Signatures Of Deposits Research Articles

Coals peripheral to opening and closure of the northern Western Interior Seaway: depositional signatures recording interplay between Milankovitch orbital forcing and salt dissolution collapse-subsidence structures with responsiveness to craton deformation and eustatic sea-level

Aptian paralic and Paleocene intracratonic mires that bracket the northern reaches of the Western Interior Seaway preserve Milankovitch orbital forcing cycle signals and record the dynamic interplay with craton-linked salt dissolution tectonism in the Alberta Basin foreland and the adjoined intracratonic Williston Basin to the southeast. These mires, now lignite-bearing strata, along the northern Seaway provide a proxy record for Milankovitch orbital signals that would not have been otherwise recorded because of the voluminous clastic sediments accumulated along the western Canadian portion of the Cretaceous Seaway, in contrast to pervasive orbital forcing signals recorded by organic-rich cyclic carbonate deposits accumulated along the central and southern reaches of the Seaway. Prior to opening of the northern segment of the Seaway, the base levels of the paralic mires of the lower McMurray Formation (Aptian) were hydraulically linked to the rising sea-level fluctuations resulting from southward transgression of the Boreal Sea. In contrast, inland fluvial-margin peats of the Paleocene Ravenscrag and Fort Union formations accumulated following the closure of the Cretaceous Seaway. These mires were increasingly sensitive to aquifer-eustatic controls on the water table–mire relationship. Orbital forcing signals were preserved in the central Williston Basin mires in contrast to their masking by salt dissolution collapse-subsidence structures that exerted dominant control on peat mire configurations across the northern Williston Basin. Paralic mires accumulated at the top of the lower McMurray Formation record short-term orbital forcing, possibly representing an obliquity cycle recorded by lithotype sequencing in petrographic profiles. Mires accumulated along the coastal areas during the transition from a lowstand system tract to an early transgressive system tract. The linkage of sea level with coastal water table fluctuations permitted pervasive accumulations of paralic mires at the top of the lower McMurray Formation. Internal mire cycles resulted in coal lithotype profiles responsive to Milankovitch fifth-order and higher sub-cycles. In contrast, concurrent cataclysmic sinkhole collapses and differential fault block displacements were responsive to underlying salt removal patterns. The sea level/water table linkage was punctuated by the salt tectonism and orbital forcing signals were masked, resulting in sinkholes infilled with different coal lithotype sequences. Following the closure of the Western Interior Seaway, inland Paleocene fluvial-margin mires across the northern intracratonic Williston Basin were responsive to larger scale salt removal events in the subsurface. The collapse-subsidence structures were propagated several km up-section, diffused but sufficient to control mire configurations and mask orbital forcing signals. Southward into the central and southwestern Williston Basin, these strong salt dissolution tectonism patterns were out-of-phase with the repetitive mire accumulations, permitting the recognition of the markedly weaker signals of the 100 kyr orbital eccentricity cycle.

Isotopic range of bioavailable strontium on the Peloponnese peninsula, Greece: A multi-proxy approach

Sr isotopes are a powerful tool used for provenancing in many disciplines, but their successful application requires the availability of robust Sr baselines of potential target areas. This study presents 87Sr/86Sr signatures and Sr concentrations of water, plants and soil leachates from the Peloponnese peninsula, Greece, to establish the first comprehensive bioavailable Sr isotope baseline for this region. Additionally, this study aims to evaluate which proxy is most suitable to characterise bioavailable Sr in a geologically complex area also exposed to foreign aeolian Sr sources. Our recorded bioavailable Sr isotope signatures correspond well with the surface lithologies characteristic of the Peloponnese. Unradiogenic 87Sr/86Sr ratios and a narrow isotope range (0.70779–0.70955) characterise the bioavailable Sr signatures of the sedimentary deposits and more radiogenic and isotopically variable values (0.70791–0.72370) were measured for metamorphic and igneous rock outcrops. The differences in 87Sr/86Sr values measured between proxies of one site are comparatively low for samples from the sedimentary and igneous deposits, while the overall spread in 87Sr/86Sr values is wider for samples from metamorphic deposits. We propose to define bioavailable 87Sr/86Sr baseline ranges as the average bioavailable 87Sr/86Sr ratio of all proxies of each lithology ± its double standard deviation (x̅ ± 2σ). This results in narrow baselines for the sedimentary outcrops of 0.70832 ± 0.00053 (n = 58) for clastic sediments and 0.70835 ± 0.00089 (n = 29) for chemical sediments. The metamorphic deposits are characterised by wider bioavailable 87Sr/86Sr baselines of 0.70906 ± 0.00116 (n = 4) and 0.71429 ± 0.01133 (n = 13) for marble and schist, respectively. The bioavailable Sr baseline for igneous rock outcrops is also characterised by a comparatively wide range with 0.70950 ± 0.00259 (n = 7). The wide range in inter- and intra-site specific bioavailable 87Sr/86Sr variation observed in this study emphasise the need for comprehensive multi-proxy sampling strategies within geologically-complex areas.

