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Lacustrine Environment (lacustrine + environment)

Distribution by Scientific Domains

Earth and Environmental Science	80%

Selected Abstracts

FRESHWATER ALGAE FROM THE UPPER TRIASSIC CUYANA BASIN OF ARGENTINA: PALAEOENVIRONMENTAL IMPLICATIONS

PALAEONTOLOGY, Issue 6 2006
ANA M. ZAVATTIERI
Abstract:, Diverse freshwater aquatic palynomorphs are present among rich terrestrial palynofloras from the upper part of the Potrerillos and Cacheuta formations outcropping in the Cuyana Basin at the southern extremity of the Precordillera of Argentina. The Potrerillos/Cacheuta sequence includes fluvial, deltaic and lacustrine facies deposited in the early Late Triassic fault-bounded syn-rift Cacheuta hemigraben. The phytoplankton described and illustrated consist of representatives of colonial chlorococcalean algae belonging to the Hydrodictyaceae (Plaesiodictyon) and Botryococcaceae (Botryococcus), a diverse group of Zygnemataceae zygospores, a freshwater dinoflagellate cyst (Bosedinia) and acritarchs (sphaeromorphs). Zygnematacean zygospores are represented by species of Gelasinicysta?, Lecaniella, Mougeotia, Ovoidites, Peltacystia and Schizocystia. Of these, Gelasinicysta? cuyanensis sp. nov. is newly described. Quantitative analysis of the palynofloras permits interpretation of changes in the local vegetation and phytoplankton communities controlled by changes in environmental setting. Two algal associations are recognized as belonging to different stages in the evolution of the basin. Diverse zygnematacean assemblages along with hydrodictyacean, botryococcacean and leiosphere/sphaeromorph algae are common in fluvial,deltaic environments of the upper part of the Potrerillos Formation and the lowermost part of the Cacheuta Formation. Higher in the sections (in the Cacheuta Formation) Botryococcus dominates, in association with amorphous kerogen, representing deposition in a relatively deep, quiet-water, lacustrine environment where anoxic conditions prevailed. In the Cacheuta hemigraben the lacustrine shales have average TOC values of 4 per cent (locally reaching 20 per cent), with some terrigenous components (Type II/III kerogen), but dominant amorphous, algal-like, organic matter (Type I/II kerogen). Oils derived from these source rocks are predominantly waxy. [source]

Late Cenozoic structural and stratigraphic evolution of the northern Chinese Tian Shan foreland

BASIN RESEARCH, Issue 3 2010
Honghua Lu
ABSTRACT Three successive zones of fault-related folds disrupt the proximal part of the northern Tian Shan foreland in NW China. A new magnetostratigraphy of the Taxi He section on the north limb of the Tugulu anticline in the middle deformed zone clarifies the chronology of both tectonic deformation and depositional evolution of this collisional mountain belt. Our ,1200-m-thick section encompasses the upper Cenozoic terrigenous sequence within which ,300 sampling horizons yield an age span of ,8,2 Ma. Although the basal age in the Taxi He section of the Xiyu conglomerate (often cited as an indicator of initial deformation) is ,2.1 Ma, much earlier growth of the Tugulu anticline is inferred from growth strata dated at ,6.0 Ma. Folding of Neogene strata and angular unconformities in anticlines in the more proximal and distal deformed zones indicate deformation during Miocene and Early Pleistocene times, respectively. In the Taxi He area, sediment-accumulation rates significantly accelerate at ,4 Ma, apparently in response to encroaching thrust loads. Together, growth strata, angular unconformities, and sediment-accumulation rates document the northward migration of tectonic deformation into the northern Tian Shan foreland basin during the late Cenozoic. A progradational alluvial,lacustrine system associated with this northward progression is subdivided into two facies associations at Tugulu: a shallow lacustrine environment before ,5.9 Ma and an alluvial fan environment subsequently. The lithofacies progradation encompasses the time-transgressive Xiyu conglomerate deposits, which should only be recognized as a lithostratigraphic unit. Along the length of the foreland, the locus of maximum shortening shifts between the medial and proximal zones of folding, whereas the total shortening across the foreland remains quite homogeneous along strike, suggesting spatially steady tectonic forcing since late Miocene times. [source]

First Late Triassic Record of a Paleoentomofauna from South America (Malargüe Basin, Mendoza Province, Argentina)

