Stratigraphic Geometries Research Articles

Structural Variability and Rifting Process of the Segmented Cenozoic Pearl River Mouth Basin, Northern Continental Margin of the South China Sea

AbstractThe Pearl River Mouth Basin (PRMB) is an important area for studying the evolution of continental marginal basins in the northern South China Sea (SCS), but the structural variability and spatiotemporal rifting process remains poorly understood. This study investigates the differential structural features of the eastern, middle and western PRMB, as well as the extensional deformation laws in operation during the rifting stage, according to an integrated analysis of geometric characteristics and kinematic parameters, i.e., horizontal displacement and stretching factors of basin and crust. The PRMB underwent at least three phases of intense extension, which varied in time and space. (1) During the middle Eocene, most sags in the PRMB were intensely stretched and high‐angle planar to listric boundary faults controlled the wedge‐shaped stratigraphic geometry. (2) During the late Eocene‐to‐early Oligocene, the stratigraphic geometry of the sags was slightly wedge‐shaped and continuously controlled by boundary faults, however, the extensional strength decreased relatively in the Northern depression zone, but increased in the Southern depression zone. (3) During the late Oligocene, the extension was extremely weak in the northeast PRMB, but relatively strong in the southwest PRMB, leading to tabular stratigraphic geometry in the northeast PRMB, but localized slightly wedge‐shaped stratigraphic geometry in the southwest. The southwest PRMB still underwent relatively strong extension during the early Miocene. The southwest PRMB that was induced by a small‐scale localized mantle convection system constantly rifted during the late Oligocene, controlled by the weak lithosphere, westward (southwestward) diachronous opening and southward jump of the ocean ridge. The applied quantitative parameters and spatiotemporal rifting process may be used as a reference with which to study the segmented continental margin rifts.

The role of flat slab subduction, ridge subduction, and tectonic inheritance in Andean deformation

AbstractConvergent plate boundaries show sharp variations in orogenic width and extent of intraplate deformation. Analysis of late Cenozoic contractile deformation along the Andean mountain front and adjacent foreland highlights the contrasting degrees of deformation advance toward the plate interior. The retroarc positions of the Andean topographic front (marked by frontal thrust-belt structures) and foreland deformation front (defined by isolated basement block uplifts) range from 300 to 900 km inboard of the trench axis. Over the ~8000 km arcuate length of the Andes (10°N to 55°S), four discrete maxima of inboard deformation advance are spatially co-located with the Peruvian (5°S–14°S) and Pampean (27°S–33°S) zones of flat slab subduction, the subducted Chile Ridge (45°S–48°S), and the anomalously thick Paleozoic stratigraphic wedge of Bolivia (17°S –23°S). The spatial correspondence of retroarc shortening with specific geodynamic configurations demonstrates the mechanical role of flat slab subduction, slab window development, and combined structural and stratigraphic geometries in shaping the orogenic architecture of Cordilleran margins, largely through lithospheric strengthening, weakening, and/or tectonic inheritance.

Depositional Architecture of Eocene and Cretaceous Reservoirs (Blending Seismic and Well Data) in Mubarik Area, Central Indus Basin, Pakistan

Structural and stratigraphic geometries in the Central Indus Basin of Pakistan are subjects of interest for explorers. The thickening and thinning sequences in conjunction with structural anomalies often provide an environment for hydrocarbon accumulation. Hydrocarbons play a vital role in the energy mix of Pakistan. With the help of well correlation, work on seismicity can be confirmed by marking terminations, depositional extent and thickness variation at different stratigraphic levels. Therefore, this work presents depositional geometries of Eocene and Cretaceous reservoirs [Sui Main Limestone (SML) and B-sand interval of Lower Goru Formation], keeping in view the vertical and lateral resolution of the seismic data and the distance between the wells for correlation. The Eocene SML and Cretaceous B-sand are extensively mapped as isochrones using 20 seismic lines and correlated with ten wells. The results give evidence of a building up of shallow marine shelf carbonate in Eocene SML. The isochrone maps also indicate that Eocene SML has untapped potential east of the study area. The Cretaceous B-sand demonstrates sheet-like geometry of sand-filled delta progrades in the west to southwest direction in a ramp setting. This geometry suggests an updip hydrocarbon potential in Cretaceous B-sand reservoir in the north of the study area. Because of the pinching out behavior and excellent stratigraphic trapping system, these two reservoirs can be considered future exploration targets.

