Ravinement Surface Research Articles

Accommodation space in a high-wave-energy inner-shelf during the Holocene marine transgression: Correlation of onshore and offshore inner-shelf deposits (0–12 ka) in the Columbia River littoral cell system, Washington and Oregon, USA

The Columbia River Littoral Cell (CRLC), a high-wave-energy littoral system, extends 160km alongshore, generally north of the large Columbia River, and 10–15km in across-shelf distance from paleo-beach backshores to about 50m present water depths. Onshore drill holes (19 in number and 5–35m in subsurface depth) and offshore vibracores (33 in number and 1–5m in subsurface depth) constrain inner-shelf sand grain sizes (sample means 0.13–0.25mm) and heavy mineral source indicators (>90% Holocene Columbia River sand) of the inner-shelf facies (≥90% fine sand). Stratigraphic correlation of the transgressive ravinement surface in onshore drill holes and in offshore seismic reflection profiles provide age constraints (0–12ka) on post-ravinement inner-shelf deposits, using paleo-sea level curves and radiocarbon dates. Post-ravinement deposit thickness (1–50m) and long-term sedimentation rates (0.4–4.4mka−1) are positively correlated to the cross-shelf gradients (0.36–0.63%) of the transgressive ravinement surface. The total post-ravinement fill volume of fine littoral sand (2.48×1010m3) in the inner-shelf represents about 2.07×106m3year−1 fine sand accumulation rate during the last 12ka, or about one third of the estimated middle- to late-Holocene Columbia River bedload or sand discharge (5–6×106m3year−1) to the littoral zone. The fine sand accumulation in the inner-shelf represents post-ravinement accommodation space resulting from 1) geometry and depth of the transgressive ravinement surface, 2) post-ravinement sea-level rise, and 3) fine sand dispersal in the inner-shelf by combined high-wave-energy and geostrophic flow/down-welling drift currents during major winter storms.

Submerged shoreline preservation and ravinement during rapid postglacial sea-level rise and subsequent “slowstand”

Submerged shorelines hold much potential for examining the interplay between the rate of sea-level rise and geomorphic setting, and informing the development of models of contemporary shoreline behavior. This paper describes the sedimentary architecture of a submerged barrier shoreline complex off Durban, South Africa. The complex consists of several barriers that have survived the postglacial transgression and associated erosive ravinement processes. The main shoreline sequence (–60 m) dates to 11,690 ± 90 calibrated (cal) yr B.P. and rests on a Pleistocene lagoonal deposit (35,395 ± 592 cal yr B.P.). The entire barrier shoreline complex is truncated by a strongly diachronous wave ravinement surface. The ravinement surface seaward of the main –60 m shoreline is steep, but the gradient declines across and landward of the subordinate landward shoreline complexes. The steep ravinement surface is attributed to fast sea-level rise (possibly associated with meltwater pulse 1B), while the gentle ravinement surface is associated with a subsequent slowing of the rate of sea-level rise (to 0.15 mm yr –1 ). Preservation of the main barrier and back-barrier deposits through overstepping is associated namely with rapid transgression and increased back-barrier accommodation. The smaller barriers landward of the main barrier were preserved through overstepping related to gentle antecedent gradients, despite more intense ravinement during a very slow rise in sea level (slowstand). This process was assisted by a sediment deficit. The resulting post-transgressive drape is also influenced by antecedent topography created by the barriers themselves; damming along the landward sides of overstepped barriers thickens the drape and creates a temporal disconnect between the migration of the shoreface and more landward elements of the littoral system. In examining the rates of shoreline migration associated with the overstepped barrier system, these are far greater than those calculated for the predicted rates of shoreline change on similarly steep coastal profiles. Future rates of shoreline migration appear to be insufficient to cause overstepping, and rollover or erosional responses are more likely.

