GEOLOGY OF THE LOTHIDOK RANGE,

NORTHERN KENYA

by

Harold Bradley Boschetto

A thesis submitted to the faculty of the University of Utah partial fulfillment of the requirements for the degree of

Master of Science

in

Geology

Department of Geology and Geophysics

The University of Utah

August 1988 Copyright © Harold Bradley Boschetto 1988

All Rights Reserved THE UNIVERSITY OF UTAH GRADUATE SCHOOL

SUPERVISORY COMMITTEE APPROVAL

of a thesis submitted by

Harold Bradley Boschetto

This thesis has been read by each member of the following supervisory committee and by major vote has been found to be satisfactory.

Chair: Francis H. Brown

Maijorie A. Chan

’ t_feennis M. Bramble THE UNIVERSITY OF UTAH GRADUATE SCHOOL

FINAL READING APPROVAL

To the Graduate Council of the University of Utah:

I have read the thesis of______Harold Bradley Boschetto______in its final form and have found that ( 1 ) its format, citations, and bibliographic style are consistent and acceptable; (2 ) its illustrative materials including figures, tables, and charts are in place; (3) the final manuscript is satisfactory to the Supervisory Committee and is ready for submission to the Graduate School.

/• X / ? / Date Francis H. Brown

Chair, Supervisory Committee

Approved for the Major Department

Francis H. Brown

Chair/Dean

Approved for the Graduate Council

B. Gale Dick

Dean of The Graduate School ABSTRACT

This study provides a stratigraphic and geochronologic context for fossils collected from the Lothidok Range, northern Kenya, that include three new hominoid genera. Age control on the strata is provided by 22 potassium-argon determinations at 11 stratigraphic levels. Three fossil bearing horizons in the Lothidok Range are: (1) pre-17.7 Ma, (2) between 17.7 and 16.6 Ma and (3) between 13.6 and 12.0 Ma. Fauna from the lower two horizons are equivalent to the collections from Rusinga and the highest horizon is equivalent to, or slightly younger than, those from Fort Teman.

Exposed Tertiary strata of the Lothidok Range are 1500 m thick, but the base is not exposed. In ascending order the section comprises 785 m of basalts and intercalated sedimentary rocks (Kalakol basalts, > 17.7 Ma), 540 m of interbedded sedimentary, pyroclastic and basaltic rocks (lower and upper Lothidok Formation, 17.7 - 12.0 Ma), 120 m maximum of basalts (Loperi basalts, 12.0 - 10.9 Ma) and 50 m minimum of undifferentiated Tertiary sedimentary strata (< 10.9 Ma).

Sedimentary rocks of the lower Lothidok Formation consist primarily of polymictic conglomerates and conglomeratic litharenites with southwest paleotransport directions, and are interpreted as braided stream-alluvial fan deposits. The upper Lothidok Formation contains polymictic conglomerates and conglomeratic and feldspathic litharenites near its base, with finer grained lithic to arkosic sandstones near the top. The lower strata are interpreted as braided stream-alluvial fan deposits, and the higher strata are interpreted as meandering stream deposits. All upper Lothidok Formation strata have easterly paleotransport directions. Pyroclastic rocks of the Lothidok Formation consist of mafic alkaline and trachytic tephra and lahars. A disconformity within the Lothidok Formation represents a time gap from -16.4 to

~13.7 Ma. An angular unconformity exists between the Lothidok Formation and the

Loperi basalts, and a third unconformity may exist within the undifferentiated Tertiary strata.

The structure is dominated by late Tertiary normal faults. Only one fault predates deposition of the Lothidok Formation. Other faults postdate the undifferentiaited Tertiary strata and the range front fault postdates 4.1 Ma.

No source terrains for the sediments and pyroclastic rocks are known. The sedimentary and pyroclastic rock record, however, provides evidence for a proximal, early

Miocene volcanic highland and mafic alkaline volcanic center close to and east of the

Lothidok Range as well as middle Miocene trachytic center(s) to the south.

v To my mother, Roberta,

for her continual support and encouragement,

and

to Anne Pasch, for sparking my interest in this fascinating science. TABLE OF CONTENTS

ABSTRACT...... iv

LIST OF F I G U R E S ...... ix

LIST OF TA BLES...... xii

A CK N O W LED G M EN TS...... xiii

IN T R O D U C T IO N ...... 1

Geographic N a m e s ...... 1 Previous Work in th Region...... 6 Methods of S t u d y ...... 8 Potassium-Argon Age Determination...... 11

REGIONAL GEOLOGY OF THE TURK ANA D E P R E S S IO N ...... 14

Regional S tructure...... 14 Regional S tratig rap h y ...... 14

STRATIGRAPHY OF THE LOTHIDOK RANGE...... 20

Stratigraphy...... 20 Kalakol Basalts - T k b ...... 30 Lothidok Formation - T 1 ...... 35 Loperi Basalts - T i b ...... 80 Undifferentiated Tertiary Deposits - T u ...... 83

INTERPRETATION OF DEPOSITIONAL E N V IR O N M E N T S ...... 8 8

I n tr o d u c tio n ...... 8 8 Lithologic A s s o c ia tio n s ...... 90

ORIGINS OF THE LOTHIDOK D E P O S IT S ...... 100

Sedimentary Provenance...... 100 Pyroclastic Source ...... 101

UNCONFORMITIES IN THE LOTHIDOK ST R A T IG R A PH Y ...... 105

Base of the Lokipenata C onglom erate...... 105 Base of the Loperi B a sa lts...... 109 Base of the Tertiary Arkosic S a n d s to n e s ...... 110 STRUCTURE OF THE LOTHIDOK R A N G E ...... 113

General S tr u c tu r e ...... 113 Normal F a u lts ...... 113 Grabens and H o rs ts ...... 119 Temporal Fault R elations ...... 122 Fault G o u g e ...... 122

FAUNA OF THE LOTHIDOK R A N G E ...... 123

I n tr o d u c tio n ...... 123 Eragaleit Beds, Kalakol B a s a l t s ...... 134 Lower Lothidok Formation ...... 134 Upper Lothidok Formation...... 136

NGAKORINGORA RIDGE STRA TA ...... 138

D escription...... 138 In te rp re ta tio n ...... 140

GEOLOGIC HISTORY AND PALEOGEOGRAPHY...... 142

I n tr o d u c tio n ...... 142 -25 to 17.7 M a ...... 142 17.7 to 17.3 M a ...... 143 17.3 t o -16.4 M a ...... 149 -16.4 t o -13.7 M a ...... 152 -13.7 t o -13.0 M a ...... 152 -13.0 to 10.9 M a ...... 157 10.9 to -5 Ma...... 157

CONCLUSIONS...... 163

APPENDICES

A: LIST OF MEASURED SECTIONS IN THE LOTHIDOK RANGE . .166

B: FIELD DESCRIPTIONS OF THE COMPOSITE STRATIGRAPHY OF THE LOTHIDOK RA NG E...... 167

REFERENCES ...... 197

viii LIST OF FIGURES

Figure Page

1. Location map for the Lothidok Range, northern Kenya...... 2

2. Geographic references for the Lothidok Range...... 4

3. Location map for sections measured in the Lothidok Range...... 9

4. Relation of the Lothidok Range to regional geographic and structural features of the Turkana Depression...... 15

5. Sequence and terminology used for the composite stratigraphy of the Lothidok Range...... 21

6 . Component sections for the stratigraphy of the Lothidok Range...... 23

7. Generalized north-south cross-section and lateral distribution of the strata exposed in the Lothidok Range...... 26

8 . Generalized east-west cross-section and lateral distribution of the strata exposed in the Lothidok Range...... 28

9. The sequence and terminology used for the composite-stratotype of the Lothidok Formation...... 36

10. Correlation and lateral variation in the Kalodirr Tuffs...... 42

11. Typical sequence of the Kanukurinya tuff beds from section 33...... 45

12. Photograph of leaf imprints in the Kalodirr T u f f s ...... 48

13. Typical sequence of the Naserte Tuffs from sections 21 and 39...... 52

14. Photograph of accretionary lapilli in airfall tuffs of the Naserte Tuffs near at section 39...... 54

15. Photograph of fluid escape structures in airfall tuffs of the Naserte Tuffs near section 8 ...... 56

16. Photograph of a palm leaf imprint in an unassigned trachytic tuff at section 27 (photo by F. H. Brown)...... 64 17. Photograph of a burrow in an unassigned trachytic tuff taken near section 19 (photo by F. H. Brown)...... 65

18. Paleocurrent rose diagrams for the Basal Conglomerate Member of the lower Lothidok Formation for data sets from near section 1 and from section 20. All data were corrected for tectonic tilt...... 67

19. Paleocurrent rose diagrams for unassigned strata of the lower Lothidok. Formation for data sets from multiple outcrops. All were corrected for tectonic tilt...... 69

20. Paleocurrent rose diagrams for the Lokipenata conglomerate of the upper Lothidok Formation for data sets from sections 3 and 19. All were data corrected for tectonic tilt...... 78

21. Paleocurrent rose diagrams for unassigned strata of the upper Lothidok Formation for data sets from sections 27 and 29. All were corrected for tectonic tilt...... 81

22. Paleocurrent rose diagrams for undifferentiated Tertiary strata taken along the Kalodirr River. All data were corrected for tectonic tilt. . . . 8 6

23. Typical sequence of the Conglomerate-Sandstone-Mudstone (CSM) lithologic association...... 92

24. Typical sequence of the Sandstone-Siltstone-Mudstone (SSM) lithologic association...... 95

25. Paleocurrent rose diagrams illustrating the complete reversal of sediment transport direction above the disconformity relative to that below. Each diagram represents the combination of every measurement taken in the respective level and all measurements were corrected for tectonic tilt. . . .106

26. Schematic geologic cross-sections of the Lothidok Range (no vertical exaggeration)...... 114

27. Simplified structure map of the Lothidok Range illustrating location of major structural features and geologic cross sections...... 117

28. Fossil localities of the Lothidok Range: 1) Lothidok north, 2) Lothidok south, 3) Kalodirr, 4) Kanukurinya, 5) Moruorot south, 6 ) Moruorot north, 7) Esha, and 8 ) Atirr...... 124

29. Fossil levels and isotopic ages of the Lothidok Range...... 127

30. Paleogeographic reconstruction of the Lothidok region at ~ 18 Ma and explanation of symbols used for paleogeographic reconstructions of the Lothidok region (Figures 31-36)...... 144

31. Paleogeographic reconstruction of the Lothidok region at 17.7Ma...... 147

x 32. Paleogeographic reconstruction of the Lothidok region at 16.8 Ma . . . 150

33. Paleogeographic reconstruction of the Lothidok region at~ 13.7 Ma . . . 153

34. Paleogeographic reconstruction of the Lothidok region at~13.0 Ma . . . 155

35. Paleogeographic reconstruction of the Lothidok region at ~ 12.0 Ma . . . 158

36. Paleogeographic reconstruction of the Lothidok region between 10.9 and ~ 5 Ma...... 160

xi LIST OF TABLES

Table Page

1. Analytical data for potassium-argon a g e s ...... 12

2. Lithologic types of the Lothidok Formation, equivalent lithofacies codes, prominant sedimentary structures and interpretation of depositional settings. . 89

3. Measured sections, maximum and minimum ages for the fossil localities...... 126

4. Miocene fauna of selected East African fossil localities. The locations given are listed oldest to youngest; B - Bukwa, K - Karungu , S - Songhor, Le - Lothidok (Eragaleit beds), R - Rusinga, N - Napak, LI -lower Lothidok Fm.,Bu - Buluk, FT - Fort Teman and Lu - upper Lothidok Formation...... 129 ACKNOWLEDGEMENTS

I would first like to thank my advisor, Frank Brown, who spent seemingly limitless

personal and professional time answering my questions and guiding my studies. His

insight and knowledge of the Turkana Basin proved invaluable to this study. My other

comittee members, Drs. M. A. Chan and D. M. Bramble, reviewed my manuscript and provided valuable comments.

I would especially like to thank Richard E. and Meave G. Leakey for their kind help, gracious hospitality and logistical support in Nairobi and in the field.

Funding for the 1986 field work was provided by Amoco. Field assistants, gear and vehicles were supplied by the Kenya National Museums. The 1987 field work was supported wholly by the Kenya National Museums. Logistical support and exceptional hospitality was provided by Mike Renolds while in Kenya and by Denise Stone while in

Houston, Texas. Amoco also provided airphotos, LandSat images and a suite of thin

sections for which I am grateful.

I am deeply indebted to my field assistants, John Musa Kyeva and Paul Ngoleni

Muthieni, for their impeccable assistance and for making my work in Turkana a pleasure.

My sincerest appreciation is due to Musa for seeing the carpet viper that I didn't. Special

thanks are also due to Ian McDougall, Tony Ekdale and William Anyonge for their

invaluable help in the field.

Age determinations for all 1986 samples were made by F. H. Brown under the

careful guidance of Ian McDougall. Ian McDougall made all age determinations for the 1987 samples and is greatly appreciated. Terry Davies, Robin Maier and Andrew Sienko provided technical assistance for all dates.

Paul Onstatt is credited for his excellent drafting. Other people who deserve special thanks for reveiwing my manuscript include Kent Wheeler, Wanda Taylor, and Rip

Langford.

xiv INTRODUCTION

The Lothidok Range (Lothidok Hills) has been of interest to paleontology since the

discovery of vertebrate fossil bearing strata there in 1932 (Arambourg, 1943). The

Lothidok Range is located in the Turkana district, Northern Kenya, approximately halfway

between Lake Turkana (formerly Lake Rudolf), and the town of Lodwar (Figure 1).

Recently, the Kenya National Museums reported three new species of hominoids (Leakey

and Leakey, 1986a, 1986b,1987) and collected numerous other vertebrate specimens in the

Lothidok Range. The Miocene fossil sites in the Lothidok Range are comparable to several

others in East Africa including Rusinga, Napak and Buluk.

The principal objectives of this study are to establish the stratigraphy of the

Lothidok Range, to place the collected fossils in a stratigraphic context, and to provide numerical age control for these fossils. The study area covers approximately 200 km2 and lies between the southern end of the Lothidok Range and the Kalakol River (Figure 2).

Geographic Names

Numerous variations in the names and spelling of specific geographic features in the study area are cited in the literature. The names of features referred to in this text are those of the native (Turkana) people, and were verified in the field whenever possible.

Three important geographic terms have been used in many earlier publications with varied spelling and usage, and to prevent misunderstanding these are reviewed below.

The first of these is 'Lothidok', which is used here to refer to both the entire range and the highest peak in the range, Lothidok Hill (Figure 2). This spelling is used on most 2

Figure 1. Location map for the Lothidok Range, northern Kenya. 3 Figure 2. Geographic references for the Lothidok Range. 5 6 published maps, including the 1:500,000 Geologic Sheet No. 10 (Walsh and Dodson,

1969) and the 1:250,000 Lodwar sheet (NA-36-4) issued by the Survey of Kenya.

