Depositional elements associated with a basin floor channel-levee system: case study from the Gulf of Mexico

The seismic geomorphology and seismic stratigraphy of a deep-marine channel-levee system is described. A moderate to high-sinuosity channel trending southeastward across the northeastern Gulf of Mexico basin floor, and associated depositional elements are well imaged using conventional 3D multi-channel seismic reflection data. Depositional elements described include channels, associated levees, a channel belt, avulsion channels, levee crevasses, frontal splays, sediment waves, and mass transport complexes. Distinguishing morphologic and stratigraphic characteristics of each depositional element are discussed. These deposits are presumed to be associated with repeated deep-marine turbidity flows and other mass transport processes.     

Fine-grained Pleistocene deepwater turbidite channel system on the slope of Qiongdongnan Basin, northern South China Sea

Recently, as oil exploitation has become focused on deepwater slope areas, more multi-channel high resolution 2D and 3D seismic data were acquired in the deepwater part of the Qiongdongnan Basin, northern South China Sea. Based on 3D seismic data and coherence time slice, RMS and 3D visualization, a series of deepwater channels were recognized on the slope that probably developed in the late Quaternary period. These channels trend SW–NE to W–E and show bifurcations, levees, meander loops and avulsions. High Amplitude Reflections (HARs), typical for channel–levee complexes, are of only minor importance and were observed in one of the channel systems. Most of the detected channels are characterized by low-amplitude reflections, and so are different from the typical coarse-grained turbidite channels that had been discovered worldwide. The absence of well data in the study area made it difficult to determine the age and lithology of these channels. Using a neighboring drill hole and published data about such depositional systems worldwide, the lithology of these channels is likely to be dominated by mudstones with interbedded thin sandstones. These channels are formed by turbidity currents originated from the little scale mountain river of mid-Vietnam in SW direction and were probably accompanied by a relative sea level drop in the last glacial age. These channels discovered on the northern South China Sea slope are likely to be fine-grained, mud-dominant and low N:G deposits in a deepwater paleogeographic setting.     

Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea

High-resolution multichannel 2-D and 3-D seismic data, primarily from upper fan reaches of near-seafloor channel-levee systems on the Niger Delta slope and in the Arabian Sea, reveal a high level of detail and architectural complexity. Several architectural elements are common to each system examined in this study. They include inner levees, outer levees, erosional fairways, channel-axis deposits, rotational slumps blocks, and mass transport deposits. Although the scale of individual systems varies significantly, similarities in first-order architectural elements and their configurations suggest that common depositional processes are involved regardless of scale differences.Most of the channel-levee systems examined in this study are characterized by a basal erosional fairway that is bordered by outer levees of varying thickness. Together these elements define the base and margins of the channel-belt, where channel-axis deposits and inner levees are the dominant architectural elements. Vertical, sub-vertical, and lateral stacking patterns of sinuous and/or meandering channels create seismic facies that range from narrow to wide zones of high amplitude reflections (HARs) with chaotic to continuous and shingled to horizontal reflections. Some HARs appear as isolated or stacked asymmetric to symmetric u- and v-shaped reflections, referred to here as channel-forms. Channel-belts evolve within the confines of the scalloped erosional fairway walls (flanked by outer levee), and are similar in morphology to meander-belts in fluvial systems, but commonly have a greater component of vertical aggradation. Detailed study of one particular channel-levee system on the Niger Delta slope shows a period of incision followed by three distinct phases of channel development during its aggradational history. Each fill phase corresponds to a different channel stacking architecture, planform geometry, and nature of terrace development, with important implications for reservoir architecture. In some cases, multiple phases of inner levee growth are observed, each intimately linked to the channel migration and aggradation history. Channel sinuosity evolves dynamically, with some meander loops undergoing periods of accelerated meander growth at the same time that others show little lateral migration.     

