琼东南盆地构造沉降史及其主控因素

以28口钻井、3条主干地震剖面最新解释成果资料为依据的构造沉降史恢复结果表明：琼东南盆地新生代存在三期快速沉降过程和一期缓慢沉降过程：第一期为始新世,第二期为渐新世—早中新世,第三期存在时空差异：在盆地西部乐东凹陷为晚中新世以来,在盆地中东部陵水凹陷和松南-宝岛凹陷为上新世以来;缓慢沉降过程亦存在时空差异,盆地西部中中新世沉降缓慢,盆地中东部中—晚中新世沉降缓慢.第一期快速沉降过程受东亚陆缘扩张和红河断裂左旋走滑共同影响,第二期快速沉降受南海海底扩张和红河断裂左旋走滑联合作用,第三期快速沉降主要受红河断裂右旋走滑控制,缓慢沉降过程与南海海底扩张停止以及红河断裂构造活动处于宁静阶段相耦合.     

南海北部珠江口与琼东南盆地构造-热模拟研究

珠江口盆地和琼东南盆地位于南海北部的大陆边缘,本文在此地区选取了13条典型剖面,进行了构造沉降史和热史的模拟,初步探讨了其新生代以来的构造-热演化历史.其研究结果表明:珠江口盆地存在两次热流升高过程,分别为始新世(56.5～32 Ma)和渐新世(32～23.3 Ma).琼东南盆地存在三期加热和两期冷却过程,始新世盆地热...     

Morphology,sedimentary characteristics,and origin of the Dongsha submarine canyon in the northeastern continental slope of the South China Sea

The Dongsha submarine canyon is a large canyon belonging to a group of canyons on the northeastern South China Sea margin. Investigation of the Dongsha canyon is important for understanding the origin of this canyon group as well as the transport mechanism of sediments on the margin, and the evolution of the Taixinan foreland basin and the associated Taiwan orogenic belt. In this study, the morphology, sedimentary characteristics, and origin of the Dongsha canyon were investigated by integrating high-resolution multi-channel seismic reflection profiles and high-precision multibeam bathymetric data. This is a slope-confined canyon that originates in the upper slope east of the Dongsha Islands, extends downslope in the SEE direction, and finally merges with the South Taiwan Shoal canyon at a water depth of 3000 m. The total length and average width of the canyon are around 190 and 10 km, respectively. Eleven seismic sequence boundaries within the canyon fills were identified and interpreted as incision surfaces of the canyon. In the canyon fills, four types of seismic facies were defined: parallel onlap fill, chaotic fill, mounded divergent facies, and migrated wavy facies. The parallel onlap fill facies is interpreted as alternating coarser turbidites or other gravity-flow deposits and fine hemipelagic sediments filling the canyon valley. The chaotic fill facies is presumed to be debrites and/or basal lag deposits filling the thalwegs. The mounded divergent and migrated wavy seismic facies can be explained as canyon levees consisting mainly of overspilled fine turbidites and sediment waves on the levees or on the canyon-mouth submarine fans. Age correlation between the sequence boundaries and the ODP Site 1144 data suggests that the Dongsha canyon was initiated at approximately 0.9 Ma in the middle Pleistocene. Mapping of the canyon indicates that the canyon originated at the upstream portion of the middle reach of the modern canyon, and has been continuously expanding both upstream and downstream by retrogressive erosion, incision, and deposition of turbidity currents and other gravity transport processes. The ages of the sequence boundaries representing major canyon incision events are in good agreement with those of global sea-level lowstands, indicating that sea-level changes may have played an important role in the canyon's development. The Dongsha canyon developed in a region with an active tectonic background characterized by the Taiwan uplifting and the development of the Taixinan foreland basin. However, no evidence suggests that the canyon formation is directly associated with local or regional faulting and magmatic activities. Turbidity currents and other gravity transport processes(including submarine slides and slumps) may have had an important influence on the formation and evolution of the canyon.     

