Early Highstand Systems Tract Research Articles

Haynesville and Bossier mudrocks: A facies and sequence stratigraphic investigation, East Texas and Louisiana, USA

Upper Jurassic Haynesville and Bossier shale-gas plays of the northwestern Gulf of Mexico (GOM) Basin produce from organic-rich, marine transgressive to highstand mudrocks within mixed carbonate-clastic depositional systems. Modern wireline-log suites from ∼200 deep wells were used for detailed correlations, and 10 + cores throughout the upper Kimmeridgian to lower Tithonian basin were incorporated into detailed facies, stratigraphic, and lithologic analyses. The Haynesville Shale lies within a 2nd-order transgressive systems tract (TST) that encompasses back-stepping ramp carbonates (proximal) and marine shales (distal) below a maximum flooding surface (MFS). This shale onlaps retrogradational carbonates and basement highs and is capped by the 2nd-order MFS. Bossier shales and local sandstones prograde basinward above the Haynesville shale and downlap the 2nd-order MFS. They grade upward and updip into the thick, highstand systems tract (HST), fluvio-deltaic sandstones of the Cotton Valley Group. Distally, organic-rich facies developed in restricted Bossier environments, creating another productive gas-shale opportunity. Several preexisting basement highs in the south and carbonate platforms in the northwest and west restrict and partition the basin, thus influencing deposition of highly organic versus nonorganic and siliciclastic-dominated versus carbonate-dominated lithologies. Haynesville and Bossier shales each compose three upward-coarsening cycles that probably represent 3rd-order sequences within the larger 2nd-order TST and early HST, respectively. Most of the three Bossier 3rd-order cycles are dominated by varying amounts of siliciclastics. Deposition of the Haynesville mudrocks occurred below storm-wave base under mostly dysoxic conditionsm allowing settlement of benthic bivalve communities and bioturbating organisms and oscillating periodically to more anoxic conditions on the seafloor. However, most of the sparse faunae found in these mudrocks are of planktonic origin. Bossier mudrocks exhibited more oxygenated and clastic-rich lithologies related to progradation of the clastic shoreline and sea-level highstand. Knowledge of lithologic, stratigraphic, geochemical, and faunal variations across mudrock basins needs to be detailed so that shale-gas basins might be successfully explored.

Depositional systems and evolution of the Upper Carboniferous Huanglong Formation in the eastern Sichuan Basin

Abstract New research on basic geology has been done for Upper Carboniferous carbonate lithologic gas pools in the Huanglong Formation in the eastern Sichuan Basin. Based on the development characteristics of terrigenous clastic deposit and the development location of suspending gravels with some psephicity, the transgression direction and tidal depositional system distribution are deduced. It is believed that, two transgression channels were developed in southeast of Yunyang and northwest of Dazhou in the area, two tidal-flat depositional systems in Chongqing-Wanzhou and southeast of Dazhou respectively were formed, and a suit of complete type-I sequence composed of lowstand systems tract (C 2 h 1 ), transgressive systems tract (C 2 h 2 ) and early highstand systems tract (C 2 h 3 ) was developed. Sedimentary microfacies features of each member of the Huanglong Formation were delineated finely. In the middle stage of late Carboniferous, transgressive systems tract was developed in eastern Sichuan, where shallow shoal and algal dolomite-flat microfacies were developed, which are favorable for the development of carbonate lithologic traps. In contrast, in the early and late stages of late Carboniferous, lowstand systems tract and highstand systems tract were developed, respectively, where shallow shoal microfacies were poorly developed, which limited the large scale development of carbonate lithologic traps.

PALEOECOLOGICAL DYNAMICS OF FURONGIAN (LATE CAMBRIAN) TRILOBITE-DOMINATED COMMUNITIES FROM NORTHWESTERN ARGENTINA

Abstract The Cambrian–Ordovician boundary interval is a critical moment in the ecology of trilobite communities. To understand this transition, we studied—at three different spatial scales—changes in the structure of olenid-dominated communities included in the Parabolina fauna, which flourished in the latest Cambrian, largely storm-dominated, successions of northwestern Argentina. At the local (∼meter) scale, species-poor communities occur in shoreface deposits. Relatively flat species-abundance distributions (SADs) and high evenness characterize upper offshore to offshore transition settings of the early highstand systems tract (HST), whereas uneven SADs in species-poor communities are typical of the lower offshore and shelf environments of the transgressive systems tract (TST). This pattern is unlikely to be caused by a change in time averaging and is consistent with the intermediate disturbance hypothesis predicting unimodal diversity gradients. The pattern is thus interpreted to be related to a trend...

