Fourth-order Depositional Sequences Research Articles

A high-resolution sequence stratigraphic framework for the eastern Ellis Bay Formation, Canada: A record of Hirnantian sea-level change

Anticosti Island (Québec, Canada) contains one of the thickest Ordovician/Silurian boundary sections in the world, providing a fossiliferous and well-preserved record of the Late Ordovician mass extinction; however, the absence of a comprehensive correlation framework for the island prevents the full incorporation of data from across Anticosti into a global understanding of the extinction event. Here, we combine sedimentological, stratigraphic, and chemostratigraphic data to develop a sequence stratigraphic framework for the Ellis Bay and lowermost Becscie formations along the northeastern shore of Anticosti Island. These formations record six fourth-order (∼100−400 k.y.) depositional sequences bounded by regionally traceable unconformities. Evidence of subaerial weathering and exposure at many of these unconformities is subtle and has long gone unrecognized, which highlights the complexity of correlation in this and other stratigraphically thin icehouse records. Quartz pebble lags that mantle these surfaces punctuate the otherwise fine-grained, mixed carbonate−siliciclastic strata of the Ellis Bay Formation and provide the means to trace unconformities across localities; these surfaces are also marked by depleted carbon isotopic values, meteoric cement, microkarst, and incised valleys. This sequence stratigraphic framework provides the first correlation framework for exposures from the eastern end of this classic Ordovician/Silurian boundary section, and thus enables the integration of paleobiological, stratigraphic, and geochemical data into a basin-wide perspective of the Late Ordovician mass extinction. Critically, comparison to sequence stratigraphic frameworks from coeval sections suggests that other regions may be incomplete at the level of the fourth-order cycles recorded on Anticosti Island and may contain similarly cryptic unconformities.

Gamma-ray, stable carbon and oxygen isotope chemostratigraphy and sequence stratigraphy of the Lower Mahil Formation (KS-1 Khuff-Equivalent), Northern Oman

This research presents findings from a study focused on the Lower Triassic (Induan) Lower Mahil KS-1 Formation, situated on a homoclinal carbonate platform in Northern Oman. The sequence stratigraphy of this formation is characterized by a considerable thickness variation, slumps, and breccia deposits related to active normal faults coupled with intra-basin growth faults. The main objective was to establish a reliable stratigraphic framework for the Lower Mahil KS-1 Formation by integrating high-resolution carbon isotope data along with high-resolution spectral and total gamma-ray data. To achieve this, whole-rock samples were analyzed for δ13C and δ18O isotopes. Spectral and total gamma-ray records were obtained for the formation. Isotope sampling is conducted every 0.5 m in the Saiq Plateau and Wadi Sahtan sections. Furthermore, spectral gamma-ray measurements were taken at intervals of 10 cm from the logged sections. Within the third-order sequence, the spectral gamma-ray data revealed a distinct sea-level trend, leading to the division of KS1 into two different parts. Five fourth-order depositional sequences were identified by analyzing stable carbon isotopes, uranium, and total gamma-ray profiles. Four of these sequences displayed complete patterns, reflecting transgression and regression phases, while the fifth sequence was incomplete and solely comprised a transgressive phase. An essential outcome of the study is the correlation of the δ13C curve of the Lower Mahil KS-1 Formation with other similar formations around the Tethys region. This correlation indicates that the Lower Mahil KS-1 Formation captures the near-primary signal of carbon isotope variations in coeval seawater. As a result, it holds promise as a reference section for future investigations and studies in this field. Compared to the prior investigation, this study utilizes data with higher precision, capturing spectral gamma-ray measurements at 10 cm intervals and isotope measurements at 50 cm intervals. Furthermore, the study’s focus is confined explicitly to KS1.

Cyclicity and sequence stratigraphy of the Neoproterozoic uppermost Haut Shiloango-Lukala carbonate ramp system in the Lower Congo region (Democratic Republic of the Congo): Example of tectonostratigraphic control versus climatic changes