The depositional signature of cyclic steps: A late Quaternary analogue compared to modern active delta slopes

AbstractCyclic‐step bedforms typifying a Froude‐supercritical flow regime are a recurrent component of depositional/erosional turbiditic systems. Over modern delta slopes, cyclic steps have been inferred from observations of upslope‐migrating crescent‐shaped bedforms. However, the recognition in the sedimentary record of the resulting stratal pattern and depositional facies remains challenging. In this study, the depositional facies observed in exposed late Quaternary glaciofluvial upper delta‐slope sands (Portneuf‐Forestville, Québec) are compared to those cored from a modern analogue consisting of sediment waves interpreted as cyclic steps (Southwind Fjord, Baffin Island). The fossil and modern delta slopes share similar context, morphology and stratigraphic record. The clinoform foresets dip 2 to 6° and consist of prevailing sand‐sized material including subhorizontal to upslope‐dipping top‐cut‐out turbidites. Individually, the latter are 5 to 20 cm thick and massive to planar laminated (prevailing TA and TB subdivisions). In the fossil delta slope, related successions form relatively thick, well‐bedded suites, which are truncated downcurrent by, and onlap upstreamward on, inclined composite erosion surfaces here referred to as pseudo‐foresets. Pseudo‐foresets are regularly spaced (10 to 30 m) and have dips greater than the clinoform foresets (ca 20°). Large composite scours form pseudo‐channel structures filled in by structureless pebbly sand deformed by sheared flame structures, in association with coarse sand showing undulating lamination and rip‐up clasts. Similar depositional facies are observed on the modern delta slope. The stratal pattern is best compatible with upslope‐migrating bedforms and structureless sand indicates hydraulic jump deposits typical of cyclic steps. Cyclic‐step flow events, encompassing a succession of genetically linked erosional cyclic steps, depositional cyclic steps and subsequent waning‐flow conditions, were associated with the dense basal layer of high‐density (stratified) turbidity currents. They are specifically associated with pseudo‐channel incision and infill. The deposition of well‐bedded suites on the stoss side, occasionally also on the lee side, is interpreted to result from a different type of turbidity current, which decelerated over the bedform stoss and accelerated over the lee sides of pre‐existing cyclic‐step morphologies. Antidunal flow conditions in expanded (non‐stratified) turbidity currents are tentatively suggested, deposition being linked, in this case, to interactions between inherited bedform morphologies and a near‐bed tractive layer. In the resulting dual flow model, the bulk of delta‐slope sands was mainly deposited from turbidity currents not developing a cyclic‐step instability, yet cyclic steps were instrumental in shaping and/or re‐organizing the delta‐slope morphology, bedforms and resulting stratal patterns. In some delta systems, the upslope migration of supercritical crescentic bedforms may not only be due to repetitive cyclic steps but could also result from antidunal turbidity current conditions remobilizing an inherited cyclically‐stepped morphology.

Geochemical signatures of recent bar deposits in the Tista river, Bangladesh: Implications to provenance, paleoweathering and tectonics

Petrography and geochemical composition of sediments is a sensitive indicator which archives the signature of a previous record of a source rock and depositional environments in a basin. This study deals with the elemental geochemistry of recent bar deposits of the Tista river in the north western part of Bangladesh to evaluate their provenance, paleoweathering and tectonic setting. Petrographically, the sediments are rich in quartz (70%), whereas feldspar and lithic fragments are found about 8% and 3%, respectively. The geochemical composition of the samples exhibits dominantly quartzose litharenites with low grade sedimentary and metasedimentary lithics, low feldspar indicates tectonic provenance field of recycled orogeny. Discrimination functions revels that the sediments of the Tista river are the derivation of active continental margin. The analyses also reflect that the sediments are dominantly a mixture of felsic (e.g., granitic/Gneiss, quartzite, amphibolite, granulite facies rock types) and some of mafic source (e.g., alkali-basalt/greenschist facies). It may occur, since 60% of the sedimentary flux carried out by the river from low-grade metamorphic rock and the rest from high-grade rock (in the west and north Sikkim Himalaya respectively). The weathering indices highlight that the source rocks have not undergone significant chemical weathering. The immature sorting status and petrographic evidences indicate that the sediments deposited in the Tista river basin are simply the product of mechanically weathered rocks.

