Sequence Stratigraphic Context Research Articles

Depositional and diagenetic heterogeneities in sequence stratigraphic framework of a mixed carbonate-siliciclastic reservoir: A case study from Oligocene–Miocene Asmari Formation in the Persian Gulf

Mixed siliciclastic–carbonate reservoirs, such as the Asmari Formation in the Zagros Basin, present significant heterogeneities that influence hydrocarbon exploration and production. This study examines the depositional and diagenetic heterogeneities of the Asmari Formation in the northwest Persian Gulf through a multi-methodological approach, including petrographic analysis, petrophysical measurements, core CT-scanning, X-ray diffraction, and sieve analysis. The primary goal is to identify the origins, scales, and predictability of these heterogeneities within a sequence stratigraphic framework to enhance reservoir characterization. Facies analysis reveals depositional environments ranging from sabkha to shoreface, peritidal, lagoonal, and shoal settings, with carbonate sedimentation interrupted by clastic influxes associated with sea-level fluctuations. Diagenetic processes, including dolomitization, dissolution, cementation, and fracturing, were identified through petrographic analysis, highlighting their effects on reservoir quality. To quantify these heterogeneities and delineate reservoir flow units, methods such as Winland classification, hydraulic flow unit (HFU) analysis, and Lorenz zonation were applied. Results show that depositional heterogeneities, predominantly driven by sea-level changes, range from small to large scales and can be predicted within a sequence stratigraphic context. Diagenetic modifications substantially affect porosity and permeability, with dolomitization improving reservoir potential, while compaction and cementation tend to reduce it. Fracturing, particularly in the lower Asmari carbonates, enhances permeability. The study concludes that accurately recognizing and modeling reservoir heterogeneities in three dimensions can significantly improve exploration and development strategies in the Persian Gulf. A better understanding of the spatial distribution of high-quality reservoir zones and impermeable baffle layers will facilitate more precise well placement, optimize hydrocarbon recovery, and mitigate exploration risks.

Coupled δ56Fe and δ18O analysis of the Frere iron formation, Western Australia, and Paleoproterozoic ocean circulation

The Paleoproterozoic (ca.1.89 Ga) Frere Formation is a ca. 300-m-thick sedimentary succession of five stacked, Fe- and silicate-rich shallowing upwards cycles or parasequences that were deposited during marine transgression. Deposition in an oxygen-stratified water column is supported by the occurrence of magnetite-rich mudstones (anoxic and distal) beneath shallower, hematite- and chert-rich grainstones (suboxic and proximal). Minor (LREE/HREE)PAAS depletion and positive Ce anomalies in grainstones support this interpretation. Sequence stratigraphic relationships suggest that the Frere Formation accumulated on an unrimmed continental shelf associated with coastal upwelling of Fe.Large variations in oxygen isotope ratios (δ18O) of magnetite (–0.6 ‰ ≤ δ18O ≤ +2.8 ‰, n = 12) and quartz (+8.8 ‰ ≤ δ18O ≤ +20.1 ‰, n = 11) from lithofacies composing the Frere Formation indicate that pristine δ18O of minerals were overprinted by diagenesis. Highly variable δ18O values calculated at 200 °C in equilibrium with quartz (–3.0 to + 8.3 ‰) and magnetite (+8.0 to + 12.7 ‰) suggest that fluids with heterogeneous δ18O evolved through oxygen isotope exchange between minerals and porewater in accumulating sediment during diagenesis.The Fe isotope ratios (δ56Fe) of bulk rock (–0.50 ‰ ≤ δ56Fe ≤ +1.55 ‰, n = 20) and magnetite separates (–0.09 ‰ ≤ δ56Fe ≤ +2.06 ‰, n = 11) from deeper anoxic and shallow suboxic paleoenvironments define a decreasing trend through stratigraphically stacked parasequences. The high δ56Fe values of magnetite (+0.7 ‰ to + 2.06 ‰) characterizing the base of the Frere Formation is interpreted to reflect equilibrium isotopic fractionation between dissolved Fe2+aq and freshly precipitated Fe-(oxyhydr)oxide (Fe(OH)3) resulting from sluggish recharge of Fe2+aq from deeper shelf environments. These high δ56Fe values of magnetite from shallow paleoenvironments may also reflect microbial dissimilatory Fe reduction at the seafloor. As sea level rose, continued deepening and encroachment of a well-mixed water mass increased the oxidation of Fe2+aq, which ultimately increased the influence of kinetic isotopic fractionation between Fe2+aq and Fe(OH)3. Such a process produced δ56Fe compositions of magnetite that gradually decrease through the stratigraphic thickness of the Frere Formation (+2‰ to −0.1 ‰). Thus, Fe isotopic data framed in a sequence stratigraphic context provides greater insight into water column processes on Fe-rich, oxygen stratified Paleoproterozoic and Mesoproterozoic shelves, as well as geologically younger restricted basins with a redox gradient.

