Delta-front Deposits Research Articles

Activity of the Yoro Fault System Determined from Coseismic Subsidence Events Recorded in the Holocene Delta Sequence of the Nobi Plain, Central Japan

The Holocene delta sequence of the Nobi Plain, central Japan, records coseismic subsidence in the western part of the plain caused during the past 6000 years by movement on the Yoro fault. We identified four episodes of upward‐fining deposition and increasing electrical conductivity in delta front deposits that occurred 600–200, 1300–900, 4200–3800, and 5600–4700 years ago, suggesting that episodes of sea‐level rise interrupted progradation of the Kiso River delta. Considering the general trend of relative sea‐level fall during the middle to late Holocene under the influence of eustasy and hydroisostasy, the temporal correspondence of rises of relative sea level with activity on the Yoro fault suggests they reflect coseismic subsidence due to Yoro fault activity. The timing of the coseismic subsidence events identified in this study also corresponds with that of previously reported faulting events at the Kuwana fault to the south of the Yoro fault. These results are consistent with the previous interpretation that the Yoro and Kuwana faults comprise a behavioral segment within the Yoro fault system. Our off‐fault paleoseismic study shows that coseismic subsidence events detected in Holocene delta sequences can provide further evidence of faulting events inferred from the on‐fault paleoseismic studies.

Architecture analysis of a river flood-dominated delta during an overall sea-level rise (early Pliocene, SE Spain)

The sedimentary record of delta-front to prodelta transition that developed during the early Pliocene (Bajo Segura Basin, peri-Mediterranean Betic Cordillera Basin) represents an unusual infilling of an incised valley cutting downwards into late Messinian pelagic marls. The complete delta succession, less than 65 m thick, consists of four upwardly fining and thinning sequences several metres thick. Each sequence consists of basal conglomeratic and coarse-grained sandstone beds (delta-front deposits) evolving upwards to alternating sandstones and silty-marl beds (prodelta deposits). The retrogradational stratal succession pattern is interpreted as having been deposited during an overall sea-level rise. Facies association is characterised by quasi- to planar-laminated sandstones and pebbly sandstones alternating with clast-supported boulders to pebbles, sheet-like climbing cross-laminated sandstones with abundant plant remains, and epsilon cross-stratified and channelized gravels and sandstones. These are interpreted as having been deposited by highly concentrated flows interpreted as hyperpycnal turbidity flows from high (cohesionless debris flows and sinuous submarine channels) to moderate (sheet-like climbing cross-laminated sandstones) discharge river-flood events. Slabs of sediment from unstable banks, outsized clasts which roll onto thin beds from channel mouths, slumps and intraformational breccias in delta front are other signatures (including a lack of bored clasts) which indicate high-energy and rapid deposition at the river mouth, encouraging the delta-front gravitational instability failures. Major changes in the discharge of the river-fed delta could have been caused by brief catastrophic, heavy-precipitation events coupled with quasi-stationary convective orographic rainfall typical of the modern Mediterranean.

Along-Strike and Down-Dip Variations in Shallow-Marine Sequence Stratigraphic Architecture: Upper Cretaceous Star Point Sandstone, Wasatch Plateau, Central Utah, U.S.A.

Abstract The sequence stratigraphic architectures of shallow-marine deposits in the upper Cretaceous Star Point Sandstone are analyzed over a large (c. 100 km), nearly continuous outcrop section aligned oblique to depositional strike. The unit consists of five parasequences that predominantly comprise wave-dominated shoreface–shelf deposits. Two parasequences contain riverdominated delta-front deposits locally. Within each parasequence, wave-dominated shoreface–shelf deposits record 7–45 km of ESE- to ENE-directed progradation of a linear to moderately lobate shoreline that was supplied with sediment by longshore drift and subjected to strong offshore sediment transport by storms. Wave-dominated shoreface sandstones in each parasequence thin and wedge out over short distances ( 800 km2) river-dominated delta-front complex (Panther Tongue). This highly focused fluvial sediment flux probably occurred via a structurally controlled sediment entry point between two active thrusts.