Previous workers use either Losodok' (Arambourg, 1935, 1943; Jeremine, 1935), or

'Losidok' (Fuchs, 1939; Dixey, 1945) to refer to the same area and hill. The Lothidok

Range is labeled 'Muruarot' on the 1:250,000 Lodwar sheet.

The second problematic term is 'Moruorot', which, following the native usage, is used here to distinguish a second large hill near the eastern edge of the range, approximately 11 km south of Lothidok Hill (Figure 2). Previous spellings in the literature include 'Muruaret' (Fuchs, 1939), 'Muruarot' (Walsh and Dodson, 1969), and both Moruaret and Moruarot (Leakey and Leakey, 1986a). Arambourg (1943) refers to this hill as Losodok’.

The third problem exists with the terms used for the Kalatum and Alomonet Rivers in the southern end of the Lothidok Range, both of which refer to parts of the same stream course. In accordance with the Turkana people, this streamcourse is called the Kalatum

River above its confluence with the Naserte River, and the Alomonet River below the confluence in this report (Figure 2). This river has been previously called the Lopi River

(Arambourg, 1935; 1943; Walsh and Dodson, 1969) and the Lopi River Pass (Fuchs,

1939).

Previous Work in the Region

The first important scientific expeditions along the western margin of the lake began in the 1930s with the Lake Rudolf Expedition led by V. E. Fuchs of Cambridge

University. Fuchs (1939) produced a report on the geologic history of the area based on observations during this expedition. A. M. Champion, the Turkana District

Commissioner, made the first detailed geologic observations and produced the first reliable physiographic map of the region in 1937 (Walsh and Dodson, 1969). W. C. Smith (1938) 7 provided petrographic descriptions of samples collected by Champion, most of which are well-located, and of considerable value.

C. Arambourg led an expedition through Turkana to the Omo Valley, Ethiopia, in

1932-33. Arambourg (1943) briefly discussed Miocene through Quaternary sediments, fossils, stratigraphy and structure of the Lothidok Range. Although Arambourg (1943) referred to the location of fossiliferous strata as Losodok Hill, the actual site is believed to be at Moruorot Hill. Evidence for this is provided by Arambourg's map (1943) and rediscoveiy of his original quarry at Moruorot Hill by a University of California expedition in 1948 (Madden, 1972). Arambourg (1943) regarded the fauna collected from this locality as earliest Burdigalian in age (ca 22 Ma). Jeremine (1935) provided detailed petrographic descriptions and chemical analyses of some samples collected by Arambourg.

The Kenyan Geological Survey has completed numerous regional geological reports on several areas within the Turkana Basin. The work most applicable to the

Lothidok Range is the report by Walsh and Dodson (1969), which includes cursory discussions of the sedimentary and volcanic rocks in the Lothidok Range.

Madden (1972) analyzed the Miocene fauna of the Lothidok Range based on previously collected specimens. Arguing from published descriptions of the fossiliferous sediments, from the associated gastropod fauna , and the presence of hyracoids, proboscideans, anthracotheres, and primates, he suggested that the paleoenvironment was open and semiarid, with shallow swampy lakes.

Zanettin et al. (1983) measured potassium-argon ages for samples of volcanic rocks collected in the Lothidok Range. Their results are integrated into the discussions of the relevant strata. Bellieni et al. (1987) reported chemical analyses for a few of these volcanic rocks. 8

Methods of Study

Field work during the summers of 1986 and 1987 consisted of mapping on aerial photos flown by the Royal Air Force in 1972 at a scale of approximately 1:50,000. Maps from individual photographs were compiled to produce the final map (Plate, in pocket). All adjustments for distortion were made visually. In addition, 36 stratigraphic sections

(Figure 3; Appendix A ; Boschetto, 1988) were measured with a Jacob's staff or taping methods. Each bed in every measured section was described in detail and sampled for representative lithologies.

The sections were numbered in sequence of examination. Occasionally, areas with either more than one section in a small region, or those that had no sections but were sampled, were also given numbers. Sections measured in these areas are labeled with the area number first and the section number, following a decimal, second (i.e., sections 6 .1 and 6.2 are the first and second sections of area 6 ). Areas were not numbered in the 1987 field season and all sections were numbered consecutively. Samples numbers for the 1986 collections consist of the general location abbreviation (Los = Lothidok), and the section number followed by the sample number. For the 1987 collections, the samples are labeled with a K87 for Kenya , 1987, and are followed by a sample number. All sample numbers preceeded with 'K8 6 ' were collected by F. H. Brown.

Paleocurrent measurements on sediment transport direction were taken at several localities allowing sufficient three-dimensional exposure of imbricated clasts, trough crossbed axes, and foresets of cross- and ripple stratification. Measurements of imbricated clasts were taken by measuring the azimuth of the long axis of the clast in the direction opposite to clast inclination. Measurements were taken on the azimuth of the dip of the foresets and trough crossbeds axes. The paleocurrent data were corrected for tectonic tilt by using a stereonet computer program written by Adolf Yonkee (University of Utah,

1988). A computer program (Stereonet, version, 2.6 by R. W. Allmendinger for the 9

Figure 3. Location map for sections measured in the Lothidok Range. Sections followed by an asterisk represent combined sections. 1 0 11

Macintosh) was used to calculate the mean direction and to plot rose diagrams for each data set. The results are included in the discussions addressing the stratigraphy. Due to limited exposures, a few data sets contain less than 25 measurements and may not be statistically valid (Miall, 1984). However, these measurements still serve as useful indicators of sediment dispersal.

Potassium-Argon Age Determination

Potassium-argon ages, determined from sanidine, plagioclase, amphibole, biotite, and whole rock basalt, provide 2 1 new ages from 11 levels to establish the geochronologic framework for the strata of the Lothidok Range. Table 1 provides all ages and the analytical data relevant to these determinations.

Sanidine (Or5 5 ) was separated from pumice clasts collected from tuffs. The glass of these pumices has been totally altered to phillipsite, analcite and/or montmorillonite, and the vesicles within the pumice have been filled with calcite. Weathered exterior surfaces of the pumice and large pieces of adhering detrital material were removed with a rocksaw.

The calcite was dissolved in dilute (~5%) HNO3 and the remaining material was separated into heavy, intermediate, and light fractions using heavy liquids (s.g. = 2.85 g/cm3 and 2.5 g/cm3 ). The intermediate fraction was sieved (+35 mesh) and the best sanidine phenocrysts were picked by hand under a binocular microscope from the +35 mesh fraction. These crystals were crushed by hand in an agate mortar to -45 +100 mesh fraction to ensure homogeneity between splits taken for argon and potassium analysis.

Amphiboles (kaersutite) were separated by similar methods. For small crystals from primary airfall tephra beds, dilute (-5%) HNO 3 was used to disaggregate samples.

The disaggregated samples were washed to remove fine material and dried. A heavy fraction was obtained using heavy liquids (s.g. = 2.85), and further concentrated using a

Frantz isodynamic separator. The best phenocrysts were picked from this concentrate by 1 2

Table 1. Analytical data for potassium-argon ages.

K 40 Ar* %4 uAr* Calculated Age Sample Sample number W t% 1 0 - 11 mol/g Ma ± 1 s.d. Location

Kalodirr Tuff Member Amphibole Los 6 -8 C 1.345, 1.401 4.153 56.7 17.4 + 0.5 Section 6 .1 Los 6 -8 C 1.385, 1.380 4.242 45.3 17.6 + 0.2 Section 6 .1 Los 6 -8 A 1.446,1.486 4.513 70.9 17.7 + 0.4 Section 6 .1 K87-3108 1.449, 1.451 4.365 76.1 17.3 + 0.2 Section 6 .1 Los 3A 1.433, 1.436 4.398 53.5 17.6 ± 0.2 Section 1

Biotite Los 6 -8 B 7.125,7.168 21.64 60.0 17.4 + 0.2 Section 6.1 21.58 61.9 17.3 + 0.2 Section 6.1 K87-3107 6.72, 6.81 20.42 49.0 17.3 ± 0 .3 Section 6.1 20.84 30.4 17.7 + 0.3 Section 6.1 K86-2742 7.067, 6.976 2 1 . 2 1 42.8 17.4 ±0.2 Section 6.1

Naserte Tuff Member Sanidine separate from pumice K87-3465 7.76, 7.83 22.78 76.1 16.8 + 0 . 2 Section 31 K87-3468A 7.85, 7.86 23.03 78.3 16.8 + 0 . 2 Section 31 Los 4-23B 7.639, 7.639 22.41 8 6 . 1 16.8 ± 0 . 2 Section 4

Unnamed tuff, lower Lothidok Fm Sanidine separate from pumice Los 8-2B 7.907, 7.893 22.80 87.8 16.6 ± 0 . 2 Section 8

Kamurunyang lahar Sanidine separate from pumice K87-3411 5.89, 5.85 13.37 81.2 13.1 + 0.2 Section 25 K87-3437B 5.78, 5.72 13.45 62.3 13.4 + 0.2 Section 29 K87-3437C+D 5.59, 5.53 13.52 70.4 14.0 + 0.2 Section 29

Kalatum basalt Plagioclase separate from crushed sample K86-2743 0.431,0.437 1.013 76.2 13.6 ±0.2 Section 4

Kalakol basalts Whole rock Los 6.2-0 0.923, 0.917 2.839 55.5 17.7 ± 0.2 Section 6.2

Loperi basalts Whole rock B-20 1.522, 1.520 3.185 75.5 12.0 + 0.1 Section 40 B-31 1.411, 1.409 2.668 80.9 10.9 + 0.1 Section 29 1 3

hand under a binocular microscope. A single large crystal was also prepared by crushing,

sieving and washing.

Biotites were separated from crushed material using an elutriation tube. Biotite was further concentrated by scattering grains on paper and vibrating the paper while holding it at an angle at which the biotite flakes would remain and the heavier grains would roll off.

The final material used for dating was picked by hand under a binocular microscope from the concentrate.

Thin sections of each basalt sample collected for isotopic dating were examined for the presence of alteration products such as zeolites and clay minerals. Samples that contained abundant alteration minerals or secondary minerals were excluded, and only one sample (Los 6.2-0) in which such materials were sparse was dated as a whole rock specimen. Even this sample contained minor amounts of alteration products, and was therefore treated with weak acetic acid (5%) to remove carbonate minerals before dating.

Plagioclase phenocrysts were separated from another basalt (K86-2743) by magnetic methods following crushing, sieving and washing.

All samples were dated in the potassium-argon laboratory in the Research School of

Earth Sciences at the Australian National University , Canberra, under the direction of Dr.

Ian McDougall. The procedure for determining the potassium-argon age is outlined in

McDougall et al. (1980), and McDougall and Watkins (1986). Briefly, potassium was measured by flame photometry and argon by isotope dilution. Analytical precision is about

1 % at the level of one standard deviation, confirmed by satisfactory replication of potassium analyses and the agreement between duplicate argon analyses. Measurements were made on many separates from the same samples or bed with excellent agreement. To insure complete extraction of radiogenic argon from the phenocrysts, temperatures of 1600°

C were maintained for a minimum of 40 minutes (F. H. Brown, pers. comm.). Decay constants used are those recommended by Steiger and Jaeger (1977). REGIONAL GEOLOGY OF THE TURKANA DEPRESSION

Regional Structure

The Turkana Depression (Figure 4), a triangular lowland in northwest Kenya, is bounded on the west by the Turkwel and Ugandan Escarpments (Baker et al., 1972). The

Turkwel Escarpment is the oldest and most dissected rift fault escarpment in Kenya (Baker et al., 1972) and the Ugandan Escarpment is interpreted as an eroded monoclinal flexure

(Walsh and Dodson, 1969). To the south, the Turkana Depression merges with the

Baringo-Suguta graben, a well-defined graben at the northern end of the Gregory Rift. The

Kinu-Sogo Fault Zone, essentially the northern continuation of the Baringo-Suguta

Graben, forms the eastern boundary of the Turkana Depression (Baker et al., 1972). To the northeast, the depression merges with the well-defined graben presently occupied by

Chew Bahir (Lake Stephanie) (Williamson and Savage, 1986). Chew Bahir lies roughly

40 km west of the southern end of the Ethiopian Rift, which is presently occupied by Lake

Chamo.

Regional Stratigraphy

Four major lithostratigraphic units are defined in the Turkana Depression

(Williamson and Savage, 1986). These are (1) the Precambrian and ?lowermost Paleozoic gneisses of the Mozambique fold belt; (2) a thick sequence of coarse, immature clastic sedimentary rocks, named the Turkana Grits or Laburr Series; (3) a sequence of Oligocene through Miocene volcanic and interbedded sedimentary rocks; and (4) a thick, heterogeneous assemblage of Plio-Pleistocene volcanic and sedimentary rocks. 1 5

Figure 4. Relation of the Lothidok Range to regional geographic and structural features of the Turkana Depression. 1 6

/ 38°E L. Chamo ETHIOPIA

,L. Turkana, LOTHIDOK ^ RANGE

Toror

Moroto

Kadam o' Aden

Sutfon C v V Uganda p^-

Indian Ocean L./Baringo Kenya

Tanzania

L. Victoria 1 7

An early geologic study in northwest Kenya defined a sequence of sedimentary strata as the Turkana Grits (Murray-Hughes, 1933). Subsequently, nearly all sedimentary sequences in the Turkana depression have been collectively referred to as the Turkana

Grits' (Dixey, 1934; Fuchs, 1939; Walsh and Dodson, 1969; Madden, 1972). The presence of angiospermous fossil wood (Dryoxylon) prompted numerous workers to consider the 'Turkana Grits' as Oligocene to Miocene in age. Arambourg (1943) alone doubted that the presence of Dryoxylon proved a mid-Tertiary age, and suggested that the wood-bearing sediments might be of Eocene or Cretaceous age. Arambourg (1943) was thus the first to suggest that two distinct episodes of sedimentation occurred in the Turkana depression, the first in the Mesozoic, and the second in the Miocene. Discovery of a sauropod humerus (Arambourg and Wolff, 1969) in the Laburr Series of the Lapurr Range proved Arambourg correct.

The regional survey of sedimentary strata in the Turkana Depression by Williamson and Savage (1986) is in strong agreement with Arambourg's original interpretation and division of the sedimentary sequences into two distinct groups. The lower group includes the clastic sequences of Lodwar/Muruanachok, Lapurr (Cretaceous) and lower strata at

Kajong (Figure 4). Strata at Lodwar, Muruanachok, and Kajong lie nonconformably on basement, and are unconformably overlain by Miocene deposits. Oligocene basalts (Walsh and Dodson, 1969) overlie the Cretaceous sedimentary rocks in the Lapurr Range. The coarse fraction of these older sediments lacks volcanic minerals and volcanic rock fragments. The upper group includes the sedimentary rocks of Lothidok, Loperot, and upper strata at Kajong (Figure 4). This sequence includes a significant volcaniclastic component and has been shown paleontologically and isotopically to be Miocene in age

(Williamson and Savage, 1986).

