Rapid Subsidence Rates Research Articles

Diachronous basin evolution along northern South China Sea: Result of a migrating Hainan plume?

The diachronous basin evolution from west to east along the northern margin of the South China Sea (SCS) is closely related to the formation of the SCS, but in details remains ambiguous. We focused our study on the crustal structure and Cenozoic subsidence and sedimentary evolution of the Pearl River Mouth Basin (PRMB) and the Qiangdongnan Basin (QDNB) to reveal the divergences between the western and eastern segments of this margin. The back-stripping method and gravity modeling were employed based on drilling wells and geophysical data. We revealed that the crustal structure of the PRMB has a wedge shape, thinning monotonously from the coastline to the continent ocean transition (COT), while that of the QDNB has a symmetrical shape, thinning intensively in the Central Canyon. The characteristics of multi-episodic sedimentation and subsidence evolution were revealed in the PRMB, and its rapid subsidence rates were roughly synchronized with the decrease in the seafloor spreading rate of the SCS basin. However, the sedimentary and subsidence rate curves mainly featured accelerated sedimentation and subsidence in the western QDNB and an accelerated subsidence rate curve but a slow sedimentation rate curve in the eastern QDNB after 11.6 Ma. Some low-velocity conduits, which might be the crustal magmatic footprints of the Hainan mantle plume, were revealed in the middle and eastern QDNB. One possible dynamic model that can cause accelerated subsidence after 11.6 Ma in the eastern QDNB is that the Hainan mantle plume has migrated northwestward which may be derived from the contemporaneous southeastward movement of the South China Block. This conjecture coincides with magmatism and subsidence rate variations in the northwestern margin and is evidenced by the low-velocity conduits in the eastern QDNB.

Post-breakup deformations in the Bay of Bengal: Response of crustal strata to the sediment load

Passive continental margins are tectonically inactive, but a few of them including the East India Passive Margin (EIPM) show evidences for post-breakup deformations. This unusual process prompted us to investigate the post-rift deformations on EIPM and adjacent deep-water region for understanding the possible mechanisms. Seismic reflection images of the Bay of Bengal reveal a post-rift deformation with a manifestation of extensional faults in Krishna–Godavari (K–G) basin and on flanks of the 85°E Ridge. In K–G basin, one of the rift-related faults reactivated during the Early Miocene time (~16 Ma), while on flanks of the 85°E Ridge new normal faults originated at about 6.8 Ma. From detailed analysis of fault throws, it is observed that the fault in K–G basin recorded a cumulative throw of about 900 m between the basement and Early Miocene horizon (~16 Ma), later the fault was reactivated at 6.8 Ma and continued the activity progressively until 0.3 Ma before cessation. The fault system on the margin spatially extends for about 300 km between offshore extensions of the Pranahita–Godavari graben and Nagavali–Vamshadhara shear zone. The faults on 85°E Ridge, initiated at 6.8 Ma and continued until 0.8 Ma, have cumulative throws of about 60 and 110 m on western and eastern flanks of the ridge, respectively. Back-stripping analysis of the fault from the K–G basin discloses two distinct phases of subsidence history: (i) during the first phase (120–23 Ma) the basement subsided at a rate of 46–18 m/Myr due to thermal cooling of the lithosphere, (ii) during the second phase (23 Ma–Present) rapid subsidence rate (69.56 m/Myr) of basement is noticed as a consequence of deposition of copious amounts of Bengal Fan sediments. The thick sedimentary strata exerted vertical load on underlying heterogeneous crust that led to build excessive internal stress and release through weak zones (lying at intersecting planes of heterogeneous crustal blocks). The stress, thus released through fault planes has caused the deformation of crust as well as overlying sedimentary strata.

Altered surface hydrology as a potential mechanism for subsidence in coastal Louisiana

Abstract. The natural wetlands of coastal Louisiana are experiencing rapid subsidence rates averaging 9±1 mm yr−1. Recent measurements based on GPS data and CRMS surface elevation tables (SETs) have shown that most of the subsidence is shallow and occurs in the uppermost 5 meters. Sources of subsidence and the origin of their spatial variability are strongly debated. Here we use CRMS SETs together with historic maps of coastal Louisiana to explore two hypotheses: (i) shallow subsidence is a result of accommodation created by (long-term) deep subsidence processes and self-weight consolidation, and (ii) changes in marsh hydrology (groundwater and surface water flows) have led to a recent increase in shallow subsidence. First, we find that, although self-weight consolidation would result in generally high observed shallow subsidence rates, it does not explain the rates nor the spatial variability of the CRMS SET data. Second, based on historic maps, we find that shallow subsidence rates are significantly higher for CRMS sites where shipping canals have reduced their distance to the marsh edge. This is potentially a result from increased sediment deposition, but CRMS data also show altered groundwater levels near the marsh edge. We find some indication that prolonged periods of low water could have led to increases in effective stresses that explain some of the rapid rates of shallow subsidence observed along Louisiana's coastline.

