abundant titaniferous phlogopite with 1 6 bao fluoroapatite with 2 5 f and dynamically shaped glass spherules now altered to clay the spherules and clay matrix constitute 45 of the bed and compositionally define an illite 90 smectite i s structure with about 7 5 k 2 o age dating by ar ar analysis of the phlogopite crystals yielded a younger than expected 440 445 ma age this difference along with evidence of localized chloritization of phlogopite likely reflects known post ordovician hydrothermal activity within the basin on the basis of several geochemical proxies the magmatic source of the russell k bentonite falls within the trachyandesite field and was ba enriched comparison of geochemistry and mineralogy with older middle to late ordovician and younger early silurian k bentonites within the taconic orogen along eastern laurentia and baltica reveals that the potential source magma for the russell bed was more mafic more alkaline and less fractionated than sources typical of older platform bentonites instead it is more similar to the younger llandovery bentonites of scandinavia and scotland it remains uncertain if it signals local or regional compositional change in volcanic source in the northern appalachians

paleozoic intraplate escape tectonics in gondwanaland and major strike slip duplication in the lachlan orogen of southeastern australia intraplate tectonic escape along the eastern margin of gondwanaland in late silurian to middle devonian time was caused by the eastward movement of antarctica along a major intraplate tear resulting collisions caused a crustal block from the southern part of the lachlan orogen of southeastern australia to become detached from its lower lithosphere and escape to the northwest as a crustal flake this escaping block then became incorporated into the western part of the lachlan orogen as a separate terrane

basin dimensions and morphology as controls on amplification of tidal motions the early miocene north hungarian bay following the late aquitanian sea level fall tide influenced deposition started in the north hungarian bay an embayment in the paratethys open to the north east the relatively narrow funnel shape of the bay supported amplification of tidal movements resulting in the generation of strong tidal currents the length and the depth of the north hungarian bay and the connecting seaway through east slovakia fell into the tidal amplification window and thus fulfilled the conditions needed for resonant amplification of semidiurnal m2 tides tide influenced deposits were formed at both sides of the north hungarian bay they reflect dominant currents in opposite directions and of different strengths at either side of the basin this difference was the result of bottom tide interactions cyclonic anticlockwise residual currents were induced above the asymmetrical central depression in the bay and were superimposed upon the tidal currents producing an anticlockwise time and velocity asymmetrical current system the north hungarian bay and other examples show that amplification of tidal motions and formation of tide influenced deposits may occur if basin dimensions pass through the tidal amplification window this window represents ideal conditions for resonant or amphidromic amplification of tidal currents it determines an ideal length depth or width depth ratio relative to the wavelength of the astronomical tides thus signs of strong tidal influence in fossil basin fills can be used to reconstruct the dimensions length depth and width of such basins

comparative sequence stratigraphy of two classic upper ordovician successions trenton shelf new york ontario and lexington platform kentucky ohio implications for eustasy and local tectonism in eastern laurentia abstract comparison of the classic upper ordovician mohawkian to lower cincinnatian caradoc to lower ashgill black river and trenton groups in new york state southern ontario with the tyrone lexington formations exposed in the jessamine dome southern cincinnati arch in north central kentucky and southern ohio reveals striking similarities previous emphasis on complex local facies mosaic has obscured widespread regional patterns biostratigraphy and k bentonites provide broad constraints on inter regional correlations however an allostratigraphic approach permits higher resolution correlations and a partial test of eustatic vs strictly local tectonic models to explain stratigraphic patterns upper mohawkian to lower cincinnatian 455 449 ma depositional sequences previously recognized for the jessamine dome and nashville dome areas are correlated between the two main study areas and further refined chatfieldian rocklandian to shermanian or cobourgian of traditional terminology sequences m5 and m6 of previous workers are interpreted to be composite sequences and are each subdivided into three smaller scale sequences which also have counterparts in the new york ontario strata in turn these correlations indicate at least partial allocyclic control on sedimentary cycles complex lateral variations within depositional sequences especially in the late shermanian to edenian indicate that tectonically controlled patterns of basinal subsidence and uplift of crustal blocks perhaps reflecting forebulge migration exerted a strong influence on the local facies and motif of depositional sequences these tectonic features however did not obliterate the underlying allocyclic pattern indeed high resolution sequence stratigraphy enables detailed resolution of shifting patterns of minor uplift and subsidence

grain size areal thickness distribution and controls on sedimentation of the 1991 mount pinatubo tephra layer in the south china sea the june 15 1991 climactic eruption of mt pinatubo produced an extensive largely co ignimbrite derived airfall ash layer on luzon island and across the central south china sea the layer covers an area of 4 105 km2 with a volume of 5 5 km3 near the coast of luzon the deposit consists of two units a normally graded basal ash bed unimodal in grain size and a finer grained internally structureless upper ash bed showing grain size bimodality with increasing distance from the source the coarse particle populations of the two units merge and migrate towards a near constant fine population 11 m the distal region is covered by a fine mode dominated virtually ungraded single ash layer the reversal of the winds from easterly directions at upper tropospheric and stratospheric levels to westerly directions in the middle and lower troposphere indicates that both the coarse and fine mode components fell out from high altitude eruption clouds the high velocity upper level winds however would have transported fine grained ash particles far beyond the south china sea which suggests that their settling was accelerated by aggregation the boundary between the units thus marks a change from fallout of predominantly discrete pyroclasts to simultaneous fallout of aggregated fines and freely falling coarse grained particles the particle populations composing the upper ash bed were almost completely removed from the proximal areas by the upper level winds at lower elevations the counterclockwise circulation of a typhoon over the coastal area advected the ash south and eastward producing a thickness maximum in the medial region at about 160 km from source the strong displacement of fines possibly aided by wind turbulence led to a break in bulk tephra thinning rates close to the coastline in the distal region outside the influence of the typhoon southwest monsoonal winds caused a distinct lobe axis inflection and thickness asymmetry within this region at about 420 km from source fallout of particle aggregates created a second thickness maximum comparison of the field data with previous experimental observations and tephra flux records in the deep sea wiesner et al 1995 carey 1997 mccool 2002 implies that the transport of ash in the water column was largely determined by vertical density currents differences in the reaction of coarse and fine particles to turbulence in the descending plumes probably suppressed the segregation of fines but allowed the coarser pyroclasts to maintain their initial order of arrival at the sea surface considering typical fall rates of convective plumes modifications of the initial fallout position of the particles by the south china sea current system are on the order of only a few kilometers the results suggest that convective sedimentation processes ensure the preservation of atmospheric particle transport directions distances and fallout modes in the deep sea

