Genetic Stratigraphic Sequences Research Articles

Organic petrographic, geochemical, and sequence stratigraphic analyses for evaluating the hydrocarbon potential of Middle Jurassic–Lower Cretaceous rocks in Shushan Basin, northwestern Egypt

Integrated organic petrographic and geochemical analyses were made on organic-rich marine carbonate and mixed clastic-carbonate rocks of Middle–Late Jurassic and Early Cretaceous age from the Shushan Basin, Egypt to evaluate their hydrocarbon potential. Analyses allowed the identification of depositional settings, paleoclimate, and three third order genetic stratigraphic sequences (SQ) with deposits assigned to highstand (HST), lowstand (LST) and transgressive systems tracts (TST). Deposition of the source rocks in the rifting Shushan Basin resulted from the interaction between Neotethyan sea level changes, tectonic, and climate. The good reducing conditions developed during the Neotethyan Middle–Late Jurassic (Bajocian–Kimmeridgian) second order sea level rises and the climatically induced carbonate sedimentation resulted in the deposition of the organic-rich carbonates of the Khatatba Formation (SQ 1, early–middle TST) in inner–middle shelf settings under anoxic–dysoxic conditions. The Late Jurassic (late Kimmeridgian) uplifting resulted in the deposition of the organic-lean mixed clastic–carbonate strata of the Masajid Formation (SQ 1, latest TST) in the same shelfal and reducing conditions, which experienced a notable dilution of organic matter. The late TST deposits of SQ 1 are good to very good oil-producing source rocks, where they show average good to very good generative potential of late mature (late oil-to early wet gas-window) highly oil-prone organic matter. The Early Cretaceous (Valanginian–Albian) uplifting associated with the rifting of the Shushan Basin overprinted the Neotethyan late Valanginian–Hauterivian second order sea level rises, Aptian second order highstand sea level, and Albian second order sea level rise. The coeval climatic shift toward more humid conditions resulted in the clastic-dominated deposition of the organic-lean regressive units of SQ 2 (HST and LST of Alam El Bueib, Alamein, and Dahab formations) and SQ 3 (HST and LST of the lower–upper Kharita Formation) in marginal marine settings under anoxic–dysoxic to oxic conditions. The HST and LST deposits of the SQ 2 and SQ 3 show poor to good organic richness of early–mid mature (early–peak oil-window) oil/gas-prone and gas/oil-prone organic matter, respectively and exhibit average fair oil source rock potential with no gas generation.

Chapter 2. Sequence stratigraphic concepts and methodologies

Abstract This chapter reviews sequence stratigraphic concepts and methodologies and presents an approach that is most applicable to the North Sea Jurassic, based on the concept of genetic sequence stratigraphy. The concept of depositional sequences, comprising rock units bounded by unconformities, has been developed from the late nineteenth century up to the present day. Many different studies have been carried out on North Sea Jurassic sequence stratigraphy, from the early 1980s to the present day and involving a range of different approaches. Many authors have adopted the J sequence approach that was first published in the early 1990s; however, a number of alternative North Sea Jurassic sequence schemes have also been described. A close relationship existed between tectonics and sequence boundary development, particularly during the Middle–Late Jurassic in the North Sea region. Several of the major unconformities that are known to be of regional extent can be directly related to significant tectonic phases. Other sequence boundaries, for which a tectonic control is not evident, for example, particularly in the Early Jurassic, were potentially driven by glacio-eustatic cycles, which may have been controlled by orbital forcing cycles.

Tectonostratigraphic Framework and Depositional History Pattern of the Cretaceous Successions Period in Southern Iraq