Volcanogenic sedimentation in the Iceland Basin: influence of subaerial and subglacial eruptions

Cores recovered from the Iceland Basin show evidence of transport and deposition of volcaniclastic sediment from the Eastern Volcanic Zone of Iceland during the Holocene and last glacial period. Three types of deposits have been identified: tephra fall, sediment gravity flows, and bottom-current-controlled deposits. Tephra fall layers contain basaltic glass of composition that suggests Katla volcano as the major source. A chronology of the volcano activity is reconstructed, back to isotopic stage 5d (120,000 yr). Glass chemistry of tephra in sediment gravity flows deposited south of Myrdalsjökull Canyon indicates a source in the Grímsvötn–Lakagígar volcanic system. These volcaniclastic gravity flows were most likely derived from jökulhlaups or large glacial floods, at a time of a more extensive ice cover over the volcanic zone. Deposition of the sediment gravity flows has created a deep-sea fan south of the canyon. Basalt glass composition, age, and depositional environment suggest that one early Holocene turbidite sequence was derived from a large jökulhlaup of the Grímsvötn area. The volcanogenic sediment gravity flows were influenced by a strong contour current, moving across the Katla sediment ridges. The contour current has winnowed the silt fraction and transported it downstream as suspended load. The recovery of numerous silty volcaniclastic layers, enriched in detrital crystals, indicates that they contributed to the sedimentation of contourite drifts.     

Failure of Marine Deposits and their Redistribution by Sediment Gravity Flows

— SedFlux simulates the fill of sedimentary basins, and can be used to examine the location and attributes of sediment failure on continental margins and the runout of their associated sediment gravity flows. Numerical experiments show how the evolving boundary conditions of sea-level fluctuations, floods, storms, tectonic and other relevant processes control the rate and size of slope instabilities. By tracking deposit properties (pore pressures, grain size, bulk density, porosity), a finite-slope factor-of-safety analysis of marine deposits examines failure potential. A decider routine is used to determine whether the failed material will travel down slope as turbidity current or a debris flow. Examples provided insight into: (i) why fjords dominated by turbidity current deposition often contain debris flow deposits; (ii) how glaciated margins prograde seaward through shallow failures of low yield strength material; and, (iii) how large-scale basin subsidence can control the onset of canyon formation across continental slopes.     

Decadal record of sediment export to the deep sea via Eel Canyon

An active upper-canyon system, Eel Canyon, was studied to determine its role as a conduit and/or sink for terrigenous material over decadal timescales and to assess the sedimentary record preserved by transport processes. These data are used to (1) link seasonal fluctuations in sediment transport and deposition to preserved stratigraphic signatures, and (2) assess sediment storage and removal in the upper Eel Canyon (100–850 m water depth) over decadal timescales. Previous research has shown that upper thalwegs commonly experience gravity-driven flows during winter (November–March), due to increased sediment supply from Eel River flooding and intense storms that produce energetic wave/current conditions. Thick winter deposits composed of recently discharged fluvial sediment are formed in upper thalwegs, with distinct short- and long-lived radioisotopic and textural signatures (detectable ⁷Be and ²³⁴Th_xs, lowered ²¹⁰Pb activity, elevated clay content, and physical structures). Box and kasten cores were collected in the upper canyon (thalwegs and walls) to measure these signatures in recent and preserved winter deposits, and to calculate 100-yr accumulation rates. Non-bioturbated deposits (that have signatures indicative of rapid accretion by gravity-driven flows during the winter) are common in the upper canyon thalwegs. Short-lived radioisotopes (⁷Be and ²³⁴Th) show that sediment delivery to the upper thalweg varies temporally, sometimes beginning at the onset of river flooding, and at other times beginning during fall/early winter dry-storm events. In contrast, bioturbated deposits (which do not have signatures indicative of rapid deposition) are found on canyon walls.Non-bioturbated winter deposits are easily identified in the decadal record of thalwegs by decreases in ²¹⁰Pb activity and increases in clay content. Stacking of multiple years of winter deposits (∼10 cm preserved per winter) results in non-steady-state ²¹⁰Pb profiles and high decadal accumulation rates. However, down-core changes in ²¹⁰Pb profiles show that slope failures are actively redistributing these winter deposits. Partial or total removal of multiple winter deposits appears to happen periodically (every ∼13 yr), which will inhibit preservation of the longer decadal record. 100-yr accumulation rates were calculated in the thalwegs from the resulting ²¹⁰Pb profiles (i.e., the result of winter accretion and decadal removal by failures). Accumulation rates are much higher in thalwegs (1–6 cm/yr) than walls (0.1–0.8 cm/yr), which is likely the result of differing sediment delivery processes (via gravity-driven flows and nepheloid layers, respectively). At least 2.6±1.4% of the Eel River sediment budget is accumulating in the upper canyon over 100-yr timescales. However, this value greatly underestimates the total amount entering the canyon system because minimum accumulation rates were used in many areas (due to limited core length) and slope failures are moving sediment out of the budget area.     

