Incised Valley Fills Research Articles

Tectonically Controlled Nearshore Deposition: Cozzette Sandstone, Book Cliffs, Colorado, U.S.A

Abstract The Book Cliffs of eastern Utah and western Colorado have been pivotal in the development of outcrop-based sequence stratigraphic concepts for nonmarine to shallow marine siliciclastic depositional settings. Prior studies in this area, and more generally in the Cretaceous western interior foreland basin of North America, have concluded that nearshore accumulation is controlled for the most part by the interaction between oscillatory eustatic change and longer-term regional patterns of flexural subsidence. New outcrop and subsurface evidence reported here from the eastern Book Cliffs suggests that three-dimensional tectonic tilting at length scales of up to ∼ 50 km (31 mi) and timescales of less than ∼ 200 kyr also strongly influenced sedimentation. Continental ice sheets are thought to have been small at the time. Documented patterns of accumulation are inconsistent with those expected from interactions of eustasy and regional flexure alone. The upper Campanian Cozzette Sandstone Member of the Mount Garfield Formation consists of twelve lithofacies arranged into six lithofacies assemblages, inferred to have been deposited in shallow marine, marginal marine, and nonmarine depositional environments. Shallow marine facies are organized into six wedge-shaped units < 40 m thick, bounded by flooding surfaces, and separated into three larger-scale cycles, which display along-strike and time-equivalent aggradational and progradational stacking. These successions are interpreted as shoreface–foreshore–swamp parasequences, and are erosionally overlain by fluvial and estuarine deposits. Both fluvial and estuarine accumulations are underlain by composite erosional surfaces, belonging either to two distinct incised-valley fills or to one composite fill. The proposed interpretation is based on high-resolution correlations made from digital video and continuous photographs acquired during a helicopter survey of the member. This interpretation differs significantly from published cross sections of the Cozzette, in which the sandstone is inferred to consist of at least two sheet-like shallow marine parasequences, truncated by sequence boundaries. Facies variations, stratigraphic thickness trends, and geometrical relationships reveal that three basinward–landward cycles of syndepositional to postdepositional tectonic tilting in the southern Piceance basin controlled accumulation in the Cozzette Sandstone. Differential subsidence to the southeast led to the development of a northeast-trending clinoform rollover. Subsequent tilting to the north resulted in renewed accommodation creation, and reorientation of the rollover to an eastward trend. Following deposition of six shallow marine parasequences, tilting towards the northeast resulted in bypass and the development of a sequence boundary. The main conclusion of our study, that nearshore sedimentation was modulated by local tectonism at timescales of less than hundreds of thousands of years, casts doubt on the generally accepted eustatic paradigm at a location that was among the most important in the development of sequence stratigraphic concepts.

Sedimentary facies and depositional environments of the Pleistocene to Holocene incised valley fills in Funabashi city, Chiba prefecture

Sedimentary facies and depositional environments of the Pleistocene to Holocene incised valley fills in Funabashi city, Chiba prefecture

Recognition of Tidal-Bore Deposits In the Rock Record: Towards A Facies Model

Abstract A facies model for the deposits of tidal bores (upstream-propagating hydraulic jumps associated with the flood tide in estuarine rivers) has not yet been developed, despite the publication of data from laboratory experiments and some modern estuaries. Moreover, only one example of tidal-bore deposits has hitherto been identified in the rock record. Herein, we document evidence for tidal bores in an Upper Pennsylvanian (Virgilian, Gzhelian) incised-valley fill in northeastern Kansas, USA. Across the flanks and crest of a bank-attached bar remnant in this fill are thin (< 10 cm) lenses of muddy, massive sandstone and massive, plant-debris-rich sandstone bounded by erosion surfaces exhibiting symmetrical wave ripples and irregular scouring. This interval is enclosed by sandstones with bidirectional cross-stratification, rhythmites, mud drapes, flaser bedding, and related sedimentary structures that together record appreciable tidal influence. The erosion surfaces bounding the massive sandstone lenses, and the massive sandstone lenses themselves, are evidence for scour and sediment suspension by upstream-propagating bores followed by deposition from suspension after their passage. Not only are these deposits the first sedimentary record of tidal bores to be documented in the Pennsylvanian of the USA, they are only the second example of such deposits to be documented in the global rock record. On the basis of our observations, we propose a provisional facies model for tidal-bore deposits that can be refined by future work.

