Delta Top Research Articles

Balancing deformation in NW Borneo: Quantifying plate-scale vs. gravitational tectonics in a delta and deepwater fold-thrust belt system

Recent GPS measurements demonstrate that NW Borneo undergoes 4–6 mm of plate-scale shortening a year, which is not accommodated by plate-scale structures. The only geological structure in NW Borneo described to accommodate on-going shortening is the Baram Delta System located on the outer shelf to basin floor. Delta toe fold-thrust belts are commonly thought to be caused by margin-normal compressional stresses generated by margin-parallel upslope gravitational extension. The Baram Delta System is divided into three neotectonic provinces: 1) an inactive onshore and inner shelf region of inversion superimposed on an older extensional deltaic province, 2) an outer shelf region of present-day deltaic extension, and; 3) a compressional delta toe. However, it is uncertain whether compression in the delta toe area is purely driven by gravity (i.e. shortening is less than upslope extension) or if there is a component of regional shortening involved, which can only be demonstrated if shortening is greater than upslope extension. In order to quantify the balance between the shortening related to the upslope gravitational extension and the shortening related to the regional geodynamic framework in the Baram Delta System, we have used a geomechanical code based on the Finite Element Method and conservation of mass and momentum, Dynel 2D. This enabled us to reconstruct the tectonic evolution of the delta and to demonstrate for the first time that the total shortening observed in the delta toe does not balance against the active extension in the delta top; with the system exhibiting >1.8% shortening overall (or ∼2.0 km). This additional shortening is therefore attributed to plate-scale shortening across NW Borneo produced by far-field compression. Using a convergence rate of 4 mm y −1, demonstrated by GPS measurements, the delta toe has accommodated far-field compression only in the last 0.5 My.

Present-day stress and neotectonic provinces of the Baram Delta and deep-water fold–thrust belt

Abstract: We present the first present-day stress orientation measurements across a delta and deep-water, delta toe fold–thrust belt. Stress and neotectonic data for NW Borneo reveal three discrete neotectonic provinces: an inverted, inner shelf province (where present-day maximum horizontal stress, σ H , is oriented margin-normal); an extensional, delta top province (σ H is margin-parallel), and a compressional, delta toe province (σ H is margin-normal). We interpret the delta top and delta toe data to reflect that compression in the fold–thrust belt is coupled to delta top extension and that the inverted, inner shelf province reflects the location of this system on an active margin.

Controls on the sedimentology of an ice-contact jökulhlaup-dominated delta, Kangerlussuaq, west Greenland

This paper characterises the sedimentary impact of a glacial outburst flood or ‘jökulhlaup’ on an ice-contact delta topset at Russell Glacier, Kangerlussuaq, west Greenland. Rapid drainage of an ice-dammed lake in July 1987 generated a jökulhlaup with a peak discharge of ∼ 1300 m 3 s − 1 , which drained across a 500-m-wide, 200-m-long, delta top into a proglacial lake. The delta topset comprises boulder clusters, ice block obstacle marks with relief of up to 4 m, and is graded to lake levels up to 6 m higher than those during typical non-jökulhlaup conditions. The delta top was dissected by the 1987 jökulhlaup causing a fan-shaped extension of the delta front by 30 m. Surface grain size on the delta decreases rapidly away from the main flood flow direction, reflecting rapid downstream reduction in sediment transport capacity. The 1987 jökulhlaup was predominantly fluidal and turbulent and had peak stream powers of 2846 W m − 2 proximally and < 400 W m − 2 distally. Delta topset sedimentation can be characterised by four lithofacies associations in order of decreasing flow energy: (A) coarse-grained deposits related to a flow expansion; (B) finer-grained peripheral deposits located at the margins of the main flow; (C) lobate bars and delta fronts deposited within distal locations and (D) fine-grained deposits at distance from the delta front associated with slackwater conditions. Jökulhlaup-dominated delta topsets are controlled by the geometry of the channel expansion into the proglacial lake, jökulhlaup hydrograph form, the sediment availability and character, proglacial lake basin depth and surface area, lake outflow spillway erodibility and cross-sectional area, and history of previous jökulhlaups.

