Sedimentary features of a Weichselian glaciolacustrine delta

Inclined beds of sand, granules, pebbles and cobbles resembling a glacio-tectonically tilted sequence were shown by sedimentological studies to constitute the 10–12 m thick foreset beds of a glaciolacustrine Gilbert-type delta in Kyndby, North Sjælland, Denmark. The foreset beds are slightly curved, dip 10–28 SE, and display a bundlewise structure with alternating coarse-grained cobble-rich and fine-grained sandy units. The occurrence of ascending megaripple cross-bedding and climbing ripple cross-lamination in the sandy foresets can be ascribed to strong backflow currents formed by the lee-side vortice. The foreset beds are underlain by flat-lying fine-grained sand, silt and clay (bottomset beds), and are overlain transitionally or erosively by 2–3 m of flat-lying sand, pebbles and cobbles (topset beds). In the transition zone between foreset beds and topset beds, various delta distributary channel units occur. The delta probably formed in a partly ice-dammed lake in connection with the general retreat of a Weichselian advance from the north ('Norwegian ice')     

Geometries of Gilbert-type deltas and large channels in the Meteora Conglomerate, Meso-Hellenic basin (Oligo-Miocene), central Greece

The Meteora conglomerate is a thick, coarse-grained unit of Oligo-Miocene age in the Meso-Hellenic basin consisting of Gilbert-type delta deposits (wedge-shaped bodies) and deep channel fills. Gilbert-type deltas prograded from the eastern margin basinward, perpendicular to the NW-SE basin axis. Delta bodies are thicker in the distal portion, where the dip of the foresets is lower than in the proximal area. Organized and disorganized conglomerates coexist, suggesting a mixture of mass- and turbulent-flow processes. The mass flow deposits seem to prevail in the uppermost part of the foresets. Channels (up to 25 m deep) are cut into the Gilbert-type delta deposits, and have axes perpendicular to the progradational trend of the deltas. Giant bars displaying accretion of the downstream side formed inside the channels. The channels were probably cut into Gilbert-delta deposits due to uplift of the source area or to a fall in sea-level. Although large cross-sets of both Gilbert delta and channels have been formed by progradation, they can be distinguished by: (i) angle of foreset bed dips, (ii) facies and textures and (iii) vertical sequences. Differences in these features aid in distinguishing Gilbert-type deltas from in-channel bars.     

Depositional systems and sequence stratigraphy of coarse-grained deltas in a shallow-marine, strike-slip setting: the Bohemian Cretaceous Basin, Czech Republic

Deposits of coarse‐grained, Gilbert‐type deltas showing varying degrees of reworking of foresets by basinal currents were identified in Middle Turonian to Early Coniacian sandstones of the Bohemian Cretaceous Basin. The progradation of the deltaic packages, earlier interpreted as large‐scale subaqueous dunes, shelf ridges or subaqueous fault‐scarp ‘accumulation terraces’, was controlled by high‐ and low‐frequency, relative sea‐level changes in a relatively slowly subsiding, intracontinental strike‐slip basin. End‐member types of the Bohemian Cretaceous coarse‐grained deltas are deep‐water deltas, characterized by thick (50–80 m) foreset packages with steep (10–30°) foresets, and shallow‐water deltas, which deposited thin (<15 m) packages with foresets typically between 4° and 10°. The differences in thickness and foreset slope angle were controlled predominantly by the accommodation available during progradation. The depositional regime of the deltas was governed by (i) the fluvial input of abundant sand bedload, with a minor proportion of gravel; (ii) gravity flows, most probably caused by liquefaction of the upper part of the unstable foreset slope; and (iii) migration of sandy bedforms on the foreset slopes. The bedform migration was driven by unidirectional currents of possible tidal origin. Individual foreset packages represent systems tracts, or parts of systems tracts, of depositional sequences. A variety of stacking patterns of high‐frequency sequences exists in the basin, caused by low‐frequency relative sea‐level changes as well as by local changes in sediment input. Because of generally low subsidence rates, fluvial or beach topset strata were not preserved in the cases studied. The absence of preserved fluvial facies, which has been one of the main arguments against the fluvio‐deltaic origin of the sandstone bodies, is explained by erosion of the topsets during transgression and their reworking into coarse‐grained lags of regional extent covering ravinement surfaces.     

