Congxian Li

Sedimentary facies of the tide-dominated paleo-Changjiang (Yangtze) estuary during the last transgression

Abstract A large estuary was formed by marine inundation of the paleo-Changjiang (Yangtze) incised valley during the transgression after the Last Glacial Maximum. This paper presents the sedimentary facies and architecture of the estuary fill, based on the analysis of three sediment cores (CM97, JS98, and HQ98) obtained from the present Changjiang delta plain. Estuary fill deposits showing an upward-fining succession were grouped into five depositional facies: tidal river, distributary channel, muddy intertidal to subtidal flats, transgressive lag, and estuary front, based on sedimentary textures, lithology, and physical sedimentary structures. Sand–mud couplets are common in these deposits, indicating that tides played a significant role in producing these sedimentary structures and that the estuary type was tide-dominated. Some of the successive sand–mud couplets probably recorded neap-spring cycles as well as semidiurnal tidal cycles. The nature of the estuary was very different from other representative tide-dominated estuaries in sediment facies, its distribution, and sediment source for estuarine fill. Unlike the other estuaries that receive sediments mainly from the sea, the paleo-Changjiang estuarine fill deposits were supplied largely from the river. This difference would also have a great influence on the sedimentological and morphological component in the estuary. The sediment distribution of the estuary showed fining-seaward and estuary-mouth sand bodies fed by marine-source sand were absent. The architecture model of tide-dominated estuaries should be divided into two types by the degree of fluvial sediment supply. The paleo-Changjiang estuary shows a good example for an estuary of large rivers.

Sedimentary facies and Holocene progradation rates of the Changjiang (Yangtze) delta, China

Abstract The Changjiang (Yangtze) River, one of the largest rivers in the world, has formed a broad tide-dominated delta at its mouth during the Holocene sea-level highstand. Three boreholes (CM97, JS98, and HQ98) were obtained from the Changjiang delta plain in 1997–1998 to clarify the characteristics of tide-dominated delta sediments and architecture. Based on sediment composition and texture, and faunal content, core sediments were divided into six depositional units. In ascending order, they were interpreted as tidal sand ridge, prodelta, delta-front, subtidal to lower intertidal flat, upper intertidal flat, and surface soil deposits. The deltaic sequence from the prodelta deposits to the delta front deposits showed an upward-coarsening succession, overlain by an upward-fining succession from the uppermost part of the delta front deposits to the surface soil. Thinly interlaminated to thinly interbedded sand and mud (sand–mud couplets), and bidirectional cross laminations in these deposits show that tide is the key factor affecting the formation of Changjiang deltaic facies. Sediment facies and their succession combined with AMS 14C dating revealed that isochron lines cross unit boundaries clearly, and delta progradation has occurred since about 6000 to 7000 years BP, when the rising sea level neared or reached its present position. The average progradation rate of the delta front was approximately 50 km/kyear over the last 5000 years. The progradation rate, however, increased abruptly ca. 2000 years BP, going from 38 to 80 km/kyear. The possible causes for this active progradation could have been an increase in sediment production in the drainage basin due to widespread human interference and/or decrease in deposition in the middle reaches related to the channel stability caused by human activity and climatic cooling after the mid-Holocene.

Stratigraphy and paleoenvironmental changes in the Yangtze Delta during the Late Quaternary

The Yangtze Delta area may be subdivided into paleo-valley and paleointerfluves of the last glaciation. The postglacial transgressive sedimentary cycle (PTSC) on the front zones of the paleointerfluves is composed of marsh-nearshore and barrier-lagoon, shallow marine and nearshore-tidal flat units, with a basement in stiff clay. The PTSC in back zones of the paleointerfluves consists of lacustrine-marsh deposits. The PTSC in the incised valley contains river channel, floodplain-estuary, estuarine-shallow marine and deltaic units, with an erosional surface at its bottom. The stiff clay and the erosional surface constitute the PTSC lower boundary. The stiff clay, consisting of a paleosol, experienced deposition alternating with pedogenesis during the falling of sea-level during δ18O stage 3; ongoing pedogenesis in the sea-level lowstand of δ18O stage 2; and early diagenesis after paleointerfluve inundation by the sea-level rise of δ18O stage 1. The climate during the paleosol formation was temperate with more than 500-mm a−1 rainfall and frequently fluctuating groundwater. The Yangtze River incised its course during falling sea level of stage 3, and a huge incised valley was formed during the sea-level lowstand (stage 2). The filling of the incised valley took place during the postglacial sea-level rise, and delta formation occurred at a late stage of PTSC development.

