Henry W. Posamentier

Seismic Geomorphology and Stratigraphy of Depositional Elements in Deep-Water Settings

Analyses of 3-D seismic data in predominantly basin-floor settings offshore Indonesia, Nigeria, and the Gulf of Mexico, reveal the extensive presence of gravity-flow depositional elements. Five key elements were observed: (1) turbidity-flow leveed channels, (2) channel-overbank sediment waves and levees, (3) frontal splays or distributary-channel complexes, (4) crevasse-splay complexes, and (5) debris-flow channels, lobes, and sheets. Each depositional element displays a unique morphology and seismic expression. The reservoir architecture of each of these depositional elements is a function of the interaction between sedimentary process, sea-floor morphology, and sediment grain-size distribution. (1) Turbidity-flow leveed-channel widths range from greater than 3 km to less than 200 m. Sinuosity ranges from moderate to high, and channel meanders in most instances migrate down-system. The high-amplitude reflection character that commonly characterizes these features suggests the presence of sand within the channels. In some instances, high-sinuosity channels are associated with (2) channel-overbank sediment-wave development in proximal overbank levee settings, especially in association with outer channel bends. These sediment waves reach heights of 20 m and spacings of 2-3 km. The crests of these sediment waves are oriented normal to the inferred transport direction of turbidity flows, and the waves have migrated in an up-flow direction. Channel-margin levee thickness decreases systematically down-system. Where levee thickness can no longer be resolved seismically, high-sinuosity channels feed (3) frontal splays or low-sinuosity, distributary-channel complexes. Low-sinuosity distributary-channel complexes are expressed as lobate sheets up to 5-10 km wide and tens of kilometers long that extend to the distal edges of these systems. They likely comprise sheet-like sandstone units consisting of shallow channelized and associated sand-rich overbank deposits. Also observed are (4) crevasse-splay deposits, which form as a result of the breaching of levees, commonly at channel bends. Similar to frontal splays, but smaller in size, these deposits commonly are characterized by sheet-like turbidites. (5) Debris-flow deposits comprise low-sinuosity channel fills, narrow elongate lobes, and sheets and are characterized seismically by contorted, chaotic, low-amplitude reflection patterns. These deposits commonly overlie striated or grooved pavements that can be up to tens of kilometers long, 15 m deep, and 25 m wide. Where flows are unconfined, striation patterns suggest that divergent flow is common. Debris-flow deposits extend as far basinward as turbidites, and individual debris-flow units can reach 80 m in thickness and commonly are marked by steep edges. Transparent to chaotic seismic reflection character suggest that these deposits are mud-rich. Stratigraphically, deep-water basin-floor successions commonly are characterized by mass-transport deposits at the base, overlain by turbidite frontal-splay deposits and subsequently by leveed-channel deposits. Capping this succession is another mass-transport unit ultimately overlain and draped by condensed-section deposits. This succession can be related to a cycle of relative sea-level change and associated events at the corresponding shelf edge. Commonly, deposition of a deep-water sequence is initiated with the onset of relative sea-level fall and ends with subsequent rapid relative sea-level rise.

Sequence Stratigraphy and Facies Model of an Incised Valley Fill: The Gironde Estuary, France

ABSTRACT The Gironde estuary was formed by the Holocene drowning of a fluvial valley incised during the Wurm global sea-level fall. A depositional sequence accumulated in the valley during the eustatic lowstand, the Holocene rise, and the post-Holocene highstand. The sequence comprises a diverse assemblage of lithofacies that can be grouped into lowstand, transgressive, and highstand systems tracts. The lowstand systems tract comprises a continuous unit of relatively thin fluvial gravel and coarse sand in the thalweg of the incised valley. The transgressive systems tract comprises the bulk of the incised valley fill and forms a landward-thinning wedge of tidal-estuarine sands and muds. In the estuary mouth these are overlain by a thick unit of coarse-grained, estuary-mouth tidal-inlet and tida -delta sands. The highstand systems tract, initiated at about 4000 BP, forms a seaward-prograding, tide-dominated estuarine bayhead delta that has been gradually filling the estuary since the post-Holocene stillstand. Coeval with this filling of the estuary, the adjacent oceanic shoreline has been starved of sediment and is being eroded by waves, indicating that transgressive and highstand systems tracts locally can be synchronous. Several important stratigraphic surfaces punctuate the valley fill: the sequence boundary, the transgressive surface, the tidal ravinement surface, the wave ravinement surface, and the maximum flooding surface. The stratigraphic expression of the sequence boundary depends on its position within the valley. In the thalweg it separates lowstand fluvial deposits from underlying Tertiary carbonates; on the valley walls it is directly overlain by transgressive estuarine sediments. On the interfluves the unconformity continues to be enhanced by modern subaerial erosion. As the interfluves are progressively transgressed by the eroding shoreline, the sequence boundary is expressed as a wave ravinement surface with transgressive marine sediments unconformably overlying Pleistocene or Tertiary ubstrates. The stratigraphic expression of the transgressive surface in the valley thalweg is characterized by onlap of transgressive tidal-estuarine sediments onto lowstand fluvial deposits. On the valley walls the transgressive surface merges with the sequence boundary. Tidal scour at the estuary mouth forms a deeply erosional tidal ravinement surface overlain by thick estuary-mouth wave- and tide-reworked sands. Subsequently these tidal-inlet sands are eroded by waves associated with the passage of the transgressing shoreline to produce a wave ravinement surface. In the distal part of the estuary the maximum flooding surface is expressed as a downlap surface where the regressive highstand estuarine muds prograde over transgressive tidal-estuarine muddy sands or estuary-mouth sands. In the upstream or proximal end of the estuary the maximum flooding surface separates identical facies, i.e., transgressive and regressive tidal-estuarine point bars, and would be very difficult to identify.

