Changes In Depositional Environment Research Articles

Late Glacial and Holocene development of Lake Donggi Cona, north-eastern Tibetan Plateau, inferred from sedimentological analysis

Sediments of Lake Donggi Cona on the northeastern Tibetan Plateau were studied to infer changes in the lacustrine depositional environment, related to climatic and non-climatic changes during the last 19kyr. The lake today fills a 30×8km big and 95m deep tectonic basin, associated with the Kunlun Fault. The study was conducted on a sediment-core transect through the lake basin, in order to gain a complete picture of spatio-temporal environmental change. The recovered sediments are partly finely laminated and are composed of calcareous muds with variable amounts of carbonate micrite, organic matter, detrital silt and clay. On the basis of sedimentological, geochemical, and mineralogical data up to five lithological units (LU) can be distinguished that document distinct stages in the development of the lake system.The onset of the lowermost LU with lacustrine muds above basal sands indicates that lake level was at least 39m below the present level and started to rise after 19ka, possibly in response to regional deglaciation. At this time, the lacustrine environment was characterized by detrital sediment influx and the deposition of siliciclastic sediment. In two sediment cores, upward grain-size coarsening documents a lake-level fall after 13cal ka BP, possibly associated with the late-glacial Younger Dryas stadial. From 11.5 to 4.3cal ka BP, grain-size fining in sediment cores from the profundal coring sites and the onset of lacustrine deposition at a litoral core site (2m water depth) in a recent marginal bay of Donggi Cona document lake-level rise during the early to mid-Holocene to at least modern level. In addition, high biological productivity and pronounced precipitation of carbonate micrites are consistent with warm and moist climate conditions related to an enhanced influence of summer monsoon. At 4.3cal ka BP the lake system shifted from an aragonite- to a calcite-dominated system, indicating a change towards a fully open hydrological lake system. The younger clay-rich sediments are moreover non-laminated and lack any diagenetic sulphides, pointing to fully ventilated conditions, and the prevailing absence of lake stratification. This turning point in lake history could imply either a threshold response to insolation-forced climate cooling or a response to a non-climatic trigger, such as an erosional event or a tectonic pulse that induced a strong earthquake, which is difficult to decide from our data base.

A Cromerian Complex palaeolake sediment sequence from the Zidini site, south-eastern Latvia

Abstract This study is based upon palaeobotanical reinvestigation of the Pleistocene deposit strata revealed by test drillings in the buried valley at the Zidini site, southeastern Latvia. Up to 100 m thick lake sediment strata covered by three different till layers have been found in three 120–171 m deep test drillings. New palynological and plant macroremain analysis for sediment samples from core No. 43 have been done. These results and data from the earlier investigations at Zidini were compared. Clearer insights into depositional environment and vegetation change during the accumulation of sediments in the palaeolake have been acquired. According to recent studies and correlation of obtained palaeobotanical data, the palaeolake sediments were accumulated from the Elsterian 1 Late Glacial, through the Cromerian Complex, including two interglacials (Cromer III and Cromer IV) separated by cooling of Glacial C and hiatuses. The upper part of the palaeolake sediment sequence accumulated during Elsterian 2 Early Glacial stadial and interstadial conditions.

Waterford Formation in the south-eastern Karoo: Implications for basin development

Extensive research on the rocks of the Karoo Supergroup has shown that this sequence, which contains an unsurpassed record of Permian–Jurassic tetrapods, records a largely unbroken stratigraphic succession from 300 Ma to 180 Ma. This Gondwanan succession was deposited in a changing environmental setting reflecting glacial marine through deltaic to fluvial and aeolian desert conditions. The contact between the Ecca and Beaufort Groups (at the top of the Waterford Formation of the Ecca Group) in the southern and western Karoo represents a change in depositional environment from a subaqueous to a subaerial delta plain. By contrast, the Waterford Formation has not yet been recognised in the south-eastern Karoo Basin, which might imply that a major unconformity is present between the Fort Brown Formation of the Ecca Group, deposited in a prodelta environment, and the overlying fluvially deposited Koonap Formation of the Beaufort Group. From careful documentation of lithofacies and sedimentological data, it can be demonstrated that the Waterford Formation is indeed present in the south-eastern part of the basin and that no major unconformity is present – a fact that has implications for the mapping of Karoo rocks in the Eastern Cape Province of South Africa, for understanding the depositional environment of ’reptilian‘ fossils from the lowermost Beaufort in this part of the Karoo basin, and for basin development models.