The Kregnes moraine in Gauldalen, west-central Norway: anatomy of a Younger Dryas proglacial delta in a palaeofjord basin*

The Kregnes “moraine” ridge in Gauldalen, a north-trending valley south of Trondheim, is a Gilbert-type delta formed at a Younger Dryas glacier terminus. The gravelly delta consists of a north-dipping foreset, 150 m thick, comprised of turbidites, debrisflow beds and debrisfall deposits. The bottomset consists of turbiditic sand and mud layers. The topset, 2-3 m thick, is a braided-river alluvium with local beach deposits, matching the marine limit of 175 m a.s.l. The fjord-wide delta front had an extent of 3 km and prograded over a distance of 1.5 km, in probably less than 100 years, with the delta toe climbing by 50 m against the basin's rapidly aggrading muddy floor. The delta advanced through the alternating episodes of its toe aggradation and progradation, related to the increases and decreases of the delta-slope gradient. Slope steepening led to intense sediment sloughing by chutes and occasional large-scale failures. The fjord's wave fetch was low and the wave base no deeper than 1.5-2 m, but strong storm waves occasionally reworked the delta front to a depth of 6 m. Glacitectonic deformation was limited to the system's upfjord end. Allostratigraphic analysis suggests that the proglacial system commenced its development as an ice-contact submarine fan that was deformed, quickly aggraded to the sea surface and turned into an ice-contact delta, which further evolved into the large glaciofluvial delta. The Kregnes ridge represents an episode of the ice-front re-advance due to climatic deterioration and is tentatively correlated with the Hoklingen substage.     

Architecture and sedimentology of the Kerinitis Gilbert‐type fan delta,Corinth Rift,Greece

The Kerinitis Delta in the Corinth Rift, Greece, is a footwall derived, coarse‐grained, Gilbert‐type fan delta deposited in the hangingwall of a linked normal fault system. This giant Gilbert‐type delta (radius 3·8 km, thickness > 600 m) was supplied by an antecedent river and built into a brackish to marine basin. Although as yet poorly dated, correlation with neighbouring deltas suggests that the Kerinitis Delta was deposited during a period of 500 to 800 ka in the Early to early Middle Pleistocene. Facies characterizing a range of depositional processes are assigned to four facies associations (topset, foreset, bottomset and prodelta). The dominantly fluvial topset facies association has locally developed shallow marine (limestone) and fluvial‐shoreface sub‐associations. This delta represents a subsidence‐dominated system in which high fault displacement overwhelmed base‐level falls (creation of accommodation predominantly ≥ 0). Stratal geometries and facies stacking patterns were used to identify 11 key stratal surfaces separating 11 stratal units. Each key stratal surface records a landward shift in the topset breakpoint path, indicating a rapid increase in accommodation/sediment supply. Each stratal unit records a gradual decrease in accommodation/sediment supply during deposition. The cyclic stratal units and key stratal surfaces are interpreted as recording eustatic falls and rises, respectively. A 30 m thick package of foresets below the main delta records the nucleation of a small Proto‐delta probably on an early relay ramp. Based on changes in stratal unit geometries, the main delta is divided into three packages, interpreted as recording the initiation, growth and death of the controlling fault system. The Lower delta comprises stacked, relatively thin, progradational stratal units recording low displacement on the young fault system (relay ramp). The Middle delta comprises vertically stacked stratal units, each recording initial aggradation–progradation followed by progradation; their aggradational component increases up through the Middle delta, which records the main phase of increasing rate of fault displacement. The Upper delta records pure progradation, recording abrupt cessation of movement on the fault. A major erosion surface incising basinward 120 m through the Lower and Middle delta records an exceptional submarine erosion process (canyon or delta collapse).     

Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.     

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

An integrated study of the stratigraphy, structure, sedimentology, and geomorphology of the Akrata–Derveni region (southern coast of the Gulf of Corinth, Greece) forms the basis for a tectono-stratigraphic model for the evolution of the Plio-Pleistocene central Corinth Rift.

The syn-rift sediments exposed on the uplifted southern coast of the Gulf of Corinth comprise three stratigraphic groups. Maximum total thickness of the syn-rift sediments can reach 2800 m in the middle of the studied area. The Lower Group is made of fluvio-lacustrine deposits. The Middle Group corresponds to thick alluvial fan conglomerates and their equivalent Gilbert-type fan deltas that built toward the north. The Upper Group is composed of uplifted terrace deposits, slope breccias and small Gilbert-type deltas. These groups have been subdivided into informal formations and depositional systems. Restoration of the stratigraphic architecture along a N–S transect provides a linked structural and depositional model for this part of the rift. Reconstruction of the latest phases of uplift is based on a study of geomorphological features.

Evolutionary phases include, (1) an overall increase in accommodation space during deposition of the Lower and Middle Groups followed by (2) a drastic decrease in accommodation space during deposition of the Upper Group. Sedimentary signals indicate that most of the major normal faults were active during deposition of the Lower Group. The depocentre was located in the middle part of the study area and paleocurrents were predominantly toward the ENE. The main depositional system shifted south at the onset of deposition of the Middle Group, recording a widening and deepening of the rift. This major event also corresponds to a change in paleocurrent direction to a clear northward polarity. The southernmost border fault, the Killini Fault, was sealed during deposition of the Middle Group. A northward migration of fault activity was associated with northward progradation of giant Gilbert-type fan deltas that record water depths up to 500 m. Finally, the fan delta system was abandoned as progressive tilting to the south and uplift of the margin induced a reversal of the drainage system with the development of an endorheic depression. Sediment supply to the basin thus decreased and a forced regression took place during deposition of the Upper Group recording a northward shift of more than 5 km and a 600 m relative sea-level drop. As no major eustatic sea-level falls of such amplitude are documented during the Pleistocene, the uplift is linked to regional tectonics. Uplift and fault reactivation gave the present day configuration of the southern coast of the Gulf.     

Significance of fan deltas without toe-sets within rift and piggy-back basins: examples from the Corinth graben and the Mesohellenic trough, Central Greece

Trapezoidal-type fan deltas, lacking bottomset deposits, were studied in two different tectonic settings: extensional (rifted), and compressional (piggy-back) basins. In both cases studied fan deltas were characterized by: (i) an absence of bottomsets; (ii) development in protected or narrow basins and sub-basins confined by intrabasinal basement highs or by topographic highs, respectively; (iii) coarse-grained sediment fluxes, dominated by mass-flows forming fan deltas that prograded from steep nearshore slopes basinwards; and (iv) a high-energy environment, with powerful underflows that probably bypassed the basins and transported fine-grained sediments outside the basins. The location of channels cut by such underflows is influenced by local tectonic style. When the supplied sediments in the rifted basins overstepped the intrabasinal basement highs, trapezoidal fan deltas were replaced upwards by Gilbert-type deltas, with bottomsets.     

Gilbert‐type deltas recording short‐term base‐level changes: Delta‐brink morphodynamics and related foreset facies

