Shallow Sedimentary Structures Research Articles

Basement morphology of the fold-and-thrust belt and foreland basin of the United Arab Emirates: Evidence for thick-skinned tectonics in the northern Oman-UAE Mountains

We determined the basement depths in the fold-and-thrust belt (FTB) and the associated Aruma and Pabdeh foreland basins of the United Arab Emirates. Additionally, we investigated the relationship between shallow sedimentary structures and deep basement features. The Late Cretaceous Aruma basin is associated with Semail ophiolite thrusting and loading, while the late Oligocene to Miocene Pabdeh basin is associated with the culmination of Musandam and early Zagros collision. In the northern portions of the study area, seismic reflection profiles and derivative maps of gravity and magnetic data show NNW–SSE striking thrusts, backthrusts, and folds. The southern region around Jabal Hafit is dominated by the Tarabat backthrust, which cuts across the Upper Cretaceous and Cenozoic successions. Moreover, the Mesozoic platform carbonates are dissected by thrust faults. These thrusts may have originated from Jurassic extensional faults that were reactivated during the Late Cretaceous ophiolite obduction, as well as the late Oligocene to the early Miocene culmination of Musandam resulting from the collision of the Arabian Plate and Central Iran along the Zagros Suture. Results of the 3D gravity inversion indicate that the basement depths range from 11.5 km along uplifted areas of the FTB to 18.8 km within the deeper parts of the foreland basin. Significant northward increases in both basement depth and the thickness of the late Oligocene-early Miocene successions indicate a northward increase in the severity of thrusting and crustal loading along the Khusub thrust. The basement uplifts are found to be correlated with shallower anticlinal features, indicating a direct relationship between basement uplifts and shallow sedimentary structures. Therefore, we suggest that the basement structures were reactivated during the Late Cretaceous ophiolite obduction and the late Oligocene-early Miocene Zagros collision.

Shear‐Wave Velocity Structure of Sediments on Cascadia's Continental Margin From Probabilistic Inversion of Seafloor Compliance Data

AbstractSeveral seismic techniques, both passive and active, exist for estimating the shear‐wave velocity structure of shallow sedimentary structures. In particular, passive compliance signals recorded by broadband ocean‐bottom seismometers (OBSs) can be used to invert for structure. While compliance‐based imaging studies have been carried out at several locations across the Cascadia Subduction Zone, such an approach has not been extensively applied to OBSs deployed on the continental shelf and slope. In this study, we measure compliance and coherence signals at 13 broadband OBSs deployed along Cascadia's continental shelf and upper slope. We then use a recently developed technique to probabilistically invert compliance signals for shallow structure that makes use of mixture density neural networks. Finally, we compare and contrast our inverted profiles and derived properties obtained using this method with previous studies focused on the properties of basin sediments.

The Late-Quaternary deposits of the Piedra Buena Terrace (Patagonian continental slope, SW Atlantic): An example of interaction between bottom currents and seafloor morphology

The Piedra Buena Terrace (PBT), situated in the Patagonian continental slope, is swept by the vigorous flow of Antarctic-sourced bottom water masses, and intersected by a considerable submarine canyon system. It thus represents a world-class natural laboratory to investigate the sedimentary dynamics of deep-ocean settings influenced by both, along-slope, and down-slope sedimentary processes. In the framework of the Pampa Azul project, the PBT was surveyed with swath bathymetry and a high-resolution parametric sub-bottom seismic profiler and sampled at several locations with a 7 m long gravity corer. Through the integration of the acoustic data, grain size results from a sediment core and two AMS 14C ages this work attempts to: (1) describe for the first time the detailed geomorphology of a poorly known region of the Patagonian continental slope; (2) depict the principal sub-surface seismic units, illustrating their internal structure, geometry, and distribution; (3) establish a correlation between sub-surface seismic facies and sediment deposits using the first age constraints for this region; (4) propose a model with the depositional history of the PBT, focusing on the interaction between along-slope and down-slope processes and seafloor morphology. The acquired bathymetric data imaged with unprecedent detail the geomorphology of the southern part of the PBT revealing the presence of a field of kilometric-sized, sub-circular, crater-like depressions, and a submarine canyon. The analyzed seismic sub-bottom profiles showed that the shallow sedimentary structure of the study area is represented by patchy-mounded deposits characterized by three distinctive features: cyclic stacks of two seismic facies (FA and FB), mounded shape, and preferential location northwards the morphological depressions and the submarine canyon. Integration of a seismic sub-bottom profile with one sediment core indicates that the alternation of seismic facies reflects variations in the hydrodynamic conditions. FA is related to a high to moderate energy medium with input of terrigenous material and FB is related to a low energy environment with a limited contribution of sediments from the continent. The patchy distribution, mounded shape and preferential location are associated with hydrodynamic processes occurring between the geomorphological features and the north-flowing Antarctic bottom currents, which result in the deposition of mounded contourites in the northern flank of depressions and hybrid along-slope down-slope deposits northwards a submarine canyon. The 14C ages constrain the chronological framework of these sediments to the Late-Pleistocene. This is in contrast with previous investigations that have suggested much older ages (Late Miocene) for the sub-surface sediments of PBT. Finally, a conceptual model is here proposed to illustrate the Late Quaternary sediment depositional history of the PBT where the interaction between north-flowing bottom contouritic currents and seafloor relief played an important role in the deposition and preservation of sediments in this area.

