Post-depositional Deformation Research Articles

Genesis of high-grade lode gold shoot dominated by ore fluid overprinting during a ductile-brittle shear event

Lode gold deposits hosted by ductile-brittle shear zones account for more than one-third of the world’s gold production. High-grade ore shoots from this type of deposit are the most critical exploration targets. The ore shoots can form through the post-depositional deformation of auriferous sulfides or overprinting of ore fluids accompanied by coupled dissolution-reprecipitation (CDR) reactions. However, the mechanism that dominates ore shoot genesis remains unknown, primarily due to the controversial single progressive or polyphase nature of ore-bearing shear zones. Here, we report on geological and geochemical analyses we conducted at the large Hetai goldfield, South China, to construct an accurate gold upgrading model for the formation of ore shoots. Stages 1−3 of mineralization at Hetai show features typical of ductile shearing, while Stage 4 is characterized by quartz-sulfide veinlets in brittle fractures. 40Ar/39Ar ages of ca. 184 Ma and 157 Ma for the mineralization of stages 1 and 4 overlap with the regionally dextral ductile-brittle shear that occurred during ca. 210−162 Ma. Thus, the gold event at Hetai should have been controlled by a single progressive ductile-brittle shear episode, rather than polyphase structural events. The auriferous fluids at Hetai precipitated minor invisible gold in pyrites (mean 0.173 ppm) produced during stages 1−4 through fluid-rock interaction. The systematic increase of elements Au, As, Sb, Bi, Ag, and Cu and δ34S values in ductile-deformed pyrites from stages 1−3 indicate that early invisible gold upgrading should be the result of the post-depositional remobilization of auriferous sulfides during the long-lived ductile-brittle transition. Cataclastic pyrites hosting invisible gold from Stage 4 have zoned and porous mantles with elevated invisible gold (mean 0.503 ppm) and Sb, Bi, Pb, Co, Ni, and Ti contents. These pyrites are further replaced by chalcopyrite and pyrrhotite with increasing invisible gold, Co, and Ni contents. In addition, numerous visible native gold grains in Stage 4 are included in sulfides formed by replacement and develop along the microfractures and grain boundaries of these sulfides. We suggest the late invisible and visible gold upgrading events in Stage 4 can be attributed to the auriferous fluid superposition and subsequent replacement of pyrite via CDR reactions in a brittle regime. Therefore, the gold upgrading process at Hetai is jointly caused by the early remobilization induced by ductile-brittle deformation and the late ore fluid superposition with accompanying CDR reactions within a brittle domain. As the ore fluid superposition and CDR reactions in Stage 4 produce a significant amount of visible gold, they exert a first-order control on the genesis of ore shoots at Hetai. The refined model may be widely applicable to lode gold deposits elsewhere and can be used to identify regions with promising exploration targets.

Integrating facies, mineralogy, and paleomagnetism to constrain the age and provenance of Paleozoic siliciclastic sedimentary rocks along the northern Gondwana margin: insights from the Araba and Naqus formations in western Gulf of Suez, Egypt

The depositional ages and provenance of the Paleozoic Araba and Naqus Formations along the northern Gondwanan margin in Egypt have remained uncertain due to a lack of index fossils. Resolving this issue is crucial for understanding regional geology during deposition and subsequent tectonic development. We integrate detailed facies analysis, anisotropy of magnetic susceptibility (AMS), paleomagnetism, and mineralogical data to elucidate the genesis and depositional ages of the Araba and Naqus Formations. Petrographic analyses identified seven distinct facies types, providing insights into sedimentary textures, maturity, and sources, with contributions from igneous and metamorphic sources indicated by heavy minerals. X-ray diffraction (XRD) analysis identified accessory minerals such as quartz, goethite, kaolinite, hematite, and anatase. Paleomagnetism isolated two magnetic components (CA and CN) providing the first robust paleo pole positions at Lat. = 70.8° N, Long. = 308.2° E and Lat. = 37.8° N, Long. = 233.1° E, indicating Cambrian and Carboniferous ages for the Araba and Naqus formations, respectively. Thermal demagnetization constrained these dates using established polarity timescales. Mineralogical data indicated that the Araba Formation originated from an igneous source, while the Naqus Formation had a mixed metamorphic-igneous provenance. The integrated AMS and paleomagnetic data reveal evidence of post-depositional deformation. Specifically, the clustering of maximum AMS axes in the NW–SE direction for both formations, suggests the initial presence of a primary depositional fabric. However, prevalent tectonic activity during the Cenozoic appears to have overprinted and modified this fabric through deformation related to rifting of the Gulf of Suez region. Through this novel multi-proxy approach, we have resolved long-standing uncertainties regarding the formations' depositional ages. Our study thereby provides the first chronostratigraphic framework for these strategically important sedimentary units, significantly advancing understanding of regional Paleozoic geology.Graphical abstract

