Decrease In Accommodation Space Research Articles

Multi-Proxies Analysis of Organic Matter Accumulation of the Late Ordovician–Early Silurian Black Shale in the Lower Yangtze Region, South China

The evolutional process of palaeoceanic environment and its effect on the accumulation of organic matter during the Ordovician–Silurian transition in Lower Yangtze region has been overlooked compared to that in Upper Yangtze region of South China, although their paleogeographic settings were expected to be discrepant. This paper documents the marine depositional environment, paleoclimate, and sediment supply changes, and discusses their roles in controlling the organic matter enrichment in sedimentary rocks within the Ordovician–Silurian transition of the Lower Yangtze region, using the latest geochemical data of the continuous drilling core. The stratigraphic framework of the Ordovician–Silurian transition in the Lower Yangtze region is composed of two third-order sequences, each of which can be subdivided into a lower TST (transgressive systems tract) and an upper RST (regressive systems tract). TST1 represented an evident depositional transition stage which was marked by the ending of the underlying carbonate sediments and the initiation of the terrigenous clastic-dominated sediments. Geochemical proxies indicate that the relatively low productivity, dysoxic water column condition, and high sediment supply flux collectively resulted in inadequate organic matter hosted in deposits of the TST1. During the depositional period of RST1, the global sea level declined due to the Hirnantian glaciation age. The icehouse also caused the decrease in overall river flux and, thus, the terrigenous clastic sediment supply. The icehouse also strengthened the upwelling that occurred in the Lower Yangtze sea. The upwelling boosted the marine algae explosion through the delivery of abundant nutrients, which not only enhanced paleoproductivity but also led to an anoxic environment by oxygen consumption. Such high paleoproductivity, anoxic water column environment, and low sediment supply flux caused the deposition of organic-rich shale. The sea level rose during the TST2 due to the ending of an ice age. The relatively large water depth and high paleoproductivity associated with volcanic eruptions are the main factors that caused the enrichment of organic matter during this stage. During the deposition of RTS2, the increase of sediment supply flux resulted in a decrease in accommodation space and water depth and the dilution of organic matter in deposits, which was the primary constraint of organic matter accumulation.

Sequence stratigraphy of the Miocene siliciclastic–carbonate sediments in Sadat Area, north‐west of Gulf of Suez: Implications for Miocene eustasy

The Miocene sequences in the north‐west of the Gulf of Suez represent transition zones between carbonate‐dominated and siliciclastic‐dominated strata. These rocks were greatly influenced by the Gulf of Suez rifting, which resulted in lateral variations of facies and hence presented a complicated stratigraphic setting. Here, we interpret the sequence stratigraphy of the Sadat Area to the north‐west of the Gulf of Suez by integrating biostratigraphic data and microfacies analysis to contribute primarily to the geology of the studied successions and incorporate the existing knowledge of syn‐rift deposition. We determined the ages of the studied formations by biostratigraphic examination of the integrated calcareous nannoplankton, planktic, and benthic foraminifera. Our results distinguish four genetic Miocene sequences. These sequences are defined by four major sequence boundaries that outline sedimentation cycles and were chiefly deposited in a lagoon to outer‐shelf settings. The proposed Miocene eustacy is well‐correlated with global curves, implying a rise in sea level in the lower parts of Zones M4, M5, and M8. In contrast, the reduction of sea‐level rise resulted in the decrease of accommodation space in the upper part of Zone M5. The studied Miocene deposits mainly formed in a shallower marine environment, as shown by a comparison between data from the eastern side of the Gulf of Suez and the data presented in this paper. Furthermore, we demonstrate that the local tectonic activity linked with the Gulf of Suez rifting and the eustatic sea‐level changes were significant factors in the deposition of the Miocene in the studied successions.

The spatial and temporal evolution of the Portland and Tualatin forearc basins, Oregon, USA