THE OCCURRENCE OF VERTEBRATE AND INVERTEBRATE FOSSILS IN A SEQUENCE STRATIGRAPHIC CONTEXT: THE JURASSIC SUNDANCE FORMATION, BIGHORN BASIN, WYOMING, U.S.A

Previous studies of the sequence stratigraphic distribution of fossils have focused on the record of relatively abundant marine invertebrates. Only a handful of studies have examined how sequence stratigraphic architecture influences the occurrence of vertebrates, particularly large and rare tetrapods. The Jurassic Sundance Formation of the Bighorn Basin, Wyoming, USA, contains a rich suite of invertebrate and vertebrate fossils, including large and rare marine reptiles, and this allows the sequence stratigraphic controls on the distribution of these groups to be compared. The Sundance Formation consists of four depositional sequences, with the lower two being carbonate dominated and the upper two siliciclastic dominated. Two incised valley fills are also present. The presence of multiple depositional sequences and strongly erosional sequence boundaries is the likely cause of the complicated lithostratigraphic nomenclature of the Sundance. Invertebrates (mollusks and echinoderms) in the Sundance conform to well-established patterns of occurrences, including strong facies control and fossil concentrations at maximum flooding surfaces, in the upper portion of parasequences, and within lags overlying sequence boundaries. As expected from their rarity, marine reptiles (ichthyosaurs, plesiosaurs, and pliosaurs) show a weaker connection to sequence stratigraphic architecture. Nonetheless, they do display facies control and are found primarily in offshore mudstone, rather than shoreface and estuarine sandstone. They are also more common at hiatal surfaces, including a zone of concretions at the maximum flooding surface and in lag deposits overlying sequence boundaries. These associations suggest that sequence stratigraphic architecture may be a useful approach for discovery of marine vertebrates and that sequence stratigraphic context should be considered when making paleobiological interpretations of marine vertebrates as well as invertebrates.

Paleocene sequence stratigraphy and depositional systems in the Lishui Sag, East China Sea Shelf Basin

The East China Sea Shelf Basin (ECSSB) is a typical back-arc rift basin and is divided further by a series of sags and basement ridges. The Lishui Sag on the southwest margin of the ECSSB, is underlain by a productive, gas-prone, marine Paleocene section. In combination with sequence stratigraphy and seismic geomorphology, an integrated approach was applied to map the high-frequency sequences and systems tracts of the Lower E1m (57.2–56.5 Ma) and Upper E1l strata (60–57.2 Ma) by using well, two-dimensional and three-dimensional seismic data. Our approach involves documentation of seismic stratal geometries, seismic facies and core lithofacies. Seismic truncation, offlap, onlap and downlap geometries were used for identifying two depositional sequences with four systems tracts in the Upper E1l and Lower E1m strata. Two sedimentary facies were recognized from core and log data and identified as tidal sand ridges and delta mouth bar deposits. The depositional facies were imaged from a series of strata slices obtained by 3D survey. Five types of depositional systems were recognized based on the geometry change expressed on strata slices. In stratigraphic order these are (a) incised channels on the inner shelf and subaqueous fans on the middle shelf, (b) diffuse, elongated linear tidal sand ridges on the continental shelf and turbidity flows on the slope, (c) incised valley, prograding shelf edge delta on the continental shelf and shelf edge and delta front turbidites on the slope, (d) incised valley fills, fluvial aggradation and fluvial dominated mouth bar on the continental shelf, and (e) shoreface barrier bar and sand ridges that are parallel to the shoreline. This integrated analysis provides us an opportunity to evaluate the exploration significance in the stratigraphic framework and improve the accuracy of petroleum plays prediction.

