The sedimentary record of the exhumation of a granitic intrusion into a collisional setting: the lower Gonfolite Group,Southern Alps,Italy

The clastic wedge of the Gonfolite Lombarda Group (GLW) accumulated during Oligocene–Miocene times in the Southern Alps foreland basin, formed on the southern, inner side of the Alpine belt. It represents the depositional counterpart of the exhumation and erosion of the Central Alps metamorphic–magmatic units.Among the Central Alps units, the Tertiary Bergell Intrusion (TBI) is one of the principal sources of pebbles occurring within the GLW. Geochronologic data, both from intrusive pebbles and present-day outcrops of intrusive rocks, document the rapid uplift history of the GLW source area.The lower Gonfolite clastic wedge (Como Conglomerate and Val Grande Sandstone Formations, Oligocene–Early Miocene) has been investigated through the study of sandstone and conglomerate petrology for detecting the effects in the sedimentary record of this collision-related event.The main results are: (i) sandstone petrology of the Como Conglomerate records an evolution from feldspatholithic to feldspathic sandstones; (ii) the related Q/F–F/L ratios suggest an evolution from a mixed plutonic–metamorphic to a mainly plutonic source; (iii) consistently, conglomerate petrology records a progressive increase of plutonic pebbles (from nearly 0–50% of the total), a corresponding decrease of metamorphic clasts (from nearly 80 to nearly 50%) and the disappearance of cover rock fragments. Considering the high relief/short transport setting of the GLW clastic routing system, these values probably resemble the real proportions of such rocks in the Gonfolite catchment area.During the Aquitanian, the return to a metamorphic-rich source is recorded both by sandstones and conglomerates at the top of the Como Conglomerate and in the Val Grande Sandstone. This last signal is interpreted as the result of the reorganisation of the Gonfolite source area, possibly related to the northward shift of the main Alpine divide.     

The Monte Orfano Conglomerate revisited: stratigraphic constraints on Cenozoic tectonic uplift of the Southern Alps (Lombardy, northern Italy)

The Monte Orfano Conglomerate (MOC), exposed in the foothills of the Southern Alps (northern Italy), is one of the few outcrops of sediments documenting the Cenozoic tectonic evolution of the Alpine retrowedge. Calcareous nannofossil biostratigraphy allowed us to constrain the upper part of the MOC, formerly attributed to the Early-Middle Miocene in the type-locality, to the earliest Miocene (Neogene part of the NN1 nannofossil zone). A likely latest Oligocene age is therefore suggested for the bulk of the underlying conglomerates, whose base is not exposed. Deposition of the MOC can be placed within the post-collisional tectonic uplift of the Alps, documented in the Lake Como area by the Como Conglomerate (CC) at the base of the Gonfolite Lombarda Group, and supports the correlation with Upper Oligocene clastic sediments cropping out further to the East, in the Lake Garda and in the Veneto-Friuli areas (“molassa”). The remarkable difference in petrographic composition between the western (CC) and eastern (MOC) clastics deposited in the Alpine retro-foreland basin highlights the synchronous tectonic activity of two structural domains involving different crustal levels. Whilst the bulk of the CC, that straddles the Oligocene/Miocene boundary, records largely the tectonic exhumation of the Alpine axial chain crystalline complexes, the coeval MOC consists of detritus derived from the superficial crustal section (Triassic to Paleogene sedimentary rocks) of the Alpine retrowedge and constrains the onset of the post-collisional deformation phase of the Southern Alps as not younger than the Late Oligocene.     

Petrographical, geochemical and geochronological constraints on igneous clasts and sediments hosted in the Oligo-Miocene Bakony Molasse, Hungary: evidence for a Paleo-Drava River system

The provenance of igneous clasts and arenitic sediment enclosed within the Bakony Molasse was studied using geochemical and geochronological methods. The majority of igneous clasts were eroded from the Oligocene Periadriatic magmatic belt. A part of the andesite material has Eocene formation age. Rhyolitic pebbles originated from Permian sequences of the Greywacke zone or the Gurktal Alps. Apatite fission track (FT) ages from the sandstone matrix (age clusters at ~75 and ~30 Ma) are typical for the Austroalpine nappe pile and for the cooling ages of Periadriatic magmatic belt. Variscan detrital zircon FT ages indicate source areas that had not suffered Alpine metamorphism, such as the Bakony Mountains, Drauzug and the Southern Alps. Another group of detrital zircon grains of Late Triassic-Jurassic FT age (mean: ~183 Ma) marks source zones with Mesozoic thermal overprint such as the Gurktal Alps and some Austroalpine regions. Zircon grains with Oligocene FT age (mean: ~34.7 Ma) were derived from the Periadriatic intrusives and their contact zones. On the basis of the new data, we propose that the ancestor of the recent Drava River had already existed in Oligo-Miocene time and distributed eroded material of the southern Eastern Alps to the east.     

