base level buffers and buttresses a model for upstream versus downstream control on fluvial geometry and architecture within sequences abstract the effects of downstream base level control on fluvial architecture and geometry are well explored in several broadly similar sequence stratigraphic models cretaceous dakota group strata u s western interior have characteristics reflecting combined downstream and upstream base level controls that these models cannot address particularly three layers of amalgamated channel belt sandstone within this group thicken and are continuous for distances 300 km along dip that stretch the reasonable lengths for which these models are intended to apply as well architecture in up dip reaches records repeated valley scale cut and fill cycles this contrasts with equivalent strata down dip which record channel scale lateral migration with no such valley scale cycles apparent we here introduce the concept of buffers and buttresses to address these observations we assume that river longitudinal profiles are each anchored down dip to some physical barrier e g the sea strand etc that we refer to as a buttress buttress shift is considered the primary downstream control on base level profiles extrapolated up dip from the buttress over any modeled duration of buttress shift can range widely because of high frequency variability in upstream base level controls e g discharge etc all these potential profiles however are bounded above by the profile of highest possible aggradation and below by the profile of maximum possible incision these upper and lower profiles are buffers and they envelop the available fluvial preservation space thickness of the buffer zone is determined by variability in upstream controls and should increase up dip to the limit of downstream profile dominance dakota valley scale surfaces record repeated cut and fill cycles driven by up dip controls and are confined between thick stable buffers equivalent strata down dip record lateral reworking within a thinner channel scale buffer zone that was positioned by downstream controls regression exposed slopes similar to the buffer zone thus buffers were stable for long distances and durations this prompted dip extensive lateral reworking of strata into upstream valley scale and downstream channel scale sheets buffers and buttresses provide a broadly applicable model for fluvial preservation that captures upstream vs downstream base level controls on geometry and architecture the model lends general insights into dip oriented variations in fluvial architecture production of sheet vs lens geometry total preservation volumes for fluvial systems and variations in these factors related to contrasting climatic conditions and basin physiography the model can be amended to existing sequence stratigraphic approaches in order to capture dip oriented variations in sequence architecture

forces driving late pleistocene ca 77 12 ka landscape evolution in the cimarron river valley southwestern kansas abstract this study presents stratigraphic geomorphic and paleoenvironmental 13 c data that provide insight into the late pleistocene landscape evolution of the cimarron river valley in the high plains of southwestern kansas two distinct valley fills t 1 and t 2 were investigated three soils occur in the t 2 fill and five in the t 1 fill all indicating periods of landscape stability or slow sedimentation of particular interest are two cumulic soils dating to ca 48 28 and 13 12 5 ka 13 c values are consistent with regional paleoenvironmental proxy data that indicate the prevalence of warm dry conditions at these times the cimarron river is interpreted to have responded to these climatic changes and to local base level control specifically aggradation occurred during cool wet periods and slow sedimentation with cumulic soil formation occurred under warmer drier climates significant valley incision 25 m by ca 28 ka likely resulted from a lowering of local base level caused by deep seated dissolution of permian evaporite deposits

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

a permian kuroko type hydrothermal system afterthought ingot area shasta county california lateral and vertical sections and geochemical evolution the afterthought and smaller kuroko deposits in the east shasta mining district formed at the culmination of a late permian mafic to felsic volcanic cycle they were succeeded by triassic elastic sediments and reworked tuff in a presumed rifting environment with local evidence of renewed mafic volcanism and large scale slumping the afterthought host horizon is repeated in outcrop by folding fold closures are tight east of ingot where preore volcanic rocks are more altered they are open basins and domes west of ingot where the preore volcanic rocks are less altered igneous pyroxene and titanomagnetite and secondary pumpellyite occur only in preore andesite and basalt more than 2 5 km from known kuroko deposits in outcrops closer to the deposits disseminated pyrite type e is locally abundant in preore dacite the kuroko deposits comprise three ore types a banded fine grained sulfides and gangue b recrystallized coarser grained sulfides and gangue with a consistent paragenetic sequence and c coarse grained pyrite and barite b and c appear to replace type a a further type of mineralization type d consists of fine grained pyrite occurring locally within silicified dacite on the afterthought host horizon the paragenetic sequence of type b implies a sequential introduction of metals in a fluid evolving toward higher h 2 s activity cu was introduced early as chalcopyrite and bornite later fluids altered preexisting cu minerals rather than introducing significant new cu type b fluids probably cooled from 300 degrees c chalcopyrite stage to about 270 degrees c sphalerite barite stage the last type b fluids dissolved much base metal sulfide and deposited calcite quartz and sericite types a b c and e sulfides have a delta 34 s value range of 5 to 12 per mil implying a common origin barites from type a and b ore have delta 34 s values near 30 per mil partially reduced contemporaneous marine sulfate 10 12ppm is a possible sulfur source but a concealed rock sulfur source of high delta 34 s value is more probable type d pyrite with delta 34 s values between 28 and 0 per mil contains sulfur of a different probably biogenic origin hydrothermal carbonates formed from fluids with delta 13 c values from near 0 to 12 per mil consistent with oxidation of organic c in the hydrothermal system hydrothermal and postore carbonates have delta 18 o values of 17 to 19 per mil and probably equilibrated with a single fluid after the deposition of the hosselkus limestone