Clinoform nucleation and growth in coarse‐grained deltas, Loreto basin, Baja California Sur, Mexico: a response to episodic accelerations in fault displacement

AbstractWe investigate the controls on the architecture of coarse‐grained delta progradational units (PUs) in the Pliocene Loreto basin (Baja California Sur, Mexico), a half‐graben located on the western margin of the Gulf of California. Dorsey et al. (1997b) argued that delta progradation and transgression cycles in the basin were driven by episodic fault‐controlled subsidence along the basin‐bounding Loreto fault. Here we test this hypothesis by a detailed analysis of the sedimentary architecture of 11 exceptionally well‐exposed, vertically arranged fluvio‐deltaic PUs, each of which shows lateral facies transition from proximal alluvial facies palaeo‐seaward into distal pro‐delta facies. Of these 11 PUs, seven exhibit a lateral transition from a shoal water to Gilbert‐delta facies associations as they are traced palaeo‐seaward. This transition is characterised by down‐transport development of foresets, which grow in height up to 35 m. Foreset units thicken in a basinward direction, with initially an oblique topset–foreset geometry that becomes increasingly sigmoidal. Each delta is capped by a shell bed that records drowning of the delta top. This systematic transition in delta architecture records increasing water depth through time during individual episodes of progradation. A mechanism that explains this transition is an accelerating rate of fault‐controlled subsidence during each PU. During episodes of low slip rate, shoal‐water deltas prograde across the submerged topography of the underlying delta unit. As displacement rate accelerates, increasing bathymetry at the delta front leads to steepening of foresets and initiation of Gilbert deltas. Subsequent delta drowning results from sediment starvation at the shoreline at high slip rates because of sediment trapping upstream. The observed delta architecture suggests that the long‐term (&gt;100 kyr) history of slip on the Loreto fault was characterised by repetitive episodes of accelerating displacement accumulation. Such episodic fault behaviour is most likely to be because of variations in temporal and spatial strain partitioning between the Loreto fault and other faults in the Gulf of California. A physical explanation for the acceleration phenomenon involves evolving frictional properties on the episodically active Loreto fault.

Grain‐size sorting in grainflows at the lee side of deltas

AbstractThe sorting of sediment mixtures at the lee slope of deltas (at the angle of repose) is studied with experiments in a narrow, deep flume with subaqueous Gilbert‐type deltas using varied flow conditions and different sediment mixtures. Sediment deposition and sorting on the lee slope of the delta is the result of (i) grains falling from suspension that is initiated at the top of the delta, (ii) kinematic sieving on the lee slope, (iii) grainflows, in which protruding large grains are dragged downslope by subsequent grainflows. The result is a fining upward vertical sorting in the delta. Systematic variations in the trend depend on the delta height, the migration celerity of the delta front and the flow conditions above the delta top. The dependence on delta height and migration celerity is explained by the sorting processes in the grainflows, and the dependence on flow conditions above the delta top is explained by suspension of fine sediment and settling on the lee side and toe of the delta. Large differences in sorting trends were found between various sediment mixtures. The relevance of these results with respect to sorting in dunes and bars in rivers and laboratory flumes is discussed and the elements for a future vertical sorting model are suggested.