Controls on the development of valleys,canyons, and unconfined channel–levee complexes on the Pleistocene Slope of East Kalimantan,Indonesia

Shallow 3D seismic data show contrasting depositional patterns in Pleistocene deepwater slopes of offshore East Kalimantan, Indonesia. The northern East Kalimantan slope is dominated by valleys and canyons, while the central slope is dominated by unconfined channel–levee complexes. The Mahakam delta is immediately landward of the central slope and provided large amounts of sediments to the central slope during Pleistocene lowstands of sea level. In the central area, the upper slope contains relatively straight and deep channels. Sinuous channel–levee complexes occur on the middle and lower slope, where channels migrated laterally, then aggraded and avulsed. Younger channel–levee complexes avoided bathymetric highs created by previous channel–levee complexes. Levees decrease in thickness down slope. Relief between channels and levees also decreases down slope.North of the Mahakam delta, siliciclastic sediment supply was limited during the Pleistocene, and the slope is dominated by valleys and canyons. Late Pleistocene rivers and deltas were generally not present on the northern outer shelf. Only one lowstand delta was present on the northern shelf margin during the upper Pleistocene, and sediments from that lowstand delta filled a pre-existing slope valley complex and formed a basin-floor fan. Except for that basin-floor fan, the northern basin floor shows no evidence of sand-rich channels or fans, but contains broad areas with chaotic reflectors interpreted as mass transport complexes. This suggests that slope valleys and canyons formed by slope failures, not by erosion associated with turbidite sands from rivers or deltas. In summary, amount of sediment coming onto the slope determines slope morphology. Large, relatively steady input of sediment from the Pleistocene paleo-Mahakam delta apparently prevented large valleys and canyons from developing on the central slope. In contrast, deep valleys and canyons developed on the northern slope that was relatively “starved” for siliciclastic sediment.     

Knickpoint migration in submarine channels in response to fold growth,western Niger Delta

Several knickpoints have been identified along the present-day thalweg of a sinuous submarine channel–levee system (CLS) on the slope of the western Niger Delta using 3D seismic data. The knickpoints form as a result of gradient changes caused by the uplift of a thrust and fold belt orthogonal to the CLS. The channel gradient is lower locally upstream of folds causing turbidity currents within the channel to decelerate and deposit the coarsest sediment load. The basinward dipping fold limb causes local steepening of the gradient, which leads to increased flow velocity and turbulence within the turbidity currents. This enhances erosion at the base of the channel and leads to the formation of a knickpoint. If preserved, e.g., as a result of channel avulsion or abandonment, the deposits upstream of the knickpoints could constitute an important hydrocarbon reservoir element. They can, however, also be partially eroded by headward-migrating knickpoints, as the channel strives to regain its equilibrium profile, leaving remnant sand pockets preserved on channel margins. Although knickpoints are difficult to recognise from subsurface seismic or outcrop data, it is anticipated that they can form at any stage of the evolution of a channel–levee system and may be particularly important in controlling 3D channel architecture where channels intersect dynamically changing seabed bathymetry.     

Geophysical evidence for cyclic sediment deposition on the southern slope of Qiongdongnan Basin,South China Sea

Gravity flow deposits form a significant component of the stratigraphic record in ancient and modern deep-water basins worldwide. Analyses of high-resolution 3D seismic reflection data in a predominantly slope setting, the southern slope of Qiongdongnan Basin, South China Sea, reveal the extensive presence of gravity flow depositional elements in the Late Pliocene−Quaternary strata. Three key elements were observed: (1) mass transport deposits (MTDs) including slumps and debris flows, (2) turbidity current deposits including distributary channel complexes, leveed channel complexes and avulsion channel complexes, and (3) deep-water drapes (highstand condensed sections). Each depositional element displays a unique seismic expression and internal structures in seismic profiles and attribute maps. Based on seismic characteristics, the studied succession is subdivided into six units in which three depositional cycles are identified. Each cycle exhibits MTDs (slump or debris) at the base, overlain by turbidities or a deep-water drape. The genesis of these cycles is mainly controlled by frequent sea-level fluctuations and high sedimentation rates in the Late Pliocene–Quaternary. Moreover, tectonics, differential subsidence, and paleo-seafloor morphology may have also contributed to their formation processes. The present study is aimed to a better understanding of deep-water depositional systems, and to a successful hydrocarbon exploration and engineering-risk assessment.     

