Fan Conglomerates Research Articles

Along-strike tectonic evolution of the Neogene Bermejo foreland basin and Eastern Precordillera thrust front, Argentina (30-32°S)

The Bermejo retroarc foreland basin system formed in flexural response to Cenozoic crustal thickening in the Andean orogenic system, specifically, the eastward propagation of the Precordillera fold-thrust belt and the basement-involved uplift of the Sierras Pampeanas. Previous work in the region has mainly focused on the mechanisms and expression of flat slab subduction and the structural geometry of the basement-involved Sierras Pampeanas and east-directed Precordillera fold-thrust belt at depth, advancing our understanding of the Bermejo basin history north of 31°S. However, the along strike evolution of the basin system to the south remains unresolved and contrasting tectonic models have proposed the Bermejo basin evolved synchronously versus asynchronously through time. Our study seeks to constrain the along-strike Neogene tectonics of the Bermejo basin via well-exposed Miocene fluvial stratigraphic intervals along the Eastern Precordillera, integrated with detrital zircon U–Pb geochronology provenance and detrital apatite (U–Th)/He thermochronology datasets. New data from two Neogene stratigraphic sections in the southern Bermejo basin constrain deposition between 13 and 6 Ma. Dominant fluvial-lacustrine mudstones, siltstones, and sandstones transition into fluvial-megafan deposits capped by alluvial fan conglomerate facies, tracking the eastward migration of Precordillera deformation. The cessation of sedimentation, and thermal history models of apatite (U–Th-Sm)/He thermochronology ages, indicates basin incorporation into the orogenic wedge by 6 Ma. When we compare our southern datasets with previous constraints from the northern Bermejo basin, we observe, from north to south: (1) a time-transgressive trend in basin initiation, (2) a ∼3 km decrease in stratigraphic thicknesses, and (3) older exhumation along the thrust front. These trends highlight the asynchronous nature of the Bermejo foreland basin system and along-strike variability of Precordillera thrust-front evolution.

Cretaceous uplift history of the Tibetan Plateau: Insights from the transition of marine to terrestrial facies in central Tibet

The uplift of the Tibetan Plateau has been widely associated with India-Asia continental collision during the Cenozoic. However, recent studies have proposed that a proto-Tibetan Plateau in central Tibet was elevated during the late Mesozoic. This paper improves that understanding by focusing on the Cretaceous terrestrial strata in the Gerze area of central Tibet. Our studies suggest that the Qushenla Formation in the Nare area contains alluvial fan conglomerate intercalated with rhyolitic ignimbrite and vesicular rhyolite. Zircon UPb dating of rhyolitic ignimbrite (103.9 ± 1.4 Ma) and two granitoid boulders of conglomerate (106.0 ± 1.6 and 106.1 ± 1.5) indicate that the Qushenla Formation was deposited during the late Albian. The compositions of the Qushenla Formation conglomerate were dominated by hypabyssal intrusions on the southern Qiangtang terrane, which recorded a rapid uplift and denudation process in the late Albian, accompanied by intense volcanic eruptions. The second set of terrestrial strata is the Upper Cretaceous Jingzhushan Formation alluvial fan-braided river deposits, which can be divided into a lower conglomerate member (Member 1) and a upper sandstone member (Member 2). The large amount of bioclastic limestone fragments in the Member 1 show that they come from the Upper Cretaceous Langshan Formation in the central and northern Lhasa subterranes. While in the Member 2, abundant limestone debris, detrital zircon yielding Jurassic–Cretaceous ages with εHf(t) values between −18.6 and + 8.4, and occurrence of volcanic and ancient zircon grains indicate a volcanic-sedimentary source located in the central and northern Lhasa subterranes, with some input from the Bangong-Nujiang suture zone. This suggests that the central and northern Lhasa subterranes experienced rapid crustal uplift in the Late Cretaceous, thus ending the marine sedimentary environment since the Jurassic. Based on the temporal and spatial sedimentary characteristics of the Qushenla and Jingzhushan formations as well as the regional data, our integrated study suggests that the central plateau (Lhasa and southern Qiangtang terranes) experienced a complex uplift history during the Aptian-Turonian, with southwards stepwise growth in time and space. Furthermore, the uplift mechanism of the central plateau is complex, which was controlled by the diachronous closure of the Bangong-Nujiang Tethyan Ocean from east to west during the Early Cretaceous, and by the far-field forces associated with northwards subduction of the Neo-Tethys Ocean during the Late Cretaceous.

