Late Quaternary stratigraphy and sedimentology of the inner part of southwest Joseph Bonaparte Gulf

Joseph Bonaparte Gulf is a large embayment on the northwestern continental margin of Australia. It is approximately 300 km east‐west and 120 km north‐south with a broad continental shelf to seaward. Maximum width from the southernmost shore of Joseph Bonaparte Gulf to the edge of the continental shelf is 560 km. Several large rivers enter the gulf along its shores. The climate is monsoonal, sub‐humid, and cyclone‐prone during the December‐March wet season. A bedrock high (Sahul Rise) rims the shelf margin. The sediments within the gulf are carbonates to seaward, grading into clastics inshore. A seaward‐thinning wedge of highstand muds dominates the sediments of the inner shelf of Joseph Bonaparte Gulf. Mud banks up to 15m thick have developed inshore. Coarse‐grained sand ridges up to 15 m high are found off the mouth of the Ord River. These overlie an Upper Pleistocene transgressive lag of mixed carbonate and gravelly siliciclastic sand. Four drowned strandlines are present on the inner shelf at depths of 20, 25, 28 and 30 m below datum. These are interpreted as having formed during stillstands in the Late Pleistocene transgression. Older strandlines at great depths are inferred as having formed during the fall in sea‐level following the last highstand. For the most part the Upper Pleistocene‐Holocene marine sediments overlie an erosion surface cut into older Pleistocene sediments. Incised valleys cut into this erosion surface are up to 5 km wide and have a relief of at least 20 m. The largest valley is that cut by the Ord River. Upper Pleistocene sediments deposited in the incised valleys include interpreted lowstand fluvial gravels, early transgressive channel sands and floodplain silts, and late transgressive estuarine sands and gravels. Older Pleistocene sediments are inferred to have been deposited before and during the 120 ka highstand (isotope stage 5). They consist of sandy calcarenites deposited in high‐energy tide‐dominated shelf environments. Still older shelf and valley‐fill sediments underlie these. The contrast between the Holocene muddy clastic sediments and the sandy carbonates deposited by the 120 ka highstand suggests that either the climate was more arid in the past, with less fluvial transport, or that mud was more effectively trapped in estuaries, allowing development of carbonate depositional environments inshore.     

Late-Quaternary paleoenvironmental evolution of Lesina lagoon (southern Italy) from subsurface data

Integrated sedimentological and micropaleontological (foraminifers and ostracods) analyses of two 55 m long borehole cores (S3 and S4) drilled in the subsurface of Lesina lagoon (Gargano promontory—Italy) has yielded a facies distribution characteristic of alluvial, coastal and shallow-marine sediments. Stratigraphic correlation between the two cores, based on strong similarity in facies distribution and AMS radiocarbon dates, indicates a Late Pleistocene to Holocene age of the sedimentary succession.Two main depositional sequences were deposited during the last 60-ky. These sequences display poor preservation of lowstand deposits and record two major transgressive pulses and subsequent sea-level highstands. The older sequence, unconformably overlying a pedogenized alluvial unit, consists of paralic and marine units (dated by AMS radiocarbon at about 45–50,000 years BP) that represent the landward migration of a barrier-lagoon system. These units are separated by a ravinement surface (RS1). Above these tansgressive deposits, highstand deposition is characterised by progradation of the coastal sediments.The younger sequence, overlying an unconformity of tectonic origin, is a 10 m-thick sedimentary body, consisting of fluvial channel sediments overlain by transgressive–regressive deposits of Holocene age. A ravinement surface (RS2), truncating the transgressive (lagoonal and back-barrier) deposits in core S4, indicates shoreface retreat and landward migration of the barrier/lagoon system. The overlying beach, lagoon and alluvial deposits are the result of mid-Holocene highstand sedimentation and coastal progradation.     