Modification of sediment characteristics during glacial transport in high-alpine catchments: Mount Cook area, New Zealand

The Mount Cook area in the Southern Alps of New Zealand is heavily glacierized with numerous peaks over 3000 m a.s.l. feeding several large valley glaciers. The region is subject to rapid tectonic uplift and heavy precipitation (up to 15 m per year). This paper describes the clast roundness, clast shape and textural characteristics associated with five glaciers (Fox, Franz Josef, Hooker, Mueller and Tasman) in terms of inputs to the glacier system, transport by the glaciers and reworking following glacial deposition. Inputs include rockfall, alluvial fan and avalanche material delivered to the surface of valley glaciers. Basal debris, where observed at the terminus of two glaciers, consists mainly of incorporated fluvial material. Following deposition, reworking is mainly by subglacial and proglacial streams. The dominant facies are (i) boulder gravel with mainly angular clasts on the steep slopes above the glaciers, (ii) sandy boulder gravel, with mainly angular and subangular clasts, forming lateral and end moraines, and (iii) sandy boulder/cobble gravel with mainly subrounded clasts, and sand, which represent glacially transported sediment reworked by braided rivers. Diamicton is rare in the contemporary glacial environment. Since most sediment associated with glaciers in the Southern Alps lacks unambiguous indications of glacial transport, interpretation of similar sediments in the geological record should not necessarily exclude the involvement of glacial processes.     

The Corsica–Sardinia Massif as source area for the early northern Apennines foredeep system: evidence from debris flows in the "Macigno costiero" (Late Oligocene, Italy)

Large isolated gravity flows (debrites) are widely present in the stratigraphic record of the northern Apennines foreland-basin system. These strata may be useful for provenance signals and dispersal pathways during foreland evolution. This paper examines a cohesive debris flow bed interbedded with turbidite strata of the Macigno Formation (Late Oligocene, Tuscany, Italy), in order to obtain new data on the provenance of the clastic material. Clasts in the debris flow are predominantly plutonic (granodiorite, tonalite, and S-granite) and subordinately metamorphic (gneiss and schist) and sedimentary calcareous clasts. The composition of the clasts within the debris flow is similar to the clastic composition of the interbedded turbidite sandstones of the "Macigno costiero." The depositional features of the debris flow suggest that it traveled for a short distance within the basin before it was deposited not far from the slope. The absence of a high-pressure/low-temperature (HP/LT) paragenesis in the plutonic and metamorphic clasts of the debris flow indicates a provenance from a crystalline basement not involved in the high-pressure phases of the Alpine Orogenesis. Previous studies have indicated the Central-Western Alps as potential source areas for the Macigno Formation sediments. The lack of HP/LT metamorphic signatures in our studied samples excludes the Pennidic and Austroalpine nappes of the Western Alps as possible sources for the debris flows of the "Macigno costiero." These new data (sedimentological, petrographical, and microstructural) suggest that the Corsica-Sardinia Hercynian basement, lacking a HP/LT paragenesis, is the more accredited source area of the debris flow and of the related turbidite sandstones of the "Macigno costiero" succession. These foredeep-feeding sediments were probably before deposited within an episutural basin developed close to the northern Apennines orogenic wedge.     