Sequence stratigraphy in a marine moraine at the head of Hardangerfjorden, western Norway: evidence for a high-frequency relative sea-level cycle

The depositional architecture and stratigraphy of the Hæreid moraine, a sedimentary ridge ca. 1.5 km long and up to 113 m in height at the head of Hardangerfjorden, West Norway, have been reconstructed in a high-resolution sequence stratigraphic framework. Information from ground-penetrating radar (GPR) profiles, outcrop sections and borehole data have been used in the analysis. The basal, forced regressive deposits comprise relicts of a Gilbert-type delta that commenced its development at the front of a grounded glacier. On the seaward (distal) side of the ridge, there is a major channel-fill/mouth-bar complex covering large parts of the delta's former frontal slope and prodelta zone. The complex occurs in the seaward lengthening of a fluvially incised channel and is interpreted as a lowstand to early transgressive succession of fluvial and fluvial-estuarine facies. The channel-fill/mouth-bar complex and the dissected delta top have been planed off through transgressive ravinement and the delta top is overlain by an upvalley-thinning sedimentary wedge, 3–8 m thick and with a general landward onlapping and propagating internal bedform geometry. This wedge is thought to be composed mainly of transgressive, marine (estuary–mouth barrier) facies. On the landward (proximal) side of the ridge, the transgressive deposits include a retrograding landward-dipping foreset interpreted as relicts of a flood-tidal delta overlain by possible washover fan deposits. These deposits are again conformably overlain by laminated clay, interpreted as the remains of a highstand mud drape. The depositional sequence delineated in this study, up to 20 m thick and comprising deposits of a lowstand-wedge, a transgressive and a highstand systems tract, developed in response to a relative sea level (RSL) rise of 12–15 m. This RSL rise, which interrupted the general RSL fall following the deglaciation, is correlated with the Younger Dryas transgression, a regionally recorded sea-level event. The highstand mud drape was deposited as the rising RSL ultimately led to drowning of the moraine. The highstand and transgressive deposits are erosively truncated by a thin gravelly lag, deposited during final emergence in the early Holocene.

Ichnofabrics and sedimentary facies of a tide-dominated delta: Jurassic Ile Formation of Kristin Field, Haltenbanken, Offshore Mid-Norway

Abstract Tide-dominated deltas are poorly known from the stratigraphic record and are notoriously complex, owing to the wide spectrum of facies encountered and their spatial/temporal variability. The tide-dominated deltaic palaeoenvironment combines the ecological harshness of brackish-water settings with complex tidal channel/tidal-flat type facies architecture on the delta top, in association with more classic deltaic facies-stacking patterns. The Ile Formation is interpreted herein as a tide-dominated delta deposited in a microtidal setting. Its palaeoenvironments are interpreted based on a combination of ichnology, ichnofabric analysis and sedimentology. Ichnofabric stacking patterns are used to elucidate the internal architecture of the notoriously problematic aggradational multi-storey tidal channel units. The tide-dominated deltas of the Ile Formation have a distinctive ichnological signature that may be used to characterize tide-dominated deltas. In comparison to typical river-dominated deltas the Skolithos ichnofacies is less well developed and ichnodiversity is lower than expected in wave-dominated deltas. The ichnofabric model presented has potential to be used, with modification, in other tide-dominated deltaic settings.

Interactions between Topography and Channel Development from 3D Seismic Analysis: an Example from the Tertiary of the Flett Ridge, Faroe-Shetland Basin, UK

Abstract Interpretation of 3D and 2D seismic data in the Faroe-Shetland Basin (FSB) has revealed the important role that structurally controlled bathymetry had in controlling sedimentary dispersal during Early Cenozoic thermal subsidence. The Flett Ridge was a major NE-SW structural high during some of the Palaeogene, actively growing and influencing adjacent sedimentary systems. During the Palaeogene this area of the FSB was a key entry point for siliciclastic sediment with a major deltaic system prograding towards the NW during the Middle Eocene. Prior to delta development, the Flett Ridge was onlapped during the Late Palaeocene and subsequently blanketed and drowned in Early Eocene times. Major periods of fluvial incision cutting up to 100 m into the Middle Eocene strata are identified and a variety of channel networks with differing trends documented. Broad channels or valleys of earliest Middle Eocene age inherited the palaeotopography created by the Flett Ridge, whereas subsequent later Middle Eocene meandering channels trend perpendicular to the shelf edge and traverse the Flett Ridge structure. Seismic amplitude maps suggest that a complex and variable channelized drainage system developed across the coastal plain and delta top in the Middle Eocene. These channels influenced sediment supply creating an area of bypass to the more distal fan systems preserved at the base of slope. Later faulting on the ridge crest may also have affected the channel network pattern.

