Postglacial Transgression Research Articles

Ediacaran-Cambrian microbialites of the Southern Amazon Craton: relation with the metazoan rise, sea-level changes, and global tectonics

Microbialites are the most abundant life evidence in Precambrian sedimentary rocks. They are produced by microbial interaction activity and sedimentary processes reflecting paleoenvironmental conditions. The Ediacaran-Cambrian carbonate and siliciclastic successions in the Southern Amazon Craton in Central Brazil, provide a key opportunity to understand how the metazoan life coexisted with the microbial communities. The spatial and temporal distribution of microbialites as well as morphological and paleoenvironmental changes have been assessed, reinterpreting previous works and including new data from the Araras-Alto Paraguai and Corumbá basins. The deposition was controlled by subsidence and sea-level changes that affected these basins, considered extensions of epicontinental seas during the Gondwana assembly. The stromatolites are restricted to coastal deposits and experienced thriving flourishment intervals after the Marinoan Glaciation (635 Ma). Post-glacial transgression was marked by microbial colonization in shallow platforms represented by stratiform and giant domical stromatolites in the Araras-Alto Paraguai Basin. The continuity of the transgression generated a moderately deep aragonite sea at about 622 Ma. A progressive sea-level fall caused the implantation of coastal environments under greenhouse conditions with tidal flat and sabkha settings colonized by centimetric-scale stromatolites. The sea retreat was accompanied by progressive uplift, causing a moderate inversion of the basin and erosion of the succession until ~560 Ma with the deposition of the last preserved tidal flat deposits with the occurrence of thrombolites. The subsiding Corumbá Basin was the site of microbially-induced deposition of carbonates in a shallow platform connected to an offshore setting with the proliferation of metazoan straddling the Ediacaran-Cambrian boundary. Microbial communities were restricted to lagoon deposits during the Lower Cambrian transgression in the Araras-Alto Paraguai Basin and the last phase refers to the sea retreat towards southeast, developing a fluvial system connected with the arid and evaporitic tidal flats colonized by microbialites that lasted until the upper Cambrian. Except for the post-glacial stromatolites, the columnar and domal microbialite indicate that the coastal settings dominated the Ediacaran-Cambrian transition. The preservation of microbialites in the post-glacial intervals can be associated with the Mg-Ca-CO3 oversaturation in dolomitic platforms. The rapid calcification and ability to resist the dissolution and replacement have increased the stromatolites’ preservation potential reported here, where its well-preserved occurrence in tidal flats and sabkha occurs due to intense early diagenetic silicification. The change from carbonate accumulation to siliciclastic-rich environments contributed to the demise of microbially-induced strata. In general, the scarce coexistence between coastal stromatolite and metazoan-bearing marine deposits makes it challenging to establish a competitive relationship between these organisms, as previously postulated.

Sedimentary Response to the Early Silurian Post-Glacial Transgression and Kwangsian Orogeny in the Upper Yangtze Region, South China

Sedimentary Response to the Early Silurian Post-Glacial Transgression and Kwangsian Orogeny in the Upper Yangtze Region, South China

Holocene coastal evolution of Colorado River Delta based on diatom assemblages, Northern Patagonia, Argentina

Diatom assemblages and coastal sedimentary facies succession in the deltaic plain of the Colorado River (Argentina) were studied in order to reconstruct the environment conditions in response to Holocene eustatic sea-level changes and delta progradation. Samples were selected from a 200-cm core (39°35'52" S, 62°6'43" W). Chronology was based on radiocarbon datings in bulk samples and mollusk shells. Nine lithological units were recognized where fine and very fine sand predominate. The core sediments were dominated by tychoplanktonic coastal-marine diatoms. The accompanying flora included stable marine-coastal taxa and allochthonous fluvial fresh-brackish species. Three diatom zones (DZ) were established based on cluster analyses. The LCH site was significantly affected by Late Pleistocene and Holocene sea-level fluctuations, and the progradation of the deltaic lobe during the last ca. 7,000 14C yr. BP. Ancient tidal channels were flooded in the early Holocene postglacial transgression. Erosive and re-working processes associated with the formation of estuarine channels, affected valves preservation, yielding with barren sections. Tidal flats and marshes have developed in the area during the late Holocene regression phase. The comparison of the sedimentary sequence with deltaic facies evolution models, confirm the presence of a hiatus, which masks the Holocene Maximum and highstand.

