Sea-level Fall Research Articles

Intrasediment gypsum in subtidal offshore carbonates as a witness of basin-wide evaporitic precipitation. The case of the southern Variscan foreland basin (mid-Carboniferous)

Widespread calcite and quartz pseudomorphs, interpreted as originally gypsum crystals, occur within hemipelagic calci-mudstone accumulated in subtidal offshore environments in the broad marine foreland basin developed on the southern flank of the Ouachita–Alleghanian–Variscan Orogen during mid-Carboniferous times, which acted as a marine corridor connecting the Panthalassa and Palaeo-Tethys Oceans during the Mississippian, and progressively narrowed during the assembly of Pangea. In this study, 67 outcrops of radiolaria-bearing calci-mudstone deposits that contain calcite and quartz pseudomorphs located in northern Spain and southern France were studied to constrain the gypsum spatial distribution and sedimentological features. The recognised microfacies indicate intrasediment gypsum precipitation, accompanied by less abundant bottom-grown precipitates and gypsum cumulates, in extensive offshore, probably several ten to a few hundred metres deep, basinal environments. Gypsum precipitation took place during a short-lived temporal episode during the early Bashkirian time (Voznesenkian), which can be correlated, on the basis of benthic foraminifera, with the coastal (inter- to supratidal) gypsum evaporites identified in NW Africa (Tindouf and Reggan successions in Morocco and Algeria) that would represent the shallow-water counterparts. The occurrence of gypsum precipitates both in offshore hemipelagic calci-mudstones of the Variscan foreland basin and in inter- to supratidal environments of the epeiric Sahara Platform indicates that hypersaline conditions affected vast marine areas, roughly coinciding with the estimated age of closure of the Panthalassa and Palaeo-Tethys marine connection. Therefore, the studied succession represents the trace of a basin-wide evaporitic episode extending for hundreds of kilometres driven by foreland basin restriction, mid-Carboniferous sea-level fall and arid climate. This study provides new insights for the interpretation of gypsum precipitates in offshore marine environments encountered in the Phanerozoic and whose genesis is poorly understood.

Archive for the East Australian Current: carbonate contourite depositional system on the Marion Plateau, Northeast Australia

The modern East Australian Current (EAC) sweeps across the Marion Plateau, forming a widespread contourite depositional system. The history of the EAC is recorded in the contourite drifts that have evolved through time as topographic, sea level, and oceanographic changes have influenced the current flow. During the Miocene, five isolated carbonate platforms on the Marion Plateau formed obstacles for the EAC, leading to the development of a complex contourite drift system in which drift types and non-depositional/erosional features transition laterally and temporally because of continuously evolving platform-current interactions. The significant topographic changes are caused by two episodes of platform drowning in the Middle and Late Miocene. We reinterpreted 2D seismic, well, and core data from ODP Legs 133 and 194 and updated the ages of the seismic sequences. The updated ages enable the reconstruction of the evolution of the EAC since the Miocene. The influence of the EAC in the study area began at 13.5 Ma, with five significant phases of change identified by shifts in contourite drifts' geometry and thickness. During the first phase (13.5 Ma – 11 Ma), strong currents carried sediment across platforms and deposited as periplatform drifts (Marion Drift B6), previously identified as a lowstand platform. In Phase 2 (11 Ma – 7 Ma), strong EAC caused the formation of Marion Drift C, characterized by thick, confined, and elongated mounded drifts separated by the wide moats and a marginal valley along the drowned Northern Marion Platform. At 7.5 Ma, a major re-arrangement of the contourite depositional system documents the influence of EAC across the drowned platforms. The subsequent Marion Drift D smoothed out some of the antecedent topography. However, the Marion Reef and Southern Marion Platform still presented obstacles to the EAC, leading to the deposition of multi-crested drifts. Phase 4 (5.7 Ma – 3.3 Ma) is marked by decreased sedimentation rates, possibly due to sea level fall at Mi-7 and the restriction of the Indonesian Throughflow. The youngest and currently active depositional phase began at around 3.3 Ma with a change in drift geometry from multi-crested to elongated mounded, coinciding with the onset of the Northern Hemisphere Glaciation and further reduction of the Indonesian Throughflow.