Late Quaternary seismic stratigraphy in response to postglacial sea-level rise at the mid-eastern Yellow Sea

Late Quaternary seismic stratigraphy and depositional history at the mid-eastern Yellow Sea were investigated using high-resolution seismic profiles and core sediments. The results show that the shelf sequence consists of five sedimentary units formed since the LGM: incised-channel fill (SU1), estuarine deposit (SU2), thin sand veneer (SU3), tidal sand ridge (SU4), and central deltaic mud (SU5). The lowermost unit (SU1) above the sequence boundary is interpreted as channel fill deposits mainly formed during the LGM, which belongs to the lowstand systems tract. Three units (SU2, SU3, and SU4), regarded as transgressive systems tract, can be grouped into paralic and marine components separated by a ravinement surface. SU2 lying below the ravinement surface represents a paralic unit that consists of estuarine sediments left behind from shoreface erosion. The top surface of SU2 is truncated by an erosional surface and is overlain by two marine units (SU3 and SU4), which were produced by shoreface erosion that shifted landward during the transgression. SU3, mainly distributed over a wide area of the central part, is very thin, whereas SU4 on the eastern part off the Korean Peninsula forms serial sand ridges, partly modified by modern tidal currents. The uppermost unit (SU5) above the maximum flooding surface, regarded as the highstand systems tract, formed the thin deltaic mud patch derived from the Huanghe River developed after the highstand sea level approximately 7 ka BP.

Late Holocene Evolution of Currituck Sound, North Carolina, USA: Environmental Change Driven by Sea-Level Rise, Storms, and Barrier Island Morphology

ABSTRACT Moran, K.L.; Mallinson, D.J.; Culver, S.J.; Leorri, E., and Mulligan, R.P., 2015. Late Holocene evolution of Currituck Sound, North Carolina, USA: Environmental change driven by sea-level rise, storms, and barrier island morphology. The Holocene evolution of Currituck Sound, North Carolina, is investigated using geological data and a hydrodynamic model to understand how this depositional basin changed in response to sea-level rise and regional climate patterns. Five depositional units (one Pleistocene and four Holocene) are defined based on geophysical surveys, lithofacies, biofacies, and geochronological data. The earliest Holocene unit (ca. 5000 cal YBP) represents a midsalinity (10–25‰) sand shoal above the transgressive ravinement surface. This unit is overlain, successively, by (1) a freshwater swamp forest deposit, (2) a relatively saline (25–35‰) back-barrier estuarine deposit associated with increased inlet activity, and (3) a mid- to low-salinity (<10‰) deposit that is typical of modern ...

Evidence for erosion and deposition by the 2011 Tohoku-oki tsunami on the nearshore shelf of Sendai Bay, Japan

Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.

High‐resolution reconstruction of a coastal barrier system: impact of Holocene sea‐level change

AbstractThis study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr−1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr−1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply.

Modification of the Quaternary stratigraphic framework of the inner-continental shelf by Holocene marine transgression: An example offshore of Fire Island, New York

Abstract The inner-continental shelf off Fire Island, New York was mapped in 2011 using interferometric sonar and high-resolution chirp seismic-reflection systems. The area mapped is approximately 50 km long by 8 km wide, extending from Moriches Inlet to Fire Island Inlet in water depths ranging from 8 to 32 m. The morphology of this inner-continental shelf region and modern sediment distribution patterns are determined by erosion of Pleistocene glaciofluvial sediments during the ongoing Holocene marine transgression; much of the shelf is thus an actively forming ravinement surface. Remnants of a Pleistocene outwash lobe define a submerged headland offshore of central Fire Island. East of the submerged headland, relatively older Pleistocene outwash is exposed over much of the inner-continental shelf and covered by asymmetric, sorted bedforms interpreted to indicate erosion and westward transport of reworked sediment. Erosion of the eastern flank of the submerged Pleistocene headland over the last ~ 8000 years yielded an abundance of modern sand that was transported westward and reworked into a field of shoreface-attached ridges offshore of western Fire Island. West of the submerged headland, erosion of Pleistocene outwash continues in troughs between the sand ridges, resulting in modification of the lower shoreface. Comparison of the modern sand ridge morphology with the morphology of the underlying ravinement surface suggests that the sand ridges have moved a minimum of ~ 1000 m westward since formation. Comparison of modern sediment thickness mapped in 1996–1997 and 2011 allows speculation that the nearshore/shoreface sedimentary deposit has gained sediment at the expense of deflation of the sand ridges.

