Rim Syncline Research Articles

Implications of Salt Diapirism in Syn-Depositional Architecture of a Carbonate Margin-to-Edge Transition: An Example from Plataria Syncline, Ionian Zone, NW Greece

The present study examines the imprint of salt tectonics on carbonate depositional patterns of the Ionian zone platform edge to slope transition. The study area is part of an overturned rim syncline adjacent to a salt diapir. The Ionian zone is made up of three distinct stratigraphic sequences (pre-, syn- and post-rift sequences) represented by evaporites and shallow water carbonates at the base that pass gradually to a sequence consisting of pelagic limestones with shale intervals. In the study area, six cross sections were constructed, mainly covering the edge-to-slope overturned succession of Early Cretaceous to Eocene carbonates (post-rift stage) in the northern limb of the syncline. In the measured sections, abrupt changes in sediment texture resulted in the formation of distinct, thick-bedded carbonate layers, identified as packstones to grainstones–floatstones, with abundant fossil fragments, indicating deposition by debrites in a platform slope or slope-toe environment. Planar and ripple cross-lamination also suggest the involvement of turbidity currents in the depositional process. In the upper levels of the Lower Cretaceous carbonates, chert bodies with irregular shapes indicate soft sediment deformation due to instability of the slope triggered by salt intrusion. Internal unconformities identified in the field and in the available seismic data combined with the vertical to overturned dipping of the strata correspond to a basal megaflap configuration. Syn-sedimentary deformation resulted in the accumulation of debritic and turbiditic layers, while the compressional regime established in the area from the Late Cretaceous to Early Eocene enhanced the fracture porosity of carbonates, which could eventually affect the reservoir properties.

Geologic map of central Panamint Range, California, USA

This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex deformational history and reactivation relationships of the Panamint Range (California, USA), from active transtension of the Walker Lane belt, Miocene extension of the Basin and Range, to multiple Mesozoic events related to subduction, and Neoproterozoic extension. This collection of map data focuses on the geometry, kinematics, and relative timing of deformation to understand the deformation history and effects of structural reactivation. A minor portion of this geologic mapping data was presented in the analysis and figures of Andrew and Walker (2009). The Neogene extension and subsequent dextral transtension deformation has created a complex network of faults via partial reactivation of Mesozoic and Neoproterozoic structures. Structural data show oblique normal slip overprinting earlier normal slip along the western range flank fault of the western Panamint Range. Jurassic and Cretaceous deformation is localized along the western range on the Goldbug fault. The hanging wall of this fault preserves migmatitic fabrics and intense deformation due to Jurassic contraction. The Goldbug fault places Paleoproterozoic to Mesoproterozoic rocks over Neoproterozoic rocks. The Jurassic contraction has top-to-the-northeast relative transport and the more discrete Cretaceous thrust faulting has top-to-the-east transport. A set of Late Cretaceous plutonic rocks and mylonitic gneisses derived from them, occur along the Goldbug fault and demonstrate the reactivated nature of this fault in the Late Cretaceous. New data for the Butte Valley fault show that this fault cuts Late Jurassic plutonic rocks and has normal slip. The Butte Valley fault ends northward at the linked sinistral slip Warm Spring Canyon fault, which was previously interpreted to be an intrusive contact. A previously unrecognized rim syncline structure occurs along the boundary of the Late Jurassic Manly Peak quartz monzonite. Neoproterozoic deformation is difficult to discern due to the overprinting deformations. Numerous Neoproterozoic deformation-related mass wasting deposits can be seen within this formation, including a set of conspicuous allochthonous deposits and clasts of older Beck Spring Dolomite that appear to be frozen in the process of breaking away from intact, normal thickness beds in the Surprise–Happy Canyons divide. This detailed geologic mapping and collection of structural data for the rocks in the central Panamint Range were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer combined with an earlier set of field data that were digitized into the digital georeferenced database. This map is a simplification of detailed geologic mapping data collected at 1:2000–1:6000 scales and reduced to 1:20000 scale. Structural data include kinematic and relative timing of deformation information.

