Regional Unconformity Research Articles

The Relationship between Jurassic Coal Measures and Sandstone‐type Uranium Deposits in the Northeastern Ordos Basin, China

AbstractOutcrop and drill hole data show that the Jurassic coal measures in the northeastern Ordos Basin are composed mainly of the Yan'an Formation and the lowstand system tract of the Zhiluo Formation, and there is a regional unconformity between them. The Dongsheng uranium deposit is associated with the Jurassic coal measures. Research data indicate that the Jurassic coal measures in the study area have a certain hydrocarbon‐generating capacity, although the metamorphic grade is low (Ro=0.40%–0.58%). In the Dongsheng region alone, the accumulative amount of generated coalbed methane (CBM) is about 2028.29 × 108 −2218.72 × 108 m3; the residual amount is about 50.92 × 108 m3, and the lost amount is about 1977 × 108 m3. Analysis of the burial history of the host rocks and the evolutionary history of the Dongsheng uranium deposit suggests that the Jurassic coal measures generated hydrocarbon mainly from Middle Jurassic to Early Crataceous, which is the main mineralization phase of the Dongsheng uranium deposit. By the Late Cretaceous, a mass of CBM dissipated due to the strong tectonic uplift, and the Dongsheng uranium deposit stepped into the preservation phase. Therefore, the low‐mature hydrocarbon‐containing fluid in the Jurassic coal measures not only served as a reducing agent for the formation of sandstone‐type uranium deposits, but also rendered the second reduction of paleo‐interlayer oxidation zone and become the primary reducing agent for ore conservation. Regional strata correlation reveals that the sandstone‐type uranium reservoir at the bottom of the Zhiluo Formation is in contact with the underlying industrial coal seams in the Yan'an Formation through incision or in the form of an unconformity surface. In the Dongsheng region with poorly developed fault systems, the unconformity surface and scour surface served as the main migration pathways for low‐mature hydrocarbon‐containing fluid migrating to the uranium reservoir.

New plays on the Greater East Shetland Platform (UKCS Quadrants 3, 8-9, 14-16) – part 1: Regional setting and a working petroleum system

Despite significant discoveries within Palaeogene-age reservoirs (e.g., Mariner) the East Shetland Platform (ESP) is underexplored, with only ten wells per 1000 km2. Mesozoic units are thin or absent whilst Paleozoic reflectors resemble acoustic basement on legacy seismic. Recent 3D dual-sensor broadband surveys (GeoStreamer) covering 17,200 km2 over parts of Quadrants 3, 8-9, 14-16 have allowed for clearer imaging. Here, this dataset is interpreted, leading to new insights into this large frontier region. The ESP petroleum system comprises multiple proven reservoir and source intervals, with viable play fairways. Up to four regional unconformities are interpreted, merging into fewer erosional surfaces on persistent highs. Elsewhere on the ESP, predominantly subsiding Permo-Triassic depocentres contain a nearly continuous Paleozoic-Mesozoic succession. The most prominent of these, to the south and south-west of the Beryl Embayment, is here referred to as the ‘Crawford-Skipper Basin’. Existing hydrocarbon discoveries on the ESP are in the vicinities (<7 km) of intra-platform Permo-Triassic basin margins. Exploration close to such basins is inherently less risky due to possible positive influences of deep-seated structures on the petroleum system. These include: (1) formation of Meso-Cenozoic closures; (2) Devonian source maturity and presence of simple fault-related migration pathways; (3) viability of sub-Cretaceous reservoir-trap-seal configurations.

Mechanisms initiating fluid migration at Snøhvit and Albatross fields, Barents Sea

The southwestern Barents Sea, part of the epicontinental Barents Sea located between Norway and the Svalbard archipelago, hosts several mature petroleum systems. Large gas chimneys, leaking faults and seabed pockmarks are proof that hydrocarbon leakage has taken place previously in this part of the Barents Sea. Glacial erosion, being one of the major episodes of erosion and uplift, has probably caused fluid leakage out of proven hydrocarbon reservoirs and fluid migration along permeable carrier beds. We use 3D seismic data from the Snohvit gas field to focus on the nature of the fluid transport systems and the origins and potential mechanisms for how fluids migrated from deep-seated reservoirs. Mapping of the trap of the reservoirs at Snohvit has allowed us to determine potential spill point locations, coinciding with major gas chimneys and several faults that extend into shallow strata. We explain how uplift and erosion caused gas expansion in the reservoirs allowing the gas to reach below the spill point located at the gas–water contact level at the bottom of the closure. Fluids, such as hydrocarbons, may have escaped through these spills points, leading to the emplacement and development of the aforementioned gas chimneys. Similarly, migration of fluids may have taken place along faults, reactivated by glaciotectonic processes and connected with spill points at reservoir depth. Both gas chimneys and faults have allowed gas to migrate into and be trapped in shallow gas accumulations within the Torsk Formation and below the upper regional unconformity.