Exploring Offshore Sediment Evidence of the 1755 CE Tsunami (Faro, Portugal): Implications for the Study of Outer Shelf Tsunami Deposits

Outer shelf sedimentary records are promising for determining the recurrence intervals of tsunamis. However, compared to onshore deposits, offshore deposits are more difficult to access, and so far, studies of outer shelf tsunami deposits are scarce. Here, an example of studying these deposits is presented to infer implications for tsunami-related signatures in similar environments and potentially contribute to pre-historic tsunami event detections. A multidisciplinary approach was performed to detect the sedimentary imprints left by the 1755 CE tsunami in two cores, located in the southern Portuguese continental shelf at water depths of 58 and 91 m. Age models based on 14C and 210Pbxs allowed a probable correspondence with the 1755 CE tsunami event. A multi-proxy approach, including sand composition, grain-size, inorganic geochemistry, magnetic susceptibility, and microtextural features on quartz grain surfaces, yielded evidence for a tsunami depositional signature, although only a subtle terrestrial signal is present. A low contribution of terrestrial material to outer shelf tsunami deposits calls for methodologies that reveal sedimentary structures linked to tsunami event hydrodynamics. Finally, a change in general sedimentation after the tsunami event might have influenced the signature of the 1755 CE tsunami in the outer shelf environment.

Quantifying the three‐dimensional stratigraphic expression of cyclic steps by integrating seafloor and deep‐water outcrop observations

AbstractDeep‐water deposits are important archives of Earth’s history including the occurrence of powerful flow events and the transfer of large volumes of terrestrial detritus into the world’s oceans. However the interpretation of depositional processes and palaeoflow conditions from the deep‐water sedimentary record has been limited due to a lack of direct observations from modern depositional systems. Recent seafloor studies have resulted in novel findings, including the presence of upslope‐migrating bedforms such as cyclic steps formed by supercritical turbidity currents that produce distinct depositional signatures. This study builds on process to product relationships for cyclic steps using modern and ancient datasets by providing sedimentological and quantitative, three‐dimensional architectural analyses of their deposits, which are required for recognition and palaeoflow interpretations of sedimentary structures in the rock record. Repeat‐bathymetric surveys from two modern environments (Squamish prodelta, Canada, and Monterey Canyon, USA) were used to examine the stratigraphic evolution connected with relatively small‐scale (average 40 to 55 m wavelengths and 1.5 to 3.0 m wave heights) upslope‐migrating bedforms interpreted to be cyclic steps within submarine channels and lobes. These results are integrated to interpret a succession of Late Cretaceous Nanaimo Group deep‐water slope deposits exposed on Gabriola Island, Canada. Similar deposit dimensions, facies and architecture are observed in all datasets, which span different turbidite‐dominated settings (prodelta, upper submarine canyon and deep‐water slope) and timescales (days, years or thousands of years). Bedform deposits are typically tens of metres long/wide, <1 m thick and make up successions of low‐angle, backstepping trough‐shaped lenses composed of massive sands/sandstones. These results support process‐based relationships for these deposits, associated with similar cyclic step bedforms formed by turbidity currents with dense basal layers under low‐aggradation conditions. Modern to ancient comparisons reveal the stratigraphic expression of globally prevalent, small‐scale, sandy upslope‐migrating bedforms on the seafloor, which can be applied to enhance palaeoenvironmental interpretations and understand long‐term preservation from ancient deep‐water deposits.

A geochemical approach for identifying marine incursions: Implications for tsunami geology on the Pacific coast of northeast Japan

Potential tsunami inundation areas can be predicted by the distribution of paleo tsunami deposits on land, which are mainly composed of marine-derived sands and muds. These exotic sandy and muddy layers have been identified by multiple approaches. However, there still remain uncertainties regarding the sources and weathering trends over the long term, and other useful proxies need to be investigated to identify of tsunami deposits. We examined the geochemical signatures of modern tsunami deposits from the Pacific coast of northeast Japan (2011 Tohoku tsunami) and those paleo tsunami deposits taken from the Tohoku District (Jogan, ~1080 calibrated ages before present [cal BP] and Yayoi, ca. 2000 cal BP). A geochemical ternary diagram (Seawater-Rock-(As + Metals)) showed the weathering trends of tsunami deposits over approximately 2000 years in the Sendai plain in the Tohoku. In the tsunami layers from the Tohoku, the Na/Ti atomic ratios markedly increased to 23.4 with an average value of 19.3 ± 3.0 (Jogan and Yayoi tsunami deposits), which was clearly higher than those of other layers (soil deposits, 10.3 ± 3.5 on average). These results show that Na/Ti ratios and other geochemical proxies are useful indicators of marine incursions in our case, and may contribute to the detection of muddy tsunami deposits close to the limit of inundation.

