the nature of shallow water carbonate lithofacies thickness distributions carbonate lithofacies thickness distributions are of fundamental importance to understanding shallow water carbonate deposystems because they record evidence of the lateral distribution and migration of lithofacies elements as well as evidence for the various other intrinsic and extrinsic controls on stratal geometries and accumulation rates previous analyses of lithofacies thickness data led to the suggestion that exponential distributions are ubiquitous in the ancient record this has been interpreted to indicate deposition by stochastic poisson process lithofacies mosaics to further investigate these ideas statistical analysis of 56 outcrop and core examples was performed the kolmogorov smirnov test was used to identify the degree to which measured lithofacies thicknesses are well represented by a theoretical exponential distribution results from this analysis show that 16 of the 56 examples can be confidently shown to be exponential while 28 are very probably not exponential this indicates that stochastic poisson processes are a plausible explanation for many carbonate successions but they do not explain all of those tested here suggesting that other non poisson processes either stochastic or deterministic in nature or both must also be important thus lithofacies planform geometries and the processes controlling vertical stacking in ancient carbonate platform top deposystems were likely more diverse than has been suggested requiring significant further quantitative analysis and numerical forward modeling to properly understand

facies and beddding sequences in shelf storm deposited carbonates fayetteville shale and pitkin limestone mississippian arkansas abstract the fayetteville shale miss is a black marine shale that was deposited on a southward deepening ramp in northern arkansas it is conformably overlain by skeletal oolitic grainstones of the pitkin limestone and together they form a shoaling upward succession of 1 deep muddy shelf 2 storm dominated muddy shelf and 3 ooid skeletal shoal and shoreface facies rhythmically bedded limestones of the upper fayetteville shale and some lower pitkin ooid grainstones are interpreted to be shelf storm deposits because of their stratigraphic position and bedding sequences upper fayetteville limestone beds are transitionally positioned between cephalopod bearing black shales deep muddy shell underneath and cross bedded ooid grainstones shoal and shoreface above fayetteville and pitkin beds are similar to other reported shelf storm deposits in that they contain hummocky cross stratification hcs and unidirectional climbing ripple cross stratification thought to have been laid down by combined flows six principal types of bedding sequences were noted in order of increasing water depth and or distance from the inferred shore 1 cross bedded and planar laminated beds with some keystone vugs beach 2 graded units of coarse skeletal ooid grainstone shoreface 3 planar laminated and hummocky cross stratified limestones lower shoreface 4 hummocky cross stratified limestones and thin shales storm dominated muddy shelf 5 fine grainstones with parallel laminae in lower half followed above by climbing ripple cross stratification storm dominated muddy shelf and 6 structureless lime mudstones and black shale deep muddy shelf reconstruction of the ancient shelf suggests that it sloped southward at 0 08 to 0 14 and the water depth at 30 km estimated width of storm dominated shell was 40 to 70 m the paleolatitudinal position of northern arkansas 5 15 s indicates that strong tropical storms or hurricanes rather than intense winter storms were responsible for eroding carbonate sediment from lower shoreface environments and depositing it as much as 30 km offshore in the muddy shelf

magnetic stability of oceanic gabbros from odp hole 735b11measurements were performed while at the bundesanstalt fur geowissenschaften and rohstoffe hannover germany abstract ocean drilling program odp hole 735b was drilled to a depth of 1 5 km in a tectonic window of gabbroic lower oceanic crust created at the southwest indian ridge the gabbros have a very stable natural remanent magnetization nrm of reversed polarity with most unblocking temperatures slightly below the curie temperature of magnetite the nrm includes a drilling induced overprint but its intensity decays strongly towards the interior of the drill core the demagnetization data yield no or only a very small secondary magnetization component acquired during the present brunhes chron or an earlier normal chron suggesting cooling through most of the blocking temperature range during chron c5r and a strong resistance against the acquisition of thermoviscous magnetization a novel furnace has been designed to measure magnetizations and their time dependences at high temperatures up to 580 c inside a commercial squid magnetometer magnetic viscosity experiments have been conducted on the gabbros at temperatures up to 550 c to determine the time and temperature stability of remanent magnetization viscosities are generally small and increase little with temperature below the main blocking temperature where the increase becomes almost an order of magnitude extrapolations to geological times infer viscous acquisitions that would be 5 25 of a thermoremanence in 100 kyr and at temperatures of 200 500 c at ocean bottom temperature the predicted magnetization of one sample acquired in the present brunhes chron should be 10 of the nrm however this is not recognized during nrm demagnetization and partial thermoremanent magnetization ptrm acquisitions at 250 c are also much smaller than predicted it thus appears that the nrms are generally magnetically harder than magnetizations acquired after heating to 570 c in the laboratory susceptibility changes during heating are small

geometry spatial arrangement and origin of carbonate grain dominated scour fill and event bed deposits late jurassic jubaila formation and arab d member saudi arabia

magnetization modeling in the north and equatorial atlantic ocean using magsat data we have analysed the shorter wavelength component of the magnetic field observed by magsat over the north and equatorial atlantic ocean with the idea of trying to determine the possible sources of this component of the field in most cases inversion techniques produce both normally and reversely magnetized sources and we show that this is not appropriate for the possible sources in the oceanic crust which at the wavelengths observed by magsat need to be entirely normally magnetized or with a magnetization of zero in order to do this we have applied an annihilator to the solutions produced by our inversion technique we show how the annihilator that we use is convenient for matching up magnetization solutions for areas that have been inverted separately one problem with the inversion technique is that the best spacing of the sources can only be found by trial and error we show that much of the final variation in source spacing is produced by a latitudinal effect which seems to require sources spaced further apart the lower the latitude this may be due to contamination of the anomalous field by noise which might be expected to be worse at lower latitudes we have also examined the possibility that much of the magnetic source in the oceanic crust is due to induced magnetization the values of susceptibility necessary are much higher than those measured directly on samples taken from the ocean basins remanent magnetization is a more likely candidate but the values especially after application of the annihilator are still large compared with observed values only considerable thickening of the crust combined with remanent magnetization can explain the high intensities of magnetization that we have modeled viscous magnetization is a possible source for the high intensities of magnetization of our model one problem with an explanation of the field from crustal sources is that in many cases there is little correlation with the geological history of the region an alternative is that noncrustal sources contribute to the anomalous magnetic field derived from magsat data this field derived by subtracting degrees of harmonic 1 through 13 from a spherical harmonic model may be contaminated with core field signals from leakage of high amplitude short wavelength anomalies into longer wavelengths and by external fields

a 22 000 year record of vegetation change in the north central sonoran desert abstract plant macrofossils from eleven packrat neotoma sp middens provide a history of vegetation change over the last 22 140 years at eagle eye mountain near the northern border of the sonoran desert in arizona this site is located in the arizona upland subdivision departures and arrivals of various perennial plant species during the holocene were similar to other records from more southerly sites and support earlier generalizations of van devender van devender t r 1990 late quaternary vegetation and climate of the sonoran desert united states and mexico in betancourt j l van devender t r martin p s eds packrat middens the last 40 000 years of biotic change university of arizona press tucson pp 134 165 regarding the tempo and mode of late quaternary changes in the arizona upland evergreen woodland containing pinus monophylla singleleaf pinyon juniperus osteosperma utah juniper and quercus turbinella shrub live oak occupied the site during full glacial times the latter two species but not p monophylla persisted into the early holocene and were joined by a limited number of desertscrub species including acacia greggii catclaw carnegiea gigantea saguaro and several other succulents with the arrival of many other species during the middle holocene a desertscrub community that differed considerably from the modern vegetation developed acacia greggii and cercidium floridum blue paloverde were the principal large woody species on exposed hillslopes assemblages similar in composition to the modern desertscrub did not appear until approximately 4000 years ago when the more xerophytic cercidium microphyllum foothills paloverde arrived and c floridum disappeared from hillslopes the delayed arrival of c microphyllum in comparison to other sonoran desertscrub species with similar climatic tolerances is attributed to the limited dispersal distance of its seeds by seed eating heteromyid rodents at 4540 and 6425 yr b p the cold intolerant shrub encelia farinosa was present on northeasterly aspect exposures where it is absent today suggesting slightly warmer winter conditions during the middle holocene the presence of the tree cercidium floridum and the perennial c 4 grass setaria leucopila on exposed slopes during the middle holocene indicates substantially moister conditions during that time than in the late holocene or today at sites on the xeric limits of the modern distributions of both species average annual precipitation is from 50 to 100 greater than that presently received at eagle eye mountain in particular the responses of s leucopila and cacti during the middle holocene indicate substantial increases in warm season precipitation

seismic architecture of a lower cretaceous platform to slope system santa agueda and poza rica fields mexico two three dimensional seismic data sets over the albian western golden lane margin and time equivalent basinal deposits of poza rica field allowed us to investigate the linked architecture of a steep sided carbonate platform el abra formation and a thick accumulation of redeposited carbonate sediment at the toe of the slope and in the basinal area tamabra formation regional seismic cross sections show that the most aggrading albian platform has an eroded platform top a scalloped margin and a channelized slope that are equivalent to a 20 km 12 4 mi wide westward thinning thick toe of slope apron made of chaotic contorted mounded moderate to high amplitude reflections detailed reflection geometries in the albian toe of slope and basinal deposits consist of chaotic to short discontinuous low amplitude reflection at the toe of the slope of the golden lane platform laterally changing to a discontinuous mounded shingling reflection which ultimately turns into high amplitude parallel reflections we interpret this lateral change to reflect the seismic signature of the change from the block and debris flow dominated toe of slope area to debris flow and concentrated density flow deposits in the basin that ultimately grade laterally into pelagic deposits on a flattened seismic slice mounded reflections correspond to lobate to fan shaped seismic events several kilometers wide that are interpreted as a carbonate basin floor fan comparison between core and seismic data shows a dominance of debris flows in the lower two albian sequences albian 1 and albian 2 that grade vertically into more lobate concentrated density flows and turbidites in the upper two albian sequences albian 3 and albian 4 seismic data used in this study combined with core observations do not support the interpretation of the albian tamabra formation being of shallow water origin seismic features identified as basin floor fan channel and debris flow deposits have a shape and size that are similar to those of other redeposited basinal carbonate deposits elsewhere the seismic architecture shows that the poza rica field is a typical example of thick accumulation of grainy porous carbonate deposits in a basinal setting this example shows the potential of a large hydrocarbon accumulation in a tectonically modified stratigraphic trap around shallow water carbonate platforms