Deformation geometry and timing of theWupoer thrust belt in the NE Pamir and its tectonic implications

The Pamir region, located to the northwest of the Tibetan Plateau, provides important information that can aid the understanding of the plateau’s tectonic evolution. Here we present new findings on the deformation geometry and timing of the Wupoer thrust belt at the northeastern margin of Pamir. Field investigations and interpretations of seismic profiles indicate that the eastern portion of the Wupoer thrust belt is dominated by an underlying foreland basin and an overlying piggy-back basin. A regional unconformity occurs between the Pliocene (N2) and the underlying Miocene (N1) or Paleogene (Pg) strata associated with two other local unconformities between Lower Pleistocene (Q1) and N2 and between Middle Pleistocene (Q2-4) and Q1 strata. Results of structural restorations suggest that compressional deformation was initiated during the latest Miocene to earliest Pliocene, contributing a total shortening magnitude of 48.6 km with a total shortening rate of 48.12%, most of which occurred in the period from the latest Miocene to earliest Pliocene. These results, combined with previous studies on the Kongur and Tarshkorgan extensional system, suggest an interesting picture of strong piedmont compressional thrusting activity concurrent with interorogen extensional rifting. Combining these results with previously published work on the lithospheric architecture of the Pamir, we propose that gravitational collapse drove the formation of simultaneous extensional and compressional structures with a weak, ductile middle crustal layer acting as a decollement along which both the extensional and compressional faults merged.

Genesis of the Northern Adriatic Sea (Northern Italy) since early Pliocene

The Northern Adriatic Sea is a shallow and very flat shelf area located between the northern Apennines, the southern Alps and the Dinarides; its present day physiography is the result of the filling of a relatively deep Quaternary foredeep basin, developed due to the northeastward migration of the Apennine chain. Multichannel seismic profiles and well data have allowed documenting the stratigraphic architecture, the depositional systems and the physiographic evolution of the Northern Adriatic sea since early Pliocene time. In particular, three main depositional sequences bounded by regional unconformities were recognized. The Zanclean Sequence 1 documents first the drowning of late Messinian incised valleys and then the southward progradation of a shelf-slope system, which is inferred to be related to a tectonic phase of the Apenninic front. The Piacenzian-Gelasian Sequence 2 records a relatively rapid transgressive episode followed by minor southward progradation; the top of the sequence is associated with a major late Gelasian drowning event, linked to the NE-ward migration of the Apennine foredeep. The Calabrian to upper Pleistocene Sequence 3 testifies the infilling of accommodation previously created by the late Gelasian drowning event, and it initially accumulated in deep-water settings and then in shallow-water to continental settings. The upper part of Sequence 3, consisting of the paleo-Po deltaic system, is composed of seven high-frequency sequences inferred to record late Quaternary glacio-eustatic changes. These high-frequency sequences document the stepwise filling of the remaining accommodation, resulting in the development of the modern shelf.

Geologic controls on submarine slope failure along the central U.S. Atlantic margin: Insights from the Currituck Slide Complex