AbstractThe Portland and Tualatin basins are part of the Salish-Puget-Willamette Lowland, a 900-km-long, forearc depression lying between the volcanic arc and the Coast Ranges of the Cascadia convergent margin. Such inland seaways are characteristic of warm, young slab subduction. We analyzed the basins to better understand their evolution and relation to Coast Range history and to provide an improved tectonic framework for the Portland metropolitan area. We model three key horizons in the basins: (1) the top of the Columbia River Basalt Group (CRBG), (2) the bottom of the CRBG, and (3) the top of Eocene basement. Isochore maps constrain basin depocenters during (1) Pleistocene to mid-Miocene time (0–15 Ma), (2) CRBG (15.5–16.5 Ma), and (3) early Miocene to late Eocene (ca. 17–35 Ma) time. Results show that the Portland and Tualatin basins have distinct mid-Miocene to Quaternary depocenters but were one continuous basin from the Eocene until mid-Miocene time. A NW-striking gravity low coincident with the NW-striking, fault-bounded Portland Hills anticline is interpreted as an older graben coincident with observed thickening of CRBG flows and underlying sedimentary rocks. Neogene transpression in the forearc structurally inverted the Sylvan-Oatfield and Portland Hills normal faults as high-angle dextral-reverse faults, separating the Portland and Tualatin basins. An eastward shift of the forearc basin depocenter and ten-fold decrease in accommodation space provide temporal constraints on the emergence of the Coast Range to the west. Clockwise rotation and northward transport of the forearc is deforming the basins and producing local earthquakes beneath the metropolitan area.

Sequence stratigraphic and petrological analyses of the Cambrian oncoids exposed in the Liaoning Province, North China Platform

The Cambrian Miaolingian Series strata are continuously exposed in the North China Platform and contain diverse sedimentary phenomena, including substantial with oncoid-rich units. Limestone samples from the Zhangxia Formation in the Sandaogou section were collected to characterise unique macroscale to microscale sedimentary characteristics of the marine carbonate oncoids. In the oncolitic beds (∼4.5 m thick), the individual size of oncoids gradually increases from bottom to top. The oncoid morphology and the presence of a nucleus and laminae in thin-sections are used to classify these oncoids into three types (concentric laminar, lateral growth and multicore oncoids). As evidenced by scanning electron microscopy and energy-dispersive X-ray observations, Girvanella are abundant inside these oncoids and are associated with nanospheres, framboidal pyrite, and spherical and filamentous microbial fossils, confirming the biogenicity of the studied oncoids. These results suggest that the oncolitic–oolitic limestone formed in an upward-shallowing marine environment caused by a forced regression process that resulted in a decrease in accommodation space in a fourth-order sequence. Thus, the conditions became increasingly suitable for the development of cyanobacteria-dominated microbial mats and large oncoids. The late stage of the depositional setting and elevated solar radiation resulted in the formation of the growth termination surface. KEY POINTS Evidence of microbial origin from the oncoids of the Cambrian Miaolingian Series in the North China Platform. Vertical variation in oncoid size and distribution coincide with geochemical data. A shallowing depositional environment is interpreted as the reason for variation in oncoid size and formation of oncoid growth termination surfaces.

A new interpolation method to measure delta evolution and sediment flux: Application to the late Holocene coastal plain of the Argens River in the western Mediterranean

Rapid environmental changes along the Mediterranean coasts influenced the sedimentary dynamics, shoreline position and human settlements in deltaic areas over the last millennia. An innovative and multiproxy approach using geostatistical modelling was developed to estimate geomorphic evolution and sediment fluxes in deltaic areas, while palaeoecological conditions and environmental changes were assessed from geochemistry and cluster analyses of molluscan fauna. This method was applied to the coastal plain of the Argens River in southern France. Depositional environments of prodelta, delta front, river channel, floodplain, marsh and abandoned channel were identified from the study of facies associations. The late Holocene sediment flux in the Argens River bayhead delta ranged from 15,800 ± 2300 to 52,000 ± 8500 m3·yr−1. The highest rate of sediment deposition between 2500 and 2000 cal yr BP was associated with increased river flooding in southeastern France. A general decrease in subaqueous sediment flux over the past 2500 years was primarily controlled by a decrease in accommodation space and delta slope. From 500 to 0 cal yr BP, a sharp increase in subaerial sediment flux probably due to hydroclimatic change was contemporaneous with a period of high flood frequency during the Little Ice Age. A southward shift of the North Atlantic westerlies combined with either a negative phase of the East Atlantic/West Russian pattern or a negative phase of the East Atlantic pattern coupled to a northward migration of the intertropical convergence zone could have led to increased fluvial activity in the lower Argens valley during the late Holocene.