Twenty oil wells were selected to study the tectonostratigraphic of the Cretaceous in southern Iraq, in order to develop a comprehensive description of the petroleum system in the region. That was conducted through an interpretation of the technical reports and the available information of the wells, which include sedimentary, stratigraphy, tectonic reports, and oil reservoir studies of the Cretaceous. Stratigraphically, a third order cycle was identified in the Cretaceous succession in southern Iraq, which also comprises seven and half cycles of the fourth order. Eight genetic stratigraphic sequences were also identified, as well as eight maximum flooding surfaces. The concept of the tectonostratigraphic boundary (TSB) and the tectonostratigraphic unit (TSU) has been adapted in this study. In the present study, the Cretaceous period in southern Iraq considers one tectonostratigraphic system (TSS) consisting of four main tectonostratigraphic categories. Each category consists of a set or group of secondary tectonostratigraphic units; these are TSU1A, TSU1B, TSU1C, TSU1D-TSU2A, TSU2B-TSU3A, TSU3B, TSU3C-TSU4A, TSU4B, and TSU4. These units are separated by five tectonostratigraphic boundaries presented from TSB1 to TSB5 by Sulaiy, Shuaiba, Mauddud, Khasib, lower part of Tanuma, and Shiranish. The lateral extensions of the TSUs that are close to the passive margin (northeast part of the study area) are hydrocarbon reservoirs. The lateral extensions TSUs that are far from the passive margin (southwest part of the study area) are hydrocarbon generator source. The intermediate unite is characterized as both a generator source and reservoir hydrocarbon. Vertically, the TSUs are characterized by improved reservoir properties with reduced depth due to the lack of compressional tectonic force, which leads to forming a good primary porosity. The transfer from north to south of the study area represents a trend of improvement in reservoir characteristics for the same reason as mentioned previously. Finally, the TSB represents a source generator hydrocarbon more than a reservoir.

The Turonian-Coniacian stage boundary in an expanded siliciclastic succession: Integrated stratigraphy in deltaic through offshore facies, Bohemian Cretaceous Basin

During the latest Turonian – early Coniacian time, a succession of sand-rich, Gilbert-type deltas was deposited along the faulted margin of the Bohemian Cretaceous Basin (BCB, Central Europe), fining along depositional dip into prodeltaic heterolithic facies and offshore mudstones to marlstones. The active tectonic setting with accelerated subsidence and supply rates contributed to deposition and preservation of an expanded record showing otherwise insufficiently known parts of the Turonian-Coniacian (T–C) boundary interval. In this study a combination of detailed biostratigraphy, genetic sequence stratigraphy, and carbon isotope chemostratigraphy is employed to characterize the T–C boundary in the northern part of the BCB, in both nearshore and offshore facies. The basis of the biostratigraphic framework was the establishment of the same succession of inoceramid bivalves and other molluscan marker taxa and bioevents as that in Salzgitter-Salder (Germany) and Słupia Nadbrzeżna (Poland). All the faunal markers from the Mytiloides scupini through Cremnoceramus crassus crassus inoceramid zones were found both in the nearshore and offshore facies. The linkage of biostratigraphic and carbon isotope-stratigraphic data to a regional stratigraphic picture, as well as to individual outcrop and core sections, provides an important new database for further study of the boundary interval, with a direct link to the transgressive-regressive history of the nearshore depositional systems. Therefore it is proposed here that the T–C interval in the BCB complements the Salzgitter-Salder and Słupia Nadbrzeżna sections and together with them constitutes a broader type region for definition of the T–C boundary.

Genetic sequence stratigraphy on the basis of ichnology for the Middle Jurassic basin margin succession of Chorar Island (eastern Kachchh Basin, western India)

Abstract Synrift basin margin successions are greatly influenced by eustatic sea level changes, tectonics and accommodation space filled in by sediments. The Middle Jurassic (Bajocian–Callovian) of Chorar Island (western India) comprises a ~109-m-thick synrift basin margin succession of clastic, non-clastic and mixed siliciclastic-carbonate rocks which are here analysed and categorised into nine lithofacies. The succession is bioturbated to varying intensities; 16 identified ichnogenera can be assigned to environmentally related groups of five trace fossil assemblages, which include Gyrochorte, Hillichnus, Rhizocorallium, Skolithos and Thalassinoides. These ichnoassemblages document the Skolithos and Cruziana Ichnofacies which marks a change in energy conditions, sedimentation dispersal patterns and bathymetry in a shallow-marine environment. The Bajocian–Callovian succession is further analysed on the basis of sedimentological and ichnological data that show two genetic sequences consisting of Transgressive Systems Tract and Highstand Systems Tract bounded by Maximum Flooding Surface. The synrift basin margin succession of the Middle Jurassic of Chorar Island shows cyclicity in deposition; the Bajocian–Bathonian succession represents progradational to retrogradational coastlines, while the Callovian succession documents an aggrading progradational coastline.