Active synsedimentary tectonism on a mixed carbonate-siliciclastic continental margin: third-order sequence stratigraphy of a ramp to basin transition, lower Sekwi Formation, Selwyn Basin, Northwest Territories, Canada

The lower part of the Early Cambrian Sekwi Formation in the Selwyn Basin of the Northwest Territories, Canada, is composed of two regional, unconformity-bounded sequences, S0 and S1, which record the first widespread carbonate deposition during the initial Palaeozoic transgression onto the western margin of Laurentia. These Early Cambrian sequences are unique to the western North American Cordillera, representing the only record of primarily deep-water deposition on a tectonically active, mixed carbonate–siliciclastic ramp during this period. More specifically, the geometry of the Sekwi ramp changed during deposition of S0 and S1, from a shallowly dipping homoclinal ramp during the S0 transgressive systems tract to a steeply dipping tectonically modified ramp during the early highstand systems tract of S0. The steeply dipping ramp profile of S0 was preserved into the early transgressive systems tract of S1. The Sekwi ramp returned to a gently sloping ramp during the late highstand systems tract of S1 and remained so throughout the remainder of Sekwi deposition. The evolving shape of the Sekwi ramp is attributed to syndepositional ‘down to the basin’ faulting during deposition of both S0 and S1 and is recorded by: (i) the westward thickening, irregular geometries of S0 and S1; (ii) geographical restriction of deep-water facies (including sediment gravity flow deposits); (iii) the presence of large allochthonous blocks; and (iv) the clast composition of sediment gravity flow deposits. Sediment gravity flow deposits play an unusually important role in the sequence stratigraphic interpretation of the lower Sekwi Formation, as they delineate depositional packages, including the maximum flooding zone, the transitions between portions of systems tracts, and the inferred locations of syntectonic extensional faults. Syntectonic faults increased accommodation basinward of an extensive ooid-shoal complex that developed along the Sekwi ramp crest, greatly influencing sequence geometry and initiating the downslope motion of sediment gravity flows. The syndepositional faulting probably was a continuation of extension that began during the latest Neoproterozoic rifting of western Laurentia. The composition of sediment gravity flow deposits track changing accommodation space on the lower Sekwi ramp and can be used to differentiate systems tracts that probably were related more to tectonism than eustasy.

MISS in Mesoproterozoic Nonstromatolitic Limestones: A Case Study of the Third Member of Gaoyuzhuang Formation at Qiangou Section in Beijing

The Precambrian carbonate rocks are commonly characteristic of stromatolitic carbonate succession. However, in the western segment of Yanshan area, the third member of the Mesoproterozoic Gaoyuzhuang Formation at the Qiangou section in Yanqing, Beijing, is made up of a set of particular limestone succession with the sparseness of stromatolites. This limestone succession has been defined as the nonstromatolitic carbonate succession, which is a typical example in which three third-order sequences can be discerned according to the cyclicity reflected by successions of sedimentary facies. Within these third-order sequences, there are many subtidal carbonate cycles made up of both the medium- to thin-bedded aphanitic micrites and the thin-bedded marls in the transgressive and the early high-stand system tracts, and many grotesque sedimentary structures are widespread in the bedding planes of aphanitic micrites. These sedimentary structures, including the domal structure, the irregular reticular and chaotic tepee ridges, and the palimpsest ripple, make up an association of the microbial-induced sedimentary structure (MISS). Therefore, both the special MISSs and the particular lithological types make the third member of the Mesoproterozoic Gaoyuzhuang Formation at the Qiangou section to become the representative of the Precambrian nonstromatolitic carbonate succession. More importantly, this succession was formed during the golden age of stromatolite and could provide useful information for the further understanding of the evolving carbonate world of the Precambrian.

Volcanic Tephra Bed Formation and Condensation Processes: A Review and Examination from Devonian Stratigraphic Sequences

The formation and preservation of airfall volcanogenic tephras is the result of various interacting factors. Explanations of the depositional history of individual tephra beds lie along a spectrum between two end-member models of a primary single event and a complex depositional history. Nonrecognition of the history of a tephra bed may lead to errors in geochronology, correlation, and interpretation of the record of explosive volcanism in sedimentary successions. Increased understanding of sequence stratigraphic principles and condensation processes favors the model of tephras with a complex depositional history. This is supported by studies that indicate many tephras in the rock record result from amalgamation and/or reworking in the depositional environment. A prediction of increased tephra bed formation/preservation within transgressive systems tracts due to condensation processes is tested through 11 Lochkovian to Eifelian (Lower to lower Middle Devonian) depositional sequences. The character and distribution of over 80 Devonian K-bentonites and tuffs in the Appalachian Basin (eastern United States) indicates a complex history of tephra deposition and preservation. Of 11 sequences, four have numerous tephra beds, one sequence yields one tephra bed, and six sequences yield no tephra beds. In total, 38 individual beds cumulatively occur within transgressive systems tracts, 39 in early highstand systems tracts, one in late highstand systems tracts, and one in lowstand systems tracts, noting that lowstands are poorly preserved or not recognized along the outcrop belt. This indicates that airfall tephras are concentrated in the transgressive and early highstand systems tracts, not just in condensed intervals of transgressive systems tracts; they do not consistently occur through all sequences. A simple sequence stratigraphic condensation model for the deposition and preservation of volcanic tephras is not applicable.

