青海巴颜喀拉山三叠系复理石沉积粒度概率累积曲线的特征

巴颜喀拉山群为碎屑岩复理石沉积。粒度概率累积曲线分析结果表明,完全为悬浮总体的浊积型曲线很少．曲线主要由牵引和悬浮总体组成。碎屑流是巴颜喀拉山群砂岩主要的成因类型。粒度参数特征表明,巴颜喀拉山盆地由早到晚平均粒径变粗、分选变好。风暴岩序列不同阶段粒度概率累积曲线形态不同,粒度曲线特征与被改造砂体的组成有关。     

松潘-甘孜褶皱带复理石火山岩夹层时代及其地质意义

在四川北部松潘西北和甘肃南部合作市一带的上三叠统(深海沉积)复理石中发现了数套火山(碎屑)岩夹层。利用UPb同位素测年,获得4件安山岩、凝灰岩样品锆石的年龄加权平均值分别为205.9±1.6Ma、208.9±1.8Ma、210.4±1.6Ma和212.3±1.5Ma,时代属于晚三叠世诺利期晚期—瑞提期早期。此系第一次较为精准地报道松潘-甘孜褶皱带晚三叠世复理石火山岩夹层及其可能代表的火山喷发年龄。研究结果表明,直到瑞提期早期松潘-甘孜古残留海盆还未停止接受沉积物供给。该套火山(碎屑)岩的测年结果为松潘-甘孜褶皱带上三叠统复理石岩石地层单元的划分和对比提供了重要参考,同时为进一步约束华南与华北板块碰撞-造山事件时限提供了新证据。     

Conceptual model of fracture-limited sea cliff erosion: Erosion of the seaward tilted flyschs of Socoa,Basque Country,France

Sea cliff morphology and erosion rates are modulated by several factors, including rock control that reflects both lithology and rock structure. Erosion is anticipated to preferentially exploit ‘fractures’, broadly meant as any discontinuity in an otherwise continuous medium, where the rock mass is weakest. Unpicking the direct control of such fractures on the spatial and temporal pattern of erosion remains, however, challenging. To analyse how such fractures control erosion, we monitored the evolution of a 400 m-long stretch of highly structured sedimentary cliffs in Socoa, Basque Country, France. The rock is known as the Socoa flysch formation. This formation combines decimetre-thick turbidites composed of repeat triplets of medium to strong calcareous sandstone, laminated siltstones and argillaceous marls. The sequence plunges at 45° into the sea with a shore-parallel strike. The cliffs are cross-cut by two normal and reverse fault families, with 10–100 m alongshore spacing, with primary and secondary strata-bound fractures perpendicular to the bedding, which combined delimit the cliff rock mass into discrete blocks that are exploited by the erosion process. Erosion, and sometimes plucking, of such beds and blocks on the cliff face was monitored using ground-based structure-from-motion (SfM) photogrammetry, over the course of 5.7 years between 2011 and 2017. To compare with longer time change, cliff-top retreat rate was assessed using SfM-orthorectified archive aerial photographs spanning 1954–2008. We show that the 13,250 m² cliff face released 4500 blocks exceeding 1.45 × 10⁻³ m³, removing a total volume of 170 m³. This equates to an average cliff erosion rate of 3.4 mm/year, which is slightly slower than the 54-year-long local cliff-top retreat (10.8 ± 1.8 mm/year). The vertical distribution of erosion reflects the height of sea water inundation, where the maximum erosion intensity occurs ca. 2 m above high spring-tide water level. Alongshore, the distribution of rockfall scars is concentrated along bed edges bounding cross-cutting faults; the extent of block detachment is controlled by secondary tectonic joints, which may extend through several beds locally sharing similar mechanical strength; and rockfall depth is always a multiple of bed thickness. Over the longer term, we explain block detachment and resultant cliff collapse as a cycle. Erosion nucleates on readily exploitable fractures but elsewhere, the sea only meets defect-free medium-strong to strong rock slabs offering few morphological features for exploitation. Structurally delimited blocks are quarried, and with sufficient time, carve semi-elliptic scars reaching progressively deeper strata to be eroded. Lateral propagation of erosion is directed along mechanical weaknesses in the bedding, and large episodic collapses affect the overhanging slabs via sliding on the weak marl beds. Collapse geometry is confined to one or several triplets of turbidite beds, but never reaches deeper into the cliff than the eroded depth at the foot. We contend that this fracture-limited model of sea-cliff erosion, inferred from the Socoa site dynamics and its peculiar sets of fractures, applies more broadly to other fractured cliff contexts, albeit with site-specific geometries. The initiation of erosion, the propagation of incremental block release and the ultimate full failure of the cliff, have each been shown to be fundamentally directly controlled by structure, which remains a vital control in understanding how cliffed coasts have changed in the past and will change in the future.     