Variations in extent, distribution and impact of dolomitization on reservoir quality of Upper Cretaceous foreland-basin carbonates, Abu Dhabi, United Arab Emirates

Petrography, petrophysics, and geochemistry of an Upper Cretaceous, foreland-basin carbonate reservoir, Abu Dhabi, United Arab Emirates, are used to constrain the spatio-temporal variations in the extent of dolomitization and its impact on reservoir quality. Dolomitization in highstand systems tracts (HST) is attributed to repeated tidal and evaporative pumping as well as seepage reflux of penesaline brines during restriction of the platform due to 4th and 5th order cycles of relative sea-level fall and particularly below parasequence boundaries. This interpretation is supported by the presence of rare poikilotopic gypsum cement and scattered laths and micro-nodules of calcitized gypsum in the dolomitized peritidal dolostones. Dolomitization along bioturbation sites, which is most common in the transgressive systems tracts (TST), is attributed to the development of suitable localized geochemical conditions (e.g., microbial sulfate reduction and related increase in carbonate alkalinity). Porosity and permeability of the dolostones are strongly controlled by depositional textures of precursor limestones and by subsequent diagenetic evolution. Dolomitization of mudstones, wackestones and matrix-rich packstones has resulted in the formation of micropore-dominated microcrystalline dolostones whereas dolomitization of the shoal grainstones resulted in the formation of coarse-crystalline dolostones with abundant well-connected intercrystalline and moldic macropores. Mesogenetic alterations of the dolostones, which are attributed to the flow of hot basinal brines along steep faults, include dolomite cementation and, subsequently, dissolution and calcitization of dolomite (dedolomitization) and calcite cementation. The lack of systematic differences in porosity and permeability of dolostones between the oil and water-saturated dolostones suggest that most diagenesis occurred prior to completion of oil emplacement and/or reflect the shallow maximum burial depths of the formation (around 1.3 km). This study demonstrates that variations in the distribution and extent of dolomitization within a sequence stratigraphic context across an oilfield should be considered as primary control on the spatio-temporal reservoir lithology and heterogeneity in carbonate successions.

The influence of taphonomy and time on the paleobotanical record of the Permian–Triassic transition of the Karoo basin (and elsewhere)

Terrestrial fossil assemblages preserved in the upper Permian–Lower Triassic strata of the Karoo Basin, South Africa, have played a central role in the interpretation of ecosystem patterns and end-Permian extinction models. However, these models need to be carefully reconsidered because of the limitations of the rock record. Four lessons learned from a multidisciplinary approach to the rocks, lithology, stratigraphy, and dating are relevant to other paleofloras in large continental basins. In reality, the Karoo paleofloral record is very sparse. Hence, reports of a near continuous fossil record in this basin should be considered as the near continuous record of erosion and time lost with sporadic plant-fossil assemblages.A review of the debate over the rate and timing of the Permian–Triassic in the Karoo Basin reinforces the need for extensive stratigraphic mapping, the analysis of depositional environments of the plants, as well as the application of a variety of dating methods. First, Late Permian to Early Triassic paleobotanical assemblages are extremely rare in the basin with only a handful of sites in the Free State and Eastern Cape Provinces. These fossil data originate from >3750 m of total measured section wherein megafloral remains are preserved in <1% of the available rock record (0.9% all megafloral elements; permineralized wood = 0.1%, adpressions = 0.8%), with spore-and-pollen assemblages only slightly more frequently encountered at 1.3%. This low occurrence is comparable with other basins. Thus, any continental fossil assemblage represents a very short temporal window into the paleobiosphere because of taphonomic effects of the soils, pedogenesis, and controls on depositional environments. Second, geochronometric and rock magnetic data, developed in a sequence stratigraphic context, are critical to constrain time and biological trends in continental successions. The missing time, diastems and hiatuses, are critically important. Third, the spatial relationships of plant-fossil assemblages are not easily correlated across the basin without an extensive dataset of the paleolandscape. In general, the Late Permian Beaufort rocks represent channels, floodplains, and braided streams rather than lakes and oxbows that are conducive to the preservation of plant parts. Finally, the temporal distribution of paleobotanical assemblages is complicated by the missing time (sediments) that has resulted in the apparent scarcity of vegetation before and after the end Permian extinction. The reported uncharacteristic diversity and abundance of plants in the Carnian–Norian Molteno Formation is most likely due to an environment conducive to preserving fossil-plant assemblages combined with a record of intensive collecting. Overall, the large inland Karoo Basin, without any marine influence or extensive volcanic deposits, has favored the preservation of vertebrate assemblages.

