living on the edge evaluating the impact of stress factors on animal sediment interactions in subenvironments of a shelf margin delta the mayaro formation trinidad abstract integrating sedimentological and ichnological characteristics of the gelasian mayaro formation along the se coast of trinidad allows recognition of a river dominated and wave influenced shelf edge deltaic succession of the paleo orinoco river developed under strong slope instability four main sedimentary settings and twelve subenvironments developed on and beyond the outer shelf i e at the shelf margin have been identified extreme paleoenvironmental conditions make the succession rarely and sporadically colonized ichnologic evidence suggests that shelf edge deltas are among the most stressful marine environments due to a combination of physicochemical factors in response to the intrinsic sedimentary processes and the relative hierarchy of their influences specific to every subenvironment within any particular subenvironment the relative dominance of the fluvial feeder system the action of waves and rare influence of tides and slope instability determine the combinations and ranking of stress factors river dominated shelf edge delta subenvironments demonstrate the extreme influence of stress factors related to channel activity e g salinity changes high sedimentation and erosion rates flocculation of mud seasonal variations of fluvial flux whereas subenvironments dominated by waves storms and oceanic swells away from the axial feeder s are mostly influenced by barform morphodynamics and its effects on local physiography the paleo orinoco delta is represented not only by the delta lobe s developed at the shelf edge but by delta lobe s formed on the outer shelf as well outer shelf deltaic subenvironments manifest typical ichno sedimentological signatures of a normal wave influenced in this case inner shelf delta being susceptible to extensive gravitational instabilities the delta system suffered from canyonization and smaller gullying and subsequent filling n b hereafter we use the term canyon for both canyons and smaller gullies the shelf edge delta system was also associated with shelf attached mass transport systems such depositional subenvironments dominated by slope instability and sediment gravity flows appear to be the most unconducive for benthic colonization resulting in almost complete absence of bioturbation characterization of the stress factors as the functions of parameters arising from specific sedimentary and or oceanographic processes chemical conditions and preservational constraints specific to each subenvironment leads to the construction of a comprehensive ichnological and depositional analog model for shelf edge deltas in general and for the accommodation driven low latitude shelf edge deltas at an active oblique foreland setting in particular

a theoretical treatment of paleointensity determination experiments on rocks containing pseudo single or multi domain magnetic particles a phenomenological theory of thermoremanent magnetization trm is applied to model the possible results of paleointensity determination experiments using the thellier method it is shown that intrinsic curvature of the arai plot for samples containing pseudo single or multi domain particles can be attributed exclusively to the presence of remanences which are unblocked below their blocking temperature thus the thermal demagnetization tail of partial trm ptrm is not a direct measure for a sample s tendency to yield a curved arai plot here it is proposed to use the advancing decay of a ptrm t1 t2 which during heating occurs already below the lower temperature t1 t2 as a more reliable pre selection criterion in addition the effect of an average magnetostatic interaction on the thellier experiment turns out to be negligible an extended thellier method which compares the thermal demagnetization of natural remanent magnetization with that of an artificial trm according to the applied trm theory should result in a perfectly linear modified arai plot even for pseudo single or multi domain samples if no alteration or viscous effects occur

