Relative Sea-level Lowstand Research Articles

Architecture and sedimentary evolution of the Ladinian Kobilji curek basin (External Dinarides, central Slovenia)

The study area is located in cental Slovenia, and geologically located at the junction between the Alps and the Dinarides. The Middle Triassic of this region is characterised by intense rifting manifested by differential subsidence and volcanism. This led to a major paleogeographic reorganisation of the region, where three paleogeographic domains formed in the Upper Triassic: The Julian Carbonate Platform in the north, the intermediate Slovenian Basin, both parts of the Southern Alps, and the Dinaric (Adriatic, Friuli) Carbonate Platform in the south, which today is a part of the External Dinarides that host the area of investigation. Prior to the installation of the Dinaric Carbonate Platfrom, i.e. in the Ladinian, the entire area of the preset-day External Dinarides broke up into numerous tectonic blocks that were exposed to either erosion or continental, shallow-marine, and deep-marine sedimentation. In this study, we analyse at small scale a complex transitional area between a local carbonate platform and the Kobilji curek basin (depositional area dominated by deeper marine sediments), located in the Rute Plateau in central Slovenia south of Ljubljana. During enhanced subsidence, the basin was filled with volcanic material (tuffs and volcanogenic clays and subordinate extrusive material), while the adjacent platform aggraded. The slope was positioned above active paleofaults. During relative sea level lowstand, the platform prograded across the basin. The study area is divided into four major tectonic paleoblocks. The NW paleoblock experienced the most enhanced subsidence, and the platform prograded twice in this area and was submerged again by the rejuvenated subsidence and/or sea-level rise. The second and third paleoblocks subsided only during discrete major subsidence events, and the carbonates of the platform and slope were soon reinstated therein. In the fourth paleoblock to the east the platform persisted during the Ladinian. In the Carnian, the entire study area became emerged, and continental clastics were deposited. These were then replaced by a uniform shallow marine/intertidal Hauptdolomit (Dolomia Principale) formation at the onset of the Norian. This study provides the first detailed reconstruction of the sedimentary evolution of small-scale Ladinian basin and platforms system in the northern External Dinarides.

Recycling effects in detrital zircon U[sbnd]Pb signatures in a foreland basin: Identifying the multicyclic sediment sources of the Eocene-Miocene Jaca basin (southern Pyrenees, Spain)

This work deals with the provenance evolution of the mid Eocene to early Miocene clastic deposits of the Jaca basin (southern Pyrenees), which record a main drainage network reorganization during the progressive tectonic uplift of the Pyrenean orogen. We present new detrital zircon UPb data which show a Variscan-dominated detrital zircon signature for the Lutetian Hecho Group turbidites, while the overlying late Eocene to early Miocene alluvial fan sediments record a Cadomian-dominated detrital zircon signature, allowing to document a major change in the source area. Moreover, a tectono-stratigraphic cyclicity can be identified in each of the turbidite systems that conform the upper Hecho Group, featuring an enrichment of the Variscan detrital zircon signal at the base of the system, which can be interpreted as the result of an efficient sediment transfer from distant Axial Zone sources during relative sea-level lowstands, while a dilution of this signal occurs during highstands, at the top of each system, due to the reduced erosion capability of the fluvial network. Integration of detrital zircon UPb data with sandstone petrography allows for improved resolution of the source-area shifts, for discrimination between first-cycle and multi-cycle zircons, and to identify that the detrital zircon signal of the alluvial fans does not fully represent the source area configuration. This study illustrates the importance of combining petrography with detrital zircon UPb data to improve provenance reconstructions and to identify the role of sediment recycling during progressive foreland evolution.

Leveraging Sequence Stratigraphy to Accelerate Site Remediation: Pliocene Citronelle Formation, Eglin Air Force Base, Florida, USA

AbstractAt Eglin Air Force Base (AFB) in the Florida Panhandle, a groundwater extraction and treatment system was installed to contain and remediate a chlorinated solvent plume. After 2 years of operation, the system was not removing the contaminant mass at the rate predicted or required to meet performance‐based contract terms. As a result, a sequence‐stratigraphic analysis was initiated to develop a strategy to improve performance. Sequence Stratigraphy methods were employed to identify a marine flooding surface (mfs) formed during a relative sea level highstand. The analysis also found that the mfs was locally eroded away, indicating that incised valleys were eroded into the formation during a relative lowstand of sea level. These valleys were backfilled with coarse‐grained fluvial and estuarine strata. The analysis concluded that the groundwater extraction system lacked an extraction well screened within the coarse‐grained valley fill. An additional extraction well was installed, which targeted the incised valley fill and resulted in a significant increase in contaminant mass removal rate without increasing system capacity or operational costs. This case study suggests that efficiency improvements are tenable at many sites where groundwater remediation is occurring within the Surficial Aquifer System of the Gulf Coast (Citronelle Formation) as well as sites in similar geologic settings worldwide.

