Quaternary alluvial terraces in an active tectonic region: the San Juan River Valley,Andean Ranges,San Juan Province,Argentina

The San Juan River, located in San Juan Province (Argentina), crosses the Precordillera and other geologic units including the Ullum tectonic valley and the La Laja Zone between latitudes 31°S and 32°S. The San Juan River is antecedent as is suggested by its two perpendicular segments linked by an almost parallel segment to the main structural trend. Along the Precordillera, the San Juan River valley has many different alluvial fans at the river junctions with its tributaries. The Quaternary alluvial fans display surfaces cut in a series of steps which we consider to be alluvial terraces generated by aggradation and repeated incision episodes. The studied sector includes one area with recent major seismic activity (La Laja Zone), another without major seismic activity (Precordillera area), and a subsident area (Ullum area) where a large lake was formed 6500 yr BP. The old San Juan River was captured by the Quebrada de Ullum valley by means of a 25-m incision, which resulted in river-gradient headward erosion. The San Juan River gradient shows some irregularities that, although unrelated to the main structures, are associated with river dynamics, which emphasizes lithologic differences. The main river valley width, the geometry and gradient of each tributary, together with the basement rock lithologies and the size of each local source area are the major factors which control the alluvial terrace generation processes. In the La Laja Zone, where the uppermost terrace is capped by travertine, dating of travertine deposits suggests that the maximum incision rate is 0.9–1 mm/yr related to episodic activity on the La Laja Fault.     

Early Paleozoic and Andean structural evolution in the Rio Jáchal section of the Argentine Precordillera

In the fold-and-thrust belt of the northern Argentine Precordillera, Early Paleozoic basin and slope sediments are affected by a folding event which was combined with a slight greenschist facies metamorphism. The structural geometries are influenced by the former normal faulted boundary towards the eastern carbonate platform. To the east of the slope, Early Paleozoic marine deposits record a ˜ W-vergent folding without a clear metamorphic overprint. This deformation probably took place in the Devonian to pre-Upper Carboniferous interval while in the west an onset during the Late Silurian is reasonable. During Andean (Late Tertiary) compression, the escarpment was again reactivated as an important, east-directed thrust fault, and the folded strata to the east were juxtaposed along distinct, east-directed high-angle reverse faults with some ˜ N-S fold structures interfering with pre-Tertiary folds. Hence, the present architecture of this part of the orogen was largely influenced by different Early Paleozoic depositional realms and structures of one pre-Tertiary compressional event. The latter can be linked with the collision of the Sierras Pampeanas basement complex at the eastern margin of the Precordillera and be related to the collision with the Chilenia Terrane in the west.     

Carbonate platform dominated by peloidal sands: Lower Ordovician La Silla Formation of the eastern Precordillera,San Juan,Argentina

The Lower Ordovician La Silla Formation of the Precordillera of west‐central Argentina is part of the west‐facing early Palaeozoic, tropical carbonate platform succession that comprises the core of the Cuyania terrane. Up to 360 m thick, it is exposed in several thrust sheets over a distance of some 250 km along and across depositional strike over a palinspastically unrestored distance of about 35 km. La Silla Formation is a strikingly pure limestone with subordinate finely crystalline dolomite and rare chert. It accumulated on a more or less uniformly subsiding passive margin. Copious precipitation of microcrystalline calcite, probably influenced by microbial activity to varying degrees, led to the generation of peloids, ooids and aggregates of these grains, as well as small amounts of lime mud, intraclasts, stromatolites and thrombolites. Rare bioclasts are limited mostly to scattered gastropods and trilobite sclerites; bioturbation is present locally. The array of carbonate rock types is grouped into eight recurring lithofacies, in order of decreasing abundance: (i) peloidal grainstone; (ii) laminated dolostone; (iii) intraclastic rudstone; (iv) microbial laminite; (v) peloidal packstone; (vi) ooidal grainstone; (vii) thrombolite boundstone; and (viii) mudstone. These facies represent sediments that formed solely in a shallow subtidal marine environment, with no evidence of restricted conditions, hypersalinity or subaerial exposure. No events of eustatic sea‐level change are recorded. By far the dominant facies is grainstone composed of well‐sorted, fine sand‐sized peloids and peloidal aggregates in homogeneous, tabular to gently undulating, medium to thick beds; cross‐lamination is scarce. Clusters of sub‐metre‐sized microbial patch reefs developed sporadically. The shallow platform is envisaged to have been covered by extensive peloidal sand flats and low‐relief banks, and little lime mud was generated. The setting was probably microtidal and may not have been affected by strong trade winds. It was washed by frequent, relatively gentle wave action but without experiencing powerful storms. In the middle member, anomalous lenses of intraclastic rudstone and laminated dolostone occur as graded beds overlying sharply downcut scoured surfaces up to 20 cm deep; these are interpreted to indicate a phase when accretion was punctuated occasionally by tsunamis generated from rift‐faulting seaward of the platform margin. The remarkably uniform peloidal grainstone composition over a broad area shows that, given the appropriate combination of climate, environmental and ecological factors, large portions of some early Palaeozoic platforms were dominated by grainy sediment and remained under well‐agitated conditions within fair‐weather wave‐base, without distinct lateral facies differentiation or tidal‐flat aggradation.     