Sediments and history of the Rottnest Shelf, southwest Australia: a swell-dominated, non-tropical carbonate margin

The Rottnest Shelf is a narrow, wave-dominated open shelf on the passive continental margin of southwest Australia, adjacent to a hinterland of low relief and sluggish drainage. High physical energy, low nutrients in cool subtropical waters, and rapid postglacial transgression have limited carbonate productivity, restricted grain types, and reworked the transgressed surface to form only a thin ( < 1 m) blanket of carbonate and relict sediment, with little terrigenous influx. Subaerial weathering of the shelf during Late Pleistocene emergence was followed by postglacial drowning, erosional shoreface retreat, and generation of a transgressive lag deposit. Establishment of the warm temperate biota, dominated by bryozoans and calcareous red algae, resulted in bioerosion of the shelf disconformity surface and generation of hardground veneers and thin skeletal carbonate sheets. Linear topographic ridges of Pleistocene limestone partition the shelf into systems with varying physical energy, biota and sediment supply. The Holocene sediments are a shallowing-upward coastal sequence; wave-ripple cross-stratified grainstone (Inner Shelf); and bioturbated bryozoan grainstone to skeletal wackestone (Outer Shelf to Upper Continental Slope), characterised by seaward fining and increasing percentages of planktic carbonate sediment.

Given sufficient time, the Rottnest Shelf could recover from drowning, and form blanket-like skeletal carbonates. Thin ( < 1 m) lags overlying disconformities, which underlie shallowing-upward coastal and shelf sediments a few metres thick, will be generated by glacio-eustatic cycles of sedimentation (10⁵ y duration). Thick (several tens of metres) sediment bodies, composed of wave-rippled to bioturbated skeletal carbonate sediment with a temperate biota, will be formed during longer term (1–10 My) sedimentation cycles. Such cycles have characterised passive margins during the Cenozoic. The Rottnest Shelf thus provides a facies model for temperate shelf sedimentation along passive continental margins.     

Holocene stratigraphy and depositional history of the Narragansett Bay System, Rhode Island, U.S.A.

ABSTRACT An extensive seismic reflection survey has been used to gain further knowledge of Holocene stratigraphy and depositional history in the Narragansett Bay System (NBS). The early Holocene stream-dissected surface beneath the NBS is interpreted as having been flooded by the Holocene sea in a manner suggested by Oldale & O'Hara's (1980) sea-level-rise curve. The sea initially is believed to have penetrated the pre-NBS East Passage trunk valley about 9000 yr BP and subsequently spread landward via the trunk valley and its branches. The Holocene sediments display stratigraphic relationships that differ spatially. At passage mouths, the basal unconformity is inferred to be covered over with some 3 m of paralic and 5 m of marine sands and silts separated, by a transgressive unconformity. In contrast the interior sequences reveal (a) a valley section up to more than 15 m thick in which the regressional unconformity is overlain by probable lower fluvial and/or estuarine sand-silt facies that commonly grades upward to an estuarine silt-clay facies and (b) an interfluve section in which a basal transgressive unconformity is blanketed by an estuarine nearshore sand-silt facies that locally may change upward to a silt-clay facies. Primarily Holocene silt-clay accretion, produced by sedimentation processes associated with net non-tidal estuarine circulation, infilled the evolving NBS. Depositional bodies, lenticular in shape and comprised of 12 m or more of sediment, developed in lowlands near Gould Island (?9000 yr BP), in Upper Narragansett Bay (?7500 yr BP), around Hope Island (?7500 yr BP) and in Mt Hope Bay (?6250 yr BP) with an average minimal sedimentation rate of 1.6–2.2 mm yr^-1. Silt-clay deposition, commonly gas-bearing, has buried the basal relief in most of the NBS upper and middle portions except for middle East Passage. A comparison of NBS sedimentation with that of Chesapeake, Delaware and Hudson Estuaries shows that the estuaries to the south have accumulated more sediment over a slightly longer period yet, with the exception of the higher rate in the Hudson Estuary, the sedimentation rates appear to be similar.     