Evidence of decoupled deformation during Jurassic rifting and Cenozoic inversion phases in the salt-rich Corbières-Languedoc Transfer Zone (Pyreneo-Provençal orogen, France)

A detailed field study of Jurassic tectono-stratigraphic architecture of the southwestern part of the Corbières-Languedoc Transfer Zone (CLTZ, NE-SW oriented), between the Pyrenean and Provençal orogenic segments (N110 oriented) in SE France, document for the first time variations in thickness and stratigraphic geometries in relation to oblique extensional cover structures (NE-SW and N110 oriented). These structures (low-dip normal faults, salt rollers, rollovers, forced folds) formed during a Jurassic extension phase with strong decoupling between basement and cover on the Keuper pre-rift salt (Carnian-Norian evaporites). Some of these structures such as the Treilles Fault, the Valdria and the Terres Noires fold pairs, were previously interpreted as compressional and Pyrenean in origin (Late Santonian-Bartonian). Our study instead shows that these are Jurassic extensional and salt related structures, which were later affected by Pyrenean compression and Oligo-Miocene extension. Evidence of Jurassic extension is still observable in the field despite later rectivations, making these good field analogs for gently inverted extensional salt structures. During the Jurassic the interference between oblique structures above Keuper, leads to the formation of three dimensional growth strata observable at kilometric scale. Despite the mechanical decoupling effect of Keuper, we infer that their formation was linked to a strong interaction between oblique basement structures (NE-SW and N110) as the area lies during the Jurassic at the intersection between the eastern part of the Pyrenean E-W trending rift system and the NE-SW trending European margin of the Alpine Tethys rift. The decoupled deformation and the strong segmentation above and below Keuper detachement are here identified as Jurassic structural inheritence for later tectonic events, notably for the Nappe des Corbières Orientales emplaced at the end of Pyrenean orogenesis. Associated to previous works, this study highlights that the CLTZ is a key area to better understand Pyreneo-Provençal system evolution along its whole Wilson cycle and to better understand the processes that govern the formation of a salt-rich transfer zone in a strongly pre-structured crust, its multiple reactivations and the decoupling role of salt.

The Bakken Formation—understanding the sequence stratigraphic record of low-gradient sedimentary systems, shale depositional environments, and sea-level changes in an icehouse world

The Bakken Formation is a major petroleum producer in the continental US. However, its deposition in an intracratonic, low-gradient setting has often been mistakenly described as “layer-cake”. This contribution is designed to highlight the time-transgressive nature of its main petroleum-producer, the middle Bakken member. Correlation of individual parasequences reveal the subtle nature of otherwise invisible low-angle stratigraphic geometries. Sequence stratigraphically-relevant surfaces occur throughout the unit and subdivide the entire Bakken into 5 third-order sequences; one of them is a hidden sequence at the base of the petroleum-producing middle Bakken indicating both a lowstand and a subsequent transgression. The organic-rich shales above and below the middle Bakken were deposited in an oxygen-deficient environment and show several burrow/fecal string types and indications of active currents during deposition. The Bakken records high amplitude sea-level changes during sequences compared to relative low amplitude sea-level changes of parasequences. This, coupled with a likely mismatch in timing of Bakken deposition relative to world-wide ice-age-induced cyclicity makes it unlikely that the Bakken sea-level fluctuations were dominated by glaciation.