Indicative Structures of Paleoseismicity in the Acequion River Valley,San Juan Province,Central-Western Argentina

Evidence for paleo-seismicity has been discovered in the Acequión river valley, in West central Argentina. Two Holocene rock avalanches have been observed; the most recent of these dammed a lake, whose sediments contain liquefaction structures. At least five paleo-earthquakes affected this region during the late Quaternary, as deduced from the succession of their secondary effects. The magnitude and the probable tectonic source of these paleo-events are discussed. The observed liquefaction features associated with slumps, joints, fractures, and faults, should be generated by M>5 earthquakes related to the nearby quaternary Cerro Salinas fault, which belongs to the Eastern Precordillera fault system. These data extend the regional seismicity record to the Holocene and highlight the high seismic hazard in this part of Argentina.     

Fault-related carbonate breccia dykes in the La Chilca area,Eastern Precordillera,San Juan,Argentina

Carbonate fault breccia dykes in the Cerro La Chilca area, Eastern Precordillera, west-central Argentina, provide clues on the probable mechanism of both fault movement and dyke injection.Breccia dykes intrude Upper Carboniferous sedimentary rocks and Triassic La Flecha Trachyte Formation. The timing of breccia dyke emplacement is constrained by cross cutting relationships with the uppermost Triassic unit and conformable contacts with the Early Miocene sedimentary rocks. This study supports a tectonic-hydrothermal origin for these breccia dykes; fragmentation and subsequent hydraulic injection of fluidized breccia are the more important processes in the breccia dyke development.Brecciation can be triggered by seismic activity which acts as a catalyst. The escape of fluidized material can be attributed to hydrostatic pressure and the direction of movement of the material establishes the direction of least pressure.Previous studies have shown that cross-strike structures have had an important role in the evolution of this Andean segment since at least Triassic times. These structures represent pre-existing crustal fabrics that could have controlled the emplacement of the dykes. The dykes, which are composed mostly of carbonate fault breccia, were injected upward along WNW fractures.     

Alluvial fan–lacustrine association in the fault tip end of a half-graben, northern Triassic Cuyo basin, western Argentina