The division of the Turkana Grits into two distinct sequences, however, falls short of solving the problems that currently plague the stratigraphic nomenclature for the 1 8

sedimentary deposits in the Turkana Depression. Because sedimentary strata contain no

substantial volcaniclastic component, lie nonconformably on basement, or are nonconformably overlain by Miocene basalts does not prove they belong to either group of

sediments. The lack of a volcaniclastic component may simply indicate there was no

volcanic source. This is clear at Loperot, roughly 100 km south of the Lothidok Range

(Figure 4). Work by Joubert (1966) and by this writer in 1986 and 1987 show that the

Miocene sedimentary sequence consists of arkosic sandstones, lying directly on basement,

which are overlain by volcaniclastic strata. The sequence is nonconformably overlain by

Miocene basalts. This is also the case for the Pliocene and Pleistocene sediments exposed

southwest of Lodwar and east of the Lothidok Range. Those near Lodwar were termed

Turkana Grits' (Walsh and Dodson, 1969), and those near Lothidok were termed the

'Laburr Series' (Arambourg, 1943) solely because they lack volcaniclastic material.

The present strati graphic morass resulted from the assignment of local stratigraphic

sequences to one regional unit (Turkana Grits). In a sense Williamson and Savage (1986)

continue this style of stratigraphy by correlating the lower sediments at Kajong (Sera

Iltomia Formation) with the Labur series even though these sediments contain no direct

evidence of deposition during the Cretaceous Period. However, by defining formations on

the basis of local sections they helped resolve some of the problems. Should their

correlations be disproved in future, the formations will remain valid.

The most complete study of Tertiary strata in the region is that of Watkins (1982),

who described a 1900 m thick sequence of volcanic rocks and intercalated sedimentary

deposits in the Suregei-Asille region northeast of Lake Turkana (Figure 4). Watkins

(1982) proposed a formal lithostratigraphic nomenclature for these units consisting of nine formations that span the interval from the early Miocene to the middle Pliocene.

Potassium-argon dating of lavas and high-temperature alkali feldspars separated from rhyolitic units in the sequence provide temporal control for these formations (McDougall and Watkins, 1988). The volcanic and sedimentary deposits of the Surgei-Asille region were deposited during the same interval of time as those at Lothidok, and have yielded an important fauna from the site of Buluk (Leakey and Walker, 1985; Harris and Watkins,

1974). The lack of clearly correlative strata between the two regions is of substantial paleogeographic importance, and is treated below. STRATIGRAPHY OF THE LOTHIDOK RANGE

Stratigraphy

Following the lead set by Watkins (1982) and by Williamson and Savage (1986), a new formation name for the sedimentary strata of the Lothidok Range is defined here on the basis of local sections. The strata are divided into the Kalakol basalts (new informal name), the Lothidok Formation (new stratotype), the Loperi basalts (new informal name), and undifferentiated Tertiary deposits (Figure 5).

No single section exposes the entire sequence of strata in the Lothidok Range, and the stratigraphy is therefore constructed from 10 partial sections (Figure 6 ). The Kalakol basalts are described from Sections, 17 and 7. Section 17 includes strata from the base of the section up to a break that results from faulting. Section 7 includes strata above the section break to the upper contact of the Kalakol basalts. The Lothidok Formation is exposed in a series of correlated sections (6.2, 19, 21, 24, 25, 29, 30 and 31; Figure 6 ).

The Loperi basalts that overlie the Lothidok Formation are described from section 29, which continues upward through the undifferentiated Tertiary rocks. The composite stratigraphic sequence of the exposed strata is approximately 1500 m thick. Detailed descriptions of these partial sections are given in Appendix B.

All correlations between sections are tightly constrained except for one within the

Kalakol basalts. A break in the section occurs approximately 200 m above the Eragaleit beds (Figure 5) where section 7 is correlated with section 17. The basalts between the

Eragaleit beds and section break were neither measured nor examined in detail, hence there may be missing or duplicated section. If section is missing, the thickness is a minimum; if 21

Figure 5. Sequence and terminology used for the composite stratigraphy of the Lothidok Range. METERS 200 300 1400 100 - J ▼ . } Missing Section Missing } Eaaet beds-Eragaleit Klkl basalts(Tkb) -Kalakol Ltio Formation -Lothidok -Loperi Basalts(Tib) -Loperi -Tertiary Undifferentiated -Tertiary k **fr- Explanation +V ^AAAA/ A ?* i- (Tu) * a a a a a V] Fault Basalts ufcos and Tuffaceous lsi sediments clastic Unconformity Lahars Tephra

22 Figure 6 . Component sections for the stratigraphy of the Lothidok Range. 24 Measured Sections Unit Tertiary Undifferentiated (Tu) -Loperi Basalts (Tib)

Lothidok Formation

6.2

Kalakol basalts (Tkb)

Explanation Missing Section ■P-'.-fc-'j Lahars Unconformity Tephra Tuffaceous and clastic sediments Eragaleit beds Basalts Fault 25 duplicated, the thickness is overestimated by 200 m or less. The generalized correlations and lateral variation are illustrated on Figures 7 and 8 .

Lithology

The lithologies encountered in the Lothidok Range consist primarily of terrestrial sedimentary and volcanic deposits common to many regions. In addition to these, however, a considerable proportion of the strata comprising the Lothidok Range consists of pyroclastic deposits. Because two types of these deposits, airfall tephra and lahars, may not be familiar to the reader, they are briefly described below.

For the purposes of this report, pyroclastic rocks are considered to be those composed of volcanic ejecta directly originating from a volcanic eruption, and consist of stratified and massive airfall tephra. These are distinguished by the presence of pumice

(recognizable at present alteration state), euhedral volcanogenic minerals (amphibole, pyroxene, biotite, and sanidine) and angular volcanic rock fragments; and by the lack of fluvial sedimentary features. Fluvially reworked tephra are discussed as clastic deposits.

The granulometric classification of pyroclasts and pyroclastic deposits is from Schmid

(1981).

Lahars are mudflows that consist chiefly of volcanic materials (Bates and Jackson,

1987). The lahars form distinctive deposits of matrix dominated/supported, massive to inversely graded conglomerates. They are distinguished from matrix supported conglomerates of fluvial origin by the lack of sedimentary structures and erosional basal contacts, and by the abundance of fine-grained (clay to sand) matrix and volcanogenic material such as pumice. 26

Figure 7. Generalized north-south cross-section and lateral distribution of the strata exposed in the Lothidok Range. Marker Horizons Kl Kamurunyang lahar Ab Akwang'a basalt Kab Kalatum basalt Lc Lokipenata conglomerate NEb Nakwel Esha beds NT Naserte Tuffs KT Kalodirr Tuffs Be Basal conglomerate Eb Eragaleit beds Section (s) ?Fault 3 5 o5 0 lE 36°E Explanation - 3 °30'N Tertiary undifferentiated (Tu) Lake ------Unconformity? Turkana Loperi Basalts (Tib) I 1.1 , — Unconformity *

NT lower Lothidok Fm. (Til) KT 00 Contact -interbedded sediments Kalakol basalts (Tkb) -3°IO' N ^A re a boundary 0 5 km » Section location 28

Figure 8 . Generalized east-west cross-section and lateral distribution of the strata exposed in the Lothidok Range. 29

Marker Horizons Kl Kamurunyang lahar Kab Kalatum basalt Lc Lokipenata conglomerate NT Naserte Tuffs KT Kalodirr Tuffs Be Basal conglomerate

Section (s) ?Fau It

Explanation

III j Tertiary undifferentiated (Tu) ------Unconformity ? Loperi Basalts (Tib) ------Unconformity Kl upper Lothidok Fm. (Tlu) Kob

Discon for mity NT lower Lothidok Fm. (T il) KT

- B c Contact -----interbedded sediments Kalakol basalts (Tkb) 30

Kalakol Basalts - Tkb

The lowest strata exposed in the Lothidok Range consist of basalt flows and intercalated sedimentary deposits, here informally named the Kalakol basalts (Tkb - Plate,

Tvbl - Walsh and Dodson, 1969). On the basis of similar isotopic ages, the Kalakol basalts apparently correlate with the Turkana Basalts of the Lodwar sequence (Zanettin, et al., 1983) and with the Lodwar Formation (Bellieni, et al., 1987). However, the respective authors do not designate a stratotype or provide descriptions for these units.

Therefore a new, informal name is used here to refer specifically to the older basalt flows of the Lothidok Range.

Most of the basaltic hills in the Lothidok Range north of the Kalatum and Alomonet

Rivers consists of Kalakol basalts. Only four minor exposures of the Kalakol basalts occur south of these rivers (Plate). The lowest flows of the Kalakol basalts crop out only between the Eragaleit and Nathuraa rivers below Lothidok Hill (Plate, Figure 2), but the base of the section is not exposed.

The Kalakol basalts are a minimum of 785 m thick, and consist of at least 20 flows ranging from 4 to 60 m thick with interbedded sedimentary and pyroclastic rocks ranging from 2 to 50 m thick. The flows are occasionally vesicular near upper contacts. The highest basalt flow ranges from 7 to 17 m thick and thins to the north where it pinches out between sections 7 and 6 .1 (Figure 7).

The Kalakol basalts consist predominantly of olivine-augite lava flows that are aphyric to coarsely phyric with an aphanitic groundmass. Phenocrysts of olivine, augite and plagioclase are normally between 1 and 3 mm in long dimension but in one flow, augite reaches 1 cm. Accessory minerals include an Fe-Ti oxide, probably magnetite, and biotite occurs occasionally as poikilitic plates in the groundmass. Fractures filled with drusy quartz and/or calcite are common. All of the basalts are weathered and altered to various degrees. Olivine phenocrysts have been altered to chlorite, or iddingsite with 3 1 limonite rims. Chlorite alteration of the groundmass is usually extensive, and calcite amygdules are common.

Thin, lenticular sedimentary deposits commonly lie between basalt flows and consist of granule to cobble conglomerates, poorly sorted to conglomeratic sandstones, stratified siltstones (rare), and massive mudstones. These deposits are poorly exposed, and generally only a few meters thick. The thickest interval, designated the Eragaleit beds, contains vertebrate fossils and is consequently discussed in detail.

Eragaleit Beds

The most important sequence of sedimentary and pyroclastic rocks interbedded with the Kalakol basalts is here informally named the Eragaleit beds. These beds are only exposed in the Eragaleit and Nathuraa Rivers near the base of Lothidok Hill and he roughly

600 m below the top of the Kalakol basalts (Figures 5 and 7). Although incomplete, the thickest exposures are found along the Eragaleit river (section 16) where the beds reach a maximum of 50 m. The section appears thicker because it is faulted against lithologically similar strata of an overlying formation (Plate). The only complete sequence of the

Eragaleit beds occurs in the small, north flowing tributary to the Nathuraa river (Figure 7).

At this location (section 17) the sequence has thinned to 37 m and continues thinning to the north. The section is about 10 m thick where it is truncated by a northwest striking, east dipping normal fault, north of which no exposures of the Eragaleit beds were found.

Approximately 500 m south of the Eragaleit River, the beds are truncated by the fault that divides the Eragaleit and Lataagur grabens (see structure discussion, p. 127). Only small exposures of the beds, the bases of which are also truncated by faulting, exist west of the

Lataagur graben.

The Eragaleit beds consist primarily of polymictic conglomerates and conglomeratic litharenites (McBride, 1963; Folk, 1968), minor siltstones and mudstones, and a stratified 32 tephra sequence. The base of the sedimentary sequence consists of a 4 to 6 m thick, massive to crudely bedded, normally graded, grey to reddish grey conglomerate. Clasts are stained with hematite, are poorly imbricated, and range in size from small boulders to large cobbles. The lower contact is an irregular erosional surface with scours up to 1 m deep cut into the underlying basalt. Broad, internal scour and fill structures are common, and large-scale trough crossbeds are present but rare.

Overlying the basal conglomerates are pale red to grey granule to cobble conglomerates that generally grade into pale red, poorly sorted, coarse-grained to conglomeratic sandstones. These contain small- to medium-scale trough crossbeds with both upward-coarsening and upward-fining sequences. Basal, pebble to cobble conglomerate lenses and mudstone rip-up clasts are common in the sandstones. Clasts of the conglomerates and conglomeratic sandstones consist of phonolite and basalt.

Sandstones of the Eragaleit beds are made up of very fine to coarse, rounded volcanic and lithic grains. The sandstones occasionally grade to ripple stratified siltstones or interfinger with massive, pale red to brown mudstones.

The siltstones are moderately to very sandy and generally too poorly exposed to display bedding structures. These fine grained sediments constitute less than 10% of the total section. The highest bed in the sequence is a dark red, well-consolidated, massive siltstone. Columnar jointing in the upper 20 cm of this siltstone indicates it was baked by the overlying basalt.

A moderately resistant conglomerate-sandstone-siltstone sequence (13 m) that forms small ridges near the top of the section (bed 18, section 16; bed 12, section 17) contains abundant vertebrate fossils (see faunal discussion, p. 129). This sequence is dark red with purple streaks and yellow limonite blotches. The sandstones are very poorly sorted and contain abundant clay. The conglomerates contain small to medium pebbles of volcanic rocks and sedimentary rip-up clasts. Medium- to small-scale trough 33 crossbedding, internal scour and fill sequences, normally graded bedding, and basal erosional contacts occur in coarse grained layers. Ripple laminations occur in the fine­ grained sandstones and siltstones. Upward-fining sequences average 1 m thick.

A tuffaceous interval within the Eragaleit beds lies about 15 m above the basalt- conglomerate contact in section 17. These tuffs are informally named the Nathuraa tuffs, because they are best exposed near the Nathuraa River. The tuffaceous interval consists of interstratified 1 to 3 cm thick, fine tuffs to coarse lapillistones and 10 to 30 cm thick, reworked tephra beds. The lapillistones contain abundant, extremely altered and flattened pumice lapilli in which the glass has been altered to a zeolite or a clay. The tuffs and lapillistone matrix have also been altered to montmorillonite.

Several small basaltic dikes intrude the Eragaleit beds in the exposures flanking the

Eragaleit River. These extremely altered dikes are generally dark greenish grey to pale olive and average less than 1 m wide by 2 0 m long.

