Synsedimentary Faults Research Articles

Sedimentary features and filling process of the Miocene gravity-driven deposits in Ledong area, Yinggehai Basin, South China Sea

The shallow marine channels and fans in the Yinggehai Basin have huge hydrocarbon exploration potential. The depositional features and filling processes of the submarine channel and fan system during 10.5–7.2 ma were studied based on seismic data, cores, well curves, and lab data. (1) Five depositional elements were recognized in the channel system, including PSC, MSC, DSC, levee, and abandonment fill, and the deposits show the typical sedimentary structure of debris flow. The filling process of the channel system was divided into three stages based on deposits supply intensity. The stages were made up of several filling units, and each unit possessed at least one complete channelized depositional succession. (2) The fan system was developed under the large-scale “U” shaped feeding canyon, and its filling process was divided into two stages, the shape and distribution were controlled by shelf break and syn-sedimentary faults in the early stage, and was influenced by the localized changes of paleogeomorphology in late stage. The influence factors for the development of gravity-driven systems were summarized into two levels: regional depositional environment changes and localized distinctive geologic settings. The former one controlled the development of shallow water gravity-driven deposits in the whole basin, mainly including rapid falling of sea-level, moderate subsidence rate, and stable deposits supply. The latter one controlled the morphology and distribution of depositional elements in gravity-driven systems. The pre-existing banded low terrain induced by blind faults controlled the shape of the confined channel, and variable faulting intensity in different segments of the syn-sedimentary fault and localized paleogeomorphology changes influenced the distribution of channels and inner fan in the fan system. The study of deposition features, internal architecture, and controlling factors may provide a reference for further exploration in this area and a deeper understanding of gravity-driven processes in shallow marine.

Petrological and micrometer-scale geochemical constraints on chert origins in the Dengying Formation, Yangtze Block, South China: Implications for Late Ediacaran hydrothermal activity and tectonic setting

The Ediacaran–Cambrian (E–C) transition was a key interval in geological history. Chert was widely distributed in the Yangtze Block at the end of the Ediacaran and may have recorded important information about the coevolution of earth systems in this period. Five lithofacies of chert were identified in the northern margin of the Yangtze Platform. Breccias with crustiform chert were formed by hydrothermal fracturing of dolomite to form breccias in vents, which were immediately immersed and crusted by a silicon-rich fluid. This silicon-rich fluid was deposited directly onto the inner walls of the conduits to form chert linings. The silicon-rich fluid filled the water-fractured fissures to form spider-veinlet chert and then mixed with residual seawater in primary pores of the bedding to form void-filling cherts. Finally, the dolomite far away from the conduits was replaced by the silicon-rich fluid, forming chert bands. In the vents, there are typical hydrothermal mineral assemblages of quartz, pyrite, barite, Fe–Mn metal microparticles, and saddle dolomite and these assemblages contain abnormally high-temperature inclusions. The rare-earth element (REE-Y) patterns of void-filling chert, crustiform chert, and saddle dolomite show that they originated from normal seawater influenced by a low-temperature hydrothermal fluid, silicon-rich hydrothermal fluid, and high-temperature hydrothermal fluid, respectively. The trace element ratios and Si isotope contents of chert also show that deposition was mainly caused by hydrothermal solutions. All these data indicate that the various types of cherts at the top of the Dengying Formation near the synsedimentary fault zone constitute typical siliceous sinter. There was massive upward migration of silicon-rich hydrothermal fluids from depth to the seafloor during the E–C transition, which resulted in abnormally silicon-rich seawater and a widespread distribution of various cherts in the Yangtze Block. The identification of siliceous sinter in synsedimentary faults indicates an intense extension of the northern margin or even of the entire Yangtze Block during the E–C transition. This may be related to the combined effects of slab pull on the western side and rollback on the northeastern side of the Yangtze Block during the convergence of Gondwana.