Paleoproterozoic (Huronian) valley-controlled deglacial-fluvial sedimentation, northern Cobalt Basin, Ontario, Canada

Detailed sedimentological and stratigraphic analysis of pre-vegetation sedimentary systems that deposited across global-scale climate change episodes is key to fully understanding past feedback responses in Earth surface systems and the details of their paleoenvironments. The Cobalt Basin, recording the upper Huronian Supergroup, captures a transition from glacial (Gowganda Formation) to post-glacial (Lorrain Formation) deposystems in parallel with Earth's Great Oxidation Event and is ideal for sedimentological/stratigraphic analysis due to its limited post-depositional deformation. Stratigraphic measurements and correlations from the northern Cobalt Basin reveal that the Gowganda and Lorrain formations are preserved within a paleovalley, interpreted as a fjord. Sub-glacial deposits of the Coleman Member (lower Gowganda Formation) are preserved at the base of the paleovalley, unconformably above Archean volcanic and granitic rocks of the Abitibi subprovince. Subsequently, the fjord was filled by glaciomarine strata sourced from a grounded tidewater icesheet prior to climate warming. Above the glaciomarine deposits, deltaic deposits of the Firstbrook Member (upper Gowganda Formation) represent a transition to nonglaciogenic deposition under a warm and humid climate. The deltaic deposits comprise three facies associations: (1) delta front/slope strata dominated by horizontally stratified siltstone; (2) interdistributary bay deposits composed of symmetrically rippled and thinly laminated siltstone; (3) distributary mouth bar deposits dominated by very fine-grained arenite. Delta front/slope deposits occur directly above glaciomarine strata, while interdistributary bay and distributary mouth bar deposits occur above delta front/slope strata and are laterally discontinuous across the study area. Prodelta deposits are not preserved in the study area, although they are reported elsewhere in the basin. At the flanks of the paleovalley, alluvial fan deposits, confined by Archean rocks, interfinger with deltaic strata. Alluvial fan sedimentation, triggered by enhanced weathering in an unvegetated warm and humid setting, led to flashy sheetflood deposition due to significant topographic variations at the flanks of the valley setting. In some instances, sandstone beds of the overlying Lorrain Formation, comparable to distal to medial reaches of the modern South Saskatchewan River, cut down through delta strata. Up-section, the braidplain strata coarsen to gravel-rich facies representing Donjek River-type deposits. Overall, the stratigraphic succession in the northern Cobalt basin study area represents a single progradation from glaciomarine through deltaic deposition, into fluvial braidplain deposition. The succession is preserved at the cratonward limit of a passive margin, suggesting that progradation was likely influenced by relative sea-level fall linked to glacio-isostatic uplift, similar to fjord-filled sedimentary successions that have developed since the Last (Pleistocene) Glacial Period.

Stratal characteristic and depositional origin of two-part (Mud-poor overlain by mud-rich) and associated deep-water strata: Components in a lateral depositional continuum related to particle settling in negligibly sheared mud-rich suspensions

AbstractDeep-marine strata consisting of a sandy basal part overlain sharply by a muddier and mud-clast-rich upper part are increasingly being recognized in the deep-marine sedimentary record, and have been termed linked debrites, cogenetic debrite–turbidite beds, hybrid event beds, transitional-flow deposits, and bipartite beds. These composite strata are generally reported to develop downflow of clean (less muddy) sand, but the variable distance (hundreds of meters to tens of kilometers) over which this lithological change is reported to take place has resulted in a variety of physical depositional models. As such, the details about the origin and spatial evolution of these admixed sand–mud strata still remain poorly understood. Part of this uncertainty is that many earlier studies, with a few exceptions, have been based on macroscopic observations in drill core or discontinuous outcrops, and therefore interpreted bed continuity is based on similarity of textural or other sedimentological attributes. However, in slope to proximal basin-floor deposits of the Neoproterozoic Windermere Supergroup, and distal basin-floor deposits of the Ordovician Cloridorme Formation, exceptional outcrop exposure permitted individual two-part (bipartite) facies and their associated strata to be continuously traced and lithological changes to be documented along the strike of the outcrop.In both the Windermere and the Cloridorme, the bipartite facies consists of a lower sandy (25–60% matrix) part overlain sharply by a muddier (40–80% matrix), commonly mud-clast-rich upper part, and occurs downflow of muddy sandstone and upflow of sandy mudstone in a hundreds-of-meters long proximal to distal depositional continuum. These strata are then overlain everywhere by a thin- to very thin-bedded, traction-structured sandstone and/or silty mudstone cap. Notably, the interface separating the lower sand-rich part from the upper mud-rich part in the bipartite part of the depositional transect is always planar in the Windermere, but often alternates between planar and irregular in the Cloridorme. The continuum is interpreted to represent deposition downflow of an avulsion node that resulted in erosion of the local mud-rich seafloor and charged the flow with fine-grained sediment, resulting in increased effective fluid viscosity and significant changes in the characteristics, intensity, and transfer of fluid turbulence needed to maintain the particle suspension. Collapse of the suspension and development of overcapacity conditions resulted in rapid particle settling in the now rapidly depleting and negligibly sheared mud-enriched suspension that formed along the margins of the avulsion-wall jet in proximal-basin-floor and slope deposits (Windermere), or at the downflow terminus of the flow in more distal basin-floor deposits (Cloridorme). Additionally, when normalized to the total length of the continuum, the transition from muddy sandstone to bipartite facies and bipartite facies to sandy mudstone are equally proportioned in all examples, suggesting that, once formed, the depleting suspension evolved in a spatially systematic and dimensionally proportionate manner—a consistent depositional evolution difficult to reconcile with two mechanistically different parts to the flow. Moreover, differences in the morphology of the bipartite interface (i.e., planar vs. irregular) is not a primary depositional feature, but instead is interpreted to be a consequence of local postdepositional deformation of a previously continuous planar surface.