Late Quaternary stratigraphic evolution on the outer shelf of the East China Sea

The East China Sea is a typical marginal sea and is characterized by strong land–sea interaction and paleoenvironmental changes during the late Quaternary with sea-level fluctuations. In this study, late Quaternary sedimentary stratigraphy and facies on the outer shelf of the East China Sea were reconstructed by using high-resolution seismic reflections and combining evidences from lithology, microfossil assemblages, element geochemistry and geochronology of borehole SFK-1. Sedimentary sequence consists of nearshore facies of forced regression formed during the late marine isotopic stage 3, tide-influenced fluvial and estuarine facies during the last glacial maximum, incised-valley fill and estuarine-tidal flat facies during the deglacial transgression, buried and quasi-active tidal sand ridge facies during the postglacial and Holocene periods.The sea-level fluctuation and paleo-river and sea interaction primarily controlled the stratigraphic framework and sedimentary facies on the outer shelf during the late Quaternary. This study confirms the occurrence of paleo-river channels and relating deposits on the outer shelf. Paleo-fluvial deposits accumulated during the last glacial maximum albeit with strong tidal reworking, and tide-dominated estuarine facies are recognized during the early deglacial transgression. The previously-recognized delta facies might not have formed on the outer shelf because of the low sediment supply and/or strong tidal reworking.

Development of An Incised-Valley Fill Under the Influence of Tectonism and Glacio-Eustatic Sea-Level Change: Valley Morphology, Fluvial Style, and Lithology

Abstract Sequence stratigraphic concepts for making predictive lithologic models are based on relative sea-level changes, which for lower-rank cycles of 1 Ma or 400 ka are considered to be driven by eustasy. This study of an incised-valley fill of the lower Miocene Addit Member, Billund Formation, Denmark, shows that this is an oversimplification and that the distribution of lithology differs from that commonly used in sequence stratigraphic models. The incised-valley fill is composed of up to 60 m of fluvial deposits with intercalated lacustrine and marine sand and mud. A regional subaerial erosional surface separates the incised-valley fill into two distinct units that together constitute a compound fill. This second boundary clearly shows a stepped erosional pattern characteristic for a regional composite scoured surface (RCS). The scouring shows a systematic migration towards the east. The main incision and the twofold development of the valley fill are interpreted to be strongly influenced by the eustatic sea-level changes that occurred during the end of the Aquitanian (early Miocene), the so-called Mi1a glacial event. The development of systems tracts also follows the standard sequence stratigraphic model. However, inversion tectonism and continued uplift of the hinterland resulted in the upstream widening of the valley morphology and the systematic eastward migration of incision of at least the second regional composite scour surface. The presence of gravel clasts (< 5 cm) indicates a relatively steep dip for the fluvial system. As only minor local climatic changes have been detected during the incision and filling of the valley, local climate is not considered to have been important.

Fluvial evolution of the Rhine during the last interglacial-glacial cycle in the southern North Sea basin: A review and look forward

This paper presents the current state of knowledge on the evolution and depositional history of the River Rhine in the southern part of the North Sea basin during the upper Middle and Late Pleistocene, and its response to climate change, sea-level oscillation and glacio-isostasy. The study focuses on the development of the Eemian interglacial lower-delta in the central Netherlands and its relation to records of climate and sea-level rise, and uses the Saalian and Weichselian pre- and postdating periods to place its development in context.The Rhine fluvial system fills the gradually subsiding North Sea basin, but its development has strongly been affected by the Saalian glaciation and its remaining topography. Ice-pushed ridges originating off the limit of maximum glaciation basically divided the central Netherlands into two sedimentary depocentres: a central depocentre within the former ice-limit, and a southern depocentre south of it.The sedimentary record of the central depocentre, including an incised-valley fill, shows a 20–40 m thick stacked sequence consisting of three units. The incised-valley fill consists of a Late Saalian to early Eemian age lower fluvial unit and a Weichselian age upper fluvial unit, both composed of coarse-grained channel deposits. Sandwiched in-between is a 5–15 m thick record composed of fine-grained fluvial and estuarine (tidal) floodbasin and shallow-marine deposits. It is of Eemian interglacial and Early Weichselian age, and comprises transgressive and highstand deposits that show the drowning of a fluvial system. Inland parts transformed from fluvial to deltaic and estuarine environments, and the most downstream parts transformed to a shallow-marine embayment. Preservation of these units occurred, despite considerable sea-level fall and climate-controlled erosion taking place in the last-glacial. Preservation potential was increased by the fact that the Rhine system avulsed away to the southern depocentre, halfway the Weichselian Pleniglacial. Consequently, the infill of the southern depocentre is of an entire different nature, and last-interglacial transgressive or highstand units are hardly preserved.Because of glaciation and resulting depocentre configuration, the Netherlands in NW Europe thus offers a very good opportunity to study the transgressive interglacial lower-deltaic records and falling-stage preservation thereof – both key elements for understanding sedimentary development over full 100-ky glacial-interglacial cycles of climate and base-level change.