Carbon isotopes, ammonites and earthquakes: Key Triassic-Jurassic boundary events in the coastal sections of south-east County Antrim, Northern Ireland, UK

A continuous succession of marine and marginal-marine sediments of Rhaetian (Late Triassic) and Hettangian (Early Jurassic) age is present in the Larne Basin in Northern Ireland. These strata cover a period in Earth's history that included the emplacement of the Central Atlantic Magmatic Province (CAMP), the End Triassic Mass extinction (ETE), the Triassic–Jurassic Boundary (TJB), and major perturbations in the global carbon cycle. The Waterloo Bay section in the Larne Basin offers a well exposed sedimentary succession that spans this interval, and it has previously been proposed as a candidate GSSP for the base of the Jurassic System. A high-resolution δ13Corg and organic carbon record for this locality is presented here, with these new data tied to previous stratigraphic descriptions, ammonite biostratigraphy, atmospheric carbon dioxide concentration (pCO2) estimates, and nearby borehole sections that do not suffer from the thermal alteration that has affected the Waterloo Bay section. Several new exposures, unaffected by thermal metamorphism, are described that could provide future palynological and micropalaentological studies across this important boundary interval. Correlation is established between the well-studied sections in north Somerset and the likely position of the TJB in the Larne Basin, and records of soft sediment deformation, synsedimentary fault movement, relative sea-level change and their likely causes are discussed.

Sedimentary Setting and Ore-Forming Model in the Songtao Manganese Deposit, Southwestern China: Evidence from Audio-Frequency Magnetotelluric and Gravity Data

The break-up of the supercontinent Rodinia in the late Neoproterozoic led to the formation of the Nanhua rift basin within the South China Block. The Datangpo-type manganese deposit, which developed in the Nanhua rift basin, is one of the most important types of manganese deposits in South China. Although it is widely accepted that deep sedimentary structures significantly affect the manganese ore system, the relationship between the manganese deposits in South China and the Nanhua rifting process is still unclear. The origin of the manganese ore layer remains controversial. In this paper, we integrated the audio-frequency magnetotelluric (AMT) data, gravity data, and comprehensive geological and borehole data analysis to characterize the structure of the Datangpo-type manganese deposit in Songtao, Guizhou Province. The resistivity and density models produced an inclined layered structure, which correlated well with the coeval sediment strata of the Nanhua rift basin. A high-resistivity cap was observed from the surface to a depth of 800 m, corresponding to the Cambrian Loushanguan (ϵ3−4ls) and Palang dolomite formation (ϵ2p), which has helped the storage of the manganese ore. The most significant low-resistivity anomaly (25–40 Ω·m) resides at a depth of 1400 m in the Nantuo (Nh3n) gravel sandstone and Datangpo (Nh2d) silty and carbonaceous shale, corresponding to the ore-forming layer. This distinct low-resistivity layer was possibly produced by aqueous fluids and pyrite in the syn-sedimentary fault and alteration zone. The accumulations of sulfide minerals in the rock samples suggest a possible anoxic-euxinic deposition environment during the manganese mineralization and precipitation. The fault revealed in the resistivity models is perhaps a previous fault zone produced by extension in the Nanhua rifting process, which provided migration and upwelling channels for ore-forming minerals. Based on our resistivity models, density models, and geological survey, the manganese ore-forming model was derived, which can help to provide geophysical evidence for the origin of the Datangpo-type manganese deposit.

Soft-sediment deformation structures in a lacustrine depositional context: An example from the Eocene Dongying Depression in the Bohai Bay Basin, East China

The study documents soft-sediment deformation structures (SSDSs) in the lacustrine Dongying Depression, a Paleogene synrift basin located in the southeastern corner of the Bohai Bay Basin, East China. The focus of this study is the lower part of the upper 4th member of the Eocene Shahejie Formation (Es4U), characterized by littoral to sublittoral deposits in the southwestern Dongying Depression. Nine lithofacies are grouped into the upper shoreface facies associations (FA1-1), lower shoreface facies associations (FA1-2), and sublittoral facies associations (FA2). Fine-grained calcareous sandstone was mainly deposited in FA1-1, with thinly alternating calcareous mudstone and siltstone or sandstone in FA1-2, while deposition in FA2 was dominated by mudstone, containing sediment gravity flow deposits. The SSDSs widely encountered in the lower Es4U can be classified into seven categories: (1) load structures, (2) water-escape structures, (3) folds and convolute lamination, (4) sediment-injection structures, (5) microfaults, (6) autoclastic breccias, and (7) slumps. Detailed observations of SSDSs from cores indicate liquefaction and fluidization as the main deformation mechanisms in the sands and muds. Moreover, rupturing of cohesive deposits also contributes to the soft-sediment deformation. The types and morphologies of the SSDSs are closely related to the rheologies of the sediments (e.g., the sand content), influenced by the facies that determine the lithologies and stratigraphic stacking patterns. Except for some deformation structures related to subaqueous sediment gravity flows (e.g., muddy debrite flows and slumps), most SSDSs are ascribed to rift-related seismicity due to the activity of the adjacent synsedimentary faults when the Dongying Depression underwent quick subsidence during the Eocene.