signatures of climate vs sea level change within incised valley fill successions quaternary examples from the texas gulf coast abstract the passive margin texas gulf of mexico coastal plain consists of coalescing late pleistocene to holocene alluvial deltaic plains constructed by a series of medium to large fluvial systems alluvial deltaic plains consist of the pleistocene beaumont formation and post beaumont coastal plain incised valleys a variety of mapping outcrop core and geochronological data from the extrabasinal colorado river and the basin fringe trinity river show that beaumont and post beaumont strata consist of a series of coastal plain incised valley fills that represent 100 kyr climatic and glacio eustatic cycles valley fills contain a complex alluvial architecture falling stage to lowstand systems tracts consist of multiple laterally amalgamated sandy channelbelts that reflect deposition within a valley that was incised below highstand alluvial plains and extended across a subaerially exposed shelf the lower boundary to falling stage and lowstand units comprises a composite valley fill unconformity that is time transgressive in both cross and down valley directions coastal plain incised valleys began to fill with transgression and highstand and landward translation of the shoreline paleosols that define the top of falling stage and lowstand channelbelts were progressively onlapped and buried by heterolithic sandy channelbelt sandy and silty crevasse channel and splay and muddy floodbasin strata transgressive to highstand facies scale architecture reflects changes through time in dominant styles of avulsion and follows a predictable succession through different stages of valley filling complete valley filling promoted avulsion and the large scale relocation of valley axes before the next sea level fall such that successive 100 kyr valley fills show a distributary pattern basic elements within coastal plain valleys can be correlated with the record offshore where cross shelf valleys have been described from seismic data falling stage to lowstand channelbelts within coastal plain valleys were feeder systems for shelf phase and shelf margin deltas respectively and demonstrate that falling stage fluvial deposits are important valley fill components signatures of both upstream climate change vs downstream sea level controls are therefore interpreted to be present within incised valley fills signatures of climate change consist of the downstream continuity of major stratigraphic units and component facies which extends from the mixed bedrock alluvial valley of the eroding continental interior to the distal reaches wherever that may be at the time this continuity suggests the development of stratigraphic units and facies is strongly coupled to upstream controls on sediment supply and climate conditions within hinterland source regions signatures of sea level change are critical as well sea level fall below the elevation of highstand depositional shoreline breaks results in channel incision and extension across the newly emergent shelf which in turn results in partitioning of the 100 kyr coastal plain valleys moreover deposits and key surfaces can be traced from continental interiors to the coastal plain but there are downstream changes in geometric relations that correspond to the transition between the mixed bedrock alluvial valley and the coastal plain incised valley channel incision and extension during sea level fall and lowstand with channel shortening and delta backstepping during transgression controls the architecture of coastal plain and cross shelf incised valley fills

domain size closure domains and the importance of magnetostriction in magnetite abstract previous micromagnetic calculations invoking magnetostatic exchange and crystalline anisotropy energies predict about an order of magnitude more domains in magnetite than have experimentally been observed domains observed on magnetite by the magneto optic kerr effect are of similar size as previously observed ones by the bitter technique simple but quite accurate domain calculations show that for rectangular parallelepipeds of magnetite it is the closure four domain configuration that should be of lowest energy for all multidomain grain sizes unless magnetostrictive energies are considered for the observed domain pattern it is shown that the domains themselves must carry significant magnetoelastic energies

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

thermally activated viscous remanence in some magnetite and hematite bearing dolomites a brunhes age viscous remanent magnetization vrm carried by magnetite and hematite in the el paso and montoya dolomite formations has unblocking temperatures significantly higher than those predicted by single domain theory the samples appear to contain a substantial fraction of pseudo single domain magnetite suggesting transdomain processes may be responsible for the stable vrm a strong inverse correlation between the occurrence of high unblocking temperatures and high isothermal remanence fractions acquired above 0 3 tesla suggests that the hematite grain size may also be important in determining the range of observed temperatures larger grains are inferred to have lower coercivities but higher unblocking temperatures an older reverse polarity component is also observed and appears to be a record of late cretaceous tertiary tectonic activity

assessment of tomography models of taiwan using first arrival times from the taiger active source experiment in 2008 the taiger project conducted an active source experiment in which seismic records from 10 explosions were collected at nearly 1400 locations on the island of taiwan in this study we manually pick the first arrival times from the explosion records to ensure reliable and accurate travel time observations and use them to examine the complex p wave structural variations in the crust beneath taiwan

properties of chemical remanence in synthetic hematite testing theoretical predictions we synthesized hematite under controlled conditions of the applied magnetic field in order to test the behavior of chemical remanent magnetization crm predicted on the basis of theories developed to explain thermoremanent magnetization trm we restrict this investigation to the remanence acquired during mineral crystallization grain growth crm and do not consider chemical remanence acquired during mineral alteration the material produced by our technique is hematite up to 10 m in diameter thermal demagnetization to 200 c removes the viscous component acquired under identical field temperature conditions to those prevalent during synthesis as predicted by analogy to trm the crm acquired parallels the orientation of the magnetic field applied during growth the intensity of chemical remanence is a linear function of the field in which it grew bcrm up to 7 5 mt the maximum growth field investigated furthermore the ratio of intensities of crm to trm acquired in the same field is independent of the strength of that field the intensity of bcrm controls the growth and orientation of hematite crystals a result not predicted from theory derived by analogy to trm the control on crystal growth by the field is reflected in the dependence on bcrm of the orientation of ellipsoids of anisotropy of isothermal remanence and the acquisition of isothermal and anhysteretic remanence