Geophysical and Sedimentological Investigations Integrate Remote-Sensing Data to Depict Geometry of Fluvial Sedimentary Bodies: An Example from Holocene Point-Bar Deposits of the Venetian Plain (Italy)

Over the past few millennia, meandering fluvial channels drained coastal landscapes accumulating sedimentary successions that today are permeable pathways. Propagation of pollutants, agricultural exploitation and sand liquefaction are the main processes of environmental interest affecting these sedimentary bodies. The characterization of these bodies is thus of utmost general interest. In this study, we particularly highlight the contribution of noninvasive (remote and ground-based) investigation techniques, and the case study focuses on a late Holocene meander bend of the southern Venetian Plain (Northeast Italy). Electromagnetic induction (EMI) investigations, conducted with great care in terms of sonde stability and positioning, allowed the reconstruction of the electrical conductivity 3D structure of the shallow subsurface, revealing that the paleochannel ranges in depth between 0.8 and 5.4 m, and defines an almost 260 m-wide point bar. The electrical conductivity maps derived from EMI at different depths define an arcuate morphology indicating that bar accretion started from an already sinuous channel. Sedimentary cores ensure local ground-truth and help define the evolution of the channel bend. This paper shows that the combination of well-conceived and carefully performed inverted geophysical surveys, remote sensing and direct investigations provides evidence of the evolution of recent shallow sedimentary structures with unprecedented detail.

Microtremor HVSR analysis of heterogeneous shallow sedimentary structures at Pohang, South Korea

AbstractWe apply the single-station microtremor horizontal-to-vertical spectral ratio (HVSR) method to image the geometry of Pohang Basin, South Korea, which experienced the greatest earthquake damage in Korea during a series of anthropogenic earthquakes between November 2017 and February 2018. We collected and analysed the ambient seismic noise at 124 temporary stations. The resonance frequencies, which vary significantly across the area (0.35–19.86 Hz), were inverted to constrain the depth of the major impedance contrast, which is interpreted to be the sediment–bedrock interface beneath each station. The sedimentary layer thickness is generally thin in the north and thickens to the southern and central parts of the basin, where the depth to bedrock is up to 340 m. We compare the HVSR results with six borehole observations in the area, whose depth to bedrock ranges from 189 to 359 m. The sediment thicknesses obtained via the direct borehole measurements and HVSR method are comparable with each other. The resultant three-dimensional shape of the sedimentary basin provides crucial information for the microzonation of the Pohang area for seismic risk mitigation. It also provides a realistic initial velocity model for three-dimensional tomographic inversions to elucidate the detailed subsurface structure of the region.

Shallow Sedimentary Structure of the Brahmaputra Valley Constraint from Receiver Functions Analysis

In this study, receiver functions from ten Broadband seismograph stations on Cenozoic sediment formations of Brahmaputra valley and its neighboring region in northeastern part of India are determined. Receiver function traces from this region show delay in peak by 1–2.5 s and associated minor peaks with the direct P-phase peak. Based on such observation, we try to image sedimentary structure of the Brahmaputra valley plain, adjacent Shillong plateau and Himalayan foredeep region. An adapted hybrid global waveform inversion technique has been applied to extract sedimentary basin structure beneath each site. The sedimentary cover of the basin is about 0.5–6.5 km thick across the valley, 0.5–1.0 km on Shillong plateau and 2.0–5.0 km in nearby foredeep region. We have found that sedimentary thickness increases from SW to NE along the Brahmaputra valley and towards the Eastern Himalayan syntaxes. The estimated sediment thickness and S wave velocity structure agree well with the results of previous active source, gravity, and deep borehole studies carried out in this region. The thick crustal low velocity sediment cover in Brahmaputra valley is expected to amplify ground motions during earthquakes and therefore important for seismic hazard assessment of the region.

