Glacio-eustatic Cycles Research Articles

Quaternary stratigraphic architecture of the Pearl River Mouth continental shelf, northern South China Sea: Unraveling stratigraphic imprints of tectonism, paleoceanography, and climate change

The complexity of continental shelf stratigraphy is determined by the interactions among various global or local factors. However, their specific roles are still poorly understood. Using a newly-acquired grid of single-channel seismic reflection profiles, combined with piston cores, shallow boreholes, and industrial wells, this study investigates the sequence stratigraphic architecture and evolution of the Quaternary continental shelf offshore the Pearl River Mouth (PRM), northern South China Sea (SCS). Three types of bounding surfaces have been identified according to their stratal terminations: erosional surfaces (ESs), flat-lying surfaces (FSs), and locally dipping surfaces (LDSs). Given the limited extent of LDSs, we utilized ES/FS couplets to subdivide the upper Quaternary section into six high-frequency sequences (SQ6–SQ1) and their corresponding systems tracts. Within each sequence, regional progradational clinoforms (RPCs) prevail across the entire shelf and are accompanied by downstepping shoreline trajectories, which we interpreted as deposits of falling-stage systems tract. RPCs are interrupted by intercalated sheeted units (ISUs) characterized by high-amplitude and continuous seismic reflections, which may consist of transgressive and lowstand deposits. In younger sequences particularly SQ2 and SQ1, progradational-aggradational clinoforms (PACs) with ascending shoreline trajectories are well developed but confined to the shelf margin, indicating efficient sedimentation during lowstand.Our results revealed significant spatial and temporal variability in stratigraphic architectures of the PRM shelf. Along the SW–NE direction, the stratigraphic pattern changes from progradation to retrogradation and back to progradation, resulting in three sectors with distinct shoreline trajectories. Retreating shoreline trajectories and retrogradational stacking patterns are preferentially present in areas adjacent to the Dongsha Rise, which are jointly influenced by several local factors such as tectonism and intense marine processes. Temporally, a dramatic change in stratigraphic patterns can be observed across a key surface ES4, dated ca. 0.8 Ma. Below ES4, fluvial incisions are scarce and RPCs are overall low-angle and capped with relatively thick ISUs. Also, no lowstand PACs were observed. Above ES4, large-scale incised valley networks appear, together with increased scale and dominance of RPCs and PACs. This stratigraphic imprint is attributed to the Mid-Pleistocene Transition (MPT), which caused a noticeable increase in periodicity, amplitude, and asymmetry of glacio-eustatic cycles. The increased scale of RPCs and PACs after the MPT may reflect increased sediment input due to high denudation rate under intensified monsoon climate.

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.

Sedimentation since 140 ka in Te Tai-o-Aorere Tasman Bay, Aotearoa New Zealand

ABSTRACT Late Pleistocene and Holocene sedimentation records changes in glacio-eustatic cycles, oceanography, active tectonics, and sediment supply. Here, we use high-resolution seismic-reflection profiles in Te Tai-o-Aorere Tasman Bay to investigate sedimentary deposit distributions associated with sea-level cyclicity over ∼140 ka. We identify nine seismic stratigraphic units below the inner continental shelf, interpret their character based on stratal relationships and acoustic properties, and infer ages using global sea level. The sequence includes stacked deposits and unconformities associated with the shifting shoreline over glacial cycles. Prominent erosional features include a lowstand ravinement surface and two transgressive surfaces formed during sea-level rise following the last two glacial maxima of ∼20 ka and ∼140 ka. The youngest surface marks the base of post-glacial sediment accumulated since ∼12 ka in the study area. Post-glacial sediment includes five seismically mappable units, covering an area of ∼1400 km2 and deposit volume of ∼9 km3. Time-averaged sediment accumulation rates are estimated to range from a maximum of ∼1.6–2.8 m/kyr to ∼0.5–0.9 m/kyr. Sequence stratigraphy interpreted in Te Tai-o-Aorere Tasman Bay is representative of accumulation in a relatively wide-shelf, sheltered inner bay setting supplied by mountainous catchments, and provides stratigraphic context necessary for future stratigraphic and tectonics studies.