Assessing tectonic and climatic causal mechanisms in foreland‐basin stratal architecture: insights from the Alborz Mountains,northern Iran

The southern foreland basin of the Alborz Mountains of northern Iran is characterized by an approximately 7.3‐km‐thick sequence of Miocene sedimentary rocks, constituting three basin‐wde coarsening‐upward units spanning a period of 10⁶ years. We assess available magnetostratigraphy, paleoclimatic reconstructions, stratal architecture, records of depositional environments, and sediment‐provenance data to characterize the relationships between tectonically‐generated accommodation space (A) and sediment supply (S). Our analysis allows an inversion of the stratigraphy for particular forcing mechanisms, documenting causal relationships, and providing a basis to decipher the relative contributions of tectonics and climate (inferred changes in precipitation) in controlling sediment supply to the foreland basin. Specifically, A/S > 1, typical of each basal unit (17.5–16.0, 13.8–13.1 and 10.3–9.6 Ma), is associated with sharp facies retrogradation and reflects substantial tectonic subsidence. Within these time intervals, arid climatic conditions, changes in sediment provenance, and accelerated exhumation in the orogen suggest that sediment supply was most likely driven by high uplift rates. Conversely, A/S < 1 (13.8 and 13.8–11 Ma, units 1, and 2) reflects facies progradation during a sharp decline in tectonic subsidence caused by localized intra‐basinal uplift. During these time intervals, climate continued to be arid and exhumation active, suggesting that sediment supply was again controlled by tectonics. A/S < 1, at 11–10.3 Ma and 9‐6–7.6 Ma (and possibly 6.2; top of units 2 and 3), is also associated with two episodes of extensive progradation, but during wetter phases. The first episode appears to have been linked to a pulse in sediment supply driven by an increase in precipitation. The second episode reflects a balance between a climatically‐induced increase in sediment supply and a reduction of subsidence through the incorporation of the proximal foreland into the orogenic wedge. This in turn caused an expansion of the catchment and a consequent further increase in sediment supply. Copyright © 2013 John Wiley & Sons, Ltd.     

南海大陆边缘动力学:科学实验与研究进展

国家重点基础研究发展计划(973)项目(2007CB411700)首次在南海南部大陆边缘及西南次海盆开展长排列大震源多道地震、海底地震仪(OBS)折射/反射地震等的综合地球物理探测,结合地质构造、地球化学、动力模拟等的综合研究,形成如下重要认识:南海海盆新生代发生了早、晚两期海底扩张.早期扩张发生于33.5～25 Ma...     

花东海盆浊流沉积的磁性特征及其环境意义

对取自台湾以东花东海盆GX168孔的浊流沉积物进行系统的岩石磁学研究,揭示其沉积学和岩石磁学特征,分析其物源和形成机制.研究结果显示,剖面上共识别出12层浊流沉积物,其分布存在规律,下部350~700cm共发育11层浊流沉积物,而0~350cm仅出现1层浊流沉积物.浊流沉积物粒径明显较背景沉积物粗,石英、长石含量更高,底部与下伏背景沉积呈突变接触,顶部与上覆背景沉积呈渐变接触,内部发育典型的正粒序韵律结构.浊流沉积物和背景沉积物具有相似的磁学特征,两者均以磁铁矿为主要载磁矿物类型,且磁铁矿颗粒均以准单畴和多畴颗粒为主.同时,两者也存在一定差异,浊流沉积物中磁铁矿较背景沉积物更为富集,磁化率和饱和等温剩磁更强,磁铁矿粒径更粗,这与浊流沉积物原始沉积区更靠近物源区有关.花东海盆浊流沉积形成的诱发机制可能是末次冰期以来频发的海平面波动造成陆坡之上沉积物重力失稳,导致陆坡沉积物向海盆搬运.     