Shallow‐water microbialite‐volcaniclastic facies association in the Cambro‐Ordovician Mt Windsor Subprovince,Australia

The Trooper Creek Formation is a mineralised submarine volcano‐sedimentary sequence in the Cambro‐Ordovician Seventy Mile Range Group, Queensland. Most of the Trooper Creek Formation accumulated in a below‐storm‐wave‐base setting. However, microbialites and fossiliferous quartz‐hematite ± magnetite lenses provide evidence for local shoaling to above fairweather wave‐base (typically 5–15 m). The microbialites comprise biogenic (oncolites, stromatolites) and volcanogenic (pumice, shards, crystal fragments) components. Microstructural elements of the bioherms and biostromes include upwardly branching stromatolites, which suggest that photosynthetic microorganisms were important in constructing the microbialites. Because the microbialites are restricted to a thin stratigraphic interval in the Trooper Creek area, shallow‐water environments are interpreted to have been spatially and temporarily restricted. The circumstances that led to local shoaling are recorded by the enclosing volcanic and sedimentary lithofacies. The microbialites are hosted by felsic syneruptive pumiceous turbidites and water‐settled fall deposits generated by explosive eruptions. The microbialite host rocks overlie a thick association (≤?300 m) of andesitic lithofacies that includes four main facies: coherent andesite and associated autoclastic breccia and peperite; graded andesitic scoria breccia (scoriaceous sediment gravity‐flow deposits); fluidal clast‐rich andesitic breccia (water‐settled fall and sediment gravity‐flow deposits); and cross‐stratified andesitic sandstone and breccia (traction‐current deposits). The latter three facies consist of poorly vesicular blocky fragments, scoriaceous clasts (10–90%), and up to 10% fluidally shaped clasts. The fluidal clasts are interpreted as volcanic bombs. Clast shapes and textures in the andesitic volcaniclastic facies association imply that fragmentation occurred through a combination of fire fountaining and Strombolian activity, and a large proportion of the pyroclasts disintegrated due to quenching and impacts. Rapid syneruptive, near‐vent aggradation of bombs, scoria, and quench‐fragmented clasts probably led to temporary shoaling, so that subsequent felsic volcaniclastic facies and microbialites were deposited in shallow water. When subsidence outpaced aggradation, the depositional setting at Trooper Creek returned to being relatively deep marine.     

The statistics of counting clasts in rudites: a review, with examples from the upper Palaeogene of southern California, USA

The relative proportions of gravel sized particles of different lithology in rudaceous sedimentary rocks are generally determined in the field by clast counting. Clast counts are usually carried out qualitatively in order to assess sedimentary provenance. However, a review of the statistical aspects of clast counting suggests that this technique also can be applied quantitatively, and to investigate a variety of other objectives during basin analysis. Geographical and stratigraphical changes in the relative proportions of clasts can be quantified statistically and used to characterize sediment dispersal patterns in space and time, respectively. Statistical comparisons between clast assemblages can be used as a tool to match up rock units. This approach may help to constrain tectonic or suspect-terrane models, or to document sediment recycling. Both counting and sampling errors contribute to the total probable error of a clast count. Sampling error results from the uneven distribution of clasts in outcrop, perhaps caused by selective sorting. Counting error arises from a count of some number less than the total number of clasts in the population. Sampling and counting errors can be minimized by counting in closely spaced subsets, and by counting a total of at least 400 clasts, respectively. Thus, a useful procedure is to count four closely spaced subsets of 100 each, and combine the results for a total of 400. Point counting should not be used because differences in particle size produce biased results. A better method is to count all clasts above some minimum size within a specified area of outcrop. Analysis of upper Palaeogene non-marine conglomerates composing part of the Sespe Formation in California, using confidence intervals, hypothesis testing, analysis of variance, ratio analysis and varietal studies, demonstrates that useful statistics can be derived by counting clasts.     

Numerical modelling of clast rotation during soft-sediment deformation: a case study in Miocene delta deposits

A numerical model for a rotated clast in a sedimentary matrix is presented, quantifying the deformation in associated soft-sediment deformation structures. All the structures occur in a southwards prograding deltaic sequence within the Miocene Ingering Formation, deposited at the northern margin of the Fohnsdorf Basin (Eastern Alps, Austria). Debris flow and pelitic strata contain boudins, pinch-and-swell structures, ptygmatic folds, rotated top-to-S reverse faults and rigid clasts, developed under different stress conditions within the same layers. The deformation around a 24×10 cm trapezoid-shaped rigid clast, resembling the δ-clast geometry in metamorphic rocks, has been modelled using a 2D finite element modelling software. Under the chosen initial and boundary conditions the rotational behaviour of the clast mainly depends on the proportions of pure and simple shear; best fitting results were attained with a dominantly pure shear deformation (~65–85%), with stretching parallel and shortening normal to the bedding. In this specific model set-up, the initial sedimentary thickness is reduced by 30%, explained by stretching due to sediment creeping and compaction. The high amount of pure shear deformation proposed is compatible with the observed layer-parallel boudinage and pinch-and-swell structures. Rotated faults and ptygmatic folds were caused by the minor component of bedding-parallel simple shear.     