Submarine Geomorphology and Sedimentary Features around the Egadi Islands (Western Mediterranean Sea)

In this paper, the physiography, geomorphological features, and sedimentary bedforms of the offshore Egadi Islands (Italy) have been illustrated and mapped through an integrated analysis of high-resolution bathymetric, seismo-acoustic, and sedimentological data. The study area is characterized by a wide, up to 25 km, continental shelf which is separated by a NNW-trending linear incision, the Marettimo Channel, along which several erosional and depositional features have been detected and mapped. Sedimentary prograding wedges were detected at water depths between 100–125 m along the shelf margin, which accumulated during the sea-level fall and lowstand stages of the last glacio-eustatic cycle (post- MIS 5.5). This study detected several slope breaks defining scarps across the continental shelf, which were interpreted as coastal cliffs that originated during the post-LGM eustatic sea-level rise. Several fields of different types of sedimentary bedforms, including 2-D and 3-D hydraulic dunes and sorted bedforms, were found across the continental shelf, providing evidence of a high hydrodynamic regime affecting the seafloor. Further on, this study recognized erosive and depositional features related to bottom currents (contourites) in the Marettimo Channel. These findings provide a better understanding of the morpho-sedimentary evolution of the Egadi Islands offshore in the latest Quaternary. Moreover, they offer essential scientific support for effectively managing the most valuable priority habitats for conservation, such as the Posidonia oceanica meadow and coralline algae bioconstructions (Coralligenous habitat).

Chapter 2. Sequence stratigraphic concepts and methodologies

Abstract This chapter reviews sequence stratigraphic concepts and methodologies and presents an approach that is most applicable to the North Sea Jurassic, based on the concept of genetic sequence stratigraphy. The concept of depositional sequences, comprising rock units bounded by unconformities, has been developed from the late nineteenth century up to the present day. Many different studies have been carried out on North Sea Jurassic sequence stratigraphy, from the early 1980s to the present day and involving a range of different approaches. Many authors have adopted the J sequence approach that was first published in the early 1990s; however, a number of alternative North Sea Jurassic sequence schemes have also been described. A close relationship existed between tectonics and sequence boundary development, particularly during the Middle–Late Jurassic in the North Sea region. Several of the major unconformities that are known to be of regional extent can be directly related to significant tectonic phases. Other sequence boundaries, for which a tectonic control is not evident, for example, particularly in the Early Jurassic, were potentially driven by glacio-eustatic cycles, which may have been controlled by orbital forcing cycles.

Trench floor depositional response to glacio-eustatic changes over the last 45 ka, northern Hikurangi subduction margin, New Zealand

ABSTRACT Glacio-eustatic cycles lead to changes in sedimentation on all types of continental margins. There is, however, a paucity of sedimentation rate data over eustatic sea-level cycles in active subduction zones. During International Ocean Discovery Program Expedition 375, coring of the upper ∼110 m of the northern Hikurangi Trough Site U1520 recovered a turbidite-dominated succession deposited during the last ∼45 kyrs (Marine Isotope Stages (MIS) 1–3). We present an age model integrating radiocarbon dates, tephrochronology, and δ18O stratigraphy, to evaluate the bed recurrence interval (RI) and sediment accumulation rate (SAR). Our analyses indicate mean bed RI varies from ∼322 yrs in MIS1, ∼49 yrs in MIS2, and ∼231 yrs in MIS3. Large (6-fold) and abrupt variations in SAR are recorded across MIS transitions, with rates of up to ∼10 m/kyr occurring during the Last Glacial Maximum (LGM), and <1 m/kyr during MIS1 and 3. The pronounced variability in SAR, with extremely high rates during the LGM, even for a subduction zone, are the result of changes in regional sediment supply associated with climate-driven changes in terrestrial catchment erosion, and critical thresholds of eustatic sea-level change altering the degree of sediment bypassing the continental shelf and slope via submarine canyon systems.

