Transgressive Events Research Articles

Modeling Marsh‐Forest Boundary Transgression in Response to Storms and Sea‐Level Rise

AbstractThe lateral extent and vertical stability of salt marshes experiencing rising sea levels depend on interacting drivers and feedbacks with potential for nonlinear behaviors. A two‐dimensional transect model was developed to examine changes in marsh and upland forest lateral extent and to explore controls on marsh inland transgression. Model behavior demonstrates limited and abrupt forest retreat with long‐term upland boundary migration rates controlled by slope, sea‐level rise (SLR), high water events, and biotic‐abiotic interactions. For low to moderate upland slopes the landward marsh edge is controlled by the interaction of these inundation events and forest recovery resulting in punctuated transgressive events. As SLR rates increase, the importance of the timing and frequency of water‐level deviations diminishes, and migration rates revert back to a slope‐SLR‐dominated process.

Transgressive Events since the Late Pleistocene in the Yellow River Delta: Grain‐size Distribution and Palynological Results

AbstractThis study deals with the relationship between sea‐level changes and paleoclimatic fluctuations based on the analysis of stratigraphy, grain sizes, palynology, and radiometric dating of the Yellow River delta since the Late Pleistocene. Evidence from the sedimentary record, grain sizes, and pollen provides a paleoenvironmental history of the Late Pleistocene from the boreholes of the delta. Based on a combination of grain‐size analysis with lithological studies, marine deposit units contain the intervals of 13.85–16.9, 18.5–19.69, 27.9–34.8, 36.4–37.2, 48.4–51.6, and 54.1–55.9 m, and transitional facies units contain the intervals of 10.25–13.85, 16.9–18.5, 19.69–27.9, 34.8–36.4, 37.2–48.4, 51.6–54.1, and 55.9–60 m, compared with fluvial (terrestrial facies) deposit units (3.36–10.25 m). Based on pollen analysis and pollen assemblages, there were three warm‐wet periods from 9.1–0.16 ka BP, 16.1–60 ka BP, and 90.1–94.6 ka BP From the top to the bottom of the borehole, the paleoclimate has an evident fluctuation: warm and moist (Holocene Optimum) —cool and dry (Younger Dryas Event)—mild semi cool—cool and dry—warm and moist. There were three warm‐wet periods from 9.1–0.16 ka BP, 16.1–60 ka BP, and 90.1–94.6 ka BP, corresponding to the Holocene Optimum stage, MIS 3, and MIS 5, respectively. The warm period allowed monsoonal evergreen and broadleaved deciduous forests that corresponded to Holocene hypsithermal climatic conditions and the Late Pleistocene climatic Optimum. Three warm‐wet periods occurred in marine deposit units from 9.1–0.16 ka BP, 60.1–16.1 ka BP, and 94.6–90.1 ka BP. These periods correspond to the Cangzhou transgression, Xianxian transgression, and Huanghua transgression, respectively. From 90.1–60.1 ka BP, 17.5–9.1 ka BP, and 0. 16 ka BP–1855 AD, three dry and cold phases are recognized. The phases indicate the fluvial (flood plain) sedimentary environment, corresponding to cooler and mild dry periods based on palynological results and grain‐size distribution.

