Siliciclastic Shelf Research Articles

Niche expansion of archaeocyaths during their palaeogeographic migration: Evidence from the Chengjiang Biota

Archaeocyaths were among the iconic metazoans of the early Cambrian seas and dominated the earliest Phanerozoic reef system before their dramatic decline toward the end of the early Cambrian time (c. 510 million years ago (Ma)). Over their almost 200-year research history, these organisms have been recognized as stenobionts adapted to a very narrow range of carbonate-dominated environments. Here, for the first time, we report three sorts of archaeocyath-like fossils from the Chengjiang Biota (c. 518 Ma), which was developed on a siliciclastic shelf devoid of any reefal buildups. These fossils are preserved in storm-induced mudstone without evidence of fragmentation during transportation. All of them share the same preservation mode involving replacement by clay minerals and iron oxides but exhibit archaeocyath-type external skeletal features. One species, which used to be interpreted as a dasycladalean alga, shows an aquiferous system and a skeletal ontogeny that is typical of archaeocyaths. It can be assigned to Propriolynthus sp. The other two species are comparable with representatives of the superfamily Ethmocoscinoidea and the family Sigmocyathidae, respectively. Propriolynthus sp. represents the first recognized one-walled archaeocyath in South China and fills a missing link in the migration route of one-walled archaeocyaths, although not in the phase of a reef-builder or dweller. This data reveals new aspects of archaeocyath evolution by niche expansion into siliciclastic-dominated seas at the acme of their diversification.

Geochemical and geochronological evidence of a Neoproterozoic island-arc on the easternmost Sierras Pampeanas. New U-Pb LA-ICP-MS data from amphibolites and metacarbonate depositional sequences

The first basement outcrops west of the Río de La Plata craton are metabasaltic and metasedimentary units of the Sierra Chica belt, part of the Eastern Sierras Pampeanas. These are orthoamphibolites and subordinate garnet-biotite gneisses and metacarbonate sequences that are intruded by Cambrian calc-alkaline and peraluminous rocks of the Pampean Orogeny. In accordance with one of the leading tectonic models for the region, new provenance data from this research suggest that the metabasaltic-metasedimentary association represents the proximal deposits of a Late Neoproterozoic island-arc. This contribution brings new geochemical and geochronological evidence from orthoamphibolites located on different sites along the Sierra Chica belt, which are interbedded with metacarbonate and gneisses outcrops, showing tholeiitic compositions, island-arc related tectonic settings, and Cryogenian to Ediacaran crystallization ages. Moreover, silicate-rich metacarbonates interbedded within the same sequences yielded a maximum depositional age of 569 Ma, reinforcing the assumed age for the sequence. All things considered, these rocks are interpreted as old basic lava flows and gabbro dikes linked to an island-arc activity, interspersed within the carbonatic and siliciclastic shelf sediments, together constituting the Neoproterozoic Córdoba Arc from the Sierra Chica belt. A depositional analogy with the present-day Caribbean island-arcs deposits suggests that these sequences could have been part of Oceanic Arc Depositional Systems (OADS) during the diachronous closure of the Goiás ocean and the final assembly of West Gondwana. Overlapping thermo-tectonic processes associated with the Pampean Orogeny are ubiquitous throughout all the studied outcrops, as evidenced by the petrography, the zircon morphology, and the U-Pb results, indicating a complex scenario towards a Late Cambrian slab break-off event.

Facies-neoichnological variability and sedimentation rates of modern continental shelves