Taming the Thunder Horse With Axes and Vectors

The Thunder Horse Field targets Middle Miocene deepwater turbiditic reservoirs. Despite being prolific, the mapping of the ~180 m thick, partly amalgamated reservoir sandstones is challenging. Seismic quality is reduced by the presence of salt structures. The salt overburden and high formation pressure require the use of heavy mud weights and oil-based drilling fluids, which limit the resolution and interpretation potential of borehole image logs (BHI). Halokinetic movements caused significant post-depositional deformation of the already complex gravity-driven sediment stack, and the reservoir beds drape against an E-W oriented salt wall. Consequently, the assessment and removal of the structural dip component are not trivial, and the evaluation of paleo-transport directions is considerably more complicated compared to undisturbed deepwater reservoirs. This paper illustrates the potential of eigenvector methods to BHI from Ruehlicke et al. (2019) for reconstructing the depositional slope and the architecture of mass transport complexes in the case of chaotic depositional settings and uncertain structural dip. Figures from Henry et al. (2018) are used wherein part axial analysis was performed on data from a group of Thunder Horse wells and presented in more detail.

Thrusting sequence in the Western Himalayan foreland basin during the late phase of continental collision defined by low-temperature thermochronology

Foreland basins associated with collisional mountain belts undergo postdepositional deformation in response to compressive stress exerted from the evolving orogen. The low thermal regime of such processes requires sensitive methods to unravel the structural evolution of such regions. In this study, we conducted apatite fission track (AFT) and (U-Th)/He thermochronology on the sedimentary succession of the Subathu basin in the Himalayan foreland of the Himachal Pradesh region in India, an area preserving the sedimentary record of the India-Asia collision since the late Paleocene-early Eocene. The results show that the Subathu basin exposes a fossilised apatite partial annealing zone (APAZ) depicted by incompletely reset AFT ages and shortened track lengths in the Paleocene-middle Miocene pre-Siwalik units and unreset ages in the Neogene Siwalik Group. We associate the partial annealing of the pre-Siwalik rocks with thickening due to internal imbrication and nappe stacking triggered by shearing along the Main Boundary Thrust and its splays since the middle Miocene. Thermal modelling of the Siwalik rocks projects a rapid heating up to approximately 90–100 °C reached between 4 and 1 Ma. Following the thermal peak, the Siwalik rocks cooled rapidly, as indicated by 2.6–1.2 Ma AHe ages. We associate this recent cooling event with SW-directed thrusting along the Main Frontal Thrust, which placed the foreland basin over the Gangetic Plains. Our results are similar to those obtained for the foreland basin in Nepal, indicating that the movement along the youngest major discontinuity of the Himalaya is synchronous along the orogen.