Beach-ridge formation as a possible indicator for an open Limfjord – North Sea connection

Raised beach ridges are prograded sequences of wave-built deposits that may provide valuable information about past relative sea-level changes, climate change and coastal evolution. In the Limfjord in northern Denmark, the Early and Middle Holocene sea-level changes are well-constrained. However, our understanding of Late Holocene sea-level fluctuations is limited, and the exact period when the coastal barrier between the Limfjord and the North Sea formed remains uncertain. In this study, we use optically stimulated luminescence (OSL) dating to determine the age of raised beach ridges at Gjellerodde in the western part of the Limfjord. The OSL ages presented here indicate that the beach ridges formed during three periods at 3.3–2.7, 1.4–1.0, 0.2–0.1 ka. In addition our data suggest a c. 0.2 mm/yr relative sea-level fall during the Late Holocene. The three distinct periods of beach-ridge formation coincide with periods when the Limfjord was open towards the North Sea as documented in historical records and marine records. This suggests that OSL dating of beach ridges can be used as a potential indicator for determining when the connection between the Limfjord and the North Sea was open in the Late Holocene.

A 1500-year multi-proxy record of subtropical mangrove dynamics in relation to sea level and climate changes on Babitonga Bay, Southern Brazil

Climatic changes and sea level variations have had a significant impact on the mangroves along the Brazilian coast during the Holocene. The present study focused on understanding the specific factors that have determined the establishment and expansion of mangroves in a subtropical estuary of southern Brazil (Babitonga Bay, State of Santa Catarina-SC), as well as their response to climate change and sea level variations during the late-Holocene. In this study, we carried out sedimentary analysis, palynology, and radiocarbon dating (C-14) to paleoenvironmental reconstruction of the last 1500 cal. year BP. Three facies associations were identified, indicating a progradational succession where a tidal flat was developed at the margin of the estuary. During the first phase, between at least 1440 and ±1286 cal. year BP, the area was characterized by a subtidal environment. The presence of Laguncularia pollen grains since ± 1390 cal. year BP indicated favorable conditions for mangrove establishment in proximity to the study site. Subsequently, around ± 1286 cal. year BP, the tidal flat developed, reaching the present-day shoreline, facilitated by the relative sea level drop. Avicennia trees were established on the tidal flat since ± 1273 cal. year BP, and the establishment of Rhizophora trees occurred in the most recent decades. This mangrove succession developed in Babitonga Bay following a temperature gradient, associated with low-temperature tolerance, and likely its establishment is associated with a temperature increase during the late-Holocene, that caused a migration of the southern limit of the mangrove in the subtropical zone to higher latitudes. Furthermore, the relatively slow expansion of mangroves upstream of Babitonga Bay also may have been controlled by the suitable salinity and substrate conditions, which were favored by the relative sea-level fall during the late-Holocene.

Taphofacies of coastal lakes based on mollusk death assemblages: a case study of Mirim Lake

Mirim Lake is the second-largest lacustrine body in Brazil, stretching for some 185 km and covering about 375 thousand hectares of water surface. This lake is part of the Patos-Mirim System, one of the world’s largest complexes of coastal lagoons, which is the result of the rise and fall of sea level over transgressive-regressive cycles triggered by glacio-eustasy during the Late Pleistocene and Holocene. Its characteristics and dynamics give it singular importance for studies on the genesis of shell accumulation in lagoons. This study aims to improve the knowledge of how the taphonomic signatures reflect the environmental characteristics and dynamics of Mirim Lake. Categorization of the arrangement and packing of death assemblages, as well as the orientation of shells, supported the biostratinomical approach. The taphonomic signatures included disarticulation, fragmentation, and corrosion of shells. Corrosion is the primary damage observed in the bioclasts, varying from a single loss of color to complete degradation. Differences in the characteristics of death assemblages observed in the dune fields and the lacustrine plain led to the recognition of two taphofacies. A more significant number of whole shells was observed in the dune field taphofacies, while sharp fragments and open-articulated bivalves in hydrodynamic unstable positions characterize the lacustrine plain taphofacies. The genesis of death assemblages and the taphonomic signatures of bioclasts were linked to three sedimentary dynamics attributed to the environment of Mirim Lake. Keywords: biostratinomy, taphonomic signatures, shell deposits, dune field, lacustrine plain.