Eastern Mississippi delta: late Wisconsin unconformity, overlying transgressive facies, sea level and subsidence

Analysis of 92 engineering core logs located in the Balize sector of the eastern Mississippi delta focuses on the late Wisconsin unconformity and lithofacies of strata lying immediately below and above this stratigraphic horizon. This major sequence boundary is a key feature used to correlate strata across shelf and slope to the basin. Observations emphasize lithofacies distributions of the latest Pleistocene sediments underlying the unconformity, the late Pleistocene to early Holocene transgressive facies, and the immediately overlying deltaic deposits. Maps and a cross-section compiled with this information highlight the critical relation between lithofacies distributions and late Wisconsin sea-level oscillations. Core analysis reveals that the transgressive facies comprises distinct environments of deposition, offshore to onshore. Findings identify criteria to determine the approximate position of the shoreline at the late Wisconsin maximum sea-level lowstand and at the extent of early Holocene maximum marine inundation in the Balize complex. Mapping of lithologies along the late Wisconsin unconformity serves to improve correlation of sediment facies with changes in acoustic response along high-resolution seismic profiles. An estimate of long-term averaged land subsidence suggests lowering of at least 1 mm/year near the shelfedge during the past 18 000 years. This mapping also refines sea-level response models for the Mississippi delta, including criteria to locate key paleogeographic features such as shelfedges and maximum flooding surfaces in other Recent marine deltas and in older, stacked deltaic sequences.     

Bed thickness characteristics of inner-shelf storm deposits associated with a transgressive to regressive Holocene wave-dominated shelf, Sendai coastal plain, Japan

Holocene inner-shelf storm deposits preserved beneath the Sendai coastal plain facing the Pacific coast of north-eastern Japan were formed during a transgressive–regressive cycle. The evolution of the Holocene wave-dominated depositional system along the Sendai coast is reconstructed using 76 AMS (accelerator mass spectrometers) ¹⁴C ages and the origin of bed thickness variations in the inner-shelf storm deposits is explored. The Holocene succession is <30 m thick and overlies latest Pleistocene to early Holocene non-marine deposits above a transgressive ravinement surface. It comprises transgressive ravinement and inner-shelf deposits, and regressive inner shelf, shoreface, and coastal plain deposits. The inner-shelf deposits comprise alternating sand and mud layers interpreted as stacked storm beds. The average preservation interval of a single storm bed is shortest during the transgression (5·7–20·6 years), and then increases to a maximum during the early regression (83·3–250·0 years), decreasing to 7·7–31·3 years with shoreline progradation. Average accumulation rates decreased during the transgression and then increased during the regression, but the sand/mud ratio varies little, reflecting inefficient sediment segregation downdip on the inner shelf. The vertical pattern of sand-layer thicknesses also shows no relationship to position within the cycle, although small-scale intervals of upward thickening and thinning probably relate to lateral switching of river mouths and/or random storm processes. The average thickness of storm beds is the highest in the interval deposited during the period from maximum flooding to early regression. This is probably because of the low preservation potential of thin beds associated with frequent, low-magnitude storms during this period of low accumulation rates and extensive reworking. This preservation bias and the nature of the Sendai inner shelf resulted in an absence of characteristic bed thickness trends in the preserved storm deposits.     

Formation and significance of a middle Silurian ravinement surface on Gotland, Sweden

A laterally extensive and conspicuously smooth erosional surface is exposed near the Wenlock–Ludlow boundary on east-central Gotland, Sweden. It occurs in the reef complex area of a carbonate platform, has an undulating topography, and separates truncated bioherms and biostromes from overlying allochthonous high energy deposits. On a basin regional scale, the surface is associated to a shift from a prograding to a retrograding platform, and to a substantial hiatus in basin marginal areas (Estonia). The significance of the surface is further indicated by: (a) clear-cut truncation of the reef complex, including m-sized stromatoporoids, along a distance of at least 20 km, (b) an erosional relief exceeding 2.08 m, (c) a conspicuously smooth nature in both palaeolows and palaeohighs, (d) present, although scarce, subaerial diagenetic indications, e.g. shallow karst features at the unconformity surface and pendant/meniscus cement in the lowermost part of overlying strata, (e) a locally occurring basal conglomeratic lag in overlying strata, and (f) peritidal indications and, as evident from at least one quarry, onlapping geometry in overlying strata.

