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Structure, Geochronology and Tectonic Significance of the Northern Suguta Valley (Gregory Rift), Kenya

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Structure, Geochronology and Tectonic Significance of the Northern Suguta Valley (Gregory Rift), Kenya Journal of the Geological Society, London, Vol. 150, 1993, pp. 751-762, 8 figs. Printed in Northern Ireland Structure, geochronology and tectonic significance of the northern Suguta Valley (Gregory Rift), Kenya W. BOSWORTH 1 & ANDRI~ MAURIN 2 1Marathon Petroleum Egypt, Ltd. PO Box 52, Maadi-l1431, Egypt *Fixed address: PO Box 1228, Houston, TX, USA 77251-1228 2 Total Compagnie Franf.aise des P(troles Cedex No. 47, 92069 Paris La D(fense, France Abstract: The Suguta Valley occupies the present axis of the Gregory Rift south of Lake Turkana. Since the Pliocene, faulting has been focused along a 35 km wide belt between the Ng'iro basement uplift and the Loriu Plateau, producing an eastward thickening asymmetric basin. The oldest exposed volcanic rocks are basalts, tufts and ashes dated 4.2-3.8 Ma. These were followed by a shift to trachytic volcanism, starting at about 3.8 Ma and continuing at least to 2.7-2.6 Ma. Feeder dykes to the trachytes strike NW-SE, suggesting a NE-SW least horizontal stress direction in the Early Pliocene. The trachytes were followed by a return to basaltic and alkali basaltic volcanism, which continues to the present. The structural evolution of the Suguta rift was similar to parts of the central and southern Gregory Rift, with early, broad basin development followed by constriction of deformation to an axial zone of grid faulting. This later structuring rotated flows and strata by as much as 35 ° in the past 3.8 Ma, or 9° per Ma. Rotational rates of this magnitude are extremely high in comparison to other continental rift settings. Formation of the Suguta basin also resulted in renewed uplift of the Ng'iro footwall and large-scale monoclinal flexuring of volcanic flows in the vicinity of the basin border faults. The intense basement structuring and accompanying footwall uplift led to the formation of large-scale land slips that carried large volumes of rock into the basin axis. Patterns of fluvial and deltaic sedimentation similarly reflect direct control by rift structures. The stratigraphic and structural histories of the Gregory Rift work completed in the Suguta Valley in 1988 and 1990. Our are well-known in comparison to those of most continental studies concentrated on the playa lake Namakat, now rift basins. This is in part because most of the syn-rift fill is commonly known as Lake Logipi, at the northern terminus volcanic in origin, and radiometric age-dating (e.g., Baker et of the valley. The purpose of our work was to document the al. 1971; Fairhead et al. 1972; Baker et al. 1988) has detailed stratigraphical evolution of a highly saline, provided an absolute chronology with a resolution that can organically productive intra-rift lake. It was believed that only be approached in marine stratigraphic sequences. such a structural/stratigraphical setting might provide a Fortunately, most structural aspects of the Gregory Rift model for preservation of organic material and the appear to be representative of continental rifts in general, generation of hydrocarbons during burial of the syn-rift despite its clearly unique igneous history (e.g., Baker et al. sequence. In order to fully utilize this information, it was 1972; Chapman et al. 1978; Bosworth et al. 1986). Gregory necessary to first understand the structural and volcanic Rift-based structural models should therefore still be useful development of the basin. We began by interpreting a for comparison with those rifts that are only known from the 1:100 000 scale SPOT image of the Suguta Valley from the subsurface, or where detailed chrono-stratigraphies are not vicinity of Lake Logipi south to a narrow restriction in the available. dry lake plain known as the 'Gap' (Kaminanguwadang; Fig. Despite the abundant information available for many 2). Based on this regional picture, it was evident that the segments of the rift, some of its basins remain relatively area on the eastern margin of the lake was most intensely unexplored, largely due to their remoteness. This is faulted and warranted mapping from the ground. This particularly evident for the rift valley north of Lake detailed study area provides most of the structural data Baringo, where access by roads is limited and permanent presented in this paper. We will also discuss new settlements are widely dispersed. This area contains a radiometric age dates that help constrain the rift-fill history variety of geological features that are not equally developed of the northern Suguta Valley, and will describe several in other parts of the rift, and warrant more extensive study. aspects of the interplay between rift sedimentation and An example of this potential for new research occurs in the structuring that