Seismic attributes and quantitative reverse reservoir simulations of shallow-marine eocene sedimentary fairways, extensional rifting Indus onshore petroleum system, southern Pakistan: Implications for hydrocarbon exploration

Quantitative reservoir characterization plays a vital role in exploring carbonate depositional systems. The subsurface vertical and lateral changes in the tectonics and sea level adversely impact the signatures of band-limited seismic data. The seismic amplitudes are very sensitive to these vertical and lateral tilts of the stratigraphy and sedimentology of the subsurface stratigraphic layers. The dynamical tectonic-based fluctuating sea creates vital seismic expressions of high amplitude. These seismic expressions can inversely infer the dense-fractured depositional parasequences, thickness, porosity, and lithological change for the accommodation space and the sedimentary influxes for the development of petroleum fairways. Hence, it becomes very challenging to predict whether the producing zone for hydrocarbons sourced from the wet-gas-bearing zones is a potential zone for hydrocarbon exploration or a light oil-bearing zone (WGZ or LOZ). Therefore, the development of reverse quantitative simulations of parasequences within the extensional regimes is always a very complex research problem during stratigraphic-based fuel exploration of thin-bedded carbonate systems. This research utilizes the spectral-based amplitudes along with the instantaneous waveform-based temperature-porosity stratigraphic simulations (WTPS) on middle-Eocene carbonates (MCARB), Pakistan. 18-Hz amplitudes detect the gas-bearing stratigraphic prospect of 230 km2 on the northern flanks. The 18-Hz-based WTPS has predicted the parasequences within the complete petroleum systems. 24 m thick progradational and laterally continuous and fractured reservoirs are deposited within the wet-gas-bearing zones (WGZ) throughout a rapidly falling sea and succeeding growth at 175 °F and 28% porosity pseudo-wells. Deposition of 15–20 m thick deep-marine transgressive shale has been completed throughout a rising sea level at a temperature of 125 °F and 12% poor porosity. There is a lateral change in the dense-fractured then low-velocity from 2.1 to 1.8 g/cc and 2670 to 1690 ms/s predicted by WTPS from the NE to SW zones of MCARB, which impacts the presence of 1.5 km of the regionally developed pure stratigraphic trap. The WGZ has a vertically resolved thickness of 24 m, which is appreciably above the resolvable magnitude threshold of 23 m. Hence, it implies that the extensive fall of sea level followed by the regional development of the accommodation space for the accumulation of WGZ carbonate stratigraphic units is at its accurate location within the sub-surface reservoir system. This workflow has robust consequences towards exploring petroleum systems, Onshore Pakistan. Hence, this workflow may serve as an exemplary move towards regional analogues aimed at world carbonate in extensional regimes.

Analysis of the late Hirnantian and early Rhuddanian unconformities of southern Ontario: evidence for far-field glacioeustatic effects

Several unconformities have been previously recognized in the Late Ordovician and early Silurian strata of southern Ontario. We examined the Georgian Bay, Queenston, Whirlpool, Manitoulin, Power Glen, and Cabot Head formations and associated unconformities. Detailed sequence stratigraphic and chemostratigraphic analysis of Late Ordovician and early Silurian outcrops between Niagara, New York, and Manitoulin Island, Ontario, reveals new insights on the timing of the erosional unconformities and the Ordovician–Silurian boundary. We recognize three significant lowstand unconformities in this interval, which are referred to as the Cherokee, S1B, and S2 unconformities. Additional small-scale surfaces are present but do not reflect any major change in sea level or pause in sedimentation. Using δ13Ccarb chemostratigraphy, biostratigraphy, and sequence stratigraphy, we tentatively correlate units and the mentioned unconformities from southern Ontario to other eastern North American sections of comparable age, showing how glacioeustasy had a widespread effect on the deposition and removal of strata in far-field, subtropical basins. The Cherokee unconformity appears to be a composite erosion surface found across eastern North America that formed during the two or more episodes of glacioeustatic sea level fall in the early to middle Hirnantian. The overlying S1B and S2 unconformities can also be found across eastern North America and appear to be the result of glacioeustatic sea level falls occurring during the early Silurian. These new insights on the timing of erosional unconformities help better constrain the placement of the Ordovician/Silurian boundary in Ontario. It appears that the boundary does not correspond to a lowstand-related disconformity as previously suggested but can be tentatively placed within the Power Glen or lower Cabot Head formations.

