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Journal Article

Sequence of tuffs between the KBS Tuff and the Chari Tuff in the Turkana Basin, Kenya and Ethiopia Available to Purchase

Francis H. Brown, Bereket Haileab, Ian McDougall
Published: 01 January 2006
Journal of the Geological Society (2006) 163 (1): 185–204.
DOI: https://doi.org/10.1144/0016-764904-165
...Francis H. Brown; Bereket Haileab; Ian McDougall Abstract This paper describes a sequence of tuffs between the KBS and the Chari Tuff of Omo Group formations in Kenya and Ethiopia. These tuffs have recently been shown to be 1.87 ± 0.02 Ma and 1.38 ± 0.03 Ma in age, respectively. The sequence...
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Journal Article

Geochronology of the pre-KBS Tuff sequence, Omo Group, Turkana Basin Available to Purchase

IAN McDOUGALL, FRANCIS H. BROWN
Published: 01 March 2008
Journal of the Geological Society (2008) 165 (2): 549–562.
DOI: https://doi.org/10.1144/0016-76492006-170
... of the KBS Tuff, which has previously been dated at 1.87 ± 0.02 Ma. The results from 17 stratigraphic levels encompassing the 2.4 Ma time interval from the base of the group to the KBS Tuff provide a numerical time framework for the geological history of the lower part of the Pliocene sequence. The new ages...
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View PDF for article title, Geochronology of the pre-<span class="search-highlight">KBS</span> <span class="search-highlight">Tuff</span> sequence, Omo Group, Turkana Basin
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Journal Article

Geologic origin of magnetic volcanic glasses in the KBS tuff Available to Purchase

Charles M. Schlinger, Rebecca M. Smith, David R. Veblen
Journal: Geology
Published: 01 November 1986
Geology (1986) 14 (11): 959–962.
DOI: https://doi.org/10.1130/0091-7613(1986)14<959:GOOMVG>2.0.CO;2
...Charles M. Schlinger; Rebecca M. Smith; David R. Veblen Abstract Volcanic glass shards and banded pumice from the KBS tuff of northern Kenya exhibit marked variations in magnetic susceptibility and color (colorless to dark brown). The darker glass shards exhibit superparamagnetism, which we now...
View PDF for article title, Geologic origin of magnetic volcanic glasses in the <span class="search-highlight">KBS</span> <span class="search-highlight">tuff</span>
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Orbital tuning of fossiliferous sediments below the KBS Tuff. (a) Simplified stratigraphic sequence for the Upper Burgi Member (UBM) (Koobi Fora Formation) below the KBS Tuff. Fish fossils extracted from these sediments were analysed for Sr isotopic compositions (Joordens et al. 2011, 2013). (b) 65° N summer insolation curve of the La2004(1,1) solution (Laskar et al. 2004) for the time interval 2000–1860 ka (maximum isolation i-cycles labelled). Wetter climatic periods are associated with insolation maxima (blue bars) and drier periods with insolation minima. Also shown are the positions of Mediterranean deep-sea sapropel units (47–53; Lourens et al. 1996). (c) North African dust records from Mediterranean and Atlantic Ocean marine records (Tiedemann et al. 1994). Elevated levels of aeolian terrestrial dust accumulations are associated with reduced Mediterranean monsoonal activity and drier climatic periods. (d) Revised tuning model (Model I) for the UBM in Areas 102 and 105 based on 87Sr/86Sr isotopic data from fish fossils (Joordens et al. 2011) and calibrated to the new KBS age of 1879.1±2.4 ka (Table S7). This tuning model indicates an age 1975±2 ka for the base of the Olduvai Subchron (bC2n), which is older than the previously reported values of 1945 ka (Horng et al. 2002) and 1925 ka (Channell et al. 2020) and links bC2n to an insolation minimum rather than the maximum inferred from Ocean Drilling Program data (Lourens et al. 2004; Lisiecki and Ramo 2005). (e) 87Sr/86Sr isotopic data for UBM fish fossils in sediments below the KBS Tuff in Area 105 (Joordens et al. 2013) relative to the GPTS bC2n age. (f) Alternative tuning model (Model II) for the UBM in Areas 102 and 105 (Joordens et al. 2011) (Table S7). 87Sr/86Sr isotopic cycles above the sandy units are referenced to the age of the KBS tuff (1879.1±2.4 ka), whereas cycles below the sandy units are referenced to a bC2n age of 1968 ka (Lisiecki and Ramo 2005). This tuning model requires a change in local or regional hydrodynamic patterns in the middle of the UBM (e.g. van der Lubbe et al. 2021). Source: part (a) adapted from Joordens et al. (2011).