Astronomical time calibration of the Upper Devonian Lali section, South China

Abstract The Late Devonian Period was a critical geological interval, affected by numerous significant global bio- and environmental events, including the Kellwasser, annulata and Hangenberg events, and a major climate transition leading to the Carboniferous ice ages. The timing and pattern of these events are unclear due to lack of the precise time scale. In this study, an analysis of calcium concentration series was carried out on the Upper Devonian cyclostratigraphic slope-facies of the Lali section, South China. Objective testing of the section indicates the presence of astronomical forcing: long orbital eccentricity, short eccentricity, obliquity, and precession index. Two independent astrochronologies were developed for the Ca series and compared with spectral analysis. The age of the Devonian–Carboniferous boundary (358.9 ± 0.4 Ma) was selected as an “anchor”, and an Astronomical Time Scale (ATS) was constructed from the late Frasnian to early Tournaisian based on 405-kyr cycles. The ATS indicates a duration of the Famennian Stage of 14.4 ± 0.28 myr, and assigns 373.3 ± 0.49 Ma to the Frasnian–Famennian boundary. The durations of the Famennian conodont zones are also indicated. The timing of Famennian bio- and environmental events also can be established by the ATS. The Upper Kellwasser horizon starts at 373.4 Ma and is constrained to a duration of 100-kyr; a sharp positive carbon isotope excursion CIE, +3‰ recognized across the Frasnian–Famennian boundary lasts 300 kyr. The Lower and Upper annulata events each last 50-kyr with positive CIEs; and the ages of two CIE peaks are 361.7 Ma and 361.3 Ma, respectively. Five transgressive events, including the Nehden, Enkeberg, Dasberg, Strunian, and Hangenberg transgressions occur at 370.6 Ma, 368 Ma, 360.4 Ma, 360.1 Ma, and 358.9 Ma, respectively. Five regressive events i.e., the linguiformis, prima, Condroz, velifer, and Derwer regressions occur at 373.4 Ma, 371 Ma, 369 Ma, 362.3 Ma, and 359.2 Ma, respectively.

Marine influence during deposition of the Kiliranjao Brown Shale, Central Sumatra, from palynology point of view

The Eocene to Oligocene succession of the Kiliranjao Brown Shale was deposited in a lacustrine environment. This study combines palynological analysis and XRD analysis technique to investigate the possibility of marine influence during deposition of Kiliranjao lacustrine shale. Two biostratigraphic zonations of Karbindo section are Middle to Late Eocene Proxapertites operculatus Zone and Oligocene Meyeripollis naharkotensis Zone. The marine flooding surface, as shown by the peak of the abundance of mangrove pollen, is used to delineate the section into five parasequences. The tripartite stratigraphic architecture of Karbindo section is consists of early rift deposit (TS1), syn-rift deposit (TS2), and syn-rift deposit (TS3). Middle to Late Eocene coal and limestone facies (parasequence I) represents the TS1. Late Eocene to Oligocene parasequence II to IV (below MFS) shows the TS2 (deepening-upward lacustrine succession), and the shallowing-upward lacustrine succession of Oligocene parasequence V (above MFS) indicate the TS3. The abundance of mangrove and back mangrove palynomorph assemblages and the high amount of carbonate mineral from XRD analysis are the evidence of marine influence to the deposition of Karbindo section Brown Shale. The marine incursion to the lacustrine environment occurred from the adjacent nearby open sea, that separated by shoal area, which can submerge below sea level during a marine transgressive event. Marine incursion events did contribute to the deposition of Kiliranjao Brown Shale though further work is still needed.

Provenance changes across the mid-Cretaceous unconformity in basins of northeastern China: Evidence for an integrated paleolake system and tectonic transformation

Abstract Detrital zircon U-Pb dating and whole-rock Nd isotopic analyses were carried out on selected stratigraphic horizons across a major unconformity between synrift and postrift stages in the Songliao Basin and Dasanjiang basin group of NE China to constrain the crustal evolution of the source area providing detritus into these basins. The strata underlying the mid-Cretaceous unconformity in the Songliao Basin show regionally distinct detrital zircon age populations and Nd isotopic compositions, which generally are characterized by Phanerozoic age peaks and relatively depleted Nd isotopic compositions, indicating derivation from nearby highlands. In contrast, the overlying strata are dominated by Proterozoic zircon ages and enriched Nd isotopic compositions, which imply that the provenance source region shifted to the northern part of the North China craton. A coeval provenance change also affected the sedimentary architecture in the eastern Dasanjiang basin group, marking the migration of erosion centers from west to east. The contribution from Lesser Xing’an–Zhangguangcai ranges was pronounced during deposition of synrift strata but became negligible afterward in the Songliao and Dasanjiang areas, which is consistent with both basin complexes temporarily forming an extensive lake system during the early Late Cretaceous. This paleolake was likely responsible for transgressive events recorded in the Late Cretaceous strata of basins in NE China. Combining observations from seismic reflection profiles and the stratigraphic record of neighboring intracontinental sedimentary basins as well as widespread contemporaneous exhumation and denudation events, we suggest that the provenance variation in basin strata was controlled by large-scale tectonic transitions in East Asia. The switch from extension to contraction during the mid-Cretaceous is attributed to the docking of the Okhotomorsk block along the East Asian continental margin. The resulting lithospheric buckling might have been responsible for reshaping the basin-and-range configuration in NE Asia.

