extensive felsic lavas and rheoignimbrites in the keweenawan midcontinent rift plateau volcanics minnesota petrographic and field recognition the 8 km thick sequence of plateau lavas north shore volcanic group in northeastern minnesota that was erupted during the formation of the 1 1 ga midcontinent rift system includes an unusually large proportion 10 25 of felsic flows rhyolites and icelandites individual flows are now recognized with estimated volumes up to 600 km3 and lateral extents up to 40 km some were probably originally much broader many are anomalously large compared to most silicic lava flows primary structures and textures of these essentially unmetamorphosed rocks indicate eruptive style for most of these large flows in particular the features of the basal meter or so and uppermost several meters show that some flows were emplaced as fluid lavas and others as strongly rheomorphic tuffs it may be essential in interpreting the origin of felsic flows elsewhere to examine petrographically the lowermost few decimeters and the top where the only evidence poorly to strongly welded shards may exist of the pyroclastic origin of a large otherwise lava like flow interior textures of these crystalline flows provide evidence of high temperature eruption especially in the form of abundant paramorphs of magmatic tridymite in the groundmass commonly arranged in a pilotaxitic flow structure based on structures textures field relations and geochemistry we conclude that a all the icelandites and some rhyolites were emplaced as large moderately to very fluid lava flows that spread widely while crystallizing tridymite and that b some large rhyolites which are only sparsely phyric were emplaced as pyroclastic flows subsequently remobilized as rheoignimbrites the mode of emplacement of one of the largest aphyric flows remains enigmatic vent areas or characteristics are unknown the low apparent viscosity of the erupted magmas is probably attributable to compositional factors f content high fe2 as well as to high temperatures and eruption rates estimated temperatures of about 1000 1100 c are associated with the low water content and perhaps great depth of origin of the magmas at the time of massive influx of hot matic melts from the mantle during continental rifting

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

perspectives on proterozoic surface ocean redox from iodine contents in ancient and recent carbonate abstract the proterozoic eon hosted the emergence and initial recorded diversification of eukaryotes oxygen levels in the shallow marine settings critical to these events were lower than today s although how much lower is debated here we use concentrations of iodate the oxidized iodine species in shallow marine limestones and dolostones to generate the first comprehensive record of proterozoic near surface marine redox conditions the iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters to assess the validity of our approach neogene quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate iodine contents despite the potential for diagenetic loss maximum proterozoic carbonate iodine levels are elevated relative to those of the archean particularly during the lomagundi and shuram carbon isotope excursions of the paleo and neoproterozoic respectively for the shuram anomaly comparisons to neogene quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends the baseline low iodine levels in proterozoic carbonates relative to the phanerozoic are linked to a shallow oxic anoxic interface oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes however the diagnostically low iodine data from mid proterozoic shallow water carbonates relative to those of the bracketing time intervals are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans this redox instability may have challenged early eukaryotic diversification and expansion creating an evolutionary landscape unfavorable for the emergence of animals

well preserved late precambrian paleosols from northwest scotland abstract two paleosol profiles were developed side by side on gneiss sheigra clay paleosol and amphibolite staca clay paleosol along the same unconformable contact of lewisian metamorphic rocks 2 7 1 7 ga overlain by torridonian sandstones 0 8 ga in northwestern scotland although altered during burial by compaction and illitization these profiles are well preserved compared with most other precambrian paleosols that are metamorphosed to at least greenschist facies in addition deformation of pegmatite veins in a manner similar to soil creep can be taken as an indication that upper horizons of these paleosols have been preserved soil development in both profiles was characterized by the oxidation of iron in biotite and magnetite and by hydrolysis of plagioclase orthoclase and hornblende clay skins soil like microfabrics and pedogenic smectite have been preserved in the profiles alteration of silicate grains other than quartz microcline and biotite has been intense but chemical analyses failed to show dramatic removal of alkalis and alkaline earths from the profiles microscopic concretions similar to pedogenic carbonate were found in subsurface horizons of the staca clay rare earth elements are more abundant within the paleosols than in their parent materials these paleosols developed on a well drained landscape that was buried by torridonian alluvial fans atmospheric oxidation was greater than revealed by studies of archean paleosols but probably still short of modern levels paleoclimate was subhumid and probably seasonal organic carbon and its light isotopic composition in the paleosol is evidence for photosynthetic microbial crusts paleosols of such great antiquity do not fit comfortably within modern soil taxonomies and are for the moment assigned to the informal category of green clays

life associated with a 2 76 ga ephemeral pond evidence from mount roe 2 paleosol dark sericitic material at and near the top of the 2 765 0 01 ga mount roe 2 paleosol in western australia contains 0 05 0 10 wt organic carbon with delta 13c values between 33 and 51 pdb peedee belemnite such negative isotopic values strongly indicate that methanotrophs once inhabited this material the textures and the chemical composition of the dark sericitic material indicate that the methanotrophs lived in or at the edges of ephemeral ponds that these ponds became desiccated and that heavy rains transported the material to its present sites the discovery of methanotrophs associated with the mount roe 2 paleosol may extend their geologic record on land by at least 1 5 b y methanotrophy in this setting is consistent with the notion that atmospheric methane levels were or 20 mu atm during the late archean the radiative forcing due to such high atmospheric methane levels could have compensated for the faint younger sun and helped to prevent massive glaciation during the late archean