Depositional process of incised-valley fills of the Pleistocene Kioroshi Formation, Shimosa Group in the Omiya and Noda districts, central Kanto Plain

Depositional process of incised-valley fills of the Pleistocene Kioroshi Formation, Shimosa Group in the Omiya and Noda districts, central Kanto Plain

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.

Sequence stratigraphic controls on reservoir characterization and architecture: case study of the Messinian Abu Madi incised-valley fill, Egypt

Abstract Understanding sequence stratigraphy architecture in the incised-valley is a crucial step to understanding the effect of relative sea level changes on reservoir characterization and architecture. This paper presents a sequence stratigraphic framework of the incised-valley strata within the late Messinian Abu Madi Formation based on seismic and borehole data. Analysis of sand-body distribution reveals that fluvial channel sandstones in the Abu Madi Formation in the Baltim Fields, offshore Nile Delta, Egypt, are not randomly distributed but are predictable in their spatial and stratigraphic position. Elucidation of the distribution of sandstones in the Abu Madi incised-valley fill within a sequence stratigraphic framework allows a better understanding of their characterization and architecture during burial. Strata of the Abu Madi Formation are interpreted to comprise two sequences, which are the most complex stratigraphically; their deposits comprise a complex incised valley fill. The lower sequence (SQ1) consists of a thick incised valley-fill of a Lowstand Systems Tract (LST1)) overlain by a Transgressive Systems Tract (TST1) and Highstand Systems Tract (HST1). The upper sequence (SQ2) contains channel-fill and is interpreted as a LST2 which has a thin sandstone channel deposits. Above this, channel-fill sandstone and related strata with tidal influence delineates the base of TST2, which is overlain by a HST2. Gas reservoirs of the Abu Madi Formation (present-day depth ∼3552 m), the Baltim Fields, Egypt, consist of fluvial lowstand systems tract (LST) sandstones deposited in an incised valley. LST sandstones have a wide range of porosity (15 to 28%) and permeability (1 to 5080mD), which reflect both depositional facies and diagenetic controls. This work demonstrates the value of constraining and evaluating the impact of sequence stratigraphic distribution on reservoir characterization and architecture in incised-valley deposits, and thus has an important impact on reservoir quality evolution in hydrocarbon exploration in such settings.

Stratigraphic signatures of punctuated rise in relative sea-level in an estuary-dominated heterolithic succession: Incised valley fills of the Toarcian Ostreaelv Formation, Neill Klinter Group (Jameson Land, East Greenland)