Basement-involved thrusting, salt migration and intramontane conglomerates: a case from the Southern Pyrenees

The northern margin of the Organyà basin (Southern Pyrenees) has a complex structure in which syn-rift Lower Cretaceous carbonates flank a wide Keuper evaporite province, featuring the leading edges of the basement-involved thrust sheets of the Pyrenean antiformal stack. Recent studies show that Keuper diapirs and salt walls grew during the Cretaceous extensional episode, conditioning the development of differentiated depocenters and minibasins. The role of salt tectonics during the Pyrenean orogeny has not been addressed in previous structural studies, but present-day cross-sections indicate a Keuper evaporite-bearing vertical thickness of up to 3000 m in the Senterada-Gerri de la Sal area. We infer that salt migration was a determinant mechanism in triggering a gentle northward tilting of the Organyà basin during the Eocene-Oligocene, recorded in the La Pobla de Segur and Gurp syn-tectonic conglomerates in a large north-directed onlap, opposite to the main sedimentary influx direction. Contemporaneously, we interpret that salt migration, promoted by conglomerate differential loading, enabled the sinking and rotation of the unrooted Nogueres thrust units (têtes plongeantes). We use new and published structural data for the Lower Cretaceous margin of the Organyà basin, combined with structural and clast provenance data from the Cenozoic alluvial fan conglomerates of La Pobla and Gurp, to understand the Lutetian to late Oligocene evolution of the northern margin of the Central South-Pyrenean Unit. The tectono-sedimentary evolution of this area and the salt evacuation patterns are closely related to the exhumation history of the stacked Paleozoic thrust sheets of the Pyrenean hinterland to the north. In this study, we correlate the movements over a mobile substratum and the paleogeographic changes of conglomeratic basins at the toe of an exhuming orogenic interior.

Reconstructing the incision of the Lancang River (Upper Mekong) in southeastern Tibet below its prominent knickzone using fluvial terraces and transient tributary profiles

The Lancang River (Upper Mekong) is a key large river in Asia with a prominent knickzone over 500 km distance (27.5‐30.7°N), as it descends the southeastern margin of the Tibetan Plateau. To explore the role of tectonics in regulating the river downcutting at regional and local scales, we reconstructed the rate and spatial pattern of incision within and below the knickzone. We combined the investigation of fluvial terraces with tributary river profile analysis for a 300-km reach near Yunlong below the knickzone (25–27.5°N). We conducted field mapping and dating of strath terraces at the heights of 60–380 m above the trunk river. The overlying terrace deposits comprise fluvial conglomerates and overbank sands as well as tributary alluvial fan conglomerates. K-feldspar post-infrared infrared stimulated luminescence (pIRIR) dating results suggest terrace formation during the Middle Pleistocene and the incision rate varying at <0.59–0.22 mm/yr. The river profiles of most tributaries in the study area are in a transient state with a knickpoint separating an upstream relict graded profile and a downstream steepened reach. These tributary profiles suggest river incision of ~50–1800 m, showing a spatial trend of increase towards the north. Knickpoint elevation and channel steepness of tributaries show a similar spatial pattern of northward increase. We suggest that the spatial variation in river incision along the Lancang River near Yunlong is likely to have been regulated by differential rock uplift in southeastern Tibet. Using the K-feldspar pIRIR dating method, we rectified the incision rate within the knickzone to vary at <0.63–0.28 mm/yr. New dating results thus suggest that terrace-derived incision rates along the Lancang River are similar to regional erosion rates on million-year and millennial time scales in terms of magnitude and spatial pattern. Coupled erosion rates on various time scales along the Lancang River and spatial pattern of river incision near Yunlong supports a primary role of tectonic forcing in shaping the southeastern Tibetan landscape.

Quaternary Thrusting in the Central Oman Mountains—Novel Observations and Causes: Insights from Optical Stimulate Luminescence Dating and Kinematic Fault Analyses

For the first time, Quaternary thrusts are documented within the Central Oman Mountains to the northwest of the Jabal Akhdar Dome. Thrusts with a throw of up to 1.1 m displace Quaternary alluvial fan conglomerates. These conglomerates have an Optical Stimulate Luminescence (OSL) age of 159 ± 7.9 ka BP and were deposited during MIS 6 (Marine Isotope Stage). The thrusts occur in two sets. Sets 1 and 2 formed during NE/SW and NW/SE shortening, respectively. Set-1-thusts correlate with the present-day stress field of NE/SW shortening which is related to subduction in the Makran Subduction Zone, and they strike parallel to the main continuous fold axis of the Jabal Akhdar and Hawasina windows. Set-2-thrusts correspond to NW/SE shortening and Plio-Pleistocene contractional structures in the southwestern Jabal Akhdar Dome. Set-2-thrusts are probably related to local variations of the present-day stress field originating from the Musandam area which is a part of the Zagros Collision Zone. Both thrust sets mimic the main thrust directions (NW/SE and NE/SW) within the Permo-Mesozoic allochthonous units (Semail Ophiolite, Hawasina napps) of the larger study area. The investigated thrusts imply some reactivation of the Hawasina and Semail thrusts due to far-field stress either from the Makran Subduction Zone and/or the Zagros Collision Zone. The ongoing tectonic activity of this part of the Oman Mountains, which has been considered of moderate activity, is for first time identified by structural data as contractional.