Taphonomy and paleoecology of the bivalve trace fossilProtovirgulariain deltaic heterolithic facies of the Miocene Chenque Formation, Patagonia, Argentina

Lower Miocene tide-influenced deltaic deposits from the Chenque Formation, Patagonia, Argentina, contain abundant and well-preserved biogenic structures attributed to locomotion of deposit-feeder protobranch bivalves. These trace fossils, assigned to the ichnogenusProtovirgularia, consist of delicate, inclined-to-horizontal, chevronate structures, mostly symmetrical with respect to a median axis. Identification ofProtovirgulariaat sandstone sole beds (hypichnion) is quite straightforward. Endichnial, exichnial and epichnial preservation in heterolithic facies, however, provides a wide variety of forms that depart from the archetypalProtovirgulariaand challenges ichnotaxonomic classification. Specimens in prodelta and delta-front facies display morphologic features controlled by substrate fluidity, toponomy, and sedimentation rate. Most specimens show sharp, closely spaced chevrons and occur along sandstone/mudstone interfaces of the proximal prodelta and distal delta-front deposits. These forms reflect how tracemakers experienced significant friction while advancing through the sediment, which resulted in relatively smaller increments of movements. In contrast, variants ofProtovirgulariaformed in muddier beds, such as in prodeltaic facies, show irregular, poorly defined and unevenly spaced chevrons, and are locally asymmetric in relation to the axis, reflecting softer, water-rich, and plastic substrates. This sediment offered relatively low friction but poor anchorage for the foot. These occurrences ofProtovirgulariain tide-influenced, marginal-marine deposits suggests that protobranchs were tolerant of fluctuations in salinity, sedimentation rates, turbidity, and oxygen depletion, displaying opportunistic strategies in stressed nearshore environments. Our evaluation of taphonomic controls and appropriate identification ofProtovirgulariacan provide valuable information for expanding our knowledge of the ethology and paleoecology of protobranch bivalves.

Taphonomy and paleoecology of the bivalve trace fossil Protovirgularia in deltaic heterolithic facies of the Miocene Chenque Formation, Patagonia, Argentina

Lower Miocene tide-influenced deltaic deposits from the Chenque Formation, Patagonia, Argentina, contain abundant and well-preserved biogenic structures attributed to locomotion of deposit-feeder protobranch bivalves. These trace fossils, assigned to the ichnogenus Protovirgularia, consist of delicate, inclined-to-horizontal, chevronate structures, mostly symmetrical with respect to a median axis. Identification of Protovirgularia at sandstone sole beds (hypichnion) is quite straightforward. Endichnial, exichnial and epichnial preservation in heterolithic facies, however, provides a wide variety of forms that depart from the archetypal Protovirgularia and challenges ichnotaxonomic classification. Specimens in prodelta and delta-front facies display morphologic features controlled by substrate fluidity, toponomy, and sedimentation rate. Most specimens show sharp, closely spaced chevrons and occur along sandstone/mudstone interfaces of the proximal prodelta and distal delta-front deposits. These forms reflect how tracemakers experienced significant friction while advancing through the sediment, which resulted in relatively smaller increments of movements. In contrast, variants of Protovirgularia formed in muddier beds, such as in prodeltaic facies, show irregular, poorly defined and unevenly spaced chevrons, and are locally asymmetric in relation to the axis, reflecting softer, water-rich, and plastic substrates. This sediment offered relatively low friction but poor anchorage for the foot. These occurrences of Protovirgularia in tide-influenced, marginal-marine deposits suggests that protobranchs were tolerant of fluctuations in salinity, sedimentation rates, turbidity, and oxygen depletion, displaying opportunistic strategies in stressed nearshore environments. Our evaluation of taphonomic controls and appropriate identification of Protovirgularia can provide valuable information for expanding our knowledge of the ethology and paleoecology of protobranch bivalves.

Controls on stratigraphic architecture in contemporaneous delta systems from the Eocene Roda Sandstone, Tremp-Graus Basin, northern Spain