The impact of tectonic activity on carbonate density-flow deposition along two sectors of the Apulia Carbonate Platform

ABSTRACTGeneration of thick megabreccias along carbonate platforms apparently relies on the establishment of overpressured zones in the margin and upper slope deposits (particularly during relative sea-level lowstands), but the main triggering mechanism is thought to be seismic tremors. Here, we present a detailed sedimentological analysis of carbonate density-flow deposits south and north of the Mattinata Fault, a major strike-slip fault zone in the Gargano Promontory (Italy). The analysis shows that in the southern sector the deposits of Albian–Cenomanian age (Monte S. Angelo Formation) are made up predominantly of thick and amalgamated debrites (megabreccias), whereas some 25 km to the north they are composed predominantly of prograding high-density turbidites. Moreover, detailed analysis of Maastrichtian slope deposits (Monte Acuto Limestones) from the southern sector shows that they make up a N–S-prograding system of coalesced lobes composed of high-density turbidites and subordinate debrites. We infer that preconditional factors (e.g., platform progradation, tectonic oversteepening, and high pore pressures in the margin and upper-slope deposits) for triggering the density flows varied along strike of the platform, but the main controlling factors were the activity of the prominent strike-slip Mattinata Fault and the propagation of tremor energy to its near- and far-field regions: large earthquakes triggered thick (up to 40 m) and amalgamated Albian–Cenomanian debrites in proximity (south of) to the fault, whereas the subdued effect of the tremors triggered thinner debrites (5–10 m) and rock falls in the far-field region north of the fault. Moreover, predominantly high-density turbidite deposits were emplaced in the far-field region during lower-magnitude earthquakes. During the Maastrichtian the thick succession of high-density turbidites and scattered thick debrites south of and adjacent to the Mattinata Fault may record an overall quiescent period of the fault. The results are of relevance for understanding the spatiotemporal distribution of density-flow deposition along carbonate platforms in tectonically active regions—in particular with respect to the activity of large strike-slip faults.

Fluvial shoal water deltas: pre‐vegetation sedimentation through the fluvial–marine transition, Lower Cambrian, English Channel region

AbstractLower Palaeozoic fluvial systems tend to be more sand‐prone than those of later eras and the nature of coastal environments less certain. Field studies are presented that characterize the fluvial to marine transition over a distance of 80 km, in the Lower Cambrian of the Cotentin Peninsula, northern France. The sedimentary rocks are divided into six facies associations which represent deposition in proximal fluvial, distal fluvial, delta plain, delta front, pro‐delta and offshore carbonate bank environments. The basin fill is sandstone‐dominated and subdivided into three stratigraphic intervals. A 200 to 300 m thick basal interval contains very coarse‐grained fluvial sandstones deposited during a relative sea level lowstand. An overlying interval, 250 to 1500 m thick, is a facies mosaic. Fluvial strata in the north‐west pass laterally south‐east into deltaic and shallow marine pro‐delta sediments. The delta front deposits show repetitively stacked, upward‐coarsening parasequences, 8 to 10 m thick, which reflect the repeated progradation of lobate, fluvially‐dominated deltas onto a shallow marine shelf. The deltas formed following marine transgression and accumulated during a period of gradually rising relative sea level. An upper unit, 130 m thick, containing offshore stromatolitic and oolitic limestones, caps the study interval and represents deposition during a relative sea level highstand. The fluvial and delta distributary channel sandstones of the middle unit contain <1% mudstone. The cohesionless substrate determined that deltaic distributaries were predominantly braided in character and subject to common bifurcations which resulted in an ordered diminution of channel size and competence in a seaward direction. Terminal distributary channels show evidence of migratory levées and mouth‐bars and consistently delivered fine to medium‐grained sand to the delta front. The study highlights an example of pre‐vegetation deltaic sedimentation that was hydraulically organized and predictable, despite being fed by braided fluvial systems with high levels of peak discharge.