Other Interbedded Sedimentary Rocks

Sedimentary deposits interbedded within the Kalakol basalts at various levels, differ from the Eragaleit beds mainly in the amount of mud occurring as matrix of coarser rocks and as discrete bodies of mudstone. Although conglomerates and sandstones typically dominate, mudstones are locally the only rocks exposed between the basalt flows. The mudstones are moderately to very sandy, dark red to brown, and grade into muddy sandstones. The mudstones contain numerous white, chalky, calcareous root casts and 1 to 5 cm thick, tabular calcrete? interbeds. Mud-rich sandstones generally contain interbedded mudstone lenses, lithic grains, volcanic mineral grains, and mudstone rip-up clasts. The upper surfaces of some basalts grade to mudstones suggesting that these may represent ancient weathering horizons. 34

The uppermost sedimentary strata within the Kalakol basalts lie 10 to 17 m below the upper contact (Figure 7). These strata are exposed in sections 1, 1.2, 3, 4, 7, 10.1,

10.3, 30, and 31, and are 10 m thick in all exposures along the west side of the Lothidok

Range. The only exposure to the east lies on the north side of Moruorot Hill (section 10.3) where this sedimentaiy interval is 80 m thick (Figure 8 ). The deposits consist of pale purple to purplish grey, polymictic conglomerates, conglomeratic litharenites and dark reddish brown mudstones. At most outcrops pebbles or cobbles dominate, but at

Moruorot, large cobbles and boulders up to 2.5 m diameter are present.

Potassium-Argon Age Determinations

The single whole-rock dated sample (Los 6.2-0), collected from the uppermost

Kalakol basalt below the Lothidok Formation at section 6.2, yielded an age of 17.7 + 0.2

Ma (Table 1). Although the determinations are replicable, the age should be regarded as a minimum until confirmed by additional work.

Zanettin et al. (1983) give an isotopic age of 24.4 +1.0 Ma for a phonolite in this area, but the location from which the dated material was collected is unclear. Coordinates provided for the sample (K 60; Zanettin, et al.,1983) place it near Kakurtua Hill, where the Loperi basalts, now known to be much younger (this report), are exposed. Zanettin et al. (1983) map the sample as a Turkana basalt' and indicates that it was taken near

Moruorot Hill, approximately 10 km north of the coordinates given. No phonolites were found in this area during this study. Volcanic rocks from this location could belong to either the Loperi or Kalakol basalts although the measured age indicates the latter.

Assuming that the sample was collected from the Kalakol basalts exposed at Moruorot Hill and that the age is correct, deposition of the Kalokol basalts in the Lothidok Range occurred from prior ~25 to -18 Ma ago. 35

Lothidok Formation - T1

The Lothidok Formation is defined here as all strata lying between the upper contact of the highest flow of the Kalakol basalts and the lower contact of the Loperi basalts

(Figure 9). The formation is a heterogenous assemblage of early and middle Miocene sedimentary and volcanic rocks.

The term 'Lothidok' has previously been used informally to designate a formation on the basis of a single dated basalt (Bellieni, et al., 1987). However, Bellieni, et al.

(1987) provide no type section, formation description or outcrop location. The

International Guide to Stratigraphic Nomenclature (Hedberg, 1976) strictly prohibits the definition of formations based of time of deposition, and requires that some description of the named units be given.

It is my intent to formally define the Lothidok Formation and to designate a stratotype for this unit in a subsequent publication; therefore, the terminology is used here in a formal sense to prevent future confusion. Descriptions of the constituent strata are given both in the text below and on columnar sections (Appendix B; Boschetto, 1988).

The term 'Lothidok' is retained as it is the most logical name for the formation because the stratotype is a composite with component sections (Figure 6 ) widely scattered throughout the Lothidok Range (Figure 3). The names of all formal and informal subsidiary units within the formation are new, and are defined with reference to particular type sections.

The Lothidok Formation is informally divided into lower (Til) and upper (Tlu) units. The general sequence, names, and isotopic ages of the strata are shown in Figure 9.

Three members within the lower Lothidok Formation are formally defined: the Basal

Conglomerate Member, the Kalodirr Tuffs, and the Naserte Tuffs; a fourth unit is informally named the Nakwel Esha beds. No units are formally defined within the upper

Lothidok Formation, but four units are informally named: the Lokipenata conglomerates, the Kalatum basalt, the Akwang'a basalt, and the Kamurunyang lahar. Units are 36

Figure 9. The sequence and terminology used for the composite-stratotype of the Lothidok Formation. Loperi Basalts (10.9-12.0 Ma) angular unconformity 500 Kamurunyang lahar (13.1 - 13.4 Ma)

Akwang'a basalt (TIu) upper

400

unassigned strata ------

- *■ A ► V 300 ___ a Kalatum basalt (13.6 Ma) P i Lokipenata Conglomerate orstt — disconformity

200 - S Nakwel Esha beds *+• -----

~ASr*l Naserte Tuff (l6.8Ma) (Til) lower unassigned strata 100 AAA A A AAA A A~) AAAAAAAAA, Alomonet tuffs Kalodirr AAAAAAAAAAAl lA A A A A A AAA A I AAAAAAAAAAAJ Kanukurinya tuffs . Tuffs (17.3­ 17.7 Ma) III Basal Conglomerate g jH i '/:S\ Kalakol basalts (?-l7.7Ma) 38 informally defined when they form useful local markers but are not widespread or well enough exposed for detailed description. The boundary between the lower and upper

Lothidok Formation is placed at the base of the Lokipenata conglomerate.

Exposures of the Lothidok Formation vary greatly. The most complete exposures occur along the southwestern edge of the Lothidok Range (sections 4 and 30). Section 30 provides a general thicknesses of 250 and 290 m for the lower and upper Lothidok

Formation respectively. All other exposures are truncated by faulting, unconformities, or erosion. The predominant trend of the outcrop is north-south (Figure 7), and east-west lateral variation can only be observed in the vicinity of the Kalatum and Alomonet Rivers

(Figure 8 , Plate). The poor exposure of sections 4 and 30 necessitates the use of eight component sections (Figure 6 ) to construct a composite-stratotype for the Lothidok

Formation (Figure 9).

Strata in the lower Lothidok Formation contain enough distinctive marker horizons that correlations are relatively straightforward. By contrast the heterogeneous, areally limited, and laterally variable nature of the upper Lothidok Formation, coupled with the lack of marker horizons and the use of an angular unconformity as the upper boundary, conspire to make correlation very difficult.

Marker horizons comprise very little of the thickness of the Lothidok Formation.

Most of the formation is composed of laterally variable sedimentary and pyroclastic deposits. Because these can only be broadly correlated on the basis of their stratigraphic position with respect to the marker horizons the Lothidok Formation cannot be exhaustively divided into subordinate stratigraphic units. The following discussions therefore emphasize marker horizons to set the stratigraphic framework and address the strata not assigned to a specific unit in a general way. Interpretations of depositional environments for the sedimentary and pyroclastic rocks are presented in the following discussion. 39

Lower Lothidok Formation - Til

Strata of the lower Lothidok Formation are best exposed in sections along or north of the Naserte and Alomonet Rivers with limited exposures farther south. The lower

Lothidok Formation consists of volcaniclastic sedimentary rocks, and altered mafic alkaline and trachytic tuffs. Four marker horizons proved useful for correlating outcrops and subdividing the lower Lothidok Formation. These are the Basal Conglomerate Member, the Kalodirr Tuffs, the Naserte Tuffs, and the Nakwel Esha beds (Figure 9).

Basal Conglomerate Member

The Basal Conglomerate Member is the lowest widespread unit of the Lothidok

Formation. Although the member crops out throughout the Lothidok Range, outcrops are generally discontinuous and incomplete, and exposures with more than 40% of the interval represented by this unit are rare. The best exposures are in sections 1 (type section), 1.2,

10.3, and 20; less complete exposures occur in sections 3, 4, 6.1, 6.2, 10.1, 22, 30 and

31 (Figure 3).

The Basal Conglomerate Member consists of 10 to 75 m of volcaniclastic conglomerates and sandstones that conformably overlie the Kalakol basalt and interfinger with the conglomerates below the highest Kalakol basalt flow. The unit is thickest in the central and southern exposures and thins to the north.

The member consists primarily of polymictic, dark greyish red to light reddish grey and purplish grey, pebble to cobble conglomerates. The largest clasts are large cobbles to medium boulders. Dark red hematite partially or wholly coats phonolite and basalt clasts that are set in a sandstone matrix with medium to very coarse, subangular to subrounded grains. The beds are poorly to moderately cemented with calcite, which may form 2 mm thick rinds on clasts where matrix is absent. 40

Clast-supported conglomerates are massive, or crudely bedded with imbricated clasts. Matrix-supported conglomerates generally exhibit medium- to large-scale trough crossbedding, internal scour and fill sequences and low-angle crossbedding. Strata sets are predominantly normally graded although inverse and symmetric (normal-reverse-normal) grading occurs. Conglomeratic strata cosets generally fine vertically and laterally. Channel fill sequences reach 1 m thick and 3 m wide. Irregular, subhorizontal erosion surfaces are common. The conglomerates grade into or interfinger with poorly sorted to conglomeratic sandstones that occasionally dominate the sequence.

The sandstones consist of medium to very coarse, subrounded to subangular lithic grains and are classified as litharenites (McBride, 1963, Folk, 1968) because they contain less than 2% (modal) quartz/feldspar. Most of the quartz grains are euhedral crystals that probably originated from cavity fillings or geodes common in the Kalakol basalt.

Small- to large-scale trough crossbeds, low-angle crossbeds and internal scours dominate the sedimentary structures in the sandstones. Thin, massive, conglomeratic

lenses commonly overlie basal and internal scours and cosets of cross strata predominantly fine upwards but occasionally coarsen upward. The sandstones occasionally grade into pale red to red siltstones, but these represent less than 1 0 % of the exposed section.

Dark red, and reddish brown to brown massive mudstones occur commonly as interbeds. Thin, lenticular mudstones primarily occur within conglomeratic cosets and tabular mudstones lie between cosets. The poorly exposed mudstones are moderately to very sandy and moderately to poorly consolidated. In a few outcrops these contain widely scattered, extremely altered, fine granule to medium pebbles. White, chalky, calcareous rootcasts and thin, tabular, calcrete horizons are common, and are interpreted as pedogenic features.

The actual amount of mudstone occurring in this sequence is not clear, although the variation appears great. In all sections except 6 .1 and 6.2, mudstones represent less than 41

20% of the actual outcrop. At sections 6 .1 and 6.2, the massive, dark reddish brown mudstones account for ~15% of the exposed strata, the remainder of the section being conglomerate.

K al ndirr Tuff Member

The Kalodirr Tuffs (new member) lie immediately above the Basal Conglomerate

Member, and are internally divided into the Kanukurinya (lower) and Alomonet (upper)

tuff beds. The type locality for the Kanukurinya tuff beds is section 6.1, located 12.75 km north of the Kalakol-Lodwar road (Figure 3). The type locality for the Alomonet tuff beds

is section 23, located along the Alomonet river (Figure 3). The thickness of the Kalodirr

Tuffs ranges from 15 m in section 10.1 to 25 m at section 6.1 (Figure 10), depending on

the number and thickness of beds comprising this member. The lower boundary bed is a

very fine, massive, pale red tuff at the base of the Kanukurinya tuff beds. The upper

boundary is the top of the Alomonet tuff beds.

Kanukurinya Tuff Beds

The Kanukurinya tuff beds consist of stratified and reworked tephra, lahar deposits

and interbedded clastic sediments (Figure 10). The Kanukurinya tuffs occur at every

outcrop of this stratigraphic level and can be traced laterally for the length of every

continuous exposure.

The stratified tephra are olive to pale green, pale red and reddish tan and range from

5 to 50 cm thick. The scale of stratification decreases upwards from thick to thin planar

laminations. The laminae commonly drape small scale, topographic irregularities such as

mounds or fossil wood, with no evidence for erosion on the basal contact. Planar

laminated strata occasionally grade into ripple marked and cross-stratified beds laterally.

Platy and elongate grains are oriented parallel or subparallel to bedding. Accretionary Figure 10. Correlation and lateral variation in the Kalodirr Tuffs. Alomonet tuff beds

Konukurinya tuff beds N Explanation Fluvial clastic deposits Scour surface Primary airfall and reworked tephra deposits - 2 5 Accretionary lapilli Non-erosional surface Syenite clasts Miscellaneous clasts Lahor deposits Plant debris - 2 0

£ Index Map

Section 6.2 -&• OJ 44 lapilli, ranging from 1 to 4 mm in diameter, occur in one 2 cm thick massive tuff.

The stratified and massive tuffs consist of crystals (65%) and lithic lapilli (35%) in an altered matrix, of fine ash. The most abundant crystals are euhedral pyroxene (diopsidic augite), kaersutite, and biotite. The pyriboles (pyroxenes and amphiboles) are up to 2 mm diameter (1 0 ) where as biotite phenocrysts reach 4 cm diameter. Nepheline in various stages of alteration is present but not common. Accessory minerals include apatite, perovskite, mafic clots, extensively altered plagioclase, and an unidentified opaque mineral.

Basaltic or basanitic lithic fragments up to 3 mm diameter (~-2 0 ) contain both plagioclase and olivine.

The stratified tuffs are predominantly normally graded, fining upwards through the cosets. Symmetric grading (normal-inverse-normal) occurs in the coarsest beds. A very thin (< 1 cm), very fine grained, pale red, thinly laminated to massive tuff commonly forms the highest layer of the beds, and locally has desiccation cracks and raindrop impressions.

Gastropods, preserved as calcite casts, are the most common fossils found in the stratified tuff beds. These include Lanistes carinatus, Pila ovata, Cerastua miocenica, and

Burtoa nilotica verdcourti (Van Damme and Gautier, 1972; see fauna discussion, p. 123).

A turtle carapace and plastron found in a laminated bed (this study) is completely filled with the tuffaceous material of the surrounding bed.

Lahar deposits overlie many of the stratified tuff sequences (Figure 11). These range from 15 cm to 4.4 m thick, with the thickest bed being the highest at sections 1 and

33. These beds generally thin to the south. The lahar deposits are pale green to olive, pale yellowish brown to pale reddish brown, and consist of 45 to 65% clay to coarse grained sand matrix, and 35 to 55% granules to boulders. Thin section analysis shows that 35% of the sand grade matrix is made up of volcanic minerals such as euhedral amphibole, pyroxene and biotite. 4 5

Figure 11. Typical sequence of the Kanukurinya tuff beds from section 33. Bed 15 Bed 14 D------Bed 13

Bed 12

Bed II i— Reworked tuffs Igneous boulders ^Soft sediment injection dikes — Lahar deposits |-Mudstones Plant debris Airfall tuffs Gastropods 6 - A A AAAAAAAAAA AAAAAAAAAAAA Bed 10 Accretionary lapilli AAAAAAAAAAAAJ Contorted bedding

Bed 9

4- A A A Bed 8 Bed 7

]Bed 6 Bed 5 A aaa a a ' a ® a^ ^ AAAAAAAAAAA/ A A A A A A A A A A A A 0 /© \A A A A A A A A ®> 1

AAAAAAAAAAAAA I- BBC] C.