Lithospheric structure beneath NW Iran using regional and teleseismic travel-time tomography

We compute a 2-D tomogram using the P wave arrival time readings from a temporary seismic experiment to study the seismic structure of the crust and upper mantle in NW Iran. The study area includes the western margins of the South Caspian Basin (SCB), and the Sahand and Sabalan post-collisional volcanoes in NW Iran. We invert 2780 regional and teleseismic relative P wave arrival times recorded by 23 stations along the seismic profile extending from the western shoreline of the Caspian Sea to Lake Urumieh. Our tomographic results show a higher-velocity region beneath the SCB. The observed higher velocities strongly correlate with the observed positive gravity anomalies over the southwestern margins of the Caspian Sea, suggesting an oceanic like nature for the SCB lithosphere. The tomographic results also show several lower-velocity anomalies in the crust. The Sabalan volcano is underlain by a low-velocity zone in the lower crust, which is most likely thermal in nature. In the Sahand region, the lower velocities are considerably shallower in depth and might be controlled by shallow sedimentary structures, as well as an anomalously warm upper crust. The shallow low-velocity regions are connected with deeper low-velocity zones 60–100km deep in the upper mantle. This pattern points to a possible mantle source of post-collisional volcanism in NW Iran, i.e. the melting of a subducted slab.

Application of waveform tomography to a crooked-line 2D land seismic data set

Onshore hydrocarbon exploration in the back-arc region of Japan suffers from seismic imaging challenges. Seismic data are typically acquired in a difficult environment: along a crooked line over a rough surface topography underlain by severe near-surface weathering layers. The area is geologically complex, and prestack depth processing is desirable. However, the data quality is suboptimal at near offsets, and it hinders the migration velocity analysis required for depth processing. Thus, a geologic interpretation needs to rely on time-processing results. Nevertheless, some data sets are rich in low-frequency components and contain clear refracted and wide-angle reflected waves, both of which are favorable conditions for the application of waveform tomography. We have used one such data set, and we determined the applicability of waveform tomography to estimate the P-wave velocity distribution. To mitigate difficulties in the data, we carefully optimized waveform tomography strategies and diligently validated the results. We evaluated the effects of the crooked line by conducting waveform tomography in 2D and 2.5D. We determined that for this data set, the effect of the crooked line was not substantial enough to require the 2.5D implementation, but it was large enough to require careful trace editing. We also minimized the effects of large variations in amplitudes due to near-surface effects and source and receiver characteristics, primarily by fitting phase components of the data during waveform tomography, but also by adjusting the amplitudes in a surface-consistent manner to stabilize the source estimation process. The final velocity model delineated known shallow sedimentary structures, but it also revealed deep large-thrust structures critical to the structural understanding of the area. The result confirmed the capability of waveform tomography to yield a reliable velocity model, and it also opened up the possibility of applications of depth processing in the region.

Formation Conditions and Sedimentary Characteristics of a Triassic Shallow Water Braided Delta in the Yanchang Formation, Southwest Ordos Basin, China.

A large, shallow braided river delta sedimentary system developed in the Yanchang Formation during the Triassic in the southwest of the Ordos basin. In this braided delta system, abundant oil and gas resources have been observed, and the area is a hotspot for oil and gas resource exploration. Through extensive field work on outcrops and cores and analyses of geophysical data, it was determined that developments in the Late Triassic produced favorable geological conditions for the development of shallow water braided river deltas. Such conditions included a large basin, flat terrain, and wide and shallow water areas; wet and dry cyclical climate changes; ancient water turbulence; dramatic depth cycle changes; ancient uplift development; strong weathering of parent rock; and abundant supply. The shallow water braided river delta showed grain sediment granularity, plastic debris, and sediment with mature composition and structure that reflected the strong hydrodynamic environment of large tabular cross-bedding, wedge cross-bedding, and multiple positive rhythms superimposed to form a thick sand body layer. The branch river bifurcation developed underwater, and the thickness of the sand body increased further, indicating that the slope was slow and located in shallow water. The seismic responses of the braided river delta reflected strong shallow water performance, indicated by a progradation seismic reflection phase axis that was relatively flat; in addition, the seismic reflection amplitude was strong and continuous with a low angle and extended over considerable distances (up to 50 km). The sedimentary center was close to the provenance, the width of the river was large, and a shallow sedimentary structure and a sedimentary rhythm were developed. The development of the delta was primarily controlled by tectonic activity and changes in the lake level; as a result, the river delta sedimentary system eventually presented a “small plain, big front” character.

