Depositional Style Research Articles

Kontrastujące style metamorficznych i metasomatycznych złóż kamieni szlachetnych w Cabo Delgado w północnym Mozambiku

From a conceptual point of view, in some ruby and garnet deposits of the Lalamo Supergroup – Cabo Delgado Nappes Complex, Northern Mozambique – the structural control of the primary mineralization seems to be spatially and geochemically related to the genesis of calcsilicate rocks, also with gemstones. Two main locations, with contrasting paragenesis, were studied in detail – Miteda area and Micute to N’Djekwa sector. Structural and paragenetic analysis, departing from high-resolution cartography, fabric microanalysis, SEM microscopy and mineral identification by X-ray diffractometry, were used with a major incidence in outcrops and small-scale mine diggings. Regional Neoproterozoic formations, especially its metacarbonate members, host Mg, B - rich skarns, bearing dravite and B-Mg-calc-silicate gemstones, especially in Micute and N’Djekwa area. Following the composition of the probable lithological precursor, these minerals, consistently, reveal a geochemical Mg - B signature that could have been inherited from an evaporite type ancestor, inter-stratified with carbonate rocks in the protolithic Lalamo sequence. The focus of hydrothermal circulation in multistage shear zones is invoked as the main pathway to interpret the complex mineralization observed in Miteda. In this case, the structure of a corundum + garnet major spot is coherent with a pull-a- part dilation associated with sinistral shear. There is a relation between red color saturation of ruby and the content of Cr in corundum. Cr can be leached from the meta-ultramafic host-rocks as a byproduct of desilication. Calcsilicate metasomatism could trigger Si sequester, producing metamorphic desilicated fluids. The focus of hydrothermal circulation in shear zones is invoked as the main pathway for the entrapment of metamorphic and metasomatic gem deposits. These occur in corridors of anomalous host-lithologies and vein swarms where peraluminous paragenesis are located inside highly modified meta-ultramafic or metacarbonate rocks. Skarn like lithologies result from reaction with a sedimentary carbonate protolith and gem deposition is mainly metasomatic. Veining affecting metaultramafic sequences of amphibolitic to granulitic facies show occasional, metasomatically affected, eclogite. For the contrasting styles of deposits represented in Micute – N’Djekwa and in Miteda, although the corresponding paragenesis are separated in space, their deposition can be situated in a conceptual model characterized by a geochemical interdependency and time proximity.

Palaeogeographic heterogeneity of large-amplitude changes in marine sedimentation rates during the Carnian Pluvial Episode (Late Triassic)

Major environmental changes marked the Carnian (Late Triassic), including global warming and enhanced hydrological cycling linked to multiple carbon cycle perturbations. These perturbations occurred in an interval called the Carnian Pluvial Episode (CPE), between the Julian 1–Julian 2 boundary and the base of the Tuvalian 2 (late early to middle late Carnian). The CPE is linked to pronounced changes in terrestrial and marine sedimentary basins, with abrupt switches in the sedimentation style and a widespread crisis of carbonate systems. In this study, we have quantified changes in sedimentation rate and the relative variations in sedimentation rate through well age-constrained successions spanning the CPE in different marine basins. Relative to background (pre-CPE) Julian 1 values, during the CPE interval sedimentation rates rose by 142%–3400% in the Julian 2, and then decreased in the Tuvalian 1 in the marine sequences of Western Tethys and Panthalassa. By contrast, in the successions of Eastern Tethys sedimentation rates dropped by 50%–100% across the CPE interval. In the Western Tethys, the high increases in sedimentation rates were coupled with a shift from prevailing carbonate to siliciclastic sedimentation, but a similar lithological shift is associated with a decrease in sedimentation rates in Eastern Tethys. In Panthalassa, higher Julian 2 sedimentation rates can be linked to mineralogical and geochemical evidence of increased input of aeolian dust under arid environmental conditions. The heterogenous changes in sedimentation rate across different basins/regions during the CPE interval indicate that sedimentation was likely controlled by the interplay of different, sometimes local phenomena such as synsedimentary tectonism, sea-level oscillations and climate. However, they could ultimately be related to palaeogeographically different responses to the pronounced environmental changes implied by the Carnian carbon cycle perturbations.

