Early Lowstand Research Articles

Large-scale turbidite systems of a semi-enclosed shelf sea: The upper Miocene of Eastern Yinggehai Basin, South China Sea

In addition to the continental slope and basin floor, gravity flow deposition on the shelf is increasingly attracting attention. In order to explore the environmental conditions and sedimentary rules of large-scale gravity flow deposition in shelf sea, paleogeomorphology restoration and identification, distribution and evolution of sediment gravity flow units in the upper Miocene Yinggehai Basin are studied by using 3D seismic, borehole and seismic inversion data. Two third-order sequences (SQhl2 and SQhl1) are identified, and the planar distribution of strata and turbidite deposition are controlled by different styles of slope breaks. In SQhl2, turbidite systems of subaqueous shelf fans and channels are identified. The shelf fans can be classified into early lowstand fans, late lowstand fans and highstand fans. In the early lowstand systems tract, the fans are the largest in scale (more than 700 km2), distributed in the subsidence center, and controlled by the basinward flexure slope break zone formed by fault activity. The shelf channels are classified into slope-parallel channels and slope-perpendicular channels according to the relationship between the strike of channels and the basin axis. The formation of the slope-perpendicular channels was controlled by large-scale global sea-level falls that happened 10.5 Ma ago. The formation of the slope-parallel channel was controlled by the flexure slope-break zone caused by activity of multiple groups of en-echelon faults. This study suggests that the development of large-scale turbidite systems in shelf sea was controlled by semi-enclosed geomorphology, fault activity and high sediment supply under sea-level falls. The semi-enclosed bay topography and local subsidence centers in the basin ensured a relatively isolated source-to-sink system, and allowed a large number of terrestrial sediments to converge intrabasin instead of being transported further to extrabasin. Fault activity formed structural slope-break zone and provided a “slope system” that facilitated the triggering of gravity flows on the shelf. Sharp increase of sediment supply under sea level falls contributed to the development of large-scale shelf turbidite systems. The study of large-scale turbidite system in shelf sea is of great significance for deepening the sedimentary law of gravity flow under different sedimentary backgrounds and opening up a new field for oil and gas exploration.

Seismic stratigraphy of late Neogene - Quaternary units and evolutionary model in the inner sector of Bahia Blanca estuary (Argentina)

The Quaternary sedimentary deposit of the inner sector of Bahía Blanca estuary (BBE), Argentina, was studied using high-resolution seismic profiles, core samples and correlation with continental – marine units outcropping in the continent. We interpreted four prominent seismic reflection surfaces and the acoustic basement which together defined five seismic stratigraphic units (S0, S1, S2, S3, S4) and four related systems tracts. Studying those units, we proposed a fourth Stage deposition evolutionary model that took place in a relative sea-level fluctuation context after the Miocene in the BBE area. The first stage contained the deposition of the regressive seismic units S0 and S1. The S0 was determined as an acoustic basement. The top of the S0 erosional surface (SU) was determined and it was formed by subaerial processes during the Late Pleistocene fall in sea-level and the following marine reworking during transgression. The unit was correlated with the oldest continental Miocene loess deposit and was associated with an Early Lowstand Systems Tract (ELST). Unit S1 was associated with a Late Lowstand Systems Tract (LLST). An irregular stratigraphic surface was observed above it. It was inferred as a Maximum Regressive Surface and the Ravinement Surface (RS/MRS) which are related to a maximum regression and subsequent transgression. The S1 unit was correlated with Pliocene-Pleistocene continental deposits. The Miocene - Pleistocene coastline movement in this sector would be associated with both Neogene Andean dynamics and the Last Maximum Glacial. After that, during the second Stage in the Late Pleistocene - middle Holocene the sea-level on the Bahia Blanca coast rose and unit S2 was formed. We interpreted unit S2 as a Transgressive System Tract which developed until the maximum sea-level of the Mid-Holocene. The coastline extension on the continent conformed the Maximum Flooding Surface. After a stabilization period to the mid Holocene, the sea-level started falling, offering conditions for the deposition of the High Stand System Tract (S3 – S4). Following stabilization, the third Stage took place and a progradation unit S3 was deposited overlying the MFS and a minor stratigraphic surface Bedset Boundary (BsB) separated this unit from S4. During the fourth phase, the sea-level continued to fall until the present level and the last HST (S4) was interpreted, which is interpreted as an aggradational – progradational deposit formed in the Middle Holocene to the present day by a change in the rate of sediment supply or energy flux.