processes facies and architecture of fluvial distributary system deposits abstract there is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down flow and transitions from proximal channelised to distal unconfined flow these deposits form fan shaped bodies several tens of kilometres in radius and their stratigraphic architecture is aggradational with no evidence of deep incision driven by base level fall the fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation a conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation proximal areas are characterised by amalgamated coarse pebbly and sandy channel deposits with little preservation of overbank facies channel dimensions are generally smaller in the medial areas but sizes are variable deposits are of braided meandering and simple channels which show varying degrees of lateral migration the channel fills may be mud or sand with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion the proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow interconnection of sandstone bodies is poor in the distal areas because channel fill bodies are sparse small and are not deeply incised the radial pattern of the sediment body forms by the repeated avulsion of channels active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas the term fluvial distributary system is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand

facies and evolution of the modern brazos delta texas wave versus flood influence abstract in order to better define the facies architecture and processes controlling the evolution of wave dominated deltas a detailed sedimentary and geomorphologic study was undertaken on the brazos delta texas the delta has formed since 1929 when the brazos river was diverted by the u s army corps of engineers the brazos delta is composed primarily of fine grained sediments prodelta clay constitutes more than half of the sediment volume thick sands are restricted to the narrow delta front environment whereas back bar lagoonal clays are a significant component of the delta plain sequence the facies architecture is not representative of the classic strandplain model for wave dominated deltas this is due to the strong influence of floods on deltaic evolution in early 1992 statewide flooding gave rise to a major constructional phase of the delta significant quantities of fine grained sediments were deposited in the prodelta one year after the onset of flooding a channel mouth bar emerged offshore of the river mouth and enabled progradation of the delta similar flood events that occurred during 1941 1957 and 1965 were recorded as ridge trough pairs in the delta headland the brazos delta is therefore fluvially influenced during floods and wave influenced during intervening periods

the geometry and internal architecture of stream mouth bars in the panther tongue and the ferron sandstone members utah u s a abstract clinothems are seaward dipping beds which are deposited during the progradation of delta front mouth bars measurements taken from clinothems in the field and from geospatially constrained photo realistic virtual outcrops were used to record clinothem dip maximum bed thickness and bed thickness every 10 m along a series of depositional dip oriented profiles from two ancient river dominated delta successions which crop out in central utah usa these data show systematic changes in bed dip and bed thinning which highlight discrete packaging of the beds into bedsets interpreted to represent the progradation of stream mouth bars in the delta front individual bedsets are characterized by a progressive steepening of beds coupled with a more rapid thinning systematic collection of 2800 locations along 73 separate clinothems from the panther tongue sandstone member and parasequence 1f from the ferron sandstone were used to study the relationship between bed thickness dip and decay gradient parameter which describes the down dip bed thinning clinothems in the panther tongue are much longer and more gently dipping than the clinothems in the ferron sandstone in both systems beds can be clustered into groups bedsets which show a systematic decrease in values and an increase in dip angle boundaries between the groups are defined by a sudden increase in values and a concurrent decrease in dip angle these bedsets are interpreted to represent individual stream mouth bars which amalgamate to form mouth bar complexes within the delta front sand bodies the use of virtual outcrops was essential for the collection of the large volumes of thickness data and the recognition of very subtle 1 changes in dip angle which have made these interpretations possible