Classifying basin‐scale stratigraphic geometries from subsurface formation tops with machine learning

AbstractPresented here is a transfer‐learning model for classifying basin‐scale stratigraphic geometries from subsurface formation tops. Support vector, decision trees, random forests, AdaBoost and K‐nearest neighbour classification models are evaluated to support this challenge. Each model is trained on labelled synthetic stratigraphic geometry data generated in Python using observable geological principles and concepts. Accuracy is measured using a weighted Jaccard similarity coefficient score, and certainty of each prediction is quantified using margin sampling. The random forest classifier has the highest initial accuracy, and the optimal hyperparameters for the model that yield 88.4% accuracy and 72.8% mean certainty via five‐fold cross‐validation and active learning are documented on a real‐world subsurface dataset. The random forest classifier with optimised hyperparameters is then used to make predictions on the real‐world subsurface formation tops dataset. The dataset consists of formation tops for the Upper Cretaceous and Palaeocene strata of the Eastern Greater Green River Basin in south‐central Wyoming. Results from model predictions include an area of truncation in the Lance Formation across the basin, and an area of onlap and truncation on the nose of the Rock Springs Uplift that previous studies in the region corroborate. It is believed that this model is most useful for guided interpretation, and identifying regions that warrant further inquiry by domain experts.

Linking continental erosion to marine sediment transport and deposition: A new implicit and O(N) method for inverse analysis

The marine sedimentary record contains unique information about the history of erosion, uplift and climate of the adjacent continent. Inverting this record has been the purpose of many numerical studies. However, limited attention has been given to linking continental erosion to marine sediment transport and deposition in large-scale surface process evolution models. Here we present a new numerical method for marine sediment transport and deposition that is directly coupled to a landscape evolution algorithm solving for the continental fluvial and hillslope erosion equations using implicit and O(N) algorithms. The new method takes into account the sorting of grain sizes (e.g., silt and sand) in the marine domain using a non-linear multiple grain-size diffusion equation and assumes that the sediment flux exported from the continental domain is proportional to the bathymetric slope. Specific transport coefficients and compaction factors are assumed for the two different grain sizes to simulate the stratigraphic architecture. The resulting set of equations is solved using an efficient (O(N) and implicit) algorithm. It can thus be used to invert stratigraphic geometries using a Bayesian approach that requires a large number of simulations. This new method is used to invert the sedimentary geometry of a natural example, the Ogooué Delta (Gabon), over the last ∼5 Myr. The objective is to unravel the set of erosional histories of the adjacent continental domain compatible with the observed geometry of the offshore delta. For this, we use a Bayesian inversion scheme in which the misfit function is constructed by comparing four geometrical parameters between the natural and the simulated delta: the volume of sediments stored in the delta, the surface slope, the initial and the final shelf lengths. We find that the best-fit values of the transport coefficients for silt in the marine domain are in the range of 300−500 m2/yr, in agreement with previous studies on offshore diffusion. We also show that, in order to fit the sedimentary geometry, erosion rate on the continental domain must have increased by a factor of 6 to 8 since 5.3 Ma.

Three-dimensional geologic mapping to assess geothermal potential: examples from Nevada and Oregon

Geologic structure plays an important role in controlling fluid flow in geothermal systems. In particular, very complex structural settings, consisting of many closely spaced and intersecting faults, host many geothermal systems. To elucidate the key geologic factors that affect fault-controlled geothermal circulation, it is critical to precisely characterize the structural and stratigraphic geometries in these complex settings. Here, we present a methodology and the results of 3D geologic analyses of two geothermal systems in the Basin and Range, USA. This methodology is a quantitative and geologically focused technique that can be used to precisely characterize geothermal areas, in a time when future geothermal growth demands increased exploration precision and efficiency. Surficial and subsurface geologic and geophysical data are synthesized in the construction of detailed 3D geologic maps of geothermal areas. Based on these 3D geologic maps, we examine several geologic attributes that control permeability development and geothermal fluid flow along faults. We use the stress state of faults and the distribution of structural discontinuities (i.e., fault intersections and fault terminations) to identify locations of upflow along faults in these geothermal systems. These results and the methodology presented herein are directly applicable to structurally controlled geothermal fields in the Basin and Range and worldwide. As development focus shifts toward blind geothermal resources, integration of precisely characterized subsurface structural information into exploration methods will be increasingly critical to continued growth in geothermal exploration and development.

