architecture of miocene overbank deposits in northern pakistan abstract understanding the geometry of sediment bodies and patterns of sediment disruption in ancient fluvial overbank successions is fundamental to interpretations of overbank deposition overbank deposits of the fluvial chinji formation siwalik group can be divided into sequences 1 20 m thick defined by the alternation of stratified sediments and paleosols preservation of stratified sequences does not support a gradual vertical aggradation of overbank sediments over the extent of the floodplain but instead indicates periods of rapid sediment deposition followed by long hiatuses and soil development rapid deposition of overbank sequences is predicted by models that hypothesize 1 episodic vertical aggradation of the entire floodplain 2 deposition controlled by the growth of alluvial ridges 3 incision and filling of local valleys 4 rapid filling of localized low areas or 5 rapid deposition associated with river channel avulsion these models also have important implications for understanding fossil preservation and floodplain paleoecology paleosol bounded sequences in the chinji formation can pinch out laterally over kilometers indicating that episodes of rapid deposition were restricted to local areas on the floodplain there is no evidence that bases of sequences are erosional and they do not appear to record valley incision and filling sequences generally do not thin and fine systematically away from the terminating margins of major channel deposits suggesting they are not related to the growth of alluvial ridges along active channels lithologic variations and patterns of sediment disruption within sequences appear to reflect the rapid tilling of low areas on the floodplain both adjacent to and distal from the major river channel this infilling of floodplain topography may reflect a continuous process whereby local areas were always being filled somewhere on the floodplain by sediments from minor tributary or crevasse channels alternatively the filling of local flood plain topography may have been caused by widespread but short lived events associated with river channel avulsion

late pleistocene holocene eolian deposits of the southern buenos aires province argentina a preliminary model abstract the aim is to reconstruct the sedimentological history of the late pleistocene holocene eolian deposits of southern buenos aires province hence a preliminary model is proposed in order to analyze the different stages of the sedimentological cycle the deposits include sandy loess fine sand sheets and dunefields the southern buenos aires province represents the depositional area whereas the source and transport environments are located in the northern patagonian andes and northern extra andean patagonia according to the mineralogical characteristics of the material explosive volcanism is considered the predominant process of particle formation particularly as generator of pyroclastic deposits which were reworked by different erosive processes glacial fluvial eolian during the late glacial maximum and the holocene alluvial deposits which were mainly generated by fluvial erosion of volcaniclastic sedimentary units were deposited along the floodplains of the colorado and negro rivers including those distal segments located in the emerged offshore platform the fluvial regime depended upon the regime of the glaciers hence it reflected the major glacial fluctuations which took place in the andes floodplains were deflated by southwest winds transporting the material towards the southern buenos aires province sandy loess and loessial sands and to la pampa province dunefields

the plio pleistocene record of the central eastern pampas buenos aires province argentina the chapadmalal case study abstract the plio pleistocene record of central eastern pampas argentina is preserved in loess like sediments exposed in the sea cliffs south of mar del plata the uplift of the andes during miocene triggered the deposition of wind blown material the source areas are located about 1000 km away in western argentina volcanic ashfalls play a significant role in the formation of eolian sediments which were reworked and redeposited by aqueous transport in a flat low relief landscape under varying climatic conditions vertebrate fossil assemblages document warm and wetter pliocene and early pleistocene climates during montehermosan and uquian land mammal ages more arid and cooler conditions alternating with humid and warmer intervals prevailed during the middle to late pleistocene in ensenadan and lujanian land mammal ages the chapadmalal locality is the best exposed and most complete section almost all the units have been modified by pedogenesis truncated paleosols as well as pedogenic and non pedogenic calcretes are very frequent throughout the stratigraphic succession several paleosurfaces with prominent paleosols and calcretes suggest that sedimentation was an episodic process with numerous depositional gaps recording relatively long lasting intervals of landscape stability the depositional events represent alternations of fluvial and eolian sedimentation low frequency catastrophic episodes played an important role in the sedimentation process

