Possible retrogressive flow slide deposits from the Kongsfjord Formation: a Precambrian submarine fan, Finnmark, N. Norway

Three thinning and fining-upward turbidite sequences are described from the Precambrian Kongsfjord Formation, a 3.5 km thick flysch succession. Their thicknesses range between about 2 and 5 m. They show a progressive upward decrease in bed thickness, bulk mean grain size and the ratio of the higher to lower energy division of the Bouma sequence. In one case, however, there is an initial upward bed thickness and grain size increase, with an increase in the proportion of the higher energy division. The absence of structureless mud of the Bouma E division and the presence of wavy interfaces between beds, together with similar palaeo-currents within each sequence suggest that these sequences resulted from related depositional events. These sequences are interpreted as the deposits of retrogressive flow slides, as an alternative to the classic mechanism of channel fill after abandonment.     

Flow, sediment transport and bedform dynamics over the transition from dunes to upper-stage plane beds: implications for the formation of planar laminae

Preliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms^-1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near-bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low-relief bedwaves on upper-stage plane beds are ubiquitous and give rise to laterally extensive, mm-thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of the bedwaves.     

Non-uniform across-shelf variations in thickness, grain size, and frequency of turbidites in a transgressive outer-shelf, the Middle Pleistocene Kakinokidai Formation, Boso Peninsula, Japan

Across-shelf variations in thickness, grain size, and frequency of sandstone beds in a transgressive outer-shelf succession were investigated from the Middle Pleistocene (ca. 0.7 Ma) Kakinokidai Formation on the Boso Peninsula, Japan. The transgressive deposits are generally muddy and contain slumps and slump scars. The intercalated sandstone beds are interpreted to have been formed from turbidity currents as a response to erosion and resuspension of sandridge-complex deposits in the southwestern upslope area during storm events. Mapping of volcanic ash beds and a transgressive surface in the base of the formation permits detailed bed-by-bed correlation of the outer-shelf sandstone beds. Although, overall, thickness, grain size, and frequency of sandstone beds decrease in the downslope direction, some sandstone beds locally thin out and coarsen in association with slump scars in the surrounding muddy deposits. These sandstone beds subsequently thicken and fine, and finally thin out in the farther downslope area. In addition to the local thinning of sandstone beds, the frequency of sandstone beds first decreases and then increases in the farther offshore direction. From this evidence, we concluded that these non-uniform patterns of across-outer-shelf variations in thickness, grain size, and frequency of sandstone beds were caused by the local increases in flow speeds and subsequent expansion and reduced speeds of turbidity currents, along with a local increase in the seafloor gradient that was induced by the development of slump scars in the transgressive outer-shelf floor. These physiographic features in the outer shelf are interpreted not to have permitted monotonous downslope thinning and fining of sandstone beds, compared with the bed-shape models of depletive turbidity currents and with the proximality trend of shelf sandstones from modern and ancient highstand-stage shelf systems.     

辽西地区义县组底部沉积环境粒度分析与对比

对分布于金岭寺-羊山盆地与阜新-义县盆地义县组底部陆家屯层、老公沟层沉积环境的粒度分析与对比,不仅揭示了两个盆地初始期冲积扇-河流相沉积环境,而且表明这两个沉积层可以进行对比.由于受到火山作用的影响,两个沉积层体现出粒度分析结果的不完全一致性.     

Criteria for the instability of upper-stage plane beds

Calculations of the critical dimensionless bed stresses that obtain when upper-stage plane beds should revert to ripple and dune bed forms are presented. Strong support is given to the Bagnold ‘universal’ plane-bed instability criterion and to a modified criterion suggested by Allen over a wide range of solids grain size. A reinterpretation of the mechanism of plane bed instability is based upon the extent to which significant grain concentrations in plane bed flows increase apparent fluid viscosity and decrease turbulence production over potential bed defects, thereby preventing ripple or dune propagation and growth.     