Environmental Impact Assessment of Deep Pit Project in floodplain near Yangtze

The construction of deep pit has a great impact on the surrounding environment, which is mainly reflected in the subsidence of surrounding soil. Because of the particularity in geological and hydrogeological conditions of riverside floodplain area, environmental impact assessment is needed. According to the monitoring data of practical engineering, the deep pit in the floodplain area near Yangtze River may cause a large subsidence of the surrounding soil with a rapid subsidence rate and a wide range of subsidence effects. The subsidence of the surrounding soil increases with the depth and time of excavation, and the post-construction subsidence effect is not obvious. The environmental effects of the deep foundation pit project in this project have provided valuable experience for guiding the implementation of the deep foundation pit near the floodplain in the future.

Source rock evaluation and thermal maturity modeling of Jake oil fields, Fula sub-basin, Muglad Basin, Sudan

Comprehensive source rock evaluation, burial history modeling, and thermal maturity of oil generation have been investigated in petroleum system of Jake oil fields. The consideration of geological, geochemical, and palynofacies data was used for source rock and thermal maturity. Basin Mod® software was applied to construct the burial and thermal histories and to determine the timing of hydrocarbon generation and expulsion. The modeling was achieved through transient heat flow and collaborated with measured %Ro. The Lower Cretaceous Abu Gabra Formation was believed to be organic-rich shale and characterized by source rock of excellent abundance and potential that possesses total organic carbon (TOC) ranging from 0.37 to 6.27 wt% and production yield 1.31–49.23 mg HC/g rock. Kerogens can be classified as oil-prone types I–II. Vitrinite reflectance (Ro) and Tmax values suggested that most samples were of early maturation for oil generation. Palynofacies and geochemical combination indicated that amorphous phytoclasts of plant debris were dominated by high TOC and hydrogen index (HI), whereas assemblage kerogens confirmed the presence of oil source rock. Geohistory models showed the highest rapid subsidence rates during the Early-Late Cretaceous. The results demonstrated that petroleum could be generated in the upper (1950–2500 m) and the lower intervals (2550 total depth) of Jake South field. Maturity and hydrocarbon generation modeling indicated that the source rocks reached the early maturation stage in the Early Cretaceous (127 Ma) at a temperature of 90 °C. The middle maturation stage began in the Late Cretaceous (64 Ma) with a temperature of 110 °C. Essential oil generation started approximately 63 Ma ago and continues to the present day.

Measuring Deformation in Jakarta through Long Term Synthetic Aperture Radar (SAR) Data Analysis

Jakarta as a home for more than 10 millions habitant facing complex environmental problems due to physical development that cause physical deformation. Physical deformation issues such as decreasing environmental carrying capacity, land cover changes and land subsidence have occurred. Recent studies shows that the long of shoreline changes in a span of 13 years from 2002 to 2015 around 14 km due to land reclamation in Jakarta bay. Previous studies also concluded that Jakarta suffer a sinking phenomena due to its rapid subsidence rate, approximately 260 mm/year in northern part of Jakarta. During the 2007 to 2011, the land subsidence phenomena in Jakarta was observed by InSAR based on ALOS-PALSAR data and found that the subsided areas only occurred in certain areas, mainly in Pluit and Cengkareng regions, with a subsidence of approximately 70 cm for 4 years. Land subsidence is generally related to geological subsidence i.e. sediment consolidation due to its own weight and tectonic movements; or related to human activities such as withdrawal of ground water and geothermal fluid, oil and gas extraction from underground reservoirs, and collapse of underground mines. The amount of subsidence or uplift can be estimated from the number of concentric fringes that appear in the interferogram. This research utilizes Synthetic Aperture Radar (SAR) data observed from ALOS-2 (L-band) and Sentinel-1 (C-band) satellites. By interfering two single look complex (SLC) images from different observation epoch, it is found that the subsided area that has been identified before continues to subside. This occurs especially in Pluit region and has been revealed by interfering ALOS-2 data up to year 2016. The deformation in this area is approximately 12 cm from November 2015 to September 2016. The process of land reclamation also clearly identified by Sentinel-1 image by series data processing in Sentinels Application Platform (SNAP) software.