The Ostreaelv Formation (latest Pliensbachian–Toarcian) of the Neill Klinter Group is exposed along a >105 km wide, ENE-trending section in Jameson Land, East Greenland. Deposition took place in a large embayment (Jameson Land Basin) that was connected to the proto-Norwegian-Greenland Sea. Lithofacies in the Ostreaelv Formation range from clean sandstone to muddy heterolithic facies typified by strong grain-size contrasts.The Ostreaelv Formation is divided into four distinct and overall retrograding allostratigraphic units each composed of a characteristic set of tide-influenced, tide-dominated and wave-influenced facies associations. The allostratigraphic units are bounded by subaerial unconformities, interpreted as sequence boundaries, and are up to 75 m thick and 16 to >20 km in width. The allostratigraphic units include a sandy heterolithic estuary bay-head delta succession overlain by two sandy tide-dominated estuary fill successions, interbedded with a muddy heterolithic offshore marine succession. Each of the three estuarine allostratigraphic units was accumulated in an incised valley formed during fall in relative sea level and filled during successive transgressions with sediment supplied from marine and reworked fluvial deposits.In the three incised valleys fluvial sediments were deposited on top of an initial subaerial unconformity surface (SU) and were later reworked by succeeding transgressive ravinement along a transgressive surface (TS), thus creating combined SU/TS sequence boundaries. The data from the Ostreaelv Formation also provides knowledge and conceptual understanding of valley infill processes (tidal current, wave and fluvial energy), and both lateral and vertical variations in lithofacies architecture within incised valleys.Moreover, the study provides quantitative input data, such as incised valley dimensions, sand-containing capacity, and geometry to subsurface reservoir characterisation and modelling efforts of estuary fill successions.

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.

Three-dimensional analysis of a stratigraphic surface model by the borehole data interpolation in consideration of the formation process of buried geomorhic surfaces : a case of the basal surface model of the latest Pleistocene to Holocene incised-valley fills in the northern Tokyo lowland and the southern Nakagawa lowland, central Japan

東京低地北部から中川低地南部に分布する沖積層を対象として，地形・地質の形成過程を考慮したボーリングデータ補間による沖積層基底面モデルの三次元解析方法を示す．作成手順は以下の通りである．（1）沖積層基底面はボーリングデータから，そして沖積層の地表境界線と台地面は地質図と国土地理院の数値地図5 mメッシュ（標高）から，それぞれ実ポイントデータを作成する，（2）埋没平坦面と侵食崖との境界，支谷筋を明確にするために補填ポイントデータを作成する，（3）形成年代の異なる埋没平坦面毎に実ポイントデータから面モデルを求めそのポイントデータを作成する，（4）これらのポイントデータから，沖積層基底面モデルを空間補間処理計算によって求める．得られた沖積層基底面モデルは，ボーリングデータの不足・偏在の影響を補い，沖積低地に伏在する最終氷期最盛期頃に形成された古地形面を近似するものとなる．

Sequence stratigraphic hierarchy of the Upper Devonian Foreknobs Formation, central Appalachian Basin, USA: Evidence for transitional greenhouse to icehouse conditions

The Foreknobs Formation (Upper Devonian; Upper Frasnian to basal Famennian) comprises the uppermost marine strata of the progradational “Catskill clastic wedge” of the south-central Appalachian Mountains (Virginia-West Virginia; USA). The Foreknobs Formation consists of 14 lithofacies arranged in four facies associations which record the following depositional settings: 1) storm-dominated distal to proximal offshore to shoreface (facies association A); 2) sharp-based conglomeratic shoreface (facies association B); 3) fluvial redbed (facies association C); and 4) incised-valley fill (IVF; facies association D). Vertical juxtaposition and stacking patterns of lithofacies and facies associations permit recognition of a hierarchy of three scales of cyclicity. Up to 70 short-term 5th-order cycles, each averaging ~65kyr, consist of coarsening-upward parasequences of storm-dominated offshore marine facies in the distal setting which correspond to high frequency (unconformity bound) sequences (HFS) of fluvial redbed strata overlain by offshore marine strata in the proximal setting. These facies relationships are a consequence of 10–15m of sea-level fluctuations. Up to 12 intermediate-term 4th-order cycles, each averaging ~375kyr, consist of stacked 5th-order cycles. The 4th-order cycles are bounded by regressive surfaces of marine erosion (RSME) at the base of sharp-based conglomeratic shoreface sandstones in the distal setting that correspond with paleosols in the proximal setting. In some cases, the 5th-order cycles within each 4th-order cycle exhibit stacking patterns indicative of increasing or decreasing accommodation space. These facies relationships are a consequence of 25–35m of sea-level fluctuations. Three complete and portions of two additional 3rd-order cycles, each averaging ~1.12Myr, consist of stacked 4th-order cycles. The 3rd-order sea-level trends reflected in the Foreknobs Formation are nearly identical to published eustatic sea-level curves. Incised-valley fills are present at one of the 3rd-order cycle boundaries and are a consequence of a 35–45m sea-level fluctuation. The amplitudes of the inferred sea-level fluctuations are comparable to the expansion and contraction of ice volumes within the current Greenland and Antarctic ice sheets which suggests glacioeustasy was the primary control on sea-level fluctuations and cyclicity within the Foreknobs Formation. Such an interpretation is consistent with knowledge of Devonian climate, transitioning from Middle Devonian greenhouse to Late Devonian icehouse, as indicated by evidence of glaciation during parts of the Late Devonian in South America and the Appalachians.