Mechanisms of muddy clinothem progradation on the Southwest Louisiana Chenier Plain inner shelf

In both modern and ancient shelf settings, mud-dominated successions commonly contain complex stratigraphic geometries in which low-gradient clinothems feature prominently. Despite their ubiquity, the full range of mechanisms responsible for sediment dispersal and clinothem progradation in such settings is not well understood. Using sediment core data (210PbXS, 137Cs, grain size, porosity, X-radiography) and shallow seismic observations, this study examines the mechanisms of across-shelf sediment transport and clinothem progradation on the muddy Southwest Louisiana Atchafalaya Chenier Plain inner shelf. Observations indicate that rapid transfer of organic matter-rich sediment to the outer topsets and clinothem rollover occurs mainly via hydrodynamic fluid-mud processes during times of high wave-current bed shear stress (e.g., during the passage of storms). Rapid sedimentation, wave perturbation, and the development of biogenic methane within the shallow seabed result in the generation of large internal pore water pressures such that the clinothem rollover and foreset sediments are inherently in a condition of incipient failure. Subsequent basinward sediment transfer to the foresets occurs largely in association with low-gradient (<0.02°) mass-failure events, evidenced by widespread scarping and mudflows on the seabed. These represent an important and as yet unattributed mechanism for clinothem progradation in the study area and are likely to drive basinward sediment transport in other muddy shelf clinothem systems, both modern and ancient.

Geometry and kinematics of Neogene allochthonous salt systems in the Mississippi Canyon, Atwater Valley, western Lloyd Ridge, and western DeSoto Canyon protraction areas, northern deep-water Gulf of Mexico

The structural history of the Mississippi Canyon, Atwater Valley, westernDeSoto, and western Lloyd Ridge protraction areas in the northeastern deep-water Gulf of Mexico is of a basin influenced by complex salt tectonics that controlled the formation of intraslope minibasins and sediment distribution. Large volumes of Middle Jurassic autochthonous salt (Louann Salt) were successively mobilized upward into younger sediments, forming three distinctive allochthonous salt layers: At the top Barremian, at the top Cretaceous, and within the Neogene interval. Four types of Neogene allochthonous salt systems are identified based on the geometry of the allochthonous salt bodies and associated faults, folds, and minibasins: (1) basement-controlled, (2) counterregional, (3) roho, and (4) fold-belt-related salt systems. The allochthonous salt systems are defined based on salt-body geometry, salt-stem geometry, associated fault network, and associated stratigraphic geometries. The distribution of the Neogene allochthonous salt systems is controlled by the original autochthonous salt thickness, the basement configuration, and the regional sediment-loading pattern. The basement-controlled Neogene salt systems are present in the eastern and northern part of the study area where little basinward gravitational gliding occurred. The counterregional and roho allochthonous salt systems are associated with the basinward evacuation of salt in response to extensive sediment loading. The fold-belt-related allochthonous salt systems are present in the southern part of the salt province where extensive shortening remobilized salt into and onto contractional structures. The detailed study of those Neogene allochthonous salt systems is used to build conceptual kinematic models for each style of salt system. Copyright © 2017 The American Association of Petroleum Geologists. All rights reserved.

Recent development in stratigraphic forward modelling and its application in petroleum exploration

In order to better predict the geometry and spatial distribution of reservoir facies and permeability barriers, it is critical to understand stratigraphic geometries and architecture at all scales within a basin. Stratigraphic forward modelling (SFM) allows geologists to forward project the deposition and evolution of sedimentary facies within a stratigraphic framework with given prior boundary conditions. The method can approximate depositional processes while taking into consideration a range of factors that affect basin evolution and sedimentation. This paper presents an overview of recent developments in stratigraphic forward models that are currently available and have been successfully employed in simulating stratigraphic development, including geometric models, diffusion models, Fuzzy Logic models and hydraulic models. In addition, the paper discusses SFM used in the simulation of the behaviour of natural sedimentary systems and verification of the potential for hydrocarbon entrapment and accumulation in a basin using an example from the Ordos Basin, western China.