The unconformity has implications for the analysis of the middle Silurian Baltic basin evolution as well as for the interpretation of erosional surfaces on carbonate platforms in general. Based on the above characteristics, the formation of the unconformity is attributed to a relative sea-level fall, causing subaerial exposure, followed by transgressive abrasion in a rocky shore environment. The unconformity hence constitutes a ravinement surface which, based on the associated basin regional sedimentary changes, is interpreted as coinciding with a regional exposure surface (sequence boundary). It thus increases our understanding of the hitherto poorly understood palaeogeographic evolution of the middle Silurian Baltic basin. Further, the transgressive erosion was significant as well as recurrent, as indicated by the clear-cut truncation of large stromatoporoids at the unconformity surface and by truncated marine fossil fragments in upper edges of karst infills. The unconformity therefore constitutes a good example of the capability of transgressive erosion in creating stratigraphic incompleteness, and hence in removing subaerial indications, on previously exposed carbonate platforms.     

东昆仑东段新生代高原隆升重大事件的沉积响应

根据新生代沉积盆地的地层序列、环境演变及地貌和水系变迁分析将研究区新生代高原隆升过程划分为5个重大事件.首次在研究区内海拔5400m的主夷平面上发现古土壤、岩溶角砾岩及钟乳石等, 认为沱沱河组上段细碎屑沉积及五道梁组石膏沉积(中新世) 是主夷平面形成期的沉积响应, 提出了主夷平面的高程具有东西向排列的盆岭地貌特点, 这种高程差异反映了后期构造隆升的不均衡, 应该是上新世以来差异断块抬升的结果; 早更新世中期(1525.5ka后) 一套河流砂砾卵石沉积是青藏运动C幕的沉积响应, 体现在布尔汗布达山的强烈上升和成山; 早更新世晚期(1113.9~836.3ka) 先湖滨砾石沉积, 后转为河流砂砾卵石沉积, 是昆黄运动在研究区的沉积响应, 并体现在马尔争-布青山的强烈上升和成山, 中更新世早期冰碛物的发育说明昆黄运动后研究区已隆升达到"水汽冻结高度"; 研究区T5阶地沉积前的强烈下蚀, 柴达木盆地内陆水系溯源侵蚀切过布尔汗布达山主分水岭, 袭夺了昆南断裂带原由西向东流向共和古湖的东西向水系并到达阿拉克湖一带, 晚更新世洪冲积角度不整合于中更新世洪冲积之上等是共和运动的具体表现.      

Late Quaternary stratigraphic signature, offshore from the Dutch barrier shoreline

More than 400 km of high-resolution, partially digitally acquired seismic data were collected in a 30 × 60 km area offshore from the Dutch mesotidal barrier island shoreline, in order to analyze the Holocene transgressive sequence.

The investigated area was subaerially exposed during the most recent (Weichselian) Pleistocene lowstand and glaciated during the previous (Saale/Illinoian) glacial period. Sea level was probably at a level comparable to that of the present during the intervening Eem/Sangamonian highstand.

The area's seismic signature is characterized by several levels of channelization, separated by major, area-wide high-amplitude reflectors. The lowermost aerial reflector represents the Saale boulder clay. The largest channel system, which cuts this reflector, was incised during the Older Dryas cooling event and filled during the Allerød warming period. It was subsequently re-incised during the Younger Dryas and refilled during the early Holocene. The most pronounced and extensive reflector represents the base of the Holocene, and almost everywhere equals the planed-off top of the Eem highstand section. This reflector is overlain by aggradational facies, separated by one level of channelization and one planed-off (ravinement?) surface. The most prominent type of seismic facies above the lowermost level of channelization is a set of progradational clinoforms, which is interpreted as a preserved ebb-tidal delta system. This system is associated with the first Holocene shoreline of ca. 7500 yrs BP.

The eastern part of the study area contains sand ridges with a complex pattern of internal cross-stratification. Ridge-crest sediments are very well-sorted with a mean grain size of 2.8-2.6 φ (ca. 0.15 mm), whereas sediments in the troughs are well-sorted with a mean grain size of 1.8-0.8 φ (0.25–0.5 mm). Origin of these ridges is tentatively thought to be related to reworking since 5000 yrs BP, when the dynamic regime was more or less similar to that of the present and sediment supply had decreased substantially.     