The Emptiness of the Stratigraphic Record: A Preliminary Evaluation of Missing Time In the Mesaverde Group, Book Cliffs, Utah, U.S.A

Abstract Observations of modern sedimentation rates in nonmarine and shallow-marine clastic environments indicate that deposits formed in comparable settings in the ancient record could accumulate in a fraction of the time that would appear to be available according to conventional chronostratigraphic dating methods. Typically as little as 10% of elapsed time is represented by sediment, the remainder by breaks in sedimentation, many of which are inconspicuous. An examination of the chronostratigraphy and sedimentary history of the Mesaverde Group of the Book Cliffs, Utah, U.S.A., confirms this pattern. Shoreface, deltaic, and nonmarine successions are considered to be particularly fragmentary. Ravinement surfaces may each account for up to 105 years of missing time. In the sedimentary record of shallow-marine and nonmarine deposits stratigraphic continuity is not to be expected, and calculation of time-related issues, such as mass-balance transport rates and the trajectories of shoreline transgression and regression, must take this into account.

Late Quaternary transgressive deposits in a low-gradient environmental setting: Korea Strait shelf, SE Korea

Analysis of high-resolution seismic profiles and sediment data from the Korea Strait shelf reveals that the late Quaternary deposits in this area consist of five sedimentary units deposited during transgression phases of sea-level changes between about 15 and 6 ka BP: ancient beach/shoreface complex (unit P1), estuarine deposits (unit P2), mid-shelf sand sheet (unit M1), sand ridge system (unit M2), and inner-shelf sand sheet (unit M3). They are paralic and marine, separated by a ravinement surface. The lower paralic component below the ravinement surface consists of two sedimentary units (P1 and P2) preserved from shoreface erosion. The top surface of the paralic unit is truncated by a sharp erosional surface. This surface is overlain by three sedimentary units (M1, M2, and M3), which were produced by shoreface erosion that shifted landward during transgression.The transgressive deposits in this area, considering geometries and distribution patterns, can be divided into three types (I, II, and III). Type I overlying the lowstand systems tract is confined to the shelf margin, and consists of a thick paralic unit P1 and a relatively thin marine unit M1. Type II on the mid shelf has no paralic component and the marine units M1 or M2 directly overly the sequence boundary. Type III, found in the inner shelf, includes a thick paralic (unit P2) and a thin marine (unit M3) component. It is completely covered by the highstand systems tract.

Impact of relative sea level and rapid climate changes on the architecture and lithofacies of the Holocene Rhone subaqueous delta (Western Mediterranean Sea)