How did subterranean amphipods cross the Adriatic Sea? Phylogenetic evidence for dispersal–vicariance interplay mediated by marine regression–transgression cycles

AbstractAimFreshwater subterranean amphipods with low dispersal abilities are known from both sides of the impermeable barrier, the Adriatic Sea. We tested the hypothesis that historical marine regression–transgression cycles shaped the distribution patterns of subterranean amphipods through repeated cycles of dispersal and vicariance against the hypothesis that subterranean amphipods colonized both sides of the Adriatic Sea independently.LocationWestern Balkan Peninsula, Adriatic Sea Islands and Apennine Peninsula, Europe.TaxonGenus Niphargus, a clade of freshwater subterranean amphipods (Crustacea: Amphipoda).MethodsThe taxonomic structure of the studied clade was revised using unilocus species delimitation methods. The timeframe of cladogenetic events was inferred using a multi‐locus time‐calibrated phylogeny and compared to the main regression–transgression events in the Miocene and Pleistocene. The geographical origin of the studied clade, species range expansions and contractions, as well as vicariance events were assessed through modelling of historical biogeography.ResultsSubterranean amphipods of the genus Niphargus, found on both sides of the Adriatic Sea, form a monophylum. The reconstructions of ancestral ranges suggest that the clade emerged in the Balkan Peninsula, dispersed three times independently to the Apennine Peninsula and once back to the Balkans. Adriatic Islands were colonized multiple times, predominantly from the Balkan Peninsula. The dispersal–vicariance events correspond to historical regression–transgression cycles in Miocene and Pleistocene.Main conclusionsMarine regression–transgression cycles apparently shaped the distribution patterns of subterranean amphipods while the alternative hypothesis received no support. The actual distribution of subterranean faunas apparently reflects old biogeographical events.

Bioerosion pattern in a nearshore setting as a tool to disentangle multiphase transgressive episodes

Palaeoichnology is known to be a powerful tool for the characterisation of unconformity surfaces. In a Neogene post-orogenic basin of the Betic domain (Alicante, SE Spain), a complex Miocene transgressive event on an exposed Eocene succession is recorded. The resulting unconformity surface as well as the overlying beds resulting from the establishment of a nearshore environment are ichnologically and palaeoecologically analysed. Different boring structures related to the Entobia ichnofacies were observed on a variety of substrates, including i) the rocky Eocene substrate, ii) Tortonian conglomerates and breccias, iii) oyster and coral patches (mainly represented by Tarbellastraea). Boring structures are mainly represented by Gastrochaenolites, Entobia and, more rarely, Trypanites and Spirolites? Different colonization patterns were identified, including truncated Gastrochaenolites and Entobia that are extensively developed on the Eocene rockground, small-sized Entobia, and superficial bioerosion on conglomerates and breccias, and Gastrochaenolites hospitium on corals; a selective colonization of G. hospitium was observed, with Tarbellastraea much more intensively bioeroded than Porites. Finally, the integration of stratigraphic, palaeoecological and ichnological data allowed for the identification of three different phases in the Miocene transgression: i) slow transgression with the consequent colonization of the Eocene rockground, the production of a wide planar surface and the truncated Gastrochaenolites-Entobia assemblage; ii) the breaking of this uniform coastal setting into areas characterized by different hydrodynamics, with coral development where turbulence was higher; this second phase was of limited temporal extension, with a limited accommodation space; iii) a final phase characterized by a new transgressive pulse leading to the establishment firstly of a restricted inner platform and secondarily of a slightly deeper sandy platform; in this phase, bioerosion was limited to shells, while soft- and loose-ground burrows as Ophiomorpha developed within the substrate.