provenance of cambrian conglomerates from new zealand implications for the tectonomagmatic evolution of the se gondwana margin the oldest rocks in new zealand are the mid to late cambrian intra oceanic island arc rocks of the takaka terrane devil river arc the provenance of cambrian conglomerates stratigraphically above the exposed arc succession was studied to constrain the late stages of arc evolution and its accretion to continental crust the dead goat conglomerate contains two distinct groups of igneous clasts 1 intermediate to felsic volcanic clasts with moderately enriched light rare earth element lree and high field strength element hfse contents and positive nd500 2 1 that were derived from a medium k calc alkaline source probably the main sequence of the devil river arc 2 dioritic to metagranitic plutonic clasts strongly enriched in lree and hfse and with nd500 of 3 5 to 5 9 that were derived from a high k arc source probably the uppermost units of the devil river arc this is consistent with a new u pb sensitive high resolution ion microprobe age of 496 6 ma the lockett conglomerate also contains two distinct groups of igneous clasts 1 ultramafic to intermediate igneous clasts identified as boninitic to transitional low k calc alkaline arc related rocks based on depleted ree and hfse abundances 2 i type metagranitoid clasts derived from a distinct andean type continental margin as indicated by nd500 as low as 7 1 both conglomerates contain sandstone clasts derived from a common old multi cycle continental source with nd500 of 14 2 to 15 7 and no suitable source has been found in present day new zealand the new provenance data from these conglomerates constrain the time of accretion of the devil river arc to the palaeo pacific gondwana margin and provide new information on the structural evolution of the accretionary event

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

a felsic mash zone of crustal magmas feedback between granite magma intrusion and in situ crustal anatexis abstract magma mixing and mingling are described from different tectonic environments and are key mechanisms in the evolution of granitoids the literature focuses on the interaction between mafic and felsic magmas with only limited research on the interaction between similar magmas here we investigate instead hybridization processes between felsic magmas formed during the 500 ma delamerian orogeny on the south coast of kangaroo island field relations suggest that a coarse megacrystic granite intruded and interacted with a fine grained diatexite that resulted from combined muscovite dehydration and water fluxed melting of kanmantoo group turbidites the two magmas hybridized during syn magmatic deformation explaining the complexity of relationships and variability of granitoids exposed we suggest that granite intrusion enhanced melting of the turbidites by bringing in heat and h 2 o with rising melt fraction intrusive magmas became increasingly unable to traverse the partially molten terrane creating a positive feedback between intrusion and anatexis this feedback loop generated the exposed mid crustal zone where magmas mixed and homogenized thus the outcrops on kangaroo island represent a crustal and felsic melting assimilation storage homogenization felsic mash zone where instead of having direct mantle magma involvement as originally proposed these processes developed in a purely crustal environment formed by felsic magmas

early middle pleistocene transitions linking terrestrial and marine realms abstract marked by a progressive increase in the amplitude of climate oscillations an evolving waveform and a shift towards a quasi 100 ky frequency the early middle pleistocene transition empt previously known as the mid pleistocene transition or mid pleistocene revolution 1 4 0 4 ma represents a fundamental transformation in the earth s climate state the empt began with a substantial change in climate dynamics and ended with the mid brunhes event signaling the establishment of a new steady state the reasons for the empt while uncertain appear to involve a non linear response of the earth climate system the physical and biotic responses to this transition amplified by the growth of northern hemisphere ice sheets have been profound two important chronostratigraphic markers characterize the empt the jaramillo subchron 1 070 0 988 ma and the matuyama brunhes chron boundary 773 ka the latter has been chosen as the primary guide for the lower middle pleistocene subseries boundary as it lies at the approximate midpoint of the empt and aids in global recognition both in marine and terrestrial deposits the jaramillo subchron has received less attention but the late early pleistocene is important in europe because it saw the progressive transition from the villafranchian to galerian mammal faunas and expansion of hominins into western and northern europe the jaramillo subchron is represented by marine isotope stages mis 31 to 28 with mis 30 already showing the asymmetrical sawtooth pattern characteristic of the middle pleistocene indeed while variation in the 40 ky band obliquity remains strong throughout the empt low frequency variability begins at around 1250 1200 ky which coincides with a progressive increase in global ice volume against a backdrop of increasingly severe glacial cycles notably during mis 36 34 24 22 the so called 0 9 ma event 16 and 12 pronounced phases of warming are also documented globally including the super interglacial mis 31 the early phase of the empt is characterized by important glaciations beginning with mis 36 and continuing to mis 24 22 a major intensification of the east asian monsoon system intensification of loess deposition in northern europe development of open landscapes in western siberia increased fluvial incision higher amplitude sea level change and spread of large mammals across northern eurasia and a strong reduction in the north atlantic thermohaline circulation in europe the loss of thermophilous plant taxa during the empt and indeed throughout the quaternary is a reminder of the progressive cooling that took place here and elsewhere