Appareils gravitaires et évolution mégaséquentielle du Numidien dans le Nord et I’Est de la Grande Kabylie

Résumén

L’analyse séquentielle et la sédimentologie de faciès sont appliquées aux grès numidiens. Ces méthodes fournissent des données nouvelles concernant l’origine de ces dépôts. Ceux-ci présentent des associations de faciès proximales et résultent de transports sur des distances relativement courtes. On n’observe pas de dépôts distaux. Plusieurs dépo-centres sableux. vraisemblablement des éventails radiaux, de vraient être répartis tout le long d’un bassin. Ces données rendent difficile de situer ce bassin au Sud des « noyaux » kabyles, ceux-ci fournissant à cette époque un matériel terrigène déposé en discordance ou alimentant les grès micacés » périphériques; la bordure « tellienne » ne semblant pas non plus avoir pu laisser transiter des sables (transports continentaux puis marins). Il est donc logique de placer le bassin au nord des « noyaux » kabyles. Cependant, la question de l’origine continentale du matériel quart/eux n’étant pas réglée (Afrique ou continent septentrional), celte position pose le problème paléogéographique de la disposition des terres ou des deltas ayant permis le drainage continental de ces sables jusqu’aux points d’entrée sous-marine.     

Electron microprobe study of detrital amphiboles from Famennian synorogenic clastic sediments of the Saxothuringian belt (Erbendorf Paleozoic,NE-Bavaria,Germany): Consequences for provenance and geotectonic development

Abstract

Detrital amphiboles from the Famennian, Saxothuringian greywackes of the “Erbendorf Paleozoic” in Bavaria were analysed with an electron microprobe. The results were compared with recalculated literature data of amphiboles from potential source rocks to obtain more detailed information about the provenance of the oldest preserved synorogenic sediments in the Saxothuringian belt.

All of the detrital amphiboles show very similar, homogeneous chemical compositions. All of them are Ca-amphiboles (with (Ca+Na)_B ≥: 1.34; Naf_B < 0.67; (Na+K)_A < 0.50 and Ti < 0.50), mostly inagnesio-hornblende and tschermakitic hornblende. The comparison with amphiboles from rocks of potential provenance areas reveals that the Randamphibolit-Series of the Miinchberg Massif – or an equivalent, already eroded unit – can be regarded as source rock of the detrital amphiboles.

The presented data are the first evidence, that the Randamphibolit of the Miinchberg Massif was exhumed as early as in the Famennian, shortly after its metamorphism. It can be concluded that before Famennian time the complete Saxothuringian oceanic crust and large parts of the Saxothuringian continental crust had been subducted at the active margin. This implies that the collision of the Saxothuringian plate with the Tepla-Barrandian microplate, leading to the accretion of the Saxothuringian plate, happened 15–20 Ma earlier than previous authors had supposed. Accordingly, the collisional stage must have been reached not later than in the middle Devonian (approx. 380 Ma).     

Transected folds with opposite patterns in Terena Formation (Ossa Morena Zone,Portugal): anomalous structures resulting from sedimentary basin anisotropies

The Terena Formation is located in the central part of the Ossa-Morena Zone (OMZ) and outcrops in the core of a latter (D3) first order syncline. This Formation is a Lower Devonian flysch and shows an unusual “Z” shape, with a central sector trending nearly N-S, and the tips trending NW-SE. This central sector is crossed by the cleavage (NW-SE) showing an apparent dextral (clockwise) transection pattern, anomalous and opposite to the regional widespread sinistral (anti-clockwise) transpression. The same sector with cartographic dextral transection, shows at outcrop scale, mesoscopic folds with a sinistral transection. During the Lower Devonian a N-S trending basin was developed as an effect of an early tectonic deformation phase. This trough was filled with turbidites and its elongated geometry determined the shape of the main syncline. We propose that the dextral transection pattern, at cartographic scale, result from the superposition of the NW-SE upright S3 cleavage on this major regional structure controlled by a sedimentary trough. The mesoscopic folds, observed on the upper levels of the sedimentary sequence were not influenced by the topographic anisotropy of the basin, and therefore they developed a left transection, according to the regional deformation mechanisms.

The “Z” shape of the syncline could be explained as a consequence of two major tectonic shear zones situated along the north and south boundaries of the OMZ, respectively the Tomar-Badajoz-Cordoba Shear Zone and the South Iberian Suture, lined by the Beja-Acebuches Ophiolitic Complex. Both shear zones have a sinistral transpressive character and were active during late Variscan tectonic events.     