Fluvio-Marine Sediment Partitioning As A Function of Basin Water Depth

Progradational fluvio-deltaic systems tend towards but cannot reach equilibrium, a state in which the longitudinal profile does not change shape and all sediment is bypassed beyond the shoreline. They cannot reach equilibrium because progradation of the shoreline requires aggradation along the longitudinal profile. Therefore progradation provides a negative feedback, unless relative sea level falls at a sufficient rate to cause non-aggradational extension of the longitudinal profile. How closely fluvio-deltaic systems approach equilibrium is dependent on their progradation rate, which is controlled by water depth and downstream allogenic controls, and governs sediment partitioning between the fluvial, deltaic, and marine domains. Here, six analogue models of coastal fluvio-deltaic systems and small prograding shelf margins are examined to better understand the effect of water depth, subsidence, and relative sea-level variations upon longitudinal patterns of sediment partitioning and grain-size distribution that eventually determine large-scale stratigraphic architecture. Fluvio-deltaic systems prograding in relatively deep-water environments are characterized by relatively low progradation rates compared to shallow-water systems. This allows these deeper water systems to approach equilibrium more closely, enabling them to construct less concave and steeper longitudinal profiles that provide low accommodation to fluvial systems. Glacio-eustatic sea-level variations and subsidence modulate the effects of water depth on the longitudinal profile. Systems are closest to equilibrium during falling relative sea level and early lowstand, resulting in efficient sediment transport towards the shoreline at those times. Additionally, the strength of the response to relative sea-level fall differs depending on water depth. In systems prograding into deep water, relative sea-level fall causes higher sediment bypass rates and generates significantly stronger erosion than in shallow-water systems, which increases the probability of incised-valley formation. Water depth in the receiving basin thus forms a first-order control on the sediment partitioning along the longitudinal profile of fluvio-deltaic systems and the shelf clinoform style. It also forms a control on the availability of sand-grade sediment at the shoreline that can potentially be remobilized and redistributed into deeper marine environments. Key findings are subsequently applied to the literature of selected shelf clinoform successions.

New imaging of the salinity crisis: Dual Messinian lowstand megasequences recorded in the deep basin of both the eastern and western Mediterranean

Through several examples we show that following sea-level fall and marginal erosion during the Messinian salinity crisis (MSC), clastic inputs into the eastern and western Mediterranean Sea are not distributed evenly in space and time but are mainly limited to the lower section of the Messinian salinity crisis depositional megasequence. Significant similarities around the basin allow us to propose a Mediterranean Messinian salinity crisis depositional episode that can be divided into two seismic megasequences: the Messinian lower megasequence (MLM) and the Messinian upper megasequence (MUM). Their distinctive seismic facies correspond to systems tracts deposited during three main stages that represent a complete sea level cycle. (1) A falling stage systems tract including mass transport deposits and forced regressive clinoforms deposited in the early part of the falling stage, and related to the increasing rate of relative sea level fall. This stage is characterized by a marked shift in the depocenter towards the deep basins. (2) An early lowstand characterized by massive clastic inputs from major Messinian rivers (the Rhone, Nile, and Antalya Gulf rivers) or smaller river systems (offshore south Lebanon). These clastics were deposited in an oversaturated basin, as evidenced by the interfingering chaotic and transparent seismic facies of the Messinian lower megasequence (MLM). (3) A late lowstand, starting with rapid deposition of massive halite, with no detrital inputs into the deep basin. The upper part of the evaporites clearly onlaps the Messinian erosional surface at the margins and is evidence for a transition between a late lowstand stage and an early transgressive stage. These deposits belong to the Messinian upper megasequence (MUM). We interpret the transition between the two megasequences as the peak of the “salinity” crisis, the end of the relative sea level fall, and the maximum dispersal of sands into the deep Mediterranean basins.

Lowstand Turbidites and Delta Systems of the Itararé Group in the Vidal Ramos region (SC), southern Brazil

Turbidites have captioned the attention of sedimentologists during the last decades due their importance as hydrocarbon reservoirs. However, their relationship to delta systems still deserves further studies. This paper presents examples from a late deglacial to early post-glacial deltaic and turbidite strata exposed in the surroundings of Vidal Ramos (Santa Catarina State, Brazil), southern portion of the Parana Basin. There, the uppermost part of the Mafra Formation and the Rio do Sul Formation onlap the Proterozoic basement and comprises an up to 360 m thick package. It includes (base to top) black shales, mass transport deposits (MTD) and sandy turbidites (Mafra Formation) as well as thin bedded turbidites (tbt), including one interval of black shales and sandy turbidites, overlain by proximal delta front sandstones (Rio do Sul Formation). The analysis of the succession shows two more than 150 m thick coarsening-upwards deltaic successions composed of turbidite sand sheets at their base (prodelta), followed by partially collapsed thin bedded turbidites (delta slope wedge) and delta front sandstones. Both turbidite sand-sheets abruptly overlay black shale intervals related to maximum flooding surfaces and therefore record correlative conformities. A detailed stratigraphic section elaborated from the correlation of four logs (1/100) suggests that distal delta front sands includes both thin bedded turbidites and wave reworked sands whereas the proximal delta front was dominated by long-lived underflows (hyperpycnal flows). The succession suggests that the most expressive turbidite beds (base of the delta systems) have resulted from relative sea-level falls (early lowstand) whereas the thin-bedded turbidites were related to the development of the late lowstand wedge. Black shales represent the transgressive systems tract and HST were not deposited or preserved in the area. High sediment supply associated with lowstand tracts could explain the occasional (Vidal Ramos) to common occurrence of slope failures (slumps and diamictites) involving thin bedded turbidites and delta front sandstones. This situation is quite logical in terms of deglacial periods, and resulting high sediment supply, within a long-term icehouse context, with prevalence of lowstand to transgressive settings.