Anatomy of a submarine channel–levee: An example from Upper Cretaceous slope sediments,Rosario Formation,Baja California,Mexico

To date, facies architecture models of submarine channel–levees have largely been derived from seismic data, isolated core data and limited field studies. We report field observations of an Upper Cretaceous submarine channel–levee complex within the Rosario Formation, Baja California, Mexico, which provide high-resolution data of lithofacies and ichnofacies distribution, and levee depositional thickness decay along transects perpendicular to the channel axis. Within the levee, both sandstone thickness and the overall proportion of sandstone decrease according to a power law away from the channel axis. Spatial variation in sedimentary structures away from the channel axis is predictable and provides an important link to the depositional flow regime. In channel-proximal locations, structureless sands, parallel lamination, overturned ripples, and ripple cross-lamination (including climbing ripple cross-lamination) are common; in channel-distal localities starved ripples are abundant. Sandstone bed thickness generally increases up stratigraphy within the levee succession, which is interpreted to indicate increasing turbidity current magnitude and/or contemporaneous channel floor aggradation reducing relative levee relief. However, in the most channel-proximal location sandstone bed thickness decreases with height; combined with evidence from both facies and palaeocurrent analysis this allows the position of the levee crest to be inferred. The thickest beds occur at higher levels with increasing distance from the channel axis, using this evidence we present a model for levee growth and migration of the crest.Quantitative analysis of ichnofacies distribution reveals that traces typical of the Cruziana and Skolithos ichnofacies are superimposed over the ‘normal’ background Nereites ichnofacies, forming a ‘bioturbation front’ which is indicative of proximity to the channel. By analogy with modern canyons and channels, the association of Cruziana and Skolithos ichnofacies with the channel may be attributed to oxygen and nutrient enrichment and possible turbidity current transport of organisms responsible for these ichnofacies.     

Migrated hybrid turbidite-contourite channel-lobe complex of the late Eocene Rovuma Basin,East Africa

Analysis of 3 D seismic data and well log data from the Rovuma Basin in East Africa reveals the presence of a late Eocene channel-lobe complex on its slope. The first two channels, denoted as channel-1 and channel-2, are initiated within a topographic low on the slope but come to a premature end when they are blocked by a topographic high in the northwest region of the basin. New channels migrate southeastward from channel-1 to channel-6 due to the region's sufficient sediment supply and stripping caused by bottom currents. The primary factors controlling the development of the channel complex include its initial paleo-topographic of seafloor, the property of gravity flows, the direction of the bottom current, and the stacking and expansion of its levees. The transition zone from channel to lobe can also be clearly identified from seismic sections by its pond-shaped structure. At a certain point, thest systems record a transiton from erosive features to sedimentary features, and record a transition from a confined environment to an open environment. Channels and lobes can be differentiated by their morphologies: thick slump-debris flows are partly developed under channel sand sheets,whereas these slump-debris flows are not very well developed in lobes. Well log responses also record different characteristics between channels and lobes. The interpreted shale volume throughout the main channel records a box-shaped curve, thereby implying that confined channel complexes record high energy currents and abundant sand supply, whereas the interpreted shale volume throughout the lobe records an upward-fining shape curve,thereby indicating the presence of a reduced-energy current in a relatively open environment. Within the Rovuma Basin of East Africa, the average width of the Rovuma shelf is less than 10 km, the width of the slope is only approximately 40 km, and the slope gradient is 2°–4°. Due to this steep slope gradient, the sand-rich top sheet within the channel also likely contributes to the straight feature of the channel system. It is currently unclear whether the bottom current has any effect on its sinuosity.     

Turbidite deposition and the development of canyons through time on an intermittently glaciated continental margin: The Bonanza Canyon system,offshore eastern Canada