South-Central Barents Sea Composite Tectono-Sedimentary Element

Abstract The south-central Barents Sea today comprises a shallow continental shelf with water depths mainly in the 200–400 m range, straddling the Norway–Russia marine boundary. Geologically, it consists of a stable platform (the Bjarmeland Platform) dissected by rifts of probable Late Carboniferous age, with a significant and geologically persistent basement high (the Fedynsky High) in its southeastern part. The rifts are the ENE–WSW-trending Nordkapp Basin, the similarly trending but less clearly demarcated Ottar Basin and the NW–SE Tiddlybanken Basin. The varying rift trends appear to reflect the orogenic grain patchwork of the basement (Caledonide and Timanide), and these basins were infilled with a variable facies assemblage including substantial Carboniferous–Permian halites. A massive sedimentary influx of fluvio-deltaic to shallow-marine sediments took place in the Triassic, from the east and SE (Urals, Novaya Zemlya and western Siberia) and the south (Baltic Shield), resulting in doming and diapirism in the areas of thickest salt, particularly in the rifts. The succeeding Jurassic, Cretaceous and Cenozoic successions are generally thin, locally thickening in rim synclines and in the NE of the area towards the deep basins flanking Novaya Zemlya. Reactivation of the halokinetic structures took place in the early Cenozoic, probably associated with the development of the NE Atlantic–Arctic Ocean linkage. Marine source rocks of Triassic and Late Jurassic age are present in the area, along with Carboniferous and Permian source rocks of uncertain effectiveness. Petroleum has been found in Jurassic and Triassic clastic reservoirs, including recent shallow Jurassic oil and gas discoveries. Although none is currently in production, near-future oil development is likely in the Wisting discovery, on the western margin of the area. New exploration, including drilling, has taken place in the east of the area as a result of recent Norwegian and Russian licensing.

Improved seismic interpretation of a salt diapir by utilization of diffractions, exemplified by 2D reflection seismics, Danish sector of the North Sea

Development of salt diapirs affects the hydrocarbon trapping systems in the Danish sector of the North Sea, where the reservoirs mainly consist of chalk. Seismic imaging and interpretation of the salt structures are challenging, primarily due to the complex geometry of the salt bodies and typically strong velocity contrast with the neighboring sediment layers. The quality of seismic imaging in the North Sea is highly dependent on the quality of the estimated velocity model. We have studied diffracted arrivals originating from the salt flanks and adjacent sedimentary structures using a diffraction imaging technique. The diffracted waves carry valuable information regarding seismic velocity and the location of geologic discontinuities, such as faults, fractures, and salt delimitations. We apply a plane-wave destruction method to separate diffractions from our stacked data. We optimize imaging based on diffraction analysis by using a velocity continuation migration technique, which leads to an estimation of the optimum focusing velocity model. We determine that the diffraction-based approach significantly improves the seismic imaging adjacent to the salt diapirs and the neighboring layers when compared with a standard approach in which we mostly ignore the diffractions. The new poststack time-migrated results provide detailed information that optimizes our interpretation of the salt diapir itself (e.g., the width of the salt neck) as well as the sediment layers related to the rim synclines. Processing schemes such as prestack depth migration and full-waveform inversion may potentially provide high-resolution images of the salt structures. We only account for diffractions in nonmigrated stacked data to better constrain seismic velocity and improve imaging around the salt diapir. The obtained results are critical for reservoir characterization.

End-Cretaceous Quartz Arenite Formation in an Estuarian Environment under Brine Influence, N. Germany; Linked to both Deccan Volcanism and Chicxulub Impact Degassing during Climate Change

Maastrichtian off-shore carbonate sediments and transitional estuarian quartz arenite (primarily subarkosic/arkosic) deposited in N Germany, underwent indirect effects by end-Cretaceous plume volcanism (Caribbean Arc/Antilles, Amirante Arc/Seychelles, Deccan Traps) and by the Chicxulub impact during climate change. In addition, brines of local salt structures had increasing influence on pore water chemistry of siliciclastics deposited in rim synclines and sub-rosion bowls during the transition from salt pillow state (Upper Campanian/Maastrichtian) to diapirism since the KPgB (66.043 Ma) until end-Paleocene. As main drivers degassing (CO2, SO2 et al.), temperature rise, acid rain/metal toxicity of both volcanic and impact origin caused kill effects by acidification (pH-drop) of sea water resp. dissolution processes on land initiated by complex acid mixtures onto both marine and continental sediments; all in all, leaving a remarkable reduction of the clastics’ primary mineral content, accompanied by kaolinite in-situ neoformation (quartz, kaolinite). Furthermore, driving effects even controlled lithofacies and sequence-analytical patterns (LST, TST, HST). Around the Lower/Upper Maastrichtian B. (MFS) radiolarian ooze was deposited across flat estuarian mouth channels during an ingression (tsunami), originally as soft pebbles, then diagenetically slightly consolidated. Surprisingly, the radiolarian skeletons normally composed of opal or celestite, were identified as β-quartz and elementary silicon. The latter hitherto unknown in nature, demands extreme reducing conditions (in Industries: by elementary A1, Mg, C) in pore water as possibly given by brines (see Atlantis II-Deep, Red Sea). The top portions of the uppermost Maastrichtian deposits of N Germany were eroded by the KPgB-convulsive events. However, recent publications (i.e. from Seymour Island, Antarctica) make evident that Deccan volcanism played obviously a prime role versus the Chicxulub impact during reversal magnetization (Chron 29 R). Thus, there exists a high probability that plume volcanism had important influence on the quartz arenite in-situ formation by degassing and related acid in combination with brines in trap position of the ascending salt diapirs. Accordingly, Price’s concept (2001) major impacting may cause plate motion, has to be modified towards the version plume mechanism and may have the same or even stronger effect, thereby relating to recent studies on the Arabian Platform, Jordan. A synopsis of Phanerozoic loss of biodiversity-events caused by both plume volcanism and impacting comparatively exposes Homo sapiens since the Industrial Revolution as a geological force in biotic as well as abiotic processes in Earth History.

