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Reconnaissance Geology of the Amaro Horst, Southern Ethiopian Rift

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Reconnaissance Geology of the Amaro Horst, Southern Ethiopian Rift DUBI LEVITTE Geological Survey of Israel, Jerusalem, Israel JOHN COLUMBA Geological Survey of Ethiopia, Addis Ababa, Ethiopia PAUL MÖHR Smithsonian Astrophysical Observatory, Cambridge, Massachusetts 02138 Reconnaissance Geology of the Amaro Horst, Southern Ethiopian Rift ABSTRACT roughly equal to the 75-km width of the rift farther north (Baker and others, 1972, Fig. 14). The Amaro horst is a 90-km by 25-km b iock of sialic crust uplifted 1.5 km above the axis of the main Ethiopian rift, near its southern STRATIGRAPHY termination. The Amaro horst resembles the famous Ruwenzori horst of the Western rift, but additionally the former has had a Precambrian Rocks Neogene-Quaternary volcanic history ths.t reveals episodic uplift of Precambrian rocks are exposed along a narrow, upfaulted crustal the horst from the axis of a bilaterally downwarping-downtilting rift sliver forming the summit ridge of the northern part of the Amaro floor. Vertical forces that produced normal faults uplifted the horst horst (Fig. 2). In the central and southern part, Precambrian rocks to stratigraphic elevations higher than the plateaus outside the rift. are exposed across the width of the horst, although the summit ridge and other high ground are capped by Tertiary volcanic rocks. INTRODUCTION Shallow-dipping mica schists form most of the Precambrian rocks (De Angelis d'Ossat and Millosevich, 1899; Kazmin, 1971). In the The main Ethiopian rift valley divides south of Lake Abaya (or western part of the Amaro horst, muscovite schist overlies biotite Margherita), near lat 6.5° N. (Fig. 1). The north-trending Amaro schist, with foliation having southwest to northwest dips between Mountains (Amaro horst) separate the Ganjuli graben to the west 10° and 30°. This thick, monotonous sequence is cut by abundant from the Galana graben to the east. The lorst and both grabens die intrusions of quartz-feldspar pegmatite. In the northern part of the out near lat 5° N. on the southern fringe of the Ethiopian swell; the horst, biotite schist with aplitic gneiss and quartzite bands is injected rift is transposed en echelon west into the southward-continuing by coarse feldspathic pegmatite and quartz veins. Less common Stefanie graben (Baker and others, 1972). rock types within the schist sequence include serpentinite (possibly The only previous work on the geology of the Amaro region is that from altered sills) and chlorite schist. A prominent inselberg in the of Maurizio Sacchi, a member of the ill-fa ted second Bottego expedi- west-central part is formed of gneissose granodiorite, again injected tion. Sacchi's traverse observations, published by De Angelis with pegmatite (Fig. 2). d'Ossat and Millosevich (1899), showed Precambrian rocks at high The regional foliation of the Precambrian rocks was thus imposed elevations on the Amaro Mountains, higher indeed than on the after a localized intrusive episode. No unmetamorphosed granite of neighboring plateaus east and west of the rift valley. the type occurring southeast of Yavello (Rogers and others, 1965; Dainelli (1943) considered the Amaro Mountains a plateau see Fig. 1) has yet been discovered in Amaro. remnant, left standing within a collapsed rift valley, and drew his The Precambrian sequence of the Amaro horst can be correlated provocative map of basement isohypsals accordingly. Reconnais- with the "Lower Group" of Chater and Gilboy (1970), who mapped sance surveys in the late 1950s (Mohr, 1960) showed, however, that in detail the Precambrian rocks of the Shakisso-Arero area —100 km the Amaro Mountains are formed by a horst uplifted from the rift east of the Amaro horst on the Bali plateau. At the southern end of floor. the Ganjuli graben floor is a similar sequence of Precambrian rocks (Mohr and Gouin, 1968) that consist of regionally extensive biotite PHYSIOGRAPHY schist and subordinate biotite gneiss and marble with foliation The Amaro horst trends N. 5° E. for a length of 90 km. The typical dipping south to southeast between 0° and 20° and with profuse width is —25 km but increases to the south. A continuous, intrusions of feldspathic pegmatite. Rogers and others (1965) have knife-edged summit ridge runs close to the eastern, steeper margin of dated two tectonothermal events affecting the Precambrian rocks of the horst at elevations of —2,600 m. The northernmost segment of southern Ethiopia: an event at least 600 m.y. ago presumably the horst is stepped down from the central sector and has the form of associated with the regional foliation and an event 500 m.y. ago a narrow, linear, bladed ridge, similar to the northern "nose" of the associated with the intrusion of posttectonic