Gerakan struktur dan kaitannya dengan faktor kendali tektonik, berdasarkan analisis stratigrafi; Studi kasus geologi kuarter terhadap fase perkembangan Danau Tondano purba sepanjang Remboken - Kakas, Kec. Remboken dan Kec. Kakas, Kab. Tomohon, Sulawesi Ut

Http://dx.doi.org/10.17014/ijog.vol2no3.20075Studies of stratigraphy on Quaternary deposits in the southwest region of Lake Tondano, Tomohon Regency, South Sulawesi, revealed six facies of deposition environments. These facies consist of Tondano tuff, young volcanic eruption, lake, sand lake, tidal lake, and swamp deposits. Based on the correlation of the lateral and vertical variation of the Quaternary deposits, three unit of the depositional facies were recognized (UFP I-III). Each of the depositional facies unit is characterized by variation of the depositional environment changes which were controlled by tectonics, and was followed by the activity of Sonder main fault. The Sonder main fault activity, which controlled the depositional environment changes was recorded as an UFP. Probably, the character of this stratigraphy could be called as cyclo-tectonostratigraphy. The study was based on analyses of sedimentology and stratigraphy of six borehole information obtained along the E-NW traverse which is approximately parallel to the shorelines of the southwestern part of Lake Tondano. The penetration of the borehead varied from 1.20 to 8.7 m.

Magnetostratigraphy of the Suerkuli Basin indicates Pliocene (3.2 Ma) activity of the middle Altyn Tagh Fault, northern Tibetan Plateau

The left-lateral strike-slip Altyn Tagh Fault (ATF) forming the northern boundary of the Tibetan Plateau accommodates parts of the overall convergence between the colliding Indian and Eurasian plates. Precise dating of the ATF activity is essential for understanding possible mechanisms of Tibetan Plateau deformation and uplift. Here we report a magnetostratigraphic study of the Suerkuli Basin deposits recording depositional changes during the ATF activity. Field investigations reveal a remarkable and widespread change in depositional environment in the Suerkuli Basin, i.e. a transformation from low-energy lacustrine deposits (grayish-green mud-siltstone and brown mud-siltstone) into high-energy alluviul fan deposits (poorly sorted gray pebble and cobble conglomerates). Detailed magnetostratigraphy of the 390-m-thick Daban section, at the southeastern margin of the Suerkuli Basin (38°43.09′N, 90°58.84′E), shows that this change in depositional facies occurred at ∼3.2 Ma, accompanied by a remarkable increase in sediment accumulation rate. We attribute this depositional change to the Piocene tectonic activity of the middle ATF although the contribution of the Pliocene global climate deterioration cannot be excluded.

The Miguasha Fossil-Fish-Lagerstätte: a consequence of the Devonian land–sea interactions

The evolution of vertebrate assemblages in terms of fluctuating environments has rarely been investigated for the Devonian period. Variation of biodiversity (richness, abundance and species composition) in the diverse Devonian fish assemblage of the Escuminac Formation (Quebec, Canada) is analysed in response to changes in lithofacies, depositional environment and taphonomy through time. Five sequences within an inner wave-dominated estuary show shifts in continentalisation. Although a ubiquitous fish assemblage is identified throughout the formation, species are more diversified and species composition is better structured during relative sea-level rise than during still-stand and relative sea-level fall. Konservat and Konzentrat Fossil- Lagerstatte horizons occur in the transgressive phase of the sequences.

Climate-controlled mass extinctions, facies, and sea-level changes around the Devonian–Carboniferous boundary in the eastern Anti-Atlas (SE Morocco)