Chapter 11. North Sea Basin Jurassic lithostratigraphy

Abstract The most important North Sea Jurassic–lowermost Cretaceous lithostratigraphic units, as developed in the UK, Norway and Danish sectors, are summarized in this chapter (55 units from the UK, 25 from Norway and 10 from Denmark). Some significant issues remain with the use and application of lithostratigraphic terminology in the Jurassic of the North Sea Basin. In particular, there are inconsistencies in unit definition and nomenclature changes across country sector boundaries that obscure the recognition of regional stratigraphic patterns that exist across the region. To aid clarity and to overcome some issues of definition, some revisions are made to the existing lithostratigraphic schemes. Several informal lithostratigraphic units are described, a number of unit definitions are revised and various formerly informal units are formalized (Buzzard Sandstone Member, Ettrick Sandstone Member and Galley Sandstone Member). It is recommended that use of the Heno Formation in offshore Denmark is discontinued. In addition, four new lithostratigraphic member terms are introduced (Home Sandstone Member, North Ettrick Sandstone Member, Gyda Sandstone Member and Tambar Sandstone Member). All described units are placed into a sequence stratigraphic context. All significant lithostratigraphic boundaries conform with key sequence stratigraphic surfaces.

Sequence stratigraphy of the Middle Jurassic Yan’an Formation (NE Ordos Basin, China), relationship with climate conditions and basin evolution, and coal maceral’s characteristics

The Yan’an Formation of the Ordos Basin in North China is among the largest and most extensively studied Jurassic coal reservoirs in the world. The lacustrine Yan’an Formation was investigated near Dongsheng (Ordos Basin, China) in its sequence-stratigraphic context, to understand the factors that controlled the peat accumulation and the cyclicity in the coal-bearing strata. Nine facies, grouped into two facies associations, jointly composed two third-order sequences. These represent lowstand system tracts, extended (lacustrine) system tracts and highstand system tracts. The sequence stratigraphic framework could be established on the basis of correlations of cores and logging from several wells. It appears that the coal development was controlled partly by fluctuations of the lake level. The sequence development controlled the vertical distribution of the coal seams, which mainly developed during extended lacustrine system tracts (= during transgressive stages of the lake) and the early stage of highstand system tracts. Sequence stratigraphy and maceral analysis indicate that basin subsidence and climate were the main controlling factors for the development of the coal seams and the coal-bearing succession. A generic model was established to show the lateral and vertical distribution of coal seams in the large, subsiding lacustrine coal basin with no significant folding or faulting. Special attention is paid to the changing climate conditions. The findings are considered to help deepen the theory of coal formation; they will also help increase the efficacy of coal exploitation in basins such as the Ordos Basin.

Characterization and evolution of a carbonate basin floor fan on a leeward side of an isolated carbonate platform, Sirte Basin, Libya

The recognition of carbonate basin floor fans and the interpretation of their formation and evolution are extremely crucial to the hydrocarbon exploration since they form important oil and gas reservoirs. There has been little research on the geometries and controlling factors of the carbonate basin floor fans from subsurface three-dimensional (3D) seismic and wireline log data. Most studies have been conducted on siliciclastic basin floor fans. Thus, this study characterizes the internal configuration and processes controlling the deposition of a lower Eocene carbonate basin floor fan system in the Sirte Basin in a sequence stratigraphic context through integrated 3D seismic and wireline log interpretations. The carbonate fan complex shows hummocky to undulated seismic geometries on vertical seismic sections and bounded by an unconformable sequence boundary at the base and a downlap surface at the top. It has an aerial extension of around 9 km from south to north along depositional dip and at least 8 km from east to west along strike. In well data, the fan system predominantly comprises limestone units intercalated with thin shale intervals. Log curves display an overall funnel-shaped trend, suggesting progradational stacking pattern. The vertical thickness of the fan complex varies from 120 to 165 m. The evolution of the carbonate fan was influenced by the interplay of accommodation space and sediment supply. A pre-existing topography, tectonic subsidence, and sea-level rise generated accommodation space for the deposition of the fan system, whereas sediment-gravity flows transported the fan sediments from the platform interior and margin to the basin floor. This study provides sedimentological and stratigraphic criteria to recognize and interpret the evolution of basin floor fans along isolated carbonate platforms from 3D seismic and wireline log data and may serve as an analog for ongoing and future studies on carbonate basin floor fans elsewhere.

Paleoenvironment reconstruction and peat-forming conditions of Neogene paralic coal sequences from Mukah, Sarawak, Malaysia

Eight coal seams containing Neogene paralic coals from the Mukah coalfield, Sarawak, Malaysia, were investigated using petrographical, palynological, and organic geochemical analyses to describe coal-forming vegetation, paleoclimatic, and paleoenvironment conditions during peat development and precursor mires, as well as their associations within a sequence-stratigraphic context. The petrographic and geochemical data of the coals imply the existence of oxygen-deficient and water-saturated conditions in the precursor mires. The reducing conditions in the mires were followed by biomass loss. The Mukah coals are suggested to be deposited in freshwater peat swamps, and the rich preservation of angiosperm pollen indicates that the organic matter in dense and lowland forest vegetation was mostly terrigenous. The overwhelming presence of Casuarina and Calamus types suggest the paleomires were closely linked to the Kerapah/Kerangas peat forest and were marginally bordered by rattan, which was supported by biomarker data. Rheotrophic–ombrotrophic mires temporarily formed due to water table fluctuations, which were strongly dependent on ever-wet climate changes and syn-depositional tectonics during the Neogene, and resulted in the balanced to high peat accumulation and preservation. A maximum thickness of 35 m of peat deposits is suggested to form between 10,000 and 175,000 years ago based on the peat:coal ratio. The coals are proposed to be influenced by transgressive to initial highstand cycles within the paralic setting.