The timing of the postglacial marine transgression in the Ría de Ferrol (NW Iberia): A new multiproxy approach from its sedimentary infill

The Ría de Ferrol is a confined tide-dominated incised valley in the mesotidal passive Atlantic margin of NW Iberia. A new multidisciplinary approach enables a high-resolution reconstruction of the main environmental changes affecting this area during the Lateglacial and the Holocene. We defined the main seismic and sedimentary facies in the infill and selected different key points to be cored. A number of multiproxy analyses were performed on four sections of sediment (sedimentology, X-ray fluorescence, radiocarbon dating and palynological analyses). Physiography exerted a strong control on the evolution of the basin, as a rock-bounded narrow channel in the middle of the basin influenced the flooding of the ria during the postglacial marine transgression. Depositional environments shifted towards the east as the sea level rose and fluvial facies at the base were covered by facies from a tide-dominated estuary, which were predominant during most of the Holocene. Several layers of sediments from stagnant freshwater areas dominated by Juncus sp. were recovered at ca. −20.5 to −21, ca. −13.1 to −13.5, −12.5 to −12.8 and −12.1 to −13.5 m NMMA along the central axis of the embayment, and dated to be older than 10190–10290; 10790–10970; 7510–8220 and 7130–7830 cal yr BP. They may respectively represent a local stage of lowering sea-level dated at the end of the Lateglacial (i.e. the Younger Dryas), and different episodes of deceleration/stabilization of sea-level rise occurred during the early Holocene (i.e. the 11.4, 10.5, 9.3, 8.2 kyr Henrich events). Thus, for the first time in this region, we were able to generate a high-resolution spatial–temporal interpretation of environmental changes linked to the relative sea-level variations using subtidal sediments from the same sub-basin (and thus free of substantial/differential post-depositional deformations) that describes in detail the flooding of the ancient coastal plains of this region at the beginning of the Holocene.

High-resolution late Paleozoic cyclostratigraphy and tectonic evolution of the Keeler Basin, California, southwest Laurentia

Abstract Cyclic strata exposed in the Inyo Mountains of eastern California contain a continuous 6 m.y. record of deep marine deposition that spans the Pennsylvanian–Permian boundary. To better understand the geologic evolution of southwest Laurentia and the role of glacially driven eustasy in upper Paleozoic stratigraphy, we measured two detailed ∼600 m composite stratigraphic sections of the Keeler Canyon Formation and collected a handheld spectral gamma ray log. Post-depositional deformation complicates field relationships, but 1:5000 scale mapping of faults and folds permits assembly of two continuous sections. Measured strata alternate at the 5–20 m scale between intervals of fine-grained laminated marlstone and intervals of mixed carbonate and siliciclastic turbidites and debrites. Based on facies characteristics and a prominent marker horizon, we reassign the Pennsylvanian-Permian age upper Salt Tram unit of the upper Keeler Canyon Formation to a new Estelle Member. We estimate sediment accumulation rates within the Keeler Canyon Formation using published conodont biostratigraphy and correlative U-Pb geochronology from Eastern Europe combined with spectral analysis and timescale optimization using the Astrochron R package. Evolutive harmonic analysis of gamma ray-derived element concentrations reveals prominent bundled periodicities that are consistent with both long and short eccentricity cycles. Average sediment accumulation rates calculated using the time scale optimization function of Astrochron suggest a gradual increase from 40–60 m/m.y. to ∼120 m/m.y. during the late Pennsylvanian and then a minima of ∼50 m/m.y. near the Pennsylvanian–Permian boundary, which is followed by an increase to ∼175 m/m.y. into the Early Permian. This trend in sediment accumulation rates and subsequent Permian contractile deformation are compatible with flexural subsidence in a SW-migrating foreland basin related to the southern part of the late Antler orogenic system.

Tectonic–sedimentary interplay of a confined deepwater system in a foreland basin setting: the Pennsylvanian lower Atoka Formation, Ouachita Mountains, U.S.A.

ABSTRACT Submarine fans deposited in structurally complex settings record important information on basin evolution and tectonic–sedimentary relationships but are often poorly preserved in outcrops due to syndepositional and post depositional deformation. This study aims to understand the influence of tectonics on the deposition of the synorogenic Pennsylvanian lower Atoka submarine fan system deposited in a structurally complex foreland basin during the Ouachita orogeny. This study is a synthesis of new outcrop stratigraphic data as well as published stratigraphic and structural data. The lower Atoka crops out in the Ouachita Mountains and the southern Arkoma Basin and is divided into three structural–depositional zones: the foredeep, the wedge top, and the continental foreland. The mean paleoflow is axial, and each zone exhibits unique patterns in facies distribution. The foredeep consists of two fan systems, a large westward-prograding fan that exhibits significant longitudinal and lateral facies changes, and a small eastward-prograding fan on the western part. The wedge top consists of a westward-prograding fan that exhibits subtle longitudinal facies change. The continental foreland consists of small slope fan systems along the northern and western margins. By comparing to basin morphology and structural styles, we interpret the facies distribution patterns in the three zones as the result of different combinations of lateral structural confinement, axial and lateral sediment supply, and paleogeography. This study provides an improved and comprehensive understanding of the lower Atoka deepwater system and has implications for deciphering the tectonic–sedimentary relationships in laterally confined submarine fan systems.