Holocene Depositional History of Low-Lying Reef-Rim Carbonate Islets of Fakarava Atoll, Northwest Tuamotu, Central South Pacific

Assumptions about the fate of low-lying coral reef islands (atolls) facing global warming are poorly constrained, due to insufficient information on their depositional history. Based on the U/Th dating of 48 coral clasts, the chronostratigraphic analysis of excavated sections through rim islets (motu) at the windward and leeward sides of Fakarava Atoll (Tuamotu, French Polynesia) reveal that the deposition of coral detritus started approximately 2000 years ago. Most of these deposits lie on conglomerate pavements or reef flat surfaces, and are about 4500 to 3000 years old. The islet expansion at the windward sites seems to have operated coevally across the reef rim, from the ocean-facing shore lagoonwards. Meanwhile, well-developed, continuous, elongated, vegetated islets mostly occur along the windward, northeast to southeast coast, and isolated islets, vegetated or not, associated with the dense networks of conglomerates, are common on the leeward, partly submerged, western rim. Islet accretion on the windward rim sides is believed to have been mainly triggered by winter storms and occasional cyclonic events, whilst the leeward atoll parts were most likely shaped by distant-source swells from mid to high latitudes. The projections of the accelerated sea level rise in the future suggest that the long-term islet stability at Fakarava could be altered because the islets have accreted under the conditions of the falling sea level.

Dynamic evolution of marine productivity, redox, and biogeochemical cycling track local and global controls on Cryogenian sea-level change

The Cryogenian period of the Neoproterozoic was marked by two Snowball Earth glaciations that bookended a non-glacial interval that lasted roughly 10 million years. The break-up of Rodinia in the early Neoproterozoic gave rise to numerous rift basins whose time-varying redox dynamics, linked to marine productivity and biogeochemical cycling, were likely controlled by varying degrees of basin restriction in response to sea-level change. Those drivers were associated with both global glaciations (Sturtian deglaciation and the possibly early onset of the Marinoan) and local tectonics (regional basin uplift/subsidence). To explore these relationships, we focus on the Datangpo Formation, an exceptional shale sequence deposited in a marginal rift basin in South China. The Datangpo succession comprises lower organic-rich black shales and upper organic-lean gray shales/siltstones. As elucidated by our integrated data, the former were deposited in H2S-rich, deep-water settings underlying highly productive surface waters. We attribute this productivity to elevated nutrient supplies replenished from the open ocean following a sea-level rise, as augmented by high terrestrial input of phosphate in the wake of the Sturtian deglaciation. By contrast, the gray shales/siltstones were associated with relatively shallow deposition in an oxic/suboxic water column with suppressed primary production. The major driver of this dramatic shift in water chemistry may lie with a greater extent of basin isolation and the corresponding reduction in nutrient fluxes from the global ocean due to a significant sea-level fall—possibly linked to regional basin uplift. At this time, the basin experienced an elevated influx of detrital clays and organic material, perhaps triggered by lower sea level. Surprisingly, we found that euxinic conditions prevailed in the water column for a substantial period following the onset of more isolated conditions. This relationship can be attributed to redox-dependent phosphorus cycling. Specifically, H2S-replete conditions facilitated regeneration of bioavailable phosphorus, initiating a positive feedback that allowed the persistence of anoxic waters. This study advances our understanding of nutrient cycling and biogeochemical evolution during the Cryogenian under the influence of large-scale sea-level fluctuations and associated time-varying connectivity to the open ocean that must have been common in many basins at this time. Furthermore, our study sheds novel light on the iron proxy and early sulfur cycling. Almost all of the terrestrial silicate-bound iron was likely converted into highly reactive phases due to strong chemical weathering, while sulfur data revealed isotopic stratification in the wake of Sturtian deglaciation, with important messages for interpretations of analogous data in the aftermath of other climate upheavals in Earth history.

