Sea-level Fall Research Articles

Controls on organic matter accumulation on the early-Cambrian western Yangtze Platform, South China

Abstract The origin of widespread organic-rich shales on the early Cambrian Yangtze Platform of Nanhua Basin (South China) has been attributed to high primary productivity. To test this hypothesis, we generated multiple paleoproductivity (OCAR and PAR), paleoredox (Corg/P ratios), upwelling/restriction (CoEF × MnEF, Co (ppm) × Mn (%), Mo/TOC, and Cd/Mo ratios), and terrigenous flux proxies (Al and unreactive Fe (FeU)) for the lower Cambrian Niutitang Formation in the outer-shelf Jinsha section, which records a typical transition from the organic-rich black shales to overlying organic-lean mudstones or siltstones. These new data were integrated with published paleoredox proxies (iron speciation and redox-sensitive trace metals) for the same section. Our results suggest that the organic-rich (TOC = 4.6 ± 1.8%) lower member (LM; 0–37 m) of the Niutitang Formation records ferruginous conditions, high productivity levels, and low siliciclastic fluxes, and that the organic-poor (TOC = 0.1 ± 0.1%) upper member (UM; 37–329 m) was characterized by dominantly oxic conditions, declining productivity levels, and high siliciclastic fluxes. Our results suggest a redox shift from ferruginous to oxic conditions and enhanced terrigenous fluxes in response to falling sea level from the LM to the UM. Secular variations in primary productivity and redox conditions are strongly correlated, which is consistent with a positive feedback: high primary productivity led to increasing water-column anoxia, which enhanced organic P recycling, stimulating further productivity. Variations in primary productivity may reflect variable upwelling intensity in a semi-restricted shelf setting linked to sea-level changes. Taken together, these findings suggest that a combination of primary productivity, redox conditions, and terrigenous fluxes driven by sea-level changes played a key role in organic matter accumulation on the early-Cambrian western Yangtze Platform.

Tracing sea-level changes during the mid-Holocene in the southern Mediterranean coast: An example from the Gulf of Gabes

Mid-Holocene sea-level emergence was recorded along a mid-latitude coastal site in the Gulf of Gabes, Tunisia, in beach rock deposits. However, the post-emergence sea-level curve and associated hydro-sedimentary processes have rarely been documented. This study presents new records of sea-level changes along the Tunisian coast since the mid-Holocene. Each phase of the sea-level curve was evidenced and supported by lithostratigraphic, morphological and archaeologic markers. Post-emergence sea-level fall due to postglacial isostatic rebound was recognized in regressive sequences which vary depending on available accommodation space, from a small pocket beach to a well developed dune system. Sea-level stabilization following isostatic rebound has promoted the end phase deposits of the regressive sequences. However, this stability was interrupted during the late Holocene by a high-energy storm event that accumulated a transgressive boulder unit at 2 m above present. Shape and disposition of the boulder ridge behind a prograding modern shoreline suggests that sea level has not changed during the late Holocene. The existence of a Roman fish pond on the upper foreshore confirms stability of sea level during the last two millennia. The combination of lithostratigraphic, morphological and archaeologic markers represents a tool for improving the accuracy of dating of erosional and depositional periods in mid-coastal sections.

Pleistocene Coastal Evolution in the Makran Subduction Zone

Along the coast of the Makran subduction zone (SE Iran and SW Pakistan), active uplift combined with efficient erosion and vigorous sediment transport have led to marine terraces with unique morphology and sedimentology. These terraces are characterized by the systematic presence of an extensive 1–10Cm thick sandstone layer capping their wave-cut base. Our investigation of thirty-six sedimentary logs of the terrace deposits revealed a general prograding trend from nearshore to beach deposits moving upsection. The presence of a thick marine sedimentary succession above the erosive platform suggests continued creation of accommodation space following carving of the platform by wave-erosion (i.e., erosion of the platform occurred before the peak of the highstand). Deposition of prograding beaches above the platform is interpreted to have occurred during the sea-level stillstand and the start of sea-level fall and was favored by a high sedimentary supply. While some terraces evolve into a classic staircase morphology, others are found as flat-topped platforms bounded by steep cliffs, isolated within the low-lying coastal plain. We find that this morphological difference results from a contrast in bedrock erodability (resistant sandstone versus soft marl, respectively). The flat-topped isolated marine terraces with marl bedrock share morphological and sedimentological similarities with Holocene crenulated beaches currently developing in low-lying bays between headlands. As indurated beaches are uplifted into headlands, they influence the development of following generations of beaches before being eroded by surficial erosion and wave action. Our study shows that the coastal geomorphology of the Makran coast is dictated by the interaction between tectonics (providing relative sea-level fall and juxtaposing units of different erodability at the same structural level by faulting), differential erosion between hard and soft rock (responsible for the presence of isolated headlands) and coastal sedimentary transport processes (permitting accumulation of extensive beach deposits).