Miospore distribution and sedimentological facies distribution as an insight to changing terrestrial palaeoequatorial floral communities during a Pennsylvanian glacio-eustatic sea level cycle

Systematic trends in miospore assemblages are known to characterise systems tracts in Carboniferous successions and could relate either to changing delivery mechanisms or to the influence of fluctuating sea levels on coastal plain vegetation. A succession of deltaic Pennsylvanian mudstones exposed in the UK Pennine Basin provides a record of continuous sedimentation at a location that remained submerged through a complete glacio-eustatic sea level cycle. Changing sedimentological facies, identified from thin sections enable the succession to be linked to an existing sequence stratigraphic framework and provide a context for the miospore trends through the succession. An abundant and diverse microflora is present in all samples examined. Detailed palynological and palaeoecological analyses have identified some significant trends and key taxa at important stratigraphic surfaces. This succession has provided a rare opportunity to study miospore assemblages in the late highstand and the lowstand systems tracts that are normally either removed by erosion or represented by sandstones. In the mid-late highstand, forest mire abundances (dominated by Lycospora) peak at 81.75% falling to 39.5% in the early lowstand, suggesting that lycopsid-dominated forests characterised coastal plains in the highstand and that there was a major floral turnover during significant sea level falls. The early lowstand sand- and silt-bearing mudstones are also associated with an abrupt increase in Florinites, Calamospora, Punctatisporites and Spelaeotriletes arenaceus. Maximum flooding surfaces, characterised by clay-rich lenticular mudstone facies, have some similar miospore characteristics to the sequence boundaries, including low abundances of the forest mire group and peaks in Punctatisporites, but are distinguished by high abundances of upland miospores. Changes in the diversity and abundance of miospores through the studied section appear to relate to their location within the stratigraphic framework, rather than directly to the facies and sedimentary processes. Therefore the greatest control on miospore trends and distributions appears to be sea level changes influencing vegetation in the coastal areas rather than the depositional mechanisms transporting sediment on to the shelf.

The transition from wave-dominated estuary to wave-dominated delta: The Late Quaternary stratigraphic architecture of Tiber River deltaic succession (Italy)

This paper presents a detailed description of the stratigraphic architecture of the Late Pleistocene/Holocene Tiber delta succession in order to document the passage from wave-dominated estuary to wave-dominated delta in the broader context of Late Quaternary sea level fluctuations.This succession constitutes a sequence-stratigraphic unit known as Tiber Depositional Sequence (TDS), which was deposited during the last glacial–interglacial cycle (last 120ka). Our study is based on the examination of an enormous amount of data derived from the stratigraphy of about 300 wells, petrographical and paleontological data (foraminifera, ostracoda, pollen, and plant macrofossils), 14C dating, and from the integration of geomorphological and geoarcheological data. Recently a 100m deep core (Pesce Luna well) was studied through a multidisciplinary approach and a detailed description of sedimentary facies, foraminifer and ostracod assemblages, pollen and 14C dating is presented in this paper. The new data allowed to produce three new correlation panels and to describe in more detail, with respect to previous interpretations, the stratigraphic-depositional architecture of the TDS, which internally shows the preservation of sediment deposited during the early and late lowstand, the transgressive and the highstand systems tracts. Alluvial and coastal depositional systems characterize the early lowstand phase of the TDS, which developed during the eustatic sea-level fall between about 120 and 30–26yrBP. During the late lowstand phase, which is characterized by stillstand and slow eustatic sea-level rise a prograding delta and an aggrading incised-valley fluvial fill developed. The Tiber incised valley was transformed into a wave-dominated estuary during the transgressive phase (TST), whereas a coastal-shelf sedimentation took place during the subsequent highstand phase (HST). This study confirms the lithofacies distribution resulting from transgression and infilling of the wave-dominated estuaries, but also shows how the transition to a wave-dominated delta, prograding at the time of sea-level highstand occurred. Changes in sediment input, climatic variations and, more recently, human activities played a major role in the development of the Tiber delta during the last 20,000yrBP. In the last 3000years a relationship between progradational phases of the delta and flood events of the Tiber river has been highlighted, suggesting also the formation and merging of barrier-spits to the mainland.