depositional patterns following the 1870s avulsion of the saskatchewan river cumberland marshes saskatchewan canada abstract the avulsion of the saskatchewan river cumberland marshes east central saskatchewan canada has resulted in new floodplain deposition over an area of more than 500 km2 since beginning in the 1870s the avulsion deposits are predominantly fine grained and could be otherwise classifiable as overbank sediments in alluvial floodplains presumed to be supplied by intermittent flooding of dominant master channels the north angling and cadotte channels are older pre existing channels that were intercepted and appropriated by the expanding avulsion following annexation of the north angling channel stable cohesive banks inhibited channel enlargement and crevassing forcing the increasing discharges to be accommodated by overbank flow and in the process enhancing new levee aggradation these levee deposits coarsen upward and thin downstream as a result of greater competence and higher proximal deposition rates brought on by the larger flows in contrast the cadotte channel responded to increasing discharges by levee breaching giving rise to splay progradation and floodbasin filling the cadotte splay complex formed from the avulsively annexed cadotte channel represents a small but typical area of the avulsion belt facies characteristics and interrelationships examined from auger borings and vibracores are presented to illustrate patterns of deposition and floodplain development avulsive flooding initially raised local water surface elevations deepening small existing lakes and creating new shallow 1 2 m deep lakes on the floodplain surface deposition began when levees of the cadotte channel were breached allowing water and coarser sediment to spread as sheet like flows while finer sediment was carried and deposited farther into the basin later as the proximal splay sheets coalesced and grew basinward flows became channelized and progradation of mouth bars into marshes and shallow lakes together with deposition of interchannel fines became dominant forming coarsening upward sequences further floodplain aggradation developed by overbank flooding when splay channels became stabilized and bordered by levees gradual abandonment was accompanied by deposition of organic rich fine grained sediment on top of the progradational sequences throughout the avulsion the north angling channel has stood as an older alluvial ridge separating two broad areas of avulsive deposition its recent levee deposits now grade basinward into these new floodplain sediments composed of thicker heterogeneous predominantly upward coarsening and rapidly deposited facies of the avulsion

effect of multi cycle heat treatment and pre history dependence on partial thermoremanence ptrm and ptrm tails abstract we test two fundamental assumptions embedded in thellier experiments the initial state dependence and the effect of multi cycle heat treatment we observe that the magnitude of partial thermoremanent magnetizations ptrms imparted on an initial state of thermal demagnetization is larger than those of ptrms in the presence of a trm when the field used to impart ptrm is equal in magnitude and parallel to that used to produce trm a multi cycle thellier analysis on coarse grained magnetites progressively produces more intense ptrms and progressively erases more of the ptrm tails both pre history and multi cycle dependence will likely enhance the non linear features of the arai plot for coarse grained magnetites

sediment accumulation in a modern epicontinental shelf setting the yellow sea sediment accumulation in the yellow sea epicontinental shelf environment is investigated on 100 yr and 1000 yr time scales using 210pb and 14c geochronologies the distribution of modern 210pb accumulation rates in the yellow sea reveals that the loci of modern huanghe sediment accumulation are the topset 1 2 mm yr foreset 4 9 mm yr and proximal bottomset deposits 2 4 mm yr of the shandong subaqueous delta which extends south from the shandong peninsula 210pb rates in the distal bottomset deposits of the subaqueous delta in the central and southern yellow sea are generally low 0 3 0 9 mm yr a sediment budget demonstrates that between 9 15 of the annual huanghe discharge is accumulating in the yellow sea about two thirds of this sediment is accumulating in the topset foreset and proximal bottomset deposits of the shandong subaqueous delta with the remaining third accumulating as widespread distal bottomset deposits 14c age dates of subaqueous delta sediments indicate that the thick 40m clinoform structure formed predominantly between 6200 and 4060 yrs b p at rates approaching 20 mm yr observations in the yellow sea as well as on the amazon and ganges brahmaputra shelves demonstrate that subaqueous deltaic stratigraphy is the general rule where major rivers enter energetic shelves whether epicontinental or pericontinental the development of extensive bottomset deposits may be restricted to epicontinental shelf environments and may be diagnostic of sedimentation in this type of setting

the demagnetization temperature necessary to remove viscous remanent magnetization recent experimental studies have shown that the temperature necessary to demagnetize viscous remanent magnetization vrm in magnetite bearing rocks is significantly higher than predicted by standard neel single domain theory the walton time temperature equation which has been invoked by some to explain the enhanced thermal stability actually relates intensities of vrms acquired at different temperatures rather than describing the unblocking of a previously acquired vrm we show that the low field approximation lfa used in all previous formulations which predicts equal vrm acquisition and decay rates is not valid for a magnetite assemblage in the earth s field a medium field approximation mfa predicts significantly faster vrm acquisition and therefore greater thermal stability in decay for the larger particles in the assemblage conventional theory does apply to red bed acquisition decay experiments since the lfa is valid for hematite