Sedimentary characteristics of shallow-marine fans of the Huangliu Formation in the Yinggehai Basin, China

Abstract In recent years, shallow-marine fans have attracted considerable attention because they may serve as valuable targets in hydrocarbon exploration. By integrating 3-D seismic, well log, core, and other measured data, this study analyzed the sedimentary characteristics and factors controlling the deposition of shallow-marine fans in Member #1 of the Huangliu Formation in the Yinggehai Basin. Four depositional elements were recognized in the channel-lobe complexes (CLCs), channel-fill sandy debrites, channel-fill turbidites, sand-rich lobe deposits and mud-rich lobe deposits. The channel-fill sandy debrites, which are characterized by low root-mean-square (RMS) amplitude in plan view and deep V-shaped reflections in seismic profile, are commonly located in the internal regions of CLCs. They have widths ranging from 1 to 2 km and single sandbody thicknesses exceeding 25 m. A cored interval from a channel-fill sandy debrite comprising several ungraded sand units revealed massive fine-grained sandstones with floating mud clasts and muddy rip-up clasts. In contrast, the channel-fill turbidites are characterized by low or high RMS amplitude in plan view and shallow V-shaped or vermiform-shaped reflections in profile. They have a small-size frontal splay at the terminus, thus forming a lobe-channel-lobe depositional style. The width of a channel-fill turbidite is usually less than 0.5 km, and the single sandbody thickness is no more than 20 m. A cored interval of channel-fill turbidite consisted of very fine-grained sandstones containing incomplete Bouma subdivisions Ta–Tb. The sand-rich and mud-rich lobe deposits are characterized by high RMS amplitude and low RMS amplitude with continuous sheet reflection patterns, respectively. The deposition of the shallow-marine fans was significantly influenced by sea-level fluctuations and paleogeomorphology. When sea level fell rapidly, gravity flows with higher erosive energy and velocity tended to form widely developed large-scale channel-fill sandy debrites and sand-rich lobe deposits. When sea level rose, small-scale channel-fill turbidites were dominant because of relatively decreased gravity flow energy. Distinct variations of reservoir characteristics, including mean grain size, mud content, and pore-throat radius, were controlled by sea-level fluctuations. From the period of rapid sea-level fall to the period of rising sea level, the gravity flows may have been increasingly diluted due to sufficient mixing with ambient seawater, resulting in a gradual transformation from sandy debris flows into turbidity currents. In addition, changes in gradient caused by irregular paleogeomorphology may have significantly triggered different depositional styles. With a steeper gradient, the CLC is dominated by a channelized-lobe depositional style; however, a lobe-channel-lobe depositional style prevails under a gentler gradient condition. Taking the reservoir characteristics and depositional styles into consideration, it is suggested that the sandy debrites, with better physical properties, constitute favorable hydrocarbon reservoir intervals. The shallow-marine fans in this study may serve as a distinctive analogue to similar gravity flow deposits in other basins worldwide.

Holocene sea-level variability from Chesapeake Bay Tidal Marshes, USA

We reconstructed the last 10,000 years of Holocene relative sea-level rise (RSLR) from sediment core records near Chesapeake Bay, eastern United States, including new marsh records from the Potomac and Rappahannock Rivers, Virginia. Results show mean RSLR rates of 2.6 mm yr−1 from 10 to 8 kilo-annum (ka) due to combined final ice-sheet melting during deglaciation and glacio-isostatic adjustment (GIA subsidence). Mean RSLR rates from ~6 ka to present were 1.4 mm yr−1 due mainly to GIA, consistent with other East Coast marsh records and geophysical models. However, a progressively slower mean rate (<1.0 mm yr−1) characterized the last 1000 years when a multi-century-long period of tidal marsh development occurred during the ‘Medieval Climate Anomaly’ (MCA) and ‘Little Ice Age’ (LIA) in the Chesapeake Bay region and other East Coast marshes. This decrease was most likely due to climatic and glaciological processes and, correcting for GIA, represents a fall in global mean sea level (GMSL) near the end of Holocene Neoglacial cooling. These pre-historical climate- and GIA-driven Chesapeake Bay sea-level changes contrast sharply with those based on Chesapeake Bay tide-gauge rates (3.1–4.5 mm yr−1) (back to 1903). After subtracting the GIA subsidence component, these rates can be attributed to long-term (millennial) global factors of accelerated ocean thermal expansion (~1.0 mm yr−1) and mass loss from alpine glaciers and Greenland and Antarctic Ice Sheets (1.5–2.0 mm yr−1).