Impact of the early and synsedimentary cretaceous fracturing on the final structuring of the Atlas folds of the Central-Southern Atlas, case of Jebel Meloussi (Central Tunisia)

The aim of this work is to study the combined effects of early fracturing and synsedimentary tectonic activity on the current structuring in the Tunisian Atlas. The fieldwork carried out on Jebel Meloussi anticline, imposing structure in the Central-Southern Atlas, reveals significant variations in thicknesses and facies in the different Cretaceous units. These variations result from the reactivation of an ante-Cretaceous NE-SW to E-W preexisting fault, which has controlled the sedimentation of the Cretaceous series in an N–S direction. Likewise, this tectonic inheritance is associated to a well pronounced synsedimentary tectonic activity, which is clearly involved in the structuring of Upper Cretaceous mega-grabens. It can be deduced from this study that the current structuring of Jebel Meloussi is governed essentially by a set of WNW-ESE to submeridian synsedimentary faults, which are connected to the preexisting quasi-directional fault on which the main fold was developed during compressional events. The study of the activation and reactivation of these fractures since the Cretaceous provides details on lithostratigraphic features and tectonic movements.

Classification of slope area types and hydrocarbon accumulation rules in Wuerxun-Beier Depression

After more than 50 years of exploration and development, Wuerxun-Beier depression has entered a high exploration stage. It is more and more difficult to find large-scale structural reservoirs. Therefore, structural-lithologic reservoirs in slope areas have gradually become the focus of exploration. Based on the overall study and dissection of the depression, the geological differentiation characteristics of the large-scale slope area are discussed. According to the occurrence of the slope, the intensity of tectonic activity, the intensity of source supply and the pattern of synsedimentary fault combination, the slope zone in the study area can be divided into six types, namely, the steep slope with strong source along the fault step type, the compound fault step type of medium slope with strong source, the reverse fault step type of the gentle slope with strong source, and the weak supply type. Different types of slope have different sand control mechanisms and reservoir forming characteristics, such as the source gentle slope forward fault terrace type, the weak source gentle slope reverse fault terrace type and the weak source middle slope forward fault terrace type. Source supply intensity and slope dip control the development type of sedimentary system, the combination of synsedimentary faults and valleys controls the distribution characteristics of sedimentary system. Through exploration practice, significant discoveries have been made on the southwestern, southeastern and northern slopes of Beixi, Wuerxun-Beier depression, which confirms that the slope area is the main breakthrough direction for oil and gas exploration and large-scale reservoir increase in continental fault-depression lake basins.

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part I: Syndiagenetic Base Metal Sulfide Mineralization Related to Dynamic Subbasin Evolution