Scale dependence of drilling predation in the Holocene of the northern Adriatic Sea across benthic habitats and nutrient regimes

AbstractPredation has strongly shaped past and modern marine ecosystems, but the scale dependency of patterns in drilling predation, the most widely used proxy for predator–prey interactions in the fossil record, is a matter of debate. To assess the effects of spatial and taxonomic scale on temporal trends in the drilling frequencies (DFs), we analyzed Holocene molluscan assemblages of different benthic habitats and nutrient regimes from the northern Adriatic shelf in a sequence-stratigraphic context. Although it has been postulated that low predation pressures facilitated the development of high-biomass epifaunal communities in the eastern, relatively oligotrophic portion of the northern Adriatic shelf, DFs reaching up to 30%–40% in the studied assemblage show that drilling predation levels are comparable to those typical of late Cenozoic ecosystems. DFs tend to increase from the transgressive systems tract (TST) into the highstand systems tract (HST) at the local scale, reflecting an increase in water depth by 20–40 m and a shift from infralittoral to circalittoral habitats over the past 10,000 years. As transgressive deposits are thicker at shallower locations and highstand deposits are thicker at deeper locations, a regional increase in DFs from TST to HST is evident only when these differences are accounted for. The increase in DF toward the HST can be recognized at the level of total assemblages, classes, and few abundant and widespread families, but it disappears at the level of genera and species because of their specific environmental requirements, leading to uneven or patchy distribution in space and time.

Late Maastrichtian-early Danian sea-level changes at Gebel Tawil Shihaq in Central Jordan: Implications of chemostratigraphy and foraminiferal biostratigraphy

Integrated foraminiferal and geochemical analyses of the upper Maastrichtian-lower Danian succession of the Gebel Tawil Shihaq section in the Central Jordan Plateau has been conducted to interpret the nature of that succession within a sequence stratigraphic context. The studied succession covers the interval of the CF4-P1c planktonic foraminiferal zones. The incorporation of foraminiferal and geochemical proxies led to the recognition of four 3rd order depositional sequences, three in the Maastrichtian (M-Sq1, M-Sq2, and M-Sq3) and one in the Danian (D-Sq1). These depositional sequences are bounded by four sequence boundaries, detected based on abrupt faunal (e.g., zonal interruption, foraminiferal turnover) and geochemical (e.g., dolomitization, Sr/Ca and nearby Ti/Al maxima, Mn and CaCO3 minima) shifts. Based on the paleodepth variation revealed from the foraminiferal analyses and the consistent behavior of the different geochemical proxies, a relative sea-level curve was established and consequently, the systems tracts of each depositional sequence were recognized. The development of the inferred relative sea-level curve reveals the twofold control of both global sea-level changes and regional tectonics. The latter resulted in amalgamation and elimination or modification of some sea-level cycles relative to the previously recognized eustatic schemes. At the Cretaceous/Paleogene (K/Pg) boundary, a highly agitated phosphatized event bed is intercalated and exposes distinctive unique lithological, geochemical, and biostratigraphical characteristics denoting a long-lasting period of non-deposition and erosion commonly interrelated to the tectonic impact of the Syrian Arc Orogeny (SAO).

Stratigraphic architecture and paleosols as basin correlation tools of the early Paleogene infill in central–south Patagonia, Golfo San Jorge Basin, Argentinean Patagonia

The Paleogene infill of the eastern Golfo San Jorge Basin, Patagonia, Argentina, is composed of marine (Salamanca Formation) and terrestrial (Grupo Río Chico and lower Sarmiento Formation) deposits. The Río Chico Group is a fluvial, pedogenically modified succession interlayered with eolian volcaniclastic deposits during the Eocene. Several authors have highlighted the stratigraphic significance and usefulness of strongly developed paleosols in the definition of sequence stratigraphic studies. Even though the area hosts abundant geological and paleopedological data, no large-scale (i.e., basin-scale) stratigraphic architectural correlation including paleosols and relationships within the sequence stratigraphic context had hitherto been carried out. By integrating previously published and unpublished data sets, this paper proposes a sequence stratigraphic framework for the middle Danian–middle Eocene successions of the eastern Golfo San Jorge Basin. Here, spatio-temporal changes in fluvial/alluvial architecture of the Paleogene infill allow us to define four depositional sequences (S), limited by sequence boundaries (SB) that internally presents a low-accommodation system tract (LAST), and a high-accommodation system tract (HAST). Part of these sequences occur as fining-upwards fluvial successions that are pedogenically modified on top by strongly developed paleosols, or are erosively overlain by the coarse-grained base of the following sequence without the development of well-developed paleosols. The sedimentological and paleopedological analysis of the four sequences identified for the early Paleogene infill of the basin indicates that the interplay between subsidence, base level, and climate controlled both fluvial style and landscape evolution, as well as soil development. Volcaniclastic supply also played a significant role, especially during the Eocene.