Redating the earliest evidence of the mid-Holocene relative sea-level highstand in Australia and implications for global sea-level rise.

Reconstructing past sea levels can help constrain uncertainties surrounding the rate of change, magnitude, and impacts of the projected increase through the 21st century. Of significance is the mid-Holocene relative sea-level highstand in tectonically stable and remote (far-field) locations from major ice sheets. The east coast of Australia provides an excellent arena in which to investigate changes in relative sea level during the Holocene. Considerable debate surrounds both the peak level and timing of the east coast highstand. The southeast Australian site of Bulli Beach provides the earliest evidence for the establishment of a highstand in the Southern Hemisphere, although questions have been raised about the pretreatment and type of material that was radiocarbon dated for the development of the regional sea-level curve. Here we undertake a detailed morpho- and chronostratigraphic study at Bulli Beach to better constrain the timing of the Holocene highstand in eastern Australia. In contrast to wood and charcoal samples that may provide anomalously old ages, probably due to inbuilt age, we find that short-lived terrestrial plant macrofossils provide a robust chronological framework. Bayesian modelling of the ages provide improved dating of the earliest evidence for a highstand at 6,880±50 cal BP, approximately a millennium later than previously reported. Our results from Bulli now closely align with other sea-level reconstructions along the east coast of Australia, and provide evidence for a synchronous relative sea-level highstand that extends from the Gulf of Carpentaria to Tasmania. Our refined age appears to be coincident with major ice mass loss from Northern Hemisphere and Antarctic ice sheets, supporting previous studies that suggest these may have played a role in the relative sea-level highstand. Further work is now needed to investigate the environmental impacts of regional sea levels, and refine the timing of the subsequent sea-level fall in the Holocene and its influence on coastal evolution.

Holocene reef growth in the tropical southwestern Atlantic: Evidence for sea level and climate instability

The Holocene epoch offers a potential analogue for understanding future sea-level variability as both SST's and Global Mean Sea Levels (GMSL) were at times higher than observed today. However, GMSL can differ significantly from Relative Sea Level (RSL), even at far-field sites remote from margins of former ice sheets. Much of this spatial variability has been shown to be consistent with the predictions of glacial isostatic adjustment (GIA) models. Whilst it is generally accepted that RSL at far-field sites reached its maximum during the mid-Holocene, there have been many interpretations of sea level fall following the highstand from ∼6 ka. Here, we present a RSL history from several tectonically stable, far-field sites in eastern Brazil, derived from 17 microatoll and 45 fossil reef flat ages. Our results show evidence for two periods of RSL instability during the Holocene which differ from GIA predictions, including a hiatus in reef growth ∼3.7–2.5 ka. These results are broadly synchronous with several other locations in the Southern Hemisphere suggesting global rather than regional climatic forcing mechanisms are responsible. Variations in SST and southern hemisphere ice sheet dynamics are proposed as possible controlling mechanisms for the observed RSL oscillations beginning at ∼3.7 ka and 2 ka respectively. We suggest that these global processes combined with increased precipitation (and higher sediment flux) from several regional climatic forces created inhospitable conditions for reef growth, contributing to the observed hiatus and reduced reef flat accretion during the late Holocene (∼2 ka to present).

The Messinian Ebro River incision

Abstract Morphological sills condition sedimentary, water and fauna exchanges between different domains. In particular, sills are crucial factors to consider during the Messinian Salinity Crisis (MSC) palaeogeographic evolution (5.97–5.33 Ma) of the NW Mediterranean area. Here we focus on the Ebro River and its up to now unexplained short Messinian onshore length (~100 km) compared to that of the Messinian Rhone River (~480 km) despite similar present-day drainage basins. Thanks to an extensive seismic and borehole dataset, we present a new interpretation of a complete 270 km long Messinian Ebro incised-valley system course underneath the present-day continental margin and bathyal basin and its related distal detrital deposits. These results favour a syn-MSC or pre-MSC opening of the (endorheic) Ebro Basin to the Mediterranean. We propose a mechanism of retrogressive erosional process, localized at structural knickpoints that shift seaward through time. This mechanism resulted in the development of the complete incised-valley system and the falling stage system tract (FSST) during the MSC sea level fall with negligible or even null retrogressive inland erosion beyond the Catalan Coastal Range. The shifting of erosion-deposition is controlled by the pre-Messinian stepwise morphology and segmentation in the Valencia, Menorca and Liguro-Provence Basins. By comparison, the Rhone system is simpler, characterized by the key role of a single knickpoint (at the shelf-break) and a steeper continental slope. Both cases highlight the relationship between kinematic, segmentation and their relative morphologies, base-level fall and erosional/depositional response particularly well expressed during the outstanding MSC associated with a huge relative sea-level drop that we measured down to -1100 m below present day sea-level.