Late Quaternary incised-valley fill of the Yangtze delta (China): its stratigraphic framework and evolution

A Late Quaternary stratigraphic framework of the Yangtze delta, China, has been established based on the analysis and correlation of over 600 cores. Thirty cores were drilled and analyzed during this study, and the rest were collected from numerous publications. The Late Quaternary stratigraphic framework of the Yangtze delta consists of incised-valley fill and two interfluvial sequences. The incised valley roughly coincides with the modern Yangtze delta. From bottom to top, the incisedvalley fill sequence is typically composed of channel, floodplain–estuary, estuary–shallow marine, and modern deltaic facies. The interfluvial sequences occur along the two flanks of the incised-valley fill. On the southern flank, seaward of the maximumtransgression line, the interfluvial sequence is typically composed of marsh–nearshore, shallow marine, and tidal flat facies with predominantly muddy deposits. On the northern flank, the interfluvial sequence is typically composed of barrier–lagoon, tidal sand ridge, and littoral–tidal flat facies with predominantly sandy deposits. The interfluvial sequence landward of the maximum-transgression line on both flanks is composed mainly of lacustrine muddy deposits. The post-glacial fining-upward incised-valley fill sequence was formed by retrogressive aggradation in the lower reach of the Yangtze River, induced by sealevel rise. The upstream extent of the retrogressive aggradation exceeds the reach of flood tidal currents, which explains the lack of marine fossil and tidal sedimentary structures in the channel sand deposits. A huge estuary was formed in the present Yangtze delta area when the post-glacial transgression reached maximum at about 7500 years BP. The shallow marine mud was deposited broadly in the estuary and the maximum flooding surface, which separates the underlying transgressive sequence from overlying regressive sequence, located within the muddy stratum. An erosional surface between the channel sand unit and deltaic sand unit exists at the apical area of the Yangtze delta. The modern Yangtze delta developed after the post-glacial transgression maximum was reached. The delta development was dominated by the abundant fluvial sediment supply. Six subdeltas with a southward migrating trend have developed. D 2002 Elsevier Science B.V. All rights reserved.

Discrimination of geochemical compositions between the Changjiang and the Huanghe sediments and its application for the identification of sediment source in the Jiangsu coastal plain, China

Concentrations of 25 elements in the fine-grained fraction (6 63 Wm) of bottom sediments of the Changjiang and the Huanghe were determined to characterize the geochemical compositions of each riverine origin. Most of the elements, except for alkali and alkaline earth elements (Na, Ca, Sr and Ba), are highly enriched in the Changjiang sediments compared to those in the Huanghe sediments. Differences in source rocks and weathering mechanisms between the two drainage basins are the major factors controlling those compositional characteristics. The provenance index of core sediments from the northern Jiangsu coastal plain, calculated with nine elements with distinct compositional differences between both river sediments, indicates that the Changjiang had prevailed in the coastal plain sedimentation during the early stage of Holocene, while the Huanghe dominated over the area during the late Holocene. However, neither of the signatures of the modern Changjiang- and Huanghe-derived sediments is remarkable in the present-day coastal plain. 5 2002 Elsevier Science B.V. All rights reserved.

Characteristic and paleoenvironmental evolution of subaerial tidal sand body in Subei coastal plain

The subaerial tidal sand area in the northern Jiangsu Province (Subei), stretching from Dongtai towards east with a fan shape, is an early developing stage of radial sand ridges distributed in the South Yellow Sea. Since 5000–6000 a BP, after the Holocene transgression maximum in the northern Jiangsu Province, subaqueous tidal sand bodies were exposed and changed into land gradually. The environmental magnetism analysis shows that subaerial tidal sand strata are formed by the convergent-divergent paleo-tidal current field. The sediment source of tidal sand strata came early from the Changjiang River and late from the Yellow River. Sea floor erosion by tidal currents also served as an important sand source. Drilling cores and ground-penetrating profile show that there exists no probability of sand supplying directly by a large river through the apical area of tidal sand ridges either on land or in the sea. Fluvial deposits supplied the tidal sand bodies by alongshore transportation, which corresponds to the conclusions obtained by the analyses of provenance and paleocurrent field.

Rhythmic deposition on mudflats in the mesotidal Changjiang Estuary, China

ABSTRACT The mesotidal Changjiang Estuary is characterized by high suspended sediment concentration, broad mudflats, no barrier islands, and a semidiurnal tidal regime controlled by the tropical period. Study mudflats are located at the south flank of the Changjiang Delta, facing the East China Sea. Thickness and number of tidal couplets deposited on the mudflats were measured daily and fortnightly over three neap-spring cycles. Changes in mudflat elevation were monitored from May (pre-typhoon season) to August (mid-typhoon season). Daily observation of sedimentary features shows that short-term deposition on the mudflats during calm weather is controlled mainly by tides. Neap-spring cyclicity can be extracted from the variation of sand-mud couplet thickness. High suspended sediment concentration allows deposition of two couplets per semidiurnal tide. However, number of couplets preserved per tidal cycle is reduced because of reworking and erosion by tidal currents and waves. Over a fortnightly period, tidal cyclicities are obscured, with ca. 80% of the couplets reworked. On a seasonal scale, sand-dominated layers reflect deposition during storms and mud-dominated layers reflect deposition from tides during calm weather. Two orders of sand-mud lamina alternations were examined in the tidal-flat succession. The first-order alternations are in the range of centimeters to decimeters (consisting of 16-26 couplets) and related to storm-calm weather variation; the second-order, millimeter-thick alternations are related to tidal rhythm. Deposition rates decrease exponentially as time scales increase. The open-coast tidal-flat succession is truncated with numerous depositional hiatuses.