Current ichnofacies models for shelf environments are largely based on analysis of ancient sedimentary deposits and have rarely been applied to studies of modern siliciclastic shelves. Siliciclastic shelf margins are key components of source rocks and unconventional shale plays that can be used as modern analogs to ancient systems. This paper examines the sedimentology and ichnology of shelf margin cores of the Mississippi River Delta containing strata formed during the 2004 (Ivan) and 2005 (Katrina and Rita) hurricane events in the Gulf of Mexico, together with those from the Kikori-Fly-Purari rivers in the Gulf of Papua, New Guinea and the Tūranganui-a-Kiwa/Poverty Bay shelf deposits of the Waipaoa River basin, offshore New Zealand. The cores provide examples of modern-day traces created by marine organisms on muddy shelves and are used to evaluate ichnological models that have been applied to determine sedimentation rates and depositional processes in ancient settings.In total, six facies were recognised based on lithology and ichnological characteristics: (1) laminated very fine sand, produced by waxing and waning currents, displaying scarce to moderate bioturbation; (2) laminated silty mud, formed by waxing and waning currents, exhibiting scarce, very low to moderate bioturbation; (3) normally graded sand and silt, created by suspension settling following a wave-enhanced sediment-gravity flow (WESGF), which reveals low to moderate bioturbation; (4) fluid mudflow/suspension settling deposits, characterised by structureless mud with low to high bioturbation; (5) biogenically mottled silty mud deposited by suspension settling which show high bioturbation intensity; and, (6) storm-generated facies of laminated sand formed by rapid sediment accumulation on the shelf.The different study areas show remarkably similar traces with variable intensity, diversity, abundance and distribution. The Mississippi River Delta cores are typified by Helminthopsis, Chondrites, Schaubcylindrichnus, Skolithos, Siphonichnus, navichnia, fugichnia and abundant bivalve shells. The deposits from the Gulf of Papua consist of Phycosiphon, Chondrites, Schaubcylindrichnus, Skolithos, Helminthopsis, Arenicolites, worm-like traces and abundant bivalve shell fragments. In addition to the traces and bivalves, Tūranganui-a-Kiwa/Poverty Bay shelf includes Asterosoma, Thalassinoides and gastropods. These traces are consistent with the newly defined Phycosiphon Ichnofacies (MacEachern and Bann, 2020), which characterise muddy prodelta settings. This research demonstrates that the neoichnology of modern shelf sediments match models developed from studies of ancient systems, and provides a robust tool for analysing and interpreting ancient shelf strata.

Developing sediment provenance discrimination tracers using major elements in East China Sea mud deposits

Discriminating shelf sediment provenances using sediment trace metal and rare earth element (REE) concentration and isotope ratios and clay mineral contents has several unavoidable hurdles, such as the effect of sediment grain size on metal concentrations, ubiquitous inclusion of REE-rich heavy minerals in sediments, and meager and inconsistent compositional differences. Furthermore, to date, no study has analyzed and determined a convincing tracer that can differentiate between Huanghe (HH) and Changjiang (CJ) sediment provenances, especially quantitatively. In this study, we quantitatively discriminated the provenances of the southwestern Cheju Island mud deposit (SWCIM) in the northern East China Sea (ECS) shelf, using seven major elements (Si, Al, Fe, K, Na, Ca, and Mg), and further determined the spatial variations of the contributions from the HH and CJ rivers to the northern ECS sediments. The results indicate that the SiO2/Al2O3 and Fe2O3/Al2O3 ratios of the HH, CJ, and Korean (KR) river sediments can be considered as reliable tracers for differentiation of the northern ECS shelf sediment provenance. Furthermore, the quantitative contributions of HH, CJ, and KR to the northern ECS sediment budget were approximately 80, 20, and 0%, respectively. The systematic spatial variations of the HH and CJ river contributions to the sediment provenances in the study area, such as the significant contributions of the HH river in the northwestern part of the SWCIM and the CJ river in the southwestern part, support the availability and applicability of the Fe2O3/Al2O3 ratio as a sediment provenance discrimination tracer. We propose that these major element concentration ratios can be applied to distinguish other siliciclastic shelf sediment provenances.

Facies character and evolution of a mixed carbonate–siliciclastic shelf: Upper Triassic–Lower Jurassic succession in the eastern Northern Calcareous Alps (Stumpfmauer, Austria)