During the Cryogenian-Ediacaran, tectonically- and climatically-driven perturbations created sea level changes. Snowball Earth-type glaciations suggest ~500–800 m sea level fall or at least 1,000–1,500 m in eustatic sea-level change as a result of severe climate changes. However, in Central Africa, geologic evidence of such processes is lacking. In the Lower Congo region (Democratic Republic of the Congo), detailed facies analysis and sequence stratigraphy of the Neoproterozoic uppermost Haut Shiloango-Lukala carbonate ramp system allowed to reconstruct the multiple order relative sea-level changes. Two hundred twenty-three fifth-order elementary parasequences, grouped into eleven fourth-order depositional sequences or parasequence sets, recorded a severe sea-level fall up to 15 m for slope-outer-to-outer ramp facies of the uppermost Haut Shiloango Subgroup-Upper Diamictite Formation followed by two distinct sea-level rises for the slope-outer-to-inner ramp facies of the Lukala Subgroup. First marine transgression shows a cumulative ~60 m of sea-level rise throughout C1 to C3 (Lukala Subgroup) sediment accumulations. Second marine transgression shows a cumulative ~50 m sea-level rise throughout C4 to C5 sediment accumulations. The transition from uppermost Haut Shiloango to Upper Diamictite ramp system points to an isostatic rebound and uplift of the rift flanks of the Congo Craton, creating paleoreliefs, potentially allowing local glaciation and periglacial sedimentation. This rebound can be ascribed to a diachronous far-field effect of ~660 Ma Macaúbas Basin opening in Brazil. First marine transgression is interpreted as a consequence of syn-to post-rifting related to sea-floor spreading, which initiated the subsidence of the basin. A ~30 m of sea-level rise drowned the C1 formation carbonate ramp, which turned on the oversupply and progradation of offshore to nearshore marine deposits on the wedges of the basin. An abrupt change from both marine transgressions points to ~25 m of sea-level rise that is interpreted as the consequence of the development of ~585-560 Ma Araçuaì-West Congo Orogen, which significantly increased the siliciclastic supply in the basin. Our results show that no anomalous climatic or eustatic events such as those proposed in the Snowball Earth model are recorded in DRC. On the contrary, relative sea-level changes result from long-term overriding regional tectonic processes controlling diachronous sedimentation along the western passive margin of the Congo Craton.

Marsili and Cefalù basins: The evolution of a rift system in the southern Tyrrhenian Sea (Central Mediterranean)

Abstract The Marsili Basin (in the southern Tyrrhenian Sea), whose mode of extension is still a controversial issue, is the youngest bathyal basin of the Central Mediterranean. A thin sedimentary cover in the basin permits to image basement fabric and structure by swath mapping and seismic reflection data. We investigate the crustal structure of the southern Tyrrhenian Sea, extending from the bathyal Marsili Basin to the adjacent Sicily continental margin. Interpretation of seismic reflection profiles (calibrated by well and dredge data) and crustal cross-sections were used to identify the stratigraphic infill, structural pattern and large-scale crustal features of the region. We recognized three basins in the southern Tyrrhenian Sea: (1) the Termini basin that is an overfilled sedimentary basin on the continental shelf area; (2) the Cefalu basin, located on the continental slope area and filled by thick deep water turbidite deposits; (3) the distal Marsili basin, filled by hemipelagic and thin distal turbidite deposits. The sequence stratigraphy interpretation permitted us to recognize fourth-order depositional sequences and the stratigraphic signature of the rift stages. An important increase in the sedimentary supply from the continental shelf to the bathyal basin occurred approximately over the last 0.5 Ma and is related to the uplift of the coastal area. The stratigraphic constrains indicate a Lower Pleistocene age for the opening of the southern Tyrrhenian Sea basin. The structural map reveals a complex fault pattern, in which coeval normal faults disclose a triangular basin from the Marsili bathyal basin to the Sicily continental margin, associated to an Euler pole in the northern Sicily. Tacking in account the faults pattern that developed in the whole Southern Tyrrhenian Sea during the Lower Pleistocene, we reconstructed two opposite triangular basins separated by a perpendicular rift. For analogy with the contiguous Vavilov basin, we propose that the extension in the Marsili basin reached the mantle exhumation stage.