Prediction of stratigraphic compartmentalization in marginal marine reservoirs

Abstract Marginal marine depositional systems exhibit stratigraphic reservoir compartmentalization potential at three hierarchical scales. At each of these scales, stratigraphic compartmentalization potential can be related to the dominant depositional processes and accommodation:coarse sediment supply ratio (A/S) that are acting at the time of deposition. All three orders of compartmentalization potential must be considered in order to define optimal field development plans and completion strategies. The lowest order of compartmentalization is usually at the inter-parasequence scale. The parasequence is represented by a conformable succession of strata separated by marine flooding surfaces and as such it generally defines the basic flow unit in marginal marine systems. In systems tracts associated with relatively high A/S ratios, for example late Lowstand, Transgressive and early Highstand (steeply rising shoreline trajectories), vertical compartmentalization potential is relatively high because of the enhanced preservation potential of flooding surface shales under these conditions. In systems tracts associated with relatively low A/S ratios, for example late Highstand, Falling-stage and early Lowstand (flat, slightly rising and falling shoreline trajectories), vertical compartmentalization potential of parasequences is reduced because the potential for erosion of flooding surface shales by overlying deposits is high and hence potential for vertical sand–sand contact between parasequences is enhanced. The second level of compartmentalization hierarchy is the inter sand-body scale. Individual sand bodies are defined within parasequences. The lateral connectivity of these sand bodies is a product of the dominant depositional processes active at the time of their deposition (wave, tidal, fluvial). Wave-dominated systems tend to produce more laterally continuous sand bodies, fluvial-dominated systems more laterally restricted sand bodies and tide-dominated systems both laterally continuous and laterally restricted sand bodies. Vertical compartmentalization potential of these reservoir sand bodies is related to A/S regime. In high A/S regimes, sand bodies are more likely to be disconnected or compartmentalized. In low A/S regimes, erosional amalgamation of sand bodies is more likely thereby leading to lower compartmentalization potential. The third order of potential stratigraphic compartmentalization is the intra sand-body scale. This scale is represented by intra sand-body heterogeneities such as dipping or horizontal shales, carbonaceous-rich beds or laminae, shale abandonment plugs of channels and carbonate concretions. In high A/S regimes the preservation potential of these heterogeneities is relatively high leading to an enhanced potential for intra sand-body compartmentalization. Lower A/S regimes result in a greater likelihood of lateral and vertical erosion of these heterogeneities leading to a higher potential for reservoir connectivity.

Distribution and origin of hybrid beds in sand-rich submarine fans of the Tanqua depocentre, Karoo Basin, South Africa

This study documents the stratigraphic and palaeogeographic distribution of hybrid event beds that comprise both debris-flow (cohesive) and turbidity current (non-cohesive) deposits. This is the first study of such beds in a submarine fan system to combine outcrop and research borehole control, and uses a dataset from the Skoorsteenberg Formation of the Tanqua depocentre in the Karoo Basin, South Africa. Three types of 0.1–1.0 m thick hybrid beds are observed, which have a basal weakly graded fine-grained sandstone turbidite division overlain by a division of variable composition that can comprise 1) poorly sorted carbonaceous-rich material supported by a mud-rich and micaceous sand-matrix; 2) poorly sorted mudstone clasts in a mud-rich sand-silt matrix; or 3) gravel-grade, rounded mudstone clasts in a well sorted (mud-poor) sandstone matrix. These upper divisions are interpreted respectively as: 1) the deposit of a debris-flow most likely derived from shelf-edge collapse; 2) the deposit of a debris flow, most likely developed through flow transformation from turbidity current that eroded a muddy substrate; and 3) from a turbidity current with mudstone clasts transported towards the rear of the flow. All three hybrid bed types are found concentrated at the fringes of lobes that were deposited during fan initiation and growth. The basinward stepping of successive lobes means that the hybrid beds are concentrated at the base of stratigraphic successions in medial and distal fan settings. Hybrid beds are absent in proximal fan positions, and rare and thin in landward-stepping lobes deposited during fan retreat. This distribution is interpreted to reflect the enhanced amounts of erosion and availability of mud along the transport route during early lowstands of sea level. Therefore, hybrid beds can be used to indicate a fan fringe setting, infer lobe stacking patterns, and have a sequence stratigraphic significance.

Termination of the Last Glacial Maximum sea-level lowstand: The Sunda-Shelf data revisited

