New insight into the evolution of large-volume turbidity currents: comparison of turbidite shape and previous modelling results

The Marnoso Arenacea Formation provides the most extensive correlation of individual flow deposits (beds) yet documented in an ancient turbidite system. These correlations provide unusually detailed constraints on bed shape, which is used to deduce flow evolution and assess the validity of numerical and laboratory models. Bed volumes have an approximately log‐normal frequency distribution; a small number of flows dominated sediment supply to this non‐channelized basin plain. Turbidite sandstone within small‐volume (<0·7 km³) beds thins downflow in an approximately exponential fashion. This shape is a property of spatially depletive flows, and has been reproduced by previous mathematical models and laboratory experiments. Sandstone intervals in larger‐volume (0·7–7 km³) beds have a broad thickness maximum in their proximal part. Grain‐size trends within this broad thickness maximum indicate spatially near‐uniform flow for distances of ∼30 km, although the flow was temporally unsteady. Previous mathematical models and laboratory experiments have not reproduced this type of deposit shape. This may be because models fail to simulate the way in which near bed sediment concentration tends towards a constant value (saturates) in powerful flows. Alternatively, the discrepancy may be the result of relatively high ratios of flow thickness and sediment settling velocity in submarine flows, together with very gradual changes in sea‐floor gradient. Intra‐bed erosion, temporally varying discharge, and reworking of suspension fallout as bedload could also help to explain the discrepancy in deposit shape. Most large‐volume beds contain an internal erosion surface underlain by inversely graded sandstone, recording waxing and waning flow. It has been inferred previously that these characteristics are diagnostic of turbidites generated by hyperpycnal flood discharge. These turbidites are too voluminous to have been formed by hyperpycnal flows, unless such flows are capable of eroding cubic kilometres of sea‐floor sediment. It is more likely that these flows originated from submarine slope failure. Two beds comprise multiple sandstone intervals separated only by turbidite mudstone. These features suggest that the submarine slope failures occurred as either a waxing and waning event, or in a number of stages.     

Turbidite Reservoirs in the Dongying Sag,Bohai Bay Basin:Interactions between Carbonate Cementation and Hydrocarbon Charge

Turbidite sandstones have become increasingly significant in hydrocarbon exploration.Carbonate cementation occur commonly in turbidite reservoirs developing within the Paleocene lacustrine basins,Northeastern China.This study utilizes core data,thin section data and production data to investigate the interaction between the carbonate cementation and hydrocarbon charge within turbidite reservoirs in the Niuzhuang Sub-sag of the Dongying Sag,Bohai Bay Basin, East China.The results reveal that the carbonate cementation is mainly developed at the top and bottom of the turbidite sandbodies,and even forms carbonate cement shells.Three stages of hydrocarbon accumulation are identified based on fluid inclusion analysis: stage I(27.5–24.6 Ma),stage II(14.0–5.0 Ma),and stage III(5.0–0 Ma).The interaction between the carbonate cementation and hydrocarbon charge has significant controls on the formation of a turbidite reservoir.The temporal relations and intensity relations between the two factors should be considered significantly.Moreover,hydrocarbon charge during the early stage can inhibit the carbonate cementation, favoring the hydrocarbon accumulation in turbidite reservoirs.Many deep-lacustrine turbidite sandbodies surrounded by source rocks with abnormal high pressure,are also favorable for hydrocarbon accumulation.These results suggest that some deeply buried turbidite sandbodies with similar geological settings have high potential for hydrocarbon exploration.     

利用能量半衰属性识别涠西南凹陷砂泥薄互层组合中浊积砂体

涠西南凹陷是我国南海北部湾盆地重要的油气聚集地,浊积砂体是其重要的储量贡献地质体,显示了巨大的油气勘探潜力.随着勘探开发力度的不断推进,在砂、泥岩地震特征叠置严重的背景下,常规的地球物理技术手段难以识别真正的浊积储层.为此,基于浊积砂体强振幅反射形成的地质背景,剖析了非储层形成异常强振幅的两个主要成因:砂、泥岩薄互层组...     

巴里坤红井子地区中石炭统阿什喀腊组浊积岩的发现及其意义

阿什喀腊组浊积岩野外识别标志明显，如层面构造的底部冲刷面、工具模和层内构造的粒序层理、包卷层理、同生叠瓦状泥砾构造、水平层理、沙纹交错层理等鉴别特征清楚；生物丰富，尤其深海一半深海遗迹化石组合十分明显；浊积岩的剖面结构类型虽然复杂多样，但总体分析，以远源浊积岩为主。鲍马序列清晰，一般为小型。该浊积岩的发现对红井子地区和博格达地区的大地构造分析具有重要意义。     