glaciations in patagonia and tierra del fuego during the ensenadan stage age early pleistocene earliest middle pleistocene while the ensenadan stage age ca 2 1 to 0 5 ma was characterized by certain mammal associations in the pampean plains the andean ranges of patagonia and tierra del fuego their piedmont areas and in some cases the adjacent lowlands were undergoing a well defined alternating sequence of glacial and interglacial stages driven by the regional climate of southernmost south america and antarctica this paper reviews the available geographical geological and chronological evidence of glaciation during the ensenadan stage age with perhaps more than 15 distinct glacial events including the great patagonian glaciation gpg and other cold episodes that took place before and after it finally a tentative correlation of patagonian glacial episodes and the pampean biostratigraphic stages based on radiometric dating and paleomagnetic studies is herein presented

palaeoclimatic interpretation of clay minerals in marine deposits an outlook from the continental origin abstract as distribution of clay minerals in modern oceans seems to be controlled by contemporary climates marine clays in ancient deposits have been widely used to reconstruct palaeoclimates in this review we examine this question mainly from the continental origin of the clay minerals and show the difficulties of retrospective analysis of the influence of climate on common marine deposits we examine successively how soils develop and behave are reworked and palaeoweatherings are preserved focusing the review on kaolinitic soils indicative of tropical wet climates several stages intervene between the time of clay mineral development in soils and their final deposition in the basin 1 the persistence over geological times of huge amounts of kaolinitic palaeoweathering materials in the landscapes may seriously alter the palaeoclimatic signal of kaolinite in the sedimentary record although the kaolinite deposited in recent sediments of the world oceans is consistent with wet climatic zones this is mainly because the major kaolinitic and bauxitic palaeosurfaces coincide with the tropical areas where kaolinite is still developing at present 2 possible development of deep kaolinitic profiles at high latitudes and cold conditions under co 2 enriched palaeoatmospheres or even by unusual acidic weathering linked with dry climates is misleading palaeoenvironments for palaeoclimatic reconstruction 3 erosion and transport processes always cloud the signal to some extent because frequently erosion products from the bedrock and different soil horizons will be mixed and major drainage systems may include soils of different climatic zones 4 the arrival of the soil clay assemblages in the basin inevitably lags against their formation on the continent and may occur several million years after formation of the soils on the continent and climate may have considerably changed at that time it is pointed out that sedimentologists and palaeoclimatologists should consider the complexity of the relations between soil and sedimentary clay minerals soil formation rates are slow and therefore the resolution of the palaeoclimatic record in marine clay may not be closer than 1 or 2 ma at present the palaeoclimatic interpretations of marine clay assemblages are yielding at best nothing more than rather broad palaeoclimatic information this approach cannot achieve the same degree of resolution as other techniques such as isotope or microfossil studies clay mineral assemblages may provide integrated records of overall climatic impacts whereas other techniques are more likely to reveal local or temporary climates

distal alluvial deposits in a foreland basin setting the lower freshwater molasse lower miocene switzerland sedimentology architecture and palaeosols the lower freshwater molasse untere susswasser molasse crops out over a wide area of the swiss molasse basin coarse grained alluvial fan conglomerates dominate in proximal basin areas along the alpine front these conglomerates pass northwards into sandstones and mudstones of an extensive northeastward draining meandering river system which ran parallel to the basin axis sedimentological study of outcrops quarry exposures and boreholes in the basal miocene aquitanian has permitted detailed facies analysis of this distal alluvial sequence the distal aquitanian is made up of distinct architectural elements characterized by their geometries and sedimentary structures each may be assigned to a particular depositional setting meander belt levees crevasse channels and splays overbank fines and palaeosols and lacustrine meander belt sandstones were deposited in mixed load channels with a dominant bedload component sandstones commonly comprise amalgamated and locally stacked ribbon bodies 2 15 m thick and 150 1500 m wide interbedded rippled laminated and mottled fine grained levee sandstones and siltstones form lenticular packages up to 3 m thick and 30 100 m across small scale crevasse channel sandstones 2 4 m thick and 5 10 m across pass laterally into metre scale medium to fine grained crevasse sandstone sheets rare laminated lacustrine siltstones occur only in the north east part of the basin floodplain mudstones and marls make up the remainder of the succession these display a variety of pedogenic features recording cyclical palaeosol development palaeosols show strong variations in morphology and maturity both laterally across the floodplain and downstream along the basin axis reflecting local variation in aggradation rate associated with proximity to alluvial channel courses as well as regional variation in subsidence and floodplain drainage analysis of the organization and distribution of the various sediment bodies permits reconstruction of the fluvial system and allows development of a model for the sedimentary architecture of the lower freshwater molasse in the study area integration of palaeosol studies into a well defined architectural framework assists recognition of areal facies belts and may aid location of sand prone sequences in the subsurface