Using Gamma Emissions to Identify Cycles in the Lower Cambrian Monkton Formation (NW Vermont): Implications for Identifying Sea Level Variation on the Iapetus Margin

The Monkton Formation is described as a Lower Cambrian regressive sandstone unit containing shallowing-up cycles, called parasequences, which record tidal flat progradation. Spatial variation and limited outcrops of continuous stratigraphy have made it difficult to characterize how cycles change in architecture and thickness through the entirety of the Monkton Formation. This study seeks to identify stratigraphic trends in the parasequence architecture and thickness from the successful recognition of facies in the subsurface, information that will clarify how the sea level changes and impacts accommodation space. New geophysical data allow this research project to explore the stratigraphy of the Monkton at higher levels of resolution than previously achieved. Initial statistical study of gamma ray data from a well through the Monkton suggests that meter-scale parasequences are identifiable in geophysical logs. Using continuous gamma log data, this work identified clastic carbonate ratios from a 1034’ deep geothermal well drilled at Champlain College in Burlington, VT. Although cycles were identified in the gamma log, the spatial variation, complexity and variety of parasequence types within the Monkton made it difficult to correlate the sequences with specific environments or changes in sea level. To see if specific architectural elements of parasequences and lithologies could be characterized by gamma values, outcrops of the Monkton were surveyed containing both parasequences and lithologies that were useful paleobathymetric indicators. The survey determined gamma ray patterns and relative values of a common Monkton parasequence representing tidal flat progradation and a carbonate lithology indicating an abrupt sea level rise. Gamma data from a second well on the University of Vermont campus, along with accompanying borehole camera video, confirmed that the outcrop survey results could be useful in subsurface interpretations of the Monkton. Intervals representing the parasequences and carbonate lithologies were identified in the Champlain College Well and were combined with the UVM Fleming Well findings to create a composite stratigraphic section of the Monkton. The parasequences and lithologic trends identified in the composite section indicated that, over the entirety of the Monkton, accommodation space decreased. The decrease in accommodation space is interpreted to represent a change in the rate of sea level rise. The Monkton Formation deposition occurred in a transgressive systems tract with varying rates of sea level rise, which implied that the overlying Winooski Formation would represent highstand systems tract deposition.

Provenance of the Cenozoic siliciclastic intramontane Amagá Formation: Implications for the early Miocene collision between Central and South America

The late Oligocene-middle Miocene Amagá Formation is a continental siliciclastic succession deposited along several semi-isolated intramontane sedimentary basins in the northwestern the Andes. Despite the potential information that this sedimentary record may bear on the evolution of tropical intramontane siliciclastic successions along active Andean type convergent margins, little is known about its provenance. Here we document petrographic, heavy mineral and detrital zircon U-Pb ages data from the Amagá Formation, cropping out along the Santa Fe de Antioquia - San Jerónimo Sub-Basin, from which we investigate the evolution and controlling factors leading to changes in its provenance. Sandstones from the late Oligocene Lower Member of the Amagá Formation display compositional modes and U-Pb detrital zircon ages which suggest sediment sources exclusively associated to the continental South American Plate. The compositional modes and detrital zircon U-Pb ages of sandstones from the early - middle Miocene Upper Member suggest, instead, sediment sources associated to both, the South American Plate and the allochthonous Panamá-Chocó Block. In particular, the Upper Member documents the arrival of detrital zircons younger than 50 Ma (18.7, 21.0, 25.3, 31.9, 40.7, 45.1 Ma), which are not present in the South American Plate. This change in provenance parallels a change in sedimentary environments, from meandering (Lower Member) to braided (Upper Member) rivers, which have been interpreted as a major decrease in sediment accommodation space along the northern Andes. This decrease in accommodation space is associated to a regional accelerated uplift in the northern Andes resulting from the early Miocene accretion of the Panamá-Chocó Block to northern South America.

A unified framework for modelling sediment fate from source to sink and its interactions with reef systems over geological times

Understanding the effects of climatic variability on sediment dynamics is hindered by limited ability of current models to simulate long-term evolution of sediment transfer from source to sink and associated morphological changes. We present a new approach based on a reduced-complexity model which computes over geological time: sediment transport from landmasses to coasts, reworking of marine sediments by longshore currents, and development of coral reef systems. Our framework links together the main sedimentary processes driving mixed siliciclastic-carbonate system dynamics. It offers a methodology for objective and quantitative sediment fate estimations over regional and millennial time-scales. A simulation of the Holocene evolution of the Great Barrier Reef shows: (1) how high sediment loads from catchments erosion prevented coral growth during the early transgression phase and favoured sediment gravity-flows in the deepest parts of the northern region basin floor (prior to 8 ka before present (BP)); (2) how the fine balance between climate, sea-level, and margin physiography enabled coral reefs to thrive under limited shelf sedimentation rates after ~6 ka BP; and, (3) how since 3 ka BP, with the decrease of accommodation space, reduced of vertical growth led to the lateral extension of reefs consistent with available observational data.