Extracting Mg isotope signatures of ancient seawater from marine halite: A reconnaissance

The Mg/Ca ratio of seawater fluctuated remarkably over the Phanerozoic. The processes that controlled the temporal variation in seawater Mg/Ca ratio may also have caused the secular changes in Mg isotope ratios of seawater. Therefore, the Mg isotope composition of seawater may be useful to understand the long-term variation in seawater chemistry. Different sedimentary carbonates have been proposed as archives of ancient seawater Mg isotope signatures, but each type of carbonate has its limitations. Halite is a common mineral in evaporite deposits and their fluid inclusions may be used to infer the Mg isotope composition of contemporary seawater. In this study, we developed a method to measure the isotope composition of Mg in halite with an accuracy of better than ±0.1‰ in δ26Mg. The accuracy of the method was verified systematically, using synthetic NaCl solutions, as well as Holocene lacustrine halite samples from the Dead Sea. The δ26Mg values of marine halite samples show a large variation, ranging from −0.1‰ to −1.6‰ relative to the DSM3 international Mg isotope standard. The δ26Mg values of some ancient salt samples are lower than a published model curve for δ26Mg of seawater, and such inconsistency could be explained by 1) isotopic evolution of brines due to mineral precipitation, 2) entrapment of evaporite minerals into halite during halite precipitation or post-depositional deformation, 3) recrystallization and dissolution of pre-existing Mg minerals, and 4) limitations of the model curve of seawater δ26Mg values. Collectively, we suggest that Mg isotopes in halite are a new geochemical proxy to study the secular variations in seawater chemistry, syndepositional and post-depositional processes of evaporite deposits. Our data highlight the importance of understanding the sedimentary background and textural details of evaporites for appropriate interpretations of Mg isotope data from halite.

Regional Structural Orientation of the Mount Sharp Group Revealed by In Situ Dip Measurements and Stratigraphic Correlations on the Vera Rubin Ridge

AbstractGround‐based bedding orientation measurements are critical to determine the geologic history and processes of sedimentation in Gale crater, Mars. We constrain the dip of lacustrine strata of the Blunts Point, Pettegrove Point, and Jura members of the Murray formation using a combination of regional stratigraphic correlations and bed attitude measurements from stereo Mastcam images taken by the Mars Science Laboratory Curiosity rover. In situ bed attitude measurements using a principal component analysis‐based regression method reveal a wide range of dips and dip azimuths owing to a combination of high stereo errors, postdepositional deformation of strata (e.g., fracturing, rotation, and impact cratering), and different primary depositional dips. These constrain regional dips to be within several degrees of horizontal on average. Stratigraphic correlations between targets observed in the Glen Torridon trough and at the Pettegrove Point‐Jura member contact of Vera Rubin ridge (VRR) constrain dips to be between 3°SE and 2°NW, consistent with nearly flat strata deposited horizontally on an equipotential surface. The Jura member is determined to be stratigraphically equivalent to the northern portion of the Glen Torridon trough. Rover‐based dip magnitudes are generally significantly shallower than the orientation of VRR member contacts measured from High Resolution Imaging Science Experiment‐based traces, suggesting the sedimentary strata and VRR member contacts may be discordant.

The evolution of the Dowsing Graben System: implications for petroleum prospectivity in the UK Southern North Sea

Upper Permian (Zechstein Supergroup) evaporites have a major control on structural styles and prospectivity in the UK Southern North Sea (SNS). They form the regional super-seal for the main Rotliegend Group (Leman Sandstone Formation) reservoir play fairway immediately beneath. The evaporites have highly variable thicknesses due to the syndepositional basin architecture, differential loading and post-depositional deformation through diapirism and salt withdrawal. The halokinetic activity leads to touchdown (welding) of the supra-salt section onto the sub-salt strata and the development of narrow (up to 15 km-wide) graben systems. The interpretation and depth conversion of well-calibrated, high-quality, 3D post-stack time-migrated (PSTM) seismic data along the southwestern margin of the basin show that a NW–SE-striking elongate extensional Dowsing Graben System transects the area. The graben is defined by a series of large, overlapping, en echelon listric growth faults, with oblique secondary planar faults, which sole-out on two main (deep and shallow) décollement levels in the Zechstein Supergroup and the Middle Triassic Röt Halite Member. Whilst its initial formation was related to Mesozoic extension, the graben system also displays a contractional overprint resulting from regional compression and structural inversion during the Cenozoic. Detailed mapping of the Zechstein Supergroup has revealed that the evolution of the extensional system was influenced by the ESE–WNW-striking anhydrite–carbonate Zechstein shelf-margin. The occurrence of variable-thickness, low-velocity sediments within the graben impacts seismic imaging and depth conversion, leading to prospective structures being overlooked; something that has implications for prospectivity in the SNS and other evaporite basins where similar graben occur.