Overview of potential geothermal reservoirs in Denmark

AbstractThe Danish onshore subsurface contains very large geothermal resources that have the potential to make a significant contribution to transforming Danish energy consumption toward a more sustainable energy mix. Presently, only a minor fraction of this green energy is exploited in three small plants. The main factors that have hampered and delayed larger-scale deployment are related to uncertainties in the geological models, which inevitably lead to high economic risks that are difficult for smaller district heating companies to mitigate without support from a compensation scheme. To facilitate and stimulate much wider use of the Danish geothermal resources, the Geological Survey of Denmark and Greenland (GEUS) and other research institutes have conducted several regional research projects focusing on the geological and geochemical obstacles with the principal objective of reducing the exploration risks by selecting the best geological reservoirs.One of the most important geological factors causing uncertainty is the quality of the reservoirs and their ability to produce the expected volume of warm geothermal brine. Thus, great emphasis has been placed on investigating and understanding the relationships between reservoir sandstone, porosity, permeability, petrography, diagenetic processes and alterations related to variable sediment sources, basin entry points, depositional systems and climate, burial and thermal history. Mesozoic sandstones comprise the most important geothermal reservoirs in Denmark. Details concerning the reservoir quality are compiled and compared for the Lower Triassic Bunter Sandstone, Triassic Skagerrak, Upper Triassic – Lower Jurassic Gassum and Middle Jurassic Haldager Sand formations. The Bunter Sandstone Formation contains extensive aeolian and more confined fluvial sandstones with high porosity and permeability. However, highly saline formation water could be unfavourable. The Skagerrak Formation comprises well-sorted braided stream sandstones in the centre of the basin, and is otherwise characterised by muddy sandstones and alluvial fan conglomerates. An immature mineralogical composition has caused intensive diagenetic changes in the deepest buried parts of the basin. The Gassum Formation consists of shoreface, fluvial and estuarine sandstones interbedded with marine and lacustrine mudstones. In the upper part of the formation, the sandstone beds pinch out into mudstones towards the basin centre. Pervasive siderite- and calcite cement occurs locally in shallowly buried sandstones, and with burial depth the maximum abundances of quartz and ankerite cement increase. Sandstones of shallow burial represent excellent reservoirs. The relatively coarse grain size of the Haldager Sand Formation results in high porosity and permeability even at deep burial, so the formation comprises a high-quality geothermal reservoir.Substantial progress has been made, and a well-established regional geological model combined with reservoir quality is now available for areas with cored wells. This has enabled an improved estimation of reservoir quality between wells for exploration of geothermal reservoirs.

Paleocene initial indentation and early growth of the Pamir as recorded in the western Tarim Basin

It is presently unclear as to when and how the Pamir were uplifted to their current elevation, because a Cenozoic sedimentary record is lacking on the Pamir Plateau, and the Pamir domes only record metamorphism associated with crustal shortening since ∼37 Ma. Studies of the tectonic and geomorphological evolution of the Pamir in the early stages of the India–Asia continental collision are also lacking. The western Tarim Basin at the northeastern margin of the Pamir provides an ideal opportunity to investigate their formation and evolution. Four Cretaceous–Paleogene stratigraphic sections were examined in this study. From these, we infer that the West Kunlun Mountains were topographic highs during the Mesozoic that divided the adjacent Central Pamir–Tianshuihai terrane from the Tarim Basin. This paleo-topographic configuration lasted until a marine transgression occurred in the Aksai Chin and Central Pamir areas in the latest Cretaceous. Provenance analysis indicates that sediments in the western Tarim Basin were derived solely from the West Kunlun Mountains until the Paleocene. The appearance of Pamir-derived detrital zircons in Paleocene and Eocene strata of the Tarim Basin suggests that the Pamir was experiencing uplift and reached an elevation similar to, or even higher, than the West Kunlun Mountains. At that time, the sandstone composition documents a provenance change from a stable continental block to an active thrust belt of the proto-West Kunlun Mountains, indicating that the western Tarim Basin evolved from an intraplate to a foreland basin setting. During the Paleocene, alluvial fan conglomerates were deposited along the northeastern margin of the Pamir, and a thick gypsum unit began to be deposited in the Tarim Basin. All these lines of evidence indicate that the north-verging thrusting, foreland basin subsidence, uplift, and northward indentation of the Pamir began in the Paleocene (62–58 Ma).

Proveniência e paleocorrentes de conglomerados e arenitos do Grupo Santa Bárbara (Ediacarano) no Vale do Piquiri, Sub-bacia Camaquã Oriental, RS: implicações tectônicas