The Triassic deposits of Cerro Puntudo in the San Juan province of western Argentina constitute the northernmost exposures of the northern portion of the nonmarine Cuyo rift basin, also known as the Las Peñas-Tamberías half-graben. The local column, with an exposed thickness of approximately 400 m, consists of abundant basal and topmost coarse alluvial fan conglomerates and breccias (facies associations I and II) and a relatively thin (approximately 50 m) intervening sequence of marginal, shallow lacustrine deposits characterized by stromatolitic (domal) limestones, tuffaceous mudstones, and fine-grained sandstones (facies association III). Subaerial exposure in the lacustrine deposits is evidenced by desiccation cracks and brecciation. A very thin (0–6 m), laterally, discontinuous succession of lacustrine deposits with similar characteristics is interbedded with the basal conglomerates. Laterally, this lacustrine interval was eroded by overlying conglomerates. The basal conglomerates commonly show crude normal grading, faint cross-bedding, and b-axis clast imbrication. The predominance of coarse deposits and paleocurrents from NW to SE, indicative of an axial flow pattern, suggest that these exposures correspond to the northern end of the Cuyo basin, which is characterized by a shallow, alluvial fan-encased, carbonate-rich lake margin. This lacustrine interval can be correlated with the thicker lacustrine section exposed to the south at Quebrada del Tigre and Ciénaga Larga along the border fault margin of a nonmarine half-graben. The correlation with these sections suggests that the Cerro Puntudo lacustrine deposits are the shallowest equivalent of more profundal, organic-rich lacustrine sediments exposed in the deepest segment of the border fault margin. These thickness and facies variations are the result of differential subsidence along the border fault margin between the low accommodation, fault tip end represented by the Cerro Puntudo section and the high accommodation, central segment located to the south.     

Holocene landscape evolution and geoarcheology of low-order streams in the Rio Grande basin,San Juan Mountains,Colorado, USA

This geoarcheological study investigates soil stratigraphy and geochronology of alluvial deposits to determine Holocene landscape evolution within the Hot Creek, La Jara Creek, and Alamosa River drainage basins in the San Juan Mountains of Colorado. Geomorphic mapping and radiocarbon dating indicate synchronicity in patterns of erosion, deposition, and stability between drainage basins. In all three basins, the maximum age of mapped alluvial terraces and fans is ~ 3300 cal yr BP. A depositional period seen at both Hot Creek and the Alamosa River begins ~ 3300 to 3200 cal yr BP. Based on soil development, short periods of stability followed by alluvial fan aggradation occur in the Alamosa River basin ~ 2200 cal yr BP. A period of landscape stability at Hot Creek before ~ 1100 cal yr BP is followed by a period of rapid aggradation within all three drainages between ~ 1100 and 850 cal yr BP. A final aggradation event occurred between ~ 630 and 520 cal yr BP at La Jara Creek. These patterns of landscape evolution over the past ~ 3300 yr provide the framework for an archeological model that predicts the potential for buried and surficial cultural materials in the research area.     

全新世古洪水事件光释光测年研究——以渭河下游临潼段为例

通过对渭河下游河谷进行深入的考察,在临潼段发现全新世黄土-古土壤层里夹有古洪水滞流沉积层。在剖面系统采样并进行磁化率和粒度成分分析,从沉积学角度证明该地层记录了全新世以来古洪水事件的信息。同时,在剖面采集OSL测年样品,应用单片再生剂量法(SAR)进行红外后蓝光(Post-IROSL)释光测量,获得了7个OSL绝对年龄值,建立了全新世剖面年代深度关系曲线,确定渭河在3200～3000aB.P.之间发生特大洪水事件。综合地层学对比以及剖面磁化率和粒度等气候替代性指标分析,揭示了全新世中期向晚期转折过渡时期,气候向干旱化转折,气候不稳定,频繁波动,降水变率大,是造成渭河流域特大洪水事件发生的主要原因。     

Environmental history since 11,000 C yr B.P. of the northeastern Pampas, Argentina, from alluvial sequences of the Luján River