the double mer formation and the lake melville rift system eastern labrador the double mer formation is a sequence of redbed arkosic sandstone conglomerate siltstone and shale in the grenville province of eastern labrador the formation is closely associated with a rift system extending inland for at least 300 km from the labrador coast substantial thicknesses of strata of double mer formation are confined to two basins the lake melville graben and the double mer half graben strata correlated with the double mer formation are found elsewhere in southeast labrador one isolated outcrop occurs within the sandwich bay graben a separate smaller basin 100 km to the southeast that trends parallel to the lake melville graben the only definite age constraint on the double mer formation is that it postdates grenvillian deformation previously the formation was assigned a latest precambrian to early cambrian age based on lithological similarity with the lower cambrian bradore formation of the southeast coast of labrador on the basis of a structural model proposed here that sugges

paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite abstract rare earth element concentrations in biogenic apatite of conodonts fish debris and inarticulate brachiopods were determined in over 200 samples from cambrian to modern sediments tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory interexperiment or interspecies related variations clearly show that no resolvable fractionation of ree occurs incorporation of ree in biogenic apatite of recent fish debris occurs near the sediment water interface soon after deposition and reflects characteristics of seawater the original ree signature apparently survives subsequent burial and diagenesis cerium variations in fish debris from modern environments are controlled by redox potential ce is fractionated by co precipitation with metallic oxides under oxidizing conditions this fractionation produces a negative ce anomaly ce anom in seawater that is reflected in the sedimented fish debris conversely no fractionation of ce occurs under reducing conditions resulting in ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions extrapolation of observations of the redox control of ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower paleozoic and lower triassic oceans whereas in the upper paleozoic the world ocean was generally oxidizing

petrology and geochemistry of ordovician k bentonites in new york state constraints on the nature of a volcanic arc altered volcanic ashes k bentonites in the late ordovician caradocian utica shale of new york state are the product of explosive arc volcanism most of the 30 k bentonites examined in this investigation contain fragmental crystals and rock fragments microliths up to 600 mu m in diameter that generally are neither detrital contamination from the surrounding black shale nor igneous phenocrysts the dominant phases are garnet two groups gr sub 15 20 alm sub 45 75 py sub 35 0 sp sub 2 5 gr sub 2 5 alm sub 55 85 py sub 10 40 sp sub 1 plagioclase feldspar an sub 80 10 alkali feldspar or sub 99 0 clinopyroxene wo sub 50 40 en sub 50 30 fs sub 0 30 and orthopyroxene wo sub 1 2 en sub 77 32 fs sub 32 66 accompanied by lesser quantities of hornblende aluminosilicate quartz sphene fe ti oxides apatite and zircon most of the fragmental crystals appear to be derived from the same source as the metamorphic microliths which possess minerals with similar compositions both crystals and microliths are interpreted as xenocrysts and xenoliths from the ancient continental crust on which the ordovician arc was constructed they became entrained in the volcanic plume during explosive eruptions a precambrian age acquired on k feldspars from one k bentonite using the sup 40 ar sub 39 ar method shows that these xenocrysts were derived from depths of less than 5 10 km in the microcontinent at the time of late ordovicianmore volcanism the occurrence of xenocrysts and xenoliths in these k bentonites underscores the importance of performing detailed petrology on ash layers prior to the onset of more sophisticated tasks e g isotopic age determinations regional stratigraphic correlations of k bentonites based upon chemical compositions less

tectonic development and metallogeny of the kanmantoo trough in south australia abstract the kanmantoo trough is the sedimentary and tectonic link between the adelaide geosyncline composed of late proterozoic sediments and volcano sedimentary sequences of the tasman fold belt system in eastern australia cambrian flyschoid sediments of the kanmantoo trough were likely deposited on a marginal shelf reflecting tectonic activity to the west and a continental margin to the east the sediments may extend eastwards under the murray basin but the tectonic significance of the padthaway ridge and of mafic volcanics northeast of that region are poorly understood copper mineralisation is scattered throughout the kanmantoo trough and is commonly associated with the intersection of northwest trending fracture zones and pyritic sedimentary horizons

ordovician arc collision and foredeep evolution in the gaspe peninsula quebec the taconic orogeny in canada and its bearing on the grampian orogeny in scotland the middle to upper ordovician deslandes cloridorme and garin formations in the eastern gaspe peninsula quebec are some of the most extensively exposed foredeep successions along the northern appalachian portion of the taconic caledonian orogen their depositional and biostratigraphic frameworks n gracilis to c spiniferus zones approximate correlatives to the n gracilis to d clingani british zones indicate that foredeep evolution spanned c 10 ma and was essentially synchronous with the convergence and accretion of the popelogan arc to the laurentian margin during the caradoc c 458 449 ma these results when combined with metamorphic and radiometric age data synthesized from across the laurentian segments of the northern appalachian and northwestern ireland taconic caledonian orogen further substantiate that orogenesis consisted of several short lived c 10 20 ma deformational episodes confined to arenig through ashgill time c 485 440 ma this indicates that the total duration of orogeny as recorded by arc magmatism metamorphism and laurentian foredeep development spanned no more than c 45 ma in that the scottish caledonides are a consanguineous part of this orogen their tectonostratigraphic evolution i e grampian orogeny should be similar accordingly the results detailed above are geodynamically compatible with and thereby support tectonic models that define the grampian orogeny in scotland as a mostly early to mid ordovician c 480 460 ma tectonic episode of short rather than long duration