Development of Predictive Geoarchaeological Models to Locate and Assess the Preservation Potential of Submerged Prehistoric Sites Using Remote Sensing, Palaeoenvironmental Analysis, and GIS

Using the Mesolithic site of Tudse Hage in the Great Belt of Denmark, this paper proposes a generic stepwise process to create geoarchaeological models that output seamless morphology maps in a GIS. This was achieved using remote sensing databases and the collection of marine geophysical data, above and below the seabed. On the basis of these data, key areas, with sediment sequences representative of the postglacial transgression surfaces, were identified. Core samples were taken for palaeoenvironmental analysis and dating that enabled a reconstruction of the relative sea-level changes. Using this information, palaeogeographic coastline maps of the Kongemose, late Kongemose, Ertebølle, and Neolithic periods in the Tudse Hage area were prepared, and potential hotspots for archaeological sites were proposed. Since their inundation, submerged prehistoric archaeological sites have been, and are, dynamic, with anthropogenic and natural processes affecting their stability and preservation. With the advocation of in situ preservation as a means of managing underwater cultural heritage, predicting where sites have survived these processes, and where they can be found, in advance of subsea development or other anthropogenic exploitation, is essential. Future natural threats to sites preserved in situ were determined through the modelling of seabed currents and sediment erosion.

Bedform evolution and dynamics of a geostrophic current-swept shelf, northern KwaZulu-Natal, South Africa

When compared to tide-dominated shelves, shelves swept by geostrophic flows are relatively understudied, yet geostrophic currents have the potential to construct substantial current-generated shelf bedforms. This paper examines the evolution of a series of bedforms encountered along the narrow Agulhas Current-swept northern KwaZulu-Natal shelf. Their evolution is placed in the context of progressive current impingement and variation in flows associated with the postglacial transgression and culminating in the present-day highstand.Ultra-high resolution seismic reflection, multibeam, side-scan sonar and single beam data sets reveal several bedform scales and morphologies; wave ripples and shoreface-connected ridges are associated only with the inner shelf, and 2D and 3D very large dunes are more cosmopolitan and span the entire shelf sector. The inner shelf is marked by rock outcrop (aeolianite), surrounded by sandy sediment, grading seaward into sediment starved bedforms associated with bioclastic gravels. Where sufficient sediment exists in local depocenters, very large dunes form discontinuous fields along the outer shelf.The seismic stratigraphy of the outer shelf reflects the effects of postglacial flooding by rising sea levels and increasing current impingement by the Agulhas Current. The Holocene wave ravinement surface is overlain by flat-lying strata (early dune development and dune amalgamation with first current exposure), in turn covered by hummocky, sub-horizontal aggrading beds (amalgamation), overlain by inclined cross-bedded packages (lee faces of the bedforms formed during migration and full current interaction).Morphometric analyses show that for both the inner and outer shelf, no relationships exist between water depth, wavelength and spacing. Height to spacing (H/L) relationships are weak but nevertheless show a broadly positive trend. Bedform heights are lower on the inner shelf compared to the outer shelf, but bedform spacing is greater on the inner shelf, with a 40% overlap in H/L indices observed between the two areas. The departures in overlap can be linked to the competing offshore Agulhas Current and the inshore wave-dominated processes. Bedforms of the inner shelf plot below the global H/L mean and can be related to the more infrequent incursions of the Agulhas Current core. At the time of survey, the Agulhas Current was likely situated well offshore, resulting in reduced current activity in the survey area resulting in rounding of the dune crests, degradation of the dune crests and trough infilling. On the outer shelf, the H/L values plot above the mean global trend, suggesting vertical accretion due to the faster currents. Crest rounding and downlapping of the upstream lee faces onto the downstream stoss faces indicate dune degradation, which is related to a seaward location of the Agulhas Current at the time of survey.

Bayhead delta evolution in the context of late Quaternary and Holocene sea-level change, Richards Bay, South Africa.

Richards Bay is part of a back-barrier lagoon fronted by high coastal dunes on the NE, Indian Ocean coast of South Africa. In the early 1970s, a berm was constructed, dividing the original Mhlathuze Estuary into two separate systems; the Richards Bay Harbour and the new Mhlathuze Estuary. This study investigates the stratigraphic evolution of the incised valley system and bayhead delta in the Richards Bay Harbour segment. Seven seismic units (Units 1–7) were imaged. A single regionally developed sequence boundary (SB) along with two tidal ravinement surfaces (tRS1 and tRS2) were identified. Surface SB is associated with the LGM lowstand which developed when sea levels were ~ 130 m below present, until ~18,000 year BP. Cretaceous age siltstones (Unit 1) form the basement. Transgressive material overlying SB (Unit 2) reflects the filling of an incised valley located in the middle segment of a wave-dominated back-barrier system. It is overlain by a bayhead delta (Unit 3), the geometry and seismic signature of which indicate alternating periods of aggradation/progradation and backstepping. The behaviour is attributed to episodic jumps in sea-level, and is tentatively (on the basis of elevations in relation to the regional sea-level curve) linked to periods of rapidly rising sea-level (8.2 ka event and Meltwater Pulse (MWP)-1d). These intervals of rapidly rising sea-level, combined with relatively low gradient settings facilitated backstepping of the delta. Fills (Unit 4) occur within minor incisions along the delta top. These are interpreted as distributary channels that fed sediment to the seaward edge of the bayhead delta system. Elongated mounds on the seafloor (Unit 5) are interpreted as spoil from contemporary port dredging. Slump deposits (Unit 6) along the delta front are attributed to a combination of oversteepening of the delta by dredging, as well as deposition of modern sediments brought into the system by tidal currents. The system is capped by fine-grained, tidally redistributed and deposited sediments (Unit 7) which were possibly sourced from older organic material of an indeterminate source. This site is especially sensitive to episodic rates of sea-level change due to the relatively small Glacial Isostatic Adjustments (GIA) during the postglacial transgression and the flat antecedent gradients of both the subaerial unconformity and the overlying tidal ravinement.