holocene vegetation and climate in the puerto blanco mountains southwestern arizona abstract plant macrofossils from 21 pack rat neotoma sp middens at 535 605 m from the puerto blanco mountains southwestern arizona provide and excellent history of vegetation and climate for the last 14 120 yr b p in the sonoran desert a late wisconsin juniper joshua tree woodland gave way to a transitional early holocene desertscrub with sparse juniperus californica california juniper by 10 540 yr b p important sonoran desert plants including carnegiea gigantea saguaro and encelia farinosa brittle bush were dominants riparian trees such as acacia greggii catclaw acacia prosopis velutina velvet mesquite and cerdicium floridum blue palo verde grew on dry south facing slopes in a middle holocene sonoran desertscrub in a warm wet summer climate with frequent winter freezes modern subtropical sonoran desertscrub formed about 4000 yr b p as summer rainfall and winter freezes declined cercidium microphyllum foothills palo verde sapium biloculare mexican jumping bean olneya tesota ironwood and stenocereus thurberi organ pipe cactus became dominant as riparian trees retreated to wash habitats the inferences of a latest wisconsin early holocene summer monsoonal maximum by j e kutzbach 1983 modeling of holocene climates in late quaternary environments of the united states vol 2 the holocene h e wright ed pp 271 277 univ of minnesota press minneapolis are not supported for the southwest apparently the persistence of late wisconsin circulation patterns offset any increases in insolation

reconstruction of past desert vegetation along the colorado river using packrat middens abstract the paleoecological reconstruction of pleistocene deserts along the colorado river of western north america is attempted using data from fossil packrat middens the colorado river drainage is set into a physiographic context along a gradient from the hyperarid colorado desert to the moist high elevations on the colorado plateau the pleistocene and modern distributions of individual plant species along this corridor are compared emphasizing records from the picacho peak area of the colorado desert and the eastern grand canyon in general plant species are now distributed 700 900 m higher in elevation and 400 700 km further up river than they were during the late wisconsin however some plant species have not conformed to the general pattern emphasizing the individualistic nature of species distributions in time

origin of the absence of magnetic lineations in the yamato basin of the japan sea magnetic properties of mafic rocks from ocean drilling program hole 794 d we investigated magnetic properties of mafic rocks recovered from ocean drilling program hole 794 d in the yamato basin of the japan sea in contrast to other back arc basins in the western pacific lineated magnetic anomaly patterns have not been recognized in the yamato basin our objective is to clarify the reason for the absence of lineated magnetic anomalies by direct magnetic measurements of crustal rocks the dominant magnetic mineral of hole 794 d is titanomaghemite as commonly observed for typical oceanic basalts but the grain size is much larger greater than several tens of microns remanence properties i e low natural remanent magnetization intensities 1 5 a m low koenigsberger ratios 1 0 high viscous remanent magnetization and mixed magnetic polarities indicate that the mafic rocks cannot be a source of lineated magnetic anomalies while curie temperature saturation magnetization and susceptibility values are not significantly different from those of typical oceanic basalts magnetic properties related to stability are greatly reduced the large grain size of the mafic rocks from hole 794 d plays a primary role in differentiating the magnetic properties from those of typical oceanic basalts and is caused by the formation of sill sediment complexes beneath the yamato basin sill sediment complexes rather than extrusive basalts in typical oceanic crust are responsible for the absence of lineated magnetic anomalies in the yamato basin

origin of shore normal channels from the shoreface of sable island canada a suite of modern shore normal channels was discovered on the southern shoreface of sable island canada repetitive multibeam and sidescan surveys of these features as well as box coring and sampling showed that the depressions were perennial features of the shoreface they occurred in fine and medium sand and were most abundant in depths less than 20 m they were up to 1 m deep and 50 m wide increased in size seawards and generally followed the seabed slope the channels dominated the inner shoreface becoming wider and deeper seawards to depths in excess of 40 m there is a continuum of scales in width and depth of these channels the smallest of which are equated with gutters preserved in the geological record the internal structure of the shoreface sediments where the gutters are found is characterised by normally graded tempestites composed of cut and fill structures overlying an erosion surface thus the environmental setting internal structure and scale are similar to ancient counterparts gutters have been interpreted to result from downwelling over palaeo shorefaces during ancient storms myrow 1992b the composition of gutter infill as well as the stability evolution associated bedforms and hydrodynamic conditions under which they formed were examined to verify genetic interpretations of the ancient counterparts the multi sensor benthic lander ralph was deployed within a field of shore normal channels for 17 days during late winter 1998 ralph burst sampled flow at hourly intervals whilst imaging the seabed resulting sequential imagery revealed the genesis of a group of five gutters during storm spin down that was coincident with the formation of large scale wave ripples this was followed by infilling of the gutters and wave ripple destruction within 20 hours by longshore sand transport the gutters were located within and parallel to a larger channel 50 m wide and 0 50 m deep they were formed by coastal set up and subsequent downwelling that was triggered by the rotation of storm winds from seawards onshore wind to landwards offshore wind the gutters formed within 2 to 3 hours during spin down of a winter storm under live bed conditions of sand transport wave height and mean tidal flows were well below those of the storm peak and so they were not directly responsible for creating the gutters the mechanism we propose for the evolution of gutters supports the geological interpretations that is a storm driven downwelling event as described by swift and niedoroda 1985 however the downwelling event appeared to take place suddenly rapid acceleration in flow briefly few hours duration and within narrow corridors guided by wider and deeper channels of the shoreface high turbidity during the time of gutter formation leads us to believe that the phenomenon of inner shelf turbidity currents as suggested by walker 1985 exists and may be instrumental in channel formation

deglacial climatic oscillations in the gulf of california a high resolution accelerator radiocarbon dated climate record of the interval 8 000 18 000 years b p from deep sea drilling project site 480 guaymas basin gulf of california shows geochemical and lithological oscillations of oceanographic and climatic significance during deglaciation nonlaminated sediments are associated with cooler climatic conditions during the late glacial up to 13 000 years b p and from 10 300 to 10 800 years b p equivalent to the younger dryas event of the north atlantic region we propose that the changes from laminated varved to nonlaminated sediments resulted from increased oxygen content in pacific intermediate waters during the glacial and the younger dryas episodes and that the forcing for the latter event was global in scope prominent events of low 18o are recorded in benthic foraminifera from 8 000 to 10 000 and at 12 000 years b p evidence for an earlier event between 13 500 and 15 000 years b p is weaker maximum 18o is found to have occurred 10 500 13 500 and 15 000 years ago and beyond oxygen isotopic variability most likely reflects changing temperature and salinity characteristics of pacific waters of intermediate depth during deglaciation or environmental changes within the gulf of california region several lines of evidence suggest that during deglaciation the climate of the american southwest was marked by increased precipitation that could have lowered salinity in the gulf of california recent modelling studies show that cooling of the gulf of mexico due to glacial meltwater injection which is believed to have occurred at least twice during deglaciation would have resulted in increased precipitation with respect to evaporation in the american southwest during summertime the timing of deglacial events in the gulf of mexico and the gulf of california supports such an atmospheric teleconnection

origin of the rudstone floatstone beds in the upper jurassic arab d reservoir khurais complex saudi arabia abstract detailed description and analysis of cores from more than 50 wells through the kimmeridgian arab d reservoir arab d member and upper part of the jubaila formation in khurais abu jifan and mazalij oil fields are used to establish a consistent depositional model that represent a shift in the conceptual understanding of the upper jurassic carbonates of saudi arabia the jubaila fm and arab d mb conform to a shallowing upward carbonate evaporite succession in which three main carbonate facies belts occur capped by anhydrite the lower facies belt corresponds to the jubaila formation and consists of a series of eventites triggered by breaking internal waves interbedded peloidal wackestones represents the background sedimentation where neritic lime mud was churned by pellet producing benthonic fauna eventites consist of a lower intraclastic rudstone floatstone interval a middle grainstone packstone interval and an upper barren mudstone interval the middle facies belt is made of microbial coral stromatoporoid buildups the upper facies belt consists of the following shallow water lithofacies with cladocoropsis green algae foraminifers peloids and oolitic grainstones and oolitic cryptomicrobial mudstone to grainstone the stacking patterns of the internalites internal wave deposits in the jubaila fm suggests alternating periods with different internal waves activity namely different strength and bathymetric position of the pycnocline and intermittent production and accumulation of the neritic lime mud this interpretation represents a change to previous interpretations turbidites and tempestites and has important implications in both the perceptions of depositional processes in play and the criteria behind stacking lithofacies into parasequences and sequences

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

tropical climates at the last glacial maximum a new synthesis of terrestrial palaeoclimate data i vegetation lake levels and geochemistry palaeodata in synthesis form are needed as benchmarks for the palaeoclimate modelling intercomparison project pmip advances since the last synthesis of terrestrial palaeodata from the last glacial maximum lgm call for a new evaluation especially of data from the tropics here pollen plant macrofossil lake level noble gas from groundwater and delta 18 o from speleothems data are compiled for 18 2 ka c 14 32 degrees n 33 degrees s the reliability of the data was evaluated using explicit criteria and some types of data were re analysed using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the coldest month mtco mean annual temperature mat plant available moisture pam and runoff p e cold month temperature mat anomalies from plant data range from 1 to 2 k near sea level in indonesia and the s pacific through 6 to 8 k at many high elevation sites to 8 to 15k in s china and the se usa mat anomalies from groundwater or speleothems seem more uniform 4 to 6 k but the data are as yet sparse a clear divergence between mat and cold month estimates from the same region is seen only in the se usa where cold air advection is expected to have enhanced cooling in winter regression of all cold month anomalies against site elevation yielded an estimated average cooling of 2 5 to 3 k at modern sea level increasing to approximate to 6 k by 3000m how ever neotropical sites showed larger than the average sea level cooling 5 to 6 k and a non significant elevation effect whereas w and s pacific sites showed much less sea level cooling 1 k and a stronger elevation effect these findings support the inference that tropical sea surface temperatures ssts were lower than the climap estimates but they limit the plausible average tropical sea surface cooling and they support the existence of climap like geographic patterns in sst anomalies trends of pam and lake levels indicate wet lgm conditions in the w usa and at the highest elevations with generally dry conditions elsewhere these results suggest a colder than present ocean surface producing a weaker hydrological cycle more arid continents and arguably steeper than present terrestrial lapse rates such linkages are supported by recent observations on freezing level height and tropical ssts moreover simulations of greenhouse and lgm climates point to several possible feedback processes by which low level temperature anomalies might be amplified aloft