Chapter 6 Summary

Summary The structures that controlled the Carboniferous tectonic and stratigraphic development of northern England were inherited from the earlier Caledonian orogeny which imparted a strong NW-SE and NE-SW tectonic grain that is evidenced on surface and subsurface data throughout northern England. The subsequent Variscan plate cycle, which involved the closure of the Rheic and Rheno-Hercynian oceans, controlled the development of syn-rift, post-rift and inversion megasequences from late Devonian to early Permian times. Regionally extensive, seismically resolvable sequences developed within the Carboniferous were controlled by episodic rifting and fault reactivation, with eustatic sea-level changes providing a high frequency control on depositional sequences that are generally below seismic resolution. In the south of the province during the Dinantian, carbonate environments were extensively developed in syn-rift basins starved of terrigenous elastics. The north of the region was dominated from early Dinantian times onward by a southward prograding terrigenous clastic delta system. The carbonate environments in the south were finally drowned in the early post-rift (Pendleian) when the supply of terrigenous clastic sediments outpaced subsidence for the first time. Following this, rapid southward progradation of the delta systems occurred as a series of pulses largely controlled by eustatic sea-level changes, but with important local controls, namely antecedent rift physiography. By the early Westphalian, delta top conditions had been established over most of northern England. These conditions were progressively disrupted from late Westphalian C times by inversion related to the Variscan orogeny. Over 70 years of exploration in northern England has resulted in the discovery of 75 million barrels of recoverable reserves. All hydrocarbon discoveries to date within the Carboniferous have shown some element of Variscan deformation in their trap geometry. Tectonics have also exerted a subtle, but important control on play fairway evolution (Fig. 62). The main source rocks, developed in distal pro-delta environments, are restricted to the syn-rift depocentres. Syn-rift siliciclastic reservoirs, associated with the early rift phase are located within the isolated, rifted, half graben. Carbonate reservoirs are controlled by the rift topography and the most prospective rim the margins of the deeper half graben in the south of the province. Delta top channel and mouth bar reservoirs are best developed where they axially infill remnant rift bathymetry. Mesozoic burial, ensuring hydrocarbon generation post-dating Variscan trap formation, is considered to be the main control on the present day distribution of hydrocarbons in northern England (Fig. 62). Areas such as the central Pennines and Northumberland and Stainmore basins which received limited Mesozoic burial are considered to have poor hydrocarbon potential. The Cleveland, Manx-Furness and Cheshire basins where Mesozoic burial was excessive have gas as the major hydrocarbon phase. The East Midlands, where Mesozoic burial and post Variscan trap modification have been moderate, has therefore emerged as the most successful oil province in northern England.

Stratigraphy and sedimentology of the Permian of the Falkland Islands: lithostratigraphic and palaeoenvironmental links with South Africa

Comparison of the facies, ichnology and palaeocurrent patterns of the Permian of the Falkland Islands and the Dwyka, Ecca and Beaufort (part) groups in South Africa supports the hypothesis that the Falkland Islands lay east of South Africa in a rotated position prior to continental break-up. Key sections in the Falklands (NW Lafonia) and the Eastern Cape (Ecca Pass) would have lain about 200 km apart in Permian time. In the absence of accurate chonostratigraphic and biostratigraphic information from the Permian of the Falkland Islands, emphasis is placed on lithostratigraphic comparison. The Permian successions in both areas pass from diamictite through basin floor sediments, including organic-rich shale, rhythmites and turbidites. Mud-dominated delta front deposits culminating in channel deposits of a generally subaqueous delta top overlie the basin floor sediments. Points of close lithostratigraphic comparison are: (1) the diamictites have identical appearance, facies, and compatible palaeocurrents; (2) glacial sections are overlain by organic-rich black shales; (3) K-bentonites occur in basin-floor successions; (4) presence of basin-floor turbidite sandstone units with interbedded rhythmite units, and a distinctive Umfolozia/Undichna ichnofauna; (5) coarsening-up delta-front sequences are overlain by fining-up channel sandstone facies; (6) there is a close petrographic similarity indicating derivation of sandstones from a contemporaneous volcanic arc. The Permian successions of the two areas form parts of the fill of the eastern end of the main Karoo Basin, and were subject to the same influences of tectonic development, sediment derivation, eustatic variation and climatic change.