Genetic diversity and population structure of three Hydroides species (Sedentaria, Serpulidae) in the Persian Gulf and Gulf of Oman, with the possible indication of heteroplasmy

The Persian Gulf, an enclosed sea connected to the Gulf of Oman through the narrow Strait of Hormuz, is known as the world’s hottest sea. Owing to its harsh environmental conditions, genetic isolation has been proposed for the Gulf’s biota. During the present study, population genetic and phylogeographic analyses were carried out focusing on potential barriers to dispersal in some polychaete populations between the Persian Gulf and the Gulf of Oman. Ninety-three specimens from three defined spatial populations of three Hydroides species, H. bulbosa, H. elegans and H. operculata, were sequenced for regions of two nuclear ribosomal (18S and ITS2) and two mitochondrial (COI and Cyt-b) genes to compare the genetic structure among the sampled populations. Haplotype networks of the three species showed no significant genetic differentiation and were consistent with their geographic distribution patterns. Population genetic analyses such as pairwise FST, nucleotide diversity and neutrality tests revealed some levels of intraspecific genetic diversities, suggesting different demographic histories. The genetic diversity observed in the more conservative genes (18S, Cyt-b, COI) in populations of H. elegans and H. operculata could reflect a more diverse genetic structure along their entire distribution range unlike the endemic species, H. bulbosa that presented a unique haplotype for each of those genes. Using Extended Bayesian Skyline plot analyses, populations of H. bulbosa and H. operculata showed signs of recent demographic expansion within the post-glacial transgression time frame of the Persian Gulf, while populations of H. elegans remained relatively constant. Another finding of the present study is the possible indication of heteroplasmy in H. elegans where two distinct mitochondrial lineages with about 11% of genetic divergence seem to coexist in the same individuals. We discuss also the alternative explanation of NUMTs (nuclear mitochondrial DNA), but regard it as substantially less likely.

Carbon isotope chemostratigraphy and sea-level history of the Hirnantian Stage (uppermost Ordovician) in the Oslo–Asker district, Norway

AbstractWe present a δ13Ccarb chemostratigraphy for the Late Ordovician Hirnantian Stage based on 208 whole-rock samples from six outcrops in the Oslo–Asker district, southern Norway. Our data include the Norwegian type section for the Hirnantian Stage and Ordovician–Silurian boundary at Hovedøya Island. The most complete record of the Hirnantian Isotope Carbon Excursion (HICE) is identified in a coastal exposure at Konglungø locality where the preserved part of the anomaly spans a c. 24 m thick, mixed carbonate–siliciclastic succession belonging to the upper Husbergøya, Langåra and Langøyene formations and where δ13Ccarb peak values reach c. +6 ‰. Almost the entire HICE occurs above beds containing the Hirnantia Fauna, suggesting a latest Hirnantian age for the peak of the excursion. The temporal development of the HICE in southern Norway is associated with substantial shallowing of depositional environments. Sedimentary facies and erosional unconformities suggest four inferably fourth-order glacio-eustatically controlled sea-level lowstands with successively increased exposure and erosion to the succession. The youngest erosional unconformity is related to the development of incised valleys and resulted in cut-out of at least the falling limb of the HICE throughout most of the Oslo–Asker district. The fill of the valleys contains the falling limb of the HICE, and the postglacial transgression therefore can be assigned to the latest part of the Hirnantian Age. We address the recent findings of the chitinozoan Belonechitina gamachiana in the study area and its relationship to the first occurrence of Hirnantia Fauna in the studied sections, challenging identification of the base of the Hirnantian Stage.