Discovery of the Messinian Eratosthenes Canyon in the deep Levant Basin

The Messinian Salinity Crisis (MSC) was a major disruptive event in Earth's history involving all Earth's systems - hydrosphere, biosphere, lithosphere, and atmosphere. Approximately six million years ago, the entire Mediterranean Sea turned into a hypersaline basin when the Gibraltar gateway to the Atlantic Ocean narrowed and evaporation exceeded fresh water input. Consequently, marine fauna died and massive deposition of salt on the seafloor had led to the formation of the world's youngest salt giant.A fundamental observation related to this enigmatic period is a remarkable erosional surface, associated with 100 s-of-m deep canyons that were incised across the Mediterranean's continental margins during the crisis. Such canyons across the Levant margins were described in the 1970s and, later, in the 2000s they were shown to be the gateway for shallow, sinuous, submarine channels that continued towards deeper parts of the basin. Though it could have been assumed that these channels end as terminal sedimentary lobes in the Levant Basin, we show here that they contributed to a much wider sedimentary system in the Eastern Mediterranean Sea: farther west, they merge into a nearly 500-m-deep and ∼ 10-km-wide canyon, which we name after the nearby Eratosthenes Seamount. This remarkable canyon was excavated at the beginning of the MSC prior to salt deposition. We suggest that incision took place during Stage 1 of the MSC, but we cannot rule out faster incision (10s of ky) at the end of Stage 1 (∼5.60 Ma), or at its very beginning (∼5.97 Ma).Regardless of its exact age, we show that the Eratosthenes Canyon was excavated subaqueously and suggest that incision was caused by gravity currents, such as sediment-laden turbidites and/or dense brines. We argue that the increased salinity at the beginning of the crisis and a possibly limited sea level fall - whose amplitude should be investigated - had triggered these gravity currents, destabilizing the continental margin and carving the seabed wherever it was steep enough.

Late holocene progradation in a mixed siliciclastic-carbonate beach ridges system, Northeast Brazil

A morpho-sedimentary analysis based on grain size, texture, composition, luminescence ages, and morphologic pattern was performed in a beach ridge system located on the Brazilian semiarid coast to understand the siliciclastic and bioclastic mixtures during a progradation of strandplain. Moreover, the rhodolith beds were adopted as a new sea-level proxy to analyze the coastline displacement trajectory of the mixed siliciclastic-carbonate sedimentary environment. The beach ridges system shows a succession of ridges and swales interrupted in the central part by a lagoon plain, the western sector exhibits the crest relief related to the ancient foredune on top and underlying foreshore deposits, and the eastern sector shows a great presence of transgressive dunes overlying beach ridges. Foredune/aeolian facies and the foreshore facies (with rhodolith beds associated) show siliciclastic and bioclastic mixture related to the late Holocene coastal progradation with a progressive increase in carbonate influence, mainly calcareous algae, foraminifera, and molluscs. Moreover, comparing the modern counterpart of the older beach ridges, finer and better-sorted sediments were found in the ancient deposits than in the modern ones. Thus, the progradation was marked by an eminently dissipative beach system. This coeval sedimentation is controlled by key local settings, such as the semi-arid climate, the absence of local fluvial input, the shallow shelf, and the carbonate factory for bioclastic supply. The presence of rhodolith beds in the foreshore facies with an elevation lowering trend towards the modern coastline reinforces the hypothesis that the mixed progradational pattern was accompanied by a small fall in sea level, which corroborates with late Holocene regional curves and provides pioneering preliminary contributions for the studied area.

Aptian major changes in accommodation. New sedimentary evidence from the Maestrat Basin (E Iberia)