The Trent Gas Field: correlation and reservoir quality within a complex Carboniferous stratigraphy

The Trent Field gas accumulation in the UKCS (United Kingdom Continental Shelf) Southern Gas Basin is reservoired in Carboniferous age sediments. It is contained within a NW–SE trending base Permian structural feature that straddles Blocks 43/24a, 43/23 and 43/25. The proven reserves within the core area lie solely within the 43/24a block. Field appraisal and development encountered three main areas of complexity: structure, reservoir continuity and reservoir property variations. The 20 km long base Permian feature is controlled by NW–SE trending post-Rotliegendes reverse faults which produced a positive inversion. Antithetic faults to these reverse faults also occur within the structure. A second WSW–ENE normal fault trend comparmentalizes the field, and is also thought to have influenced sedimentation during Carboniferous times. Similar fault-influenced deposition is documented at equivalent times in the UK onshore. Ultimately this structural complexity controls the Carboniferous subcrop, which is seen to increase in age from late to early Westphalian A, in a southeasterly direction, although dramatic changes can occur on a more local scale in some fault compartments. The Trent Field accumulation is contained within an interval ranging in age from Marsdenian to early Westphalian A. It encompasses a number of facies associations in delta top and lower delta plain environments, which vary rapidly both vertically and laterally. Reservoir facies include incised valley-fills, transgressive reworked sandstones and distributary channels, which vary markedly in their lateral extent. Reservoir quality of these facies ranges from very poor to excellent and is controlled primarily by the depositional environment in which grain size and sediment maturity are the most important factors.

Exploration in the Gulf of Thailand in deltaic reservoirs, related to the Bongkot Field

Abstract Statoil has been involved in exploration in the Gulf of Thailand since 1985, following an invitation by PTTEP (National Exploration and Production Oil Company of Thailand) to assist in the evaluation and possible development of the Bongkot gas and condensate Field. Statoil and PTTEP jointly studied the field in 1985 and 1986. In 1990 PTTEP farmed 60% out to a group consisting of Total, British Gas and Statoil. Development work on the field started in 1990 with the acquisition of 3D seismic data. The field came on stream in June 1993, and production increased from the original planned 150 10 6 ft 2 /day to 350 10 6 ft 2 /day at present. In the joint PTTEP/Statoil study the Oligocene to Middle Miocene reservoirs were described as fluvial point bar sandstones, deposited in a meandering river system located on a delta top or coastal plain. Based on available data it was not possible to map the individual reservoirs. 22 wells found intermingled gas- and water-bearing sandstones without obvious relation to structure. No field fluid contacts were found, and the pressure regime was thought to be hydrostatic. Statoil simulated a reservoir model consisting of numerous pointbar deposits on a muddy delta plain. The pointbars were assumed to be isolated sandbodies which favoured stratigraphic trapping. Structural mapping of existing 2D and old 3D data sets was thought to give reliable structural maps, although it was difficult to correlate single events. Stratigraphic traps were interpreted as the main traps in the field because no relationship was found between structural closure and gas pools. Since 1990 when field development started with Total as operator around 2500 km 2 of 3D data has been acquired, and approximately 20 exploration wells and 45 development wells have been drilled. These data resulted in a much more accurate description of the field, especially through the use of high-resolution 3D seismic data. The well information allowed the identification of high GR intervals with a continuous seismic expression. Also individual sand reservoirs could be correlated. It became evident from the detailed 3D seismic interpretation that structural traps form the main trapping mechanism, although a minor part of the reserves are found in isolated channel sands, which could be detected from amplitude maps. It has been difficult to identify stratigraphic traps but it is hoped that ongoing seismic attribute analyses will help. In 1993 Statoil farmed in to licence B10/32, operated by Ampolex. The Bongkot model of isolated fluvial channels, with traps formed by several faults with or without structural closure was accepted as an attractive play type. Not withstanding this, the first wildcat well, Karawake-1 was a structural test. The well failed to find hydrocarbons because it intersected a series of overlapping alluvial fans; these lack internal seal and would have precluded hydrocarbon migration and entrapment. Ampolex made a reinterpretation of the block based on the existing 2D seismic, and concentrated on seismic facies mapping. The mapping resulted in two prospect areas where fluvial facies could be recognized above a lacustrine source rock; and also the results of the Karawake-1 well could be better explained. A second well should prove the model.