Detailed sedimentological and biostratigraphical studies of successions in the eastern Anti-Atlas (SE Morocco) focused on changing depositional environments and reconstructions of sea-level changes during the late Famennian to middle Tournaisian. The investigations cover a transect from the central Maider to the easternmost and southern Tafilalt regions near the Moroccan/Algerian border. The Moroccan data are compared with the standard succession of the German Rhenish Massif and with the regional and global stratigraphic record. The results testify to high-frequency sea-level changes, including two global transgressive and anoxic events: the main Hangenberg Event (Hangenberg Black Shale level) at the top of the Wocklumeria Zone (Upper Devonian VI-E), and the Lower Alum Shale Event at the lower/middle Tournaisian (Lower Carboniferous I/II) boundary. Regionally, both anoxic intervals coincided with mass extinctions, followed by rapid radiation, especially of pelagic biota. A significant shallowing and regression occurred just below the Devonian–Carboniferous boundary at the northern margin of Gondwana and caused the sudden N/NW progradation of a large deltaic complex over 300–500 km. Thick siliciclastic successions filled fast subsiding synsedimentary troughs of the Maider and Amessoui Syncline (southern Tafilalt). Correlation with the glacial phases of southern Gondwana (South America, South Africa) indicates a glacial-eustatic origin of sea-level changes and a major sea-level fall in the scale of 100 m or more, coinciding with the main regressive phase of the extended Hangenberg Crisis interval. The new results are in accord with the hypothesis of an episode of cool-humid climate at the end of the Devonian that allowed contemporaneous glacial buildup in high latitudes and extensive clastic discharge in the middle latitudes of NW Africa.

Identification of Lower Pleistocene tephras under Tokyo and reconstruction of Quaternary crustal movements, Kanto Tectonic Basin, central Japan

The Kanto Tectonic Basin (KTB) originated from a paleo-forearc basin, and is filled with the Pleistocene Kazusa Group, Quaternary marine sediments with a thickness of more than 3000 m. In order to understand general process of emerging paleo-forearc basin development on active plate margins, an attempt was made to establish the tephrochronological framework of the Kazusa Group under the Tokyo metropolitan area, southwest KTB, and reconstruct changes in depositional environments and geological structure. Chemical compositions of major, trace and rare earth elements of glass shards, and refractive indices of glass shards and phenocrysts, were used to identify 12 Early Pleistocene tephras collected from five sediment cores, including seven tephras newly recognized under the Tokyo metropolitan area: Kd8B, YM, KK, Ob4e-Kd23A, Ob4d-Kd23B, Ob4c-Kd24, Ob4b-6, Ob4b-5, Ob4b-4, Ob4b-1, Omn-SK110, and SYG, in descending order. Calcareous nannofossil biostratigraphic and magneto-stratigraphic positions of 24 tephras discussed in this paper, including 17 known tephras described by previous studies, indicate that their ages range from ca. 0.9 Ma to ca. 2.0 Ma. Northward-dipping strata with a gradient of about 0.015 of the base of the Johoku Sand and Gravel Bed (1.219–1.343 Ma) indicates that south-side was uplifted, caused by crustal movements with a mean ratio of 1.2 × 10 −5/ky since its deposition. This movement has continuously occurred, deforming MIS 5.5 marine terrace surfaces. The depths of SYG and Ob4c-Kd24 suggest that the deformation of the southwestern part of the NE-SW trending Mizonokuchi Syncline is significantly lower than that of the northeast part. This syncline extends northeastward to the southeastern part of the Musashino Upland. Activity of the Mizonokuchi Syncline probably was terminated at the end of the Early Pleistocene to Middle Pleistocene. The Tachikawa Active Fault (TAF), trending NW-SE, has been active since the Middle Pleistocene. The termination of the Mizonokuchi Syncline and the activation of TAF simultaneously occurred roughly around 1.0–0.5 Ma. These tectonic changes most likely resulted from a change in motion direction of the Philippine Sea Plate relative to the North American Plate.

Late Neogene environmental changes in the central Himalaya related to tectonic uplift and orbital forcing

The linkage between tectonic forces and climate evolution remains a matter of much debate and speculation. Here we present high-resolution oxygen and carbon isotope data from an ancient lake basin in the central Himalaya. These data, together with sedimentologic evidence, reveal major changes in drainage systems and depositional settings at ∼7.2, ∼5.5 and ∼3.2 Ma. These environmental changes appear to be driven by regional-scale tectonics. The oxygen isotope record also reveals alternating wet and dry climates with periodicities of 24 and 100 kyr that were likely controlled by orbital forcing. Paleo-temperatures, estimated using a fossil-based oxygen isotope “paleo-thermometer”, are 21 ± 6 °C at ∼7 Ma, which is ∼19 ± 6 °C higher than the present-day mean annual temperature in the same area. The much warmer environment inferred here is consistent with fossil mammalian and pollen assemblages and sediment clay mineralogy as well as carbon isotope data from fossil tooth enamels and paleosol carbonates. The estimated temperature difference would require the study area to have been raised by ∼2–2.5 km since ∼7 Ma to its current elevation of 4100–4500 m above sea level. This result can be interpreted as either indicating the presence of a low-altitude intermountain basin in the hanging wall of the already formed Main Central Thrust or a protracted development of a north-trending rift basin that has experienced changes in drainage system and depositional environment through time.