PALYNOLOGY AND CHEMOSTRATIGRAPHY OF MIDDLE TRIASSIC SUCCESSIONS IN NORTHERN SWITZERLAND (WEIACH, BENKEN, LEUGGERN) AND SOUTHERN GERMANY (WEIZEN, FREUDENSTADT)

New Anisian to Ladinian palynology, palynofacies and stable carbon isotope records are reported for the Middle Triassic from deep Swiss wells (Weiach, Benken, Leuggern), well B3/13 (Weizen, S-Germany) and the type area of the Freudenstadt Formation in southern Germany. A wide spectrum of moderately to well-preserved palynomorphs represent a high Middle Triassic plant diversity. Based on the distribution of diverse spore-pollen assemblages five Anisian Palynozones (A–E) and two Ladinian ones (F, G) are differentiated. Throughout these Palynozones the occurrence of spores and pollen, assigned to plant groups so far known only from the Palaeozoic, shed a new light on the evolution of plant assemblages during the Triassic. The comparison of Palynozones A–G with palynostratigraphic schemes from the central part of the Germanic Basin and from the Tethyan realm demonstrate the regional variability of marker species ranges – especially for the Anisian. n agreement with the lithological record, two prominent transgressive events (Lower Muschelkalk, Upper Muschelkalk) are indicated by increased abundances of marine particulate organic matter in palynofacies data. Marginal marine influence is documented at the base of the studied interval, comprising the Buntsandstein and the base of the Lower Muschelkalk. Carbonate carbon isotopes data show a negative shift at the boundary between lower and middle Muschelkalk. Coincidently, a prominent change in relative abundances of climate-sensitive plant groups (e.g. Triadispora spp. vs. Pteridophytes) indicate a change to relatively dryer climatic conditions during the middle Muschelkalk.

Postglacial transgressive shales of Upper Devonian–Lower Carboniferous boundary of the Parnaíba Basin

Upper Devonian-Lower Carboniferous deposits in north Brazil show a postglacial transgressive event characterized by the formation of thick layers of gray to black shales. In the Parnaíba Basin, the shales of the Longá Formation overlap the Upper Famennian non-marine glacial deposits of the Cabeças Formation marked by a ravinement surface. The Longá shales exhibit wavy-lenticular beddings in their base, grading to gray-black shales locally interbedded with thin layers of sandstones with hummocky cross-stratification and megaripple bedding. These deposits were interpreted as having formed in shallow marine upper offshore to lower shoreface with episodic storm events. The organic matter analysis of the outcropping Longá shales using Total Organic Carbon (TOC), Rock-Eval pyrolysis, and biomarkers provide several geochemical parameters for the interpretation of the paleoenvironment, the origin of the organic matter and its thermal maturation. The Longá shales presented low TOC contents (less than 1%), very low numbers of free hydrocarbons (S1), and potential generators (S2). The maximum pyrolysis temperature (Tmax), as well as the thermal maturation biomarker parameters, indicate that the Longá shales are immature. The values of pristane/n-C17, phytane/n-C18, Hydrogen Index (HI) and Oxygen Index (OI) suggest kerogen types III and IV, which are formed from conical land leaves, needles, and plant waxes; this interpretation is also supported by total sulfur (TS) < 0.2 wt%, terrigenous/aquatic ratio (TAR), C29-sterol, hopane/sterane and Methylphenanthrene (MPs). This organic matter was deposited in an epeiric sea with shallow water, oxidizing conditions, normal salinity, and low temperatures prevalence.

