Etude stratigraphique, pétrographique et diagénétique des grès d ...

tel-00534181, version 1 - 9 Nov 2010
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Chapter I
sive system visible within the upper parts of the LSF. These two lavas are petrographically and geochemically
similar. They are intensively altered basaltic rocks showing mainly secondary calcite and brown
clays. Plagioclase, clinopyroxene and olivine phenocrysts can still be recognized within KER 63/05 sample
whereas KER 64/05 is totally altered (Tables 5 and 6).
Many other dykes of various trends (N0°, N50-70°, N90°, N160°) and widths (from a few decimeters
up to 10 m) as well as sills have been observed to crosscut the LSF and to intersect each other at
Keniroliom, Kerral, Lokitaung Gorge areas (Fig. 16c-f). In the Lapur Peak area, a few narrow basaltic dykes
(max. 0.80 m wide) were seen intruding the amphibolitic basement (Fig. 16g, h). Dating such dykes has been
difficult because of the deep alteration of the rocks. Nevertheless, in the Kerral area, one of the largest dykes
- ~10 m wide, several hundreds of m long, trending N60° - and which is seen crosscutting the middle and upper
terms of LSF (Fig. 16c, d), is a relatively fresh lava (sample KER 65/05 collected at lat. 04°15’01”N; long.
35°48’23”E) which has been geochemically characterised as a tephrite. It has been dated 27.03 ± 0.57 Ma
(Tables 5, 6 and 7b). This lava is of similar age to that of the 15-m thick tephrite flow sampled in the upstream
part of the Lokitaung Gorge, near the town of Lokitaung at lat. 04°15’43”N; long. 35°45’23”E (sample LAP
02/13D), which is dated 27.81 ± 0.65 Ma (Fig. 14; section LOK A) (Tables 5, 6 and 7a). Other volcanics of
similar age have been characterised within the Turkana Volcanics south of the Lokitaung Gorge (Nathura Laga
section; lat. 04°10’02’’N; long. 35°47’32’’E; Fig. 9a), where they are represented by rhyolitic sills and dykes
(samples NATH 08/04 and 09/04) that have been dated 28.73 ± 0.64 Ma and 27.40 ± 0.66 Ma, respectively
(Clément, Bellon et al., work in progress) (Table 7a). These data suggest volcanic phases of upper Oligocene
age marked by the emplacement at the base of Lapur sandstones of intrusive bodies (dykes and sills) and overlying
lava flows, both characterised by silica undersaturated (tephrites) or silica supersaturated lavas. New
geochemical studies will confirm whether or not those lavas may belong to the same magmatic phase
(Clément, Bellon et al., work in progress).
4.2. Structural framework of LSF at the scale of the Turkana depression
(Figs. 9b and 17)
The Lapur Range (Fig. 8), an impressive 50 km-long range, is the only place where, at outcrop level,
the Lapur Sandstone Formation can be seen and its proper stratigraphic position identified. Due to its general
dip to the W-SW, exposures of the LSF disappears rapidly below the overlying Turkana Volcanics (Figs. 8 and
9b). Farther to the west (Fig. 7), in the Gatome and Lotikipi Basins, poorly imaged sub-volcanic strata have
Figure 13. Photographs of various sites and geological sections showing the different types of contacts between the Lapur
Sandstone Formation and the Turkana Volcanics. (a) General view from the left side of the Lokitaung Gorge of the upper
parts of LSF (white arrow) overlain by a ~500-m thick pile of Turkana Volcanics (black arrow). The small white box on
the right side of the photograph shows our Land-Rover, for scale. (b) Detailed view of the stratigraphic contact between
the upper terms of the LSF and the lowermost flows of the Turkana Formation. Right bank of the Lokitaung Gorge at
04°16’04”N, 35°47’34”E (LOK A2 section; Fig. 14). (c) Detailed view of LOK A3 section (see Fig. 14). At the base of this
conglomeratic sandstone bed, subangular to well-rounded pebbles and cobbles of grey green basalt (white arrow, 1) similar
to the underlying lava (white arrow, 2) are seen to be reworked within quartz pebbles. (d)-(f) Residual basaltic flows
(black arrows) unconformably overlying the upper terms of the Lapur Sandstone Formation at Lapur Peak and northward.
The fine-grained basalt that unconformably overly the upper terms of LSF at the north end of the Lapur Range has been
dated (Ar-Ar) 35.6 ± 0.80 Ma. (g) Malachite-rich breccia between the two first basaltic flows that overlain the upper-most
terms of LSF, dated 33.57 and 32.77 Ma, respectively (See Fig. 14; LOK A section). (h) Quartz + calcite-rich (white arrow)
geodes in basaltic lava, Lokituang Gorge section (See Fig. 14; LOK A section).
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