the flin flon paleosol and the composition of the atmosphere 1 8 bybp within the 1800 to 1900 my old flin flon snow lake greenstone belt amisk group volcanics are overlain by missi group fluvial sediments several localities along the missi amisk contact the volcanics show evidence of subaerial weathering field relationships mineralogical evidence and chemical analyses confirm that this alteration zone is a paleosol pedogenic fabrics and mineralogy were somewhat obscured by greenschist grade metamorphism associated with the hudsonian orogeny 1750 my this is especially true in the upper meter of the paleosol where metamorphic paragonite and sericitic micas developed in a crenulated fabric this metamorphism did not however obliterate the imprint of weathering on the amisk volcanics features characteristic of well drained modern soils are evident in the paleosol corestones of spheroidally weathered pillow lavas occur at depth within the paleosol cr horizon the corestones decrease in size upward and eventually disappear into a hematite rich horizon at the top of the paleosol these macroscopic changes are accompanied by a decrease in cao and mgo and by an increase in al2o3 tio2 and total iron toward the paleosol missi contact ferrous iron decreases upward toward the contact feo was apparently oxidized to ferric iron and retained within the paleosol during weathering the oxidation and retention of iron within the flin flon paleosol indicates that po2 was probably or 10 2 p a l at the time of weathering the behavior of iron in the flin flon paleosol contrasts sharply with its behavior in the 2200 my hekpoort paleosol which is strongly depleted in iron this difference suggests that a significant increase in the ratio of po2 pco2 in the atmosphere took place between 2200 and 1800 mybp

evidence from early proterozoic anhydrite for sulphur isotopic partitioning in precambrian oceans relative to meteoritic sulphur sulphur in the contemporary exogenic cycle is marked by a depletion of 34s in biogenic sulphide that is balanced by excess 34s in sulphate of the oceans and evaporites1 2 the onset of this isotopic partitioning during the precambrian required sulphate in the ocean in sufficient concentration to permit significant isotopic fractionation during biogenic reduction3 4 and the evolution of sulphate reducing bacteria5 7 the conclusion that partitioning existed at 2 300 myr has been derived from strongly 34s depleted sedimentary sulphide of this age from south africa4 i report here on 34s enriched stratiform nodular anhydrite of similar age from canada which supports partitioning at this time strontium isotope data on these samples indicate that the anhydrite was derived from continental brines possibly in the inland portion of a continental sabkha this requires that the reported enrichment in 34s be considered a minimum for seawater sulphate at 2 300 myr

sulfur isotopic studies of archean slate and graywacke from northern minnesota evidence for the existence of sulfate reducing bacteria abstract sulfur isotopic studies of pyrite from metasediments in the 2 6 byr old deer lake green stone sequence minnesota have been conducted in order to evaluate the possible importance of sulfate reducing bacteria in sulfide formation pyrite occurs as ovules up to 2 cm in diameter within graphitic slates and as fine disseminations in metagraywacke units sem studies indicate the pyrite is framboidal in morphology 34 s values of pyrite from the deer lake sediments range from 2 3 to 11 1 with a peak at 2 isotopic data are consistent with either high temperature inorganic reduction of circulating seawater sulfate or low temperature bacterial reduction however the lack of sulfide bands or massive occurrences in the sediments the restriction of pyrite mineralization to the sediments and the absence of evidence for hot spring activity suggest that a diagenetic origin of pyrite is more feasible sulfide in such an environment would be produced principally by the action of sulfate reducing bacteria results of the study are in agreement with those of goodwin et al 1976 who suggest that dissimilatory sulfate reduction was operative in the archean ocean some 2 75 byr ago

