Perched lobe formation in the Gulf of Cadiz: Interactions between gravity processes and contour currents (Algarve Margin, Southern Portugal)

The Gulf of Cadiz is swept by the strong saline Mediterranean Outflow Water (MOW). On the Algarve Margin (South Portugal), this current has constructed fine-grained contourite drifts. This margin is dissected by the Portimao Canyon and three short channels that only incise the upper slope, and are absent on a terrace located at mid-slope depths along the Algarve Margin. High-resolution seismic profiles and sediment cores highlight the original architecture of the sedimentary deposits on this terrace. Coarse-grained lenticular chaotic bodies formed during major relative sea-level lowstands are intercalated within the drift. The lobate shape and sandy nature of the lenticular chaotic bodies and their location at the mouths of the three short channels suggest they are gravity-generated deposits that are perched on the middle continental slope.In the Gulf of Cadiz, the interaction between contour current and gravity processes is strongly controlled by climatic variations and relative sea-level changes during the late Quaternary. During cold periods when sea-level was low, erosion intensified on the continental shelf and the deepest part of MOW was active. Sediment was transported downslope through the channels and deposited on sedimentary lobes perched on the mid-slope terrace. During warm periods when relative sea-level was high, the supply of sediment from the shelf was shut off and the shallowest part of MOW was more active. Contourite drifts fill the channels and bury the sandy lobes.     

Simulation of the interactions between gravity processes and contour currents on the Algarve Margin (South Portugal) using the stratigraphic forward model Sedsim

The margin of the Gulf of Cadiz is swept by an intermediate current the Mediterranean Outflow Water (MOW) flowing from the Mediterranean to the Atlantic. On the northern margin of the Gulf (Algarve Margin, South Portugal) the MOW intensity is low, and fine-grained contourite drifts are built up with an alongslope alignment. Recent sedimentological studies emphasize the presence of complex process interactions resulting in the formation of a unique depositional architecture. Alongslope processes related to contour currents generate contourite drift, while downslope processes form canyons and channels aligned on deep faults. This paper uses a combined oceanographic and geological dataset to simulate the different types of interactions between gravity processes and contour currents, which were evidenced on this margin. An extrapolation of the contour current intensity has been used based on the present day velocity field and sea-level fluctuations over the simulated geological time-scale. According to our model results, the construction of the contourite drift is closely linked to contour current velocities and directions, the types of sediments transported and the existing topography. Using modern sedimentological understanding of the area, we have correlated gravity flow's strongest activity to sea level lowstand periods mainly due to a closer connection between canyon's mouth and river or deltaic systems. The simulated gravity flows are initialized at different locations and times on the margin depending on the preserved lobes retrieved from seismic analysis. Their resulting morphological features are identified as perched-lobes with volumes and forms close to the ones observed on Portimão and Lagos Drifts. This study provides a process-based understanding of the construction of contourite system and a physical evaluation of the interactions between gravity flows perpendicular to the slope, and alongslope processes. In addition, it shows the influence of autocyclic factors in the construction of contourite sedimentation, which is important to consider in future sedimentary paleo-reconstruction interpretations.     

Hyperpycnal-fed turbidite lobe architecture and recent sedimentary processes: A case study from the Al Batha turbidite system, Oman margin

The main sediment depocenter along the Oman margin is the Al Batha turbidite system that develops in the Gulf of Oman basin. It is directly connected to the wadi Al Batha, and forms a typical sand and mud rich point source system that acts as regional sediment conduit and feeds a ~ 1000 km² sandy lobe.The Al Batha lobe depositional architecture has been investigated in detail using very high-resolution seismic, multibeam echosounder data and sediment cores. Several scales of depositional architecture can be observed. The Al Batha lobe is composed of several depositional units, made of stacked elementary sediment bodies (thinner than 5 m) that are each related to a single flow event. The lobe is connected to the feeder system through a channel-lobe transition zone (CLTZ) that extends on more than 25 km. The lobe can be divided into proximal, middle and distal lobe areas. The proximal lobe is an area of erosion and by-pass with small axial feeder channels that rapidly splay into several small distributaries. They disappear in the mid-lobe area where deposits consist of vertically stacked tabular to lens-shaped sediment bodies, with a lateral continuity that can exceed 10 km. The distal lobe fringe shows a classical facies transition towards thin-bedded basin plain deposits.Sub-surface deposits consist of sandy turbidites and hyperpycnites, interbedded with fine-grained deposits (thin turbidites, hyperpycnites, or hemipelagites). Although these distal deposits are mainly related to flow transformations and concentration evolution, they highlight the importance of flooding of the wadi Al Batha on the sediment transfer to the deep basin. The thick sandy hyperpycnites recovered in such a distal area are also possibly related to the initial properties of gravity flows, in relation to the flooding characteristics of mountainous desert streams.Finally, the Al Batha lobe depositional architecture is typical of sand-rich lobes found within “small”, sand and mud rich turbidite systems fed by mountainous “dirty” rivers. Turbidite sedimentation in the Al Batha system appears to be primarily controlled by the strong climatic and geomorphic forcing parameters (i.e. semi-arid environment with ephemeral, mountainous rivers subjected to flash-flooding).     

Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic

Abstract A study of the seafloor of the Gulf of Cadiz west of the Strait of Gibraltar, using an integrated geophysical and sedimentological data set, gives new insights into sediment deposition from downslope thermohaline bottom currents. In this area, the Mediterranean Outflow (MO) begins to mix with North Atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components. Changes in bedform morphology across the study area indicate a decrease in the peak velocity of the MO from >1 m s^?1 to <0·5 m s^?1. The associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves. A series of downslope‐oriented channels, formed by the MO, are found where the MO starts to descend the continental slope at a water depth of ≈700 m. These channels are up to 40 km long, have gradients of <0·5°, a fairly constant width of ≈2 km and a depth of ≈75 m. Sand waves move down the channels that have mud wave‐covered levees similar to those seen in turbidite channel–levee systems, although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems. The channels terminate abruptly where the MO lifts off the seafloor. Gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels. Each gravity flow system has a narrow, slightly sinuous channel, up to 20 m deep, feeding a depositional lobe up to 7 km long. Cores from the lobes recovered up to 8·5 m of massive, well‐sorted, fine sand, with occasional mud clasts. This work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel–levee systems. The distribution of sand within these systems is of particular interest, with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings.     

渤海湾盆地东营凹陷碎屑流主控型深水体系沉积过程及模式

【研究目的】碎屑流是深水环境沉积物搬运和分散的重要机制,其相关的砂岩储层是含油气盆地重要的勘探目标,然而,与经典浊流及浊积系统相比,对碎屑流主控型深水体系的发育规律目前仍知之甚少。【研究方法】本文基于岩心、测井及全三维地震资料,通过系统的岩心观察描述、测井及地震资料解释,对渤海湾盆地东营凹陷始新统沙三中亚段深水体系沉积过程及模式开展研究。【研究结果】结果表明,沙三中深水体系发育九种异地搬运岩相,可概括为四大成因类型,反映了块体及流体两种搬运过程。岩相定量统计表明,该深水体系主要由碎屑流沉积构成,浊流沉积很少,碎屑流中又以砂质碎屑流为主。重力流在搬运过程中经历了滑动、滑塌、砂质碎屑流、泥质碎屑流及浊流等5个阶段演变,发育5类主要的深水沉积单元,包括滑动体、滑塌体、碎屑流水道、碎屑流朵体及浊积薄层砂。从发育规模及储层物性上,砂质碎屑流水道、朵体及砂质滑动体构成了本区最重要的深水储层类型。【结论】认为沙三中时期充足的物源供给、三角洲前缘高沉积速率、断陷期频繁的断层活动以及较短的搬运距离是碎屑流主控型深水体系形成及演化的主控因素,最终基于沉积过程、沉积样式及盆地地貌特征综合建立了碎屑流主控型深水体系沉积模式。本研究将进一步丰富深水沉积理论,为陆相深水储层预测提供借鉴。     

Structural and physiographic control on the Holocene marine sedimentation in the bay of Cadiz (SW Spain)

The Quaternary tectonic activity in the Gulf of Cadiz has considerably influenced the depositional regime and distribution of Holocene marine deposits. The aim of this work is to determine the nature of the recent sedimentary filling in the Bay of Cadiz sea bottom and adjacent continental shelf and to establish the main controlling factors on the Holocene marine sedimentation.