Slope failures and stability analysis of shallow water prodeltas in the active margins of Western Greece, northeastern Mediterranean Sea

Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment failures on the low-angle (0.5°–2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80–150 m long and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10–15 m below seafloor. Shear planes develop in the lower part of muddy and/or gas charged HST foresets. Deeper basal transparent muddy layers of the early HST bottomset, together with the late Pleistocene transgressive systems tract sequences (TST), are mostly unaffected. On the steeper (2°–6°) fan delta slopes of the western Gulf of Corinth debris flows and avalanches with a significant retrogressive component dominate slope destabilisation. Sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments thereby indicating that failure planes are in the late HST foresets/upper part of the early HST bottomsets gas charged zone. The foresets of the HST prodelta deposits display high water content (30–80%), low bulk density (1.4–1.9 g cm−3) and relatively low values of undrained shear strength (3–20 kPa). The water content of the HST distal muddy bottomsets is relatively higher (50–110%) and bulk density relatively lower (1.3–1.7 g cm−3) with low values of shear strength (2–10 kPa). The shear strength of the gas releasing sediment layer displays lower values (2–9 kPa) relative to the overlying, post failure, muddy sediments of the late 100–300 years. Slope stability was calculated using the normalised soil parameter (NSP) method under undrained conditions for normally consolidated prodelta sediments. This analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26.6–29.6% g for the HST foresets and 12.4–14.1% g for the basal transparent layer belonging to the early HST bottomsets. Consequently the early HST bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations (PGAs) for the next 50 years are expected to range from 19 to 30% g. Moreover, our results show that new glide planes in the prodeltaic sediment bodies of the seismically active continental margins of Western Greece will likely develop from the gas charged sediments of the lower part of the HST foresets to the upper part of early HST bottomsets.

Seismic stratigraphy and Quaternary sedimentary history of the northeast Aegean Sea

Seismic profiles show that the upper Quaternary–Recent stratigraphic architecture of the NE Aegean Sea is characterized by several vertically stacked depositional sequences which are developed within three NE–SW-trending basins. The depositional sequences are separated from one another by prominent shelf-crossing unconformities, formed during periods of falling and low sea level and reworked further during the subsequent sea-level rise by shoreface erosion. Each depositional sequence consists of a lower mainly parallel-stratified subunit, and an upper oblique to sigmoid-oblique prograded subunit. The lower subunit represents hemipelagic and prodeltaic deposition of the transgressive and early highstand systems tracts. The upper subunit represents deltas of the forced regressive systems tract. The chronology of the depositional sequences is constrained by radiocarbon and U/Th dates on cores. An average sedimentation rate of ∼ 30 cm/ka is calculated for basinal muds of the region. Correlations with global sea-level curves based on oxygen-isotope stratigraphy reveal that the seaward-prograded delta sequences developed on a rapidly subsiding shelf during successive glacio-eustatic sea-level falls associated with late Quaternary glaciations. A set of paleogeographical maps shows changing basin configuration since ∼ 330 ka. Volumes of the depositional sequences compare well with the volumes of sediment estimated to have been eroded from the incised valleys of the Tuzla, Karamenderes, and Dümbek rivers draining the Biga Peninsula.

Sedimentary Features and Implications for the Precambrian Non‐stromatolitic Carbonate Succession: A Case Study of the Mesoproterozoic Gaoyuzhuang Formation at the Qiangou Section in Yanqing County of Beijing