granite. The original Ruwenzori horst (Holmes, 1965). age of the sediment now occurring as schist remains unknown (see West of the Amaro horst, the Ganjuli graben contains the Kazmin, 1972). southern end of Lake Abaya (1,175-m elev), which overflows a narrow land bridge, the Tosa Sucha ( 'Bridge of God"), into Lake Mesozoic(P) Rocks Chamo (1,130-m elev). The Ganjuli graben is 25 to 30 km wide and Thin beds of gritstone and conglomerate occur in patches between continues the south-southwest trend of the main Ethiopian rift. West the Precambrian schist and the Amaro flood basalt (see below). The of the graben is an abrupt rise to the extensive plateau whose summit outcrops are too narrow to be indicated on Figure 2; however, they is the Gughe Mountains (—3,500-m elev). are well exposed on the upfaulted Golole ridge, bordering the East of the Amaro horst, the Galana graLen trends nearly due eastern shores of Lake Chamo south of the Sagan effluence, and also north. Its eastern side rises regularly to the edge of the Bali south of Kelyi. Variegated, subhorizontal fine gritstone, 2 to 5 m (Somalian) plateau, a major watershed whose elevation ranges thick, rests unconformably on muscovite schist in the Golole region between 2,000 and 2,500 m. and was more or less transformed into pisolitic lateritite before The divergence from the main Ethiopian rift is slight. The total extrusion of the Amaro flood basalt. Farther east, in the upper Sagan width of the two grabens plus that of the intervening horst remains valley, equivalent gritstone and angular conglomerate are <1 m Geological Society of America Bulletin, v. 85, p. 417-422, 2 figs., March 1974 417 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/3/417/3433393/i0016-7606-85-3-417.pdf by guest on 26 September 2021 418 LEVITTE AND OTHERS thick; still farther east, on the summit ridge of the horst, the An:aro flood basalt rests on nonlateritized schist without any intervening sedimentary rocks. The age of the gritstone and conglomerate is uncertain, but the lateritization suggests a late Mesozoic age by analogy with the well-dated lateritites of central Ethiopia (Dainelli, 1943; Mchr, 1962). Mohr and Gouin (1968) concluded that the 20-m-thick sequence of the Gatto Grits, at the southern end of the Gan uli graben, was probably of Mesozoic rather than Tertiary age. The age of the lithologically similar Turkana gritstone of the Lake Rudolf region has recently been revised from Miocene to Cretaceous by Arambourg and Wolff (1969). Upper Tertiary to Quaternary Rocks Amaro Flood Basalt. A thick sequence of flood basalt is developed over much of the Amaro horst. On the eastern and western fringes of the south-central part, the Amaro flood basalt does not exceed 100 m in thickness (10 to 15 flows), although in places this may represent a minimum owing to denudation. In the Baraka valley, thicker flood-basalt flows total 200 to 250 m; a similar thickness, veneered with silicic tuff, caps the summit ridge of the Amaro horst southwest of Kelyi (Amaro basalt on Fig. 2). South of Kelyi, ~200 m of flood basalt occurs on the tilted blocks of the faulted eastern escarpment of the horst; this sequence is absent from the summit ridge immediately to the west. The Amaro flood basalt thickens northward and exceeds 1,000 :n on the northern part of the horst, where the full sequence is last exposed. This northward thickening matches the general pattern on the plateaus west and east of the rift (Mohr and Gouin, 1968). Tbe Amaro flood basalt is commonly olivine-phyric, particularly the lower flows. Light-colored, amygdaloidal (agate and calcite) basalt, interspersed with feldspar-phyric flows, characterizes the upper part of the sequence. Numerous feeder dikes for the Amaro flood basalt are exposed on the western flank of the horst. In the Sagan valley (~10 km south of the southern limit of Fig. 2), a swarm of dikes cuts the Precambrian rocks of the graben axis. The swarm is 2 to 3 km wide, and the dikes trend between north and north-northeast and dip steeply west. Figure 1. Location map of the Amaro horst near the southern termination of the Typical dike lengths are 600 to 1,200 m, and right en echelon main Ethiopian rift (elevations higher than 2,000 m indicated by stippling). transpositions occur. On the southeastern part of the horst, north-trending dikes dip steeply east and are slightly but definitely part. The tuff shows a tendency to be draped on the existing oblique to the N. 10° E. strike of the dipping lava flows. In the topography of the horst, indicating an unconformity with the Baraka valley, thin (<1 m) vertical dikes trend north to underlying basalt. north-northeast. One of these dikes, sampled from the upper part ol The age of the tuff is uncertain but must be later than early the flood-basalt sequence, has yielded a radiometric age of 21 m.y.
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