Western Tethys sedimentary successions constitute fundamental archives of Late Triassic–Early Jurassic environmental, carbonate production and tectonic changes. During the Late Triassic, the Northern Calcareous Alps (Austria) belonged to the Western Tethys passive margin, characterised by the deposition of the early-dolomitized peritidal Hauptdolomit (Norian) adjacent basinward to the Dachstein carbonate shelf and passing upward to the mixed carbonate–siliciclastic Kössen Formation (Upper Norian–Rhaetian). The Kössen Fm. was subdivided into the lower shallow-water Hochalm Member and the upper Eiberg Member, accumulated in an intraplatform basin coeval to shallow-water carbonates (Upper Rhaetian Limestone). The Eiberg Mb. and overlying Jurassic strata were extensively studied as a continuous marine record across the Triassic/Jurassic boundary. In contrast, shallow-water successions, time-equivalent to the Eiberg Mb. and Upper Rhaetian Limestone, located North of the Eiberg Basin, are poorly investigated. This study focusses on the approximately 350 m thick Norian to Lower Jurassic succession cropping out in the eastern Northern Calcareous Alps (Stumpfmauer). The 32 distinguished lithofacies compose seven, vertically superimposed, sedimentary units (A–G), belonging to the Hochalm Mb. of the Kössen Fm. (Unit A peritidal cyclothems, Unit B claystone/marlstone with fossiliferous beds, Unit C coral boundstone to floatstone), Upper Rhaetian Limestone (Unit D subtidal cyclothems with claystone), shallow-water carbonate strata transitional to Lower Jurassic (Unit E ooidal coated grain peloidal grainstone with basal transgressive lag, Unit F bivalve-rich, microbialite and oncoidal lithofacies, previously attributed to the Upper Rhaetian Limestone) and Hettangian–Sinemurian Kalksburg Fm. (Unit G cross-laminated coated grain peloidal grainstone with quartz and chert). The detailed lithofacies characterisation presented in this study contributes to the knowledge on the Northern Calcareous Alps stratigraphy and depositional environments across the Triassic/Jurassic boundary. The identified sedimentary units can be framed in the evolution of Western Tethys and share similarities with depositional systems from the Western Carpathians, Transdanubian Range, Southern Alps and Dinarides suggesting coherent sedimentary response and environmental, climate and tectonic controls in different palaeogeographic domains.

Facies analysis and depositional environment of a late Cambrian mixed carbonate-siliciclastic ramp from the Zard Kuh Mountain (Zagros Basin, south-western Iran)

Mixed carbonate-siliciclastic rocks were particularly widespread during the Furongian (late Cambrian), a critical time interval marked by biotic turnovers, extinction events and carbon cycle perturbations. In this context, the Zagros Basin (SW Iran) is a key area for the study of this time interval due to the well-exposed mixed carbonate-siliciclastic successions. Nevertheless, these are still poorly constrained from a sedimentological point of view. The current study aims to contribute to the palaeoenvironmental reconstruction of the Zagros Basin during the Furongian through high-resolution facies and microfacies analysis of the Tang-e-Ilbeyk section. The latter includes, in ascending order, the upper Mila C Member and Ilbeyk Formation. The facies and microfacies analysis recognized six facies associations, representing different settings of a mixed carbonate-siliciclastic ramp at the north-eastern Gondwanan margin. The vertical stacking pattern of these facies associations revealed five medium-scale Transgressive-Regressive cycles, superimposed by larger-scale sequences. In particular, during the latest Miaolingian (formerly late middle Cambrian) to early Furongian, a climate change from arid to humid conditions caused a major regressive event with the progradation of clastic shorelines and the following pollution and drowning of the Mila C Member's carbonate factory. The latter was replaced by a storm-dominated siliciclastic shelf (Ilbeyk Formation) that, throughout middle/late Furongian, experienced a large-scale transgressive event, culminating with a maximum flooding interval. A significant forced regression occurred in the latest Furongian when shallow water conditions were re-established (uppermost Ilbeyk Formation). The palaeoenvironmental reconstruction of this study allowed correlation with other coeval depositional events at regional (Arabian plate) and global (Laurentia and Baltica) scales, contributing to new constraints for the lower Palaeozoic evolution of north-eastern Gondwana.