Orbital tuning of fossiliferous sediments below the KBS Tuff. ( a ) Simplif... Open Access

Published: 15 June 2023
Fig. 10. Orbital tuning of fossiliferous sediments below the KBS Tuff. ( a ) Simplified stratigraphic sequence for the Upper Burgi Member (UBM) (Koobi Fora Formation) below the KBS Tuff. Fish fossils extracted from these sediments were analysed for Sr isotopic compositions ( Joordens et al
Image
Estimates of hominin fossil ages from strata above and below the KBS Tuff in the Koobi Fora Formation. Coloured symbols represent age estimates from the current study. Grey bars indicate age estimates from previous studies (Feibel et al. 1989, 2009; Leakey et al. 2012; Joordens et al. 2013).

Estimates of hominin fossil ages from strata above and below the KBS Tuff i... Open Access

Published: 15 June 2023
Fig. 11. Estimates of hominin fossil ages from strata above and below the KBS Tuff in the Koobi Fora Formation. Coloured symbols represent age estimates from the current study. Grey bars indicate age estimates from previous studies ( Feibel et al. 1989 , 2009 ; Leakey et al. 2012
Journal Article

Precise 40 Ar/ 39 Ar geochronology for the upper Koobi Fora Formation, Turkana Basin, northern Kenya Available to Purchase

Ian McDougall, Francis H. Brown
Published: 01 January 2006
Journal of the Geological Society (2006) 163 (1): 205–220.
DOI: https://doi.org/10.1144/0016-764904-166
...Ian McDougall; Francis H. Brown Abstract The Plio-Pleistocene Koobi Fora Formation, about 560 m thick, crops out east of Lake Turkana and is part of the much larger depositional system of the Omo–Turkana Basin. The upper half of the Koobi Fora Formation from just below the KBS Tuff to above...
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Journal Article

40 Ar/ 39 Ar eruption ages of Turkana Basin tuffs: millennial-scale resolution constrains palaeoclimate proxy tuning models and hominin fossil ages Open Access

David Phillips, Erin L. Matchan, Andy J. Gleadow, Frank H. Brown, Ian A. McDougall, Thure E. Cerling, Meave G. Leakey, Janet M. Hergt, Louise N. Leakey
Published: 15 June 2023
Journal of the Geological Society (2023) 180 (4): jgs2022-171.
DOI: https://doi.org/10.1144/jgs2022-171
...Fig. 10. Orbital tuning of fossiliferous sediments below the KBS Tuff. ( a ) Simplified stratigraphic sequence for the Upper Burgi Member (UBM) (Koobi Fora Formation) below the KBS Tuff. Fish fossils extracted from these sediments were analysed for Sr isotopic compositions ( Joordens et al...
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View PDF for article title, 40 Ar&#x2F; 39 Ar eruption ages of Turkana Basin <span class="search-highlight">tuffs</span>: millennial-scale resolution constrains palaeoclimate proxy tuning models and hominin fossil ages
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Journal Article

K-Ar and 40 Ar/ 39 Ar dating of the hominid-bearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya Available to Purchase