Bottomset and foreset sedimentary processes in the mixed carbonate-siliciclastic Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Picún Leufú Area, Argentina

The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation at the Picún Leufú area constitutes bottomset and foreset marine deposits comprising open bay, siliciclastic basin and mixed carbonate-siliciclastic shelf facies. Detailed sedimentological, ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences. Depositional Sequence 1 (DS1) begins with lowstand eolian deposits included in the Quebrada del Sapo Formation, and continues with thin, retrograding, transgressive open bay facies of the Vaca Muerta Formation encompassing coastal sand sheet and dune, marginal marine, bay margin and distal bay facies. Above the transgressive deposits, the regressive hemicycle of DS1 consists of bottomset deposits representing a siliciclastic basin facies association. Lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen-deficient environment. A relative sea-level fall generated a sequence boundary, which coincides with an angular unconformity that marks the base of Depositional Sequence 2 (DS2). This sea-level fall triggered the formation of an extensive lowstand channel-fill and lobe complex at the base of the slope, followed by retrogradational lobe facies during the subsequent transgression. The regressive hemicycle of DS2 represents foreset mixed carbonate-siliciclastic shelf facies, forming a slope mud belt and slope sand bodies. Near the top, sedimentation in a foreset-topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits. Sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary. The combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition, underscoring the role of wave-influenced hyperpycnal flows, and the effect on organic matter dilution in one of the most important unconventional reservoirs from Argentina.

Long-term regional trend and variability of mean sea level during the satellite altimetry era

The rise and fall of mean sea level are non-uniform around the global oceans. Their long-term regional trend and variability are intimately linked to the fluctuations and changes in the climate system. In this study, geographical patterns of sea level change derived from altimetric data over the period 1993-2015 were partitioned into large-scale oscillations allied with prevailing climatic factors after an empirical orthogonal function analysis. Taking into account the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillations (PDO), the sea level change deduced from the multiple regression showed a better estimate than the simple linear regression thanks to significantly larger coefficients of determination and narrower confidence intervals. Regional patterns associated with climatic factors varied greatly in different basins, notably in the eastern and western regions of the Pacific Ocean. The PDO exhibited a stronger impact on long-term spatial change in mean sea level than the ENSO in various parts of the Indian and Pacific Oceans, as well as of the subtropics and along the equator. Further improvements in the signal decomposition technique and physical understanding of the climate system are needed to better attain the signature of climatic factors on regional mean sea level.

The Schandelah Scientific Drilling Project: A 25-million year record of Early Jurassic palaeo-environmental change from northern Germany

With the aim to understand prolonged and repeated marine anoxia after the Triassic-Jurassic mass- extinction event, a continuously cored, 338 metre thick succession of Rhaetian to Toarcian sediments was retrieved close to the village of Schandelah near Braunschweig (Lower Saxony, northern Germany). Here, preliminary biostratigraphical, lithological, sedimentological, geochemical, and geophysical borehole data are presented and discussed. Based on the presence of ammonites, ostracods, benthic foraminifers, calcareous nannofossils, and palynomorphs, all major Late Triassic and Early Jurassic stage boundaries and many of the standard Lower Jurassic ammonite zones could be defined. The deltaic Rhaetian sand- and siltstone succession (Exter Fm) contains evidence for seismic activity probably related to large-scale geodynamic processes. The Hettangian (Lias Alpha) is represented by a thick heterolithic succession composed of shallow marine sandy and silty beds with intercalated organic-rich shale, representing deposition on a shoreface with frequent storm activity. Progressive deepening during the Sinemurian to Toarcian resulted in repeated deposition of laminated organic-rich facies. Periods of relative sea-level fall likely occurred during the Late Hettangian, the Late Sinemurian, and Early Pliensbachian, where a series of hardgrounds occur indicating erosion on the sea floor followed by sea level rise leading to omission, and increased reworking. One of the most conspicuous features of the Lower Jurassic in the Schandelah-1 core is the presence of abundant authigenic carbonates (glendonites,concretions, beef-calcite) within the Upper Pliensbachian with partly very negative C-isotope values (down to –37‰ V-PDB) suggesting the anaerobic oxidation of methane. A high-resolution organic carbon isotope record based on 485 analyses shows two major negative carbon isotope excursions (CIEs) within the lowermost Hettangian and Lower Toarcian, respectively. Both excursions coincide with the onset of black shale deposition and are well-known features of these time-intervals in other regions. Despite the fact that black shale deposition also characterizes parts of the Sinemurian and Pliensbachian in similar facies, no large negative CIEs are apparent. The Schandelah-1 core thus provides an unique archive of sedimentary, biotic and geochemical records of long-term Triassic-Jurassic palaeo-environmental change in the European Epicontinental Seaway.