Reply to Molina-Garza et al. (2019) “Discussion of: Ortega-Flores et al. (2018) provenance analysis of Oligocene sandstone from the Cerro Pelón area,southern Gulf of Mexico”

ABSTRACT

The origin of the Oligocene turbidites from the Cerro Pelón area in south Gulf Mexico proposed by Ortega-Flores et al. (2018) is in disagree with the interpretations made by Molina-Garza et al. (2019), which main criticism is based on U-Pb ages of detrital zircons from the matrix of a conglomerate unit, which they refer to as ‘Nanchital Conglomerate’, as well as on the presence of limestone, gabbros, and mafic protolith-derived clasts. Molina-Garza et al. (2019) basically interpret the Nanchital Conglomerate as Miocene in age, which was sourced mainly from metamorphic complexes including their sedimentary covers located to the west and south of the Cerro Pelón area. For some reason, Molina-Garza et al. (2019) suppose that the Nanchital Conglomerate should have the same provenance sources that the Oligocene turbidites from Cerro Pelón area, reported by Ortega-Flores et al. (2018). Based on the foregoing, we strongly disagree with Molina-Garza et al. (2019) considering that, from the beginning, they intend to compare two units of different age. Additionally, the scarce data reported from both the matrix and the clasts of the Nanchital Conglomerate are not determinant for interpreting the provenance of this conglomeratic unit and subsequently, to consider the same rock sources from the Oligocene through Miocene time.     

Anatomy of a Forearc Submarine Fan: Upper Eocene — Oligocene Andaman Flysch Group,Andaman Islands,India

Detailed facies analysis in the upper Eocene—Oligocene Andaman Flysch Group reveals fourteen different facies, grouped into three different associations. These facies associations represent two anatomical divisions of a submarine fan, viz. inner fan and depositional lobe in middle fan. Paleocurrent pattern, high percentage of quartz in the sandstones and outer arc derived clasts in the inner fan channel conglomerates suggest dual sediment supply in this fan. A longitudinal geometry of the basin has been inferred where juxtaposition of sediments of different proximality deterred development of any definite sequence pattern. A tectonically induced active sediment supply in a rising sea level stand is thought to be responsible for development of the fan.     

Boulder beds in the glaciogenic Permo-Carboniferous Dwyka Formation in South Africa

Single-layer and massive boulder beds, which include boulder pavements, are sporadically distributed in the glaciogenic Permo-Carboniferous Dwyka Formation. These matrix-supported beds consist of moderately to poorly sorted, rounded boulders, cobbles and pebbles with a clast composition similar to those in the underlying or overlying diamictite. Alternatively, the clasts are composed of monolithic basement rock-types. The clasts show a long-axis orientation which, in the case of the boulder pavements, is parallel to the striae on the pavements. The various types of boulder beds have a similar mode of deposition and their subglacial origin is evidenced by the clast orientation, clasts with stoss and lee sides, stacking of clasts, and the development of a cleavage in the matrix due to horizontal stresses exerted by the boulders in the subglacial sediment. Subglacial streams, kame mounds, subaqeously winnowed till, or boulder beaches supplied the coarse debris which was entrained in the basal ice by plastic flow and regelation. Selective lodgement of the transported boulders occurred down-glacier when the basal thermal conditions changed from cold-freezing to warm-melting. The formation of the different types of boulder beds is thought to depend primarily on the concentration of coarse debris in the basal ice.     