Postglacial relative sea-level histories along the eastern Canadian coastline

We have assembled a database of Relative Sea Level (RSL) data points from the eastern coast of Canada from Hudson Bay to the border with the USA. In compiling this database we have critically reviewed 1092 radiocarbon dated samples from raised beaches, isolation basins, intertidal and marine deposits, and archaeological indicators to produce 405 sea-level index points and 687 sea-level limiting points. Our comprehensive, systematic, and quality-controlled RSL database allowed for the reconstruction of the postglacial evolution of 34 regions of eastern Canada providing new basin-scale insights into the processes driving RSL changes in the last ∼16 ka. The combination of a database of sea-level index points with an innovative empirical-Bayesian spatio-temporal statistical model provided new insights into rates and magnitude of the spatially-variable glacial isostatic adjustment (GIA), which dominated the postglacial RSL evolution in this sector of North America. A continuous postglacial RSL fall is observed at latitudes ≥ ∼50° N with higher rates (up to 35 mm a−1) recorded in southeastern Hudson Bay. At lower latitudes, the evolution is non-monotonic with RSL that dropped to a spatially variable early-Holocene lowstand, followed by a mid-Holocene highstand and, eventually, a gradual drop to present RSL. This pattern is particularly evident in the St Lawrence corridor. Along the majority of the Newfoundland, New Brunswick and western Nova Scotia coasts, a late-Pleistocene/early-Holocene RSL lowstand was followed by a continuous rise through the Holocene. At the margin of the former ice-sheet (i.e. eastern Nova Scotia), our data identify a continuous RSL rise through the Holocene. These records are characterized by decreasing rates of RSL rise through time, commencing with a rapid rise during the early Holocene (up to ∼17 mm a−1), a slowdown in the mid-Holocene (average rates ≤ ∼9 mm a−1), and a further reduction in the late Holocene (average rates < 2 mm a−1). Finally, our database allowed the identification of regions, including the Labrador coast and part of the St Lawrence corridor, where further investigations are required to better constrain the RSL evolution and improve our ability to assess the variability of RSL histories.

Reconstruction of repeated Quaternary slope failures in the northern South China Sea

A series of Quaternary mass transport deposits (MTDs) with a total areal extent of ~11,000 km2 and a conservative volume estimate of ~1035 km3 are observed in the Pearl River Mouth Basin (PRMB), northern South China Sea (SCS), based on integrated analysis of extensive 2-D/3-D seismic data, multibeam bathymetric data and ODP/IODP wells. Unlike other large MTDs worldwide, the MTDs in this study are not directly sourced from slopes built on major deltaic depositional systems. Moreover, they are remarkable in that the whole PRMB is a giant debris flow corridor that has transected a broad subsided continental margin. The MTDs in this study have a large evacuation zone on the slope and a depositional zone in the ocean basin connected by a narrow pathway. Six events of MTDs (MTDa - MTDd on the slope and MTDo1 - MTDo2 in the ocean basin) have been identified as being separated by basal erosional surfaces defined by strong negative seismic reflections. Similar ages and volumes (depleted volume of MTDb on the slope and the depositional volume of MTDo2) suggest that MTDo1 and MTDo2 correlate with MTDa and MTDb, respectively. The major factor controlling the occurrence of slope failures is argued to be the presence of a key reflection package. This package is interpreted to consist of gas-charged, coarse-grained sediments deposited during a relative lowstand of sea level, overlain by a unit of fine-grained sediments. Other contributing factors that influenced the morphology and position of the MTDs include the presence of free gas, normal faulting in the slope sediments, and seafloor relief related to igneous activity. From MTDd to MTDb, slope failures are shown to have retrogressed upslope with a decreasing time interval between events. The events, dimensions, susceptible factors and formation processes of slope failures in the study area are demonstrated for the first time in the petroliferous basin of northern SCS. Their recognition is important for geo-hazard assessment (e.g. tsunami) in this area. Moreover, this study highlights that the lithostratigraphy of the slope succession, especially the coarse-grained sediments in the deepwater area, can be an important component for the occurrence of slope failures, and may be a key factor in slope stability on other continental margins too.

Tidal sedimentation preserved in volcaniclastic deposits filling a peripheral seaway embayment (early Miocene, Sardinian Graben)