Origin and evolutionary processes of deep groundwater salinity in southwestern coastal region of the Ganges-Brahmaputra-Meghna Delta, Bangladesh

Study RegionSouthwest coastal region of Bangladesh.Study FocusDeep groundwater in the southern Ganges-Brahmaputra-Meghna delta, the coastal region of Bangladesh, is partially salty whereas the drivers behind this salinity are not well understood. In this study, we explored the origin and evolutionary processes of the salinization in deep groundwater by analyzing geochemistry and environmental isotopes, paleogeography, and sedimentation processes.New Hydrological Insights for the RegionThe deep groundwater in the paleo-channel area exhibited higher salinity and lighter stable isotopes of water while that in the paleo-interfluve area lower salinity and heavier stable isotopes. Based on the Cl−/Br− molar ratios, the seawater and the coastal rainwater were surmised to be the ultimate source of the observed deep groundwater salinity. Evolution of the deep groundwater salinity can be explained by the paleogeography and associated paleo- and recent processes. The brackish estuarine water formed in the paleo-channels during the postglacial transgression was possibly intruded into the fresh deep groundwater due to the density contrast. The brackish deep groundwater in the paleo-channel area was subsequently mixed with the fresh groundwater in the paleo-interfluve area through lateral flow, driven by both anthropogenic (municipal pumping) and natural processes. These new insights about the occurrence and distribution of the fresh and the brackish deep groundwater can facilitate the management of potable water supply in the Ganges-Brahmaputra-Meghna delta or other similar regions.

Holocene coastal landscape development in response to rising sea level in the Central Wadden Sea coastal region

AbstractThe Holocene sea-level rise has led to significant changes in present-day coastal zones through multifold retrogradational and slightly progradational displacements of the mainland coastline. During the course of this postglacial transgression, sediments characteristic of coastal environments accumulated first in palaeovalleys of the pre-Holocene landscape and later on the subsequently developed coastal plain. Based on a compilation of sedimentological, lithological and litho-chronostratigraphical data of more than 1200 sediment cores, we generated four palaeogeographic maps of the coastal zone of the central Wadden Sea to document with a high spatial resolution the landscape changes during characteristic phases of the Holocene sea-level rise, i.e. the periods 8600–6500 cal BP, 6500–2700 cal BP, 2700–1500 cal BP and 1500–1000 cal BP. Along three cross-sections, representing different hydrodynamic conditions and exposure, we exemplify how the Holocene landscape development and sedimentary facies types are controlled by the local palaeorelief, sea-level changes, sediment supply, accommodation space, the morphodynamic impact of channel shifts, and their erosion base. This leads to a better understanding of main factors controlling the local depositional processes of the coastal landscape along the central Wadden Sea during the Holocene transgression.

The late Paleozoic Ice Age along the southwestern margin of Gondwana: Facies models, age constraints, correlation and sequence stratigraphic framework

The southwestern margin of South America offers a complete record of the Late Paleozoic Ice Age (LPIA) that affected the Gondwana supercontinent. The tripartite division of LPIA glacial episodes has been refined with the help of new radiometric dates and biostratigraphic (flora and fauna) zonations in recent years to five shorter-lived discrete events: 1. Latest Devonian-earliest Tournaisian, 2. Tournaisian, 3. Visean, 4. Serpukhovian-Early Bashkirian, 5. late Pennsylvanian-earliest Permian. These glacial events are capped by postglacial transgressive deposits with marine fauna. The unbalanced preservation potential of the glacial deposits, skewed toward the glaciomarine sediments, provides an uneven stratigraphic record with few cases of continental glacial sedimentation, confined to the Serpukhovian-Early Bashkirian event, and numerous examples of glacial sedimentation in marine environments. Glacial sedimentation in marine settings has been grouped in two main facies associations: a valley-glacier-retreat (fjord) facies association and a submarine-retreat (glaciomarine apron) facies association in open-marine areas.Transitional facies, correspondent to those formed by the flooding of valleys during postglacial transgressions, are widely distributed along the Protoprecordillera in western Argentina, where paleofjord successions are well exposed particularly in western Paganzo Basin, and mapped in subsurface in the Tarija basin. A general paleofjord model includes (from base to top) the following stages: (i) Incision of paleovalley and deposition of subglacial diamictites in ice contact deltas, (ii) Early Transgressive stage characterized by resedimentation of subglacial material by subaqueous sediment gravity flows and slumps in proglacial settings, (iii) Maximum flooding (late transgressive stage) dominated by black shales or laminated mudstones related to a marine incursion that flooded valleys; normal marine or brackish conditions may dominate this stage and (iv) Highstand: progradation of a fluvial-deltaic system including in some cases Gilbert-type deltas. In glaciomarine apron environments, the basal facies includes massive clast-supported conglomerates, with few striated and polished clasts, followed by fining-upward successions including thinly bedded diamictites with ice-rafted debris (IRD) and locally contorted sandstone masses in diamictite beds, indicative of mass-emplacement mechanisms. The presence of inter- and intratill pavements suggests glacial advance/retreat fluctuations along the basin margins.Deglaciation sequences, reflecting deposition mainly during the retreat of ice sheets, ice caps and alpine glaciers and successive deglaciation, can be used as operational tools for the analysis of glacial successions in southwestern Gondwana. They are characterized as rather simple upward-fining successions in open marine settings as exemplified in most of the Calingasta-Uspallata Basin,Claromecó Basin (and adjacent Ventana foldbelt, VFB) and central portions of the Paraná and Karoo basins. In more proximal areas (i.e., paleofjords) this vertical trend is commonly punctuated with deltaic wedges fed by nearby provenance areas.The late Paleozoic glacial-related successions of southwestern Gondwana exhibit a common tripartite motif, equivalent to second-order sequences with estimated durations of 10–80 Myr. The lower section corresponds to glacial and glacially-influenced diamictites; the middle interval is initiated with postglacial transgressions. The lower and middle intervals correspond to the deglaciation sequence as described and identified in several basins of Gondwana. Finally, the upper term includes coastal progradation, followed in some places by continentalization, accompanied in many sectors by increasing aridization. Examples of second-order sequences can be identified in the thick late Paleozoic successions of the Paraná and Karoo basins and in the VFB. Thinner second order sequences can be identified in the Calingasta-Uspallata, Rio Blanco, Paganzo and San Rafael basins. In the Paganzo and San Rafael basins the middle interval is also punctuated by short lived marine ingressions. The basal sequence boundary is commonly an abrasion surface (glacial erosion surface, GES) developed on bedrock. Deglaciation sequences are assigned to third order sequences made up, when present, of a thin lowstand system tract (LST) of subglacial deposits followed upward by thick glaciomarine and glacially influenced sediments. These facies are part of a thick transgressive systems tract (TST) that culminates with marine shales that reflect interglacial or postglacial conditions during ice retreat. Thus, the deglaciation sequences are proposed to be third order sequences made up of LST-TST or exclusively TST.