Stratigraphy of the Younger Dryas Chronozone and paleoenvironmental implications: Central and Southern Great Plains

The Great Plains of the United States was the setting for some of the earliest research in North America into patterns and changes in the character of late Pleistocene environments and their effects on contemporary human populations. Many localities in the region have well-stratified records of terminal Pleistocene and early Holocene human (Paleoindian) activity and past environments. These have proven important in debates over the character of the Younger Dryas Chronozone (YDC; 11,000–10,000 14C BP; 12,900–11,700 cal BP) in the continental interior. This paper reviews the lithostratigraphic record of the YDC on the Central and Southern Great Plains and summarizes paleobiological records (largely isotopic). The goal is to determine if there is any uniformity in the timing, character, direction and/or magnitude of changes in depositional environments or broader geomorphic systems before, during or after the YDC in order to address the question of the character of environments through this time. The stratigraphic records of the late Pleistocene to early Holocene transition, and in particular, the stratigraphic records of the YDC vary through time and space. The data clearly show that a host of geomorphic processes produced the terminal Pleistocene and early Holocene stratigraphic records of the Great Plains. Moreover, the YDC is not necessarily manifest as a distinct lithostratigraphic or biostratigraphic entity in these different types of deposits and soils. The various geomorphic systems of the Great Plains did not behave synchronously in response to any common climate driver. These stratigraphic records reflect local environmental conditions and probably a complex response to the reorganization of mid-latitude climates in the terminal Pleistocene and early Holocene.

Hydrogeology, sequence stratigraphy and diagenesis in the Paleoproterozoic western Thelon Basin: Influences on unconformity-related uranium mineralization

The Paleoproterozoic (Statherian) Thelon Formation consists of unmetamorphosed sandstone and conglomerate that was deposited in the Thelon Basin, a continental basin in northern Canada. This study presents the first detailed sedimentological and sequence stratigraphic analysis of the expansive and under-studied western Thelon Basin, and its relation to uranium mineralizing processes in the vicinity of the Boomerang Lake unconformity-related U prospect. Four lithofacies comprise the Thelon Formation in the western Thelon Basin, consisting of breccia, conglomerate, sandstone, and minor mudstone. These are grouped into four facies associations that represent deposition in alluvial fans and braided fluvial streams. Three stratigraphic sequences were identified based on the recognition of widespread surfaces that mark significant periods of subaerial exposure or changes in depositional environment. These unconformity-bounded sequences are composite in nature and can be subdivided into six high-frequency sequences (HFS). Low-accommodation and high-accommodation systems tracts are delineated, instead of the traditional systems tracts that are linked to changes in relative sea level, as no marine deposits are recognized in the Thelon Formation. Two significant strataform paleohydrogeological units were distinguished in the Thelon Formation, based on the relative timing of key diagenetic events such as compaction, grain dissolution, and cementation. Diagenetic aquicludes became isolated from basinal fluid flow due to stylolite-catalyzed early diagenetic quartz cementation, and occlusion of secondary intergranular macroporosity by kaolinite and muscovite, and minor dickite during later peak diagenesis. Diagenetic aquifers conducted U-bearing basinal fluids throughout diagenesis and are marked by an abundance of peak diagenetic dickite. Sequence stratigraphic correlation indicates that diagenetic aquicludes, which were not effective at conducting U-bearing basinal fluids, overlie the unconformity at the Boomerang Lake prospect and extend along the entire strike length of a related exploration trend. In contrast, thick diagenetic aquifers lie approximately 10 km to the north of the Boomerang Lake prospect along a different exploration trend. The western and eastern portions of the Thelon Basin experienced depositional environments and diagenetic conditions distinct from one another, suggesting that they were separate sub-basins. The western Thelon Basin contains fewer of the well-cemented facies that compartmentalized basinal fluid flow than the eastern Thelon Basin, and contains a kaolin-dominated diagenetic mineral assemblage rather than a muscovite-dominated assemblage that is common in the eastern Thelon Basin. In these regards, the western Thelon Basin evolved more similarly to the eastern portion of the Athabasca Basin, where thick, regional, kaolin-rich diagenetic aquifers were fluid conduits for the movement of the U-rich fluids that formed high-grade unconformity-related U deposits.