Abstract: In the long Precambrian period, stromatolitic carbonate successions were very common. However, the non‐stromatolitic carbonate succession that is marked by subtidal deposits shows a sharp contrast to the stromatolitic carbonate succession. Both the non‐stromatolitic and the stromatolitic carbonate successions are important clues for the further understanding of the evolving carbonate world of the Precambrian. The Mesoproterozoic Gaoyuzhuang Formation at the Qiangou section in northwestern suburb of Beijing is a set of more than 1000 m‐thick carbonate strata that can be divided into four members (or subformations), in which a non‐stromatolitic carbonate succession marked by the scarcity of stromatolites makes up the third member of the formation. This non‐stromatolitic carbonate succession can further be subdivided into three third‐order sequences that are marked by the regular succession of sedimentary facies. In third‐order sequences, a lot of subtidal carbonate meter‐scale cycles made up of medium‐bedded leiolite limestones and thin‐bedded marls constitute their transgressive system tracts (TSTs) and the early high‐stand system tracts (EHSTs), a lot of meter‐scale cycles made up by thin‐bedded limestones and marls constitute their condensed sections (CSs), and thick‐bedded to massive dolomitic limestones or lime dolomites make up the late high‐stand system tracts (LHSTs). The particularly non‐stromatolitic carbonate succession making up the third member of the Mesoproterozoic Gaoyuzhuang Formation at the Qiangou section might be the representative of the non‐stromatolitic carbonate succession of the Precambrian because of its special lithological features and particular sedimentary structures, and its general sedimentary features are helpful and meaningful for the further understanding of the evolution rules of the sophisticate and evolving carbonate world of the Precambrian. The time scale of the Gaoyuzhuang Formation is deduced as that from 1600 Ma to 1400 Ma; thus, the non‐stromatolitic sedimentary succession making up the third member of the Gaoyuzhuang Formation may demonstrate a stromatolite decline event occurring at ca. 1450 Ma of the Proterozoic besides other three events that occurred respectively at ca. 2000 Ma, ca. 1000 Ma and ca. 675 Ma. The forming duration of this non‐stromatolitic sedimentary succession of the third member of the Gaoyuzhuang Formation can be generally correlated with a similar sedimentary succession in North America, i.e. a non‐stromatolitic sedimentary succession of the Helena Formation of the Belt Supergroup, which suggests that the stromatolite decline occurring at ca. 1450 Ma might be a global event. Therefore, the non‐stromatolitic sedimentary succession discussed in the paper provides an important example for further understanding of carbonate sedimentology in the Precambrian.

Sequence stratigraphy and orbital cyclostratigraphy of the Mooreville Chalk (Santonian–Campanian), northeastern Gulf of Mexico area, USA

Cycles from the depositional sequence level to decimeter-scale rhythmic alteration of chalk and marl beds in the Mooreville Chalk (Santonian–Campanian) in the northeastern Gulf of Mexico area are studied employing an integrated biostratigraphic, cyclostratigraphic, and sequence stratigraphic approach. Two dominant frequencies, identified from the natural gamma-ray log from the Alabama Power Selma Site Test well, site 3 hole 3, are attributed to two of the Milankovitch cycles, the long eccentricity (413 ka) and eccentricity (100 ka) cycles. Parasequences, the building blocks of systems tracts, appear to be modulated by long eccentricity cycles. Individual eccentricity cycles were counted to establish a high-resolution orbital chronology, which agrees with established biostratigraphic chronology. A high-resolution record of temporal changes in sedimentation rate is established on the basis of cyclostratigraphic chronology. Two condensed sections are identified. The lower condensed section is found to correspond to a downlap surface recognized in seismic data in the offshore Alabama and Mississippi area. This condensed section and downlap surface are interpreted to represent the maximum flooding event of a third-order depositional sequence. The maximum flooding event is also represented by a marine flooding surface in eastern Mississippi and by the Mooreville tongue in eastern Alabama. The upper condensed section is found to correspond to a faunal abundance peak recognized in data from outcrop studies, and is interpreted to represent the maximum bathometric surface that was achieved as a result of a condition of rising sea level in association with low sediment input during the deposition of the early highstand systems tract.

Top seal development in the shale-dominated Upper Devonian Catskill Delta Complex, western New York State

The preferential generation of vertical natural hydraulic fractures at the contact of the Upper Devonian Hanover gray shale and overlying Dunkirk black shale of the Catskill Delta Complex, western New York State, suggests that the latter served as a hydraulic top seal to formation fluids migrating upward from deeper in the sediment pile. Petrophysical properties and small-scale textural characteristics of these siliceous fine-grained rocks confirm the crucial role of depositional environment and sequence stratigraphic position of a shale lithotype in determining its sealing capacity. The especially high sealing capacity of the basal interval of the Dunkirk shale, inferred early high-stand systems tract (HST) strata, reflects the anoxic depositional environment of these deposits that favored the preservation of their abundant organic matter and finely laminated depositional texture. The absence of bioturbation enabled the undisrupted sediment, notably carbonaceous clay-rich laminae, to undergo rapid mechanical compaction, platy grain reorientation, and porosity reduction. Compaction-induced squeezing of ductile organic matter into void spaces further reduced pore throat diameters. Immediately underlying heavily bioturbated deposits of the organic-lean Hanover shale, inferred upper HST or low-stand wedge sediments, accumulated in a dysoxic depositional environment. Disruption of layering and homogenization of sediment by burrowing organisms produced a more porous and permeable microfabric through which formation fluids moved only to be arrested by the high capillary entry pressures at the base of the Dunkirk shale. Natural hydraulic fractures, some of which propagated into the Dunkirk shale, formed when fluid pressure at the top of the Hanover shale reached the fracture gradient. The high sealing capacity of the basal Dunkirk shale was probably enhanced by its finely laminated nature and the generation of biogenic methane, both of which contributed to the formation of a near-impermeable gas capillary seal.