High-Cd sphalerite in the Pangxidong Pb-Zn-Ag deposit (Yunkai Domian, South China): Insight for physicochemical condition of orogenic-type deposit

The Pangxidong Pb-Zn-Ag-(Au) deposit is one of the representative orogenic-type Au-bearing polymetallic deposits in the Yunkai Domian, Cathaysia Block, South China, and both its vein- and altered rocks-type orebodies are controlled by the regional NE-trending Bobai-Cenxi Fault and its subordinate faults. Metallogenic potential of critical metals, such as Cd, In, Ga and Ge, hosted in these orogenic-type polymetallic deposits, have never been evaluated. We, therefore, selected the Pangxidong to conduct a series of field survey, microscopic observation, electron probe micro-analyzer (EPMA) and laser ablation inductively coupled plasm mass spectrometer (LA-ICP-MS) investigation. We identified two generations of sphalerite with different mineral assemblages and mineralogical features, and both sphalerite generations have the relatively higher Cd, Cu, Ag and Au contents, against the lower Ga, Ge, In and Sn contents. Notably, an extremely elevated Cd content was detected in the earlier SpA (4869–47194 ppm, mean = 15976 ppm), against those in the later SpB (5629–9980 ppm, mean = 7224 ppm). Further temperature evaluation using GGIMFis geothermometer indicates that the earlier SpA (229–367 ℃, mean = 239 ℃) have a slightly higher crystallization temperature than the later SpB (228–240 ℃, mean = 233 ℃). Collectively, we highlight that the Pangxidong sphalerite have different minor and trace element associations and concentrations against most other Zn-Pb deposit styles such as MVT and skarn system, and therefore represents a novel sphalerite type from the orogenic-type deposit.

Unidirectional and combined transitional flow bedforms: Controls on process and distribution in submarine slope settings

ABSTRACTMixed grain‐size bedforms comprise alternating sand‐rich and poorly sorted mud‐rich laminae and bands. These bedforms have been identified in distal submarine settings formed underneath unidirectional flows. This study documents mixed grain‐size bedforms in a proximal submarine slope setting formed beneath both unidirectional and combined flows. Core and outcrop data with well‐constrained palaeogeographical context are used to describe two types of mixed grain‐size bedform. Type A bedforms comprise mud‐rich current ripples and low‐amplitude bed‐waves with alternating concave and planar sandstone–mudstone foresets that pass into mud‐rich troughs, and aggradational sinusoidal laminasets. Type B bedforms consist of sandstone–mudstone laminasets that comprise rounded, biconvex ripples with sigmoidal‐shaped foresets and swale and hummock‐like laminasets and banded sets. These bedforms occur in channel‐margin, internal‐levée and external‐levée, intraslope and disconnected lobe environments, and represent 27 to 63% by stratigraphic thickness of the studied successions. They are interpreted as deposits of clay‐rich transitional flows, whose depositional style is governed by the balance of cohesive and turbulent forces, and the rate of flow deceleration. Type B bedforms are further interpreted as combined transitional flow deposits, resulting from flow deflection and ponding processes by seabed topography. Upward and lateral transitions between different bedforms create distinct bedform sequences, demonstrating progressive spatio‐temporal transformations in flow properties and their topographic interactions. By using a well‐constrained palaeogeographical setting, mixed grain‐size bedforms are shown to be situated close to sites of erosion into muddy substrates, abrupt losses in confinement, and/or changes in slope gradient. These bedforms demonstrate that flow transformation and transitional flow behaviour are not restricted to distal submarine settings. Furthermore, mixed grain‐size bedforms are not a diagnostic criterion for bottom currents, because such flows cannot account for the high mud content in laminasets, or the interlamination of sand and mud.

Basin topography and depositional styles controlled by collisional tectonics in the Alps-Apennines junction (Tertiary Piedmont Basin, NW Italy)

Basin topography and depositional styles controlled by collisional tectonics in the Alps-Apennines junction (Tertiary Piedmont Basin, NW Italy)

Surface grain size of alluvial fans on Mars from thermal inertia, as an indicator of depositional style

