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The Cambrian Tonto Group of the Grand Canyon was used by Edwin McKee in 1945 to make an insightful visual representation of how sedimentary facies record transgression across a craton--a common conceptual framework still used in geologic education. Although the tenets of McKee's facies diagram persist, the integration of new stratigraphy, depositional models, paleontology, biostratigraphy, and other data is refining the underlying dynamics of this cratonic transgression. Instead of McKee's interpretation of one major transgression with only minor regressions, there are at least five stratigraphic sequences, of which the lower three are separated by disconformities. These hiatal surfaces likely represent erosion of previously deposited Cambrian sediments that were laid down on the tropical, pre-vegetated landscape. Rather than being fully marine in origin, these sequences were formed by a mosaic of depositional environments including braided coastal plain, eolian, marginal marine, and various shallow marine environments. McKee, not having the insights of sequence stratigraphy and plate tectonics, concluded that the preservation of these sediments were due to predepositional topography and subsidence of the "geosyncline." Our modern interpretation is that accommodation space was a result of eustasy and differential subsidence on the continental margin. Our modified depositional model provides a more effective teaching tool for fundamentals and nuances of modern stratigraphic thinking, using the Tonto Group as a stillinfluential type location for understanding transgressive successions. [ABSTRACT FROM AUTHOR]

The article offers information on the 55th annual meeting of the Palynological Society to be held in Lexington, Kentucky, USA, from June 6 to June 10, 2023.

Allogenic controls are frequently cited as key factors influencing basin evolution; however, fewer studies perform paleotopographic reconstructions to examine the impact of topography in the development of stratigraphic sequences. Disentangling how allogenic versus autogenic controls affect the stratigraphic succession within a basin affected by salt tectonics is particularly challenging because decoupling the stratigraphic signature of lithospheric induced uplift and subsidence from salt tectonics is not a trivial exercise. We tackle this problem by integrating physical modeling results with a landscape numerical model and compare results with a case scenario from the subsurface. The physical model provides surface displacement data that are then used as inputs into the landscape numerical model to simulate the surface and stratigraphic evolution of a salt tectonic basin during a 25-m.y. timespan and within the context of a continental-scale source-to-sink (S2S) system. Results show that the evolution of salt structures impact the development and diversion of sedimentary routing systems within salt basins, thus influencing the character of the stratigraphic record independently of allogenic factors such as lithospheric induced uplift. Modeling results highlight the importance of reconstructing the paleo-topography of ancient depositional systems affected by salt tectonics to truly understand the nature of the final stratigraphic record. [ABSTRACT FROM AUTHOR]

Widespread, geologically fast (<500 kyr) shallow-marine glaucony formation during the Cretaceous greenhouse world has no recent analogue. A detailed investigation from the Upper Turonian Danubian Cretaceous Group (southern Germany) links authigenic marine glaucony precipitation (Großberg Formation) to the intense chemical weathering in the catchment of a supplying river system (Seugast Member, Roding Formation). Nine glauconitic facies types within the Großberg Formation reflect a mixed shallow-marine depositional environment with land-derived siliciclastics, marine-carbonates and authigenic glaucony. X-ray diffraction analyses confirm these authigenic green grains and matrix as glauconitic minerals. The very high values of the modified chemical index of alteration (mean CIX between 86 and 92) demonstrate that the catchment area of the Seugast River was deeply chemically weathered and leached. In comparison to the hydrochemistry of modern tropical streams, massive Late Cretaceous nearshore glaucony formation was promoted by the riverine flux from the deeply chemically weathered, wet-continents amply supplying essential elements (Fe, Si, K and Al), fuelling shallow-marine glauconitization. The sequence stratigraphic calibration of the shallow-marine to continental transect of the Großberg Formation and the Seugast Member, corresponding to the Late Turonian depositional sequence DS Tu 5, further demonstrates that Late Cretaceous glaucony formation was a fast process (<500 kyr). Supplementary material: Table S1 provides contents of total carbon (TC), total inorganic carbon (TIC), total organic carbon (TOC), major elements and Zr as well as element/Al ratios, calculated excess K (Kxs), silicate K (Ksil or K2Osil), molar element oxides and CIX values, and is available at https://doi.org/10.6084/m9.figshare.c.6297404 [ABSTRACT FROM AUTHOR]

A recent study conducted by researchers at the University of Chicago explores the preservation of vertebrate fossils and its implications for paleontologists. The study focuses on the analysis of marine mammal bones from the Eocene siliciclastics of Wadi al-Hitan, Egypt. The researchers found that bones can provide valuable information about their preservational history and the post-mortem environment. Despite being fractured and infilled by gypsum, the bones retained early diagenetic signatures, offering insights into their original depositional context. This research contributes to a better understanding of fossil preservation and its impact on the study of biological and ecological change over time. [Extracted from the article]

Sequence stratigraphic interpretations are limited in their prediction of sedimentary architecture and changes in sea-level when used in systems characterized by marked fluctuations in sediment supply, particularly in carbonate-dominated environments. Pronounced variations in sediment supply occurred during the Pliensbachian–Toarcian transition, when significant perturbations of the carbon cycle and intense environmental disturbances led to a global shutdown of carbonate production. We studied the impact of this event on sedimentation and stratal stacking patterns in the Moroccan central High Atlas. We reconstructed the lateral variations in facies and sedimentary geometries along a carbonate platform margin and slope in the field by tracking six key discontinuity surfaces covering the Pliensbachian–Toarcian transition from the platform margin to the toe-of-slope. This work highlights the difficulties in sequence stratigraphic interpretations in cases of neritic carbonate factory collapse without assessments of shoreline movement because the stacking patterns in open marine environments do not necessary reflect the regional variation in base level in disturbed carbonate systems. This study also emphasizes the uncertainties associated with focusing solely on lower offshore strata to assess the history and causes of palaeoenvironmental perturbations because decreases in the rate of carbonate production dampen neritic carbonate shedding and therefore favour the creation of a hiatus in deep water settings. [ABSTRACT FROM AUTHOR]