Gilbert‐type deltas are sensitive recorders of short‐term base‐level changes, but the delta‐front record of a base‐level rise tends to be erased by fluvial erosion during a subsequent base‐level fall, which renders the bulk record of base‐level changes difficult to decipher from the delta‐front deposits. The present detailed study of three large Pleistocene Gilbert‐type deltas uplifted on the southern coast of the Gulf of Corinth, Greece, indicates a genetic link between the delta‐front morphodynamic responses to base‐level changes and the delta‐slope sedimentation processes. Sigmoidal delta‐brink architecture signifies a base‐level rise and is accompanied by a debrite‐dominated assemblage of delta foreset deposits, thought to form when the aggrading delta front stores sediment and undergoes discrete gravitational collapses. Oblique delta‐brink architecture tends to be accompanied by a turbidite‐dominated assemblage of foreset deposits, which are thought to form when the delta‐front accommodation decreases and the sediment carried by hyperpycnal effluent bypasses the front. This primary signal of the system response to base‐level changes combines further with the secondary ‘noise’ of delta autogenic variation and possible allogenic fluctuations in fluvial discharge due to regional climatic conditions. Nevertheless, the evidence suggests that the facies trends of delta foreset deposits may be used to decipher the delta ‘hidden’ record of base‐level changes obliterated by fluvial topset erosion. Early‐stage bayhead deltas may be an exception from the hypothetical model, because their narrow front tends to be swept by river floods irrespective of base‐level behaviour and their subaqueous slope deposits are thus mainly turbidites.     

断陷湖盆中强制湖退沉积作用及其成因机制

断陷湖盆中相对湖平面下降引起的湖退作用称之为强制湖退作用。强制湖退作用时期若存在沉积物供给盆地,将加速湖退作用,并形成一套具有明显前积结构的强制湖退前积体。强制湖退前积体的顶积层不发育,地震剖面上具斜交前积反射结构。强制湖退前积体的顶界面为层序界面,且伴随有下切谷的形成。气候变化、构造抬升和边界断层的幕式断陷活动引起的盆地缓坡带基底差异沉降运动均可引起断陷湖盆中相对湖平面的下降,也是引起强制湖退作用的三大成因机制。强制湖退前积体具有良好的油气成藏条件,是隐蔽油气藏有利勘探目标。     

箕状湖盆萎缩阶段两种湖泊三角洲的沉积特征及成因探讨--以平庄、铁法盆地为例

以平庄、铁法盆地为例，首先阐述了箕状湖盆萎缩阶段的河口坝型湖泊三角洲发育于盆缘断裂一侧的扇前河流入湖的湖缘带，具有与海岸河控三角洲相似的沉积特征；Gilbert 型湖泊三角洲发育于沉积-侵蚀盆缘一侧河流入湖的湖缘带，具有由底积层、前积层、顶积层构成的垂向层序及朵状平面砂体形态，它与扇体没有成因联系和空间上的毗邻关系。在此基础上，论述了这两种湖泊三角洲形成的地质背景，并对它们的发育条件及进积方式等成因问题进行了较深入的探讨。     

Ripple/dune to upper stage plane bed transition: Some observations from the ancient record

In two Proterozoic sandstones, of the Indian shield cross-stratification and cross-lamination are observed to grade continuously into parallel-laminations often bearing parting lineations. These are interpreted as having resulted from a gradual transition from ripple/dune to upper stage plane bed structures. During the transition the inclination of the cross-strata diminishes and their shape changes from concave-up to sigmoidal. The sigmoidal cross-strata are characterized by well defined topsets, foresets and toesets and the topsets bear parting lineations. In the course of the transition sigmoidal cross-strata may give way either to horizontal parallel-lamination or inclined parallel-lamination. In the former the toesets of the successive sigmoidal cross-strata thicken and the thickness of the cross-strata beyond the brink point (the junction between the topset and foreset) tends to become uniform until a plane-bed state is reached and horizontal parallel-laminations are formed. In the latter the topset of the successive sigmoidal cross-strata increases in length at the expense of the foreset and toeset until the brink point ceases to exist so that only the topset laminations prevail and appear as inclined parallel-laminations. These transitions presumably result from a gradual increase in flow intensity beyond the stability limits of ripples/dunes. The progressive morphological changes of the cross-stratification over the transition are attributed to changing fallout patterns on the lee face of the bedforms in response to increasing flow intensity. Preservation of the records of such transitions suggests an abundant supply of sand grade sediments from suspension during the transition, shaping the sediment concentration profile over the bedforms and facilitating turbulence suppression. The variation in the pattern of transition from sigmoidal cross-stratification to parallel-lamination may be the result of different rates of sediment feed from the prevailing suspended sediment load in the two instances.     