A Sub-bacia Camaquã Oriental compreende exposições da borda leste da Bacia do Camaquã, como o Vale do Piquiri, pertencentes ao Grupo Santa Bárbara (Ediacarano). Estudos estratigráficos de fácies, sistemas deposicionais, proveniência e paleocorrentes conduziram à subdivisão do Grupo Santa Bárbara na Sub-bacia Camaquã Oriental em duas unidades: (i) Formação Passo da Capela, formada por turbiditos de leques subaquosos; e (ii) Formação Rincão dos Mouras, constituída por arenitos e conglomerados de leques aluviais e sistemas fluviais entrelaçados. Este artigo propõe formular um modelo de evolução de altos estruturais durante o preenchimento de uma bacia rifte. Conglomerados de leques subaquosos da Formação Passo da Capela apresentam composição do arcabouço derivada da própria bacia (arenitos), indicando autofagia, além de rochas do embasamento (quartzitos, xistos, metarriolitos). Análises de paleocorrentes indicam dispersão dos sedimentos segundo NNE, compatível com litotipos do Complexo Porongos (quartzitos, metarriolitos, milonitos). A norte, análises de proveniência dos conglomerados indicam derivação principalmente do Sienito Piquiri, e também de outras litologias. Esse fato sugere que a proveniência reflete contribuição do embasamento adjacente (clastos do Sienito Piquiri), com pouco ou nenhum transporte. Conglomerados aluviais da Formação Rincão dos Mouras contam com proveniência diversificada. Na base ocorrem clastos metamórficos do Complexo Porongos (metarriolitos) e da própria bacia (arenitos). Análises de proveniência sugerem denudação do Complexo Porongos (altos de embasamento) e da bacia. A porção intermediária apresenta clastos de granitos róseos/muscovita leucogranitos correlacionáveis ao Batólito Pelotas, e litologias do Complexo Porongos. Sienitos e traquitos aparecem em pequena proporção, assim como arenitos. Análises de proveniência, aliadas às de paleocorrentes, indicam que áreas a leste estavam sendo soerguidas e erodidas, como Batólito Pelotas e Complexo Porongos, comportando-se como altos de embasamento fornecedores de sedimentos para a bacia. No topo predominam fragmentos do Complexo Porongos (mármores). Análises de paleocorrentes realizadas na Formação Rincão dos Mouras sugerem controle tectônico importante durante o preenchimento da unidade, com mudanças de fontes de detritos para a bacia e soerguimento de altos internos. As análises de proveniência levadas a efeito em todas as unidades (i) indicaram sempre fonte adjacente aos depósitos sedimentares; (ii) demonstraram as fontes de detritos; e (iii) auxiliaram na diferenciação entre as formações Passo da Capela e Rincão dos Mouras. A ocorrência na Formação Rincão dos Mouras de fragmentos de mármore e o aumento considerável da proporção de clastos do Granito Encruzilhada do Sul, situado a ENE da região do Vale do Piquiri, foi mais um dos critérios utilizados na separação das unidades, sobretudo quando conglomerados estratificados de ambas se acham contíguos.

The position and age of flysch deposits in the Crimean Mountains (Southern Ukraine).

The Crimean Mountains (CM) are regarded as part of the Alpine-Himalaya orogenic belt related to the collision of the Eurasian and African plates. Our research in the CM has allowed confirming the existence of at least two flysch formations of different ages: the Taurida Flysch Formation (Upper Triassic/Lower–? Middle Jurassic) and the Sudak Formation (uppermost Jurassic/Lower Cretaceous) in the western and eastern sectors of the CM, respectively. After the Middle Jurassic volcanism, the freshwater claystones with coal-bearing intercalations, as well as local alluvial fan conglomerates were deposited. Then, following the Oxfordian/Kimmeridgian marine transgression, three separated Tithonian/Berriasian carbonate platforms developed: Baydarska, Chatyr-Dag and Demerji/Karabi. At the turn of the Late Jurassic, the deep-water Sudak Basin (eastern sector of the CM) began to develop in the eastern periphery of the Demerji/Karabi carbonate platform. This basin, dominated by turbiditic deposition, was probably connected with the Great Caucasus sedimentary system. Finally, distal flysch sedimentation in the Sudak Basin was followed by debris-flow deposits, with huge blocks of Upper Jurassic limestones. These blocks were derived from destruction of the Karabi carbonate platform. During the Berriasian/Valanginian, the eastern sector of the CM began to subside. As a result, both carbonate platforms (western CM) and the debris flow fans of the Sudak Basin (eastern CM) were overlain by marly shales and/or distal turbidites. This type of deposition lasted until the Aptian/Albian. In the post-Albian period, the Alushta-Salgir tectonic zone was transformed into a SE-dipping thrust fault with at least 10 km amplitude of overthrusting.

Upper Cretaceous alluvial fan deposits in the Jianglangshan Geopark of Southeast China: implications for bedrock control on Danxia landform evolution

The Jianglangshan Geopark in the western Zhejiang Province of Southeast China is well-known for its spectacular red-colored sandstone landforms. Little is known about the depositional processes of the conglomerate-dominated Fangyan Formation, the lithologic base of the Danxia landforms in this geopark. Based on detailed field investigation of lithology, sedimentary structures, bed thickness and geometry, five facies are recognized: facies A (matrix-supported cobble conglomerate), facies B (pebble conglomerate), facies C (pebbly sandstone), facies D (fine-grained sandstone) and facies E (mudstone). The results show that streamflow-dominated fans were the main depositional processes responsible for the accumulation of the Fangyan Formation along the mountain fronts. These fan conglomerates form the base for the evolution of the Danxia landscapes owing to the uplift and erosion of the study area. In contrast, the fine-grained sedimentary successions produced by fluvial floodplains in the distal part of the fans were thinner and more easily weathered. Such sedimentary facies distribution patterns were thought to be similar during Late Cretaceous across Southeast China. The Danxia landforms are largely the geographical expressions of the conglomerate-dominated redbeds in the proximal-middle fans.