The sea-level history around the last glaciation is in the focus of recent, controversial debates. A profound understanding of sea-level changes during this time interval is, however, essential since sea level is a central parameter in the climate system as well as a major force on continental margin sedimentation. Here, we present a seismic record together with augmented data from the Sunda Shelf [Hanebuth, T.J.J., Stattegger, K., Saito, Y., 2002. The architecture of the central Sunda Shelf (SE Asia) recorded by shallow-seismic surveying. Geo-Marine Letters 22, 86–94.] and compare our results in a careful evaluation with the sparse existing data sets of global validity, i.e. the Bonaparte Gulf record [Yokoyama, Y., Lambeck, K., DeDeckker, P., Johnston, P., Fifield, L.K., 2000. Timing of the Last Glacial Maximum from observed sea-level minima. Nature 406, 713–716.; Yokoyama, Y., De Deckker, P., Lambeck, K., Johnston, P., Fifield, L.K., 2001. Sea-level at the Last Glacial Maximum: evidence from nortwestern Australia to constrain ice volumes for oxygen isotope stage 2. Paleogeography Paleoclimatology Paleoecology 165, 281–297.], the Barbados coral record [Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record: influence of glacial melting dates on the Younger Dryas event and deep ocean circulation. Nature 342, 637–642.; Peltier, W.R., Fairbanks, R.G., 2006. Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record. Quaternary Science Reviews 25 (23–24), 3322–3337.] and the latest numerical model of continental deglaciation [Peltier, W.R., Fairbanks, R.G., 2006. Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record. Quaternary Science Reviews 25 (23–24), 3322–3337.]. Sea level seems to have been lower shortly prior to the conventional Last Glacial Maximum (LGM; 21–19 cal kyr BP). The time interval around this glacial lowstand is not covered by ages from the Sunda Shelf, but documented by an ancient barrier — tidal-flat system. These palaeo-coastal relict forms indicate such an early lowstand some 5 m deeper than sea level was during LGM times. The LGM sea level on the Sunda shelf is recalculated to − 123 ± 2 m modern water depth. This depth fits nicely with the lowstand data derived from Barbados and the Bonaparte Gulf. The recently assumed 19-kyr sea-level rise is supported by the Sunda and Bonaparte data sets combined, although it might have started already as early as at 19.6 cal kyr BP lasting for some 800 kyr with an amplitude of at least 10 m. This early pulse-like rise might have played a crucial role in the physical preservation of the high-glacial to early deglacial deposits on the Sunda Shelf. The modelled sea-level history is, thus, supported with respect to an initial high-glacial lowstand prior to the LGM, which might be in apparent contrast to observations from Bonaparte. Nevertheless, field data suggest a glacial sea-level evolution about 10 m deeper than the model. Also, the gradual rising trend from 26 to 16 cal kyr BP, as deduced from the model, can definitively not be approved by any field data. However, our knowledge is still unsatisfactory and an expansion of field data from suited areas is urgently needed.

Hyperpycnal flow variability and slope organization on an Eocene shelf margin, Central Basin, Spitsbergen

Identification of bypass at the shelf margin is critical to deep-water exploration. We examine the shelf margin of an early Eocene fourth-order sequence with an attached basin-floor fan in the Spitsbergen Central Basin. Turbidity currents were fed mainly by hyperpycnal flow emerging from shelf-edge deltas. The life span of any turbidity current was determined primarily by the sediment concentration of the flow and the duration of the river flood. High-density hyperpycnal flows created sand-filled slope-channel complexes 10–15 m (33–49 ft) thick and 100–200 m (328–656 ft) wide that served as conduits for bypass to the basin floor. Low-density hyperpycnal flows were unconfined and deposited heterolithic lobes on the slope. Shelf-margin accretion of about 1.5 km (0.9 mi) during the falling stage gave way abruptly to bypass in the early lowstand. Most of the basin-floor fan growth was achieved after shelf-edge incision and before relative sea level rise. Coastal-plain aggradation in the late lowstand sequestered sediment from the shelf-edge distributaries, effectively diminishing high-density hyperpycnal flow output. The late lowstand was therefore marked by a second phase of shelf-margin accretion with only limited bypass to the basin floor, and a heterolithic, prograding complex downlapped the early lowstand channels. Transgression ultimately led to the abandonment of the shelf-edge delta complex and the accumulation of mainly mudstone on the margin. The shelf-margin architecture exhibited by this sequence should serve as a type example of a deep-water feeder system in which hyperpycnal flow is the primary initiator of turbidity currents for sand accumulation on the slope and basin floor.

Late Holocene lake-level fluctuations in Walker Lake, Nevada, USA

Walker Lake, a hydrologically closed, saline, and alkaline lake, is situated along the western margin of the Great Basin in Nevada of the western United States. Analyses of the magnetic susceptibility ( χ), total inorganic carbon (TIC), and oxygen isotopic composition (δ 18O) of carbonate sediments including ostracode shells ( Limnocythere ceriotuberosa) from Walker Lake allow us to extend the sediment record of lake-level fluctuations back to 2700 years B.P. There are approximately five major stages over the course of the late Holocene hydrologic evolution in Walker Lake: an early lowstand (> 2400 years B.P.), a lake-filling period (∼ 2400 to ∼ 1000 years B.P.), a lake-level lowering period during the Medieval Warm Period (MWP) (∼ 1000 to ∼ 600 years B.P.), a relatively wet period (∼ 600 to ∼ 100 years B.P.), and the anthropogenically induced lake-level lowering period (< 100 years B.P.). The most pronounced lowstand of Walker Lake occurred at ∼ 2400 years B.P., as indicated by the relatively high values of δ 18O. This is generally in agreement with the previous lower resolution paleoclimate results from Walker Lake, but contrasts with the sediment records from adjacent Pyramid Lake and Siesta Lake. The pronounced lowstand suggests that the Walker River that fills Walker Lake may have partially diverted into the Carson Sink through the Adrian paleochannel between 2700 to 1400 years B.P.