low temperature behavior of single domain through multidomain magnetite abstract we have investigated the low temperature behavior of a suite of grown synthetic and natural magnetites that span single domain sd and multidomain md behavior synthetic samples had been grown in the laboratory either in an aqueous medium or in glass natural samples included sd magnetites occurring in plagioclase and truly md magnetites in the form of large octahedra in all experiments a sample was first given a saturation remanence at room temperature next moment was measured continuously during cooling and warming between 230 k and 60 k similar to results reported earlier by other workers magnetic memory is large in sd samples whereas truly md samples are almost completely demagnetized by cycling between room temperature and 60 k pseudo single domain samples exhibit behavior that is intermediate with respect to that of the sd and truly md states when data from this study are combined with data obtained by hartstra 10 from sized natural magnetites it is found that the percentage of total remanence that survives cycling between room temperature and 60 k decreases linearly with the logarithm of grain size and thus with increasing number of domains this relation suggests that memory can provide a reasonable estimate of grain size in those magnetite bearing rocks for which these samples provide good analogues remarkably some of the large natural octahedra provide a magnified view of md response to low temperatures and thus reveal two surprising and intriguing types of behavior first below approximately 180 k these octahedra demagnetize through a series of large barkhausen jumps second near 117 k these same octahedra exhibit a wild zone where magnetic moment executes large random excursions we interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and possibly the broadening of domain walls as temperatures drop toward the isotropic point one implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by md grains even without passage through the isotropic point of magnetite

kinematic evolution of the patagonian retroarc fold and thrust belt and magallanes foreland basin chile and argentina 51 30 s the kinematic evolution of the patagonian fold and thrust belt and cogenetic magallanes retroarc foreland basin is reconstructed using new geologic mapping two dimensional 2 d seismic reflection data and zircon u pb geochronology results span an 160 km wide transect of the thrust belt and magallanes basin near 51 30 s and highlight the influence of inherited extensional structures on basin paleogeography syntectonic sedimentation and late cretaceous neogene foreland shortening south of 50 s the patagonian fold and thrust belt developed on oceanic and attenuated crust of the predecessor late jurassic rocas verdes rift basin resulting in a collisional nature to early fold and thrust belt development and foreland sedimentation atop rifted south american crust we identify six principal stages of development between late cretaceous and neogene time a palinspastic restoration indicates 32 40 km 19 23 of retroarc shortening following closure of the rocas verdes basin and incipient growth of the thrust belt more than half of the estimated crustal shortening occurred synchronously with the deep water phase of late cretaceous foreland basin sedimentation subsequent deformation migrated into the foreland accounting for 12 km of shortening across the cretaceous early miocene basin fill thick skinned thrust faulting along multiple detachment levels in paleozoic metamorphic basement resulted in 5 km of foreland uplift and exposure of preforeland basin deposits the final phase of early miocene deformation ca 21 18 ma may reflect enhanced coupling between the continental and oceanic lithospheres causing foreland basement uplifts as the chile ridge spreading ridge approached the trench we speculate that neogene foreland shortening was accommodated by reactivation of mesozoic normal faults zones and accounts for broad uplift of the patagonian fold and thrust belt