late ordovician platform foundering its paleoceanography and burial as preserved in separate eastern michigan basin ottawa embayment basins southern ontario comparison of litho bio and chemostratigraphy in two cores from the northeastern margin of the michigan basin manitoulin island and from within the ottawa embayment eastern ontario identifies interbasinal differences of late ordovician platform foundering linked to taconic orogenesis graptolite biostratigraphy defines an east to west younging late edenian to early maysvillian of platform burial a regional unconformity likely caps the platform succession in both basins an increased supply of mafic material appears during the final stages of platform collapse with the accumulation of organic rich 8 petroliferous shales collingwood member michigan basin eastview member ottawa embayment both units preserve evidence for deposition coincident with increased dysoxic to possible anoxic bottom water conditions but the collingwood member accumulated under a relatively stable paleoceanographic environment rhythmic interbedding with platform limestone in eastern ontario combined with ev

geophysical testing of balanced cross sections of fold thrust belts with potential field data an example from the fleurieu arc of the delamerian orogen south australia abstract the delamerian south australia and ross antarctica orogens once formed a continuous chain along the paleo pacific margin of gondwana correlations between the two are based on many characteristics including style of shortening and deformation constraining structural features in one of these orogens impacts on interpretations of tectonic processes for the other balanced cross sections have provided crucial information on the internal architecture of the delamerian orogen south australia and form a key piece of evidence in some widely circulated models for the deformational style these sections have not been constrained by geophysical investigations which can provide significant information about the subsurface geometry we present magnetic forward models for two balanced cross sections of the fleurieu arc in the delamerian orogen the forward models require a larger volume of magnetic basement at shallower depths than predicted by balanced sections requiring a revision of currently proposed shortening mechanisms the shortening mechanism inferred from the models requires less imbrication of upper crustal units and more folding above thrusts that involve both basement and cover sequences this style of shortening compares favorably with published observations from the ross orogen in antarctica

correlation of north american ordovician bentonites by using apatite chemistry middle ordovician bentonites in the central and southeastern united states have been correlated on the basis of rare earth element ree concentrations of primary apatite phenocrysts ratios of light to heavy ree such as la tb or ce yb produce the best separation of the four studied bentonites the deicke bentonite has been correlated over a distance of more than 1000 km from southern minnesota to central tennessee and the millbrig bentonite has been correlated over 750 km from central minnesota to central missouri the successful discrimination of closely spaced bentonites over a long distance demonstrates that apatite chemistry is a powerful correlation tool on a continental scale

continental margins orogenic belts and intracratonic structures angular bends promontories and embayments in rifted and passive continental margins are interpreted to be the precursors of curves recesses and salients respectively in orogenic belts convergence at an irregularly shaped continental margin results in along strike diachroneity of closing and thus of orogeny and in a variable trajectory of stress into continental crust both timing and sense of movement of intracratonic structures apparently have complex relationships to the progress of convergence at irregular continental margins

a paleozoic suture in eastern gondwanaland a new tectonic interpretation of southeastern australia tasmania and north victoria land antarctica is given in which a major n s trending lineament representing a suture zone is recognized separating essentially shallow water shelf and continental deposits of latest cambrian or early ordovician age from deeper water ordovician flysch deposits lateral offset of this lineament which is interpreted as a suture along which thrust telescoping and major strike slip faulting occurred could adequately explain the present distribution of major rock types in the three areas and also removes a number of problems previously encountered in correlation

influence of convective sedimentation on the formation of widespread tephra fall layers in the deep sea the formation of widespread volcanic ash fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra 20 180 m diameter as it travels from the atmosphere into water using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of mount st helens 0 2 g cm 2 hr the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents the currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large at the air water interface the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface the implication of the experiments is that deposition of distal tephra fall layers in deep sea sediments may be dominantly controlled by diffuse vertical gravity currents as opposed to passive settling of individual particles through the water column this process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers support for this mechanism comes from observations of greatly accelerated tephra settling rates in the south china sea following the 1991 eruption of mount pinatubo in the philippines

detrital zircon ages in neoproterozoic to ordovician siliciclastic rocks northeastern australia implications for the tectonic history of the east gondwana continental margin u pb detrital zircon ages in variably metamorphosed dominantly fine grained clastic successions are used in northeastern australia to identify two major successions along the east gondwana margin the older succession is of probable late neoproterozoic age and is considered part of a passive margin associated with rifting at c 600 ma most detrital zircons have ages in the range 1000 1300 ma and were probably derived from an extension of a late mesoproterozoic 1050 1200 ma orogenic belt from the central australian musgrave complex located 1500 km to the west no evidence has been found for 600 800 ma rifting of a rodinian supercontinent and therefore it is suggested that breakup must have occurred well outboard of the present early palaeozoic east gondwana margin the younger succession is of early palaeozoic age and contains the distinctive 500 600 ma detrital zircon signature that is widespread in east gondwana in addition to some samples with ages in the range 460 510 ma consistent with local igneous sources the younger succession is related to the active margin of gondwana that developed on the former passive margin in a back arc setting and the source of 510 600 ma zircons is considered to be a composite of rift related and back arc volcanic sources

using black shales to constrain possible tectonic and structural influence on foreland basin evolution and cratonic yoking late taconian orogeny late ordovician appalachian basin eastern usa