The Sardinian Graben System was a part of a NE-SW-oriented extensional basin, rotated counter-clockwise into a N-S-elongate basin, as consequence of the eastward migration of the Apennine orogenic front, in the western Mediterranean during the Neogene.Starting from the early Miocene, the Sardinian Graben was inundated by marine waters, turning progressively into a seaway, characterized by a tidal circulation as consequence of the connection between the Atlantic Ocean to the west and the Paratethys Ocean to the east.In this work, we investigate an area located marginally to the mid-seaway, whose well-exposed volcaniclastic deposits record the local expression of a tidal amplification occurring in a coastal peripheral embayment of the wider Sardinian Seaway.The studied succession is ca. 140 m thick and includes three main units: (i) the 20-m-thick lowermost unit consists of fluvio-lacustrine sandstones and conglomerates belonging to lower delta-plain and delta-platform environments; (ii) the second unit is 60–70 m thick and includes heterolithic sandstones and mudstones, exhibiting a variety of tidal sedimentary structures, and lies on the previous deposits through a tidal ravinement surface; these two units are mostly volcaniclastic in composition, reflecting the dominance of a magmatic source over other extrabasinal components; (iii) the uppermost unit is ca. 50 m thick, erosionally overlies the previous deposits and is made up of shoreface sandstones and open-shelf mudstones, whose composition indicates even less volcaniclastic elements and the prevalence of other clastic alongshore-derived components.Based on the results of the facies analysis, the study succession is interpreted as the infill of an incised valley along the southern flank of a structural high. The valley was excavated during a phase of relative sea-level lowstand (Aquitanian?) preceding a subsequent stage of major transgression (Burdigalian). Initially, a fluvial system impinged the valley from the west favoring the progradation of a deltaic system in a shallow-marine embayment. During an early stage of transgression, the isolation of a part of this coastal area generated by the building of a barrier island, produced the onset of a tidal-flat sedimentation over the previous deposits. A late transgression occurred through the inundation of this coastal area by marine waters and the consequent back-stepping of beach-barrier and open-shelf strata.The sedimentological features of this stratigraphic succession indicate as this valley was filled in a tectonic setting with a high rate of accommodation, where the tidal influence progressively increased during sediment accumulation, possibly due to the marginal position respect to a wider tide-dominated marine conduit.The present paper thus: (i) documents for the first time a tidal signature in the lower Miocene strata of Sardinia; (ii) indicates new possible relationships with other, coeval seaway successions of the western and northern Mediterranean area; (iii) suggests constrains for palaeogeographic reconstructions; (iv) and throws the basis for future researches on the Sardinian Seaway.

Coralgal buildups associated with the Bolca Fossil-Lagerstätten: new evidence from the Ypresian of Monte Postale (NE Italy)

The Monte Postale and Pesciara di Bolca sites are well-known Eocene Fossil-Lagerstatten. The origin of these deposits has been previously related to a lagoon environment, protected from the open sea by a barrier formed by some kind of reef. However, no detailed description of a bioconstructed system has ever been reported in the area. This study provides the first detailed characterization of the upper Ypresian Monte Postale reef, made of in situ corals, calcareous algae, and encrusting foraminifera. Three main facies associations have been recognized: (1) a coralgal rim, roughly E/W oriented, (2) lagoon deposits in the south and (3) fore-reef sediments towards the north. The coralgal rim is represented by a wave-resistant framework made of heavily calcified organisms associated to Alveolina accumulations; the adaptation of most of these organisms to the euphotic zone promoted their growth in proximity to the sea level, allowing this structure to act as an effective barrier. In particular, during relative sea-level lowstands or highstands, the coralgal rim could have limited the water exchange between the lagoon and the open sea, reducing the oxygenation in the lagoon and allowing the formation of the Fossil-Lagerstatten. During relative sea-level transgressions, the water could have risen well above the margin, establishing normal oxygenation and hampering the fossils’ preservation. The Monte Postale succession represents one of the few examples of lower Eocene bioconstructions. The description of this biotic assemblage provides new data for the characterization of reef communities during a crucial climatic phase, near the end of the early Eocene climatic optimum global warming.

Palaeovalleys in foreland ramp settings: what happens as accommodation decreases down dip?

AbstractRecent advances in our understanding of palaeovalleys are largely guided by examples from passive margins, in which accommodation increases down depositional dip. This study tests these models against a dataset from the Pennsylvanian Breathitt Group of the central Appalachian foreland basin, USA. This fluvio‐deltaic succession contains extensive erosionally based fluvio‐estuarine sand bodies that can be tracked over 80 km down depositional dip from a proximal zone of high accommodation close to the orogenic margin to a distal, lower accommodation zone close to the cratonic margin of the basin. The sand bodies are up to 25 m thick, multi‐storey and characterized in their lower parts by strongly amalgamated storeys containing sandy fluvial to estuarine bar accretion elements, and in their middle to upper parts by more fully preserved storeys up to 10 m thick and laterally extensive over 100s of metres. The upper storeys include abandonment channel‐fills of heterolithic marine or marginal marine deposits or muddy to sandy point‐bar elements. Three major regional‐scale architectures include: (i) Tabular sand bodies that everywhere incise open marine prodelta and mouth bar facies and are interpreted as palaeovalleys formed during falling stage and lowstand systems tracts, when eustatic sea‐level fall outpaced tectonic subsidence across the entire study area. (ii) Sand bodies that incise genetically related floodplain lake and/or bay‐fill minor mouth bar deposits up depositional dip and open marine prodelta and mouth bar facies down dip. These stacked distributary channel deposits map down dip into palaeovalleys and formed when up dip subsidence rate resulted in positive, but reduced rate of accommodation creation, while lower tectonic subsidence rate down‐dip resulted in incision. (iii) Sand bodies that incise genetically related floodplain, lake and/or bay‐fill minor mouth bars up dip and pass down‐dip into genetically related unconfined floodplain, prodelta and mouth bar deposits. These sand bodies represent stacked distributary channel fills and channel amalgamation was the product of high rates of lateral migration, typical of the behaviour of channels above their backwater reach. Case (2) sand bodies demonstrate that in rapidly subsiding foreland basins, cross‐shelf palaeovalleys may form down depositional dip from aggradational, distributive fluival strata. Additionally, the genetic relationship between stacked distributary channels and palaeovalleys supports recent models for palaeovalley formation that emphasize diachronous, cut‐and‐fill during falling stage and lowstands of relative sea level.