Influences of Storm Erosion and Deposition on Rhythmites of the Upper Wenchang Formation (Upper Ordovician) Around Tonglu, Zhejiang Province, China

ABSTRACT Tonglu rhythmites (Upper Ordovician) in Zhejiang Province, east-central China, display three orders of cyclicity in sandstone and mudstone layer thickness. Millimeter-thick alternations of sandstone and mudstone laminae are ascribed to single tidal cycles. Centimeter-thick alternations of sand-dominated layers (SDLs) and mud-dominated layers (MDLs) are interpreted to be related to alternation of storm and calm weather conditions with a periodicity longer than that forced by neap-spring tidal cyclicity. The SDLs are interpreted as storm deposits on the basis of presence of scour structures, abundant intraformational mud pebbles, oscillation ripples, and thinning-upward trends in the sandstone laminae. A third, meter-thick cycle of variations in sandstone-lamina thickness is interpreted as a reflection of cross-shore changes in coastal dynamics and water depth in the subtidal-intertidal environment. Storm waves, usually considered to be random destructive factors to normal cyclic deposits, are here highlighted as effective agents of sediment transport and deposition of the sand-dominated layers. This study aims at improving our facies-level understanding of the genesis and preservation of storm-related tidal-flat rhythmites on open coasts, and highlights the fact that storm related facies can mimic the cyclicity that is commonly ascribed to neap-spring tidal variation.

Temporal Distribution of Diastems in Deposits of an Open-Coast Tidal-Flat With High Suspended Sediment Concentrations

Abstract Based on field observations of the preservation potential and genesis of couplets and small sequences, diastems occur within tidal flat deposits at scales of semi-tidal, daily, monthly, and yearly intervals. Thus, tidal flat deposition is riddled with more diastems of different scales than previous assumptions. Units that are apparently complete when examined at longer time scales actually contain many diastems when examined at a shorter time scale. Diastems within tidal flat deposits are generated not only by high-magnitude storm waves, which occur during typhoon seasons, but also by daily erosion produced by tides and weak waves during calm-weather seasons. The missing intervals in diastems generated by storm waves can be several days to several years. Preservation potential of storm-generated small sequences is of the order of one-third. Diastems generated by erosion, by tides, and weak waves can have spans from a semidiurnal cycle to several days. They are easily overlooked because of the low-magnitude destruction and lack of distinctive sedimentary features. Net erosion by tides and weak waves is unexpectedly increased by day-to-day processes. Alternations of sandy and muddy laminae are related to tidal rhythm, but individual laminae can be separated by a significant diastem. Documentation of erosion by tides and weak waves under normal (non-typhoon) conditions is the principal focus of this report.

Development of the Volga Delta in Response to Caspian Sea-Level Fluctuation during Last 100 Years

Abstract The Volga Delta developed in the closed Caspian basin can be classified as a river-dominated type. The subaqueous part of the delta (avandelta) is characterized by an extremely gentle slope and may be subdivided into shallow and deep parts. The very low-angle slope of the shallow avandelta is a major factor to mitigate impacts of Caspian sealevel (CSL) changes on the Volga Delta during the last 100 years. The CSL has fluctuated 3.46 m in the last 100 years, with a high of −25.55 m relative to the Baku Datum (BD) in 1903 and a low of −29.01 m BD in 1977. Sea-level changes can be grouped into three stages: (1) slowly falling at an average rate of 1.1 cm yr−1 in 1900–1929; (2) rapidly falling at an average rate of 6.3 cm yr−1 in 1930–1977; (3) rapidly rising at an average rate of 8.8 cm yr−1 in 1978–1999. During the rapid fall stage of sea level, the Volga delta-plain expanded at a rate of 2.3 km2 yr−1 to 180 km2 yr−1 because of alluvial accumulation and exposure of the shallow avandelta. Flourishing subaqueous vegetation promotes this expansion by trapping suspended sediments on the avan-delta. The shallow avandelta is primarily covered by freshwater and its geomorphic and sedimentary features are controlled more by the river flow than by the CSL changes, waves or wind-induced currents. The shallow avandelta decelerated delta progradation during sea-level fall and delayed delta retreat during sea-level rise. Significant changes in the delta-plain area have not occurred in the rapid CSL rise stage. Moreover, the distributaries and their distal underwater channels underwent erosion by the river flow at least until the end of the last century. The CSL has risen with an average annual rate of 8.8 cm during the last 20 years, yet coastal erosion, coastal flooding, wetland loss and saltwater intrusion, observed in other deltas, have not occurred here. This may have resulted from the existence of the shallow avandelta and show that the worlds deltas respond to sea-level rise in different ways.