Aging brine-dependent deposition of crude oil components onto mica substrates, and its consequences for wettability

While wettability alteration is widely accepted as the dominant mechanism underlying enhanced oil recovery (EOR) in sandstone reservoirs under low salinity water flooding (LSWF) conditions, the effectiveness of LSWF still varies substantially between laboratory experiments and the field indicating our incomplete understanding and control of the process. We use simple mica substrates to investigate the impact of aging conditions on the macroscopic contact angle and the microscopic properties (morphology, composition, stiffness, surface charge) of the surface. Mica samples pre-aged in brines of variable salt content prior to aging in crude oil at elevated temperature displayed macroscopic wettabilities ranging from complete water wetting to intermediate contact angles around 90° in ambient brines of variable salinity. Compared to the macroscopic wettability, variations of the microscopic properties as probed by Atomic Force Microscopy and Confocal Raman microscopy were found to be rather subtle. Clear signatures of asphaltenic deposits were observed ubiquitously, usually consisting of a combination of a thin film and larger aggregates. The deposits typically displayed a salinity-dependent elasticity and (predominantly negative) surface charge. Selective removal of ions from the pre-aging brine lead to strong variations of both macroscopic wettability and microscopic properties, including the response to low ambient salinity: For the same crude oil, reducing the salinity of the ambient brine can either increase or decrease the macroscopic oil contact angle, depending on the composition of the formation brine used to pre-age the mineral surface. These results clearly highlight the complex balance of interactions during brine-oil-rock aging and the subtle role of the connate water composition in extracting and binding different types of crude oil components and thereby setting the initial and boundary conditions for the oil recovery process.

First documentation of seismic stratigraphy and depositional signatures of Zhongsha atoll (Macclesfield Bank), South China Sea

Carbonate platforms form informative archives for paleoclimates and their internal structures can also hold crucial information about the tectonic history and carbonate evolution of the ocean basins. The Zhongsha atoll (Macclesfield Bank) forms the largest atoll system in the South China Sea with a surface area of 23500 km 2 . However, the internal structure and evolution of this atoll system is completely unknown. 2D multichannel seismic reflection data were acquired in 2017 over the Zhongsha atoll in the South China Sea to unravel the stratigraphy, geomorphology, depositional processes, and seismic facies of one of the world's largest atoll for the first time. This Neogene carbonate platform comprises more than 1 km thick carbonate sequence and overlies a metamorphic basement. The southeastern part of the atoll comprises a fault-controlled graben system, which was formed during the Cenozoic rifting stage of the South China Sea. Most of the faults trend NE-SW and E-W and terminate at or slightly above the top of Middle Miocene strata. Atolls and abundant organic reefs initiated on the positive relief and closely mimicked the underlying topography during the Early Miocene. Shallow-water carbonates continued growing through Middle Miocene to present times. Regional uplift led to subaerial exposure, termination of platform growth and karstification during the Miocene. We also reveal a number of fluid-flow features such as vertical sub-bottom venting features (chimneys and pipes), chaotic reflection zones, which provide the first evidence of active fluid venting in the area of Zhongsha atoll. The Neogene sedimentation history of Zhongsha atoll further provide an important paleoenvironmental context for future scientific drilling to better constrain the evolution of Asia Monsoon. • First documentation of seismic stratigraphy and deposition of Zhongsha atoll (Macclesfield Bank), South China Sea. • Reefs and lagoons developed during the post-rift phase, whose onset is marked by a turning point around the time of the formation of reflector T50. • Three drowning events are identified to tentatively imply three episodes of fast sea floor spreading in the study region.

Characterizing deposits emplaced by cryovolcanic plumes on Europa

In the absence of direct observations of Europa's particle plumes, deposits left behind during eruptive events would provide the best evidence for recent geological activity, and would serve as indicators of the best places to search for ongoing activity on the icy moon. Here, we model the morphological and spectral signatures of europan plume deposits, utilizing constraints from recent Hubble Space Telescope observations as model inputs. We consider deposits emplaced by plumes that are 1 km to 300 km tall, and find that in the time between the Galileo Mission and the arrival of the Europa Clipper spacecraft, plumes that are <7 km tall are most likely to emplace deposits that could be detected by spacecraft cameras. Deposits emplaced by larger plumes could be detected by cameras operating at visible wavelengths provided that their average particle size is sufficiently large, their porosity is high, and/or they are salt-rich. Conversely, deposits emplaced by large plumes could be easily detected by near-IR imagers regardless of porosity, or individual particle size or composition. If low-albedo deposits flanking lineated features on Europa are indeed cryoclastic mantlings, they were likely emplaced by plumes that were <4 km tall, and deposition could be ongoing today. Comparisons of the sizes and albedos of these deposits between the Galileo and Europa Clipper missions could shed light on the size and frequency of cryovolcanic eruptions on Europa.