Abstract Divergent genetic models have been proposed for clastic dominant (CD-type) massive sulfide Zn-Pb mineralization in the Proterozoic Carpentaria Zn Province. Due to varying degrees of tectonic overprint, it has been difficult to accurately constrain structural and paragenetic timing aspects of the CD-type genetic model, and the most basic timing constraints (syngenetic vs. epigenetic, synextension vs. syninversion) remain debated. The recently discovered Teena Zn-Pb deposit is hosted by an exceptionally well preserved subbasin that permits relative timing relationships to be well defined. Using a combination of geophysical, structural, sedimentological, and petrographic datasets, a new model for subbasin development and syndiagenetic hydrothermal replacement mineralization is developed for the Teena mineral system. At Teena, sulfide mineralization was deposited from lateral fluid flow beneath an impermeable seal several hundred meters below the paleosurface and maximum flood surface, after formation of fine-grained diagenetic pyrite (py1) and dolomite nodules. Sulfide mineralization resulted from syndiagenetic carbonate replacement and pore space cementation where thermochemical sulfate reduction took place. The sulfide mineralization is therefore partly cospatial but not cogenetic with its thick pyritic hanging wall, and its lateral alteration footprint is much smaller than predicted by sedimentary exhalative (SEDEX) models. An additional zone of low-grade Zn-Pb mineralization in the footwall W-Fold Shale Member represents a different style of mineralization not previously reported for Carpentaria CD-type Zn deposits: it is associated with strata-bound lenses of hydrothermal dolomite (HTD) that formed by both replacement and carbonate dissolution and infill, which yielded diverse cavity-infill textures that include coarse-bladed dolomite fans cemented by interstitial sphalerite, dolomite, and quartz. Volumetrically minor Zn mineralization is also present in a fault conduit hydrothermal breccia and in hanging-wall synorogenic vein sets derived by hydrothermal leaching and remobilization of Zn from the underlying mineralized zones. Whereas both the Teena and nearby McArthur River Zn-Pb deposits are located along the northern margin of the 3rd-order Hot Spring-Emu subbasin, they formed in separate 4th-order subbasins in association with local extensional growth faults. Growth fault movement in the Teena subbasin was initiated during deposition of the W-Fold Shale Member and persisted episodically until a weak structural disconformity associated with sedimentary facies regression developed in the Upper HYC unit. Shifting patterns in depocenter location, sedimentary facies development, mineralization, and alteration zonation are attributed to progressive growth and linkage of segments of a regionally anomalous ENE-trending, synsedimentary fault zone. Similar patterns of extensional subbasin development were repeated in other Zn-mineralized subbasins throughout the River supersequence across the northern Carpentaria Zn Province, and formed in response to a short-lived episode of north-northwest–south-southeast regional extension around ca 1640 ± 5 Ma, triggered by far-field subduction events.

Megabeds in Istrian Flysch as markers of synsedimentary tectonics within the Dinaric foredeep (Croatia)

Istrian Flysch was deposited during the Eocene in the Dinaric foredeep and is composed of hemipelagic marls and various gravity flow deposits. The latter are predominantly 5-40 cm thick turbidites, developed mostly as laminated and cross-rippled sandstone beds (Tb-e, Tc-e and Td-e Bouma sequences). In addition to the turbidites, there are deposits characterized by a significant thickness, occasionally more than 10 m, described as complex (bipartite) megabeds. The megabeds are composed of debrites in the lower part (Division I), and high-density turbidites in the upper part (Division II). The distinct clast composition of each megabed indicates that the lithoclasts were derived from tectonically active slopes and fault scarps along which collapses of the different parts of the Cretaceous to Palaeogene neritic carbonate succession, that underlie the Flysch, occurred. The Division II deposits are well cemented, normally graded calcirudite/calcarenites composed mostly of orthophragminids, nummulitids, and red algae, originating from outer ramp environments. Redeposited marl, observed in the matrix of the debrites and as intraclasts in some megabeds, implies that the collapses along the synsedimentary fault scarps and steep slopes also occurred within the foredeep itself, during the rapid tectono-sedimentary evolution of the Dinaric foreland basin.

Sedimentary facies, depositional environments and conceptual outcrop analogue (Dam Formation, early Miocene) Eastern Arabian Platform, Saudi Arabia: a new high-resolution approach

This paper presents the facies and depositional environment of the early Miocene Dam Formation, Eastern Arabian platform, Saudi Arabia. Deposition of Dam Formation (Fm.) was considered as a restricted shallow marine deposition. Few studies suggest the role of sea-level change in its deposition but were without decisive substantiation. Here, we describe the facies and high-resolution model of Dam Fm. under varying depositional conditions. The depositional conditions were subjected to changing relative sea level and tectonics. High-resolution outcrop photographs, sedimentological logs, and thin sections present that the mixed carbonate–siliciclastic sequence was affected by a regional tectonics. The lower part of Dam Fm. presents the development of carbonate ramp conditions that are represented by limestones and marl. The depositional conditions fluctuated with the fall of sea level, and uplift in the region pushed the siliciclastic down-dip and covered the whole platform. The subsequent rise in sea level was not as pronounced and thus allowed the deposition of microbial laminites and stromatolitic facies. The southeast outcrops, down-dip, are more carbonate prone as compared to the northwest outcrop, which allowed the deposition of siliciclastic-prone sedimentation up-dip. All facies, architecture, heterogeneity, and deposition were controlled by tectonic events including uplift, subsidence, tilting, and syn-sedimentary faulting, consequently affecting relative sea level. The resulting conceptual outcrop model would help to improve our understanding of mixed carbonate–siliciclastic systems and serve as an analogue for other stratigraphic units in the Arabian plate and region. Our results show that Dam Fm. can be a good target for exploration in the Northern Arabian Gulf.