The sedimentary record indicates siliciclastic sedimentation supplied from the continent, with alternating episodes of high and low sedimentation rates. The recent sedimentary evolution of this marine area was controlled by the Late Quaternary eustatic fluctuations. Bathymetric, geophysical and drilling data have been employed to prepare a detailed isopach map of the non-consolidated recent sedimentary cover. Thickness distribution shows significant variations related to the infilling of former fluvial palaeochannels incised during the Late Pleistocene lowstand, and highly controlled by the structural neotectonic trends of faults and joints: NNW-SSE, NNE-SSW and ENE-WSW. The general distribution of isopachs in this area is clearly influenced by these morphostructural lines, which controlled the sedimentary processes during the Holocene. These results are coherent with the main regional neotectonic structures previously described in the nearby continental area, and confirm their prolongation towards the marine domain.     

海底扇规模优质储集体的分布模式与成因分析——以上新世—更新世孟加拉扇为例

利用高品质3D地震资料搜索刻画深水水道—朵叶复合体的沉积构成并揭示油气储集体的分布模式对于深水油气勘探具有重要的指导意义。利用PaleoScan全三维智能解释手段和三色(RGB)混相分频技术识别解剖了孟加拉扇上新统—更新统深水水道—天然堤体系6种富砂沉积单元,包括补给水道、分支水道、似点坝、决口扇、漫溢扇和末端朵叶。研究认为:补给水道、决口扇、末端朵叶是潜在的规模且优质深水油气储集体类型;而漫溢扇、分支水道和似点坝分别是潜在的规模非优质、非优质非规模以及优质非规模深水油气储层类型。补给水道主要分布在水道—朵叶复合体的上游和中游;决口扇、漫溢扇、分支水道和似点坝主要发育在水道—朵叶复合体的中游;而分支水道和末端朵叶主要分布在水道—朵叶复合体的下游。漫溢扇向供源水道一侧楔状增厚,形成“砂盖泥”的格局;而决口扇向供源水道一侧楔状减薄直至尖灭形成“泥包砂”的格局,相应形成海底扇岩性圈闭。     

http://www.sciencedirect.com/science/article/pii/S1674987114000255

Geophysical studies point to a complex tectonic and geodynamic evolution of the Alboran Basin and Gulf of Cadiz.Tomographic images show strong seismic waves velocity contrasts in the upper mantle.The high velocity anomaly beneath the Alboran Sea recovered by a number of studies is now a well established feature.Several geodynamic reconstructions have been proposed also on the base of these images.We present and elaborate on results coming from a recent tomography study which concentrates on both the Alboran and the adjacent Atlantic region.These new results,while they confirm the existence of the fast anomaly below the Alboran region,also show interesting features of the lithosphere-asthenosphere system below the Atlantic.A high velocity body is imaged roughly below the Horseshoe Abyssal plain down to sub-lithospheric depths.This feature suggests either a possible initiation or relic subduction.Pronounced low velocity anomalies pervade the upper mantle below the Atlantic region and separate the lithospheres of the two regions.We also notice a strong change of the upper mantle velocity structure going from south to north across the Gorringe Bank.This variation in structure could be related to the different evolution in the opening of the central and northern Atlantic oceans.     

Stratigraphic record in the transition from basin floor to continental slope sedimentation in the ancient passive-margin Windermere turbidite system

Well-exposed, vertically dipping, glacially polished outcrops of the Neoproterozoic Windermere Supergroup in the southern Canadian Cordillera include basin-floor deposits of the Upper Kaza Group overlain by slope channel complexes of the Isaac Formation. Within the 2·5 km thick Kaza and Isaac succession is an up to 360 m thick interval composed of diverse deep-water stratal elements including scour and interscour deposits, distributary channels, fine-grained turbidites, terminal splays, mass-transport deposits, erosional and levéed channels and avulsion splays, which collectively were formed during the development of an ancient passive-margin channel-lobe system. The proportion and vertical and lateral arrangement of stratal elements reveal three distinct complexes. The lower complex, consisting mostly of distributary channels and small and large scours, is interpreted to represent the detachment of lobes from an upflow levéed channel, wherein a well-developed channel-lobe transition zone was formed by efficient, siliciclastic flows during a period of sustained transport bypass and limited deposition coincident with the onset of falling relative sea level. The middle, comparatively thicker and more sandstone-rich complex, comprises distributary channel fills, fine-grained turbidites and lesser terminal splays that are interspersed with small scours, capped by a slope levéed channel filled with coarser-grained siliciclastic sediment. The abundance of basin-floor elements suggests negligible separation between the levéed channel and lobe, and therefore a poorly-developed channel-lobe transition zone, resulting from inefficient, siliciclastic-rich depositional flows that became dominant during lowstand and/or ensuing transgression. The stratal makeup of the upper complex resembles the lower detached complex, suggesting a return to efficient flows, and an abrupt change to mixed carbonate–siliciclastic sediments associated with highstand conditions. Accordingly, the stratigraphic architecture and stacking pattern of the Kaza–Isaac interval, which relate to the formation of multiple channel-lobe transition zones, were controlled by temporal changes in sediment supply and flow characteristics during the long-term progradation of the Laurentian continental margin.     