structure and origin of the fleurieu and nackara arcs in the adelaide fold thrust belt south australia salient and recess development in the delamerian orogen abstract the regional map pattern of structural trends in the early paleozoic adelaide fold thrust belt of south australia resembles an upright letter s in general structures in the fold thrust belt verge toward the gawler craton to the west so the northern curve nackara arc of the s is a salient whereas the southern curve fleurieu arc is a recess the map view axial trace of the nackara arc coincides with the thickest portion of the adelaidean late proterozoic basin and the map view axial trace of the fleurieu arc coincides with the intersection between the adelaidean and kanmantoo cambrian basins our structural analysis of the adelaide fold thrust belt demonstrates that the cross sectional structural geometry of the nackara arc contrasts markedly with that of the fleurieu arc in the nackara arc the belt consists of open detachment folds that probably formed above an evaporite hosted decollement whereas in the fleurieu arc the belt consists of an imbricate fan of basement involved thrusts further the foreland edge of the nackara arc coincides with a blind ramp whose strike parallels fold axes of the arc whereas the foreland edge of the fleurieu arc coincides with emergent thrusts that cut obliquely across earlier formed folds of the arc in the transition region between the two arcs the fold thrust belt consists of an intensely sheared basal duplex overlain by open folds structural contrasts between the nackara and fleurieu arcs indicate that the two curves did not form in the same way we present a model for development of these curves based on a comparison of their structural features to those of curves in other fold thrust belts and based on previous studies of curve formation using sandbox analogs we suggest that curvature of the nackara arc reflects the control of basin stratigraphy on the width of a fold thrust belt while curvature of the fleurieu arc reflects oroclinal bending of the fold thrust belt when it impinged on the southeastern corner of the gawler craton our model explains the structural contrast between the two arcs and illustrates correlations between fold thrust belt curve formation and pre deformational basin geometry

sm nd isotopic evidence for the provenance of sediments from the adelaide fold belt and southeastern australia with implications for episodic crustal addition abstract in south australia late proterozoic and cambrian sediments were deposited in basins formed within archaean to early mid proterozoic basement these sequences were subsequently deformed and uplifted by the cambro ordovician delamerian orogeny to form the adelaide fold belt in using the adelaide fold belt to address models for lithospheric evolution it is necessary to understand the mechanisms of basin formation and whether the sequences merely reflect reworking of existing cratonic material or if there were new crustal additions this study presents geochemical and isotopic data on the sediments and basement rocks to complement existing data on the delamerian igneous rocks as a means of investigating these questions the data reveal that the archaean mid proterozoic basement rocks of the gawler craton have nd depleted mantle model ages which average about 2 6 ga and nd values at 800 ma in the range 9 to 24 or at 500 ma from 11 to 28 the late proterozoic adelaidean sediments however have an average model age of 1 9 ga and initial nd values in the range 5 to 11 the sequence can be divided into lowermost rift phase sediments derived from erosion of local gawler craton material and a subsequent widely distributed thermal sag phase sedimentation whose isotopic signature is not compatible with the basement this implies that a more primitive or juvenile source was accessed via broadening of the provenance region the cambrian sediments have slightly older model ages 2 1 ga and lower nd 9 to 13 and can be interpreted as mixtures of basement and reworked adelaidean detritus the early palaeozoic sedimentary sequences of the lachlan fold belt to the east have isotopic compositions very like the south australian late proterozoic and cambrian sequences they have an average model age of 1 85 ma but much higher k 2 o na 2 o this indicates that the widespread flysch of eastern australia is the result of recycling of the late proterozoic and cambrian sediments by erosion of the adelaide fold belt with some modification by the mixture of the cambro ordovician mafic and felsic magmatic rocks in this orogen our data suggest regional intracontinental crustal evolution combined with recycling of older crustal material with new additions from the mantle during orogenesis and to a lesser extent episodes of extension and sedimentation

discovery of the ordovician millbrig k bentonite bed in the trenton group of new york state implications for regional correlation and sequence stratigraphy in eastern north america the ordovician chatfieldian millbrig k bentonite bed is a key stratigraphic marker horizon that is regionally synchronous over much of eastern and central north america this prominent marker is an independent source of correlation among the major chronostratigraphic and sequence stratigraphic units in this region the general stratigraphic position of the millbrig k bentonite has suggested to some authors that it is identical with the hounsfield k bentonite at dexter new york the traditional type area of the middle ordovician or mohawkian series in north america but previously available geochemical and biostratigraphical evidence has been insufficient to confirm this correlation analyses of apatites and melt inclusions in quartz phenocrysts from the millbrig k bentonite at eight localities in kentucky missouri virginia and pennsylvania and the hounsfield k bentonite at its type locality at dexter new york using high precision electron microprobe analysis techniques shows that the millbrig and the hounsfield have identical apatite and melt inclusion chemistry indicating a geochemical correlation this correlation is supported by conodont biostratigraphy and 13c isotope chemostratigraphy the new data demonstrate that the millbrig k bentonite and therefore the base of the chatfieldian stage of the north american mohawkian series by definition lies very close to the base of the traditional rocklandian stage of new york furthermore the millbrig k bentonite bed lies in close proximity to the base of the taconic supersequence over much of the midcontinent region and in particular lies just below the m5 sequence boundary recognized in kentucky and tennessee our results permit extension of the chatfieldian sequences into new york state and southern ontario and contributes to the resolution of the long standing uncertainty about the position of the base of the trenton group in ontario furthermore viewed in a broad context of regional stratigraphic relations we conclude that our results suggest that the persistent correlation difficulties reflect diachronous effects of widespread changes in oceanographic circulation patterns that emerged during taconic orogeny finally regional differences in the timing and character of sequence bounding surfaces and facies similarities summarized here suggest that the causes of relative sea level change during the chatfieldian may have been primarily tectonoeustatic mechanisms