Depositional environments and cyclicity of the Early Ordovician carbonate ramp in the western Tarim Basin (NW China)

During the Early Ordovician, the Tarim Basin (NW China) was mainly occupied by an extensive shallow-water carbonate platform, on which a carbonate ramp system was developed in the Bachu-Keping area of the western part of the basin. Three well-exposed typical outcrop sections of the Lower Ordovician Penglaiba Formation were investigated in order to identify the depositional facies and to clarify origins of meter-scale cycles and depositional sequences, thereby the platform evolution. Thirteen lithofacies are identified and further grouped into three depositional facies (associations): peritidal, restricted and open-marine subtidal facies. These lithofacies are vertically stacked into meter-scale, shallowing-upward peritidal and subtidal cycles. The peritidal cycles are mainly distributed in the lower and uppermost parts of the Penglaiba Formation deposited in the inner-middle ramp, and commonly start with shallow subtidal to intertidal facies followed by inter- to supratidal facies. In contrast, the subtidal cycles occur throughout the formation mostly in the middle-outer ramp and are dominated by shallow to relatively deep (i.e., intermediate) subtidal facies. The dominance of asymmetrical and incomplete cycles suggests a dominant control of Earth’s orbital forcing on the cyclic deposition on the platform. On the basis of vertical facies and cycle stacking patterns, and accommodation changes illustrated by the Fischer plots from all studied sections, five third-order depositional sequences are recognized in the Penglaiba Formation. Individual sequences comprise a lower transgressive part and an upper regressive one. In shallow-water depositional environments, the transgressive packages are dominated by thicker-than-average subtidal cycles, indicating an increase in accommodation space, whereas regressive parts are mainly represented by thinner-than-average peritidal and subtidal cycles, denoting a decrease in accommodation space. In contrast, in intermediate to deep subtidal environments, transgressive and regressive packages display an opposite trend in accommodation space changes. Sequence boundaries (except the basal and top boundaries of the Penglaiba Formation) are usually represented by laterally traceable, transitional boundary zones without apparent subaerial exposure features. Good correlation of the long-term changes in accommodation space (or sea-level) inferred from vertical stacking patterns of facies and cycles suggests an overriding eustatic control on the formation of meter-scale cycles and third-order depositional sequences as well as platform evolution superimposed with local and/or regional tectonic influence during the Early Ordovician. This study would help understand the controls on the tempo-spatial facies distribution, stratal cyclicity and carbonate platform evolution in the western Tarim Basin during the Early Ordovician, facilitating prediction for favorable subsurface carbonate reservoirs and future hydrocarbon exploration and production in the Penglaiba Formation.

The effect of land reclamations and sediment extraction on the suspended sediment concentration in the Ems Estuary

Natural sediment sinks have been removed in many estuaries and tidal basins, and the resulting loss in accommodation space likely led to an increase in the suspended sediment concentration (SSC). The effect of land reclamations and changes in other sediment sinks on SSC has been investigated using the Ems Estuary as a well-documented example. Coastline reconstructions and stratigraphic observations in the Ems Estuary, on the Dutch–German border, have been used to quantify the long-term (centennial) changes in sediment sinks. These data indicate that approximately 2–3·106dry weight tons of fine-grained sediments annually accumulated in the estuary from the end of the 16th century up to the beginning of the 20th century. In the 20th century and especially from 1960 to 1994, large amounts of sediment were dredged and removed from the Ems Estuary. Since the 1990s, sediment removal rates through dredging activities sharply decreased, while available monitoring data show that on average SSC in the estuary increased during this period. The potential impact of sediment sinks on SSC has been investigated using a simple mass balance and a complex numerical model. Both methodologies indicate that the reduction in sediment sinks lead to an increase in suspended sediment concentration of several 10s of mg/l averaged over the estuary. The numerical model also resolves the spatial variation in changing suspended sediment concentrations, suggesting that locally the increase may exceed 100mg/l. This work reveals that a decrease in accommodation space for fine-grained sediments by land reclamations or by reducing fine-grained sediment extraction will lead to an increase in suspended sediment concentrations, and provides methodologies to estimate the impact of interventions on SSC of estuaries and tidal basins.