chemistry of modern soils as a guide to interpreting precambrian paleosols precambrian paleosols are potentially one of our best sources of information about the composition of the atmosphere early in earth s history to be used this way however an alteration profile must be shown to be an in situ product of vadose processes data from modern soils show that the ratios of less mobile elements such as zr ti and al can be used as a preliminary screen to test for the in situ nature of a profile if the ti zr ratio departs from that of the host rock by more than 40 or the ti al ratio departs by more than 50 for moderate degrees of weathering then a significant allochthonous contribution to the profile is indicated this technique correctly separates in place from allochthonous horizons when applied to independent data from two modern profiles a review of published precambrian profiles identifies two often cited examples denison and pronto as suspect based on these criteria in the interpretation of paleoatmospheres it is also necessary to show that fe is controlled not b

extreme element mobility during transformation of neoarchean ca 2 7ga pillow basalts to a paleoproterozoic ca 1 9ga paleosol schreiber beach ontario canada abstract this study reports petrographic major and trace element and oxygen isotopic data for a subaerial weathering profile at schreiber beach ontario canada the weathering profile developed on neoarchean ca 2 7 ga pillow basalts and is unconformably overlain by the paleoproterozoic ca 1 88 ga gunflint chert and basal conglomerates this stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the gunflint formation rocks at schreiber beach have been classified as pillow cores pillow rims hyaloclastites weathered red basalts and weathered brown to green basalts there are gradual textural mineralogical and geochemical transitions from unweathered basalts to intensely weathered hematite bearing basalts with stratigraphic height rocks at schreiber beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the neoarchean following the subaerial weathering they were buried and re metamorphosed to greenschist facies in the paleoproterozoic given the preservation of primary igneous textures and trace element patterns in pillow cores they are used as a protolith to estimate element mobility during seafloor alteration metamorphism and subaerial weathering chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major si ca na fe ti k and trace lile ree hfse and transition metals elements during neoarchean seafloor alteration alteration of these rocks at relatively low temperatures is indicated by enrichment in 18 o 18 o 10 4 to 12 7 relative to fresh basalt 5 5 subaerial weathering resulted in significant textural mineralogical e g hematite precipitation and geochemical variations in metamorphosed basalts causing additional redistribution of most major and trace elements and further enrichment in 18 o the highest 18 o values 16 1 to 17 1 occur at the silica rich base of the weathering profile many elements including lile ree and hfse are strongly enriched 100 or depleted up to 96 relative to their original abundances alkaline metasomatism which is a characteristic of precambrian paleosols is also evident at schreiber beach where extreme enrichment in k 280 to 1000 rb 390 to 660 and cs 1160 to 2560 occurs in the weathering profile

sensitivity of coastally trapped disturbance dynamics to barrier height and topographic variability in a numerical model abstract the sensitivity of a coastally trapped disturbance ctd to topographic height is examined using simulations of the 15 18 may 1985 ctd these simulations include three with uniform topography in which the north american west coast mountains are represented by a three piece uniform ramp at the coast leading to a constant plateau inland and three with realistic topography in each trio of uniform and realistic topography simulations there is a control case in which the terrain height closely approximates reality and two variations in which the topography is multiplied everywhere by a topographic multiplication factor tmf of 1 5 and 0 5 to assess the sensitivity of the simulation to the barrier height average propagation speeds increased decreased by 15 20 with the increased decreased tmf it was found that the position of the ctd leading edge generally followed close behind a leading pressure trough minimum which propagated northward along the coast in a manner similar to a topographi

conditions of subaerial weathering of basalts in the neoarchean and paleoproterozoic abstract precambrian weathering profiles or regoliths were formed on the earth s surface in contact with meteoritic water this signifies that their mineral and bulk composition should reflect the environmental conditions at the time of weathering the aim of this study is to reconstruct the main mineralogical and chemical trends involved in the formation of precambrian weathering regoliths by means of geochemical modeling verified through laboratory experimentation we have demonstrated that in a wide range of modeling conditions weathering profiles of basalts consist mostly of clay minerals smectites and illite carbonate minerals may also be deposited within the profile at the initial stage of weathering which are later dissolved during a further process the time necessary for complete dissolution of primary minerals and crystallization of authigenic minerals is between 100 and 140 kyr modeling results indicate that neoarchean and paleoproterozoic 2 8 2 0 ga weathering profiles were formed under an atmosphere with pco2 not much higher than 25 pal and climatic conditions close to those found in modern times weathering under co2 rich pco2 1 and 10 bar and ch4 rich atmospheres as well as high temperature 50 c and 75 c conditions do not produce compounds corresponding to the analyzed samples

abrupt chemical weathering increase across the permian triassic boundary previous studies have suggested a variety of causes for the end permian extinction event including a bolide impact flood basalt volcanism methane clathrate dissociation or some combination of catastrophic processes one common feature of these hypotheses is the prediction of an enhanced earliest triassic greenhouse new high resolution geochemical results from graphite peak antarctica support an abrupt increase in chemical weathering in the earliest triassic among otherwise genetically similar paleosols of similar provenance relative to the latest permian paleosols the earliest triassic paleosol exhibits greater leaching greater accumulation of immobile rees and evidence of lower soil po2 with no evidence of an erosional unconformity between the paleosols which are separated by 15 cm stratigraphically these results support a rapid shift perhaps 10 000 years to an earliest triassic greenhouse and a role for methane release in the extinction event and its aftermath