The depositional history of alluvial fans on Mars provides insight into the climatic conditions during the time of fan formation in the late Hesperian to early Amazonian. However, traditional stratigraphic analysis of the alluvial fan deposits is not possible across most of Mars. This study assesses the use of thermal inertia data as a tool for sedimentologic interpretation of 437 Mars alluvial fans. Based on previous work demonstrating the relationship between depositional style, grain size, and thermophysical properties, this study uses analysis of the thermal inertia of alluvial fan surfaces across the global population of fans on Mars to make an assessment of depositional styles that built the alluvial fans. The thermal inertia values across the global population of fans are indicative of sand- to pebble-sized sediment. The variability of grain sizes across the global population is more homogenous than expected based on comparisons to terrestrial alluvial fans. Nearly all Mars alluvial fans have an average thermal inertia that corresponds to pebble and smaller grain size, and < 1% of Mars alluvial fans have an average thermal inertia that corresponds to cobble-sized grains. Spatial patterns of thermal inertia variability on alluvial fan surfaces show a small number of fans with evidence for either downslope fining or channelization, but the majority of fans show no recognizable geologic patterns in surface thermal inertia. The interpretation of the thermal inertia-derived grain size suggests that either there is widespread mantling of unconsolidated sand across the surface, or that Mars alluvial fans were built by primarily sand-sized sediment, which may be indicative of lower energy sediment transport events.

Facies Variability in Paleogeographic Trends from Core Data in the Lower and Middle Frio Formation in South Texas

This core-based study of the Oligoene lower and middle Frio formation in South Texas documents facies variability, depositional systems, and sandstone-body architecture in the lower and middle Frio formation in South Texas. A previous study by the Bureau of Economic Geology (BEG) in 1982 characterized the Frio Formation in terms of depositional systems, structural framework, hydrocarbon origin, migration, and exploration potential. Paleogeographic trends in the 1982 study were delineated for the lower, middle, and upper Frio Formation, based mostly on wireline-log data. Core data in this study augment and refine facies interpretations and clarify depositional styles in four paleogeographic trends from the 1982 study in the lower and middle Frio Formation. These trends include (1) lower delta plain/margin, (2) aggradational fluvial, (3) aggradational (multistoried) barrier, and (4) floodplain. With minor exceptions, this study validates and refines interpretations of the regional distribution of lower and middle Frio paleogeography in South Texas in the 1982 study. Four (4) main results of this study include (1) documenting lithology and facies types in these paleogeographic trends, (2) demonstrating that these paleogeographic trends are a composite of multiple transgressive-regressive (T–R) cycles, (3) providing evidence that these fluvial systems are mixed-load, meander belt in origin rather than coarse-grained bedload in origin, based on vertical grain-size profiles and the presence of paleosols that indicate stable, well-developed floodplains, and (4) identifying anomalous occurrences of sandstone-rich fluvial-channel systems in areas previously mapped as sandstone-poor and interpreted to be primarily floodplain in origin. Finally, this study serves as an example of the value of core data in refining interpretations of paleogeography and depositional systems in stratigraphic units that are a composite of multiple T–R episodes.

Three‐dimensional stratigraphic architecture, secondary pore system development and the Middle Pleistocene transition, Sandy Point area, San Salvador Island, Bahamas

AbstractThe Bahamian Archipelago has been the subject of extensive studies of stratigraphy, carbonate island morphology and diagenetic overprinting for many decades. A recently‐acquired dataset comprised of high‐resolution light detection and ranging, tightly spaced cored boreholes with image logs, thin sections, porosity and permeability measurements, and isotopic geochemical analyses provides a unique opportunity to perform detailed sequence stratigraphic analysis. This multi‐faceted study first establishes a new high‐resolution sequence stratigraphic framework for the Sandy Point area of San Salvador Island, encompassing six unconformity bound sequences (exposure surfaces) representing varying lengths of missing time. The stratigraphically lowest sequence is Late Pliocene in age and is dominated by reef facies capped by an extensive laminar caliche that represents 1.5 million years of exposure. The overlying Early Pleistocene is split into two sequences, EP1 and EP2, which are both dominated by low‐energy subtidal facies. The upper three sequences are tied to the Marine Isotope Stage 11, 9 and 5e interglacials, and are distinctly different in facies composition and architecture, being dominated by higher energy facies distributed in a more complex mosaic. The new stratigraphic framework highlights significant changes in depositional style and facies architecture that can be linked to increasing sea‐level amplitude oscillations and greater climate gradient at the Mid Pleistocene Transition. The well‐constrained framework is then used to provide key context for observed complex diagenetic evolution through repeated sea‐level inundation and subaerial exposure. These results suggest that well‐developed subaerial exposure surfaces drive increases in non‐matrix features in the subsequent stratigraphic package following exposure as the well‐cemented exposure surface acts to concentrate fluid flow and dissolution. Integration of non‐matrix features within this framework highlights the importance of dissolution/cementation patterns on formation of these modified unconformity surfaces for focusing later meteoric diagenesis and creating enhanced fluid flow pathways for continued multicyclic diagenetic events.