Variability of rock mechanical properties in the sequence stratigraphic context of the Upper Devonian New Albany Shale, Illinois Basin

Abstract Rock mechanical properties of tight shale reservoirs are important parameters in the exploration and development of unconventional shale oil and gas. Understanding the stratigraphic variability of rock mechanical properties in black shale successions is important for identifying promising intervals for hydraulic stimulation. High-resolution hardness tests (6 cm spacing) with an Equotip Bambino 2 hardness tester were conducted on a New Albany Shale core to study the stratigraphic variability of rock mechanical properties. Combined with high-resolution geochemical analyses (8 cm spacing) acquired with a portable X-ray fluorescence spectrometer, it is possible to relate hardness variability to shale composition. The results of our study suggest an average quartz content of about 28% for the entire succession. Approximately half of that amount (13%) is interpreted to be of biogenic origin. Biogenic quartz, mainly derived from dissolution of radiolaria, occurs as authigenic quartz precipitated in Tasmanites cysts, and as microcrystalline quartz precipitated in the shale matrix. Petrographic observations indicate that the hardness of the New Albany Shale is critically controlled by biogenic quartz content, not by total quartz content as suggested in multiple prior studies. Biogenic quartz forms an interconnected stiff framework which increases the hardness of shales. In contrast, dispersed “hard” grains (e.g., detrital quartz, feldspar, and dolomite) do not contribute to brittle behavior. When examined under a scanning electron microscope, “hard” shales are rich in biogenic quartz and contrast visibly with “soft” shales that are characterized by high contents of clay minerals. Negative correlation between hardness and Al2O3 content, as well as differential compaction of clay minerals around “hard” grains, indicate that clay minerals are an important ductile component in the New Albany Shale. Although the general softness of kerogen suggests that there should be an influence of organic matter on hardness, this effect is counteracted by biogenic quartz cementation which has a comparable stratigraphic distribution pattern. We propose a new mineral-composition-based brittleness index on the basis of biogenic quartz content to characterize the brittleness of tight shale reservoirs. Within a sequence stratigraphic context, the hardness of the New Albany Shale increases in transgressive systems tracts, reaches a maximum at maximum flooding surfaces, and decreases in highstand systems tracts with the exception of the Blocher Member. Maximum flooding surface intervals have the highest potential to develop brittle fractures and should be potential targets for hydraulic stimulation.

Non-depositional and erosional “offshore” bioeroded hardgrounds from the Lower Cretaceous of the Neuquén Basin, Argentina: Insights into their sequence-stratigraphic implications and controls

Abstract Hardgrounds are important surfaces for basin analysis and oil exploration, representing in some cases significant markers of the sedimentary basin fill. In the Neuquen Basin, bioerosion structures are common on the shell surfaces of benthic organisms, but bioeroded hardgrounds are comparatively uncommon. In the Pilmatue Member of the Lower Cretaceous Agrio Formation (Valanginian-Hauterivian) two examples of bioeroded hardgrounds are described in detail, allowing to recognize contrasting palaeoenvironmental and sequence-stratigraphic contexts, and in turn, discussing the potential major controlling factors in each example. In both cases Trypanites isp. and Gastrochaenolites isp. are described, interpreted as belonging to a Trypanites ichnofacies. The two cases are developed in different environmental conditions: one in non-depositional offshore conditions at the top of a parasequence (i.e., transgressive surface); and the other in offshore erosional settings with the omission surface located in an intra-parasequence position, with no obvious sequence-stratigraphic connotation. These marked differences in the palaeonvironmental conditions (non-depositional vs erosive) and sequence-stratigraphic context (inter vs intra-parasequence occurrence) interpreted for these example allowed us to propose different controlling factors related to the generation of the hardgrounds. Despite these differences, namely long periods of non deposition and low-energy conditions vs shorter time and high-energy (erosive) conditions, in both cases a Trypanites ichnofacies is developed, representing an important aspect to be considered in the characterization of the basin evolution based on omission surfaces.