Multiple styles of failure, ranging from densely spaced, mass transport driven canyons to the large, slab-type slope failure of the Currituck Slide, characterize adjacent sections of the central U.S. Atlantic margin that appear to be defined by variations in geologic framework. Here we use regionally extensive, deep penetration multichannel seismic (MCS) profiles to reconstruct the influence of the antecedent margin physiography on sediment accumulation along the central U.S. Atlantic continental shelf-edge, slope, and uppermost rise from the Miocene to Present. These data are combined with high-resolution sparker MCS reflection profiles and multibeam bathymetry data across the Currituck Slide Complex. Pre-Neogene allostratigraphic horizons beneath the slope are generally characterized by low gradients and convex downslope profiles. This is followed by the development of thick, prograded deltaic clinoforms during the middle Miocene. Along-strike variations in morphology of a regional unconformity at the top of this middle Miocene unit appear to have set the stage for differing styles of mass transport along the margin. Areas north and south of the Currituck Slide are characterized by oblique margin morphology, defined by an angular shelf-edge and a relatively steep (>8°), concave slope profile. Upper slope sediment bypass, closely spaced submarine canyons, and small, localized landslides confined to canyon heads and sidewalls characterize these sectors of the margin. In contrast, the Currituck region is defined by a sigmoidal geometry, with a rounded shelf-edge rollover and gentler slope gradient (<6°). Thick (>800m), regionally continuous stratified slope deposits suggest the low gradient Currituck region was a primary depocenter for fluvial inputs during multiple sea level lowstands. These results imply that the rounded, gentle slope physiography developed during the middle Miocene allowed for a relatively high rate of subsequent sediment accumulation, thus providing a mechanism for compaction–induced overpressure that preconditioned the Currituck region for failure. Detailed examination of the regional geological framework illustrates the importance of both sediment supply and antecedent slope physiography in the development of large, potentially unstable depocenters along passive margins.

Depositional environment, sequence stratigraphy and sedimentary mineralization mechanism in the coal bed- and oil shale-bearing succession: A case from the Paleogene Huangxian Basin of China

It has been a long dispute in the forming mechanism of coal and oil shale in coal-and oil shale-bearing succession because the depositional environments between coal and oil shale are distinctly different. In this study, the Huangxian Basin was chosen as a typical example to study the mechanism of sedimentation of the coal and oil shale. Eleven facies and four facies associations have been recognized from petrologic, sedimentological and paleogeographic characteristics. The four facies were formed in alluvial fan, fan delta, braided stream, braided stream delta, shore-shallow lake-lacustrine bog, shallow lacustrine and deep lacustrine environments. Two types of sequence boundaries (regional unconformities and regional exposed non-sedimentary surfaces) and double systems tract boundaries (maximum flooding and first flooding surfaces) are identified. Sequence stratigraphic frameworks with two sequences (each including three systems tracts) are established to analyze the depositional evolution. We find that the Huangxian Basin is a paralic continental faulted lacustrine basin with four different types of coal bed and oil shale combinations: oil shale/coal bed/oil shale (OS-CB-OS), coal bed/oil shale (CB-OS), coal bed/mudstone/oil shale (CB-M-OS) and oil shale/coal bed (OS-CB). The formation mechanisms of the coal bed and oil shale associations are interpreted to be: a) the basin first experienced transgression, giving rise to the CB-OS or CB-M-OS combinations where oil shale was overlain by coal beds, and b) continental, non-lacustrine deposition developed in the basin and was terminated by subsequent transgression, forming lacustrine deposits. Later, the basin was silted into a peat swamp that was subsequently interrupted by transgression or terrigenous clastic deposition, eventually forming OS-CB-OS or CB-OS combinations. Through analyzing depositional evolution of the basin we conclude that lowstand system tracts tend to develop CB-OS associations; expanding system tracts tend to develop CB-OS, CB-M-OS and OS-CB-OS combinations; and highstand system tracts tend to develop OS-CB combinations.

Why are there no Messinian evaporites in the Black Sea?

An unconformity has been observed along the Black Sea shelf on seismic reflection profiles and wells which is broadly similar to the one associated with that formed during the Messinian salinity crisis (MSC) in the Mediterranean. Therefore, this intra- (or Middle) Pontian unconformity has been traditionally interpreted as the manifestation of the MSC in the Black Sea Basin. However, the magnitude of the sea-level fall associated with this erosive surface does not appear to be nearly as significant as was assumed previously. Also, the inferred MSC surface itself cannot be easily followed into the palaeo-deep-water basin as a regional unconformity in the same manner as in the Mediterranean. Moreover, around the Black Sea, there is no evidence of major river incisions during the MSC, unlike the well-documented cases in the Mediterranean region. If the MSC evaporites in the Mediterranean indeed deposited in a subaerial setting at the basin floor, the lack of a major drawdown in the Black Sea explains why there are no Messinian evaporites in the Black Sea. Owing to the approximately 500 m MSC sea-level drop the Black Sea basin system, this basin did not even get close to the conditions required for the formation of evaporites in the basin centre. As the magnitude of the sea-level drop and the overall impact of the MSC in the Black Sea is interpreted to be less significant than in the Mediterranean, the risk of breaching pre-existing hydrocarbon traps during the MSC is less than has been suggested before.