Reefal and mud mound facies development in the Lower Devonian La Vid Group at the Colle outcrops (León province, Cantabrian Zone, NW Spain)

In the locality of Colle (Cantabrian Zone, NW Spain), the upper part of the Valporquero Shale Formation (Emsian, La Vid Group) contains an interval of shales and marlstones (barren, greenish-grey shales and fossiliferous, greenish-grey or reddish shales/marlstones) with beds and packages of homogeneous and cross-bedded skeletal limestones. Metre-scale mud mounds and coral biostromes occur encased in the fossiliferous reddish and greenish-grey shale/marlstones, respectively, with the coral biostromes overlying conspicuous skeletal limestone bodies. These rocks were deposited on a carbonate ramp, ranging from above storm wave base for the cross-bedded skeletal limestones to below the storm wave base for the remaining deposits, organic buildups included. The vertical stacking of these facies and the occurrence of the two types of buildups are interpreted to reflect the interplay among several (possibly 4th and 5th) orders of relative sea-level variations, during a 3rd-order highstand. Coral biostromes occur in early 5th-order transgressive system tracts developed within late 4th-order highstand, and are interpreted to have thrived on a stable granular substrate (skeletal limestones) in non-turbid waters, being later aborted by the onset of muddy sedimentation. Biostrome features suggest that they developed under environmental conditions essentially different from those related to the sedimentation of their granular substrate. Mud mounds occur in 5th-order transgressive and early highstand system tracts tied to early 4th-order sea-level rise. Field relationships suggest that mud mounds grew coevally with muddy sedimentation, with high-frequency variations in carbonate vs. terrigenous mud sedimentation influencing their development.

The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy

Diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs. Variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability. Linking the types and distribution of diagenetic processes to the depositional facies and sequence-stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity. The heterogeneity patterns of sandstone reservoirs, which determine the volumes, flow rates, and recovery of hydrocarbons, are controlled by geometry and internal structures of sand bodies, grain size, sorting, degree of bioturbation, provenance, and by the types, volumes, and distribution of diagenetic alterations. Variations in the pathways of diagenetic evolution are linked to (1) depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; (2) detrital sand composition; (3) rate of deposition (controlling residence time of sediments at specific near-surface, geochemical conditions); and (4) burial thermal history of the basin. The amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and, therefore, can be predicted in the context of sequence stratigraphy, particularly in paralic and shallow marine environments. Changes in the relative sea level exert significant control on the types and extent of near-surface shallow burial diagenetic alterations, which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs. Carbonate cementation is more extensive in transgressive systems tract (TST) sandstones, particularly below parasequence boundaries, transgressive surface , and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter, bioturbation, and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates, which also enhance the formation of glaucony. Eogenetic grain-coating berthierine, odinite, and smectite, formed mostly in TST and early highstand systems tract deltaic and estuarine sandstones, are transformed into ferrous chlorite during mesodiagenesis, helping preserve reservoir quality through the inhibition of quartz cementation. The infiltration of grain-coating smectitic clays is more extensive in braided than in meandering fluvial sandstones, forming flow barriers in braided amalgamated reservoirs, and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution. Diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars, micas, and mud intraclasts under wet and warm climates, whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers. Turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls, as well as along layers of concentration of carbonate bioclasts and intraclasts. Commonly, hybrid carbonate turbidite arenites are pervasively cemented. Proximal, massive turbidites normally show only scattered spherical or ovoid carbonate concretions. Improved geologic models based on the connections among diagenesis, depositional facies, and sequence-stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies, as well as for CO2 geologic sequestration, but also support more effective hydrocarbon exploration through reservoir quality prediction.

Small rivers contribution to the Quaternary evolution of a Mediterranean littoral system: The western gulf of Lion, France