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

a middle holocene vegetation record from the mojave desert of north america and its paleoclimatic significance abstract packrat middens from the mccullough range in the southeastern mojave desert of nevada record vegetation conditions during the periods 6800 to 5060 yr b p and 1250 yr b p to present their age and location are ideal to test for middle holocene aridity the period between 6800 and 5060 yr b p does appear to have been more arid than during the late holocene period of record this agrees with other evidence for middle holocene aridity from the southwest arguments for strengthened middle holocene monsoons in the region are based chiefly on packrat midden samples older than 7500 yr b p or on samples that are younger than 5500 yr b p simulations of 6000 yr b p climate that fail to show strong subtropical flow into the southwest are therefore supported by evidence for enhanced aridity in that region

onset of submarine debris flow deposition far from original giant landslide submarine landslides can generate sediment laden flows whose scale is impressive individual flow deposits have been mapped that extend for 1 500 km offshore from northwest africa these are the longest run out sediment density flow deposits yet documented on earth this contribution analyses one of these deposits which contains ten times the mass of sediment transported annually by all of the world s rivers understanding how this type of submarine flow evolves is a significant problem because they are extremely difficult to monitor directly previous work has shown how progressive disintegration of landslide blocks can generate debris flow the deposit of which extends downslope from the original landslide we provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide encased within deposits of a more dilute flow type called turbidity current very little sediment was deposited across the intervening large expanse of sea floor where the flow was locally very erosive sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea floor gradient from 0 05 degrees to 0 01 degrees this debris flow was probably generated by flow transformation from the decelerating turbidity current the alternative is that non channelized debris flow left almost no trace of its passage across one hundred kilometres of flat 0 2 degrees to 0 05 degrees sea floor our work shows that initially well mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean

late quaternary arthropod remains from sonoran desert packrat middens southwestern arizona and northwestern sonora abstract a total of 50 arthropod taxa were identified from 41 fossil packrat neotoma sp middens from 160 to 625 m elevation in three study areas in the lower colorado river valley subdivision of the sonoran desert radiocarbon dates associated with the middens range from 43 200 to 610 yr b p the fauna in the tinajas altas mountains southwestern arizona was relatively modern by ca 10 000 yr b p in the early holocene although a california juniper woodland persisted in the area until 8970 yr b p in contrast the fauna of the hornaday mountains northwestern sonora increased dramatically in species richness after 4000 yr b p although we are limited by poor taxonomic resolution and by insufficient knowledge of current distributions the arthropod fauna may have been much more conservative than the regional flora during the last glacial interglacial climatic cycle decreasing differences between modern and glacial climates both temperature and precipitation at lower latitudes and elevations may have resulted in minimal changes in the arthropod fauna of the sonoran desert lowlands

stratal thickness frequencies and the prevalence of orderedness in stratigraphic sequences assuming the existence of discernable hierarchies in the thickness and or temporal recurrence of stratal units within sedimentary sequences has become an increasingly important axiom of sequence and cyclostratigraphic studies and multiple orders of stratigraphic cyclicity are now commonly associated with inferred durations and magnitudes of rhythmic variation in global sea level beyond the desire to establish an informal nomenclature relating stratigraphic thickness to periodicity of eustatic change an assumption of stratigraphic orderedness also supports the inference that groups of sedimentary units with distinct modal dimensions in fact do occur with some generally distinct frequency in stratigraphic sequences in contrast to such perceived patterns of stratigraphic order many groupings of sedimentary units exhibit lognormal frequency distributions wherein most of the population has an exponentially increasing frequency of occurrence with linearly decreasing class size such thickness distributions

late triassic and early jurassic palaeogeography of the world abstract a new version of global and regional palaeogeographic maps is presented for two time intervals these maps depict the plate tectonic configuration palaeoenvironment and lithofacies during the late triassic carnian norian and early jurassic hettangian toarcian time the individual maps illustrate the general conditions present during the maximum marine transgressions of higher frequency cyclicity within the absaroka sequence of sloss during triassic time pangaea began to stretch initiating the rifting and future break up of the supercontinent the continued northward drift of the cimmerian continent corresponded with the progressive closure and consumption of palaeotethys oceanic crust and the opening of the neotethys ocean the most significant late triassic convergent event was the indosinian orogeny occurring as a result of the consolidation of south china and north china blocks also indochina and indonesia were sutured to south china at the same time the qiangtang block approached the eurasian margin the consolidation of the north chinese and amurian blocks left open a large embayment of panthalassa between amuria and laurasia the so called mongol okhotsk ocean active subduction existed along the margin of this ocean dipping cratonwards towards east siberia the last collisional events of the uralian orogeny took place during the triassic and early jurassic time the conclusion of the uralian orogeny was accompanied by uplift of the adjacent areas of eastern europe and western siberia during the early jurassic the palaeotethys ocean was finally closed and the cimmerian continent collided with asia causing the cimmerian orogeny the time around the triassic jurassic boundary marked an important biotic extinction event plate tectonic activity caused palaeogeographic and palaeoclimatic change which may have contributed to the mass extinction from the plate tectonic and palaeogeographic point of view the following events could have influenced the extinction 1 the closure of palaeotethys and assembly of the asian part of pangaea 2 the break up of pangaea in the future central atlantic area and transition from rifting to drifting and 3 the very extensive basaltic volcanism of the central atlantic magmatic province

carbonate apron models alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration sediment gravity flow deposition along the deep water flanks of carbonate platforms typically does not produce submarine fans rather wedge shaped carbonate aprons develop parallel to the adjacent shelf slope break the major difference between submarine fans and carbonate aprons is a point source with channelized sedimentation on fans versus a line source with sheet flow sedimentation on aprons two types of carbonate aprons may develop along relatively gentle 4 platform margin slopes aprons form immediately adjacent to the shallow water platform and are referred to as carbonate slope aprons along relatively steep 4 15 platform margin slopes redeposited limestones accumulate in a base of slope setting by passing an upper slope via a multitude of small submarine canyons and are referred to as carbonate base of slope aprons both apron types are further subdivided into inner and outer facies belts inner apron sediments consist of thick mud supported conglomerates and megabreccias facies f as well as thick coarse grained turbidites facies a interbedded with subordinate amounts of fine grained peri platform ooze facies g outer apron sediments consist of thinner grain supported conglomerates and turbidites facies a as well as classical turbidites facies c with recognizable bouma divisions interbedded with approximately equal proportions of peri platform ooze facies g seaward aprons grade laterally into basinal facies of thin base cut out carbonate turbidites facies d that are subordinate to peri platform oozes facies g carbonate base of slope aprons grade shelfward into an upper slope facies of fine grained peri platform ooze facies g cut by numerous small canyons that are filled with coarse debris as well as intraformational truncation surfaces which result from submarine sliding in contrast slope aprons grade shelfward immediately into shoal water platform margin facies without an intervening by pass slope the two carbonate apron models presented here offer alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration for mass transported carbonate facies

storm deposits and storm generated coarse carbonate breccias on a pelagic outer shelf south east basin france uppermost jurassic limestones of the south east basin france are organized into four facies associations that were deposited in four distinct zones 1 peritidal lagoonal limestones 2 bioclastic and reefal limestones 3 pelagic lime mudstones 4 lime mudstones calcarenites coarse breccias calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave induced combined flow in subzone 4a both vertical and lateral transitions from lime mudstone calcarenite to breccia indicate in situ brecciation under wave cyclic loading breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor deposits in subzone 4a record relatively long periods 400 kyr of sedimentation below wave base alternating with periods of deposition under wave induced currents and periods of in situ deformation in this zone storm waves were attenuated by wave sediment interaction and wave energy was absorbed by the deformation of soft sediment with reference to present day wave attenuation water depths in this zone ranged between 50 and 80 m landwards of the attenuation zone in zone 3 storm waves were reduced to fair weather wave heights storm wave base was not horizontal and became shallower landwards as a consequence water depth and wave energy were not linearly related on a small area of the seaward edge of subzone 4a cobbles were removed by traction currents and redeposited in subzone 4b there they formed a 100 m thick wedge which prograded over 3 km and was built up by the stacking of 5 to 20 m thick cross stratified sets of coarse breccia this wedge records the transport and redeposition of cobbles by a high velocity unidirectional component of a combined flow the increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel like structure of the downwelling in the gulf formed by the basin in more distal subzone 4c the hydrodynamic effect of wave induced currents was quasi permanent and brecciation by wave sediment interaction occurred only episodically this indicates that seawards of the attenuation zone hydrodynamic storm wave base was deeper than mechanical storm wave base uppermost jurassic carbonates were deposited and soft sediment deformed on a hurricane dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration located seawards of a zone of sedimentation below wave base

marine magnetic anomalies the origin of the stripes the results of recent observational and theoretical investigations of lineated magnetic anomalies on the ocean floor are summarized in tables graphs and diagrams and analyzed topics addressed include early lineation models inversions of magnetic anomalies to obtain source functions deep tow studies of magnetic anomalies evidence from the long wavelength component of the magnetic field including magsat observations and direct measurements of the magnetic properties of oceanic rocks it is concluded that the source of the lineated anomalies must reside in most of the oceanic crust not just in the pillow lavas of layer 2a

storm wave reactivation of a submarine landslide submarine landslides move large amounts of sediment and are often hazardous to offshore installations marine surveying techniques effectively define landslide locations and geometries for example refs 1 4 in various underwater geological settings including deltas fjords and earthquake prone areas1 5 movement rates and mechanics are generally inferred from indirect evidence such as cable breaks and damage to sea floor structures for example refs 6 8 but actual measurements of landslide behaviour are sparse using bottom deployed pressure sensors tiltmeters and accelerometers we have collected data on sea floor landslide reactivation involving sediment collapse and remoulding on the submarine slopes of the huanghe delta china the sensors were emplaced into sea floor clayey silts on 4 october 1987 and recovered on 16 october after the passage of three severe storms

significance of channel belt clustering in alluvial basins the distribution of channel deposits in alluvial basins is commonly used to interpret past changes in climate tectonics and sea level here we present preliminary evidence that longtime scale 10 3 10 5 yr self organization in fl uvial systems may generate structured stratigraphic patterns spontaneously in the absence of or independent from changing basin boundary conditions a physical experiment and an ancient alluvial succession ferris formation latest cretaceous paleogene south central wyoming both show stratigraphy where clusters of many closely spaced channel deposits are separated from each other by extensive intervals of overbank mudstones analysis using spatial point process methods shows that channel deposits in both basins are statistically clustered over intermediate basin length scales in the experiment external controls base level subsidence rate and sediment water supplies were not varied and therefore not factors in cluster formation likewise the ancient system lacks stratigraphic and sedimentologic evidence of external controls on channel clustering we propose that channel clusters as seen in this study refl ect a scale of flself organization that is not usually recognized in ancient deposits this type of internally generated stratigraphy should be considered when reconstructing tectonic climate and sea level changes from

carbonate filled channel complexes on carbonate ramps an example from the peerless park member keokuk limestone visean lower carboniferous mississippian st louis mo usa abstract the peerless park member keokuk limestone lower carboniferous visean mississippian exposed in the st louis mo area includes a carbonate filled channel complex with lithology and faunal elements distinct from under and overlying strata the complex is confined within a channel of 100 m apparent width that is under and overlain by cherty glauconitic crinoid bryozoan skeletal wackestone packstone and grainstone with abundant hardgrounds and little evidence for wave or current energy laterally the valley includes two subchannels separated by a subtle high the deeper subchannel truncates up to 5 m of strata the maximum thickness of the channel fill and associated deposits is 6 m the channel fill includes a matrix supported conglomerate locally at its base non cherty quartz silty foraminifer peloid crinoid bryozoan skeletal grainstone with spectacular lateral accretion foresets up to 5 m thick and cherty skeletal packstone a capping foraminifer skeletal grainstone is erosionally overlain by a 10 cm thick quartz siltstone that grades up into glauconitic crinoid bryozoan skeletal wackestone and packstone on the channel flanks correlative strata of the peerless park member are less than 1 m thick and include a lower non cherty crinoid foraminiferal skeletal grainstone a middle cherty packstone and an upper skeletal grainstone the upper grainstone is in turn erosionally overlain by the thin quartz siltstone that abruptly grades into the crinoid bryozoan skeletal wackestone and packstone of the upper part of the keokuk limestone the strata of the peerless park member are interpreted to be the result of a relative fall of sea level that places shallow water high energy inner ramp grainstone immediately above deeper water outer ramp packstone and grainstone there is no evidence for gradual downward or basinward migration of facies belts instead shallow water facies appear to have abruptly jumped downdip likewise no field or petrographic evidence for subaerial exposure has been recognized so the relative fall in sea level may not have subaerially exposed the area this occurrence of an isolated downdip carbonate grainstone filled channel complex not accompanied by progradational forced regressive deposits appears to be unique in carbonate ramp depositional systems and the channel includes fill with character different from many previously documented ramp carbonate filled channels