Organic matter content and type variation in the sequence stratigraphic context of the Upper Devonian New Albany Shale, Illinois Basin

Organic matter quantity and type are important parameters in conventional source rock evaluation and unconventional shale oil/gas reservoir characterization. Understanding the stratigraphic distribution of organic matter content and type in black shale successions is critical for identifying potentially productive intervals, because organic matter can adsorb large amounts of oil and gas. Detailed examination of total organic carbon content, organic petrographic composition, and high-resolution (8 cm spacing) geochemical proxies in a sequence stratigraphic framework were conducted on an early mature (0.55% Ro) New Albany Shale core of the Illinois Basin to study the influence of relative sea-level fluctuations on paleoproductivity, clastic supply, bottom-water redox conditions, and their combined control on total organic carbon distribution pattern and organic maceral variation.Marine organic matter including amorphous organic matter and alginite (mainly derived from Tasmanites cysts) is the dominant organic matter in the New Albany Shale. Terrestrial organic matter accounts for <10% of total organic matter. Within a sequence stratigraphic context, total organic carbon content increases in transgressive systems tracts, reaches a maximum before the maximum flooding surface, and shows relatively low values in highstand systems tracts. Comparatively, low total organic carbon contents at maximum flooding surfaces reflect a combination of low sedimentation rates, elevated bottom-water oxygenation, and high biogenic silica dilution.Stratigraphically, amorphous organic matter content increases in transgressive systems tracts, reaches a maximum near the maximum flooding surface, and decreases in highstand systems tracts. Enrichment of broken Tasmanites cysts and their detrital infills are an indicator of high-energy environments and could possibly indicate lowstand systems tracts. The stratigraphic distribution of organic matter content and type may result in cyclic stratigraphic variations of hydrocarbon generation potential and oil saturation, and influence the development of secondary organic pores when the New Albany Shale is within the oil and gas windows.

Trace fossils and fluvial-lacustrine Ichnofacies of the Eocene Uinta and Duchesne River Formations, northern Uinta Basin, Utah

Trace fossil assemblages in a fluvial-lacustrine sequence stratigraphic context hold significant poten-tial for expanding our understanding of environmental controls and continental basin-fill history. The succession of the Eocene Uinta Formation and four members of the Duchesne River Formation is ex¬tremely well-exposed in the Uinta Basin of northeastern Utah, revealing a robust stratigraphic framework to document broad-scale fluvial-lacustrine facies architectures and associated trace fossil assemblages. Greenish- and gray-colored mudstone beds with interbedded tabular sandstone representing lacustrine environments contain the trace fossils Arenicolites and Gordia (= Haplotichnus). In contrast, red mudstone beds with interbedded channelized sandstone representing upstream fluvial and alluvial environments contain a variety of insect trace fossils, including Scoyenia, Ancorichnus, and nest structures. Transitional, interfingering lithologies of wetland or shallow, short-lived lacustrine environments on the alluvial plain contain the trace fossil Steinichnus. Although there are many small-scale (bed-scale) physical sedimen¬tary structures and trace fossils from continental subenvironments, this study focuses on the large-scale (member-scale) change in trace fossil assemblages, with results indicating that the ichnofacies corroborate continental sequence stratigraphic interpretations in a fluvial-lacustrine setting.

Trace fossils and fluvial-lacustrine ichnofacies of the Eocene Uinta and Duchesne River Formations, northern Uinta Basin, Utah

Trace fossil assemblages in a fluvial-lacustrine sequence stratigraphic context hold significant poten-tial for expanding our understanding of environmental controls and continental basin-fill history. The succession of the Eocene Uinta Formation and four members of the Duchesne River Formation is ex¬tremely well-exposed in the Uinta Basin of northeastern Utah, revealing a robust stratigraphic framework to document broad-scale fluvial-lacustrine facies architectures and associated trace fossil assemblages. Greenish- and gray-colored mudstone beds with interbedded tabular sandstone representing lacustrine environments contain the trace fossils Arenicolites and Gordia (= Haplotichnus). In contrast, red mudstone beds with interbedded channelized sandstone representing upstream fluvial and alluvial environments contain a variety of insect trace fossils, including Scoyenia, Ancorichnus, and nest structures. Transitional, interfingering lithologies of wetland or shallow, short-lived lacustrine environments on the alluvial plain contain the trace fossil Steinichnus. Although there are many small-scale (bed-scale) physical sedimen¬tary structures and trace fossils from continental subenvironments, this study focuses on the large-scale (member-scale) change in trace fossil assemblages, with results indicating that the ichnofacies corroborate continental sequence stratigraphic interpretations in a fluvial-lacustrine setting.

Stratigraphic signatures of mass extinctions: ecological and sedimentary determinants.