Current-independent paleomagnetic declinations in flysch basins: a case study from the Inner Carpathians

Paleomagnetic declinations from the Inner Carpathian Paleogene Basin imply that the area rotated counterclockwise about 60°, during the Miocene[1]. The question may arise if the paleomagnetic declination could have been biased by the W-E directed turbidity currents prevailing in the basin causing an apparent counter-clockwise rotation of the paleomagnetic direction.

The paleomagnetic results were obtained for fine grained strata, deposited in relatively calm water. Nevertheless, to confirm the paleomagnetic rotation, we needed evidence that flow activity on the magnetic grains was indeed insignificant in the beds yielding paleomagnetic results. Therefore, we carried out magnetic anisotropy measurements.

Results of AMS (representing para and ferromagnetic minerals together) measurements, compared with paleomagnetic observations, demonstrate that well-clustered lineations at locality level and failure to define a paleomagnetic direction are coupled. Lineation, when observable, is flow parallel, suggesting that magnetic lineation in the Inner Carpathian flysch basins may be regarded as a good proxy for turbidity current direction. It is remarkable, however, that the well-defined paleomagnetic directions are observed for localities, where the magnetic fabric is not showing lineation on locality level. Moreover, the lineation direction of the ferromagnetic minerals alone (obtained by measuring the anisotropy of the remanence) is independent of that of the turbidity currents. Thus we can safely conclude that the Inner Carpathian flysch basin indeed was affected by 60° tectonic rotation, and the paleomagnetic vectors were not biased by paleocurrents.     

Tectonics of the western part of the Polish Outer Carpathians

The western part of the Polish Outer Carpathians is built up from the thrust, imbricated Upper Jurassic-Neogene flysch deposits. The following Outer Carpathian nappes have been distinguished: Magura Nappe, Fore-Magura group of nappes, Silesian, Subsilesian and Skole nappes. Interpretation of seismic and magnetotelluric survey from the region South of Wadowice, allows observation of relationship between basement and flysch nappes in the Outer Carpathians. It also allows identification of dislocation cutting both flysch nappes and their basement. All the Outer Carpathian nappes are thrust over the southern part of the North European Platform. The platform basement is composed of older Precambrian metamorphic rocks belonging to the Bruno-Vistulicum terrane. Sedimentary cover consists of Paleozoic, Mesozoic and Neogene sequences. The characteristic features of this boundary are horsts and troughs of general direction NW-SE, turning W-E. Faults cutting only the consolidated basement and the Paleozoic cover were formed during the Hercynian Orogeny in the Carboniferous and the Early Permian. Most of the older normal faults were covered by allochtonous flysch nappes forming thus the blind faults. During the last stage of the geodynamic development the Carpathians thrust sheets moved towards their present position. Displacement of the Carpathians northwards is related to development of dextral strike-slip faults of N—S direction. The orientation of this strike-slip fault zones zone more or less coincides with the surface position of the major faults perpendicular to the strike of the Outer Carpathian thrustsheets. The huge fault cuts formations from the Paleozoic basement through the flysch allochton between the boreholes in Sucha Beskidzka area. The displacement of nappes of the Carpathian overthrust and diapiric extrusion of plastic formations of the lower flysch units occurred along this fault.     

滇西南景洪南光附近泥盆系沉积建造特征 及形成环境

1976年的1∶20万勐腊幅、尚勇幅区调报告报导了景洪澜沧江边南光附近泥盆系的发现并将其时代确定为中—晚泥盆世.中泥盆统为海相沉积(未建组),上泥盆统为陆相沉积,并建“南光组”,对其地层层序及沉积环境和空间关系缺少深入研究.该剖面清楚地记录了复理石沉积建造与磨拉石沉积建造(由海到陆)的连续变化关系,沉积物以含陆源碎屑和火山碎屑物为特点,代表了该区中—晚泥盆世经历了与火山喷发活动有关的造山运动.这一发现将为研究澜沧江河谷地带的地质构造演化增加新的内涵.     

试论西藏南部上三叠统复理石郎杰学群与涅如组

根据新资料结合前人的研究成果对西藏南部仁布以东上三叠统复理石两套地层郎杰学群和涅如组的岩层特征、化石面貌、碎屑组成、沉积特性、地球化学等方面进行了总结及分析.对比研究结果显示,除亚相、岩层厚度分布、砂岩与板岩比例、重矿物ZTR指数变化趋势有一定的差异外,郎杰学群和涅如组的其他十余项特性(颜色、岩性、碎屑组分、重矿物组成...