Late Paleozoic coal beds and coaly mudstones in northwestern basins of Argentina: Paleoenvironmental context and paleoclimatic significance

Coals and coaly mudstones are found in the late Paleozoic Paganzo Basin, forming a key stratigraphic level deposited during the Bashkirian – Moscovian interval of relative tectonic quiescence. Coal beds vary from bituminous high volatility type C to sub-bituminous type A and, the coaly mudstones are kaolinitic-rich and presently used in the ceramic industry. Taking in account the stratigraphic position, facies associations, and the characteristics of the organic-rich levels, three domains were recognized within the basin: 1) The western domain (Precordillera) where paralic coals formed in transitional (mainly estuarine) environments, 2) The central domain (western Sierras Pampeanas) with limnic coals formed in floodplain areas, and 3) The eastern domain (eastern Sierras Pampeanas and Nevados del Famatina), in which coals are almost absent and are replaced by kaolinitic-rich coaly mudstones. Sea-level changes and climatic conditions related to the end of the Gondwanan glaciation played critical roles in the formation of coals and coaly mudstones. Regarding sea-level changes, three main transgressive events were identified in the Paganzo Basin. The oldest one corresponds to the “Namurian” postglacial transgression that produced no coal beds. The second event is the Pennsylvanian transgression 1, which favored the formation of coals and coaly mudstones in both coastal (i.e estuarine) and inland (i.e fluvial) environments. The third transgression, Pennsylvanian transgression 2, produced only very thin and scarce coal seams. During the transition from glacial (late Serpukhovian) to early postglacial climates (early Bashkirian), the vegetation was scarce and not suitable for the development of extensive mires. The post-glacial (late Bashkirian-early Moscovian) climatic amelioration, characterized by humid and temperate conditions, facilitated the proliferation of the NBG flora composed of hygrophilous elements as equisetalean, arborescent to sub-arborescent lycophytes and pteridosperms fronds, together with some gymnosperm that produced large amounts of organic detritus. On the contrary, the transition from subhumid to semiarid climate (late Moscovian-Kasimovian) led to the declination of the arborescent floras and the disappearance of coal beds. Although climate can be assumed to have simultaneously affected the whole basin, fluvial systems needed time to adjust to the sea-level changes and, therefore, at least two possibilities should be considered regarding the chronostratigraphic significance of the coal horizon. Firstly, that sea-level rise effectively controlled the equilibrium profile of channels well inland and coals and coaly mudstones across the Paganzo Basin form a time-transgressive horizon with a lag time not yet quantified based on the available paleontological and radiometric data. Secondly, that the growing of the riparian vegetation led by the ameliorating climatic conditions resulted in the almost synchronic formation of coals in the three domains.