Do soft sediment deformations in the Late Triassic and Early Jurassic of the UK record seismic activity during the break-up of Pangea?

The lagoonal and shallow marine sediments of the Penarth Group in the UK span the Triassic–Jurassic boundary. These sediments contain several disturbed levels with soft sediment deformations (SSDs), such as synsedimentary faults, injective domes, recumbent folds and slumps that are recognised in most basins from SW England and South Wales to NW Northern Ireland. Field observations, notably the close link of the SSDs to active faults, attest an earthquake origin of the SSDs. Fluids, faults, overpressure and lithology guided the style of the SSDs and their distribution in the sedimentary sections. Analysis of the directional data relating to SSDs in each disturbed level shows preferred orientations of deformation, which correspond to the local state of stress at the time. We favour a series of earthquakes, rather than a single mega-event as a trigger of the observed features. The active local extensional tectonic context was driven by the opening of the Permo-Triassic basins in Western Europe. The data from the SSDs in the UK suggest the development of a multi-directional, mosaic-style extensional context to occur during this early phase of the break-up of Pangea. Our integrated tectonic/sedimentary study suggests that directional data from faults, injective domes, recumbent folds and slumps preserved in sediments are reliable to reconstruct past seismic activity and basin geodynamics.

From marginal to axial tidal-strait facies in the Early Pleistocene Siderno Strait

This geological guide presents the description of locations associated with a two-day field trip arranged in relation to the 10th International Congress of Tidal Sedimentology (Tidalites), Matera, Italy. The field guide describes sedimentological features of the largest among a series of tectonically controlled tidal straits that dissected the Calabrian Arc in southern Italy during the Early Pleistocene. The WNW-ESE trending, 50x20 km- wide Siderno Strait connected the Tyrrhenian with the Ionian seas. Due to tidal phase opposition between the two basins, continuous water-mass exchanges occurred through the strait, leading to powerful, bi-directionally flowing tidal currents. Sediments filling the Siderno Strait derived from both fluvial supply from the margins and intra-basinal autochthonous carbonate-factory debris. The main objective of the two-day field trip is to guide the visitor through a cross-section of the ancient strait, starting from one of the margins, ending in the deeper axial zone. The focus during the day one is on strait-margin deltaic fluvial-dominated deposits, shed from the tectonically-controlled, northern border and reworked by tidal currents in their distal reaches (delta front). Erosively-based, 4–5 m-thick pebbly-sandstone strata intercalated with 2–3 m-thick tidally-generated cross strata stack into a ca. 170 m-thick succession, exposed in a series of outcrops progressively located down-current with respect to the inferred entry point to the north. The focus of the day two is a ca. 150–190 m-thick succession consisting of cross-stratified mixed (bioclastic-siliciclastic) deposits, forming a series of WNE-ESE-oriented, elongated ridges that accumulated in the south-eastern axial zone of the Siderno Strait. The selected stops offer panoramic views of exceptionally continuous sections and close-up observations, revealing different scales of depositional architectures and a variety of sedimentary structures and trace fossils that record the development of these tidal sand ridges during the strait lifespan. The interplay between the tectonic uplift of a central bedrock sill and a number of syn-sedimentary faults and high-frequency relative sea- level changes (induced by glacio-eustacy and active tectonics) can be deciphered from the architecture of the tidally-generated cross strata composing the main body of the ridges.

Microfacies and stable isotope features of the Lower–Middle Jurassic carbonate rocks of Western Saharan Atlas (Aïn Ouarka area, Algeria)