Facies Associations, Paleoenvironment, and Base-Level Changes in the Upper Cretaceous Wahweap Formation, Utah, U.S.A.

The Wahweap Formation is an ~ 400-m-thick succession of fluvial and estuarine channel sandstones and floodbasin mudstones divided into lower, middle, upper, and capping sandstone members. Facies analysis of the Wahweap Formation on the Kaiparowits Plateau reveals the presence of ten facies associations grouped into channel and floodbasin deposits. Facies associations (FAs) from channels include: (1) single-story and (2) multistory lenticular sandstone bodies, (3) major tabular sandstone bodies, (4) gravel bedforms, (5) low-angle heterolithic cross-strata, and (10) lenticular mudrock, whereas floodbasin facies associations include: (6) minor tabular sandstone bodies, (7) lenticular interlaminated sandstone and mudrock, (8) inclined interbedded sandstone and mudrock, and (9) laterally extensive mudrock. The lower and middle members are dominated by floodbasin facies associations. The lower member consists dominantly of FA 8, interpreted as proximal floodbasin deposits including levees and pond margins, and is capped by a persistent horizon of FA 3, interpreted as amalgamated channel deposits. FAs 4 and 6 are also present in the lower member. The middle member consists dominantly of FA 9, interpreted as distal floodbasin deposits including swamp, oxbow-lake, and waterlogged-soil horizons. FAs 1, 2, 5, 6, 7, 8, and 10 are present in the middle member as well, which together are interpreted as evidence of suspended-load channels. The upper member is sandstone-dominated and consists of FAs 1, 2, 3, 5, 7, and 8. FAs 5 and 7, which occur at the base of the upper member, are interpreted as tidally influenced channels and suggest a marine incursion during deposition of the upper member. The capping sandstone is characterized by FAs 3, 4, and 6, and is interpreted to represent a major change in depositional environment, from meandering river systems in the lower three members to a low-accommodation, braided river system. Improved age constraints on the Wahweap Formation indicate that the middle and upper members were deposited during the eustatic Claggett transgression (T8 of Kauffman 1977) in the adjacent Western Interior Seaway. Additionally, facies analysis of the Wahweap Formation has revealed an increase in both the sand:mud ratio and the degree of sandstone amalgamation from the upper part of the middle member to the base of the capping sandstone, suggesting a gradual reduction in generation of accommodation in the basin. Following recent alluvial sequence stratigraphic models, the middle member is interpreted as the isolated fluvial facies tract, while the upper member represents the tidally influenced and highstand facies tracts. Maximum transgression occurred during deposition of the lowest part of the upper member, synchronous with the putatively eustatic Claggett highstand in other parts of the Western Interior Basin. The sequence boundary is placed at the base of the overlying capping sandstone member, diagnosed by a major shift in petrography and paleocurrent direction, as well as up to 4 m of fluvial incision into the underlying upper member. The capping sandstone member is interpreted as the amalgamated fluvial facies tract, and the sequence boundary at the base of the capping sandstone is regarded as tectonically induced. Improved geochronology in the formation now permits high-resolution correlation with marine units to the east and along-strike correlation with contemporaneous alluvial units up and down the western margin of the Western Interior Basin.

Depositional processes of the Zhushadong and Mantou formations (Early to Middle Cambrian), Shandong Province, China: roles of archipelago and mixed carbonate–siliciclastic sedimentation on cycle genesis during initial flooding of the North China Platform