Evolution from Carbonate Platform to Siliciclastic Shelf Driven by Relative Sea–Level Changes: Lower Cretaceous of the Zagros Area, Southwest, Iran

Evolution from Carbonate Platform to Siliciclastic Shelf Driven by Relative Sea–Level Changes: Lower Cretaceous of the Zagros Area, Southwest, Iran

Oxfordian plug-shaped trace fossils from northwestern Algeria

Plug-shaped trace fossils B. perata, B. hemispherica, Bergaueria isp., Calycraterion samsonowiczi, Conostichus ornatus and Conostichus isp. are found in the middle-upper Oxfordian fine-grained sandstone of the Argiles de Saïda Formation (northwestern Algeria). Except for the trace Calycraterion which is interpreted as an annelid trace, these traces correspond to distinct behaviours of sea anemones, and their occurrence within tempestite deposits is influenced by a stress factors in a storm-dominated environment. The abundance of these plug-shaped trace fossils suggests that the upper Jurassic siliciclastic shelves with fine-grained soft clastic substrate were colonised by actinarian sessile benthos.

Unfathomable: The shifting sand of wave base

ABSTRACTNotions of fair-weather, storm, and swell wave bases are ubiquitous in interpretations of wave-dominated siliciclastic shelves, carbonate ramps, and mixed-systems deposits that are present throughout the geologic record. A review of literature, observations of several modern and ancient depositional systems, and numerical hydrodynamic models reveal the roles of sediment grade, bathymetric irregularities, and depositional gradient on the variable depths to which waves leave a sedimentologically discernible record. They reveal that wave-induced horizontal particle velocities and estimates of wave effectiveness form a continuous spectrum with depth, with a lack of distinct subdivision into fair-weather and storm conditions. Although commonly ascribed to “fair-weather wave base,” the depth above which sediment is persistently agitated or winnowed also is shaped by bathymetric gradient, direction of wave approach, tides, and currents, as well as sediment grade. Similarly, even with identical waves, the maximum depth of initiation of sediment movement, e.g., effective wave base, is not directly comparable among shelves, or even within the same shelf through time, because hydrodynamic processes are encoded differently by shelves of variable morphology and sedimentology.The numerical models further suggest possible geomorphology–hydrodynamics–sediment linkages. Relative to steeper shelves impacted by identical waves, shelves of shallower gradient favor lower-energy seafloor conditions, likely accompanied by accumulation of finer or muddier sediment, thinner sandy shoreface accumulations, or both. Given that many stratigraphic accumulations from the parasequence to composite-sequence scale steepen upward with time, this conceptual model predicts an apparent increase in wave energy on the seafloor through time for individual progradational shorelines, even with constant waves.

Unravelling hidden glacial effects in the Cryogenian marine depositional settings of the Tandilia Basin, Argentina

The Cryogenian Period (720–635 Ma) is wide known by the occurrence of two global glaciations, Sturtian and Marinoan, responsible of dramatic climate and sea level changes as well as seawater oxygenation events. Constrained 87Sr/86Sr ratios from 0.7064 to 0.7071 and δ13C excursion are consistent with Cryogenian cap carbonates as documented in the upper Villa Mónica Formation. This unit constitutes the basal sedimentary cover of the Río de La Plata Craton in Central Argentina which begins its deposition in a shallow marine siliciclastic shelf, turning to a carbonate succession with exceptionally well-preserved stromatolites. The detailed facies analysis added to stromatolite morphologies included in paleoenvironmental models of deposition remain unknown. The later are considered necessary to establish the stratigraphic framework and the regional correlation of the unit across the basin as a piece for the Cryogenian assembly. The entire Villa Mónica Formation in the Tandilia Basin was deposited on a shelf with sedimentary facies variations observed from the shallow margins to the offshore transition zone. The transition from the lower to the upper sections implies the conquer of microbial communities to paleogeographically controlled chemocline fluctuations where the stromatolites evidence the development from a post-glaciation phosphorus-rich ocean. This drastic shift in paleoenvironments represented on the two distinctive marker sections was influenced by changes in climatic and oxygenation conditions. In this context, presumed glacial influence is predictable in a partially ice-covered Cryogenian Earth surface.