Plio-Quaternary sequences and tectonic events in the northern Adriatic Sea (northern Italy)

The analysis of multichannel seismic profiles and their correlation with well data in the northern Adriatic Sea (Northern Italy) has allowed to document Plio-Quaternary episodes of tectonic subsidence and uplift. They affected the sedimentation above the Mesozoic to lower Eocene Friuli-Dinaric Carbonate Platform (FDCP) and the unit composed of the Eocene Trieste Flysch and the Miocene Molassa (TFM), as well as the sedimentary succession lying SW of the FDCP. In particular, the early Zanclean reflooding that postdated the Messinian Salinity Crisis, was followed by the SW-ward progradation of a highstand to forced regressive shelf-slope system (Sequence 1). Forced regression was controlled by a main episode of tectonic uplift that occurred at the end of the Zanclean. This tectonic event also led to the formation of the sequence boundary separating the two main sequences that compose the Plio-Quaternary succession. The lower part of Sequence 2 (Piacenzian to late Pleistocene) recorded a two-step subsidence phase, which led to drowning episodes during Piacenzian and in particular late Gelasian (transgressive systems tract of Sequence 2). They were associated with the partial flooding of the TFM and the Miocene deposits overlying the FDPC, and with a downward bending of the pre-Calabrian succession toward the Apennine chain. The accommodation created during the late Gelasian drowning event was filled in part by an Alpine-sourced, SW-ward prograding system, and in part by the NE-ward paleo-Po prograding system (highstand systems tract of Sequence 2). The most recent part of Sequence 2 documents the last glacio-eustatic cycles in a physiographic context characterized by a very low gradient.

Multiple glacio-eustatic cycles and associated environmental changes through the Hirnantian (Late Ordovician) in South China

The Hirnantian glaciation, which was the first in the Phanerozoic to occur with elevated CO 2 levels, caused severe mass extinctions and disrupted biogeochemical cycling in the latest Ordovician. Previous research that aimed at exploring the triggers and consequences (both abiotic and biotic) of this glacial event have largely been based on condensed or incomplete sections. However, there is much debate over the detailed glacio-eustatic processes, and especially over the possible existence and characterization of multiple glacial cycles. Herein we conduct high-resolution geochemical analyses (via XRF core scanning and ICP-OES/MS) and a sedimentological study of three continuous upper Katian – lower Rhuddanian drill cores (Well Huangge-1, Well Shuanghe-1 and Well Yiwan-1), which represent a combination of nearshore and offshore facies in South China. The geochemical and sedimentological evidences reveal for the first time three glacio-eustatic cycles encompassed within a major sea level cycle through the upper Katian and Hirnantian. Each glacio-eustatic cycle comprises a combination of regressive and transgressive systems tracts, with the third one denoting the largest sea level fall and the glacial maximum. This pattern of sea level changes can be correlated with both the glacial sedimentary records and geochemical signals in North Gondwana, Laurentia and Baltica. These glacio-eustatic changes likely led to perturbations in regional marine nutrient cycling and formed a redox landscape that is depicted herein as somewhat different from previous models. We also present the glacio-eustatic effects at a basin scale, and a refined constraint for the tempo of polar ice sheet advances and retreats. It also calls for a re-evaluation of the mechanisms whereby a multi-cycle glaciation led to the Hirnantian biotic crisis, a relationship that may be much more complicated than the simplified cause-and-effect relationship proposed before. • Core scanning data revealed three glacio-eustatic cycles for the Hirnantian glaciation. • New pattern of glacial cycles correlates well with other records among different blocks. • Glacio-eustatic changes formed the redox landscape both temporally and spatially.