Sedimentology, clay mineralogy and palaeosols of the Mid-Carnian Pluvial Episode in eastern Spain: insights into humidity and sea-level variations

This study examines rainfall variations of the Mid-Carnian Pluvial Episode (CPE) based on the continental fluvial sedimentology, palaeosol and clay mineralogy records of the Stable Meseta (eastern Spain). In the formation examined, the Manuel Fm or K2 Fm, the CPE is represented by three regressive–transgressive sequences, or subunits K2.1, K2.2 and K2.3, from base to top. Each subunit broadly consists of a genetic stratigraphic sequence bearing well-developed highstand, lowstand and transgressive systems tracts. Hydromorphic features in the palaeosols suggest changes in the activity of both groundwater and surface water. The clay mineral assemblage is dominated by illite, with a minor presence of kaolinite and traces of smectite in some samples. After ruling out tectonism in the study area, climate and eustatism emerge as the main allogenic controls in the sedimentary record. Differentiated sedimentary facies and architectural elements in the K2.2 subunit were probably controlled by both a more humid climate and source area, whereas K2.1 and K2.3 were more related to base-level changes and eustatic control. The presence of more waterlogged pedotypes and of kaolinite and traces of smectite in the clay mineral assemblage of K2.2 also indicates increased humidity. Notwithstanding, our data do not point to intense rainfall periods for the CPE in eastern Spain.

Barremian synrift sedimentation in the Oliete sub-basin (Iberian Basin, Spain): palaeogeographical evolution and distribution of vertebrate remains

A review of the onset of the synrift sedimentation and synsedimentary extensional tectonics of the Oliete sub-basin (northwestern Maestrazgo basin, East Spain) is presented here based on new data acquired after extensive sedimentological, structural and palaeontological analysis of the Barremian Blesa Fm. The lower boundary of the Blesa Fm is a prominent basal synrift unconformity overlying Jurassic units. This formation has been divided into three genetic stratigraphic sequences bounded by sub-basin-wide unconformities. The lower Blesa sequence (LBS) is characterized by distal alluvial to palustrine marls/clays grading upward to palustrine–lacustrine limestones. The LBS is bounded on top by a planar to irregular transgressive, hardened ferruginous surface, locally encrusted by oysters. Above this discontinuity, the middle Blesa sequence consists of oyster-rich limestones and marls deposited in a shallow restricted bay, which grade to distal alluvial and palustrine–lacustrine marls/clays and limestones towards the marginal areas of the basin. The boundary between the middle and upper Blesa sequence (UBS) is a regressive surface outlined by the local presence of an erosive conglomeratic bed. The UBS generally starts with red clays deposited in distal alluvial fan environments, which grade upwards to palustrine and lacustrine carbonates and marls/clays. The local presence of heterolithic alternations of clay with fine-to-medium sandstone and of cross-bedded sandstones indicates the local occurrence of siliciclastic coastal environments in the UBS. The upper boundary of the Blesa Fm is marked by widespread transgression, giving rise to the bioclastic limestones of the Alacon Fm. In the present paper, the stratigraphic position and palaeoenvironmental context of the abundant vertebrate remains found across the defined sequences within the Blesa Formation is reviewed. The results obtained are relevant for a further understanding of the tectosedimentary evolution of the studied basin. Successive stages of evolution are distinguished, including the initial uplift, breakup and erosion of the earlier Jurassic carbonate platform that took place during the Tithonian–Hauterivian; the onset of synrift sedimentation during the early Barremian, which was highly controlled by extensional faulting and differential block subsidence; the homogenization of the basin subsidence accompanied by the incursion of marine waters (sourced from southeastern areas) during the middle part of the Barremian; and the significant fall in base level, of possible climatic origin and also involving significant siliciclastic input in the northern areas of the Oliete sub-basin around the middle part of the late Barremian.

Stratigraphic architecture of incised valleys and unincised channel systems in the Carbonera Formation (C6-C1 members: Upper Oligocene – Lower Miocene), Llanos basin, Colombia