do red beds indicate paleoclimatic conditions a permian case study abstract terrestrial red beds have long been interpreted as desert deposits by comparison with modern red deserts more recently red beds have been interpreted as evidence of seasonally dry conditions and a permo triassic pangean monsoon red beds of cala viola sardinia are identified as paleosols and used to reconstruct late permian paleoclimatic conditions reconstruction of paleoenvironmental conditions based on the paleosols of the cala viola indicates warm humid conditions with no evidence of dry conditions as in a desert or of extreme seasonality as in a monsoon instead it is suggested that the red color of the paleosols is a result of former good drainage and that red color in general does not indicate specific paleoclimatic conditions

equation for compaction of paleosols due to burial existing empirically derived compaction curves for marine sediments fail to estimate the compaction of paleosols flood plain sediments and peats as indicated by deformation of clastic dikes dinosaur footprints and sandstone paleochannels yet compaction estimates are vital to reconstructing paleoclimate and geochemical profiles of paleosols to modeling sediment accumulation rates of nonmarine rocks and to tectonic flexural modeling of molasse facies this paper presents a compaction equation and physically derived constants that make geologically realistic estimates of compaction of paleosols and other nonmarine sediments a protocol for the application of the equation is suggested that would allow the following equation to solve for burial compaction c as a fraction of original thickness given depth of burial d in km as c s i f e d k 1 this equation can be applied to nonmarine sedimentary rocks or paleosols given appropriate empirical data on the physical constants in the equation such as initial solidity s i initial porosity f and the corresponding curve fitting constant k data on physical constants useful for this equation are compiled here for a range of paleosol and nonmarine sediment types

the 1 1 ga midcontinent rift system central north america sedimentology of two deep boreholes lake superior region abstract the midcontinent rift system mrs of central north america is a 1 1 ga 2500 km long structural feature that has been interpreted as a triple junction rift developed over a mantle plume as much as 20 km of subaerial lava flows mainly flood basalts are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds this rock sequence known as the keweenawan supergroup has been penetrated by a few deep boreholes in the search for petroleum in this paper two deep boreholes in the upper peninsula of michigan are described in detail for the first time both the amoco production 1 29r test herein referred to as the st amour well and the nearby hickey creek well drilled by cleveland cliffs mining services were 100 cored the former is 7238 ft 2410 m deep and the latter is 5345 ft 1780 m deep the entirety of the stratigraphic succession of the hickey creek core correlates very well with the upper portion of the st amour core as determined by core description and point counting of 43 thin sections selected out of 100 studied thin sections two lower paleozoic units and two keweenawan red bed units the jacobsville sandstone and the underlying freda sandstone are described the jacobsville is largely a feldspatholithic sandstone and the freda is largely a lithofeldspathic sandstone below the freda the remaining footage of the st amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate sandstone siltstone and shale black shale individual basalt flows and a basal ignimbritic rhyolite this lower portion of the st amour core presents an enigma as it correlates very poorly with other key boreholes located to the west and southwest while a black shale sequence is similar to the petroleum bearing nonesuch formation farther west there is no conglomerate unit to correlate with the copper harbor conglomerate other key boreholes are distributed over a 1300 km distance along the better known southwest arm of the triple junction mrs and can be correlated rather well with the units that are exposed in the lake superior region however a definitive explanation of the anomalous deeper st amour stratigraphy is elusive and any explanation is tenuous a possible explanation for this anomalous stratigraphy may be the geographic proximity of the st amour borehole to the keweenawan hot spot mantle plume the suggested thermal force behind the development of the mrs similarly a drastic change in structural architecture may be explained by this geographic relationship thus within the locale of this rifting center complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm

eocene hydromorphic paleosols significance for interpreting ancient floodplain processes abstract pervasive mottling iron oxide nodules and slickensides indicate that alluvial paleosols of the lower eocene willwood formation in the elk creek area of the bighorn basin wyoming were hydromorphic two basic groups of hydromorphic paleosols are present cumulative which show evidence for concurrent deposition and pedogenesis and simple noncumulative which show evidence for a single episode of deposition and pedogenesis lithologic packages consisting of vertically stacked simple paleosols and crevasse splay sandstones alternate with cumulative paleosols the simple paleosols are weakly developed indicating rapid deposition with relatively little time for pedogenesis cumulative paleosols show greater pedogenic development indicating significantly longer periods of pedogenic m dification and slower more episodic sediment accumulation differences between the simple and cumulative paleosols reflect differences in the deposition of their parent materials the packages of simple paleosols are similar to avulsion belt deposits described from the saskatchewan river and suggest that avulsion was significant in floodplain construction the upper parts of cumulative paleosols were formed on true overbank deposits which accumulated gradually after avulsion and establishment of a new channel the lower parts of many cumulative paleosols resulted from a prior episode of avulsion deposition and simple soil formation because hydromorphy is associated with texture these different depositional processes influenced lateral variations in hydromorphy of the cumulative paleosols paleosols in the elk creek area provide a simple means of identifying avulsions sediment accumulation rates suggest that avulsions occurred with a regular period of about 20 000 years and may have been induced by climatic cycles