The modern Rhone delta in the Gulf of Lions (NW Mediterranean) is a typical wave-dominated delta that developed after the stabilization of relative sea level following the last deglacial sea-level rise. Similar to most other deltas worldwide, it displays several stacked parasequences and lobes that reflect the complex interaction between accommodation, sediment supply and autogenic processes on the architecture of a wave-dominated delta. The interpretation of a large set of newly acquired very high-resolution seismic and sedimentological data, well constrained by 14C dates, provides a refined three-dimensional image of the detailed architecture (seismic bounding surfaces, sedimentary facies) of the Rhone subaqueous delta, and allows us to propose a scenario for delta evolution during the last deglaciation and Holocene. The subaqueous delta consists of “parasequence-like” depositional wedges, a few metres to 20–30m in thickness. These wedges first back-stepped inland toward the NW in response to combined global sea-level rise and overall westward oceanic circulation, at a time when sediment supply could not keep pace with rapid absolute (eustatic) sea-level rise. At the Younger Dryas–Preboreal transition, more rapid sea-level rise led to the formation of a major flooding surface (equivalent to a wave ravinement surface). After stabilization of global sea level in the mid-Holocene, accommodation became the leading factor in controlling delta architecture. An eastward shift of depocentres occurred, probably favoured by higher subsidence rate within the thick Messinian Rhone valley fill. The transition between transgressive (backstepping geometry) and regressive (prograding geometry) (para)sequences resulted in creation of a Maximum Flooding Surface (MFS) that differs from a “classical” MFS described in the literature. It consists of a coarse-grained interval incorporating reworked shoreface material within a silty clay matrix. This distinct lithofacies results from condensation/erosion, which appears as an important process even within supply-dominated deltaic systems, due to avulsion of distributaries. The age of the MFS varies along-strike between ca. 7.8 and 5.6kyrcal. BP in relation to the position of depocentres and climatically-controlled sediment supply. The last rapid climate change of the Holocene, the Little Ice Age (1250–1850AD), had a distinct stratigraphic influence on the architecture and lithofacies of the Rhone subaqueous delta through the progradation of two deltaic lobes. In response to changes in sediment supply linked to rapid climate changes (and to anthropic factors), the Rhone delta evolved from wave-dominated to fluvial dominated, and then wave dominated again.

Late Quaternary inner shelf deposits in response to late Pleistocene–Holocene sea level changes: Nakdong River, SE Korea

The high-resolution seismic profiles from the inner shelf off the Nakdong River in SE Korea can be directly correlated with the lithologic characteristics in the borehole data. These correlations offer an opportunity to document the evolution of the inner shelf deposits with sediment supplies in response to the late Quaternary sea level changes. The inner shelf sequence in this area consists of four sedimentary units, which comprise a set of lowstand, transgressive, and highstand systems tracts deposited since the last glacial maximum: incised-channel fill (SU1), estuary (SU2), sand veneer (SU3), and subaqueous delta (SU4). The lowermost unit (SU1), which overlies the sequence boundary, is interpreted as fluvial deposits formed during the last glacial period and the early stage of transgression, and belongs to the lowstand to transgressive systems tract. The lower middle unit (SU2) lying below the ravinement surface represents a paralic component that consists of estuarine sandy mud or muddy sand developed between approximately 13 and 8 cal ka BP, whereas the upper middle unit (SU3) above the ravinement surface corresponds to a marine component that consists of sand veneer produced by the shelf erosion during the ensuing sea-level rise. These two units (SU2 and SU3) belong to the transgressive deposits. The uppermost unit (SU4), regarded as the highstand systems tract, formed the Nakdong subaqueous delta including the proximal and distal systems developed after the highstand sea level at approximately 6 cal ka BP. The lateral transition from the proximal to distal facies suggests a prograding delta system in the Nakdong River.

Sedimentary characteristics of relict deposits on the western South Yellow Sea

Integrated studies of vertical sedimentary sequences, grain sizes, and benthic foraminifera and ostracoda, in combination with AMS 14C dating, and 210Pb and 137Cs analysis were carried out in three vibracores taken from the area of relict deposits on the western South Yellow Sea. The relict sands, which are about 0.4 m thick, overlie on the Early Holocene coastal marsh or tidal flat deposits with an evident erosional interface in between. The middle and upper parts or sometimes the whole of the relict sands have been reworked under the modern dynamic environment. The sedimentation rate varies between 0.20–0.30 cm year−1. The relict sands show a bimodal grain-size distribution pattern in frequency curves, with a sharp peak in the coarse fraction between 3Φ and 4Φ and a secondary peak in the fine fraction of about 7Φ. Of the benthic foraminiferal and ostracod assemblages, the reworked relict sands are characterized by the mixing of the nearshore euryhaline shallow-water species and deeper water species. The erosional interface at the bottom of the relict sands is considered as a regional ravinement surface formed during the transgression in the Early Holocene due to shoreface retreating landwards. The relict sands were accumulated on the ravinement surface during the transgression in the deglaciation period as lag deposits after winnowing and reworking by marine dynamic processes. And the secondary peak of fine fraction in the frequency curve for the relict sands suggests the input of fine-grained sediments during the reworking process. As the conclusion, the relict sands in the study area are interpreted as a type of reworked relict sediments.