3D structural and stratigraphic model of the Perth Basin, Western Australia: Implications for sub-basin evolution

The history of rifting and breakup of eastern Gondwana is recorded by the development and fill of the Perth Basin in Western Australia. Knowledge of the structural architecture and stratigraphic geometry of the Perth Basin is essential to understand the evolution of the Western Australian margin and its applications to hydrocarbon and geothermal prospects, and effective aquifer management. This study integrates existing, publicly available geological, gravity, magnetic and digital elevation data to develop the first refined, regional structural and stratigraphic interpretation of the entire onshore and offshore Perth Basin, Western Australia. This new 3D model offers formation depth and thickness predictions in areas of sparse or no data. The model shows significant heterogeneity in the preserved formation thicknesses and depths at both local and regional scales. These differences may have resulted from differential subsidence and/or differential exhumation, but the formation geometries alone cannot distinguish between these two models if any erosion has occurred. Only the Lower to Middle Jurassic stratigraphy has been minimally eroded and thereby records the net subsidence. This stratigraphic interval shows that subsidence was broadly hinged from south to north, with a greater subsidence rate in the southern and central Perth Basin. Localised differences in thicknesses across adjacent sub-basins were likely controlled by differential displacement along sub-basin bounding faults during subsidence and, subsequently, during exhumation episodes. This new 3D model of the entire Perth Basin provides a framework for numerical simulations of fluid and heat flow and large-scale tectonic analysis, such as stratigraphic forward modelling of the southwestern Australian margin.

Three-dimensional unified geological model of the Milk River Transboundary Aquifer (Alberta, Canada – Montana, USA)

The Milk River Transboundary Aquifer (Canada–USA) has been so intensively used over the 20th century that concerns have risen about the durability of this resource since the mid-1950s. This aquifer actually corresponds to the middle Virgelle Member of the Upper Cretaceous Milk River Formation (called Eagle Formation in Montana). To assess the conditions needed for a sustainable use of the aquifer, a comprehensive and unified portrait of the aquifer is needed across its international boundary. The stratigraphic framework and geometry of geological units on both sides of the international border were thus unified in a 50 000 km2three-dimensional (3D) geological model. The Virgelle Member is 0–60 m thick and it subcrops near the border and along both sides of the Sweetgrass Arch. It dips away from the subcrop areas in a semi-radial pattern. The Medicine Hat gas field hosted by the Alderson Member (Alberta), which is separated from the other members by a regional unconformity, and the Tiger Ridge gas field near the Bears Paw Mountains (Montana) limit the extent of the aquifer. The unified 3D geological model forms the necessary basis for conceptual and numerical hydrogeological models of the Milk River Aquifer.

Late Quaternary alluvial sequence and uplift-driven incision of the Urumqi River in the north front of the Tian Shan, northwestern China

This work focuses on the driving force behind the late Quaternary river incision and terrace formation of the Urumqi River in the north piedmont of the Tian Shan, northwestern China. Field investigations on geomorphic surfaces, terrace deposits, and its underlying bedrock identify four most significant features, which creates a local applicable framework for subdivision of the late Quaternary terrace sequence in the study area. Nine stepped river terraces are defined and designated as T1 to T9 increasing systematically in elevation. Morphologically, the highest T9 correlates with the oldest alluvial fan F1 of the Urumqi River. River incision and the resultant abandonment of fan F1 are chronologically constrained at ca. 550ka. The stratigraphic geometry of the Saerqiaoke anticline, a structure developing at the fan end of F1, reveals the existence of growth strata, implying continuous growth of this fold when the F1 alluvial sediments were deposited. In the range front of the Urumqi River, growth of the Saerqiaoke anticline has derived tectonically from uplift and basinward thrusting of the Tian Shan range. Such thrusting and basinward extension of the range are expected to force rock uplift of the headwater of the Urumqi River with respect to the Chaiwopu basin to the north and thus river incision occurring at ca. 550ka. During the subsequent period, several younger terraces have been formed in response to the further uplift of the Saerqiaoke anticline as well as climate changes during glacial–interglacial transitions. In the present study area, the total incision during Quaternary comes close to 400m, with about 85% contribution likely attributed to rock uplift of the Saerqiaoke anticline.