Along the Mediterranean coasts and the Atlantic French coast, former and actual research programs focused on major river systems, estuaries and deltas, characterized by a last Glacial relative sea level lowstand and incision and a well developed sedimentary incised valley infilling deposited during the following sea level rise and highstand. This paper presents the preliminary results of a program focused on a particular area of the Gulf of Lion coast with a thin sedimentary cover over the substratum and only minor rivers with non-apparent and important sedimentary contribution during the Late Quaternary. The results show that in this area the best Pliocene to Actual sedimentary record is preserved. The paper rests on the analysis of an extensive database of recent high resolution and very high resolution seismic reflection lines and previously published core data. Seismic data show that a major complex of paleovalleys connected with the Orb, Aude and Agly rivers is preserved on the inner shelf and adjacent coastal plain. On the inner shelf, the separated incised valleys merged in a unique broad and shoreparallel incision dipping southward. At the southward extremity, the incision turns eastward and seaward. The basal surface of this incision extends seawards under six to seven Late Quaternary depositional sequences preserved on the mid and outer shelf. The infilling of the complex of paleovalleys is characterized by aggrading deposits attributed to periods of relative sea level rise (transgressive systems tracts), organized into several subunits bounded by internal discontinuities locally deeply incising. The discontinuities are amalgamated surfaces, including successive sequence boundaries (indicative of phases of relative sea level falls) merged with transgressive surfaces (indistinct tidal and wave ravinement surfaces). The subunits are the part of Late Quaternary depositional sequences preserved in estuarine environments. They are the lateral equivalents of landward fluvial terraces and seaward coastal and prodeltaïc deposits on the shelf. Using borehole dataset, the underlying and eroded deposits below the basal unconformity are correlated with Pliocene deposits outcropping landward in the hinterland. The top of the incised valleys complex is capped by the last Glacial lowstand surface of erosion (18 ky B.P.) reworked by the postglacial transgressive surface (TS), dissociated near the shoreline into a tidal and a wave ravinement surfaces. Above the TS, the very high resolution (VHR) seismic data in the lagoons, the tidal channels and cores, reveal in details the stratigraphic architecture of the deposits. At the base, a small wedge constitutes the postglacial transgressive systems tract (TST) locally thinning in the areas distant of the sediment point sources. The TST is capped by a flat surface of wave reworking (maximum flooding surface or mfs) prolongating under the Leucate lagoon and merging offshore at the seafloor. Boreholes and VHR seismic lines trough the coastal barrier and in the lagoon show that the shoreline probably migrated far landward at the end of the transgression. When the rate of sea level rise decreased strongly, the shoreline migrated seaward and prograding and aggrading sandy material, with landward muddy lateral equivalent facies, deposited early highstand systems tract (HST) above the MFS. Offshore, fine material deposited as a sigmoidal blanket of mud originating in part from the north-east and Rhône river under oceanic circulation (equivalent to a subaqueous prodelta). Subsequently, the modern beach barrier built up by wave reworking of the early HST. This new study of the western part of the Gulf of Lion inner shelf and littoral illustrates an incised valley complex and thus presents the best preserved example of the sedimentary record of the effects of the relative sea level changes during the Pliocene to Actual period. For the first time, the land to sea transition is preserved and the Late Quaternary depositional sequences are in a great part observed. The last post Glacial deposits present a simplified but very different organisation compared to a record front of the adjacent Rhône river. Consequently a synthesis is now possible.

Isolated highstand shelf sandstone body of turbiditic origin, lower Kenilworth Member, Cretaceous Western Interior, Book Cliffs, Utah, USA

An isolated highstand shelf sandstone body of turbiditic origin has been identified in the lower Kenilworth Member, Hatch Mesa area, Book Cliffs, Utah. Relative to most other marine mudstone-encased isolated sandstone bodies, the Hatch Mesa succession has an abundance of Bouma-like turbidite beds, a greater proportion of mudstone and siltstone, is thinner (6–20 m), and is weakly coarsening-upward. These attributes, in combination with locally abundant hummocky cross stratified sandstones, a paucity of wave ripples in the background fair-weather deposits, carbonaceous matter in all facies, a lack of clinoforms, and a low diversity/abundance trace fossil suite, suggest deposition in a prodelta environment, between fair weather and storm wave base. A three-component shoreface to shelf model, consisting of delta-front, subaqueous channel and prodelta turbidite deposits, is proposed to explain the depositional environment and setting of the Hatch Mesa succession. Regional correlation has demonstrated that the Hatch Mesa succession is time equivalent with the second parasequence in the Kenilworth (KPS2), which is characterized by inner shelf turbidite bodies at a number of other localities. Paleogeographic reconstructions reveal that the Hatch Mesa succession was deposited at least 16 km basinward of the KPS2 lower shoreface, in approximately 20 m of water. The KPS2/Hatch Mesa sandstone body package is part of the early highstand systems tract, indicating that isolated or stray sandstone bodies are not solely restricted to the falling stage or lowstand systems tracts.

Taphonomy and sequence stratigraphic significance of oyster-dominated concentrations from the San Julián formation, Oligocene of Patagonia, Argentina