fluvial responses to climate and sea level change a review and look forward summary fluvial landforms and deposits provide one of the most readily studied quaternary continental records and alluvial strata represent an important component in most ancient continental interior and continental margin successions moreover studies of the long term dynamics of fluvial systems and their responses to external or allogenic controls can play important roles in research concerning both global change and sequence stratigraphy as well as in studies of the dynamic interactions between tectonic activity and surface processes these themes were energized in the final decades of the twentieth century and may become increasingly important in the first decades of this millennium this review paper provides a historical perspective on the development of ideas in the fields of geomorphology quaternary geology vs sedimentary geology and then summarizes key processes that operate to produce alluvial stratigraphic records over time scales of 103 106 years of particular interest are changes in discharge regimes sediment supply and sediment storage en route from source terrains to sedimentary basins as well as changes in sea level and the concept of accommodation late quaternary stratigraphic records from the loire france mississippi usa colorado texas usa and rhine meuse the netherlands rivers are used to illustrate the influences of climate change on continental interior rivers as well as the influence of interacting climate and sea level change on continental margin systems the paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea level change at present empirical field based research on fluvial response to climate and sea level change lags behind a the global change community s understanding of the magnitude and frequency of climate and sea level change b the sequence stratigraphic community s desire to interpret climate and especially sea level change as forcing mechanisms and c the modelling community s ability to generate numerical and physical models of surface processes and their stratigraphic results a major challenge for the future is to catch up which will require the development of more detailed and sophisticated quaternary stratigraphic sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings there is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter 102 103 years and longer 104 106 years time scales than has commonly been the case to date as well as in larger river systems from source to sink studies of quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre quaternary rock record

sedimentary response to a tectonically induced sea level fall in a shallow back arc basin the mulichinco formation lower cretaceous neuquen basin argentina this study examines the sedimentary response to a tectonically driven relative sea level fall that occurred in the neuquen basin west central argentina during the late early valanginian early cretaceous at this time the basin lay behind the emergent andean magmatic arc to the west following the relative sea level fall sedimentation was limited to the central part of the neuquen basin with the deposition of a predominantly clastic continental to shallow marine wedge on top of basinal black shales this lowstand wedge is called the mulichinco formation and consists of a third order sequence that lasted about 2 myr and contains high frequency lowstand transgressive and highstand deposits significant variations in facies depositional architecture and internal organization of the sequence occur along depositional strike these variations are attributed mainly to tectonic and topographic controls upon sediment flux basin gradient fault tilting and shifting of the depocentre through time these controls were ultimately related to asymmetrically distributed tectonic activity that was greater towards the magmatic arc in the west the superposition of fluvial deposits directly upon offshore facies provides unequivocal evidence for a sequence boundary at the base of the mulichinco formation however the mulichinco sequence boundary is marked by shallow low erosional relief and widespread fluvial deposition the surface lacks prominent valleys traditionally associated with sequence boundaries this non erosive sequence boundary geometry is attributed to the ramp type geometry of the basin and or rapid uplift that limited stratigraphic adjustment to base level fall significant along strike facies changes and a low relief sequence boundary are attributes that may be common in tectonically active semi enclosed basins e g shallow back arc basins foreland basins

thermal demagnetization of partial thermoremanent magnetization three well characterized glass ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations ptrms in the temperature interval 400 350 c for varying lengths of time 10 min to 5 days and in magnetic fields from 0 5 to 5 oe 0 05 0 5 mt the thermal demagnetization behavior of the ptrms changes with domain state in the sense that with decreasing coercivity increasing grain size demagnetization curves become less steep and may not completely demagnetize until heated to the curie temperature numerical calculations based on neel s single domain sd theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures we observe that experimental results do not match any of the theoretical predictions there is no discernible field dependence of demagnetization curves for the sd and pseudo single domain samples and even for the soft multidomain md sample in the field range investigated an important consequence for thermally derived paleointensity data is that they may not be entirely reliable especially if the sample remanence is due to md grains

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

stratigraphic evolution of an outcropping continental slope system tres pasos formation at cerro divisadero chile depositional slope systems along continental margins contain a record of sediment transfer from shallow water to deep water environments and represent an important area for natural resource exploration however well preserved outcrops of large scale depositional slopes with seismic scale exposures and tectonically intact stratigraphy are uncommon outcrop characterization of smaller scale depositional slope systems i e 600 m thick succession units 3 and 4 which is interpreted as reflecting increased gradient conditions as the system accreted basinward