pedogenesis and geochemical alteration of the picture gorge subgroup columbia river basalt oregon reddened clay rich horizons between basalt flows have historically been identified as baked zones or as zones of lateral groundwater movement the reddening has been attributed to thermal or chemical alteration of the permeable portion of the basalt flow many of these red horizons are instead paleosols that record significant hiatuses in basalt flow emplacement paleosols were identified in the field on the basis of features such as horizonation ped morphology and root traces terrestrial gastropods fossil plants and peats also occur locally plots of weathering indices such as base loss are consistent with a pedogenic origin for these horizons ca na mg fe 21 and p have been extensively lost from the a and bt horizons of the paleosols consistent with modern weathering patterns k and rb have been added to the upper portions of the profiles and lost in the lower portions of the profiles relative to the parent material with rb added disproportionately historically this pattern of enrichment has been cited as evidence of k metasomatism the apparent metasomatism can be explained using a new model in which there was extensive felsic volcanism contemporaneous with pedogenesis and k and rb were likely added by windborne ash the apparent discordance between k and rb contents of the paleosols can be explained by the presence of land plants because k is an important electrolyte and rb does not serve any major biological role this model could account for apparent k metasomatism in other paleosols following the advent of land plants and may be applicable to older paleosols as well

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

tellurium and selenium in mesoproterozoic red beds abstract samples of red bed sediments in several mesoproterozoic successions contain enrichments of tellurium te including discrete telluride minerals the tellurides were neoformed during redox controlled diagenesis rather than representing mechanical concentrations of heavy minerals tellurium is enriched relative to selenium which may reflect erosion of archean and palaeoproterozoic rocks with high te contents high continentality during the mesoproterozoic would have limited the delivery of te and other trace elements to the oceans so accumulated te on the continents

quaternary glacial interglacial climate cycles in hawaii abstract the upper kilometer of the hawaiian scientific drilling program core consists of lavas that were emplaced subaerially burying paleosols these paleosols were preserved with minimal alteration and are used to construct a 330 kyr quaternary climate record within the temporal resolution of this record there appears to be a 100 kyr climate oscillation between cool dry and warm wet conditions indicating that tropical regions such as hawaii have responded to the same global climate forcings as have higher latitude areas during at least the past three glacial interglacial cycles it is also suggested that hawaiian climate may have become progressively warmer and wetter over the same period

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

the sturgeon falls paleosol and the composition of the atmosphere 1 1 ga bp a paleosol is exposed along the north bank of the sturgeon river some 25 km sw of baraga michigan the paleosol was developed on hydrothermally altered keweenawan basalt and is overlain by the jacobsville sandstone textures mineralogy and chemical composition change gradually upwards from unweathered metabasalt through the paleosol to the contact of the paleosol with the jacobsville sandstone many of these changes are similar to those in modern soils developed on basaltic rocks however k has clearly been added to the paleosol probably by solutions which had equilibrated with k feldspar in the jacobsville sandstone the keweenawan basalt was oxidized quite extensively during its conversion to greenstone during weathering the remaining fe2 was oxidized to fe3 and was retained in the paleosol the composition of the parent greenstone and its change during weathering can be used to define an approximate lower limit to the ratio of the o2 pressure to the co2 pressure in the atmosphere during the formation of the paleosol formula see text free o2 must have been present in the atmosphere 1 1 ga ago but its partial pressure could have been 10 3 times lower than in the atmosphere today

denitrification and mixing in a stream aquifer system effects on nitrate loading to surface water ground water in terrace deposits of the south platte river alluvial aquifer near greeley colorado usa had a median nitrate concentration of 1857 mol l 1 median nitrate concentrations in ground water from adjacent floodplain deposits 468 mol l 1 and riverbed sediments 461 mol l 1 both of which are downgradient from the terrace deposits were lower than the median concentration in the terrace deposits the concentrations and 15n values of nitrate and n2 in ground water indicated that denitrifying activity in the floodplain deposits and riverbed sediments accounted for 15 30 of the difference in nitrate concentrations concentrations of cl and sio2 indicated that mixing between river water and ground water in the floodplain deposits and riverbed sediments accounted for the remainder of the difference in nitrate concentrations river flux measurements indicated that ground water discharge in a 7 5 km segment of river had a nitrate load of 1718 kg n day and accounted for about 18 of the total nitrate load in the river at the downstream end of that segment this nitrate load was 70 less than the load predicted on the basis of the median nitrate concentration in the terrace deposits and assuming no denitrification or mixing in the aquifer water exchange between the river and aquifer caused ground water that originally discharged to the river to reenter denitrifying sediments in the riverbed and floodplain thereby further decreasing the nitrate load in this stream aquifer system results from this study indicated that denitrification and mixing within akluvial aquifer sediments may substantially decrease the nitrate load added to rivers by discharging ground water

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

pedogenic translocation of fe in modern and ancient vertisols and implications for interpretations of the hekpoort paleosol 2 25 ga abstract many interpretations of oxygen levels in the precambrian atmosphere use interpretations of fe distributions measured in paleosols this article addresses the current lack of knowledge concerning baseline fe translocations in modern soil analogs for the 2 25 ga hekpoort paleosol at waterval onder south africa as well as the lack of understanding of diagnostic petrographic features related to fe translocation preserved in the precambrian paleosols petrographic features related to fe translocations include redox related fe depletions and enrichments and possibly fe mn nodules which compare favorably with similar pedogenic features in paleozoic paleo vertisols and modern analog vertisols additionally preservation of sepic plasmic microfabrics in the hekpoort paleosol which are characteristic of clay soils experiencing shrink swell supports previous paleo vertisol interpretations total fe losses in vertisols are 10 50 losses increase with increasing soil age and with increasing mean ann