Tracking the isostatic and eustatic signals in an end Ordovician–early Silurian deglacial sedimentary record (Algeria – Libya border)

To elucidate the signature of isostatic and eustatic signals during a deglaciation period in pre‐Pleistocene times is made difficult because very little dating can be done, and also because glacial erosion surfaces, subaerial unconformities and subsequent regressive or transgressive marine ravinement surfaces tend to amalgamate or erode the deglacial deposits. How and in what way can the rebound be interpreted from the stratigraphic record? This study proposes to examine deglacial deposits from Late‐Ordovician to Silurian outcrops at the Algeria–Libya border, in order to define the glacio–isostatic rebound and relative sea‐level changes during a deglaciation period. The studied succession developed at the edge and over a positive palaeo‐relief inherited from a prograding proglacial delta that forms a depocentre of glaciogenic deposits. The succession is divided into five subzones, which depend on the topography of this depocentre. Six facies associations were determined: restricted marine (Facies Association 1); tidal channels (Facies Association 2); tidal sand dunes (Facies Association 3); foreshore to upper shoreface (Facies Association 4); lower shoreface (Facies Association 5); and offshore shales (Facies Association 6). Stratigraphic correlations over the subzones support the understanding of the depositional chronology and associated sea‐level changes. Deepest marine domains record a forced regression of 40 m of sea‐level fall resulting from an uplift caused by a glacio‐isostatic rebound that outpaces the early transgression. The rebound is interpreted to result in a multi‐type surface, which is interpreted as a regressive surface of marine erosion in initially marine domains and as a subaerial unconformity surface in an initially subaerial domain. The transgressive deposits have developed above this surface, during the progressive flooding of the palaeo‐relief. Sedimentology and high‐resolution sequence stratigraphy allowed the delineation of a deglacial sequence and associated sea‐level changes curve for the studied succession. Estimates suggest a relatively short (<10 kyr) duration for the glacio‐isostatic uplift and a subsequent longer duration transgression (4 to 5 Myr).     

Quaternary stratigraphy of the Koratallaiyar-Cooum basin,Chennai

In this paper we present Quaternary stratigraphy of the area around Chennai based on archaeological findings on the ferricrete surface, geomorphological observations supplemented by radiocarbon dating. The coastal landscape around Chennai, Tamil Nadu, has preserved ferruginised boulder gravel deposits, ferricretes and fluvial deposits of varying thickness. The area studied is approximately 150 km east to west and 180 km north to south with a broad continental shelf towards the seaward. Several rivers enter the Bay of Bengal along its shores like the Koratallaiyar, Cooum and the Adyar. Precambrian charnockite and Upper Gondwana sandstone and shale bedrock rim the shelf margin. For the most part, the Upper Pleistocene-Holocene fluvial sediments overlie an erosion surface that has cut into older Pleistocene sediments and ferricrete surface. Incised valleys that cut into this erosion surface are up to 5–6 km wide and have a relief of at least 30 m. The largest valley is that cut by the Koratallaiyar River. Holocene sediments deposited in the incised valleys include fluvial gravels, early transgressive channel sands and floodplain silts. Older Pleistocene sediments are deposited before and during the 120-ka high stand (Marine isotope stage 5). They consist of ferricretes and ferricrete gravel formed in nearshore humid environments. Muddy and sandy clastic sediments dated to the ca. 5 ka highstand suggest that the climate was semi arid at this time with less fluvial transport. The coarsening up sequence indicates deposition by high intensity channel processes. Pedogenic mottled, clayey silt unit represents an important tectonic event when the channel was temporarily drained and sediment were sub aerially exposed. Uplift of the region has caused the local rivers to incise into the landscape, forming degradation terraces.     

Late Pliocene–Quaternary tectonics in the frontal part of the SE Carpathians: Insights from tectonic geomorphology