The Eocene Roda Sandstone of the Spanish Pyrenees is comprised of two coeval coarse-grained delta systems which share correlatable surfaces but show a markedly different internal facies architecture. The two deltas developed contemporaneously with the differences in internal architecture attributed to basin morphology and the role of two, distinct sediment sources, while the correlatability of the larger-scale packages indicates an external, allocyclic control. As such the Roda Sandstone provides the possibility to address the relative roles of different parameters known to interact and control stratal architecture. The Roda Sandstone crops out at the NE-margin of the Tremp-Graus Basin, a “piggyback” basin initiated in Palaeogene times in the foreland of the Southern Pyrenees. The study area (∼ 30 km 2) is composed of the well-understood and well-exposed parts of the Roda Sandstone cropping out along the Isabena Valley, and the less-worked and less-exposed outcrops to the north along Barranco de Ricans and Barranco de Codoñeras. Data for this study include a combination of conventional sedimentology data coupled with analyses of 3D Virtual Outcrop Models. Based on these methods 8 different facies associations have been distinguished. Their distribution suggests that the Roda Sandstone is composed of two different but coeval delta systems and their linked subenvironments; a Gilbert-type delta outcropping along Isabena Valley, and a shoal-water fan delta along Barranco de Ricans and Barranco de Codoñeras. Along the Isabena Valley the outcrops of the Gilbert-type deltaic system are composed of at least 6 distinct packages (Roda U–Z) characterized by texturally mature, delta-front sandstones separated by offshore mudstones. The stacked basinward-stepping architecture of the Roda U–Y units here represents a minimum of 3 major phases of SW-directed progradation, capped by hardground horizons formed during the subsequent flooding phases. Tidally-modified delta-front deposits found in the distal exposures of Roda Y represent the redistribution of the delta toeset by W–NW directed ebb-tidal currents, active during advanced stages of progradation of the Gilbert-type delta. The more gentle sloping shoal-water fan-delta system found in Barranco de Ricans and Barranco de Codoñeras, is characterized by progradational to aggradational stacking of texturally immature wave-modified mouth-bar sandstone packages separated by offshore mudstones. The zone of interference between the Gilbert-type and shoal-water delta is dominated by wave-reworked sediments deposited as shoreface, barrier and spit deposits. Biostratigraphic dating suggests that the Roda Sandstone was deposited during a 3rd order relative sea level cycle as a shelf-margin systems tract. However, the stacking of the individual Roda U–Y units demarcated by offshore mudstones, both within the Gilbert-type and the shoal-water deltaic systems, suggest that 4th order fluctuations occurred. These 4th order units are physically correlatable between the two deltaic systems, which implies that both deltas responded synchronously to a combination of regional subsidence and eustatic sea level fluctuations. The differing internal architecture, such as the steeply-dipping, textural mature, foreset beds of the Gilbert-type delta versus the more gentle sloping, textural immature mouth-bars of the shoal-water fan-delta, are controlled by different hinterland characteristics and intrabasinal topography.

Tetrapod tracks in a marginal lacustrine setting (Middle Triassic, Argentina): Taphonomy and significance

Fossil tetrapod footprints not only provide valuable information about trackmaker paleobiology but also to give insight into details of the depositional conditions of the substrate at the time of imprinting. Therefore, in the present study the mode of formation and taphonomy of footprints in different substrates was used to investigate the gait and walking dynamics of the trackmakers as well as a source of additional information on the environmental conditions of the track-bearing beds during imprinting. The analyzed section corresponds to thick Middle Triassic lacustrine/deltaic deposits of the Ischichuca/Los Rastros Formation (Ischigualasto–Villa Unión Basin) that crops out at the Quebrada de Ischichuca in northwestern Argentina. Part of the track-bearing surfaces correspond to the top of sandy distributary channel mouth bars in a distal delta front setting that were exposed along the lake margin during a lake level fall. Cross-cutting relationships observed among ripple-marks, the footprints, and invertebrate traces of a softground suite of the Scoyenia ichnofacies suggest an omission surface. Measured trackway orientations in the sandstones are perpendicular to the paleo-shoreline, with the animals coming and going along the exposed top of the bars, probably for drinking. Laterally, the distal delta front deposits interfinger with track-bearing wackestone beds of palustrine origin deposited in a restricted local embayment lateral to the delta influenced environment. Trackway orientations in the wackestones are, in contrast, consistent with the animals moving nearly parallel to the lake border, probably along a preferred route. Evidences of a relative high groundwater table at the time of imprinting in the track-bearing surfaces are revealed by the well developed rims of extruded sediment and collapsed digits in the studied tracks and the nearly absence of associated desiccation cracks on the same surfaces. Nevertheless, temporary emergence cannot be ruled out when paleosoil formation was probably promoted as can be observed in the microstructure of both sandstones and wackestones. Moreover, footprint preservation in the wackestones might have been enhanced by partial hardening of the trampled surface during subaerial exposure. Combining ichnofossil content and taphonomy with facies analysis we identified in the lower part of the Ischichuca/Los Rastros succession a relatively rapid withdrawal of the water basinward that was probably due to a forced regression during early rifting of basin evolution. Footprints can also provide valuable information about locomotion dynamics and trackmaker behavior. Thus, the sideways deformation observed in the studied footprints, attributed to basal archosaurs and putative basal dinosaurs, can be related to an outward rotation of the foot during the step cycle, a condition that might allied to the development of the parasagittal posture in Archosauria. Besides, the densely trampled surface described herein constitutes the first documented evidence of putative social behavior among therapsid dicynodonts, the most important group of herviborous animals in the early Mesozoic terrestrial ecosystems throughout Gondwana.