Sedimentological, malacological, and pollen analyses from ¹⁴C-dated alluvial sections from the Luján River provide a detailed record of environmental changes during the Holocene in the northeastern Pampas of Argentina. From 11,200 to 9000 ¹⁴C yr B.P., both sedimentary and biological components suggest that the depositional environment was eutrophic, alkaline, and freshwater to brackish shallow water bodies without significant water circulation. During this time, bioclastic sedimentation was dominant and the shallow water bodies reached maximum development as the climate became more humid, suggesting an increase in precipitation. Short-term fluctuations in climate during the last stage of this interval may have been sufficient to initiate changes in the water bodies, as reduction of the volume alternated with periods of flooding. The beginning of the evolution of shallow swamps in the wide floodplain or huge wetlands was contemporaneous with a sea level lower than the present one. From 9000 and 7000 ¹⁴C yr B.P., mesotrophic, alkaline, brackish, probably anoxic swamps existed. Between 7000 and 3000 ¹⁴C yr B.P., anoxic calcareous swamps were formed, with subaerial exposure and development of the Puesto Berrondo Soil (3500-2900 ¹⁴C yr B.P.). A trend to a reduction of water bodies is recorded from 9000 to ca. 3000 ¹⁴C yr B.P., with a significant reduction after ca. 7000 ¹⁴C yr B.P. A shift to subhumid-dry climate after 7000 ¹⁴C yr B.P. appears to be the main cause. During this time, an additional external forcing toward higher groundwater levels was caused by Holocene marine transgression causing changes in the water bodies levels. The climate became drier during the late Holocene (ca. 3000 yr B.P.), when clastic sedimentation increased, under subhumid-dry conditions. Flood events increased in frequency during this time. From ca. A.D. 1790 to present, the pollen record reflects widespread disturbance of the vegetation during the European settlement.     

萨拉乌苏河流域MGS1层段微量元素记录的全新世千年尺度的气候变化

本文以毛乌素沙漠东南缘萨拉乌苏河流域米浪沟湾剖面MGS1层段为代表,试图通过对其的微量元素分析结果探讨该地全新世千年尺度的气候变化.在分析的14种微量元素中,除Cr和Zn以外的12种元素P、Pb、Rb、Nb、Zr、V、Sr、Cu、Ni、As、Ba、Co含量在MGS1剖面的垂直方向上具有这样的分布规律:元素含量值在沙丘砂...     

Irrigated area determination: a case study in the Province of San Juan,Argentina

Much of the central-western region of Argentina, where San Juan Province is located, experiences arid to semi-arid climatic conditions with low average annual rainfall accompanied by substantial evapotranspiration. Consequently, a viable crop industry depends to a large extent upon irrigation from major river systems. Increasing demand for water in the lower basin of the San Juan River is emphasizing the need for more accurate estimates of water used for irrigation. Since the water demand for a particular crop is very closely related to crop area, monitoring the area of crop under irrigation is considered a proxy for the amount of water used. Landsat 5 imagery for the growing season, field data and aerial photographs were used to evaluate crop area.     

Architecture and development of the alluvial sediments of the Upper Jurassic Tordillo Formation in the Cañada Ancha Valley, northern Neuquén Basin, Argentina

The Upper Jurassic Tordillo Formation at Cañada Ancha area, northern Neuquén Basin, Argentina, comprises a multi-stage suit of predominantly alluvial sediments that is heterolithic in nature. In that suit, several lithofacies, architectural elements, and bounding surfaces of different order have been identified and their lateral and vertical distribution characterized. This analysis allowed the differentiation of 3 main units (lower, middle and upper), 20 subunits (C-1 to C-20), and the characterization of their alluvial styles.The lower unit (which comprises subunits C-1 to C-4) is mainly formed by fine- to medium-grained sandstones, which become medium- to coarse-grained towards the top. These sandstones characterize settings ranging from floodplains with isolated, unconfined flows, to more complex, vertically stacked, multi-storey sheet sandstones of braided fluvial systems. The middle unit (C-5 to C-10) is dominated by pale brown-grey fine-to coarse-grained sands and medium size subangular to angular conglomerates, which reflect amalgamated complexes of sandstone sheets and downstream accretion macroforms. Remarkably, this alluvial sedimentation was episodically punctuated by volcaniclastic flows. The upper unit (C-11 to C-20) consists of finer sediments, mainly pink to white fine-to medium grained sandstones and red to green siltstones. Towards the top, bioturbation becomes important, and also the presence of volcanosedimentary flows is noticeable. Fluvial settings include braided sheet sandstones with waning flood deposits evolving to isolated high-sinuosity fluvial systems, with flash flood deposits. At the top of this unit, facies may suggest marine influence.Vertical changes in the fluvial style result from both climatic and tectonic controls. A semiarid to arid climate and the active tectonism linked to the eastward migration of the Andean volcanic arc determined major bounding surfaces, fluvial style evolution and the presence of the volcano-sedimentary deposits. Different stages of high and low subsidence rates has been deduced from the vertical stacking of sediments.     