Tectono-sedimentary signature of the second rift phase in multiphase rifts: A case study in the Lufeng Depression (38–33.9 Ma), Pearl River Mouth Basin, south China sea

Compared to first rift phase (RP1) or single rift basins, the relationship between fault behaviours and depositional architecture during the second rift phase (RP2) remains insufficiently understood in multiphase rifts. This study attempts to provide implications for tectono-sedimentary signatures during the RP2 in multiphase rift evolution, according to an integrated analysis of 3D seismic reflections, wire logs and cores from the Lufeng Depression in the Pearl River Mouth Basin, South China Sea. The four recognised third-order sequences, ESQ1-4, define a two-stage tectono-sedimentary evolution, including an early (ESQ1-2) and a late stage (ESQ3-4) of the RP2 (38–33.8 Ma) in the Lufeng Depression. Fan-deltaic, braided deltaic sandstone-prone and lacustrine mud-prone deposits are interpreted from the basin margin towards to the central areas. During the early stage of the RP2, two relatively steep dipping E-W striking faults in the northern area were very active, undergoing rapid reactivation with a high displacement, while other faults were less active, had low displacements with respect to the extension direction and possibly represent pre-existing fault geometries. Two tectono-depositional signatures are identified during the early stage of the RP2: 1) facilitation of newly captured axially-dominated deltaic systems in the rapidly reactivated fault propagated areas, as a more uniform distribution of connected and elongated depocentres establishes an axial-supply drainage catchment; 2) sustained previous drainages from the end of the RP1 showed an increase of dip-slope sourced systems in low-displacement faulted-controlled areas, i.e. these areas lack fault-controlled topographic re-organisation but show establishment of a hanging dip-slope. The late stage of the RP2 is characterised by a low-accommodation basin as faulting activities totally waned, and thus large-sized shallow-water deltaic systems and long-distance transported sandstones are promoted. This study highlights the distinctive stratigraphic architectures and associated sediment dispersal patterns induced by progressive fault activity during the RP2 cycle. Furthermore, the proposed depositional signatures may be used as a reference when studying less data-rich multiphase rifts elsewhere.

Linking Mesozoic lode gold deposits to metal-fertilized lower continental crust in the North China Craton: Evidence from Pb isotope systematics

The source(s) of lode gold deposits formed in Precambrian cratons related to accretion/collision and cratonic reactivation, formerly attributed to either supercrustal volcano-sedimentary sequences or deep crustal/sub-crustal origin, remain controversial largely because gold deposits are spatially related to metamorphosed rocks, but geochemical data somewhat indicate a poorly understood deep source. Reconciling such conflicts is important to better understand the main factor controlling the formation of ore deposits and their genetic link with specific tectonic settings. Giant Late Mesozoic lode gold provinces in the North China Craton (NCC) were formed ca. 1.7 Ga later than cratonization metamorphism, and contemporaneous with intensively felsic to mafic magmatism related to cratonic reactivation. In this study, we conduct a comprehensive Pb isotope study on major gold deposits from the eastern Yanshan belt, northern margin of the NCC. In order to constrain the source(s) of gold, we attempt to map regional Pb isotope variations of lower continental crust (LCC), and develop a two stage quantitative model (punctuated by three prominent geological events at 2.80 Ga, 1.85 Ga and 0.16 Ga) to reproduce time-integrated Pb isotopic signatures of deep-seated lithospheric reservoirs to make a comparison with Pb isotopic signatures of the gold mineralization. Gold-bearing pyrites within different types of host rocks have relatively uniform Pb isotopic ratios, which are significantly different to high-grade metamorphosed host rocks, but similar to those of spatially associated Late Mesozoic granitic rocks. Our data show that the Pb isotopic signatures of gold deposits vary consistently with presumed regional Pb isotopes of the LCC during the Late Mesozoic. Lead isotopic heterogeneity of the LCC was likely caused by underplating of mafic magmas derived from mantle sources. During underplating, highly chalcophile elements (e.g. Au, Ag and Cu) were concentrated at the base of the LCC due to sulfide saturation from mafic magmas. Integrating petrological, geochemical, geochronological, and considering chalcophile element solubility, we propose a new genetic model to describe the formation of Late Mesozoic gold deposits in the NCC: (1) early formation of sulfide-bearing cumulates with high Au/Cu ratios during magma differentiation at the base of the LCC; (2) subsequent fluid-fluxed remelting of these cumulates at the onset of lithospheric extension and release and ascent of ore-forming fluids to the site of precipitation. Considering the metallogenic characteristics of other Au ore-forming systems, we suggest that metal fertilization in deep-seated reservoirs and subsequent tectonic decompression are important factors controlling the development of Au-rich ore deposits worldwide. This study demonstrates how Pb isotope can be employed to trace source(s) of gold deposits.