The sedimentology of Miocene shallow marine clastics of the Sandakan Formation of eastern Sabah

The Sandakan Basin lies on the east coast of Sabah and contains some excellent exposures, resulting in part from the steep topography of the region and from the recent development of housing estates and road construction. These often spectacular outcrops allow examination of the Miocene succession, which has not previously been feasible. A total of 4 months has been spent collecting sedimentological data throughout the c. 25 km by 15 km basin. Over 70 measured sections have been logged across the basin and this, in conjunction with numerous macrofossil, microfossil and ichnofacies studies, has allowed the differentiation of the interbedded sands and clays, previously described as a marginal marine succession, into facies associations ranging from continental to shallow shelfal in origin. Seven main facies groups are identified which range from mangrove deposits cut through by fluvial channels, through delta top with channels into facies deposited in a lagoonallinterdistributary bay environment. These pass into shoreface trough or planar crossbedded sands with common Ophiomorpha or Skolithos trace fossils and finally into offshore marine muddy facies. Palaeoseismicity is suggested by synsedimentary deformation, often confined in discrete horizons which can be traced over several kilometres. Detailed palaeontological studies ofthese rocks will allow development ofthis facies scheme and further division of these systems into sub-environments. These well exposed marginal marine facies present an exceptional opportunity to develop a sequence stratigraphic framework of the Sandakan Basin through the Miocene. Examination of the eustatic/ tectonic control of the system will then be applied to hydrocarbon-bearing strata in adjacent basins. Furthermore this study will constrain palaeogeographic reconstructions of the region.

Global sea-level and the (pen-)insularity of late Cenozoic Britain

Abstract During mid-Pliocene times, Britain was surrounded by warm temperate seas. In the south, these received relatively little clastic input, accumulated mainly biogenic sediments, and appear to have allowed free circulation of marine waters between the North Sea and the Atlantic Ocean, either around or across southern Britain. In late Pliocene times, commencing at about 2.5 Ma bp , the first major global sea-level falls associated with northern hemisphere glaciation occurred. At about the same time, climatic changes appear to have accelerated the rate of headwater extension and bedload transport of the principal rivers flowing towards the southern North Sea. The combination of sea-level fall and progressive deltaic progradation into the southern North Sea converted Britain into a European peninsula. By the mid-Quaternary Cromerian stage, the Great European (Ur-Frisia) Delta top was sufficiently moribund, and subject to subsidence, to allow interglacial sea-levels to encroach southwards across it. During the following Anglian Glaciation, impounded glacio-marine or glacio-lacustrine waters appear to have extended southwards, across both the former delta top and the bounding Weald-Artois chalk ridge, into the English Channel. Subsequently a marine connection between the North Sea and the Atlantic Ocean, via the English Channel, was alternately established and broken, as global sea-level rose and fell (synchronously with interglacial and glacial periods), cyclically converting Britain from island to peninsula and back again.