Terrace styles and timing of terrace formation in the Weser and Leine valleys, northern Germany: Response of a fluvial system to climate change and glaciation

In glaciated continental basins accommodation space is not only controlled by tectonics and sea-level but also by the position of ice-sheets, which may act as a regional base-level for fluvial systems. Although the Pleistocene terrace record of major river systems in northwestern Europe has been investigated by many authors, relatively little attention has been paid to base-level changes related to glacier advance–retreat cycles and how these regional changes in base-level interacted with river catchment processes. This study provides a synthesis of the stratigraphic architecture of Middle Pleistocene to Holocene fluvial terraces in the upper Weser and middle Leine valley in northern Germany and links it to glaciation, climate and base-level change. The depositional architecture of the fluvial terrace deposits has been reconstructed from outcrops and high-resolution shear wave seismic profiles. The chronology is based on luminescence ages, 230Th/U ages, 14C ages and Middle Palaeolithic archaeological assemblages.The drainage system of the study area developed during the Early Miocene. During the Pleistocene up to 170 m of fluvial incision took place. A major change in terrace style from strath terraces to cut-and-fill terraces occurred during the early Middle Pleistocene before Marine Isotope Stage MIS 12, which may correlate with climate deterioration and the onset of glaciation in northern central Europe. During this time a stable buffer zone was established within which channels avulsed and cut and filled freely without leaving these vertical confines. Climate was the dominant driver for river incision and aggradation, whereas the terrace style was controlled by base-level changes during ice-sheet growth and decay. A major effect of glacio-isostatic processes was the post-Elsterian re-direction of the River Weser and River Leine.The Middle Pleistocene fluvial terraces are vertically stacked, indicating a high aggradation to degradation ratio, corresponding with a regional base-level rise during glacier advance. At the beginning of the Late Pleistocene the terrace style changed from a vertical to a lateral stacking pattern, which is attributed to a decrease in accommodation space during glacier retreat. The formation of laterally attached terraces persisted into the Holocene.Major incision phases took place during MIS 5e, 5d, 5c, and probably early MIS 4, early MIS 3 and MIS 2 (Lateglacial). During MIS 5e and the Lateglacial the braided river systems changed into meandering rivers, indicated by preserved organic-rich flood-plain and point bar deposits. The Late Pleistocene braided river systems (MIS 5c to MIS 3) are characterized by a high sinuosity, which may be a direct effect of an increased downstream gradient after deglaciation when the channel lengthened and the river adjusted to the increased gradient by increasing sinuosity. These Middle Pleniglacial fluvial deposits are unconformably overlain by Lateglacial to Holocene meandering river deposits, which form laterally attached terraces, recording millennial-scale channel shifts. The lack of Late Pleniglacial deposits might be related to Late Weichselian forebulge formation.

Depositional facies and stratal cyclicity of dolomites in the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, NW China

The Upper Cambrian Lower Qiulitag Group in the Tarim Basin, NW China, is overwhelmingly composed of cyclic dolomites. Based on extensive field investigations and facies analysis from four outcrop sections in the Bachu-Keping area, northwestern Tarim Basin, four main types of facies are recognized: open-marine subtidal, restricted shallow subtidal, intertidal, and supratidal facies, and these are further subdivided into ten lithofacies. In general, these facies are vertically arranged into shallowing-upward, metre-scale cycles. These cycles are commonly composed of a thin basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: peritidal and shallow subtidal cycle, are further identified. The peritidal cycles, predominating over the lower-middle Lower Qiulitag Group, commence with shallow subtidal to lower intertidal facies and are capped by inter-supratidal facies. In contrast, the shallow subtidal cycles, dominating the upper Lower Qiulitag Group, are capped by shallow-subtidal facies. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, six larger-scale third-order depositional sequences (Sq1–Sq6) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thicker-than-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average peritidal cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lower-order sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq1–Sq3, is characterized by peritidal facies-dominated sequences and a progressive decrease in accommodation space, indicating a longer-term fall in sea level. In contrast, the upper sequence set (Sq4–Sq6) is characterized by subtidal facies-dominated sequences and a progressive increase in accommodation space, indicating a longer-term rise in sea level.

Quaternary structural partitioning within the rigid Tarim plate inferred from magnetostratigraphy and sedimentation rate in the eastern Tarim Basin in China

It has been proposed that within the Tarim Basin tectonic activity has been limited since Triassic time. However, on the basis of magnetostratigraphy from the eastern Tarim Basin, which defines the chronology of sedimentation and structural evolution of the basin, we show that the basin interior has been uplifted and partitioned during Quaternary. The magnetostratigraphy was constructed from 2228 samples that yielded acceptable inclination values. Characteristic remnant magnetization (ChRM) with both normal (N1–N11) and reversed (R1–R11) polarity was isolated by thermal demagnetization. The data correlate best with polarity chrons C3r to C1n, which range from 5.39Ma to recent on the geological time scale 2004 (GTS2004). An abrupt decrease in the sedimentation rate is observed at 1.77Ma in the Ls1 core. This change does not overlap with known Pleistocene climate-change events. We attribute this sedimentation rate decrease to a structurally controlled local decrease in accommodation space where basin basement uplifts occur. This period of sedimentary environmental change reveals that structural partitioning in the basement of the Tarim Basin occurred since ~1.77Ma, and we speculate that tilting of the Southeast Uplift (a sub-basin unit) within the Tarim Basin began in early Pleistocene time.