Integrated biostratigraphy and chemostratigraphy of the Maastrichtian–Danian succession at Abu Minqar, Egyptian Western Desert: Implications for sequence stratigraphy and tectonic influence

We investigated the Maastrichtian–Danian succession at the Abu Minqar section in the Western Desert of Egypt (part from the southern Tethyan margin) using an integrated analysis of biostratigraphic (calcareous nannofossils, foraminifers, macrofauna) and chemostratigraphic data to determine the controlling factors on depositional history. The integrated microplankton biostratigraphic framework identified two prolonged hiatuses at the lower/upper Maastrichtian and the Maastrichtian/Danian boundaries. Four depositional sequences were identified and subdivided into systems tracts based on paleodepth variation evidenced by foraminiferal investigations and the comparable response of the various geochemical indices. The study also highlighted the presence of two alternated depositional styles in the study area, influenced by relative sea level, with the first style involving the deposition of aluminosilicate-enriched intervals that yield non-calcareous foraminifers in lagoonal and coastal depositional settings induced by relative sea-level fall and river rejuvenation, and the second one involving the deposition of calcareous intervals containing abundant megafossils, calcareous foraminifers, and nannofossils during a sea-level rise. Despite the occurrence of certain resemblances to the global eustatic curves, the inferred relative sea level pattern suggests that regional tectonics had a substantial role in developing the depositional history and paleoenvironmental circumstances of the southern Tethys margin. These findings help build a more comprehensive picture of the geological evolution of the southern Tethyan margin during the Late Cretaceous to Paleogene.

Depositional architecture and sequence stratigraphic framework of the fluvio‐lacustrine Ash Shumaysi Formation, Jeddah‐Makkah Region, Saudi Arabia: Implications for climatic and tectonic changes in a local‐scale sub‐basin

AbstractThis study aims to interpret and document the depositional architecture styles and sequence stratigraphic framework of the Ash Shumaysi Formation in the Jeddah‐Makkah region, the west‐central part of Saudi Arabia, and presents an example of rarely discussed, local‐scale sub‐basins (half grabens). It also shows the relationships between synchronous sedimentary processes and pre, syn and post‐rift conditions. The described lithofacies and their facies associations indicate the presence of seven architectural depositional styles: proximal‐distal braided fluvial, meandering fluvial (point bar), crevasse splay, floodplain, estuarine and lacustrine. A proposed depositional model for the Ash Shumaysi Formation is drawn. The Ash Shumaysi Formation forms a second order depositional sequence, which is organised into two third order depositional sequences (sequences I and II) bounded by three sequence boundaries. Each third‐order sequence encloses the low accommodation systems tract and high accommodation systems tract. The low accommodation systems tract represents the coarse‐grained, braided‐distal fluvial facies developed during low accommodation space associated with high sediment supply (high discharge). The high accommodation systems tract encloses the fine‐grained deposits of point bar, estuarine and lacustrine facies that reflect the creation of significant accommodation space and low sediment supply (low discharge). Vertical and lateral variations of the inferred depositional architectural styles, sequences and systems tracts reflect that tectonic forces and climate are the main controlling factors during deposition of the Ash Shumaysi Formation, although base‐level changes in response to sea‐level changes cannot be ruled out.