Dolomite and dolomitization of the Permian Khuff-C reservoir in Ghawar field, Saudi ArabiaSea

ABSTRACT The Permian Khuff-C reservoir in Saudi Arabia is known for its lateral and vertical heterogeneity caused mainly by dolomitization. Detailed petrographic analysis of 600 thin sections, from six cored wells, revealed three main replacive dolomite fabrics: (1) fabric-preserving mimetic (FPM), (2) fabric-preserving nonmimetic (FPNM), and (3) fabric-destructive (FD) dolomites. Crystal sizes are mostly less than or equal to 20 μm for FPM dolomite, less than or equal to 50 μm for FPNM dolomite, and less than or equal to 100 μm for FD dolomite. The FPM dolomite decreases in abundance, and FPNM dolomite increases in abundance, with increasing grain content of the facies. The δ 18 O values of dolostones (although considered an obsolete term, dolostone is used here to mean rock containing ≥80% dolomite) indicate early dolomitization at low temperatures in Permian seawater or evaporated seawater, with landward facies (mudstone and wackestone) generally dolomitized by more evaporated waters and seaward grainy facies generally dolomitized by less evaporated, more normal marine seawaters. Stratigraphic variations in the dolostones’ δ 18 O values track with facies variations through fourth-order depositional sequences and indicate that different stratigraphic bodies of dolomite formed from seawaters with different degrees of evaporation. The δ 13 C values of the dolostones exhibit temporal trends inherited from the precursor limestones. Variations in the lateral and vertical abundance of dolomite and dolomite fabrics, in the propensity for each facies to be dolomitized, and in the dolomites’ oxygen isotopic values all suggest that multiple dolomitization events occurred in the Khuff-C reservoir as depositional cycles accumulated, with some dolostones overprinted by younger events. Average porosities of grain-rich dolostones are greater than those of mud-rich dolostones, indicating that depositional facies preordained porosity distribution within the dolostones. However, the more evaporated the dolomitizing fluid, the more likely dolomitization resulted in lower porosity regardless of facies.

General characteristics of quartz arenite types and their role in the recognition of sequence stratigraphic boundaries in ancient coastal and near shore sediments. A case study from Egypt and Saudi Arabia

Quartz arenites are useful in the recognition of depositional sequences and cycle boundaries in ancient coastal and near shore facies. In this study two types of quartz arenite are recognized: 1) depositional quartz arenite (calcareous, dolomitic, ferruginous and kaolinitic quartz arenites), and 2) diagenetic quartz arenite (orthoquartzite, siliceous quartz arenite, compact quartz arenite, and dedolomitic quartz arenite). Depositional quartz arenite often occurs on the tops (upper boundaries) of the depositional sequences that are bounded by sequence boundaries (with or without evidence of subaerial exposure) or correlative conformity surfaces that mark the change from forced regression to lowstand normal regression. Therefore, depositional quartz arenite can define the upper boundaries of third and fourth-order depositional sequences, cycle bases and tops (boundaries) within lowstand, transgressive and highstand systems tracts. Diagenetic quartz arenite (orthoquartzite, siliceous quartz arenite and dedolomitic quartz arenite) usually occurs at the tops (upper boundaries) of depositional sequences that have subaerial sequence boundary and have been subjected to prolonged subaerial weathering and hence is closely associated with subaerial unconformity sequence boundary surfaces and consequently indicates a sharp drop in sea level. Thus, diagenetic quartz arenite types can be used to recognize the tops (upper boundaries) of first and second-order depositional sequences. Compact quartz arenite that is considered the fourth type of diagenetic quartz arenite consists entirely of packed quartz grains, but lacking cement, occurs at the base of each fining-upward cycle (lower boundaries) in lowstand systems tracts and may define the bases of some different-order depositional sequences.

Facies analysis and depositional sequences of a siliciclastic–carbonate deposit: the Late Paleocene–Early Eocene succession, Fariman, Central Iran

A mixed carbonate–siliciclastic succession in Central Iran was investigated using detailed field and laboratory studies. The succession represents a shallow marine to coastal plain setting and a fluvial depositional system. Based on sedimentological analysis, six facies are identified: (1) pelagic and shelf-to-lagoon sediments (lime/silt/clay mudstone), (2) shoal sediments (thin-bedded limestone), (3) shelf-to-lagoon sediments (limestone), (4) tidal flat sediments (lime mudstone/dolomite/evaporites), and (5/6) fluvial deposits (sandstone and conglomerate). The Kariz-Bolagh strata contain parasequences as part of fourth-order depositional sequences. Uplift, compaction, and rate of deposition were the main factors that controlled the vertical facies transitions in these parasequences. This succession provides an excellent opportunity to study the interplay between shallow-water sedimentation and syndepositional subsidence. The effects of these processes caused relative sea level changes, variation in depositional parasequences, and formation of sequence boundaries. The Kariz-Bolagh strata contain type 1 sequence boundaries. Due to local structural events in the basin, the interpreted sea level curve during deposition of the Late Paleocene–Early Eocene strata cannot be correlated with the world sea level curve.