Small carbonaceous fossils (SCFs) from North Greenland: new light on metazoan diversity in early Cambrian shelf environments

AbstractThe Sirius Passet Lagerstätte of North Greenland is one of the oldest records of soft‐bodied metazoan‐dominated ecosystems from the early Cambrian. The Lagerstätte site itself is restricted to just a singlec. 1‐km‐long outcrop located offshore from the shelf margin, in an area affected by metamorphic alteration during the Ellesmerian Orogeny (Devonian – Early Carboniferous). The recent recovery of small carbonaceous fossils (SCFs) to the south, in areas that escaped the effects of this deformation, has substantially expanded the known coverage of organic preservation into shallower water depositional settings in this region. Here, we describe additionalSCFassemblages from the siliciclastic shelf succession of the Buen Formation (Cambrian Series 2, stages 3–4;c. 515 Ma), expanding the previously documentedSCFbiota. Newly recovered material indicates a rich diversity of non‐mineralizing metazoans, chiefly represented by arthropod remains. These include the filtering and grinding elements of a sophisticated crustacean feeding apparatus (the oldest crustacean remains reported to date), alongside an assortment of bradoriid sclerites, including almost complete, 3D valves, which tie together a number ofSCFs previously found in isolation. Other metazoan remains include various trilobite cuticles, diverse scalidophoran sclerites, and a range of metazoan fragments of uncertain affinity. This shallower water assemblage differs substantially from the Sirius Passet biota, which is dominated by problematic euarthropod stem‐group members and sponges. Although some of these discrepancies are attributable to taphonomic or temporal factors, these lateral variations in taxonomic composition also point to significant palaeoenvironmental and/or palaeoecological controls on early Cambrian metazoan communities.

The Hushoot Shiveetiin gol section (Baruunhuurai Terrane, Mongolia): sedimentology and facies from a Late Devonian island arc setting

A Late Devonian to (?)Early Mississippian section at Hushoot Shiveetiin gol in the Baruunhuurai Terrane of the Central Asian Orogenic Belt (CAOB) exposes large parts of cyclic Famennian shallow-water siliciclastic shelf deposits composed of siltstones, sandstones, shales, volcaniclastics, and intercalated autochthonous carbonates. The youngest part of the section, possibly Early Mississippian, is represented by arkosic sandstones with large plant remains. The facies reflects a range from shallow-intertidal to outer ramp settings. In terms of conodont stratigraphy, the Hushoot Shiveetiin gol section ranges from the Palmatolepis minuta minuta Biozone to at least the Palmatolepis rugosa trachytera Biozone. Hiatuses of several conodont biozones occur due to the facies setting (erosion and reworked sediments which are recognized by reworked conodonts) rather than thrusting or folding. The environmental setting was characterized by coeval subaerial volcanism resulting in numerous pyroclastic deposits. The depositional environments and intense volcanic activity at the Hushoot Shiveetiin gol section limited the stratigraphic distribution, abundance, and diversity of many elements of the fauna such as brachiopods. Ostracods were very abundant and diverse through many parts of the section. Although limited in stratigraphic distribution, the crinoid fauna is the most diverse Palaeozoic fauna collected from Mongolia to date and supports the hypothesis that the CAOB was a biodiversity hotspot in the aftermath of the Frasnian–Famennian extinction event.

Alluvial fan to shallow marine sedimentation record in the ~3.0 Ga Keonjhar Quartzite, Singhbhum Craton, India: An example of Phanerozoic style passive margin sedimentation from the Mesoarchean

The arenite-dominant Mesoarchean Keonjhar Quartzite (>700 m) succession unconformably overlies the granitoid-greenstone core of the Singhbhum craton in the north and west of the Keonjhar town in eastern India. The Keonjhar Quartzite is characterized by eleven lithofacies. The lower part of the succession represents proximal subaerial fan association dominated by cohesive and cohesionless mass-flow conglomerates deposited in a falling stage to lowstand systems tracts (FSST-LST). The channelized conglomerates are likely to be incised valley fills (IVF) from the FSST. The differentiated mass-flow deposits interbedded with tractional deposits such as trough cross-stratified sandstone with ubiquitous evidence of wave-reworking represent subaqueous delta front setting. The upper 3/4th of the succession is dominated by mature wavy bedded arenites and is arranged in aggradational stacking pattern representing TST to early stage of HST of a tectonically driven sea level rise cycle. The Keonjhar Quartzite is a rare example of well preserved record of stable siliclastic shelf development during the Mesoarchean time. The arenite-dominated Keonjhar Quartzite is comparable to only a few well studied examples of such Mesoarchean stable siliciclastic shelf sedimentation, for example, the arenite-dominated units of the Mozaan Group of the Pongola Supergroup and its correlatable Westrand and Centralrand Groups of the Witwatersrand Supergroup in the Kaapvaal craton. The presence of thick mature arenite and wave-tide reworking implies existence of Mesoarchean stable shelf depositional setting similar to the modern day passive continental margins at least in the Singhbhum and in the Kaapvaal cratons.

