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Middle Miocene to Pleistocene sedimentary record of rift evolution in the southern Albert Rift (Uganda) 总被引：2，自引：1，他引：1
S. Roller J. Hornung M. Hinderer I. Ssemmanda 《International Journal of Earth Sciences》2010,99(7):1643-1661
This study presents an almost complete Middle Miocene to Pleistocene sequence of synrift sediments in the western branch of
the East African Rift. The studied succession is exposed in several patches on an eastward tilted block between the northern
tip of the Rwenzori Block and the eastern shoulder of the Albert Rift. In this position, it reaches a maximum thickness of
600 m of which 350 m have been logged systematically by analysing lithofacies and sediment architecture. Stratigraphic subdivision
of the succession relies on published biostratigraphic data of endemic mollusc associations and their correlation across East
Africa. The synrift sediments encountered are siliciclastics ranging from clay to coarse gravel with gypsum and ferrugineous
interlayers or impregnations. Lithofacies and architectural analysis indicate alluvial plain, delta plain, nearshore, delta
front, or lacustrine depositional environments. Based on the vertical stacking pattern, prograding and retrograding trends
of the depositional environments, and climatic indicators (e.g. conservation of feldspar, gypsum, and/or iron hydroxide precipitation),
four evolutionary phases can be distinguished: (i) a first phase between ca. 14.5 and 10.0 Ma is characterised by bedload-dominated
fluvial environment with massive sandy to gravelly bedforms, feldspar-rich sands, rare iron impregnations and relatively low
accommodation space. This phase is interpreted as pre- and early synrift sedimentation under a semiarid climate. (ii) From
ca. 10.0 to 4.5 Ma predominantly fine-grained siliciclastics were deposited in a distal fluvial plain to lacustrine setting
characterised by limited accommodation space. Fluctuation of thin beds, dominance of clay and frequent iron impregnations
point to a more humid climate with seasonality and weak tectonic activity. (iii) During the third phase between 4.5 and 2.0 Ma
delta plain and nearshore deposits with frequent ferrugineous impregnations and rich mollusc associations occurred, indicating
a humid period with lake-level highstands and accelerated subsidence. (iv) During the final sedimentary interval between 2.0
and 1.5 Ma gravel units reoccurred with less iron- but more carbonate and gypsum impregnations, and arkosic sandstones. This
phase recorded a general aridisation trend most probably caused by the upcoming rain barrier of the Rwenzori Mountains together
with accelerated rift-flank uplift and strong subsidence of the rift floor. The results of this study are of particular importance
for delineating key controls on sedimentation in the Albert Rift. 相似文献
2.
V. N. Panizzo A. W. Mackay I. Ssemmanda R. Taylor N. Rose M. J. Leng 《Journal of Paleolimnology》2008,40(1):325-338
Environmental change in many tropical, alpine habitats remains poorly resolved due to an absence of proximate and sustained
observations. In the Rwenzori Mountains of East Africa, glaciers have receded rapidly over the last century, and here we assess
the impact of this recession through palaeolimnological analyses of a 45 cm sediment core (Buju3) from Lake Bujuku which is
closest to the ice-fields and partly supplied by melt water in-flows. 210Pb and 137Cs suggest that Buju3 has an average sedimentation rate of 2.9 mm year−1 and the base of the core can be dated to 1864 ± 20 years. Contemporary diatom taxa found in the lake are dominated by Tabellaria flocculosa and Synedra spp., but also include Achnanthes minutissima and Fragilaria pinnata. However, the diatom flora for Buju3 is less diverse and dominated by small, tychoplanktonic species of Fragilaria. Over the period associated with glacial recession, organic carbon isotope analysis (δ13C) suggests a small but distinct increase in within-lake productivity, which increases in rate since the mid 1970s up to the
present day, in line with a shift towards increased algal productivity (as highlighted by C/N ratios). However, the diatom
and pollen records appear rather insensitive to changes in glacier recession since the late 19th century. 相似文献
3.
Thomas Christian Brachert Gerhard B. Brügmann Dieter F. Mertz Ottmar Kullmer Friedemann Schrenk Dorrit E. Jacob Immaculate Ssemmanda Heinrich Taubald 《International Journal of Earth Sciences》2010,99(7):1663-1675
The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon
intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean
Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall
patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data
(18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control
the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma
to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants
of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the
rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently,
we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5–2.5 Ma to maximum aridity and/or maximum hydrological isolation of
the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence
of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well
constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst.