Factors controlling organic-matter accumulation in the Upper Ordovician-Lower Silurian organic-rich shale on the northeast margin of the Upper Yangtze platform: Evidence from petrographic and geochemical proxies

Organic-rich shales are widespread in the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations on the northeastern margin of the Upper Yangtze platform, and these shales have been recognized to be the hydrocarbon source rocks and shale gas reservoirs with the greatest development potential in China. However, organic matter accumulation in these shales needs further research, especially in the areas which experienced an extensive upwelling and volcanism. Here, this work integrates detailed petrographic and geochemical proxies, namely redox-sensitive trace elements (RSTEs) and ratios of Si/Al & Ti/Al, from two Ordovician-Silurian sections on the northeast margin of Upper Yangtze platform, in order to understand the depositional environment and identify the major factors controlling the organic matter accumulation during the Ordovician-Silurian transition. Data of Si/Al and Ti/Al ratios indicate that the deposition of black-gray siliceous mudstones in the Lower Wufeng Formation commenced with a transgression triggered by the Boda warming event which led to a reduced detrital input. Based on RSTEs, the black carbonaceous shales of the Upper Wufeng Formation are organic-rich owing to an elevated primary productivity caused by seasonal upwelling driven by the vigorous thermohaline circulation. Abundant nutrients supplied by the post-glacial transgression and a mass biotic recovery of marine microorganisms stimulated productivity and further strengthened the depletion of oxygen in the water column, which lead to high concentrations of organic matter in Lower Longmaxi Formation. Although the water column conditions were not conducive to the accumulation of organic matter, it was possible that the increased productivity from upwelling resulted in relatively high organic matter contents in the middle Longmaxi Formation. The RSTEs (especially Cd/Mo, Co × Mn) reveal that the organic matter accumulation of the Wufeng-Longmaxi Formation during Ordovician-Silurian transition may have been primarily controlled by the preservation conditions and also influenced by the marine primary productivity and the detrital input.

Anatomy of the transgressive depositional system in a sediment-rich tide-dominated estuary: The paleo-Yangtze estuary, China

Sediment-rich tide-dominated estuary is generally considered nonexistent or ephemeral under the stable sea-level condition, given the rapid transition to a deltaic system after massive sediments dumping at the river mouth. The well-preserved transgressive sequence in the Yangtze paleo-valley represents an excellent location to examine the interplay between sediment supply, tide action, and sea-level change. To study the Yangtze paleo-valley geomorphological evolution and facies model, four 60-90-m long cores (YD0901, YD0902, YD0903, and CX03) were obtained. Lithological description, grain-size analysis, XRF core scanning, and AMS 14C dating were carried out. The results show that the Yangtze incised valley evolved from a river channel system (before 14.6 ka), through a tidal river (14.6-13 ka), and a tide-dominated estuary (13-8 ka), to a shallow marine (after 8 ka). The paleo-Yangtze estuary displays inner section dominated by muddy heterolithic deposition and the outer section occupied by mud-rich tidal bars and channels, distinctly differing from the well-down small estuaries with sand predominance. A fining-upward facies succession was produced by migration of mud-rich tidal bars in response to post-glacial sea-level rise. These bars are potentially the predecessor of tidal sand ridges on the East China Sea shelf after they were progressively abandoned during the post-glacial sea-level rise and exposed to tidal and storm-wave reworking. This is the first facies model developed from a sediment-rich tide-dominated estuary. This study provides a best example to study evolution of tide-dominated mega-river systems in response to the post-glacial transgression.

The Silurian glaciation in the eastern Parnaíba Basin, Brazil: paleoenvironment, sequence stratigraphy and insights for the evolution and paleogeography of West Gondwana

Glacial strata are excellent stratigraphic markers in Paleozoic basins worldwide. The eastern Parnaíba Basin in northern Brazil is a highly favorable setting for studying episodes related to the Ordovician-Silurian glaciation. Previous studies in this region led to the recognition of unequivocally glaciogenic sediments associated with pre- and postglacial deposits, favoring the understanding of lower Paleozoic events that occurred in West Gondwana. Outcrop-based facies and stratigraphic analyses of the ~170 m-thick siliciclastic succession of the Serra Grande Group indicate fluvial, glacial, and coastal depositional systems. Seventeen sedimentary facies are grouped into seven facies associations (FAs): FA1, interpreted as intermediary sheet braided plain deposits that consist of tabular cross-bedded coarse-grained sandstones and conglomerates; FA2-FA4, which correspond to subaqueous glacial deposits comprising massive to stratified diamictites, cross-laminated sandstones, diamictites and organic matter-rich black shales with dumpstone and dropstone structures; FA5-FA6, constituting organic matter-rich black shales, fine- to medium-grained sandstones with sigmoidal cross-bedding and occurrences of the ichnotaxon Arthrophycus interpreted as delta front deposits; and FA7, composed of medium- to coarse-grained pebbly sandstones interpreted as channelized braided plain deposits. This interpretation partially confirms the previous paleoenvironmental interpretation discounting the presence of subaerial alluvial fan and outwash plain deposits for this unit. The sequence stratigraphy of the Serra Grande Group is refined mainly using the coherent interpretation of the key surfaces and comparing the systems tracts with the global sea-level curve, providing a more robust third-order sequential evolutionary model that includes three depositional sequences. The Middle Ordovician sheet braided deposits are the lowstand sediments of Sequence 1 truncated by an extensive unconformity that possibly removed the transgressive and highstand strata. This unconformity generated by glacial dynamics represents 23 Myr during the Late Ordovician to early Silurian. The second sequence started with the advance of ice sheets in marginal areas of the basin, causing the deposition of ablation diamicton during the lowstand. The postglacial transgression is marked by ice-rafted debris from icebergs deposited onto organic matter-rich mud in the shoreface-offshore zone. The maximum flooding during the middle to late Silurian was succeeded by the highstand phase marked by the progradation of deltaic deposits. Biological activity is marked by the presence of polychaetes that reworked the marine seafloor. The Early Devonian featured extensive sea-level fall and renewed continental drainage with the formation of a braided plain marking a lowstand phase of Sequence 3. These new interpretations allow the global correlation of the Serra Grande Group and provide an improved understanding of the role of ice sheets and postglacial transgressions that affected West Gondwana during the Ordovician-Silurian periods.