微量元素记录的毛乌素沙漠全新世气候波动--以萨拉乌苏流域DGS1层段为例

选取毛乌素沙漠东南缘萨拉乌苏河流域具有代表性的滴哨沟湾剖面全新统(DGS1层段),结合微量元素与测年分析结果,探讨了该地全新世千年尺度的气候波动.结果表明,P、V、Cu、Zn、Sr、Ni、As、Cr、Pb属于易迁移-较稳定元素,其含量在沉积相中主要呈现为古土壤< 沙丘砂 <湖沼相,Co、Rb、Nb、Ba、Cl、Zr属于稳定-惰性元素,主要呈现为古土壤 >沙丘砂 >湖沼相,说明沙丘砂是冬季风主导下的沉积,古土壤和湖沼相是夏季风主导下的沉积.以Rb和Sr两种元素在垂向上峰谷交替的变化特征为依据,将DGS1记录的全新世气候分为升温期(11 000~10 130 a B.P.)、大暖期(10 130~6 590 a B.P.)、降温期(6 590~3 760 a B.P.)和寒冷期(3 760~0 a B.P.)四个阶段,这与国内外的相关研究结果一致,表明萨拉乌苏河流域全新世的气候变化是全球环境变化影响下的结果.     

On the origin of multi‐layer coronas between olivine and plagioclase at the gabbro–granulite transition,Valle Fértil–La Huerta Ranges,San Juan Province,Argentina

Troctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW‐Argentina exhibit multi‐layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at <840 °C. Diffusion controlled metamorphic solid‐state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro–granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO–μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion‐free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion‐free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at <770 °C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ‘closed’ system although in all cases the gain and loss of elements within the multi‐layer coronas (except H₂O, Na₂O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P–T path at the gabbro–granulite transition at 5–6.5 kbar and temperatures below 900 °C.     

A multi‐proxy study of Holocene lake development,lake settlement and vegetation history in central Ireland

Stratigraphical investigations, geomorphological mapping, and diatom, plant macrofossil and pollen analyses were undertaken in and around two lakes in central Ireland to establish correlations between changes in lake conditions and catchment vegetation throughout the Holocene. Similar investigations of an adjacent mire reveal early Holocene changes in lake level and area. The palaeoecological data show high correlations related to variations in lake depth and area, catchment vegetation type, organic inputs and trophic status. Catchment‐scale deforestation is gradual and occurs through the Bronze and the Iron Ages, and the construction of a crannog in the early Medieval period (seventh century AD ) appears to be associated with a widespread increase in deforestation and mixed agriculture in the catchment. Both pollen and plant macrofossils suggest that one of the crannogs was used for crop storage in addition to domestic and any other activities. In the early to middle Holocene similarities in the proxy‐data appear to be climatically driven through changing lake levels and areal extent whereas the later Holocene record is clearly dominated by anthropogenic changes within the catchment and the construction of crannogs in the lakes. The advantages of combining multi‐proxy indicators of lake hydroecology with the vegetation record are illustrated. Copyright © 2005 John Wiley & Sons, Ltd.     

Holocene environmental change in the West African Sahel: sedimentological and mineral‐magnetic analyses of lake sediments from Jikariya Lake,northeastern Nigeria