The Romanian East Carpathians display large-scale heterogeneities along the mountain belt, unusual foredeep geometries, significant post-collisional and neotectonic activity, and major variations in topography, mostly developed in the aftermath of late Miocene (Sarmatian; 11 Ma) subduction/underthrusting and continental collision between the East European/Scythian/Moesian foreland and the inner Carpathians Tisza-Dacia unit. In particular, the SE corner of the arcuate orogenic belt represents the place of still active large-scale differential vertical movements between the uplifting mountain chain and the subsiding Focşani foredeep basin. In this key area, we have analysed the configuration of the present day landforms and the drainage patterns in order to quantify the amplitude, timing and kinematics of these post-collisional late Pliocene–Quaternary vertical movements. A river network is incising in the upstream a high topography consisting of the external Carpathians nappes and the Pliocene–Lower Pleistocene sediments of the foreland. Further eastwards in the downstream, this network is cross-cutting a low topography consisting of the Middle Pleistocene–Holocene sediments of the foreland. Geological observations and well-preserved geomorphic features demonstrate a complex succession of geological structures. The late Pliocene–Holocene tectonic evolution is generally characterised by coeval uplift in the mountain chain and subsidence in the foreland. At a more detailed scale, these vertical movements took place in pulses of accelerated motion, with laterally variable amplitude both in space and in time. After a first late Pliocene uplifting period, subsidence took place during the Earliest Pleistocene resulting in a basal Quaternary unconformity. This was followed by two, quantifiable periods of increased uplift, which affected the studied area at the transition between the Carpathians orogen and the Focşani foreland basin in the late Early Pleistocene and the late Middle to late Pleistocene. Both large-scale deformation events affected the western Focşani basin flank, tilting the entire structure with 9° during the late Early Pleistocene and uplifted it as a block during the early Late Pleistocene. The late Early Pleistocene tilting resulted in 750 m uplift near the frontal monocline and by extrapolation in a presumed 3000 m uplift near the central parts of the Carpathians. The late Middle to late Pleistocene cumulative uplift reaches 250 m and correlates with a contemporaneous progradation of the uplifted areas towards the Focşani Basin. The uplifting events are separated by a second Quaternary unconformity. On the whole, the late Pliocene–Quaternary evolution of the Carpathians orogen/Focşani basin structure indicate large-scale differential uplift during the latest stages of a continuous post-collisional orogenic evolution.     

Quaternary stratigraphy and geoarchaeology of the colorado and concho rivers,west texas

Geoarchaeological investigations in an area surrounding the confluence of the upper Colorado and Concho Rivers, Edwards Plateau of West Texas, have produced a detailed landscape evolution model which provides a framework for discussion of the influences of geomorphic processes on the development, preservation, and visibility of the archaeological record. Field mapping within the study area has differentiated six allostrati-graphic units of fluvial origin in both valleys, as well as extensive eolian sand sheets along the Colorado River. Early to middle Pleistocene terrace remnants cap many upland areas, whereas two distinct late Pleistocene terrace surfaces are widespread within the study area at somewhat lower elevations. Fluvial activity during the time period of human occupation is represented by an extensive Holocene terrace and underlying valley fill, which is up to 11 m in thickness. Valley fill sediments can be subdivided into allostratigraphic units of early to middle Holocene (ca. 10,000–5000 yr B.P.) and late Holocene age (ca. 4600–1000 yr B.P.), which are separated by a buried soil profile. The modern incised channels and very narrow floodplains represent the last millennium. Eolian sand sheets of early to middle Holocene age overlie limestone- and shale-dominated uplands, Pleistocene terraces, and in some cases the Holocene valley fill along the Colorado River. Pleistocene terraces have been stable features in the landscape and available for settlement through the time period of human occupation. Archaeological materials of all ages occur at the surface, and the record preserved in individual sites range from that associated with discrete periods of activity to longer-term palimpsests that represent repeated use over millennia. Sites within early to middle Holocene and late Holocene fills represent short-term utilization of constructional floodplains during the Paleoindian through early Archaic and middle to late Archaic time periods respectively. By contrast, those that occur along the buried soil profile developed in the early to middle Holocene fill record middle to late Archaic cultural activity on stable terrace surfaces, and represent relatively discrete periods of activity to longer-term palimpsests that represent repeated use over the 3000–4000 years of subaerial exposure. Late Prehistoric sites occur as palimpsests on soils developed in late Holocene alluvium or stratified within modern floodplain facies. Paleoindian through Late Prehistoric sites occur stratified within eolian sand sheets or along the unconformity with subjacent fluvial deposits. The landscape evolution model from the upper Colorado and Concho Rivers is similar to that developed for other major valley axes of the Edwards Plateau. This model may be regionally applicable, and can be used to interpret the geomorphic setting and natural formation processes for already known sites, as well as provide an organizational framework for systematic surface and subsurface survey for new archaeological records. 0 1992 John Wiley & Sons, Inc.     