Late Devonian (Famennian) phacopid trilobites from western Mongolia

The early Famennian phacopid trilobites Feistops mongoliensis gen. nov. sp. nov., Feistops khovdensis gen. nov. sp. nov., ?Feistops sp. A, and ?Houseops olonbulagensis sp. nov. are recorded and illustrated from the western Mongolia Hushoot Shiveetiin gol section from the Baruunhuurai Terrane of the Central Asian Orogenic Belt (CAOB). In this section, exposing large parts of cyclic Famennian shallow-water siliciclastic shelf deposits, Feistops mongoliensis gen. nov. sp. nov., Feistops khovdensis gen. nov. sp. nov., ?Feistops sp. A, and ?Houseops olonbulagensis sp. nov. are the first known and described representatives of trilobites from Late Devonian deposits of western Mongolia. Their affinities with known other sighted species of the same age are discussed.

Age Constraints on the Portezuelo Del Tontal Formation (Middle–Upper Ordovician) of the Western Argentine Precordillera: First Insights from Conodont Biostratigraphy

The first conodont fauna recovered from the lower levels of the Portezuelo del Tontal Formation (PTF) in the Sierra del Tontal, San Juan Province, is analysed, providing a precise biostratigraphic control for the scarcely fossiliferous, thick Ordovician siliciclastic sequences of the Western Argentine Precordillera. In the studied Cerro Condores stratigraphic section, the PTF is characterized by sandstone beds, with few with minor calcareous reaction, deposited under medium energy, open-marine, siliciclastic shelf conditions. The recovered assemblage consists of 50 conodont elements, including Ansella jemtlandica (Lofgren), Costiconus costatus (Dzik), Drepanodus arcuatus Pander, Parapaltodus simplicissimus Stouge, Paroistodus originalis (Sergeeva), Periodon macrodentatus (Graves and Ellison), Protopanderodus graeai (Hamar), and Spinodus spinatus (Hadding) as most conspicuous species. After the biostratigraphic assessment of species of the Paroistodus horridus complex, the PTF conodont asemblage is referred to the Darriwilian (Middle Ordovician) Yangtzeplacognathus crassus Zone, a critical interval in the tectonostratigraphic history of the Precordillera. The collection also contains reworked conodont specimens whose biostratigraphic ranges coexist during the latest Floian (Early Ordovician) Oepikodus intermedius Zone. The conodont assemblage from the PTF allows for a precise correlation with the eastern and central domains of the Precordillera, as well as other regions of the world, improving the knowledge on the dynamics of the Ordovician Precodilleran basin and the palaeogeographic distribution of its faunas.

Detrital zircon U–Pb LA‐ICPMS ages from the Kolhan Group, Singhbhum Craton, eastern India: Implications for terminal Mesoproterozoic palaeogeography between Columbia and Rodinia along the Central Indian Tectonic Zone

The Kolhan Group in the Singhbhum Craton of eastern India is a passive margin siliciclastic shelf to deep‐water carbonate ramp platform succession. It occupies a unique position in respect of the Central Indian Tectonic Zone (CITZ), widely considered as the suture joining the northern and southern Indian cratonic blocks at the time of amalgamation of the Mesoproterozoic supercontinent Columbia. The Kolhan succession is closely comparable to other Late Proterozoic ‘Purana’ successions of India including the siliciclastic‐carbonate Suasar Group from the central part of the CITZ. We report here the first U–Pb detrital zircon ages from the Kolhan Group. The youngest age population obtained from basal siliciclastics suggests it is not older than ca. 1,158 Ma. The absence of any concordant detrital zircon <1,000 Ma which is well represented in the adjoining Singhbhum Shear Zone and CITZ extensions, suggests that the sedimentation took place prior to ca. 1,000 Ma. The Kolhan Group, therefore, records an extensional event between two major episodes of supercontinent assembly (i.e., Columbia and Rodinia). A similar motif of extension and passive margin sedimentation across Mesoproterozoic Peninsular India together with the widespread Mesoproterozoic lamproite and kimberlite emplacement event suggests an episode of extension and major basin development across the Indian Shield in between the two supercontinent amalgamation cycles of Columbia and Rodinia.