Constraining the Intracontinental Tectonics of the SW Central Asian Orogenic Belt by the Early Permian Paleomagnetic Pole for the Turfan‐Hami Block

AbstractIn order to better understand the kinematics and geodynamics of the late Paleozoic to early Mesozoic transition from accretionary to intracontinental tectonics in the southwest Central Asian Orogenic Belt, we performed a paleomagnetic study on Lower Permian strata in the Turpan‐Hami Basin, NW China. Magnetite and hematite are shown to be the principal magnetic remanence carriers. A typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study. Stable characteristic remanent magnetizations, all of reverse magnetic polarity, are revealed by step demagnetizations. Remanence age is determined to be of Early Permian affinity, based on a fold test, remanence behavior of minerals, and geochronologic data. With inclination shallowing corrections, the first Early Permian paleomagnetic pole for the Turpan‐Hami Block is acquired as λ = 75.7°N, φ = 276.3°E, A95 = 5.7°, and N = 10 sites. Comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region. The Turpan‐Hami and Yili blocks have kept their relative positions since the Early Permian. Significant latitudinal movement of 14.7° ± 7.2° occurred between the Turpan‐Hami and South Junggar blocks and could be mainly accommodated by late Paleozoic crustal shortening of at least ~530 km across an intra‐arc basin in the Bogda belt. The Turpan‐Hami Block has experienced large‐magnitude relative rotations of −41.1° ± 7.1° and 21.2° ± 9.4° with respect to Tarim and South Junggar since the Early Permian, respectively, corresponding to dominant intracontinental tectonics characterized by large‐magnitude strike‐slip displacements along megashear zones.

Architecture and paleogeography of the Early Paleozoic carbonate systems in the east-central Tarim Basin (China): Constraints from seismic and well data

Abstract The evolution of the Cambrian-Middle Ordovician carbonate systems in the east-central Tarim Basin, northwestern China, has been investigated using 2D seismic profiles and well data. Interpretation was performed on seismic profiles flattened along an originally horizontal surface (base of the Cambrian), to remove the effects of post-depositional deformations. Six seismic facies differently arranged in six seismic stratigraphic units record four major stages of architectural evolution, from ramp to flat-topped systems, with differences from north to south: (1) the onset of a homoclinal ramp over a clayey unit at the beginning of Early Cambrian; (2) the development of a prograding carbonate platform characterized by restricted (evaporitic) facies in the inner platform domain during Early Cambrian to Late Cambrian; (3) the development of a first prograding and a aggrading (retrograding to the north) platform during Lower Ordovician; (4) the development of a prograding/retrograding to aggrading final platform during Middle Ordovician. The basinward (eastward) progradation of the carbonate platform edge has been reconstructed in time steps, highlighting differences from the north to the south of the study area in terms of amount of progradation, platform-to-basin relief and depositional surfaces, likely controlled by differences in subsidence and environmental factors. The identification of the four major stages in the evolution of the lower Paleozoic succession of the Tarim Basin was likely controlled by changes in the rate of creation/destruction of accommodation space, due to the interplay between eustatic changes and tectonic events. The demise of the Cambrian to Middle Ordovician carbonate system in the study area is associated with a major tectonic event, recorded by a tilting of the succession toward the east (onlapped by sediments coming from the east) and an uplift (bulge) of the western part of the study area.

Geology of the Northwestern Krania Basin

We present a new map of 30 km2 of the northwestern Krania Basin at 1:10,000 scale, including rocks of the Pindos Ophiolite Group and associated units, and the sedimentary fill of the Krania Basin. The Krania Basin is a flexural basin developed in the Middle – Late Eocene and filled first with alluvial fan conglomerates and later with turbidite sandstones and siltstones, following a deepening of the basin. Analysis of the clasts within the sediment, combined with paleoflow analyses, suggest sediment input from the eroding Pindos Ophiolite to the west. The Pindos Ophiolite Group is represented in the area by pillow lavas, sheeted dykes and serpentinized harzburgites of the Aspropotamos Complex. The ophiolite forms imbricated, thrust bounded blocks which show two phases of thrusting, corresponding to Late Jurassic and Eocene stages of ophiolite emplacement. We identify five stages of deformation within the basin itself, starting with Early - Middle Eocene syndepositional extensional faulting associated with the formation of the basin. This was followed by four stages of post-depositional deformation, starting with Late Eocene compression associated with basin closure, which caused thrust faulting and folding of the sediments. Oligocene dextral faulting with a thrust component affected the basin margins. Finally, two normal faulting events with different orientations have affected the basin since the Miocene.