caves as sources of biotic remains in arid western north america caves have been a valuable source of paleoenvironmental information since the eighteenth century in arid portions of western north america caves have replaced small lakes as the primary source of biotic remains of quaternary age packrat middens dung of extinct animals pollen and archeological artifacts are excellently preserved aridity of the sediment which is influenced by the cave s topographic position internal morphology and the regional climate is the primary cause of good preservation for stratigraphic pollen analysis the best sites are those without internal moisture sources and without extensive bioturbation pollen samples should be taken near the center of the chamber where eolian deposition is most rapid pollen concentrations in cave sediment are lower 2000 355 000 grains g 1 than in lake sediment and pollen percentages in cave sediment differ from those in packrat middens due to additional transport mechanisms on plant tissue and on the packrats themselves for middens pollen diagrams for bechan cave utah and organ pipe cactus national monument arizona are presented as examples of cave sediment and packrat midden analysis

late quaternary lacustrine history and paleoclimatic significance of pluvial lake cochise southeastern arizona abstract during the latest quaternary freshwater pluvial lakes intermittently formed in the topographically closed willcox basin arizona a lacustrine sequence of six separate high stands of lake cochise is documented by stratigraphic studies 19 radiocarbon ages and supplementary evidence two stands of pluvial lake cochise older than 14 000 yr b p reached elevations above 1290 m the prominent 1274 m shoreline of lake cochise which circumscribes the basin was largely created during a high stand between 13 750 and 13 400 yr b p during the holocene water filled the willcox basin three times to an elevation slightly below the crest of the 1274 m shoreline this occurred once during the early holocene around or before 8900 yr b p and twice during the later part of the middle holocene since the middle holocene only shallow ephemeral lakes have occupied the deflated central portion of ancient lake cochise a depression known as the willcox playa the lacustrine sequence of lake cochise provides an independent evaluation of late quaternary paleoclimatic reconstructions for southern arizona and the american southwest

magnetite rods in plagioclase as the primary carrier of stable nrm in ocean floor gabbros abstract the ubiquitous occurrence of magnetite rods in plagioclase may account for the stable intense magnetization of ocean floor gabbros these single domain grains are formed by the subsolidus exsolution of fe 2 and fe 3 incorporated into the plagioclase at higher temperatures we expect that this process will be particularly important in the upper portions of the oceanic gabbro unit because of the relative paucity of femic minerals at these levels the geometry of accretion prohibits a constructive contribution to marine magnetic anomalies by the lower olivine rich gabbros as a result of the presence of these rods ocean floor gabbros may constitute an important source for marine magnetic anomalies

a holocene climatic record for the sonoran desert from pollen analysis of montezuma well arizona usa abstract pollen and macrofossil analyses of a radiocarbon dated core 1125 cm long from montezuma well elev 1125 m yavapai county arizona provide a record of summer precipitation similar to that of other monsoonal regions of the world precipitation was least from 4000 to 5000 yr b p and generally was greater than today before 8400 yr b p during the early holocene moist period oak and grass pollen are abundant and the best modern analogs are within the arizona monsoon boundary so increased summer precipitation is indicated climatic parameters are estimated with the technique of best modern analogs for fossil samples temperature was highest ca 3400 and 5700 yr b p and generally was cooler than today before 6800 yr b p brief cold intervals in the holocene match periods of global cooling and of positive 14 c and 10 be anomalies

fossil packrat middens and the tandem accelerator mass spectrometer analyses of well preserved plant remains from ancient packrat neotoma middens have yielded much information on the history of vegetation fauna and climate from the more arid portions of north america over the past 40 000 years1 2 in most of the modern deserts woodland or forest communities were present during the last glacial age the wisconsin radiocarbon dating of packrat midden assemblages from a single cave or from several nearby sites in a homogeneous local area can yield a detailed local vegetation chronology for many thousands of years from within 30 50 m on the rocky slopes in front of the dry rock shelters3 4 the development of radiocarbon dating using the tandem accelerator mass spectrometer tams allows direct dating of very small samples here we present 20 examples of tams radiocarbon dates on packrat midden materials to illustrate their usefulness in testing anomalous mixtures of species in ice age communities understanding the details of plant migrations and refining wisconsin and holocene time boundaries based on range changes of important plant species table 1 fig 1

seafloor geomagnetic vector anomaly of the intersection of the mid atlantic ridge and the kane transform fault implications for magnetization of the oceanic crust we investigated the fine scale magnetic structure of the oceanic crust and upper mantle around the western intersection of the mid atlantic ridge and the kane transform fault using the geomagnetic vector field measured at the seafloor the strikes of magnetic boundaries were found to reflect of the local tectonics represented by fault strikes and dips the ridge transform intersection is characterized by magnetic boundary strikes rotated eastward the strike directions reflect the direction of recent magmatic activity these indicate that a few traces of faults extending obliquely from the ridge axis to the transform fault are associated with neovolcanism magnetic boundary strikes in the lower part of the transform valley wall and near a linear trace of the transform fault indicate deformation or tilting of the crust associated with transform faulting it is also indicated that the crustal block around the median ridge is tilted by uplift accompanied by injection or emplacement and serpentinization of peridotites the nodal basin consists of extrusive basalts in the e w direction and n s trending magnetic strikes submersible observation of in situ magnetization of oceanic crust shows that extrusive basalts are typically strongly magnetized the most recent neovolcanic ridge has a mean magnetization of 50 a m the mean magnetization of the ridge axis is 20 a m the magnetization of the 3 ma ridge flank in the inside corner high is about 10 a m the gabbroic layer in the transform valley has a magnetization less than 2 a m the serpentinized peridotites in the median ridge have magnetization less than i a m this magnetization records a reversal in polarity which is important because the serpentinized peridotites contribute to the marine geomagnetic anomaly pattern

three dimensional modelling and sequence stratigraphy of a carbonate ramp to shelf transition permian upper san andres formation this study highlights three dimensional variability of stratigraphic geometries in the ramp crest to basin of mixed carbonate siliciclastic clinoforms in the permian san andres formation standard field techniques and mapping using ground based lidar reveal a high degree of architectural complexity in channellized scoured and mounded outer ramp stratigraphy development of these features was a function of location along the ramp profile and fluctuations in relative sea level deposition of coarse grained and fine grained turbidites in the distal outer ramp occurred through dilute and high density turbidity flows and was the result of highstand carbonate shedding within individual cycles in this setting high frequency cycles of relative sea level are interpreted on the basis of turbidite frequency lateral extent and composition submarine siliciclastic sediment bypass during lowstand cycles resulted in variable degrees of siliciclastic preservation abundant siliciclastic material is preserved in the basin and distal outer ramp as point sourced lowstand wedges and line sourced early transgressive blankets in mounded topography of the outer ramp siliciclastic preservation is minimal to absent and rare incised channels offer the best opportunity for recognition of a sequence boundary growth of mounded topography in the outer ramp began with scouring followed by a combination of bioherm construction fusulinid mound construction and isopachous draping intermound areas were then filled with sediment and continued mound growth was prevented by an accommodation limit mound growth was independent of high frequency cycles in relative sea level but was dependent on available accommodation dictated by low frequency cyclicity low angle ramp clinoforms with mounded topography in the outer ramp developed during the transgressive part of a composite sequence mound growth terminated as the ramp transformed into a shelf with oblique clinoform geometries during the highstand of the composite sequence this example represents a ramp to shelf transition that is the result of forcing by relative sea level fluctuations rather than ecologic or tectonic controls

magnetic properties of the bay of islands ophiolite suite and implications for the magnetization of oceanic crust rock magnetic properties opaque mineralogy and degree of metamorphism were determined for 101 unoriented samples from the north arm and blow me down massifs of the bay of islands ophiolite complex newfoundland the weathered and metamorphosed extrusive basalt samples have a weak secondary magnetization arising from oxidation and exsolution of ilmenite of unknown origin the initial magnetization of the underlying sheeted dike complex appears to have been destroyed by hydrothermal alteration soon after formation the magnetic intensity of the gabbroic samples increases as the degree of alteration increases with the highly altered upper metagabbros having an average intensity of 3 10 3 emu c3 because magnetization of the metagabbro samples is related to nonpervasive variable alteration these crustal units are unlikely to make a significant contribution to lineated magnetic anomalies a compilation of our results and other studies suggests a model in which oceanic crust magnetization results from an upper extrusive basalt source layer roughly 600 m thick with no contribution from a deeper source layer recognizable from these bay of islands data

typhoon vamei an equatorial tropical cyclone formation 1 due to the diminishing coriolis effect the belt 300 km either side of the equator has been considered tropical cyclone free typhoon vamei which developed near singapore on 27 december 2001 was the first recorded tropical cyclone formation within 1 5 degrees of the equator the development was the result of two interacting systems a weak borneo vortex that drifted into the southern tip of the south china sea and remained there for four days and a strong and persistent cold surge that created the large background cyclonic vorticity at the equator the probability of a similar equatorial development is estimated to be once every 100 400 years

ultra thick triassic dolomites control the rupture behavior of the central apennine seismicity evidence from magnetic modeling of the l aquila fault zone high resolution tomography from the 2009 l aquila extensional seismic sequence has shown that the mw 6 1 main shock and most of the aftershocks occurred within a high velocity body 6 6 vp 6 8 km s located between depths of 3 and 12 km the nature of the high vp body has remained speculative although exhumed mafic deep crustal and upper mantle rocks serpentinites have been favored we used 3d magnetic anomaly modeling to investigate the plausibility of these favored sources for the l aquila body the modeling does not support the presence of high velocity serpentinites with a 30 50 serpentinization degree and gabbros accordingly we conclude that the high vp body may represent non magnetic upper triassic and possibly lower liassic dolomites that have been drilled in neighboring wells for 2 4 km this conclusion is also consistent with the lack of a coherent gravity anomaly for the body we speculate that ultra thick triassic dolomites reaching a thickness of 8 km may have been deposited in syntectonic wedges formed at the northern margin of the ionian sea where oceanic spreading occurred in mid late triassic times