The Polonez Cove Formation of King George Island, Antarctica: stratigraphy, facies and implications for mid-Cenozoic cryosphere development

The middle to late Oligocene Polonez Cove Formation, exposed on south‐eastern King George Island, South Shetland Islands, provides rare evidence of mid‐Cenozoic West Antarctic cryosphere evolution. A revised lithostratigraphy and facies analysis and a review of the palaeoenvironmental significance of the formation are presented here. The diamictite‐dominated basal member of the formation (Krakowiak Glacier Member) records the presence and retreat of marine‐based ice on a shallow continental shelf. Five overlying members are recognized. These consist of basaltic‐sourced sedimentary rocks and lavas and represent a variety of shoreface and shallow continental shelf environments in an active volcanic setting. These units contain diverse reworked and ice‐rafted exotic clasts that become sparse towards the top of the formation, suggesting a continuing but waning glacial influence. New ⁴⁰Ar/³⁹Ar dates from interbedded lava flows indicate a late Oligocene age (25·6–27·2 Ma) for the Polonez Cove Formation, but are slightly younger than skeletal carbonate Sr‐isotope ages obtained previously (28·5–29·8 Ma). There is evidence for wet‐based subice conditions at the base of the Polonez Cove Formation, but no sedimentary facies to suggest substantial meltwater. This may reflect a subpolar setting or may result from lack of preservation or a high‐energy depositional environment. A northern Antarctic Peninsula/South Shetland Islands provenance is probable for most non‐basaltic clasts, but certain lithologies with possible origins in the Transantarctic and Ellsworth Mountains also occur sparsely throughout the formation. There is evidence to suggest that the presence of such far‐travelled clasts within subglacially deposited facies at the base of the formation reflects the advance of a local ice cap across marine sediments containing the clasts as ice‐rafted material. The presence of these clasts suggests that extensive marine‐based ice drained into the southern Weddell Sea region and that a strong Weddell Sea surface current operated both before and during deposition of the Polonez Cove Formation.     

A non-glacial origin for the ordovician (middle caradocian) cosquer formation,veryarc'h,crozon peninsula,brittany, France

The presence of a dispersed clast fraction in strata near the base of the Cosquer Formation in west Brittany, does not support a glacial origin for this unit. The upper 25 to 30 m of the underlying Kermeur Formation consists of a prograding sequence of very fine to fine sandstones deposited in a mid to distal current swept shelf setting. This sequence shows signs of slope instability, as do the supposed ‘glacial strata’ which overlie it. The upper two thirds of the Cosquer Formation contain spectacular slump-breccias. Smaller clasts within the laminated mudrocks at the base of the formation are associated with thin graded and non-graded sandstone laminae. They show no evidence of active penetration into underlying laminae other than can be explained by compaction. Larger clasts are confined to thicker massive beds, or disrupted units with marked internal contorted lamination. This, along with the abundance of slump features within the sequence suggests lateral emplacement by sediment gravity flows in a distal shelf-slope setting. Surface textures of sand grains within the formation are related to rock disaggregation along fractures developed during post-depositional deformation and are not related to glacial processes. Distinctive mineralogically immature, poorly-sorted aggregate sediment pellets, which have been considered as positive proof of glaciation, are not present.     

Limestone pseudoconglomerates in the Late Cambrian Gushan and Chaomidian Formations (Shandong Province, China): soft-sediment deformation induced by storm-wave loading

This paper focuses on the formative processes of limestone pseudoconglomerates in the Gushan and Chaomidian Formations (Late Cambrian) of the North China Platform, Shandong Province, China. The Gushan and Chaomidian Formations consist mainly of limestone and shale (marlstone) interlayers, wackestone to packstone, grainstone and microbialite as well as numerous limestone conglomerates. Seventy‐three beds of limestone pseudoconglomerate in the Gushan and Chaomidian Formations were analysed based on clast and matrix compositions, internal fabric, sedimentary structures and bed geometry. These pseudoconglomerates are characterized by oligomictic to polymictic limestone clasts of various shapes (i.e. flat to undulatory disc, blade and sheet), marlstone and/or grainstone matrix and various internal fabrics (i.e. intact, thrusted, edgewise and disorganized), as well as transitional boundaries. Limestone pseudoconglomerates formed as a result of soft‐sediment deformation of carbonate and argillaceous interlayers at a shallow burial depth. Differential early cementation of carbonate and argillaceous sediments provided the requisite conditions for the formation of pseudoconglomerates. Initial deformation (i.e. burial fragmentation, liquefaction and injection) and subsequent mobilization and disruption of fragmented clasts are two important processes for the formation of pseudoconglomerates. Burial fragmentation resulted from mechanical rupture of cohesive carbonate mud, whereas subsequent mobilization of fragmented clasts was due to the injection of fluid materials (liquefied carbonate sand and water‐saturated argillaceous mud) under increased stress. Storm‐wave loading was the most probable deformation mechanism, as an external triggering force. Subsequent re‐orientation and rounding of clasts were probably prolonged under normal compactional stress. Eventually, disrupted clasts, along with matrix materials, were transformed into pseudoconglomerates by progressive lithification. Soft‐sediment deformation is prevalent in alternate layers of limestone and mud(marl)stone and/or grainstone, regardless of their depositional environments.     