Miocene to Quaternary tectonostratigraphic evolution of the middle section of the Burdur-Fethiye Shear Zone, south-western Turkey: Implications for the wide inter-plate shear zones

The Burdur-Fethiye Shear Zone (BFSZ) is a 75- to 90- km wide and 300-km-long transtensional left-lateral shear zone which is located in one of the most tectonically active regions in south-western Turkey. A considerable number of studies suggested contradictory models of the evolution and Neogene stratigraphy of the BFSZ and in most cases, the local river and alluvial fan deposits were mapped together with the lacustrine sediments and assigned a Pliocene age. We present new field data, fault kinematic analyses, and DEM and earthquake data to characterize the tectonic controls and extent of the middle section of the BFSZ including Acıpayam, Çameli and Gölhisar basins. Our field observations revealed two distinct sedimentary sequences that unconformably overlie the pre-Neogene basement. The first sequence begins with middle-upper Miocene meandering- and braided-river sediments of the Gölhisar Formation, which transition upward into lacustrine sediments of the upper Miocene-lower Pliocene İbecik Formation. This sequence is overlain by upper Pliocene-lower Quaternary alluvial fan conglomerates, mudstones and claystones of the Dirmil Formation. The basin deposits located in the middle section of the BFSZ consist of lacustrine sediments of a late Miocene lake that likely evaporated due to the Messinian salinity crisis. Fault kinematic analysis and DEM and earthquake data indicate that the middle BFSZ can be characterized as a heterogeneous left-lateral transtensional shear zone rather than a major fault system. Our findings suggest that the middle section of the BFSZ developed under the influence of progressive counter clockwise rotation of south-western Turkey, the Aegean graben system and the Cyprus and Hellenic arcs since the middle Miocene.

Stratigraphic and sedimentary characteristics of the Upper Jurassic-Lower Cretaceous strata in the Junggar Basin, Central Asia: Tectonic and climate implications

The Junggar Basin, located in northwestern China, provides important records for investigating Mesozoic tectonic and climate evolution in Central Asia. The Upper Jurassic-lowermost Cretaceous strata in the basin include the Qigu, Kalazha, and Qingshuihe Formations in ascending order. By synthesizing outcrop and well data and previous research, we conducted a detailed sedimentologic and stratigraphic correlation analysis for the three formations. Our study shows that a sharp lithofacies change from intermittent braided-river red fine-grained sediments of the Qigu Formation to alluvial fan conglomerates of the Kalazha Formation, is widespread along the basin margins. From bottom to top, the Kalazha alluvial fan conglomerates can be divided into six sequences showing a progradation to retrogradation cycle, and they finally retrograde into the lake sediments of the lowermost Cretaceous Qingshuihe Formation. However, within the Junggar Basin, the Kalazha Formation is commonly considered to be absent and the Qingshuihe Formation possesses a basal fluvial coarse-grained sandstone bed (50–100m), beneath which a widespread unconformity is observed. Stratigraphic correlation suggests that the alluvial fan conglomerates of the Kalazha Formation along the basin margins are equivalent to the basal sandstone bed of the Qingshuihe Formation within the basin. Finally, we conclude that progressive tectonic uplift and climate aridification occurred in the Junggar Basin during deposition of the Qigu Formation in the Late Jurassic, while the basin began to experience a relatively humid climate and underwent rapid subsidence during deposition of the Kalazha Formation in the latest Jurassic-earliest Cretaceous, probably driven by the far-field effects of the short-lived but significant Mongol-Okhotsk collisional orogeny.

Mechanism of diagenetic trap formation in nearshore subaqueous fans on steep rift lacustrine basin slopes—A case study from the Shahejie Formation on the north slope of the Minfeng Subsag, Bohai Basin, China