沖積層ボーリングコアＧＳ‐ＳＳＳ‐１（埼玉県さいたま市）の堆積相と堆積物物性

The stratigraphy and transitions of sedimentary environments in the latest Pleistocene to Holocene incised valley fills in the Arakawa Lowland are revealed based on sedimentary facies, physical properties and chemical elements contents in the GS-SSS-1 core, Saitama City, Saitama Prefecture.The sedimentary successions are composed of Pleistocene Shimosa Group (Unit 1), gravelly fluvial deposits (Unit 2), sandy fluvial deposits (Unit 3), salt marsh deposits (Unit 4), inner bay to delta front deposits (Unit 5) and salt marsh to back marsh deposits (Unit 6) in ascending order. The depositional units excepting Unit 1 are the latest Pleistocene to Holocene incised valley fills, indicating a transgression (Unit 2 to the middle part of Unit 5) followed by a regression (the upper part of Unit 5 to Unit 6). In a transverse section, borehole logs correlated with the GS-SSS-1 core show that the gravelly fluvial deposits, sandy fluvial deposits and salt marsh deposits fill the narrow axis of the incised valley under the Arakawa Lowland, the inner bay deposits extend over buried terraces, and the delta front deposits and the salt marsh - back marsh deposits cover the entire lowland.

相対的海水準の急激な上昇イベントが示唆する養老断層系の完新世活動―濃尾平野西縁におけるボーリングコアの解析から―

A 6000-year history of relative sea-level change on the western Nobi plain, at the footwall side of the Yoro fault, was reconstructed from three drilling cores based on analyses of sedimentary facies, grain-size distributions and electrical conductivity (EC), as well as 20 14C age determination. The Yoro fault comprises the northern part of the Yoro fault system. These cores show a prograding delta sequence. Two intervals characterized by fine deposits, mainly composed of fine sand, and high EC value are identified in sandy delta front deposits showing a general coarsening upward sequence. In synchronous with these changes in the delta front, a rapid rise of EC value is recognized in the abandoned channel fill deposits on the delta plain. These data suggest that temporal sea-level rise events occurred during the progradation process of the delta system. A probable cause of these events is coseismic subsidence. These subsidence events are dated at around 5600-4700 cal BP and 4600-3800 cal BP, respectively. The above-mentioned subsidence events broadly correspond with previously known faulting events at the Kuwana fault to the south of the Yoro fault. Evidence for the faulting of the Yoro and Kuwana faults associated with two historical earthquakes (AD 745 and AD 1586) accompanied by coseismic subsidence and synchronicity of the subsidence events identified in this study and faulting events at the Kuwana faults suggest that subsidence events identified in this study are caused by faulting at the Yoro fault. These results are consistent with the notion that the Yoro and Kuwana faults comprise a behavioral segment in the Yoro fault system.

Intra-parasequence architecture of an interpreted asymmetrical wave-dominated delta

Although modern wave-dominated shorelines exhibit complex geomorphologies, their ancient counterparts are typically described in terms of shoreface-shelf parasequences with a simple internal architecture. This discrepancy can lead to poor discrimination between, and incorrect identification of, different types of wave-dominated shoreline in the stratigraphic record. Documented in this paper are the variability in facies characteristics, high-resolution stratigraphic architecture and interpreted palaeo-geomorphology within a single parasequence that is interpreted to record the advance of an ancient asymmetrical wave-dominated delta. The Standardville (Ab1) parasequence of the Aberdeen Member, Blackhawk Formation is exposed in the Book Cliffs of central Utah, USA. This parasequence, and four others in the Aberdeen Member, record the eastward progradation of north/south-trending, wave-dominated shorelines. Within the Standardville (Ab1) parasequence, distal wave-dominated shoreface-shelf deposits in the eastern part of the study area are overlain across a downlap surface by southward prograding fluvial-dominated delta-front deposits, which have previously been assigned to a separate ‘stranded lowstand parasequence’ formed by a significant, allogenic change in relative sea-level. High-resolution stratigraphic analysis of these deposits reveals that they are instead more likely to record a single episode of shoreline progradation characterized by alternating periods of normal regressive and forced regressive shoreline trajectory because of minor cyclical fluctuations in relative sea-level. Interpreted normal regressive shoreline trajectories within the wave-dominated shoreface-shelf deposits are marked by aggradational stacking of bedsets bounded by non-depositional discontinuity surfaces. Interpreted forced regressive shoreline trajectories in the same deposits are characterized by shallow incision of fluvial distributary channels and strongly progradational stacking of bedsets bounded by erosional discontinuity surfaces that record enhanced wave-base scour. Fluvial-dominated delta-front deposits most probably record the regression of a lobate delta parallel to the regional shoreline into an embayment that was sheltered from wave influence. Wave-dominated shoreface-shelf and fluvial-dominated delta-front deposits occur within the same parasequence, and their interpretation as the respective updrift and downdrift flanks of a single asymmetrical wave-dominated delta that periodically shifted its position provides the most straightforward explanation of the distribution and relative orientation of these two deposit types.