Global sea-level fluctuations with amplitudes of tens of metres and durations between 0.5 and 3 Myr are well-known oceanographic events of the Cretaceous Period. During recent years, major Cretaceous changes in accommodation recorded in different basins have been mainly interpreted as eustatically driven. Two episodes of major relative sea-level fall and rise of late early and early late Aptian age were investigated in the Maestrat Basin (E Iberian Peninsula). Previously documented in eight outcrops, these Aptian records of major changes in accommodation have now been identified in five additional localities within the basin, suggesting at least a basin-wide significance of these relative sea-level fluctuations. The records of relative sea-level fall analysed are associated with the local development of incised valleys and forced regressive wedges. Two upper lower Aptian incised valleys, which are downcutting highstand platform carbonates dominated by rudists and corals, were studied and show depths of approximately 10 and 80 m. The incision depths measured indicate amplitudes of relative sea-level variations in the order of tens of metres. During subsequent base-level rise, the lower part of these incised valleys was respectively infilled with orbitolinid limestones and high-energy carbonate deposits influenced by tides. Further evidence of major relative sea-level drop and rise during the late early Aptian includes a detached backstepping carbonate platform of reduced extension, underlain and overlain by basinal marly deposits, which flourished basinwards of a highstand platform margin. Although incised valleys or smaller-scale incisions are local features, evidence for relative declines in sea level can be identified from other evidence. For example, by the development of palaeokarst or by the presence of sharp or slightly erosive stratigraphic surfaces covered by deposits of higher energy influenced by tides, as observed in the late Aptian examples investigated herein. Comparable Aptian sedimentary features of subaerial exposure and subsequent transgressive genetic types of deposits have been observed in other basins across different tectonic plates, indicating that eustasy likely played a role in controlling accommodation changes during this stage. The discovery of new evidence for major relative Aptian sea-level fluctuations in various outcrops within the Maestrat Basin calls for further investigation in other underexplored Aptian platform carbonate successions in this basin. These investigations would contribute to a more accurate understanding of oceanographic dynamics during the Aptian Stage along the western margin of the Alpine Tethys.

Quaternary evolution of the Meiji Atoll (Mischief Reef) in the southern South China Sea

This paper describes the Quaternary evolution of Meiji Atoll (also known as Mischief Reef) in the southern South China Sea. Findings are based on a sedimentological and paleontological investigation of the uppermost 150 m of core NK-1. Through the Quaternary, open-lagoon sediments initially developed from ca. 2.6 to 1.6 Ma, which were succeeded by backreef sediments from 1.6 to 1.1 Ma, and subsequently by reef-flat sediments from 1.1 Ma to present, as the atoll evolved. The deepest depositional environments occurred between 2.0 and 1.6 Ma as a result of significant rates of subsidence in the Dangerous Grounds area. Coral abundance steadily increased during the Quaternary due to the secular eustatic sea-level drop and continuous high temperatures in the southern South China Sea. After the start of the Mid-Pleistocene Transition from 1.25 Ma, there was an increase in the amplification of short-eccentricity glacio-eustatic cycles, which resulted in two impacts. First, more frequent exposure horizons developed over the atoll due to an increase in amplitude of short-term sea-level fall events. Second, the occurrence of the thickest coral framestone units developed at around 0.8 Ma, perhaps linked to prominent short-term sea-level highstands (i.e., 125-ka cycle) and moderate subsidence rates. In summary, we document the Quaternary evolution of the Meiji Atoll, and show how variations in sea surface temperature, eustatic sea level, and tectonics have influenced its development. These findings have broader implications for understanding the Quaternary evolution of similar tropical carbonate atolls in the region.

Global sea-level fall triggered Ediacaran–Cambrian unconformity in North China craton

The Great Unconformity that separates the Cambrian and the underlying Proterozoic strata has been interpreted as recording a composite of multiple diachronous unconformities, among which the Ediacaran–Cambrian unconformity represents the last stage of these erosional processes. Whilst the major denudational episode of the Great Unconformity was strictly restricted to happen before or during the Cryogenian and considered to be associated with the Rodinia supercontinent cycle and “Snowball Earth” event, the cause(s) for the relatively temporary exhumation in late Neoproterozoic remains elusive. Here we present a synthetic geochronological study for the late Ediacaran to early Cambrian strata on the southern North China craton (NCC), where the Ediacaran–Cambrian unconformity occurs. The absence of the latest Ediacaran to earliest Cambrian age zircons of these strata may suggest a significant tectonic quiescence in NCC. A similar phenomenon can also be recognized in other blocks isolated from the Gondwanaland during the late Neoproterozoic–early Cambrian interval. This world-wide Ediacaran–Cambrian unconformity thus was more likely caused by a global sea-level fall, possibly associated with the assembly of the Gondwana supercontinent during the Precambrian–Cambrian transitional period.