分叉波痕在广西上泥盆统钙质浊积岩中的发现及意义

小型不对称分叉波痕(简称分叉波痕)发现于桂林杨堤剖面上泥盆统弗拉斯阶linguiformis牙形石带，桂林碳酸盐台地东南缘斜坡相钙质浊积岩鲍马序列C段。波长7．5～8．0cm，波高0．5～0．8cm，波痕指数15～10；向流面长5．0～7．0cm，背流面长2．5～4．0cm，波痕对称指数2．0～1．8；背流面向东倾；波脊较圆滑、缓曲，且具明显的分叉现象。是浊流流速减缓，密度流转化为牵引流后在低流态条件下，并叠加有推进型风暴浪作用形成的复合成因波痕。根据该波痕和寄主地层特征，推断含分叉波痕的阳朔碳酸盐盆地的最大水深约100m，极限水深小于200m。这一估计值应能代表广西乃至华南板块泥盆纪广泛发育的含牙形石动物群和钙质浊积岩碳酸盐沉积盆地的定量水深，可能也代表了弗拉斯阶一法门阶之交受集群绝灭事件重创的浅水海相生物与基本未受影响的深水海相生物的水深分界线。     

西藏山南地区琼结南部上三叠统郎杰学群的沉积学岩相特征与浊积扇模式

西藏山南地区琼结南部上三叠统郎杰学群由轻微变质的泥质岩和碎屑岩组成,其中,砂岩主要为(岩屑)长石(杂)砂岩和(长石)岩屑(杂)砂岩,碎屑的分选和磨圆度较差,成分和结构成熟度较低,经历了压溶、交代及胶结等成岩作用;发育与浊积岩相关的层面、层理构造及沉积旋回发育。研究表明,按照宋热组、江雄组和姐德秀组地层层序,郎杰学群岩相总体呈现外—中扇砂板岩相向中—内扇砂板岩含砾岩相演化,向上变粗变浅,中源和近源/分支水道亚相增加;以姐德秀组下段中部为界可分为A和B两个层序,它们分属浊积扇演化的两个不同阶段;推测研究区及邻近地区可能存在一个较大规模向南迁移进积的舌状浊积扇。     

Trapping of sustained turbidity currents by intraslope minibasins

Depositional turbidity currents have filled many intraslope minibasins with sediment creating targets for petroleum exploration. The dynamics of sustained turbidity currents and their depositional characteristics are investigated in a scaled physical model of a minibasin. Each turbidity current deposited a downstream thinning wedge of sediment near the inlet. Farther downstream the turbidity current was ponded by a barrier. The ponded part of the turbidity current was separated from the sediment‐free water above by a relatively sharp, horizontal settling interface indicating highly Froude‐subcritical flow. The very slow moving flow within the ponded zone created conditions for the passive rainout of suspended sediment onto the bed. In the lower part of the ponded zone, the concentration and mean grain‐size of the sediment in suspension tended to be relatively uniform in both the vertical and streamwise directions. As a result, the deposit emplaced in the ponded zone showed only a weak tendency toward downstream fining and was passively draped over the bed in such a way that irregularities in the inerodible bed were accurately reflected. The discharge of suspended sediment overflowing the downstream end of the minibasin was significantly less than the inflow discharge, resulting in basin sediment trapping efficiencies >95%. A simple model is developed to predict the trapping of sediment within the basin based on the relative magnitudes of the input discharge of turbid water and the detrainment discharge of water across the settling interface. This model shows a limiting case in which an intraslope basin captures 100% of the sediment from a ponded turbidity current, even through a succession of sustained flow events, until sediment deposition raises the settling interface above the downstream lip of the minibasin. This same process defines one of the mechanisms for minibasin filling in nature, and, when this mechanism is operative, the trap efficiency of sediment can be expected to be high until the minibasin is substantially filled with sediment.     

东非鲁武马盆地海底水道-朵体体系粗粒浊流沉积物波特征及主控因素

海底水道-朵体体系内粗粒沉积物波的研究可以深化浊流搬运过程的认识。利用先进的地球物理成像技术,通过地震地貌分析,对东非鲁武马盆地海底水道-朵体体系内这种后期易被改造的特殊沉积体进行识别和解释,结合粗粒沉积物波的形态、尺度、移动方式、厚度变化、平面分布等特征,探讨其成因和影响因素。鲁武马盆地近海底水道-朵体体系内的粗粒浊流沉积物波具有多变的地貌和逆行砂丘的底形。水道内粗粒沉积物波规模较小,分布范围局限;水道-朵体过渡带的粗粒沉积物波规模大,波高约45~110 m,波长可达一千余米,总体规模大于其他地区已识别出的粗粒深水沉积物波。构造活动、超临界流产生的水跃作用、地形地貌的变化以及底流作用是鲁武马盆地粗粒浊流沉积物波形成的主控因素。     

南盘江印支期前陆盆地中上三叠统深水浊积岩沉积特征

南盘江印支期前陆盆地,于中晚三叠世发育了一套厚度较大,分布广泛的陆屑浊积岩。按照沉积结构,构造和岩性特征,可划分出四种浊积岩相类型。其沉积模式为海底扇,并具三个幕次的沉积活动,其物源来自盆地南端,即为印支板块与扬子板块碰撞形成的前陆褶冲带。经研究,百蓬期（Ｔ２１）浊积盆地位于ＣＣＤ面之下,而河口期（Ｔ２２）和法郎期（Ｔ１３）烛积盆地位于ＣＣＤ面之上。