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The lahar deposits contain crystals up to 3 cm diameter (-50) of the same mineralogy as the laminated beds, but platy and elongate particles are randomly oriented in most of these beds. Reverse grading occurs when the beds contain a wide range of clast grades. Basal contacts of these beds are sharp but nonerosional, which is apparent from intact mud flakes of very fine bedded tuffs lying along the basal contact. The curled mud flakes are occasionally overturned and imbricated but rarely broken.

Lithic clasts of basalt, phonolite, and nephelinite reach 20 cm in diameter in the lower beds and up to 2.2 m in diameter in the highest bed. The basalt and phonolite clasts were extensively altered and were not collected. Nephelinites are rare and have not been examined in detail. Near sections 1, 1.2 and 33 the highest bed contains numerous well- rounded nepheline-bearing syenite boulders up to 1.2 m in diameter. At sections 4 and

10.1, the boulders are up to 50 cm in diameter. The roundness has been attributed to prolonged periods of water abrasion (Walsh and Dodson, 1969), but may have resulted from magmatic processes (Holmes, 1965). These syenites consist of 75% potassium feldspar, 5-15% aegirine augite, 5-15% alkali amphibole, < 5% nepheline, an Fe-rich biotite and minor apatite. Petrologically similar boulders are discussed by Jeremine (1935) and Smith (1939), but differ from those of the Kalodirr Tuffs by their higher percentage of nepheline and presence of sphene.

The lahars also contain abundant plant debris consisting of wood and leaves generally present in the upper parts of individual beds. No preferred orientation of this material was observed. The wood is preserved as calcite filled casts of logs, branches and stumps with very well-preserved exterior surfaces. The largest examples reach 30 cm in diameter and 2 m in length. Leaf and grass imprints are commonly rolled around matrix material (Figure 12), and are most abundant in sections 6.1 and 31. The majority of the leaf imprints have entire margins and are very well-preserved. Some leaf imprints have been tentatively identified as fossils of a broad leafed bamboo (B. Jacobs, pers. comm.). 48

Figure 12. Photograph of a leaf imprint in the Kalodirr Tuffs at section 6.1. The knife above and to the right of the imprint is 9 cm in length. 49

A calcite cast of a fossil fruit collected from these beds is believed to belong to the

Dicotlyedonae, possibly Burseraceae, Canarum sp. nov. (C. Kabuye, pers. comm.).

Thin, fluvially reworked, tephra beds typically overlie the stratified tephra and lahar deposits (Figure 11). These 20 to 50 cm thick beds contain subrounded to rounded lithic and volcanic mineral grains, volcanic rock fragments and minor bedded-tephra intraclasts.

Sedimentary structures include basal erosion surfaces, small-scale trough crossbeds and planar to asymmetric ripple stratification.

Soft sediment deformation structures are very common in these beds and include contorted and convolute stratified tephra beds. Other features include soft sediment dikes of stratified tephra material injected into overlying lahar deposits, and deformed rip-up clasts (up to 1 m long) of stratified tephra beds within the overlying lahars.

Alomonet Tuff Beds

The Alomonet tuff beds range from 1.5 to 2.8 m thick, thin to the north and lie less than 4 m above the Kanukurinya tuffs where both occur at the same outcrop. The reddish to yellowish brown color of the Alomonet tuffs is very distinct from the pale to dark olive green to rare pale red colors of the Kanukurinya tuffs. A second difference between the

Kanukurinya and Alomonet tuffs is the significantly smaller size and lower abundance of phenocrysts in the Alomonet tuff. Near sections 1, 1.2, 30 and 33 the Alomonet tuffs contain numerous well-preserved footprints of unidentified birds and artiodactyls, and vertebrate fossils are also associated with these beds (see faunal discussion, p. 129).

Stratified tuffs comprise roughly 15-25% of the total thickness. The tuffs are primarily very fine to coarse, very thinly laminated and planar to ripple stratified. The beds consist of less than 2 0 % pyroxene, amphibole and biotite phenocrysts,and less than 1 0 % lithic fragments. Pyriboles are < 1 mm and biotites are ~ 2 mm in diameter. 50

The massive tuffs range from 2 to 15 cm thick and overlie laminated beds with

sharp, nonerosional contacts. These are very fine to coarse tuffs with biotite flakes from 1 fragments. The wood, preserved as calcite casts, ranges from 3-5 mm diameter and 2 to 5 cm long. Some fine grained massive beds are extensively bioturbated, and the thinly bedded tuffs exhibit various degrees of bioturbation.

Several lenticular, 10 to 15 cm thick, fluvially reworked tuffs are interbedded with the tephra beds discussed above. These tuffs are generally very poorly sorted with grains ranging from very coarse sand to small rounded lithic pebbles. They contain abundant

'pyriboles' and very little or no biotite. The 'pyriboles' are commonly abraded and account for ~25-35% of the grains. Sedimentary structures include sharp basal erosion surfaces, internal scours, small-scale trough crossbeds and normal grading.

Interbedded Sedimentary Rocks

The Kalodirr Tuffs contain minor interbedded conglomerates, sandstones and siltstones (Figure 10). Maximum clasts range from medium pebbles in the conglomeratic sandstones to small cobbles in the conglomerates. At section 31, the sandstones between the Kanukurinya and Alomonet tuffs contain rip up clasts of the underlying tuffs in addition to lithic grains and minor pyroxene and amphibole. The depositional structures are very

similar to those in the Basal Conglomerate Member. Small- to medium-scale trough crossbed sets in sandstones fine upward, whereas overall the cosets tend to coarsen upward. In a few instances, the sandstones grade to very sandy, dark reddish brown, lenticular mudstones that reach 30 cm in thickness and contain abundant calcareous root casts. In section 6 .1 the interbedded clastic layers coarsen upward from sandstones in the

lower part to conglomerates in the upper part. 5 1

Potassium-Argon Age Determinations

Potassium-argon age determinations on biotite and ampohibole phenocrysts separated from the Kanukurinya tuff beds yielded ages ranging from 17.3 ± 0.3 to 17.7 ±

0.3 Ma (Table 1). The majority of the dates are from samples collected at section 6.1 including Los 6 - 8 A, B and C, K86-2472 , K87-3107, and K87-3108. An additional date is from sample Los 3A from section 1.

Naserte Tuffs

The Naserte Tuffs are named for the Naserte River, a small southern tributary of the

Kalatum, which drains the area of the type locality. The type sequence consists of 40 cm of stratified tephra overlain by a 12 m thick lahar deposit (Figure 13). The best exposures of a complete sequence of the Naserte Tuffs is section 21 (Figure 3), which serves as the type locality but section 39 offers the best exposures of the tephra beds.

The interstratified, thinly to medium bedded, stratified and massive tuffs are pale yellow to pale orange. Red to dark red, orange and pale purple liesegang banding is common. The interstratified massive and laminated tuffs range from 2 to 40 cm thick.

Accretionary lapilli ranging from 2 mm to 2 cm in diameter account for 5 to 45% of individual massive beds (Figure 14). The accretionary lapilli are cored by fine ash or fragmented accretionary lapilli debris and rimmed with concentric bands of fine ash. Sand sized debris consists predominantly of fine to coarse, angular lithic grains and minor fine to coarse, euhedral to subhedral crystals of sanidine, biotite and amphibole. Altered, fine pumice lapilli occur rarely in a matrix of fine altered ash. Bed geometry is usually tabular, but changes laterally to irregular pinch and swell. The tuffs are bounded by sharp, nonerosional contacts. Massive tuffs dominate the sequence at section 39 where laminated beds occur only once. Sedimentary structures in the stratified beds are dominated by planar laminae with minor ripple stratification. Figure 13. Typical sequence of the Naserte Tuffs from sections 21 and 39. Section 21 Explanation

iM1230 Breccia deposits "Stratified tephra — Massive tephra ■—Accretionary lapilli Grading of lithic component ■ None ▼ Inverse

Section 39 54

Figure 14. Photograph of accretionary lapilli in airfall tuffs of the Naserte Tuffs near at section 39. 55

A lahar deposit overlies the tephra beds in most sections, and ranges from 40 cm to over 12 m thick (Figure 13). At section 39 the bed is a minimum of 8.5 m thick but is truncated by a normal fault. The deposit exhibits crude inverse grading with altered pumice clasts, up to small boulder size lying on the exposed upper surfaces of lahar deposits.

Smaller pumice clasts, 1 to 2 cm in long dimension, have been altered to a clay or zeolite and frequently weather out to produce a vesicular texture over the entire outcrop. Minor amounts of small, calcite casts of wood, and rounded clasts up to large cobbles occur throughout this bed. A discontinuous, massive, coarse granule to coarse pebble, clast supported breccia lies along the contact at sections 19 and 21. The breccia consists of angular phonolite and basalt clasts with imprints of these clasts in the underlying bed. This bed rarely exceeds 3 cm in thickness and overlies a sharp, basal surface.

Pumices in these lahar deposits contain abundant euhedral, sanidine phenocrysts and accessory euhedral amphibole and biotite phenocrysts. The acicular amphiboles have been tentatively identified as arfvedsonite. On the basis of phenocryst mineralogy, these tuffs are considered trachytic. The glass of larger pumice clasts has been altered to analcime, and the vesicles in the pumice are filled with calcite. The original matrix of these deposits has been entirely altered to analcime.

Fluid escape structures and compaction deformation features are very common in the stratified tephra beds. The fluid escape structures cut across 10 to 25 cm of interbedded stratified and massive beds (Figure 15). Compaction deformation, resulting from the deposition of the overlying lahar deposits, consists primarily of contorted bedding and an irregular pinch and swell bed geometry. Soft-sediment injections of underlying material into overlying lahar deposits are also common. At section 21, the lahar deposit contains a fragment of the slightly deformed stratified tephra about 1 m long and 15 to 25 cm thick. Figure 15. Photograph of fluid escape structures in airfall tuffs of the Naserte Tuffs near section 8 . The brunton compass is 7 cm wide. 57

Potassium-Argon Age Determinations

Potassium-argon age determinations on sanidine phenocrysts separated from pumices of the Naserte Tuffs yielded an average age of 16.8 + 0.2 Ma (Table 1). The samples, Los 4-23 B and K87-3468, were collected at sections 4 and 31.

Nakwel Esha Beds

The Nakwel Esha beds are important for correlation purposes, but are not well enough exposed to be formally defined as a member. This sequence is composed of stratified and massive tephra; and volcanic sandstones and conglomerates. The best exposures, albeit poor, occur at section 19, and consist of 13 m of tuffs overlain by a minimum of 8 m of conglomerates.

The tuffs of the Nakwel Esha beds are grey to pale yellowish grey and thinly to medium bedded with sharp, nonerosional basal contacts. Strata sets consist of 4 to 60 cm thick basal pumiceous conglomerates with normally graded pumice clasts. Elongate pumice lapilli ranging from 2 to 2 0 cm in long dimension lie parallel to bedding surfaces, and constitute up to 45% of some beds. Bed cosets fine vertically as massive beds grade to thinly laminated beds. A few stratified beds appear to have low-angle trough crossbedding. Coarse pumice lapilli are crudely graded (although widely scattered) with very large pumice lapilli in the higher, finer beds. The pumiceous conglomerates grade into dark brown, massive mudstones that contain abundant fine, altered pumice lapilli. Both the groundmass and phenocrysts of these pumices consist of alkali feldspar. The pumice lapilli are primarily yellow or pink and up to 4 cm long, but one deposit contains slate green pumice up to 20 cm long. Pink pumice bearing beds are most common near the top of this unit and can be correlated from the type section to section 3, and to similar beds in section

6 . 2 . Above the lower tephra layers of the Nakwel Esha beds lies a yellow, medium to coarse, thinly bedded, horizontally stratified sandstone with a scoured basal contact interpreted as reworked material from tephra and lahar deposits. The sandstone is overlain by a clast supported, phonolite pebble to cobble lag deposit with an irregularly scoured contact. The bed grades to a very coarse, poorly sorted trough cross bedded sandstone.

In section 19, the pink pumice beds are overlain by very coarse conglomerates, conglomeratic sandstones and sandstones. These sediments are moderately well-exposed along the east side of Lokipenata Ridge from the Kalakol-Lodwar road south to approximately section 4, and additional exposures occur at the top of section 19. These correlate with the very poorly exposed lahar deposit in section 21 (bed 53).

The base of the sedimentary sequence consists of yellow to pale yellow, matrix supported, medium to coarse pebble, clast supported conglomerates consisting of phonolite, minor amounts of basalt and abundant pumice. Sandstones contain discontinuous small cobble to medium boulder lenses overlying irregular, basal scour surfaces. These upward fining lenses are roughly 5 to 8 m long and up to 1 m thick composed predominantly of phonolite clasts. The yellow to pale yellow sandstones occasionally contain abundant altered pumice. Bedding structures are poorly preserved, but a few sandstones grade to dark brown, massive, very sandy mudstone.

Unassigned Strata of the Lower Lothidok Formation

Two principal stratigraphic intervals, one between the Kalodirr Tuffs and the

Naserte Tuffs and the second between the Naserte Tuffs and the Nakwel Esha beds, consist of heterogeneous clastic sedimentary and pyroclastic rocks. Because individual beds are laterally discontinuous, they cannot be correlated from one local section to another and are not assigned a specific stratigraphic rank. For this reason, only the principal 59 lithologic types of these intervals are given below. Detailed descriptions of these strata are given in the measured sections (Appendix B; Boschetto, 1988).

Sedimentary Rocks

Conglomerates average 1 m in thickness, and are moderately resistant so that they crop out as low ridges. Their colors include pale red to dark reddish brown, yellow to yellowish brown, and pale orange to brownish orange. In some exposures dark red conglomerates contrast strikingly with interbedded pale yellow tephra. They are similar in clast lithology, contact relationships, and bedding structures to the conglomerates described in the Basal Conglomerate Member and within the Kalodirr Tuffs. Etheria elliptica (fresh water oysters) mounds are found in situ near the base of some conglomerates in these intervals. These typically grade laterally and vertically to sandstones.

Sandstones generally form low outcrops, but a few form small ridges. Colors are simlar to those of the conglomerates described above, but a few are reddish gray, grey, or pale olive. The variety of colors results from varying amounts of tephra reworked into the sandstones. They range from litharenites to feldspathic litharenites (McBride, 1963; Folk,

1968), and are cemeted with calcite, or, less commonly, hematite. These rocks grade laterally to either coarser or finer sedimentary rock, and also interfinger with other sedimentary beds. Individual beds range from 20 cm to 2.5 m thick, averaging less than 1 m. Most beds are primarily irregular in form (rarely tabular), and basal contacts are predominantly gradational. Where conglomeratic, the clast lithology is the same as that of associated conglomerates.