Diagenetic traps in conglomerate in nearshore subaqueous fans in the steep slope zones of rift basins have been important exploration targets for subtle reservoirs in eastern China. However, the mechanism of how those traps were formed is not clear, which inhibits further exploration for this type of reservoir. In order to solve the problem, we take as an example nearshore subaqueous fans in the upper part of the fourth member of the Shahejie Formation (Es4s) on the north slope of the Minfeng Subsag in the Dongying Sag. Combining different research methods, such as core observation, thin section examination, scanning electron microscope (SEM) observation, fluid-inclusion analysis, carbon and oxygen isotope analysis of carbonate cements, and analysis of core properties, we studied the genetic mechanisms of diagenetic traps on the basis of diagenetic environment evolution and diagenetic evolution sequence in different sub/micro-facies. Conglomerate in Es4s in the north Minfeng Subsag experienced several periods of transition between alkaline and acidic environments as “alkaline-acidic-alkaline-acidic-weak alkaline”. As a result, dissolution and cementation are also very complex, and the sequence is “early pyrite cementation / siderite cementation / gypsum cementation / calcite and dolomite cementation—feldspar dissolution / quartz overgrowth—quartz dissolution / ferroan calcite cementation / ankerite cementation / lime-mud matrix recrystallization / feldspar overgrowth—carbonate dissolution / feldspar dissolution / quartz overgrowth / pyrite cementation”. The difference in sedimentary characteristics between different sub/micro-facies of nearshore subaqueous fans controls diagenetic characteristics. Inner fan conglomerates mainly experienced compaction and lime-mud matrix recrystallization, with weak dissolution, which led to a reduction in the porosity and permeability crucial to reservoir formation. Lime-mud matrix recrystallization results in a rapid decrease in porosity and permeability in inner fan conglomerates in middle-to-deep layers. Because acid dissolution reworks reservoirs and hydrocarbon filling inhibits cementation, reservoirs far from mudstone layers in middle fan braided channels develop a great number of primary pores and secondary pores, and are good enough to be effective reservoirs of hydrocarbon. With the increase of burial depth, both the decrease of porosity and permeability of inner fan conglomerates and the increase of the physical property difference between inner fans and middle fans enhance the quality of seals in middle-to-deep layers. As a result, inner fan conglomerates can be sealing layers in middle-to-deep buried layers. Reservoirs adjacent to mudstones in middle fan braided channels and reservoirs in middle fan interdistributaries experienced extensive cementation, and tight cemented crusts formed at both the top and bottom of conglomerates, which can then act as cap rocks. In conclusion, diagenetic traps in conglomerates of nearshore subaqueous fans could be developed with inner fan conglomerates as lateral or vertical sealing layers, tight carbonate crusts near mudstone layers in middle fan braided channels as well as lacustrine mudstones as cap rocks, and conglomerates far from mudstone layers in middle fan braided channels as reservoirs. Lime-mud matrix recrystallization of inner fan conglomerates and carbonate cementation of conglomerates adjacent to mudstone layers in middle fan braided channels took place from 32 Ma B.P. to 24.6 Ma B.P., thus the formation of diagenetic traps was from 32 Ma B.P. to 24.6 Ma B.P. and diagenetic traps have a better hydrocarbon sealing ability from 24 Ma B.P.. The sealing ability of inner fans gradually increases with the increase of burial depth and diagenetic traps buried more than 3,200 m have better seals.

Oligocene-Miocene deformational and depositional history of the Andean hinterland basin in the northern Altiplano plateau, southern Peru

Cenozoic basin fill of the northern Altiplano plateau records the tectonic development of the flanking Western Cordillera magmatic arc and Eastern Cordillera fold-thrust belt. The Ayaviri hinterland basin of southern Peru contains a ~2300 m thick succession of fluvial sandstones and overbank siltstones (upper Oligocene Puno Group and lower Miocene lower Tinajani Formation) capped by ~400 m of alluvial fan conglomerates (middle Miocene upper Tinajani Formation). New U-Pb zircon chronostratigraphic constraints from ~30 to 15 Ma yield sediment accumulation rates of 110–660 m/Myr. Newly dated growth strata highlight the genetic role played by thrust displacement in basin evolution. A several phase accumulation history derived from chronostratigraphic, provenance, and structural data reveals Oligocene basin filling by fluvial sand and mud that changes provenance from Western Cordillera Mesozoic-Cenozoic volcanic rocks to Paleozoic-Mesozoic Eastern Cordillera sedimentary rocks driven by deformation along the southwest directed, northeastern basin margin Ayaviri thrust at 28–26 Ma. Continued early Miocene fluvial deposition was sourced solely from the Eastern Cordillera. An abrupt middle Miocene shift to coarse alluvial fan deposition sourced from the Western Cordillera was driven by out-of-sequence deformation along the northeast directed, southwestern basin margin Pasani thrust at 18–16 Ma. This northern Altiplano out-of-sequence deformation was coincident with increased Eastern and Western Cordillera exhumation and thrusting and may be symptomatic of changes in critical wedge dynamics. The overall record of basin sedimentation and syndepositional fold-thrust deformation emphasizes the role of regional shortening in governing crustal thickening and basin evolution in the central Andes during the Oligocene to Miocene.