are particularly well-displayed in this part of Suguta Valley, the region immediately south of Lake the Gregory Rift. Turkana (Fig. 1). This area has been recognized as a region of intense micro-seismicity (Pointing et al. 1985), and Stratigraphy and geochronology corresponds to a marked narrowing of the present-day geomorphological rift valley. It was anticipated prior to our The northern Suguta Valley was originally mapped by the field studies that the structural style of the Suguta Valley Kenya Geological Survey in conjunction with studies of the might therefore differ significantly from broader segments of South Horr region (Dodson 1963). More recently this area the rift, and that this could have a corresponding influence was included in a reconnaissance survey of the Loiyangalani on the patterns of syn-rift sedimentation. (Ochieng' et al. 1988) and Maralal (Key 1987) areas. This paper presents partial results of reconnaissance field Basement in the region is assigned to the Mozambique 751 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/4/751/4892564/gsjgs.150.4.0751.pdf by guest on 30 September 2021 752 W. BOSWORTH & ANDRI~ MAURIN exposed at Lowoi east of the Gap, and by Truckle (1976) at Kalakopon west of the Gap. We therefore attempted to obtain K-Ar dates for several of the lithologies outcropping in the detailed study area, and for additional samples from the Gap for comparison with the existing dates. The results LAKE 1 of these analyses are summarized in Table 1. TURKANA 4"N KENYA Volcanic sequence east of the Suguta Valley The oldest volcanic rocks in the northern Suguta area are exposed in the deep dry gorge of the Losergoi River (Fig. ~o ~ NORTH 2). This sequence consists of intercalated fine-grained HORR basalts, ashes, tufts and agglomerates and is referred to as O LODWAR the Parkati volcanics by Ochieng' et al. (1988); (Fig. 3). Dodson suggested a tentative correlation with the Samburu Series of the Maralal region (Shackleton 1946) and the Tertiary porphyritic basalts and tufts of Baragoi (Baker STUDY "~ ../] ' _ 1963). The Samburu basalts are now known to be of Early AREA : . Miocene age (23-18.5 Ma; Baker et al. 1971). The basalt SOUTHHORR sample LL-V1 was collected from the Losergoi gorge near 2"N 2"N the inferred border fault at the base of the Ng'iro range ' ° "OC (Fig. 2). The sample contained large quantities of secondary O BARAGOI magnetite, goethite, limonite and calcite. An analytically " ~ ¢)D acceptable whole rock age of 4.2-I-0.3 Ma was obtained, c$ ' " suggesting a tentative position within the Lower Pliocene s I (Table 1). If this date is confirmed through future sampling, , , 0 t~ MARALAL the areal extent and correlations of supposedly 'Miocene' . hi J ~ , oo,~ basalts in the northern rift valley must be reconsidered. I i The lowermost basalts are overlain by a thick sequence of highly porphyritic basalts and very minor tufts and ashes L.BAR)NGO i (Dodson 1963). This unit is part of the Longipi shield volcanic rocks (Ochieng' et al. 1988), which extend from the ot:: b western slopes of Ng'iro north to the Longipi Hills. The o >-" ~ ~ o" Longipi shield is itself overlapped in the northeast by the 35"E 38"E Upper Pliocene Kulal shield (Rundle 1985). The Longipi volcanic rocks were not sampled in the present study. As Fig. 1. Location of general study area and localities referred to in the text. Small box south of Lake Turkana corresponds to SPOT our sample of the Parkati volcanic rocks yielded an Early image used in the interpretation of Fig. 2. Basement rocks are Pliocene age, and the overlying Longipi volcanics are stippled. After the Geologic Map of Kenya, 1969. probably Late Pliocene or older (Ochieng' et al. 1988), we believe that the Parkati-Longipi sequence, at least along the Orogenic Belt and is dominated by north-south-trending Losergoi gorge, might best be referred to one unit. The structures in predominantly gneissic lithologies (Shackleton Longipi Hills are also located more than 30 km north of the 1945; Baker 1963; Dodson 1963; McConnell 1972; Key et al. Losergoi River gorge. Rather than utilize a name that 1989). The basement complex is well-exposed east of the mistakenly implies any reliable correlations, we will simply Suguta Valley in the Ng'iro range, which peaks at an refer to the basalts in the Losergoi River, including both elevation of 2811 m near Mowo Engosowan (elevations and Dodson's 'fine-grained' and 'porphyritic' basalts, as the place name spellings are from HMSO topographic map, Losergoi volcanic rocks (VL) (Figs 3 & 4). 1:100 000 scale Ng'iro Sheet 53, 1977 ed., unless otherwise The Losergoi basalts are overlain by olivine basalts of noted). The crystalline rocks are progressively on-lapped by the Tirr Tirr Series on the Emurua Kirim Plateau (Baker a sequence of basalts and trachytes, which Dodson (1963) 1963; Dodson 1963). Some confusion exists in the published sub-divided into five Tertiary units in the South Horr area.
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