Facies architecture and depositional model of a macrotidal incised-valley succession (Qiantang River estuary, eastern China), and differences from other macrotidal systems

Filling of the modern coastal-zone portion of the Qiantang River (QR) in eastern China was initiated by marine inundation during the transgression after the Last Glacial Maximum and has continued during the Holocene sea-level stillstand. The early part of the fill is transgressive in character, while the younger part is regressive. This paper deals with the sedimentary facies, surfaces, architecture, and depositional model of the QR incised-valley fill based on the detailed analysis of the newly drilled core SE2 and its correlation with more than 800 boreholes. The incised-valley deposits are grouped into five stacked facies: amalgamated channel, floodplain and channel, paleo-estuary, offshore shallow marine, and present-day estuary. A paleo-estuary facies had never been identified before, making these observations novel. This facies is characterized by a sedimentary succession and sediment distribution that are distinct from those of the present-day estuary because of a change in sediment supply from the sea floor beyond the estuary mouth. It also contains large numbers of tidal-channel sand bodies that are significant reservoirs for shallow biogenic gas. Since the last glaciation, there have been three stages in the development of the QR incised-valley fill sequence: (1) a formation stage associated with sea-level fall, (2) a filling stage during the early transgression with rapid sea-level rise, and (3) a burial stage corresponding to the slowing of sea-level rise and the onset of progradation. This fill consists of four longitudinal segments, each of which is distinguished by a distinct stratigraphic succession and different degrees of marine and fluvial influence. The basal erosion surface and sidewalls of the incised valley, the top of stiff clay on the interfluves, and the top of falling-stage fluvial-terrace deposits compose the sequence boundary. The initial flooding surface and maximum flooding surface are located within the amalgamated channel and offshore shallow-marine sediments, respectively. These observations indicate that: (1) relative sea-level changes determine the stratal stacking patterns, but local environmental factors, such as physical processes, accommodation, sediment supply, and coastal configuration control the nature of the facies, surfaces, and sediment-distribution patterns; and (2) the tripartite facies organization and wave ravinement surfaces typifying wave-dominated or mixed-energy (wave and tide) incised-valley fills are absent in the macrotidal QR succession, and the tidal ravinement and erosion surfaces of the macrotidal QR incised valley are more extensive and numerous than those of the wave-dominated or mixed-energy succession.

Carbon isotope evidence for sedimentary discontinuities in the lower Toarcian of the Lusitanian Basin (Portugal): Sea level change at the onset of the Oceanic Anoxic Event