Sources and origins of eolian dust to the Philippine Sea determined by major minerals and elemental geochemistry

AbstractWe investigated the microscopic mineral characteristics of modern eolian dust particulates and the trace-element compositions of the siliciclastic fractions of these samples, collected from the Philippine Sea in 2014 and 2015, and conducted an air mass backwards trajectory analysis of dust particulates in the spring and winter of 2015, to better constrain the provenances and transport dynamics of dust delivered to this region. The microscopic minerals show obvious signatures of dust deposition and physical abrasion, indicating long-distance wind transport from the Asian deserts. The trace-element compositions (Zr–Th–Sc) display a binary mixture of eolian materials derived from the eastern Asian deserts and the central Asian deserts, which is similar to the result of the Sr–Nd isotopic compositions of modern sediment trap sediments collected on the Benham Rise in 2015. We demonstrate that modern dust sediments in the Philippine Sea primarily originate from the Ordos Desert (generally &gt; 80%), while the contributions of the Taklimakan Desert and the Badain Jaran Desert are small. Eolian dust particulates raised from source regions are predominantly transported to the Philippine Sea by the East Asian winter monsoon, but not by the westerlies. In addition, our results indicate that increased precipitation in the source regions can result in relatively low dust fluxes in the Philippine Sea, and there is a period of 6–7 days for eolian dust originating from source areas to be delivered to the Philippine Sea.

Constraints of lead isotopes on regional metallogeny of sediment-hosted sulfide deposits in the Langshan-Zhaertai ore belt, northern China

The Langshan-Zhaertai ore belt in northern China includes a dozen sediment-hosted Zn-Pb-Cu deposits, which are traditionally considered to have been formed during sedimentary exhalative (SEDEX) activities in different subbasins in the same Proterozoic marginal rift system. However, this assumption has never been systematically tested. Herein, we re-evaluate the regional metallogeny by the analysis and comparison of Pb isotope data obtained in this study and published in literatures. The Pb isotope signatures of several representative deposits from this ore belt are heterogeneous and do not lie along similar secondary or anomalous lead evolution lines, suggesting that these deposits belong to different metallogenic systems. The Pb isotope ratios of the bulk of the sulfide ores in south ore belt are characterized by unradiogenic isotope compositions and Proterozoic Pb-Pb model ages, whereas the various types of sulfides from the Huogeqi deposit in north ore belt show a very radiogenic lead signature (206Pb/204Pb > 17.000), which is broadly consistent with common lead of associated Indosinian magmatic rocks, indicating an magmatic-hydrothermal origin. The Dongshengmiao and Tanyaokou massive-type ore sulfides have similar and exceedingly unradiogenic isotope compositions grouped around 15.200 (206Pb/204Pb). Their vein-type sulfides show scattered Pb isotope signatures that fall along a well-correlated linear trend towards the right of the massive-sulfide array, indicating some modification during metamorphism. The similarity of lead isotope compositions of the Jiashengpan vein-type sulfides to the common lead of Hercynian igneous rocks indicates that the proximal source of the ore metals in its epigenetic hydrothermal system was Hercynian igneous activity.

A geochemical evaluation of the genetic relationship between Ouachita Mountains Paleozoic rocks and the Mississippi Valley-type mineralization in the southern Ozark Region, USA