南海北部神狐海域天然气水合物钻探区第四纪以来的沉积演化特征

南海北部神狐海域是我国首次获取海洋天然气水合物实物样品的海域.然而,陆坡区深水水道和海底峡谷的侵蚀以及频发的沉积物失稳,将会加剧地层对比和沉积相识别的难度,导致目前该区域典型地震相-沉积相特征、沉积体类型、成因机制和空间匹配关系等方面还缺少精细的研究,特别是第四纪以来的沉积演化涉及较少,区域内水合物形成和分布的沉积地质条件尚不清晰.基于海底地形特征的描述、层序地层格架的对比和地震资料的综合解释,本次研究在第四纪以来的沉积充填序列中识别出5种典型的地震相类型,并分析了对应的沉积体类型:进积型的陆坡、第四纪早期发育的小型浊积水道、沉积物失稳(滑移和滑塌)、海底峡谷和伴生的沉积物变形、以及深海沉积-块体流沉积的复合体.通过沉积单元的空间匹配关系,将沉积演化划分为3个阶段:浊积水道侵蚀-沉积物再沉积阶段、陆坡进积-沉积物失稳阶段、海底峡谷的侵蚀-充填阶段.研究结果表明,受第四纪早期小型浊积水道的侵蚀,再沉积的沉积物将在中-下陆坡以"近源"的方式堆积下来,可能具有相对较好的物性条件,从而可被视为适于水合物赋存的有利沉积体.进积型陆坡带来的沉积物易于发生失稳,在研究区内广泛分布,因其具有较小的沉积物颗粒粒度和较好的垂向连续性,可被认为是水合物的区域盖层.大量发育的海底峡谷及伴生的沉积物变形,将会侵蚀和破坏先前沉积的有利沉积体,使其呈现为"斑状/补丁状"的平面展布特征,进而影响了神狐海域水合物的分布.因此,神狐海域第四纪以来的沉积演化是钻探区水合物不均匀性分布的关键控制因素之一.     

U–Pb zircon ages of the Nakanogawa Group in the Hidaka Belt,northern Japan: Implications for its provenance and the protolith of the Hidaka metamorphic rocks

Zircon U–Pb ages of two acidic tuff and two turbidite sandstone samples from the Nakanogawa Group, Hidaka Belt, were measured to estimate its depositional age and the development of the Hokkaido Central Belt, northeast Japan. In the northern unit, homogeneous zircons from pelagic acidic tuff from a basal horizon dated to 58–57 Ma, zircons from sandstone from the upper part of the unit dated to 56–54 Ma, and zircons from acidic tuff from the uppermost part dated to 60–56 Ma and 69–63 Ma. Both of the tuff U–Pb ages are significantly older than the youngest radiolarian fossil age (66–48 Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58 Ma and the younger age limit, estimated from the fossil age, is 48 Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58–57 Ma. Thus the depositional age of this turbidite facies was interpreted to be 66–56 Ma from the fossil age, probably close to 57 Ma. Most of the zircon U–Pb ages from the Nakanogawa Group are younger than 80 Ma, with a major peak at 60 Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80 Ma. Older zircon ages (120–80 Ma, 180–140 Ma, 340–220 Ma, 1.9 Ga, 2.2 Ga, and 2.7 Ga) give information about the provenance of other rocks in the Hidaka Belt. It is inferred that the Nakanogawa Group comprises protoliths of the upper sequence of the Hidaka Metamorphic Zone, which therefore has the same depositional age as the Nakanogawa Group (66–48 Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.     