The Elements of Fold Style Analysis in the Khaftar Anticline, Zagros, Iran

The Study area is located in the Zagros Simply Folded Belt of Iran and in the interior Fars sub-basin. The Khaftar anticline is located in the West, North-West of Jahrom city in the Fars province (148 km distance from Persian Gulf). The trend of Khaftar anticline has three orientations, consist of North-Northeast, East-West and South-Southwest. This anticline has asymmetric structure and some faults with large strike separation observed in this anticline. In the study area, stratigraphic units are affected by many faults in this area. Also one salt plug cropping out in the middle part of the Khaftar anticline. Maybe this salt plug affected on the stratigraphic units and geometry of structure. Description of fold geometry is important because it allows comparisons within and between folds and allow us to recognize patterns in the occurrence and distribution of fold systems. The main aim of this paper is determination of fold style elements and folding pattern of the Khaftar anticline. This paper presents a part of the results of a regional study of the Fars province in the Zagros Simply folded belt, based on satellite images, geological maps, and well data. Some data, such as geological maps and geological regional data were prepared and provided by the National Iranian Oil Company (NIOC). Because of the Khaftar anticline, has complex structure, the analysis of fold style elements is seems necessary. Therefore, in further studies on this structure the changes of fold style elements will be analyzed and investigated from east to west in the different parts of this anticline. The activity of Nezamabad sinistral strike slip fault in the Khaftar anticline causes changes of axial plane characteristics and fold axis. Some of the results such as folding style analysis, how position of salt plug, changes of fold type and main structural changes (rotation of fold axis and 2.5 km displacement in this anticline) show main changes in the middle parts of the Khaftar anticline. It seems that, these changes have formed by activity of the Nezamabad fault and this fault’s activity same as fault zone.

High-Resolution Sequence Stratigraphic Division and Monosandbody Isochronic Correlation of Meandering River Delta of Fuyu Oil Layer in the Northern Honggang

By using the cores, drilling and logging data, Fuyu oil layer in the northern Honggang is divided into 2 long-term base level cycle, 4 medium-term base level cycles, 20 short-term base level cycles, and monosandbody high-resolution sequence strata framework is established, monosandbody isochronic correlation is achieved in Isochronic strata framework at the same time. Through high--resolution sequence stratigraphic analysis, sedimentary evolution, monosandbody distribution, sand geometry and distribution characteristics of reservoir are controlled by base-level changes in Fuyu oil layer of the northern Honggang. The lower Fuyu oil layer is dereliction prograding sequence stratigraphic modes in fall range of base-level. Space stacked form of the sand body is controlled by A/S ratio or base level changes. The symmetry of cyclic changes with the geographical location and distance from the sedimentary source.