Sedimentary environment and geological significance of the Upper Devonian Wutong Formation in Northern Yangtze Plateau, China

There is an ongoing debate about the evolution and geological significance of sediments deposited during the Upper Devonian period in the Wutong Formation on the northern margin of Yangtze Plate. Although the deposition of the Wutong Formation is reported to have coincided with the period of global sea-level changes, the sedimentary record has not been clearly interpreted. We utilise the lithofacies, sedimentary petrology and paleogeography of the Wutong Formation to determine the sedimentary environment and its significance in relation to the northern Yangtze Plate and global sea-level fluctuations. Detailed sedimentary petrology shows that the Wutong Formation was deposited in the barrier-free coastal environment, and stratigraphic analyses indicate the presence of two intervals of transgressions. Lithofacies and paleontological records found in the Wutong Formation show that the two transgressive events correspond to periods of global sea-level rise and allow new insights into impacts of the Frasnian global sea-level changes. KEY POINTS Six lithofacies and five lithofacies assemblages can be identified from the Devonian Wutong Formation in the Chaohu area. We find that the Wutong Formation in the Chaohu area is formed in a coastal sedimentary environment. The transgression and regression of the late Devonian in the Chaohu area correspond to the sea-level changes of global Frasnian–Famennian events.

Molecular Organic Geochemical Characteristics and Coal Gas Potential Evaluation of Mesozoic Coal Seams in the Western Great Khingan Mountains

AbstractCoal‐bearing strata are widespread in the western Great Khingan Mountains. Abundant coal resources have been found in the Jurassic Alatanheli Groups, the Cretaceous Bayanhua Groups, the Damoguaihe Formation and the Yimin Formation. The organic geochemical characteristics were analyzed in combination with hydrocarbon source rock evaluation and molecular organic geochemistry experiments, and the coal gas potential of coal seams was evaluated. The source rock evaluation results indicated that the Mesozoic coal samples have the characteristics of high organic matter abundance (TOC&gt;30%), low maturity (Ro values of approximately 0.6%), and type III composition. The hydrocarbon generation potentials of the Alatanheli Groups and Bayanhua Groups are high, while the generation potentials of the Damoguaihe Formation and the Yimin Formation are low. The results of geochemistry show that the depositional environment of the coal seam was a lacustrine, oxidizing environment with a low salinity, and the source of the organic matter was mainly higher plants. Affected by weak degradation, the coal seams mainly formed low‐maturity gas of thermal catalytic origin. The Cretaceous coal seams contain a large amount of phytoplankton groups deposited in a low‐stability environment affected by a transgression event, and the potential range varied widely. For the Jurassic coal seams, the depositional environment was more stable, and the coal seams feature a higher coal‐forming gas potential.

The Ordovician iron ore of the Anti-Atlas, Morocco: Environment and dynamics of depositional process

In Morocco, iron deposits of sedimentary origin are mainly developed in the Ordovician where they are interbedded at different levels of the stratigraphic column. A detailed analysis of the sedimentary facies shows that the iron ores are not randomly distributed but are concentrated at omission surfaces of the sequences thus raising the question of their sedimentological significance within the sequences that shelter them. This study determines three morphological types of ore, which provide a possible avenue for economic exploitation. These morphological types are: firstly, from metric to decametric bodies of regional extension (examples are “Caid Rami” and “Tachilla”), consisting of a set of detrital facies organized in two stratigraphic stages that cyclically recur to form genetic sequences. Within each sequence, the maximum mineralization is at the top of the prograding basal stage and is expressed by diagenetic iron cement and at the base of the regressive stage in the form of strata of iron oolites, or in massive beds. Secondly, lenses with a thickness up to 30 m over a kilometer width (example of “Imi n’Tourza” field), corresponding to incisions cut along the ravinement surfaces. The maximum mineralization of this second type is within the median sandstone stage in connection with a large transgressive event. Thirdly, there is a type of reworked material consisting of discontinuous layers of ferruginous conglomerates and bioclastics lining the ravinement surface that separates the higher order sequences. It is this third type that has generated the most economic interest. The iron-bearing mineral phases identified are: magnetite, hematite, iron chlorite, maghemite, iron hydroxides (goethite, lepidocrocite) and siderite. The ultimate source of iron would probably be the large reserves of banded iron formation of the West Africa Craton (Mauritania in particular). Intense tropical weathering would have leached the silica and left a highly enriched iron residuum that was carried as particulate iron to shallow marine shelf environments where Fe3+ was converted to Fe2+ during early diagenesis. This reduced iron was recycled into the water column by tidal currents or storm waves where it precipitated as ferric oxide or ferrous silicate ooliths. This depositional mechanism accounts for the positioning of ironstone beds at times of stasis and reworking.