Graptolite biostratigraphy of the Ordovician Almelund and Sularp Shale formations of the Fågelsång-3 drill core, Scania, Sweden

ABSTRACTThe upper Middle to basal Upper Ordovician succession (Darriwilian to Sandbian) of the Fågelsång-3 drill core provides new important information on the graptolite biostratigraphy of Scania, southern Sweden. The Scanian succession is known largely from drill cores and a few scattered outcrops exposing only parts of the interval. The Darriwilian Almelund Shale Formation overlies the Komstad Limestone Formation and has a thickness of more than 42 m in the Fågelsång-3 drill core. A seemingly complete succession through the Holmograptus lentus, Nicholsonograptus fasciculatus, Pterograptus elegans, Pseudamplexograptus distichus and Jiangxigraptus vagus biozones can be recognized. Some intervals are poorly documented by their respective graptolite faunas or show extensive post-depositional tectonic deformation. The Nemagraptus gracilis Biozone of earliest Sandbian age cannot be positively identified, even though fragments of Nemagraptus have been discovered in the uppermost part of the Almelund Shale below the Fågelsång Phosphorite Bed.

Sedimentary compared to tectonically-deformed serpentinites and tectonic serpentinite mélanges at outcrop to petrographic scales: Unambiguous and disputed examples from California

This paper compares features of unambiguous tectonic serpentinite mélanges (TSM) or serpentinite shear zones in the Coast Range ophiolite, Franciscan subduction complex, of coastal California and Sierra City Mélange of the northern Sierra Nevada of northeastern California with undisputed sedimentary serpentinite mélange (SSM) of the Great Valley Group (GVG) forearc basin deposits of coastal California, and with Franciscan serpentinite mélanges of disputed (sedimentary versus tectonic) origin. The GVG sedimentary serpentinite mélanges and disputed Franciscan serpentinite mélanges share strongly similar matrix textures and block-matrix relationships at scales from tens of meters or more to petrographic scale but differ significantly from serpentinite shear zones and TSM. This comparison suggests shared (non-diagnostic) and distinguishing features of TSM versus SSM. Internal bedding or foliation in blocks is oriented subparallel to mélange boundaries and matrix foliation for both TSM and SSM both may have strongly foliated matrix and both may feature localized shearing in matrix around block borders, especially if an SSM underwent significant post-depositional deformation. The same holds true for deformation and dismemberment of blocks, which is the block-forming and mixing mechanism in TSM but variably exhibited in SSM. In contrast only SSM have blocks or clasts whose internal foliation or bedding terminates abruptly along clast/block boundaries with a mismatch in mineralogy and/or lithology across such boundaries. Matrix foliation cuts blocks/clasts in TSM but not in SSM. SSM may show block/grain size grading but not TSM. SSM have exotic blocks and blocks may span a range of metamorphic grade, whereas TSM lack exotic blocks and blocks are isofacial.

Radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late Mesoproterozoic Borden Basin, Arctic Canada

Abstract The ca. 1050 Ma Bylot Supergroup in Arctic Canada is one of the best-preserved archives of late Mesoproterozoic geochemistry and biology and offers evidence that this period of Earth history may have been more biogeochemically dynamic than previously appreciated. The Bylot Supergroup was deposited in the Borden Basin and is the most thoroughly studied stratigraphic succession from a series of broadly contemporaneous late Mesoproterozoic intracratonic basins known as the Bylot basins. This ∼6-km-thick mixed carbonate-siliciclastic succession has undergone minimal postdepositional deformation and is now exposed on Baffin and Bylot Islands, Nunavut, Canada. Deep-water and tidally influenced carbonate facies, traditionally interpreted as marine, have yielded important insights into the evolution of Proterozoic seawater chemistry; however, more recent studies indicate that the Borden Basin was restricted marine or lacustrine for portions of its depositional history. Here, we present new multiproxy radiogenic isotope chemostratigraphic data spanning the Bylot Supergroup. A comparison of stratigraphic trends in radiogenic isotope data from hydrogenous (black shale 187Os/188Os and limestone 87Sr/86Sr) and detrital (whole-rock mudstone 143Nd/144Nd) sedimentary phases elucidates the complex hydrologic history of the Borden Basin and reconciles these disparate interpretations. Episodic coupling and decoupling between the composition of basin waters (from Os and Sr isotopes) and the local weathering input to the basin (from Nd isotopes) indicate that depositional environments within the Borden Basin fluctuated between marine and nonmarine (i.e., lacustrine). Variations in basin hydrology controlled secular sedimentation patterns through changes in basin water chemistry. These interpretations help to characterize the environment in which the early red algal fossil Bangiomorpha pubescens evolved. Episodically restricted epeiric seaways, such as within the Borden Basin, were relatively widespread within Rodinia and may have exerted unique selective pressures on eukaryotic evolution in the Mesoproterozoic Era. Hydrogenous and detrital radiogenic isotope chemostratigraphy, as implemented in this study, may provide a useful paleoenvironmental framework for future paleontological studies aimed at testing the role of freshwater environments in eukaryotic evolution. In addition, 87Sr/86Sr compositions from 81 new middle Bylot Supergroup marine limestone samples, calibrated by recent Re-Os geochronology, contribute to the terminal Mesoproterozoic marine 87Sr/86Sr curve. These data display a rise from ∼0.705 to 0.706 that reflects weathering of the active Grenville orogenic belt and demonstrates a global increase in chemical weathering during the amalgamation of Rodinia.