Pliocene subaqueous fans and Gilbert-type deltas in maar crater lakes, Hopi Buttes, Navajo Nation (Arizona), USA

Maar craters of the Mio-Pliocene Hopi Buttes volcanic field of Arizona formed within a broad playa system, and accumulated a variety of lacustrine sedimentary deposits. Many craters initially held isolated, groundwater-fed lakes. Ephemeral streams crossing the playa entered some of the lake-filled craters, and built coarse grained Gilbert-type deltas and subaqueous fans along the margins of these craters. The small, coarse grained fans and deltas have many features in common with much larger coarse grained deltaic and fan deltaic deposits. However, the local production of coarse grained volcanic sediment, low gradients in the local stream catchment, steep subaqueous relief and the small size of the receiving ‘basins’resulted in a unique combination of features. Cone-shaped subaqueous fans initially formed at the mouths of incised feeder streams. The fans are small accumulations of steeply dipping gravelly tephra that consist almost entirely of overlapping lobes constructed by density-modified grain flows. Gravelly Gilbert-type tephra deltas formed in brimfull craters fed by a freely migrating feeder stream. They are concave lakeward, mimicking the underlying crater wall topography. Complex deltaic geometries are defined by topset strata that steeply onlap tall foreset beds. They suggest that feeding stream floods caused rapid and comparatively large variations in lake level within the small crater lakes. Bed-specific carbonate alteration is common, and probably resulted from both influx of detrital carbonate across the playa and alteration of tephra beds by carbonate-saturated lakewaters during between flood periods of high net evaporation.     

中国东部箕状断陷湖盆扇三角洲与辫状河三角洲对比研究

中国东部箕状断陷湖盆短轴方向,常常发育扇三角洲和辫状河三角洲,二者均可形成良好的油气储集体,但在岩芯以及地球物理资料的识别上常常容易混淆。通过对它们沉积特征、地球物理资料特征、含油气性和控制因素的对比研究和分析认为:扇三角洲是陡坡带冲积扇直接入湖形成,地震反射特征为楔形前积,沉积物成分成熟度和结构成熟度均低于辫状河三角洲,沉积特征反映重力流和牵引流双重作用;辫状河三角洲是缓坡带辫状河入湖形成的三角洲,向湖盆方向推进较远,地震反射特征为叠瓦前积,沉积特征反映牵引流作用。扇三角洲易形成构造-岩性油气藏和岩性油气藏,有利相带为扇三角洲前缘水下分流河道、席状砂以及前扇三角洲容易发育的滑塌浊积扇;辫状河三角洲易形成地层-岩性油气藏和岩性油气藏,有利相带为辫状河三角洲平原辫状分流河道,辫状河三角洲前缘水下分流河道、河口坝、席状砂以及半深湖中的浊积岩体。古气候变化、构造-古地貌以及湖平面变化速率三者共同作用于扇三角洲和辫状河三角洲的沉积特征,其中构造-古地貌是主控因素。结合二者沉积特征差异,建立了中国东部箕状断陷湖盆扇三角洲和辫状河三角洲沉积模式图,以期能对相似构造背景下扇三角洲和辫状河三角洲的分析与研究提供一定的指导作用。     

A gravel lobe deposit in the prodelta of the Doumsan fan delta (Miocene), SE Korea