Stratigraphy and depositional environment of late Pleistocene Sunakothi Formation in Kathmandu Basin, central Nepal

Thick sandy sequences were recognised between the central and southern part of the basin, it was named as the Sunakothi Formation. We designate the type locality of this formation at Sunakothi, a village lying 3.0 km south of Patan. This formation is extensively distributed in the Nakhu, Kodku, and Godavari Kholas ranging in altitude from ca 1420 m in the southern margin to 1300 m in the central part. The average thickness of this formation ranges from 34 to 60 m. The sedimentary strata are gently inclined towards the north. On the basis of geological mapping this formation is located between the muddy part of the Kalimati Formation of the ancient lake and the terrace gravel deposits. It is further divided into the following four parts: (1) muddy rhythmic basal part, (2) sandy lower part, (3) muddy, sandy, and gravelly middle part, and (4) laminated silty upper part. The basal part shows a transition from the lacustrine to fluvial environment in the south and the prodeltaic environment towards the basin centre. Its lower part contains sandy fluvial to lacustrine delta front deposits whereas the middle part comprises sandy bar, muddy floodplain, and gravelly channel-fill deposits. The upper part of this formation is restricted only to the southern end of the basin and shows marginal shallow lacustrine environments. The sedimentological evidence indicates that the Palaeo-Kathmandu Lake was drained out from the south due to tectonic activities as well as late Pleistocene climatic changes.

Subsurface control on seafloor erosional processes offshore of the Chandeleur Islands, Louisiana

The Chandeleur Islands lie on the eastern side of the modern Mississippi River delta plain, near the edge of the St. Bernard Delta complex. Since abandonment approximately 2,000 years b.p., this delta complex has undergone subsidence and ravinement as the shoreline has transgressed across it. High-resolution seismic-reflection, sidescan-sonar, and bathymetry data show that seafloor erosion is influenced by locally variable shallow stratigraphy. The data reveal two general populations of shallow erosional depressions, either linear or subcircular in shape. Linear depressions occur primarily where sandy distributary-channel deposits are exposed on the seafloor. The subcircular pits are concentrated in areas where delta-front deposits crop out, and occasional seismic blanking indicates that gas is present. The difference in erosional patterns suggests that delta-front and distributary-channel deposits respond uniquely to wave and current energy expended on the inner shelf, particularly during stormy periods. Linear depressions may be the result of the sandy distributary-channel deposits eroding more readily by waves and coastal currents than the surrounding delta-front deposits. Pits may develop as gas discharge or liquefaction occurs within fine-grained delta-front deposits, causing seafloor collapse. These detailed observations suggest that ravinement of this inner shelf surface may be ongoing, is controlled by the underlying stratigraphy, and has varied morphologic expression.

沖積層ボーリングコアＧＳ‐ＴＫＴ‐１（埼玉県戸田市）の堆積相と堆積物物性

The stratigraphy and transitions of sedimentary environments in the latest Pleistocene to Holocene incised valley fills in the Arakawa Lowland are revealed based on sedimentary facies, fossil shell assemblages, physical properties, chemical element contents and radiocarbon dating in the GS-TKT-1 core, Toda City, Saitama Prefecture.The sedimentary successions are composed of Pleistocene Shimosa Group (Unit 1), gravelly fluvial deposits (Unit 2), sandy fluvial to salt marsh deposits (Unit 3), inner bay to delta front deposits (Unit 4) and sandy fluvial deposits (Unit 5) in ascending order. Unit 2 and upper units are the latest Pleistocene to Holocene incised valley fills, indicating a transgression (Unit 2-3) followed by a regression (Unit 4-5). Depositional ages are 10000 cal yBP or before for Unit 2, 10000-9000calyBP for Unit 3, 9000-5000calyBP for Unit 4 and 5000calyBP to present for Unit 5.In a transverse section, the gravelly fluvial deposits and the sandy fluvial deposits fill the narrow axis of the incised valley under the Arakawa Lowland, the salt marsh deposits and lower part of the bay deposits extend over a buried terrace south of the valley axis, and upper part of the bay deposits, the delta front deposits and the fluvial deposits cover the entire lowland.