Sedimentary structures in the sandstones are dominated by medium- to small-scale trough crossbeds and internal scour and fill sequences, the latter generally overlain by thin, discontinuous, pebble-grade conglomerates. The beds are normally or inversely graded with cosets that primarily fine upward. Irregular, subhorizontal erosional surfaces 60 commonly cut across subordinate bed boundaries. Less common sedimentary structures include low-angle, planar and epsilon cross-stratification, horizontal stratification, and ripple stratification. Some finer-grained sandstones contain downward branching traces of irregular diameter filled with a white, chalky calcareous material that are interpreted as rootcasts. Vertical traces of constant diameter with a meniscate backfill of darker and finer grained sedimentary material also occur and are interpreted as burrows.

Siltstones are most common in southern exposures of the lower Lothidok

Formation (sections 10.1, 10.3 and 21). Outcrops of these pale red, reddish tan, and pale yellowish tan strata are generally low but also occur as interbeds in cliff forming, upward fining sequences. They resemble altered tephra deposits, but are distinguished by associated clastic sedimentary rocks and lack of euhedral volcanic mineral grains.

The siltstones are moderately to well-consolidated, average 60 cm thick, and are usually lenticular with gradational lower and upper contacts. These are thinly laminated to thinly bedded, or planar to ripple stratified. Some appear massive, perhaps as the result of extensive bioturbation. Vertically and occasionally laterally, the siltstones grade to silty mudstones. Flattened pumice clasts are abundant in these beds, and resemble rip-up clasts.

Lenticular (rarely tabular), dark brown to reddish brown, or tan to yellowish brown mudstones crop out poorly. These commonly interfinger with, or are interbedded with coarse, clastic sedimentary rocks. They are generally massive, but some exhibit ripple stratification or lamination. Basal contacts are either sharp or gradational, whereas upper contacts are generally sharp and erosional. White, chalky, calcareous root casts and thin tabular calcretes are common.

Tan to dark brown, waxy mudstones commonly overlie tephra beds. These contain angular to subrounded lithic fragments, volcanic mineral grains (sanidine, biotite) and altered pumice lapilli similar in petrology and abundance to the underlying tephra. Fine- to 6 1 very coarse-grained sand occurs in these beds as thin lenses or scattered throughout.

Lighter colored, flattened pumice lapilli give the appearance of intraclasts.

Lahar deposit outcrops range from well-defined ridges to unconsolidated piles of angular boulders. Beds range from 30 cm to 15 m thick, and are thickest and are more numerous in the southern part of the Lothidok Range. The lahar deposits commonly overlie tephra beds, with sharp, nonerosional basal contacts. They are massive to inversely graded and contain 10 to 35 % granules to large boulders (up to 2.5 m diameter) in a yellow clay to sand size matrix. Phonolites, coarsely porphyritic and aphyric basalts, altered pumice, and indurated blocks of tephra are the prevalent clast lithologies. The pumice clasts range from 10 to 30 cm in length and 2 to 10 cm in diameter, and have been altered to zeolites with calcite-filled vesicles. Leaf impressions and calcite casts of wood fragments a few centimeters long and a few millimeters across are common constituents of these deposits.

Pyroclastic Rocks

Numerically, these are the most frequently occurring beds, although they constitute less than 20% of the thickness of strata between defined members. The pyroclastic rocks encompass two petrologically distinct types, one that is mafic-alkaline, and another that is trachytic. The mafic alkaline tephra are confined to the lower levels of the lower Lothidok

Formation and consist primarily of the Kalodirr Tuffs. Higher in the section, the pyroclastic rocks consist of trachytic tuffs that can only be broadly correlated by stratigraphic position. Further study based on detailed stratigraphy, clast petrology and alteration geochemistry may provide stronger correlations

Directly above the Kalodirr Tuffs, is a sequence of massive, pale red, fine tuffs that have been altered to a zeolite or montmorillonite. The tuffs contain less than 10% euhedral amphibole and pyroxene phenocrysts and 15-25% medium to coarse, angular to subrounded lithic grains. Basal contacts are sharp, but do not appear to be erosional.

All pyroclastic deposits higher in the section are stratified and massive trachytic tuffs commonly interbedded on a centimetric to decimetric scale. These tuffs are pale yellow, tan to yellowish brown, and yellowish grey, which makes them very distinct visually. Cumulative bed thicknesses average 10 to 30 cm and rarely exceed 2 m. The best exposures of these trachytic tephra are at sections 18 and 19 (Figure 3).

The stratified tephra layers are thinly to thickly laminated with fine to coarse, planar to ripple stratification. Trough cross beds occur in some of these stratified tuffs. The lower contacts are sharp and nonerosional, and the basal strata commonly drape small topographic irregularities.

Massive tephra layers occur as thin to thick, tabular interbeds between well- stratified tephra layers. Accretionary lapilli and pumice lapilli are commonly associated with these massive beds. The accretionary lapilli range from 2 to 4 mm in diameter and may account for 15 to 35% of some beds.

Very fine to fine euhedral amphibole, sodic sanidine, biotite and angular lithic grains comprise up to 20% of these tuffs. The matrix is now composed of either analcime or montmorillonite believed to represent altered volcanic ash. Very fine to coarse pumice lapilli comprise up to 50% of the tephra. The original glass of the pumice has been altered to clays and/or zeolites, and the pumice clasts are generally flattened and confined 1 to 5 cm thick layers. Calcite fills the vesicles of nonflattened pumice, which is recognizable only by the ghost outlines of the original glass. Pumice replaced by clays and zeolites is recognised by the presence of volcanogenic minerals and/or distinct color differences wtih the matrix. Crude normal grading of lithic fragments and inverse grading of pumice lapilli is common, but symmetric (normal-inverse-normal or inverse-normal-inverse) grading also occurs. 63

Asymmetric ripple cross-stratification with indices of about 0.6 is seen in some

tuffs, but at two locations (sections 1 and 31) the ripples appear to be symmetric. Planar

and trough cross-stratification, and soft sediment deformation also occurs, but individual

thin beds and thick laminae appear structureless. Raindrop imprints and desiccation cracks

occur on the upper surfaces of fine tuffs. Contorted and convolute bedding formed by

compaction deformation, and water escape structures are common in these beds. The tuffs

also contain well-preserved imprints of grass and palm leaves (Figure 16 ).

Several types of burrows are commonly well-preserved in the yellow tuffs that have

alternate laminated (1-8 cm) and massive (3-8 cm) layers. Vertical, meniscate filled

burrows of uniform diameter up to 8 mm across and 8 cm long (Figure 17) typically cut across two to three layers of sediment. These are usually simple, but some are branched, and are best observed in the laminated strata. Similar burrows oriented horizontally are also present. Other trace fossils consist of irregular networks of variable diameter, and horizontal to subvertical tunnels and shafts. The organism(s) that produced the traces is unknown.

Fluvially reworked tephra (volcanic sandstones and conglomerates) overlie bedded tephra deposits in many sections. These consist of an altered ash matrix with fine to coarse, rounded to subrounded lithic grains that are petrologically similar to underlying or correlative beds. These beds are lenticular, and commonly have small- to medium-scale trough crossbeds, sharp basal scours, rip-up clasts, abraded volcanic phenocrysts and fine to very coarse pumice pebbles. Pumice is most abundant at the top of reworked deposits where it may constitute as much as 85% of a bed. These beds commonly fine upward to ripple stratified fine sandstones and silts tones. 64

Figure 16. Photograph of a palm leaf imprint in an unassigned trachytic tuff at section 27 (photo by F. H. Brown). 65

Figure 17. Photograph of a burrow in an unassigned trachytic tuff taken near section 19 (photo by F. H. Brown). 66

Paleocurrents - Lower Lothidok Formation

Paleocurrent measurements from the Basal Conglomerate Member (Figure 18)

indicate sediment transport was to the southwest (means = 230° and 237°). The

measurements were taken from the upper 30 m of outcrops near section 1 in the north and

from two outcrops within the lower 25 m of section 20 in the south.

Additional measurements on strata between the Kalodirr and Naserte Tuffs indicate

a south (mean = 172°) to southwest (mean = 245°) sediment transport direction (Figure

19). In the north, groups of measurements were taken from five beds in section 1.2 over a

total vertical thickness of approximately 50 m. Data for the southern part of the region

were collected at one outcrop in section 10.1 from three continuous sequences of sediments

over a stratigraphic interval of roughly 2 0 m.

Upper Lothidok Formation - Tlu

The upper Lothidok Formation is best exposed south of the Kalatum and Alomonet

Rivers. Additional exposures exist in the Nakwel Esha graben (see structure discussion, p.

126), along the Nakwel Esha River, and west of Lokipenata Ridge. There are no complete, well-exposed, continuous sections of the upper Lothidok Formation requiring the use of four sections to construct the sequence. Four marker horizons, the Lokipenata conglomerate, the Kalatum basalt, the Akwang'a basalt, and the Kamurunyang lahar

(Figure 9), are discussed in detail and the unassigned strata between these levels are also briefly addressed.

Lokipenata Conglomerate fNew Informal Name')

The Lokipenata conglomerate, best exposed along the east side of Lokipenata

Ridge, forms the base of the upper Lothidok Formation. Exposures of this conglomerate extend from the north side of the Kalakol-Lodwar road south to section 4, and also Figure 18. Paleocurrent rose diagrams for the Basal Conglomerate Member of the lower Lothidok Formation for data sets from near section 1 and from section 20. All data were corrected for tectonic tilt. VV>' 68 69

Figure 19. Paleocurrent rose diagrams for unassigned strata of the lower LothidokFormation for data sets from multiple outcrops. All were corrected for tectonic tilt. - 3°30'N - • N 3°I0 tA V'J.' V» 3°20'N Lothidok 0 Range 5 km 35°50' E 35°50' l 1 Mean direction direction Mean f eiet transport sediment of ubr of Mumber measurements N- 36°0‘E

1 70 7 1 occur at the top of section 19. The unit is poorly exposed in the road cut through

Lokipenata Ridge and at the tops of sections 21, 30 and 31 (Figure 3).

The Lokipenata conglomerate is a minimum of 40 m thick at section 30 but is not well enough exposed at other sections to determine thicknesses for comparison. The presence of the conglomerate at section 19 and along the Lokipenata Ridge suggests reasonable lateral continuity, but the overall bed geometry is indeterminate. The lower contact is a broad, irregular erosional surface with intermittent deep, narrow scours.

The conglomerate consists of very coarse pebbles to very coarse cobbles in clast

(rarely matrix) support, and is pale red to dark red, dark reddish grey, or reddish orange.

Clasts are imbricated and crudely graded, extremely weathered, and typically stained with^ hematite. Rip-up clasts from the underlying bed are common. The matrix consists of very fine to very coarse, subrounded to subangular sand grains of quartz, pyroxene and minor potassium feldspar. This marks the first occurrence of significant amounts of basement material in the section. Soil carbonates are common near the base of the contact.

Kalatum Basalt ("New Informal Unit)

The Kalatum basalt consists of at least two and possibly three flows with a minimum total thickness of 20 m. This basalt is exposed in a series of small hills west of and parallel to the southern end of Lokipenata Ridge (sections 3 and 4), and also farther south where the basalt is offset by small normal faults. Small exposures occur as hillocks in the stream bed of the Kalatum River. In section 30, the Kalatum basalt lies approximately 15 m above the highest exposure of the Lokipenata conglomerate. The thickest exposures lie just north of Ngaletiti Hill (Figure 3), where the basalt is truncated by a normal fault.

The lowest part of the Kalatum basalt is an agglomerate (12 m) with blocks up to 2 m in diameter consisting of plagioclase phyric basalt in a green, aphanitic groundmass. 72

The basal agglomerate is overlain by a thin, green, aphyric flow (8 m) that is either overlain by, or gradational into, a coarse plagioclase phyric flow with a similar groundmass. The porphyritic flow contains coarse plagioclase and amphibole phenocrysts up to a few millimeters long.

Potassium-Argon Age Determinations

Potassium-argon age determinations on plagioclase phenocrysts separated from the basalt matrix of the Kalatum basalts yielded an average age of 13.6 ± 0.2 Ma (Table 1).

The samples, K86-2743, were collected near section 4.

Akwang'a Basalt (New Informal Name)

The Akwang'a basalt, interpreted to he above the level of the Kalatum Basalt, crops out only in sections 24 and 25 (Figure 3). It is extremely weathered and poorly exposed.

In outcrop the basalt is dark grey to olive grey and is coarsely porphyritic with olivine, plagioclase, and pyroxene phenocrysts. Calcite amygdules and cavities filled with euhedral calcite crystals reach 3 cm in diameter. Other cavities are lined or filled with analcime crystals.

Kamurunvang Lahar (New Informal Name)

The Kamurunyang lahar forms a continuous yellowish brown ridge of east dipping strata at the type locality (sections 24 and 25). Blocks over 4 m in diameter have weathered from this ridge and litter the scree slope to the west. This bed is here informally named the

Kamurunyang lahar after Kamurunyang Hill which lies between sections 24 and 35. At sections 8 and 29, the lahar forms a small continuous ridge. At section 35 the lahar also forms a ridge but is partly buried by recent alluvium derived from the overlying Loperi 73 basalts. The correlations between the type locality and sections 8 , 29 and 35 are not completely secure (see below) and should be used with caution.

The Kamurunyang lahar consists of 15 to 40 % pebble to boulder size clasts, of predominantly phonolite but with minor basalt and indurated lahars, in a matrix presently consisting of analcime. Lensoidal, clast supported conglomerates occur above sharp, nonerosional, basal contacts and as discrete lenses within these beds. These exhibit no obvious vertical or lateral gradation. The lahar deposits also contain abundant pumice clasts up to 20 cm in length and 8 cm in diameter weathering from the upper surface. The pumices are very similar in mineralogy and alteration to the lahars from which they are derived. Smaller pumices have altered to clay and weather to producing a vessicular texture of the outcrop.

Correlation of Lahar Deposits

Correlation of the lahar deposits of the upper Lothidok Formation is important because they overlie fossil localities (see fauna discussion, p. 142) and pumice clasts within them are the only material available for potassium-argon age determinations. These deposits, however, are virtually indistinguishable on the basis of outcrop morphology, rock texture, pumice petrology, or alteration geochemistry, and the lateral relationships are therefore problematic. The lack of associated airfall tephra and proximal marker horizons requires the correlations be based on the stratigraphic position of the deposit relative to the upper boundary of the Lothidok Formation (the base of the Loperi basalts). Because this contact is an angular unconformity (see below), its level is difficult to determine and these correlations are tenuous.