Hybrid event beds dominated by transitional‐flow facies: character,distribution and significance in the Maastrichtian Springar Formation,north‐west Vøring Basin,Norwegian Sea

Hybrid event beds comprising clay‐poor and clay‐rich sandstone are abundant in Maastrichtian‐aged sandstones of the Springar Formation in the north‐west Vøring Basin, Norwegian Sea. This study focuses on an interval, informally referred to as the Lower Sandstone, which has been penetrated in five wells that are distributed along a 140 km downstream transect. Systematic variations in bed style within this stratigraphic interval are used to infer variation in flow behaviour in relatively proximal and distal settings, although individual beds were not correlated. The Lower Sandstone shows an overall reduction in total thickness, bed amalgamation, sand to mud ratio and grain size in distal wells. Turbidites dominated by clay‐poor sandstone are at their most common in relatively proximal wells, whereas hybrid event beds are at their most common in distal wells. Hybrid event beds typically comprise a basal clay‐poor sandstone (non‐stratified or stratified) overlain by banded sandstone, with clay‐rich non‐stratified sandstone at the bed top. The dominant type of clay‐poor sandstone at the base of these beds varies spatially; non‐stratified sandstone is thickest and most common proximally, whereas stratified sandstone becomes dominant in distal wells. Stratified and banded sandstone record progressive deposition of the hybrid event bed. Thus, the facies succession within hybrid event beds records the longitudinal heterogeneity of flow behaviour within the depositional boundary layer; this layer changed from non‐cohesive at the front, through a region of transitional behaviour (fluctuating non‐cohesive and cohesive flow), to cohesive behaviour at the rear. Spatial variation in the dominant type of clay‐poor sandstone at the bed base suggests that the front of the flow remained non‐cohesive, and evolved from high‐concentration and turbulence‐suppressed to increasingly turbulent flow; this is thought to occur in response to deposition and declining sediment fallout. This research may be applicable to other hybrid event bed prone systems, and emphasizes the dynamic nature of hybrid flows.     

四川名山——丹棱地区青衣江流域的砾石层特征及水系演化

对四川名山、邛崃砾石层的砾石成分、砾度和砾向进行统计,分析了该砾石层的物质来源、古流向及阶地特征;并将其与丹棱-思蒙的砾石层进行对比,认为两砾石层是青衣江流域不同地质历史时期的产物;据古流向的分布、砾石层形成年代以及构造运动特征,讨论了青衣江水系总体上由北往南演化变迁的历史过程。     

On how thin submarine flows transported large volumes of sand for hundreds of kilometres across a flat basin plain without eroding the sea floor

Submarine gravity currents, especially long run‐out flows that reach the deep ocean, are exceptionally difficult to monitor in action, hence there is a need to reconstruct how these flows behave from their deposits. This study mapped five individual flow deposits (beds) across the Agadir Basin, offshore north‐west Africa. This is the only data set where bed shape, internal distribution of lithofacies, changes in grain size and sea floor gradient, bed volumes, flow thickness and depth of erosion into underlying hemipelagic mud are known for individual beds. Some flows were 30 to 120 m thick. However, flows with the highest fraction of sand were less than 5 to 14 m thick. Sand was most likely to be carried in the lower 5 to 7 m of these flows. Despite being relatively thin, one flow was capable of transporting very large volumes of sediment (ca 200 km³) for large distances across very flat sea floor. These observations show that these relatively thin flows could travel quickly enough on very low gradients (0·02° to 0·05°) to suspend sand several metres to tens of metres above the sea floor, and maintain those speeds for up to 250 km across the basin. Near uniform hemipelagic mud interval thickness between beds, and coccolith assemblages in the mud caps of beds, suggest that the flows did not erode significantly into the underlying sea floor mud. Simple calculations imply that some flows, especially in the proximal part of the basin, were powerful enough to have eroded hemipelagic mud if it was exposed to the flow. This suggests that the flows were depositional from the moment they arrived at a basin plain location, and that deposition shielded the underlying hemipelagic mud from erosion. Reproducing the field observations outlined in this exceptionally detailed field data set is a challenge for future experimental and numerical models.     