Very high-resolution seismic stratigraphy of Pleistocene eustatic minima markers as a tool to reconstruct the tectonic evolution of the northern Latium shelf (Tyrrhenian Sea, Italy)

This paper analyzes depth variations of seismically detected lowstand features (i.e., paleo–shelf break and lowstand submerged depositional terraces [LSDTs]), in order to define vertical movements along a continental margin. Narrowly spaced, high-resolution two-dimensional reflection seismic data were used, collected along a segment of the continental shelf of Latium (eastern Tyrrhenian Sea, Italy). Seismostratigraphic analysis allowed us to identify the six most-recent fourth-order depositional sequences formed in the past ∼500 k.y. They have a different degree of preservation in the outer and middle shelf, with almost no continuation in the inner shelf. Some wedge-shaped bodies interpreted as LSDTs can be identified in these units. The depth of paleo–shelf break and LSDTs of marine isotope stage 12 (ca. 435 ka) was measured, and gradually varies along the margin between ∼220 m and ∼131 m below current sea level. Differential subsidence rates have been estimated for the area, varying from 0 to −0.21 mm/a. The spatial variations agree with studies of uplifted coastal terraces on land, controlled by localized volcanic activity along the Latium section of the Tyrrhenian coast. The comparison of deformation onshore and offshore has proven to be a promising approach for understanding the evolution of uplifting continental margins.

Succession of rudistid lithosomes along the western coastal margin of the Iberian Basin (Coniacian, Castrojimeno Section, central Spain)

Rudistid lithosomes cropping out near Castrojimeno, at the northern margin of the Central System in north-central Spain, provide detailed information on their composition and structure, on their development and succession, and about their relationship with the Coniacian sequence stratigraphy framework of the Iberian Basin. Most rudist assemblages are oligospecific, with a dominant species, or monospecific. The radiolitids Biradiolites angulosus, Praeradiolites requieni, and Radiolites sauvagesi and the hippuritids Hippurites incisus and Vaccinites giganteus were identified. Radiolitids demonstrate wide intraspecific morphological variability. The following Riding’s structural categories of organic reefs are represented: segment reefs, spaced and close cluster reefs, and close cluster/frame reefs. Bioclastic beds of reworked rudist fragments occur below or in between the rudist reefs. The vertical succession of all five types of rudistid lithosomes distinguished evidences a shallowing-upward trend. Rudistid lithosomes developed on the coastal margin during the superposition of the highstand sea-level stage of third- and fourth-order depositional sequences.

北海道幾春別背斜東翼部の蝦夷層群三笠層〜羽幌川層における堆積相とシーケンス層序―セノマニアン－コニアシアンの第3～4 オーダー堆積シーケンス

北海道幾春別背斜東翼部の蝦夷層群三笠層〜羽幌川層における堆積相とシーケンス層序―セノマニアン－コニアシアンの第3～4 オーダー堆積シーケンス

SEQUENCE STRATIGRAPHIC ANATOMY OF DIVERSITY PATTERNS: LATE QUATERNARY BENTHIC MOLLUSKS OF THE PO PLAIN, ITALY

The influence of sequence stratigraphic (base-level driven) processes on patterns derived from the fossil record is receiving increasing attention. This study explores the stratigraphic anatomy of diversity patterns across two late Quaternary fourth-order sequences deposited on the Po Plain (Italy) over the last 150 ky (i.e., the two most recent glacial-interglacial cycles). The rich mollusk fauna, dominated by extant forms, preserved as a part of well-understood eustatic cycles, offers a testing ground for exploring how climate-driven sea-level changes influence sample diversity, diversity turnover, and higher-order diversity patterns within and across systems tracts and sequences. These two fourth-order depositional sequences were densely sampled from three cores. The data (152 species and 22777 specimens from 29 Holocene and 19 Pleistocene samples) were analyzed using single-sample and multisample rarefaction techniques. In all three cores and for both cycles, sample-level diversity decreased upward within sequences: the late transgressive systems tract samples displayed the highest equitability and richness, and the highstand systems tract samples displayed the lowest diversity (the trend primarily reflects the increase in the dominance of most common species in highstand systems tract samples). This pattern is likely due to a combination of ecological, environmental, and taphonomic processes. Multisample rarefaction indicates that species turnover is more limited in transgressive phases of both depositional cycles. This trend may reflect increasing environmental heterogeneity of marginal habitats averaged within shallowing-upward successions or decreasing time averaging associated with increasing sedimentation rates during highstand systems tract phases of the cycles. The sequence and multisequence diversity levels are lower than those observed within individual late transgressive systems tracts, indicating that species turnover was minimal both within as well as across the last two glacial-interglacial cycles. This study shows that species richness and equitability patterns of the most common mollusk species track closely the sequence stratigraphic architecture of late Quaternary successions of the Po Plain.