enhanced bioturbation on the down drift flank of a turonian asymmetrical delta implications for seaway circulation river nutrients and facies models extant models predict delta front environments down drift of river mouths as unfavourable for organisms because of the physico chemical stresses caused by sediment and fresh water influx this study however finds evidence for near optimal living conditions down drift of contemporaneous mouth bars and distributary channels as well as at the tops of abandoned lobes in part of the asymmetrical notom delta complex of the ferron sandstone turonian south eastern utah usa presented herein is a sedimentological and ichnological model using thirty two detailed measured sections along a 16 km transect through two continuously exposed ca 10 m thick allomembers containing delta front mouth bar and distributary channel facies azimuths from sedimentary structures show south eastward deflection of near shore palaeocurrents relative to the inferred north south shoreline as well as minor reversal of flow two end member trace fossil suites are recognized in delta front sandstones i a stressed suite of low abundance low diversity diminutive traces reflecting mobile deposit feeding resting and locomotion behaviours and ii a comparatively unstressed high abundance moderate diversity suite with a regular heterogeneous distribution of deep vertical or u shaped suspension feeding burrows which in places thoroughly homogenize the sandstones the down drift side of the delta was colonized by suspension feeders during seasonal reversal of the seaway gyre when mud plumes were swept northward or when river derived nutrients were sufficiently concentrated relative to fresh water and sediment input during normal seaway circulation very high sedimentation rates and mud laden wave dampened waters down drift of the river mouths heightened the preservation potential of the pervasively bioturbated facies up drift of the river mouths these bioturbated facies were either not preserved or not developed until the lobe was abandoned this alternative model for delta planform asymmetry contributes to the refinement of facies models for deltaic systems and provides a framework for predicting the distribution of bioturbation enhanced porosity and permeability in lobe deflected deltas

characteristic trace fossils in nearshore to offshore sequences upper cretaceous of east central utah more than 20 trace fossil species occur in marine facies of the cretaceous star point and blackhawk formations in the book cliffs and wasatch plateau provinces of utah major genera include ancorichnus arenicolites aulichnites chondrites conichnus cylindrichnus medousichnus ophiomorpha palaeophycus planolites rosselia schaubcylindrichnus scolicia skolithos teichichnus teredolites and thalassinoides newly named taxa include ancorichnus capronus medousichnus loculatus and rosselia chonoides most trace fossils occur in characteristic albeit intergradational ichnofacies correlative with major lithofacies of regressive nearshore to offshore sequences the latter include foreshore foreshore shoreface transition shoreface and offshore facies landward facies are typified by clean well sorted well stratified sparsely burrowed sandstones seaward facies except where interrupted by hummocky bedded sandstones exhibit successively less pure less well sorted and stratified more intensely

ichnology of prodeltaic hyperpycnite turbidite channel complexes and lobes from the upper cretaceous prairie canyon member of the mancos shale book cliffs utah usa

ichnological signatures from wave and fluvial dominated deltas the la anita formation upper cretaceous austral magallanes basin patagonia abstract depositional processes operating in deltaic shorelines create a variety of physico chemical stressful conditions that impact directly on benthic fauna the ichnological signatures of deltaic successions reflect these physico chemical stresses such as rapid sediment influx elevated water turbidity periodically reduced oxygenation salinity fluctuations and wave and tidal energy in the campanian la anita formation of the austral magallanes basin two examples of deltaic successions each showing different relative dominance of wave and fluvial processes are described in detail allowing the recognition of contrasting stresses that acted differentially in a wave dominated fluvial influenced delta and a fluvial dominated delta variations in the burrowing intensity and ichnodiversity allowed us to discriminate the different physico chemical stresses in the wave dominated fluvial influenced deltaic deposits the alternation between fair weather waves and storm events acted as the main controlling factor on benthic fauna in the facies of the prodelta and distal delta front displaying facies crossing and horizontal elements attributable to the skolithos and cruziana ichnofacies such as arenicolites isp chondrites isp cylindrichnus isp diplocraterion isp gyrochorte isp ophiomorpha isp palaeophycus isp planolites isp rhizocorallium isp rossellia isp schaubcylindrichnus isp and skolithos isp water turbidity and periodically reduced bottom water oxygenation conditions also played an important but subordinate role the proximal delta front and distributary and terminal distributary channel deposits show low burrowing intensities and ichnodiversities with structures attributable to the depauperate skolithos ichnofacies with few diplocraterion isp gordia isp lockeia isp macaronichnus isp ophiomorpha isp schaubcylindrichnus coronus and siphonichnus isp showing stress conditions associated with rapid sediment influx high energy and reduced salinity conditions related to fluvial discharges as the most important controlling factors periodically fluctuations in salinity conditions due to river discharges and channel avulsion may have also played important roles the fluvial dominated deltaic deposits show that the controlling factors were associated with fluvial discharge and river sediment influx with no clear influence of wave or tidal action the facies of the distal delta front were controlled by episodic sedimentation elevated water turbidity periodically reduced oxygenation and salinity fluctuations the associated trace fossil suite consists of a monospecific assemblage of ophiomorpha isp and sparse palaeophycus isp the proximal delta front and the distributary channel deposits are mostly unburrowed locally displaying a few arenicolites isp lockeia isp ophiomorpha isp psilonichnus isp and teredolites isp the development of these suites was controlled mainly by heightened sedimentation rates influx triggered by high fluvial discharges elevated water turbidity due to heightened suspended load and buoyant plumes periodic fluctuations in salinity conditions caused by river discharges and channel avulsion interdistributary areas are characterized by facies preserving evidence that reduced oxygenation was the most important controlling factor in this subenvironment fluvial processes generate the most extreme stressful conditions for benthic colonization by tracemakers wave processes tend to buffer the stresses produced by river discharge the effect of waves on fluvial stress is reflected in the higher bioturbation intensities and ichnodiversities that the wave dominated deltaic deposits of the la anita formation delta show in relation to the fluvial dominated ones