Stratigraphic patterns of last occurrences (LOs) of fossil taxa potentially fingerprint mass extinctions and delineate rates and geometries of those events. Although empirical studies of mass extinctions recognize that random sampling causes LOs to occur earlier than the time of extinction (Signor–Lipps effect), sequence stratigraphic controls on the position of LOs are rarely considered. By tracing stratigraphic ranges of extant mollusc species preserved in the Holocene succession of the Po coastal plain (Italy), we demonstrated that, if mass extinction took place today, complex but entirely false extinction patterns would be recorded regionally due to shifts in local community composition and non-random variation in the abundance of skeletal remains, both controlled by relative sea-level changes. Consequently, rather than following an apparent gradual pattern expected from the Signor–Lipps effect, LOs concentrated within intervals of stratigraphic condensation and strong facies shifts mimicking sudden extinction pulses. Methods assuming uniform recovery potential of fossils falsely supported stepwise extinction patterns among studied species and systematically underestimated their stratigraphic ranges. Such effects of stratigraphic architecture, co-produced by ecological, sedimentary and taphonomic processes, can easily confound interpretations of the timing, duration and selectivity of mass extinction events. Our results highlight the necessity of accounting for palaeoenvironmental and sequence stratigraphic context when inferring extinction dynamics from the fossil record.

Impact of glacial-lake paleofloods on valley development since glacial termination II: A conundrum of hydrology and scale for the lowstand Brahmaputra-Jamuna paleovalley system

Research Article| August 08, 2018 Impact of glacial-lake paleofloods on valley development since glacial termination II: A conundrum of hydrology and scale for the lowstand Brahmaputra-Jamuna paleovalley system J.L. Pickering; J.L. Pickering † 1Department of Earth and Environmental Science, Vanderbilt University, Nashville, Tennessee 37235, USA †Present address: Shell International Exploration and Production, Inc., Houston, Texas 77082, USA; jennifer.pickering@shell.com. Search for other works by this author on: GSW Google Scholar M.S. Diamond; M.S. Diamond 1Department of Earth and Environmental Science, Vanderbilt University, Nashville, Tennessee 37235, USA Search for other works by this author on: GSW Google Scholar S.L. Goodbred; S.L. Goodbred 1Department of Earth and Environmental Science, Vanderbilt University, Nashville, Tennessee 37235, USA Search for other works by this author on: GSW Google Scholar C. Grall; C. Grall 2Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar J.M. Martin; J.M. Martin 3Shell International Exploration and Production, Inc., Houston, Texas 77082, USA Search for other works by this author on: GSW Google Scholar L. Palamenghi; L. Palamenghi 4Faculty of Geosciences, University of Bremen, Bremen 28359, Germany Search for other works by this author on: GSW Google Scholar C. Paola; C. Paola 5Department of Earth Sciences and St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, Minnesota 55455, USA Search for other works by this author on: GSW Google Scholar T. Schwenk; T. Schwenk 4Faculty of Geosciences, University of Bremen, Bremen 28359, Germany Search for other works by this author on: GSW Google Scholar R.S. Sincavage; R.S. Sincavage 1Department of Earth and Environmental Science, Vanderbilt University, Nashville, Tennessee 37235, USA Search for other works by this author on: GSW Google Scholar V. Spieß V. Spieß 4Faculty of Geosciences, University of Bremen, Bremen 28359, Germany Search for other works by this author on: GSW Google Scholar GSA Bulletin (2019) 131 (1-2): 58–70. https://doi.org/10.1130/B31941.1 Article history received: 08 Oct 2017 rev-recd: 02 Mar 2018 accepted: 15 Jun 2018 first online: 08 Aug 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation J.L. Pickering, M.S. Diamond, S.L. Goodbred, C. Grall, J.M. Martin, L. Palamenghi, C. Paola, T. Schwenk, R.S. Sincavage, V. Spieß; Impact of glacial-lake paleofloods on valley development since glacial termination II: A conundrum of hydrology and scale for the lowstand Brahmaputra-Jamuna paleovalley system. GSA Bulletin 2018;; 131 (1-2): 58–70. doi: https://doi.org/10.1130/B31941.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract To better define the base of the Brahmaputra River paleovalley, we analyzed an extensive borehole data set from the subaerial Bengal delta and a 255-km-long multichannel seismic survey along the modern river. The data reveal that the paleovalley floor is defined by a gravel unit containing boulder-sized clasts up to 30 cm in diameter, deposited after ca. 30 ka but before ca. 9 ka. Paleohydrology during that time and the previous glacial maximum was characterized by a weak monsoon and reduced river discharge, both of which are inconsistent with large valley formation. However, our work indicates that glacial-lake outburst floods sourced from the Tibetan reaches of the Brahmaputra were routed through the lowstand valley, producing megaflood-scale discharge capable of transporting gravel and cannibalizing the valley margins. The timing of these glacial-lake outburst flood–driven discharge events was coincident with valley development and explains the anomalously large width of the valley and basal gravel surface. Despite the underfit scale of Brahmaputra discharge following the last glacial period, a strengthening monsoon and high sediment discharge in the early Holocene subsequently contributed to the efficient infilling of the massive paleovalley by the mid-Holocene. In a sequence stratigraphic context, this work provides an example of a major unconformity that developed late in the eustatic cycle (i.e., during early transgression rather than an earlier, protracted response to sea-level lowering) and in response to a perturbation originating in the catchment instead of changing accommodation in the basin. As such, it represents a geologically instantaneous time surface that can be used as a marker for stratigraphic correlation but one that is not in phase with eustatic sea-level fall. You do not currently have access to this article.