Sequence stratigraphy: methodology and nomenclature

The recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply, which correspond to sequences in the rock record. These cycles may be symmetrical or asymmetrical, and may or may not include all types of systems tracts that may be expected within a fully developed sequence. Depending on the scale of observation, sequences and their bounding surfaces may be ascribed to different hierarchical orders. Stratal stacking patterns combine to define trends in geometric character that include upstepping, forestepping, backstepping and downstepping, expressing three types of shoreline shift: forced regression (forestepping and downstepping at the shoreline), normal regression (forestepping and upstepping at the shoreline) and transgression (backstepping at the shoreline). Stacking patterns that are independent of shoreline trajectories may also be defined on the basis of changes in depositional style that can be correlated regionally. All stratal stacking patterns reflect the interplay of the same two fundamental variables, namely accommodation (the space available for potential sediment accumulation) and sediment supply. Deposits defined by specific stratal stacking patterns form the basic constituents of any sequence stratigraphic unit, from sequence to systems tract and parasequence. Changes in stratal stacking patterns define the position and timing of key sequence stratigraphic surfaces. Precisely which surfaces are selected as sequence boundaries varies as a function of which surfaces are best expressed within the context of the depositional setting and the preservation of facies relationships and stratal stacking patterns in that succession. The high degree of variability in the expression of sequence stratigraphic units and bounding surfaces in the rock record means ideally that the methodology used to analyze their depositional setting should be flexible from one sequence stratigraphic approach to another. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture. The purpose of this paper is to emphasize a standard but flexible methodology that remains objective.

Variability of the sequence stratigraphic model: effects of local basin factors

Abstract The basic parameters that control stratal architecture in depositional sequences are sediment flux, rate of change of sediment accommodation (i.e., eustasy and sea-floor subsidence/uplift) and physiography. Of these parameters, only the eustatic component associated with the generation of sediment accommodation is globally significant. Consequently, local factors within a particular basin can play a relatively dominant role, vis-a-vis eustasy, in determining the internal organization of depositional sequences. In general, eustasy and sea-floor subsidence/ uplift determine the timing of sequence bounding surfaces, whereas sediment flux and physiography are most effective in determining the stratal architecture between those bounding surfaces. Basin physiography exerts an important control on the internal organization of sequences by determining, for example, whether deep-water turbidite systems will be a major part of the lowstand systems tract and whether or not fluvial incision will occur. On ramp-type basin margins with low shelf gradients, little or no fluvial incision and no deep-water turbidite deposition will occur. Lowstand deposits in this setting will consist of shoreline sands scattered at many locations within the basin. If, however, relative sea-level fall exposes a shelf/slope break, deeper fluvial incision as well as deep-water turbidite deposition will be likely. Basin physiography also plays an important role in the stratal architecture of the transgressive systems tract. The relatively deeper water setting that lies just seaward of the last clinoform of the underlying progradational wedge is the site of a depositional unit referred to herein as the “healing phase”. These early transgressive systems tract deposits commonly are relatively sand-poor and are derived primarily from eroded delta plain/coastal plain sediments. The role of sediment supply varying as an indirect response to relative sea-level change can be significant in deep water environments. Within lowstand systems tracts that form in physiographic settings characterized by discrete shelf/slope breaks, the stratigraphic expression of depositional units that accumulate in deep-water settings primarily will be a function of sand/mud ratio of sediments supplied, as well as the physiography of the slope and basin. Commonly, the highest sand/mud ratio delivered to the deep water occurs early within the lowstand systems tract. This results in stratigraphic successions that typically grade from relatively widespread tabular-bedded sand-rich turbidite deposits early within the lowstand systems tract, to confined flow leveed-channel turbidite deposits late within the lowstand systems tract when the incoming sand/mud ratio is lower.