Tidal strait to embayment: Seismic stratigraphy and evolution of a rock-bounded embayment in the context of Holocene sea level change

Elongate, rock-framed Santa Catarina Island partly encloses a low-energy marine embayment that communicates with the ocean via narrow, rock-bounded inlets with ebb and flood-tide deltas. The bay shoreline consists of low-energy sandy beaches, strandplains, mangroves and bayhead deltas while the exposed Atlantic shoreline comprises a series of headland-embayment littoral cells in a linked, north-directed longshore drift system.New and previously published seismic stratigraphic investigations enable assessment of the palaeogeographic/geomorphic evolution of the area since the early Holocene. At a sea level of ca. −50 m a sandy mainland-attached shoreline complex (dune and beach) developed on a coastal plain setting some 6–7 km offshore of the present shoreline. North of the island, a barrier island and tombolo were formed in a coastal re-entrant adjacent to the northern entrance to Florianopolis Bay. The open coast beach/dune system was overstepped by rapid sea-level rise during MWP-1B whereas the barrier island and tombolo continued to accrete until MWP-1C when they too were overstepped.The back-island embayment contains up to 40 m of sediment overlying an irregular bedrock surface. It was initially flooded during MWP-1C and the early muddy sedimentation suggests a distal, sheltered marine embayment. The subsequent development of many large bioherms on bedrock and mud suggests a change to more fully marine conditions as sea-level continued to rise. In the North Bay, these were subsequently enveloped by tidal dunes migrating north from the mid-bay. The vertical stacking of 3-D and 2-D sandy bedforms suggests that a tidal strait had developed with strong tidal currents radiating from a constriction in the vicinity of modern Florianopolis City. The strong tidal currents may be linked to the Holocene sea-level high that is well documented for this region.A fall in sea level from this highstand, coupled with accumulation of tidal strait sediments, reduced the tidal prism and strength of tidal currents in the bay. At the same time, regressive beachridge plains developed in open coast and bay environments. This coincided with development of the contemporary tidal inlet complexes at the north and south outlets of the bay. As a consequence, the upper surface of the tidal strait sand deposits experienced tidal ravinement and the bay switched to a sheltered back-barrier environment dominated by deposition of mud and silt which blanket the contemporary bay floor. The tidal inlets sequestered sand from the littoral drift system in ebb and flood-tide deltas, which created breaks in the longshore drift system, causing littoral sediment to accumulate at the north of the island in a series of spits, and promoting strandplain development south of the island.

Análise de fácies e ambientes deposicionais do Grupo Vazante na região da mina de Morro Agudo, Paracatu, noroeste do estado de Minas Gerais

Rochas do Grupo Vazante afloram no noroeste do estado de Minas Gerais, em uma faixa contínua de aproximadamente 250 km, da região de Vazante, ao sul, até Unaí, ao norte. Nessa região está localizada a mina de zinco e chumbo de Morro Agudo, que juntamente com os depósitos de Vazante, Ambrósia e Fagundes, todos hospedados no Grupo Vazante, compõe a principal província zincífera do país. Foram descritas, em seis testemunhos de sondagem e afloramentos de campo na região, 12 fácies sedimentares e quatro associações de fácies (AFs), que indicam deposição em ambiente de margem passiva mista carbonática-siliciclástica. A associação mais basal (AF01) é composta por pelitos e ritmitos silto-arenosos carbonosos, representando sedimentação em plataforma profunda de baixa declividade, em regime de transgressão marinha. Após a inundação máxima, em trato de mar alto, depositou-se a AF02 em ambiente de recife com barreiras em borda de plataforma, composta por dolomitos laminados com estromatólitos, dolarenitos e brechas dolareníticas, e ainda pelitos carbonáticos carbonosos com fácies diamictito. O abaixamento do nível relativo do mar gerou uma regressão forçada e a deposição da AF03 em trato de sistema do estágio de queda. É composta, na base, por diamictito argilo-carbonoso com fragmentos de AF02, passando a pelito e ritmito silto-arenoso. A retomada da subida do nível relativo do mar proporcionou a deposição da AF04 e ainda de rochas da AF03 em posição mais distal. A AF04 é composta por dololutitos argilosos laminados e maciços, e dolarenitos intraclásticos depositados em plataforma carbonática. O padrão de empilhamento observado sugere que a deposição de todas as unidades ocorreu progressivamente, em processo de subida do nível relativo do mar