Large fluctuations of shallow seas in low-lying Southeast Asia driven by mantle flow

The Sundaland continental promontory, as the core of Southeast Asia, is one of the lowest lying continental regions, with half of the continental area presently inundated by a shallow sea. The role of mantle convection in driving long-wavelength topography and vertical motion of the lithosphere in this region has often been ignored when interpreting regional stratigraphy, including a widespread Late Cretaceous-Eocene unconformity, despite a consensus that Southeast Asia is presently situated over a large-amplitude, dynamic topography low resulting from long-term post-Pangea subduction. We use forward numerical models to link mantle flow with surface tectonics, and compare predicted trends of dynamic topography with eustasy and regional paleogeography to determine the influence of mantle convection on regional basin histories. A Late Cretaceous collision of Gondwana-derived terranes with Sundaland choked the active margin, leading to slab breakoff and a ∼10-15 Myr-long subduction hiatus. Slab breakoff likely resulted in several hundred meters of dynamic uplift and emergence of Sundaland between ∼80 and 60 Ma, and may explain the absence of a Late Cretaceous-Eocene sedimentary record. Renewed subduction from ∼60 Ma reinitiated dynamic subsidence of Sundaland, leading to submergence from ∼40 Ma despite falling long-term global sea levels. Our results highlight a complete ‘down-up-down' dynamic topography cycle experienced by Sundaland, with transient dynamic topography manifesting as a major regional unconformity in sedimentary basins.

Optimal aquifers and reservoirs for CCS and EOR in the Kingdom of Saudi Arabia: an overview

An overview on the tectono-stratigraphic framework of the Arabian plate indicates obvious differences between two distinct areas: the hydrocarbon-prolific sector and non-hydrocarbon-prolific sector. These differences resulted from the interplay of a variety of factors; some of which are related to the paleo-geographic configuration (eustatic sea level fluctuations, climatic conditions, and salt Basins), others to differential subsidence (burial) and structural inversions. During the Paleozoic, the regional compression was caused by far field effects of the Hercynian orogeny. This led to major folded structures in central and eastern Saudi Arabia (e.g. Ghawar anticline). During the Mesozoic, the most important tectonic factor was the stretching of the crust (extension), accompanied with the increase in temperature, resulting in an increase of the accommodation space, and thicker sedimentary successions. Regional unconformities are mostly found where folded structures are dominant, and they acted as a carrier systems for the accumulation of hydrocarbon and groundwater. A good understanding of the stratigraphy and tectonic evolution is, thus, required to develop carbon capture and storage (CCS) and to design efficiently enhanced oil recovery (EOR) in both sectors. Oil and gas reservoirs offer geologic storage potential as well as the economic opportunity of better production through CO2-EOR. The world greatest hydrocarbon reservoirs mainly consist of Jurassic carbonate rocks, and are located around the Arabian Basin (including the eastern KSA and the Arabian Gulf). The Cretaceous reservoirs, which mainly consist of calcarenite and dolomite, are located around the Gotnia salt Basin (northeast of KSA). Depleted oil and gas fields, which generally have proven as geologic traps, reservoirs and seals, are ideal sites for storage of injected CO2. Each potential site for CO2-EOR or CCS should be evaluated for its potential storage with respect to the containment properties, and to ensure that conditions for safe and effective long term storage are present. The secured deep underground storage of CO2 implies appropriate geologic rock formations with suitable reservoir rocks, traps, and impermeable caprocks. Proposed targets for CCS, in the non-hydrocarbon-prolific sector, are Kharij super-aquifer (Triassic), Az-Zulfi aquifer (Middle Jurassic), Layla aquifer (Late Jurassic), and Wasia aquifer (Middle Cretaceous). Proposed targets for EOR are Safaniya oil field (Middle Cretaceous) (Safaniya, Wara and Khafji reservoirs), Manifa oil field (Las, Safaniya and Khafji reservoirs) (Late Jurassic), and Khuff reservoir (Late Permian-Early Triassic) in central to eastern KSA.