MIDDLE EAST GEOLOGIC TIME SCALE 2013

ABSTRACT During the Aptian 28 to possibly 34 transgressive-regressive “fourth-order” sequences were deposited on the Arabian Plate. The sequences were controlled by sea-level fluctuations with a relative amplitude of 5–20 m. The fluctuations are interpreted as the glacio-eustatic response to orbital-forcing and assumed to have an average duration of 405 Kyr corresponding to the long-eccentricity orbital cycle. The sequences are referred to as “stratons” and calibrated in the orbital time scale of Matthews and Al-Husseini (2010, abbreviated M&amp;H-2010). An independent study by Huang et al. (2010) counted nearly 33 cycles of 405-Kyr in a deep-marine Aptian succession in the Piobicco core in central Italy. The Italian cycles and Arabian stratons can be correlated in GTS 2004 by the position and age of the oceanic anoxic event OAE1a (Selli Interval, ca. 124.5–123.1 Ma). Two lowermost Aptian stratons and at least nine upper Aptian ones show stratigraphic geometries that imply 40–50 m box-like drops in relative sea level. They provide evidence for the formation of an ice sheet, mainly in Antarctica, that held several 10s of meters sea-level equivalent. The ca. 5-Myr-long late Aptian drop started at Global SB Apt 5 (ca. 117.9 Ma), which correlates to a major eccentricity minimum predicted at 118.2 Ma in the M&amp;H-2010 scale. Similar minima are predicted to recur every 14.58 Myr (36 × 405 Kyr), and to cause major glacio-eustatic drops and regional sequence boundaries (SB). The youngest SB 0 is predicted at 1.586 Ma, and SB 8 (118.2 = 1.586 + 8 × 14.58 Ma) is interpreted to have triggered the late Aptian glaciation. The M&amp;H-2010 scale was tested against the high-resolution sea-level curve derived from benthic foraminiferal δ18O isotopes for the late Miocene to Holocene (9.25– 0.0 Ma, Miller et al., 2005, abbreviated Metal-2005). Antarctica’s glacio-eustatic signature is interpreted as high-frequency sea-level fluctuations with a period of 41 Kyr (obliquity) above -20 m relative to present-day sea level. The fluctuations ride up-and-down on longer-period sea-level cycles (transgression-regression) with amplitudes of 20–40 m. The cycles are bounded by prominent lowstands, have durations of 325–545 Kyr, and an average duration of 405 Kyr. Sequence Boundary SB 0 (predicted at 1.586 Ma) is interpreted at 1.54 Ma, and correlated to Calabrian Global sequence boundary Cala1 (1.54 Ma).

Shelf-edge deltas along structurally complex margins: A case study from eastern offshore Trinidad

A 15,000-km2 (5792-mi2) three-dimensional seismic data survey that covers the shelf-slope transition of the eastern offshore Trinidad continental margin reveals the geometry and depositional history of the last maximum glacial lowstand shelf-margin succession. Despite the lack of well information at these shallow depths, the quality and continuity of the seismic data allow us to pursue a detailed seismic stratigraphic interpretation of the last lowstand margin succession. The basin-fill stratal architecture of the studied stratigraphic interval shows a great deal of lateral and vertical variability along the continental margin during the Pleistocene to Holocene. Three geomorphological elements controlled the character of the accommodation within the basin and were crucial in transporting, delivering, and storing sediment supply from shelf to slope areas: (1) the Columbus sedimentary pathway on the shelf, (2) bypass zones in the shelf-break region, and (3) deep-water depocenters. The location and geometry of these geomorphological elements within the basin are clearly controlled by underlying structures, transpressional to the north and gravity driven to the south. Migration of the paleo-Orinoco delta system across the shelf was also a key factor in defining the stratigraphic geometries that are observed within the shelf break. Development of shelf-edge versus outer-shelf deltaic systems on the continental margin was controlled by the nature of sediment supply at specific times, as well as by the availability of accommodation, although, to a lesser extent, to relative sea level fluctuations. The interpretation also showed that, for time-equivalent units, parts of the shelf-edge region could develop as an erosional margin (sediment bypass zones), whereas other parts of the shelf edge could behave as an accretionary margin (sediment accumulation). The sequence-stratigraphic interpretation that was attempted in this work also demonstrated that the characteristics of systems tracts can abruptly change along strike in the shelf-edge region for time-equivalent units. These changes can be misleading if a genetic interpretation is pursued only on the basis of the definition of system tracts in the shelf-edge region without the consideration of a complete regional framework.