Chemical and physical responses to deformation in micaceous quartzites from the Tauern Window, Eastern Alps

Micaceous quartzites from a subvertical shear zone in the Tauern Window contain abundant quartz clasts derived from dismembered quartz‐tourmaline veins. Bulk plane strain deformation affected these rocks at amphibolite facies conditions. Shape changes suggest net shortening of the clasts by 11–64%, with a mean value of 35%. Quartz within the clasts accommodated this strain largely via dislocation creep processes. On the high‐stress flanks of the clasts, however, quartz was removed via solution mass transfer (pressure solution) processes; the resulting change in bulk composition allowed growth of porphyroblastic staurolite + chlorite ± kyanite on the clast flanks. Matrix SiO₂ contents decrease from c. 83 wt% away from the clasts to 49–58% in the selvages on the clast flanks. The chemical changes are consistent with c. 70% volume loss in the high‐stress zones. Calculated shortening values within the clast flanks are similar to the volume‐loss estimates, and are greatly in excess of the shortening values calculated from the clasts themselves. Flow laws for dislocation creep versus pressure solution imply large strain‐rate gradients and/or differential stress gradients between the matrix and the clast selvages. In a rock containing a large proportion of semirigid clasts, weakening within the clast flanks could dominate rock rheology. In our samples, however, weakening within the selvages was self limiting: (1) growth of strong staurolite porphyroblasts in the selvages protected remaining quartz from dissolution; and (2) overall flattening of the quartz clasts probably decreased the resolved shear stress on the flanks to values near those of the matrix, which would have reduced the driving force for solution‐transfer creep. Extreme chemical changes nonetheless occurred over short distances. The necessity of maintaining strain compatibility may lead to significant localized dissolution in rocks containing rheologic heterogeneities, and overall weakening of the rocks may result. Solution‐transfer creep may be a major process whereby weakening and strain localization occur during deep‐crustal metamorphism of polymineralic rocks.     

The Palaeocene-early Oligocene Zacatecas conglomerate,Mexico: sedimentology,detrital zircon U–Pb ages,and sandstone provenance

ABSTRACT

This article presents detailed mapping results and the first U–Pb zircon dating and sedimentological characterization of the Zacatecas Conglomerate, which belongs to the Palaeogene red beds of central Mexico, deposited in fault-bounded basins during the Late Cretaceous to Eocene Laramide orogeny. The conglomerate was divided into five depositional facies associations according to their clast-type abundances and interlayered volcanic rocks. The lowermost member has a maximum depositional age based on young zircon grain ages varying from ca. 63 to 81 Ma. It is unconformably overlain by a continuous sequence characterized by a conglomerate rich in granite clasts at the bottom, with an interlayered tuff dated at 37.64 ± 0.36 Ma. Near the top, another tuff was dated at 30.84 ± 0.47 Ma, and a sandstone has a maximum depositional age of ca. 31.5 Ma. Normal grading, massive textures, channels, channel-form sandstone bodies, and upward-finning successions suggest that the Zacatecas Conglomerate is of fluvial origin. Late Jurassic to Early Cretaceous ages from zircons in plutonic rocks and sandstones bracket possible source regions for the Zacatecas Conglomerate. One possible source is Late Jurassic-Early Cretaceous granite derived from the Alisitos-Guerrero arc of western Mexico. Another possible source is the Tuna Manza Diorite, now exposed 250 km southeast of the study area. The lack of pre-Jurassic grains implies that possible sources such as the Nazas arc or the Potosí fan were not cropping out at that time, or at least that these areas were not affected by the fluvial system feeding the Zacatecas Conglomerate. It is possible that during the Palaeocene-early Oligocene the fluvial systems drained from west to east and from southeast to north, according to the above-mentioned constraints.     