Shelf-margin architecture and deposition variability across the mid-Pleistocene climate transition, northeastern South China Sea

The shelf-margin prism of the Upper Pliocene to Pleistocene in the northeastern South China Sea has recorded significant signatures of glacio-eustatic cycles and paleo-climatic conditions. Integrated analysis of 3D seismic, well-logging, and borehole dating data in the study area provided a detailed depiction of the high-resolution sequence architecture and depositional evolution as they responded to the glacio-eustatic cycles and climate changes. The shelf-margin section of the Upper Pliocene to Pleistocene is comprised of a composite sequence bounded by a regional unconformity, which can be further divided into five sequences confined by subordinate unconformities. The two lower sequences (S1-S2) with similar and simple internal structures, are displayed as having roughly equal thicknesses (ca. 50 m) and gently seaward-dipping foresets. In contrast, the upper three sequences (S3-S5) are characterized by the development of high (200–600 m) and steep (2–5°) shelf-margin clinoforms, which can be further divided into fifteen clinothems. The sequences generally embody four systems tracts, showing different stratal stacking patterns, and the linking contemporaneous depositional systems including fluvial channels, deltaic and slope fan systems are documented. Based on the chronostratigraphic framework, comparisons have been made between the sequence cycles and the global oxygen isotopic records. The results suggest that the formation of different order sequences may have been in response to the glacio-eustatic cycles paced by the Milankovitch orbital cyclicity. The abrupt changes in the sequence and depositional architectures between S1-S2 and S3-S5 are assumed to be related to the significant increases in the amplitudes and durations of the glacio-eustatic fluctuations across the mid-Pleistocene climate transition (MPT). Intensively variable climatic conditions with strengthened monsoons during and after the MPT could have potentially led to an abundant sediment supply, which may have enhanced the development of shelf to shelf-edge delta systems with the large-scale prograding clinoforms.

The Quaternary stratigraphic architecture of a low-accommodation, passive-margin continental shelf (Santee Delta region, South Carolina, U.S.A.)

ABSTRACTThe Quaternary stratigraphy of the continental shelf offshore of South Carolina consists of stratigraphic units deposited in coastal-plain, shallow marine, and shelfal environments bounded by composite erosional surfaces that developed in response to numerous glacioeustatic cycles and were overprinted by regional uplift. These units are commonly distributed laterally, rather than stacked vertically, a function of the long-term low shelf gradient and the resulting lack of accommodation. Additionally, marine processes such as waves and geostrophic currents can rework both relict and modern sediments across the continental shelf.This study integrates high-resolution geological and geophysical datasets acquired offshore and onshore with existing data onshore into a comprehensive conceptual model describing the Quaternary geologic evolution of the coastal plain and continental shelf within a study area of approximately 8,000 km2. We use seismic facies and core analysis to define stratigraphic units associated with transgressive, regressive, and lowstand systems offshore. Regressive systems include progradational wave- and river-dominated deltaic and shoreface deposits. Lowstand systems consist of a complex network of paleo-incisions produced by regional, Piedmont-draining fluvial systems and smaller coastal plain rivers. Transgressive systems include paleochannel-fill successions dominated by mud-rich, tidally influenced backbarrier deposits, cuspate and linear shelf sand ridges, and transgressive sand sheets and shoals.The low-accommodation setting of the continental shelf influences the stratigraphic record in several ways: 1) the geometry of progradational coastal lithosomes, 2) the development of composite allogenic erosional surfaces, 3) the deposition of widespread, thin transgressive sand sheets, and 4) the restriction of thicker transgressive deposits to paleo-incisions. In this setting, the use of a bounding surface scheme that is hierarchical is preferable to the more common sequence stratigraphic or allostratigraphic convention for several reasons: 1) major erosional bounding surfaces are commonly amalgamated; 2) lower-order surfaces capture internal variability, which is key to the genetic interpretation of stratigraphic units, and 3) stratal stacking patterns typically used to define a sequence stratigraphic framework are rare.

The evolution of coastal-plain incised valleys over multiple glacio-eustatic cycles: Insights from the inner continental shelf of Delaware, U.S.A.