Ichnology and sedimentology of a tide-influenced delta, Lower Miocene Chenque Formation, Patagonia, Argentina: Trace-fossil distribution and response to environmental stresses

Combined sedimentologic and ichnologic analysis of the Lower Miocene Chenque Formation allows recognition of a tide-influenced deltaic succession exposed along the coast of Caleta Olivia city, Patagonia, Argentina. Two main subenvironments were identified, prodelta and delta front, stacked forming a progradational coarsening-upward succession, up to 10 m thick. The prodelta deposits are mainly characterized by heterolithic strata (lenticular and wavy bedding), with low bioturbation intensity and sporadic distribution of trace fossils. The trace-fossil assemblage is dominated by deposit-feeder structures (e.g. Planolites montanus, Protovirgularia isp. , and Teichichnus rectus), constituting an impoverished expression of the Cruziana ichnofacies, with respect to their fully marine counterparts. A transition zone between the prodelta and the delta front is discontinuously distributed along this outcrop. This interval consists mainly of flaser-bedded sandstone, almost completely obliterated by equilibrium/adjustment trace fossils of large bivalves ( Atrina), and subordinately, by the trace fossils Nereites missouriensis, Phycosiphon incertum, T. rectus, Thalassinoides isp ., and Schaubcylindrichnus freyi. The delta-front deposits consist of sigmoidal cross-stratified sandstone with mud drapes. The trace-fossil assemblage is dominated by large Rosselia socialis and Macaronichnus segregatis in the sandier beds, whereas the mud drapes blanketing the sandstone foresets commonly contain N. missouriensis and Protovirgularia isp. Ichnologic characteristics (e.g. shallow-tiered communities, impoverished trace-fossil assemblages, dominance of deposit-feeder structures, and inhibition of suspension-feeder elements) suggest that different paleoenvironmental stresses, such as changes in salinity, water turbidity, and fluctuations in energy and in sedimentation rates, affected the infaunal communities of these tide-influenced delta settings.

Benthic foraminiferal evidence of sediment supply changes and fluvial drainage reorganization in Holocene deposits of the Po Delta, Italy

Detailed benthic foraminiferal analyses performed on the Holocene subsurface deposits of modern Po Delta evidenced a complex palaeogeographic evolution. Hierarchical R- and Q-mode cluster analyses allowed to distinguish four assemblages indicative of different marine environments and sub-environments. Temporal and spatial distribution patterns of benthic foraminifers reflect changes in Po River discharge during delta evolution. The capability of foraminiferal assemblages to track nutrients and sediment supply oscillations permitted to recognize four main evolutionary phases (A–D), which took place after the maximum marine transgression (ca. 5500 cal yr BP). Prodelta and delta front deposits are accumulated in the western part of the study area between the first stages of progradation and the late Middle Ages (ca. 10th century AD); these are paralleled, in a more distal position, by open-marine sedimentation, replaced upward by prodelta sediments (phases A and B). Owing to the combined effect of a major avulsion (Rotta di Ficarolo) and the Adriatic circulation patterns, an organic-rich zone developed in the eastern part of the present Po Delta at the transition to the Modern Age (phase C), around 600 cal yr BP. For the fist time a fossil mud-belt, although its marginal fringe, along with its peculiar microfossil content ( Nonionella turgida assemblage), is recorded beneath a modern delta plain. During the recent past an outstanding rate of sedimentation affected the Po Delta, as clearly evidenced by the deposition of 14 meters fine-grained prodelta sediments between 600 and 380 cal yr BP. Prodelta deposits were finally replaced by delta front sands after the Porto Viro cut (ca. 350 cal yr BP), which marked the onset of Modern Delta (phase D).

Braid-delta deposits from a broad shallow-marine setting: the middle member of the Kalpingtag Formation in the central Tarim Basin

During the early Silurian, a transgressive and vast shallow sea with flat sea-floor covered the central Tarim Basin (the Tazhong area). The depositional environment of the middle member of the Kalpingtag Formation is controversial. In order to provide a basis for the prediction of reservoir sand, the sedimentary facies are recognized according to abundant core observations and de- scriptions combined with well-log analysis, isograms, seismic interpretations and regional sedimentary background. The middle member of the Kalpingtag Formation, which shows a retrograding sequence, is interpreted as braid-delta deposits influenced by mi- nor tidal reworking. The sources of clasts are from the southern uplift. The subaqueous braid-delta deposits in the study area have some characteristics quite different from the common deltas that generally deposit in marginal seas. Four facies grouped to a delta front association are recognized, ranging from distributary-channel (Facies A), front bar (Facies B), sand sheet (Facies C) and inter- distributary bay (Facies D). The distributary channels construct the sandbody framework of the delta front. Front bar deposits, which are fine-grained with low depositional dips, display a near-continuous sand strip around the entire periphery of the delta. Sand sheet deposits are mainly found in front of Facies B, gradationally contacting with the prodelta. The interdistributary bay is essentially the uppermost unit capping the channel sequence and generally made up of laminated and massive mudstones. The delta front deposits display extensive sheet-like bodies contrasting with the characteristic wedge shapes of common subaqueous delta bodies. The bi- modal cross-stratification and mud drapes in the fine- to medium-grained sandstone in the distal area are inferred to reflect high-energy tidal processes.