The Toarcian Oceanic Anoxic Event (T-OAE), Early Jurassic, was an episode of rapid warming and deep perturbation of the carbon cycle, as suggested by the large carbon and oxygen isotope excursions recorded by various carbonate and organic materials of this age. Previous studies have shown that strata deposited immediately below the T-OAE are marked by widespread discontinuities, but their duration and synchronicity remain uncertain. In this study, we use the carbon isotope profiles of twelve sections from the Lusitanian Basin, Portugal, to correlate shallower and deeper settings and identify the durations and possible causes of these discontinuities. The obtained correlation suggests the existence of three major discontinuities. The first occurs close to the Pliensbachian–Toarcian boundary and locally corresponds to an interval of condensation due to rapid transgression. The second and most marked discontinuity occurs in the mid-upper part of the Polymorphum ammonite Zone. Depending on the location in the Lusitanian Basin, this discontinuity is interpreted to reflect either marine erosion (in deeper parts of the basin) or subaerial exposure (in the shallow parts of the basin), both induced by a high-amplitude sea-level fall. In intermediate parts of the basin, this discontinuity is interpreted as a transgressive ravinement surface due to subsequent sea-level rise. This latter transgression induced the formation of a third discontinuity of sedimentary condensation (uppermost Polymorphum Zone) that corresponds to the interval of highest rate of sea-level rise just before the onset of the T-OAE. The comparison of the carbon isotope record of the Lusitanian Basin with that of other European basins indicates that these discontinuities are present on a wide paleogeographical scale within the western Tethys, and correspond to marked changes in seawater temperatures and CO2 levels, implying a control by high-amplitude, likely glacio-eustatic sea-level changes.

Allostratigraphy of the Peace River Formation (Albian) in north-western Alberta and adjacent British Columbia

Abstract The middle to upper Albian Peace River Formation was deposited at the southern end of the ‘Hulcross Sea’, an embayment of the Boreal Ocean that occupied the foredeep of the Western Canada foreland basin. The Peace River Formation can be allostratigraphically divided by numerous, regionally-mappable erosion surfaces, recognizable both in wireline logs, core, and outcrop. We propose recognition of the informal Harmon, Cadotte and Paddy alloformations, bounded by four regionally-mappable marine transgressive surfaces, HE0, CE0, PE0 and VE3 in ascending order. The Harmon alloformation consists predominantly of marine mudstone, and is divided into five allomembers bounded by marine flooding surfaces and/or thick bentonites. The Harmon forms a wedge thinning from >145 m in the SW to 5 m in the NE over approximately 300 km. There is no geometric evidence of a sigmoidal geometry, or that allomembers downlap onto a single, distinct surface. Deposition took place on a very low-gradient and low-relief marine ramp, strongly influenced by storms. The Cadotte alloformation consists of well-sorted swaley cross-stratified sandstone and was deposited on a wave-dominated strandplain shoreface. Ravinement surfaces divide the Cadotte into three, sheet-like allomembers, each typically about 10 m thick. Allomembers CA and CB are mainly fine-grained sandstone whereas allomember CC is dominantly medium-grained sandstone with abundant pebble conglomerate. Towards the north, sandstone grades laterally into heterolithic facies indicative of an offshore setting. The Paddy alloformation lies unconformably on the Cadotte, and comprises an array of facies, with alluvial and lagoonal deposits predominant in the south. Further north, sandstone indicative of a wave-influenced strandplain shoreline forms a belt trending WSW-ENE across the middle of the area; heterolithic offshore marine deposits lie to the north. The Paddy alloformation comprises nine allomembers, PA to PI, that onlap progressively onto the basal unconformity PE0 from SW to NE. The pronounced wedge shape and muddy lithology of the Harmon implies rapid flexural subsidence and largely vertical aggradation whereas the sheet shape of the Cadotte suggests a lower rate of subsidence when high-frequency eustatic change may have driven rapid regressions and transgressions. The lower Paddy allomembers PA to PF are strongly wedge-shaped, suggestive of renewed tectonically-driven subsidence whereas upper allomembers PG to PI are more sheet-like, suggesting that a tectonic load exerted a less important influence and instead, more uniform regional isostatic subsidence took place, driven by the load of the eastward-encroaching sediment body. Sandstone-rich Paddy strata are abruptly overlain, at the VE3 unconformity, by a blanket of dark marine mudstone that forms the basal part of the lithostratigraphic Shaftesbury Formation; the mudstone between the VE3 and VE4 surfaces is of upper Viking age, and has a pronounced wedge geometry which implies a major new pulse of flexural subsidence in the west.