Hydrochemical reactions and origin of offshore relatively fresh pore water from core samples in Hong Kong

Summary The existence of relatively fresh pore water offshore has been well recognised over the globe but studies on the chemistry of the pore water from offshore geological formations are extremely limited. This study aims to characterize the hydrochemistry of the submarine groundwater body in Hong Kong. It looks into the major ion concentrations and the stable isotopic compositions of pore water extracted from core samples from an offshore 42.30-m vibrocore in the southwestern Hong Kong waters. A minimum Cl − level of about one-third of that in typical seawater was noted in the terrestrial sediments, suggesting the presence of offshore relatively fresh water. Unexpectedly high NH 4 + levels are attributed to organic matter decomposition in the terrestrial sediments. The leaching of shells due to exposure of marine sediments at sea-level low stands raises the Mg 2+ and Ca 2+ concentrations. Base Exchange Indices show weak cation exchange reactions in which Na + and K + are released while Mg 2+ and Ca 2+ are adsorbed. Isotopic compositions of pore water reveal that the low-salinity water is probably the relic water sequestered in fluvial systems during relative sea-level low stands. Cores properly stored in a freezer for a long time has been used to study the pore water chemistry. For the first time, this study introduces an approach to correct the measured data by considering the possible evaporation effect during the transportation and storage of the samples. Corrections for evaporation were applied to the major ion concentrations and the stable isotopic compositions of pore water measured. It is found that the corrections determined by the Cl − mass balance approach are more reliable. The corrected measurements give more reasonable observations and hence allow sensible conclusions on the hydrochemical reactions and the origin of pore water.

Paraglacial landscapes in St George's Bay, Newfoundland, Canada

The Late Wisconsinan submarine moraine in St George's Bay, Newfoundland, was exposed to erosional processes during and after an early Holocene −25 m lowstand of relative sea level. Primary morphology has been largely removed, resulting in a gently sloping platform veneered with mobile sand and gravel, terminating on the landward side in progradational ‘spillover’ lobes. Erosion of coastal Quaternary deposits has produced systems of gravel-dominated spits and barriers constructed upon subaqueous platforms that have buried glacimarine deposits and a lowstand delta. Paraglacial processes have therefore created a complex submarine terrain with few parallels in Atlantic Canada. A gently sloping platform-like shoal (average depth 20 m) extends across inner St George's Bay (Fig. 1a), enclosing several basins to the east (maximum depth 100 m). The platform surface comprises sand waves, areas of gravel ripples and patches of immobile, bouldery gravel with relief of 2–3 m (e.g. A in Fig. 1a). This mosaic of terrains is reflected in a backscatter image (Fig. 1b). As a result of wave action, sand and gravel have migrated to the NE, spilling into the adjacent basins to form lobate, prograding sediment wedges (e.g. B in …

Spongy-like porosity in peritidal carbonates: An interaction of cyclic sea-level oscillations, fresh water supply and sediment texture

This paper focuses upon the analysis of a complex paleokarstic system recorded within uppermost Triassic peritidal cycles in northwestern Sicily. Besides documenting spectacular karstification at the Triassic/Jurassic boundary, it provides an example of stratabound ‘spongy’ or ‘swiss-cheese’ dissolution. On the base of field observations, microfacies analysis, transmitted-light and cathodoluminescence petrography and stable-isotope analyses we put forward an original model for the formation of this peculiar stratabound dissolution. It implies a complex interaction of several controlling factors at the interface between the marine and meteoric diagenetic realms during the relative cyclic oscillations of sea-level.The presence of a fresh water supply from an adjacent emerged area is the key for the periodic formation of a mixing water lens during the relative sea level lowstand that brought about the subaerial exposure of the platform. The resulting dissolution pattern in the subtidal unit of a specific cycle is strongly controlled by the textural features of the sediments. In the case of bioturbated wackestones the ‘spongy’ or ‘swiss-cheese’ pattern develops, while in mollusk-rich beds biomoldic porosity occurs. In well-sorted subtidal members, such as algal grainstones, the dissolution originates as randomly distributed vuggy porosity. During periodic flooding of the platform, a new subtidal unit is formed and the dissolution stops as fully marine phreatic conditions are re-established.