Triggers of sand remobilization in deep marine deposits

Abstract Sand injections are overrepresented in deep marine deposits and other settings with slope instabilities. We have examined seismic data from the North Sea to reveal their main trigger mechanisms. This was performed by inspection of local seismic observations in the Oligocene to Miocene sediments above the Utsira High and analyses of regional distribution of mounds at the top Hordaland Group surface in the northern North Sea. We make the observations that (1) the uplift of jack-up folds is in most cases larger than the sand thickness within the mounds, (2) onlap to mounds is present in periods with sand deposition, but no such onlap is seen in mudstones that were deposited in periods with little or no sand input, (3) onlapping to several Oligocene surfaces is seen above the Utsira High, (4) rim synclines are not present adjacent to Oligocene and Miocene mounds and (5) mounds that affect the top Hordaland Group surface are present in basin flanks and at basin-flank transitions but not in basin centres. We suggest that triggering by incipient slab sliding can explain these observations.

Sand pipe formation by liquefaction and sand injection: examples from Kodachrome Basin State Park, Utah, USA

Abstract The Kodachrome pipes were developed in a porous-sand host medium which is very unusual. The sub-vertical cylindrical sand pipes have previously been interpreted as injection structures with material derived from source beds near the base of the pipes. However, the large pipe formation (diameter >4 m) was probably caused by the disruption and partial liquefaction of the host sediments which have been broken into blocks by upward fluid streaming, with injection of only small amounts (<20% of pipe volume) of fluidized sand, derived from a source below. The strongest evidence for this mechanism is the sub-horizontal layering of large tabular blocks up to 4 m in length, which occurs in the largest pipes, and the preservation of traces of bedding extending from the host rock across the pipes. There is also a notable absence of rim synclines and upturned bedding around most of the pipes, which would be expected with local removal of highly viscous material containing large blocks derived from a source bed below the pipes.

Between plate and salt tectonics—New stratigraphic constraints on the architecture and timing of the Dead Sea basin during the Late Quaternary

AbstractThe Dead Sea is an extensional basin developing along a transform fault plate boundary. It is also a terminal salt basin. Without knowledge of precise stratigraphy, it is difficult to differentiate between the role of plate and salt tectonics on sedimentary accumulation and deformation patterns. While the environmental conditions responsible for sediment supply are reasonably constrained by previous studies on the lake margins, the current study focuses on deciphering the detailed stratigraphy across the entire northern Dead Sea basin as well as syn and post‐depositional processes. The sedimentary architecture of the late Quaternary lacustrine succession was examined by integrating 851 km of seismic reflection data from three surveys with gamma ray and velocity logs and the stratigraphic division from an ICDP borehole cored in 2010. This allowed seismic interpretation to be anchored in time across the entire basin. Key surfaces were mapped based on borehole lithology and a newly constructed synthetic seismogram. Average interval velocities were used to calculate isopach maps and spatial and temporal sedimentation rates. Results show that the Amora Formation was deposited in a pre‐existing graben bounded by two N‐S trending longitudinal faults. Both faults remained active during deposition of the late Pleistocene Samra and Lisan Formations—the eastern fault continued to bound the basin while the western fault remained blind. On‐going plate motion introduced a third longitudinal fault, increasing accommodation space westwards from the onset of deposition of the Samra Formation. During accumulation of these two formations, sedimentation rates were uniform over the lake and similar. High lake levels caused an increase in hydrostatic pressure. This led to salt withdrawal, which flowed to the south and southwest causing increased uplift of the Lisan and En Gedi diapirs and the formation of localized salt rim synclines. This induced local seismicity and slumping, resulting in an increased thickness of the Lisan succession within the lake relative to its margins. Sedimentation rates of the Holocene Ze'elim Fm were 4–5 times higher than before. The analysis presented here resolves central questions of spatial extent and timing of lithology, deposition rates and their variability across the basin, timing of faulting at and below the lake floor, and timing and extent of salt and plate tectonic phases and their effect on syn and post‐depositional processes. Plate tectonics dictated the structure of the basin, while salt tectonics and sediment accumulation were primarily responsible for its fill architecture during the timeframe examined here.