Age of the earliest transgressive event in the Krishna-Godavari Basin, India: evidence from dinoflagellate cysts and planktonic foraminifera biostratigraphy

A combined biostratigraphic study of dinoflagellate cysts and foraminifera was carried out on Early Cretaceous subsurface well cutting sediments from well A (DNG) (2800–2746 m depth) from the Krishna-Godavari Basin, India. The last appearance datum of marker species of dinoflagellate cysts and planktonic foraminifera was considered for the construction of the biostratigraphic framework. The study shows dominance of Early Cretaceous marker dinoflagellate cysts Cassiculosphaeridia magna, Cribroperidinium perforans, Hystrichodinium voigtii, Kleithriasphaeridium eoinodes, and planktonic foraminifera Hedbergella aptiana, Hedbergella mitra, Hedbergella praelippa, Hedbergella tardita, Microhedbergella miniglobularis and Hedbergella mitra species. In addition to this, the dinoflagellate cyst data were compared with the dinoflagellate biozones of Austral and Tethyan provinces. Based on earlier micropalaeontological records from the Krishna-Godavari Basin and the present study, a latest Barremian-early Aptian age has been determined for the earliest marine transgression in the Krishna-Godavari Basin. The early marine incursion during late Barremian-earliest Aptian in the Krishna-Godavari Basin compared to Albian age in Cauvery Basin suggests the opening of east coast from north to south.

A Cambrian age for the upper Bambuí Group, Brazil, supported by the first U-Pb dating of volcaniclastic bed

The Serra da Saudade Formation is part of the Bambuí Group, a cratonic cover from the western São Francisco craton. It is composed by siliciclastic sediments deposited in a foreland basin during a transgressive event. The Serra da Saudade Formation is mainly represented by rhythmites, siltstones (locally glauconitic) and rare sandstones. The glauconitic siltstone was deposited during a maximum flooding surface and sediment starvation, which allowed high concentrations of probable volcanic-sourced potassium. The interpretation of the depositional age of the Bambui Group had multiple turnovers in the last decades. Finding and dating synsedimentary volcanic rocks is essential to solve this conundrum. This paper presents the first U-Pb direct dating of the Bambuí Group from zircon grains recovered from a volcaniclastic layer interbedded with glauconitic siltstones from the upper Serra da Saudade Formation. A total of 107 U-Pb ages were obtained by LA-ICP-MS and plotted into a Concordia diagram. The provenance signature comprises a wide range of detrital zircon ages (1.0–2.8 Ga). These data represent inherited zircon grains incorporated into the magma during magmatic activity that produced the volcanic sediment, or to dilution and reworking of volcanic ashes that mixed to fine-grained clastic sediments prior to deposition. A significant amount of 10 highly concordant, prismatic zircon grains clustered into a well constrained age of 520.2 ± 5.3 Ma, which represent the explosive volcanic eruption age, and by extent, the depositional age of the upper Serra da Saudade Formation. Since the currently final thrust stacking ages from the Brasilia Belt and the Araçuaí Belt are dated ca. 600 and 540 Ma, respectively, one must carefully look for the real age of the deformation event that affected the Bambuí Group over the São Francisco craton.

The Berriasian–Aptian of the Western Gydan Peninsula (West Siberia): Biofacies and Lithofacies Models

Abstract —Well logs and core analyses of Berriasian–Aptian petroliferous sediments from the western Gydan Peninsula are used to model their biofacies and lithofacies and to reconstruct the respective sequence stratigraphy. The sedimentological and palynological data, along with logging results, provide constraints on the history of transgressive and regressive events and make a basis for paleogeographic reconstructions for the Lower Cretaceous deposition of reservoir sand beds.