clay mineral distribution patterns in late neogene fluvial sediments of the subathu sub basin central sector of himalayan foreland basin implications for provenance and climate abstract the late neogene 6 0 5 ma fluvial succession of the subathu sub basin a part of the himalayan foreland basin comprises a 2 4 km thick pile of conglomerate grey and buff sandstone and mudstone representing middle and upper siwalik subgroup this basin is filled mainly by major trunk and piedmont drainage which are nearly perpendicular to each other the clay mineral assemblages of this sedimentary succession have illite 7 82 smectite 0 90 chlorite 2 23 and kaolinite 1 13 the grey sandstones have moderate to abundant smectite 23 90 whereas the buff sandstones have abundant illite 66 79 and low to absent smectite 0 14 the mudstones that dominates the succession 50 have clay mineral assemblages similar to grey and buff sandstones or intermediate proportion the temporal distribution of clay minerals in mudstones shows occasional intense zigzag pattern with either smectite 3 81 or illite 15 82 abundance the smectite rich grey sandstones and mudstones are deposited by trunk drainage and the illite rich buff sandstones and mudstones are deposited by piedmont drainage the intense zigzag distribution pattern of clay minerals in mudstone indicates interfingering of floods from trunk and piedmont drainages the interfingering was severe ranging between 4 8 and 3 36 ma and between 2 60 and 1 77 ma related to tectonic activity the association of smectite 36 bearing mudstones and piedmont source derived buff sandstone and conglomerate towards the upper part of the section above 1 77 ma suggests either floodwater of trunk drainage over spill on the fringe of piedmont alluvial fan or derivation from smectite bearing middle siwalik rocks exposed due to the activity of an intra foreland thrust ift in the piedmont zone the occurrence of smectite and its variable proportion with time suggests its probable derivation not only from the sparsely exposed basic rock in the catchment area but also from siliceous and metamorphic rocks under favourable climatic conditions between 6 and 0 5 ma

high frequency controls on alluvial successions an integrated sedimentological and palaeopedological approach to the plio pleistocene of argentina abstract plio pleistocene palaeosol bearing alluvial strata are continuously exposed along the marine cliffs of southeastern buenos aires province argentina the study interval was deposited by a mixed predominantly suspended load fluvial system outcrops are dominated by floodplain siltstones and mudstones that exhibit a cyclic alternation between calcic and vertic palaeosols this cyclicity is represented by meter scale alluvial cycles that show as a general trend a gradual increase in the proportion of channel fill deposits up section cyclic variations in palaeosols facies and architectural elements can be related to allogenic drivers such as climate and eustasy for the lower punta san andres alloformation the stratigraphic arrangement of the recognised palaeosols and the clay mineral distribution indicate that the palaeoclimate was subhumid average mean annual precipitation map 840 mm seasonal and temperate average mean annual temperature mat 7 85 c for at least 1 ma from ca 2 9 ma to ca 1 8 ma late pliocene early pleistocene with several intervals in which conditions became drier and probably colder even though climate was a major control on palaeosol development it is suggested here that it was not the main control on high frequency cycles variations in the orbital parameters more specifically in insolation were interpreted as having controlled the sedimentation and architectural evolution of the lower punta san andres alloformation depositional systems