On the frequency distribution of turbidite thickness

The frequency distribution of turbidite thickness records information on flow hydrodynamics, initial sediment volumes and source migration and is an important component of petroleum reservoir models. However, the nature of this thickness distribution is currently uncertain, with log‐normal or negative‐exponential frequency distributions and power‐law cumulative frequency distributions having been proposed by different authors. A detailed analysis of the Miocene Marnoso Arenacea Formation of the Italian Apennines shows that turbidite bed thickness and sand‐interval thickness within each bed have a frequency distribution comprising the sum of a series of log‐normal frequency distributions. These strata were deposited predominantly in a basin‐plain setting, and bed amalgamation is relatively rare. Beds or sand intervals truncated by erosion were excluded from this analysis. Each log‐normal frequency distribution characterizes bed or sand‐interval thickness for a given basal grain‐size or basal Bouma division. Measurements from the Silurian Aberystwyth Grits in Wales, the Cretaceous Great Valley Sequence in California and the Permian Karoo Basin in South Africa show that this conclusion holds for sequences of disparate age and variable location. The median thickness of these log‐normal distributions is positively correlated with basal grain‐size. The power‐law exponent relating the basal grain‐size and median thickness is different for turbidites with a basal A or B division and those with only C, D and E divisions. These two types of turbidite have been termed ‘thin bedded’ and ‘thick bedded’ by previous workers. A change in the power‐law exponent is proposed to be related to: (i) a transition from viscous to inertial settling of sediment grains; and (ii) hindered settling at high sediment concentrations. The bimodal thickness distribution of ‘thin‐bedded’ and ‘thick‐bedded’ turbidites noted by previous workers is explained as the result of a change in the power‐law exponent. This analysis supports the view that A and B divisions were deposited from high‐concentration flow components and that distinct grain‐size modes undergo different depositional processes. Summation of log‐normal frequency distributions for thin‐ and thick‐bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power‐law trend. Thus, the occurrence of both log‐normal and segmented power‐law frequency distributions can be explained in a holistic fashion. Power‐law frequency distributions of turbidite thickness have previously been linked to power‐law distributions of earthquake magnitude or volumes of submarine slope failure. The log‐normal distribution for a given grain‐size class observed in this study suggests an alternative view, that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters, in addition to the settling velocity of the grain‐sizes present.     

Aerodynamic grain‐size distribution of blown sand

Aeolian sand entrainment, saltation and deposition are important and closely related near surface processes. Determining how grains are sorted by wind requires a detailed understanding of how aerodynamic sand transport processes vary within the saltating layer with height above the bed. Grain‐size distribution of sand throughout the saltation layer and, in particular, how the associated flux of different grain size changes with variation in wind velocity, remain unclear. In the present study, a blowdown wind tunnel with a 50 cm thick boundary layer was used to investigate saltating sand grains by analyzing the weight percentage and transport flux of different grain‐size fractions and the mean grain size at different wind velocities. It was found that mean grain size decreases with height above the sand bed before undergoing a reversal. The height of the reversal point ranges from 4 to 40 cm, and increases with wind velocity following a non‐linear relationship. The content of the finer fractions (very fine and fine sand) initially increases above the sand bed and then decreases slightly with height, whereas that of the coarser fractions (medium and coarse sand) exhibits the opposite trend. The content of coarser grains and the mean grain size of sand in the saltation layer increase with wind velocity, indicating erosional selectivity with respect to grains in multi‐sized sand beds; but this size selectivity decreases with increasing wind velocity. The vertical mass flux structure of fine sand and very fine sand does not obey a general exponential decay pattern under strong wind conditions; and the coarser the sand grain, the greater the decrease rate of their transport mass with height. The results of these experiments suggest that the grain‐size distribution of a saltating sand cloud is governed by both wind velocity and height within the near‐surface boundary layer.     