formation and reactivation of the cambrian kanmantoo trough se australia implications for early palaeozoic tectonics at eastern gondwana s plate margin the kanmantoo group is a thick and largely metamorphosed sedimentary succession that filled an isolated arcuate cambrian basin kanmantoo trough which formed within continental gondwana and now lies on the southern margin of the present australian continent kanmantoo group sediments unconformably overlie neoproterozoic and older cambrian rocks we consider that the geometry of the southern part of this trough was influenced by strike slip movement along an intra continental tear fault to the north the basin changes to a style dominated by orthogonal extension and eventually tapers and dies out normal to the tear fault balanced sections show that the kinematic style and strain distribution developed during early palaeozoic inversion was controlled by the specific architecture of the kanmantoo trough early palaeozoic tear faulting could have linked contrasting subduction polarities along the then contiguous palaeo pacific margin of gondwana the kanmantoo trough is considered to have formed at a passive margin related to east directed subduction in what is now the australian continent in contrast west directed subduction formed an active margin at contiguous parts of current antarctica kanmantoo group sediments were derived from the south by erosion of a grenvillean province mixed with sediments eroded from the emergent active margin of gondwana the inception localization and sedimentation in the kanmantoo trough reflects a complex interaction of tectonic processes along the encroaching ross delamerian orogen

some geodynamic and compositional constraints on postorogenic magmatism postorogenic magmatic suites are common to many orogens in many cases apparently just postdating the cessation of deformation they differ from preceding orogenic suites in that they have higher temperatures more primitive isotopic signatures and bimodal natures and thus are compositionally similar to suites found in extensional regimes we propose that thinning of the lithospheric mantle which may be an automatic response to orogenic lithospheric thickening is responsible for these magmatic suites mantle lithospheric thinning moves the asthenospheric lithospheric thermal boundary higher in the lithospheric column thereby raising the overall thermal budget of the orogen and the likelihood of basaltic magmatism this removal of much of the dense lithospheric mantle root of the orogen also invokes uplift capable of producing horizontal orogenic buoyancy forces that will oppose and potentially terminate deformation extreme fractionation of the magmas is promoted by high temperatures and a noncompressional or tensional lithospheric stress regime to produce accompanying felsic a type magmas this model explains why the cessation of deformation is coincident with high temperature bimodal magmatism and inferred uplift as well as the long noted similarity and confusion between postorogenic and extensional magmatic suites this arises because both reflect thinning of the lithospheric mantle not because both reflect extensional tectonics we suggest that these magmatic episodes may be important in transferring lithospheric mantle material and its compositional signatures into the crust

brief sea level fall event and centennial to millennial sea level variations during marine isotope stage 19 in osaka bay japan detailed sea level variation was investigated for marine isotope stage mis 19 based on diatom and grain size analyses of a marine sequence in a core of the osaka group diatom sea level proxies represent precession related signals correlated with highstands mis 19 3 and 19 1 and lowstand mis 19 2 astronomical tuning shows the marine sequence has a uniform accumulation rate of about 60 cm ka 1 a rapid sea level fall event was found in the earliest mis 19 demonstrated by several independent sea level proxies of diatom and grain size this event began with a rapid sea level drop followed by a gradual recovery at about 783 782 ka a maximum abundance of pelagic diatom taxa at a core depth of 402 20 m evidently shows the highest sea level peak in mis 19 supported by many other proxies based on the diatom data sea level change across mis 19 1 is characterized by centennial to millennial fluctuations the sea level fall event began just after the onset of a cooling event previously reported from the same core observations of a comparable sea level fall signal in many deep sea core records suggest the event is global

influence of basin architecture on the style of inversion and fold thrust belt tectonics the southern adelaide fold thrust belt south australia abstract the southern adelaide fold thrust belt involves neoproterozoic adelaidean strata deposited during protracted rift and sag phases and an overlying cambrian kanmantoo sequence represented by a thick wedge of turbidites rapidly deposited in an eastwards deepening basin deeply incised into the underlying rocks the belt was deformed during the delamerian orogeny developed at the southeastern margin of the australian craton during early palaeozoic nw directed displacement during this shortening event the margin of the kanmantoo basin was reactivated and formed a significant strain guide that controlled key features of the fold thrust belt strain integrated and restored sections reveal contrasting geometric and kinematic styles along the strike of the belt a transpressional zone is developed at the steep basin margin in the south oriented oblique to the structural grain the central parts of the belt are dominated by strong buttressing growth fault reactivation and basement involved footwall shortcut thrusting in the northern part of the belt shortening is accommodated by homogeneous folding above a master de collement these variations in regional structural style and internal strain partitioning reflect significant variations in the original kanmantoo basin margin geometry rheological parameters control the partitioning of strain associated with pervasive fabrics for example cleavage and transposed bedding folding is accompanied by negative line length changes shortening in the northern part of the belt in its thrusted central part pelites commonly show positive line length changes psammites were least affected by line length changes and thus provide key beds for section balancing pervasive strain makes a full restoration of cross sections impossible however incorporating regional strain data assists establishment of balanced and restorable sections which provide a powerful tool in understanding both pre deformational basin geometry and its shortening

towards understanding the early gondwanan margin in southeastern australia abstract this review synthesizes the proterozoic and early paleozoic geology of tasmania bass strait and western and central victoria we examine the many different conflicting hypotheses that have been proposed to solve the paradoxical relationships between tasmanian geology and that of mainland australia most notably the prevalence of proterozoic basement of western and central tasmania while immediately across bass strait evidence of proterozoic rocks is much more cryptic we conclude that the selwyn block model is the most satisfactory hypothesis to date since it fits best with the obvious patterns in the magnetic and gravity data this model proposes that the central victorian melbourne zone is underlain by the northern extension of thin tasmanian proterozoic and cambrian crust under bass strait and that the silurian to middle devonian melbourne zone was shortened along a decollement during the tabberabberan orogeny the ordovician rocks of eastern tasmania correlate more closely with the tabberabbera zone than the melbourne zone in victoria however the silurian and devonian correlations are less certain major unresolved issues are the origins of the proterozoic and early cambrian lithostratigraphic packages tectonic models for their assembly during the tyennan orogeny and how these models fit with those for mainland australia