Oligocene siliciclastic shallow marine sediments of the San Julián Formation in southeastern Patagonia (Argentina) are interpreted as a deepening–shallowing cycle that represents a depositional sequence in which lowstand system tract deposits are not present. There are no significant compositional differences in the assemblages, all are dominated by the large oyster Crassostrea? hatcheri. However, four types of shell concentrations can be distinguished by differences in stratigraphic, sedimentologic, palaeoecological, and taphonomic features. These four types of shell concentrations are found in distinct positions within the depositional sequence: as a transgressive lag at the early phase of the transgressive system tract (TST), at the top of the TST, at the early phase of the high stand system tract (HST), and at the top of the HST. Oyster-dominated concentration in the early TST has a distinct basal erosional surface (ravinement surface), and the shells are completely disarticulated, fragmented and abraded with a chaotic orientation, suggesting reworking of previous deposited sediments, and deposition in a shoreface environment. Concentration at the end of the TST is a hiatal concentration formed from continued sea-level rise in the context of backlap deposition. It is characterized by high percentages of articulated oysters in life position, with high grades of bioerosion and encrustation. This concentration was accumulated during times of high rates of production of biogenic hard parts and low sedimentation rates, below fair weather wave base in an offshore environment. Concentrations over the maximum flooding surface in the early HST are in situ event concentrations (Census assemblages), characterized by high percentages of individual articulated adult specimens and clusters of juvenile oysters with well-developed endolithic bioerosion and encrustation, formed by rapid sedimentation in an offshore to lower shoreface environment. Concentrations at the top of the HST are multiple-event concentrations formed by densely fossiliferous deposits with completely disarticulated, convex-up oysters with low grades of abrasion and high grades of bioerosion and encrustation, intercalated with repeating beds with benthic colonizers. They were originated below fair weather wave base, in an offshore to lower shoreface environment, by sporadic high-energy events. The basic pattern of condensed deposits formed in the context of onlap, backlap, downlap, and toplap can be applied to the studied deposits. However, some differences are recorded, and these are attributed mostly to differences in the basin characteristics and to the high productivity and high preservation potential of large, thick-shelled oysters.

04/02234 Geochemical analysis and familial association ofRed River and Winnipeg reservoired oils of the Williston Basin, Canada: Smith, M. and Bend, S. Organic Geochemistry, 2004, 35, (4), 443–452

The upper Katian (Upper Ordovician) Red River Formation is a carbonate-evaporite supersequence that formed in the epicratonic Williston Basin of western North America. This study focuses on the upper part of the formation in the southern, US part of the basin. The studied succession is made up of three transgressive-regressive sequences (RR1, RR2, and RR3), each ∼0.5–∼1 m.y. in duration and composed of relatively few meter-scale high-frequency cycles (parasequences). Sequence boundaries are characterized by intraclastic breccia-laminated dolomite successions that have been truncated and capped by evaporite deposits. Subaqueous anhydrites within each sequence represent evaporite Lowstand Systems Tracts (LSTs) that formed during relative sea-level lows that lead to the precipitation of gypsum (later recrystallized into anhydrite) in a quiet, salina-type environment. Transgressive (TST) and early Highstand Systems Tracts (HST) are composed of subtidal facies of predominantly skeletal mudstone to wackestone-packstone with abundant burrow mottling. Late HSTs are predominantly composed of peritidal, porous laminated dolomite. The study illustrates how the sequence stratigraphic framework of carbonate-evaporite deposits from the Williston Basin relates to the Late Ordovician glacio-eustasy, transitional climates, and low basin subsidence in an arid setting.

Controls on organic accumulation in late Jurassic shales of northwestern Europe as inferred from trace-metal geochemistry

Abstract In the Kimmeridge Clay Formation of the Wessex-Weald Basin, five organic-matter-rich intervals (or ORIs), dated from Kimmeridgian-Tithonian times, can be correlated from distal depositional environments in Dorset and Yorkshire (UK) to the proximal environments in Boulonnais, northern France. The ORIs are superimposed on a meter-scale cyclic distribution of organic matter (OM), referred to as primary cyclicity, which is commonly interpreted to result from Milankovitch climate forcing. The present work addresses the distribution of redox-sensitive and/or sulfide-forming trace metals and selected major elements (Si, Al and Fe) in Kimmeridge Clay shales from the Cleveland Basin (Yorkshire) and the Boulonnais cliffs with two objectives: 1) to determine whether the ORIs formed in similar paleoenvironments, and 2) to identify the mechanism(s) of OM accumulation. High-resolution geochemical data from primary cycles in the Yorkshire boreholes (Marton and Ebberstone boreholes), were studied and the results are then applied with lower resolution sampling at the ORI scale in the Flixton borehole and Boulonnais cliff. Good correlations are found between total organic carbon (TOC) vs Cu/Al and Ni/Al, but relationships between TOC and Mo/Al, V/Al and U/Al are more complex. Cu and Ni enrichment is interpreted to have resulted from passive accumulation with OM in an oxygen-deficient basinal setting, which prevented the subsequent loss of Cu and Ni from the sediment. Mo and V were significantly enriched only in sediments where considerable amounts of OM (TOC>7 %) accumulated, the result of strongly reducing conditions and OM burial. At the scale of the Flixton ORIs, the samples with the highest Mo and V concentrations also show relative Fe enrichment, suggesting pyrite formation in the water column (combination of euxinic conditions and presumably low sedimentation rates). Samples from all ORIs were slightly enriched in Si relative to Al, interpreted as reflecting decreased sediment flux during transgressive and early-highstand systems tracts. The data show that in some ORIs, OM accumulation proceeded while productivity was not particularly high and sediments were not experiencing strong anoxia. In other ORIs, OM accumulation was accompanied by widespread anoxia and possibly euxinic conditions in distal settings. Though somewhat different from each other, the ORIs have all developed during episodes of reduced terrigenous supply (transgressive episodes). The common feature linking these contrasted episodes of enhanced OM storage (ORIs) must be the conjunction of productivity coupled with a decrease in the dilution effect by the land-derived supply, in a depositional environment prone to water stratification and, therefore, favorable to OM preservation and accumulation.