Formation and preservation of an overstepped segmented lagoon complex on a high‐energy continental shelf

AbstractThe duration of shoreline occupation at a given sea‐level, coastal response to sea‐level change and the controls on preservation of various shoreline elements can be recognized by detailed examination of submerged shorelines on the continental shelf. Using bathymetric and seismic observations, this article documents the evolution and preservation of an incised valley and lithified barrier complex between −65 m and −50 m mean sea‐level on a wave‐dominated continental shelf. The barrier complex is preserved as a series of aeolianite or beachrock ridges backed by laterally extensive back‐barrier sediments. The ridges include prograded cuspate lagoonal shoreline features similar to those found in contemporary lagoons. The incised valley trends shore‐parallel behind the barrier complex and records an early phase of valley filling, followed by a phase of extensive lagoonal sedimentation beyond the margins of the incised bedrock valley. Sea‐level stability at the outer barrier position (ca −65 m) enabled accumulation of a substantial coastal barrier that remained intact during a phase of subsequent slow sea‐level rise to −58 m when the lagoon formed. These lagoonal sediments are stripped seawards by bay ravinement processes which caused the formation of several prograded marginal cuspate features. An abrupt rise in sea‐level to −40 m, correlated with melt‐water pulse 1B, enabled the preservation of thick lagoonal sediments at the top of the incised valley fill and preservation on the sea bed of the cemented core of the barriers. This situation is unique to subtropical coastlines where early diagenesis is possible. The overlying sandy sediment from the uncemented upper portion of the barriers is dispersed by ravinement, partly burying the ridges and protecting the underlying sediments. The high degree of barrier or shoreline preservation is attributed to rapid overstepping of the shoreline, early cementation in favourable climatic conditions and the protection of the barrier cores by sand sheet draping.

Correlation of the latest Pleistocene-Holocene incised valley fills between the Arakawa and Menuma Lowland and the Nakagawa and Watarase Lowland, Central Kanto Plain

Correlation of the latest Pleistocene-Holocene incised valley fills between the Arakawa and Menuma Lowland and the Nakagawa and Watarase Lowland, Central Kanto Plain

テクトニックな沈降域における沿岸河口低地の地層形成

There have been fewer studies on strata formation in tectonically subsiding areas than in tectonically stable and uplifting areas. In this study, (1) 1000 year scale sediment stacking patterns, (2) seismic subsidence and meltwater pulse (MWP) 1B, and (3) event stratigraphy derived from seismic subsidence are revealed on the basis of sedimentary facies and 162 radiocarbon dating of three sediment cores obtained from the latest Pleistocene to Holocene Shinano River incised-valley fill (Alluvium) in the Echigo Plain. Aggradation is dominant in sediment stacking patterns of the incised-valley fill due to tectonic subsidence from the Nagaoka Plain Western Margin Fault Zone. Two continuous layers with abundant marine diatoms and rapid sediment accumulation rates are identified in transgressive salt marsh sediments. These layers may be due to seismic subsidence, but the latter layer (11.2-11.6 cal kyr BP) coincides with MWP 1B. If MWP 1B exists, the thickness of the latter layer indicates that the magnitude of the relative sea-level jump is less than 4 m. The event stratigraphy derived from seismic subsidence in salt marsh sediments is characterized by grading, dominant marine diatoms, and intense sediment accumulation rates. The presence of this event stratigraphy in the Echigo Plain suggests that seismic events strongly affect strata formation in tectonically subsiding areas in Japan.