The Pb and/or Zn deposits of the Tri-State and the Northern Arkansas Mississippi Valley-type (MVT) districts in the southern Ozark Region, located north of the Arkoma Basin and the Ouachita fold-thrust belt in North America, are genetically connected to the Pennsylvanian-Permian Ouachita orogeny. This orogenic event triggered a south-north topographic gradient flow of basinal brines, leaching metal-rich sediments en route. To establish whether the organic-rich shales of the Ouachita Mountains and the Ozark Region provided metals during the mineralization event, the Pb isotope compositions of the ores (sphalerite) have been compared to those of the sedimentary rocks (whole-rocks and leachates) of the Ouachita Mountains (Cambrian Collier Shale, Early Ordovician Mazarn Shale, Middle Ordovician Womble Shale, Late Ordovician Polk Creek Shale, Mississippian Stanley Shale, and Pennsylvanian Jackfork Sandstone) and the Ozark Plateau (Devonian/Mississippian Chattanooga Shale and Mississippian Fayetteville Shale). The Pb isotope compositions of ores in both districts show a broad and a linear range of isotopic values. This trend suggests mixing of Pb from two distinct end-member components. One end-member must be highly radiogenic, with Pb isotope ratios equal to or higher than the highest noticed value for ores. The other end-member must be less radiogenic, with Pb isotope ratios equal to or lower than the lowest value recorded for ores. The Pb isotope compositions of the analyzed whole rocks and leachates indicate that the best candidates for contributing the less radiogenic Pb to the ores are the sandstone members of the upper and middle Jackfork Sandstone and the lowermost section of the Chattanooga Shale.The depositional environment of the potential source rocks and their ability to sequester metals has been appraised using redox sensitive trace elements (Cd, Co, Cr, Cu, Mo, Ni, Pb, Th, U, V, Zn). Paleoredox proxies (e.g., enrichment factors of redox sensitive trace elements and V/V + Ni ratios) indicate deposition of the analyzed rocks mainly under anoxic and euxinic conditions, which are favorable for metal enrichment. However, oxic-suboxic conditions are also indicated by other geochemical proxies, suggesting a possible first order control of primary production that has resulted in an increased flux of organic matter. A disagreement in the redox conditions yielded by the paleoredox proxies has been noticed in several cases. This inconsistent behavior may be associated with several factors, including redox potential, pH, sulfur abundance, chemical availability, TOC values, and sediment provenance. The authigenic flux is the only fraction needed to unravel past redox conditions; however, the analyzed bulk sediments consist of authigenic, biogenic, and siliciclastic components, which may also have led to the observed disagreement in the redox conditions. Additional factors such as the basin geometry, the rates of sediment accumulation, the post depositional alteration processes associated with diagenesis and low-grade metamorphism during the Ouachita orogeny might have overprinted the depositional signatures.

Mesozoic tectonic in the deposition and evolution of Cretaceous sedimentary packages of the Parecis Basin, center-western Brazil

The Cretaceous record of the Parecis Basin (central western Brazil) consists of two sedimentary packages with distinct depositional signatures, physically discontinuous, and deposited in two depocenters in the east and west of Serra Formosa Arch. The basin evolved during the Mesozoic age and includes a regional record with volcanism, tectonism, deposition and uplift with erosion. During the Upper Triassic and Lower Jurassic, volcanism and sedimentation occurred, which were followed by uplift and erosion until the Lower Cretaceous, moment in which basic and alkaline magmatism was present. Since the Upper Cretaceous, with the beginning of the compressive phase of Andean Orogeny and opening of the Atlantic Ocean, tectonism and sedimentation were developed in the Juruena Sub-basin during the Cenomanian and in the Xingu Sub-basin from the upper Coniacian. Based on facies association and regional surface recognition, a differential depositional signature was identified for each sedimentary package, influenced by the activity and behavior of the internal sub-basins. In the Juruena Sub-basin, fluvial and aeolian sedimentary package were deposited with sedimentation rates higher than that of subsidence. In the Alto Xingu Sub-basin, a sedimentary package is here named the Rio Tapirapé Formation and includes a lacustrine system that was established with an overpass of the subsidence rate in relation to the sedimentation rate. Toward the top, it was replaced by a fluvial and deltaic sedimentary system, with the decrease in the subsidence rate.

Geomorphic complexity and sensitivity in channels to fire and floods in mountain catchments

Fires and floods are important drivers of geomorphic change. While the hydrologic and geomorphic effects of fires have been studied at the hillslope scale, we have much more limited data on post-fire runoff, channel changes, and inferred or measured sediment storage and delivery at larger scales. In this study we intensively documented channel changes over four years in two very similar ∼15 km2 catchments in the northern Colorado Front Range, Skin Gulch and Hill Gulch, that were burned primarily at high and moderate severity in June 2012. Ten and 11 cross sections and longitudinal profiles along the lower channel network were repeatedly surveyed in Skin Gulch and Hill Gulch, respectively. Summer thunderstorms generally caused deposition in the valley bottoms while incision resulted from the intervening baseflows and spring snowmelt, but a very high intensity summer thunderstorm in Skin Gulch just one week after burning caused much more deposition than in Hill Gulch. Fifteen months after burning both watersheds experienced an exceptional, long-duration flood resulting from a multi-day rainstorm with a several hundred year recurrence interval. This removed nearly all of the deposited post-fire sediment along with some of the older valley bottom deposits. The expansion and coarsening of the channel has greatly decreased the geomorphic sensitivity of both watersheds to future floods, and eliminated the more typical and persistent post-fire depositional signature. The channel changes in Skin Gulch after the fire and long-duration flood were much greater than in Hill Gulch, and this can be attributed to the much greater sediment deposition in Skin Gulch shortly after the fire, reduced geomorphic sensitivity in Hill Gulch resulting from a large erosional flood in 1976, and the spatial distribution of burn severity and storm rainfalls leading to lower peak flows in Hill Gulch. These results suggest that fires in the Rocky Mountains can trigger significant and dynamic hillslope and channel changes over sub-decadal timescales, but unusually long or intense rainstorms can cause larger and more persistent changes regardless of whether a catchment has recently burned.