Structural Characteristics and Formation Dynamics: A Review of the Main Sedimentary Basins in the Continent of China

AbstractThe formation and evolution of basins in the China continent are closely related to the collages of many blocks and orogenic belts. Based on a large amount of the geological, geophysical, petroleum exploration data and a large number of published research results, the basement constitutions and evolutions of tectonic–sedimentary of sedimentary basins, the main border fault belts and the orogenesis of their peripheries of the basins are analyzed. Especially, the main typical basins in the eight divisions in the continent of China are analyzed in detail, including the Tarim, Ordos, Sichuan, Songliao, Bohai Bay, Junggar, Qiadam and Qiangtang basins. The main five stages of superimposed evolutions processes of basins revealed, which accompanied with the tectonic processes of the Paleo–Asian Ocean, Tethyan and Western Pacific domains. They contained the formations of main Cratons (1850–800 Ma), developments of marine basins (800–386 Ma), developments of Marine–continental transition basins and super mantle plumes (386–252 Ma), amalgamation of China Continent and developments of continental basins (252–205 Ma) and development of the foreland basins in the western and extensional faulted basin in the eastern of China (205–0 Ma). Therefore, large scale marine sedimentary basins existed in the relatively stable continental blocks of the Proterozoic, developed during the Neoproterozoic to Paleozoic, with the property of the intracontinental cratons and peripheral foreland basins, the multistage superimposing and late reformations of basins. The continental basins developed on the weak or preexisting divisional basements, or the remnant and reformed marine basins in the Meso–Cenozoic, are mainly the continental margins, back–arc basins, retroarc foreland basins, intracontinental rifts and pull–apart basins. The styles and intensity deformation containing the faults, folds and the structural architecture of regional unconformities of the basins, responded to the openings, subductions, closures of oceans, the continent–continent collisions and reactivation of orogenies near the basins in different periods. The evolutions of the Tianshan–Mongol–Hinggan, Kunlun–Qilian–Qinling–Dabie–Sulu, Jiangshao–Shiwandashan, Helanshan–Longmengshan, Taihang–Wuling orogenic belts, the Tibet Plateau and the Altun and Tan–Lu Fault belts have importantly influenced on the tectonic–sedimentary developments, mineralization and hydrocarbon reservoir conditions of their adjacent basins in different times. The evolutions of basins also rely on the deep structures of lithosphère and the rheological properties of the mantle. The mosaic and mirroring geological structures of the deep lithosphère reflect the pre–existed divisions and hot mantle upwelling, constrain to the origins and transforms dynamics of the basins. The leading edges of the basin tectonic dynamics will focus on the basin and mountain coupling, reconstruction of the paleotectonic–paleogeography, establishing relationship between the structural deformations of shallow surface to the deep lithosphère or asthenosphere, as well as the restoring proto–basin and depicting residual basin of the Paleozoic basin, the effects of multiple stages of volcanism and paleo–earthquake events in China.

STUDY OF JURASSIC AND NEOCOMIAN AGE BARRIER TRAPS TO INCREASE THE PETROLEUM POTENTIAL OF THE YAMAL PENINSULA

In the article the geologic structure of the Jurassic and Neocomian age sedimentary cover in the Yamal Peninsula is considered. The regional unconformities of the north-east strike where the traps of barrier type are formed and the petroleum potential of which has been proved in Novoportovskoye, Rostovtelecoye and in a number of other fields have been mapped.

Gold nugget morphology and geochemical environments of nugget formation, southern New Zealand

Gold nuggets (centimetre scale) have formed in a supergene alteration zone on hydrothermal gold deposits, and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals, and along fractures in quartz and host rocks. The supergene alteration was driven by groundwater-driven water-rock interaction near to a regional unconformity beneath fluvial sediments, and involved clay alteration and oxidation that extended up to 50m below the unconformity. Oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals. The nuggets have some crystalline form, and internally they consist of anhedral grains, elongated gold plates, and intimate intergrowths of gold and iron oxyhydroxide. Nugget surfaces have further micron scale overgrowths of microparticulate gold, gold plates, and gold crystals. Nuggets were eroded and recycled into nearby proximal Miocene quartz pebble conglomerates, where they concentrated in placers near the basal unconformity. Later recycling transferred gold into Pleistocene fluvial channels. Gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles, with little change in mass. All groundwater maintained high pH throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation. Hence, gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets, albeit with lower dissolved sulphur concentrations. Despite aridification of the climate in the late Cenozoic, with resulting localised high dissolved chloride concentrations, chloride complexation did not contribute to gold mobility.