IAN MCDOUGALL
Journal: GSA Bulletin
Published: 01 February 1985
GSA Bulletin (1985) 96 (2): 159–175.
DOI: https://doi.org/10.1130/0016-7606(1985)96<159:KAADOT>2.0.CO;2
.... Stratigraphically higher tuffs and their ages include the Ninikaa Tuff, 3.06 ± 0.03 Ma; the KBS Tuff, 1.88 ± 0.02 Ma; the Malbe Tuff, 1.86 ± 0.02 Ma; the Chari Tuff, 1.39 ± 0.02 Ma, and the Silbo Tuff, 0.74 ± 0.01 Ma, near the top of the sequence. The geochronology and geological data indicate at least three...
View PDF for article title, K-Ar and 40 Ar/ 39 Ar dating of the hominid-bearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya
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 Stratigraphic relations between tuffs of Group 1 that lie between the KBS Tuff and the Morutot Tuff. M, mollusc-rich layer. The schematic stratigraphic sections are left unornamented, but the width of the column is a rough guide to grain size; the wider the column, the coarser the material. The section for Area 104 is after Section 104-13 of Tindall (1985); for Area 131, after Brown &amp; Feibel (1985); for Area 10, after Section PNG-10.1 of Gathogo (2003; with additions), for Ilgwa and Errum, after Haileab (1995); and for the Shungura Type Area, after figures 45 and 46 of de Heinzelin (1983). Others are from field notes of F.H.B.

Stratigraphic relations between tuffs of Group 1 that lie between the KBS T... Available to Purchase

Published: 01 January 2006
Fig. 3.  Stratigraphic relations between tuffs of Group 1 that lie between the KBS Tuff and the Morutot Tuff. M, mollusc-rich layer. The schematic stratigraphic sections are left unornamented, but the width of the column is a rough guide to grain size; the wider the column, the coarser
Image
 Generalized stratigraphic columns for the Nachukui, Koobi Fora and Shungura Formations from the level of the KBS Tuff ( H-2) to the Silbo Tuff. Named tuffs, discussed in the text, are shown in the stratigraphic columns in black. Secure correlations between sections are indicated. The age of each dated tuffaceous horizon is shown, based upon pooled single-crystal analyses. The age is given as the arithmetic mean and standard deviation of the population. Width of the unfilled stratigraphic column indicates the dominant detrital sediment, with widest to narrowest representing sand, silt and clay, respectively. A dashed line indicates the absence of sediment in the section.

Generalized stratigraphic columns for the Nachukui, Koobi Fora and Shungura... Available to Purchase

Published: 01 January 2006
Fig. 3.  Generalized stratigraphic columns for the Nachukui, Koobi Fora and Shungura Formations from the level of the KBS Tuff ( H-2) to the Silbo Tuff. Named tuffs, discussed in the text, are shown in the stratigraphic columns in black. Secure correlations between sections are indicated. The age
Journal Article

New single crystal 40 Ar/ 39 Ar ages improve time scale for deposition of the Omo Group, Omo–Turkana Basin, East Africa Available to Purchase

Ian McDougall, Francis H. Brown, Paulo M. Vasconcelos, Benjamin E. Cohen, David S. Thiede, Michael J. Buchanan
Published: 01 March 2012
Journal of the Geological Society (2012) 169 (2): 213–226.
DOI: https://doi.org/10.1144/0016-76492010-188
.... The new age data on feldspars from the pumice clasts range from 4.02 ± 0.04 Ma for the Naibar Tuff of the Koobi Fora Formation to 1.53 ± 0.02 Ma for Tuff K of the Shungura Formation. The Orange Tuff in the KBS Member of the Koobi Fora Formation was dated at 1.76 ± 0.03 Ma, providing good control...
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View PDF for article title, New single crystal 40 Ar/ 39 Ar ages improve time scale for deposition of the Omo Group, Omo–Turkana Basin, East Africa
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Representative major element variation diagrams for tuff and pumice glass shards from current KBS (= H2), Malbe (= H4), Chari (= L) and Gele Tuff samples. Tuff glass samples are ETH86-279 (KBS), K80-225 (Malbe), K03-0069 (Chari) and K82-835 (Gele). The remaining sample numbers refer to pumice glass fragments. The electron microprobe analysis source data are listed in Table S3.