The evolution of coastal-plain incised valleys over multiple glacio-eustatic cycles: Insights from the inner continental shelf of Delaware, U.S.A.

Late Ordovician obliquity-forced glacio-eustasy recorded in the Yangtze Block, South China

A high-precision time framework is critical for improving our understanding of three major events: the Late Ordovician mass extinction, the Hirnantian glaciation, and the Hirnantian isotopic carbon excursion (HICE). In the present study, we conducted detailed cyclostratigraphic analysis of a high-resolution (1–2-cm intervals) anhysteretic remanent magnetization series obtained from the Wanhe section in South China, covering the lower Katian through Hirnantian. Power spectra and derived average spectral misfit results show that the section records the ensemble of Milankovitch cycles with 405-kyr (long-eccentricity), 93–125-kyr (short-eccentricity), 33.8-kyr (main obliquity), and 17–22-kyr (precession) periods. The obliquity period of 33.8 ± 0.46 kyr indicates a precession constant k of 57.19 ± 0.53 arcsec/yr, a length of day of 22.37 ± 0.12 h, and an Earth–Moon distance of 375,330 ± 722 km (vs. 384,000 km today) during the Late Ordovician (445 Ma). The ratios of obliquity band power to total power reveal periodicities of 1.32 Myr for the s4–s3 term and 173.4 kyr for the s3–s6 term. Aided by the highly fluctuating magnetic susceptibility (MS), we recognized a series of 1.32-Myr obliquity-forced glacio-eustatic cycles with three intervals of sea-level rise and climate warming trend characterized by high MS values and two episodes of sea-level fall and climate cooling trend before the Hirnantian. The 405-kyr and subsequent 33.8-kyr calibrations produce an ~7.36-Myr floating astronomical time scale (FATS) for the studied succession, indicating that the estimated durations for the Hirnantian Stage, the worldwide Hirnantian glaciation, and the first and second phases of the Late Ordovician mass extinction were ~1225 kyr, ~830 kyr, ~440 kyr, and ~540 kyr, respectively. This work provides a reliable high-resolution time framework for further investigation of the major geological events occurred in the Late Ordovician, and improves our knowledge of Solar System behaviour and Earth–Moon history during that period.

The Permo-Carboniferous Dwyka Group of the Aranos Basin (Namibia) – How detrital zircons help understanding sedimentary recycling during a major glaciation

The Permo-Carboniferous glacial Dwyka Group deposits in Namibia provide the first evidence for high sedimentation rates caused by a Carboniferous/Permian extensional stress regime in southern Gondwana, which eventually led to the opening of the southern Atlantic Ocean in the Early Cretaceous. Besides palaeocurrent direction measurements, the use of U–Th–Pb data in combination with morphometrics of detrital and magmatic zircons is a not well studied approach to determine fluxes and reworking processes of glacial and marine clastic sediments. Roundness values of 7–8 for the investigated zircon grains are very common, suggesting long transport distances and/or multiple recycling. In addition, abundant scratches and collision marks indicate high energy environments during transport. Only one ash bed and one sandstone sample featured low roundness values and smooth zircon surfaces, indicating low transport energies. Zircon ages from all concordant analyses can be clustered into several groups: Cryogenian to early Cambrian (c. 0.7–0.5 Ga; 15%), major early Neoproterozoic to late Mesoproterozoic (c. 0.95–1.25 Ga; 46%), late (1.9–1.7 Ga; 8%) as well as a middle Palaeoproterozoic (2.1–1.9 Ga; 5%) and scarce Archaean (3%). The first age group may be confined to Pan-African magmatic events occurring around the Kalahari Craton margin, whereas the protosource of Mesoproterozoic ages may be the Namaqua Metamorphic Complex. Palaeoproterozoic structural units folded in the course of the Namaqua Orogeny as well as the late Palaeoproterozoic Rehoboth Belt, are possible source areas for the Palaeoproterozoic ages. The youngest zircons from an ash bed were dated at 296.2 ± 2.3 Ma and suggest an Asselian age for the upper Zwartbas Formation (DS II), pointing to a duration of 12 Ma for DS II-IV, assuming a duration of 3–6 Ma for the three uppermost glacio-eustatic cycles. The data suggests that the siliciclastic successions of the Dwyka Group mainly consist of reworked material, with minor input of freshly eroded material. An initial diamictite deposit, possibly induced by a southerly directed ice advance, occurring in the southern part of the Aranos Basin, recycled parts of the Nama Basin fill and covered the latter strata based on the difference in zircon age spectra. All subsequent deglaciation sequences indicate prevalent westward ice movement. A recycling of the detrital zircon grains over several generations is also implied by an increase in mean zircon width/length ratio and width as a function of age. The Permian evolution of southern Gondwana was confined by the occurrence of pre-Dwyka erosional plains and deep glacial scoured valleys reaching into its interior, which were successively covered by extensively recycled material of various protosources and consequently eroded by the Dwyka aged tillites.