Transgressive systems tract of a ria-type estuary: The Late Holocene Vilaine River drowned valley (France)

The understanding of control parameters of estuarine-infilling at different scales in time and space is important for near-future projections and better management of these areas facing sea-level change. During the last 20 years, the stratigraphy of late Quaternary estuaries has been extensively investigated leading to the development of depositional models which describe the sedimentary facies distribution according to changes in sea level, tidal range and wave activity but few of them describe in detail the influence of the estuarine geomorphology. On the Atlantic coast, recent studies provide a detailed understanding of the sedimentary infill succession of incised valleys during the late Holocene transgression with recent emphasis on the macrotidal bay of the Vilaine River in South Brittany. The latter has shown the importance of the inherited morphology in controlling the preservation of the sedimentary deposits. However, until now, there has been little information available on the geometry of the underlying bedrock of the estuary and its immediate approaches including the Holocene history of the most proximal part of the Vilaine River estuary. The recent acquisition of very-high resolution seismic reflection (Chirp) and sediment core data in the Vilaine Estuary provides the opportunity to address several of these issues leading to the identification of the controlling factors that played a role in the sedimentary filling of the estuary. The estuary of the Vilaine River is one of the most sheltered estuaries of the Atlantic coast and thus shows an excellent record of the successive transgressive phases since the last glacial period. The valley of this ria-type estuary is incised in the Hercynian magmatic and metamorphic rocks of the Armorican Massif along the striking branches of the South Armorican Shear Zone and filled with well-stratified sediments. The study of the morphology of the substratum in the estuary reveals narrow paleovalleys (1 km wide) with a maximum depth of 20 m. The shapes and dimensions of these paleovalleys are comparable with those observed upstream in the present-day Vilaine River valley. Facies analysis of seismic and sedimentary data, complemented by 21 AMS radiocarbon age dating, enables the reconstruction of the infilling of the estuarine valley. The valley is filled by Pleistocene to early Holocene alluvial, coarse-grained, pebbly sands overlain by mid- to late Holocene marine sands, silts and muds. From 10,000 to 5000 cal. BP, the marine transgression shows a rapid initial phase of transgression followed by a progressive slowing down which can be recognised over more than 90 km in an upstream direction. The rapid transgression developed homogeneously over the entire length of the estuary with a widespread expression of tidal currents (tide-dominated estuary). The slow phase of the transgression gives rise to a differentiated expression of the hydrodynamic factors according to the geomorphological setting. The sedimentary deposits of the innermost and narrow part of the estuary reflect a strong control of tidal and river currents when the influence of tidal currents increases at the expense of fluvial control in a downstream direction. The sedimentary filling of the outer reaches of estuary shows a strong influence of tidal currents modulated by changes in sea level where, from 5700 to 3000 cal. BP, the progressive decrease in accommodation space promotes the expression of wave action (wave-dominated estuary). Finally, from 5000 years BP to present day, the stabilisation of sea level gives birth to scour and lag sedimentary morphologies (scour and lag estuary). The sedimentary record appears thus strongly marked by the Holocene transgression and the geomorphological setting where hydrodynamic parameters superimposed on the geomorphological and eustatic contexts expressed in different ways in time and space govern the evolution of the estuarine system.

Sedimentology of geomorphologically controlled Quaternary tufas in a valley in southern Spain

Four vertical facies sequences characterizing different environments have been identified in relation to two phases of tufa deposition (probably Pleistocene and Holocene) within a valley located in the central sector of the Betic Cordillera (southern Spain): (1) alluvial-fluvial (non-tufa) deposits, (2) fluvial tufa, (3) fluvio-lacustrine facies, and (4) spring with waterfall tufa. The first three created a stepped axial depositional system and the spring with waterfall tufa formed a transverse system. During the first phase of tufa construction, the vertical evolution in the axial system from alluvial-fluvial (non-tufa) to fluvio-lacustrine tufa deposits is interpreted as the consequence of geomorphological control. The evolution through time from a high-gradient and stepped fluvial system to a low-gradient fluvio-lacustrine system occurred upstream from where the progradation of the transverse, perched spring tufa system became narrower and finally dammed the valley. The tufa barrage caused an increase in the vertical accumulation within the axial system and a consequent upstream decrease of accommodation space and slope of the longitudinal profile. After a period of fluvial incision, tufa formed again during the Holocene in a high-gradient and stepped fluvial system.