Submerged barrier shoreline sequences on a high energy, steep and narrow shelf

Abstract Late Pleistocene–Holocene shoreline sequences and associated shelf stratigraphy are described from a high gradient, high wave-energy shelf. Although shoreline preservation by overstepping is considered unlikely in such an environment as a result of intense ravinement during shoreline translation, the development and preservation of two distinct shorelines at − 100 m and − 60 m are evident and attributed to century to millennial scale periods of stillstand. During these periods shoreline equilibrium forms developed, characterised by early diagenesis of beachrock and aeolianite. Between these features the shelf is dominated by erosional surfaces reflecting ravinement during periods of slowly rising sea-level. Where shorelines are not preserved, areas of scarping in the ravinement surface, at similar depths to the adjoining shorelines, represent rocky headlands separating the sandy coastal compartments where the depositional shorelines formed. Shoreline preservation by overstepping was facilitated by rapid relative sea-level rise associated with meltwater pulses 1A and 1B, although the features preserved are likely to represent only the cemented cores of the coastal dunes and linear beachrock outcrops formed. The steep antecedent shelf gradient fostered the erosion of significant portions of the shorelines by ravinement, with loose sediment re-organised into a thick post-ravinement drape. The removal of shorelines during ravinement was countered by the rapid rate of sea-level rise and early sub-tropical diagenesis. It is clear that due to the combined effects of subtropical diagenesis, long periods of stillstand and drowning by meltwater pulses, the governing controls on shoreline preservation can be over-ridden by combinations of local (climatic) and global (eustatic) factors.

The Messinian succession of the Crotone Basin (southern Italy) II: Facies architecture and stratal surfaces across the Miocene–Pliocene boundary

The upper Messinian to lower Zanclean succession of the Crotone Basin, a forearc basin located along the Ionian side of the Calabrian Arc (southern Italy), consists of a turbidite-bearing succession (the Petilia Policastro Formation) overlying evaporites and abruptly passing upward into coarse-grained continental deposits (the Carvane Conglomerate), which are in turn overlain by distal shelf to slope mudstones (the Cavalieri Marl). The marked environmental changes from subaqueous to continental and vice versa documented by the studied succession are inferred to be the expression of basin-scale tectonics, leading to uplift and exposure, superposed on the Mediterranean-scale base-level rise that followed the acme of the Messinian Salinity Crisis. The upper part of the succession documents a transgressive trend probably initiated within the Carvane Conglomerate, and further testified by the presence of a ravinement surface marked by a transgressive lag at the base of the Zanclean Cavalieri Marl, pointing to a non-catastrophic deepening occurred near the end of the Messinian.

Large-volume gravity flow deposits in the Central Carpathian Paleogene Basin (Orava region, Slovakia): evidence for hyperpycnal river discharge in deep-sea fans

Abstract The deep-water clastic systems of the Central Carpathian Paleogene Basin contain megabeds, which are developed in distinctive stratigraphic horizons and can be traced over long distances. These beds are characterized by great individual thickness (4-13 m), uniform lithology and internal structures. On the basis of their lithology, sedimentary structures and sequence development, the megabeds are characterized by 15 individual facies and interpreted from the viewpoint of flow hydrodynamics. The grain-size distribution and internal structures of the megabeds point to their deposition from uniform turbulent flows. The main controlling factor for generation of such large voluminous flows is inferred in the sea-level changes, when a relative rising of sea level during the Eocene/Oligocene boundary was responsible for long-lasting accumulation of the clastic supply at the basin margins. The large volume of detritus from river discharge and ravinement surfaces of flooded land was accumulated on the shore and in the conduit heads where the sediment was remobilized by other triggers. The flows generated by catastrophic floods during the early Rupelian sea-level lowstand are thought to be the most probably triggering mechanism. The large highly erosive hyperpycnal flows from flooding rivers could erode accumulated deposits in the conduit or on steeper basin-margin slopes and could cause progressive increase of the sand volume in the flow. Conduit flushing appears to be the most probable source of sediment for the very large voluminous flows that were responsible for deposition of the Orava megabeds