soil geomorphology and wet cycles in the holocene record of north central mexico abstract the distinction between the impact of climatic periodicities or land use practices on soil erosion is an important issue for pre hispanic and colonial mexico that question can best be addressed by first documenting the dynamics of changing wet cycles during the holocene in the central mexican region between the northern limits of pre hispanic agriculture and its southern margins in northwestern chihuahua consequently the laguna project targeted a 125 000 km 2 sector of north central mexico 250 km from north to south and 500 km from east to west from saltillo to durango some 40 sedimentary profiles with multiple cumulic soils were studied in the field and laboratory supported by 163 conventional 14 c and ams dates on charcoal and soil humates we distinguish 1 wet floodplains with humic paleosols redox phenomena reflecting high water tables channel ponding sequences and interbedded tufas that imply aquifer recharge sustained base flow and mainly low energy conditions and 2 high energy pulses of discharge that mobilized cobble gravels or forced channel entrenchment gullying and were tied to episodic excessive rains that promoted valley and slope instability in between such wet cycles and recurrent disequilibrium events climate was similar to today probably less humid with limited geomorphologic change or slow soil formation wet cycles were rare at the end of the pleistocene but prominent during the holocene disequilibrium proxies became common and dramatic after 2500 bp the drainages from the eastern and western sierra madres responded in phase but varied in detail around ad 1050 1200 natural erosion led to loss of soil organic carbon as alternating severe droughts and heavy rains destroyed the ground cover and led to ecological aridification well before arrival of spanish miners and settlers the evidence that human activity triggered pre hispanic or colonial erosion in central mexico should therefore be re evaluated global comparisons and interpretations are discussed but with caution since no single theory can explain the whole of the record the soil geomorphology geoarchive of north central mexico primarily is an environmental history of alternating wet cycles rather than of sustained wet or dry climates the critical differences between soaking and excessive rains with their respective impacts may be due to switching between winter and summer storm categories

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

further sulfur and carbon isotope studies of late archean iron formations of the canadian shield and the rise of sulfate reducing bacteria sulfur and carbon contents and isotope ratios are reported for five archean iron formations helen nakina and finlayson lumby and bending lake areas distributed across 850 km of the canadian shield all 2 7 ga old a 34s profile through a complete stratigraphic column oxide facies excluded of the helen iron formation shows a 34s range of 30 2 mean 34s value of 2 5 and a standard deviation i of 7 3 in sharp contrast to the sulfide and siderite facies the oxide facies in the column shows a uniform 34s value close to zero the 34s values obtained for the other four iron formations are again wide ranging highly variable in the sulfide and pyrite siderite facies but uniform and close to zero for the oxide facies the carbon in the oxide siderite chert facies has 13c values of 2 3 to 1 1 in the range of phanerozoic marine carbonates however the carbonates in the graphite rich sulfide facies have 13c values as low as 7 6 the mixing of reduced carbon with marine carbonate is suggested to explain the light carbonate values the reduced carbon associated with the light carbonate is also relatively light at up to 13corg 33 5 but is in the range of other precambrian values distal high temperature abiogenic sulfate reduction as a source of highly fractionated sulfides in the archean iron formations is ruled out on the basis of both isotopic and geologic evidence it is concluded that only the bacterial reduction of sulfate at low temperatures could produce the wide ranging highly variable 34s values exhibited by these sulfides over large areas

laser microprobe sulphur isotope analysis of arsenopyrite experimental calibration and application to the boliden au cu as massive sulphide deposit abstract metamorphic remobilization of arsenopyrite rich ores is a globally important process which can lead to significant concentrations of gold in order to understand this and related processes relations of sulphur isotopes can give a number of important clues to resolve such relations in detail we have successfully calibrated and applied a laser combustion system for in situ analysis of sulphur isotopic compositions of arsenopyrite experimental calibration of the laser fractionation factor 0 4 was obtained by using compositionally and isotopically homogeneous natural samples from ore deposits at boliden sweden and freiberg germany subsequent to detailed microscopic study the s isotope ratios of these samples were measured by conventional and laser combustion techniques the present application to different types of arsenopyrite in the palaeoproterozoic metamorphosed vhms ores of the boliden au cu as deposit skellefte district northern sweden shows that the sulphur isotope composition of arsenopyrite is essentially unmodified during medium grade metamorphic recrystallization and remobilization here massive arsenopyrite ore is crosscut by later veins that carry a complex quartz sulphosalt sulphide assemblage the latter ore type is markedly au rich compared to the host ore and thus of significant economic interest we find that both ore types exhibit very similar sulphur isotope compositions ca 2 to 3 v cdt which is similar to most massive sulphide deposits in the skellefte district thus the crosscutting au rich vein ore has inherited the sulphur isotope composition from sulphur liberated by metamorphic reactions affecting the massive ore and most likely also inherited the au through this mechanism the latter finding clearly has important implications both for the general discussion on and the prospecting for similar high grade au ores in this world class mining region

isotope paleoecology of episodic mid to late holocene bison population expansions in the southern plains u s a abstract we used a xad purified ams radiocarbon method to date 62 bison specimens from different contexts on the very southern extent of the great plains of north america to produce a precise chronology of bison population expansions spanning the last 6000 years sixty one of these samples provide stable carbon and nitrogen isotope data indicating relative temperature and moisture conditions during intervals defined by the presence of bison within this time span this chronology indicates climatic conditions favorable to bison were present in the greater central texas area including the uplifted edwards plateau and extending to the coastal plain during periods from 5955 to 5815 3290 to 3130 2700 to 2150 and 650 to 530 cal bp however isotope results suggest climatic conditions differed for each period the earliest calf creek period was characterized by cool but dry conditions the later late archaic 1 2 periods were increasingly warmer and wetter and the latest toyah period was cooler and drier than the late archaic periods but warmer and wetter than calf creek both the calf creek and toyah periods had higher variability within these overall trends comparison with regional records suggests that these periods represent variation within generally cool dry climates human adaptive response to increased bison availability resulted in significant cultural changes across all four periods