A study on the predominant period of long-period ground motions in the Kanto Basin, Japan

We analyzed seismic waveforms of moderate to large shallow earthquakes recorded in the Kanto Basin, Japan, to clarify the characteristics of long-period ground motions in deep sedimentary basins. Our analysis revealed a tendency of long-period ground motions to develop in limited periods up to around 7 s. In addition, we found a specific relation between the predominant period of long-period ground motions and the depth of bedrock. The predominant period tends to increase in proportion to bedrock depth until it reaches approximately 7 s; however, it becomes almost constant for bedrock depths greater than 2 km. These observations, in conjunction with numerical experiments performed in this study using realistic sedimentary basin structure models, indicate that the characteristic of long-period ground motions, excited by the basin-induced fundamental Love waves, is controlled strongly by the shallow sedimentary structure, if the bedrock depth exceeds approximately 2 km. This result might also be important in understanding and evaluating the characteristics of long-period ground motions in other deep sedimentary basins of the world for moderate to large shallow earthquakes.

Seabed morphology and shallow sedimentary structure of the Storfjorden and Kveithola trough-mouth fans (North West Barents Sea)

This study aims to present an overview of the seafloor morphology and shallow sedimentary structure of the Storfjorden and Kveithola Trough Mouth Fans (TMFs) on the northwestern Barents Sea continental margin. Data have been compiled from two International Polar Year (IPY) cruises (SVAIS, of the BIO Hespérides and EGLACOM of the R/V OGS-Explora) that yielded 15,340 km 2 of multi-beam bathymetry and 9500 km of sub-bottom seismic profiles. In this area, the continental shelf edge defines three wide and subdued sedimentary lobes forming Storfjorden TMF, one single lobe on Kveithola TMF, and three inter-TMF areas on the continental slope. The two northernmost lobes of Storfjorden TMF (Lobes I and II) are composed by thick (up to 50 m) sequences of glacially derived debris flow deposits interbedded with thin a few metres de-glacial and interglacial deposits. A network of upper slope gullies incises these debris flow deposits as a consequence of subglacial meltwater release at or near the shelf break. Gullies evolve into channels whose morphologic evidence disappears midslope, leaving place to a subdued chevron-like morphological pattern inherited by the preceeding glacial maximum debris flow deposits. A drastic change occurs on the continental slope of Storfjorden TMF Lobe III and Kveithola TMF, where are several translational submarine landslides mostly originated in the upper slope, the majority of which detach at the contact between Middle Weishelian glacigenic debris flows and the overlying acoustically laminated plumites. Dendritic canyon systems only develop in inter-TMF areas. The data suggest that TMF continental slope progradation depends on short-lived episodes of extreme sedimentation during glacial maxima and during the early deglaciation phase, and that an important controlling factor is the mechanism of ice stream retreat from the continental shelf edge. We suggest that the two northern Storfjorden sub-ice streams were composed of thicker and perhaps faster ice progressively draining a distal and larger ice source mainly located on Svalbard. Conversely, the southernmost Storfjorden sub-ice stream and the Kveithola ice stream were fed by a local, smaller marine-based ice dome grounded on Spitsbergenbanken. The ice dome persisted after the LGM, maintaining a local ice drainage system close to the shelf edge whose sedimentary evidence can be found on the continental slope of the southern lobe of Storfjorden TMF and Kveithola TMF. The high degree of lateral variability in the style of sedimentation on TMF slopes suggests that ice stream dynamics may vary considerably within the same glacial trough, and that such variability affects the long-term development of the architecture of TMFs.