Representative major element variation diagrams for tuff and pumice glass s... Open Access

Published: 15 June 2023
Fig. 3. Representative major element variation diagrams for tuff and pumice glass shards from current KBS (= H2), Malbe (= H4), Chari (= L) and Gele Tuff samples. Tuff glass samples are ETH86-279 (KBS), K80-225 (Malbe), K03-0069 (Chari) and K82-835 (Gele). The remaining sample numbers refer
Image
Scatter plots for the four most discriminating elements (based on F ratios for KB-6, KB-7, and KB-9). Also plotted for comparison (stars) are the mean concentrations for previously discriminated tuffs KB-1 through KB-4. KB-1 through KB-3 are distinct from the others based on TiO2 alone; they all have TiO2 &gt; 1.39 wt.% (Table 4). KB-9 forms a tight cluster of points (marked by shaded ellipse) separate from other points. Only KB-6 and KB-7 cannot be distinguished from one another and largely overlap the mean composition of KB-4 regardless of which elements are considered.

Scatter plots for the four most discriminating elements (based on F ratio... Available to Purchase

Published: 10 November 2004
Fig. 8. Scatter plots for the four most discriminating elements (based on F ratios for KB-6, KB-7, and KB-9). Also plotted for comparison (stars) are the mean concentrations for previously discriminated tuffs KB-1 through KB-4. KB-1 through KB-3 are distinct from the others based on TiO 2
Image
Examples of differences in the 95% confidence intervals about the mean concentrations of six elements in tuffs KB-1 through KB-4. Number of samples is in parentheses on the vertical axis. Elemental concentrations are scaled in ppm (e.g., 20–120 ppm for vanadium). Numbers in parentheses below each chemical symbol are values of the corresponding F ratio (Table 5). The elements with the least overlap of confidence intervals, and highest F ratios, are the best for discriminating between tuffs on the basis of geochemistry.

Examples of differences in the 95% confidence intervals about the mean conc... Available to Purchase

Published: 10 November 2004
Fig. 6. Examples of differences in the 95% confidence intervals about the mean concentrations of six elements in tuffs KB-1 through KB-4. Number of samples is in parentheses on the vertical axis. Elemental concentrations are scaled in ppm (e.g., 20–120 ppm for vanadium). Numbers in parentheses
Image
Ternary feldspar diagrams with field lines after Deer et al. (2013). The electron microprobe analysis source data are listed in Table S4. (a) KBS/H2 Tuff pumice feldspar compositions. (b) Malbe/H4 Tuff pumice feldspar compositions. (c) Chari/Tuff L pumice feldspar compositions. (d) Gele Tuff pumice feldspar compositions.

Ternary feldspar diagrams with field lines after Deer et al. (2013) . Th... Open Access

Published: 15 June 2023
Fig. 4. Ternary feldspar diagrams with field lines after Deer et al. (2013) . The electron microprobe analysis source data are listed in Table S4 . ( a ) KBS/H2 Tuff pumice feldspar compositions. ( b ) Malbe/H4 Tuff pumice feldspar compositions. ( c ) Chari/Tuff L pumice feldspar compositions
Image
Previous age data for the (a) KBS/H2 and Malbe/H4 tuffs and (b) Chari/Tuff L and Gele tuffs. Ages recalculated using the decay constants of Min et al. (2000) and fluence monitor ages of Phillips et al. (2022). F&amp;M'70, Fitch and Miller (1970); F&amp;M'76, Fitch and Miller (1976); B&amp;N, Brown and Nash (1976); McD'80, McDougall et al. (1980); McD'85, McDougall (1981); McD'85, McDougall (1985); McD&amp;B'06, McDougall and Brown (2006). Error bars shown with 2σ uncertainties.