Sedimentary response to a tectonically induced sea-level fall in a shallow back-arc basin: the Mulichinco Formation (Lower Cretaceous), Neuquén Basin, Argentina

This study examines the sedimentary response to a tectonically driven relative sea‐level fall that occurred in the Neuquén Basin, west‐central Argentina, during the late Early Valanginian (Early Cretaceous). At this time the basin lay behind the emergent Andean magmatic arc to the west. Following the relative sea‐level fall, sedimentation was limited to the central part of the Neuquén Basin, with the deposition of a predominantly clastic, continental to shallow marine wedge on top of basinal black shales. This lowstand wedge is called the Mulichinco Formation and consists of a third‐order sequence that lasted about 2 Myr and contains high frequency lowstand, transgressive, and highstand deposits. Significant variations in facies, depositional architecture, and internal organization of the sequence occur along depositional strike. These variations are attributed mainly to tectonic and topographic controls upon sediment flux, basin gradient, fault tilting, and shifting of the depocentre through time. These controls were ultimately related to asymmetrically distributed tectonic activity that was greater towards the magmatic arc in the west. The superposition of fluvial deposits directly upon offshore facies provides unequivocal evidence for a sequence boundary at the base of the Mulichinco Formation. However, the Mulichinco sequence boundary is marked by shallow, low erosional relief and widespread fluvial deposition. The surface lacks prominent valleys traditionally associated with sequence boundaries. This non‐erosive sequence boundary geometry is attributed to the ramp‐type geometry of the basin and/or rapid uplift that limited stratigraphic adjustment to base‐level fall. Significant along‐strike facies changes and a low‐relief sequence boundary are attributes that may be common in tectonically active, semi‐enclosed basins (e.g. shallow back‐arc basins, foreland basins).     

珠江口盆地白云凹陷渐新世/中新世地质事件的碎屑组分响应

南海北部渐新世/中新世之交发生了物源突变地质事件。利用珠江口盆地白云凹陷渐新统珠海组和中新统珠江组砂岩薄片镜下计点法统计技术,详细研究了碎屑组分的变化特征。结果表明:珠海组砂岩碎屑组分中,岩屑以深成的岩浆岩屑占绝对优势,长石和沉积岩屑含量也相对较高,与该期物源主要来自华南沿海近源以燕山期花岗岩为主的母岩区相一致;而珠江组砂岩中,岩屑以变质岩屑占优势,同时也含有一定量的岩浆岩屑、沉积岩屑和较多的长石,显示珠江组具有更多的物源供给区。随着渐新世/中新世之交各种地质事件发生,尤其是伴随青藏高原隆升引起的大陆风化及侵蚀程度增强,珠江口盆地沉积物源区向华南古陆内部古老的沉积-变质岩区和青藏高原东麓扩展,碎屑组分变化应该是该期各种地质事件的综合作用结果。     

祁连山党河盆地二道泉剖面新近纪沉积物磁性矿物与古水流及其对盆地水系演化的启示

二道泉剖面(39°01' 34″N, 95°52' 49″E) 位于党河盆地腹地, 呈北南向展布, 长790m, 党河盆地位于祁连山腹地, 是研究青藏高原北部构造沉积与地貌演变的理想地区。本研究在野外地层观测、古流水重建及砾石组分分析的基础上, 对党河盆地中西部二道泉剖面新近系白杨河组顶部-玉门砾岩底部河湖相沉积物进行磁化率测量与等温剩磁实验分析。结果显示, 剖面沉积物的磁性矿物主要有磁铁矿和赤铁矿, 其中赤铁矿相对含量较高。通过对比发现剖面中黄色调粗粒沉积物中的赤铁矿含量显著高于红色调细粒沉积物。砾石成分和向南的古流向结果显示, 白杨河组晚期至玉门砾岩早期地层沉积物主要源自于二道泉剖面北部的野马南山地区。磁化率及沉积相揭示出源区物理风化作用逐渐增强, 赤铁矿是从源区直接搬运沉积的, 而非自生形成; 这一过程未受外界气候环境的影响, 赤铁矿多为碎屑, 岩石颜色发黄。综合分析认为, 二道泉剖面疏勒河组到玉门砾岩组早期沉积时党河盆地发育一条规模逐渐变大、水动力逐渐变强的由北向南流动的河流, 现今流向为北西向的党河水系可能最早在新近纪玉门砾岩沉积中期以后才形成。