Formation of the Yangtze Shoal in response to the post-glacial transgression of the paleo-Yangtze (Changjiang) estuary, China

The Yangtze shoal lies east of the Yangtze (Changjiang) River estuary, China, at water depths of ~25–55 m and is one of the largest subaqueous, sheet-like sand banks in the world. Previous studies have not reached a consensus regarding its origin and timing of formation, mainly due to the scarcity of coring and high-resolution seismic profiling in the shoal. We recovered a 70.20-m-long sediment core (CRE-1402) through the sand shoal and acquired 1586 km of high-resolution shallow seismic profiles across the estuary and shoal. We conducted systematic analyses of the core, including its sedimentary characteristics, down-core changes in benthic foraminiferal assemblages, and 14C and optically stimulated luminescence (OSL) dating. We also analyzed the seismic stratigraphy and integrated these results with data from other previously well-studied cores onshore and offshore of the Yangtze River mouth. Our results indicate that the stratigraphic architecture in the paleo-Yangtze estuary (i.e., the area from the present-day delta plain to the subaqueous delta and Yangtze Shoal) was controlled mainly by changes in sea level during the post-glacial transgression. The post-14 ka sedimentary successions in the paleo-estuary rest unconformably on fluvial or coastal deposits that formed mainly before MIS 1, and are separated by three distinctive bounding surfaces corresponding to MWP-1A, MWP-1B (?) or the rapid sea-level rise at ca. 11.5 ka shortly after the Younger Dryas cold period, and the 8.2 ka event, respectively, and the maximum marine flooding surface (MFS). The depositional unit of the Yangtze Shoal ranges in thickness from 6 to 32 m. The shoal exhibits large-scale, SW-dipping (generally 0.5°–2°) foresets with local cross-cutting of foresets, which are composed mainly of very fine- to medium-grained sands. The shoal is underlain by muddy intertidal to subtidal flat deposits that formed during the Younger Dryas, with an erosional surface corresponding to MWP-1B (?) or the rapid sea-level rise at ca. 11.5 ka between these and the shoal deposits. The Yangtze Shoal came into being mainly between ca. 11.5 and 7.0 ka in the form of tidal sand ridges actively moving southwestwards under intense and reciprocal tidal currents on a tidal shelf. The tidal sand ridges have evolved into sheet-like tidal sand banks with relatively low relief as a result of tidal remobilization under rotary tidal currents since ca. 7.0 ka when the MFS formed. Therefore, the Yangtze Shoal can be considered to comprise quasi-active tidal sand banks.

El registro más austral de la Fauna Aseptella-Tuberculatella / Rhipidomella-Micraphelia (Serpukhoviano tardío-Bashkiriano) en la cuenca Calingasta-Uspallata, Argentina

The Agua del Jagüel Formation crops out in the southernmost part of the Calingasta-Uspallata basin, in central western Argentina. The lower part of the unit is characterized by a glacigenic sequence with diamictites and mudstones with dropstones. In the latter, elements of the Aseptella-Tuberculatella/Rhipidomella-Micraphelia Fauna (AT/RM Fauna), such as the brachiopods Rhipidomella discreta Cisterna, Micraphelia indianae Simanauskas and Cisterna and Orbiculoidea? sp., the gastropods Murchisonia? sp., and Glabrocingulum (Stenozone)? sp., the bivalve Nuculanidae indet., rugose corals, and indeterminate fragments of nautiloids and hyolithids, have been identified. The importance of this fauna mainly resides in its paleoenvironmental and biostratigraphic implications. AT/RM Fauna is characteristic of restricted environments with relatively low concentrations of oxygen and nutrients in the seafloor, which is consistent with the glaciomarine sequences in fjord-type coasts suggested for the Agua del Jagüel Formation. The relatively low diversity of the fauna in this unit compared to that defined in the El Paso Formation, located further north in the basin, might suggest more restricted sectors for benthic colonization, related to the paleovalle’s isolation from oceanic waters. The postglacial mudstones with marine invertebrate faunas of late Serpukhovian-Bashkirian age would have been deposited in relatively restricted (palaeofjord) part of the Uspallata-Calingasta basin as well as in open shelf environments. The marine flooding over drastically different coast configurations and the availability of nutrient and oxygen in the water column would have propitiated the development of faunas with important differences in the taxonomic composition and the paleoecological structure (AT/RM and Levipustula Faunas) occurring at the same time interval. Radiometric data in Agua del Jagüel Formation and paleontological records in the glacial-postglacial sedimentary succession in the basin (marine invertebrate faunas, palynomorphs and plants) are the most important tools to adjust the timing of the postglacial transgression. This information herein presented complements the scheme proposed for the Carboniferous sequences throughout the central western of Gondwana but is not sufficient to assign a more precise age of the fauna studied within the late Serpukhovian-Bashkirian interval.