reappraisal of a 2200 ma old paleosol near waterval onder south africa abstract a fossil soil paleosol about 2200 ma old in a deep roadcut between the towns of waterval boven and waterval onder transvaal here called the waterval onder clay paleosol was formerly reported to have formed on the hekpoort basalt it is here thought to have developed on a thick formerly at least 5 5 m upward fining and thinning sequence of sandstone siltstone and shale derived from pre existing soils on hekpoort basalt within the basal portion of the fluvial dwaal heuvel formation in contrast to paleosols developed on hekpoort basalt the waterval onder clay paleosol is little deformed lacks corestones and shows compositional variation and berthierine rich laminae interpreted as relict bedding the paleosol has a ridge and swale topography linear or wavy gilgai and a subsurface structure mukkara typical of modern vertisols since there is considerable relict bedding it probably formed over a short period of time less than 7000 years and was not self plowing to the extent seen in modern vertisols clastic dikes filled with sandstone from the overlying rocks are evidence that the profile was periodically dry to depths of at least 2 m more drab colored rocks below that may represent the level of permanent water table iron and manganese stain of layers and claystone granules within the surface horizon of the profile have some characteristics of modern rock varnish and could be taken as evidence for life in the soil the high ratio of ferric to ferrous iron near the surface of the paleosol together with evidence from clastic dikes that this horizon was not waterlogged is an indication of an oxidizing atmosphere although probably one in which oxygen was much less abundant than it is today the very low amounts of acid titratable bases in the shaly parent material of this paleosol and surficial loss of these bases within the paleosol appear to have been produced by decarbonation reactions comparable to those found today in soils of vascular land plants a likely source of weathering acid during the precambrian is an elevated abundance of atmospheric carbon dioxide considering these likely atmospheric differences together with the distribution of comparable modern vertisols and other geological evidence the climate under which this soil formed was probably semiarid to subhumid seasonally dry and temperate

geology and geochemistry of paleosols developed on the hekpoort basalt pretoria group south africa the hekpoort paleosols comprise a regional paleoweathering horizon developed on 2 224 0 021 ga basaltic andesite lavas at the top of the hekpoort formation of the pretoria group transvaal supergroup south africa in five separate profiles from outcrops along road cuts near waterval onder and the daspoort tunnel and in three drill cores from the bank break area bb3 bb8 and bb14 the top of the paleosol is a sericite rich zone the sericite zone grades downward into a chlorite rich zone in core bb8 and in the road cut at the daspoort tunnel we sampled the underlying or parent basaltic andesite into which the chlorite zone grades we did not obtain samples of the parent material at waterval onder and in cores bb3 and bb14 but chemical analyses indicate that the chlorite and sericite zones in these profiles derive from underlying lavas similar to the ones we sampled in core bb8 and at the daspoort tunnel the presence of apparent rip up clasts of the paleosol in the overlying ironstones of the strubenkop formation in the cores from bank break makes it very unlikely that most of the alteration was a result of interactions with hydrothermal fluids desiccation cracks at the top of the paleosol that were filled with sand during the deposition of the overlying sediments at waterval onder point to a subaerial weathering origin very little if any al ti zr v or cr moved a discernible distance during weathering of any of the five profiles the vertical distribution of fe mg mn ni and co indicates that these elements were largely removed from the top of the soil during weathering the overall abundance of these elements in each of the profiles indicates that a significant fraction of the complement lost from the top subsequently reprecipitated in the lower portion of the soil as constituents of an fe 2 rich smectite the loss of fe from the top of the soil during weathering of the hekpoort paleosols indicates that atmospheric p o 2 was less than 8 x 10 4 atm about 2 22 ga fe 2 rich smectite should only precipitate during soil formation if atmospheric p co 2 is less than or equal to 2 x 10 2 atm rye kuo and holland 1995 ca and na were largely lost during weathering some na was apparently added to the sericite zone in cores bb3 bb8 and bb14 after weathering all five profiles are enriched in k and rb and most are enriched in ba the distribution of these elements indicates that they all were added during post weathering hydrothermal metasomatism rb sr analysis of the paleosol at the daspoort tunnel indicates that metasomatism last affected that profile 1 925 0 032 ga macfarlane and holland 1991

physical volcanology of mid proterozoic plateau lavas the keweenawan north shore volcanic group minnesota the north shore volcanic group a thick sequence of plateau lavas of keweenawan proterozoic 1 100 m y age was erupted during the development of the midcontinent rift of north america it contains many well exposed physical features that imply a range of eruptive styles and flow mechanisms these include a high volume and high effusion rate floods of basalt and basaltic andesite which ponded and crystallized largely after flow had ceased b moderate to small volume and effusion rate flows and small floods some probably building low shields and c small flows to very large flows and ignimbrites of icelandite and rhyolite lacking visible vent areas viscosities calculated from 183 chemical analyses correlate well with the physical features the style of volcanism combined characteristics of the columbia plateau and the snake river plain plains volcanism and was very similar to that of the tertiary of eastern iceland