Salt tectonics and its effect on the structural and sedimentary evolution of the Jeffara Basin, Southern Tunisia

The Jeffara Basin, as part of the North African passive margin, provides a specific type of salt-wall diapirism in Tunisia. A comprehensive study, integrating seismic, gravity, drill wells and surface geological data is performed to model the induced salt geometry and delineate its kinematics. Salt bodies, originated from the Triassic Norian series, are characterized by chaotic seismic response, higher seismic velocity and gravity mass deficit. The architecture of the marine Jeffara Basin results from the complex evolution initiated by Tethyan Ocean rifting.The reconstructed palinspastic sections together with analogic halokinetic models show the successive stages of salt movement and the development of peripheral rim synclines and associated minibasins. Arid climate Triassic developed prolific upper intertidal to supratidal coastal sabkhas bordering the Tethyan Ocean enabled the deposition of thick Triassic salt within the marine Jeffara Basin. During the Early Hettangian extensional phase, associated with the opening of the Tethys Ocean, initiated a series of normal faults. Onlap reflection terminations of Late Hettangian horizons indicate the early Triassic salt rising activity. Large depocenters occurred during Santonian to Eocene; their lateral migration indicates the main active period of salt. Salt pierced the entire sedimentary cover, reached advanced salt diapir stage and set up salt walls during the Cenozoic period. Salt movement mechanism is dictated by the combined effect of tectonism and salt rheology. Salt dynamics controlled the sedimentation and the organization of depositional environments of sediments.Structural interpretation and modeling of the available seismic sections are of critical importance to get a better understating of the evolution of salt structures within the study area. Salt pillows are located at the central part of the basin. To the northeastern part, where thick salts accumulate exhibits the most mature and extended salt-walls. These geometries have an important impact on petroleum exploration.

Polyphasic evolution of the Jeffara basin in southern Tunisia, influence of halokinesis on the passive margin structuration in the Mesozoic and the Cenozoic

During the Mesozoic and Cenozoic rifting, the Pelagian Sea recorded the consequences of the African and European plate’s rapprochement. The interpretation of surface and subsurface data that is the 2D seismic reflection and petroleum well data show new ideas on the geodynamic evolution and halokinesis of the Jeffara basin during the Mesozoic and Cenozoic period. Seismic lines interpretations of the subsurface mainly reveal normal syn-sedimentary NW-SE faulting and where the Jeffara fault seems to be the major play. This syn-sedimentary faulting induced horst and graben structures materialized by major sedimentary sequences thicknesses as well as depths variations on the seismic profiles from the Jeffara fault zone overall towards the East of Jeffara basin. After the Hercynian event of the Permian - Carboniferous age, a general extension took place, which gave rise to the Tethyan opening. This extension has favored the individualization of the Jeffara basin in the South East of Tunisia, characterized by a structuring in Horst and Graben with a Permian carbonate subsidence. During the Triassic - Middle Jurassic period, the Jeffara basin is marked by a pronounced subsidence of essentially evaporate sedimentation accompanied by the birth of normal syn-sedimentary NW-SE faults following an NE-SW extension. This subsidence continuing during the Upper Jurassic period, the accentuation of which is towards the NE of the study zone at the Jerba and El Bibane sub-basin with a dominance of bioclastic limestone and dolomites sedimentation in the same extensive NE-SW direction, during this period, the Jeffara basin was characterized by a beginning of salt activity indicated by the appearance of salt nuclei at the base of the preexisting NW-SE normal faults. During the Lower Cretaceous, we are witnessing an individualization of salt complexes in the SE of the study area at Rass Ajil sub-basin, where this reactive diapirism has produced high zones and erosions in the crest above the salt bodies. During the mid Cretaceous period, the Zebbag formation, hatched by the Gattar carbonate bar, recorded a subsidence inversion phenomenon between the three sub-basins Jerba, El Bibane and Rass Ajil sub-basin, and showed the change of movement of the African plate relative to the European plate related to the opening of the North Atlantic and the beginning of the drift towards the North of Africa. This phenomenon is concretized towards the Upper Cretaceous, where we witness a strong subsidence towards the southeastern part of the study area at Rass Ajil sub-basin with sandstone, marl and clay sedimentation under a regional extensive regime and the individualization of high zones at Jerba sub-basin. The salt movements present an active aspect by piercing their cover and inducing rim synclines in the surroundings. The Cenozoic period is characterized by a strong subsidence of sandstone, clays and carbonates along the Jeffara basin, the salt activity shows a passive aspect at the beginning of this period which slows down and eventually stopped at the late Miocene period, thus indicating the probable exhaustion of the source of the salt material.