Bonanza Canyon is a complex canyon system on the slope from the intermittently glaciated Grand Bank on the south side of Orphan Basin. A 3D seismic reflection volume, 2D high-resolution seismic reflection profiles and ten piston cores were acquired to study the evolution of this canyon system in relation to glacial processes on the continental shelf and the effects of different types of turbidity currents on the development of deep water channels. Mapped reflector surfaces from the 3D seismic volume show that the Bonanza Canyons developed in a depression created by a large submarine slide of middle Pleistocene age, coincident with the onset of glacigenic debris flows entering western Orphan Basin. Two 3–5 km wide, flat-floored channels were cut into the resulting mass-transport deposit and resemble catastrophic glacial meltwater channels elsewhere on the margin. Both channels subsequently aggraded. The eastern channel A became narrower but maintained a sandy channel floor. The western channel, B, heads at a spur on the continental slope and appears to have been rather passively draped by muds and minor sands that have built 1500-m wave length sediment waves.Muddy turbidites recorded by piston cores in the channel and on the inter-channel ridges are restricted to marine isotope stage (MIS) 2 and were deposited from thick, sheet-like, and sluggish turbidity current derived from western Orphan Basin that resulted in aggradation of the channels and inter-channel ridges. Sandy turbidites in channels and on inner levees were deposited throughout MIS 2–3 and were restricted to the channels, locally causing erosion. Some coincide with Heinrich events. Channels with well-developed distributaries on the upper slope more readily trap the sediments on Grand Bank to form sandy turbidity currents. Channel B dominated by muddy turbidity currents has wide and relatively smooth floor whereas channel A dominated by sandy turbidity currents has a sharp geometry.     

Morpho-acoustic and sedimentologic characteristics of the Indus Fan

The upper Indus Fan is characterized by an average 1∶500 gradient, chanels with 100 m high levees, several continuous subbottom reflectors on 3.5-kHz records, and generally fine-grained sediments. Multichannel seismics show the levee complexes typified by overlapping wedge-shaped reflection sets and channel axis by high-amplitude discontinuous reflections. The middle fan has 1∶500–1∶1000 gradients and channels with ≈20 m high levees. The lower fan has gradients less than 1∶1000, channels with 8–20 m high levees, few or no subbottom reflectors on 3.5-kHz records, and high sand content. Besides the dominant unchannelized turbidity currents, channelized and overbank flows also played a significant role in the sedimentation of the lower fan.     

Morphologic,acoustic, and sedimentologic characteristics of the Magdalena Fan

The Magdalena Fan can be divided into: upper fan—1:60–1:110 gradients, channels with well-developed levees, generally several subbottom reflectors on 3.5-kHz records, and fine-grained sediments; middle fan—1:110–1:200 gradients, channels with very subdued levees, several to few subbottom reflectors on 3.5-kHz records, and chaotic and discontinuous reflections on multichannel seismic (MCS) records; lower fan—<1:250 gradients, small channels and relatively smooth seafloor, generally coarsegrained sediments, few or no subbottom reflectors on 3.5-kHz records, and flat continuous reflections on MCS records. In addition to the turbidity currents, slumping along the continental slope and elsewhere also influenced sedimentation in the fan.     

Morpho-acoustic and sedimentologic characteristics of the Indus Fan

The upper Indus Fan is characterized by an average 1∶500 gradient, chanels with 100 m high levees, several continuous subbottom reflectors on 3.5-kHz records, and generally fine-grained sediments. Multichannel seismics show the levee complexes typified by overlapping wedge-shaped reflection sets and channel axis by high-amplitude discontinuous reflections. The middle fan has 1∶500–1∶1000 gradients and channels with ≈20 m high levees. The lower fan has gradients less than 1∶1000, channels with 8–20 m high levees, few or no subbottom reflectors on 3.5-kHz records, and high sand content. Besides the dominant unchannelized turbidity currents, channelized and overbank flows also played a significant role in the sedimentation of the lower fan. Margin setting represents fan and/or source area     

Tectonic control over topography and channel sedimentation across the forearc slope of the southern Kurile Trench