A series of ecostratigraphic events across the Langhian/Serravallian boundary in an epicontinental setting: the northern Pannonian Basin

The SO-1 core situated in the NE part of the Pannonian Basin represents a parastratotype of the Badenian stage (regional Central Paratethys stage corresponding to the Langhian and Serravallian). A 150-m-thick succession was deposited between ~ 14.2 and 13.5 Ma (dated by the last common occurrence of Helicosphaera waltrans to the last occurrence of Sphenolithus heteromorphus) and spans the Langhian/Serravallian boundary. The section was subdivided into four units: (1) A transgressive carbonate–siliciclastic complex (core depth ~ 150–122 m) deposited in an inner to mid carbonate ramp setting. Carbonate deposition was episodically interrupted by the input of terrigenous phytodetritus as a result of increased precipitation. (2) Tuffaceous siltstone (core depth ~ 122–48 m) corresponding to a sea-level highstand. This unit starts with a Pteropoda immigration event (14.5–14.2 Ma), which could be related to the closing of the Indian-Mediterranean corridor that triggered a change from Indian- to Atlantic-controlled surface circulation in the Mediterranean-Paratethys basin system. An enhanced proportion of high-nutrient markers and a decreased abundance of warm-water plankton might be indicative of eutrophication and cooling of Paratethyan waters at this time. (3) Lowstand sandstone (core depth ~ 48–26 m) indicating stabilization of sea-grass meadows with eutrophic and hypoxic conditions within their root system. A sea-level fall at this time is correlated with the Ser 1 event. (4) A Serravallian transgressive event (core depth ~ 26–5 m) associated with an increase in diversity of benthic assemblages and stabilization of stenohaline assemblages.

Factors controlling frequency of turbidites in the Bengal fan during the last 248 kyr cal BP: Clues from a presently inactive channel

The seafloor of the Bay of Bengal is covered by thick sediment deposits that constitute the largest turbiditic system in the world. This system is fed primarily by the Ganges and Brahmaputra rivers, which drain the high Himalayan ranges. Sediment transfers from the delta to the deep-sea fan take place as turbidity currents in channel-levee systems. Previous studies have shown that, during high sea-level stand periods, the sediments were being mainly stored in the Ganges-Brahmaputra delta, and turbiditic transfer was occurring through active channels. Most of these channels are now inactive and sealed by hemipelagic deposits. However, the evolution of the inactive channels during the last sea-level variations has never been described in detail. Sedimentation in the currently active channel, the Active Valley, was particularly important during the last sea-level rise, which suggests a very good connection between the fluvial systems and the deep turbidite system at this time. During the MONOPOL cruise (2012), we retrieved a giant piston core (MD12-3412) near the currently inactive E4 channel. Previous studies have hypothesized that this channel is connected to the Swatch of No Ground canyon on the upper fan. The upper part of the core covers the last ~250 kyr. It reveals that, contrary to what is known about the Active Valley, the turbidite activity in E4 took place mainly during low sea-level phases (glacial stages), and stopped around 11.8 kyr cal BP. This different mode of activity suggests that (i) E4 was not abandoned but served as a secondary channel, and (ii) that the supply of turbidite material at the site of core MD12-3412 was not related to past changes in summer monsoon strength. Periods of activation of the E4 channel observed on core MD12-3412 were previously identified on the shelf area, by Hubscher and Spiess (2005), as thick Forced Regression System Tracts (FRST) after a displacement of deltaic edifices. High turbidites record on the deep basin are mainly synchronous with sea-level fall and rise conditions, but mostly during low sea-level periods. This could be explained by a residual connection between the coastal system and the E4 channel during sea level low stands.