Facies and sequence stratigraphic modeling of a Upper Pliocene–Lower Pleistocene fluvial succession (Valdelsa Basin, central Italy)

This paper illustrates the results of sedimentologic and stratigraphic analyses of the upper Piacenzian–Gelasian fluvial succession exposed in the Neogene–Quaternary Valdelsa Basin (central Italy). The succession shows a cyclothemic stacking of gravelly, sandy and muddy lithofacies organized into four monogenic facies associations (A–D). These record depositional environments ranging from braided to low-sinuosity river channels to flood basins. Associations A–D attest to lowstand (A–B), transgressive and high-stand (C–D) depositions in a full cycle of base-level variations. In each association, internal erosional surfaces separate early transgressive association C from the late lowstand association B. The systematic B/C channel scouring is interpreted as the result of a high water/sediment discharge ratio determined by a decrease of coarse-grained sediment supply to the fluvial systems during rise of base level. This erosive surface is conceptually analogous to the ravinement surface sculpted by wave erosion during the transgressive, landward migration of a shoreface. The late transgressive and highstand mud-dominated association D records the flood basin, a depositional environment indicative of a high base level which transformed a former channel belt in a plain dominated by fine-grained sediment settling, bio- and pedoturbation. The studied succession records rhythmic variations of base level and sediment supply to the fluvial systems, in turn regulated by different-rank relative fluctuations of Piacenzian sea level. In this perspective, concepts of sequence stratigraphy and facies analysis are exploited for producing a reliable fluvial sequence stratigraphic model.

Facies model of a fine‐grained, tide‐dominated delta: Lower Dir Abu Lifa Member (Eocene), Western Desert, Egypt

AbstractExisting facies models of tide‐dominated deltas largely omit fine‐grained, mud‐rich successions. Sedimentary facies and sequence stratigraphic analysis of the exceptionally well‐preserved Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) aims to bridge this gap. The succession was deposited in a structurally controlled, shallow, macrotidal embayment and deposition was supplemented by fluvial processes but lacked wave influence. The succession contains two stacked, progradational parasequence sets bounded by regionally extensive flooding surfaces. Within this succession two main genetic elements are identified: non‐channelized tidal bars and tidal channels. Non‐channelized tidal bars comprise coarsening‐upward sandbodies, including large, downcurrent‐dipping accretion surfaces, sometimes capped by palaeosols indicating emergence. Tidal channels are preserved as single‐storey and multilateral bodies filled by: (i) laterally migrating, elongate tidal bars (inclined heterolithic strata, 5 to 25 m thick); (ii) forward‐facing lobate bars (sigmoidal heterolithic strata, up to 10 m thick); (iii) side bars displaying oblique to vertical accretion (4 to 7 m thick); or (iv) vertically‐accreting mud (1 to 4 m thick). Palaeocurrent data show that channels were swept by bidirectional tidal currents and typically were mutually evasive. Along‐strike variability defines a similar large‐scale architecture in both parasequence sets: a deeply scoured channel belt characterized by widespread inclined heterolithic strata is eroded from the parasequence‐set top, and flanked by stacked, non‐channelized tidal bars and smaller channelized bodies. The tide‐dominated delta is characterized by: (i) the regressive stratigraphic context; (ii) net‐progradational stratigraphic architecture within the succession; (iii) the absence of upward deepening trends and tidal ravinement surfaces; and (iv) architectural relations that demonstrate contemporaneous tidal distributary channel infill and tidal bar accretion at the delta front. The detailed facies analysis of this fine‐grained, tide‐dominated deltaic succession expands the range of depositional models available for the evaluation of ancient tidal successions, which are currently biased towards transgressive, valley‐confined estuarine and coarser grained deltaic depositional systems.