Multiple controls on rift basin sedimentation in volcanic settings: Insights from the anatomy of a small Early Cretaceous basin in the Yanshan belt, northern North China

This paper deals with a small continental rift basin in a volcanic setting and tries to decipher potential controls on its synrift sedimentation. The Luanping basin, which is a well-exposed Early Cretaceous rift basin in the Yanshan belt on the northern periphery of the North China block, was chosen as a case study. We show that extensive and vigorous volcanism took place just prior to the onset of basin formation and continued into a synrift period. Basin fills consist predominantly of lacustrine, fan-delta, braid-delta, and volcaniclastic deposits. Lacustrine facies are composed mostly of deep-lake deposits that appear to have been established soon after basin initiation. Fan-delta systems developed along two high-angle border faults, the master northern Hongqi fault and the secondary western Xiaobaiqi fault. Fan-delta fronts were built up primarily by gravelly and sandy debrites and turbidites. Braid-delta deposits are restricted to the eastern corner of the basin and are characterized by an aggregation of braid-plain conglomeratic deposits. Underwater extrusions of magma and pyroclastic flows are inferred from the presence of volcanic breccias, ignimbrites, andesitic pillows, and peperites, which are enclosed by host lacustrine deposits. Deep-lake and fan-delta systems persisted throughout the synrift period, indicating a rapid subsidence rate, with both adequate sediment and water supply. The high-angle border faults are thought to have played a critical role in creating large accommodation space, and simultaneous displacements of the Hongqi and Xiaobaiqi faults led to the localization of a major depocenter in the western part of the basin. One-dimensional backstripping analysis of three stratigraphic sections shows that the Luanping basin experienced marked synrift tectonic subsidence up to 2 km at a subsidence rate close to 0.2 mm yr −1 . We propose that the high rate of basin subsidence might have been caused mainly by left-lateral transtensional faulting and was enhanced by the superposition of basement downsag induced by underlying magma withdrawal. A predominance of volcanic particles in both debrites and sandstones suggests that source areas were erodible volcanic edifices. A humid paleoclimate is inferred, which not only provided enough water supply, but also promoted weathering of volcanic rocks in source areas, thus maintaining both the deep-lake environment and sediment influx. We conclude that relatively small and isolated rift basins in volcanic settings can experience both rapid subsidence and receive thick lacustrine deposits if they are under the combined control of favorable structure, climate, and magmatism. The model proposed here is believed to be applicable to other rift basins in similar volcano-tectonic settings.

Impact of anthropogenic subsidence on relative sea-level rise in the Fraser River delta

Subsidence is a common cause of amplified relative sea-level rise, flooding, and erosion in coastal environments. In particular, subsidence due to sediment consolidation can play a significant role in relative sea-level rise in large deltas. We use a combination of InSAR (interferometric synthetic aperture radar), leveling, and global positioning system data to map absolute vertical land motion in the Fraser River delta, western Canada. We show that primary consolidation of shallow Holocene sediments is the main cause for the slow subsidence (−1 to −2 mm/a) affecting the delta lowlands. In addition, parts of the delta undergo increased anthropogenic subsidence. Rapid subsidence rates (−3 to −8 mm/a) are associated with recent artificial loads and exhibit a first-order exponential decrease with a time constant of ~20 years, consistent with the theory of consolidation. Assuming two sea-level rise scenarios of 30 or 100 cm by the end of the twenty-first century, natural subsidence will augment relative sea-level rise in the Fraser Holocene lowlands by ~50% or ~15%. Anthropogenic subsidence will augment relative sea-level rise by ~130% or ~40%, potentially raising it to as much as 1–2 m. In deltaic, lacustrine, and alluvial environments, anthropogenic sediment consolidation can result in significant amplification and strong spatial variations of relative sea-level rise that need to be considered in local planning.

The history of Post‐Miocene sea level change: Inferences from stratigraphic modeling of Enewetak Atoll

The history of post‐Miocene sea level change has been investigated using a quantitative, one‐dimensional stratigraphic forward model. The stratigraphic model produces synthetic stratigraphies, including mineralogy and sediment age versus depth, in response to changes in sea level, subsidence, sedimentation, and diagenesis. Model outputs, using sea level curves inferred from passive margin sequence stratigraphy and deep‐sea foraminiferal oxygen isotope stratigraphy, were compared to the post‐Miocene stratigraphy of Enewetak Atoll. Modeling results support high‐frequency (104 to 105 years) fluctuations of post‐Miocene sea level. Post‐Miocene sea level elevations significantly greater than modern sea level elevation are not easily reconciled with the stratigraphy of Enewetak Atoll. Model/data fit is maximized when a rapid subsidence rate for Enewetak Atoll is used. Alternatively, model/data fit may be maximized using a lower subsidence rate for Enewetak and having post‐Miocene sea level elevations significantly lower than modem sea level elevation. Given the present state of knowledge, much work is still needed to accurately decipher the record of post‐Miocene sea level change.