Остракоды пограничных отложений девона и карбона из опорного разреза на р. Ыджыд-Каменка (Печорская гряда)

This article presents new data on ostracodes and their stratigraphic distribution in the boundary deposits of the Devonian and Carboniferous (D3-C1) in the reference section Kamenka River (Pechora Uplift). The positions of the Kamenka River section and stratotypic sections of the southern Urals relative to the transgressive-regressive sequence are considered. The ostracode assemblage of the Ps. venulosa — Cor. alba — Cr. primaris zone, which occurs in the section on the Kamenka River and appears in sections of the southern Urals from the base of this zone, is represented by the following taxa: Pseudoleperditia venulosa (Kummerov), Coryellina alba Kotschetkova, Bairdia zaninae Posner, Blessites feluyensis Tschigova, Shishaella alekseevae Tschigova, Knoxiella complanata (Kummerov), Cavellina subeichwaldi Buschmina. The taxonomic characteristics of the upper part of the Pseudoleperditia venulosa — Coryellina alba — Cribroconcha primaris ostracode zone (Gumerovian Regional Substage (Horizon)) are supplemented Acratia subordinaria Buschmina, Healdianella ex gr. alba Lethiers, Phlyctiscapha ? pusilla Gurevitsсh, Pustulobairdia ex gr. confragosa (S. et Sm.), Acanthoscapha pechorica Sobolev, Acratina romboiformis Sobolev, and Rectobairdia eleganta Sobolev first appear at this level A gradual and insignificant change in the ostracode association is observed during the transition from the Gumerovian Horizon to the Malevkian Horizon, which precludes drawing the boundary between them. It is suggested that it would have been more appropriate to draw the boundary at the base of the global Hangenberg transgressive event near the base of the Ps. venulosa — Cor. alba — Cr. primaris ostracode zone (South Urals), which coincides with the initial phase of this transgression.

Post‐Last Glacial Maximum evolution of a “fjord‐type” lake based on high‐resolution seismic data: the Lago Roca/Acigami (southern Tierra del Fuego, Argentina/Chile)

ABSTRACTLago Roca/Acigami is a “fjord‐type” lake located in the southernmost part of South America, in the proximity of the Beagle Channel. A high‐resolution seismic survey was carried out to analyse the seismic stratigraphy of the lake and to shed some light on the post‐Last Glacial Maximum history of the area. Six seismic units were recognised, and their nature and depositional context were interpreted using seismic stratigraphy and acoustic facies analysis. A buried large ridge was identified within the glacial unit (SU1), interpreted as a frontal moraine that indicates a stabilisation phase. After retreat of the glacier from the basin, the trough was flooded by meltwater and a lake developed (SU2). The seismic facies, from bottom to top, depict a transition from ice‐contact (SU2; SU3) to ice‐distal proglacial conditions (SU4). A thick draping unit (SU5) marks a marine transgressive event and the instauration of a fjord environment in the basin. The marine transgression was a rapid event preceded by a fall in the lake level that caused an erosional unconformity. During the fjord phase the sedimentation remained controlled by meltwater discharge. Sea level fall, and subsequent disconnection from the Beagle Channel was accompanied by a progradation of the glaciofluvial deltaic sediments and the occurrence of several mass‐wasting deposits (SU6). © 2019 John Wiley &amp; Sons, Ltd.

High-resolution paired carbon isotopic records from the Meishucun section in South China: Implications for carbon cycling and environmental changes during the Ediacaran-Cambrian transition

Several prominent negative carbon isotope (δ13C) excursions are observed during the Ediacaran-Cambrian (E-C) transition, reflecting large perturbations of the carbon cycle in the ocean–atmosphere system. These negative δ13C excursions have been linked to dramatic paleoenvironmental changes and been widely used as regional and global chemostratigraphic correlation tools. However, there is still much debate about their causes and mechanisms. In this study, we report high-resolution δ13C records of both carbonate (δ13Ccarb) and organic carbon (δ13Corg) from the Meishucun section in South China. Combined with lithological variations and intrabasinal correlations, we find that the negative δ13C excursions during the E-C transition in South China are tightly linked with transgressive events. The existence of a large vertical (surface-to-deep water) δ13C gradient in a stratified ocean, and episodic incursions of 13C-depleted anoxic/euxinic deep waters into shallow water environment through enhanced upwelling or shoaling of the chemocline, may have contributed to these negative δ13C excursions. In addition, our results also imply a large reduction in the vertical δ13Corg gradient during the E-C transition, which may reflect the ventilation and oxygenation of the deep ocean. The corresponding expansion of oxic seafloor conditions and a large increase in the extent of habitable space for animals may have facilitated the radiation of metazoans during the Cambrian explosion.