Slurry-flow deposits in the Britannia Formation (Lower Cretaceous), North Sea: a new perspective on the turbidity current and debris flow problem

The Lower Cretaceous Britannia Formation (North Sea) includes an assemblage of sandstone beds interpreted here to be the deposits of turbidity currents, debris flows and a spectrum of intermediate flow types termed slurry flows. The term ‘slurry flow’ is used here to refer to watery flows transitional between turbidity currents, in which particles are supported primarily by flow turbulence, and debris flows, in which particles are supported by flow strength. Thick, clean, dish‐structured sandstones and associated thin‐bedded sandstones showing Bouma T_b–e divisions were deposited by high‐ and low‐density turbidity currents respectively. Debris flow deposits are marked by deformed, intraformational mudstone and sandstone masses suspended within a sand‐rich mudstone matrix. Most Britannia slurry‐flow deposits contain 10–35% detrital mud matrix and are grain supported. Individual beds vary in thickness from a few centimetres to over 30 m. Seven sedimentary structure division types are recognized in slurry‐flow beds: (M₁) current structured and massive divisions; (M₂) banded units; (M₃) wispy laminated sandstone; (M₄) dish‐structured divisions; (M₅) fine‐grained, microbanded to flat‐laminated units; (M₆) foundered and mixed layers that were originally laminated to microbanded; and (M₇) vertically water‐escape structured divisions. Water‐escape structures are abundant in slurry‐flow deposits, including a variety of vertical to subvertical pipe‐ and sheet‐like fluid‐escape conduits, dish structures and load structures. Structuring of Britannia slurry‐flow beds suggests that most flows began deposition as turbidity currents: fully turbulent flows characterized by turbulent grain suspension and, commonly, bed‐load transport and deposition (M₁). Mud was apparently transported largely as hydrodynamically silt‐ to sand‐sized grains. As the flows waned, both mud and mineral grains settled, increasing near‐bed grain concentration and flow density. Low‐density mud grains settling into the denser near‐bed layers were trapped because of their reduced settling velocities, whereas denser quartz and feldspar continued settling to the bed. The result of this kinetic sieving was an increasing mud content and particle concentration in the near‐bed layers. Disaggregation of mud grains in the near‐bed zone as a result of intense shear and abrasion against rigid mineral grains caused a rapid increase in effective clay surface area and, hence, near‐bed cohesion, shear resistance and viscosity. Eventually, turbulence was suppressed in a layer immediately adjacent to the bed, which was transformed into a cohesion‐dominated viscous sublayer. The banding and lamination in M₂ are thought to reflect the formation, evolution and deposition of such cohesion‐dominated sublayers. More rapid fallout from suspension in less muddy flows resulted in the development of thin, short‐lived viscous sublayers to form wispy laminated divisions (M₃) and, in the least muddy flows with the highest suspended‐load fallout rates, direct suspension sedimentation formed dish‐structured M₄ divisions. Markov chain analysis indicates that these divisions are stacked to form a range of bed types: (I) dish‐structured beds; (II) dish‐structured and wispy laminated beds; (III) banded, wispy laminated and/or dish‐structured beds; (IV) predominantly banded beds; and (V) thickly banded and mixed slurried beds. These different bed types form mainly in response to the varying mud contents of the depositing flows and the influence of mud on suspended‐load fallout rates. The Britannia sandstones provide a remarkable and perhaps unique window on the mechanics of sediment‐gravity flows transitional between turbidity currents and debris flows and the textures and structuring of their deposits.     

The spatial and temporal distribution of grain‐size breaks in turbidites

Grain‐size breaks are surfaces where abrupt changes in grain size occur vertically within deposits. Grain‐size breaks are common features in turbidites around the world, including ancient and modern systems. Despite their widespread occurrence, grain‐size breaks have been regarded as exceptional, and not included within idealized models of turbidity current deposition. This study uses ca 100 shallow sediment cores, from the Moroccan Turbidite System, to map out five turbidite beds for distances in excess of 2000 km. The vertical and spatial distributions of grain‐size breaks within these beds are examined. Five different types of grain‐size break are found: Type I – in proximal areas between coarse sand and finer grained structureless sand; Type II – in proximal areas between inversely graded sand overlain by finer sand; Type III – in proximal areas between sand overlain by ripple cross‐laminated finer sand; Type IV – throughout the system between clean sand and mud; and Type V – in distal areas between mud‐rich (debrite) sand and mud. This article interprets Types I and V as being generated by sharp vertical concentration boundaries, controlled by sediment and clay concentrations within the flows, whilst Types II and III are interpreted as products of spatial/temporal fluctuations in flow capacity. Type IV are interpreted as the product of fluid mud layers, which hinder the settling of non‐cohesive grains and bypasses them down slope. Decelerating suspensions with sufficient clay will always form cohesive layers near to bed, promoting the generation of Type IV grain‐size breaks. This may explain why Type IV grain‐size breaks are widespread in all five turbidites examined and are commonplace within turbidite sequences studied elsewhere. Therefore, Type IV grain‐size breaks should be understood as the norm, not the exception, and regarded as a typical feature within turbidite beds.     

Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, western Norway

Sedimentary successions in small coastal lakes situated from 0 to 11 m above the 7000 year BP shoreline along the western coast of Norway, contain a distinctive deposit, very different from the sediments above and below. The deposit is interpreted to be the result of a tsunami inundating the coastal lakes. An erosional unconformity underlies the tsunami facies and is traced throughout the basins, with most erosion found at the seaward portion of the lakes. The lowermost tsunami facies is a graded or massive sand that locally contains marine fossils. The sand thins and decreases in grain size in a landward direction. Above follows coarse organic detritus with rip-up clasts, here termed ‘organic conglomerate’, and finer organic detritus. The tsunami unit generally fines and thins upwards. The higher basins (6–11 m above the 7000 year shoreline) show one sand bed, whereas basins closer to the sea level 7000 years ago, may show several sand beds separated by organic detritus. These alternations in the lower basins may reflect repeated waves of sea water entering the lakes. In basins that were some few metres below sea level at 7000 years BP, the tsunami deposit is more minerogenic and commonly present as graded sand beds, but also in some of these shallow marine basins organic-rich facies occur between the sand beds. The total thickness of the tsunami deposit is 20–100 cm in most studied sites. An erosional and depositional model of the tsunami facies is developed.     

Markovian models in stratigraphic analysis

A stratigraphic section may be divided into lithologic units which in turn may be divided into beds. This paper gives a mathematical formulation of stratigraphic sections that takes these two levels into account and uses bed properties to yield the thickness and number of beds in lithologic units. The model is a semiMarkov chain in which the succession of lithologic bed types forms a Markov chain and is an independent random variable. The model is tested against stratigraphic data obtained from micrologs. There is close agreement between the observed and calculated thicknesses of lithologic units. Tests for the degree of agreement between observed and calculated numbers of beds in lithologic units are hampered by inability to observe thin beds on micrologs. Some implications of this limitation to stratigraphic analysis are noted.     

试论锗在煤层中分布的两种类型

关于锗在煤层中的分布已有较多研究,但各家见解殊多矛盾,目前尚未见有将锗在煤层中分布的复杂现象,统一于比较合理的、系统的解释中;这是需要不断地探索、总结以求解决的问题。本文试图根据笔者工作地区的资料,并参考其它成果,就锗在煤层中分布的某些特点提出几点看法,与同志们讨论。     

Bed-thickness and grain size of turbidites

In measured sequences of limestone- and greywacke-turbidites the bed-thickness is found to vary proportionally with the fall velocity of the maximum grain size, found at the base of the bed. A simple theoretical model, based on the decay of isotropic turbulence, suggests that bed-thickness should be a function not only of this fall velocity, but also of bottom slope, flow depth and the concentration and grain-size distribution of the sediment in the turbidity current. The field data do show some influence of these additional factors. Nevertheless, for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values. Thus, grain size remains the primary variable and the modal regression curve of bed-thickness on maximum grain size is well defined and resembles a fall-velocity curve. Relatively steep basin floors near to source can, theoretically, cause these modal regressions for distal and very proximal parts of a turbidite to diverge, introducing a crudely parabolic appearance in the form of the total regression curve. The form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds. Such beds do occur. They are different from the thin, but relatively fine, proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee-forming processes.     