violent volcanism stagnant oceans and some inferences regarding petroleum strata bound ores and mass extinctions past changes from a well mixed aerated ocean to a stratified stagnant ocean are indicated by decreasing 13c of limestone opposite to prior conclusions as well as by increasing 34s of marine sulfates the proposed stagnant ocean model includes a warm co2 enriched atmosphere and a bathyal to abyssal marine system dominated by bacterial components in the food web and by consequent light carbon in organic material and biogenic carbonates culminations of prolonged stagnant episodes correspond with marine faunal extinctions of late permian and late cretaceous time and the protracted changes and correlations provide evidence against any hypothesis of mass extinction by sudden worldwide catastrophe including the asteroid impact hypothesis the cretaceous stagnant ocean taken as the prime example is attributed to climatic warming triggered by volcanic co2 greenhouse effect and several feedbacks including decreased earth albedo and increased sinking of warm evaporite brines instead of aerated polar waters marine extinctions are attributed to upward expansion of the oxygen minimum zone and to catastrophic mixing of surface waters with poisonous sulfidic waters of the deeps the stagnant ocean provided a counterbalance between deep reduction and shallow oxidation conditions that favored maximal formation of black sulfidic shales protopetroleum and sedimentary sulfide ores and shallow to mid depth barite phosphorite iron ore cherty iron formation and sulfate bearing red beds trace elements concentrated in stagnant ocean sediments include chalcophile and pt group metals negating claims that ir provides a unique fingerprint of meteorite impact and cosmic accretion

channel trenching and climatic change in the southern u s great plains fifteen alluvial sequences in texas and oklahoma exhibit the same late holocene record of channel trenching at 1 ka the erosion was preceded by slow alluvial sedimentation in most stream valleys resulting in the formation of a cumulic organic rich flood plain soil previously named the copan soil the soil formed during a period of regionally moister climate as indicated by pollen spectra molluscan faunas vertebrate faunas sedimentary structures and high alluvial water tables at 1 ka the regional climate changed from moist to dry coinciding with an episode of channel incision of valley floors throughout the southern great plains channel trenching occurred simultaneously in both small and large streams in drainage basins of the arkansas red trinity brazos and colorado rivers the sequences are the first documented example of widespread holocene incision accompanied by firm evidence for a synchronous change in regional climate

genetically related alluvial deposits across active fault zones an example of alluvial fan terrace correlation from the upper quaternary of the southern po basin italy integrated studies of alluvial fan systems and related intramontane valley deposits provide potentially useful tools for understanding the character of alluvial sedimentation and its control in fault controlled piedmont zones an example is reported from the uppermost quaternary alluvial deposits of the northern apennines and the southern margin of the po basin a distinctive cyclicity characterizes both the outcropping and the buried units in the study area in intramontane valleys individual cycles consist of regular alternations of sections displaying high and low terrace preservation in the subsurface a rhythmic alternation of coarse grained bodies and laterally extensive pelitic horizons occurs at various scales and represents the basic cyclic motif of the depositional system the correlation between the terrace sequences and the depositional cycles in the subsurface allows the differentiation of distinct sequences with a characteristic internal architecture in the lower part of sequences poorly preserved terrace deposits are laterally correlatable with alluvial fan gravel bodies whereas highly preserved terrace deposits are correlated laterally with predominantly floodplain sediments in the upper part of sequences reliable correlation lines have been constructed using 14c dating thrust tectonics and climate exerted a major control on sequence development deep valley excavation with subsequent low terrace preservation occurred in response to relief rejuvenation during successive thrusting events on the other hand decreasing erosion related to periods of minor tectonic activity led to terrace preservation in the intramontane valleys the coeval episodes of alluvial fan growth and abandonment recorded at the basin margin are interpreted to reflect changes in the efficiency of the drainage system due to variations of tectonic uplift in the adjacent areas alternating phases of deglaciation and renewed glaciation however are inferred to have played a major role in controlling the amount and type of sediment delivered to the alluvial fan complexes

late quaternary vegetation and climate changes in central texas based on the isotopic composition of organic carbon stable carbon isotope analysis of organic carbon in alluvial deposits and soils of three streams in central texas reveals significant shifts in the ratio of c3 to c4 plant biomass production during the past 15 000 yr these temporal changes in vegetation appear to be in response to changes in climate during the late pleistocene c4 plants comprised only about 45 to 50 of the vegetative biomass in this area suggesting that conditions were cooler and wetter than at any time during the past 15 000 yr the time between 11 000 and 8000 yr b p is interpreted as transitional between late pleistocene conditions and warmer and drier holocene conditions based on a slight increase in the abundance of c4 plant biomass during the middle holocene between approximately 6000 and 5000 yr b p mixed c3 c4 plant communities were replaced almost completely by c4 dominated communities indicating prairie expansion and warmer and drier climatic conditions by 4000 yr b p the abundance of c4 plant biomass decreased to levels similar to the early holocene transitional period suggesting a return to cooler and wetter climatic conditions no significant shift in the ratio of c3 to c4 productivity has occurred during the last 4000 yr except for a slight increase in the abundance of c4 plant biomass around 2000 yr b p the results of this investigation correlate well with other regional late quaternary climatic interpretations for central and north texas the southern plains region and with other portions of the great plains