The Doumsan fan-delta system in the Pohang Basin (Miocene), SE Korea comprises tripartite components of Gilbert-type topset, foreset, and bottomset environments with an extended prodelta in the deeper part (a few hundred metres deep). The present study documents sedimentologic features and origin of a gravel body (here interpreted as a gravel lobe) formed in the prodelta region of the Doumsan fan delta. The gravel lobe lies on sandy mud deposits and is capped by a thick massive sand bed. It extends for more than 1.5 km with a height of up to 30 m and shows a narrow tongue-like geometry. Eight sedimentary facies have been distinguished to describe characteristic features of the gravel lobe and the associated deposits. Of these, three types of gravelly sedimentary facies are important with regard to volumetric contribution and depositional processes: (1) crudely stratified pebble-grade conglomerate; (2) disorganized, clast-rich pebble(-to-cobble)-grade conglomerate; and (3) matrix-rich, bimodal cobble-grade conglomerate. The former two types dominate the central part of the lobe where they are not accompanied by sand beds, whereas the latter, as subordinate units, is prevalent in the fringe which otherwise is dominated by thick sandy mud deposits. The stacked successions of crudely stratified pebble-grade conglomerate are representative of the active aggradational phases of the gravel lobe, whereas the occurrence of channels within the lobe reflects that the gravel lobe prograded under the influence of subaqueous channel systems. The gravel lobe resulted from catastrophic disturbance (slumping) on the foreset region that further caused the development of channel systems, promoting efficient transport of gravelly sediments. This type of deposit may represent an important additional category of low-efficiency subaqueous fans.     

Rapid development of gravelly high-density turbidity currents in marine Gilbert-type fan deltas, Loreto Basin, Baja California Sur, Mexico

The Pliocene Loreto basin is an asymmetrical half graben located on the eastern margin of Baja California Sur, Mexico, which formed by rapid subsidence along the dextral-normal Loreto fault. The southern Loreto basin contains numerous, well exposed coarse-grained Gilbert-type fan deltas that were derived from the footwall of the Loreto fault. Detailed sedimentological study of individual foreset beds provides information about down-slope flow transformations of cohesionless sediment gravity flows in shallow water. Deposits of Gilbert-delta foresets consist of ungraded, normal-graded, inverse- to normal-graded, and bipartite conglomerate and sandstone. Lateral transitions in sorting, grading style and internal structure are commonly observed within individual beds, both across and down slope, suggesting heterogeneity within flows and a close relationship between high-density turbidity currents and gravel traction carpets. A conceptual model for flow transformation and deposition of high-density turbidity currents on Gilbert-delta foreset slopes is developed for Pliocene strata in the Loreto basin. In this model, ungraded cohesionless debris flows evolved rapidly down-slope into normal-graded gravelly turbidity currents. With continued down-slope transport, the gravel fraction collapses and becomes concentrated into a basal traction carpet undergoing laminar shear, and is over-ridden by a sandy turbulent suspension. The short distances (10–20 m) over which lateral transitions within single beds are observed indicate very rapid flow transformations (10–20 s) and rapid deposition of gravel traction carpets by frictional freezing on and near the base of the foreset slope.     

Architecture and evolution of a fjord‐head delta,western Vancouver Island,British Columbia

The architectural framework and Holocene evolution of the Zeballos fjord‐head delta on west‐central Vancouver Island was established through a multidisciplinary field‐based study. The Zeballos delta is a composite feature, consisting of an elevated, incised, late Pleistocene delta and an inset Holocene delta graded to present sea level. Both deltas have a classic Gilbert‐type tripartite architecture, with nearly flat topset and bottomset units and an inclined foreset unit. Time domain electromagnetic (TDEM) and ground‐penetrating radar (GPR) surveys, borehole data, and gravel pit exposures provided information on the internal form, lithologies and substrate of both deltas. Both sets of deltaic deposits coarsen upward from silt in the bottomset unit to gravel in the topset unit. The TDEM survey revealed a highly irregular, buried bedrock surface, ranging from 20 m to 190 m in depth, and it delineated saltwater intrusion into the deltaic sediments. Late Quaternary sea‐level change at Zeballos was inferred from delta morphology and the GPR survey. The elevated, late Pleistocene delta was constructed when the sea was about 21 m higher relative to the land than it is today. It was dissected when sea‐level fell rapidly as a result of glacio‐isostatic rebound. Relative sea‐level reached a position about 20 m below the present datum during the early Holocene. Foreset beds that overlap and progressively climb in a seaward direction and topset beds that thicken to 26 m landward imply that the delta aggraded and prograded into Zeballos Inlet during the middle and late Holocene transgression. Sea‐level may have risen above the present datum during the middle Holocene, creating a delta plain at about 4 m a.s.l. Remnants of this surface are preserved along the valley margins. Copyright © 2004 John Wiley & Sons, Ltd.     