However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding
effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley.
On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern
branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate
reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The
results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain
range uplift. 相似文献
4.
David B. Ryves Keely Mills Ole Bennike Klaus Peter Brodersen Angela L. Lamb Melanie J. Leng James M. Russell Immaculate Ssemmanda 《Quaternary Science Reviews》2011,30(5-6):555-569
The last millennium is a key period for understanding environmental change in eastern Africa, as there is clear evidence of marked fluctuations in climate (effective moisture) that place modern concern with future climate change in a proper context, both in terms of environmental and societal impacts and responses. Here, we compare sediment records from two small, nearby, closed crater lakes in western Uganda (Lake Kasenda and Lake Wandakara), spanning the last 700 (Wandakara) and 1200 years (Kasenda) respectively. Multiproxy analyses of chemical sedimentary parameters (including C/N ratios, δ13C of bulk organic matter and δ13C and δ18O of authigenic carbonates) and biotic remains (diatoms, aquatic macrofossils, chironomids) suggest that Kasenda has been sensitive to climate over much of this period, and has shown substantial fluctuations in conductivity, while Wandakara has a more muted response, likely due to the increasing dominance of human activity as a driver of change within the lake and catchment over the length of our record. Evidence from both records, however, supports the idea that lake levels were low from ～AD 700–1000 AD, with increasing aridity from AD 1100–1600, and brief wet phases around AD 1000 and 1400. Wetter conditions are recorded in the 1700s, but drought returned by the end of the century and into the early 1800s, becoming wetter again from the mid-1800s. Comparison with other records across eastern Africa suggests that while some events are widespread (e.g. aridity beginning ～ AD 1100), at other times there is a more complex spatial signature (e.g. in the 1200s to 1300s, and from the 1400s to 1600s). This study highlights the important role of catchment-specific factors (e.g. lakemorphometry, catchment size, and human impact) in modulating the sensitivity of proxies, and lake records, as indicators of environmental change, and potential hazards when regional inference is based on a single site or proxy. 相似文献
5.
Determining hydrological regimes in an agriculturally used tropical inland valley wetland in Central Uganda using soil moisture,groundwater, and digital elevation data 
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下载免费PDF全文 Geofrey Gabiri Bernd Diekkrüger Constanze Leemhuis Sonja Burghof Kristian Näschen Immaculate Asiimwe Yazidhi Bamutaze 《水文研究》2018,32(3):349-362
Inadequate knowledge exists on the distribution of soil moisture and shallow groundwater in intensively cultivated inland valley wetlands in tropical environments, which are required for determining the hydrological regime. This study investigated the spatial and temporal variability of soil moisture along 4 hydrological positions segmented as riparian zone, valley bottom, fringe, and valley slope in an agriculturally used inland valley wetland in Central Uganda. The determined hydrological regimes of the defined hydrological positions are based on soil moisture deficit calculated from the depth to the groundwater table. For that, the accuracy and reliability of satellite‐derived surface models, SRTM‐30m and TanDEM‐X‐12m, for mapping microscale topography and hydrological regimes are evaluated against a 5‐m digital elevation model (DEM) derived from field measurements. Soil moisture and depth to groundwater table were measured using frequency domain reflectometry sensors and piezometers installed along the hydrological positions, respectively. Results showed that spatial and temporal variability in soil moisture increased significantly (p < .05) towards the riparian zone; however, no significant difference was observed between the valley bottom and riparian zone. The distribution of soil hydrological regimes, saturated, near‐saturated, and nonsaturated regimes does not correlate with the hydrological positions. This is due to high spatial and temporal variability in depth to groundwater and soil moisture content across the valley. Precipitation strongly controlled the temporal variability, whereas microscale topography, soil properties, distance from the stream, anthropogenic factors, and land use controlled the spatial variability in the inland valley. TanDEM‐X DEM reasonably mapped the microscale topography and thus soil hydrological regimes relative to the Shuttle Radar Topography Mission DEM. The findings of the study contribute to improved understanding of the distribution of hydrological regimes in an inland valley wetland, which is required for a better agricultural water management planning. 相似文献
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