Piggy-back basins are characterized by growth stratigraphic sequences bounded by growth unconformities and record the complete deformational history of the related structures. The Biertuokuoyi piggy-back basin, located in the hanging wall of the Pamir frontal thrust (PFT), provides a remarkable example documenting the geometry, kinematics and mechanism of growth unconformities. High-resolution seismic data reveal that multiple growth sequences were folded with constant-dip kink band migration and their dips are equal to the change in the fault-bend angle of the PFT, indicating their kinematic coupling. Divided by a basal Cenozoic detachment, the PFT system contains five lower thrust ramps that cut Paleozoic–Mesozoic strata and an upper thrust ramp that cuts Cenozoic strata. The growth strata and unconformities record c. 4425 m of dip-slip and six stages of episodic thrusting for the PFT. We propose a coeval thrusting style for the five lower thrust ramps. The present seismic section reveals at least 31.1 km of total shortening at the northeastern Pamir front. In particular, the latest shortening of c. 8.0 km is recorded by the Biertuokuoyi Basin and the Mushi anticline. Our results show that multiple growth sequences bounded by growth unconformities are powerful resources for documenting the evolution of deformation. Supplementary material: Supporting figures and data, including seismic profiles and kinematic simulation results, are available at https://doi.org/10.17605/OSF.IO/DNQBS. Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts [ABSTRACT FROM AUTHOR]

The article provides an update on Region 9 (Mid-Atlantic) AEG activities, emphasizing local chapter events and challenges, including efforts to revive the Greater Pittsburgh Chapter, while highlighting upcoming meetings and advocacy for geology licensure in Virginia. Topics include chapter meetings, membership statistics, and initiatives to support professional development and legislative advocacy within the geoscience community.

The article highlights the New York-Philadelphia Chapter's fall meetings featuring talks on groundwater models, geohazards, and asbestos indicators, and their involvement in a major site remediation conference with sponsored courses.

The article discusses sequence stratigraphy has used to analyze sedimentary rock packages in terms of base-level changes and depositional trends arise as a result of the interplay between space that has available for sediments to be deposited. Topics include the formation represents a third-order depositional sequence within the overall second-order Devonian transgressive-regressive package; and carbonate-evaporite units of the Three Forks Formation.

The Late Paleozoic Ice Age (LPIA) has been well recorded in the uppermost Mississippian–Pennsylvanian of Gondwana. Nevertheless, little is known about the temporal and geographic dynamics, particularly during the early Mississippian. We report on exceptional Tournaisian glaciomarine stratigraphic sections from central Argentina (Río Blanco Basin). Encompassing c. 1400 m, these successions contain conspicuous glacigenic strata with age constraints provided by palaeontological data and U/Pb detrital zircon age spectra. A variety of marine, glaciomarine and fan-deltaic environments indicate relative sea-level variations mainly associated with tectonism and repetitive cycles of glacial activity. Provenance analysis indicates a source from the Sierras Pampeanas basement located to the east. Fifteen sequences were grouped into three depositional models: (1) Transgressive Systems Tracts (TST) to Highstand Systems Tracts (HST) sequences unaffected by glacial ice; (2) Lowstand Systems Tracts (LST) to TST and then to HST with glacial influence; and (3) non-glacial Falling-Stage Systems Tracts (FSST) to TST and HST. The glacial evidence indicates that the oldest Mississippian glacial stage of the LPIA in southwestern Gondwana is constrained to the middle Tournaisian. In contrast with previous descriptions of Gondwanan coeval glacial records, our sequence analysis confirms complex hierarchical climate variability, rather than a single episode of ice advance and retreat. Supplementary material: Detailed stratigraphic sections, palaeocurrents and compositional analysis and U/Pb detrital Zr methodology and data are available at: https://doi.org/10.6084/m9.figshare.c.5011424 [ABSTRACT FROM AUTHOR]

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The Ordovician-Silurian organic-rich shales in the Wufeng and Longmaxi Formations are widespread in southern China and are the most important shale gas producers in China. The shales display varying total organic carbon content ranging from 0.2% to 6% and are composed of black siliceous mudstone, shelly limestone, siliceous clay-rich mudstone, gray silty-shaly interlaminated mudstone, gray argillaceous mudstone, and calcareous or dolomitic mudstone. Detailed stratigraphic correlation of these formations was achieved, and a high-resolution sequence stratigraphy was constructed based on the integration of welllog data, mineralogical, geochemical, and petrophysical analyses. The Wufeng Formation and the Guanyinqiao Bed were interpreted to be a condensed section with four cycles of sea-level fluctuation. The Longmaxi Formation consists of three members. The Long-1 Member was interpreted to be a thirdorder depositional sequence that can be subdivided into a transgressive systems tract, an early highstand systems tract, and a late highstand systems tract. Within the constructed isochronal stratigraphic framework, the characteristics and genetic mechanism of different shale lithofacies were studied. The deposition of black siliceous mudstone of high total organic carbon in the Wufeng Formation and the Long-1 transgressive systems tract was likely influenced by volcanic activity. The Guanyinqiao shelly limestone or calcareous mudstone was associated with the Ordovician-Silurian transition when glaciation and mass extinction took place. The gray silty-shaly interlaminated mudstone of moderate total organic carbon in the Long-1 early highstand systems tract was the product of bottom flow intrusion. The deposition of gray argillaceous mudstone and calcareous or dolomitic mudstone with low total organic carbon in the Long-1 late highstand systems tract resulted from enhanced terrigenous input from peripheral uplifts. The distribution pattern of different lithofacies varies greatly in the Upper Yangtze area, and this difference is a result of dissimilar sediment provenances and variable hydraulic restriction levels that were controlled by basin geometry. A better understanding of the distribution pattern and depositional history of the Wufeng and Longmaxi Formations might provide guidance for future profitable shale gas exploration. [ABSTRACT FROM AUTHOR]

Researchers from Helwan University in Cairo, Egypt have conducted a study on petroleum research in the Abu Madi/El Qar'a/Khilala gas fields in the onshore Nile Delta. The study aimed to create a comprehensive model of the depositional features of the gas fields using geological and geophysical data. The researchers established a depositional pattern for the main Level III of the Abu Madi Formation, which recorded the progressive drowning of the incised valley. Gas-bearing traps were found in the Lowstand Systems Tracts of both sequences. The study provides valuable insights into the petroleum potential of the area. [Extracted from the article]