paleomagnetism and paleothermometry of the sydney basin 2 origin of anomalously high unblocking temperatures the milton monzonite of southeastern australia was thermoviscously remagnetized as a result of cretaceous burial and uplift thermal demagnetization separates the low unblocking temperature lt overprint from the high unblocking temperature ht primary remanence with a relatively sharp junction between lt and ht components in vector projections for single domain grains the junction temperature tl between two such vectors corresponds to the maximum blocking temperature tr reactivated in nature apart from a correction for the difference between natural and laboratory timescales however measured tl values are distributed over an implausibly wide range 250 c for burial remagnetization of an untilted intrusion like the milton monzonite furthermore many tl values are anomalously high compared to the predictions of single domain theory multidomain grains are the cause of these anomalies samples pretreated before thermal demagnetization by zero field cycling to liquid nitrogen temperature so as to erase multidomain remanence and isolate single domain remanence do have the theoretically expected tl values in these samples realistic remagnetization time and temperature tr tr conditions in nature are predicted using the t t contours of pullaiah et al 1975 the anomalously high tl values before low temperature treatment are due to multidomain grains which carry 50 of the lt overprint the lt thermal demagnetization curve in samples dominated by multidomain grains is quasi exponential in shape with a high temperature tail extending almost to the curie point as predicted by multidomain theory these high lt unblocking temperatures which are much greater than plausible remagnetization temperatures reached in nature overlap and mask the lower part of the ht unblocking temperature spectrum driving up tl values and leading to inflated estimates of tr although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the milton monzonite chemical overprinting may be a factor in other lithologies and tectonic settings

thermoremanent magnetization of nonuniformly magnetized grains a simple and elegant interpretation of thermoremanent magnetization trm in uniformly magnetized single domain sd grains was given by neel 50 years ago but the trm acquisition processes in larger nonuniformly magnetized grains are more varied and difficult to describe theoretically sd trm is a frozen high temperature partition between two microstates spins parallel or antiparallel to an applied magnetic field nonuniformly magnetized grains have a much greater choice of microstates local energy minimum or lem states and partitioning among various lem states continues to change during cooling these changes may involve barkhausen jumps of domain walls between positions of minimum local energy or nucleation of new domains and walls because of the lower remanence capacity of nonuniform microstates compared to the uniform sd state trm intensity decreases as grain size increases although certain microstates e g single vortex states seem to contribute little to trm thermal demagnetization of trm begins just above room temperature and continues to the curie point quite unlike the sharp unblocking of sd trm this continuous demagnetization resulting from changes in microstates driven by the changing internal demagnetizing field during heating profoundly affects the separation of different components of natural remanent magnetization and the determination of paleomagnetic field intensity