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.     

塔里木盆地塔河地区中下奥陶统鹰山组碳酸盐岩层系内幕不整合识别特征*

为论证T⁶₇界面性质并探索碳酸盐岩层系不整合的识别方法,文章综合利用柯坪—巴楚地区露头和塔河地区钻井、测井和地震资料,研究了塔里木盆地塔河地区中下奥陶统鹰山组内幕不整合面T⁶₇。在露头上, T⁶₇界面表现为岩相转换面,但在不同剖面的暴露强度存在差异。在地化数据上, T⁶₇不整合面之下碳酸盐岩地层碳氧同位素值出现明显偏负;Sc,Co,Ni,Ba,U,Sr等微量元素表现出同样的特点,而Li和V在T⁶₇界面之下富集。在自然伽马测井与自然伽马能谱测井曲线上, T⁶₇不整合面之下的鹰山组下段自然伽马测井曲线明显较界面之上的鹰山组上段齿化剧烈,经过滤波处理的Th/U曲线在T⁶₇界面处呈现出明显的低值。在地震剖面上,难以从地震同相轴反射终止现象中识别出T⁶₇界面,但“串珠”状反射沿T⁶₇不整合面发育,这些串珠状反射被解释为与喀斯特作用相关的岩溶洞穴。T⁶₇界面属于海平面相对下降形成的短暂暴露面(<1,Ma),其主要受控于海平面升降和古地貌,在塔河不同区域的暴露、剥蚀强度不同,强暴露区主要位于中东部,弱暴露区域主要位于西北部。T⁶₇不整合面对塔河地区鹰山组内幕碳酸盐岩储集层发育具有重要意义,既可为层间岩溶的发育提供条件,亦可与断裂构成连通体系为大气淡水提供通道,从而对塔河地区深部碳酸盐岩储集层起到积极的改造作用。     

Tectonics,sea-level changes and palaeoenvironments in the early Pleistocene of Rome (Italy)

The historical site of the Monte Mario lower Pleistocene succession (Rome, Italy) is an important marker of the Pliocene/Pleistocene boundary. Recently, the Monte Mario site was excavated and restudied. A spectacular angular unconformity characterizes the contact between the Monte Vaticano and the Monte Mario formations, which marks the Pliocene/Pleistocene boundary. Biostratigraphical analyses carried out on ostracod, foraminifer, and calcareous nannofossil assemblages indicate an Early Pliocene age (topmost Zanclean, 3.81–3.70 Ma) for the underlying Monte Vaticano Formation, whereas the Monte Mario Formation has been dated as early Pleistocene (Santernian, 1.66–1.59 Ma). Palaeomagnetic analyses point to C2Ar and C1r2r polarity chrons for the Monte Vaticano and the Monte Mario formations, respectively. The Monte Mario Formation consists of two obliquity-forced depositional sequences (MM1 and MM2) characterized by transgressive systems tracts of littoral marine environments at depths, respectively, of 40–80 m and 15–20 m. The data obtained from foraminifer and ostracod assemblages allow us to reconstruct early Pleistocene relative sea-level changes near Rome. At the Plio/Pleistocene transition, a relative sea-level drop of at least 260 m occurred, as a result of both tectonic uplift of the central Tyrrhenian margin and glacio-eustatic changes linked to early Pleistocene glaciation (Marine Isotope Stage 58).     

洞庭盆地澧县凹陷第四纪沉积特征与古地理演化

澧县凹陷内第四系厚度达200m左右,由早更新世-中更新世中期沉积和全新统组成。早更新世-中更新世中期沉积除顶部为粘土外,主体为砾石层,在凹陷中央夹粉砂、细砂和少量粘土。全新统分布于凹陷南部,为湖冲积粘土。早更新世澧县凹陷区断陷沉降,于河道和冲积扇环境下接受沉积;中更新世凹陷逐渐扩张,沉积范围扩展至凹陷外缘,凹陷主体部位自边缘向中央依次为冲积扇和扇前浅湖环境。中更新世中后期因盆地扩张,自凹陷中央至边缘地带均为静水湖泊环境。中更新世晚期至晚更新世,凹陷及周缘因构造抬升而遭受剥蚀而缺失沉积。全新世凹陷南部受澧水及区域湖面上升影响而发育湖冲积沉积。     