Late Albian cactus in shallow carbonate shelf, West Texas

Well-preserved vertical molds of a stalk-like fossil are in growth position in the upper part of the Upper Albian Fort Lancaster Formation exposed in West Texas. The Fort Lancaster Formation is the uppermost Albian carbonate unit deposited on the shallow water Barremian-early Cenomanian Comanche shelf. The Fort Lancaster represents one long-term transgressive-regressive cycle capped by a regional unconformity overlain by lower Cenomanian deeper water Del Rio Shale. The erect stalk-like molds are up to 1 m tall and about 6 cm in diameter. The mold walls are composed of six pairs of angular ridges and grooves evenly spaced at an angle of 60° to each other. The ridge crests are dotted by paired ringed depressions with small central nodes; these negative impressions indicate the presence of oval rings with small depressions inside of each ring. The diameter of the stalk is the same at its base as at its top indicating that growth did not expand the organism but extended it upward. These stalk-like organisms appear to be rooted on disconformable bedding surfaces that were exposed to the atmosphere. Carbon isotope data shows the classic negative shift below subaerial contacts. Subsequently sea level rose and muddy carbonate sediment buried the stalk. The absence of original wall material suggests that the wall was composed of readily decayed or dissolved material such as organic matter or aragonite. Stalk-like organisms of this size and shape are rare and potential types are cacti, plant reeds, Porifera, or Scleractinian Cnidaria.

Stratigraphy and evolution of the Galve sub-basin (Spain) in the middle Tithonian–early Barremian: Implications for the setting and age of some dinosaur fossil sites

A review of the stratigraphy of the Galve sub-basin (western Maestrazgo Basin, eastern Spain) around the Jurassic–Cretaceous transition is presented here, based on new data acquired after extensive geological mapping and logging complemented with facies analysis, new biostratigraphic data and a revision of the published information available. The results obtained are relevant for a more detailed understanding of the tecto-sedimentary evolution of the studied basin during the transition between two stages of rift evolution (i.e., syn-rift sequences 1 and 2). In addition, new information on the age and setting of numerous dinosaur fossil- and track-sites found across the Galve sub-basin and in the northern part of the nearby Penyagolosa sub-basin is provided here. Two new lithostratigraphic units are defined and characterized, the Aguilar del Alfambra and the Galve formations. The previous stratigraphic framework considered only two lithostratigraphic units (the Villar del Arzobispo and El Castellar formations) bounded by a single regional unconformity, and this resulted in significant misinterpretations. The whitish limestones, red lutites and cross-bedded sandstones of the Aguilar del Alfambra Formation were deposited in transitional environments, ranging from coastal lutitic plains to restricted lagoons. Of particular interest are the laminated micritic-peloidal limestones with abundant fenestral porosity (supratidal ponds to intertidal flats), which preserve common dinosaur footprints. This unit is bounded by widespread unconformities and is of very variable thickness (0–450 m), controlled by extensional tectonics operating at the climax of syn-rift sequence 1 during the latest Tithonian–middle Berriasian. The overlying Galve Formation is of variable thickness (from 0 to 100 m) and is also bounded by regional unconformities described in detail here. It consists of red lutites with cross-bedded and tabular-burrowed sandstones representing channel and overflow deposits in an alluvial floodplain. The sauropod dinosaur Aragosaurus ischiaticus found in this unit has a controversial age assignment. The age of the Galve Formation is poorly constrained from late Berriasian to Hauterivian, but new biostratigraphic data presented here, combined with the correlation with the nearby Penyagolosa and Salzedella sub-basins, suggest a possible equivalence to the upper Berriasian–lower Valanginian sequence deposited during the initial stage of syn-rift sequence 2.