Depositional environment and hydrocarbon exploration potential based on sedimentary facies and architectural analysis of the Upper Cretaceous Shendi Formation in Musawwarat-Naga area, Shendi-Atbara Basin, Sudan

This study examines both vertical and lateral facies changes as well as petrographic characteristics of the outcropping section of the Upper Cretaceous Shendi Formation in the Musawwarat-Naga area with a view of interpreting the depositional environment, provenance, and tectonic setting that prevailed during the deposition of the sediments. Field studies revealed the presence of eight different sedimentary lithofacies. The predominant is the trough cross-bedded sandstone facies (St), representing 75% of the total succession. Four architectural elements were recognized: sandy bedforms (SB), channel fill (CH), gravelly bars and bedforms (GB), and overbank fines (OF). Moreover, the mean value of paleocurrent directions was 285.3°, suggesting a southeasterly located source area. The majority of the sandstones were classified as quartz arenites with subordinate sublithic-arenites. Ternary plots of quartz-feldspar-lithic fragments (QFL) revealed that the tectonic setting is predominantly within the interior craton. It can be suggested that the sediments were sourced from the southeast region and associated with a braided, proximal, fluvial depositional environment. The results of this study provide outcrop analog data that might be useful in exploring hydrocarbons. Specifically, it can enhance the understanding and prediction of the proximal fluvial depositional styles in the subsurface deposits in the Shendi-Atbara Basin and similar rift basins in Sudan and Africa.

Genetic link between orogenic Au and porphyry Cu ([sbnd]Au) mineralization in the Dananhu Arc, NW China: Evidence from geochronology, geochemistry, and Sr–Nd–Hf isotopes of the Tudunbei Au deposit

The late Paleozoic Chinese Tianshan is characterized by large volumes of arc-related magmatic rocks, and numerous associated orogenic Au and porphyry Cu (Au) deposits. The link between these two deposit styles, however, is ambiguous. Here, we present zircon U–Pb–Hf isotopes, bulk-rock geochemistry, and sericite 40Ar/39Ar geochronology from the newly discovered Tudunbei Au deposit located east of the Tuwu–Yandong porphyry Cu (Au) ore district in the Dananhu Arc, Chinese North Tianshan, and combine this with previously published geochronological and geochemical data to characterize the genetic relationship between orogenic Au and porphyry Cu (Au) mineralization. Our new UPb zircon ages indicate that the wall rocks of the Tudunbei Au deposit formed during the Carboniferous between 337 and 314 Ma (K-feldspar granite: 334.8 ± 1.9 Ma, basaltic tuff: 336.6 ± 4.0 Ma, quartz diorite: 316.0 ± 1.8 Ma, gabbroic diorite: 318.3 ± 1.8 Ma, and gabbro: 314.3 ± 1.8 Ma). These wall rocks are enriched in light rare earth elements and large ion lithophile elements (e.g., Ba, U, and Pb), and depleted in high field strength elements (e.g., Nb, Ta, and Ti). Given the absence of Precambrian basement rocks in the Chinese North Tianshan, and the depleted NdHf isotope compositions (εHf(t) = 12.12–14.98; εNd(t) = 5.77–7.06) of the wall rocks, their young Hf model ages (350–510 Ma), the lack of inherited zircons, and their enrichment in Na2O (2.84–4.73 wt%) relative to K2O (0.43–2.40 wt%), it is suggested that they formed in an oceanic arc environment. One sericite sample selected from an auriferous quartz vein yielded 40Ar/39Ar plateau ages of 260.6 ± 1.1 Ma, which is representative of the gold mineralization age. Given the abundance of gold in the Carboniferous wall rocks of the Tuwu–Yandong porphyry Cu (Au) ore district, the similarity in the timing of its magmatism with Tudunbei, and the geochemical–isotopic (Sr–Nd–Hf) similarity of their wall rocks, it is inferred that the gold in the Tudunbei deposit was remobilized from pre-existing gold in the Carboniferous wall rocks during regional metamorphism and deformation.

Alluvial development during climate fluctuations depicted with spectral decomposition of the Middle Triassic to Lower Jurassic post‐rift succession in the Horda Platform (Norway) and the impact on reservoir properties