Late Paleocene pipe swarm in the Great South – Canterbury Basin (New Zealand)

We have identified a deeply buried fluid escape pipe province in Cretaceous - late Paleocene sediments of the Great South–Canterbury Basin (NZ). The seismic observations and interpretations point to an unusually vast fossil system of pipes. These features are exceptional in number (>2000 edifices) and appear to have formed from a common root zone. The areal extent of the analysed pipe system (2500 km 2 ) is among the largest systems of fluid expulsion features ever observed in three-dimensional seismic data. The unclustered distribution of the pipes suggests no specific link to faults or buried sedimentary features and, at their maximum vertical development, the pipes are equally distributed above depocentres or structural highs. The majority of pipes terminate at two discrete levels in the late Paleocene. Based on the geometrical relationship of the pipe edifices to the overburden, and the basinal setting of the hosting units, we interpret these horizons as representing the seabed at the time of pipe formation. This interpretation allows us to date the timing of pipe formation as prominently late Paleocene. We envisage that the pipes originated during discrete episodes of fluid venting in this time interval, disrupting the typical progressive basinal compaction-driven pore fluid expulsion. The pipes are associated with biogenic gas expulsion. We discuss their triggers, mechanical processes, and global significance for understanding fluid flow processes in sedimentary basins. Creation of the initial overpressure could be related to in situ biogenic gas generation in the stratigraphic interval coincident with the root zone of the pipes. We hypothesise that the main episode of gas release and pipe formation was associated with a sea-level fall during the contemporaneous late Paleocene Carbon Isotope Maximum. This case study shows a unique example of a preserved Paleogene fluid migration system and highlights how the observation and the interpretation of massive fossil systems of fluid-escape features provides invaluable information in terms of overpressure generation and basin fluid expulsion history. • Unusually vast fossil system of fluid escape pipes discovered offshore New Zealand. • The >2000 pipe edifices formed in the late Paleocene and are at present deeply buried. • They are associated to overpressure development linked to biogenic gas generation. • Gas release likely associated with late Paleocene Carbon Isotope Maximum sea-level fall.

High-frequency sequences, paleogeography, and syn-depositional tectonism on a shallow clastic ramp: Doe Creek and Pouce Coupe members of the Late Cenomanian Kaskapau Formation, Western Canada Foreland Basin

AbstractThe great Late Cenomanian transgression of the Greenhorn Sea is recorded in western Alberta and adjacacent British Columbia by shallow-marine deposits of the lower Kaskapau Formation that blanket underlying deltaic deposits of the Dunvegan Formation. Overlying the basal ‘A-X unit’ of the Kaskapau are the allostratigraphically-defined Doe Creek and Pouce Coupe units, each of which forms a SW-thickening wedge up to about 100 m thick. The Doe Creek and Pouce Coupe units comprise, respectively, 10 and 8 allomembers, each defined by a marine flooding surface. Doe Creek facies are organized in upward-shoaling successions less than about 10 m thick. Shallow-water marginal-marine facies prevail in the west and north, with transgressive mudstones overlain by thin, sharp-based shoreface sandstones that, in places, contain roots and dinosaur tracks. In some instances, marine mudstone grades directly upward into a rooted, muddy paleosol. Mapped regionally, these facies define a broad, shallow, SE-facing embayment approximately 200 x 200 km. Trace and molluscan fossils suggest that salinity was below normal marine level, and benthic fauna were, at times, stressed by high turbidity and sedimentation rate. The central part of the embayment is dominated by offshore mudstone that encloses isolated, sharp-based lenticular bodies of clean sandstone up to 9 m thick. Sandstones are ovoid to elongate in plan view, and commonly extend many tens of km N-S or NE-SW. These sandstones are interpreted as top-truncated lowstand delta and strandplain deposits that became isolated on the outer ramp following marine transgression. The sharp base of most shoreface sandstones implies that deposition took place during relative sea-level fall. All Doe Creek allomembers thicken into the foredeep, with the tectonically-flexed surface inclined to the SW. Palaeogeographic maps show, however, that lowstand shorelines were oriented approximately NE-SW, and that the depositional surface sloped to the SE. This geometric relationship implies that, despite tectonic subsidence increasing to the SW, the rate of sediment supply and efficiency of redistribution were able to maintain a wave-graded surface sloping to the SE. Because shorelines migrated across, rather than up and down tectonic dip, allomembers are interpreted to have been controlled primarily by high-rate, high-frequency eustatic cycles, rather than by tectonic movements. The Pouce Coupe unit represents a complete reversal of basin paleogeography relative to the Doe Creek. Pouce Coupe shoreface sandstones are stacked in a linear belt just west of 120°W and grade westward into offshore muddy facies that thicken to the WSW. The entire unit is truncated eastward by the K1 unconformity and is absent over most of Alberta. It is postulated that the Proterozoic Kiskatinaw domain acted as a crustal weak zone, forming a hinge that defined the eastern margin of the Pouce Coupe flexural depocentre.