渤海海域埕岛低凸起东部南区新生代断裂系统及油气分布的控制作用

埕岛低凸起东部南区新生代受控于伸展与走滑作用,断裂构造复杂,传统认为中深层北东向与近东西向断层属于同期同沉积断层。针对这一观点及引起的问题,利用钻井和地震资料,运用构造地质理论解析断裂系统。研究区主要发育正断层、走滑正断层两种类型,断开层位有基底—东营组、平原组—东营组、平原组—基底三种情况,现今断裂以东营组与馆陶组之间的区域不整合面为时限划分为两期断裂系统。早期断裂主要切开基底—东下段,属于同沉积断层;晚期断裂主要切开平原组—东营组,可断达基底,其发育受早期断裂制约。北东向与近东西向断层分别属于早晚两期断裂系统,对油气分布各起关键性控制作用:先期基底升降引起的伸展作用形成北东向断层,控制洼槽地貌与深水重力流沉积环境,发育了连片的层状砂质碎屑流;后期郯庐断裂右行走滑派生了近东西向雁列断层,断层面既充当储层上倾方向的遮挡条件,又在东西向挤压时封闭性变差而变成油气垂向运移通道,断层及断层作用控制了圈闭分布与油气聚集的有序性,自东向西,圈闭及油水界面依次升高且充满度变小,呈全充满、欠充满、半充满等状态。断裂系统研究将地质体置于一定的构造应力场中,分析断层组合的空间排列和交切关系以及断层的力学机制和位移特征等,探究时空演化对油气分布的控制作用。断裂系统研究方法在构造作用叠合区具有适宜性,对该区及类似地区的勘探开发具有现实意义。     

济阳坳陷下第三系陡岸沉积模式

济阳坳陷下第三系沉积时构造运动以断陷为主要特征,在其陡岸形成了以近物源、重力流为主的沉积体系,不同的陡岸类型控制了沉积体系的发育与展布。本文在研究济阳坳陷的陡岸沉积特征的基础上归纳提出了断蚀陡岸沉积模式、间歇陡岸沉积模式和持续陡岸沉积模式三种主要类型.     

酒西白垩纪盆地沉积构成及盆地演化动力学分析

酒西白垩纪盆地是酒西含油气盆地的一个单型盆地，发育了冲积扇、水下重力流扇、扇三角洲、湖泊、河流沉积体系。在湖盆演化的不同阶段形成冲积扇—扇三角洲—滨浅湖-砾质辫状河、近岸水下重力流扇。中深湖—扇三角洲和扇三角洲—中浅湖—河流沉积体系组合型式。依据答时界面，将湖盆充填序列划分为3个构造层序，相对应于初始裂陷、扩张裂陷—热衰减沉陷和湖盆萎缩关闭3个演化阶段。地幔热柱的形成和衰减、燕山运动等远程应力作用控制着盆地的演化。     

沉积物重力流流体转化沉积-混合事件层

随着浊流和碎屑流理论体系日臻成熟,重力流的流体转化过程逐渐受到重视,而与其相关联的混合事件层概念也应运而生。混合事件层是单次碎屑流或浊流流体转化中的沉积记录,是多种流变学特征的垂向沉积组合。典型混合事件层沉积序列具有五段式的特征（即纯净砂岩段H₁、条带状砂岩段H₂、黏性碎积岩段H₃、波状层理段H₄、块状泥岩段H₅）,其内部通常存在岩性突变界面。混合事件层发育于粗粒三角洲内部、海底扇和水道与舌状体过渡区、舌状体侧缘、远端及限制性的微型盆地边缘地区,其垂向叠置厚度可达数十米。混合事件层的发现对重力流流体转化、重力流沉积物空间流变学性质研究具有重要意义,同时也推动了油气储层构型和非均质性研究,为进一步寻找深水有利储集砂体提供了新思路。混合层地球物理识别方法的建立及其相关概念在湖泊重力流研究中的灵活应用将是下一步的研究方向。     