Nannofossil age constraints for the northern KwaZulu-Natal shelf-edge wedge: Implications for continental margin dynamics, South Africa, SW Indian Ocean

Samples collected from the shelf-edge wedge using surface grab samples and the Jago submersible constrain the KwaZulu-Natal shelf-edge wedge to a late Pliocene age on the basis of the absence of Gephyrocapsa oceanica s.l. and Discoaster brouweri, and the presence of Calcidiscus macintyrei. This correlates with proposed Tertiary sea-level curves for southern Africa and indicates relative sea-level fall during the late Pliocene coupled with hinterland uplift. Exposed failure scarps in the upper portions of submarine canyons yield sediment samples of early Pleistocene ages, indicating the uppermost age of deposition of clinoform topsets exposed in the scarp walls. Partially consolidated, interbedded silty and sandy deposits of similar age outcrop in the thalweg of Leven canyon at a depth of 150 m. These sediments provide an upper age limit of the shelf-edge wedge of early Pleistocene, giving a sedimentation rate of this wedge of 162–309 m/Ma. The distribution of widespread basal-most Pleistocene sediments on the upper slope indicates that these sediments escaped major reworking during sea-level falls associated with Pleistocene glaciations and remain as relict upper slope veneers. The absence of more recent sediments suggests that this area has been a zone of sediment bypass or starvation since the early Pleistocene. Areas where younger sediments mantle deposits of early Pleistocene ages represent areas of offshore bedload parting, re-distributing younger Holocene sediment offshore and downslope.     

Origin of Taiwan Canyon and its effects on deepwater sediment

The continental slope of the Taiwan Shoal, which has cultivated numerous submarine canyons, is located in a passive continental margin environment. However, the trend of the Taiwan Canyon, with its 45° intersection angle, is obviously different from that of the erosion valley downward along the continental slope. A distinct break is present in the lower segment of the Taiwan Canyon, which then extends from west to east parallel to the continental slope until finally joining the Manila Trench. By utilizing multiple-beam water depth data, high-resolution seismic data, and sediment cores, this study describes the topographic characteristics of the Taiwan Canyon and provides a preliminary discussion on the origin of the Taiwan Canyon and its effect on deepwater sediment. The terrain, landform, and sediment of the Taiwan Canyon exhibit segmentation characteristics. The upper segment is characterized primarily by erosion, downward cutting with a V shape, and wide development of sliding, slumping, and other gravity flow types. The middle segment is characterized mostly by U-shaped erosion-sedimentation transition and development of an inner levee. The lower segment is characterized primarily by sedimentation and development of a sediment wave. The bottom current has a significant reworking effect on the interior sediments of the canyon and forms reworked sands. The formation and evolution of the Taiwan Canyon is closely related to sediment supply, gravity sliding(slumping), faulting activities, and submarine impaling. Given the sufficient terrigenous clastic supply, the sediments along the continental shelf edge continuously proceed seaward; gliding and slumping in the front edge provide driving forces for the formation of the canyon. Faulting activities result in stratum crushing, and the gravity flow takes priority in eroding the relatively fragile stratum. Thus, the direction of the extension of the canyon crosses the surrounding erosion valley obliquely. Seamounts are formed through submarine impaling. Owing to seamount blocking, the lower segment of the canyon is turned toward the east–west direction. Large amounts of sediments overflow at the turning, forming sediment waves.     

Sediment delivery,resuspension, and transport in two contrasting canyon environments in the southwest Gulf of Lions