basement cover interaction in the adelaide foldbelt south australia the development of an arcuate foldbelt abstract the upper proterozoic to cambrian aged sedimentary and volcanic rocks comprising the adelaide foldbelt were deformed and in places metamorphosed during the cambro ordovician delamerian orogeny tectonic fabrics developed in the central portion of the foldbelt mt lofty ranges demonstrate westward transport during the orogeny the sigmoidal shape outlined by the kangaroo is mt lofty ranges olary portion of the foldbelt is interpreted to have been the result of dextral wrench faulting in the lower to mid proterozoic basement thus cover rocks overlying such basement wrench fault zones would have suffered a transpressional stress regime giving rise to the observed fold axis oriented at an oblique angle to the thrust boundary in the northern portion of the foldbelt northern flinders ranges wrench faulting is interpreted to have accommodated considerable basement shortening which initiated a basement cover decollement and resulted in thrust bound pop up structures in the cover

is the taconian orogeny of southern quebec the result of an oman type obduction the taconian orogeny is generally considered to have resulted from an arc continent collision however geochronological data indicate that much taconian deformation predates arc magmatism we propose that the late ordovician early silurian tectonic environment in southern quebec resembled the configuration of the oman iran transect where subduction took place in the final stage of a large obduction

rb sr dating of differentiated cleavage from the upper adelaidean metasediments at hallett cove southern adelaide fold belt abstract upper adelaidean psammites from the foreland part of the late proterozoic adelaide fold belt at hallett cove have suffered one major deformation which produced upright folds that plunge gently south axial to these folds is a well developed differentiation cleavage inferred to have resulted from constant volume plane strain deformation by solution transfer and now defined by alternating p phyllosilicate rich and q quartz rich domains geochemical analyses show that cleavage development resulted in a 2 4 fold fractionation of rb sr between the p and q domains thus the age of deformation is datable and isotopic analyses show the p and q domains form a 531 32 ma isochron this age is statistically older than the pervasive deformation and metamorphism in the more interior parts of the fold belt inferred from intrusive ages to have occurred at 485 6 ma and thus suggests that deformation either propagated across the fold belt from external to interior zones or that it occurred episodically the age data allow but do not demand that cleavage formation predated the deposition of the overlying cambrian kanmantoo group sediments

an interpretation of the tectonostratigraphic framework of the murray basin region of southeastern australia based on an examination of airborne magnetic patterns abstract new pixel map representations of regional total magnetic intensity data reveal previously unknown characteristics of the basement concealed beneath thin cainozoic sediments of the murray basin in southeastern australia interpretations of magnetic patterns in terms of structural features allow a revised interpretation of the nature of the tectonostratigraphic framework underlying and flanking the basin the magnetic data indicate that arcuate or curvilinear structural trends under the murray basin do not conform with those of the exposed lachlan fold belt to the east and suggest that the basement concealed beneath the basin together with that exposed in the victorian highlands to the south forms a distinct composite tectonostratigraphic terrane beneath the southwestern murray basin proterozoic lower cambrian metasediments of the padthaway ridge of the kanmantoo fold belt display a northwesterly trending structural grain and a previously unsuspected continuity of structural trend with adelaidean cambrian rocks of the mount lofty ranges to the west in the south cambrian volcanics of the black range and stavely greenstone belts have similar magnetic response and appear to be components of a single elongate and strongly magnetic domain which extends to the northwest for at least 400 km stavely belt to the north a similar but entirely concealed northeasterly trending magnetic domain can also be interpreted as volcanics lake wintlow belt together these two magnetic domains appear to form an arcuate zone of volcanics with a concave to the east configuration located at a possible suture between the lachlan and kanmantoo fold belts beneath the western murray basin in the south the magnetic imagery indicates that metasediments of the cambro ordovician stawell belt produce magnetic patterns distinct from those produced by the metasediments of the adjacent ordovician bendigo belt which can itself be subdivided into a number of areas of distinct magnetic pattern magnetic domains can be interpreted in terms of concealed proterozoic and palaeozoic structural belts and tectonostratigraphic terranes reconstructions suggest that much of the murray basin may be underlain by a former microcontinental block or rifted marginal plateau of precambrian crust now welded to the precambrian craton to the west deformation of the kanmantoo and adelaide fold belts adjacent to and beneath the western and southwestern margins of the basin can be attributed to late cambrian early ordovician convergence between the microcontinental block and the craton volcanics of the arcuate stavely and lake wintlow belts are interpreted to be located at the suture between the microcontinental block and the craton in addition the presence of concave to the east curvilinear magnetic trends in the cambro ordovician metasedimentary belts beneath the murray basin suggests that these successions as well as the volcanics of the heathcote and mount wellington belts may form allochthonous sheets these were possibly emplaced from the east during late ordovician devonian deformation possibly related to convergence of a further microcontinental block underlying the adjacent lachlan fold belt to the east of the murray basin

distal sedimentation in a peripheral foreland basin ordovician black shales and associated flysch of the western taconic foreland new york state and ontario black shale is characteristic of early siliciclastic fill along the distal western flank of the middle to late ordovician taconic peripheral foreland basin this facies referred to as the utica black shale magnafacies includes at least five intervals bounded by unconformities and or condensed beds each unit records a pulse of subsidence and subsequent siliciclastic sedimentation in the foreland basin progressive episodic shifts from carbonate to organic rich mud deposition may be due to steepening and subsidence of the western carbonate ramp followed by onlap of axially dispersed sediment in general silt poor organic rich mud was deposited on a subsided carbonate ramp that sloped eastward toward the nearest siliciclastic sediment source this mud may have accumulated from the pelitic tails of axial fan turbidites derived from source areas to the east and possibly the south the resultant black shale does not necessarily represent the deepest part of the basin the deepest part of the basin the basin axis contained coarser grained axial submarine fan deposits throughout the later part of the middle ordovician and the first half of the late ordovician the basin axis migrated westward over 100 km yet the orientation of the axis and the basin margins were persistent this basin orientation generally parallel to the orogenic belt along with the large scale basin shape resulted from a combination of deformational loading of the continental margin and progressive flexure in the foreland smaller scale structural elements normal fault bounded basement blocks were superposed on large scale taconic foreland basin geometry abrupt shifts from carbonate ramp sedimentation to deeper water organic rich mud deposition may reflect movement along these faults during the orogeny