High resolution seismic profile through a Late Cenozoic delta complex in the southern North Sea

A digitally recorded and processed multichannel seismic profile between Cromer (England) and Sylt (Germany) has provided the first continuous section through the Miocene to Middle Pleistocene delta complex of the southern North Sea Basin. The profile depicts the progressive westward expansion of deltaic sedimentation that eventually filled in this gently subsiding epicontinental basin. Significant basinward shifts in seismic facies occur above all but one of 14 seismic sequence boundaries that punctuate the deposition of up to 830 m of deltaic sediments on the profile. These sequence boundaries and the morphology of prodelta, delta front, delta top and fluvio-deltaic seismic facies record the interaction between subsidence, cyclic fluctuation in sediment supply to, and cyclic change of relative sea level within the basin. Two submarine slides were developed within delta front facies in the Pliocene section and the profile has intersected a swarm of Elsterian (Middle Pleistocene) glacial palaeovalleys that have been incised into the upper part of the delta complex.

Novobiocin-dependent topA deletion mutants of Escherichia coli.

Previous reports of the transduction of topA deletions in Escherichia coli suggested that delta top A transductants grow normally only if they acquire spontaneous mutations that compensate for the topoisomerase I defect. We show that P1-mediated transduction of delta topA in the presence of sublethal concentrations of novobiocin, an inhibitor of the DNA gyrase B subunit, yields uncompensated Top- isolates which are dependent on novobiocin for optimum growth. In the absence of novobiocin these delta topA strains grow slowly, indicating that topA deletions are deleterious but not lethal to the cell. We propose that inhibitors of DNA gyrase B, presumably by lowering intracellular levels of DNA supercoiling, can phenotypically suppress a topoisomerase I defect in E. coli.

Seismic-Scale Outcrops of Oblique Clinoforms, Topatopa Mountains, Southern California: ABSTRACT

The Devil's Heart region of the Topatopa Mountains offers an unusual opportunity to describe, measure, and interpret reservoir-scale facies distributions with respect to seismic-scale stratal geometries. Seismic-scale outcrops of middle Eocene siliciclastic, progradational deltas show stratal geometries and facies distributions parallel and perpendicular to paleoshoreline. Landward-stepping, shoaling-upward cycles with parallel-concordant geometries are overlain by oblique clinoforms. The clinoforms are truncated by toplap against a sequence-bounding disconformity that represents a surface of sediment bypass and erosion. The sequence boundary, in turn is overlain by a second set of landward stepping, shoaling-upward cycles with parallel-concordant geometries. Thus, the deltas prograded episodically across a marine basin and filled the basin nearly to sea level. A deepening event ended progradation and marked a return to aggradational processes. Stratal geometries, facies distributions, and petrophysical properties were used to generate a synthetic seismic profile on which sedimentary facies and depositional environments were superimposed. This synthetic profile was then restored to a depth of 13,000 ft (2.0-2.1 sec) and used as a model to interpret the distributions of facies and depositional environments for actual seismic data from other areas. The model shows continuous reflections from basin facies to the delta top. This contrast with observations of themore » outcrop that show discontinuity of strata at the upper sequence boundary. By superimposing the facies on the synthetic seismic line, it is possible to show that delta-front turbidites and slumped strata comprising the clinoform bottomsets and foresee are not connected to wave-dominated river and/or delta mouth bars comprising the topset strata. This underscores the importance of having the outcrop model to correctly interpret the seismic.« less

Tectono-stratigraphic development and hydrocarbon habitat of the Carboniferous in northern England

