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A Palynological Study of Neogene and Holocene Sediments from Lake Albert, Uganda, with Implications for Vegetation and Climatic Changes in East Africa*
Dave Shaw1, Paul C. Logan2, and Janice Weston3
Search and Discovery Article #50180 (2009)
Posted May 13, 2009
*Adapted from extended abstract prepared for AAPG International Conference and Exhibition, Cape Town, South Africa, October 26-29, 2008. Please refer to Logan et al., 2009, Exploration on the Frontier: Towards an Understanding of the Albert Basin: Search and Discovery Article #10192 (2009).
1 Biostratigraphic Associates (UK) Ltd., Stoke-on-Trent, United Kingdom (associate biostratigrapher with RPS Energy) ([email protected])
2 Heritage Oil & Gas Ltd, London, United Kingdom ([email protected])
3 RPS Energy, Woking, United Kingdom
Samples have been analysed palynologically from four wells (Turaco 1, 2, 3 and Kingfisher 1) and adjacent field samples from Block 3 in the south of the Lake Albert graben in Uganda. The palynological assemblages from these samples have been analysed quantitatively, and a variety of pollen, spores, algae and fungi identified, together with a visual estimate of the kerogen.
The pollen and spores have been attributed to parent plants derived from the immediate and more regional surrounding areas. These include mountain, tropical forest, savannah and swamp vegetation types. In many cases, the miospores and plants have been identified to genus level, and in most cases to family level.
From the identified plants, regional vegetation trends and changing abundances are recognised which are tied to climatic cycles/changes, related to the effects of the northern hemisphere Ice Ages and, more locally, to the effects of doming in East Africa. When calibrated using occurrences of age diagnostic pollen, this gives the basis for a robust age breakdown for these sequences, enabling accurate correlation across the southern Lake Albert area.
The character of the local vegetation source, the kerogen type and the algal/fungal recovery are also integrated with lithologies and, within the well sections, wireline data to enable interpretation of depositional style within these two very different areas.
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The Turaco and Kingfisher wells are located in the southern part of the Albert Basin in Uganda (Figure 1). They comprise a variety of lake, fluvial, alluvial plain, delta plain, marsh and swamp sediments, which are entirely non-marine. The Turaco section was deposited under predominantly alluvial and delta plain conditions, while the Kingfisher section comprises predominantly lake deposits. The Turaco samples yield high palynomorph recoveries throughout, a consequence of the predominantly swampy conditions within an alluvial/delta plain depositional setting. The Kingfisher samples exhibit variable palynomorph recoveries, with deposition under predominantly lacustrine conditions, and are considered to reflect vegetation changes in the immediately surrounding area. These vegetation changes are considered directly related to climatic changes. It is notable that the assemblages show little evidence of a direct fluvial influence. The pollen and spores recovered can be related to parent plants and thence to specific environments. Warmer and wetter interglacial phases tend to be characterised by high abundance assemblages, whilst cooler and more arid phases are characterised by low abundance palynofloras. Specific vegetation types related to their habitat are as follows:
High Altitude (above 1500m) Mountain Forest, Arid and Cooler Climate (Figure 2) This vegetation belt is characterised by abundant Podocarpus spp., in association with Alnus spp., Ericaceae spp., Juniperus spp., Olea spp., and Ilex spp. These taxa are derived from high mountain habitats. The increase in these pollen within the study area indicates extension of the mountain forest belt towards lower altitudes as a result of lower temperatures brought on by ice-age cooling in the northern hemisphere. Mountain and Lowland Tropical Forest, Wet Climatic Phase (Figure 3) A variety of tropical forest taxa are recognised, which include Costus spp., Croton