3D geometry of a shale-cored anticline in the western South Caspian Basin (offshore Azerbaijan)

The internal structure of one of the common fold structures in the western margin of the South Caspian Basin (SCB) has been characterized in 3D using a depth-migrated seismic cube in offshore Azerbaijan. The fold corresponds to a NNW-SSE anticline with a basinward vergence; i.e., eastwards, because in this direction of the margin the SCB is floored by a probable oceanic crust. The anticline has two culminations cut by mud-diapirs and is bounded by two parallel rim synclines with contrasting sedimentary thickness. This anticline deforms congruently the thick Productive Series (PS; Messinian to Late Pliocene), whereas the most recent sequences (<3.1 Ma; e.g., Akchagyl and Apsheron units) onlap or thin towards the fold crest.We reconstruct the existence of two episodes of folding. During deposition of the uppermost PS sequences (ca. 3.5–3.4 to 3.1 Ma), fold uplift initiated with a significantly lower rate than sedimentation. During this epoch, folding was accompanied by basin tilting and by faulting in a basinward normal fault with a limited right lateral, strike-slip component. Motion along this fault zone promoted the downdip flow of a weak layer formed by fluid- and mud-rich sediments (Maykop Formation), which also migrated along strike to build-up the growing anticline. Henceforth, fold growth accelerated and sedimentary units like the Akchagyl (3.1–1.7 Ma) were deposited preferentially in the subsiding flanks. Seafloor upwarping due to folding conditioned the sediment transport, and large deltas adapted their prograding pattern to the growing anticline crest.This structure resembles a detachment fold with a leading, East-vergent forelimb. Nevertheless, the occurrence of progressive tilting accompanying sedimentation and folding, or the mud inflation of the fold core by deep flow parallel to the anticline axis, make this example in the SCB a special example of this fold type.

Response of salt structures to ice-sheet loading: implications for ice-marginal and subglacial processes

During the past decades the effect of glacioisostatic adjustment has received much attention. However, the response of salt structures to ice-sheet loading and unloading is poorly understood. Our study aims to test conceptual models of the interaction between ice-sheet loading and salt structures by finite-element modelling. The results are discussed with regard to their implications for ice-marginal and subglacial processes. Our models consist of 2D plane-strain cross-sections, which represent simplified geological cross-sections from the Central European Basin System. The model layers represent (i) sedimentary rocks of elastoplastic rheology, (ii) a viscoelastic diapir and layer of salt and (iii) an elastoplastic basement. On top of the model, a temporarily variable pressure simulates the advance and retreat of an ice sheet. The durations of the individual loading phases were defined to resemble the durations of the Pleistocene ice advances in northern central Europe. The geometry and rheology of the model layers and the magnitude, spatial distribution and timing of ice-sheet loading were systematically varied to detect the controlling factors. All simulations indicate that salt structures respond to ice-sheet loading. An ice advance towards the diapir causes salt flow from the source layer below the ice sheet towards the diapir, resulting in an uplift of up to +4 m. The diapir continues to rise as long as the load is applied to the source layer but not to the crest of the diapir. When the diapir is transgressed by the ice sheet the diapir is pushed down (up to −36 m) as long as load is applied to the crest of the diapir. During and after ice unloading large parts of the displacement are compensated by a reversal of the salt flow. Plastic deformation of the overburden is restricted to the area immediately above the salt diapir. The displacements after unloading range between −3.1 and +2.7 m. Larger displacements are observed in models with deep-rooted diapirs, thicker ice sheets, longer duration of the loading phase, thicker salt source layers and lower viscosity of the salt. The rise or fall of diapirs triggered or amplified by ice-sheet loading are likely to affect glacigenic deformation, erosion and deposition above the diapir and within the rim synclines. Ice-load induced uplift in front of an ice sheet will provide favourable conditions for the formation of push moraines, for example by creating a topographic obstacle and inclining potential detachments. Subglacial subsidence of salt structures will enhance erosion by providing a preferential drainage pathway and fracturing of the overburden of the salt structure and thereby contribute to the incision of tunnel valleys. However, the resulting displacements are probably too low to have a marked effect on the advance or retreat pattern of the ice sheets.