Paleopedologic and Paleohydrologic Records of Precipitation Seasonality from Early Pennsylvanian "Underclay" Paleosols, U.S.A.

Underclays in the Crooked Fork Group (Lower Pennsylvanian, Atokan, Langsettian) in eastern Tennessee, U.S.A., are paleosols associated with fourth-order depositional sequences that record changing paleohydrologic conditions during their formation. Climate changes (possibly Milankovitch-driven) resulted in evolution of soil landscapes from well-drained, seasonally wet floodplains and delta plains dominated by vertic (Vertisol-like) paleosols, to very poorly drained, everwet swamps dominated by sideritic gley paleosols. Coals (paleoHistosols) cap most underclays and record peat-swamp development during everwet conditions, followed by marine submergence. Pedogenic slickensides and angular blocky ped structures, in conjunction with illuviated clay pore fillings and sepic-plasmic microfabric, indicate an initial better-drained phase of paleosol development (Stage I). Gley overprinting, characterized by drab, low-chroma paleosol colors (Fe reduction) in upper portions of paleosols, sideritic rhizocretions, sphaerosiderite and pyrite nodules, extensive leaching and translocation of alkali and alkaline earth elements, and kaolinitization of smectites and hydroxy-interlayer vermiculite (HIV), indicate a later poorly drained stage of paleosol development characterized by saturated conditions, moderate to low Eh, and low pH (Stage II), as well as peat-swamp development. Lastly, burial and juxtaposition of marine facies on top of terrestrial deposits terminated paleosol and peat development (Stage III). Paleosols likely formed under conditions of marked seasonality of rainfall, judging from both modern analog systems in Sumatra and Pennsylvanian paleoclimate models. The chemical index of alteration without potash (CIA-K) was used as a proxy for Atokan paleoprecipitation, estimated as 1300 mm/yr. This estimate is comparable with mean annual precipitation (MAP) in modern tropical to subtropical rainy environments.

東北日本の白亜系―古第三系蝦夷前弧堆積盆の地質学的位置づけと層序対比

The Cretaceous to Paleocene Yezo forearc basin was developed between the western Early Cretaceous volcanic arc and the eastern Early Cretaceous to Early Eocene accretionary complexes in the northeastern margin of Eurasia Plate. Its sediments are distributed across a 150 km-wide and 1400 km-long belt from Northeast Japan to Sakhalin including off the Pacific coast in north-south direction. It is subdivided into three subbasins on the basis of geologic setting: Hokkaido, Kitakami and Joban subbasins. In the first half of this paper, the tectonic setting is reviewed not only for the better understanding of the basin, but also in order to evaluate the distribution of carbonaceous rocks as source rocks and reservoir sandstone.In the latter half, twenty-seven stratigraphic sections including 4 onshore and 8 offshore boreholes and 15 surface sections are correlated to delineate the sedimentary history and basin evolution through the temporal and spatial distributions of facies successions and depositional sequences. Two second-order shallowing-upward cycles for the Cretaceous are recognized in Hokkaido but are obscure in northern Honshu. Shallow-marine to paralic sediments of the first cycle (early Albian to Turonian) are represented by the Mikasa Formation (late Albian to Turonian) as the western marginal facies in Hokkaido. The Campanian to Maastrichtian shallow-marine and non-marine facies of the second cycle (Coniacian to Maastrichtian) are characterized by the Hakobuchi Formation in the Hokkaido subbasin and its correlatives in the Kitakami and Joban subbasins. The two formations include several third- and fourth-order depositional sequences with coarsening-upward facies successions. The uppermost Maastrichtian to Upper Paleocene interval is missing everywhere in the Yezo basin. Furthermore, two third-order depositional sequences of the late Paleocene (uppermost Hakobuchi Formation) and the uppermost Paleocene to lower Eocene (Haboro Formation) have developed with unconformities in a few areas.