Classification of the Paleoproterozoic Gulcheru Formation: Implications on Early Paleogeographic and Tectonic Evolution of the Cuddapah Basin, India

Abstract The Papaghni sub-basin of the Cuddapah basin is an arcuate NW-SE trending intracratonic basin, stretching from Kurnool in the NW to Guvvalacheruvu in the SE, in southern peninsular India. Despite of large number of researches, an understanding of the evolution of the basin is still elusive. The sub-basin preserves records of repeated opening and closing of Paleoproterozoic rifts along the zone of NW-SE trending axis in the East Dharwar craton. Present study was carried out along the southwestern margin of the Papaghni sub-basin, in and around Tadpatri-Gooty-Dhone area, where the successive litho-units are represented by two major unconformity-bound sequences. Each of these sequences are characterized by a distinctive set of litho-assemblage, deposited under diverse tectonic environments with highly variable modes and tempos of sedimentation in an extensional basin. The basin initiated at about 2 Ga and shortly evolved into a shallow sea, with development of wide siliciclastic shelf in an open marine condition, the Gulcheru Formation. Gradually, with time the siliciclastic shelf was replaced by an extensive stromatolitic carbonate platform, the Vempalle Formation. Dedicated fieldwork exemplifies that the initial basin-fill is characterized largely by several cycles of simple, but amalgamated and stacked fanglomerates (alluvial fan to fan-delta) systems developed from several single stream breaches onto the proximal setting of the basin, in a syn-rift setting. These fanglomerates display a fining upward sequence, which progressively passes up to high-energy coastal, tide-storm dominated shelf environments of post-rift setting. This study justifies the subdivision of the Gulcheru Formation into two formal lithostratigraphic members, the Daditota Member and the Karvipalle Member. The Daditota Member directly overlies the granitoid basement and grades upward to Karvipalle Member. Karvipalle Member, comprising predominantly of medium to fine-grained, reddish brown and white quartz arenites of stable shelf deposits, distinctly differs from the conglomerate and very coarse-grained mass-flow dominated deposits of the Daditota Member. The Vempalle Formation overlies the Gulcheru Formation with a gradational contact and consists of stromatolitic dolomite, oolite and shale, developed in a ramp type carbonate platform setting. Gradual transition from siliciclastic to carbonate sequence points to passive margin setting developed during the breakup of the supercontinent Kenorland. Disposition of proximal to distal sequences and northeastern, eastern and southeastern paleocurrent of the sequences advocate a NW-SE shoreline and eastern paleo-slope of the basin.

Is Macaronichnus an exclusively small, horizontal and unbranched structure? Macaronichnus segregatis degiberti isubsp. nov.

The new ichnosubspecies Macaronichnus segregatis digiberti from Miocene deposits of Cádiz, SW Spain, is described. This ichnotaxa shows the characteristic presence of a mineralogical segregation within the tube, with a core made up of low density material, surrounded by a rim of glauconite. However significant differences in size, orientation and branching with respect to the type species of Macaronichnus are observed. Macaronichnus segregatis digiberti is characterized by a diameter between 4 mm and 12 mm, with common obliquely an even vertically oriented galleries, and the presence of frequent branching. Different types of branching can be observed, including false, primary/secondary successive, and true, simultaneous branching. Specimens occur in a wide range of palaeoenvironmental contexts, from inner to outer siliciclastic shelf. This includes deeper and more distal habitats than those usually interpreted for Macaronichnus. Macaronichnus segregatis digiberti could be produced by a new tracemaker showing a composite behaviour, pascichnia being the main strategy an dominichnia/cubichnia or repichnia, a secondary, sporadic one