Influence of the halokinesis on the clay mineral repartition of upper Maastrichtian–Ypresian in the Meknassy-Mezzouna basin, centerwestern Tunisia

The Meknassy-Mezzouna basin is affected by a fault system, assembling two main directions, northsouth and eastwest. The Triassic outcrops are widely noticeable at Jebel Jebbes El Meheri and the Mezzouna link. During the late Maastrichtian–Ypresian, the sedimentation in the basin is influenced by halokinetic events, which are clearly manifested either by the thickness of El Haria formation along the Triassic outcrops (rim syncline) or by an alteration surface at the top of the Abiod formation. Such events also confirm the emersion of the basin from the late Maastrichtian to the early Lutetian. However, the present work tries to highlight the effects of halokinetic uplift on the clay mineralogical variations at that area. In harmony with this halokinetic activity, the clay minerals of this time interval (during the late Maastrichtian–Ypresian) show a trend of variation which corresponds to the evolution from illite and kaolinite, indicating a strongly hydrolytic marine environment, to smectite, sepiolite, and palygorskite, reflecting a very rapid evolution from this marine environment to a lagoon environment, then to a more confined continental environment. In fact, the appearance of sepiolite and palygorskite on the top of El Haria formation and the Paleocene–Eocene transition can be explained by a transformation or neoformation mechanism in an alkaline environment, rich in silica and magnesium, under arid to semi-arid climatic conditions. Moreover, the palygorskite can be formed in continental deposits as well as in close marine environment, which displays a limited communication with the open sea. Under these conditions, the evaporation leads to high ionic concentration of alkaline pH, which is favorable to the formation and stability of this mineral.

Sequence stratigraphy of a Paleogene coal bearing rim syncline: interplay of salt dynamics and sea‐level changes, Schöningen, Germany

AbstractSalt rim synclines contain important hydrocarbon and coal resources in central Europe. The Schöningen salt rim syncline is filled with &gt;300 m of Early to Middle Eocene unconsolidated clastics with interbedded lignitic coal seams that are mined at the surface. In this study, 357 lithologic logs are integrated with measured outcrop sections and paleo‐botanical data to interpret the depositional environments and sequence stratigraphic framework of the rim syncline fill. As salt withdrew, it generated an elongate mini‐basin that mimicked an incised valley. The sustained accommodation and slow broadening of the syncline affected the stratigraphic architecture and contributed to the preservation of coal units. The clastic units in the syncline filled in seven depositional stages: (1) tidally influenced fluvial estuarine channels; (2) mixed tide‐ and wave‐ dominated estuaries; (3) prograding wave dominate deltas; (4) transgressive shoreline deposits; (5) braided fluvial channels; (6) estuaries; and (7) prograding tide‐dominated channels. The succession defines four 3rd order sequences and several higher order sequences that are possibly related to Milankovitch cycles. The higher order sequences are dominantly characterized by stacked transgressive cycles of thick, lowstand coals overlain by estuarine sands. The nearly continuous warm and wet Eocene climate was conducive to continuous peat production with a climatic overprint recorded in the mire type: ombrotrophic mires developed in wetter times and rheotrophic mires developed in relatively drier conditions pointing to the presence of orbitally controlled seasonality. Both mire types were impacted by the interplay of subsidence and base‐level. The continuous dropping of the mires below base‐level via subsidence protected the mires against erosion and may account for the absence of coals outside of the rim synclines in the region.

Controls on fluvial sedimentary architecture and sediment‐fill state in salt‐walled mini‐basins: Triassic Moenkopi Formation, Salt Anticline Region, SE Utah, USA