High‐frequency cycles of brachiopod shell beds on subaqueous delta‐scale clinoforms (early Pliocene,south‐east Spain)

During the early Pliocene, subaqueous delta‐scale clinoforms developed in the Águilas Basin, in a mixed temperate carbonate–siliciclastic system. The facies distribution is consistent with the infralittoral prograding wedge model. Stacking patterns and bounding surfaces indicate that the clinoforms formed during the highstand and falling sea‐level stages of a high rank cycle. Twenty‐two prograding clinothems were recognized over a distance of ≥1 km. Biostratigraphic data indicate a time span shorter than 700 kyr for the whole unit (MPl3 biozone of the Mediterranean Pliocene). Cyclic skeletal concentrations and occasional biostromes of suspension feeders (terebratulid brachiopods, modiolid bivalves and adeoniform bryozoan colonies), slightly evolved glauconite and occasional Glossifungites ichnofacies formed on the clinoforms during high‐frequency pulses of relative sea‐level rise. During such stages, increased accommodation space in the topsets of the clinoforms caused a strong reduction of terrigenous input into the foresets and bottomsets. This provided favourable conditions for the development of these suspension feeder palaeocommunities. During stillstand stages, however, reduced accommodation space in the topsets eventually resumed progradation in the foresets. There, the abundance of Ditrupa tubes indicates frequent siltation events that extirpated the terebratulid populations and other epifaunal suspension feeders in the foreset and bottomset subenvironments. The occurrence of shell beds on the clinoforms suggests that this case study represents lower progradation rates than standard examples where shell beds bound the clinobedded units at their base and top only. Importantly, the distributions of biofacies and ichnoassemblage associations contribute significantly to the understanding of the effects of relative sea‐level fluctuations on the evolution of subaqueous delta‐scale clinoform systems.     

A late Devensian glaciolacustrine fan-delta at Rhosesmor,Clwyd, North Wales

Glacigenic sediments exposed at Rhosesmor, Clwyd, North Wales, were deposited in a small proglacial lake ponded by the stagnating margin of the Late Devensian Irish Sea icesheet. Three major fades assemblages are identified and show a prograding sequence of topset, foreset, and bottomset deposits associated with two Gilbert-style deltas stacked one above the other. The lower delta is associated with a lake level at c. 180 m OD. A subsequent ice-marginal readvance raised the lake to c. 192 m OD, drowned the lower delta and caused the growth of a second delta by an iceward shift of facies. It is estimated that the life of the lake was of the order of 90 years during which average sediment in-fill amounted to some 4±10⁴ m³ per annum.     

Lacustrine deltas in a Mesozoic alluvial sequence from Camp Hill, Antarctica

Sedimentary rocks of late Mesozoic age exposed at Camp Hill, northern Antarctic Peninsula, are associated with calc-alkaline volcanic rocks. They represent deposition on a fault-controlled floodplain, with marginal alluvial fans, on a volcanic arc. Finely laminated mudstone and occasional graded laminae were deposited from suspension and by density underflow currents, respectively, in small shallow lakes. Thickening- and coarsening-upward sandstone bodies overlying the lake deposits are interpreted as lacustrine deltas of which two types are preserved: (1) Gilbert-type with steep foresets and (2) mouth-bar type which lack steep foresets. Sections through the latter type reveal the presence of sub-environments characteristic of fluvial-dominated marine deltas, i.e. prodelta, distributary mouth-bar and distributary channel. Abandoned mouth-bars resulting from avulsion are recognized. It is suggested that the processes which operated during formation of the mouth-bar deltas resulted from hyperpycnal flow. By contrast, the Gilbert-type delta is thought to be the consequence of a reduced inflow of suspended sediment causing homopycnal flow, and thorough mixing of the river and lake waters.