碎屑岩沉积构型研究若干进展*

近10年来,国内外的沉积构型研究从之前的以精细表征为主扩展到成因机制分析。文中以同生逆断层控制的冲积扇、可容空间影响下的曲流河点坝、浅水缓坡背景下的三角洲指状砂坝以及大陆斜坡微盆地背景下的重力流沉积等为例介绍相关研究进展。(1)挤压盆地边缘复杂的同生逆断裂构造活动控制了冲积扇的构型要素类型、叠置样式、分布演化及定量规模,表现出有别于构造稳定条件下的冲积扇构型模式;(2)曲流河在可容空间较小的情况下可发育顺流迁移型点坝,表现出特殊的微相类型、分布样式及旋回特征,而随着A/S值的不断增大,曲流河点坝可由鳞片状逐步演变为条带状,最后变为点状;(3)浅水三角洲中可发育类似于河控较深水三角洲中的指状砂坝沉积,其平面形态、微相组合样式、定量规模等特征受气候、沉积物供给、沉积水体等多因素的影响;(4)大陆斜坡微盆地内部可发育重力流水道、朵叶体、块状搬运体等多种构型要素类型,不同构型要素的空间分布样式、定量规模及构型演化模式受复杂地形地貌及构造活动的影响较为明显。综合原型模型分析、沉积物理模拟及数值模拟开展系统的定量化研究,建立定量的、可预测的碎屑岩沉积构型模式,是今后碎屑岩沉积构型研究的发展趋势。     

东濮凹陷黄河南地区下第三系沉积体系与油气评价

为了加速东濮凹陷黄河南地区油气勘探的步伐,本文根据现代沉积体系理论,综合运用地质、测井和地震资料,首次在东濮凹陷黄河南地区下第三系中系统地确定了冲积扇、河流、扇三角洲、三角洲—滑塌浊积扇、滩坝、近岸水下扇和轴向重力流水道等7种类型的沉积体系,并描述了这些沉积体系在地质、测井和地震等方面的沉积特征。结合盆地断裂活动特点,指出充填盆地沉积体系的演化具有明显的阶段性,沉积体系的平面展布具有不对称性,从而建立盆地沉积体系演化模式。同时,结合油气勘探实践指出富集油气的有利相带。     

The Maesan fan delta, Miocene Pohang Basin, SE Korea: architecture and depositional processes of a high-gradient fan-delta-fed slope system

The Maesan fan-delta-fed slope system in the Miocene Pohang Basin occurs between two Gilbert-type fan deltas. Detailed analysis of sedimentary facies and bed geometry reveals that the sequence is represented by 13 sedimentary facies. These facies can be organized into three facies associations, representing distinct depositional environments: alluvial fan (facies association I), steep-faced slope (facies association II), and basin plain (facies association III). Subaerial debris flows and dense, inertia-dominated currents were transformed into subaqueous sediment gravity flows in steep-faced slope environments. Further downslope, these flows were channelized and formed lobate conglomerate and sandstone bodies at the terminal edge of the channels (or chutes). Interchannel and interlobe areas were dominated by homogeneous mudstone and muddy sandstone, deposited by suspension settling of fine-grained materials. Part of the steep-faced slope deposits experienced large-scale slides and slumps. The chutes/channels, lobes and splays on the steep-faced slope of the Maesan system are similar to those in modern subaqueous coarse-grained fan-delta systems.     

水道加朵体型深水扇形成机制与模式:以白云凹陷荔湾3-1地区珠江组为例

深水扇储层的沉积特征是油气勘探开发中的一个热点问题。通过分析珠江口盆地白云凹陷荔湾3-1地区珠江组深水扇的沉积特征,明确了研究区具备深水扇形成的有利条件,受白云凹陷总体构造沉积背景影响,形成了富砂型的深水扇。本区深水扇是多种流体动力学机制共同作用的结果,碎屑流、浊流及底流相互作用与复合,形成了平面上具水道复合朵体的分布特征。在建立深水扇沉积模式的基础上,提出了水道加朵体型深水扇沉积微相划分方案,研究区储层以重力流水道、天然堤及滑塌朵体为主;而水道逐渐由相对近源的宽浅下切型变为远离物源的窄深型,其深度与天然堤宽度呈正比。     