Sedimentological and isotopic heterogeneities within a Jurassic carbonate ramp (UAE) and implications for reservoirs in the Middle East

Carbonate rocks are major hydrocarbon reservoirs in the Middle East and across the world. Capturing the spatial distribution and dimensions of carbonate sedimentary facies is thus of interest for reservoir modelling and for the understanding of process sedimentology. Here we present data from a Middle Jurassic carbonate ramp outcropping on the Musandam Peninsula (Wadi Naqab, Northern United Arab Emirate of Ras-Al-Khaima), which serves as an analogue for the hydrocarbon bearing units in the subsurface of the Middle East with respect to stratigraphic age, lithofacies types, vertical stacking patterns and palaeogeographical setting. Seven closely spaced sections were logged across a 1900 × 1200 × 120 m fault-bounded outcrop offering a pseudo-3D view of the stratigraphy. Sections were correlated bed-by-bed by walking key surfaces and/or by using photopanels, to produce a detailed lithofacies map. Stable isotopes (carbon and oxygen) were measured from micrite samples in order to provide chemostratigraphic constraints. Ten lithofacies types were identified in thin sections and hand specimens, ranging from marls and mudstones to grainy oncolites and peloidal-ooidal grainstones. Beds are organised in a ‘layer-cake’ architecture, but individual bed thicknesses vary laterally. Lateral merging between different lithofacies types within beds is very common, resulting in a complex facies mosaic at the scale of the outcrop. Lithofacies types have a maximum probability of 40% of being continuous between logged sections with the highest frequencies of lateral variations occurring in the open, shallow part of the ramp, where maximum wave and current energy are indicated by facies. The vertical stratigraphic stacking pattern of the succession is characterised by metre-scale shoaling upwards parasequences. These are frequently capped by hardgrounds, interpreted as composite surfaces with depleted carbon and oxygen isotope values being evidence for frequent exposure during relative sea-level lowstand, followed by flooding and formation of a condensed hardground. The carbon isotope curves show a large amount of vertical ‘isotope heterogeneity’ reflected by meteoric diagenesis around hardground surfaces, and these are not always traceable laterally between the closely spaced neritic sections due to bias in sampling. However, if considering only the positive envelop of the carbon isotope curve and thus filtering out the meteoric diagenetic events, carbon isotope chemostratigraphy becomes possible and confirms a Bajocian age for the section. The main conclusion of the study is that maximum heterogeneity of facies at the scale of 100 m or less is expected in an environment of deposition with high wave energy. This has important implications for reservoir applications since rock facies often template flow units.

Oceanic response to Pliensbachian and Toarcian magmatic events: Implications from an organic-rich basinal succession in the NW Tethys

The Bächental bituminous marls (Bächentaler Bitumenmergel) belonging to the Sachrang Member of the Lower Jurassic Middle Allgäu Formation were investigated using a multidisciplinary approach to determine environmental controls on the formation of organic-rich deposits in a semi-restricted basin of the NW Tethys during the Early Jurassic. The marls are subdivided into three units on the basis of mineralogical composition, source-rock parameters, redox conditions, salinity variations, and diagenetic processes. Redox proxies (e.g., pristane/phytane ratio; aryl isoprenoids; bioturbation; ternary plot of iron, total organic carbon, and sulphur) indicate varying suboxic to euxinic conditions during deposition of the Bächental section. Redox variations were mainly controlled by sea-level fluctuations with the tectonically complex bathymetry of the Bächental basin determining watermass exchange with the Tethys Ocean. Accordingly, strongest anoxia and highest total organic carbon content (up to 13%) occur in the middle part of the profile (upper tenuicostatum and lower falciferum zones), coincident with an increase in surface-water productivity during a period of relative sea-level lowstand that induced salinity stratification in a stagnant basin setting. This level corresponds to the time interval of the lower Toarcian oceanic anoxic event (T-OAE). However, the absence of the widely observed lower Toarcian negative carbon isotope excursion in the study section questions its unrestricted use as a global chemostratigraphic marker. Stratigraphic correlation of the thermally immature Bächental bituminous marls with the Posidonia Shale of SW Germany on the basis of C27/C29 sterane ratio profiles and ammonite data suggests that deposition of organic matter-rich sediments in isolated basins in the Alpine realm commenced earlier (late Pliensbachian margaritatus Zone) than in regionally proximal epicontinental seas (early Toarcian tenuicostatum Zone). The late Pliensbachian onset of reducing conditions in the Bächental basin coincided with an influx of volcaniclastic detritus that was possibly connected to complex rifting processes of the Alpine Tethys and with a globally observed eruption-induced extinction event. The level of maximum organic matter accumulation in the Bächental basin corresponds to the main eruptive phase of the Karoo-Ferrar large igneous province (LIP), confirming its massive impact on global climate and oceanic conditions during the Early Jurassic. The Bächental marl succession is thus a record of the complex interaction of global (i.e., LIP) and local (e.g., redox and salinity variations, basin morphology) factors that caused reducing conditions and organic matter enrichment in the Bächental basin. These developments resulted in highly inhomogeneous environmental conditions in semi-restricted basins of the NW Tethyan domain during late Pliensbachian and early Toarcian time.