Anatomy of turbidites and linked debrites based on long distance (120 × 30 km) bed correlation, Marnoso Arenacea Formation, Northern Apennines, Italy

Much of our understanding of submarine sediment‐laden density flows that transport very large volumes (ca 1 to 100 km³) of sediment into the deep ocean comes from careful analysis of their deposits. Direct monitoring of these destructive and relatively inaccessible and infrequent flows is problematic. In order to understand how submarine sediment‐laden density flows evolve in space and time, lateral changes within individual flow deposits need to be documented. The geometry of beds and lithofacies intervals can be used to test existing depositional models and to assess the validity of experimental and numerical modelling of submarine flow events. This study of the Miocene Marnoso Arenacea Formation (Italy) provides the most extensive correlation of individual turbidity current and submarine debris flow deposits yet achieved in any ancient sequence. One hundred and nine sections were logged through a ca 30 m thick interval of time‐equivalent strata, between the Contessa Mega Bed and an overlying ‘columbine’ marker bed. Correlations extend for 120 km along the axis of the foreland basin, in a direction parallel to flow, and for 30 km across the foredeep outcrop. As a result of post‐depositional thrust faulting and shortening, this represents an across‐flow distance of over 60 km at the time of deposition. The correlation of beds containing thick (> 40 cm) sandstone intervals are documented. Almost all thick beds extend across the entire outcrop area, most becoming thinly bedded (< 40 cm) in distal sections. Palaeocurrent directions for flow deposits are sub‐parallel and indicate confinement by the lateral margins of the elongate foredeep. Flows were able to traverse the basin in opposing directions, suggesting a basin plain with a very low gradient. Small fractional changes in stratal thickness define several depocentres on either side of the Verghereto (high) area. The extensive bed continuity and limited evidence for flow defection suggest that intrabasinal bathymetric relief was subtle, substantially less than the thickness of flows. Thick beds contain two distinct types of sandstone. Ungraded mud‐rich sandstone intervals record evidence of en masse (debrite) deposition. Graded mud‐poor sandstone intervals are inferred to result from progressive grain‐by‐grain (turbidite) deposition. Clast‐rich muddy sandstone intervals pinch‐out abruptly in downflow and crossflow directions, in a fashion consistent with en masse (debrite) deposition. The tapered shape of mud‐poor sandstone intervals is consistent with an origin through progressive grain‐by‐grain (turbidite) deposition. Most correlated beds comprise both turbidite and debrite sandstone intervals. Intrabed transitions from exclusive turbidite sandstone, to turbidite sandstone overlain by debrite sandstone, are common in the downflow and crossflow directions. This spatial arrangement suggests either: (i) bypass of an initial debris flow past proximal sections, (ii) localized input of debris flows away from available sections, or (iii) generation of debris flows by transformation of turbidity currents on the basin plain because of seafloor erosion and/or abrupt flow deceleration. A single submarine flow event can comprise multiple flow phases and deposit a bed with complex lateral changes between mud‐rich and mud‐poor sandstone.     

Bedform climbing in theory and nature

Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.     

断陷湖盆洼陷带重力流沉积特征与模式:以南堡凹陷东部东营组为例

利用岩心、粒度、测井信息和重力流沉积理论,系统研究了南堡凹陷东部洼陷带东营组重力流沉积特征和沉积模式。该区重力流沉积砂岩常夹于灰色、灰黑色泥岩中,砂岩相发育,其中正递变层理(含砾)中-细砂岩相(S-3)、粉砂岩相(S-4)和块状层理中-细砂岩相(S-2)发育层数最多,块状层理含砾砂岩相(S-1)次之;S-2沉积厚度最大,S-1和S-3次之。按支撑和沉积机制,将本区重力流分为浊流、砂质碎屑流、颗粒流和液化流,其中砂质碎屑流以基质支撑、冻结块状沉积为特征。不同重力流发育程度有明显差异。从砂岩层数看,浊流最多,砂质碎屑流次之,颗粒流和液化流最少;从单期沉积厚度看,砂质碎屑流最大,平均为1.17m,浊流沉积最小,仅平均为0.25m。为了回避取心的局限性、弱化重力流成因,突出具有油气储集意义的砂层概念,开展了测井岩性解释,结果表明该区重力流沉积为细砂岩或粉砂岩,单层平均厚度2.94m,最大厚度可达9.5m,其中单井中厚度在3m以上的砂体可达22层、累积达107.5m。本区重力流沉积为滑塌成因,除了(扇)三角洲前缘斜坡的自然滑塌外,断层(地震)活动或间歇式火山喷发是其关键的触发机制;断层活动除了提供滑塌的动力外,还影响着其堆积场所和沉积的结构。