AbstractThe Triassic Moenkopi Formation in the Salt Anticline Region, SE Utah, represents the preserved record of a low‐relief ephemeral fluvial system that accumulated in a series of actively subsiding salt‐walled mini‐basins. Development and evolution of the fluvial system and its resultant preserved architecture was controlled by the following: (1) the inherited state of the basin geometry at the time of commencement of sedimentation; (2) the rate of sediment delivery to the developing basins; (3) the orientation of fluvial pathways relative to the salt walls that bounded the basins; (4) spatially and temporally variable rates and styles of mini‐basin subsidence and associated salt‐wall uplift; and (5) temporal changes in regional climate. Detailed outcrop‐based tectono‐stratigraphic analyses demonstrate how three coevally developing mini‐basins and their intervening salt walls evolved in response to progressive sediment loading of a succession of Pennsylvanian salt (the Paradox Formation) by the younger Moenkopi Formation, deposits of which record a dryland fluvial system in which flow was primarily directed parallel to a series of elongate salt walls. In some mini‐basins, fluvial channel elements are stacked vertically within and along the central basin axes, in response to preferential salt withdrawal and resulting subsidence. In other basins, rim synclines have developed adjacent to bounding salt walls and these served as loci for accumulation of stacked fluvial channel complexes. Neighbouring mini‐basins exhibit different styles of infill at equivalent stratigraphic levels: sand‐poor basins dominated by fine‐grained, sheet‐like sandstone fluvial elements, which are representative of nonchannelised flow processes, apparently developed synchronously with neighbouring sand‐prone basins dominated by major fluvial channel‐belts, demonstrating effective partitioning of sediment route‐ways by surface topography generated by uplifting salt walls. Reworked gypsum clasts present in parts of the stratigraphy demonstrate the subaerial exposure of some salt walls, and their partial erosion and reworking into the fill of adjoining mini‐basins during accumulation of the Moenkopi Formation. Complex spatial changes in preserved stratigraphic thickness of four members in the Moenkopi Formation, both within and between mini‐basins, demonstrates a complex relationship between the location and timing of subsidence and the infill of the generated accommodation by fluvial processes.

Basin modelling of a lignite‐bearing salt rim syncline: insights into rim syncline evolution and salt diapirism in NW Germany

AbstractThe Helmstedt‐Staßfurt salt wall is 70 km long, 6–8 km wide and one of the most important diapiric structures in northern Germany, based on the economically significant lignite‐bearing rim synclines. The analysed Schöningen rim syncline, located on the southwestern side of the Helmstedt‐Staßfurt structure, is 8 km long and 3 km wide. The basin‐fill is up to 366 m thick and characterized by 13 major lignite seams with thicknesses between 0.1 and 30 m. The key objectives of this article were to expand on the classical cross‐section based rim syncline analysis by the use of 3D models and basin simulations. Cross‐sections perpendicular to the basin axis indicate that the basin‐fill has a pronounced lenticular shape. This shape varies from more symmetric in the NW to clearly asymmetric in the SE. Isopach maps imply a two‐fold depocentre evolution. The depocentre migrated over time towards the salt wall and also showed some distinct shifts parallel to the salt wall. The basin modelling part of the study was carried out with the software PetroMod®, which focused on the burial history of the rim syncline. Modelling results also show the progressive migration of the rim syncline depocentre towards the salt wall. The present‐day asymmetry of the basin‐fill was already developed in the early phases of rim syncline evolution. The extracted geohistory curve shows initial rapid subsidence between 57 and 50 Ma and more moderate subsidence from 50 to 34 Ma. This pattern is interpreted to reflect salt evacuation from the source layer into the salt wall. The initial salt‐withdrawal rate was rapid, but later decreased probably due to depletion of the source layer.

Assessment of the present-day thermal field (NE German Basin)—Inferences from 3D modelling

We use a refined 3D structural model based on an updated set of observations to assess the thermal field of Brandenburg. The crustal-scale model covers an area of about 250 km (E–W) times 210 km (N–S) located in the Northeast German Basin (NEGB). It integrates an improved representation of the salt structures and is used for detailed calculations of the 3D conductive thermal field with the finite element method (FEM). A thick layer of mobilised salt (Zechstein, Upper Permian) controls the structural setting of the area. As salt has a considerably higher thermal conductivity than other sediments, it strongly influences heat transport and accordingly temperature distribution in the subsurface. The modelled temperature distribution with depth shows strong lateral variations. The lowest temperatures at each modelled depth level occur in the area of the southern basin margin, where a highly conductive crystalline crust comes close to the surface. In general, the highest temperatures are predicted in the north-western part of the model close to the basin centre, where rim syncline deposits around the salt domes cause insulating effects. The pattern of temperature distribution changes with depth. Closely beneath the salt, the temperature distribution shows a complementary pattern to the salt cover as cold spots reflect the cooling effect of highly conductive salt structures. The predicted temperatures at depths beneath 8 km suggest that the influence of the salt is not evident any more. Similar to the temperature distribution, the calculated surface heat flow shows strong lateral variations. Also with depth the variations in thermal properties due to lithology-dependent lateral heterogeneities provoke changing pattern of the heat flow. A comparison with published heat flow and temperature data shows that the model predictions are largely consistent with observations and indicates that conductive heat transport is the dominant mechanism of heat transfer. Local deviations between modelled and observed temperatures are in the range of ±10 K and may be due to the convective heat transport. To assess the potential influence of convective heat transport we zoom in on a specific location of Brandenburg corresponding to the in-situ geothermal laboratory Groß Schönebeck. This local model is used to carry out 3D numerical simulations of coupled fluid flow and heat transfer processes. Our coupled models indicate that conduction is the dominant heat transfer mechanism below Middle Triassic strata. Above the Triassic Muschelkalk, the more than 3000 m of sediments with higher hydraulic conductivity promote the formation of convection cells. Here, especially high degrees of coupling result in remarkable convective heat transport.