AbstractFor the first time, this study presents interpretations of alluvial fans in spectral decomposition RGB Blends, analysed in seismic time slices from the Middle Triassic to Lower Jurassic stratigraphical interval of the Horda Platform (northern North Sea). The time slices record a shifting alluvial fan front, fluvial variability, uplift and erosion and reveals depositional elements that may set the common conception of the geological development of this area during the Early–Middle Mesozoic up for discussion. Results show that the Upper Triassic Lunde Formation in the eastern margin of the Horda Platform was characterized by the deposition of coalesced alluvial fans. A variable extent of the fan front through the Upper Triassic is linked to interplaying allogenic factors: uplifted source areas in the aftermath of Early–Middle Triassic rifting determined sediment availability; climate transitioning from arid to semi‐humid, with increasingly fluctuating precipitation, controlled sedimentation, and run‐off; provenance dictated bulk sedimentology and affected prevailing alluvial processes. An overall retreat of the fan system through the Late Triassic coincided with a significant change in landscape characteristics at the transition into the overlying Statfjord Group. Uplift and initial tilting of the Horda Platform caused landscape degradation and the formation of plateaus and incised valleys, contemporaneous with increased humidity and marine transgression, forming estuaries. The shift in depositional style has implications for reservoir properties, creating complexity and heterogeneity in terms of facies distribution and connectivity, which may benefit potential CO2 storage. Upper Triassic alluvial fan development in the Horda Platform (northern North Sea) depicted through spectral decomposition of seismic time slices Enhanced level of seismic stratigraphic interpretation with RGB blending Impact of allogenic factors on alluvial depositional development Implications of alluvial variability on reservoir properties

Sedimentary characteristics of sandy braided river deposits and factors controlling their deposition: A case study of the lower Shihezi Formation in the northern ordos basin, China

Fluvial sandstones have been identified as significant reservoirs due to their importance in global hydrocarbon exploration. The current study investigated the sedimentary characteristics and temporal evolution of sandy braided river systems as well as the factors controlling their deposition in the Lower Shihezi Formation (H1-1 to H1-4 sub-member) of the Xinzhao area in the northern Ordos Basin using well logs, core intervals, whole rock geochemical data, and other measured reservoir parameters. Within the braided river deposits, five architectural elements were identified: multistory braided channel, single-story braided channel, sandy braid bar, overbank, and floodplain. The evolution of architectural elements in the Lower Shihezi Formation is closely related to paleoclimatic conditions and tectonic activity. Climate influences sediment accumulation through its effect on terrestrial detrital input, and tectonism influences stacking patterns and architectural style by influencing subsidence rates and accommodation changes. High sediment deliveries and high-energy hydrodynamic conditions in warm and humid climates, combined with low subsidence rates and low accommodation/sediment supply (A/S) ratios, can result in the formation of highly amalgamated braided channels and braid bars as well as complex vertical stacking patterns and aggradational processes in the H1-2 and H1-3 sub-members. Conversely, the dry and hot climates, high subsidence rates and associated high A/S ratios are responsible for the low detrital influx input, stacking of isolated sand bodies, and high preservation of overbank and floodplain deposits in the H1-4 sub-member. Considering the accommodation-based systems tracts, we contend that climate and tectonism both have a significant impact on the evolution and architectural styles of sandy braided river deposits. It is suggested that climates control the stacking pattern of fluvial sand bodies during high accommodation systems tracts, whereas both climates and accommodation spaces influence architectural styles and the degree of sand body amalgamation during low accommodation systems tracts. Considering the thick, highly amalgamated sand bodies with better reservoir properties and high vertical and lateral connectivity in the H1-2 and H1-3 sub-members, we posit that these stacked multistory sand bodies should be the preferential targets for hydrocarbon exploration. The findings of this study can further elucidate the architectural styles and factors that influence the deposition of sandy braided river systems under complex tectonic and climatic conditions.

Deciphering influencing processes in a tropical delta system (middle-late Eocene? to Early Miocene, Colombian Caribbean): Signals from a well-core integrative sedimentological, ichnological, and micropaleontological analysis