Sea level and climate forcing of the Sr isotope composition of lateMioceneMediterranean marine basins

AbstractSr isotope records from marginal marine basins track the mixing between seawater and local continental runoff, potentially recording the effects of sea level, tectonic, and climate forcing in marine fossils and sediments. Our 110 new87Sr/86Sr analyses on oyster and foraminifera samples from six late Miocene stratigraphic sections in southern Turkey, Crete, and Sicily show that87Sr/86Sr fell below global seawater values in the basins several million years before the Messinian Salinity Crisis, coinciding with tectonic uplift and basin shallowing.87Sr/86Sr from more centrally located basins (away from the Mediterranean coast) drop below global seawater values only during the Messinian Salinity Crisis. In addition to this general trend, 55 new87Sr/86Sr analyses from the astronomically tuned Lower Evaporites in the central Apennines (Italy) allow us to explore the effect of glacio‐eustatic sea level and precipitation changes on87Sr/86Sr. Most variation in our data can be explained by changes in sea level, with greatest negative excursions from global seawater values occurring during relative sea level lowstands, which generally coincided with arid conditions in the Mediterranean realm. We suggest that this greater sensitivity to lowered sea level compared with higher runoff could relate to the inverse relationship between Sr concentration and river discharge. Variations in the residence time of groundwater within the karst terrain of the circum‐Mediterranean region during arid and wet phases may help to explain the single (robust) occurrence of a negative excursion during a sea level highstand, but this explanation remains speculative without more detailed paleoclimatic data for the region.

Late Pleistocene cyclicity of sedimentation and spreading-center structure in the Central Gulf of California

Abstract The interaction between climatic-driven fluctuations in sediment supply and dynamic changes in basin physiography is a fundamental process of rift basin evolution that is poorly understood. A high-resolution seismic profile collected across the southern Guaymas Basin spreading center reveals how cyclical changes in sedimentation interact with on-going axial rifting and accretion of oceanic crust. The 4 km-wide axial rift valley abuts the steep, canyon incised continental slope of Baja California. Alternating acoustically transparent and well-layered high-amplitude seismic units imaged on the accreted flanks record the filling and regeneration of the axial rift over the past ~ 200 ka. An approximate correlation of seismic units to nearby Deep Sea Drilling Program (DSDP) Site 478 indicates that transparent units are composed primarily of diatomaceous silty mud turbidites, whereas the intervening well-layered high-amplitude units represent mud turbidites interlayed with terrigenous silty sand. Age estimates using sparse biostratigraphy and plate stratigraphy suggest deposition of transparent units coincident with interglacial/high-stands, while deposition of well-layered high-amplitude units corresponds to glacial/low-stands. Analysis of seismic stratigraphy and imaged faults reveals that during glacial/low-stands, deposition of terrigenous-rich turbidites out-paced axial spreading and subsidence, filling the axial rift and spilling over onto the adjacent basin floor. However, during interglacial/high-stands, reduction in sediment supply resulted in reestablishment of the axial rift relief. We propose that increases in sediment supply and terrigenous material during glacial periods were caused by a combination of (i) enhanced erosion of Baja California during pluvial periods, corresponding to northern hemisphere glacial maxima, (ii) increased delivery of sediment from the Sierra Madre Occidental, and (iii) sediment bypass on the continental shelf and slope to the deep-water basins during relative sea-level low-stands.