Sequence stratigraphy and facies associations

Sequence stratigraphic concepts and principles sequence stratigraphic methods and tools quaternary applications of sequence stratigraphic concepts pre-quaternary applications of sequence stratigraphic concepts - Europe, Greenland, North America, Australia.

Variability in form and growth of sediment waves on turbidite channel levees

Fine-grained sediment waves have been observed in many modern turbidite systems, generally restricted to the overbank depositional element. Sediment waves developed on six submarine fan systems are compared using high-resolution seismic-reflection profiles, sediment core samples (including ODP drilling), multibeam bathymetry, 3D seismic-reflection imaging (including examples of burried features), and direct measurements of turbidity currents that overflow their channels. These submarine fan examples extend over more than three orders of magnitude in physical scale. The presence or absence of sediment waves is not simply a matter of either the size of the turbidite channel-levee systems or the dominant initiation process for the turbidity currents that overflow the channels to form the wave fields. Both sediment-core data and seismic-reflection profiles document the upslope migration of the wave forms, with thicker and coarser beds deposited on the up-current flank of the waves. Some wave fields are orthogonal to channel trend and were initiated by large flows whose direction was controlled by upflow morphology, whereas fields subparallel to channel levees resulted from local spillover. In highly meandering systems, sediment waves may mimic meander planform. Larger sediment waves form on channel-levee systems with thicker overflow of turbidity currents, but available data indicate that sediment waves can be maintaned during conditions of relatively thin overflow. Coarser-grained units in sediment waves are typically laminated and thin-bedded sand as much as several centimetres thick, but sand beds as thick as several tens of centimetres have been documented from both modern and buried systems. Current production of hydrocarbons from sediment-wave deposits suggests that it is important to develop criteria for recognising this overbank element in outcrop exposures and borehole data, where the wavelength of typical waves (several kilometres) generally exceeds outcrop scales and wave heights, which are reduced as a result of consolidation during burial, may be too subtle to recognise.

Turbidite systems: State of the art and future directions

The study of turbidite systems covering a wide range of physical scales has led to confusion regarding the use of certain key terms and hence a breakdown in communication between workers involved in turbidite research. There are three fundamentally different scales and types of observations derived from the study of outcrop data (ancient systems), high-resolution seismic reflection and side scan sonar data (modern systems), and multichannel seismic reflection data (modern and older buried systems). Despite the variability of scale the same terms are used to describe features that may have little in common. Consequently, turbidite system terminology has become imprecise and even misleading in some cases, thus providing impediments to developing useful predictive models for processes, depositional environments, and lateral and vertical distribution of sand bodies within turbidite systems. To address this concern, we review the principal elements critical to deepwater systems: slump scars, submarine canyons, channels, channel fill deposits, overbank deposits, and lobes and discuss some of their recognition criteria with each different type of data base. Local and regional tectonic setting, relative sea level variations, and bottom current activity are probably the main factors that control size, external geometry, internal stratal configuration, and facies characteristics of both modern and ancient turbidite systems. These factors ultimately control the timing and bounding characteristics between stages of growth of deepwater systems. If comparison of elements from different turbidite deposits using various data types is carried out at similar physical and temporal scales, predictive models eventually may be improved.

Lowstand alluvial bypass systems: Incised vs. unincised

Alluvial systems ranging in age from Miocene to late Pleistocene are observed beneath the southern Java Sea Shelf, offshore northwest Java. A combination of seismic reflection attributes, time slices, and horizon slices extracted from three-dimensional seismic volumes have enabled identification of these alluvial systems. The plan-view expression of these systems ranges from low sinuosity to high sinuosity, and incised to unincised. Widths of individual channels range from 100 to 250 m. Meander belt widths range from 2 to 6 km. In some instances, well-developed minor tributary feeder systems can be observed to be associated with major trunk valleys. Late Pleistocene alluvial systems imaged on the shelf were active during periods of lowered sea level when vast shelf areas were emergent. Of these systems only a select few are characterized by incision. Incision is inferred where trunk channels of fluvial systems are associated with minor, orthogonal, deeply etched tributary channels/valleys. The incised trunk valleys range from 0.5 to 5 km wide and contain channels within them; the incised tributary valleys are an order of magnitude narrower and are characterized by well-developed dendritic drainage patterns. Valley incision, which likely formed within a period of 3-5 k.y., can be traced more than 200 km inboard of the shelf edge. The presence of numerous unincised alluvial systems on marine shelves of the southern Java Sea suggests that valley incision likely characterizes only the lowest of lowstands. To the extent that the Pleistocene can be used as an analog to older sections, we conclude that unincised lowstand alluvial bypass systems can constitute a more common response to sea level lowering than do incised systems.