quaternary mammalian faunas of the pampean region abstract knowledge of the fossil mammals from the pampean region of argentina has greatly increased in recent years to analyze the patterns of change in regional climates and environments through the quaternary the fossil record of land mammals was used two principal factors affected mammalian diversity in the pampean region during the pleistocene glaciations and the climatic environmental and sea level changes associated with them and the large numbers of north american taxa that immigrated to south america from the pliocene the regional climatic conditions changed from being warm and wet to being colder drier and more seasonal during the pleistocene as a consequence of glacial cycles cold and dry conditions were interrupted by relatively short warmer and wetter periods several pulses of expansion and contraction of steppes and subtropical forests are recorded this change in habitat produced the provincialism that has characterized the mammal faunas from the pampean region finally the most notable characteristic of the quaternary mammal fauna of south america is the almost complete extinction of large mammals during the latest pleistocene earliest holocene the evidence presented here is most consistent with a model that explains megafaunal extinction through climatic fluctuations as it appears that only a few species of large mammals survived until the holocene nevertheless a human presence could have accelerated the process of extinction

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

tectonic and climatic control of paleogene sedimentation in the rhenodanubian flysch basin eastern alps austria abstract the paleocene to lowermost eocene formations of the rhenodanubian flysch were deposited in an abyssal environment at the continental rise to the south of the european plate the pattern of paleocurrents indicates a number of small distributary systems for the turbiditic material that entered the basin from the north and was deflected to the east and to the west heavy mineral assemblages in the turbidites suggest the erosion of medium grade metasediments in the danian and the progressive erosion of underlying metamorphic magmatic rocks in the thanetian and ypresian the most obvious sedimentary event is the breakdown in turbidite sedimentation during the late danian to the early thanetian remarkably this starvation of turbidites is associated with high values of chlorite in the clay mineral assemblages of interturbidite shales indicating increased mechanical erosion of the adjoining land areas tectonic uplift of these areas and associated block faulting and tilting is assumed to be responsible for this increase in erosion as well as for the synchronous cutting off of the basin from the source area of the turbidites this tectonic activity is related to the onset of the collision of the european and the adriatic plates a second major event documented in the paleogene record is the change from a predominantly siliciclastic system to a carbonate system in the latest thanetian associated with the global negative carbon isotope excursion cie in the upper part of calcareous nannozone np9 calcareous mudturbidites become the prevailing rock type eutrophication of surface waters is indicated by acmes of diatoms radiolaria and dinoflagellates at the level of the cie together with slightly increased values of kaolinite in the clay mineral assemblages of interturbidite shales this can be interpreted as a result of increased continental run off due to high precipitation rates in a humid climate the top of the increased kaolinite input is poorly constrained because of a very high input of smectite due to volcanic activity in sub zone np10a this igneous activity is assigned to the opening of the northern atlantic ocean and has no geodynamic relevance for the rhenodanubian basin

avulsion deposits in lower eocene alluvial rocks bighorn basin wyoming abstract eocene alluvial rocks in the northern bighorn basin can be subdivided into three major kinds fine grained deposits on which cumulative paleosols developed lithologically heterogeneous packages associated with immature paleosols and major sheet sandstone bodies the heterogeneous packages which alternate vertically with the cumulative paleosols are dominated by fine grained deposits and also contain ribbon sandstone bodies and thin 1 m sheet sandstone bodies most ribbon sandstones are less than 3 m thick and are more common in the middle and upper parts of the packages cut and fill deposits with fine grained fills are locally important because the lithologic packages are readily apparent in the field because of the immature or simple paleosols developed on the fine grain d facies they are referred to as simple paleosol packages the major sheet sandstones were deposited by meandering trunk rivers and locally overlie and truncate the other deposits contrary to classical models of meandering streams only a portion of the fine grained rocks in the study area were deposited by overbank flooding of a trunk channel on the basis of their stratigraphic position and few ribbon sandstones the upper parts of some simple paleosol packages are interpreted as ancient levee deposits associated with a trunk channel farther from the trunk channel overbank floods deposited alluvium on which the cumulative paleosols formed the greater parts of the simple paleosol packages including their fine grained deposits were deposited rapidly during channel avulsion ribbon sandstone bodies represent ancient splay channels on the avulsion belt simple paleosol packages in the willwood formation support the contention of smith et al 1989 that n many meandering river systems a significant portion of the fine grained deposits were deposited as the trunk channel avulsed deposits that resemble the simple paleosol packages may provide a means of recognizing avulsion episodes in other alluvial sequences