Previous studies have demonstrated long‐term changes in effective moisture in sub‐Saharan Africa. Here, we reconstruct Holocene environments using a ～7 m lake‐sediment sequence recovered from the northeastern Nigerian Sahel and attempt to distinguish basin‐specific changes from regional climatic variations. The sequence was analysed for sedimentological properties, mineral magnetism and pollen, and dated by ¹³⁷Cs, ²¹⁰Pb excess and ¹⁴C. Extremely arid conditions of the terminal Pleistocene ended ca. 11 500 cal. BP (calendar years) when climate ameliorated and a lake developed until the occurrence of an arid event leading to lake desiccation at ～11 200 cal. BP. Following this, climate ameliorated and a water body re‐emerged. Very wet conditions predominated 11 200–5600 cal. BP, followed by drought between 5600 and 5500 cal. BP and a return to moderate humidity from 5500 to 4000 cal. BP. After 4000 cal. BP, a marked deterioration occurred, culminating in lake desiccation at ca. 800 cal. BP. After this time the climate remained generally dry and the re‐emerging lake was shallow. Comparison of these results with other well‐dated sequences in the region demonstrates the importance of basin‐specific influences on the palaeolimnological records in addition to regional climatic controls. Disentangling these different controls, as well as the reconstruction of Holocene climate, therefore requires a multiple‐basin approach. Copyright © 2008 John Wiley & Sons, Ltd.     

Changing lake dynamics and sequence stratigraphy of synrift lacustrine strata in a half-graben: an example from the Triassic Ischigualasto–Villa Unión Basin, Argentina

Well‐exposed Triassic rift strata from the Ischigualasto–Villa Unión Basin (NW Argentina) include a 80 to ca 515 m thick lacustrine‐dominated package that can be correlated across a half‐graben using key stratigraphic surfaces (sequence boundaries, lacustrine flooding surfaces and forced regressive surfaces). The characteristics of the synrift lacustrine fill in different parts of the half‐graben have been examined and the mechanisms controlling sedimentation inferred. A variety of sedimentary environments are recognized including; volcaniclastic floodplain, mildly saline lake and playa lake, offshore lacustrine, delta front to fluvial‐dominated and wave‐dominated deltas, distributary and fluvial channel, and interdistributary bay. The succession can be divided into four stratigraphic sequences (SS1 to SS4), the oldest of which (SS1) contains volcaniclastic, fluvial and saline lake deposits; it is thickest close to the western border fault zone, reflecting more rapid subsidence here. Accommodation exceeded sediment and water input during SS1. The second and third sequences (SS2 and SS3) mark the onset of widespread lacustrine sedimentation, reflecting a balance between accommodation creation and water and sediment fluxes. Sequences SS2 and SS3 are represented by offshore meromictic lacustrine and deltaic deposits, the latter mostly sourced from the flexural and southern axial margins of the half‐graben. The presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake‐level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone. The youngest sequence (SS4) is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies, palaeocurrents and sandstone composition, suggesting a switch in sediment supply to the footwall margin to the NW. The change in the sediment source is related to reduced footwall uplift, the possible presence of a relay ramp and/or supply from a captured antecedent drainage network. During SS4, the rate of creation of accommodation was exceeded by the sediment and water discharge. The stratigraphic evolution of lacustrine strata in the half‐graben was mainly controlled by tectonic processes, including subsidence rate and the growth and evolution of the border fault zone, but changing climate (inducing changes in water balance and lake level) and autocyclic processes (delta lobe switching) were also important.     

A preliminary inventory of periglacial landforms in the Andes of La Rioja and San Juan, Argentina, at about 28°S

The Cerro El Potro and nearby mountain chains belong to the Andean Frontal Cordillera (28°S). Cerro El Potro is a glaciated mountain that is surrounded by huge valleys both on its Chilean and Argentinean flanks. Its southern limit is a steep rock wall towards the trough-shaped Río Blanco valley in Argentina, with a wide valley floor. The other sides of the mountain are characterized by well-developed Pleistocene cirques. The predominant landforms in this area have been shaped in a periglacial environment superimposed on an earlier glacial landscape. It is a region with abundant rock glaciers, a noteworthy rock glacier zone, but nevertheless, it is a relatively little known area in South America. In this preliminary inventory, the landforms surveyed were mainly gravitational in origin, including valley rock glaciers, talus rock glaciers, debris cones, landforms originated by solifluction processes and talus detrital sheets on mountain sides. Ancient moraine deposits have been found on the sides of the main rivers that cross the area form west to east, including the Blanco and Bermejo rivers. Present day fluvial activity is limited, and restricted to these main rivers. In this area of glacial valleys and small cirques, there are small lakes and other water bodies, grass covered patches and zones with high mountain vegetation. Present day glacial activity is restricted to the highest part of the area, above 5500 m a.s.l., mainly in the Cerro El Potro (5879 m) where a permanent ice field exists, as well as small mountain glaciers.     