Following recent hydrocarbon exploration activity, a new study has been carried out with the goal of defining the high-resolution biochronologic sequence stratigraphy of the Oligocene section in the offshore Nile Delta. A total of 98 samples were selected from 2 wells: Burullus 1 Memphis and Petrobel 1 Habbar. The study is based on a high-resolution microfossil biochronology including benthic and planktonic foraminifera and nannofossils as well as facies analysis and wire-line log calibration to investigate the evolution of Oligocene gas-bearing sequences in the southeastern Mediterranean region. The planktonic-benthic ratio, foraminiferal diversity, and upper depth limit of benthic foraminiferal taxa as well as the abundance of calcareous nannoplankton help in interpreting the depositional paleoenvironments of the Oligocene succession as fluvial, shallow-marine, middle- to outer-shelf, and bathyal domains. Six sequences were identified, three of which are lower Oligocene (Rupelian) (RuSeql, RuSeq2, and RuSeq3), and the others are upper Oligocene (Chattian) (ChSeq4, ChSeq5, and ChSeq6). The sequence boundaries and flooding surfaces were correlated with the global eustatic sea-level models. Three major breaks--Eocene-Oligocene, lower-upper Oligocene, and upper Oligocene-lower Miocene boundaries--with intraformational breaks were well defined. [ABSTRACT FROM AUTHOR]

Late Cretaceous-to-present-day mixed carbonate-clastic deposition along the Nicaraguan platform, western Caribbean Sea, has evolved from a tectonically controlled, rifted upper Eocene shallow-to-deep-marine carbonate-siliciclastic shelf to an upper Miocene-to-present-day tectonically stable shallow-marine carbonate platform and passive margin. By integrating subsurface data of 287 two-dimensional seismic lines and 27 wells, we interpret the Cenozoic stratigraphic sequence as 3 cycles of transgression and regression beginning with an upper Eocene rhodolitic-algal carbonate shelf that interfingered with marginal siliciclastic sediments derived from exposed areas of Central America bordering the margin to the west. During the middle Eocene, a carbonate platform was established with both rimmed reefs and isolated patch reefs. A late Eocene forced regression produced widespread erosion and subaerial exposure across much of the platform and was recorded by a regional unconformity. The Oligocene-upper Miocene sedimentary record includes a southeastward prograding delta of the proto-Coco river, which drained the emergent area of what is now northern Nicaragua. The late Miocene-to-present-day period marks a period of strong subsidence with the development of small pinnacle reefs. We describe favorable petroleum system elements of the Nicaraguan platform that include (1) Eocene fossiliferous limestone source rocks documented as thermally mature in vintage exploration wells and seen as active gas chimneys emanating from inferred carbonate reservoirs; (2] upper--tomiddle Eocene reservoirs in patch and pinnacle reefs, middle Eocene calcareous slumps, and Oligocene fluvial-deltaic fades documented in wells; and (3) regional seal intervals that consist of both regional unconformities and Eocene-Oligocene intraformational shale. [ABSTRACT FROM AUTHOR]

The term "beef' describes bedding-parallel calcite veins found commonly in the organic-rich matrix of unconventional resource plays. Although some authors have interpreted beef to be an early diagenetic feature, these calcite veins are commonly attributed to precipitation at high temperatures and localized overpressure during hydrocarbon generation. The temperature at which the beef formed is thus crucial to ascertain the process of beef genesis. We use the novel methodology of clumped isotope analysis to constrain both the temperature at which beef forms and the isotopic composition of fluids present during formation. For this study, we use beef from basinal sections of the Vaca Muerta Formation in the Neuquen Basin, where veins are commonly up to approximately 10 cm (~4 in.) thick and are laterally continuous over 1 km (0.6 mi). The calcite veins occur in isolation or in association with concretions and ash layers. Sequence stratigraphic boundaries have little influence on distribution, and only a low correlation between beef and total organic content or beef and ash layers exists. The internal crystal structure of beef varies largely, suggesting both syntaxial and antitaxial growth forms. The δ18O values of beef range from approximately -- 12%o to -9%o, and the δ13C values vary between approximately -l%o and l%o. The surrounding mudstone and concretion fracture fills (calcite) show little difference isotopically when compared to the beef itself. The δ18O values of nearby concretions range from approximately -3.5%o to l°/oo, and the δ13C values vary between approximately 6%o and 1 l%o. Clumped isotope analysis of beef in the Vaca Muerta Formation indicates temperatures between approximately 140°C and 195°C, whereas the surrounding mudstones vary from approximately 120°C to 150°C. The corresponding formation fluid δ18OW values range from 8.5 to 14.5%o. These temperature data are higher than the maximum temperatures suggested by published studies modeling the basin's thermal and burial histories. If these models are correct, the clumped isotope data indicate that the growth of beef in the Vaca Muerta Formation required the input of hydrothermal fluids from greater depths. Alternatively, the geothermal gradient or burial depth was underestimated in these models. [ABSTRACT FROM AUTHOR]

A comprehensive study on rift stratigraphy requires a solid understanding of sequence architecture along the steep margins of riff basins. This study analyzes an Eocene lacustrine sequence along the steep margin of the Dongying depression in eastern China through integrated core, well-log, and three-dimensional seismic analyses. The lacustrine sequence is bounded by unconformities and their correlative conformities at the base and top and consists of three systems tracts, namely an early expansion systems tract [EEST], late expansion-early contraction systems tract (LEECST), and late contraction systems tract (LCST), which record a lake expansion-contraction cycle. These systems tracts differ in thickness and development of depositional systems. The EEST is the thickest and contains well-developed marginal and basinal fan systems with an overall retrogradational stacking pattern. The well-developed fan systems are the most striking features within the sequence. The LEECST is the most widespread and contains dominantly profundal-sublittoral deposits. The LCST is the thinnest, with poorly developed fan systems, and is characterized by significant erosion by fluvial incision. The variable thickness and development of depositional systems in the three systems tracts are the responses to the interplay of sediment supply and accommodation space. Accommodation space establishes the framework for sedimentary infill, and sediment supply determines spatial distribution and temporal evolution of depositional systems within each systems tract. This study provides a lake expansion-contraction scheme to divide a lacustrine stratigraphic sequence into systems tracts and highlights the feasibility of applying this approach in studying sequence stratigraphy along the steep margin of a lacustrine rift basin. The results also provide understandings for the development, distribution, and evolution of depositional systems and their controlling factors along the steep margin of other rift basins in the world. [ABSTRACT FROM AUTHOR]