THE MIDDLE PLEISTOCENE DEPOSITS OF THE ROMAN BASIN (LATIUM, ITALY):AN INTEGRATED APPROACH OF MAMMAL BIOCHRONOLOGY AND SEQUENCE STRATIGRAPHY

The biochronological setting proposed for the Plio-Pleistocene large mammal faunas of the Italian peninsula is based on the definition of faunal units (FUs) and mammal ages (MAs). Many evidences suggest that a multidisciplinary approach could enable us to better understand the actual meaning of a given faunal assemblage taking into account sedimentological and physical stratigraphic studies of the sedimentary successions in which local mammal faunas occur. The Pleistocene deposits of the Roman Basin can be considered a significant model to test this integrated approach. The detailed study of this sedimentary succession, in terms of facies analysis and sequence stratigraphy, sets some physical and temporal constrains to the occurrence of faunal complexes because the allocyclic control (climate and eustatic variations) on both landscape and stratigraphical evolution can affect the association type of mammal faunas. A correlation scheme between the Roman Pleistocene sequence-stratigraphic units and the mammal biochrons has been proposed; this approach constitutes a first tentative to connect the mammal fauna remains to the sedimentary processes which are responsible of their transport, stock and potential preservation in the depositional environments and to collocate this fauna in the systems tracts of the fourth-order depositional sequences recognised in the local Roman Basin Pleistocene succession.

Revision of the Late Miocene, Pliocene and Pleistocene Sequence Cycles

ABSTRACT Wornardt and Vail (1990, 1991) published their Mio-Pleistocene sequence chronostratigraphic chart for the Gulf of Mexico based on calcareous nannofossils, planktonic and benthic foraminifers in the Gulf of Mexico. Wornardt et al. (1998) modified the numerical ages of the sequences, sequence boundaries and maximum flooding surfaces of Haq et al. (1987), based largely on the oxygen isotope curve of Haddad et al. (1992) and Abreu et al. (1998). Hardenbol et al. (1998) recognized twelve third-order depositional Sequences in the late Miocene, Pliocene and Pleistocene. In the present paper, we divide the late Miocene, Pliocene and Pleistocene into a series of eleven third-order depositional sequences from 9.26 Ma to 1.40 Ma and eight fourth-order depositional sequences from 0.8 to 0.1 Ma. This revision should provide more accurate correlations between wells, better definition of sequences on well-logs and seismic record sections.

Aggrading and prograding infill of a peri-Tyrrhenian Basin (Naples Bay, Italy)

High-resolution seismic reflection data are used in the identification of the stratigraphic succession filling Naples Bay, a tectonically active half-graben located in the extensional margin of the eastern Tyrrhenian Sea. The architecture of the basin infill is characterized by fourth-order depositional sequences correlated to Quaternary eustatic oscillations. These depositional sequences form aggrading and prograding stacking patterns bounded by tectonically enhanced unconformities. The changes that the stacking patterns undergo record variations in accommodation space rates. These variations are a result of the tectonic subsidence produced during the evolution of the half-graben.

Tectonics and stratigraphic architecture of a peri-Tyrrhenian half-graben (Bay of Naples, Italy)

The structural and stratigraphic interpretation of high-resolution seismic reflection profiles permitted the reconstruction of the tectonics and basin architecture of the Bay of Naples from shelf-slope to deep basin areas. The Bay of Naples half-graben experienced extensional tectonics related to the activity of NE-trending normal faults during the Middle Pleistocene. The half-graben displays asymmetric and multiple fault segments linked by relay zones. The basin infill is made up, from old to young, of an unconformity-bounded unit and seven fourth-order depositional sequences. The oldest unit marks incipient fault activity. Increases of the fault slip rates gave rise to the formation of a tectonically-enhanced unconformity on the shelf and coeval thick debris flows in the deep basin. In the shelf-slope area the fourth-order depositional sequences form a trangressive–regressive cycle. The transgressive trend is linked to fault activity, whereas the regressive trend recorded tectonic stability.