A new Late Cretaceous crocodyliform from the western margin of Gondwana (La Rioja Province,Argentina)

The Cretaceous witnessed the greatest diversity of Gondwanan notosuchian crocodyliforms, which displayed high levels of diversity and a notable array of specialized forms that developed in different ecological niches. Among this diversity, the advanced notosuchians are a clade of mid body sized forms which displays a remarkable abundance but is restricted to two lithological units from the Late Cretaceous of South America, the Adamantina Formation in southeastern Brazil and the Bajo de la Carpa Formation in Patagonia (Argentina). The only exceptions known so far were the Late Cretaceous Labidiosuchus from the Marilia Formation in Brazil and Yacarerani from the Cajones Formation in Bolivia. Herein we report a new Cretaceous crocodyliform, Llanosuchus tamaensis gen. nov. et sp. nov., found in the Los Llanos Formation (Campanian?) in northwestern Argentina (La Rioja Province). The small specimen includes well preserved fragments of the cranium and mandible with an estimated skull size of about 9 cm lengths. This crocodyliform shares several derived characters with Notosuchus terrestris from the Bajo de la Carpa Formation (Patagonia) and it was found in a weakly developed sandy paleosol profile formed in wet local conditions but in a region characterized by semi-arid climate and active eolian sedimentation. The presence of a new advanced notosuchian in the Cretaceous of western Gondwana, and its intermediate geographical region between the known South American species (Brazil, Argentina and Bolivia), has interesting implications, and adds another record of an advanced notosuchian in deposits with inferred warm climates and semi-arid paleoenvironments. Finally, Llanosuchus tamaensis supports a Late Cretaceous age for Los Llanos Formation with important geological implications for La Rioja Province.     

Late-glacial and Holocene sedimentation and fluctuations of river water level in the Caquetá River area (Colombian Amazonia)

The sequence of Late-glacial and Holocene alluvial sedimentation in the middle Caquetá River Basin of Colombian Amazonia is described, based on the study of the sediments and palynology of several river bank sections and on 30 radiocarbon dates. An early Late-glacial sedimentation cycle is recognised, followed by a minor late Late-glacial erosion phase. The Holocene valley fill consists of grey clays (often present in the lower part of the sections) deposited in open water and silty clays often with faint yellow mottling, deposited under a regime of seasonal flooding. The base of the Holocene sections is formed by sands, where exposed. In two places the transition of sand to open-water grey clay was dated around 10 000 yr BP and there is a suggestion that open water may have been more common at the beginning of the Holocene than later, when sedimentation by seasonal flooding became important. In many places much of the earlier Holocene sediments may have been removed by erosion and replaced by younger sediments, by a process of lateral aggradation. A considerable part of the present valley fill is younger than ca. 3500 yr. However, in several places older Holocene sediments are found, apparently only little affected by later erosion, lying below younger varzea silty clays. During the Holocene more organic sediments were formed in periods with reduced river discharge, related to drier climates in the Andes and possibly in Amazonia. These dry periods, deduced from data in the Caquetá River area, correspond well with dry phases in other parts of northwestern South America (e.g. between approximately 2700-1900 yr BP and approximately 3200-3800 yr BP). Rates of average net sedimentation, calculated from dated sections that apparently lack major hiatuses caused by erosion, were high in the lower Holocene, low during the middle Holocene and increase again in the upper Holocene. Levee deposits became coarser and the high river level of the Caquetá increased during the late Holocene. These phenomena may be explained by the increasing influence of man on the vegetation cover in the Andean headwater areas and possibly also in the Amazonian catchment area of the Caquetá River.