沖積粘性土のせん断剛性率とその堆積時環境依存性に関する研究

The effects of earthquakes in the Kanto district are strongly related to the three-dimensional distribution of the soil mechanical properties of Chuseki-so, which is the name given to incised-valley fills from the Late Pleistocene to the Holocene. To characterize the effects of the sedimentary depositional environment and sedimentary process of Chuseki-so on soil mechanical properties, PS logging test, physical-chemical test, consolidation test, cyclic tri-axial test, and bender element test were conducted on an undisturbed core sample. Based on the test results, we propose a method to estimate the elastic shear modulus of clay soil from conventional soil properties, and discuss the relationship between the sedimentary process of Chuseki-so and the three-dimensional distribution of soil mechanical properties. Through our investigation, we derived the following conclusions: (1) The delta front and inner bay deposits of the Kasukabe core in the Nakagawa Lowland comprise very soft clay and have a high water content and liquidity index; these characteristics are related to the sedimentary depositional environment such as water depth and salinity (ratio of marine diatoms in the sediment). (2) The proposed method can be used to estimate elastic shear moduli on the basis of water content, depth, and locality parameter; in particular, it can be used to evaluate very soft clay, the N value of which is less than 1, on the basis of water content, an easily measurable property. (3) Both bender element and PS logging tests show similar S-wave velocities, and elastic shear moduli measured using the respective tests, G0BE and G0PS, were also similar; therefore, elastic shear moduli can be determined through measurements using these two tests or estimations based on water content. (4) The method to determine the elastic shear modulus, G0TR, which is obtained using a cyclic test, is proposed based on the H-D model; G0TR btained correlates well with G0BE, even if though it is obtained by a different method. The proposed method can be used to determine elastic shear moduli with great accurac.

Sedimentology and stratigraphic development of the upper Nyalau Formation (Early Miocene), Sarawak, Malaysia: A mixed wave- and tide-influenced coastal system

This work presents the first detailed facies analysis of the upper Nyalau Formation exposed around Bintulu, Sarawak, Malaysia. The Lower Miocene Nyalau Formation exposures in NW Sarawak represent one of the closest sedimentological outcrop analogues to the age equivalent, hydrocarbon-bearing, offshore deposits of the Balingian Province. Nine types of facies associations are recognised in the Nyalau Formation, which form elements of larger-scale facies successions. Wave-dominated shoreface facies successions display coarsening upward trends from Offshore, into Lower Shoreface and Upper Shoreface Facies Associations. Fluvio-tidal channel facies successions consist of multi-storey stacks of Fluvial-Dominated, Tide-Influenced and Tide-Dominated Channel Facies Associations interbedded with minor Bay and Mangrove Facies Associations. Estuarine bay facies successions are composed of Tidal Bar and Bay Facies Associations with minor Mangrove Facies Associations. Tide-dominated delta facies successions coarsen upward from an Offshore into the Tidal Bar Facies Association. The Nyalau Formation is interpreted as a mixed wave- and tide-influenced coastal depositional system, with an offshore wave-dominated barrier shoreface being incised by laterally migrating tidal channels and offshore migrating tidal bars. Stratigraphic successions in the Nyalau Formation form repetitive high frequency, regressive–transgressive cycles bounded by flooding surfaces, consisting of a basal coarsening upward, wave-dominated shoreface facies succession (representing a prograding barrier shoreface and/or beach-strandplain) which is sharply overlain by fluvio-tidal channel, estuarine bay or tide-dominated delta facies successions (representing more inshore, tide-influenced coastal depositional environments). An erosion surface separates the underlying wave-dominated facies succession from overlying tidal facies successions in each regressive–transgressive cycle. These erosion surfaces are interpreted as unconformities formed when base level fall resulted in deep incision of barrier shorefaces. Inshore, fluvio-tidal successions above the unconformity display upward increase in marine influence and are interpreted as transgressive incised valley fills.