Previous age data for the ( a ) KBS/H2 and Malbe/H4 tuffs and ( b ) Chari/T... Open Access

Published: 15 June 2023
Fig. 2. Previous age data for the ( a ) KBS/H2 and Malbe/H4 tuffs and ( b ) Chari/Tuff L and Gele tuffs. Ages recalculated using the decay constants of Min et al. (2000) and fluence monitor ages of Phillips et al. (2022) . F&M'70, Fitch and Miller (1970) ; F&M'76, Fitch
Image
Field appearance of tuffs. Scale divisions to right are 10 cm. (a) K-bentonite KB-1 at section FP1 (Fig. 3). This horizon consists of four massive layers. (b) KB-8 at section SYW. This horizon consists of several layers, some with tractional structures indicating current action during deposition.

Field appearance of tuffs. Scale divisions to right are 10 cm. ( a ) K-bent... Available to Purchase

Published: 10 November 2004
Fig. 4. Field appearance of tuffs. Scale divisions to right are 10 cm. ( a ) K-bentonite KB-1 at section FP1 (Fig.  3 ). This horizon consists of four massive layers. ( b ) KB-8 at section SYW. This horizon consists of several layers, some with tractional structures indicating current action
Image
Long-distance correlation of megaturbidites (beds with gray-tone silty mudstone tops) in the vicinity of the proposed contact between the St-Hélier and St-Yvon allomembers. The geochemically fingerprinted tuff KB-3 is at the base; in all three sections, KB-4 is ∼30 m above the highest bed shown. Note the limited changes in bed thicknesses over a downcurrent (to left) distance of ∼25 km.

Long-distance correlation of megaturbidites (beds with gray-tone silty muds... Available to Purchase

Published: 10 November 2004
Fig. 5. Long-distance correlation of megaturbidites (beds with gray-tone silty mudstone tops) in the vicinity of the proposed contact between the St-Hélier and St-Yvon allomembers. The geochemically fingerprinted tuff KB-3 is at the base; in all three sections, KB-4 is ∼30 m above the highest bed
Image
40Ar/39Ar ages v. Ca/K ratios for (a) KBS/H2 feldspars, (b) Malbe/H4 feldspars, (c) Chari/Tuff L feldspars and (d) Gele feldspars. Note the broad correlation in ages with the Ca/K ratios for the Chari/Tuff L and Gele feldspars. Vertical and horizontal bars indicate 2σ uncertainties. Source data are listed in Table S5.

40 Ar/ 39 Ar ages v. Ca/K ratios for ( a ) KBS/H2 feldspars, ( b ) Malbe/H4... Open Access

Published: 15 June 2023
Fig. 7. 40 Ar/ 39 Ar ages v. Ca/K ratios for ( a ) KBS/H2 feldspars, ( b ) Malbe/H4 feldspars, ( c ) Chari/Tuff L feldspars and ( d ) Gele feldspars. Note the broad correlation in ages with the Ca/K ratios for the Chari/Tuff L and Gele feldspars. Vertical and horizontal bars indicate 2 σ
Journal Article

Revision of lithostratigraphic nomenclature in the Koobi Fora region, Kenya Available to Purchase

F. H. BROWN, C. S. FEIBEL
Published: 01 March 1986
Journal of the Geological Society (1986) 143 (2): 297–310.
DOI: https://doi.org/10.1144/gsjgs.143.2.0297
... Koobi Fora. Later Vondra and Bowen (1978) designated their section at 3"56'N, 36"15'E (BH6136) the type section of the Koobi Fora Formation. Their section depicts about 70 m of strata between the KBS Tuff and the Koobi Fora Tuff (see Vondra & Bowen 1976; Bowen & Vondra 1973; Vondra & Bowen 1978...
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