A high-resolution seismic survey covering more than 2,000 km² has revealed the processes responsible for the slope morphology and channel sedimentation across the forearc slope-basin of the Kurile Arc–NE Japan Arc collision zone, offshore from Tokachi (Hokkaido, Japan). The dominant slope contours parallel the trench but, in the middle and lower reaches of the southern slope, contours are convex-shaped with an offshore trend. This sector of the slope is traversed diagonally by the Hiroo submarine channel. The offshore-trending convex contours and the channel course have developed through the interplay of tectonic and sedimentary processes, including the development of anticlines, anticline-induced lobe sedimentation and channel avulsion. In its upper reaches, the channel is restricted by a topographic low associated with NNW–SSE-trending anticlines which developed within the upper and middle slope sectors during late Miocene uplift. The uplift timing and trend of these anticlines indicate that they resulted from collision, the channel sedimentology and slope morphology of the middle and lower slopes having been influenced by Pliocene uplift of NE–SW-trending anticlines. The trends of these anticlines parallel those of the Kurile Trench. The Pliocene and early Pleistocene strata of the middle and lower slopes consist of ponded lobe sediments deposited along the palaeo-Hiroo submarine channel on the landward side of the anticlines. As a lobe pile accumulated, the channel thalweg shifted to the north of the stack, allowing the channel to bypass the topographic high formed by the growing stack. Thick levee deposits built up along the channel course during the late Pleistocene and Holocene. These levees, along with the Pliocene and early Pleistocene lobes, are reflected in the present-day sigmoid-shaped, convex offshore-trending contours. Thus, the interplay of subduction- and collision-related anticlines, tectonic-related channel ponding, and avulsion has contributed to the slope morphology of the southern Kurile Trench.     

The Bulgheria canyon-fan: a small-scale proximal system in the eastern Tyrrhenian Sea (Italy)

The Bulgheria canyon-fan system in the eastern Tyrrhenian Sea displays well-developed, small-scale, fluvial-like features and has formed alongside the northern slope of the Sapri peri-Tyrrhenian basin. This study reveals, for the first time, the morphology and course of the present-day system as well as the buried elements based on a Digital Terrain Model and high-resolution seismic profiles interpretation. Two adjacent canyons (Infreschi and Luna) originate in the Cilento outer shelf at a short distance from each other and feed an intraslope basin fan through two main sub-parallel channels that run about 12 and 8 km, respectively. Channel and levee development seems to be controlled primarily by the local slope gradient and by Coriolis forces that induce a faster vertical growth of the right-side features, as is often observed in the Northern Hemisphere. Centrifugal forces, on the other hand, have induced episodic flow-stripping at the meander loops and bends, causing local destruction of the main channel levees rather than new levee growth at the outer bends. Overbank deposits are associated with overspill turbidite deposition in the mid fan where a topographic constraint occurs, whereas large-sediment, low-angle wave fields are mainly developed on the outer fan. Buried features and relict morphologies suggest that the Infreschi channel experienced at least two phases of re-incision since the final stages of the middle Pleistocene. Local re-adjustment of outer lobe growth due to channel avulsion and meander abandonment is possibly a consequence of relative base-level fluctuations. The sedimentary record of the mid and outer fan includes outrun mass wasting deposits from extensive failures of the Sapri slope. Indeed, a marked scar is present on the eastern side of the modern outer lobe that indicates the persistency of mass flow passages up to recent times. In addition to the environmental factors that are currently considered to cause canyon formation on the shelf margin, this study proposes the possibility that the head canyon branch close to the mainland was incised by massive and persistent underground freshwater flow from the adjacent aquifer when the sea-level was lower than at present.     

Morphologic, acoustic, and sedimentologic characteristics of the Magdalena Fan

The Magdalena Fan can be divided into: upper fan—1:60–1:110 gradients, channels with well-developed levees, generally several subbottom reflectors on 3.5-kHz records, and fine-grained sediments; middle fan—1:110–1:200 gradients, channels with very subdued levees, several to few subbottom reflectors on 3.5-kHz records, and chaotic and discontinuous reflections on multichannel seismic (MCS) records; lower fan—<1:250 gradients, small channels and relatively smooth seafloor, generally coarsegrained sediments, few or no subbottom reflectors on 3.5-kHz records, and flat continuous reflections on MCS records. In addition to the turbidity currents, slumping along the continental slope and elsewhere also influenced sedimentation in the fan. Margin setting represents fan and/or source area     