a 3 3 ma impact in argentina and possible consequences enigmatic glassy materials escorias and red bricklike materials tierras cocidas occur at a restricted stratigraphic level the top of the chapadmalal formation materials from one locality near mar del plata are attributed to a mid pliocene impact event with a radiometric and magnetostratigraphic age of 3 3 million years ago ma an extinction of endemic fauna including the glyptodonts and flightless cariamid birds correlates with the unit containing the impact glasses moreover the age of the glasses is coincident within dating uncertainties with a pulselike change in the oxygen isotope marine record in the atlantic and pacific oceans just before the late pliocene deterioration of the climate

sedimentological control on the clay mineral distribution in the marine and non marine palaeogene deposits of mallorca western mediterranean abstract during the middle eocene oligocene a marine and non marine succession about 1000 m thick was deposited on mallorca palaeoenvironmental interpretation of these deposits was obtained from sedimentological and palaeontological data in earlier studies the non marine environments recorded are alluvial fluvial channel and flood plain deposits and lacustrine prevailing terrigenous organic rich or carbonate sedimentation marine environments are represented by littoral and shelf deposits in most of these palaeoenvironments terrigenous sedimentation prevailed except in the marine shelf where mixed sedimentation was dominant and in the carbonate and organic rich lakes in general palaeogene freshwater lakes were perennial open and exorheic with carbonate waters the palaeogene deposits were derived from jurassic and early cretaceous pelagic and hemipelagic marls the clay mineral assemblage of these source area rocks was mostly smectite interstratified illite smectite and illite the weathering of these materials under high temperature and rainfall favoured the hydrolysis of clay minerals and the formation of kaolinite x ray diffraction tem and edx analysis were carried out on 108 samples of palaeogene mudstones clay minerals identified were illite interstratified illite smectite smectite chlorite and kaolinite the various palaeogene environments marine transitional and non marine can be distinguished on the basis of different clay mineral assemblages these assemblages are related to weathering and diagenetic processes as well as to the distance from the source area alluvial sediments contain smectite and kaolinite with subordinate amounts of illite all these clay minerals are of detrital origin smectite and kaolinite are present in all environments but the proportion of kaolinite to smectite increases with the distance from the source the k rich interstitial waters derived from illite hydrolysis in the source area favoured the formation of mixed layer illite smectite in the alluvial flood plain where the stagnation of waters during dry periods allowed the concentration of dissolved ions fluvial deposits contain interstratified illite smectite smectite kaolinite illite and chlorite in variable amounts variations in the proportion of illite and the occasional occurrence of chlorite was attributed to compositional differences in the source area and variations in the sediment supply input the terrigenous and carbonate lakes preserved the detrital clay mineral assemblage carried by the rivers organic rich lakes contain mainly kaolinite and smectite with less illite littoral deposits contain kaolinate and illite with minor amounts of smectite and interstratified smectite illite shelf deposits are mostly formed by illite and kaolinite