Evolution of a sedimentary infill of a palaeovalley at a distal margin of the peripheral foreland basin

The entrenched Odra palaeovalley, cut into the bedrock of the distal margin (forebulge basal unconformity) of the Moravian Carpathian Foredeep (peripheral foreland basin) is filled with an almost 300 m thick pile of Miocene deposits. The directon of the valley (NW–SE to NNW–SSE) has been controlled by faults subparallel with the system of “sudetic faults“. The sedimentary succession consists of 5 facies associations/depositional environments, which are interpreted (from bottom to top, i.e. from the oldest to the youngest) as: 1 – colluvial deposits to deposits of alluvial fan, 2 – deposits of alluvial fan, 3 – fan-delta deposits, 4 – shallow water delta to nearshore deposits and 5 – open marine deposits. This fining-up and deepening-up succession reveals the following: the formation of the new flexural shape of the basin; deep erosion connected with uplift and tilting of the forebulge and reactivation of the NW–SE trending basement faults; the Early/Middle Miocene sea level fall; alluvial deposition mostly driven by tectonics and morphology; forebulge flexural retreat; Middle-Miocene sea level rise; back-stepping of valley infill; marine invasion during the Early Badenian with shift of the coastline further landward of the pallaeovalley. Tectonics related to contemporary thrusting processes in the Western Carpathians are assumed to be the dominating factor of the studied deposition at the expense of eustatic sea level changes. Provenance studies have proven that the pre-Neogene basement (i.e. the Early Carboniferous clastic “Culmian facies” of the Moravian-Silesian Paleozoic) represents an important source for the conglomerates and sands, which volumetrically dominate in the palaeovalley infill. However, they also showed, that the deposits of the earlier Carpathian Foredeep Basin sedimentary stage (Karpatian in age?) covered the area under study and were eroded and resedimented into the palaeovalley infill.

Holocene sedimentary facies in coastal plain of the Song Ma Delta, Thanh Hoa Province

The Ma river delta located in the North Central of VietNam is the third largest ones in Vietnam following the Mekong Delta and the Red River Delta. However, there are not many researches on sedimentary. This study aims to determine the Characteristics of the Holocene sedimentary facies in coastal plain of the Song Ma Delta based on observation of sedimentary structure and analysis of grain size, diatom, flora, pollen and 14C dating collected for the drill core (LKTH2 borehole) from the coastal plain of the Song Ma river delta. Research results identify 08 facies consisting 02 facies of transgression period and 06 facies of regression period. During transgression period, 02 facies of tidal flat clayish silt and bay silty clay facies were formed. Of which, tidal flat clayish silt face was formed in 7,883 ± 43 14C yr BP. After the maximum flooding of the sea, the delta was initiated and 06 facies were formed during the regression period i.e. prodelta silty clay, delta front clayish silt, tidal flat sandy - silty clay, sand ridge, lagoonal sandy silty clay, and flood plain silty clay facies. The tidal flat sandy - silty clay facies occurred in 6.951± 38 14Cyr BP caused by sea level fall and the coastline was identified around the LKTH2 bore hole at that time. Integration of sedimentary facies of the sand ridge and lagoonal sandy silty clay facies shows that the Song Ma Delta was dominated by wave process during its evolution.

Development of the western Limfjord, Denmark, after the last deglaciation: a review with new data

This paper presents new marine evidence of Lateglacial and Holocene environmental changes in the western part of Limfjorden, and provides a review of the geological history/development of this part of northern Jylland, Denmark. Lateglacial clay without fossils is widespread in the region and is probably a glaciolacustrine deposit. Limfjorden began to form as a strait in the Early Holocene due to rising relative sea level and the oldest marine shells are dated to c. 9300 cal. years BP. We propose a new relative sealevel curve for the region based on new and published data, which appear to confirm that the relative sea-level change was not extremely rapid, which was suggested earlier. During the Mid-Holocene a wide connection existed from the western part of Limfjorden to the North Sea in the west and more narrow connections existed between Limfjorden and Skagerrak in the north. The marine fauna included several species that indicate warmer and more salty waters than at present. Gradually, the connections to the North Sea and Skagerrak closed due to long-shore sediment transport and deposition of aeolian sand combined with a fall in the relative sea level during the Middle- to Late Holocene. During the Viking Age, 800–1050 CE (Common Era), the western connection to the North Sea was still open, but around 1200 CE it was closed by a coastal sandy barrier and the western part of Limfjorden became brackish. The coastal barrier was flooded on several occasions but soon formed again. After 1825 CE the western connection from Limfjorden to the North Sea has been maintained artificially.