This work presents a sequence stratigraphic framework for the Upper Oligocene – Lower Miocene C6-C1 members of the Carbonera Formation in the Llanos basin, Colombia. In the interval corresponding to the C5 member, facies associations that comprise middle shoreface, tidal channels and bars, tidal flat deposits, brackish bay, tidal-fluvial channels and central estuary have been identified through the integration of core descriptions and wireline logs. The vertical assemblage of the recognized facies associations permits the definition of several cycles of transgression and regression, as well as episodes of relative sea-level fall and their consequent fluvial incision. Basinward shifts of facies, lateral changes in well-log motifs and seismic amplitude anomalies allow the interpretation of incised-valley systems overlying deposits associated with marginal and shallow marine environments, which also present channelized sand bodies. Consequently, the sedimentary and stratigraphic architecture is composed of an interspersed set of channels with different genetic origins and contrasting potentials in terms of petroleum production. Maximum flooding surfaces, subaerial unconformities and transgressive ravinement surfaces are the significant stratigraphic surfaces identified, which separate progradational and retrogradational stages and define sequence boundaries.In addition to the C5 member, a sequence stratigraphic model for the interval corresponding to the C6-C1 members of the Carbonera formation is presented based on cuttings, well-log stacking patterns and interpretation of 3D seismic horizon and time slices. In total, nine regressive-transgressive cycles (i.e., genetic stratigraphic sequences) are interpreted for the interval studied. The proposed stratigraphic framework relates oil occurrences and currently producing intervals with stratigraphic surfaces and permits a consistent classification of the petrophysical properties, establishing several families that are genetically related.The sedimentological descriptions of the Carbonera C5 member and the proposed sequence stratigraphic model highlight the marine influence in the deposition of the Carbonera Formation. Furthermore, this work emphasizes the role of relative sea-level fluctuations in deposition and erosion in coastal areas during tectonically active periods in the foreland Llanos basin.

STRATIGRAPHIC CORRELATION OF THE LATE SILURIAN-DEVONIAN OF THE AMAZON BASIN (BRAZIL) BASED ON GENETIC STRATIGRAPHIC SEQUENCE APPROACH

The current study consists in the stratigraphic analysis of Late Silurian-Devonian interval in the Amazon Basin (Brazil) using the concepts of the Modern Stratigraphy, more specifically the genetic stratigraphic sequence, proposed by Galloway. that uses the marine flooding surface as a boundary of sequence. This methodology is since the sediments of studied interval are related to marine transgressions which were part of the basin paleogeographic context. Thus, the maximum flood surfaces, which are presented in four sections (AA ', B-B', CC 'and D-D') throughout the basin, represent chronostratigraphic events, which stand out in gamma ray profiles and are used as correlation data in thirteen exploration wells. Analysis of these sections allowed the identification of four fourth-order sequences (AB, BC, CD and DE) bounded by maximum flooding surfaces on the top and the base. Each sequence consists of asymmetric regressive-transgressive cycles, represented by highstand systems tract and transgressive systems tract. The analysis of these sections integrated with the stratigraphic maps (isopach and isolith) allowed the identification of the depocenter and two main source areas of sandy sediments (west and south). Besides, it was possible to infer that the marine transgression during the Paleozoic induced the deposition of pelitic sediments and followed a trend from northeast to southwest.

Current models and standardization of sequence stratigraphy

Abstract Based on previous research, the paper summarized several sequence stratigraphic models (depositional sequence, genetic stratigraphic sequence and transgressive-regressive sequence) and standardized the workflow of sequence stratigraphy, and analyzed the merits and pitfalls of different sequence models. Sequence models representing stratigraphic records in different geological settings were applied to corresponding settings. The occurrence of system tracts was controlled by the interplay between accommodation and sediment supply, and was ascribed to changes such as tectonics, sea-level, sediment supply, climate and so on. The standardized workflow of sequence stratigraphy was based on the model-independent approach and model-dependent approach. Differentiating genetic types was essential for the standardization of sequence stratigraphy.

Salt tectonics and Pliocene stratigraphic framework at MC-118, Gulf of Mexico: An integrated approach with application to deep-water confined structures in salt basins

Integrated tectono-stratigraphic interpretation at MC-118 using 3D seismic, well logs and biostratigraphy reveals an area dominated by allochthonous salt and its related structures. OCS-Block MC-118 is located 130 km southeast of New Orleans on the Gulf of Mexico middle slope in ∼2600 ft of water.The area is divided into three domains based on their structural styles: (1) a western domain consisting of a basinward-dipping normal fault family and associated strata; (2) a central domain composed of a landward-plunging diapiric salt tongue canopy and associated salt welds, two flanking NE–SW trending salt-withdrawal mini-basins, and a crestal fault family; and (3) an eastern domain comprised of basinward/landward-dipping normal and listric normal fault families with their associated rollovers. These structural domains are genetically-and-kinematically related to the salt structure and extend beyond MC-118 boundaries. The salt structure is postulated to have evolved mostly passive, with punctuated active episodes, and by lateral spreading. This is part of a larger regional structure, eastern Gulf of Mexico, which involves some amalgamation between small-scale salt canopies and salt diapirs although collectively they appear mostly disconnected.A Pliocene (3.13–4.95 Ma) third-order genetic stratigraphic sequence, the focus of this study, is as much as ∼3600 ft thick within the mini-basins and contains: muddy mass transport complexes; sandy slope fans; muddy turbidites and condensed sections; and transitional facies flanking the salt structure that collectively have ponded and wedged external geometries. Mass transport complexes and muddy turbidites and condensed sections make most of the studied genetic sequence in a mud-dominated deltaic setting eastern Gulf of Mexico.Facies kinematic indicators and a matching number of genetic sequences accounted on the sea level chart support a eustatically driven mini-basin sedimentation. Nonetheless salt still plays a role in sedimentation (secondary/minor) by slumping generated during passive/active salt diapiric evolution.

Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian, Bahariya Formation) in the Salam Oil Field, north Western Desert, Egypt

Twenty-eight samples from the Bahariya Formation of the Salam-17 Well in the north Western Desert were palynologically investigated. These samples are of Cenomanian age. Fair diversity and fair to moderately preserved palynomorph assemblage has been recovered. Among them, the dinoflagellate cysts showed very poor diversity and abundance. Four miospore zones have been informally identified in the lower Cenomanian. Various palynofacies criteria, adopted from previous publications (e.g. relative particle abundance data, brown to black wood ratio, equi-dimensional to lath-shaped black wood ratio, average size of phytoclasts and spores/pollen ratio) are applied as alternative indicators to monitor the proximal–distal trends instead of the marine palynomorphs-based parameters. The method can be applied in the Egyptian Western Desert to overcome the rarity and absence of dinoflagellate cysts in the recovered organic residues. The palynofacies study of the section demonstrates a predominantly regressive phase, characterized by deltaic, distributary or tidal channels, interrupted by short-lived marine incursions. The palynofacies trends within the studied succession indicate six genetic sequences informally described as Genetic Stratigraphic Sequences A through F.

Biostratigraphy of the Lower Palaeozoic Haima Supergroup, Oman; its application in sequence stratigraphy and hydrocarbon exploration

ABSTRACTThe siliciclastic Haima Supergroup (Cambrian-Silurian) on the west flank of Al Ghabah Salt Basin in Oman is currently a target for gas exploration following the discovery of significant gas reserves. An understanding of stratigraphical and facies relationships within the Haima is crucial for their exploration and exploitation, and biostratigraphy is one of the more powerful and cost-effective tools that can be deployed to aid such understanding. The biostratigraphy of the Haima Supergroup is based primarily on palynomorphs. The pre-existing biozonation, comprising conventional interval zones, is of low resolution and incorporates misconceptions over the ranges of key species. The work reported here explicitly considered the biostratigraphy of the Haima Supergroup in the context of genetic sequence stratigraphy. It shows that each of the postulated marine-flooding events in the Haima Supergroup is characterised by a unique assemblage of marine palynomorphs. Intervening strata are characterised by low-diversity and sometimes sparse marine palynomorph assemblages, often comprising only sphaeromorph acritarchs, accompanied by terrestrial cryptospores. These low-diversity assemblages indicate proximal-marine to non-marine conditions, consistent with progradation following each marine flood. The sandy prograding deposits in the Haima Supergroup form reservoirs, which are sealed by the shales and mudstones deposited by succeeding marine-flooding events. Lateral changes in marine assemblages indicate onshore-offshore relationships, and have contributed towards mapping the extent of each marine-flooding event, critical for evaluation of the potential areal extent of seals. The result is improved biostratigraphical resolution, which enables more precise correlation between wells, and which has contributed to more detailed palaeogeographical maps and to a better assessment of the distribution of reservoir-seal pairs.