Analysis of high-resolution three-dimensional seismic data from the Cold Lake Production Project (CLPP) of central Alberta, Canada, has resulted in a new sequence stratigraphie interpretation and depositional model for the upper Albian Clearwater Formation of the Mannville Group. Specifically, we document the presence of one sequence boundary within the Clearwater Formation that (1) separates older, deltaic deposits from a younger fluvial-dominated, terraced incised valley fill succession and (2) ties to a lowstand shoreline approximately 100 km (62 mi) to the north of the CLPP. Although this interpretation is far simpler than previous stratigraphic interpretations of this area, the sedimentologic record within the Clearwater Formation remains very complex because of the vertical stacking of high-energy fluvial to fluvial-estuarine deposits that are scour based. The composite sequence boundary identified here is associated with an extended period of landscape degradation and the formation of a moderately large valley that is complexly defined by a series of terraced fluvial deposits. Because individual channels eroded vertically and migrated laterally during both the fall and ensuing rise of sea level, the resulting valley-shaped stratigraphie sand body is (1) substantially wider than the true topographic valley (i.e., landform that is constrained by subvertical to near-vertical walls, open to the air, and typically resulting from degradation of the landscape via vertical and lateral erosion by a fluvial channel or channels) within which the lowstand channels flowed, (2) formed by both fluvial and marine processes that can be allogenic and/or autogenic in nature, and (3) defined by a composite surface that formed during the descending limb of a base level cycle and was partially modified during the subsequent base level rise and is thus of minor chronologic significance. We attempt to define the time of maximum topographic valley development, but younger erosion has removed much of the record of this valley. However, we estimate that the Clearwater Formation topographic valley had a maximum incision depth of greater than 60 m (>197 ft) and a width of approximately 20 km (12 mi). These dimensions correlate very well to incised valleys observed in the Quaternary. Analysis of core and log results within our seismic stratigraphie framework indicates that a fluviodeltaic model best explains the lithofacies distributions and geometries within the CLPP. Furthermore, finer-scale seismic mapping was used to encapsulate packages of sediment--which we refer to as genetic sedimentary bodies--whose reservoir properties could then be defined using core results. A genetic body approach to defining stratal architectures has resulted in (1) a predictive model for reservoir types and distributions across the CLPP; (2) accurate paleoenvironmental interpretations; and (3) a simple, yet robust sequence stratigraphie model of this area that is aligned with recent results reported from the study of similar systems in the Quaternary, recent morphologic observations from small-scale, physical sand box experiments, and the most up-to-date models of coastal fluvial erosion, deposition, and stratigraphie surface formation. [ABSTRACT FROM AUTHOR]

Thunder Horse and Mensa are two of the largest fields of oil or gas, respectively, in the northern deep-water Gulf of Mexico. The fields are present in adjacent intraslope minibasins, located approximately 12 mi (19 km) apart in Mississippi Canyon. Both fields illustrate important complexities of deep-water sedimentation. Analysis is based on the integration of wire-line logs, biostratigraphy, and a 378-mi2 (979-km2), three-dimensional seismic data set. Thunder Horse and Mensa reservoirs were deposited during the middle to late Miocene. Changes in paleobathymetry controlled the reservoir deposition, initially as salt withdrawal and later as turtle structures. From 125 to 24 Ma, the lithologies in both intraslope basins are interpreted as dominantly deep-water marls with interbedded shales. From 24 to 14.35 Ma, major input of deep-water siliciclastic sediments began. Sands were deposited in amalgamated sheets and amalgamated channel-fill units within the two major paleobathymetric lows; by contrast, shales were deposited across paleobathymetric highs. Between 14.35 and 13.05 Ma, the Thunder Horse turtle formed, creating a paleobathymetric high. Channelized sands were diverted around and deposited on the flanks of the structure. Meanwhile, to the north at Mensa, thick channel-fill sediments continued to be deposited. From 12.2 to 8.2 Ma, the lithologies throughout the entire area are dominantly overbank shales with thin channel-fill sands, suggesting that large volumes of sand bypassed the study area farther downslope to the south. Finally, at 9.0 Ma, Mensa's sheet-sand reservoir represents a different setting; sands were deposited near the crest of the Mensa turtle, which had subtle bathymetric expression. [ABSTRACT FROM AUTHOR]

The article discusses the possible differentiation between proximal and distal settings (parallel to flow) in deep water environments. It states that the challenge is the finding of a blocky gamma-ray in both the proximal and distal domains, and that the inconclusive seismic data taken on a drilled deep-water system leaves room for interpretation.

The article features views on the complex maximum flooding surfaces in carbonate sequences in 2023. It states that the maximum flooding surfaces' correspond to both a gamma-ray high or low, and that they depend on the carbonate platform or ramp type. Also noted are a study on depositional systems with alternating clean and argillaceous carbonates, and the positioning's dependence on the specific depositional context.

The article discusses pre-drill predictions of basin-fill strata that have yet to be achieved as of March 2023. Topics covered include the complex factors that control stacking patterns and spatial distribution of strata, and the unlikelihood of published sea-level curves to be sufficiently accurate models of eustatic sea-level history. Also noted is the need to acquire sufficient data to reduce uncertainty when making predictions.

The future of sequence stratigraphy depends on stratigraphers making observations with a common method so that physical frameworks can be clearly separated from interpretations of driving mechanisms. Depositional sequence boundary selection is a well-known controversy that could be resolved with objective recognition criteria. Accommodation succession sequence stratigraphy refines traditional methods, using sedimentary facies, facies associations, vertical stacking, stratal geometries and stratal terminations as the objective record of competing rates of accommodation change and sediment fill through time. Observations are placed in context of lateral (transgression and regression) and vertical (aggradation and degradation) movement of shoreline through time, across multiple timescales in hierarchal stacks. The repeating motif consists of a subaerial unconformity and its correlative subaqueous surface overlain in coastal settings by a basinward shift in coastal onlap and strata with progradational to aggradation stacking, then retrogradation and aggradation-progradation-degradation stacking. These stacking patterns are bounded by key surfaces, recognized by stratal terminations and characteristic vertical successions of facies. This pattern is independent of time duration or position on a sea-level curve, but incorporates data resolution, regional extent and hierarchal stacking. Examples from multiple datasets show the utility and objectivity of the method and provide insights into sequence boundary formation. [ABSTRACT FROM AUTHOR]