palaeosols as indicators of geomorphic stability in two old red sandstone alluvial suites south wales palaeosols fossil soils are abundant in the alluvial late silurian early devonian moor cliffs formation and rat island mudstone freshwater west formation of south dyfed south wales the palaeosols occur in thick mudstones deposited on floodplains under a seasonal semi arid climate and represent vertisol type soils simple single profiles are rare and most of the mudstone intervals exhibit complex overprinted profiles however a major difference exists between the palaeo vertisols in each of the two units studied the moor cliffs formation contains thick floodplain intervals with evidence of frequent erosion and reactivation of earlier soil horizons the associated coarser channel deposits mainly represent ephemeral streams the alluvial system was unstable flashy and prone to extensive stripping of stored floodplain sediment arroyos unstable ephemeral stream channels probably developed during this interval evidence for any marine influence on soil development is missing and previous speculations that the formation may represent deposition in an upper intertidal or supratidal system are not supported the rat island mudstone represents a complete contrast in that there was little or no stripping of floodplain sediments the channel deposits are typically of the fining upwards type representing more stable low sinuosity channels and with ephemeral stream floods by integrating the palaeosols a more detailed picture of the geomorphic history of these units can be formulated comparison with contemporary drainage basins indicates that a change in climate was a possible cause for the differences between the two units less frequent and more severe storms occurring during the deposition of the rat island mudstone

integration of channel and floodplain suites ii vertical relations of alluvial paleosols abstract extrachannel sediments of the lower eocene willwood formation which were subdivided into different pedofacies or developmental stages exhibit two well developed orders of pedofacies sequences over which a grosser scale cyclicity is superposed simple sequences with thicknesses measured in meters are comprised of one or more soil profiles that are bounded above and below by pedogenically unmodified crevasse splay deposits on a larger scale compound pedofacies sequences are tens of meters thick and consist of several simple sequences sandwiched between channel sandstones between these sandbodies pedogenic maturity of the multistorey paleosols progressively increases and then decreases upwards in response to episodic avulsion formation of both compound and simple pedofacies seq ences was largely controlled by local patterns of deposition and erosion that produced vertical variability in both rate of sediment accumulation and parent material because paleosol maturity also reflects the effects of extrabasinal factors a pedofacies megasequence which is hundreds of meters thick developed in response to changes in tectonic activity through willwood time comparison between willwood deposits in the northern and central parts of the bighorn basin also reveals that areally differing sediment accumulation rates and thereby basin subsidence rates can be interpreted from large scale pedofacies sequences consequently paleosols are useful in determining changes in relative rates of sediment accumulation that are controlled by both intrabasinal and extrabasinal factors

the sequence stratigraphy of fluvial depositional systems the role of floodplain sediment storage abstract the currently used model for alluvial sequence stratigraphy has been criticised miall 1991 and an alternative model is offered here in this model alluviation mainly takes place during transgressive and not highstand phases when accommodation space allows sediment storage the high capacity for some river systems to store floodplain sediments is a critical consideration the effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple base level controlled sequence models and recognising higher order fourth and fifth order cycles will be particularly difficult

loess of southern south america abstract loess and loessoid loess like reworked loess sediments extend across the chaco pampean plains and the nw mountain environments of argentina and in neighbouring countries paraguay brazil uruguay bolivia loessoid sediments are much more abundant than primary loess the beginning of the loessoid sedimentation cycle was related to a phase of late miocene orogeny in the andes the plio pleistocene record is mostly composed of loessoid sediments modified by pedogenesis which produced welded palaeosols the late pleistocene holocene loess record reveals a heterogeneous composition across the region coarse textures and an andean derived volcanic composition prevail in the southern pampas finer textures and material coming from the andes and the other two sources sierras pampeanas and the parana basin characterize the loess deposits of the northern pampas and the eastern chaco a southern patagonian source is also suggested for the mountain valley loess of tucuman although a western andean provenance has been proposed it is believed that the material in the western chaco was derived from the bolivian andes loess deposition was related to a multistage transport mechanism involving fluvial and aeolian processes inferred westerly and southwesterly wind directions as dominant carriers of the aeolian deposits are in agreement with westerly palaeowind simulations using climate models however the role played by westerly tropospheric winds and northerly winds remains to be established

earthquake rupture stalled by a subducting fracture zone we showed that the rupture produced by the great peru earthquake moment magnitude 8 4 on 23 june 2001 propagated for 70 kilometers before encountering a 6000 square kilometer area of fault that acted as a barrier the rupture continued around this barrier which remained unbroken for 30 seconds and then began to break when the main rupture front was 200 kilometers from the epicenter the barrier had relatively low rupture speed slip and aftershock density as compared to its surroundings and the time of the main energy release in the earthquake coincided with the barrier s rupture we associate this barrier with a fracture zone feature on the subducting oceanic plate