Controls on internal structure and architecture of sandstone bodies within Upper Carboniferous fluvial-dominated deltas, County Clare, western Ireland

Summary Deposition of Upper Carboniferous (Namurian R1-R2) deltas in the Clare Basin, western Ireland, was influenced by a combination of processes: fluvial, basinal (notably waves), synsedimentary deformation, and rapid subsidence rates within the receiving basin. Fluvial processes predominate; consequently the major deltaic sandstone bodies are fluvial channels and fluvial-dominated mouth bars. The internal structure of single channel sandstones reflects high energy tractive processes and aggradation of sands in relatively straight channel reaches. The structure of mouth-bar sandstone bodies reflects a variety of frictional and buoyant river-mouth processes. Depositional processes within the Clare Basin are often overprinted by syndepositional deformation. Delta-front sequences exhibit abundant evidence of synsedimentary faulting, slumping, clay diapirism and gravity sliding. Deformed packets of sediment are up to 60 m thick and over half of all delta-front sequences are disturbed. These processes led to a modification of mouth-bar structure and geometry. Some fault-bounded delta-front sandstone bodies fill fault-induced depocentres. Large, multistorey and laterally composite channel sandstones in the S of the basin may reflect major deformation across the delta plain, related to rapid subsidence and compaction rates. In contrast, the architecture and structure of delta-plain sequences in the N of the basin are consistent with a more stable fluvial-dominated setting.

A numerical model for the formation of saline waters due to diffusion of dissolved NaCl in subsiding sedimentary basins with evaporites

Numerical modeling by a finite difference method incorporating the effects of vertical variations in pressure, temperature, salinity, and porosity shows that mass transport by diffusion may be a major brine-forming process in sedimentary basins containing halite beds. Vertical fluxes of dissolved NaCl are greatest when the subsidence rate is slow. At subsidence rates of 20 m m.y. −1 more than 250 m of salt can be dissolved over 150 m.y. At rapid subsidence rates, such as 500 m m.y. −1, pore waters are advected downward so rapidly that although a diffusion front develops above the evaporite, this front moves downward as the evaporite subsides, and diffusive fluxes at shallow depths are thus low. However, under such conditions, the pore water column becomes gravitationally unstable as a result of thermally-induced density inversions and the potential for convective overturn is enhanced with accompanying advective salt flux upward. Although dissolution and diffusion of NaCl are important processes in the burial history of evaporites, they are not adequate by themselves to produce the regional salinity versus depth profiles that are actually observed in some sedimentary basins. Diffusion-induced gradients progressively increase with depth whereas gradients actually observed in sedimentary basins tend to decrease with depth. The difference is presumably due to the activity of other mass transport mechanisms such as advection, convection and dispersion.

Geodynamic significance of localized spasmodic rapid-subsidence rates within the Cretaceous of central North America

On the basis of spatial-time analyses, rates of subsidence within 5 Ma intervals in Montana, Wyoming, and adjacent areas, are mapped on the basis of thicknesses of the dominantly shallow-water sedimentary rocks; this is possible by using (a) existing detailed radiometric (mainly for abundant bentonites) and biostratigraphic chronology, and (b) published details of several hundred well logs and measured outcrop sections. Nine 5 Ma-interval maps for Montana and Wyoming show that, in general, as in eastern U.S.A. and eastern Brazil, subsidence rates tend to be low (< 50 m/Ma and commonly < 25 m/Ma) and monotonous in space and time. However, a few localized domains are characterized by significantly higher sedimentation and subsidence rates (> 100 m/Ma and very locally > 200 m/Ma or even > 300 m/Ma); these domains are sharply defined in both space and time. Domains of very high rates sometimes partially overlap areally in successive 5 Ma intervals, but they do not do so in all cases. The well-documented Cretaceous global marine transgressions are complicated by these differential-downwarp events of the “basement”. The broadly analogous patterns documented in the Western Interior Basin, eastern U.S.A., and eastern Brazil militate against special localized explanations (e.g. a shallowly subducted plate). Analogies with major cratonic basins and geodynamic implications are reviewed briefly.

Geodynamic significance of spasmodic, cretaceous, rapid subsidence rates, continental shelf U.S.A.

Subsurface Cretaceous shallow-water sedimentary rocks of the exposed continental shelf between New York and southern Florida provide evidence of two periods of relatively high sedimentation and subsidence rates. North and south of the Cape Fear and Peninsula Arches, Late Aptian to Early Albian rates range from 50 to over 150 meters/ million years; in Coniacian/Santonian time, rates exceeding 30 meters/million years occur sporadically, even across the area of the Arches. A common, global phenomenon that affected both cratons and continental shelves is believed to have caused these periods of relatively rapid basin subsidence and consequent rapid sediment accumulation.