precambrian paleosols and atmospheric co2 levels abstract precambrian atmospheric pco2 levels were previously estimated using a simple thermodynamic model based on the mineral assemblage in paleosols in addition to theoretical flaws with that model recalculation of pco2 levels using more recent thermodynamic data gives significantly lower values a new model based on the mass balance of weathering in paleosols gives consistent results from three separate 2 2 ga old paleosols the calculated pco2 value of 23 3 3 times present atmospheric levels is insufficient to overcome the faint young sun paradox suggesting that another greenhouse gas such as ch4 was present at elevated levels relative to the present atmosphere to account for the discrepancy applying this new approach to other precambrian paleosols a proterozoic pco2 curve is generated which indicates consistently high pco2 from 2 5 to 1 8 ga ago and a substantial drop in atmospheric pco2 at some time between 1 8 and 1 1 ga ago

mantle temperature and lithospheric thinning beneath the midcontinent rift system evidence from magmatism and subsidence the tectono magmatic history of the midcontinent rift system can be explained by the presence of a mantle plume bringing elevated temperature mantle beneath the rift system at about 1110 ma huge volumes of extrusive and intrusive igneous rocks were generated as abnormally hot mantle decompressed beneath the lithospheric rift geochemical and isotopic data from the keweenawan volcanics show that the earliest melts were derived from small degree melting of primitive plume mantle coupled with enriched metasomatic melts derived from the continental lithosphere as rifting progressed the main bulk of the volcanics was generated primarily from the plume mantle with the melting starting at depths of about 120 km and extending to as shallow as the base of the stretched lithosphere at 45 km depth elevated mantle temperatures of 1500 1560 c approximately 150 200 c above normal are inferred from the rare earth element concentrations in the volcanic rocks further constraints on the mantle temperature come f

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

weathering and paleosol formation in the 1 1 ga keweenawan rift abstract at 1 1 ga the north american craton began to rift resulting in flood style basaltic volcanism during the phase of active tectonism some of that basalt was weathered and redistributed by fluvial processes new observations near the northwestern shore of lake superior have also revealed the presence of seven paleosols formed on weathered immature sediments derived from the basalt that are described here for the first time only three other paleosols have previously been described from this part of the geologic record so these new paleosols represent a significant new discovery the paleosols are weakly developed and exhibit physical and chemical characteristics similar to phanerozoic entisols in contrast to many precambrian paleosols the keweenawen paleosols have been subject to minimal post burial alteration among the paleosols three distinct pedofacies are recognized 1 waterlogged gleyed fluvial proximal paleosols 2 dry fluvial distal paleosols and 3 cumulative fluvial proximal paleosols despite these facies differences using a combination of physical and chemical measures of weathering it is found that the paleosols are all derived from the same parental basalt with one of the paleosols showing an additional extra basinal rhyolitic component by considering the mass balance behaviour of a variety of alkali and alkaline earth elements it is possible to rule out significant potassium metasomatism suggesting that these paleosols may be particularly useful for paleoatmospheric and paleobiologic reconstructions and that they form an important new source of data about mesoproterozoic weathering

provenance signatures for the miocene volcaniclastic succession of the tufiti di tusa formation southern apennines italy the tufiti di tusa formation a siliciclastic turbidite system of lower miocene age in southern italy is mainly composed of volcaniclastic and quartzolithic sandstones interbedded with mudrocks sandstones are subdivided into four distinctive petrofacies evolving from quartzolithic to volcaniclastic lithofeldspathic and feldspatholithic reflecting detrital evolution from growing orogen quartzolithic petrofacies to active volcanism volcaniclastic petrofacies the mineralogical composition of the associated mudrocks is predominantly characterized by phyllosilicates mainly illite smectite mixed layers i s r1 associated with minor amounts of i s r0 in the lower part of the succession and i s r3 in its upper part together with illite detrital micas and chlorite and minor amounts of chlorite smectite mixed layers and kaolinite in addition to quartz calcite and feldspars the most abundant phyllosilicates are i s mixed layers 10 a minerals illite and micas and chlorite while kaolinite and chlorite smectite mixed layers are present as a few per cent or in trace amounts x ray diffraction patterns show the occurrence of the ordered i s r1 mixed layers in most samples but at the top of the succession some samples are characterized by i s r3 mixed layers whilst in the lower part of the succession i s r1 is associated with a lower amount of i s r0 these features suggest that the tufiti di tusa formation experienced a medium diagenetic grade and the occurrence of i s r3 could be explained by k availability in samples in the upper part of the succession the lithic fragments in sandstones are metasedimentary rocks of palaeozoic age and andesite to dacite volcanic rocks of early miocene age the associated mudrocks also contain trace element ratios cr v y ni la sc th sc th co th cr cr th and eu eu consistent with a provenance containing intermediate to silicic sources with scarce or absent basic rocks the chemical index of alteration 63 2 to 71 6 suggests a moderate degree of weathering in the source furthermore the k cs ratios of sediments confirm likely moderate rather than intense weathering the index of compositional variability icv values from 1 2 to 2 5 are high enough to suggest the mudrocks are first cycle sediments with little recycling the al ti zr diagram and the th sc v zr sc plot indicate poor sorting and rapid deposition of the sediments detrital and sedimentary evolution of the tufiti di tusa formation provides constraints in terms of relations between a growing orogenic system and active volcanism in the central mediterranean to contribute to geodynamic and palaeogeographic reconstructions of the earliest collision in the southern apennines region