impact of dynamic sedimentation on facies heterogeneities in lower cretaceous peritidal deposits of central east oman this study based in the haushi huqf area of central east oman aims to characterize the controls on facies distribution and geometries of some of the best preserved examples of lower cretaceous tidal flat facies within the tethyan epeiric platform field petrographic and geochemical data were acquired from the barremian aptian jurf and qishn formations that crop out in a 500 1000 m2 butte thus allowing for pseudo three dimensional quantitative data acquisition of the dimensions and spatial distributions of discontinuity surfaces and sedimentary bodies the interpretation presented here suggests that the main processes impacting sedimentation in the lower cretaceous peritidal environment of the haushi huqf were transport and erosion processes related to storm waves and currents the vertical evolution of the carbonate system is organized into six types of metre scale depositional sequences from subtidal dominated sequences to supratidal capped sequences which are bounded by regional discontinuity surfaces at subaerial exposure and submarine erosion surfaces associated with a base level shift sedimentary horizons along the entire depositional profile are cut by scours possibly created by storm events chemostratigraphy allows correlation between the haushi huqf and the age equivalent sections logged in the interior of the platform in oman the correlation suggests that the change from subtidal to intertidal depositional sequences during the late highstand is coeval with the development of rudist dominated shoals on the shelf this study is the first to discuss the controls on lower cretaceous peritidal carbonate cyclicity of the arabian epeiric platform the results presented here also offer a unique quantitative dataset of the distribution and dimensions of peritidal carbonate shoals and storm scours in a regional sequence stratigraphic context

pluvial climatic episodes in north america and north africa types and correlation with global climate abstract differences in late quaternary pluvial chronologies among the world s deserts can be related to fundamental differences in the climatology of those pluvial episodes paleoenvironmental records and climate model simulations for two northern hemisphere desert regions are used to illustrate two types of pluvial climatic regime in the higher latitude deserts of the american southwest pluvial conditions during the last glacial maximum were caused by the southward displacement of the westerly jet stream in turn caused by the blocking effect of laurentide ice sheet and a steepened arctic equator pressure gradient winter seasonality of precipitation and substantially lower mean annual temperatures are reflected in the paleoenvironmental record and simulated by models of 18 000 yr b p climate a second pluvial climate type which strongly influences the lower latitude deserts of north africa is characterized by enhanced summer monsoons and occurs during interglacials and perhaps interstadials the primary forcing mechanism of the last pluvial episode in the sahara and libyan deserts appears to have been increased solar radiation milankovitch forcing causing thermally enhanced ocean land pressure gradients and increased flow of maritime tropical air into the interior of the continent poleward displacement of the summer monsoon is consistent with floristic evidence for summer seasonality of precipitation and replicated by model simulations of 9000 and 6000 yr b p climate chronologies from some deserts such as the southern portion of the american southwest suggest the consecutive occurrence of both types of pluvial climatic regime pluvial conditions during the last glacial maximum were brought about by the equatorward displacement of the prevailing westerlies a subsequent more subdued pluvial period occurred later between 12 500 and 8000 yr b p caused by the poleward displacement of summer monsoons characterization of the climatology of pluvial episodes generates explanations for their temporal distribution and therefore this approach may be useful in assessing the quaternary chronology of pluvial episodes in other desert regions

contribution of oceanic gabbros to sea floor spreading magnetic anomalies the contribution of oceanic gabbros representative rocks for layer 3 of the oceanic crust to sea floor spreading magnetic anomalies has been controversial because of the large variation in magnetic properties ocean drilling program odp leg 118 contains a continuous 500 7 meter section of oceanic gabbro that allows the relations between magnetization and petrologic characteristics such as the degree of metamorphism and the magmatic evolution to be clarified the data suggest that oceanic gabbros together with the effects of metamorphism and of magmatic evolution account for a significant part of the marine magnetic anomalies

elevational shifts of coleogyne ramossissima in the mojave desert during the little ice age abstract plant macrofossils from woodrat neotoma middens collected in the mojave desert of southern nevada were used to reconstruct the timing and magnitude of vegetation change during the past 1000 yr although vegetation at the site now is dominated by the shrub coleogyne ramossissima this species does not occur in fossil middens prior to 600 50 yr b p the appearance and persistence of c ramosissima in middens was coincident with the little ice age ca 1250 to 1850 a d and was probably related to an increase in precipitation

characterization of the in situ magnetic architecture of oceanic crust hess deep using near source vector magnetic data marine magnetic anomalies are a powerful tool for detecting geomagnetic polarity reversals lithological boundaries topographic contrasts and alteration fronts in the oceanic lithosphere our aim here is to detect lithological contacts in fast spreading lower crust and shallow mantle by characterizing magnetic anomalies and investigating their origins we conducted a high resolution near bottom vector magnetic survey of crust exposed in the hess deep tectonic window using the remotely operated vehicle rov isis during rrs james cook cruise jc21 in 2008 hess deep is located at the western tip of the propagating rift of the cocos nazca plate boundary near the east pacific rise epr 2 15 n 101 30 w rov isis collected high resolution bathymetry and near bottom magnetic data as well as seafloor samples to determine the in situ lithostratigraphy and internal structure of a section of epr lower crust and mantle exposed on the steep 20 dipping south facing slope just north of the hess deep nadir ten magnetic profiles were collected up the slope using a three axis fluxgate magnetometer mounted on rov isis we develop and extend the vertical magnetic profile vmp approach of tivey 1996 by incorporating for the first time a three dimensional vector analysis leading to what we here termed as vector vertical magnetic profiling approach we calculate the source magnetization distribution the deviation from two dimensionality and the strike of magnetic boundaries using both the total field fourier transform inversion approach and a modified differential vector magnetic analysis overall coherent long wavelength total field anomalies are present with a strong magnetization contrast between the upper and lower parts of the slope the total field anomalies indicate a coherently magnetized source at depth the upper part of the slope is weakly magnetized and magnetic structure follows the underlying slope morphology including a bench and lobe shaped steps imaged by microbathymetry the lower part of the slope is strongly magnetized with a gradual reduction in amplitude from east to west across the slope surface morphology and recent drilling results indicate that the slope has been affected by mass wasting but the observation of internally coherent magnetization distributions within the upper and lower slopes suggest that the disturbance is surficial we attribute the spatial differences in magnetization distribution to the combination of changes in in situ lithology and depth to the source these survey lines document the first magnetic profiles that capture the gabbro ultramafic and possibly dike gabbro boundaries in fast spreading lower crust

contribution to the understanding of the ionian basin sedimentary evolution along the eastern edge of apulia during the late cretaceous in albania abstract integrated in the peri adriatic domain the ionian basin extended along a nw se direction during the late cretaceous limited on its sides by the apulian and the kruja platforms the basinal slope succession was studied in seven outcrops exposed in the albanian fold and thrust belt sedimentological investigations supported by bio and chronostratigraphy were performed on calcareous upper cretaceous hemipelagites gravity flow deposits and slumps the western part of the basin was studied revealing a strong influence of the apulian margin alternatively shedding sediment basinward by means of a tectonically controlled edge the late albian to cenomanian period is characterized by the settling of muddy debrites along the margin a deep basinal environment characterizes this period which prolongs until the santonian with no significant influx of the platform basinward this sedimentary setting abruptly changed at the end of the santonian with an important influx derived from both platforms coarsening and thickening upward sequences show a progressive increase in sediment shedding during the campanian the late campanian early maastrichtian period points out a major change on the resedimentation processes with the settling of several slumped units reworking thick sediment packages the latter can be traced along the apulian margin testifying of instabilities along the edge of apulia

the last pluvial climatic episodes in the deserts of southwestern north america long term variations in the effective moisture of deserts are linked both to glacial interglacial climatic cycles and through the milankovitch theory to orbitally induced changes in solar radiation intensity1 4 for southwestern north america comparison of palaeobotanical data with model predictions of palaeoclimate reveals two distinct pluvial regimes during the late wisconsin and early holocene 23 8 kyr bp southward displacement of the aleutian low pressure centre and strengthened westerlies from 40 to 30 n were important features of the full glacial 18 kyr bp pluvial maximum increased cyclonic storms and weakened monsoons led to a winter precipitation regime a feature seen both in the fossil record5 6 and climate models7 8 dominantly zonal flow during this period contrasts with the latest wisconsin and early holocene 12 8 kyr bp when meridional circulation transported maritime tropical air into the desert interior we now show that plant macrofossil assemblages from packrat neotoma spp middens9 in the mojave desert provide evidence for increased temperatures and summer rainfall during this period10 12 the anomalously late persistence to 8 kyr bp of woodland in low latitude low elevation deserts of the south west13 is attributed to enhanced monsoonal circulation3

mechanisms for forming discontinuity surfaces within shoreface shelf parasequences sea level sediment supply or wave regime abstract a 2d process response model was used to evaluate potential mechanisms involved in the formation of small scale stratigraphic variability that is observed within progradational wave dominated shoreface shelf parasequences model experiments which are based on scenarios that consist of a 25 ky progradational phase suggest that discontinuity surfaces form when either sediment supply or sea level fall rapidly or when wave height regime increases over relatively short time periods 102 104 y during coastal progradation in these cases mean deposition rate near the lower shoreface drops which combined with coastal progradation results in a discontinuity surface both observed and simulated discontinuity surfaces form predominantly in lower shoreface deposits and bound meter scale stratigraphic cycles simulations show that wave regime variability affects the shoreface shelf to a much greater depth than minor sea level change therefore facies shifts along wave climate induced bounding surfaces occur over wider reaches of the shoreface shelf system than sea level induced facies shifts high frequency variability in sediment supply rate results in the formation of discontinuity surfaces across which there are no distinct facies shifts

storm and fair weather wave base a relevant distinction surface waves are an important mechanism for the redistribution of sediment on shallow marine shelves and are commonly interpreted as comprising two distinct populations fairweather waves and storm waves the latter of which are generally thought to penetrate to greater water depths here we used 2 3 10 6 spectral density estimates for the surface ocean collected between 1996 and 2008 from 32 buoys in the caribbean the gulf of mexico and the western atlantic to test the hypothesis that surface waves in the modern ocean comprise two size modes although distinct wave size classes occur in some individual measurements and over the time scales of some individual storms time averaged frequency distributions of wave size are unimodal thus there is no empirical basis for presupposing a distinct bimodal separation in the size of fair weather and storm waves or in the manifestation of such differences in stratigraphic successions instead there is a continuously increasing probability that a wave will reach the bottom with decreasing water depth and a separate probability that describes the hydrodynamic state of the sediment water interface wave size does however exhibit signifi cant geographic bimodality locations in the relatively protected gulf of mexico and caribbean regions have modal wavelengths that are 50 m less than waves at locations along the western atlantic time integrated estimates of the depth of wave penetration provide empirical constraints on the paleo water depths of ancient sedimentary deposits and highlight differences between sheltered shelf environments such as those that characterized many ancient epeiric seas and open ocean facing narrow continental shelves