AbstractTo understand the depositional processes and environmental changes during the initial flooding of the North China Platform, this study focuses on the Lower to Middle Cambrian Zhushadong and Mantou formations in Shandong Province, China. The succession in the Jinan and Laiwu areas comprises mixed carbonate and siliciclastic deposits composed of limestone, dolostone, stromatolite, thrombolite, purple and grey mudstone, and sandstone. A detailed sedimentary facies analysis of seven well‐exposed sections suggests that five facies associations are the result of an intercalation of carbonate and siliciclastic depositional environments, including local alluvial fans, shallowing‐upward carbonate–siliciclastic peritidal cycles, oolite dominant shoals, shoreface and lagoonal environments. These facies associations successively show a transition from an initially inundated tide‐dominated carbonate platform to a wave‐dominated shallow marine environment. In particular, the peritidal sediments were deposited during a large number of depositional cycles. These sediments consist of lime mudstone, dolomite, stromatolite and purple and grey mudstones. These shallowing‐upward cycles generally resulted from carbonate production in response to an increase of accommodation during rising sea‐level. The carbonate production was, however, interrupted by frequent siliciclastic input from the adjacent emergent archipelago. The depositional cycles thus formed under the influence of both autogenetic changes, including sediment supply from the archipelago, and allogenic control of relative sea‐level rise in the carbonate factory. A low‐relief archipelago with an active tidal regime allowed the development of tide‐dominated siliciclastic and carbonate environments on the vast platform. Siliciclastic input to these tidal environments terminated when most of the archipelago became submerged due to a rapid rise in sea‐level. This study provides insights on how a vast Cambrian carbonate platform maintained synchronous sedimentation under a tidal regime, forming distinct cycles of mixed carbonates and siliciclastics as the system kept up with rising relative sea‐level during the early stage of basin development in the North China Platform.

A New Zealand record of sea level rise and environmental change during the Paleocene–Eocene Thermal Maximum

The global warming associated with the Paleocene–Eocene Thermal Maximum (PETM) ca. 55.5 Myr ago is the most dramatic identified short-term temperature increase of the Cenozoic. One direct consequence of a warming world is a rise in sea level, due primarily to the thermal expansion of water as oceans warmed. The Kumara-2 core, South Island, New Zealand, spans the Paleocene/Eocene transition and provides a rare southern hemisphere continental margin record of the PETM. Lithology, palynology and compound-specific stable isotope compositions of higher plant leaf wax n-alkanes reveal a 4.85 m PETM and a carbon isotope excursion (CIE) of ca. − 4.5‰, larger than the − 2.5 to − 3.5‰ CIEs generally recorded by deep sea foraminifera. There is a shift from a terrestrial to a marine, potentially anoxic, sedimentary depositional environment at the base of the PETM, interpreted as being the result of a local sea level rise. Coincident with the onset of the CIE is the appearance of pollen associated with thermophilic conditions and the development of Nypa mangrove swamps. Moreover, there is a reorganisation of the angiosperm pollen assemblage during the PETM, and an initial increase in fern spores and decrease in gymnosperms. Crucially, all of these changes occur below the horizon characterised by the most negative δ 13C values, suggesting that: 1) the recorded negative excursion of 4.5‰ may indeed reflect the shift in atmospheric CO 2 isotopic composition; and 2) that the large input of 13C-depleted carbon into the ocean–atmosphere system was not geologically instantaneous, with at least some of the added carbon lagging warming, sea level rise and vegetation change. Furthermore, compound-specific hydrogen isotope analyses show a large degree of variability both directly before and during the CIE, suggesting that the PETM in New Zealand was characterised by complex and transient changes in the hydrological regime, similar to those reported in North America, the Arctic and Eastern Africa. Thus, the new PETM record from Kumara-2 reveals local climatic and biotic responses, driven by a combination of global warming and the consequential change in local depositional environment induced by sea level rise.

Stratigraphic development of the south Vøring margin (Mid-Norway) since early Cenozoic time and its influence on subsurface fluid flow

The Cenozoic seismic stratigraphy and geological development of the south Vøring margin are analyzed to understand their relation to fluid flow and margin stability. The regional stratigraphy and palaeomorphology of the Møre and Vøring basins indicate gradual changes in depositional environment and tectonic compression between 55 Ma to 2.8 Ma during Brygge and Kai periods, and abrupt changes associated with glacial/interglacial cycles from last 2.8 Ma during Naust period. These changes resulted in deposition of various types of sediments and led to processes such as polygonal faulting and dewatering, inter-fingering of contouritic, stratified and glacigenic sediments, and margin progradation. A gas hydrate related bottom simulating reflector (BSR) occurs at Nyegga and within the central Vøring Basin while pockmarks are observed at Nyegga only. Diagentic reflectors due to Opal A - Opal CT conversion (DBSRs) occur along a wider area beyond the shelf edge. The DBSRs are located in oozes within the Kai and late Brygge Formations. The gas hydrate BSR occurrence is concentrated above Eocene depocenters in hemipelagic and contouritic sediments deposited during Late Plio-Pleistocene. The BSR overlies polygonal faults and DBSRs but are confined to the slope of anticlines indicating its formation being related to fluid pathways from methanogenic rocks through focused fluid flow. Microbial gas production in Kai, Brygge and deeper formations may have supplied the gas for gas hydrate formation. Fluid expulsion due to DBSR formation and polygonal faulting in oozes may have created overpressure development in permeable layers belonging to the overlying Naust Formation. Slide headwalls are also located close to the anticlines in the study area, implying that over pressured oozes and focussed fluid flow may have been important in creating weak surfaces in the overlying Naust sediments, promoting conditions for failures to occur.