Precise early Cambrian U–Pb zircon dates bracket the oldest trilobites and archaeocyaths in Moroccan West Gondwana

AbstractNew U–Pb radioisotopic ages on early Cambrian volcanic zircons condition a high-resolution Bayesian age model that constrains the first occurrences and zonations of West Gondwanan archaeocyaths and trilobites in southern Morocco. The oldest archaeocyaths in the Tiout Member of the Igoudine Formation (519.71 + 0.26/− 0.35 Ma) are c. 6 Ma younger than the oldest Siberian archaeocyaths. The oldest Moroccan trilobite fragments, from the lower member of the Igoudine, are constrained to 519.95 + 0.43/− 0.40 Ma. The succeeding Issendalenian Stage (i.e. Hupetina antique – Eofallotaspis tioutensis – Fallotaspis plana – Choubertella – Daguinaspis trilobite zones) spans c. 1.5 Ma (519.78 + 0.26/− 0.37 Ma to 518.43 + 0.25/− 0.69 Ma). Identifiable Moroccan fallotaspidids and bigotinids, among Earth’s oldest trilobites, occur above a positive δ13C excursion dated with our age model at 520.27 + 0.59/− 0.57 Ma, and correlated with the IV excursion peak within the lower range of Siberian Atdabanian Stage trilobites (Repinaella Zone). This excursion is the best standard for a Cambrian Series 2 base. The oldest West Gondwana trilobite fragments are c. 1 Ma younger than those in Siberia and c. 0.5 Ma older than the oldest Avalonian trilobites (Callavia Zone). This diachrony means a trilobite first appearance datum is an inappropriate chronostratigraphic base for Cambrian Series 2. Taxonomic differences in the oldest trilobites between Cambrian palaeocontinents are in accordance with trace fossil evidence for the group’s appearance possibly as late as c. 530 Ma in the Cambrian Evolutionary Radiation. Coeval 519–517 Ma dates from Avalonia (cool-water siliciclastic shelf) and West Gondwana (tropical carbonate platform) sections with distinct macrofaunas emphasize these successions were latitudinally separate by the late Ediacaran Period.

Ancient rip current records and their implications: an example from the Cretaceous Ukra Member, Kutch, India

Poorly-sorted conglomerate patches rich in granules or sturdy fossils or both, and reddish mud matrix within the interstices stand out amidst fine-grained siliciclastic shelf sediments of the trangressive systems tract (TST) of the Lower Cretaceous Ukra Member, Kutch Basin, India. The siliciclastic shelf sediments contrast the conglomerates with their remarkable lateral extension. The fossils belong to a low-diversity group of sedentary bivalves that can be traced into the shoreface facies assemblage. The shelf sandstones are almost always sculpted by wave structures, especially hummocky cross-stratification while textures in the conglomerates suggest that the sediment settling was generally from suspensions. Textural variations in conglomerates reflect an immediate variation in flow viscosity prior to the downloading. The current structures obtained from the conglomerates record offshoreward palaeocurrent, in contrast to the shore-parallel palaeocurrent in the TST. The hummocky cross-stratified (HCS) beds are interpreted as seasonal storm deposits, while the conglomerate patches are taken as rip current deposits induced by waves of much longer periods. The glauconite-rich shale that alternates with conglomerates is probable fair-weather products. The conglomerates could not be recognized either in the coarse-grained shoreface deposits occupying the lower part of the overall fining-upward TST or in the coarsening-upward and glauconite-depleted highstand systems tract (HST). In contrast to the TST, the HST is dominantly tide-imprinted, having shore-normal palaeocurrent direction. It appears that intensification of waves and weakening of tides during transgression favored strong rip currents generation, which had presumably caused severe damage to the sea coast and to the shell banks growing preferably at the necks of the rip current channels. Rapid lateral facies transitions in the shoreface deposits at the basal part of the TST suggest enhanced irregularity in the coastline, possibly because of the mega cusps indented upon it. Frequency and intensity of storms enhanced during periods of global warming caused the transgression of the Early Cretaceous Ukra Sea.