paleoflood analysis of an alluvial channel in the south central great plains black bear creek oklahoma abstract late holocene floodplain of the black bear basin in north central oklahoma was chronologically developed based upon slackwater deposition and radiocarbon dating of associated paleosols major paleoflood events occurred 3590 80 yr bp and 1150 100 yr bp assumptions developed for application in semi arid bedrock channels were tested and modified maximum accumulation of slackwater deposits were located some 400 m up tributaries rather than at the intersection of the mouths of present day tributary junctions these sites represent the intersection with tributaries during the extreme widths of floods on an alluvial floodplain physically tracing slackwater units up tributaries was possible in some cases 8 km an accurate estimate of flood stage was determined by measuring up terrace perpendicular to the tributary during floods aggradation and degradation of the main channel were insignificant because the broad floodplain is the most impacted feature in an alluvial setting the preflood landscapes determined from paleosol elevations were recreated to estimate the manning roughness coefficients and paleoflood discharges the hec 2 water surface profile hydrologic engineering center 1982 determined that paleodischarges of 4391 cm in 3590 bp and 8782 cm in 1150 bp emplaced the slackwater deposits along the tributaries of black bear creek

isotopic composition of disseminated sulfur in precambrian sedimentary rocks abstract isotopic analyses of trace sulfur in precambrian carbonates and clastic sedimentary rocks support the results of previous studies which have shown that 34s measurements of archean sedimentary sulfur cluster mostly around mantle like values and that a significant overall dispersion of 34 s 32 s ratios commenced at about 2 7 ga ago the outstanding feature of the proterozoic record is the predominance of heavy 34ssulfide values mostly in excess of 0 cdm consideration of these data in combination with the sparse 534s record for proterozoic sulfates suggests that 34ssulfide sulfide may have been smaller than was the case during the phanerozoic a possible scenario that is compatible with the above data is based on a proposition that the terrestrial exogenic sulfur cycle evolved in a tripartite manner in the archean it was coupled to and controlled by the magmatic endogenic cycle the proterozoic for the most part represents a transitional stage into a phanerozoic like exogenic cycle the latter essentially decoupled from its endogenic counterpart and its redox balance maintained and mediated by the biosphere the indication that the mean combined 34s of proterozoic sulfate and sulfide may have significantly exceeded the phanerozoic mean value of 2 cdm is enigmatic and poses problems for balance considerations

sulfur isotopes in early proterozoic volcanogenic massive sulfide deposits new data from arizona and implications for ocean chemistry the 34s range of sulfides from early proterozoic volcanogenic massive sulfides of arizona is narrow 129 out of 133 measurements lying within a range of 2 8 to 3 2 other early proterozoic and archean deposits of comparable type and mineralogy have similar s isotope characteristics these deposits unlike phanerozoic analogues do not appear to have incorporated reduced sulfur derived from marine sulfate early proterozoic oceans are known to have contained sulfate in this paper it is a major tenet that sulfate was confined to an upper water layer it is proposed that most volcanogenic massive sulfide hydrothermal systems prior to 1 7 ga were recharged with sulfate free water from an anoxic lower layer and in general discharged into reducing environments rare deposits with exceptional mineralogy or s isotope compositions may reflect hydrothermal recharge from the oxidized upper layer or deposition from fluids discharging into and mixing with the upper water layer

late quaternary floodplain history of the brazos river in east central texas abstract the floodplain along a 75 km segment of the brazos river traversing the gulf coastal plain of texas has a complex late quaternary history from 18 000 to 8500 yr b p the brazos river was a competent meandering stream that migrated from one side of the floodplain to the other creating a thick layer of coarse grained lateral accretion deposits after 8500 yr b p the hydrologic regime of the brazos river changed the river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasionally avulse avulsion occurred four times first at 8100 yr b p then at 2500 yr b p again around 500 yr b p and finally around 300 yr b p the depositional regime on the floodplain also changed after 8500 yr b p with floodplain construction dominated by vertical accretion most vertical accretion occurred from 8100 to 4200 yr b p and from 2500 to 1250 yr b p two major and three minor periods of soil formation are documented in the floodplain sequence the two most developed soils formed from 4200 to 2500 yr b p and from around 1250 to 500 yr b p these changes on the floodplain appear to be the result not of a single factor but of the complex interplay among changes in climate sediment yield and intrinsic floodplain variables over time