Status and Trends in Research on Deep-Water Gravity Flow Deposits

Deep-water gravity flows are one of the most important sediment transport mechanisms on Earth. After 60 years of study,significant achievements have been made in terms of classification schemes,genetic mechanisms,and depositional models of deep-water gravity flows. The research history of deep-water gravity flows can be divided into five stages: incipience of turbidity current theory; formation of turbidity current theory; development of deep-water gravity flow theory; improvement and perfection of deep-water gravity flow theory; and comprehensive development of deep-water gravity flow theory. Currently,three primary classification schemes based on the sediment support mechanism,the rheology and transportation process,and the integration of sediment support mechanisms,rheology,sedimentary characteristics,and flow state are commonly used.Different types of deep-water gravity flow events form different types of gravity flow deposits. Sediment slump retransportation mainly forms muddy debris flows,sandy debris flows,and surge-like turbidity currents. Resuspension of deposits by storms leads to quasi-steady hyperpycnal turbidity currents(hyperpycnal flows). Sustainable sediment supplies mainly generate muddy debris flows,sandy debris flows,and hyperpycnal flows. Deep-water fans,which are commonly controlled by debris flows and hyperpycnal flows,are triggered by sustainable sediment supply; in contrast,deep-water slope sedimentary deposits consist mainly of debris flows that are triggered by the retransportation of sediment slumps and deep-water fine-grained sedimentary deposits are derived primarily from finegrained hyperpycnal flows that are triggered by the resuspension of storm deposits. Harmonization of classification schemes,transformation between different types of gravity flow deposit,and monitoring and reproduction of the sedimentary processes of deep-water gravity flows as well as a source-to-sink approach to document the evolution and deposition of deep-water gravity flows are the most important research aspects for future studies of deep-water gravity flows study in the future.     

Depositional framework and controls on mixed carbonate-siliciclastic gravity flows: Pennsylvanian-Permian shelf to basin transect, south-western Great Basin, USA

Mixed carbonate-siliciclastic sediment gravity flow deposits of Late Pennsylvanian to Early Permian age are exposed in the Death Valley - Owens Valley region of east-central California. The Mexican Spring unit constitutes the upper part of the Keeler Canyon Formation and is characterized by turbidites, debris flow deposits and megabreccias, all of mixed carbonate-siliciclastic composition. The mixed composition of the Keeler Canyon Formation provides an opportunity to link facies architecture to controls on depositional system development. Depositional relationships indicate that the deposits represent a non-channellized base of slope carbonate apron system with inner, outer and basinal facies associations. These gravity flow deposits are characterized by repeated stacked, small scale (<15 m) coarsening and thickening upward cycles with superimposed medium scale (>100 m) coarsening and thickening upward cycles. Contemporaneous outer shelf and upper slope deposits of the Tippipah Limestone are exposed at Syncline Ridge on the Nevada Test Site. The deposits consist of carbonate buildups directly overlain by cross bedded, quartz-rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf. Detritus was transported to the west, down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the Keeler Canyon Formation. This style of deposition differs from point-sourced siliciclastic submarine fan depositional systems. However, the Keeler Canyon system has lithofacies similar to some sandy siliciclastic turbidite systems, such as the delta-fed submarine ramp facies model, which is a line-sourced, shelf-fed system that is not supply limited. The mixed clastic apron systems of the Keeler Canyon Formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles. Controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply, relative sea level changes and/or tectonic events. Interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system. Relative sea level changes serve two important functions: (1) they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf, and (2) shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles. Therefore, an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level, which is a characteristic of Late Palaeozoic sea level history.     

鄂尔多斯盆地北部侏罗系直罗组沉积特征与演化*

鄂尔多斯盆地东北部发现的多个砂岩型铀矿床均赋存于侏罗系直罗组下段砂岩中。前人对已知铀矿床分布区直罗组的沉积学研究程度相对较高,但对盆地北部直罗组大区域沉积体系展布与演变、物源供给特征等的研究仍较为薄弱。文中在大量钻井资料分析、野外剖面实测等基础上,将盆地北部砂岩型铀矿含矿层段直罗组下段细分为2个亚段。在直罗组中识别出河流和三角洲相沉积,认为直罗组下段下亚段主要发育砾质、砂质辫状河沉积,东北部地区发育辫状河三角洲沉积;直罗组下段上亚段主要发育砂质辫状河和曲流河沉积;直罗组上段则以曲流河沉积为主。结合前人研究工作,认为源岩物质组成、有利沉积相带和气候条件对鄂尔多斯盆地北部砂岩型铀矿的成矿均具有重要控制作用。对盆地北部直罗组沉积特征及其演化的整体认识,可为该区砂岩型铀矿床的进一步勘查提供重要的沉积学依据。