the lower colorado river valley a pleistocene desert abstract a chronological sequence of plant macrofossil assemblages from twenty five pack rat middens provides a record of desert scrub vegetation for most of the last 13 380 yr b p from a hyperarid portion of the lower colorado river valley at the end of the late wisconsin and probably during much of the quaternary the picacho peak area imperial county california supported a typical mohave desert association of larrea divaricata creosote bush coleogyne ramosissima blackbrush yucca brevifolia joshua tree and y whipplei whipple yucca recent arrivals of sonoran desert plants such as olneya tesota ironwood and fouquieria splendens ocotillo suggest that the area supported relatively modern sonoran desert scrub species for relatively short periods during interglaciations

stable carbon and oxygen isotopes of pedogenic carbonates ajo mountains southern arizona implications for paleoenvironmental change abstract in this study we compare the stable isotope composition of late holocene pedogenic carbonate from a late holocene soil developed on the pediment below the ajo mountains in southern arizona usa with measurements of the 18 o of the soil water throughout the seasonal cycle and with 18 o and 13 c measurements on older dated soils the comparison with the soil water composition shows that the pedogenic carbonate is forming in equilibrium with highly evaporated soil water at the highest summer temperatures the 13 c of the carbonate apparently reflects equilibrium with co 2 of an isotopic composition consistent with the present mixture of c3 desert shrubs and cam cactus comparison with glacial period soil carbonates shows that there has been little change in the 18 o but that the 13 c was about 4 5 heavier independent evidence indicates that glacial period summer temperatures were probably much cooler and that the 18 o of precipitation was also lighter the increase in the carbonate water fractionation factor with decreasing temperature probably counteracted the decrease in the 18 o of precipitation producing little net change in the 18 o of the soil carbonate we attribute the decrease in the 13 c to replacement of a glacial period c4 grassland by holocene c3 cam desert shrubs and succulents this replacement was probably in response to an increase in temperature and reduction in summer precipitation at the end of the last glacial period the 13 c of older soil carbonates indicates that landscape was dominated by grassland for most of the past 700 ka and that the present vegetation likely represents a relatively brief anomaly

vegetation dynamics during the last deglaciation southeastern great basin u s a abstract vegetation changes in the southeastern great basin between 17 500 and 8000 yr b p can be reconstructed from plant macrofossil assemblages in ancient packrat neotoma spp middens vegetation instability after ca 16 000 yr b p involved a dispoportionate number of local immigrations arrivals relative to apparent local extinctions departures at a site in modern woodland less than 50 of the arrivals were of transients species that subsequently went locally extinct but at a low elevation desert site almost all arrivals before 10 000 yr b p were transient species thus by the close of the last glacial age higher elevations supported vegetation that resembled present woodland while lower elevations supported desertscrub that bore little resemblance to modern thermophilous desertscrub differences in the pace of plant community modernization in different elevational zones can be attributed to migrational lag woodland species survived the last glacial age at low elevations nearby and began arriving at sites within current woodland before 11 700 yr b p warm desert plants were displaced far to the south and began arriving after 9500 yr b p moreover their staggered arrival times suggest that low winter temperatures did not inhibit their migration during the early holocene these data suggest that biotic factors particularly variable dispersal distances and consequent migrational lag did affect deglacial vegetation change

statistical averaging of marine magnetic anomalies and the aging of oceanic crust visual comparison of mesozoic and cenozoic magnetic anomalies in the north pacific suggests that older anomalies contain less short wavelength information than younger anomalies in this area to test this observation magnetic profiles from the north pacific are examined from crust of three ages 0 2 1 29 3 33 1 and 64 9 70 3 m y b p for each time period at least nine profiles were analyzed by 1 calculating the power density spectrum of each profile 2 averaging the spectra together and 3 computing a recording filter for each time period by assuming a hypothetical seafloor model the model assumes that the top of the source is acoustic basement the source thickness is 0 5 km and the time scale of geomagnetic reversals is according to ness et al 1980 the calculated power density spectra of the three recording filters are complex in shape but show an increase of attenuation of short wavelength information as the crust ages these results are interpreted using a multilayer model for marine magnetic anomalies in which the upper layer corresponding to pillow basalt of seismic layer 2a acts as a source of noise to the magnetic anomalies as the ocean crust ages this noisy contribution by the pillow basalts becomes less significant to the anomalies consequently magnetic sources below layer 2a must be faithful recorders of geomagnetic reversals

quantitative analysis of the dimensions and distribution of channelized fluvial sandbodies within a large outcrop dataset upper cretaceous blackhawk formation wasatch plateau central utah u s a abstract conceptual and quantitative models of fluvial stratigraphy typically argue that alluvial architecture is driven by a combination of allogenic processes e g tectonic subsidence sea level sediment supply and autogenic behaviors e g avulsion we test these models via analysis of a large c 200 m thick by 100 km depositional strike extent alluvial to coastal plain succession that records shoreline progradation in response to decreasing tectonic subsidence late cretaceous blackhawk formation wasatch plateau central utah u s a oblique aerial photographs and architectural panels of six nearly linear nearly vertical cliff face windows were used to characterize the dimensions apparent widths thicknesses and spatial distributions of channelized fluvial sandbodies two statistical measures raster based lacunarity and point based l function are used to investigate whether the distribution of channelized sandbodies show significant regularity randomness or clustering over 490 channelized fluvial sandbodies are identified in the six cliff face windows from base to top of the blackhawk formation sandbodies broadly increase in width and decrease in overall abundance per unit area clustering of sandbodies occurs relatively frequently in lower coastal plain strata 50 km from the coeval shoreline a strong negative correlation between lacunarity and stratigraphic position is also observed such that a wider range and greater variety of spatial gaps occurs between sandbodies in lower coastal plain strata relative to upper coastal plain and alluvial strata comparison with numerical modeling studies of fluvial stratigraphic architecture implies the predominance of an avulsion generated pattern of sandbody distribution that includes an element of compensational stacking on the upper coastal plain and alluvial plain for a range of distances from the coeval shoreline c 40 130 km and tectonic subsidence rates c 40 700 m myr on the lower coastal plain 50 km from the coeval shoreline localized clustering of channelized sandbodies is interpreted to have formed by avulsion of deltaic distributary channels downstream of delta apex avulsion nodes possibly modulated by low amplitude 30 m sea level fluctuations

biogeographic implications of a packrat midden sequence from the sacramento mountains south central new mexico abstract thirteen packrat neotoma spp and two porcupine erethizon dorsatum middens from 1555 to 1690 m elevation from the sacramento mountains new mexico provide an 18 000 yr vegetation record in the northern chiuahuan desert the vegetation sequence is a mesic wisconsin fullglacial 18 000 16 000 yr b p pinyon juniper oak woodland a xeric early holocene ca 11 000 8000 yr b p juniper oak woodland a middle holocene ca 8000 4000 yr b p desert grassland and a late holocene ca 4000 yr b p to present chihuahuan desertscrub the frequency of spring freezes and summer droughts in the late wisconsin probably set the northern limits of pinus edulis and juniperus monosperma at about 34 n or 6 south of today s limit rising summer tempratures in the early holocene eliminated pinyon and other mesic woodland plants from the desert lowlands and allowed the woodland to move upslope at this time pinyon juniper woodland and pine forest dominated by pinus ponderosa probably began their spectacular holocene expansions to the north continued warming in the middle holocene led to very warm summers with strong monsoons relatively dry cold winters and widespread desert grasslands desertscrub communities in the northern chihuahuan desert did not develop until the late holocene when the biseasonal rainfall shifted slightly back toward the winter catastrophic winter freezes decreased and droughts in all seasons increased the creosote bush desertscrub corridor across the continental divide between the chihuahuan and sonoran deserts was probably connected for the first time since the last interglaciation

climatic influences on holocene variations in soil erosion rates on a small hill in the mojave desert a detailed study of a small hill in ne mojave desert in eastern california was conducted to elucidate the effect of climate on the variations in soil erosion rates through holocene field surveys and sampling were carried out to obtain information on topography geomorphology soil and vegetation conditions seismic refraction sediment deposition and hillslope processes integration of this information allowed reconstruction of the hill topography at the end of the pleistocene deduction of the evolution of the hill from the end of the pleistocene to the present and estimation of total soil losses resulting from various hillslope processes the estimates are consistent with the premise that early holocene climate change resulted in vegetation change soil destabilization and topographic roughening current very slow hillslope transport rates e g 5 mm ky 1 by rodent burrowing a presently important transport form appear inconsistent with the inferred total soil loss rate 31 mm ky 1 packrat midden studies imply that the ne mojave desert experienced enhanced monsoonal precipitation in the early holocene presumably accentuating soil loss water erosion on one slope of the hill was simulated using water erosion prediction project wepp a process based erosion model using 4 and 6 ky of precipitation input compatible with an appropriate monsoonal climate and the present climate respectively the wepp predicted soil losses for the chosen slope were compatible with inferred soil losses identification of two time periods within the holocene with distinct erosion characteristics may provide new insight into the current state of mojave desert landform evolution

late glacial and holocene record of climatic change in the southern rocky mountains from sediments in san luis lake colorado usa abstract large rapid climate changes occurred over the last glacial cycle in the southwestern united states and elsewhere in many regions of the world some of these changes were attributed to alternations between stadial and interstadial conditions in the north atlantic but intense debate exists on how climate anomalies in the north atlantic transmit to the southwest here we report a sediment record from san luis lake in southern colorado through analyses of grain size magnetic susceptibility mg ca total inorganic carbon 18 o and 13 c to indicate climatic and environmental changes in the southern rocky mountains over the last 16 5 ka we found that san luis lake remained hydrologically closed most of the time but overflowed during the second half of the mystery interval the big wet 15 7 14 9 ka and the latter part of the mid holocene the neopluvial 4 3 ka over the course of the last deglaciation san luis lake underwent a series of large millennial scale hydroclimatic changes such as the big dry 16 5 15 7 ka the big wet the bolling allerod dry 14 9 12 7 ka and the younger dryas wet 12 8 11 6 ka corresponding to warm cold phases in the high latitude northern hemisphere the north american monsoon waxed during the pre boreal interval 11 6 10 5 ka and waned through the holocene in phase with northward and southward displacement of the intertropical convergence zone itcz the san luis lake basin was relatively dry in the early holocene 10 5 6 7 ka wet and fluctuating in the mid holocene 6 7 2 6 ka and dry and less variable in the late holocene 2 6 0 ka we found evidence that extreme pluvial episodes of the southern rocky mountains and elsewhere in the american southwest were coeval with cold phases of the north pacific our results highlight the role of the north pacific in modulating atmospheric circulations over the region on millennial timescales