climatic implications of the late quaternary alluvial record of a small drainage basin in the central great plains four late quaternary alluvial fills and terraces are recognized in wolf creek basin a small 163 km2 drainage in the kansas river system of the central great plains two terraces were created during the late pleistocene the t 4 is a fill top terrace underlain by sand and gravel fill fill i and the t 3 is a strath terrace cut on the cretaceous dakota sandstone both fill ii early holocene and fill iii late holocene are exposed beneath the t 2 a holocene fill top terrace the t 1 complex consisting of one cut and three fill top terraces is underlain by fills iii and iv a poorly developed floodplain t 0 has formed within the past 1000 yr as valleys in wolf creek basin filled during the early holocene an interval of soil formation occurred about 6800 yr b p early holocene fill has been found only in the basin s upper reaches indicating that extensive erosion during the middle holocene removed most early holocene fill from the middle and lower reaches of the basin valley filling between 5000 and 1000 yr b p was interrupted by soil formation about 1800 1500 and 1200 yr b p as much as 6 m of entrenchment has occurred in the past 1000 yr holocene events in wolf creek basin correlate well with those in other localities in the central great plains indicating that widespread changes in climate along with adjustments driven by complex response influenced fluvial activity

depositional controls in a bedload dominated fluvial system internal architecture of the lee formation kentucky abstract sandstones of the early pennsylvanian lee formation central appalachian foreland basin were deposited in a low sinuosity bedload dominated fluvial system the internal architecture of the sandstones consists of a six fold hierarchy of bounding surfaces which are evaluated with respect to depositional controls minor first second and third order surfaces generally less than a few hundred meters in extent separate deposits of small to intermediate scale bedforms third order surfaces which dip gently in the downflow direction represent reactivation surfaces of downstream accreting macroforms major surfaces fourth fifth and sixth order are hundreds of meters to kilometers in extent fourth order surfaces separate deposits of individual macroforms fifth order bounding surfaces are erosional boundaries of major system scale channels individual sandstone members of the lee formation are bound by sixth order surfaces erosional at the base and gradational at the top deposits overlying major surfaces contain fining upward sequences the trends are more apparent with increased scale of the deposit these sequences represent responses of the fluvial system to various mechanisms that controlled sedimentation both fourth and fifth order surfaces are interpreted as results of autocyclic controls the nature of the lee formation fluvial system was such that a solitary major channel was relatively stable in one position within the alluvial plain for a significant period of time during this time the channel aggraded with the passage of one or more downstream accreting macroforms creating a fourth order erosional surface the channel eventually diverted its flow to another portion of the alluvial plain by avulsion creating a fifth order surface a gross fining upward trend and upward change in fluvial style for individual sandstone members both characteristic of paleovalley fills indicate that sixth order surfaces were controlled by allocyclic processes these characteristics and the stair step positioning of the sandstone members younger away from the alleghenian mountains to the east suggest that tectonic processes were the ultimate control on the system

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

genesis of proterozoic iron formation sulphur isotope evidence seven units of carbonaceous shale or sulphide facies iron formation have been sampled they are associated with proterozoic iron formations that range in age from 1 9 to 2 5 ga sokoman and gunflint canada riverton united states penge south africa and brockman australia sulphur isotope ratios have been determined on the sulphides removed from these shales by both physical and chemical means the mean 34s composition of the seven units varies between 4 9 and 6 6 and the sample variance is low within each unit these distributions are more characteristic of hydrothermal sulphide than sulphide produced by biogenic reduction this hydrothermal sulphide is believed to have originated from high temperature reduction of seawater sulphate and from magmatic sulphide a model is suggested whereby this sulphide was exhaled into stratified anoxic oxic basins the sulphide and associated base metals were deposited in the reduced sediments beneath the anoxic waters while some iron and manganese was deposited on oxygenated shelves the data support but do not prove a hydrothermal exhalative origin for lower proterozoic iron formation

reconstruction of prehistoric landfall frequencies of catastrophic hurricanes in northwestern florida from lake sediment records abstract sediment cores from western lake provide a 7000 yr record of coastal environmental changes and catastrophic hurricane landfalls along the gulf coast of the florida panhandle using hurricane opal as a modern analog we infer that overwash sand layers occurring near the center of the lake were caused by catastrophic hurricanes of category 4 or 5 intensity few catastrophic hurricanes struck the western lake area during two quiescent periods 3400 5000 and 0 1000 14 c yr b p the landfall probabilities increased dramatically to ca 0 5 per yr during an hyperactive period from 1000 3400 14 c yr b p especially in the first millennium a d the millennial scale variability in catastrophic hurricane landfalls along the gulf coast is probably controlled by shifts in the position of the jet stream and the bermuda high