Using relative sea-level data to constrain the deglacial and Holocene history of southern Greenland

This paper presents new Holocene relative sea-level (RSL) data collected from isolation basins close to the town of Paamiut in south west Greenland. The data shows a rapid fall from a marine limit of c. 52 m asl at c. 10.9 cal. ka BP to close to present by c. 9.5 cal. ka BP, at rates of up to c. 32 mm/yr, falling below present for the majority of the Holocene before rising to present in the last 2000 years. The elevation of the RSL lowstand is not well constrained, but was at least below −3 m. This pattern of rapid RSL fall during the early Holocene matches the pattern seen at other southern Greenland locations suggesting rapid, largely simultaneous ice retreat from the area surrounding the Qassimiut Lobe at the start of the Holocene, occurring c. 2000 years after the initial deglaciation of the extreme southern tip of Greenland. The RSL histories from this and other southern Greenland locations are distinct to those recorded further north along the west coast, and are in broad agreement with a pattern of vertical land motion and RSL predicted by the Huy2 model (Simpson et al., 2009), which predicts an 80 m drop in the contribution of vertical land motion to RSL at 10 cal. ka BP between Sisimiut and Paamiut on the west coast. Despite this broad-scale spatial agreement between the RSL data and the Huy2 model, it fails to satisfactorily predict the Holocene RSL histories at Paamiut and other southern Greenland locations. Sensitivity tests indicate that the data-model misfits are most likely due to an over-estimate of the forcing during the Holocene Thermal Maximum (or the response to this forcing) in southern Greenland and error in the North American ice sheets component of the background deglaciation model. Our new data suggests that much of the southern part of the ice sheet acted differently to the area further north. However RSL changes at Paamiut are also largely impacted by regional and larger-scale processes including a bulls-eye of uplift centred on the west, the impact of the Holocene Thermal Maximum and the influence of the collapse of the North American ice sheets.

Stratigraphic architecture of back-filled incised-valley systems: Pennsylvanian–Permian lower Cutler beds, Utah, USA

The Pennsylvanian to Permian lower Cutler beds collectively form the lowermost stratigraphic unit of the Cutler Group in the Paradox Basin, southeast Utah. The lower Cutler beds represent a tripartite succession comprising lithofacies assemblages of aeolian, fluvial and shallow-marine origin, in near equal proportion. The succession results from a series of transgressive–regressive cycles, driven by repeated episodes of climatic variation and linked changes in relative sea-level. Relative sea-level changes created a number of incised-valleys, each forming through fluvial incision during lowered base-level. Aeolian dominance during periods of relative sea-level lowstand aids incised-valley identification as the erosive bounding surface juxtaposes incised-valley infill against stacked aeolian faces. Relative sea-level rises resulted in back-flooding of the incised-valleys and their infill via shallow-marine and estuarine processes. Back-flooded valleys generated marine embayments within which additional local accommodation was exploited. Back-filling is characterised by a distinctive suite of lithofacies arranged into a lowermost, basal fill of fluvial channel and floodplain architectural elements, passing upwards into barform elements with indicators of tidal influence, including inclined heterolithic strata and reactivation surfaces. The incised-valley fills are capped by laterally extensive and continuous marine limestone elements that record the drowning of the valleys and, ultimately, flooding and accumulation across surrounding interfluves (transgressive surface). Limestone elements are characterised by an open-marine fauna and represent the preserved expression of maximum transgression.

Assessment of Hirnantian synglacial eustacy and palaeogeography in a tectonically active setting: the Welsh Basin (UK)

AbstractThe Hirnantian stratigraphy of Wales is critically assessed against the global evidence for two major Hirnantian synglacial eustatic lowstands. The evidence for two separate lowstands of relative sea level is locally suggestive in shelf sequences but generally only permissive in the basin. Nowhere can the possible candidate lowstands be separated biostratigraphically and thus basinwide correlation that might rule out local tectonic control is impossible to prove. Consistent with palaeolatitude evidence and a situation far removed from the Gondwana margin, there is no facies evidence (e.g. dropstones) for local glacial conditions and the diamictites are of mass-flow origin. Sequence stratigraphic reasoning proves helpful in the correlation of the basinal succession in Central Wales with the shelf sequences along the Tywi Lineament and the Variscan Front, and suggests that the first appearance ofNormalograptus persculptusin Wales does not correspond with the base of thepersculptusBiozone. Extensive new palaeocurrent data and sedimentological restudy is combined with review of earlier knowledge to argue for significant revisions to previous palaeogeographic maps for ‘generalized lowstand conditions’, notably along the Bala Lineament, the Variscan Front and the Central Wales Inliers.