Dynamic topography and vertical motion of the U.S. Rocky Mountain region prior to and during the Laramide orogeny

Dynamic topography of Earth’s surface occurs in response to hydrodynamic stresses due to mantle flow beneath a flexible lithosphere. Here, we compare the predicted dynamic topography from an inverse-convection model that includes flat slab subduction with the known geologic history of the western United States from Late Cretaceous through Paleocene time to evaluate the validity of the model results. Downwarping behind the evolving Cordilleran volcanic arc took place after its inception in Late Triassic time, culminating in the formation of the Western Interior Seaway in Late Cretaceous time. Subsidence behind arcs is a consistent prediction of most models of dynamic topography. A more-detailed convection model is evaluated here by comparing it to the geologic history of the central Rocky Mountain region from 100 to 50 Ma. The match of predicted and observed tectonic subsidence is good up until the time that local deformation by the Laramide orogeny begins. This is most likely due to local flexural effects of mountain building overwhelming regional dynamic effects. Maps for successive time intervals show that the model matches the geologic history quite well through time and space; in particular, the subduction of the previously postulated oceanic plateau—the conjugate Shatsky Rise—has a significant impact on surface movements. (1) From 95 to 88 Ma, the locations of most significant regional unconformities match the eastward migration of a zone of high topography. (2) From 90 to 85 Ma, the zone of maximum subsidence coincides with the motion of the leading edge of the conjugate Shatsky Rise. (3) From 78 to 60 Ma, the site of initiation of Laramide deformation migrates coincidently with the position of the center of the conjugate Shatsky Rise. (4) From 70 to 60 Ma, the time-transgressive deposition of thin fluvial conglomerate units in the southern part of the study area is generally coincident with surface uplift caused by the trailing part of the conjugate Shatsky Rise. These results suggest that the inverse model approximates the vertical motion history quite well. Synchroneity of the position of the center of the passing conjugate Shatsky Rise, the landward limit of the evolving Cordilleran volcanic arc, and the initiation of Laramide deformation suggests that the Farallon plate became coupled with the overlying North American plate as the subducted oceanic plateau passed beneath. Progressively enhanced mechanical coupling between the plates was likely the impetus for Laramide shortening. This comparison of model results with surface geologic history provides a means to validate, but not verify, predictions of dynamic mantle-flow models.

Major unconformities/termination of extension events and associated surfaces in the South China Seas: Review and implications for tectonic development

The distribution of unconformities and end of Cenozoic rifting events in the South China Seas (SCS) reflects both the modes of rift development, and the effects of driving mechanisms. Continental rifting began in the eastern basins during the Paleocene, and propagated westwards to the Vietnam basin margin in the Late Eocene. Continental breakup around 32–28Ma caused a regional reduction or cessation in extensional activity, particularly affecting basins furthest from the spreading centre. Basins in the slope and deepwater area north of the spreading centre exhibit reduced fault activity until 21–20Ma. Propagation of oceanic crust westwards between ∼25 and 23Ma, and termination of seafloor spreading sometime between 20.5 and 16Ma affected fault activity in the Qiongdongnan, and Nam Con Song basins. In the Phu Khanh Basin and South, in the Dangerous Grounds area, extension continued until about 16Ma, ending at the Red Unconformity. The end of seafloor spreading around 20.5Ma reflects loss of extensional driving force as thinned continental crust entered the NW Borneo subduction zone. Controversially, a key component of the driving force maybe attributed to slab-pull. A transitional period of about 5–7my between the onset of subduction of continental crust, and final jamming of the subduction zone (Deep Regional Unconformity, DRU) is inferred. The last pulse of extension was focussed in the western SCS, and terminated around 10.5Ma. Detailed understanding of proto South China Seas development remains uncertain and controversial.

Carbon isotope stratigraphy and correlation of depositional sequences in the Upper Ordovician Ely Springs Dolostone, eastern Great Basin, USA