stable carbon isotopes c3 c4 vegetation and 12 800years of climate change in central new mexico usa abstract a thick sequence of fine textured alluvium in central new mexico has provided an opportunity to reconstruct vegetation type and climate at the western edge of the great plains grassland the analysis of thirty eight ams radiocarbon ages 13 c values and modern weather data has produced a 12 800 cal yr record of changing c 3 c 4 vegetation temperature and precipitation the record begins with the younger dryas that was characterized by c 3 plants and was about 2 4 c cooler with over 100 mm rainfall than today after 11 000 cal yrs bp the climate became less cool and less wet reaching present day conditions by 9000 yrs the middle holocene was c 4 warm and dry although the 13 c record is incomplete for this interval from 3300 to 1400 cal yrs bp the climate was cool and wet with c 3 plants averaging 0 5 c cooler and 22 mm greater rainfall than today matching other records in the region for a wetter climate after 1400 cal yrs bp the local climate became warm and dry shifting to c 4 vegetation the severe second century drought first observed in tree rings in southern colorado is supported by the 13 c data and occurred during the interval a d 40 to a d 180 hiatuses in the local alluvial and 13 c record extend from 9000 to 6000 cal yrs bp and from 1000 cal yrs bp to present

impact crater morphology the origin of the mertz and ninnis glaciers antarctica east antarctica like west antarctica poses several questions about its geologic geomorphic and glaciologic history among these are questions regarding the origins of the wilkes subglacial basin the gamburtsev mountains the wilkes land anomaly subglacial lakes subglacial topography and the enigmatic mertz and ninnis glaciers located immediately inland of george v coast at the northern extremity of the wilkes subglacial basin the mertz and ninnis glaciers display characteristics that have posed questions of their sizes their accompanying subglacial topography their existence in tandem within the continental ice sheet and their extensions as glacier tongues offshore in the southern ocean the present study examines in particular the underlying craterform morphology as the potential explanation for these several features based upon surveys of the subglacial topography which include ground based geophysical survey airborne radiosound survey airborne geophysical survey and satellite remote sensing of the geomorphology beneath the continental ice sheet on the basis of these investigations we propose that the mertz and ninnis glaciers are the result of parallel channelizing subglacial valleys and that the entire mertz and ninnis glacier complex is a function of meteorite crater morphology beneath the east antarctic continental ice sheet

a sedimentological model for the continental upper triassic tadrart ouadou sandstone member recording an interplay of climate and tectonics argana valley south west morocco the continental upper triassic tadrart ouadou sandstone member was deposited in an extensional setting on the pangaean continent strongly influenced by a low latitude climatic regime 10 to 20 north complex interaction of basin subsidence and climatically driven processes led to high facies variability and a lack of correlatable units across the argana valley exposures a process orientated approach integrating detailed facies with architectural element analysis was undertaken which resulted in a multistage depositional model for the tadrart ouadou sandstone member the basin scale model shows that basal alluvial fan and braided river systems are confined to the centre of the argana valley exposures aeolian deposits occur throughout the sequence but dominate in the north after a phase of playa deposition prominent basin wide fluvial incision of up to 8 m marks the onset of perennial fluvial flow these well sorted internally complex and locally highly amalgamated fluvial sandstones are widespread throughout the basin and are focused in a north to south south west flowing channel system after a final stage of aeolian sedimentation sandstone deposition of the tadrart ouadou sandstone member in the argana valley is terminated rapidly by the onlap of lacustrine mudstones of the sidi mansour member the study revealed that except for one pronounced period of perennial conditions sedimentation is controlled largely by ephemeral fluvial flow alternating ground water tables deflation processes and periods with limited periodic local run off the study highlights that facies architecture in the basin is the result of complex interaction of local syn sedimentary tectonics and the climatic regime within the basin but also the climate of the catchment area to the east the data suggest a proximal to mid distal basin setting in the rain shadow to the west of a mountain range massif ancien which exerted a strong control on the depositional environments of triassic deposits exposed in this part of south west morocco