The major Late Albian transgressive event recorded in the epeiric platform of the Langshan Formation in central Tibet

Abstract Global sea-level changes strongly impact within-basin depositional patterns and the evolution of palaeoclimate, palaeogeography and palaeoecology. During the long, worldwide ice-free period in the mid-Cretaceous greenhouse time interval, high-frequency global sea-level changes were recorded in sedimentary archives. However, the causes of these global sea-level changes are still debated. In central Tibet, the 1 km-thick Langshan Formation has been dated to the late Aptian to early Cenomanian based on larger benthic foraminifera and accumulated in an epeiric seaway, thus, it provides a good opportunity to reconstruct the sea-level change and their controlling factors. Eleven distinct microfacies corresponding to three sedimentary environments have been identified in the Langshan Formation. Calcispheres marlstone and bioclastic wackestone with calcispheres were deposited in an open marine environment; coral rudstone, rudist rudstone and benthic foraminifera–rudist wackestone characterize were deposited in a rudist bank environment; and orbitolinids floatstone–rudstone, green algae packstone, bioclastic grainstone, orbitolinids wackestone with small benthic foraminifera, spicules wackestone and small benthic foraminifera wackestone–mudstone were deposited in a lagoonal environment. The Langshan Formation accumulated on an epeiric platform. This unit documents a sudden deepening event from a rudist bank to an open marine environment during the late Albian ( c. 107 Ma). Integrating these findings with regional data from the literature, we infer that this deepening event was a widespread, roughly synchronous feature across the globe, and was controlled by a global sea-level rise related to the decay of polar ice sheets or the release of water from continental aquifers.

Lithostratigraphy of the Prince Albert Formation (Ecca Group, Karoo Supergroup)

Abstract The Prince Albert Formation is a mudstone-dominated unit, which is located in the central and southwestern part of the Main Karoo Basin and the southernmost part of the Kalahari Basin in South Africa. It is Early Permian (Artinskian to early Kungurian) in age and is stratigraphically located between the underlying Dwyka Group and the overlying Whitehill Formation. In the Main Karoo Basin, its regional extent is limited to the cut-off boundary of the Whitehill Formation along a line from Hertzogville in the Free State to Coffee Bay in the Eastern Cape. Northeast of this boundary, it correlates with the lower part of the undifferentiated Ecca Group known as “Ecca Shales” and in the northern part of the basin, north of a line from Bloemfontein (Free State) to Harding (KwaZulu-Natal), with the Vryheid and Pietermaritzburg formations. The Prince Albert Formation is generally between 50 and 200 m thick, including the type area around Prince Albert, where a thickness of about 145 m was measured. It is thicker (230 to 497 m) in the region between Brandvlei and Jansenville and thins northeastwards to between about 30 and 60 m in the area between Kimberley and East London. In the Kalahari Basin, the formation is only 25 to 50 m thick, due to post-Karoo erosion of its upper part. The Prince Albert Formation contains fossils of marine invertebrates, palaeoniscoid fish, sharks, sponge spicules, foraminiferans, radiolarians, acritarchs, fragments of wood and leaves, together with ichnofossils in the form of fish trails, arthropod trackways and invertebrate burrows. High Rb/K ratios in the mudstones also indicate a marine shelf environment with suspension settling of mud being the predominant depositional process. Sedimentation was initiated during a major transgressive event following final melting of Dwyka Group associated ice sheets in southern Gondwana in the Late Palaeozoic. Water depths in the basin were probably about 400 m, but shallowed northeastward. In the proximal part of the basin northeast of Kimberley, sandstones in coarsening-upward successions represent prograding deltaic deposits derived from an adjacent source area to the north.