magnetostratigraphy and magnetic parameters of a sedimentary sequence in punta san andres buenos aires argentina abstract this contribution focuses on polarity changes and magnetic parameters obtained in late cenozoic sediments exposed on the cliffs in san andres chapadmalal buenos aires province the sedimentary section studied comprises the vorohue san andres miramar and arroyo seco formations the upper layers of vorohue and the base of san andres showed records of normal polarity and were attributed to the gauss normal polarity chron 2 6 ma the main portion of san andres formation appears to have been deposited during lower and middle matuyama polarity chron including the normal polarity subchron olduvai 1 78 2 02 ma miramar formation contains polarity changes that should be assigned to middle matuyama jaramillo 1 05 ma and to jaramillo upper matuyama 0 99 ma the upper portion of the miramar formation and the lower one of the arroyo seco formation show reverse polarity levels and were assigned to the upper matuyama 0 78 ma finally the upper part of arroyo seco formation seems to have been deposited during the brunhes polarity chron

illite in aridic soils desert dusts and desert loess abstract in some desert loess deposits notably those of central asia illite dominates the clay fraction in others such as in the near east and north africa illite is a minor component the interaction between desert loess deposits and aridic desert soils via desert dusts is used to explain the difference in illite contents vertical profile distribution of illite as well as vertical distributions of soluble and exchangeable potassium are taken to indicate that illite forms pedogenetically in the surface horizon of aridic soils once formed it is stable and accumulates it is postulated that the potassium required for the illitazation process is introduced by aeolian dusts the process is facilitated by wetting and drying cycles dusts originating in deserts where they are presently produced by desert weathering of various rocks are defined as primary or juvenile dusts they have low illite contents such are the dusts that are carried from the eastern sahara and sinai peninsula northeast towards the middle east where they form local loess deposits the central asian dusts on the other hand have been subjected to numerous cycles of deflation and deposition resulting in relatively long residence times in the pedogenic environment of aridic soils this results in the enrichment of these dusts with illite the central asian desert loess deposits that interact with these dusts are consequently enriched with illite similar processes of illite enrichment may have operated in other areas such as australia

fossil soils as grounds for interpreting long term controls on ancient rivers abstract the main factors controlling fluvial systems are time initial topographic relief geology climate vegetation base level upland runoff upland drainage network hillslope morphology downstream deliveries channel behavior and pattern of deposition similar factors control soil formation in drainage basins climate organisms topographic relief parent material and time effects of these factors may be preserved as well or better in fossil soils of alluvial sequences as in sedimentary structures facies and mineralogy of paleochannels for example former rainfall may be interpreted from depth to the top of the calcic horizon from percent clay from percent of different kinds of clay minerals from quartz feldspar ratio from hue and from identification of fossil soils interpretation of the role of these factors in producing a particular stratigraphic section should take into account its resolution the proportion of geological time represented temporal completeness of original sediment preserved lithological completeness and of original fossils preserved paleontological completeness temporal and lithological completeness can be estimated by comparison with the most likely rates of sediment accumulation and most likely thicknesses respectively for pertinent sedimentary environments and time scales paleontological completeness can be estimated by comparing the preserved fossils with expectations based on former ecosystems indicated by fossil soils likely controls and resolution of a sequence of eocene and oligocene fossil soils from badlands national park south dakota were studied as a guide to the periodicity and causes of episodic sedimentation there short term episodicity and filtering of sedimentation events of particular magnitude were apparent from superposition of paleosols but resolution of the sequence was not sufficient to permit a detailed analysis of their individual causes long term paleoenvironmental changes and four episodes of erosional downcutting could be recognized all four occasions of landscape instability and erosion were times of drier climate and sparser vegetation than usual local uplift and supply of volcanic ash were contributing factors in some episodes of erosion when allowances are made for time scales and completeness of sequences of fossil soils they can be useful indicators of changes in former environment biota and fluvial regimen