Facies and sequence stratigraphy of a Late Barremian wave-dominated deltaic deposit, Agadir Basin, Morocco

The late Barremian succession in the Agadir Basin of the Moroccan Western High Atlas represents wave-dominated deltaic deposits. The succession is represented by stacked thickening and coarsening upwards parasequences 5–15 m thick formed during fifth- or fourth-order regression and building a third-order highstand systems tract. Vertical facies transitions in parasequences reflect flooding followed by shoaling of diverse shelf environments ranging from offshore transition interbedded mudstones, siltstones and thin sandstones, lower shoreface/lower delta front hummocky bedforms to upper shoreface/upper delta front cross-bedded sandstones. The regional configuration reflects the progradation of wave-dominated deltas over an offshore setting. The maximum sea-level fall led to the development of a sequence boundary that is an unconformity. The subsequent early Aptian relative sea-level rise contributes to the development of an extensive conglomerate lagged transgressive surface of erosion. The latter and the sequence boundary are amalgamated forming a composite surface.     

Sediment distribution and evolution of tidal deltas along a tide-dominated shoreline, Wachapreague, Virginia

Borings from the barrier island/lagoon system of the Eastern Shore of Virginia penetrated an unconformity which separates Pleistocene barrier island and offshore marine sediments from the overlying Holocene tidal delta and barrier island sediments. Offshore marine sediments and deposits within the flood-tidal delta (marsh, tidal flat-bay, inlet-mouth bar complex) are recognized on the basis of sediment color, composition, grain-size changes in the vertical sequence, presence of organic matter, and faunal suite. Subsurface data, historical records, and morphology of lateral accretion on barrier islands suggest that major inlets in the vicinity of Wachapreague have been relatively stable throughout Holocene time; they appear to be located where Pleistocene stream valleys previously existed. Holocene barrier islands apparently developed on drainage divide areas following post-Wisconsin transgression of the sea.

The initial phase of tidal delta development was characterized by vertically accreting, fan-shaped, inlet-mouth bars; tidal channels stabilized after bar crests had shoaled sufficiently for marsh to form. With landward progradation across the lagoon, sand-rich deposits graded laterally away from the inlets and vertically into clayey sand and silty clay of the tidal flat-bay and marsh environments.

Ebb inlet-mouth bars developed asymmetrically southward in response to littoral drift. Flood tidal deltas also built preferentially toward the south as indicated by: (1) sand distribution of the inlet-mouth bar complex; and (2) greater development of marsh south of the inlets.     

Holocene Environmental Change and River-Mouth Sedimentation in the Baie des Anges, French Riviera

River mouths on the steep, high-relief coast of the French Riviera exhibit thick sequences of Holocene marine, estuarine, deltaic, and river channel-floodplain sediments that overlie basal fluvial Pleistocene gravel. Gravel is uncommon in most of the early to middle Holocene aggradational-progradational marine, estuarine, deltaic sediments, despite an ample supply from rock units in the steep adjoining uplands. River-mouth gravel is common only in late Holocene river channels and in barrier beaches perched on finer-grained nearshore sediments. Neither downslope grain-size fining on alluvial fans nor sediment stacking patterns during sea-level (base-level) rise readily account for the lack of early to middle Holocene gravel in the river-mouth sediment wedges. Holocene sea-level rise led to the storage of fine-grained sediments in shallow marine, estuarine, and deltaic environments in the present coastal zone. We infer that humid temperate conditions, a dense forest cover, landscape stabilization, and a regular quiescent river flow regime associated with the Atlantic climatic optimum limited gravel supply in the adjoining catchments and gravel entrainment downstream during the early Holocene. Sea-level stabilization in the middle and late Holocene coincided with a marked change in bioclimatic conditions toward the present Mediterranean-type regime, which is characterized by a less dense forest cover, soil erosion, and episodic catastrophic floods. The late Holocene was thus a time of downstream bedload channel aggradation, fine-grained floodplain and paludal sedimentation, and seaward flushing of clasts leading to the formation and consolidation of the gravel barrier beaches that bound the rivermouths and embayments.