Depositional environments in an extensive tide‐influenced delta plain, Middle Devonian Gauja Formation, Devonian Baltic Basin

AbstractThe Middle Devonian Gauja Formation in the Devonian Baltic Basin preserves tide‐influenced delta plain and delta front deposits associated with a large southward prograding delta complex. The outcrops extend over 250 km from southern Estonia to southern Lithuania. The succession can be divided into 10 facies associations recording distributary channel belts that became progressively more tide influenced when traced southwards towards the palaeo‐shoreline, separated by muddy intra‐channel areas where deposition was characterized by crevasse splays, delta plain lakes, abandoned channel deposits and tidal gullies. Tidal currents influenced deposition over the entire delta plain, extending up to 250 km from the contemporary shoreline. Tidal facies on the upper delta plain differ from those on the lower delta plain and delta front. In the former case, deposition from river currents was only occasionally interrupted by tidal currents, e.g. during spring tides, resulting in mica and mudstone drapes, and distinctive graded cross‐stratification. The lower delta plain was dominated by tidal facies and tidal currents regularly influenced deposition. There was a change from progradation to aggradation from the lower to the upper part of the Gauja Formation coupled with a vertical decrease in tidal influence and a decrease in coarse‐grained sediment input. The Gauja Formation contrasts with established models for tide‐influenced deltas as the active delta plain was not restricted by topography. The shape of the delta plain, the predominant southward (basinward)‐directed palaeocurrents, and the thick sandstone succession, show that although tidal currents strongly influenced deposition at bed scale, rivers still controlled the overall morphology of the delta and the larger‐scale bedforms. In addition, there are no signs of wave influence, indicating very low wave energy in the basin. The widespread tidal influence in the Devonian Baltic Basin is explained by changes in the wider basin geometry and by local bathymetrical differences in the basin during progradation and aggradation of the delta plain, with changes in tidal efficiency accompanying the change in basin geometry produced by shoreline progradation.

A retreating fan-delta system in the Neoproterozoic Chattisgarh rift basin, central India: Major controls on its evolution

A thick wedge of immature siliciclastics characterizes the basal part of the basin-fill succession in the eastern part of the Neoproterozoic Chattisgarh Basin. It comprises a mosaic of mass-flow–deposited conglomerate and pebbly sandstone, coarse-grained sandy braided stream deposits, wave-lag pebble sheets, high-energy, wave-dominated foreshore and upper shoreface facies, and wave- and tide-dominated lower shoreface facies. The succession is characterized by strong lateral and vertical facies variation. The facies associations and stacking pattern point to the development of an alluvial-fan to fan-delta complex, with outbuilding of delta lobes onto a high-energy shallow-marine shelf via a high-gradient, narrow delta plain. The delta-front deposits of coarse-grained sand interfinger and grade into prodelta mud in the seaward direction. The dominance of coarse-grained sandy detritus with a high content of fresh feldspar grains points to mechanical weathering of granites and gneisses of the cratonic basement and a semiarid climate. The sediment caliber and the climate exerted a major control on the facies and on processes of sediment transfer from the fan to the shelf. A critical balance between progradation, aggradation, and retrogradation controlled the evolution of delta successions within an overall transgressive regime related to the extension of a cratonic rift basin. Repeated fault-controlled uplift of the source, followed by subsidence and transgression, generated multiple fining-upward cycles and a retreating fan-delta system. The marked variations in thickness of the delta succession and the stacking pattern in different measured profiles reflect the overriding tectonic controls on delta evolution. The accumulated fault displacement in active sectors created higher accommodation and thicker delta sequences. Intermittent uplift of fault blocks exposed fresh bedrock to mechanical weathering, generated a large amount of detritus, and resulted in forced regressions, repeatedly disrupting the fining-upward pattern. The controls of source rock lithology or climate were of secondary importance to tectonic effects. Retreating fan deltas are rarely reported and may be a stratigraphic response of marine-connected rift basins at the early stage of extension. The delta stratigraphy is analogous to the submarine fan stratigraphy. In the case where basin shales act as hydrocarbon sources, the fan deltas are potential hydrocarbon reservoirs. The coarse-grained lobes can be excellent reservoirs, with the enclosing prodelta muds acting as seals to vertical migration of hydrocarbon. The coarse-grained deposits of the delta plain and proximal delta front, occurring as laterally coalesced, large sand bodies blanketed by transgressive prodelta mud, might also form good reservoirs.