Linked lowstand delta to basin-floor fan deposition, offshore Indonesia: An analog for deep-water reservoir systems

In offshore East Kalimantan, Indonesia, three-dimensional seismic reflectors can be traced downslope from a lowstand delta to a basin-floor fan, giving insight into depositional processes controlling the distribution of sands that serve as hydrocarbon reservoirs in many ancient deep-water settings. The studied interval includes the last three Pleistocene cycles (10–330 ka; each 110 k.y. in duration). Cycles on the shelf are dominated by progradational packages deposited during highstands and falling eustatic sea level. Progradational packages are separated by parallel reflectors and carbonate buildups of the transgressive systems tracts. During the last two lowstands of sea level (18 and 130 ka), coarse clastics were not deposited in deep-water environments because growth faults and regional subsidence prevented lowstand deltas from reaching the slope. During the lowstand of sea level that ended at about 240 ka, a delta prograded over the previous shelf edge, and sand-rich sediments spilled onto the slope.Strata on the slope and basin floor show how a deep-water depositional system evolved during a single cycle of eustatic sea level. A canyon on the slope connects the 240-ka lowstand delta to a coeval basin-floor fan. The canyon has a sinuous, bipartite fill that consists of a lower, amalgamated channel complex and an upper channel-levee complex. The basin-floor fan formed at the toe of the slope also has two parts. The stratigraphically lower part of the basin-floor fan has broad lobes with relatively continuous reflectors. The stratigraphically higher part has a sinuous channel-levee complex that prograded over the lower fan and fed sheetlike lobes on the outermost fan. The amalgamated channel fills on the slope and sheetlike lobes on the basin-floor fan have moderate- to high-amplitude reflectors and are inferred to represent sand-rich, early lowstand deposits. The channel-levee complexes on the slope and basin floor are dominated by low-amplitude reflectors and are inferred to be mud-rich strata deposited during the late lowstand. Unlike classic sequence-stratigraphic models, these lowstand strata do not onlap the slope; instead, deep-water clastics extend from the last clinoforms of lowstand deltas. In this system, lowstand deltas determined when and where sand-rich sediments entered preexisting canyons on the slope to feed basin-floor fans.

ABSTRACT: A Comparison of Two Lower Miocene Carbonate Margins: The Zhujiang Carbonate Platform (Subsurface, South China Sea) and The Pirin&amp;ccedil; Platform (Outcrop, Southern Turkey)

Early Miocene carbonate platform complexes have been the subject of extensive exploration efforts in Southeast Asia (for example, the Gunung Putih carbonate complex, East Java Sea, and the Zhujiang and Liuhua platforms, Pearl River Mouth Basin). However, few outcrop equivalents of these early Miocene platforms have been published to date. In this study, we present an outcrop that is an excellent analog for this petroleum province. The Pirin outcrop is an early Miocene carbonate platform-to-basin transition exposed on the northern flank of the Mut Basin in southern Turkey. The carbonates were deposited during a time of relative tectonic quiescence in the early Miocene (Burdigalian) over a relict paleotopography inherited from Oligocene tectonic activity. The interval described here is interpreted to be influenced by three large, relative sea level cycles with 100–150-m amplitude. These generated a steep-edged platform with prograding slope and margin geometries during the highstands, whereas during the lowstands, thin carbonate platforms developed with onlapping geometries against the older underlying slump deposits. Stratigraphic relationships indicate that collapse of the platform margin occurred during late highstand to early lowstand times and was coincident with major sea level falls. During transgression, an isolated platform complex developed. Carbonate production on these platforms shut down around the maximum flooding, leading to terminal drowning. This may have been the result of eutrophication caused by the arrival of nutrients from an encroaching deltaic system, combined with a period of rapid relative sea level rise. The subsurface Zhujiang Platform (Pearl River Mouth Basin, China) is an early Miocene carbonate platform that developed along the flank of a basement high in the basin. A steep, prograding, carbonate-shelf margin developed with large-scale slumping and collapse features in places. Pinnacles and small isolated platforms grow across a zone seaward of the margin. A strikingly similar stratigraphic organization (steep shelf margin, slumps, and isolated platforms) is observed in the Pirin outcrop. These similarities are attributed to a common tectonostratigraphic setting (the rapid marine flooding of a complex relict topography during a time of relative tectonic inactivity) and a Burdigalian eustatic sea level curve with cycles of sufficiently large amplitude to have a significant impact on the platform morphology. At the exploration scale, the outcrop allows us to test three play concepts. From outcrop observations, the slump packets (play concept 1) do not seem to be sufficiently interconnected to make a viable prospect. The development of an enhanced porosity wedge (play concept 2) at the shelf margin will be testable with future outcrop studies. In addition, with future outcrop work, a predictive model for the development of porosity in the isolated platforms can be developed (play concept 3) to more successfully predict porosity prior to drilling. At the reservoir scale, observations of subseismic bedding patterns from the outcrop will help us construct more accurate reservoir models. Additionally, outcrop observations of the dimensions of facies belts and their lateral and vertical variability will improve the way in which we populate a reservoir model with facies and reproduce variability on a scale finer than the interwell spacing.