A multi-proxy approach to paleoenvironmental changes in the southwestern Río de la Plata area (Argentina) during Late Pleistocene

In the southwestern margin of the Río de la Plata (Argentina), mining works have uncovered deposits that correspond to the last glacio-eustatic cycle (<120 Ky). Because deposits of this cycle are rare in the area, the studied succession provides an opportunity to study the paleoenvironmental and paleoclimatic evolution of the area during that time. Sedimentary analysis reveal an inner estuarine environment impacted by storm surges, followed by fluvial and eolian depositional environments. The paleontological record of the terrestrial deposits indicates open grasslands and semi-arid climatic conditions that can be biostratigraphically assigned to the Lujanian Stage (late Pleistocene). Nevertheless, the chronology of the succession is hereby modified based on OSL ages and geomorphological evidence suggesting that the lower levels of the studied succession are older than previously thought (i.e. MIS 5e instead of MIS 3). Relative changes of sea level are inferred: the estuarine deposits are related to a period of relative high sea-level (i.e. MIS 5e), and the subsequent fluvial system is associated to a global sea-level fall that reached its lowest point during MIS 2.

Integration of static and dynamic data and high-resolution sequence stratigraphy to define reservoir architecture and flow units within a ‘super giant’ gas condensate and oil field, Kazakhstan

Abstract The Upper Serpukhovian (Mississippian) interval of the Karachaganak reservoir comprises stratified platform interior carbonates, which are characterised by relatively high porosity and matrix permeability (maximum 23.8% and 2198 mD, respectively), and comprise high frequency (fourth-order) glacio-eustatic cycles. Based on static (geological) and dynamic (well test and production) data, this cyclicity exerts a strong control on gas migration pathways, and reservoir quality is predictable within a third- and fourth-order sequence stratigraphic framework that defines two discrete flow units. The contribution to flow made by fourth-order cycles within flow units is dependent on their position within third-order transgressive or highstand systems tracts. An upper flow unit comprises thick (up to 40 m) fourth-order cycles within a third-order transgressive systems tract. These fourth-order cycles contain large scale, clinoformal bedforms, can be correlated for up to 6 km, and are the main pathways of injected gas through the Upper Serpukhovian reservoir. A lower flow unit comprises fourth-order cycles (within a third-order highstand systems tract) that are thinner, and less laterally continuous, than those in the upper flow unit, and contribute far less to horizontal gas flow. Pressure data indicate that vertical permeability baffles are developed at the third-order sequence boundary that separates the two flow units (linked to diagenetic alteration during prolonged subaerial exposure), and also within the low porosity, mud-rich, transgressive bases of thick fourth-order cycles of the upper flow unit, that formed during periods of increased accommodation space across the platform top during third-order transgression. The scale at which this data set is observed is critical to understanding the dynamic behaviour of the reservoir. Relatively small differences in the composition of microfacies of cycles within both flow units do not explain the variations in dynamic data. This variation is better explained within the broader context of the sequence stratigraphic framework as defined from analysis of cycle stacking patterns. Thicker high frequency cycles deposited in increased accommodation space on the platform top during third-order transgressive intervals are key to developing thick intervals of net pay.