Basic Building Blocks and Process Variability of a Cretaceous Delta: Internal Facies Architecture Reveals a More Dynamic Interaction of River, Wave, and Tidal Processes Than Is Indicated by External Shape

Abstract Although delta-building processes and the resultant facies and facies associations have been studied for over a century, there remains a lack of well-documented facies architectural studies of ancient deltas. In this study, architectural-element analysis is applied to determine the basic building blocks of a Cretaceous delta at the top of the upper Turonian Wall Creek Member exposed along the western flanks of the Powder River Basin, Wyoming. We document the bed-scale basic building blocks of this ancient delta, and show them in the context of their facies and stratal variability, and their hierarchical spatial arrangement within a single deltaic parasequence. We also test the idea that external sand-body shape may not reflect the internal facies complexity, as has recently been suggested in studies of several modern deltas, including the Burdekin in Australia, the Brazos in the Gulf of Mexico, and the Baram–Trusan in Borneo. Five orders of bounding surfaces separate six facies architectural elements in the prodelta and delta front deposits. These elements are prodelta fines (PF), frontal splay (FS), channel (CH), storm sheet (SS), tidally modulated deposit (TM), and bar accretion (BA). Seasonal to decadal river floods are thought to represent the main building phases of the delta, producing channels, bars, and frontal splay elements. During intervening periods, the delta was reworked by waves, storms, and tides, producing tidally modulated and storm sheet elements. Due to the complex interactions of river effluents with waves, storms and tides, the delta is interpreted as mixed-influenced. Regional sandstone isolith, facies, and paleocurrent maps help to reconstruct the paleogeography and show how the various architectural elements varied across the delta front. The channels, bars, frontal splays, and tidally modulated elements are found only near the distributary mouth, whereas the storm sheets occur extensively away from the distributary mouth. The plan-view morphology of the studied delta shows a smooth-fronted, arcuate to cuspate shape, which should indicate wave dominance. In contrast, our analysis of the internal facies architecture shows that the delta was constructed rapidly during major river floods with significant tidal reworking. Additional outcrop mapping shows that storm-wave-reworked sands are attached to the flanks of the system. Previously published delta models predict incorrectly that the tidal reworking of a river-flood dominated system should result in a more bird-foot shape versus the lobate geometry that is actually mapped. Our analysis of internal facies versus external shape matches similar observations on several modern deltas and suggests that external shape is a poor indicator of internal facies complexity.

Extensive thin sequences spanning Cretaceous foredeep suggest high-frequency eustatic control: Late Cenomanian, Western Canada foreland basin

Research Article| August 01, 2007 Extensive thin sequences spanning Cretaceous foredeep suggest high-frequency eustatic control: Late Cenomanian, Western Canada foreland basin A. Guy Plint; A. Guy Plint 1Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada Search for other works by this author on: GSW Google Scholar Michael A. Kreitner Michael A. Kreitner 1Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (8): 735–738. https://doi.org/10.1130/G23662A.1 Article history received: 08 Jan 2007 rev-recd: 06 Mar 2007 accepted: 23 Mar 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. Guy Plint, Michael A. Kreitner; Extensive thin sequences spanning Cretaceous foredeep suggest high-frequency eustatic control: Late Cenomanian, Western Canada foreland basin. Geology 2007;; 35 (8): 735–738. doi: https://doi.org/10.1130/G23662A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Early-late Cenomanian rocks of the Doe Creek unit are near the base of the Kaskapau Formation and were deposited in a broad, very shallow embayment that occupied the foredeep of the Western Canada foreland basin. The Doe Creek unit consists of 13 upward-shoaling sequences, typically 3–10 m thick. Subsurface correlation shows that sequences can be traced for 100–350 km from the fore-deep toward the forebulge. On the proximal and distal margins of the embayment, each sequence is capped by a flooding surface, commonly underlain by a paleosol developed in marine mudstone. In the center of the embayment, most sequences are capped by one or more isolated, sharp-based sandstone bodies interpreted as lowstand delta-front deposits. Flooding surfaces on top of each sequence are continuously traceable in well logs from the foredeep to the vicinity of the forebulge, implying simultaneous transgression across the basin. Every sequence shows an abrupt upward shallowing, usually to emergence, that was caused by accommodation loss, not sediment accumulation. A vertical relative sea-level excursion of ∼10 m on a time scale spanning a maximum range of ∼9–74 k.y. is estimated for each sequence. Because transgressions and regressions appear to be in phase across the basin, sequences are better explained by eustasy rather than by flexural reciprocal sedimentation: a glacioeustatic mechanism provides the most likely explanation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.