Abstract Over 70 years of exploration in the Carboniferous of northern England has resulted in the discovery of rather modest recoverable reserves totalling 75 million barrels of oil and 27 billion cubic feet of gas. Nevertheless during this time the petroleum industry has amassed a substantial quantity of borehole and seismic information. Integrating this essentially subsurface database with information derived from outcrop studies has permitted a hitherto unachievable understanding of the Carboniferous in terms of its tectonostratigraphic development and hydrocarbon habitat. The strong NW-SE and NE-SE structural trends developed in the northern England Carboniferous were inherited from the late Palaeozoic Caledonian orogeny. These fault trends were consistently reactivated throughout the Carboniferous in both an extensional and compressional sense. The main influence on Carboniferous basin evolution in northern England was the Variscan collision-type orogeny. The Variscan plate cycle controlled the development of syn-rift, post-rift and inversion megasequences from late Devonian to early Permian times. Sequences developed within these Carboniferous megasequences are primarily controlled by episodic rifting and periodic fault reactivation with eustatic sea-level changes providing only minor control at the subsequence level. The late Carboniferous-early Permian culmination of the Variscan orogeny is seen to be the main trap forming event. All hydrocarbon discoveries to date display some element of Variscan inversion in their geometry. Variscan tectonics have also exerted a subtle but important control on play fairway evolution. The main source rocks (pro-delta shales) are confined to isolated, syn-rift depocentres. Syn-rift siliciclastic reservoirs are also restricted to the rifted half graben. Carbonate grainstone reservoirs rim the margins of the deeper half graben where terrigenous input has been limited. Delta top channel and mouth bar reservoirs are best developed where they axially infill remnant syn-rift bathymetry. Mesozoic burial, ensuring hydrocarbon generation post Variscan trap formation, is, however, the main control on the present day distribution of hydrocarbons in the Carboniferous of northern England. Several key areas where Carboniferous source rocks have generated significant hydrocarbons during the Mesozoic have been identified; the East Midlands being the most significant in terms of produced hydrocarbons and perceived future potential.

The sedimentology and development of Terravecchia Formation carbonates (Upper Miocene) of North Central Sicily: Possible eustatic influence on facies development

On at least two occasions during the Late Tortonian, siliciclastic deposition ceased and carbonates accumulated at the northern margin of the “Caltanissetta Basin” in north-central Sicily. Bioclastics dominated in the shallower proximal areas, while coral reef facies developed where shallow-water sand and cobble substrates existed or where sea-floor highs has created local shallows. The genetic model proposed is that siliciclastic deposition was repeatedly terminated during the Late Tortonian by rises in sea level which transgressed fluviatile and deltaic environments. Carbonate deposition was widespread, but, later erosion and reworking removed much of the sediments except within abandoned delta distributary channels and minor sea-floor depressions. Reef developments, associated with channel mouths and the shelf slope break, have been preserved in situ, whereas channels in more proximal locations are filled with siliciclastic, bioclastic and intraclastic rubble from elsewhere on the delta top. The wide lateral extent of the truncated tops of the carbonate beds argues for eustatic, rather than tectonic controls of erosion and subsequent reworking. These events may be closely related in time to other recorded Late Tortonian eustatic events in the foreland zones, such as the Hyblean Plateau and the Maltese Islands further south. Collectively they give some support to a model involving multiple eustatic fluctuations associated with the onset of the Messinian desiccation of the Mediterranean.

Hyperpycnal plumes and plume fronts over the Huanghe (Yellow River) delta front

The Huanghe (Yellow River) discharges extremely high suspended sediment concentrations (25 to 220 g/l) which favor sustained hyperpycnal plumes (underflows). Observations of weakly hyperpycnal unchannelized plumes and indirect evidence of strongly hyperpycnal channelized underflows over the delta front indicate the importance of these modes of sediment dispersal. The weakly hyperpycnal plumes occupy the entire water column over the shallow (<5 m) delta top. From a pronounced front near the break in slope at about 5 m depth, they descend over the delta-front slope as wide-spread underflows. Evidence of strongly hyperpycnal underflows was shown from subaqueous valleys partly filled with low-density mud.