Stratigraphic architecture of the Plio-Pleistocene infill of the Corinth Rift: Implications for its structural evolution

An integrated study of the stratigraphy, structure, sedimentology, and geomorphology of the Akrata–Derveni region (southern coast of the Gulf of Corinth, Greece) forms the basis for a tectono-stratigraphic model for the evolution of the Plio-Pleistocene central Corinth Rift. The syn-rift sediments exposed on the uplifted southern coast of the Gulf of Corinth comprise three stratigraphic groups. Maximum total thickness of the syn-rift sediments can reach 2800 m in the middle of the studied area. The Lower Group is made of fluvio-lacustrine deposits. The Middle Group corresponds to thick alluvial fan conglomerates and their equivalent Gilbert-type fan deltas that built toward the north. The Upper Group is composed of uplifted terrace deposits, slope breccias and small Gilbert-type deltas. These groups have been subdivided into informal formations and depositional systems. Restoration of the stratigraphic architecture along a N–S transect provides a linked structural and depositional model for this part of the rift. Reconstruction of the latest phases of uplift is based on a study of geomorphological features. Evolutionary phases include, (1) an overall increase in accommodation space during deposition of the Lower and Middle Groups followed by (2) a drastic decrease in accommodation space during deposition of the Upper Group. Sedimentary signals indicate that most of the major normal faults were active during deposition of the Lower Group. The depocentre was located in the middle part of the study area and paleocurrents were predominantly toward the ENE. The main depositional system shifted south at the onset of deposition of the Middle Group, recording a widening and deepening of the rift. This major event also corresponds to a change in paleocurrent direction to a clear northward polarity. The southernmost border fault, the Killini Fault, was sealed during deposition of the Middle Group. A northward migration of fault activity was associated with northward progradation of giant Gilbert-type fan deltas that record water depths up to 500 m. Finally, the fan delta system was abandoned as progressive tilting to the south and uplift of the margin induced a reversal of the drainage system with the development of an endorheic depression. Sediment supply to the basin thus decreased and a forced regression took place during deposition of the Upper Group recording a northward shift of more than 5 km and a 600 m relative sea-level drop. As no major eustatic sea-level falls of such amplitude are documented during the Pleistocene, the uplift is linked to regional tectonics. Uplift and fault reactivation gave the present day configuration of the southern coast of the Gulf.

Sub-ice shelf deposition during the late Devensian glaciation in western Ireland

Glacigenic sediments preserved around the margins of Clew Bay, western Ireland, record deposition in a tidally-influenced, sub-ice shelf environment during the late Devensian glaciation (∼ 21–13 14C kyr). Thick, massive to normally-graded glacial diamictons (tills) are interbedded with laminated to massive silt and clay beds. These sediments interbed with each other on scales ranging from millimetres to metres in a generally shallowing-up sequence associated with a decrease in accommodation space. Isolated dropstones and a downward-going clastic dike are also observed. These structures and sediments reflect a waterlain environment with rhythmically-varying levels of water energy and sediment supply. Tidally-driven uplift and downdraw of a retreating ice shelf is a likely control on sedimentation patterns, and glacial dynamics, of this sensitive, Atlantic-facing ice margin.