Migration of a synclinal depocentre from turbidite growth strata: the Annot syncline, SE France

AbstractThe Annot Sandstone turbidites of the Alpine foreland basin in SE France (Eocene-Oligocene: 40-32 Ma), provide an excellent case-study of tectono-sedimentary relations in a deepwater compressional system. The Annot outlier is a synclinal remnant previously interpreted as a static depocentre. A multi-disciplinary approach including geometrical and kinematic analyses and modelling demonstrates instead that this was a tectonically active turbidite depocentre where gentle thrust related folding controlled turbidite architecture.Stratigraphic and new structural field data are integrated with previous sedimentological studies to build a 3D geometric model of the Annot depocentre. Derived thickness maps associated with paleocurrent measurements clearly illustrate three main phases in the evolution of depocentre topography. (1) Early turbidite flows were mainly trapped by oblique intrabasinal inherited structures. (2) Once these structures were buried, the NNW-SSE active syncline constituted the main topographic control. (3) Decreasing activity of this syncline is recorded by filling and flow bypass. The progressive stages of the accepted depositional model (flow ponding, flow stripping and flow bypass), for the Annot depocentre, may therefore have a tectonic origin.The kinematic evolution of the synclinal depocentre was defined at different scales. Stratigraphic architecture records a decrease in bedding dips up through the turbidite succession on the western synclinal limb. Comparison with idealized case studies of the interaction of sedimentation with an active syncline indicates that this geometrical pattern corresponds to progressive westward migration of the synclinal hinge and depocentre. This tends to promote lateral rather than vertical stacking of sand bodies during turbidite sedimentation. Trishear kinematic modelling was used to simulate (in 2D) the rolling synclinal hinge. Stratigraphic surface geometries and turbidite depocentre migration define thrust and fold geometries at depth. The synclinal depocentre developed between two alternating or coeval fault propagation anticlines that exploited two detachment levels (Triassic evaporites and Cenomanian marls) in the underlying succession.

MIDDLE EAST GEOLOGIC TIME SCALE 2010: Early Cambrian Asfar Sequence

ABSTRACT This paper is the first in a series dedicated to the Phanerozoic Cambrian Period, and Neoproterozoic Ediacaran and Cryogenian periods, as represented in the Middle East Geologic Time Scale (ME GTS, see enclosed Chart). It introduces the term Asfar Sequence to represent a regional Early Cambrian time-rock unit, consisting mainly of continental quartz-rich arkosic sandstone, shale and siltstone, which attain a thickness of at least 750 m in Jordan and more than 700 m in Oman. The term “Asfar”, meaning yellow in Arabic, was chosen because it is the standard color for sandstone in ME GTS. To describe its stratigraphy, four representative formations are reviewed in lexicon format: Salib Arkosic Sandstone of Jordan, Siq Sandstone of Saudi Arabia, Amin Formation of Oman and Lalun Sandstone of Iran. The stratigraphic geometry of the lower boundary of the Sequence varies considerably by locality. In some regions in Iran it is conformable above the shales of the Zaigun Formation. In other regions, such as western Jordan, it is an onlap surface over Proterozoic and/or Lower and Middle Cambrian paleohighs, or a pronounced angular unconformity (e.g. central and southern Saudi Arabia). The paleo-relief represented by the unconformity surface, in many regions, forms a regional peneplain (e.g. central and eastern Jordan) implying erosion; in other paleohigh regions, the Sequence is absent by non-deposition. The age of the base Asfar Sequence is estimated at ca. 530 Ma, based on radiometric data and depositional rates in basinal areas. The top boundary of the Sequence, in Iran, Jordan, and northern and northeastern Saudi Arabia, is represented by a sequence boundary (or its correlative unconformity), above which marine, fine-grained siliciclastics and carbonates of late Early to early Mid-Cambrian age were deposited: Mila Formation in Iran, and Burj Formation in Jordan and Saudi Arabia, implying an age older than ca. 510 Ma in GTS 2004. In Oman, however, continental rather than marine deposition (Miqrat and coeval Mahwis formations) continued above the unconformity in ?Middle Cambrian. For the purpose of regional correlations it is proposed that the Angudan Unconformity of Oman be taken as the name for the basal boundary of the Sequence and the Burj Sequence Boundary for its top.