The Jurassic Lower Carbonate Group in the Aïn Ouarka area of Western Saharan Atlas, Algeria, is represented by six formations, from base to top: Hettangian Chemarikh Dolostone, Early Sinemurian–Pliensbachian Aïn Ouarka Limestone, Toarcian Aïn Rhezala Limestone, Aalenian–Late Bajocian Raknet El Kahla Limestone Breccia, Late Bajocian Theniet El Klakh, and Late Bajocian–Bathonian Tifkirt Limestone formations. From the microfacies analysis, six microfacies types (MF1–MF6) have been recognized and grouped into three associations developed during a transgressive–regressive cycle: (a) inner ramp facies association; (b) middle ramp facies association; and (c) outer ramp facies association. The mineralogical analysis of the carbonate rocks reveals that they contain mostly low‐Mg calcite associated with ankerite, pyrite, and other detrital minerals such as quartz, chlorite, illite, feldspar (albite), and a few clay minerals. These minerals could be related to the deep fluid and hydrocarbon circulations during deposition. The isotopic data display a variation of δ13C isotopic values between −5.14‰ and +2.21‰ (VPDB) and between −8.12‰ and −4.95‰ for δ18O values (VPDB). The set of δ13C values is similar to the signature of marine dissolved inorganic carbon. First of all, the positive values of δ13C show that the origin of carbon is not from the organic‐rich zone (microbial zone), but probably derived from pore‐water and/or biogenic carbonate precursors. On the other hand, the negative values of δ13C indicate that the carbon may result from organic sources linked to the sulphate reduction bacteria activity, or by a heightened volcanic and/or hydrothermal activities releasing light carbon (12C). The negative δ18O values are not consistent with marine water ambient temperature, but with a possible influence of diagenesis or increasing of temperature by hydrothermal water. This hydrothermal activity is controlled by synsedimentary faults during the Early Jurassic and related to a late pulse of the Central Atlantic Magmatic Province (CAMP) volcanism and by regional volcanism during the Middle Jurassic (Bajocian–Bathonian).

Constraining the Passive to Active Margin Tectonics of the Internal Central Apennines: Insights from Biostratigraphy, Structural, and Seismic Analysis

The polyphase structural evolution of a sector of the internal Central Apennines, where the significance of pelagic deposits atop neritic carbonate platform and active margin sediments has been long debated, is here documented. The results of a new geological survey in the Volsci Range, supported by new stratigraphic constraints from the syn-orogenic deposits, are integrated with the analysis of 2D seismic reflection lines and available wells in the adjacent Latin Valley. Late Cretaceous syn-sedimentary faults are documented and interpreted as steps linking a carbonate platform to the adjacent pelagic basin, located to the west. During Tortonian time, the pelagic deposits were squeezed off and juxtaposed as mélange units on top of the carbonate platform. Subsurface data highlighted stacked thrust sheets that were first involved into an initial in-sequence propagation with top-to-the-ENE, synchronous to late Tortonian foredeep to wedge-top sedimentation. We distinguish up to four groups of thrust faults that occurred during in-sequence shortening (thrusts 1–3; about 55–60 km) and backthrusting (thrust 4). During Pliocene to recent times, the area has been uplifted and subsequently extended by normal faults cross-cutting the accretionary wedge. Beside regional interest, our findings bear implications on the kinematic evolution of an orogenic wedge affected by far-traveled units.

Mass collapse and resedimentation on a Brigantian–Early Namurian platform margin, Halkyn–Mold area, North Wales, UK

Observations on Late Mississippian strata are summarized including: the Brigantian Cefn Mawr Limestone and Minera formations; and the Namurian Pentre Chert and Cefn‐y‐Fedw Sandstone formations. Many of those observations relate to transitory, commercial quarry exposures, especially in the Cefn Mawr Limestone Formation. During the Late Mississippian, the area was located in proximity to an unstable northeast‐facing platform edge and slope on which earlier workers postulated a megaslide, probably related to the Nant Figillt Fault zone and affecting the Cefn Mawr Limestone and Pentre Chert formations. This study reinforces that general model and highlights new evidence indicative of mass wastage along the platform margin including: synsedimentary faults, slides and slumps. Penecontemporaneous submarine crevassing occurred in the Pentre Chert Formation. Limestone beds previously described as tidal or storm‐related sand bars in the Cefn Mawr Limestone Formation are instead considered as gravity‐driven resedimented deposits. Channelling in relatively deep water facies of the Cefn Mawr Limestone, Pentre Chert, and Cefn‐y‐Fedw Sandstone formations occurred.