STRATIGRAPHY AND SEDIMENTOLOGICAL EVOLUTION OF THE NEOPROTEROZOIC IKORONGO GROUP OF NORTHEASTERN TANZANIA.

On the northern side of the Ikorongo basin, the Neoproterozoic Ikorongo Group, comprising arenites, arkosic arenites, shales and siltstones, unconformably overlies the Archaean Nyanzian Supergroup of the Tanzania craton. In the study area, the Ikorongo Group is subdivided into three stratigraphic formations namely the Kinenge, Sumuji and Masati Formations. Two new stratigraphic members for the Sumuji Formation are proposed: The lower unit is the shale member which comprises of four alternating horizons of brown and green shales containing mudcracks and ripples and is here reported for the first time to overlie, through a tectonic contact, basement rocks. The upper unit above the shale member is the cross-laminated micaceous siltstone member. Sedimentological studies reveal seven depositional litho-facies and highlight the possibility of hydrodynamic and wave energy fluctuations during deposition of the Ikorongo sedimentary units. The fining upwards trend from the Kinenge sandstones towards the overlying Sumuji shales suggests an energy reduction episode responsible for the deposition of these units whilst the coarsening in grain size, from the Sumuji siltstones upstratigraphy to the Masati sandstones attests to an energy increase. The color variations in the shale member reveal intermittent changes in redox conditions and/or depositional environments during when these strata were laid down. The presence of mud cracks and ripple marks in the rocks of the Sumuji Formation signifies deposition in shallow water environments which had experienced intermittent sub-aerial exposure and desiccation. Fluctuations in hydrodynamic energy, the presence of reddish brown beds and pristine palaeocurrent structures preserved both in space and time within the Ikorongo strata suggest an overall terrestrial depositional environment.

Evaluating foreland basin partitioning in the northern Andes using Cenozoic fill of the Floresta basin, Eastern Cordillera, Colombia

AbstractThis paper addresses foreland basin fragmentation through integrated detrital zircon U–Pb geochronology, sandstone petrography, facies analysis and palaeocurrent measurements from a Mesozoic–Cenozoic clastic succession preserved in the northern Andean retroarc fold‐thrust belt. Situated along the axis of the Eastern Cordillera of Colombia, the Floresta basin first received sediment from the eastern craton (Guyana shield) in the Cretaceous–early Palaeocene and then from the western magmatic arc (Central Cordillera) starting in the mid‐Palaeocene. The upper‐crustal magmatic arc was replaced by a metamorphic basement source in the middle Eocene. This, in turn, was replaced by an upper‐crustal fold‐thrust belt source in the late Eocene which persisted until Oligocene truncation of the Cenozoic section by the eastward advancing thrust front. Sedimentary facies analysis indicates minimal changes in depositional environments from shallow marine to low‐gradient fluvial and estuarine deposits. These same environments are recorded in coeval strata across the Eastern Cordillera. Throughout the Palaeogene, palaeocurrent and sediment provenance data point to a uniform western or southwestern sediment source. These data show that the Floresta basin existed as part of a laterally extensive, unbroken foreland basin connected with the proximal western (Magdalena Valley) basin from mid‐Paleocene to late Eocene time when it was isolated by uplift of the western flank of the Eastern Cordillera. The Floresta basin was also connected with the distal eastern (Llanos) basin from the Cretaceous until its late Oligocene truncation by the advancing thrust front.