Models for simulating the fossil record

Computer modeling of the distribution of fossils within depositional sequences provides a means for generating hypotheses and increasing understanding of the fossil record. The simulation presented here joins the basin-scale stratigraphic modeling program STRATA, a random-branching model of species origination and extinction, and a model relating the probability of species occurrence to sedimentary facies. Model results indicate that facies sensitivity of species has the strongest control on range offset, i.e., the time between last appearance in a stratigraphic column and the time of extinction within a basin or the time between first appearance in a column and time of origination in a basin. Model results also predict the distribution of zones of increased range offset within depositional sequences. In the results presented here, increased range offset occurs within the transgressive systems tract of the deep basin, in far updip settings, and near the shelf break of the late highstand to early lowstand. The model also demonstrates how incompleteness of the fossil record is partitioned among sampling bias, facies bias, and unconformity bias. In this simulation, sampling bias and facies bias are roughly equal in magnitude and constant across the basin, but unconformity bias increasingly dominates over these two in updip positions.

Event deposits: the role of sediment supply and relative sea-level changes—overview

This overview on event deposits is based on (1) a brief summary on denudation rates in regions of various relief and climate as derived from the suspended and bed loads of rivers, (2) the fractions of sand and mud present in the fills of various basins, and (3) the mechanisms controlling sediment remobilization. In continental settings, size and frequency of event deposits (debris flows on alluvial fans, avalanching on fan deltas and overbanking on floodplains) directly reflect erosional processes in the source areas and the ratio of denudation area/accumulation area. In marine environments, both sediment supply and the change in near-shore accommodation space largely control the nature of stratigraphic sequences. Under conditions of high sediment supply and low-frequency sea-level changes, the thick systems tracts tend to show only minor differences in the presence of event deposits, including tempestites. With decreasing sediment supply, event deposits are increasingly concentrated in the lowstand systems tract. As shown by a number of models (with differential subsidence or uplift of the basin margin), rapid relative sea-level fall accentuates both coastal and submarine sediment remobilization (rich in sand), particularly during the early lowstand phase, as well as delayed valley incision. The resulting submarine fans tend to be sand-dominated, whereas large fans fed by major rivers are dominated by turbidite muds. In regions of coastal uplift, valley incision persists longer than the lowstand period, and sea-level changes may cause ‘pulses of uplift’ and phases of punctuated cliff erosion. Along carbonate buildups, lowstands of third-order or higher frequency sea-level changes are often recorded by coarse skeletal debris and megabreccias and/or, in the case of mixed systems, by siliciclastic turbidites. In rapidly closing foreland basins, high-frequency sea-level cycles only tend to affect both the proximal and distal basin margins, whereas third-order sea-level changes have a limited potential to directly control depositional sequences and event deposits close to the overthrust front. With high sediment supply, individual event deposits (such as debris flows, sandy and calcareous tempestites and turbidites) mostly form at intervals of tens to hundreds up to some thousands of years. Longer recurrence intervals occur in settings with low sediment supply or characterize very large mass flows and megaturbidites.

Applications of High Resolution Sequence Stratigraphy in North Sea Syn-Rift Reservoir Correlation and Development: ABSTRACT

Tectonically active basins may host a spectrum of sequence stratigraphic expressions previously considered to be spatially mutually exclusive. In low accommodation areas with high sediment supply, fourth order eustatic cyclicity results in high frequency sequence sets while within rapidly subsiding areas, time-equivalent Type-2 sequences are expressed by highly asymmetrical coarsening upward successions, resembling large parasequences. In the shallow marine Fulmar Formation, of the U.K. North Sea Central Graben a sequence boundary and overlying lowstand deposits, which illustrate the effects of laterally variable subsidence rate and intrabasinal topography on the expression of a eustatic sea-level fall, lie between the Glosense and Serratum (J54a and J54b) maximum flooding surfaces. The syn-rift physiography comprises major tilted fault blocks, with the Central Graben dipping parallel to the major faults, simulating a ramp setting. Where the throw of the faults were greatest (SE), the structure acted as a local shelf-slope break. Adjacent to the basin margin, incised valley were cut at fluvial input points (structural transfer zones) and laterally, interfluvial sequence boundaries developed. During early lowstand, sand bypassed the footwall shelf and was deposited as lowstand fan sediments within the deepest part of the hangingwall, with the fault zone acting as a local shelf slopemore » break. Within the shallower water areas of the hangingwall a localised ramp geometry existed parallel to the fault zone. Forced regression deposits developed here were coeval but not physically related to the deep water lowstand turbidite fan deposits.« less

Marine depositional events controlled by sediment supply and sea-level changes

Under conditions of high fluvial or coastal sediment supply, individual sandy storm layers (tempestites) and turbidites form after significantly shorter time intervals than Milankovitch cycles. To provide the material for mass flows of very high volume, large, mountainborne rivers require about ten thousand years. With little sediment supply, however, all event deposits become rare or are missing. Third-order sequences (about 1–4 Ma) vary considerably in thickness depending on the sedimentation rates in the basins. Thick and widely extended mud flows and megabreccias are preferentially triggered by sea level fall below the shelf edge. Mixed siliciclastics-carbonate systems along the foot of carbonate shelves generally become coarser grained and poorer in carbonates during lowstands. Turbidite successions also frequently occur in lowstand deposits, but may be present in the other systems tracts as well if a delta is prograding continually. The position of tempestites in third or higher order shallow water sequences is less clear. In studies on the migration of coastal sands in relation to falling and rising sea level, one should distinguish, apart from sediment supply, between settings with and without substantial wave scour. The generation of sandy tempestites appears to be favoured if coastal progradation and wave scour operate simultaneously for example during late highstand and early lowstand.