Sedimentology and stratigraphic affinities of Neoproterozoic coarse clastic successions, Glenshirra Group, Inverness-shire, Scotland

Synopsis Records of ancient environments and past basin histories can be preserved in metasedimentary successions, despite their subsequent deformation and metamorphism. In the Central Scottish Highlands SE of Loch Ness, the Garva Bridge Psammite and the Glen Buck Pebbly Psammite Formations (hitherto included within the Glenshirra Subgroup at the base of the Neoproterozoic Grampian Group) represent a continuum of alluvial fan to shallow water sediments, deposited in a SE thinning fan-delta clastic wedge. These sediments, derived from an uplifted granitoid hinterland to the west, contrast with the overlying marine sedimentary rocks of the Corrieyairack Subgroup, which were deposited by sediment gravity flows within a submarine slope setting. The Glen Buck Pebbly Psammite/Garva Bridge Psammite Formations and the Corrieyairack Subgroup represent two genetic stratigraphic sequences divided by a sharp sequence boundary that records a major reorganization in basin architecture. Hence, we propose that the Garva Bridge Psammite and Glen Buck Pebbly Psammite Formations be included within a separate Glenshirra Group, genetically unrelated to either the marine deposits of the immediately overlying Grampian Group or the earlier, locally migmatized (Moinian?) basement to the Central Highlands. The Glenshirra Group thus represents the earliest phase of post-Knoydartian extension, predating the main Dalradian basin development.

Genetic sequence stratigraphy of cool water slope carbonates (Pleistocene Eucla Shelf, southern Australia)

The south Australian Eucla Shelf belongs to the world's largest cool-water carbonate sedimentary system. During the Pleistocene, it exported large amounts of sediment to the shelf edge and upper slope resulting in an expanded sedimentary wedge. Wedge-internal clinoforming seismic reflectors suggest a stacking of the deposits into genetic sequences. High-resolution stable oxygen and carbon isotope, point counting, grain size, and carbonate mineralogical XRD analyses were carried out to characterize these genetic sequences along a dip-parallel transect of three ODP Leg 182 drill holes located between the shelf edge and upper slope. Oxygen and carbon isotope fluctuations show that the genetic sequences formed as a response to sea level fluctuations. Within the genetic sequences, facies differentiation and sediment volume partitioning occur along the transect. Lowstand deposits are fine grained and contain more sponge spicules and micrite. Highstand deposits are coarse grained with tunicate spicules, brown bioclasts, as well as bryozoan and corallinacean debris. Boundaries separating highstand and lowstand deposits are triggered by sea level fall, and are expressed as abrupt grain size changes or as turning points in grain-size trends. Analyzed components vary in abundance along the transect. Genetic sequences show dip-parallel variations in thickness combined with changing relative proportions of lowstand versus highstand deposits.

Cenozoic depositional history of the Gulf of Mexico basin

A Geographic Information System (GIS) database incorporating information from 241 publications, theses, and dissertations; well logs and paleontologic reports; and interpreted University of Texas Institute for Geophysics (UTIG) deep-basin seismic lines was used to map and interpret 18 basinwide genetic stratigraphic sequences that form the Gulf of Mexico basin Cenozoic fill. Eight principal extrabasinal fluvial axes provided the bulk of the sediment infill in the basin. First-order temporal and spatial use of these axes reflects four continent-scale phases of crustal uplift. Abundant sediment supply has prograded the northern and northwestern basin margin 150 to 180 mi (240 to 290 km) from its inherited Cretaceous position. Margin outbuilding has been locally and briefly interrupted by hypersubsidence due to salt withdrawal and mass wasting. Three depositional systems tracts characterize Cenozoic genetic sequences: (1) fluvial --> delta --> delta-fed apron, (2) coastal plain --> shore zone --> shelf --> shelf-fed apron, and (3) delta flank --> submarine fan. One or more examples of the fluvial --> delta --> delta-fed apron systems tract occur in each of the major genetic sequences. Immense volumes of sand have bypassed the shelf margin to be deposited in slope and base-of-slope systems, primarily within fluvial --> delta --> delta-fed apron system tracts, during all major Paleogene and Neogene depositional episodes. Deposition and preservation of volumetrically significant coastal plain --> shore zone --> shelf --> shelf-fed apron tracts is typical of Paleogene through Miocene depositional episodes only. Fan system origin was commonly associated with major continental margin failures, but large submarine canyons occur mainly in Pleistocene sequences. Thick, potential reservoir sand bodies occur in offlapping delta-fed slope and subjacent basin floor aprons, in autochthonous slope aprons and related infills of slide scars and canyon cuts, and in submarine fans.