Evaluating the fidelity of the cerium paleoredox tracer during variable carbonate diagenesis on the Great Bahamas Bank

Abstract Inferring redox conditions for ancient marine environments is critical to our understanding of biogeochemical cycles over Earth history. Because of the redox sensitivity of cerium (Ce) relative to other rare earth elements (REEs) and its uptake in marine carbonates, the Ce anomaly (Ce/Ce*) is widely applied to ancient carbonates as a proxy for local redox conditions in the water column. However, carbonate sediments and rocks are particularly vulnerable to multiple stages and styles of post-depositional diagenetic alteration where the diagenetic redox conditions and fluid compositions can vary widely from overlying seawater. Evaluations of the effects of this post-depositional alteration for the Ce anomaly have mostly been limited to ancient carbonate rocks rather than recent, well-characterized analog facies. Here, we report on analyses of REE plus yttrium concentrations (REY) and Ce anomalies in bulk carbonate samples from drill cores collected in the Bahamas (Clino and Unda) that allow us to track loss or retention of primary signals of initial oxic deposition through a range of subsequent alteration scenarios mostly under anoxic conditions. Specifically, these materials have experienced well-constrained overprints linked to meteoric processes and marine burial diagenesis, including dolomitization. Our results show that, regardless of mineralogy, diagenetic fluid composition, and redox state, the REY patterns in these carbonates, including the Ce anomaly, are similar to those of modern oxic seawater, indicating that they likely record the seawater signatures of primary deposition. As such, the Ce anomaly in shallow marine carbonates has the potential to preserve records of primary deposition even when subject to multiple stages and styles of diagenetic alteration, confirming its utility in studies of ancient marine redox.

Geophysical signatures of New Zealand epithermal Au-Ag deposits, and methods for new exploration

ABSTRACTVolcanic-hosted epithermal Au-Ag quartz vein deposits in the Hauraki Goldfield are investigated using a range of geophysical methods. Tools such as airborne magnetics and ground-based resistivity have focussed on mapping alteration zones, lateral contacts in the volcanic rocks, and prospect scale faults. Reviewing the merits of each geophysical method is valuable as it provides exploration teams with the background material required to apply the most effective tools. The review compiles existing data and summarises the rock properties useful for constraining geophysical models. The Golden Cross mine in the Waitekauri Valley is used to illustrate the strengths and limitations of various techniques. Electrical resistivity is the most valuable rock property to target exploration, but magnetic surveying and gamma-ray spectrometry can reduce the ambiguity between quartz-rich mineralised zones and un-altered volcanic host formations. Density and porosity variations caused by alteration can be mapped with gravity data provided the regional field is defined.

Storm-related taphofacies in estuarine settings: An integrated analysis on the Early Permian deposits of the Rio Bonito Formation (Paraná Basin, S Brazil)

In the Gondwanan Paraná Basin, two fossil assemblages with affinity to the Eurydesma Fauna are well known in the Early Permian post-glacial deposits of the Rio Bonito Formation. Although the assemblages display similar invertebrate fossil content, the fossil concentrations were potentially accumulated in different paleo-bathymetric zones represented by the shallow marine and marginal marine settings. This paper provides a taphonomic analysis of the body-fossil concentration and the ichnological signatures of the fossiliferous deposits of the Rio Bonito Formation at the southern border of the Paraná Basin and also discusses their paleo-ecological and paleo-environmental significance. Three distinct taphofacies representing the fair-weather deposits, storm-wave deposits in the inner part of estuary, and time-averaged and storm-influenced deposits in the outer part of the estuary were recognized in the studied succession. They characterize the depositional processes that filled a shallow, marginal marine paleovalley formed during the Late Paleozoic Ice Age. The taphonomic signatures of the fossil concentrations suggest deposition under the transgressive and highstand phases of the paleo-valley infill. These new data reinforce the previous interpretation of the paleo-valley infill as estuarine depositional setting and further contribute to refining the stratigraphic framework of the post-glacial deposits at the southern margin of the Paraná Basin.