东非鲁伍马盆地深水沉积体系及油气勘探意义

利用高分辨率三维地震资料、测井和钻井数据,对东非鲁伍马盆地深水沉积特征进行了系统刻画。根据深水沉积体的地震相特征,识别出峡谷、水道、漫溢沉积、朵体、块体搬运沉积(MTDs)和凝缩段等深水沉积单元,建立了地震识别图版。分析总结了水道和朵体的岩性特征、电性特征和储层物性特征,砂岩具有低伽马(GR)和高电阻(RT)特征,厚层砂岩GR曲线呈“箱型”,有泥岩夹层的砂岩段呈叠加的“钟型”特点;储层压实程度弱,发育原生粒间孔隙,具有中—高孔、中—高渗的特征。结合成藏条件研究,认为由水道和朵体浊积砂岩储层、凝缩段和漫溢沉积泥岩盖层、天然堤和MTDs为侧向遮挡等要素构成的油气储、盖配置关系,是研究区油气成藏的一个关键因素,对深水油气勘探具有一定的指导意义。     

西沙海域碳酸盐台地周缘水道沉积体系

高分辨率地震资料显示,南海北部西沙海域碳酸盐台地周缘广泛发育水道沉积体系。礁缘水道底界面表现出强反射特征,内部充填弱-强、连续性好的地震相,可见底部杂乱反射特征;斜坡水道在地震剖面上表现为横向上连续发育的"V"型特征,且下切深度较浅。西沙隆起与广乐隆起之间的南北向低洼地带发育大型深水水道,并且受古地貌高点影响,水道分为南北两个分支。北分支水道可分为5期,且水道迁移现象明显;南分支水道可分为4期,水道以充填强振幅、连续性好的浊流沉积体和弱振幅、杂乱的块体搬运体系(Mass Transport Deposits,MTDs)为特征,每期水道均表现出侵蚀-充填-废弃的旋回性。分析认为西沙碳酸盐台地周缘水道沉积物源来自西沙隆起和广乐隆起的碳酸盐台地和生物礁碎屑及由火成作用产生的火成岩碎屑。西沙-广乐碳酸盐台地水道相互贯通,构成台地-斜坡-深水的水道沉积体系,为碳酸盐岩、生物礁及火山碎屑向台地周缘输送提供了良好的通道。     

Confined channel-levee complex development in an elongate depo-center: Deep-water Tertiary strata of the Austrian Molasse basin

A basin axial-channel belt was largely responsible for the observed distribution of coarse-grained gravity-flow deposits in the Tertiary Puchkirchen and basal Hall formations of the Molasse foreland basin in Upper Austria. Elements of this depositional system, mapped in three-dimensional (3D) seismic-reflection data, include channel-belt thalweg, mass-transport complexes, overbank wedge, overbank lobe, and tributary channel. The primary objective of this paper is to develop a comprehensive understanding of the sedimentary processes that were prevalent in the channel-belt complex through the analysis of well data, including drill cores and wireline logs, in conjunction with 3D seismic interpretations.     

琼东南盆地北礁凹陷梅山组顶部丘形反射特征及成因分析

南海琼东南盆地北礁凹陷中中新统梅山组顶部丘形反射目前引起广泛关注,前人推测为生物礁、重力蠕动与底流叠加成因、等深积丘等。本文通过钻井资料、二维、三维地震资料精细刻画丘形反射（残丘）和丘间水道特征及其成因。残丘及水道在北礁凸起不发育,在边缘斜坡中部和高地较发育,且有向高地两边规模减小趋势,不具对称性,残丘和水道呈平行-亚平行近E-W向展布,局部有合并分叉现象,与北礁凸起走向呈一小角度;丘宽562~1 223 m,丘高29~87 m,丘长10 km左右,存在丘翼削蚀,水道底蚀现象。地震属性分析表明三维工区西南部残丘间水道由砂泥岩互层充填,形成长条形强振幅,而残丘为中-低振幅;地震、钻井资料分析表明丘形反射（残丘）由钙质泥岩和泥岩组成,属于半深海沉积,且残丘内部波阻抗为5.0×106~6.5×106kg/m3·m/s,低于火山岩、灰岩波阻抗,属于砂泥岩地层范畴;根据梅山组下段水道由西向东强振幅变弱、分叉、前积反射和海山附近底流（等深流）沉积剥蚀特征综合判定底流古流向自西向东,根据海山两翼地震反射特征推测底流可追溯至晚中新世早期（11.6 Ma BP）,综合分析认为,研究区中中新统梅山组丘形反射是晚中新世早期底流切割梅山组地层形成的残丘。