crustal structure of the central graben of the midcontinent rift beneath lake superior abstract travel times and amplitudes of refracted and reflected arrivals observed during glimpce great lakes international multidisciplinary program on crustal evolution from the midcontinent rift in lake superior have been modeled using two dimensional ray tracing techniques forward modeling was used to iteratively refine an initial velocity model that was constructed from results of tau sum analysis of the travel time data at each station combined with information from a coincident common midpoint cmp reflection profile when converted to time the resulting model agrees quite well with the cmp reflection profile within the central graben of the midcontinent rift approximately 2 km of sedimentary rock 2 8 4 6 km s overlie an 8 km thick sequence of volcanics and interflow sediments 5 0 6 5 km s within the central rift graben observed on the reflection data beneath this sequence is a 6 8 km thick sequence of 6 6 7 0 km s material interpreted to represent metamorphosed volcanics the velocity of the material at the base of the rift graben is not well constrained approx 7 0 7 2 km s but probably comprises an additional 10 12 km of metavolcanic rocks and intrusions that extend to the base of the graben as imaged on the coincident seismic reflection profile the central graben is ranked by reverse faults the isle royal and keweenawan faults that probably represent normal faults associated with graben formation that were reactivated during the late precambrian a 50 100 ms time advance anomaly associated with the isle royal fault is observed at every station this anomaly has been modeled as shallow high velocity blocks located directly beneath the fault the blocks correlate well with the walls of a steep sided bathymetric trough and are believed to represent highly indurated upper keweenawan sediments resulting from hydrothermal alteration a marked decrease in the apparent velocity and amplitude of the first arrivals is observed on reversed sections at offsets exceeding 100 km and has been modeled as pre rift continental crustal rocks with a velocity of about 6 5 km s adjacent to the 7 0 km s material in the graben calculation of the gravity response of the seismic model demonstrates that the gravity high centered over the rift can be entirely attributed to high density rocks occupying the central half graben imaged on the cmp profile wide angle reflections from about 15 30 km depth beneath the flanks of the graben indicate the presence of velocity discontinuities that may represent rift related detachment surfaces and or pre rift structures within the pre rift crust additional wide angle reflections define the top and bottom of high velocity lower crustal layer the high velocity lower crustal layer is thickest beneath the rift graben and is interpreted to represent magmatic material underplated to the crust during rifting as discussed by trehu et al 1991 total crustal thickness beneath the lake varies from about 55 km beneath the graben to about 35 km beneath the southern shore and 40 km beneath the northern shore

processes controlling the distribution of ti and al in weathering profiles siliciclastic sediments and sedimentary rocks abstract the ratio of ti to al varies greatly in primary igneous source rocks these elements are considered to be relatively immobile in most weathering regimes so that tio2 al2o3 ratios have been used to investigate the provenance of sediments and sedimentary rocks in modern weathering profiles that have undergone moderate weathering ti al ratios remain fairly constant so that siliciclastic sediments derived from them should have ti al ratios like that of the source materials under extreme weathering conditions however the upper most weathered parts of profiles commonly show a marked increase in ti al ratio that is thought to be due to preferential translocation of al rich phases under steady state weathering conditions one particular level of the profile may act as the main source of detrital material and may thus control the ti al ratio of sediments produced from it in a glacio fluvial setting minimum weathering moderate sorting there is considerable variation in ti content but al is nearly constant this is due to concentration of ti bearing mafic minerals such as biotite in the fine fraction in modern sediments derived from moderately weathered and sorted material the ti al distribution reflects mixing of al and ti enriched fine grained materials with sands that are depleted in these elements when a plot of tio2 vs al2o3 is constructed for a suite of such sands and muds the resultant trend differs significantly from that shown by a similar suite of unweathered glacio fluvial sediments comparison of these results with data from sedimentary rock suites suggests that trends shown by plots of tio2 vs al2o3 may provide insight into weathering and depositional history in addition to their use as provenance indicators

calcium carbonate cement caliche in keweenawan sedimentary rocks 1 1 ga upper peninsula of michigan abstract a complex caliche profile and numerous widespread zones of carbonate cement occur in a thick sequence of fluvial siliciclastic sandstones and conglomerates that were deposited in the lake superior basin during and after continental basaltic volcanism as interflow conglomerates of the portage lake volcanics and as the oronto group in conglomerates the carbonate cement forms discontinuous zones up to 60 cm thick and several meters in length consisting of up to 25 vol carbonate whereas the caliche horizon at the centennial mine shows typical upward grading features characteristic of pedogenic caliche the widespread sparry carbonate cement is less diagnostic it is interpreted also as caliche for the following reasons 1 it predates regional epidote metamorphism 2 a caliche profile has been described from the same strata 3 textures characteristic of caliche such as carbonate rimmed pebbles pendulose calcite on pebbles sand grains floating in a carbonate matrix and exfoliated chlorite 4 possible vadoids 5 the carbonate rich zones are widespread stratigraphically and geographically in basinward inclined alluvial fans and 6 the distribution of these zones and the total quantity of contained carbonate requires other than a point source for the origin of the carbonate several lines of evidence suggest that the prevailing climate may have been arid with a fluctuating perhaps seasonal rainfall pattern required for caliche 1 a paleolatitude about 30 north 2 carbonate filled shrinkage cracks denoting high evaporation 3 intermittent flashy or ephemeral streams recorded locally in fluvial beds by numerous superimposed horizons of mud chip conglomerate enormous previously cemented mudstone rip ups and desiccation cracked mud drapes interpretation of the cement as vadose caliche is compatible with a sedimentation model for the region as a semi arid intermontane basin flanked by alluvial fans and with a central saline lake

speculations on the origin of the north american midcontinent rift cannon w f and hinze w j 1992 speculations on the origin of the north american midcontinent rift in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 49 55 the midcontinent rift is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began volcanism continued along with extension and deep rift subsidence for the ensuing 15 m y much of the basaltic magma including some of the earliest flows was formed by partial melting of isotopically primitive asthenosphere contained in the plume head the intense but relatively short duration of rifting and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head as the plume head spread beneath the lithosphere it stretched the overlying lithosphere radially away from the lake superior region the triple junction of the rift system and partially melted to form the great volume of basalt and related intrusive rocks of the region the plume arrived beneath a continent that was under compression as a result of the ongoing grenville orogeny that affected a large region east of the rift that compression prevented full continental separation and eventually returned the region to compressional tectonics as the energy of the plume head waned