High-Frequency Sequences in the Quaternary of Pelotas Basin (coastal plain): a record of degradational stacking as a function of longer-term base-level fall

ABSTRACT: The aim of this work was to analyze the sedimentary record of the coastal plain of Pelotas Basin, placing it in the context of temporal and spatial scales, and proposing a stratigraphic hierarchical framework. The coastal plain, located in southern Brazil and in northern Uruguay, is mainly formed by four Quaternary barrier-lagoon systems. Three of these systems were studied through the integration of surface and subsurface data (geomorphological and topographical mapping, outcrops description, geoprocessing, dating and Ground Penetrating Radar - GPR - records) and interpreted as the preserved, onshore portion of depositional sequences mainly controlled by glacioeustatic cycles of about 100 kyr. The stacking pattern comprising these sequences is progradational with seaward downsteping (highest sea-levels measured: 9.5, 8 and 3 m, respectively), comprehending a degradational sequence set, or the regressive/falling stage systems tract of a higher order depositional sequence identified in seismic sections, with about 0.5 Ma. The youngest sequence has diachronous systems tracts - while some sectors have already transitioned from the transgressive to the highstand/falling stage systems tract, others are still under a transgressive context. It indicates that, in this time and scale, the sea level is not the main control of coastal evolution, and autogenic factors are fundamental in the run of geological record.

Terrace formation in the upper Bengal basin since the Middle Pleistocene: Brahmaputra fan delta construction during multiple highstands

AbstractFloodplains, paleosols, and antecedent landforms near the apex of the Brahmaputra fan delta in north‐central Bangladesh preserve cycles of fluvial sediment deposition, erosion and weathering. Together these landforms and their associated deposits comprise morphostratigraphic units that define the river's history and have influenced its channel position and avulsion behaviour through the Late Quaternary. Previously, temporal differentiation within these units has not been sufficient to decipher their sequence of deposition, an important step in understanding the spatial pattern of migration of the Brahmaputra River. Holocene units in this region are fairly well established by radiocarbon dating of in situ organic material, but pre‐Holocene units are considered Pleistocene‐aged if organic material is dated &gt;48 000 year BP (the limit of radiocarbon dating) or the sediments are positioned beneath a prominent paleosol, interpreted as a buried soil horizon that developed during a previous sea level lowstand. In such cases, these morphostratigraphic units have been broadly interpreted as Pleistocene without knowing their absolute depositional ages or relative evolutionary chronology. Here we use detailed sediment analysis to better differentiate morphostratigraphic units at the Brahmaputra's avulsion node, establishing the sequence of deposition and subsequent weathering of these bodies. We then test this relative chronology by luminescence dating of the sands beneath these landform surfaces. This work provides the first absolute depositional age constraints of terrace sediments for the Middle to Late Pleistocene Brahmaputra River and upper Bengal basin. The luminescence ages are complemented by detailed compositional trends in the terrace deposits, including clay mineralogy and the degree of weathering. Together, these newly dated and carefully described morphostratigraphic units reflect eustasy‐driven cycles of terrace development by way of highstand floodplain deposition and subsequent lowstand exposure and weathering, along with active tectonic deformation. Defining this Late Quaternary history of terrace development and position of the Brahmaputra River is a first step toward an integrated understanding of basin and delta evolution over multiple glacioeustatic cycles and tectonically relevant timescales.