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2010
Carsten BRAUCKMANN
Abstract: Late Middle Triassic to early Late Triassic insects from Argentina have been previously described from the Bermejo and Cuyana Basins where they have been recovered from the Ischichuca-Los Rastros and Potrerillos-Cacheuta Formations, respectively. The insect fauna discussed herein was collected during field studies in 1986/1987 from the Llantenes section (Norian to Rhaetian? Late Triassic), which is situated in the Malargüe Basin in southern Mendoza province. The insect remains were found in the upper part of the Llantenes section (Llantenes Formation), which is built up of two coarsening-upwards cycles reflecting a deltaic progradation of a fluvial into a lacustrine environment (lower part), succeeded by repeated progradations into a floodplain-dominated environment (upper part; with finds of insects, conchostracans, fish remains, plant fragments, and drifted logs). The new finds represent the youngest Triassic insect records described from Argentina and even from South America in its entirety. There is only one contemporaneous fossil assemblage in Gondwana: in the Clarence/Moreton Basin (Aberdare Conglomerate; Late Norian) in Australia. The new Triassic insects include an impression of an isolated Mecopterida-like wing (Mendozachorista volkheimeri gen. et sp. nov.; Mendozachoristidae fam. nov.), coleopteran elytra of the Permosynidae (Ademosyne rosenfeldi sp. nov. and Ademosyne llantenesensis sp. nov.) and other isolated body fragments. This new Late Triassic entomofauna from Argentina is of considerable importance in the reconstruction of the biotic recovery of continental environments in Gondwana after the catastrophic mass extinction at the P/T boundary. [source]

Stable isotope evidence for impala Aepyceros melampus diets at Akagera National Park, Rwanda

AFRICAN JOURNAL OF ECOLOGY, Issue 4 2009
Sandi R. Copeland
Abstract Stable isotope analysis of tooth enamel was used to investigate the relative proportions of grass and browse in seasonal and overall diets of impala Aepyceros melampus at Akagera National Park, Rwanda. Bulk enamel samples suggest that on average, impala ate c. 86% C4 grass year-round, far more than in most previously studied impala populations across Africa. Intra-tooth samples show that seasonal changes in the proportion of C4 grass versus C3 browse are minimal (c. 10%), the diet being dominated by C4 grass year-round in contrast to other impala populations that consume ,50% browse during the dry season. Intra-tooth oxygen isotope values track carbon isotope changes to a moderate degree, but are not patterned clearly enough to permit identification of wet versus dry seasons. As other studies have shown that impala select high-protein diets, the foraging behaviour at Akagera is probably because of the availability of palatable grass for much of the year in the edaphic grasslands around the lacustrine environments of the eastern portions of Akagera National Park. Résumé L'analyse des isotopes stables de l'émail des dents a servi pour étudier les proportions relatives d'herbes et d'autres matières végétales dans le régime alimentaire saisonnier et général de l'impala Aepyceros melampus au Parc National de l'Akagera, au Rwanda. Des échantillons d'émail suggèrent qu'en moyenne, les impalas mangent c. 86% d'herbes de type C4 tout au long de l'année, beaucoup plus que la plupart des populations d'impalas étudiées ailleurs en Afrique. Les échantillons du centre des dents montrent que les changements saisonniers de la proportion d'herbes de type C4 par rapport au type C3 sont minimaux (c. 10%) et que le régime alimentaire est dominé par des herbes de type C4 toute l'année, contrairement aux autres populations d'impalas qui consomment ,50% d'autre végétation pendant la saison sèche. Les valeurs de l'isotope d'oxygène dans les dents permettent, dans une faible mesure, de détecter des changements de l'isotope de carbone, mais ils ne sont pas suffisamment définis pour permettre l'identification des saisons sèches ou humides. Comme d'autres études ont montré que les impalas sélectionnent une alimentation riche en protéines, le comportement alimentaire des impalas à l'Akagera est probablement dûà la disponibilité d'herbes goûteuses pendant une grande partie de l'année dans les prairies édaphiques qui entourent les environnements lacustres de l'est du Parc National de l'Akagera. [source]

Late Quaternary distribution and biogeography of the southern Lake Eyre basin (SLEB) megafauna, South Australia

BOREAS, Issue 1 2009
STEVE WEBBArticle first published online: 3 SEP 200
Understanding the population demography, species distribution and biogeography of Australia's megafauna is essential for understanding their extinction. This process is only just beginning, and this article discusses these aspects while concentrating on a particular region; the southern Lake Eyre Basin (SLEB). It is also the first detailed description of the distribution of megafauna across that region of central Australia. The data are based on an extensive longitudinal study of 41 palaeontological sites spread across 250 000 km2. Megafauna adaptation and response to extensive environmental change during the late Quaternary is reflected in the composition and distribution of 21 megafauna species found across the region. A flexible migration strategy in response to wet/arid cycles saw occupation focusing on riverine and lacustrine environments formed by episodes of southerly monsoon incursion into the northern Lake Eyre basin. The supply of animals to the area was largely derived from endemic populations inhabiting western parts of southeastern Australia, where the majority of SLEB species are also found. Diprotodon reflects the adaptive flexibility of some species owing to its particular continental distribution, with a number of semi-arid regions across the continent. A picture of how and when the last of the region's megafauna disappeared is closer to being understood, with likely terminal dates for Genyornis and Diprotodon now available. [source]