The Middle Jurassic Vardekløft Formation of East Greenland—analogue for reservoir units of the Norwegian shelf and the Northern North Sea

The Mesozoic rift basin of East Greenland was uplifted during the Tertiary, and provides excellent exposures of Mesozoic sediments of the same age and type as those occurring deeply buried in the shelf seas around the northern North Atlantic. This paper outlines the preliminary results of field work on the Middle Jurassic Vardekløft Formation, which is the best exposed analogue to the main reservoir rocks of the Northern North Sea and the Norwegian shelf. The formation is up to 600 m thick and is of Boreal Bajocian–Callovian age. The sediments were deposited during a period of sea-level lowstand followed by an overall rise. The base of the formation is an important sequence boundary and the top is a maximum flooding surface, separating the formation from the overlying downlapping Olympen Formation and its correlatives. The Vardekløft Formation consists of shallow marine sandstones of the Pelion Member, which pass basinward into offshore siltstones of the Fossilbjerget Member. The boundary between the two members is strongly diachronous, younging sourcewards toward the north along the basin axis, with a major northward displacement (&gt; 100 km) in the Pompeckji Zone. Internally, the formation is composed of up to 30 coarsening-upward units. The thickness of the units appears to vary in cyclical fashion upward through the member. The units typically consist of a lower, commonly very thin, fine-grained sandstone or siltstone, a middle fine-grained laminated to rippled sandstone, and an upper medium- to very coarse-grained cross-bedded, multi-storey sandstone, topped by a marine flooding surface. This facies succession reflects deposition during overall decreasing water depth. The widespread lateral continuity of the lower, commonly erosional, boundary of the multi-storey sandstone facies suggests that the top part was deposited during relative sea-level fall. This is supported by the high degree of reworking and abundance of coarse-grained sandstone and pebble conglomerate. Different types of flooding surfaces are recognized, showing distinct proximal–distal change in amount of winnowing and erosion. Three orders of relative sea-level changes which probably had significant influence on sandbody distribution and geometry can be recognized. The longest cycle had a duration of c . 20 Ma. It corresponds to the Vardekløft Formation and Olympen Formation which form a crude transgressive–regressive cycle deposited during initial lowstand and succeeding rise, and during highstand and early lowstand, respectively. An intermediate cyclicity is recognized in the stacking pattern of the coarsening-upward units, suggesting cyclical changes in the rate of long-term sea-level rise. A minimum rate of relative rise characterized the Middle and Upper Boreal Bathonian Ishmae–Variabile Zones. High rates of relative sea-level rise characterized the Boreal Bajocian Borealis–Indistinctus Zones and the Lower Callovian Apertum–Calloviense Zones. Finally, the individual coarsening-upward units represent short-term cycles of relative sea-level changes. Comparison with contemporaneous reservoir units such as the Rannoch, Etive and Tarbert formations of the Brent Group in the Northern North Sea shows many similarities in depositional motifs, including vertical and lateral changes in facies development and stacking patterns.

Recent Advances in Sequence Stratigraphy: The Lowstand and Transgressive Systems Tracts

On basin margins characterized by a ramp physiography, relative sea level fall induces basinally isolated, shelf-perched, forced-regression shoreline and deltaic deposition rather than deep-water submarine-fan turbidite deposition. These deposits comprise the early lowstand systems tract. In proximal locations, the sequence boundary at the base of these lowstand deposits is expressed as an erosional unconformity, whereas in distal settings this surface occurs as a correlative conformity. The stratigraphic discontinuity at the top of these lowstand deposits is a ravinement surface and in places can be more striking than the sequence-bounding surface at the base. significant erosion due to fluvial processes during forced regression and subsequent shoreface processes during transgression may be common at the tops of these lowstand deposits. The transgressive systems tract commonly comprises, proximally to distally, backstepped barrier beach deposits, sheet-like lag deposits, and [open quotes]healing phase[close quotes] deposits. This latter unit has not been described widely, but volumetrically may contain a significant part of the transgressive systems tract. Healing phase deposits comprise a wedge of sediment that onlaps the last clinoform of the underlying progradational phase. These sediments commonly are derived by cannibalization from the top or edge of the subjacent progradational phase deposits (i.e., either highstand ormore » lowstand), during and immediately after transgression. These deposits are referred to as the healing phase because their deposition tends to [open quotes]heal over[close quotes] the relatively steep clinoform slope of the underlying progradational phase after transgression has resulted in landward shift of the depocenter. Although sometimes misinterpreted as lowstand deposits, these depositional units commonly do not contain significant reservoir facies.« less