Abstract: Application of genetic sequence stratigraphy in defining coalbed-gas exploration fairways: Examples from Williams Fork Formation, Sand Wash and Piceance Basins, Colorado

Abstract: Application of genetic sequence stratigraphy in defining coalbed-gas exploration fairways: Examples from Williams Fork Formation, Sand Wash and Piceance Basins, Colorado

Facies architecture and sequence stratigraphy in glacially influenced basins: basic problems and water-level/glacier input-point controls (with an example from the Quaternary of Ontario, Canada)

Sequence architecture is controlled by two fundamental factors; the water level relative to the depositional surface (which controls accommodation space), and the points or areas of sediment injection (which control how this space is filled in). Relative water level, accommodation space and injection points normally vary together and various genetic terms are used to define systems tracts. But in glacially influenced basins, accommodation space is controlled not only by water level, but also by the glacier. Injection points are similarly dually controlled — by water level and by the terminus of the glacier. During high lake levels the injection point of the glacier may be underwater at the base of the slope. Genetic terms like `highstand' and `lowstand' therefore are meaningless in successions controlled by sediment input points which are independent of water levels and accommodation space. Distinct water-level and glacier input-point systems can occur side by side: the water-level system controlled by lake level or sea level, the glacier input-point system controlled by the position of the subaqueous glacier front. And both can form juxtaposed systems passing laterally into each other. A bewildering and incompatible terminology is needed if the terms of genetic sequence stratigraphy are used for both water-level and glacier input-point systems, because the sedimentary injection points and sequence boundaries of the two systems fluctuate out of phase with one another. Sequences developed by movement of the glacier are completely independent of those controlled by water levels. If genetic sequence stratigraphy is applied to a basin with both lake-level and glacier input-point systems, then each needs separate genetic terminologies to describe them, which leads only to confusion. However, descriptive sequence stratigraphy (allostratigraphy) can be readily applied to successions like the Quaternary of Lake Ontario. We therefore suggest that genetic sequence terms be discarded and simpler allostratigraphic terminology used in glacially influenced basins.

Sequential filling of a late Paleozoic foreland basin

ABSTRACT Through the use of an extensive data base of geophysical well logs, parasequence-scale subdivisions within a late Paleozoic synorogenic clastic wedge resolve cycles of sequential subsidence of a foreland basin, sediment progradation, subsidence of a carbonate shelf edge, diachronously subsiding discrete depositional centers, and basinwide transgression. Although temporal resolution of biostratigraphic markers is less precise in Paleozoic successions than in younger basins, parasequence-scale subdivisions provide more detailed resolution within marker-defined units in Paleozoic strata. As an example, the late Paleozoic Black Warrior basin in the foreland of the Ouachita thrust belt is filled with a synorogenic clastic wedge, the lower part of which intertongues with the fringe of a cratonic carbonate facies in the distal part of the basin. The stratal geometry of one tongue of the carbonate facies (lower tongue of Bangor Limestone) defines a ramp that grades basinward into a thin black shale. An overlying tongue of the synorogenic clastic wedge (lower tongue of Parkwood Formation) consists of cyclic delta and delta-front deposits, in which parasequences are defined by marine-flooding surfaces above coarsening- and shallowing-upward successions of mudstone and sandstone. Within the lower Parkwood tongue, two genetic stratigraphic sequences (A and B) are defined by parasequence offlap and downlap patterns and are bounded at the tops by basinwide maximum-flooding surfaces. The distribution of parasequences within sequences A and B indicates two cycles of sequential subsidence (deepening) and progradation, suggesting subsidence during thrust advance and progradation during thrust quiescence. Parasequence stacking in sequences A and B also indicates diachronous differential tectonic subsidence of two discrete depositional centers within the basin. The uppermost sequence (C) includes reworked sandstones and an overlying shallow-marine limestone, a vertical succession that reflects no tectonic subsidence, a very minor or null sediment supply, and basinwide transgression. The temporal resolution at parasequence scale significantly improves the resolution of the tectonic history of the thrust belt-foreland basin system.

Middle Oxfardian to Volgian sequence stratigraphy of the Greater Shearwater area

A Middle Oxfordian to Volgian sequence stratigraphic model for the Greater Shearwater area is presented. This model builds on the genetic stratigraphic sequences of Partington et al. (1993 b ), refining the position of key stratigraphic surfaces with an additional emphasis on sequence boundary description. Most stratigraphic surfaces are believed to be related to tectonic events superimposed on a gradual rise in relative sea-level from the Middle Oxfordian to Volgian. Biostratigraphic information drawn from throughout the UK Central Graben allied with regional log correlation and seismic data has allowed a robust sequence stratigraphic framework to be developed which is applicable to the Greater Shearwater area.