Aspects of the stratal architecture of forced regressive deposits

Abstract Forced regression refers to the process of seaward migration of a shoreline in direct response to relative sea-level fall. Recognition criteria for forced regressive deposits include: (1) presence of a significant zone of separation between successive shoreface deposits, (2) the presence of sharp-based shoreface/delta front deposits, (3) the presence of progressively shallower clinoforms going from proximal to distal, (4) the occurrence of long-distance regression, (5) the absence of fluvial and/or coastal plain/delta plain capping the proximal portion of regressive deposits, (6) the presence of a seaward-dipping upper bounding surface at the top of the regressive succession, (7) the presence of increased average sediment grain size in regressive deposits going from proximal to distal and (8) the presence of ‘foreshortened’ stratigraphic successions. The principal factors driving the stratal architecture of forced regressive deposits include: (1) the gradient of the sea floor progressively exposed by falling relative sea-level, (2) the ratio of the sediment flux to the rate of relative sea-level fall, (3) the ‘smoothness’ of relative sea-level fall, (4) the variability of sediment flux and (5) the changes of sedimentary process that occur as sea-level falls and progressively more of the shelf is subaerially exposed. Forced regressive deposits are grouped into attached v. detached, and smooth-topped v. stepped-topped. Attached deposits are defined as successive downstepped stratigraphic units whose shoreface/delta front deposits are generally in contact with each other. In contrast, detached deposits are defined as successive downstepped stratigraphic units whose shoreface/delta front deposits are generally not in contact with each other. Rather, in this instance a zone of sedimentary bypass exists. Stepped-top forced regressive deposits are characterized by a succession of horizontally topped though downstepping stratigraphic units. In contrast, smooth-topped forced regressive deposits are characterized by a seaward-dipping, albeit smooth, upper bounding surface. The bounding surfaces of forced regressive deposits commonly are expressed as a ravinement surface at the top and an unconformity to correlative conformity at the base.

Stratigraphic Organization of Late Pleistocene Deposits of the Western Part of the Golfe du Lion Shelf (Languedoc Shelf), Western Mediterranean Sea, Using High-Resolution Seismic and Core Data

Detailed analysis of shallow penetration single-channel seismic data, integrated with piston core data, reveals that the stratigraphic architecture on the Rhone shelf of the western Mediterranean Sea is characterized by a complex stratigraphy comprising both the regressive and transgressive parts of late Pleistocene depositional sequences. Several cycles of deposition are observed and are interpreted to be associated with fourth-or possibly fifth-order cycles of relative change of sea level. The regressive parts of the sequence are inferred to have been deposited either during late highstand or during periods of relative fall of sea level. These deposits are characterized, in some instances, by discrete downstepping wedges and internal downward shift surfaces; however, in other instances this evidence is not present. The transgressive parts of the depositional sequences are characterized by backstepping wedges and isolated sand bars. In general, these types of deposits are more common on the western part of the Rhone shelf and are largely absent on the extreme eastern part of the shelf. The transgressive deposits seem to be preferentially preserved at both inner and outer shelf locations and less so in the middle shelf. The exception to this is the area near the Rhone Delta depocenter, where transgressive deposits are observed across the entire shelf. Key surfaces separating stratigraphic units include ravinement surfaces, downlap surfaces, and subaerially formed erosional surfaces. These stratal discontinuity surfaces constitute the basis for analysis of the stratigraphic architecture. This area is dominated by seismic reflection geometries suggesting a high-energy depositional environment. Relatively steeply dipping seismic reflections bounded by horizontal to irregular erosional surfaces characterize most of the upper Pleistocene section in this area. The high-energy seismic facies correspond to three types of deposits: (1) thick regressive sands of lobate delta-front origin, (2) retrogradational beach barriers overlying the regressive wedge on the outer to middle shelf, and (3) late transgressive beach sands and ridges observed at the inner shelf. The stratigraphic complexity illustrated here is likely analogous to similar, although commonly undetected, complexity that characterizes petroleum fields in shallow-shelf settings. A variety of geomorphic elements have been interpreted here. These include transgressive sand bars, wave-dominated distributary mouth bars, recurved spits, isolated shelf edge shoreface/beaches, and distributary channels. These elements are consistent with an interpretation of a depositional environment characterized by wave-dominated delta deposition. The morphology of the delta in the eastern part of the shelf (i.e., near the depocenter) seems to be consistent from the outer to inner shelf location, suggesting that the position of the shoreline relative to the shelf edge and the presence of submerged shelf outboard of the shoreline had only minor impact on deltaic deposition; moreover, climatic change and changes of fluvial discharge that likely characterized this area during the End page 119 ---------------- late Pleistocene similarly had only minor effect on delta morphology.