Bouse Formation in the Bristol basin near Amboy, California, USA

Limestone beds underlain and overlain by alluvial fan conglomerate near Amboy, California, are very similar in many respects to parts of the Bouse Formation, suggesting that an arm of the Pliocene Bouse water body extended across a wide part of the southern Mojave Desert. The deposits are north of the town of Amboy at and below an elevation of 290 m, along the northern piedmont of the Bristol “dry” Lake basin. The Amboy outcrops contain the Lawlor Tuff (4.83 Ma), which is also found in an outcrop of the Bouse Formation in the Blythe basin near Buzzards Peak in the Chocolate Mountains, 180 km southeast of Amboy. Bouse exposures near Amboy are ∼3.4 m thick, white, distinctly bedded, with limestone and calcareous sandstone as well as stromatolite mounds; we interpret these as nearshore deposits. The Bouse at Amboy contains ostracodes, diatoms, and mollusks that indicate saline lake or estuarine environments with an admixture of fresh-water forms. Along with wading bird tracks and a spine from a marine fish, these fossils suggest that the deposits formed in saline waters near a fresh-water source such as a perennial stream. Beds of the outcrop dip southward and are 113 m above the surface of Bristol Playa, where similar age sediments are buried 270+ m deep, indicating significant faulting and vertical tectonics in this part of the Eastern California Shear Zone during the past 5 m.y. Confirmation of the Bouse Formation at Amboy strengthens previous assignments to the Bouse Formation for mudstones in driller logs at Danby “dry” Lake, California, and suggests that areally extensive arms of the Bouse water body were west of the Blythe basin. The Bristol basin arm of the lower Bouse basin probably was restricted from the main water body by narrow passages, but Bouse sediment there is similar to that in the Blythe basin, suggesting generally similar water chemistry and environmental conditions. Examining the degree to which Bouse deposits in the western arms differed from Bouse deposits in the Blythe basin offers an approach to test whether the southernmost Bouse water body was deposited in an estuarine or lacustrine setting.

Growth and fragmentation of the Andean foreland basin during eastward advance of fold‐thrust deformation, Puna plateau and Eastern Cordillera, northern Argentina

The fault‐bounded Cianzo basin represents a Cenozoic depocenter between the Puna plateau and Eastern Cordillera of northern Argentina. Analysis of fold‐thrust relationships, nonmarine sedimentation, and detrital provenance at 23–24°S helps constrain the origin, interconnectedness, and subsequent uplift and exhumation of the Cianzo basin, a potential analog for intermontane hinterland basins in the Andes. Structural mapping reveals a plunging syncline within the &gt;6000‐m‐thick, upward coarsening Cenozoic clastic succession in the shared footwall of the N‐striking, E‐directed Cianzo thrust fault and transverse, NE‐striking Hornocal fault. Growth strata within upper Miocene levels indicate syncontractional accumulation adjacent to the Hornocal fault. Measured stratigraphic sections show upsection changes from (1) paleosol‐rich, distal‐fluvial sandstones (∼400 m Paleocene‐Eocene Santa Bárbara Subgroup) to (2) braided fluvial sandstones and mudstones (∼1400 m upper Eocene‐Oligocene Casa Grande Formation) to (3) distributary fluvial megafan sandstones and conglomerates (∼3300 m upper Oligocene‐Miocene Río Grande Formation) to (4) alluvial fan conglomerates (∼1600 m upper Miocene Pisungo Formation). The 40Ar/39Ar geochronological results for interbedded tuffs indicate continuous Río Grande deposition from at least 16.34 ± 0.71 to 9.69 ± 0.05 Ma. Sandstone petrographic results define distinct upsection trends in lithic and feldspar content, potentially distinguishing the Western Cordillera magmatic arc from Puna‐Eastern Cordillera thrust‐belt sources. In addition to growth stratal relationships and 40Ar/39Ar ages, conglomerate clast compositions reflect distinct lithologic differences, constraining activation of the Cianzo thrust and coeval reverse slip on the reactivated (inverted) Hornocal fault. Finally, detrital zircon U‐Pb ages, paleocurrents, and facies patterns distinguish local from distal sources, revealing a systematic forelandward (eastward) advance of Eocene through upper Miocene fold‐thrust deformation.

Evolução Estratigráfica e paleogeográfica do grupo Santa Bárbara (Ediacarano) na sub-bacia Camaquã Central, RS

In the Central Camaquã Sub-basin, the Santa Bárbara Group (Ediacaran) is over 2,000 m thick, being well exposed in the Minas do Camaquã and Bom Jardim regions. The group is represented by a sucession of marine to coastal sandstones and rhythmites, and alluvial fan conglomerates, disposed in progradational-retrogrational cycles. Detailed stratigraphic studies of paleoenvironments, paleocurrents and provenance have lead to the subdivision of the Group in the central sub-basin into four formations: 1. Passo da Capela Formation: silty-sandy unit composed of outer- and inner- subaqueous fan turbidites and offshore tempestites; 2. Seival Formation: sandstones of coastal deposits, including tidal flat sistems; 3. Rincão dos Mouras Formation: alluvial fan and braided alluvial pebbly unit and 4. João Dias Formation: mainly sandstones interpreted as foreshore and shoreface coastal deposits. These studies have permitted the detailed correlation between the Minas do Camaquã and Bom Jardim regions, thus supporting the proposition of including in the Santa Bárbara Group as defined here, the deposits exposed in the Minas do Camaquã region, traditionally attributed to the Arroio dos Nobres Formation.