Control effects of the synsedimentary faults on the basin-marginal fans in the central part of the deep-water area of early Oligocene Qiongdongnan Basin, South China Sea

The synsedimentary faults and basin-marginal fans located in the central part of the deep-water area of the early Oligocene Qiongdongnan Basin have been investigated using seismic profiles, boreholes, and well-log data. Through the formations of the characterized paleogeomorphology, such as transverse anticlines, fault ditches, and step-fault belts, the synsedimentary faults are known to have controlled the development position, distribution direction, and extension scales of the basin-marginal fans. For example, at the pitching ends of two adjacent faults, transverse anticlines developed, which controlled the development positions and distributions of the fans. During the early Oligocene, the faults controlled the subsidence center, and fault ditches were formed at the roots of the faults. In the surrounding salient or low salient areas, which were exposed as provenance areas during early Oligocene, the fault ditches acted as the source channels and determined the flow paths of the clastics, where incised valley fills were obviously developed. The fault ditches which developed in the sedimentary basins were able to capture the drainage systems and influenced the distributions of the fans. The large boundary faults and the secondary faults generated two fault terraces and formed step-fault belts. The first fault terrace caused the clastics to be unloaded. As a result, fans were formed at the entrance to the basin. Then, the second fault terrace caused the fans to move forward, with the fans developing in a larger extension scale. The results obtained in this study will potentially be beneficial in the future prospecting activities for reservoirs and coal-measure source rocks in the basins located in the deep-water areas of the South China Sea.

Source‐sink System and Sedimentary Model of Progradational Fan Delta Controlled by Restricted Ancient Gully: An Example in the Enping Formation in the Southern Baiyun Sag, Pearl River Mouth Basin, Northern South China Sea

AbstractTectonic activity occurred during the depositional period of the Enping Formation in the southern Baiyun Sag in the Pearl River Mouth Basin, with a series of synsedimentary faults and a set of large fan delta reservoir bodies developing. There is no data from drilling, cores, etc. for this area, so this paper applies three‐dimensional seismic data to study the structural style of the steep slope zone, the seismic facies characteristics of fan deltas, and the source‐sink system and sedimentary model of the Enping Formation. The control action of tectonic activity on fan deltaic sedimentary systems is studied by combining interpretation of fault systems, dissection of structural styles, seismic reflection structure, seismic facies geometry, and seismic attribute analysis, together with theoretical analysis of the source‐sink deposition process. The Baiyun Sag has experienced tectonic activity since the Eocene, and a series of synsedimentary faults are developed in the southern steep slope zone. Under the common control of multiple large synsedimentary faults, a large ancient gully formed in the steep slope zone in the south, which gradually widened from south to north. The uplift area in the southern part of the sag was exposed for a long time during the deposition of the Enping Formation and consequently suffered weathering and erosion. The resulting sediments were transported through a system of provenance channels composed of slopes and an ancient gully to the depression area, where they were deposited and eventually converged to form a large fan delta. The fan delta presents the overall characteristics of NS strong wedge reflection and EW strong domal reflection—thick in the middle part and thin in the wings. It displays a lobe‐shaped distribution on the plane, with the fan root pointing to the south slope. According to differences in reflection intensity from bottom to top, it can be subdivided into three stages of progradational sedimentary bodies—the southern uplift and denudation zone, the large ancient gully, and the fan delta—which together constitute a complete source‐sink system. This represents a sedimentary model of progradational fan delta under the overall joint control of the restricted ancient gully and syndepositional faults.

Soft-sediment deformation structures (SSDS) in the Ediacaran and lower Cambrian succession of the Aksu area, NW Tarim Basin, and their implications

The Tarim Basin, the largest continental and marine superimposed basin in the northwestern part of China, is typically viewed as a stable and uninterrupted sequence during the Ediacaran-Cambrian transition. Undeformed strata, thick shallow marine sediments and thin deep marine sediments characterize the basin. In this study, we identify various types of soft-sediment deformation structures (SSDS) and an unconformity at four localities in the Aksu area, NW Tarim Basin. SSDSs include large-scale load structures, plastic intrusions and extrusions, load casts and pillow structures, liquefied breccias, ground fissures and lateral liquefaction, boudinage-like SSDSs, mound and sag structures, slumps, and small syn-sedimentary faults. The deformed layers are sandwiched between undeformed sedimentary rocks. Deformation features are found in the (a) lower Cambrian Yuertusi Formation consisting of interbedded thin black shale and phosphorite chert layers, thick-bedded shallow gray dolomite and silicate layers; (b) upper Ediacaran Qigebulak Formation composed of dark gray to gray dolomite and grayish green mudstone layers; (c) in the brown sandstone layers of the lower Ediacaran Sugaitebulak Formation. These units contain fluvial-lacustrine, nearshore, carbonate platform, shelf, and bathyal sedimentary environments. The SSDSs and more detailed stratigraphy indicate a dynamic setting that includes the heretofore unrecognized unconformities, a sub-stratigraphic sequence interface and high magnitude seismic events. The SSDS were produced during rifting, short sub-aerial exposure and frequent seismic disturbances. Combined on the carbon and oxygen isotope records, gamma radiations and the concentrations of main and trace elements, the differences between the SSDS layers and the undeformed layers are preliminary analyzed. We propose that this sequence not reflect a stable tectonic-sedimentary environment during the Ediacaran-Cambrian transition. Therefore, these units provide a unique glimpse into paleo-tectonic activity and paleo-ecological-environmental variation in the Tarim Basin during the late period of Rodinia breakup and the assembly of Gondwana.