a new geological framework for the middle ordovician carillon formation uppermost beekmantown group ottawa embayment onset of taconic foreland deposition and tectonism within the laurentian platform interior the carillon formation underlies the ottawa embayment a craton interior extension of the regional laurentian paleoshelf defined today by the ottawa bonnechere graben facies within the embayment during the middle ordovician identify a mixed sediment carbonate siliciclastic tide influenced peritidal and schizohaline basin host to a mosaic of dolomitic tidal mudflats shallow lagoons or ponds and channels skeletal metazoans were rare but bioturbation increased with episodic influx of less saline water carrying sand and mud possibly in response to regional climate change stratigraphic analysis reveals that the formation coincides with a third order eustatic sea level cycle but higher order stratigraphic patterns show no obvious eustatic link instead a stepwise onlap geometry coincided with local faulting that produced widespread syndepositional sediment deformation in particular stages of faulting and post carillon erosion are mapped along the present day trace of a northwest southeast orie

observations on the tectonic evolution of the southern adelaide fold belt jenkins r j f and sandiford m 1992 observations on the tectonic evolution of the southern adelaide fold belt in cl fergusson and r a glen editors the palaeozoic eastern margin of gondwanaland tectonics of the lachlan fold belt southeastern australia and related orogens tectonophysics 214 27 36 the mount lofty ranges fieurieu peninsula and kangaroo island regions of south australia expose a section across the west and no hwestern margin of the southern adelaide fold belt that was deformed and metamo h ed during the cambro ordovician delamerian orogeny in the external parts of the belt thrust complexes involving basement and a late proterozoic platformal succession show a characteristic asymmetry reflecting west to northwest vergence towards the australian craton in the more internal zones convergent deformation involving large scale upright folding and high t low p metamorphism of mainly cambrian sediments which were in part turbiditic occurred between c 516 and c 490 ma in this part of the belt the distribution of heat and possibly strain reflects intimately the advective movement of magmas within the erogenic belt convergent deformation in the southern adelaide fold belt was immediately preceded by rapid subsidence initiated during the de sition of the upper parts of the no anvilie group and which continued during deposition of the kanmantoo group mafic alkaline volcanism attendant with normanville group deposition is indicative of lithospheric thinning at c 526 ma the adelaide and lachlan fold belts record the spatial and temporal evolution of the southeastern margin of the australian continent through the late proterozoic and the palaeozoic in the southern adelaide fold belt comprising the motet lofty ranges fleurieu peninsulakangaroo island regions of south australia the terminal stages of sedimentation as well as the deformation metamorphism and felsic magmatism during the delamerian orogeny are restricted to the cambrian and earliest ordovician that is at the same time as the formation of the cambrian magmatic arcs and the initial accu

composition of biotite phenocrysts in ordovician tephras casts doubt on the proposed trans atlantic correlation of the millbrig k bentonite united states and the kinnekulle k bentonite sweden biotite phenocryst compositions in three thick widespread ordovician k bentonites the deicke and millbrig from big ridge alabama and the kinnekulle from mossen vastergotland sweden fall into three distinct groups and so the proposed intercontinental correlation of the millbrig and the kinnekulle is suspect because the biotites are nearly pristine compositionally electron microprobe analyses provide a precise geochemical fingerprint of each bed millbrig and kinnekulle biotites contain more feo and mno and less mgo and tio 2 than do deicke biotites millbrig biotites contain more mgo and less tio 2 than kinnekulle biotites and kinnekulle biotites contain appreciably more al 2 o 3 than either deicke or millbrig biotites each of these tephras was unmistakably the product of a gigantic explosive volcanic eruption but the differences in phenocryst chemistry point to derivation from three compositionally different magma batches

formation of corrensite chlorite and chlorite mica stacks by replacement of detrital biotite in low grade pelitic rocks transmission and scanning electron microscopy were utilized to investigate the nature and mechanisms of alteration of abundant detrital biotite of volcanic origin and progressive modification of phyllosilicate aggregates in a prograde sequence of pelitic rocks illite crystallinity index 0 19 0 58 a28 from the gasp6 peninsula in quebec detrital biotite has been diagenetically altered to form corrensite and chlorite through two mechanisms 1 layer by layer replacement gave rise to interstratification of packets of layers and complex mixed layering via several kinds of layer transitions between biotite and chlorite corrensite or smectite 2 dissolution transport precipitation resulted in the formation of relatively coarse grained aggregates of randomly orientated corrensite rich flakes and fine grained corrensite intergrown with chlorite and illite in the matrix the data show that stacks consisting of alternating packets of trioctahedral and dioctahedral phyllosilicates originated during early diagenesis when lenticular fissures in strained altering biotite were filled by dioctahedral clays subsequent prograde evolution of dioctahedral clays occurred through deformation dissolution and crystallization and overgrowth illite evolved to muscovite with k in part provided through biotite alteration and corrensite chlorite to homogeneous chlorite the alteration of detrital biotite is closely related to the formation of titanite and magnetite in diagenetic rocks and pyrite calcite and anatase or rutile in the higher grade rocks the observations demonstrate that detrital biotite of volcanic origin may be the principal precursor of chlorite in chlorite rich metapelites originating in marginal basins the mineral parageneses suggest that the transitions from corrensite to chlorite and illite to muscovite may be a function of local chemistry and time