Multiple canyons incise the continental slope at the seaward edge of the continental shelf in the Gulf of Lions and are actively involved in the transfer of sediment from shelf to deep sea. Two canyons in the southwest region of the Gulf of Lions, Lacaze-Duthiers Canyon and Cap de Creus Canyon, were instrumented with bottom-boundary-layer tripods in their heads to evaluate the processes involved in sediment delivery, resuspension and transport. In both canyons, intense cold, dense-water flows carry sediment across the slope. In the Lacaze-Duthiers canyon head (located ∼35 km from the shoreline), dense-water cascading into the canyon was episodic. Currents were highly variable in the canyon head, and responded to interactions between the along-slope Northern Current and the sharp walls of the canyon. Inertial and other high-frequency fluctuations were associated with suspended-sediment concentrations of ∼5 mg/l. In Cap de Creus canyon head (located ∼14 km from the shoreline), downslope currents were higher in magnitude and more persistent than in Lacaze-Duthiers canyon head. Greater suspended-sediment concentrations (peaks up to 20 mg/l) were observed in Cap de Creus Canyon due to resuspension of the canyon seabed during dense-water cascading events. The similarities and contrasts between processes in these two canyon heads emphasize the importance of the interaction of currents with sharp canyon bathymetry. The data also suggest that cold, dense-water flows have more potential to carry sediment to the slope on narrow shelves, and may more efficiently transfer that sediment to the deep sea where a smooth transition between shelf and slope exists.     

Micropaleontological and seismic observations for early developmental stage of the southwestern Ulleung Basin, East Sea (Japan Sea)

Abstract   The Ulleung Basin is located in the southwestern part of the East Sea (Japan Sea) and contains thick Neogene sediment. Detailed examination of the stratigraphic distribution of dinoflagellates was carried out on samples from the onshore Pohang Basin (E well) and two wells (Gorae I and Dolgorae VII) in the southwestern Ulleung Basin, to investigate the early evolution of the basin. The results show that thick syn-rift sediments mainly consist of terrestrial deposits and are widespread over the basin. This supports an extensional tectonic origin for the basin. The initiation of the deposits dates back to 17–16.4 Ma. Furthermore, well-preserved Eocene to Oligocene dinoflagellate taxa found in Miocene deposits of wells implies that the age of initial rifting might be Oligocene or earlier. Our results provide constraints for understanding the opening process of the East Sea.     

Comprehensive constraint on the tectono-sedimentary setting of Late Paleozoic turbidites of the Kamuste area, eastern Junggar, Xinjiang

Petrography and geochemistry, combined with sedimentation analyses allow for a thorough evaluation of the tectono-sedimentary setting of late Paleozoic turbidites of the Kamuste area, eastern Junggar. Sandstones of the Alabiye¹⁾ Formation are composed mostly of volcanic and sedimentary detritus with lesser amounts of plagioclase and quartz. They were derived from an undissected magmatic-arc provenance. The geochemistry of sandstone-mudrock suites indicates a fesic-intermediate igneous provenance, and constrains the Alabiye Formation to have derived from a differentiated oceanic-continental margin island-arc tectonic setting. Likewise, geochemistry and sandstone petrography of the Kamuste Formation reflect a mixed provenance signature dominated by magmatic arc, basement uplift, and subduction-complex sources of a differentiated continental-island arc. Sedimentation analysis indicates that the Alabiye and Kamuste formations are two sets of turbidite sequences deposited on a submarine slope and a submarine fan and basin plain respectively. In conclusion, submarine slope turbidite deposition of the Alabiye Formation records the main sedimentary response to the development of early Devonian back-arc basins of the northern Junggar tectonic belt. Submarine fan and basin plain turbidite and background hemipelagic deposition of the Kamuste Formation record the main sedimentary response to the late Early Carboniferous development of an inter-arc relict ocean basin of the eastern Junggar composite terrane.     

渐新世以来南海北部陆坡区沉积演化及其对构造的响应

通过对南海北部陆坡下部ODPl148站位沉积物中陆源矿物组分的含量、堆积速率、粒度、石英氧同位素及石英扫描电镜的分析,探讨南海沉积演化及其构造响应.结果显示,根据综合指标的变化特征可将南海海盆的沉积演化划分为5个阶段:扩张初期(34～28.5 Ma)、构造活动剧烈期(28.5～23 Ma)、构造活动减弱期(23～16....     