depositional cycles and source rock development in an epeiric intra platform basin the hanifa formation of the arabian peninsula abstract the upper oxfordian to lower kimmeridgian hanifa formation was deposited in an intra platform basin on the broad epeiric shallow water carbonate platform that extended over much of the middle east during the mesozoic the formation consists of organic rich carbonate mudstones bioturbated mudstones and sulphates the organic rich mudstones are considered to be one of the major hydrocarbon source rocks in the middle east oil province the correlation of sedimentary facies shows a hierarchical stacking pattern of transgressive regressive depositional cycles within the hanifa formation since these cycles occur at a basin wide scale it is assumed that they are related to relative changes of sea level the superimposed scales of cyclicity determine the occurrence of source rock and sulphates major source rock development occurs in transgressive dominated cycles sulphates on the other hand are more common during periods of overall regression

a 35 000 year vegetation and climate history from potato lake mogollon rim arizona abstract a new record from potato lake central arizona details vegetation and climate changes since the mid wisconsin for the southern colorado plateau recovery of a longer record discrimination of pine pollen to species groups and identification of macrofossil remains extend whiteside s 1965 original study during the mid wisconsin ca 35 000 21 000 yr b p a mixed forest of engelmann spruce picea engelmannii and other conifers grew at the site suggesting a minimum elevational vegetation depression of ca 460 m summer temperatures were as much as 5 c cooler than today during the late wisconsin ca 21 000 10 400 yr b p even cooler temperatures 7 c colder than today ca 800 m depression allowed engelmann spruce alone to predominate warming by ca 10 400 yr b p led to the establishment of the modern ponderosa pine pinus ponderosa forest thus the mid wisconsin was not warm enough to support ponderosa pine forests in regions where the species predominates today climatic estimates presented here are consistent with other lines of evidence suggesting a cool and or wet mid wisconsin and a cold and or wet late wisconsin climate for much of the southwest potato lake was almost completely dry during the mid holocene but lake levels increased to near modern conditions by ca 3000 yr b p

cyclic alteration of proximal and distal storm facies kope and fairview formations upper ordovician ohio and kentucky the upper ordovician kope and fairview formations and the bellevue tongue of the grant lake limestone exposed in the cincinnati ohio region represent storm dominated sedimentation on a prograding intracratonic ramp the upper kopelower fairview transition zone 25 m thick is an outer to intermediate ramp lithofacies and is composed of numerous 1 3 m thick shallowing upward subtidal cycles bedding cycles are composed of two depth controlled facies 1 a shale dominated distal facies and 2 a fossiliferous proximal facies the distal facies records low energy largely single storm events and consists of thin graded shale beds nearly in situ fragile fossil accumulations thin parautochthonous packstones and quartzose siltstone beds siltstones contain abundant storm depositional indicators tool marked soles gutter casts mud filled scours hummocky lamination and hummocky bedforms the proximal facies represents deposition in an energetic storm wave swept environment and consists of amalgamated sets of thick bedded fine to coarse grained skeletal grainstones and packstones with low angle cross lamination megaripples and shale lithoclasts sedimentary structures and paleocurrent patterns from both facies record deposition under a combined flow regime with the unidirectional current vector oriented in the offshore direction cycles are laterally continuous across the 35 km by 32 km study area cycle boundaries flooding surfaces are parallel to a unique storm generated marker bed implying that these surfaces are effective time lines their widespread character and relatively deep subtidal facies assemblage argues against autocyclic processes in cycle formation we propose that the alternation of proximal and distal facies was tied to fluctuations in storm wave base driven by short term glacio eustasy three scales or orders of sedimentary cyclicity are recognized in the study area meter scale bedding cycles are considered fifth order parasequences 20 100 ka duration and stack into genetically related parasequence sets these fourth order units have been previously described as member scale lithostratigraphic tongues analysis of fischer plots suggest that cycle thickness and subfacies composition was controlled by both accommodation and siliciclastic sediment supply these higher order stratal units are super imposed onto the highstand portion of the kope to bellevue depositional sequence and give insight into the high frequency controls on the progradation of the ramp complex

water movement in desert soil traced by hydrogen and oxygen isotopes chloride and chlorine 36 southern arizona abstract understanding soil water movement is important for water resource management and for analyzing pollutant transport in the vadose zone seasonal variations in shallow soil profiles may have profound effects on deep soil water and solutes we have investigated seasonal soil water movement in the top meter of undisturbed desert soil in the southern arizona using 2 h 18 o cl and bomb 36 c1 six soil profiles were sampled from a young terrace holocene during 1991 to 1992 on a seasonal basis one soil profile was sampled from an older terrace latest pleistocene in october 1992 and one from a yet older fan surface late pleistocene in march 1992 the results indicate that repeated seasonal cyclic movement of soil water in the top 60 80 cm active zone tends to produce a consistent stable isotope composition for the soil water below the active zone in different seasons the deep 18 o on the oldest surface is about 6 heavier than on the younger surfaces probably resulting from greater evaporative loss due to the larger proportion of fines in the older soil this deep isotopic composition is closely related to the composition of the local average annual precipitation an annual average evaporation rate of 35 mm year 1 was estimated using a steady state diffusion model whereas the actual regional evapotranspiration rate is 175 250 mm year 1 this suggests that about 80 of the soil water is lost through transpiration and first stage evaporation before quasi steady state is reached the long term average infiltration rate below the active zone is about 4 mm year 1 for the younger terrace based on both chloride mass balance and bomb 36 cl and 0 02 mm year 1 for the older terrace and the fan surface based on chloride mass balance the correlation between the deep 18 o the reduction of downward flux and the surface age suggests that increasing soil development has resulted in a significant decrease of soil water infiltration and hence an increase in evaporation and surface runoff

response of extreme floods in the southwestern united states to climatic variations in the late holocene a regional synthesis of paleoflood chronologies on rivers in arizona and southern utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations the flood chronologies were constructed using fine grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events high magnitude floods were frequent on rivers throughout the region from 5000 to 360014c yrs bp dendrocalibrated age 3800 2200 bc and increased again after 2200 bp 400 bc with particularly prominent peaks in magnitude and frequency around 1100 900 bp ad 900 1100 and after 500 yrs bp ad 1400 in contrast the periods 3600 2200 bp 2200 400 bc and 800 600 yrs bp 1200 1400 ad are marked by sharp decreases in the occurrence of large floods on these rivers in the modern record storms that generate large floods 10 year in the region fall into three categories 1 winter north pacific frontal storms 2 late summer and fall storms that draw in moisture from recurved pacific tropical cyclones and 3 summer storms mainly convective thunderstorms winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study and are the most probable causes of the paleofloods over the last 5000 years floods from both winter storms and tropical cyclones occur when deep mid latitude troughs steer storm systems into the region composite anomaly maps of daily 700 mbar heights indicate that these floods are associated with a low pressure anomaly off the california coast and a high pressure anomaly over the aleutians or gulf of alaska a strong connection exists between the negative phase of the southern oscillation index often associated with el nin o conditions and the large floods associated with winter storms and tropical cyclones the paleoflood records confirm the existence of centennial scale variations in the conditions conducive to the occurrence of extreme floods and flood generating storms in this region the episodes with an increased frequency of high magnitude floods coincide with periods of cool wet climate in the western u s whereas warm intervals such as the medieval warm period are times of dramatic decreases in the number of large floods a positive relationship between the paleofloods and long term variations in the frequency of el nin o events is evident over the last 1000 years this relationship continues over at least the last 3000 years with warm coastal sea surface temperatures indicative of el nin o like conditions

late glacial and holocene vegetation history and paleoclimate of the kaibab plateau arizona sediment cores spanning the last 13 500 calendar years cal yr were obtained from two lakes fracas lake 2518 m bear lake 2778 m on the kaibab plateau in northern arizona pollen and plant macrofossil records indicate that before 12 900 cal yr b p high elevation landscapes of the kaibab plateau near bear lake were covered by alpine tundra while lower elevations near fracas lake were occupied by picea woodland at12 900 cal yr b p picea engelmannii and abies lasiocarpa forest expanded upward to occupy the top of the plateau pinus ponderosa arrived near fracas lake 11 000 cal yr b p replacing picea forests since then pinus ponderosa forest has dominated the fracas lake area pinus ponderosa did not appear at bear lake until9730 cal yr b p mixed forests of picea mainly picea pungens abies lasiocarpa pinus ponderosa a ndpseudotsuga after8000 cal yr b p grew near bear lake for the remainder of the holocene picea engelmannii populations reexpanded near bear lake after 4000 cal yr b p charcoal records indicate that fire probably helpedpinus ponderosa to become established near bear lake climate changes on the kaibab plateau since the late glacial were inferred from lake levels and vegetation patterns the late glacial 11 000 cal yr b p was cold and probably wet the early holocene 11 000 to8000 cal yr b p was cooler than today and may have been the wettest period fracas lake and bear lake were probably deepest then during this period a strengthened summer monsoon brought in more moisture from the eastern pacific ocean and the gulf of mexico during the dry and warm mid holocene fracas lake and bear lake experienced water level declines the late holocene was relatively wet and cool again and aquatic plants were abundant in the two lakes increasing effective moisture in the late holocene was related to decreasing summer insolation 1999 elsevier science b v all rights reserved

slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the mid atlantic ridge south of the kane fracture zone mark area 1 the remanent magnetization of gabbroic material of the mid atlantic ridge south of the kane fracture zone mark area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment the remanence of these gabbroic samples is often complex with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales tens of centimeters to a few meters moreover several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components given the nominal age 1 ma of the crust we suggest that this pattern of normal reversed normal polarity most plausibly reflects emplacement and or cooling through three successive polarity intervals jaramillo normal polarity interval 1 07 0 99 ma a portion of the matuyama reversed polarity interval 0 99 0 78 ma and the brunhes normal polarity interval 0 78 ma to present a small number of samples with three well defined magnetization components have magnetic characteristics compatible with a remanence carried by fine grained possibly single domain magnetite laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the jaramillo matuyama 0 99 ma and matuyama brunhes 0 78 ma polarity transitions together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as 350 475 c for a minimum of 0 21 m y after emplacement we suggest that continued injection of liquid in the form of sills or small magma bodies over a broad region half width of 3 km is responsible for this slow cooling in addition inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole suggesting that little if any resolvable tilt occurred during uplift of these rocks to the seafloor

the galapagos axial magnetic anomaly evidence for the emperor event within the brunhes and for a two layer magnetic source the axial brunhes magnetic anomaly over the galapagos spreading center shows consistent short wavelength anomalies that are not consistent with a uniformly magnetized layer we interpret this anomaly to confirm the presence of the emperor reversed event at 0 49 0 05 mybp with a duration about 0 01 m y further this anomaly is consistent with a two layer magnetic source with the lower layer having very broad transition zones offset away from the axis from those of the upper layer by about 10 km

on climatic analogs in paleoclimatic reconstruction abstract many of the problems that have been discussed with regard to the analog approach to paleoclimatic reconstruction may be illustrated by consideration of basic principles and definitions and the logical consquences thereof specific attention is directed toward the definition of the regional macroclimate as the modal microclimate and to a series of axioms 1 there are various microclimates which depart more or less from the macroclimate in each terrestrial region 2 environment and climate change on time scales from near instantaneous to millions of years 3 climate is multidimensional a vector not a single scalar datum 4 biotic assemblage changes in response to environmental including climatic changes are expressed primarily in terms of reproductive success one may conclude from the corollaries to these axioms that perfect analogs cannot be found for any holocene climate but that partial analogs transfer equations may still be used if allowance is made for shared variance