Shallow water carbonate shelf strata of the Ely Springs Dolostone accumulated on the western Laurentian margin as relative sea level fluctuated during the Late Ordovician icehouse. The Ely Springs Dolostone is divided into four upper Katian depositional sequences and one Hirnantian sequence. Here we present new carbon isotope data from the Ely Springs in the context of the sequence stratigraphic architecture. These data derive from a shelf transect of five stratigraphic sections exposed in the eastern Great Basin, USA and provide records of carbon cycling on the Laurentian margin prior to and during the Hirnantian glacial maximum. The δ13C values range between −2‰ and +4‰ at each section, and chemostratigraphic tie points occur in all sections of the transect. Intrabasinal correlations based on the δ13C records are consistent with correlations derived from sequence stratigraphy and conodont biostratigraphy. The Ely Springs Dolostone carbon isotope curve can also be correlated with the late Katian (Cincinnatian) composite derived from epicontinental deposits of the Laurentian midcontinent. The globally documented Hirnantian positive carbon isotope excursion (commonly referred to as “HICE”) occurs within the uppermost depositional sequence immediately below a regional unconformity coinciding with peak glacial conditions on Gondwana. The late Katian Elkhorn and Waynseville δ13C excursions are also present in the Ely Springs Dolostone carbon isotope record. The peak value of the Hirnantian carbon excursion increases from 2.6‰ to 3.8‰ across the shallow shelf and reaches 7.3‰ in coeval limestones of the Hanson Creek Formation deposited in a deep dysoxic setting. This lateral gradient is not the result of changes to the δ13C of weathered carbon supplied by rivers, as previously proposed; it may be instead related to authigenic carbonate cement precipitation during early diagenesis of the Hanson Creek strata. That these pervasively dolomitized shallow water carbonates retain δ13C signatures useful for regional and global stratigraphic correlation indicates that δ13C chemostratigraphy may be an appropriate correlation tool for dolostones with few biostratigraphically useful fossils.

Ice volume and paleoclimate history of the Late Paleozoic Ice Age from conodont apatite oxygen isotopes from Naqing (Guizhou, China)

A high-resolution and continuous conodont apatite oxygen isotope record spanning the late Viséan to Middle Permian is reported from South China, which is interpreted with respect to the ice volume and/or tropical seawater temperature history of the Late Paleozoic Ice Age (LPIA). The presented δ18O record shows significant fluctuations in δ18O from the late Viséan to Middle Permian with highest values observed in the Bashkirian (Early Pennsylvanian). The δ18O maximum coincides with a major eustatic sea level fall recorded in low-latitude successions and postdates the significant increases in 87Sr/86Sr and δ13Ccarb measured on well-preserved brachiopod calcite, which can be interpreted as reflecting intensified weathering as consequence of the closure of Rheic Ocean as well as enhanced carbon burial. Both processes may have contributed to lower greenhouse gas levels and cooled down the Earth's surface, triggering the maximum glaciation. The high Bashkirian δ18O values are interpreted to represent the glacial maximum of the LPIA. A coeval change in faunal composition and a decreasing diversity in climate-sensitive marine invertebrates can be ascribed to icehouse cooling and/or loss of habitat. Despite inconsistencies with earlier interpretations that the Early Permian represented the glacial maximum of the LPIA as inferred from Gondwanan glacial sediments records, the suggested Bashkirian glacial maximum agrees well with ice extent estimates based on the regional tectonic history in Gondwana, which suggests that the Bashkirian glaciation occurred during Gondwana interior uplift promoting maximum ice cover of the entire LPIA. However, maximum glaciation is only poorly represented in the depositional record because large parts of the glacial deposits were possibly removed by erosion as outlined by a major regional unconformity.

Buried subglacial landforms in the SW Barents Sea imaged using high-resolution P-Cable seismic data

A large grounded ice sheet covered the epicontinental Barents Sea multiple times during the Late Pleistocene and left its erosional imprint (Solheim et al. 1990; Knies et al. 2009). Erosion involved the removal of large amounts of sediments from the Tertiary and Cretaceous sedimentary successions of the Barents Sea. The Upper Regional Unconformity (URU) represents the erosional base for continental-shelf glaciations (Vorren et al. 1988). Here, we present palaeo-landforms from this erosional base imaged using high-resolution P-Cable seismic data acquired along the border between the Ringvassoya Fault Complex and the Loppa High in the SW Barents Sea (Fig. 1). P-Cable seismic data have a frequency bandwidth of up to 300 Hz and spatial resolution of 6×6 m, which is much better than conventional 3D seismic data. Hence, interpretation of P-Cable data allows visualization of buried geomorphic landscapes in exceptional detail. In addition, amplitude information is readily available and complements grid-based interpretations. Fig. 1. Buried subglacial landforms in …