Abstract: Evidence for Hierarchy of Stratigraphic Forcing in the Upper Carboniferous (Virgilian, Wabaunsee Group) in the Anadarko Basin 

Analysis of Upper Carboniferous (Virgilian, Wabaunsee Group) strata cropping out in the North American Midcontinent suggests a hierarchy of stratigraphic forcing. Fourth-order depositional sequences from the Wabaunsee Group represent the latest highstand sequence sets of a composite third-order sequence (1-1 0 m.y.) that encompasses strata from the Douglas, Shawnee, Wabaunsee, and Admire Groups. The composite third-order sequence (1 10 m.y.) is composed of 15 composite fourth-order depositional sequences (0.1-1 m.y.). The composite fourth-order depositional sequences of the Wabaunsee Group contain between two and three fifth-order cycles (0.0 1-0.1 m.y.). These fifth-order cycles form retrogradational transgressive system tracks and aggradational to progradational highstand-system tracks.

シーケンス層序学 その可能性を求めて 背弧堆積盆地におけるシーケンス解析 新潟県蒲原地域の例

Sequence stratigraphy has been rapidly spreading in the world and causes various sort of debates because sequence stratigraphic interpretations related to mechanisms of deposition make possible three-dimensional estimation of facies distribution and worldwide correlation of strata. The “Exxon Model”, as a basin fill model, has been paid much attention because it displays the significance in the basic concept of sequence stratigraphy and possibilities of applications. The “Exxon Model” which has been constructed as a depositional model for passive margin basins represented by gentle subsidence and relatively stable deposition, however, might not always be applicable to some sedimentary basins without careful considerations for their tectonic and depositional settings.In the Niigata sedimentary basin, as a representative back-arc basin located on central Japan, hydrocarbon exploration for several decades has resulted in a large accumulation of subsurface geologic information. Three-dimensional studies for sequence stratigraphic framework within a back-arc tectonic setting, therefore, are available in this basin.As a result of this study, 12 third and fourth order depositional sequences, which are interpreted to have been formed by relative change of sea level, are now recognized. These depositional sequences are superimposed on a part of a second order depositional sequence considered as a tectonically controlled sequence. The features of fifth or sixth order depositional sequences are superimposed locally in the deposits of the Niigata sedimentary basin. The depositional history of the Niigata sedimentary basin can be divided into following 5 stages, based on the disclosed sequence stratigraphic framework of this basin.i) stage of sedimentary basin formationii) stage of turbidite deposition at basin flooriii) stage of slope progradationiv) stage of shelf extensionv) stage of delta progradation

Seismic Models of a Shelf-margin Depositional Sequence: Upper San Andres Formation, Last Chance Canyon, New Mexico

ABSTRACT The seismic resolution of stratal geometries and facies distributions observed in San Andres Formation (Permian) outcrops in Last Chance Canyon, Guadalupe Mountains, New Mexico, is studied by seismic modeling of a published, detailed stratigraphic cross section. The outcrops in Last Chance Canyon are composed of two fourth-order depositional sequences: an aggrading carbonate bank (upper San Andres 3; uSA3) followed by a strongly progradational, offlapping mixed carbonate-siliciclastic succession (upper San Andres 4; uSA4). Each sequence comprises a number of subsidiary high-frequency sequences (fifth-order). Two alternative impedance models were used: Model A, in which all facies transitions are reflecting boundaries, and Model B, in which only time-significant surfaces act as reflectors and lateral facies transitions are represented by horizontal velocity gradients. The vertical-incidence modeling technique was used to compute perfectly migrated time and depth sections with different frequencies. Using a low-frequency wavelet (25 Hz), the sequence boundary separating the two fourth-order cycles (uSA3 and uSA4) is poorly imaged. Instead, one is tempted to incorrectly interpret an onlap pattern generated by a high-frequency cycle within uSA4 as this major sequence boundary. In addition, the 25 Hz runs show toplap and downlap lap-out patterns in an overly oblique fashion, obscuring true asymptotic stratal relationships. Both at 35 Hz and 50 Hz, profiles based on Model B image the genetic structure of both uSA3 and uSA4 relatively well. At 50 Hz, Model A incorrectly shows a transition from a ramp to a rimmed margin within uSA4. The 35 Hz models are qualitatively compared with a published Exxon Production Research Co. seismic line, located approximately 50 km along depositional str ke to the northeast. Model A shows an unexpected good match with the Exxon seismic line, whereas Model B comes much closer to the depositional anatomy observed in outcrop. Our results show that the resolution of stratal geometries and facies distributions in Last Chance Canyon is strongly related to carbonate-sandstone alternations and the way impedance contrasts at carbonate-sandstone transitions are represented.