Sequence stratigraphy arose as a paradigm in stratigraphy after the integration of the descriptive seismic method, introduced by Exxon researchers in the 1970s, with genetic concepts linking seismic attributes to sedimentary dynamics. Since then, the sequence stratigraphy model underwent significant modification owing to the increasing scenarios of application, each with its own practical requirements. This led to the fragmentation of the original model into a plethora of sub-methods and the flourishing of redundant notions and terminology. Reviewers striving to preserve the unity and fitness of sequence stratigraphy systematically weakened the central assumption by which it stood as a novelty and a paradigm: the relevance of relative sea-level cycles in shaping strata 'sequentially'. In contrast to this attitude, the value of a model explicitly relying on the upholding control of sea-level is herein reconsidered, based on the marine record of Quaternary, climate-driven, sea-level cycles. Traditionally conceived as exceptionally short-lived and extraordinary events in the history of the Earth, these cycles are documented worldwide on modern continental margins, providing convincing evidence of how sea-level fluctuations actually shape sequences. [ABSTRACT FROM AUTHOR]

The sequence stratigraphic model, although no longer focused on eustasy and accommodation, has been until recently based largely on observation and interpretation of outcrop and subsurface data. This approach may be restrictive if the current model places limits on what is observed and how observations are interpreted. To make progress in our understanding of strata, the sequence stratigraphic model and method should be tested against and fully incorporate theoretical and experimental results that provide new knowledge of (1) autogenesis, (2) intrinsic stratigraphic responses, (3) alluvial grade, and (4) scales appropriate to single depositional systems evolving with relative sea-level changes. More extensive inclusion of analogue and numerical experimental results could lead to significant modification and refinement of existing sequence stratigraphic models. [ABSTRACT FROM AUTHOR]

Detailed stratigraphic and paleogeographic analyses of data from 72 boreholes for the Middle Jurassic intermontane fluvial- lacustrine coal-bearing sequences were conducted in the Yuqia coalfield of the northern Qaidam Basin, northwestern China. Three third-order sequences lasting in total ca. 10.6 m.y., and an internal lowstand systems tract (LST), transgressive systems tract (TST), highstand systems tract (HST), and falling-stage systems tract (FSST) have been identified. A series of sequence-specific paleogeographic maps have been constructed based on the contours of lithological parameters. The paleogeographic units include alluvial fan-braided (meandering) fluvial plain, upper delta plain, lower delta plain, subaqueous delta, shore-shallow lake, and deep lake. The preferred sites of coal accumulation are interdelta bays, upper delta plains, lower delta plains, and fluvial back swamps. The sequence stratigraphic and sedimentological analysis of the Middle Jurassic coal-bearing measures of the Yuqia coalfield provides a basis for a comprehensive coal accumulation model that involves a six-period evolution from the LST, early TST, late TST, early HST, late HST to FSST. The major coal seams were accumulated in the early and late TST of the sequences S1 and S2. These results are of practical significance for coal resources exploration and enhance geological effects of prospecting engineering in the northern Qaidam Basin. [ABSTRACT FROM AUTHOR]

The article discusses the development of residual oil zones (ROZs) and the way they increase the lateral and thickness extent of an oil field. It discusses that the ROZs are part of the original oil traps and they underwent natural waterflooding over geological time. It mentions the contributions of oil saturation in the development of ROZs.

High-resolution carbon isotope signatures were integrated with core descriptions and gamma-ray logs and used as a correlation tool for better age control to refine the sequence-stratigraphic framework of the Shu'aiba Formation in Saudi Arabia. The carbon isotope variations of the shallow carbonate Shu'aiba Formation correlate well with the Tethyan pelagic record and indicate an original marine C13 signature for the Lower Cretaceous (Aptian) Shu'aiba Formation. Carbon isotope values of the Shu'aiba Formation range from 1.5‰ to 6‰ with minimal or no diagenetic effects. Oxygen isotope values range from -2.7‰ to -6.7‰ but were reset during diagenesis and cannot be applied for chemostratigraphic analysis. The Shu'aiba strontium isotope records range from 0.707356 to 0.707454 and differ slightly from the standard Aptian record because of diagenesis. The Shu'aiba Formation platform is a large-scale composite sequence (~7 m.y.) composed of seven lower Aptian high-frequency sequences and two additional upper Aptian prograding sequences. Carbon isotope data were calibrated with core descriptions and gamma-ray logs to construct two detailed high-resolution stratigraphic cross sections. Carbon isotope data help refine the internal stratigraphic architecture of the Shu'aiba Formation especially on the slope and open-marine settings across the lower to upper Aptian boundary. The carbon isotope values of the Hawar "dense" unit in the base of the Shu'aiba Formation record major depletion corresponding to the global dissociation of methane hydrates, followed by major positive excursion associated with the deposition of Lithocodium and Bacinella facies coeval with the global oceanic anoxic event la. The rudist buildups on the platform have a value of approximately 4.5‰ at their base in most wells with a general uniform carbon isotope trend, followed by a gradual depletion to the top of the Shu'aiba Formation. Although some variations are observed in carbon isotope values associated with the lateral facies change from lagoon, margin, slope, open-marine, and basinal settings, carbon isotope trends are still similar and can be correlated fieldwide. Little evidence exists of meteoric diagenesis associated with the depletion of carbon isotope values. However, oxygen isotope records were possibly affected by meteoric diagenesis associated with subaerial exposure surfaces but did not get affected by the late Aptian hiatus, despite the massive karstification observed in cores. The good correlation between the original carbon isotope fluctuations and the third-order sequence framework of the Shu'aiba Formation fits well with the established carbon isotope curves that have been used as a proxy for global sea level changes during the Early Cretaceous. This study also shows that small-scale parasequences (fifth-order or higher) can be calibrated with carbon isotope curves, but they most likely represent relative sea level changes with local effects instead of global signatures. Application of high-resolution carbon isotope stratigraphy for the Shu'aiba Formation significantly constrain the stratigraphic framework and will lead to better geologic and simulation models for reservoir characterization and development. [ABSTRACT FROM AUTHOR]

Detailed sequence stratigraphic frameworks have been produced for most of the Cenozoic deposits around the North Atlantic and Greenland Sea. However, the sequence stratigraphic evolution of the NE Greenland shelf, a significant part of the Atlantic Ocean margin, has so far never been interpreted. This study provides, for the first time, a detailed sequence stratigraphic interpretation of the NE Greenland shelf since the late Eocene. The tectonic evolution, eustatic sea-level change and the impact of glaciations are integrated into a consistent geological history. Based on the identification of pronounced unconformities and their correlative conformities, ten stratigraphic sequences are identified. The onset of deposition is defined by a pronounced erosional event, where late Paleocene tectonic activity caused mass wasting of the uplifted footwall of the Danmarkshavn Ridge along a c. 200 km section of the ridge. A second pronounced unconformity developed across the margin during the mid-Miocene, which caused forced regressive submarine fans to prograde across the margin. The following tectonic quiet period is overlain by glaciomarine deposits, with evidence of ice deformation reaching the present day shelf break. Unknown super-regional plate tectonic forces are suggested as a driver of sequence formation based on correlation with the Canadian Arctic. [ABSTRACT FROM AUTHOR]

The article offers information on a palynologist in Great Britain. It mentions that his research interests involve using Palynology in Biostratigraphy, Chronostratigraphy, Sequence Stratigraphy, Palaeoenvironmental reconstructions and Palaeoclimatology in projects at both a regional and a reservoir scale.

The article presents a case study which aims to document the implications of one modern approach of building 3D sequence stratigraphic framework. The example was taken from the Pliocene wave-dominated delta of the southern North Sea. It provides brief history and theoritical period of sequence stratigraphy. It also introduces the HorizonGube approach, a semiautomated method which was used to avoid cumbersome mapping of each seismic event in seismic survey.

Cenozoic rifted lacustrine basins in east China display three main basin types: (1) basins with steeply dipping boundary fault, whose hanging walls tilt along pivot points; (2) basins with listric boundary faults, whose hanging walls bend along flexural bending points; and (3) basins formed by earlier extensional rifting with later strike-slip movement superimposed. The sequence development is intimately linked to the tectonic movements in the area, where second-order sequences are regionally correlatable from basin to basin and relate to the large-scale tectonic movements in the region. Third-order sequences are related to local tectonic activity and are correlatable within basins, between subbasins, and sometimes, between neighboring basins. Detailed sequence-stratigraphic analysis and mapping of depositional systems demonstrate that sand-body distribution patterns are related to sequence-stratigraphic frameworks. For the three kinds of basins, the positions of pivot point zones, flexural bend zones, and strike-slip faults plus the syndepositional faults all control the distribution of depositional systems systems tract, and sand bodies. These controlling factors can be attributed to different structural and stratigraphic features that change the accommodation. Structural elements include boundary faults, syndepositional faults, and abrupt changes in dip. Stratigraphic controls include preexisting surfaces with local channelization, paleobathymetric lows, and onlap onto clinoform slopes. The lowstand sand bodies deposited at the downdip end of these controlling factors constitute the current and future exploration targets for conventional reservoirs. [ABSTRACT FROM AUTHOR]

The Turonian Notom delta is one of the Ferron fluvial-deltaic wedges deposited in the Cretaceous foreland basin of North America and is well exposed in the Henry Mountains region, Utah. This article uses variable sampling of measured sections of the fluvial-deltaic wedge along a 19-mi (30-km) outcrop belt to address correlation uncertainties in sparse data sets, such as are common in subsurface studies. A high-resolution sequence-stratigraphic analysis based on 58 measured sections, extensive walking out of beds, and field photomosaics shows that the wedge is composed of 43 parasequences, 18 parasequence sets, and six depositional sequences. Resampling the outcrop belt with 13 sections provides a sparse data set. Correlations using the data set show considerable uncertainties. Fluvial complexity is greatly underestimated, and in the extreme case, only 40% of the fluvial bodies are identified. Overcorrelation results in as much as 400% exaggeration of sandstone width and thickness. Some of the uncertainties can be reduced by incorporating nonmarine sequence-stratigraphic models and various techniques that allow a more reasonable evaluation of fluvial body geometry. With sparse data, uncertainties also arise in marine successions. A key challenge is to identify and correlate the small-scale units, particularly forced regressive deposits. Many of their diagnostic features that can be easily recognized from outcrops and cores may not be as easily detected from well logs and on seismic data. In subsurface studies where sparse data are observed, integrating detailed facies analysis cores and sequence-stratigraphic concepts is critical to predict the occurrence and understand the architecture of such deposits. [ABSTRACT FROM AUTHOR]

In Costa Rica the karstic phenomenon affects mainly carbonate rocks, constituting 0.85% of the country's surface, with an approximate area of 431 km2. There are documented at least 9 episodes of carbonate deposition from the Late Cretaceous to Holocene, which include eleven stratigraphic units. These events are related to various geological-tectonic processes, changes in sea level, subsidence and changes in the sediment input. Ten karstic zones and four limestone sites have been defined: 1) North Pacific, 2) Tempisque, 3) South Zone, 4) Central Pacific, 5) Cabo Blanco and Malpaís, 6) Turrialba and Pacuare, 7) Talamanca, 8) Venado , 9) Central Valley, 10) Limón and the isolated karst areas: A) Garza, B) Cerro Huevo Frito, C)Cerro Morro and D) Playa Panama. These karstic features have morphologies which can be exo-karstic, including lapiaz, 'mogotes', karstic-towers, sinkholes and conical karst. The best known endokarstic phenomena are caves, in which it is common the development of speleothems. such as stalactites, stalagmites, columns, flowstones, draperies, calcareous pearls, etc. Serpiente Dormida (Sleeping Snake) cave is the deepest in the country, with -172 m, the longest is the Bruja - Rectangle - Tururún - Corredores system, with 3872 m in length. At least 258 caves in limestone have been registered, of which 41% have been mapped (100 caves). 88% of the caves in Costa Rica are found in karst areas of the Southern Zone (61%), Tempisque (17%) and Venado (10%), while the remaining 12% of caves are distributed in other karstic areas. These areas have special environmental and hydrogeological conditions that must be taken into account as to infrastructure development, and could also be exploited by tourism. [ABSTRACT FROM AUTHOR]

Many published sequence-stratigraphic frameworks lack a systematic and consistent designation for both depositional sequences and key surfaces, despite the original goal to provide a fully integrated stratigraphic architecture, including diagnostic age information. Based on a system in use for more than 10 yr at ExxonMobil, we recommend methodologies for a chronostratigraphic designation system (CDS) using more uniform and robust sequence-stratigraphic designations. After objectively defining important physical stratigraphic surfaces, biostratigraphic and other age-constraining information is used to designate surfaces and the packages of rocks they bound. This leads to the establishment of a sequence chronostratigraphic framework for a local area of investigation (outcrop section, field, region, or basin). Only after demonstrating clear well-documented ties to Phanerozoic global coastal onlaps or cycle charts are these sequences and associated surfaces considered as "global" entities and designated as such. Higher frequency sequences and surfaces are also accommodated in this CDS. Alternative designations for areas with limited or poor quality chronostratigraphic information are also discussed. The CDS has proven to have great use in all Phanerozoic strata, different tectonic settings, and depositional environments, especially when chronostratigraphic age constraints are robust. We have used this system at regional and basinal scales in many geographic locations to help reduce uncertainty in identifying and correlating reservoirs, sources, and seal rocks. Predicting the local distribution and quality of reservoirs as well as seals within a producing field and near field wildcats is also facilitated by this system. This system has demonstrated use in the correlation of outcrop sections within a basin or between basins. Rigorous use of the CDS proposed here will permit meaningful regional and/or interbasinal correlation that is difficult to carry out with the diverse systems currently in use. This uniform designation scheme will also facilitate communication within a company and between institutions, as well as among academic investigators. [ABSTRACT FROM AUTHOR]

Cretaceous low-accommodation deposits have been extensively studied in the subsurface of the Western Interior of North America because of their prolific hydrocarbon production and remaining potential. Understanding the strati-graphic complexities of these deposits in the subsurface relies strongly on detailed outcrop analogs. In this study, the Dakota Sandstone was examined along 100 km (62 mi) of semicon-tinuous outcrop between the towns of Hanksville and Ticaboo in the Henry Mountains of southeastern Utah. This region represented a low-accommodation setting located over the forebulge of the Cretaceous Western Interior Basin during accumulation of the unit. The Dakota Sandstone is 0 to 38 m (125 ft) thick, of Cenomanian age, and records multiple cycles of sediment accumulation. The Dakota Sandstone is subdivided into two condensed top-truncated stratigraphic sequences, the upper of which contains two parasequences. The basal parts of both sequences are composed of braided fluvial conglomerates and sandstone overlain by tidally influenced fluvial sandstone, inclined heterolithically stratified estuarine mudstone, carbonaceous shale, and coal. The overlying parasequences consist of coarsening-upward lower to upper shore-face mudstone, sandstone, tidal inlet deposits, and oyster shell concentrations. These facies define tripartite subdivisions of depositional environments typical of wave-dominated estuaries. The fluvial deposits may represent lowstand deposits, but overall sediments accumulated during transgressive systems tracts (TST). The parase-quences recorded in the Henry Mountains are similar to the Dakota Sandstone of northwestern New Mexico and to high-frequency sequences identified in the Kaiparowits Plateau, approximately 80 km (-50 mi) to the southwest, which suggests eustatic driving mechanisms. The best potential for hydrocarbon reservoirs occurs in fluvial sandstones and conglomerates. [ABSTRACT FROM AUTHOR]

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 ofbioturbation; (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. [ABSTRACT FROM AUTHOR]

Judy Creek, located in west-central Alberta, is one of the largest reservoirs in the Swan Hills oil field. Judy Creek is an isolated reef complex of the Upper Devonian Swan Hills Formation. This mature field has about 350 wells drilled and nearly 6000 m (19,685 ft) of cores and thus provides a good database for depositional facies analysis and testing methods for integrating descriptive and quantitative data in facies modeling. General depositional facies and sequence stratigraphy of Judy Creek have been studied quite extensively. However, no three-dimensional numeric facies model has been built to guide the field development. One of the objectives in constructing such a model for this study was to establish a close linkage between the facies depositional analysis and modeling. As a matter of fact, there have been significant disconnects between descriptive analysis and numeric modeling in the exploration and production. Depositional facies analysis has focused on conceptual understanding for prospect generation and reservoir delineation, whereas facies modeling has emphasized data mining for field development. In this study, a method that integrates the spatial propensity from the depositional conceptual models and facies data from the wells is used to bridge the gap between the depositional analysis and stochastic modeling. Such integration has proven to be critical in building realistic subsurface models in Judy Creek because it helped improve the estimation of subsurface resources. [ABSTRACT FROM AUTHOR]

A review of the article "Recognition of the Eocene-Oligocene boundary in the Hampshire Basin, Great Britain," which appeared in the "Journal of the Geological Society" is presented. It explains the basis of the arguments raised by Hooker on the placement of the Eocene-Oligocene boundary around the Nematura Bed in the Isle of Wight, England. The authors comment on the possibility of correlating sequences from a marine Tethyan to a continental Boreal without any fossil species common to both.

An integrated analysis of facies and palaeontological content of the middle to late Miocene (c. 12.4-9.1 Ma) Angostura Formation led to the identification of facies associations indicative of shoreface and inner-shelf settings. The Angostura Formation comprises eight high-frequency sequences that are stacked to form a tectonically driven lowstand sequence set. The most complete examples of sequences are bounded by transgressively modified unconformities and exhibit a threefold subdivision: (1) a basal suite of deepening-upward shoreface sediments (transgressive systems tract), including a base-of-cycle shell concentration; (2) a mid-cycle shell bed, consisting of molluscan shells dispersed in a matrix of inner-shelf muddy fine-grained sandstones; (3) a shallowing-upward unit of inner-shelf to lower shoreface sediments almost barren of mollusc fossils (highstand systems tract). Biostratigraphic constraints allowed a reasonable correlation between sequence bounding unconformities and the late middle to late Miocene high-frequency glacio-eustatic changes derived from recent δ18O studies. This correlation has far-reaching implications and leads to the following conclusions: (1) glacio-eustasy in tune with oxygen isotope changes at fourth-order frequency (200 ka-1 Ma duration) may have been the principal factor regulating stratigraphic packaging in the Angostura Formation; (2) these sequences provide an excellent shallow-marine outcrop record of late middle to late Miocene Antarctic glaciations. [ABSTRACT FROM AUTHOR]

Sequence-stratigraphic conceptual models typically focus on accommodation as a dominant control. Although useful in many ways, this approach may not address the full range of possible controls on stratal patterns, nor is it likely to fully address uncertainty in the identification and quantification of controlling processes. Consequently, predictions from sequence-stratigraphic conceptual models may be more limited than generally stated. Progress in addressing this problem can be achieved by applying the latest generation three-dimensional stratigraphic forward modeling to (1) investigate and include more of the parameters that may control stratal architectures and (2) consider multiple scenarios to help determine the impact of uncertainty in operating processes and their parameter values. Results from a three-dimensional diffusional stratigraphic forward model illustrate this approach, suggesting that relative sea level change, shelf width, and sediment-transport efficiency are important large-scale controls on the spatial and temporal distribution of deep-marine stratal volumes. If this result is sufficiently independent of model assumptions and can be replicated in other models, it suggests that all three controlling factors should be included in interpretations and predictions of outcrop and subsurface deep-marine strata. Modeling results also suggest that combining multiple forward-model scenarios to form conditional frequency. [ABSTRACT FROM AUTHOR]

Well-cuttings analysis predates modern carbonate facies analysis, sequence stratigraphy, seismic reflection surveys, and advanced geophysical logging techniques. These newer methods have resulted in well cuttings becoming less important as a major source of data for high-resolution subsurface analysis. Binocular analysis of well-indurated Paleozoic well cuttings can be used to construct detailed vertical facies successions in wells when tied to wire-line logs. Facies analysis can then be used to construct higher resolution sequence-stratigraphic frameworks and time-slice maps. This approach was tested on Mississippian carbonates in the Appalachian Basin of West Virginia. The analysis was done using the washed coarse fraction (1-2 mm; 0.04-0.08 in.) of the cuttings for each sample interval, classified according to Dunham rock type, counted to determine relative abundance, and plotted as percent lithology versus depth for each well. Digitized wire-line logs and the cuttings-percent logs were adjusted (typically 10 ft [3m] or so) to consider drilling lag, lithologic columns were produced from the combined data, and sequences were picked. Gamma-ray markers were used to correlate the sections, and sequence-stratigraphic cross sections were produced. Time-slice maps were generated that show the thickness of the individual sequences and the distribution of major facies within systems tracts. This approach generated a rock-based, high-resolution sequence framework for the reservoir and led to a much better understanding of controls on the distribution and stacking of reservoirs. [ABSTRACT FROM AUTHOR]

High-resolution (sub-seismic) stratigraphic correlation in fluvial strata is extremely difficult using conventional subsurface core and wireline-log data (e.g. gamma-ray, neutron, density, sonic logs). Spectral gamma-ray logs provide additional data on lithological composition that can aid identification of chronostratigraphic surfaces in a high-resolution sequence stratigraphic correlation framework. The application of spectral gamma-ray data in constructing such a framework is demonstrated using data from exposures and the subsurface from the Cretaceous non-marine Blackhawk Formation and Castlegate Sandstone of the Book Cliffs area, Utah, USA. These strata provide an ideal test case for the use of spectral gamma-ray data in fluvial stratigraphy, because interpretations can be compared directly with a high-resolution sequence stratigraphic framework from coeval shallow-marine strata exposed nearby. The stratigraphic abundance of coals and relatively uniform Th/K ratio, which indicates uniform clay mineralogy, in mudstone sections of the non-marine Blackhawk Formation implies a humid, subtropical climate throughout its deposition. Variations in facies architecture and stratigraphy are therefore interpreted to have been driven principally by fluctuations in relative sea level and hinterland tectonics. Fluvial and fluvio-tidal sandstones within incised valleys, which lie above erosional sequence boundaries, have a highly variable spectral gamma-ray character, but locally are marked by distinctive heavy mineral lags and concentrations (Th consistently >3 ppm). On the interfluves of incised valleys, sequence boundaries are marked by palaeosols that locally have a distinctive leached, potassium-depleted character (Th/K ratio >17). Lateral variability in spectral gamma-ray character of sequence boundaries is consistent with facies and sequence stratigraphic models of the non-marine Blackhawk Formation and Castlegate Sandstone, but sequence boundaries do not exhibit a clear, diagnostic character in every location. In addition, high Th/K ratios of the type noted at interfluve palaeosols may be produced by potassium depletion beneath coal seams as a result of acid groundwater percolation during early diagenesis, and by thorium enrichment in heavy mineral lags within foreshore sandstones. These alternative interpretations can be eliminated if high Th/K ratios occur in intervals lacking coal seams and shallow-marine sandstones, respectively. Thus, within the context of a sound understanding of detrital mineralogy, diagenetic history, facies and appropriate sequence stratigraphic models, spectral gamma-ray data allow sequence boundaries to be identified and correlated in non-marine fluvial strata at least 80 km from the interpreted coeval shoreline. [ABSTRACT FROM AUTHOR]

An integrated stratigraphical, sedimentological, geochemical and sequential analysis has been carried out on the Liassic rocks of the Basque-Cantabrian basin in northern Spain using outcrop and subsurface data. The study documents the progressive drowning and backstepping of an epeiric carbonate ramp that originated during the worldwide late Triassic-early Jurassic transgression. Two major carbonate depositional systems have been recognized within the drowning succession: (1) a late Triassic-early Sinemurian shallow-marine ramp, which reflects a progressive marine transgression over Triassic continental deposits; (2) a late Sinemurian-Toarcian set of hemipelagic ramps, which recorded the progressive drowning and backstepping of the ramp system during a second major event of marine flooding. Sequence stratigraphic analysis of the Liassic succession has revealed that transgressive-regressive trends constitute the most evident cyclicity. A major transgressive-regressive cycle and six subsidiary transgressive-regressive facies cycles have been defined in the basin and compared with those defined in other European basins. Although a good correlation exists for the major transgressive-regressive cycle, significant discrepancies have been found for the number, limits and extent of the transgressive-regressive facies cycles reflecting the effects of local tectonics. The overall backstepping within the Basque-Cantabrian basin resulted in a dominance of transgressive episodes across a range of different scales. Accordingly, regressive intervals are less well developed except during limited periods of active tectonics as occurred in the mid-Sinemurian. These progressively longer and deeper marine flooding intervals, followed by relative stillstand or minor sea-level falls, are interpreted to reflect high thermal subsidence rates coupled with a Liassic eustatic transgression. [ABSTRACT FROM AUTHOR]