Palynological and palaeoenvironmental analyses of the upper Albian-Cenomanian succession in Gindi Basin, Egypt: Implication for transgressive/regressive systems tracts

The current contribution illustrates the advantage of the use of quantitative palynological data to palaeoenvironmentally differentiate the similar-looking monotonous, thick clastic interval of the Kharita, Bahariya and Abu Roash “G” units in Gindi Basin, Egypt. The present study aims also to the utilization of the upper Albian-Cenomanian quantitative palynological and palynofacies data and their integral interpretation in a sequence stratigraphic context. The recovered palynomorph assemblages with moderately diverse sporomorphs and dinoflagellate cysts, show poor to fair preservation. Around 47 species of pollen grains, 37 of dinoflagellate cysts and 36 species of pteridophytic spores have been recorded from two sections encountered in El Sagha-1A and BRE 6-1 wells drilled in Gindi Basin; however, achritarchs, microforaminiferal test linings and freshwater algae are impoverished and sparsely documented through the studied intervals. Based on the first downhole appearances (FDAs) of the recovered marker taxa, the encountered Abu Roash “G”, Bahariya and the upper Kharita units are palynologically dated as late Cenomanian, early-middle Cenomanian and late Albian, respectively. Relative sea level fluctuation is particularly relevant and deduced from various palynological parameters mainly the terrestrial:marine (T:M) ratio and the palynological marine index (PMI). Based on the vertical stratigraphic distribution of the palynofacies along with the relative variations of the terrestrial versus marine palynomorphs, five complete (SQ1-SQ5) third order transgressive-regressive sequences and incomplete one are defined in El Sagha-1A in comparison to six complete (SQ2-SQ7) and one incomplete (SQ1) are constructed in BRE 6-1. Reliance on distinctive palynofacies assemblages, differentiated based on the relative composition of particulate organic matter constituents, four depositional palaeoenvironments were interpreted for the studied intervals. The Kharita and Bahariya formations were elucidated to be deposited in fluctuating marginal-shallow and proximal inner neritic marine environments. However, the sea level remarkably showed a continued rising which brought the inner-middle neritic conditions of the Abu Roash “G” Member during late Cenomanian.

Bed II Sequence Stratigraphic context of EF-HR and HWK EE archaeological sites, and the Oldowan/Acheulean succession at Olduvai Gorge, Tanzania

Archaeological excavations at EF-HR and HWK EE allow reassessment of Bed II stratigraphy within the Junction Area and eastern Olduvai Gorge. Application of Sequence Stratigraphic methods provides a time-stratigraphic framework enabling correlation of sedimentary units across facies boundaries, applicable even in those areas where conventional timelines, such as tephrostratigraphic markers, are absent, eroded, or reworked. Sequence Stratigraphically, Bed II subdivides into five major Sequences 1 to 5, all floored by major disconformities that incise deeply into the underlying succession, proving that simple "layer cake" stratigraphy is inappropriate. Previous establishment of the Lemuta Member has invalidated the use of Tuff IIA as the boundary between Lower and Middle Bed II, now redefined at the disconformity between Sequences 2 and 3, a lithostratigraphic contact underlying the succession containing the Lower, Middle, and Upper Augitic Sandstones. HWK EE site records Oldowan technology in the Lower Augitic Sandstone at the base of Sequence 3, within Middle Bed II. We suggest placement of recently reported Acheulean levels at FLK W within the Middle Augitic Sandstone, thus emphasizing that handaxes are yet to be found in earlier stratigraphic units of the Olduvai sequence. This would place a boundary between the Oldowan and Acheulean technologies at Olduvai in the Tuff IIB zone or earliest Middle Augitic Sandstone. A major disconformity between Sequences 3 and 4 at and near EF-HR cuts through the level of Tuff IIC, placing the main Acheulean EF-HR assemblage at the base of Sequence 4, within Upper rather than Middle Bed II. Sequence stratigraphic methods also yield a more highly resolved Bed II stratigraphic framework. Backwall and sidewall surveying of archaeological trenches at EF-HR and HWK EE permits definition of “Lake-parasequences” nested within the major Sequences that record downcutting of disconformities associated with lake regression, then sedimentation associated with lake transgression, capped finally by another erosional disconformity or hiatal paraconformity caused by the next lake withdrawal. On a relative time-scale rather than a vertical metre scale, the resulting Wheeler diagram framework provides a basis for recognizing time-equivalent depositional episodes and the position of time gaps at various scales. Relative timing of archaeological assemblage levels can then be differentiated at a millennial scale within this framework.