From intrabasinal volcanism to far-field tectonics: causes of abrupt shifts in sediment provenance in the Devonian–Carboniferous Drummond Basin, Queensland

The Drummond Basin of central Queensland preserves a large-volume succession of little studied, predominantly fluviatile, coarse-grained sedimentary rocks of mid-Mississippian age. The stratigraphy of the basin has been subdivided into three sedimentary cycles. The Cycle 1/Cycle 2 boundary records a distinct, but poorly understood change in provenance from a volcanic-dominated succession related to initial basin rifting (Cycle 1) to a quartz-rich, craton-derived succession (Cycle 2). Cycle 3 has been thought to mark a resumption of intrabasinal volcanism and related sedimentation. The purpose of this study was to enhance the understanding of the basin-wide siliciclastic sedimentation of Cycles 2 and 3, and causes for the changes in sediment provenance. This objective was achieved by constraining large-scale spatial and temporal depositional trends and investigating sediment transport pathways into and through the basin. Petrographic, QFL, paleocurrent and conglomerate clast analyses were undertaken. The observations presented here have several implications relevant to understanding the stratigraphy of the Drummond Basin and regional tectonic events at this time. Cycle 3 is revised here primarily to be a continuation of Cycle 2-style basement-derived sedimentation, rather than recording a resumption of volcanism in the area, as per prevailing models. Quartz-rich sedimentation in the Drummond Basin was, therefore, more long-lived than previously envisaged, and once established, was not significantly disrupted by volcanism. Cycle 2 formation thicknesses appear highly variable across the basin. This is unlikely to be a result of pre-existing rift-related topography as suggested in previous models. The thickness variations are more likely related to sediment bypassing and post-depositional deformation in the area. The distinctive coarse-grained, relatively quartz-rich sedimentation of Cycles 2 and 3 is unusual in its volume and extent. The sediment was transported into the basin from its southern/southwestern margin, implying long-distance transport and extrabasinal sediment supply. While the specific source terrain(s) remain unknown, one plausible tectonic driver was far-field influence of the intraplate Alice Springs Orogeny.

Cenozoic Development of the Nonmarine Mula Basin in the Southern Yidun Terrane: Deposition and Deformation in the Eastern Tibetan Plateau Associated with the India‐Asia Collision

AbstractTo advance tectonic models of plateau growth in response to the India‐Asia collision, parameters such as the timing, mechanism (s), and extent of Cenozoic deposition and deformation throughout the eastern Tibetan Plateau need to be determined. To better understand these parameters, we examine the Mula basin, located in the southern Yidun terrane, using field data, detrital zircon geochronology, εHf zircon isotopic values, and thin section petrology. The Mula basin is a NW‐SE elongate (~28 km long and 5–8 km wide) exposure of nonmarine strata ~1,000 m thick, deposited in alluvial environments. The basin is bound to the northeast by a thrust fault (327, 34°NE) that places Triassic Daocheng Pluton and Triassic Yidun Group rocks on top of the nonmarine strata. The western boundary is defined by an unconformable contact between the overlying nonmarine strata and Triassic Daocheng Pluton and Yidun Group rock. An intrabasin thrust fault, parallel to the basin‐bounding fault, places older nonmarine strata on top of younger nonmarine strata, illustrating postdepositional deformation. Provenance results indicate that sediment influx originated primarily from localized drainage catchments and lacked major, well‐organized throughgoing systems. A maximum depositional age of 45.5 ± 0.5 Ma, based on the weighted mean average of the youngest detrital zircon population (s), demonstrates an Eocene or younger age for strata. We interpret the Mula basin to have developed in a contractional deformational regime driven by a far‐field, upper‐crustal response associated with the transition from an Andean style margin to a continent‐continent collisional margin as India impinged upon Eurasia.