STRATIGRAPHIC CONTROLS ON PALAEOZOIC PETROLEUM SYSTEMS, GHADAMES BASIN, LIBYA

The distribution of oil‐ and gasfields in the Libyan Ghadames Basin is strongly influenced by the stratigraphic architecture of the Silurian‐Devonian succession. This interval encompasses part of two (second‐order) super‐sequences separated by the Caledonian unconformity: (i) a late Ashgillian‐Silurian super‐sequence, comprising the Melez Chograne‐Memouniat periglacial lowstand, the Tanezzuft transgressive to early highstand shales and the Acacus Formation late highstand systems tracts; and (ii) the lower part of a composite Devonian‐Carboniferous super‐sequence terminated by the Hercynian unconformity, which includes the lowstand Tadrart fluvial‐shallow marine sands, the Ouan Kasa (transgressive) and the Aouinet Ouinene transgressive to early highstand systems tracts of Devonian age.Hydrocarbon accumulations in Acacus, Tadrart and later Devonian reservoirs have been charged from a basal Silurian (Tanezzuft Formation) “hot” radioactive shale source rock. The Acacus and Tadrart sandstones provided regional migration conduits sealed by intra‐Acacus, intra‐Devonian (Emghaet Formation) and Permian (Bir Al Jaja Formation) shales. Regional stratigraphic continuity of these migration conduits permitted the development of low to moderate impedance petroleum systems dominated by lateral migration.Basin reconstructions and burial history modelling suggest three expulsion peaks from the basal Tanezzuft Formation source rock, preceding major periods of Hercynian, Austrian and Alpine (mid‐late Tertiary) uplift and exhumation. Hydrocarbons trapped prior to the Hercynian event were dispersed during later periods of structural deformation. Some pre‐Austrian accumulations may have survived locally in more robust traps. However, most if not all of the oil‐ and gasfields discovered in the Late Silurian ‐ Devonian succession were probably charged during the Late Cretaceous—Early Tertiary, only to suffer partial dispersal during later uplift, tilting and flushing. Three petroleum systems can be distinguished within the basin: (i) a Tanezzuft ‐Acacus system to the north; (ii) a Tanezzuft ‐ Tadrart system to the south; and (iii) a Tanezzuft ‐Acacus/Tadrart(+) system in the centre, with some leakage into overlying Devonian and Carboniferous sandstones locally.

Systems tracts sedimentology in the lagoon of Mayotte associated with the Holocene transgression

Twelve gravity cores from various settings within the Mayotte barrier reef–lagoon complex were studied to determine the sedimentology of the sequence stratigraphic systems tracts that formed during the Holocene transgression. Our studies focussed on the determination of physical, chemical, mineralogical and biological parameters of the sediments from specific systems tracts. These parameters determine the thickness and facies of each systems tracts and are controlled by the rate and amplitude of sea-level rise, lagoonal topography and environmental changes. The lowstand systems tract (LST) (before 11.5 ka BP) comprises ferralitic or organic-rich paleosoils in the proximal and middle lagoon and karstified Pleistocene reefal carbonates in the distal lagoon. The transgressive systems tract (TST) (11.5–7 ka BP) consists of a lower terrigenous and an upper mixed terrigenous–carbonate or carbonate-dominated unit. Locally, mangrove muds were deposited. The highstand systems tract (HST) can be divided into an early highstand (eHST) (7–1 ka BP) and a late highstand systems tract (lHST) (after 1 ka BP). In the proximal lagoonal wedge, the early highstand systems tract consists of terrigenous or mixed terrigenous–carbonate muds to sandy muds. In the middle lagoon, it shows carbonate mud to sandy mud and carbonate gravel to reefal carbonates in the distal lagoons. Terrigenous muds dominate the late highstand systems tract in the proximal lagoonal wedge. In the mid-lagoonal plain, mixed terrigenous–carbonate or carbonate mud to sandy mud dominates, while carbonate gravel to reefal carbonate prevails in the distal lagoon. For the last 9 ka, sedimentation in the lagoon of Mayotte has been spatially divided into a proximal terrigenous and a distal, carbonate-dominated province. Maximum carbonate concentrations between 4 and 1 ka BP coincide with the time of maximum solar insolation. After 1 ka BP, a general decrease in carbonate concentrations can be observed. This coincides with increased terrigenous sediment input, which results from a reduction in accommodation space and to some extent is of anthropogenic origin.