Timing of the Late Palaeozoic glaciation in western Gondwana: New ages and correlations from Paganzo and Paraná basins

During the Late Palaeozoic, the Gondwana supercontinent formed an extensive Southern Hemisphere landmass that was affected by multiple glacial episodes, known collectively as the Late Palaeozoic Ice Age (LPIA). This resulted in the deposition of glacial, periglacial and deglacial sediments over much of the supercontinent. The Mississippian to early Pennsylvanian phase of glaciation is widely represented along the western margin of Gondwana. This constitutes one of the largest glaciations of the Phanerozoic in terms of its recorded extent and the widespread erosional hiatus it produced in the stratigraphic record. It was this mid Carboniferous glaciation, recorded in the Paganzo Basin of NW Argentina, that carved most of the paleovalleys and paleofjords present there. We report new U-Pb zircon ages from a single glacial succession (Guandacol Formation) of 326 ± 3 Ma and 320 ± 5 Ma, that in comparison with neighbouring dated sequences allow for the first time a reliable estimate of the timing and duration of glacial cycles. Palynological studies of these glacial-deglacial events yielded palynoassemblages of the MQ (Late Visean) and DMa (Sepukhovian-Bashkirian) Palynozones. The Carboniferous glacially-related strata and glacial cycles of the Paganzo Basin are compared here with equivalent units of the Paraná Basin of SE Brazil, suggesting a similar climate record for most of western Gondwana. We propose a new correlation between these two basins. The new U-Pb zircon ages reported here indicate that a regional glacial peak occurred almost coincident with the Mississippian-Pennsylvanian boundary, suggesting that the ensuing postglacial transgression is the best regional marker to differentiate the Mississippian and Pennsylvanian as it forms an identifiable interbasinal horizon. The biostratigraphic and chronostratigraphic comparison with other LPIA successions from South America reinforces that the First Appearance Datum of monosaccate pollen grains occurs in the late Serpukhovian. Their correlation confirms that similar climate conditions prevailed across most of western Gondwana during this phase of the LPIA.

Гидроизостазия как фактор, повлиявший на ход послеледниковой трансгрессии на шельфе и побережье Приморья, по результатам численного моделирования

Гидроизостазия как фактор, повлиявший на ход послеледниковой трансгрессии на шельфе и побережье Приморья, по результатам численного моделирования

Stratigraphic revision of the El Imperial Formation (Pennsylvanian-Cisuralian), in its type locality, San Rafael Basin(Mendoza), Argentina

The El Imperial Formation (San Rafael Basin) shows a complete record of the late Carboniferous - early Permian stratigraphy in western basins of Argentina. The sedimentological and stratigraphic analysis in the type locality allowed recognizing eight facies associations. Facies association A includes diamictites and conglomerates related to the Late Paleozoic glacial event, facies B composed of shales and fine-grained sandstones, was formed during the postglacial transgression while facies C represents sandy littoral bars, that pass upward to shales and mudstones of the facies association D. The conglomerates and sandstones of the facies association E entirely correspond to fluvial sedimentation, followed by a new transgressive event represented in fine-grained sandstones and shales of the facies F. The last transgression is recorded in shales, mudstones and very fine-grained sandstones belonging to the facies G, which are covered by sandstones and conglomerates corresponding to multi-channel fluvial systems.From these facies associations, 33 palynofloras divided into three palynological associations were obtained. The palynological assemblages that characterized the basal section contain abundant trilete spores and monosaccate pollen grains, with a progressive increase of taeniate pollen towards the top of the unit. The presence of scolecodonts characterizes palynofloras from transitional to marine environments. The composition of the palynofloras allows correlations with the current biostratigraphic scheme of the center-west of Argentina. The presence of the Biozone Raistrickia densa- Convolutispora muriornata in the lower and middle part of the El Imperial Formation, together with Pakhapites fusus-Vittatina subsaccata Biozone in the upper part, allows us to refer the palynofloras to the late Serpukhovian/early Cisuralian.