spp., Euphorbia spp., Malvaceae spp., Margocolporites spp., Moraceae spp., Polygala spp., Proteacidites spp., Psychotria spp., Rostriapollenites robustus, Sapotaceae spp., Solanaceae spp., Striatricolpites catatumbus and Tournefortia spp., together with a variety of retitricolpate, retitricolporate, tricolpate, tricolporate and psilatricolporate pollen. The tropical forest will also include a variety of ferns and tree ferns. Savannah and Bushland, Humid Seasonal Climate (Figure 4) This vegetation belt is characterised by the high abundance of Gramineae pollen (grass), with associated Acacia spp., Asteraceae spp., Chenopodiaceae spp., Echitricolporites maristellae, Echitricolporites spinosus, Echiperiporites estelae, Fenestrites spp., Ipomoea spp., Mimosa spp. and the ferns Asplenium spp., Blechnum spp., and Pteris spp. In general, a drier climate with marked rainy seasons favours the development of extensive grasslands. Lowland Forest Swamp, Wet Climatic Phase (Figure 5) Typical swamp taxa include the tree Pachydermites diederixi (Symphonia), palms, and a variety of ferns. In addition, the palynofloras of this vegetation belt are characterised by an abundance of fungal hyphae and fungal spores. Lakeside and Marsh, Wet Climatic Phase (Figure 6) Abundances of the freshwater algae Botryococcus spp. and Pediastrum spp. are typical of the lakeside and marsh vegetation belt. These are associated with the aquatic and semi-aquatic ferns Laevigatosporites spp. and Magnastriatites howardii, the herbs Cyperaceae spp., Ipomoea spp., Sagittaria spp., Onagraceae (Triorites spp.), Typha spp., and Polygonum spp., and the carnivorous herb Utricularia spp. In addition, the palynofloras are generally characterised by an abundance of fungal hyphae and fungal spores. Lake, Streams, and Rivers (Figure 7) An abundance of the freshwater alga Botryococcus spp. is typical of open still-standing freshwater, whilst an abundance of the freshwater fluvial alga Pediastrum spp. is typical of input from streams and rivers. Overall changes within the total palynoflora related to changes within the climate through time enable a robust stratigraphy to be developed using these abundance variations. These palynofloral characteristics are outlined below and detailed in Figure 8. Late Pleistocene: Defined by an abundance of Podocarpus spp. pollen, in association with a variety of other high-mountain-derived pollen, such as Alnus spp., Erica spp., Ilex spp., Juniper spp., and Olea spp. Early Pleistocene: Characterised by an abundance of Gramineae spp., associated with the persistent recovery of high mountain-derived pollen. Late Pliocene: Defined by the highest occurrence of Peregrinipollis nigericus, together with other associated pollen, including Praedapollis spp., Retitricolpites amapaensis, and Rostriapollenites robustus (Figure 9). Early Pliocene: Characterised by an increase in tropical forest pollen, associated with a distinct reduction in Gramineae spp. Based on overall changes in abundance and types of vegetation, a number of climatic phases can be recognised. Warmer and wetter interglacial phases tend to be characterised by high abundance assemblages, whilst cooler and more arid phases are characterised by low-abundance assemblages. Within these phases, intervals dominated by savannah, marsh, swamp, forest, etc. can be identified. The dominant vegetation types and their relationship to stratigraphy and global temperature changes are summarised in Figure 10. Frakes, L.A., 1979, Climates throughout geologic time, in Climates throughout geologic time: Elsevier Science, Amsterdam, Netherlands: 310 p. Hardenbol, J., J. Thierry, M.B. Farley, P.C. de Graciansky, and P.R. Vail, 1998, Mesozoic and Cenozoic sequence chronostratigraphic framework of European basins in Mesozoic and Cenozoic sequence chronostratigraphic framework of European basins: Special Publication SEPM, v. 60, p. 3-13. Logan, P., S. Curd, Bob D., J. Weston, and D. Shaw, 2009, Exploration on the Frontier: Towards an Understanding of the Albert Basin: Search and Discovery Article #10192 (2009), Web accessed 5/10/09 http://www.searchanddiscovery.com/documents/2009/10192logan/index.htm.
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