O GRUPO SANTA BÁRBARA (EDIACARANO) NA SUB-BACIA CAMAQUÃ CENTRAL, RS: ESTRATIGRAFIA E SISTEMAS DEPOSICIONAIS

In the Central Camaquã Sub-basin, the Santa Bárbara Group (Ediacaran) is over 2,000 m thick, being well exposed in the Minas do Camaquã and Bom Jardim regions. The group is represented by a succession of marine to coastal sandstones and rhythmites and alluvial fan conglomerates, disposed in progradationalretrogrational cycles. Detailed stratigraphic studies of paleoenvironments, paleocurrents and provenance have lead to the subdivision of the Santa Bárbara Group, in the Central Camaquã Sub-basin, into four formations: (i) Passo da Capela Formation: silty-sandy unit composed of outer- and inner-submarine fan turbidites; (ii) Seival Formation: offshore tempestites and coastal deposits, including tidal flat deposits; (iii) Rincão dos Mouras Formation: alluvial fan and braided river pebbly unit and (iv) João Dias Formation: foreshore and shoreface coastal deposits. These studies have permitted the detailed correlation between the Minas do Camaquã and Bom Jardim regions, thus supporting the proposition of including, in the here defined Santa Bárbara Group, the deposits exposed in the Minas do Camaquã region, traditionally reported to the Arroio dos Nobres Formation.

O Grupo Santa Bárbara (Ediacarano) na Sub-Bacia Camaquã Central, RS: estratigrafia e sistemas deposicionais

In the Central Camaqua Sub-basin, the Santa Barbara Group (Ediacaran) is over 2,000 m thick, being well exposed in the Minas do Camaqua and Bom Jardim regions. The group is represented by a succession of marine to coastal sandstones and rhythmites and alluvial fan conglomerates, disposed in progradationalretrogrational cycles. Detailed stratigraphic studies of paleoenvironments, paleocurrents and provenance have lead to the subdivision of the Santa Barbara Group, in the Central Camaqua Sub-basin, into four formations: (i) Passo da Capela Formation: silty-sandy unit composed of outer- and inner-submarine fan turbidites; (ii) Seival Formation: offshore tempestites and coastal deposits, including tidal flat deposits; (iii) Rincao dos Mouras Formation: alluvial fan and braided river pebbly unit and (iv) Joao Dias Formation: foreshore and shoreface coastal deposits. These studies have permitted the detailed correlation between the Minas do Camaqua and Bom Jardim regions, thus supporting the proposition of including, in the here defined Santa Barbara Group, the deposits exposed in the Minas do Camaqua region, traditionally reported to the Arroio dos Nobres Formation.

Late Mesozoic to Paleogene stratigraphy of the Salar de Atacama Basin, Antofagasta, Northern Chile: Implications for the tectonic evolution of the Central Andes

The Salar de Atacama basin, the largest “pre-Andean” basin in Northern Chile, was formed in the early Late Cretaceous as a consequence of the tectonic closure and inversion of the Jurassic–Early Cretaceous Tarapacá back arc basin. Inversion led to uplift of the Cordillera de Domeyko (CD), a thick-skinned basement range bounded by a system of reverse faults and blind thrusts with alternating vergence along strike. The almost 6000-m-thick, upper Cretaceous to lower Paleocene sequences (Purilactis Group) infilling the Salar de Atacama basin reflects rapid local subsidence to the east of the CD. Its oldest outcropping unit (Tonel Formation) comprises more than 1000 m of continental red sandstones and evaporites, which began to accumulate as syntectonic growth strata during the initial stages of CD uplift. Tonel strata are capped by almost 3000 m of sandstones and conglomerates of western provenance, representing the sedimentary response to renewed pulses of tectonic shortening, which were deposited in alluvial fan, fluvial and eolian settings together with minor lacustrine mudstone (Purilactis Formation). These are covered by 500 m of coarse, proximal alluvial fan conglomerates (Barros Arana Formation). The top of the Purilactis Group consists of Maastrichtian-Danian alkaline lava and minor welded tuffs and red beds (Cerro Totola Formation: 70–64 Ma K/Ar) deposited during an interval of tectonic quiescence when the El Molino–Yacoraite Late Cretaceous sea covered large tracts of the nearby Altiplano-Puna domain. Limestones interbedded with the Totola volcanics indicate that this marine incursion advanced westwards to reach the eastern CD slope. CD shortening in the Late Cretaceous was accompanied by volcanism and continental sedimentation in fault bounded basins associated to strike slip along the north Chilean magmatic arc to the west of the CD domain, indicating that oblique plate convergence prevailed during the Late Cretaceous. Oblique convergence seems to have been resolved into a highly partitioned strain system where margin-parallel displacements along the thermally weakened arc coexisted with margin-orthogonal shortening associated with syntectonic sedimentation in the Salar de Atacama basin. A regionally important Early Paleocene compressional event is echoed, in the Salar de Atacama basin by a, distinctive, angular unconformity which separates Paleocene continental sediments from Purilactis Group strata. The basin also records the Eocene–Early Oligocene Incaic transpressional episode, which produced, renewed uplift in the Cordillera de Domeyko and triggered the accumulation of a thick blanket of syntectonic gravels (Loma Amarilla Formation).