Investigation of Shallow Sedimentary Structure of the Anchorage Basin, Alaska, Using Simulated Annealing Inversion of Site Response

Abstract This study deals with shallow sedimentary structure of the Anchorage basin in Alaska. For this purpose, inversion of site response [SR( f )] data in the frequency range 0.5–11.0 Hz from various sites of the basin has been performed using the simulated annealing method to compute subsurface layer thickness, shear-wave velocity ( β ), density, and shear-wave quality factor. The one-dimensional (1D) models for the aforementioned parameters were obtained with preset bounds on the basis of available geological information such that the L -2 norm error between the observed and computed site response attained a global minimum. Next, the spatial distribution of the important parameter β was obtained by interpolating values yielded by the 1D models. The results indicate the presence of three distinct velocity zones as the source of spatial variation of SR( f ) in the Anchorage basin. In the uppermost part of the basin, the β values of fine-grain Quaternary sediments mainly lie in the range of 180–500 m/sec with thickness varying from 15 to 50 m. This formation overlies relatively thick (80–200 m) coarse-grain Quaternary sediments with β values in the range of 600–900 m/sec. These two Quaternary units are, in turn, overlain on Tertiary sediments with β >1000 m/sec located at depths of 100 and 250 m, respectively, in the central and western side along the Knik Arm parts of the basin. The important implication of the result is that the sources of spatial variation of SR( f ) in the Anchorage basin for the frequency band 0.5–11 Hz, besides in the uppermost 30 m, are found to be deeper than this depth. Thus, use of commonly considered geological formations in the depth intervals from 0 to 30 m for the ground-motion interpretation will likely yield erroneous results in the Anchorage basin.

Slope failure, mass flow and bottom current processes in the Rockall Trough, offshore Ireland, revealed by deep-tow sidescan sonar

The margins of the Rockall Trough, currently the focus of oil and gas exploration interest, incorporate an environmentallyimportant and lightly studied shelf region. To address the paucity of information within this region an extensive high resolution sidescan sonar survey was carried out in the Irish sector of the Rockall Trough in June/July 1998. This valuable natural laboratory contains information on slope stability, sediment transport, nutrient upwelling, bottom current, and biological activity at a shelf/slope/basin-floor transition. Detailed information on the nature of many of these processes can be provided from shallow geophysical techniques. The TRIM (TOBI Rockall Irish Margins) project (Shannon et al., 2001), funded by the Irish Petroleum Infrastructure Programme (Rockall Studies Group), acquired 3100 line-km of TOBI sidescan sonar and 3700 km of 3.5 kHz profiler data along the margins of the Rockall Trough (Figure 1). This project followed on from the earlier reconnaissance AIRS (Atlantic Irish Regional Survey) survey conducted over the entire southern Rockall Trough region, using the lower resolution surface towed sonar side-scan system known as GLORIA (Unnithan et al., 2001). The TOBI sidescan system, developed at the Southampton Oceanography Centre (Flewellen et al., 1993), is deep-towed at 300-400 m above the seafloor. It transmits at a frequency of 32 kHz and samples the acoustic response of the seabed and shallow sedimentary structure to a centimetre to metre depth scale. This acoustic response therefore reflects not only the backscattering properties of the seabed but also the shallow detailed structure of sediments formed by recent sedimentological processes.

Acoustic reflectivity and shallow sedimentary structure in the Sea of Galilee, Jordan Valley

Mapping the floor of the Sea of Galilee (Lake Kinneret) with a shallow seismic system of 3.5 kHz resulted in interesting data that were not obtained previously with standard single-channel seismic systems. Over most of the lake acoustic penetration is not possible, probably because of the high gas content in the top sedimentary sequence. However, in a few areas, excellent penetration of about 20 m was achieved. One area is a terrace in the southern part of the lake, south of a small bathymetric escarpment at depths of 13–21 m along Israel latitudinal Grid 238. It is unclear whether the existence of gas in the sediment or other parameters are responsible for the marked difference in acoustic penetration on both sides of the scarp. Another area with acoustic penetration is in the vicinity of hot and salty submarine springs. Although there is no difference in the composition of the upper sedimentary layers between these areas and neighbouring areas, there is a marked difference in the acoustic penetration. The contact between areas with acoustic penetration to areas without acoustic penetration is very sharp. The craters of the submarine springs are usually located on the borders of the areas with acoustic penetration or even at some distance away from them. It is possible that the activity of the hot and salty submarine springs controls the acoustic penetration. However, determination of the exact mechanism for the existence of the zones of acoustic penetration must await further studies of the sediments, especially for measurements of various parameters that control the seismic response of the rock. Another discovery made with the shallow seismic profiles is the existence of some bathymetric irregularities on the floor of the Sea of Galilee. In view of the high sedimentation rate in the lake, which tends to smooth the floor, a bathymetric irregularity such as a linear bathymetric step could be a surface expression of an active fault.