Palynological implication of Holocene vegetation and environment in Pyeongtaek wetland, Korea

Vegetational and depositional environmental changes were interpreted from a trench in ca. 10 000 cal BP sediments in the Pyeongtaek wetland, Korea, based on palynomorph and grain size analysis. From ca. 10 400−8000 cal BP, cool, dry, temperate conditions encouraged Quercus-dominated deciduous broadleaved forest, with conifers and cool-tolerant birch prevailing on hills and in mountainous areas as postglacial warming began. Halophytic Chenopodiaceae pollen and marine dinoflagellates indicate the depositional environment was an intertidal flat. Conditions became warm and wet between ca. 8000 and ca. 6000 cal BP: areas still contained Quercus-dominated deciduous broadleaved forest but with decreased conifers and cool-tolerant birch. The absence of marine dinoflagellates and abundant meso- to hypersaline salt marshes of Gramineae, with a later increase of mesosaline Cyperaceae marshes, indicate a transition from intertidal flat to freshwater swamp. From ca. 6000 to ca. 4500 cal BP, Quercus-dominated forests were replaced by mixed conifer–deciduous broadleaved forests under climatic deterioration. Aquatic genera and freshwater algae indicate that the study area completely changed to freshwater swamp. The directional sequence in relative abundance of taxa (maximum percentage of pollen of Chenopodiaceae followed by Gramineae, Cyperaceae, and aquatics) suggests a gradational pattern indicative of marked environmental clines typical of intertidal habitats.

Ashtead Common, the evolution of a cultural landscape: A spatially precise vegetation record for the last 2000 years from southeast England

The results of palaeoecological (pollen and fungal spores) investigations of an abandoned clay pit on Ashtead Common, Surrey are presented and compared with archaeological and documentary evidence. The pit lies adjacent to a Roman industrial complex, with early Saxon activity recorded nearby. Prior to the early twentieth century AD the Common was managed as wood-pasture and it has a high nature conservation designation as a result of c. 2000 veteran pollarded oak trees which remain from this phase. The pit provides a record of landscape history from the Roman ( c. cal. AD 150) to the modern ( c. AD 1850) era which, in terms of spatial and temporal resolution, is unrivalled in southeast England. From the late Roman into the middle Saxon period the palaeoecological data are consistent with archaeological evidence for the continuity of settlement and land use. Subtle changes in woodland cover, probably reflecting changes in the importance of grazing against the exploitation of wood, are evident up to c. cal. AD 1200. Thereafter grazing pressure on the Common appears to have increased and the resulting shortage of underwood is likely to have led eventually (cal. AD 1490—1680) to the adoption of pollarding. Changes in depositional environment are shown to have a major impact on the fungal spore record. In general terms the stratigraphic data from the Ashtead Common pit accord well with archaeological/documentary evidence and demonstrate the utility of studies which use a combination of sources to reconstruct vegetation change in the historic era.

Composition and Provenance History of Late Cenozoic Sediments in Southeastern Bolivia: Implications for Chaco Foreland Basin Evolution and Andean Uplift

Abstract The Chaco foreland basin marks the eastern edge of the Andean orogenic system and contains up to 7.5 km of mostly nonmarine siliciclastics, thus providing an opportunity to study the detrital record of late Cenozoic Andean geodynamics and paleogeography. Sandstones from the southern Bolivian Chaco foreland basin testify to variable contributions from the craton and the Andean foreland fold-and-thrust belt in the context of an eastward-advancing orogen. Sandstone and conglomerate petrography of the Oligocene to Recent strata exposed in the Chaco Basin show an overall trend towards reduced mineralogical maturity, corresponding to diminishing transport distance, increasing topographic gradient and changing depositional environment from arid continental plain (Oligocene to middle Miocene Petaca sandstone) through marginal marine (middle Miocene to late Miocene Yecua sandstone) and braided fluvial settings (late Miocene Tariquia sandstone) to semihumid alluvial-fan environments (late Miocene to Recent Guandacay and Emborozu formations). Paleocurrent patterns of pre-tectonic sediments record westward transport. The late Miocene transitional-marine Yecua Formation sediments show a complex shoreline-related pattern. Syntectonic strata (Tariquia, Guandacay, and Emborozu formations) filling the foreland basin indicate dominant east-directed sediment transport from the advancing Andean orographic front. Because the Eastern Cordillera provided a drainage barrier to Altiplano erosion, sandstones of the Chaco Basin do not directly record Andean volcanism or Altiplano uplift; however, climatic inferences from changing facies and petrographic composition suggest major uplift and formation of orographic barriers between 14 and 7 Ma.