Late Cretaceous transtensional faulting of the Apulian Platform, Italy

This work focuses on the structural setting of a small area of the Murge Plateau, southern Italy, exposing Turonian bauxites and encompassing Upper Cretaceous carbonates of the Apulian Platform. By combining field geological and structural mapping with mineralogical and petrographic investigations of representative rock samples, we document the control exerted by Late Cretaceous, syn-sedimentary, high-angle faults and dilational fissures striking ca. N105E, N135E, N170E, and N45E. Altogether, these structural elements formed a few km-long, transtensional fault system along which karst-related shafts, dissolution caves, and sinkholes developed. Karst processes first affected the topmost limestone beds of the Valanginian-Cenomanian Bari Fm. Then, during Turonian, in a continental setting, cycles of terrigenous sediment deposition and weathering localized in the karst-related cavities, dilational fissures, and at the top of down-dropped fault blocks. Weathering of the Al–Si rich sediments formed bauxites made up of kaolinite, boehmite, goethite, hematite, and anatase mineral phases. The relative enrichment in kaolinite with respect to boehmite documented for the breccia and conglomerate matrix topping the bauxites is interpreted as due to lower degrees of weathering. Differently, the individual bauxite beds do not show any internal significant variation in terms of their mineral assemblages, but form angular unconformities due to syn-sedimentary faulting. The subsequent marine transgression also occurred under the influence of active tectonics, as displayed by growth folds at the bottom of the Conacian-Early Campanian Altamura Fm. The overall results of this study are in agreement with those previously reported for nearby areas of the Murge Plateau exposing Santonian limestone rocks of the Altamura Fm. Accordingly, we infer that Late Cretaceous transtensional faulting of the Apulian Platform was due to a far field stress state, likely associated to the orogenic processes that involved the northern edge of the Adriatic micro-plate.

Carbonate Replacement as the Principal Ore Formation Process in the Proterozoic McArthur River (HYC) Sediment-Hosted Zn-Pb Deposit, Australia

AbstractThe McArthur River (HYC) Zn-Pb-Ag deposit in the Carpentaria Zn belt, northern Australia, is one of the world’s largest and most studied sediment-hosted base metal deposits, owing to its lack of deformation and preservation of sedimentary and ore textures. However, the ore formation process (syngenetic vs. epigenetic) is still a subject of controversy. In this paper we focus on key characteristics of the HYC deposit that remain unexplained: preservation of sedimentary carbonate (dolomite) and its association with Zn, and the role of thallium (Tl) and manganese (Mn) distribution in the orebody.Our findings demonstrate a sequence of events during ore formation: Tl is hosted almost exclusively within euhedral pyritic overgrowths around early diagenetic pyrite; sphalerite mineralization occurred after Tl-bearing pyrite overgrowths, in association with acid dissolution (replacement) of laminated and nodular dolomite across the subbasin; and outer rims are enriched in Mn on preserved dolomite at the dissolution reaction front in contact with sphalerite. New thermodynamic fluid chemistry modeling demonstrates the metal distribution and paragenesis can be explained by acidic, oxidized ore fluids entering the pyrite-dolomite host lithology, allowing reduction and pH buffering by acid carbonate dissolution, resulting in stepwise metal deposition in an evolving fluid.We argue this represents strong evidence for epigenetic ore formation at HYC. Furthermore, the primary control on ore deposition is not synsedimentary faulting in the subbasin; rather, the chemical potential of sedimentary carbonate within reduced, sulfidic lithologies appears to be of critical importance to precipitation of sphalerite.