Textural characteristics and sediment transport dynamics of the sandstones of the Nkporo group, Southern Anambra Basin (Nigeria): evidence for the upper cretaceous sea-level lowstand

The Anambra sedimentary basin is one of the major inland basins in Nigeria. It covers an area of approximately 30,000 square kilometres and is named after the Anambra River, which traverses the basin. The geology of the basin is characterized by a complex sequence of sedimentary rocks that record a long history of deposition and tectonic activity. Detailed and systematic field investigation of its rock sequences exposed at Leru junction and adjoining localities revealed five dominant lithologic units consisting of massive sandstone facies, argillaceous sandstone facies, fissile black shale and mudstone facies, planar and ripple laminated sandstone facies, and massive and pebbly sandstone facies. The area is dominated by transgressive facies characterized by alternating sequences of thinly and parallel laminated fissile black shales, mudstones, and argillaceous sandstones overlain by regressive deposits dominated by massive and partly pebbly sandstones. The transgressive sequences perhaps represented shelf mud sedimentation that was probably interrupted by episodic influxes of sands rich in argillaceous materials transported down the shelf probably by shoreface waves or gravity-driven flows, while the regressive facies may have probably been emplaced by liquefied sand flows of the frontal slope of an advancing mouth bar. The erosion of the shelf deposits by the pebbly sandstones possibly reflected seaward migration of shallow distributary channels during sea-level low stand. Thus, facies transition from argillaceous sandstones to massive and pebbly sandstones within the continental shelf perhaps signified a fall in the relative sea level possibly triggered by the erosion of the shelf or perhaps a high rate of sediment supply.

A tsunami deposit in the Stonewall Formation (Upper Ordovician), northeastern margin of the Williston Basin, Canada, and its tectonic and stratigraphic implications

The dolomudstone of the Stonewall Formation was deposited on the present-day northeastern side of the low-relief, relatively shallow, tropical Williston Basin of central Laurentia during the latest Ordovician. This unit hosts a geographically widespread carbonate conglomerate, up to 0.5 m thick and consisting of rudstone to floatstone, that thins and eventually disappears westward and southward. The bed contains dolomudstone intraclasts in a variably argillaceous dolomudstone matrix and varies in composition laterally and vertically. Intraclasts range from millimetres to centimetres in size, are spheroidal to tabular to irregular in shape, and are angular to rounded. They record incipient microcrystalline calcite precipitation in localised domains in lime mud that began just below the sediment–water interface, forming lightly cemented nodules in an unconsolidated matrix, and indicate exhumation and deposition by a single, short-duration, high-energy but variable event that fragmented and abraded many of the nodules and washed in clay and lime mud. The composite nature of the bed in terms of varying proportion of intraclasts versus mud, and range of intraclast size with variable rounding and sorting, suggest that the event fluctuated in intensity and current direction. We interpret this bed as having been formed by a succession of tsunami waves and backwash that scoured the sea bottom and reworked the nodules, and brought in suspended illite from shallow water and the adjacent land surface. Despite drainage from tsunami-inundated coastal areas, the absence of allochthonous quartz sand and silt suggests that there was no sediment of this size stored there. Backwash transport may also have played a role in the formation of argillaceous marker beds which have been attributed previously to sea level fall and subaerial exposure. Other anomalous beds that punctuate shallow epeiric sea successions may also be tsunami deposits, especially if the ambient paleoclimate and bathymetry were not conducive to generating or leaving a signal of major storms, and sea level fluctuation can be discounted.

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

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

Vertical Accretion Trends in Australian Tidal Wetlands

Australian tidal wetlands differ in important respects to better studied northern hemisphere systems, an artefact stable to falling sea levels over millennia. A network of Surface Elevation Table-Marker Horizon (SET-MH) monitoring stations has been established across the continent to assess accretionary and elevation responses to sea-level rise. This network currently consists of 289 SET-MH installations across all mainland Australian coastal states and territories. SET-MH installations are mostly in mangrove forests but also cover a range of tidal marsh and supratidal forest ecosystems. Mangroves were found to have higher rates of accretion and elevation gain than all the other categories of tidal wetland, a result attributable to their lower position within the tidal frame (promoting higher rates of accretion) higher biomass (with potentially higher rates of root growth), and lower rates of organic decomposition. While Australian tidal marshes in general show an increase in elevation over time, in 80% of locations, this was lower than the rate of sea-level rise. High rates of accretion did not translate into high rates of elevation gain, because the rate of subsidence in the shallow substrate increased with higher accretion rates (r2 = 0.87). The Australian SET-MH network, already in many locations spanning two decades of measurement, provides an important benchmark against which to assess wetland responses to accelerating sea-level rise in the decades ahead.