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

keweenawan age caliche paleosol in the lower part of the calumet and hecla conglomerate centennial mine calumet michigan the calumet and hecla conglomerate a keweenawan interflow sedimentary unit that is 1 1 b y in age consists at the centennial mine of a felsitic conglomerate upper part and of 1 2 m of siltstone and shale along the base in these fine grained sediments there is a complex submature caliche soil profile represented by an upward increase in calcite and corresponding changes in calcite structures there is also caliche in conglomerate all of this calcite predates the period of regional epidote alteration the caliche is interpreted to have developed in slowly aggrading sediments that were deposited on a rock floored fluvial plain and took place during the initial stages of slope reversal of the lava plain from southerly to northerly this kind of carbonate accumulation in sediments requires a highly seasonal temperate or tropical climate and represents 15 000 yr of weathering the corresponding keweenawan paleolatitude has been reported to be 30 n

paleosols in clastic sedimentary rocks their geologic applications interest in pre quaternary paleosols has increased over the past decade in large part because they have proved to be beneficial in solving diverse geological problems the majority of paleosols are described from continental deposits most commonly from alluvial strata criteria for recognizing these paleosols have been extensively described however classifying them has proved more complicated pre quaternary paleosols are generally classified according to one or more modern soil classification systems although one new classification has been proposed exclusively for paleosols to avoid problems using the modern soil classifications in addition to taxonomic classification paleosols can be categorized according to the interplay among deposition erosion and the rate of pedogenesis when they formed paleosols can be solitary if they formed during a period of landscape stability following the development of an unconformity such paleosols are commonly thick and extremely well developed more commonly paleosols are vertically stacked or multistory because they formed in sedimentary systems undergoing net aggradation if erosion was insignificant and sedimentation was rapid and unsteady compound paleosols generally formed if the rate of pedogenesis exceeded the rate of deposition composite paleosols developed thick cumulative paleosols indicate that erosion was insignificant and that sedimentation was relatively steady both autogenic and allogenic processes can influence depositional and erosion patterns and thus affect the kinds of soils that form consequently paleosols can help to interpret the history of sediment deposition and the autogenic and allogenic processes that influenced a sedimentary basin paleosols are also helpful in stratigraphic studies including sequence stratigraphic analyses they are used for stratigraphic correlations at the local and basinal scale and some workers have calculated sediment accumulation rates based on the degree of paleosol development in addition to their stratigraphic applications paleosols can be used to interpret landscapes of the past by analyzing paleosol landscape associations at different spatial scales ranging from local to basin wide in scope at the local scale lateral changes in paleosol properties are largely the result of variations in grain size and topography at the scale of the sedimentary basin paleosols in different locations differ because of basinal variations in topography grain size climate and subsidence rate paleosols are used to reconstruct ancient climates even to estimate ancient mean annual precipitation map and mean annual temperature mat ancient climatic conditions can be interpreted from modern soil analogs or by identifying particular pedogenic properties that modern studies show to have climatic significance stable carbon and oxygen isotopes are also used to interpret ancient climate and some effort has been made to estimate mat from isotopic composition on the basis of

late oligocene early miocene paleosols of distal fluvial systems ebro basin spain abstract the study of paleosols can be a vital tool in the paleoenvironmental interpretation of continental deposits because their physical and chemical characteristics are a reflection of variations in aspects of soil formation changes in paleosol characteristics can be used to determine the architecture and nature of parent material paleosol maturity topography climate and soil forming organisms including vegetation cover on the ancient floodplain and can be utilized to recreate a palaeocatena furthermore paleosols can provide quantitative data on paleoclimate where other sources such as organic matter and pedogenic carbonate are absent late oligocene early miocene strata exposed in the ebro basin spain have been identified as proximal alluvial fan fluvial and lacustrine continental facies in parts of the distal fluvial system pedogenically altered channel and overbank deposits interfinger with ephemeral lacustrine facies three main paleosol types developed in these sediments and have been described and compared to modern soils entisol like early successional soils inceptisol like young soils and alfisol like open woodland soils paleoenvironmental reconstructions suggest a mosaic of ecotypes with areas of open woodland composed of shrubs herbs and small trees and other plants of low stature and with each ecotype being controlled primarily by fluctuating paleohydrological conditions from field observations and geochemical analysis the climate was found to be humid continental mean annual temperature mat 10 14 c 4 c mean annual precipitation map 450 830 mm yr 1 200 mm yr 1 these results indicate that climatic conditions were wetter than the present day ebro basin mat 14 c map 320 mm yr 1 and differ from previous interpretations of an arid to semi arid environment based on sedimentological criteria alone

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting