Sequence Stratigraphic Architecture Research Articles

Chapter 23 A brief synthesis of Indian Precambrian basins: classification and genesis of basin-fills

Abstract The geology, inferred evolution and classification according to widely accepted schemes of 22 basins from the Indian Precambrian record on the Arravali–Bundelkhand, Singhbhum, Bastar and Dharwar cratons are discussed in this volume. Although their classification is biased owing to all depositories having continental lithospheric substrates, most of the basins reflect divergent plate motion and thus lithospheric stretching and cooling. Convergent plate motion and concomitant lithospheric flexure owing to loading are postulated for the Kurnool Basin and the Eastern Dharwar Craton supracrustals. Transcurrent plate motion is interpreted for the Bhima and Kaladgi–Badami basins. The Cuddapah Basin suggests a complex polyhistory influenced by both mantle circulation/dynamic topography and loading-related flexure of the lithosphere. Mantle circulation and dynamic topography may have played a role in the evolution of the Dhalbhum and Dalma–Chandil basins. Examination of possible time trends indicates that pre- c. 2.0 Ga basins were mostly continental rifts, followed by some intracratonic basins and lesser rift-sag and back-arc basins; post- c. 2.0 Ga basins exhibit a much larger range of basin types. While this Memoir offers a broad sample of the application of plate-tectonic principles to the Precambrian basins of the large Indian shield, it also underscores that Phanerozoic-style plate tectonics and basin evolution histories are widely identified within the Precambrian sedimentary rock record. Analogously, the case studies in this book support the essential similarity between the features observed within the Precambrian basins of India and the norms that describe Phanerozoic successions in terms of sequence stratigraphic architecture. The Indian Precambrian basin-fill record shows that all types of sequence, systems tract and sequence stratigraphic surface that are known from the Phanerozoic record also occur within Precambrian successions. Differences between the stratigraphic architecture of Precambrian and Phanerozoic basin-fill successions can be ascribed to variable rates and intensities of the controls on accommodation and sediment supply, the changes inherent in the evolution of the hydrosphere–atmosphere system and related physical processes, and the evolution of the biosphere system and associated biogenic processes.

THE OCCURRENCE OF VERTEBRATE AND INVERTEBRATE FOSSILS IN A SEQUENCE STRATIGRAPHIC CONTEXT: THE JURASSIC SUNDANCE FORMATION, BIGHORN BASIN, WYOMING, U.S.A

Previous studies of the sequence stratigraphic distribution of fossils have focused on the record of relatively abundant marine invertebrates. Only a handful of studies have examined how sequence stratigraphic architecture influences the occurrence of vertebrates, particularly large and rare tetrapods. The Jurassic Sundance Formation of the Bighorn Basin, Wyoming, USA, contains a rich suite of invertebrate and vertebrate fossils, including large and rare marine reptiles, and this allows the sequence stratigraphic controls on the distribution of these groups to be compared. The Sundance Formation consists of four depositional sequences, with the lower two being carbonate dominated and the upper two siliciclastic dominated. Two incised valley fills are also present. The presence of multiple depositional sequences and strongly erosional sequence boundaries is the likely cause of the complicated lithostratigraphic nomenclature of the Sundance. Invertebrates (mollusks and echinoderms) in the Sundance conform to well-established patterns of occurrences, including strong facies control and fossil concentrations at maximum flooding surfaces, in the upper portion of parasequences, and within lags overlying sequence boundaries. As expected from their rarity, marine reptiles (ichthyosaurs, plesiosaurs, and pliosaurs) show a weaker connection to sequence stratigraphic architecture. Nonetheless, they do display facies control and are found primarily in offshore mudstone, rather than shoreface and estuarine sandstone. They are also more common at hiatal surfaces, including a zone of concretions at the maximum flooding surface and in lag deposits overlying sequence boundaries. These associations suggest that sequence stratigraphic architecture may be a useful approach for discovery of marine vertebrates and that sequence stratigraphic context should be considered when making paleobiological interpretations of marine vertebrates as well as invertebrates.

Microfacies models and sequence stratigraphic architecture of the Oligocene–Miocene Qom Formation, south of Qom City, Iran

The Oligocene–Miocene Qom Formation has different depositional models in the Central Iran, Sanandaj–Sirjan and Urumieh-Dokhtar magmatic arc provinces in Iran. The Kahak section of the Qom Formation in the Urumieh-Dokhtar magmatic arc has been studied, in order to determinate its microfacies, depositional model and sequence stratigraphy. The textural analysis and faunal assemblages reveal ten microfacies. These microfacies are indicative of five depositional settings of open marine, patch reef, lagoon, tidal flat and beach of the inner and middle ramp. On the basis of the vertical succession architecture of depositional system tracts, four third-order sequences have been recognized in the Oligocene–Miocene Kahak succession of Qom Formation. Based on the correlation charts, the transgression of the Qom Sea started from the southeast and continued gradually towards the north. This resulted in widespread northward development of the lagoon paleoenvironment in the Aquitanian-Burdigalian stages. Also, the sequence stratigraphic model of the Oligocene–Miocene Qom Formation has an architecture similar to those that have developed from Oligocene–Miocene global sea level changes.

Microbial mat records in siliciclastic rocks: Examples from Four Indian Proterozoic basins and their modern equivalents in Gulf of Cambay

Microbial mat-related structures (MRS) in siliciclastics have been investigated from four Proterozic formations in India, namely the Marwar Supergroup, the Vindhyan Supergroup, the Chhatisgarh Supergroup and the Khariar Group for their spectral variations, genetic aspects, palaeo–environmental significance and influence on sequence stratigraphic architecture. The maximum diversification of MRS has been experienced in shallow marine coastal Precambrian successions. Observations made from modern environment as well as Precambrian rock records clearly indicates that the features like petee ridges, sand-cracks, gas domes, multi-directed ripples, reticulate surfaces, sieve-like surfaces and setulf are most likely to form in the shallowest part of the marine basins, in upper intertidal to supratidal conditions while wrinkle structures, roll-up structures and patchy ripples had a broader range of palaeogeographic settings from the supratidal to subtidal conditions. Discoidal microbial colony (DMC) represents a special variety of the mat-layer feature in modern environment that may have diverse internal architecture, sometimes falsely resembles Ediacaran medusoids. The uniqueness in sequence stratigraphic architecture of the microbial mat-covered sediment is reflected by the presence of more amalgamated HSTs compare to that of TSTs. The preservation of forced and normal regressive deposits on low-gradient epeiric shelf under low continental freeboard indicates microbial mat-infested sea-floor impedes erosion and concomitant sediment supply may facilitate formation and preservation of regressive packages.

Long-term ecosystem stability in an Early Miocene estuary

The question of ecosystem stability is central to ecology and paleoecology and is of particular importance for estuaries, which are environmentally highly variable, considered as geologically short lived, and among the most degraded modern ecosystems of our planet. Understanding their ecological dynamics over geological time scales requires paleontological data in a sequence stratigraphic framework, which allows evaluation of paleocommunity dynamics in an environmental context. A 445-m-thick estuarine succession in a satellite basin of the Vienna Basin (Austria) shows continuous sedimentation over 700 k.y. and can be divided into two transgressive systems tracts and a highstand systems tract. In contrast to expectations, no major physical disturbances of the ecosystem involving abrupt changes in diversity and biofacies composition occurred at fl surfaces and at the sequence boundary. Accommodation space remained remarkably constant over the depositional history of the basin, and all changes between depositional environments were therefore more or less gradational. Biotic change along the studied succession can be described as a gradual faunal replacement in response to habitat tracking, a process also reported for some normal marine shelf environments. Benthic assemblages in the estuarine succession were strongly dominated by a few taxa and developed along two indirect gradients, water depth and hydrodynamic energy. These gradients show subtle long-term trends, corresponding to the sequence stratigraphic architecture. Tectonics affected the sequence architecture in this particular marginal marine setting: it controlled accommodation space and sedimentary input, and provided stable boundary conditions over hundreds of thousands of years. Our study demonstrates for the fi rst time that estuaries, which are under great environmental pressure today, are resilient to natural environmental perturbations and can persist over geological time scales.

Snapshot of a lower Pliocene Dendropoma reef from Sant Onofre (Baix Ebre Basin, Tarragona, NE Spain)

We study a vermetid–coralline algal buildup from the lower Pliocene deposits of the Baix Ebre Basin (Sant Onofre, NE Spain). The bioconstruction framework is made up by the intergrowth of Dendropoma and encrusting coralline algae, mostly Spongites fruticulosus with rare Neogoniolithon brassica-florida. Thus, it can be considered a fossil analog of the present-day Mediterranean Dendropoma reefs. The bioconstruction developed on top of a flat palaeotopographic high formed by pre-Pliocene substrate in a sheltered embayment. Submarine cliff sediments, consisting of big blocks and boulders, were deposited along the margin of the palaeohigh. Crusts of coralline algae, intergrowing with serpulids and vermetids, extend from the flat top downwards along the talus. Gray-bluish marl, yellowish silt and fine-grained sand, were deposited in the surrounding low areas. A rich and diverse dweller assemblage occurs in the buildup: bivalves (pectinids, venerids, Lithophaga), gastropods (muricids, nassarids, olivids, cancellarids), ahermatypic coral Cladocora ?, barnacles of the family Pyrgomatidae, Balanus trigonus and regular echinoids. In the talus deposits, clusters of Neopycnodonte cochlear, Hinnites ercolanianus and Balanus sp. dominate the faunal assemblage. Further, the blocks of the talus are densely bored, showing Gastrochaenolites, Entobia, Maeandropolydora and Caulostrepsis. Faunal and boring assemblages indicate that the buildup settled and developed in very shallow conditions (most likely less than 10m water depth). The whole fossil assemblage, framework and dweller community, are preserved in situ, maintaining their original growth positions. Development in a protected setting, together with rapid burial due to progradation of siliciclastics on top of the palaeotopographic high, accounts for this exceptional preservation in such a shallow environment.The vermetid bioconstruction of Sant Onofre represents the only carbonate deposits found in the lower Pliocene basins of NE Spain. Sequence stratigraphic architecture indicates that these carbonates represent the maximum flooding deposits. Thus, maximum carbonate production took place due to sediment starvation in the palaeohigh while terrigenous deposits were formed in the surrounded low areas.

Analysis of High-Frequency Sequence Stratigraphy Architecture of Putaohua Oil Layer in Sanzhao Sag of Songliao Basin

Using high-resolution sequence stratigraphy theory and its technology, based on 22 well cores and 4 256 logging data, interface types and characteristics of different grades the base-level cycle in Putaohua oil layer of Sanzhao Sag was systematically analyzed, and Putaohua oil layer is divided into 1 medium-term base level cycle, 4 short-term base level cycles, 8 super-short-term base level cycles, established high-resolution sequence strata framework, and proposed sequence stratigraphy model of middle super-short-term base level cycles meridionally grado-thin and up to vanish with syndepositional structure lifting control in the south of Putaohua oil layer. Sandstone is offlap Pinch-out in the upper of Putaohua oil layer, and Sandstone is onflap Pinch-out in the under of Putaohua oil layer from north to south.

Sequence stratigraphy of the Upper Cambrian (Furongian; Jiangshanian and Sunwaptan) Tunnel City Group, Upper Mississippi Valley: Transgressing assumptions of cratonic flooding

New data from detailed measured sections permit comprehensive analysis of the sequence framework of the Furongian (Upper Cambrian; Jiangshanian and Sunwaptan stages) Tunnel City Group (Lone Rock Formation and Mazomanie Formation) of Wisconsin and Minnesota. The sequence-stratigraphic architecture of the lower part of the Sunwaptan Stage at the base of the Tunnel City Group, at the contact between the Wonewoc Formation and Lone Rock Formation, records the first part of complex polyphase flooding (Sauk III) of the Laurentian craton, at a scale smaller than most events recorded by global sea-level curves. Flat-pebble conglomerate and glauconite document transgressive ravinement and development of a condensed section when creation of accommodation exceeded its consumption by sedimentation. Thinly-bedded, fossiliferous sandstone represents the most distal setting during earliest highstand. Subsequent deposition of sandstone characterized by hummocky or trough cross-stratification records progradational pulses of shallower, storm- and wave-dominated environments across the craton before final flooding of Sauk III commenced with carbonate deposition during the middle part of the Sunwaptan Stage. Comparison of early Sunwaptan flooding of the inner Laurentian craton to published interpretations from other parts of North America suggests that Sauk III was not a single, long-term accommodation event as previously proposed.

Sequence-stratigraphic architectures and sand-body distribution in Cenozoic rifted lacustrine basins, east China

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.

A quantitative simulation study of asymmetrical tectonic subsidence control on non-synchronous sequence stacking patterns of Eocene lacustrine sediments in Bohai Bay Basin, China

Compared with large-scale passive margin marine basins, continental basins are susceptive to local or regional tectonic, geomorphologic and climatic influences and can form complicated sequence stratigraphic architectures, configurations and sedimentary facies. We report herein a quantitative simulation study on the influences of asymmetrical tectonic subsidence on sequence stratigraphic stacking patterns in rift basins by varying tectonic subsidence, sediment supply, and intra-rift lake levels. It has been demonstrated that isochronous sequences within a rift basin can exhibit both synchronous and non-synchronous stacking patterns as a result of differential asymmetric tectonic subsidence. In sequence stratigraphy, synchronous stacking patterns refer to isochronous sequences at any parts of a basin, which are either all retrogradational or all progradational. We propose the concept of non-synchronous stacking patterns of isochronous sequence stratigraphy for rift basins. Non-synchronous stacking patterns refer to the fact that, within the same basin, some parts may concomitantly exhibit stacking patterns that are exactly opposite to that of other parts. Non-synchronous stacking patterns can form when the accommodation space increment (∆A) is less than that of sedimentary supply (∆S) in one area, but larger than or equal to that of ∆S in another. Moreover, the duration of non-synchronous stacking patterns is controlled by both ∆A and ∆S. Recognition of non-synchronous stacking patterns within isochronous sequence stratigraphy provides a conceptual model for correlation of sequence stratigraphic units in tectonically asymmetric rift basins in eastern China and similar basins around the world.

Sedimentary controls on the sequence stratigraphic architecture in intra-cratonic basins: An example from the Lower Permian Shanxi Formation, Ordos Basin, northern China

Intra-cratonic basins are characterized by stable tectonic regimes, closed depositional systems, finite sizes of receiving basins and extremely low morphological gradients. This paper examines the effect of sediment accumulation on the sequence stratigraphic architecture and quantitatively evaluates its controls on the development of intra-cratonic sedimentary sequences using numerical modelling. A well documented intra-cratonic sedimentary sequence, the Lower Permian Shanxi Formation in the Ordos Basin, northern China, was used to illustrate a sequence stratigraphic model developed for intra-cratonic sedimentary basins. The studied sequence is characterized by a typical backstepping or source-ward retrogradation. A 2-D simulation software (SEDPAK) and a 3-D simulation software (SEDSIM) were used to model the intra-cratonic sequence. The modelling results indicated that sediment accumulation alone can produce the classical retrogradational stratigraphic stacking patterns. The sediment accumulation can (1) increase the retrogradational range of the original retrogradational stacking sequences, (2) change an original aggradational stacking sequence to a retrogradational stacking one, and (3) decrease the progradational range of an original progradational stacking sequence, or (4) even change an original progradational stacking sequence to a retrogradational stacking sequence. Understanding the relationship between the sediment accumulation and the stratigraphic development in an intra-cratonic basin is essential for interpreting the sequence stratigraphic framework and stacking patterns, and for predicting the distribution of potential reservoir sand-bodies within such basins. This work enriches the classic sequence stratigraphic models by providing a new model for intra-continental basins, and offers new insight on hydrocarbon exploration in intra-cratonic basins.

A REVIEW OF THE LATE QUATERNARY SEDIMENTOLOGICAL STUDIES ON THE OUTER SHELF OF THE EAST CHINA SEA

The current study of Late Quaternary stratigraphy,paleo-river channels,channel-fillings and tidal sand ridges on the outer shelf of the East China Sea are summarized in this paper.Key questions,such as the boundaries of MIS 2 and MIS 4,the different schemes of sequence stratigraphic architectures,the existence and evolution of the paleo-Yangtze River during LGM,and the formation of the tidal sand ridges on the continental shelf of the East China Sea are highlighted and discussed.Comments and suggestions are provided in the paper for the study of the Late Quaternary sedimentation on the East China Sea in the future.

Climate‐controlled aggradation and cyclicity of continental loessic siliciclastic sediments in Asselian–Sakmarian cyclothems, Permian, Hugoton embayment, USA

AbstractSiliciclastic intervals in Lower Permian carbonate–siliciclastic cyclothems in western Kansas record climate control on facies progression, deposition and preservation. The 26 000 km2 study area comprises seven marine‐continental (carbonate–siliciclastic) cyclothems caused by glacioeustasy. Core data and a three‐dimensional geological model provide a detailed view of the sub‐surface on a gently sloping ramp. Siliciclastic intervals in the cyclothems are fine‐grained red beds with extensive pedogenic features, indicating a continental origin. Bed geometry (sheet‐like deposits that thin to the east), lateral grading, grain size (very fine‐grained sand to silt) and grain angularity (sub‐angular to angular) suggest that the sediment is loess sourced from the west, probably the Ancestral Rocky Mountains. There is a repeated record of glacial‐cycle‐scale, climate‐controlled cyclicity within siliciclastic intervals that has not been recognized previously. Aeolian silt grain size coarsens upward towards the middle, then fines upward in each siliciclastic interval. When sea‐level was high (interglacial) and carbonate production flourished, aeolian sedimentation nearly ceased, suggesting increased vegetation and rainfall at the source. As sea‐level fell, fine‐grained siliciclastic sediments were deposited under relatively dry, but seasonally wet conditions on an exposed ramp. Laterally graded coarser grained siliciclastic sediments with diagnostic fabrics indicate drier conditions with seasonal rainfall during a continued relative fall in sea‐level. The coarsest siliciclastic sediments were deposited during the lowest sea‐level and driest conditions, but still with sufficient seasonal moisture to allow vegetative cover and bioturbation. Subsequent upward fining is correlated with sedimentological indications of wetter conditions during relative sea‐level rise. Unlike common sequence stratigraphic models that relate siliciclastic sediment accumulation to base‐level rise, continental deposits were preserved because plants and pedogenesis stabilized aeolian sediment. The aggradational landscape formed by this process had several metres of positive relief that reduced accommodation for overlying marine carbonate strata. Thus, this mechanism for continental siliciclastic aggradation has a significant effect on sequence stratigraphic architecture.

On-shelf lower Miocene Oakville sediment-dispersal patterns within a three-dimensional sequence-stratigraphic architectural framework and implications for deep-water reservoirs in the central coastal area of Texas

Through integrating three-dimensional seismic and wireline-log pattern data, we found that the sequence-stratigraphic architecture of the lower Miocene Oakville Formation along the central part of the Texas Gulf Coast reveals four third-order sequences: the lower two containing lowstand (LST), transgressive (TST), and highstand systems tracts (HST); and the upper two appearing to contain only TST and HST. These sequences average 1.2 m.y. in duration. The lowstand incised valley fill sandstones are massive, as thick as 420 ft (73 m) on wireline logs. Seismic stratal slices demonstrate that the incised valleys are dip oriented and more than 20 mi (32 km) wide. Strata (reflections) within the incised valleys onlap the sides of the valley. The third-order TST comprise backstepping high-frequency sequences. Seismic stratal slices and wireline-log patterns within the TST suggest deltaic sediment dispersal patterns that appear reworked. Third-order HST comprise aggradational to progradational high-frequency sequences, and seismic stratal slices and wireline-log patterns indicate a variety of depositional environments that we interpreted as deltaic, shoreface, coastal plain, and interdistributary. The HST is significantly truncated by the overlying sequence boundary. Sandstone-rich incised-valley fills suggest that large amounts of sediment may have been transported through these channels and may have supplied lowstand deposits and associated reservoirs seaward of the previous shelf edge.

Along-Strike and Down-Dip Variations in Shallow-Marine Sequence Stratigraphic Architecture: Upper Cretaceous Star Point Sandstone, Wasatch Plateau, Central Utah, U.S.A.

Abstract The sequence stratigraphic architectures of shallow-marine deposits in the upper Cretaceous Star Point Sandstone are analyzed over a large (c. 100 km), nearly continuous outcrop section aligned oblique to depositional strike. The unit consists of five parasequences that predominantly comprise wave-dominated shoreface–shelf deposits. Two parasequences contain riverdominated delta-front deposits locally. Within each parasequence, wave-dominated shoreface–shelf deposits record 7–45 km of ESE- to ENE-directed progradation of a linear to moderately lobate shoreline that was supplied with sediment by longshore drift and subjected to strong offshore sediment transport by storms. Wave-dominated shoreface sandstones in each parasequence thin and wedge out over short distances ( 800 km2) river-dominated delta-front complex (Panther Tongue). This highly focused fluvial sediment flux probably occurred via a structurally controlled sediment entry point between two active thrusts.

Facies evolution and sequence chronostratigraphy of a “mid”-Cretaceous shallow-water carbonate succession of the Apulia Carbonate Platform from the northern Murge area (Apulia, southern Italy)

The “mid”-Cretaceous carbonate succession of the Apulia Carbonate Platform cropping out in northern Murge area (Apulia, southern Italy) is composed of shallow-water carbonate rocks and is over 400 m in thickness. This paper focuses on the lithofacies analysis of this carbonate succession, its paleoenvironmental interpretation, and its sequence-chronostratigraphic architecture. Lithofacies analysis permitted to identify deposits which can be grouped into the following three facies belts: (1) terrestrial facies belt formed by: intraclast-supported paleosoils; solution-collapse breccias; (2) restricted facies belt made up of lithofacies deposited in protected peritidal environments; (3) normal-marine facies belt made up of lithofacies formed in moderate- to high-energy subtidal environments. The detailed study both in outcrops and in thin-sections revealed that, at the bed scale, lithofacies are cyclically arranged and form shallowing-upward small-scale depositional sequences comparable to parasequences and/or simple sequences. The following three small-scale sequence types have been distinguished: (1) subtidal sequences mostly made up of lithofacies formed in the normal-marine open subtidal domain; (2) peritidal sequences made up of lithofacies formed in the restricted peritidal domain; (3) peritidal sequences showing a cap formed by paleosoils. Small-scale sequences are not randomly arranged in the compiled succession but form discrete packages, or sets, that alternate in the sedimentary record. The repetition of such small-scale sequence packages in the succession has been the key to recognize large-scale sequences comparable to third-order depositional sequences. Although sedimentological data are often fragmentary due to late dolomitization, four large-scale sequences have been distinguished. The data support a generalized landward-backstepping of facies belts during transgression, which implies a gradual gain of accommodation culminating with the deposition of a package of small-scale sequences formed by normal-marine subtidal deposits. These mark periods of maximum accommodation space and form the maximum-flooding zones of large-scale sequences. A gradual seaward progradation of facies belts is recorded during highstand conditions, which implies a gradual loss of accommodation culminating with the deposition of a package of peritidal small-scale sequences capped by paleosoils or by solution-collapse breccias. The occurrence of terrestrial deposits marks periods of minimum accommodation on the platform and determines the sequence boundary of large-scale sequences. The large-scale sequences identified in this study fit with the main transgressive/regressive cycles published in the sequence-chronostratigraphic chart of European basins. As a consequence, it is interpreted that changes of the sea level recorded at the scale of European basins played an important role in determining the sequence-stratigraphic architecture of the studied succession. In spite of this, the occurrence of solution-collapse breccias, which implies a significant gap in carbonate sedimentation in between Early and Middle Cenomanian times, may also have an alternative interpretation. In particular, this deposit may represent the local fingerprint of the well-known tectonic phase which, during Late Albian-Early/Middle Cenomanian times, determined the subaerial exposure of large parts of Periadriatic carbonate platforms producing a marked regional unconformity.

Sequence-stratigraphic interpretation of structurally controlled deposition: Middle Miocene Kareem Formation, southwestern Gulf of Suez, Egypt

ABSTRACT The structural evolution and sequence-stratigraphic architecture of the syn-rift Middle Miocene (Langhian) Kareem Formation were studied in the Western Sub-Basin (WSB) of the Gulf of Suez, Egypt. Biostratigraphic data, facies interpretations, and log data from thirteen wells were used to construct six tectono-sedimentary scenarios and lithofacies-distribution maps, which illustrate the paleogeography during six systems tracts. Abrupt thickness and facies changes reflect deep basins and adjacent high-relief areas created by differential fault-block movements. Within the WSB seven individual fault blocks record complex, tectonically controlled deposition of the Kareem Formation, which we subdivide into two third-order sequences. Earlier tectonic interpretations, facies analysis, and stratigraphic correlations from individual fault blocks were used to interpret the varying depositional settings during six systems tracts and, as a consequence, to discern the structural evolution of the WSB during the Middle Miocene.

New Insights on the Sequence Stratigraphic Architecture of the Dakota Formation in Kansas–Nebraska–Iowa from a Decade of Sponsored Research Activity

The Cretaceous Dakota Formation in the areas of Kansas, Nebraska, and Iowa contains a rich and well-preserved microflora of fossil palynomorphs. A comprehensive listing of these taxa is presented in this publication as part of a continuing effort to develop a refined biostratigraphic scheme for mid-Cretaceous terrestrial deposits in North America. The Dakota Formation in this region contains four distinctive Albian-Cenomanian palynostratigraphic zones that are used to partition the unit into successive depositional cycles, and each zone records deposition in fluvial-estuarine environments. The late Albian Kiowa-Skull Creek depositional cycle at the base of the Dakota Formation is recognized throughout the study area, and is also recognized in other parts of the Cretaceous North American Western Interior basin. The overlying newly recognized latest Albian "Muddy-Mowry Cycle" is formally defined for the first time in this paper and correlates with depositional cycles recognized by other workers in other parts of the Western Interior basin. The Cenomanian lower Greenhorn Cycle is already widely recognized by many other workers throughout the Western Interior basin. Laterally extensive thin zones of pervasive carbonate mineral cementation are noted in fluvial-estuarine deposits in the Dakota Formation. They are believed to have formed as synsedimentary cements that precipitated below estuarine marine-flooding surfaces in settings related to discharging paleoground waters. The existence of these early diagenetic cementation zones has important implications for the recognition of diagenetic barriers and baffles to modern fluid flow in the Dakota Formation. New stable isotopic data on these authigenic cements are reported in this paper and add to a body of published data on the δ18O of mid-Cretaceous paleoprecipitation in North America.

Plio-Pleistocene sequence stratigraphic architecture of the eastern Niger Delta: A record of eustasy and aridification of Africa

This study synthesizes the stratigraphic behavior of the whole eastern Niger Delta during the Plio-Pleistocene and discusses controls on deposition by eustasy, subsidence and sediment supply at various scales. The sequence stratigraphic architecture is determined by integrating the whole sedimentary system from the coastal plain down to the abyssal plain. We combined structural geology, sedimentology biostratigraphy, and seismic stratigraphy. Data included 2D regional sections connected by 3D seismic surveys and 45 well-logs for lithologic calibration and well-cuttings for biostratigraphic calibration. We identified three sequence orders in the stratigraphic architecture of the eastern Niger Delta and calibrated their biostratigraphic ages into absolute ages using two end-member models to estimate uncertainties. We discuss their causes and propose that the major influence was climate-driven eustasy for both the (i) short-duration (around 0.1–0.4 Myr in duration) and (ii) long-duration progradation/retrogradation sequences (around x1 Myr in duration). Two different modes of climatic control are suggested for the (iii) long-term stratigraphic trend over the Plio-Pleistocene: global climate-driven eustatic variations for the initial progradation until 2.5 Myr ago and a regional climate change (e.g. the warming of the East Atlantic and the aridification of western Africa) for the following aggradation/retrogradation period, controlled by a decrease in the sedimentary supply to the eastern delta counteracting the ongoing eustatic fall.

SEQUENCE STRATIGRAPHIC CONTROL ON PRESERVATION OF LATE EOCENE WHALES AND OTHER VERTEBRATES AT WADI AL-HITAN, EGYPT

Biological and physical factors govern the distribution of fossils, but it is not always clear which is more important. The preservation of late Eocene vertebrates at the UNESCO World Heritage site of Wadi Al-Hitan, Western Desert of Egypt, is controlled primarily by the physical processes responsible for sequence stratigraphic architecture on a siliciclastic shelf. Three types of stratigraphic surface, each characterized by a taxonomically and taphonomically distinct fossil assemblage, yield most of the known vertebrate fossils. Complete, partially articulated whale skeletons, primarily Basilosaurus isis, are abundant in offshore marine flooding surfaces (MFS) in the late transgressive systems tract (TST) of the first Priabonian sequence (TA4.1), where low net sedimentation rates and environmental averaging in offshore environments promoted the accumulation of carcasses on traceable stratigraphic surfaces. Complete, well-articulated whales, primarily Dorudon atrox, are more widely scattered on minor erosion surfaces in rapidly accumulating shoreface sediments of the overlying falling stage systems tract. Fragmented and abraded vertebrate remains are abundant and diverse in a discontinuous conglomerate that marks the first sequence boundary above the base of the Priabonian (Pr-2), which has not been previously recognized in Egypt, but which formed incised valleys with at least 45 m of total relief. Fossils in this variably thick lag conglomerate include skeletal elements reworked by rivers from underlying marine deposits and bones of terrestrial animals living in the fluvial environment. Marginal marine vertebrates, primarily dugongs, occur on shelly marine ravinement surfaces above Pr-2, in the early TST of the second Priabonian sequence. Most vertebrate remains in Wadi Al-Hitan occur in condensed stratigraphic intervals and taxonomic composition changes with sequence position, both important considerations in interpretation of paleobiological patterns.