Detrital zircon U–Pb analysis indicate a provenance shift on the Neogene Barreiras formation, atlantic coast of Brazil

The Neogene Barreiras Formation is composed of continental to shallow marine sedimentary rocks that fringe the Brazilian Atlantic coast. Due to the large outcropping extent, various open questions about the evolution of this unit are still under debate. We present new U–Pb detrital zircon age spectra of four samples from the southern Espírito Santo state (Marataízes section) and two samples from the southern Bahia state (Arraial section). In both of the studied outcrops, a basal succession composed of coarse-grained cross-bedded sandstone, conglomerate and arkose with mudstone and claystone intercalations is superposed by an upper succession composed of massive sandstone, conglomerate and wacke. The U–Pb detrital zircon age spectra of all samples are dominated by Cambrian and Neoproterozoic grains. However, while the lower portion of both sections is dominated by Ediacaran-Cambrian grains, their upper portions also present significant Cryogenian-Tonian and sparse Paleoproterozoic (mainly Rhyacian and Statherian) and Neoarchean grains. This marks an important provenance shift within the Barreiras Formation, with a simple, local provenance sourced from the G1 to G5 granite supersuites of the Brasiliano Araçuaí Orogen for the basal layers contrasting with exposure of older basement rocks and Neoproterozoic anorogenic suites during deposition of the superior layers. We interpret the lower coarse sandstone and arkose-rich succession as deposited in an alluvial setting, with mud/claystones related to local flooding plains. Tectonic movements related to the evolution of the Atlantic passive margin, as described in previous works, caused a renewed uplift of the basement areas, exposing and denuding older rocks. Then, the upper layers deposited through debris flows covering the first succession. Further geochronological data is needed to test if the erosional event responsible for exposure of older basement rocks and sourcing of the upper layers would correspond to the Tortonian Sea level fall, related to growth of the Antarctic ice sheet to its present size. In this case, deposition of the lower succession would have occurred during the early Miocene, and the upper succession during the Pliocene. Our results shed light in the complex evolution of the sedimentary rocks generally grouped under the Barreiras Formation, and suggest important tectonic and climatic shifts during the Neogene evolution of the Brazilian Atlantic coast.

Microfacies Analysis of Mixed Siliciclastic-Carbonate Deposits in the Early-Middle Ordovician Meitan Formation in the Upper Yangtze Platform in SW China: Implications for Sea-Level Changes during the GOBE

During the Early-Middle Ordovician, the Upper Yangtze Platform experienced extensive development of a distinctive set of mixed siliciclastic-carbonate deposits known as the Meitan Formation. To conduct a comprehensive study of the relationship between mixed sedimentation and sea-level changes, the Hailong section, situated at the southwest margin of the Upper Yangtze Platform in SW China, was selected as the study area due to its typical mixed sedimentary sequence. This section was effectively compared with sections in other regions. Clustering analysis of the point-count groups from the Honghuayuan and Meitan Formations revealed nine microfacies that developed during this period. Such a diverse range of microfacies provides the evidence of frequent sea-level changes in the Yangtze Platform throughout this period. Specifically, two sea-level rises were identified during the early TS.2b and early TS.3b, with the early TS.2b event occurring globally. Furthermore, four sea-level falls were observed in the late TS.2b, TS.2c, TS.3a, and late TS.3b periods. The late TS.2b sea-level fall was globally significant. From TS.2c onwards, distinct variations in sea-level changes among the Yangtze Platform, North China, Baltoscandia, Australia, and North America emerged due to alterations in the paleogeographic pattern. By comparing the sea-level curves in the Upper Yangtze Platform with the diversity curves of chitinozoans, acritarchs, and brachiopods, it became evident that environmental changes played a crucial role in the Great Ordovician Biodiversification Event (GOBE), especially during the Floian. The rising sea level and increased detrital materials fostered the development of diverse habitats, prompting organisms to adapt to varying environments. In general, rising sea levels favored increased brachiopod diversity, while falling sea levels favored enhanced planktonic diversity among chitinous and acritarch species. By shedding new light on the relationship between Ordovician sea-level changes and biodiversification in the Yangtze region, this study offers a fresh perspective on the subject from the microfacies analysis.