Here it is proposed to correlate the lowest lahar deposit at sections 24 and 25 with the thick lahar deposits between 40 to 60 m below the Loperi basalts in sections 8 , 29 and

35. Because section 25 contains three lahars, one might correlate the upper lahar of section 74

25 with the only lahar in section 35. However, the similarities of outcrop morphology, underlying strata and internal geometry indicate that the lower lahar in section 25 is equivalent to the only lahar in section 35. The small distance between these sections requires that the upper two lahars of section 25 were removed by erosion in section 35 prior to deposition of the Loperi basalts. Taking the correlation further, the lahar of section

35 might be equivalent to eroded remnants of a similar deposit in sections 26 and 27.

Problems with correlations between the type locality and sections 8 and 29 result mainly from isotopic age determinations, which are addressed below.

Potassium-Argon Age Determinations

Potassium-argon ages were determined for sanidine phenocrysts separated from pumice of the Kamurunyang lahar deposits at the type locality (section 25) and the tentatively correlative lahar deposit below the Loperi Hill (sections 8 and 29). Application of these ages to the deposits, however, must be made with caution, as the ages represent the time of eruption of the volcanogenic material and not necessarily the time of lahar deposition. One sample (K87-3411; section 25) yielded an age of 13.1 ± 0.2 Ma, but two other samples (K87-3437 B and K87-3437 C+D; section 29), yielded ages of 13.4 ± 0.2 and 14.0 + 0.2 Ma respectively (Table 1). Although the Kalatum basalt and the lahar deposit (Kamurunyang?) do not crop out in a continuous section, the basalt correlates to a position well below the lahar deposit. The age on the basalt (13.6 ± 0.2 Ma), determined from a plagioclase separate, is believed reliable and thus the older age of 14.0 Ma on the lahar appears discrepant. This can be accounted for by either a small amount of contamination in the sanidine separate or the pumice was from an earlier eruption. 75

TTnassigncd Strata - Upper Lothidok Formation

Three stratigraphic intervals, one between the Kalatum and Akwang'a basalts, one between the Akwang'a basalt and the Kamaurunyang lahar, and one between the

Kamaurunyang lahar and the upper contact of the Lothidok Formation (Figure 9) consist of heterogeneous clastic sedimentary and pyroclastic rocks that are not assigned specific stratigraphic rank. In most respects these strata are essentially the same as those of the lower Lothidok Formation, and therefore only the most distinctive characteristics of the strata will be discussed. Detailed descriptions of these strata are given in the measured sections (Appendix B; Boschetto, 1988).

Clastic Sedimentary Rocks

The conglomerates are best represented in the lower part of this interval, whereas sandstones and siltstones dominate the upper part. The sedimentary deposits typically fine upwards as does the upper Lothidok Formation.

The red to pale red, reddish grey, or white conglomerates of this interval are 2 to 8 m thick. These form discontinuous basal deposits in upward-fming sequences and discrete beds within finer sedimenatry strata that are bounded by sharp upper and lower contacts.

Clasts range from coarse pebbles to large cobbles composed primarily of phonolites and basalts. Quartz and perthite clasts are common only near the top of the section. Some beds contain boulders of indurated tephra and lahar deposits altered to analcime and montmorillonite as are the in situ beds of this type. The matrix of these rocks consists of very fine sand to coarse granules of volcanic and basement detritus. The amount of basement detritus increases from less than 10% to -60% upwards in the section. Etheria elliptica. mounds lie near the base of some conglomerates.

The 2 to 8 m thick sandstones are tan to grey, pale red and pale reddish tan, and occur in upward-fining sequences grading into very fine sandstones and siltstones. The 76 moderately to well-sorted sandstones consist of subrounded to subangular, fine to medium grains. Some sandstones are conglomeratic typically with fine to medium pebbles, but locally with small boulders.

Petrologically the sandstones of the upper Lothidok Formation are quite distinct from those of the lower Lothidok Formation. The quartz component of the sand fraction increases upwards in the section and constitutes 25 to 80% of the higher beds. Perthite occurs infrequently at the base of the interval but increases to about 30% upwards in the section. The general increase in abundance of basement detrital component upwards in the section is reflected by feldspathic litharenites near the base of the section, but lithic subarkoses, subarkoses, and occasional arkoses higher in the section. Amphibole and biotite grains vary from 5 to 15% of the total rock and volcanic rock fragments range from

5 to 75%. In contrast with the lower strata, epsilon crossbedding and climbing ripples are common sedimentary structures in the upper Lothidok Formation.

Tan, reddish tan, or reddish grey, 50 cm to 2 m thick siltstones are common.

These are commonly bounded by gradational contacts, and constitute 35 to 45 % of the upward-fining sequences in the upper part of the section. Horizontal and ripple stratification is well-preserved, and climbing ripples are common. Thinly interbedded massive and laminated layers are similar to those in the fine-grained sandstones in which the massive layers are interpreted to result from bioturbation. The siltstones interfinger with and are overlain by tan to brown, moderately to very sandy mudstones.

Dark brown and tan to greyish brown mudstones commonly occur at the top of upward-fining sequences. Laterally continuous mudstones generally have gradational lower and upper contacts, but lenticular mudstones have sharp basal contacts and interfmger with crossbedded sandstones and laminated siltstones. The massive, lenticular mudstones contain abundant very fine- to fine-grained angular sand. Pedogenic features including calcretes and calcareous root casts are common. 77

The numerous lahar deposits in the upper Lothidok Formation are virtually indistinguishable from the Kamurunyang lahar. These ranging from 50 cm to 15 m thick with at least three over 8 m thick. All three of the thickest deposits crop out only at section

25 where the lowest bed forms a pronounced ridge, and the upper two beds crop out very poorly in the riverbed to the east. Thick lahars deposits are also exposed in sections 8 , 24,

26, 29, and 35 (Figure 3). Sections 8 and 24 contain two such deposits while sections 26,

29 and 35 each have only one. These deposits form small, indistinct ridges at each section.

Section 27 contains boulders up to 4 m in length that appear to be remnants of an eroded lahar deposit. In contrast to similar deposits in the lower Lothidok Formation none is associated with tephra beds.

Pyroclastic Rocks

Tephra layers in the upper Lothidok Formation are virtually indistinguishable from those below. Most are interbedded with tan to brown, massive mudstones, but these crop out poorly and the relationship is unclear.

The uppermost exposed beds of the Lothidok Formation (< 5 m) consist of extensively weathered massive and stratified tuffs with interbedded mudstones. Thin soil carbonate horizons are common, and the beds contain abundant, small, altered pumice lapilli. The contact between these tuffs and the Loperi basalts is buried by basalt talus. The extreme weathering at this level may be the result of prolonged exposure with consequent pedogenesis.

Paleocurrents - Upper Lothidok Formation

Paleocurrent measurements from the Lokipenata conglomerate from imbricated clasts at sections 3 and 19 (Figure 20) imply sediment transport was to the east (means =

87° and 90°). Measurements from imbricated clasts and ripple stratification in unassigned 78

Figure 20. Paleocurrent rose diagrams for the Lokipenata conglomerate of the upper Lothidok Formation for data sets from sections 3 and 19. All data were corrected for tectonic tilt. 33’N 3°30’ -

3°I0'N VWW VOWV-* 4 0 35°50'E 1 — 1 Mean direction direction Mean f eiet transport sediment of ’ measurements of Location * * ubr of Number - measurements A/ = r~ — 36°0‘E

1 79 80 strata of sections 26 and 29 (Figure 21) also indicate sediment transport to the east (means

= 71° and 60°).

Loperi Basalts - Tib

In its southern exposures the Lothidok Formation is overlain by basalt flows (Tib -

Plate; Tvb2 of Walsh and Dodson, 1969), which are herein named the Loperi basalts

(informal) after Loperi Hill, which has the thickest exposures. These are well-exposed throughout the southern part of the Lothidok Range, and form most of the lava capped hills south of the Kalatum/Alomonet River. Exposures north of the Kalatum and Alomonet

Rivers are limited to Moruorot and Esha Hills, and a progressively thinning ridge west of

Lokipenata Ridge that terminates in small piles of basalt boulders. The Loperi basalts thin to the east, north and west (Figures 7 and 8 ), but thickness variation to the south is difficult to establish due to extensive faulting and poor exposures. Farther south, the basalt is buried by Quaternary deposits (Qal). These basalts are best exposed at Loperi Hill where they consist of three flows totalling a maximum of 121 m in thickness. Only one flow occurs at Ngaletiti and Esha Hills where it is about 30 m and 50 m thick, respectively. At

Moruorot Hill the flow is about 5 m thick. The pronounced thinning to the northeast may have resulted when the flow encountered topographic relief such as a channel margin.

The Loperi basalts contain minor (1 to 4 m) interbedded sedimentary and pyroclastic strata. These are only exposed along the north face of Loperi Hill where it is cut by the Kalatum River and were not examined in detail. The very poor exposures consist of very coarse conglomerates, sandstones, and extremely altered tephra.

The basalts are predominantly olivine-augite-plagioclase phyric. Titaniferous augite phenocrysts occasionally reach 3 cm diameter, and are strongly zoned. Plagioclase (A1145.

70) phenocrysts are rarely zoned. Olivine phenocrysts are occasionally rimmed with iddingsite. The groundmass consists of plagioclase, olivine, clinopyroxene, iron-titanium 8 1

Figure 21. Paleocurrent rose diagrams for unassigned strata of the upper Lothidok Formation for data sets from sections 27 and 29. All data were corrected for tectonic tilt. 82

1 3 5°5 0‘ E

- 3 °30 'N

Lothidok Range

3°20'N

3°I0‘N Mean direction of sediment transport 83 oxides (magnetite?), minor biotite and apatite, and possibly analcite. Partial alteration of both the groundmass and phenocryst olivine to chlorite is common. Calcite amygdules are com m on in some flows, and calcite also fills voids and fractures.

Columnar jointing, roughly parallel to dip, occurs at or less than 8 m above the unexposed sediment-basalt contact at Moruorot Hill. Excellent exposures of this jointing exist along the eastern and southern flanks of the hill.

Potassium-Argon Age Determinations

Samples of the Loperi collected from the top of Moruorot Hill (B-20) and from

Loperi Hill (B-31) yielded ages of 12.0 + 0.1 and 10.9 ±0.1 Ma respectively (Table 1).

Although these determinations are technically good, they probably represent minimum ages.

There is only one published isotopic age for the Loperi basalts. Zanettin, et al.

(1983) give an isotopic age of 14.6 ± 1.0 Ma for a basalt in this area but the sample location is unclear. The coordinates given for the sample (K64) place it between Lodwar and the Lothidok Hills where there are no exposures of basalt, but Zanettin et al.(1983) map the basalt sample as the Turkana basalt and indicates that the sample was taken near

Moruorot Hill (approximately 10 km north of the coordinates given).

Undifferentiated Tertiary Deposits - Tu

Tertiary deposits overlie the Loperi basalts in the south and the Lothidok Formation in the northern part of the Lothidok Range but were treated informally for this report.

Similar strata are poorly exposed above the Loperi basalts on Loperi, Esha, and

Konukuangna Hills, west Lokipenata Ridge and within the Kalodirr graben. The maximum measured thickness of these strata is 60 m in section 29, but the lowest beds are not exposed. Above Loperi and Esha Hills these beds consist of pebble and boulder 84 volcaniclastic conglomerates. At Ngaletiti and Kakurtua Hills the sediments directly overlying the Loperi basalts are quartz/perthite granule to pebble conglomerates and very coarse, poorly sorted arkosic sandstones.

The conglomerates overlying Loperi Hill are very poorly exposed on the north face of Loperi Hill, where massive conglomerate beds are at least 15 m thick. A small fault block south of the Alomonet River roughly 3 km south of Moruorot Hill also has very poor exposures of these sedimentary rocks overlying basalts.

The lowest decent exposures of undifferentiated Tertiary strata in a continuous section lie about 30 m above the Loperi basalts at section 29. Some of the best exposures are found in the cliff on the Kalodirr River (section 15) although their level within the

Tertiary section cannot be determined with confidence.

The deposits of the undiferrentiated Tertiary strata are dominated by very fine- to coarse-grained, poorly sorted to conglomeratic sandstones. These deposits are grey to pale reddish grey, orange, pale red, and white to grey lithic subarkoses to subarkoses with minor volcanic clasts with fine granule to very coarse pebble clasts. The sandstones commonly grade inversely to massive conglomerates with abundant tabular calcrete? horizons. Exposures of arkoses and subarkoses east of Moruorot Hill cannot be placed in the section

Sedimentary structures in the undifferentiated Tertiary strata include small- to medium-scale trough crossbedding, basal and internal scours, and minor planar stratification. These structures are generally poorly defined and are commonly disrupted, possibly from bioturbation. Laminated and disrupted beds contain uniform, 4 to 8 mm diameter, straight burrows filled with hematite-stained, muddy sandstone.

Except for minor interbedded volcaniclastic sediments, the conglomerates consist of granules to large pebbles of quartz and K-spar (perthite) with a subarkosic matrix. These grade into, or are equivalent to, the sediments overlying Ngaletiti and Kakurtua Hills. The 85 poorly consolidated matrix of these sediments weathers to leave unconsolidated sheets of quartz and perthite clasts. These cover much of the area surrounding the Lothidok Range including that east of Kakurtua Hill, and the area west of the small basalt outcrops west of

Lokipenata Ridge.

Sandstones grade vertically to tabular, sandy, massive mudstones up to 2 m thick.

The section as a whole fines upwards and lenticular mudstones become more abundant.

These mudstones are commonly interbedded with and interfinger with the sandstones, occasionally dominating the sequence. Siltstones occur in gradational sequences but account for little of the section. Pedogenic features such as calcareous root casts, blocky fabric, and tabular calcrete? in the thick, massive mudstones are common.

The undifferentiated Tertiary sediments are interpreted to directly overlie the

Lothidok Formation at all locations beyond the extent of the Loperi basalts, but the relationship is not clearly displayed. Only in section 6.2, where the contact is not well- defined, can this relationship be seen.

Paleocurrents - Undifferentiated Tertiary Deposits

Paleocurrent measurements (Figure 22) from imbricated clasts and trough crossbeds imply a sediment transport direction to the east (mean = 55°). Most of these measurements are from a small outcrop along the Kalodirr River. 86

Figure 22. Paleocurrent rose diagrams for undifferentiated Tertiary strata taken along the Kalodirr River. All data were corrected for tectonic tilt. ----- 1----- 35°50'E 36°0'E

- 3 °30 'N

Location of measurements N = Number of measurements -3°I0'N Mean direction of sediment transport INTERPRETATION OF DEPOSITIONAL ENVIRONMENTS

Introduction

Descriptions of the strata in the preceding section show that only a few sedimentary rock types are represented in the Lothidok Formation and that these lithologies occur in distinct associations. Consideration of sequential and lateral relations of genetically related lithologic types leads to a better understanding of the conditions under which these rocks were deposited. The lack of marine fossils and laminated fine sedimentary rocks coupled with the presence of terrestrial vertebrate and plant remains indicates the rocks were deposited in alluvial and fluvial systems.