Deltaic depositional systems are characterized by a complex interaction of physical, chemical, and biological factors. Although fluvial-, wave- and tidal-dominated deltaic environments have been extensively studied, evaluation of the processes in tropical mixed sedimentary systems has not been fully documented. Tropical regions with active margins are tectonic environments where these multiple factors act on the development of coastal systems. An onshore well-core from this tropical setting (Sinú-San Jacinto Basin, Colombian Caribbean) revealed that a middle-upper Eocene?-lower Oligocene coarse-grained deltaic setting is replaced by a thick coal-bearing mixed-energy fine-grained deltaic succession from the Oligocene to Early Miocene. The integrated analysis of facies associations, ichnological data, and terrestrial/marine micropaleontological assemblages (palynomorphs, foraminifera, and calcareous nannofossils) of this well-core allowed us to identify changes in dominance and influence of coastal processes (fluvial-, wave- and tide) and shoreline evolution. Using this information, as well as the sediment supply and accommodation space ratio, we were able to distinguish three intervals from the bottom to the top of the siliciclastic succession: (i) middle-late Eocene?–early Oligocene, prograding, fluvial-dominated, wave- and tide-influenced coarse-grained deltas with amalgamation of hyperpycnal-dominated mouth bars with hyperconcentrated flow input, (ii) Oligocene, retrograding to prograding, hyperpycnal-dominated heterolithic delta deposits punctuated by transgressive pulses, and (iii) late Oligocene to Early Miocene, aggradational, coal-bearing fine-grained delta plain with a higher proportion of transgressive phases over the continental environment. The complete succession represents long-term (∼14 Myr) mixed-energy nearshore siliciclastic systems, showing different lithological arrangements and sedimentation styles. A long-term evolution is observed from a middle-late Eocene? steep, short and coarse-grained sedimentary system with tropical humid lowland forest and punctual development of macrobenthic tracemaker communities (Interval I) to an Early Miocene gently (poorly drained), well-developed delta plain with mangroves and wave- and storm-influence record through trace fossils assemblages (Interval III). A combination of factors, including subsidence, relief uplifting, and possible relative sea level changes, are interpreted as the main controls on the stratigraphic evolution of sedimentary styles throughout the entire succession. Minor-order sedimentary successions into each interval (e.g., prograding distributary mouth-bar channel) revealed short-term cycles presumably controlled by an internal delta dynamic. Multidisciplinary analysis is essential for recognizing the influence of fluvial, wave, and tidal processes on tropical deltas, where high spatial and temporal variability makes it difficult to determine dominant processes for long periods of time.

Pennsylvanian-age plant macrofossil biostratigraphy in tropical Pangaea: uniformitarianism, catastrophes and the ‘Cantabrian’ problem

Abstract Upper Carboniferous stages in terrestrial strata are consistently recognizable throughout the Euramerican Realm based on the ranges of plant macrofossils rather than named biozones. Uniformitarianism is an invaluable principle used to understand much of Earth's history. However, it has been recognized that relatively short time intervals exist when major changes occurred in the biota and sedimentation style at a non-uniformitarian time scale. Many well-defined Upper Carboniferous stage boundaries are located at such events. The Cantabrian Stage was proposed in 1969 based on the assumption that the ‘Florensprung’ (‘floral jump’) of Gothan, a dramatic change in terrestrial floras at the traditional Westphalian–Stephanian boundary, indicated that strata were missing, and the postulated ‘gap’ had to be closed by finding strata representing this ‘missing’ time elsewhere. The suggestion of a ‘gap’ or hiatus at this level reflects uniformitarian thinking, but is incorrect. As recent work has shown that a drastic climatic change reached a threshold in the tropical palaeoequatorial parts of Pangaea at the Westphalian–Stephanian (approximately the Moscovian–Kasimovian) boundary as a consequence of a drying trend, floras changed rapidly, and the traditional Westphalian–Stephanian boundary is thus one of the most easily recognizable biostratigraphic boundaries in Pennsylvanian terrestrial beds. The Cantabrian Stage, proposed to fill the non-existent gap, does not exist and cannot be recognized, either in its type area or elsewhere.

Geology and geochemistry of Zn-Pb(-Ge-Ag) deposits in the Sichuan-Yunnan-Guizhou Triangle area, China: A review and a new type

Mississippi Valley Type (MVT)1 deposits represent the majority of carbonate-hosted Zn–Pb deposits globally. They typically form in an extensional tectonic setting along passive margins. However, studies on carbonate-hosted Zn–Pb deposits in the Sichuan–Yunnan–Guizhou Triangle area (SYGT) reveal differences in terms of the essential features of typical MVT deposits. We discuss the main features of these deposits, controlled by folds and faults, high-grade Zn–Pb ores and symbiotic or associated elements (i.e., abundant Ag and dispersed amounts of Ge, Ga, and Cd) of significant economic value. Based on a comparison of Pb-Zn deposits in SYGT and classical MVT deposits formed in an extensional tectonic setting, as well as a subsequent discussion on deposit classification, we propose that the Huize-style Zn–Pb deposits are a new deposit style, whose basic features can guide future ore exploration. We define the Huize-style deposits based on four factors: i) Hierarchical ore-controlling system of strike-slip fault-fold structures, ii) litho-facies associated with typical alternation and mineralisation, iii) ore-forming geochemical features revealed by typical mineral assemblages, mineralisation alteration zoning, fluid-inclusion, isotope geochemistry and metallogenic acid–alkali geochemical barriers, and iv) different prospecting methods to those typically used for MVT deposits. The Huize-style and typical MVT deposits constitute the carbonate-hosted non-magmatic epigenetic hydrothermal-type (CNHT) Zn–Pb deposits.