Lateral variability of shallow-water facies and high-frequency cycles in foreland basin carbonate platforms (Pennsylvanian, NW Spain)

The kilometer-sized and 100-meter-thick carbonate platforms of the Escalada Fm. I and II (Middle Pennsylvanian) accumulated in the foredeep of a marine foreland basin during the transgressive phases of 3rd-order sequences and were buried by prograding siliciclastic deltaic systems in the course of the subsequent highstand. The carbonate successions show a general upward trend from grain- to mud-supported carbonates, interfingering landwards with siliciclastic deposits of a mixed siliciclastic-carbonate shelf (Fito Fm.) adjacent to deltaic systems. The spatial variability of the carbonate facies and the high-frequency (4th–5th order) cycles, from the platform margin-outer platform to the deltaic systems, has been interpreted from basin reconstruction. Carbonate facies include skeletal grainstone to packstone, ooidal grainstone, burrowed skeletal wackestone, microbial and algal boundstone to wackestone forming mounds, various algal bafflestone and coral biostromes in areas with siliciclastic input. These high-frequency transgressive–regressive cycles are interpreted to record allocyclic forcing of high-amplitude glacioeustasy because they show characteristic features of icehouse cycles: thickness >5 m, absence of peritidal facies, and in some cases, subaerial exposure surfaces capping the cycles. In the mixed cycles, siliciclastics are interpreted as late highstand to lowstand regressive deposits, whereas carbonates as transgressive-early highstand deposition. The lateral and vertical variability of the facies in the glacioeustatic cycles was a response to deposition in a rapidly subsiding, active foreland basin subjected to siliciclastic input, conditions that might be detrimental to the growth of high-relief carbonate systems.

Glacial onset predated Late Ordovician climate cooling

AbstractThe Ordovician glaciation represents the acme of one of only three major icehouse periods in Earth's Phanerozoic history and is notorious for setting the scene for one of the “big five” mass extinction events. Nevertheless, the mechanisms that drove ice sheet growth remain poorly understood and the final extent of the ice sheet crudely constrained. Here using an Earth system model with an innovative coupling method between ocean, atmosphere, and land ice accounting for climate and ice sheet feedback processes, we report simulations portraying for the first time the detailed evolution of the Ordovician ice sheet. We show that the emergence of the ice sheet happened in two discrete phases. In a counterintuitive sequence of events, the continental ice sheet appeared suddenly in a warm climate. Only during the second act, and set against a background of decreasing atmospheric CO2, followed steeply dropping temperatures and extending sea ice. The comparison with abundant sedimentological, geochemical, and micropaleontological data suggests that glacial onset may have occurred as early as the Middle Ordovician Darriwilian, in agreement with recent studies reporting third‐order glacioeustatic cycles during the same period. The second step in ice sheet growth, typified by a sudden drop in tropical sea surface temperatures by ∼8°C and the further extension of a single, continental‐scale ice sheet over Gondwana, marked the onset of the Hirnantian glacial maximum. By suggesting the presence of an ice sheet over Gondwana throughout most of the Middle and Late Ordovician, our models embrace the emerging paradigm of an “early Paleozoic Ice Age.”

The ancestral Mississippi drainage archived in the late Wisconsin Mississippi deep-sea fan

The response of continental-scale drainage systems to short-term (i.e., millennial-scale) climate change is unknown but has wide implications for understanding climate feedbacks and terrestrial-marine fluxes. The late Wisconsin Mississippi River to deep-sea fan of North America was one of Earth’s largest sediment-routing networks during the most recent glacio-eustatic cycle. To understand late Pleistocene sediment production and dispersal related to the partly glaciated, ancestral Mississippi system, we sampled late Wisconsin deep-sea fan channel-fill and lobe deposits for detrital zircon U-Pb and (U-Th)/He double-dating analyses, from Deep Sea Drilling Project (Leg 96) cores and U.S. Geological Survey piston cores. Our results suggest a late Pleistocene glacial Mississippi system that forced a larger transfer of sediment from Cordilleran magmatic provinces and the Canadian Shield when compared to the modern drainage. This indicates a potentially more expansive and/or erosive ancestral Mississippi catchment, and the efficient dispersal of terrigenous sediment, nutrients, and solutes into the deep-sea via high-discharge meltwater and glacial-lake outbursts during ice retreat.