The waning stage of a greenstone belt: The Mesoarchaean Mosquito Creek Basin of the East Pilbara, Western Australia

The Mosquito Creek Basin stands apart in the Archaean granite–greenstone terrane of the east Pilbara Craton because of the rectangular outcrop but crescent-shaped subcrop, and its composition of low-grade metamorphic mainly turbiditic clastic sedimentary rocks. These sediments belong to the c. 2.9 Ga Mosquito Creek Formation. Tectonic interpretations of the basin have varied from a simple synclinorium, to an accretionary prism, an extensional passive margin, and more recently into a rift basin separating the Kurrana Terrane from the East Pilbara Terrane (EPT). Combined sedimentological and structural–geological investigations show coarse-clastic fan-delta marginal facies with stacked unconformities, rimming an E–W basin centre with predominantly finer-grained mass flow-dominated sediments. A N–S traverse therefore contains a complete cross-section of the Mosquito Creek Basin, shortened to about 50% of its original width by tight folding and thrusting. The basin fill shows one major FU sequence from fan-delta sandstone to distal turbidites, unconformably overlying an older, partially preserved clastic sequence of comparable facies. A large slump unit occupies the basin axis. The unconformities along the margins interfere with folding of the sediments which also influenced basin-centre turbiditic palaeocurrents. The basin is interpreted as a near-symmetrical, underfilled, compressional, intramontane, clastic sedimentary basin unconformably superposed on a volcanic greenstone belt rock assemblage with enclosed low-angle shear zones. Compression started during sedimentation under conditions of net subsidence within a rim syncline between adjacent domes. This occurred after phases of low-angle extensional and compressional deformation of the substrate not related to the present-day geometry of the granitoid domes. The geological setting of the Mosquito Creek Basin does not differ fundamentally from that of other adjacent E–Woriented greenstone belts of the EPT. The basin is inferred to represent the last, stratigraphically uppermost, and because of subsequent erosion rarely preserved, stage of greenstone belt evolution.

A 3D model of first-order structural elements of the Vredefort Dome, South Africa — Importance for understanding central uplift formation of large impact structures

Based on detailed structural mapping, geophysical information and structural consideration, we generated a 3D structural model of a segment of the Vredefort Dome. This model provides insight into the mechanism of mass transfer during central uplift formation of large impact structures. The model consists of geometrically and kinematically linked km-scale radial and concentric faults that affected the overturned limb of the rim syncline to the Vredefort Dome. The faults accomplished constrictional (centripetal) rock flow followed by radial spreading of uplifted and gravitationally unstable rocks in the crater centre. More specifically, concentric faults formed likely as normal faults during transient crater rim collapse, steepened and were transformed to reverse faults during central rock uplift and finally, were overturned during gravitational collapse of the central uplift. Radial faults, by contrast, formed at a later stage of convergent rock flow towards the crater centre and retained largely their orientation during central uplift formation. Displacement on individual faults is up to 2 km. Cumulative displacement, notably on concentric faults, can account for the large displacement magnitudes predicted by numerical models.

Magmatic diapirism of the Fangshan pluton, southwest of Beijing, China

Diapirism as a crustal magma ascent and emplacement mechanism has been increasingly questioned due to lack of unambiguous natural examples. In this paper we report the structure of the Cretaceous Fangshan pluton in the hinterland of the North China Craton, ca. 40 km southwest of Beijing, China. This pluton is a compositionally and texturally concentrically zoned granodiorite with an approximately circular shape. Our detailed structural and petrological investigations reveal a high-temperature shear aureole with pluton-side-up kinematic indicators and a rim syncline around the pluton. In addition, an intensely deformed Archean complex occurs at the northern and southern margins of the pluton requiring at least 4 km upward movement through the 4–5 km thick overlying cover sequence during magma ascent. Based on comparison of stratigraphic units inside and outside the aureole, the bulk wall-rock shortening by ductile flow is estimated to be about 4 km, which is roughly the same as the radius of the pluton. These features are consistent with results of numerous modeling studies and diagnostic criteria of magmatic diapirism, indicating that the Fangshan pluton is a magmatic diapir intruded into a ductile upper crust.