Editorial: Style of deformation, sedimentation, kinematic and thermal evolution of thrust-top basins: From natural examples to models

Thrust-top basins (or "piggyback", "wedge-top", or "structural" basins) are a fairly common feature in the accretionary and orogenic wedges worldwide. The formation of thrust-top basins provides information on compressional deformation history in various subductioncollisional settings, and was mainly controlled by the kinematics of competing growth faultrelated fold and thrust sequences. The aim of this research topic was to assemble geoscientists interested in thrust-top basins, spanning a broad array of different tectonic settings, geographical locations, and geological times. This research topic presents a collection of some of the diverse research that is being carried out on this topic. We believe that these studies contribute to a better understanding of orogenic processes in foreland basin systems.

The genetic link between iron oxide-apatite and iron skarn mineralization in the Beizhan deposit, Western Tianshan, NW China: Evidence from magnetite and gangue mineral geochemistry

• We identify coexisting IOA ore and iron skarn ore in the Beizhan deposit. • IOA and iron skarn ore could form from a single magmatic-hydrothermal system. Multiple types of deposits could form from a single magmatic hydrothermal system. Thus, identification of the temporal, spatial and genetic link among different styles of deposits is crucial to understand the evolution of magmatic-hydrothermal systems and helpful in ore deposit exploration. Iron oxide-apatite (IOA) ores sometimes coexist with iron skarn ores in the same region, but whether these two iron ore styles could form from the single magmatic-hydrothermal system remains poorly understood. The Beizhan deposit is the largest-scale iron deposit (468 Mt, with an average of 41% Fe) in the Awulale Metallogenetic Belt (AMB) in Western Tianshan, and was previously considered to be submarine volcanic-hosted iron deposit or iron skarn deposit. In this study, we identify coexisting IOA ore and iron skarn ore in the orebody proximal to a dioritic-granitic intrusion in the Beizhan deposit. Combined with the geochemistry of magnetite and gangue minerals, we suggest that the IOA ores are consistent with formation from a high-temperature hydrothermal fluid sourced from a dioritic magma. In contrast, the iron skarn mineralization originated from a relatively low-temperature and more evolved hydrothermal fluid compared with the IOA ores. We propose the consecutive precipitation of an evolving magmatic-hydrothermal system during the two stages of iron mineralization at Beizhan, that resulted through compositional evolution of the fluid during magma differentiation. The coexisting two ore types identified in the Beizhan deposit provides insights into the evolution of the intermediate-felsic magmatic-hydrothermal systems and the genetic model of the similar ore type associations in the other deposits.

Characteristics and Potential of Placer Gold Deposit in Lakan Bilem Block, West Kutai District, East Kalimantan, Indonesia

Kalimantan is recognized as one of Indonesian islands with huge prospect of minerals particularly gold both primary and secondary deposit styles. However, the publication of the gold resources is still restricted to be described in regional/district scale maps or exploration company internal reports, hence, a detailed study on deposit scale remains limited. This study is aimed to document the characteristics and the estimation of potential or resources of the secondary (placer) gold deposit in Lakan Bilem block, West Kutai district, East Kalimantan province, Indonesia. The study was done by field observation followed by microscopic analysis and simple resources estimation of the gold. Gold in the study area occur in the semi-consolidated paleo alluvial deposit as trace mineral. The gold morphologies show platy to angular form with coarse surface which indicates the process of gold deposition is predominantly controlled by hydrodynamic transportation at a relatively moderate regime. Estimation of the indicated resources of gold results a total of 4.96 tonnes of gold potentially occurred in the study area. Exploration drilling with 100 spacing grid is suggested to delineate lateral and vertical distribution of the gold deposit. The drilling also will upgrade confidence level of the resources from indicated to measured resources.