The origin and evolution of the Klondike goldfields, Yukon, Canada

The world famous Klondike goldfields are located in the unglaciated part of west-central Yukon, Canada. Since their discovery over 100 years ago, they have produced an estimated 311 tonnes of gold, primarily from bench and creek placers that are fluvial in origin and range from Pliocene to Holocene in age. Historically, the placers are classified into three levels of gravel with four main units. These include the high-level White Channel Gravel (Pliocene), presently the most important gold-bearing unit, which sits nonconformably on an erosional bedrock surface (i.e., the ‘White Channel strath’) and is overlain and interbedded with the glaciofluvial Klondike Gravel (Pliocene); the intermediate-level gravel (Pleistocene), the least important economically; and the low-level gravel (Pleistocene–Holocene), historically the most important gold-bearing unit, but it has been mined three or four times now. The goldfields originated from the weathering and erosion of early Cretaceous, discordant mesothermal quartz veins, and the light grey color of the matrix of the White Channel Gravel is due mainly to weathering and diagenetic alteration by groundwater flow. The concentration of placer gold is related to a hierarchy of physical scales: at the lithofacies scale (metres), bed roughness determined sites of gold deposition; at the element scale (tens of metres), gravel bars were preferentially enriched in gold; at the reach scale (hundreds of metres), stream gradient was an important factor; at the system scale (hundreds of km), braided river environments transported large amounts of gold; and at the sequence scale (thousands of km), economic placers formed initially in the high-level White Channel Gravel and later in the intermediate-level and low-level gravel. The White Channel strath is interpreted as an erosional ‘tectonic’ terrace that formed during isostatic uplift and under conditions of dynamic equilibrium. The high-level White Channel Gravel and Klondike Gravel are interpreted as a depositional ‘climatic’ terrace that formed during a reversal in the tectonically induced downcutting, which is attributed to the initial and most extensive of the pre-Reid glaciations (∼3 Ma) in the Yukon. The intermediate-level gravel is interpreted as minor erosional ‘complex response’ terraces that formed during static equilibrium when there were pauses in valley-floor degradation, which are attributed to the subsequent and less extensive pre-Reid glaciations. The low-level gravel formed also during valley-floor degradation and may represent a return to dynamic equilibrium conditions. Hence, the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the Yukon. In addition, the lowering of baselevel from high-level, to intermediate-level and finally to low-level gravel was accompanied by a decrease in accommodation space (as indicated by a decrease in gravel thickness), which resulted in an increase in the concentration of the placer gold.

Architecture and sedimentary facies evolution in a delta stack controlled by fault growth (Betic Cordillera, southern Spain, late Tortonian)

Tectonic control is revealed in various ways (synsedimentary deformation structures, facies, architecture) in the coarse-grained delta systems that developed in the southeastern margin of the Guadix Basin, an intramontane basin in the central sector of the Betic Cordillera, Spain, during the late Tortonian (Miocene). Vertical trends in the architecture of the deltaic succession (230 m thick) show changes in the stratal stacking pattern related to the variation in time in subsidence rates (in accommodation space). A period of high subsidence controlled by a normal growth fault began at the base of the succession, producing retrogradational units representing Gilbert-type delta systems onlapped by shallow platform calcarenites. A period of low subsidence followed, controlled by growth of a listric fault producing aggradational units representing shoal-water deltas capped by red algal biostromes. A non-subsidence period and decrease in accommodation space at the top of the succession (sediment supply remained constant) produced progradational deltas. The manuscript focuses on the part of the succession where the delta deposits show the effects of extensional tectonics, a listric growth fault and its rollover, on delta development. The progressive increase in accommodation space inherent in fault growth controlled the style of delta sedimentation in the river mouth. Gilbert-type deltas developed during periods of increase in subsidence rates and shoal-water deltas during periods of decrease in subsidence rates. Horizontal trends in the architecture of the delta lobes show changes in the stratal stacking pattern affected by differential subsidence and pre-existing basin-floor topography. Periods of increase in subsidence rates near the fault scarp correlate with accommodation space kept constant, thinning of the units and shoal-water deltas development over the rollover, away from the fault scarp.

Sequence Stratigraphic Features of Weathered Carbonate Residuum Reservoirs

Studies on the sequence stratigraphic features of carbonate weathered residuum reveal that the key factor which makes the karstification possible is the permeability paths that occurred, controlled by the exposure events which resulted from drops in the sea level. The formation of carbonate weathered residuum was diagenetically associated with different interfaces of syndepositional sequences. Studies on the weathering degree (diagenetic maturation) of the parasequences in the sedimentary systems have come to the conclusion that the development maturation of the weathered residuum basically represents the increase or decrease of accommodation space. Ideally, a relevant model between sequence stratigraphy and weathered residuum is established. Introducing the theory of sequence stratigraphy into the study of the carbonate reservoirs within the weathered residuum led to the belief that sequence stratigraphic feature analysis of the carbonate weathered residuum is actually a relationship between the different interface natures and reservoir physical properties. By comparing relevant information all over the world, the authors hold that parasequences of neritic carbonates usually represent a small part of sedimentation, and the exposure period makes up about half the period of cycling. Weathered residuum reservoirs occurred dominantly within two temporal ranges: 30–120 Ma, corresponding to megasequences or mesosequences; and 0.01–0.5 Ma, corresponding to parasequences.