development of spatially diverse and complex dune field patterns gran desierto dune field sonora mexico the pattern of dunes within the gran desierto of sonora mexico is both spatially diverse and complex identification of the pattern components from remote sensing images combined with statistical analysis of their measured parameters demonstrate that the composite pattern consists of separate populations of simple dune patterns age bracketing by optically stimulated luminescence osl indicates that the simple patterns represent relatively short lived aeolian constructional events since 25 ka the simple dune patterns consist of i late pleistocene relict linear dunes ii degraded crescentic dunes formed at 12 ka iii early holocene western crescentic dunes iv eastern crescentic dunes emplaced at 7 ka and v star dunes formed during the last 3 ka recognition of the simple patterns and their ages allows for the geomorphic backstripping of the composite pattern palaeowind reconstructions based upon the rule of gross bedform normal transport are largely in agreement with regional proxy data the sediment state over time for the gran desierto is one in which the sediment supply for aeolian constructional events is derived from previously stored sediment ancestral colorado river sediment and contemporaneous influx from the lower colorado river valley and coastal influx from the bahia del adair inlet aeolian constructional events are triggered by climatic shifts to greater aridity changes in the wind regime and the development of a sediment supply the rate of geomorphic change within the gran desierto is significantly greater than the rate of subsidence and burial of the accumulation surface upon which it rests

a 36 000 yr vegetation history from the peloncillo mountains southeastern arizona usa a 36 200 cal yr b p vegetation history was developed from macrofossils and pollen from 55 packrat middens from 1287 to 1442 m elevation in the peloncillo mountains of southeasternmost arizona usa today these elevations are dominated by semidesert grassland with a mixture of chihuahuan and sonoran desert shrubs including an eastern disjunct population of jojoba simmondsia chinensis from 36 200 to 15 410 cal yr b p rocky areas just above large pluvial lakes that occupied what are now dry playas supported pinus edulis juniperus osteosperma juniperus cf coahuilensis quercus cf turbinella and a rich understory of summer flowering c4 annuals and grasses indicating abundant summer rains and mild winters after 15 410 cal yr b p p edulis declined in abundance and disappeared briefly at 13 925 cal yr b p coincident with expansion of more xeric species and paleohydrological evidence for regional aridity during the bolling allerod warm interval p edulis rebounded briefly during the younger dryas at 12 405 cal yr b p before disappearing along with other mesic woodland species sometime after 12 100 cal yr b p the few middens dating from the early to middle holocene 10 000 4000 yr b p indicate wetter conditions than today at 7790 cal yr b p followed by a general drying trend the 35 middens from the late holocene detail the sequential arrival of desertscrub species as vegetation became more modern in character

the magnetic layer of the ocean crust how thick is it abstract special features of linear marine magnetic anomalies and the magnetic measurements made on samples of the ocean crust require that the thickness of the marine magnetic source layer be thicker than the 0 5 km thick pillow lava layer seismic layer 2a alone it is proposed here that the magnetic properties of the samples studied to date indicate a two layered source to be the most likely an upper 0 5 km thick pillow lava layer with a natural remanent magnetization of 5 a m and a lower 3 5 km thick dike and upper gabbro layer with a magnetization of 0 5 a m

magnetic properties of rocks of the kapuskasing uplift ontario canada and origin of long wavelength magnetic anomalies summary the sources of long wavelength magnetic anomalies lwa in the crust are poorly understood we have measured remanent and induced magnetizations of 210 samples of anorthosite tonalite and mafic gneiss from the kapuskasing uplift an exposed partial crustal crosssection in northwestern ontario canada anorthosites have generally high q ratios of remanent induced magnetization in the range 0 3 60 and their natural remanent magnetization nrm is resistant to both thermal and alternating field af demagnetization however anorthositenrms 0 001 0 3am 1 aretooweaktoexplainlwaamplitudes maficgneisses also have relatively high q ratios peaking in the range 1 10 and the nrm is resistant to thermal and af demagnetization nrm and induced magnetization im intensities are in the ranges 0 01 2 and 0 01 0 6 a m 1 respectively tonalites have a bimodal distribution of magnetization the more strongly magnetic group has both nrm and im intensities in the range 0 1 5 a m 1 and wide ranging q values from 0 1 to 10 approximately some tonalites could be an lwa source although the long term stability of their nrms at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600 c in general q values measured at surface temperatures overestimate remanence at depth in the deep crust im remains more or less constant but remanence decreases both reversibly and irreversibly leading to q ratios of 0 2 0 3 for multidomain grains and 1 3 for single domain grains near their blocking temperatures thermoviscous magnetization over the brunhes chron could add substantially to the effective induced magnetization typically induced thermoviscous magnetization in the direction of the present earth s field will outweigh remanence in the direction of an ancient field as a source of lwa originating in the deepest crust remanence may play a larger role for mid crustal sources where single domain grains are well below their blocking temperatures

formation of magnetic single domain magnetite in ocean ridge basalts with implications for sea floor magnetism although magnetic data are the primary evidence for ocean floor spreading the processes by which magnetic phases in ocean floor basalts are formed remain poorly constrained scanning transmission electron microscopic observations show that magnetic single domain magnetite in sheeted dike basalts of deep sea drilling project hole 504b formed through oxidation exsolution of ilmenite exsolution of ulvospinel lamellae and recrystallization of end member magnetite by interaction with convecting fluids the data suggest that the sheeted dike basalts with single domain magnetite as an efficient and stable magnetic carrier contribute significantly to sea floor magnetism

physical oceanographic influences on sedimentology of reef sand aprons holocene of aranuka atoll kiribati equatorial pacific abstract depositional facies are the net product of a complex set of processes that impact sediment supply and transport in geomorphic systems although the general facies motifs of many isolated platforms throughout the geologic record are well documented the details of geomorphological and sedimentological patterns and the physical oceanographical processes that can impact sedimentological differentiation are less well constrained on these platforms accumulations of reef associated debris in platform top reef sand aprons can form expansive geomorphic elements hosting prolific hydrocarbon reserves in geologic analogs to better understand the nature and scale of reef sand apron accumulations this project integrates remote sensing field petrographical and granulometrical characterization of surficial holocene sediments with physical oceanographical observations and modeling of aranuka atoll republic of kiribati in the western equatorial pacific results illustrate trends in hydrodynamics geomorphology and sedimentology from the platform margin to the platform interior current meter data and modeling illustrate how the tides 2 5 m spring tide amplitude modulate wave energy open ocean annual average swell height of 2 m distal swell can be larger to produce dominant on platform flow speeds up to 90 cm s on the northern reef sand apron the strongest currents there appear to be impacted by southward propagating waves related to fronts that pass thousands of kilometers north of the atoll not local wind generated waves these hydrodynamical influences are interpreted to have led to the development of the expansive northern reef sand apron 2000 m wide the southeastern apron with currents that reverse with the tides includes a distinct narrower sand apron 1500 m wide concomitantly the hydrodynamical patterns and platformward decrease in energy across the reef sand apron coupled with changes in biota are interpreted to control variability in sedimentary structures bottom types and sediment attributes sediment near the margin on the reef sand apron contains coral and red algal rich coarse sand and gravel passing to foraminifera rich medium to coarse sand toward the lagoon the lagoon includes even finer sediment chiefly fine and very fine sand and patch reefs collectively the results of this study illustrate that selective winnowing differentiation of sediment size type and sorting and nature and size of geomorphic elements ultimately are linked to the hydrodynamical patterns across the platform the sedimentological depositional porosity and permeability and geomorphic facies bodies variability and the processes active on this reef sand apron provide a general conceptual model that may be applicable to predict trends in ancient reservoir analogs

geomorphic evolution of a storm dominated carbonate ramp c 549 ma nama group namibia the well exposed hoogland member c 549 ma of the northern nama group kuibis subgroup namibia represents a storm dominated carbonate ramp developed in a foreland basin of terminal proterozoic age the ramp displays facies gradients involving updip grainstones which pass downdip into broad spatially extensive tracts of microbial laminites and finely laminated mudstones deposited above and below storm wave base trough cross bedded coarse grainstones are shown to transit downdip into finer grained calcarenites irregular microbial laminites and mottled laminites siliciclastic siltstones and shales were deposited further downdip platform growth was terminated through smothering by orogen derived siliciclastic deposits ramp morphology was controlled by several different processes which acted across many orders of magnitude millimetres to kilometres including in situ growth of mats and reefs scouring by wave produced currents and transport and infilling of coarse grained carbonates and fine grained carbonates and clastics at the smallest scale roughening of the sea floor through heterogeneous trapping and binding by microbial mats was balanced by smoothing of the sea floor through accumulation of loose sediment to fill the topographic lows within the upward propagating mat at the next scale up parasequence development involved roughening of the sea floor through shoal growth and grainstone progradation balanced by sea floor smoothing through shale infilling of resulting downdip accommodation as well as the metre scale topographic depressions within the mosaic of shoal water facies at even larger sequence platform scales roughening of the sea floor occurred through aggradation and progradation of thick carbonates balanced by infilling of the foreland basin with orogen derived siliciclastic sediments at all scales a net balance was achieved between sea floor roughening and sea floor smoothing to maintain a more or less constant ramp profile

carbonate platform scale correlation of stacked high frequency sequences in the arab d reservoir saudi arabia abstract the late jurassic arab formation contains a number of hydrocarbon bearing carbonates the most important of which is the lowermost arab d reservoir the reservoir lithofacies in khurais field are couplets of 1 lime mud and 2 intraclastic lithofacies representing basinal turbidites 3 pelletal lithofacies representing lower shoreface sands and silts 4 stromatoporoid lithofacies representing a reef 5 cladocoropsis and 6 dasyclad lithofacies representing a lagoon 7 peloidal and 8 oolitic lithofacies representing shore attached sand sheets 9 cryptomicrobial lithofacies representing supratidal flats 10 anhydrites representing sabkha followed by salina deposits and 11 stratigraphically reoccurring dolomite these are arranged in two partially preserved third order sequences the upper of which represents the arab d member and the lower of which represents the upper jubaila formation within these sequences lie six fourth order high frequency sequences composed of fifth order parasequences and parasequence scale cycles the preserved upward shallowing trend of the arab d reservoir is manifested laterally by a regional eastward thickening interpreted to be the result of an eastward progradation across the shallow late jurassic epeiric shelf and into the relatively deep arabian intrashelf basin this study presents a correlation model that explains the drastic thickening and downward climb of the reservoir lithofacies that is observed between the outcrops south of riyadh and the subsurface in ghawar field this model is different from the one currently used and predicts an eastward porosity improvement in the upper part of the reservoir accompanied by a porosity reduction in the lower part assuming a null diagenetic modification effect