Controls on bedrock channel incision along nahal paran,Israel

Nahal Paran drains 3600 km² of Egypt's Sinai peninsula and Israel's Negev Desert. Much of the channel is alluvial, but a canyon 10·5 km long has been incised into Late Cretaceous chert and dolomite in the lower portion of the basin. Slackwater deposits and paleostage indicators preserved within the canyon record approximately 10 floods of 200 to 2500 m³ s^?1 over a period of at least 350 years. Step-backwater simulations of flood-flow hydraulics indicate extreme variations in stream power per unit area along the length of the canyon, and associated variability in energy expenditure and sediment transport. These variations reflect channel cross-sectional morphology. The greatest values of stream power occur along the lower half of the study reach, in association with three pronounced knickpoints and an inner channel. The locations of these features reflect the exposure of thick, resistant chert layers along the channel. The presence of several similar, but buried and inactive, knickpoints along the upper study reach indicates that the locus of most active channel incision has shifted with time, probably in response to baselevel changes associated with tectonic activity along the Dead Sea Rift. Thus, the rate and manner of channel incision along the canyon of Nahal Paran are controlled by lithologic variability and tectonic uplift as they influence channel morphology and gradient, which in turn influence hydraulics and sediment transport.     

Geology of the Kodiak Shelf,Alaska: environmental considerations for resource development

Geologic features and processes pose several environmental concerns to resource development on the Kodiak Shelf in the Gulf of Alaska. Tectonism causes fault movement, strong seismic ground shaking, and changes in sea-floor elevation. Earthquake epicenters and structural deformation are areally concentrated, which implies areal variation in the severity of tectonic hazards. Exposures of bedrock appear to provide competent foundational material over broad areas of the sea floor, although locally rough topographic expression of inclined bedrock strata might influence the siting of engineering structures. Stable deposits of gravelly unconsolidated sediment, derived from Pleistocene glaciers, are also widespread. Deposits of fine-grained sediment in areas of negative relief might possess less desirable foundational properties than bedrock or coarse unconsolidated sediment, but their thickness is only a few tens of meters in most places. Evidence of sediment slides is rare on the shelf, which implies general slope stability, but large slides, whose activity is related to tectonism, are abundant just seaward of the shelf break. Local occurrences of gas-charged sediment show limited indication of overpressuring and no evidence of instability, but low sediment strength is possible. Fields of large sand waves might be sites of high-energy bed-load transport with the potential for erosion and abrasion problems. As inferred from sediment dispersal patterns, pollutants that become incorporated into bottom sediment could be concentrated and stored long-term in troughs that trend transversely across the shelf.     

Sedimentary records of the late Early Cretaceous compressional setting in the South China block: A case study of the Xingning Basin,Guangdong Province

Recently, some scholars have proposed that the South China Block (SCB) was controlled by a compressive tectonic regime in the middle–late Early Cretaceous, challenging the belief that the SCB was under an extensional setting during the Cretaceous. The Early Cretaceous tectonic setting constraint in the SCB can offer vital insight to clarify the Mesozoic subduction history of the Paleo-Pacific. Therefore, to determine the SCB tectonic regime during the Early Cretaceous, this study investigated sedimentary rocks from the Lower Cretaceous Heshui Formation in the Xingning Basin, a foreland basin located in the southeastern SCB. Provenance analysis was performed using sandstone modal analysis, sandstone geochemical characteristics, and detrital zircon geochronology. Based on the results, we discussed basin sediment sources and the SCB tectonic regime during the Early Cretaceous. The results showed that the maximum Heshui Formation depositional age was 103 Ma ± 1.6 Ma in the Early Cretaceous Albian. Detrital framework modes and geochemical characteristics of sandstone indicated that Heshui Formation's source rocks were granites and sedimentary rocks. The detrital zircon U–Pb ages could be classified into two major and four subordinate age populations. The Wuyi Terrane to the north and southeast coastal regions to the east were the primary potential Heshui Formation source areas. However, the lower and upper sandstones are different in the peak ages, ~437 and ~146 to 104 Ma, respectively, indicating that the major source area shifted from the Wuyi Terrane to the southeastern coastal regions during the late Early Cretaceous. The sandstone modal analysis results indicated that the source area comprised mainly collisional–orogenic material. The SCB was under a compressive tectonic regime during the late Early Cretaceous and this compression action continued until at least 103 Ma ± 1.6 Ma.