geochemistry of the alteration pipe at the bruce cu zn volcanogenic massive sulfide deposit arizona the bruce volcanogenic massive sulfide deposit is a conformable lens which lies at an andesite rhyolite contact within the 1 7 to 1 0 b y old yavapai series exposed south of bagdad arizona the upper part of a funnel shaped alteration pipe replaces andesite within the stratigraphic footwall of the deposit this pipe can be divided into three zones based on characteristic mineralogy a chlorite zone consisting of chlorite 40 90 wt and quartz completely replaces the andesite in the central part of the pipe a sericite zone containing subequal amounts of chlorite sericite and quartz is peripheral to the chlorite zone in the upper part of the pipe the modal abundances of alteration minerals decrease outward through a gradational zone to unaltered andesite amphibole and biotite occur as metamorphic minerals they are most abundant in the gradational zone some biotite is texturally and chemically related to the chlorite and sericite and is also probably hydrothermal in origin all of the rocks within the pipe have been depleted in 18 o with whole rock delta 18 o values ranging from 2 4 to 7 6 per mil a linear relationship exists between the oxygen isotope composition and the quantity of hydrothermal chlorite in the alteration pipe both the amount of chlorite and the 18 o depletion are functions of the extent to which the rock has reacted with the hydrothermal fluid the bulk chemistry of the altered rocks when measured as a function of increasing alteration progress decreasing delta 18 o shows strong enrichments in mgo and feo and depletions in sio 2 k 2 o na 2 o and cao the most altered and most 18 o depleted samples are nearly completely altered to chlorite their bulk chemistry approaches that of the chlorite in the sample all of the ferromagnesian minerals become more mg rich toward the center of the pipe the fe fe mg ratio of chlorite ranges from 0 8 in the least altered sample to 0 3 in the center of the chlorite zone extrapolating the linear relationship between the amount of chlorite in a sample and its delta 18 o value to 100 percent chlorite gives a value of 2 1 for the pure chlorite at 250 degrees and 300 degrees c water in equilibrium with this chlorite would be 1 1 and 2 05 per mil respectively these data are comparable to delta 18 o values of water issuing from hot springs on the east pacific rise and are consistent with a slightly 18 o shifted seawater as a source for the ore forming solutions prior to entering the alteration pipe the seawater reacted with volcanic rocks at depth lowering the ph of the fluid such that chlorite was stable when the fluid entered the pipe subsequent formation of chlorite continued to lower the ph and decreased the activity of mg in the solution such that the fluid moved into the sericite stability field

lower mississippi river tributaries contributions to the collective science concerning the father of waters abstract the geological and geomorphic information preserved in the tributary valleys of the lower mississippi river lmr contributes to our understanding of the lower valley s quaternary geological history prominent pleistocene terraces are preserved in the tributary valleys fisk first formulated his four terraces framework on the red river caution needs to be followed in projecting the red river terraces across the entire lower mississippi valley lmv the tributary system cannot be assumed to operate in a synchronized fashion in response to changes in climate and base level to compare the collective contribution of the tributaries of the lmr the streams are described in terms of 1 their characteristics 2 geomorphic development 3 process and response of the tributaries to and from the lmr and 4 engineering investigations and implications the characteristics of the tributaries are a direct function of their drainage basin size and geology the tributary system drains portions of six physiographic provinces synoptically the tributaries can be viewed as two groups the eastern and western tributaries all of the eastern tributaries are intra regional i e they drain only one physiographic province the coastal plain and therefore have a restricted sediment source generally the eastern tributaries are more numerous and shorter than the western tributaries the longer western tributaries drain outside the coastal plain the extra regional nature of the western tributaries adds to the variability of discharge and sediment types the sediment record of the tributaries reflects response to the trunk mississippi during glacial outwash flushes many of the tributaries were alluvially drowned producing alluvial cones expressed as flattened longitudinal profiles more recently a number of tributaries in the state of mississippi have experienced episodes of accelerated channel erosion the effects of navigation and flood control modification of the mississippi river on the tributaries have not been fully studied therefore fluvial geomorphic research in the tributaries is essential to understanding ways to mitigate the adverse effects of river engineering thereby designing engineering works in balance with the alluvial architecture and processes of the stream system

metamorphic ore remobilization in the hallefors district bergslagen sweden constraints from mineralogical and small scale sulphur isotope studies the marble and metavolcanic hosted pb zn ag sb as deposits of the hallefors district located in the palaeoproterozoic bergslagen ore province south central sweden comprise both stratabound sulphides and discordant ag rich sulphide sulphosalt veins the complex sulphide sulphosalt assemblages of the alfrida jan olof mines at hallefors were investigated by a combination of ore microscopy electron microprobe analysis and in situ laser sulphur isotope analysis the massive ore is characterized by positive and homoge neous d 34 s 1 4 to 2 7 v cdt whereas vein hosted sulphides and sulphosalts exhibit similar but generally less positive to slightly negative d 34 s 0 6 to 2 0 comparison of the observed ore mineral assemblages with calculated phase equilibria in the sys tem fe as s o h and isotopic fractionation as a function of temperature oxygen fugacity and ph indi cates that the vein type mineralization was formed from relatively reduced and rather alkaline hydrothermal fluids at these reduced conditions fractionation of d 34 s via changes of fo2 is insignificant and thus the isotopic signatures of the vein minerals directly reflect the com position of the sulphur source we therefore conclude that the vein type ore essentially inherited the sulphur isotope signature from the pre existing massive sulp hides via metamorphic remobilization at approximately 300 400c and 2 3 kbar scales of remobilization observable are on the order of about 5 mm to 30 cm overall the sulphide sulphosalt assemblages from the alfrida jan olof mines exhibit d 34 s values which are comparable to a majority of metasupracrustal hosted deposits in the bergslagen province thereby suggesting a common origin from ca 1 90 1 88 ga volcanic hydro thermal processes