Numerous studies have addressed fluvial depositional systems. In this study, lithofacies codes (Miall, 1978; Mathisen and Vondra, 1983) are used to define equivalent lithologies of the Lothidok Formation to help interpret depositional environments. The code system, introduced by Miall (1978), consists of a capital letter designation for the dominant grain size followed by one or two lower case letters that refer to principal sedimentary structures. Miall (1978) assigned codes to lithofacies of braided streams and

Mathisen and Vondra (1983) assigned codes for meandering stream and pyroclastic deposits. For comparison, their fades codes are given for equivalent lithologies of the

Lothidok Range (Table 2).

Interpretations of the depositional environments for the sedimentary rocks of the

Lothidok Range follow each discussion addressing lithologic associations. Detailed analysis of depositional environments proved difficult because of poorly exposed and laterally limited outcrops. The lack of any three-dimensional or decent two-dimensional exposures precludes defining the external geometry of the clastic sedimentary bodies. The 89

Table 2. Lithologic types of the Lothidok Formation, equivalent lithofacies codes, prominant sedimentary structures and interpretations of depositional settings.

Lithology Lithofacies Sedimentaiy Interpretation Code structures massive, matrix Gmsa none to crude debris flow supported gravel. inverse grading massive or crudely Gma horizontal bedding, longitudinal bar, lag or bedded gravel imbrication sieve deposits stratified gravel Gt3 medium to large scale minor channel fill trough crossbeds stratified gravel Gpa planar crossbeds lingoid bars growths from older bars sand, medium to Sta solitary (theta) to grouped dunes (lower flow regime) very coarse/pebbly (Pi) trough crossbeds sand, medium to sPa solitary (alpha) to grouped transverse bars, sand very coarse/pebbly (omikron) trough crossoeds upper flow regime) sand, very fine Sr3 Ripple marks,small-scale ripples (lower flow to coarse trough crossbeds regime) sand, silt, very fine to Sha parallel laminations planar bed flows (lower very coarse/pebbly planar bedding and upper flow regime) sand, fine to Sla low angle (<10) scour fills,antidunes medium grained trough crossbeds sand, fine to coarse Sea crude crossbedding scour fills with intraclasts erosional scours sand, fine to Ssa broad, shallow scour fills coarse/pebbly scours sand, fine grained Sc13 climbing ripple laminations, crevasse splay lenticular may appear massive coarse/pebbly sand, silt, mud Fia laminated, very small overbank or waning ripples climbing ripples flood mud, silt Fma massive overbank or drape deposits a Lithofacies defined by Miall (1978). b Lithofacies defined by Mathisen and Vondra (1983). 90 depositional environments of the sediments of the Lothidok Range are therefore interpreted from internal geometry, nature of contacts and scale of bedding observable in vertical sequences.

Allen (1983) and Miall (1985) have recently addressed problems with deciphering the nature of depositional systems from vertical sequences alone. Additional work in the

Lothidok Range may provide a more comprehensive analysis of the depositional systems of the Lothidok Formation.

Lithologic Associations

Sedimentary strata in the Lothidok Range may be grouped into two distinct lithologic associations. These are named for their characteristic lithologies with the dominant lithology listed first followed by subordinate lithologies. Acronyms are used to refer to each lithologic association.

The most common lithologic association in these rocks consists of clast or matrix supported conglomerates, trough cross-bedded sandstones, and minor beds of finer grained rocks. This normally graded assemblage is dominated by conglomerates with subordinate finer grained rocks. The acronym used for this association is CSM

(conglomerate-sandstone-mudstone). The second lithologic association in the Lothidok

Formation consists of well-sorted, trough cross-bedded sandstones, laminated siltstones and mudstones. This assemblage differs from the first mainly in the lack or paucity of conglomerates. Laminated siltstones and massive mudstones are subordinate lithologies.

The acronym used for this association is SSM (sandstone-siltstone-mudstone).

The sedimentary rocks within the Kalakol basalts and those of the lower Lothidok

Formation belong primarily to the CSM association with only minor occurrences of the

SSM association. The base of the upper Lothidok Formation consists of rocks primarily of the CSM association, but rocks of the upper part of the unit and the Tertiary undifferentiated strata belong to the CSM association.

CSM Association

The CSM association (Figure 23) consists of upward fining sequences ranging from conglomerates to mudstones that generally, but not always, contain each of the lithologies. Partial assemblages are too rare as distinct deposits to warrant establishing new associations.

Clast-supported, massive to crudely bedded conglomerates occur primarily at the base of the association and also as discrete interbeds bounded by sharp contacts. Massive bedding commonly grades vertically into medium- to large-scale trough crossbeds.

Conglomerates also occur as thin basal lenses, and as lenses interbedded within sandstones. These grade laterally and vertically into poorly sorted coarse grained or conglomeratic sandstones that are second in abundance to the conglomerates. The sandstones contain deep internal and broad basal scour surfaces, irregular, subhorizontal erosional surfaces, crude low-angle, medium- to small-scale trough and planar cross­ stratification and rare horizontal stratification. Epsilon crossbedding (representing lateral accretion surfaces) is very rare but occurs at local sections. Cosets of cross-strata contain normally and inversely graded strata sets. The coarse-grained sandstones typically grade vertically into finer grained strata.

Fine-grained sandstones and siltstones are ripple and planar stratified. Massive mudstones occur as interbeds with either sharp or gradational lower contacts, and with sharp, erosional upper contacts. These fine-grained rocks account for an extremely variable amount of this lithofacies. In northern exposures, they constitute less than 15% of the section, but some sequences in the south consist of up to 40% siltstones and mudstones. 92

Figure 23. Typical sequence of the Conglomerate-Sandstone-Mudstone (CSM) lithologic association. Facies codes are given in Table 2. 93

a> o Nature of Contact a>c Dominant Lithology (fades codes) k_w Sediment Structures - Interpretation 3 Vertical Thickness

sharp, rarely gradational Siltstone (Fm,FI) 0-1 rare burrow, lam scrams rare climbing ripples - crevasse splay f gradational

Sandstone (St, Sh, S r, Ss, Se) rare ripple stratification - lower flow regime small trough crossbeds - lower flow regime

planar beds - upper flow regime

1-6 very basal conglomerates over internal scours common - channel lag deposits medium trough crossbeds - dunes (lower flow regime) interbedded conglomerates - channel lag deposits conglomeratic

gradational and erosiona/ Conglomerate (6m, Gt)

medium trough crossbeds - channel fill deposits

internal scours 1-4 very common large trough crossbeds - channel fill deposits

gradational massive to crude normal grading, imbricated 0-1 common clasts- longitudinal bar and channel lag deposits

Particle grade 94

Interpretation

The CSM association closely resembles facies interpreted as braided stream deposits on alluvial fan complexes (Rust, 1978; Mathisen and Vondra, 1983). The sedimentary structures and dominance of coarse grain sizes of the CSM association indicate high energy stream deposition. The wide range of particle size implies substantial energy level fluctuations. These characteristics are commonly associated with braided streams. In such streams massive, clast supported, imbricated conglomerates are deposited as channel fills or as longitudinal or diagonal bars. Trough crossbedded sediments form during longitudinal bar migration; horizontally stratified and low-angle crossbedded sediments represent shallow scour fills (Rust, 1978). Planar crossbedded sandstones form by transverse bar migration (Mathisen and Vondra, 1983). Conglomeratic and poorly sorted sandstones are deposited as energy levels of the streams decreased (Rust, 1978). The fine­ grained beds originate from crevasse splays and overbank deposits (Miall, 1978).

Deposits of the CSM association are very similar to those produced by the Scott and

Donjek type braided streams (Miall, 1978). The occurrence of crude fining-upward sequences resembling both of these braided stream types may imply a proximal-distal relationship of sedimentation in the same alluvial system (Miall, 1983). These deposits are distinguished from coarse-grained meandering stream deposits by the vertical distribution of clast supported 'framework' conglomerates (Rust, 1978), and lack of epsilon cross bedding throughout the unit (Jackson, 1978).

SSM Association

The SSM association (Figure 24) characterizes the upper Lothidok Formation. This association consists primarily of medium-scale trough crossbedded sandstones that grade into ripple stratified sandstones and siltstones. Lenticular trough crossbedded conglomerates rarely occur as lenses less than 50 cm thick either at the base of fining 95

Figure 24. Typical sequence of the Sandstone-Siltstone-Mudstone (SSM) lithologic association. Facies codes are given in Table 2. 96

a> O Nature of Contact a>c Dominant Lithology (fades codes) w Sedimentary Structures - Interpretation U Vertical General Characteristics Thickness Ou sequence in meters O (variable) sharp, erosionat, rarely gradational Mudstone (F m ) 1-3 common tabular calcretes-p a le o s o ls massive- over bank deposits

sharp or gradational i m , Siltstone (F m , F !) climbing ripples- crevasse splay deposit very may appear massive-extensive bioturbation common ripple stratification- lower flow regime planar beds - lower flow regime

gradational Sandstone (St, Sr, Sc, Ss, Sh) ripple stratification-lower flow regime

small trough crossbeds-rippies (lower flow re g im e )

internal scours 4-15 very common epsilon crossbeds - lateral accretion

medium trough crossbeds -dunes (lower flow re g im e ) occasionally conglomeratic occasional interbed and basal conglomerates -channel lag deposits

gradational (Gt, Gm) Conglomerate medium to lorge trough crossbeds - channel lag 0-7 rare dep o sits may appear massive or crudely bedded

Particle grade 97 upward sequences, or interbedded within the sandstones. These grade vertically and laterally into conglomeratic and poorly sorted, trough crossbedded sandstones, some of which are moderately to well-sorted. Epsilon crossbedding is common in most beds, but tabular planar crossbedding is rare. Cosets of cross-strata grade vertically to ripple stratified sandstones and siltstones, the latter commonly with climbing ripple stratification.

Stratified siltstones are commonly thinly interbedded with massive beds that may result from extensive bioturbation. The siltstones interfinger with moderately to very sandy mudstones that are laterally discontinuous and have sharp basal contacts.

Interpretation

Based on the sedimentary structures and vertical sequences, strata of the SSM association are interpreted as deposits of a high sinuosity stream system similar to that described by Galloway (1985). In these systems, massive and crudely bedded gravels are deposited as channel lags, and ripple stratified, very fine-grained sandstones and siltstones originate as crevasse splay deposits. The predominance of fine-grained sedimentary rocks suggests deposition on a distal alluvial plain, and the interbedding of coarse- and fine­ grained layers implies periods of fluctuating stream energy. The strata of the SSM association resemble those of Mathisen and Vondra (1983), interpreted to have been deposited in meandering streams. Note, however, that Jackson (1978) and Long (1978) have shown that criteria commonly cited for evidence of meandering stream deposition are unreliable. Further work, conducted specifically on depositional systems of this type, is needed to firmly establish the nature of the depositional system.

Pyroclastic Rocks

Tephra beds in the Lothidok Range include the Nathuraa tuffs, the Kalodirr Tuffs, the Naserte Tuffs, the Nakwel Esha beds and unassigned pyroclastic rocks of both the 98 lower and upper Lothidok Formation. Detailed descriptions of these rocks have been given in the previous discussion on stratigraphy and in the measured sections (Appendix B;

Boschetto, 1988).

Interpretations

The stratified and massive tuffs are primary airfall tephra. This is suggested by the laminae that parallel irregular basal surfaces, accretionaiy lapilli, euhedral volcanogenic minerals (amphibole and sanidine) and angular volcanic rock fragments. These deposits lack features typical of tephra reworked by fluvial processes such as abraded volcanic mineral grains, basal scours, large trough crossbeds and abundant rounded grains. Most trachytic tuffs are too poorly exposed to determine lateral variation, bed geometry, or internal structures. In some cases, however, low-angle, small- to medium-scale trough crossbedding and ripple stratification indicate an eolian influence in deposition and reworking of the tuffs. Cross-stratification in some laminated mafic alkaline tephra is probably the result of local wind reworking. Deposition of fine ash continued following deposition of coarser airfall tephra or lahar beds. Rain drop impressions result from rainfall onto the surface of the ash, and in some cases desiccation cracks formed as the ashes dried.

Matrix Dominated Conglomerates (Lahars)

The features of matrix dominated conglomerates such as crude inverse grading of lithic and/or pumice clasts, extreme range in particle size, very well-preserved plant debris, and absence of a lower erosional surface are evidence for deposition by debris flows.

Deposits of this type of in the Lothidok Range consist primarily of volcanic debris and are therefore considered lahars (Bates and Jackson, 1987). These deposits are distinguished from pyroclastic flow deposits by the preservation of plant debris, which implies a low 99 temperature at time of deposition, and the lack mineral precipitation along fluid escape features, which forms during fumarolic or degassing processes in ignimbrites (Fisher and

Schmincke, 1984).

Lahars are the most plausible mechanism for deposition of the matrix dominated conglomerates. These commonly originate on wet volcano slopes following large accumulations of erupted volcanic material (Fisher and Schmincke, 1984; Smith, 1986), or by debris slumping into water bodies adjacent to a volcano (Janda, et al., 1981). In many instances rain provides the moisture that saturates the volcanic debris, which then may become unstable and mobilize as a lahar. ORIGINS OF THE LOTHIDOK DEPOSITS

In addition to documenting the duration and type of volcanic activity, sedimentary and pyroclastic rocks in volcanic terrains provide a means of inferring the location of unknown volcanic centers. Niether the sedimentary nor the pyroclastic deposits of the

Lothidok Formation has known sources.

Sedimentary Provenance

Kalakol Basalts and Lower Lothidok Formation

The depositional settings and large particle grade of the sedimentary deposits of the

Kalakol basalts and lower Lothidok Formation indicate a proximal provenance.

Paleotransport directions in the sedimentary rocks strongly imply that the source was to the northeast. These deposits show no pronounced changes in clast petrology or paleotransport directions upward in the section, indicating derivation from the same source.

Clasts of these deposits consist primarily of phonolites and basalts suggesting that the highland comprised volcanic flows. The basalt clasts were probably derived from the

Kalakol basalts, but the source of the phonolite clasts is not known. Phonolites occur as flows, dikes and plugs north, south and west of the Lothidok Range (Smith, 1938;

Dodson, 1963; Walsh and Dodson, 1969). These areas lie outside the source location implied by the paleocurrent data, and no phonolites described in the literature resemble the clasts found in the conglomerates. No phonolite flows or dikes were found in the Lothidok

Range nor are any known to the east. It is possible that the highland from which the basalts were derived contained phonolite dikes that do not extend into the Lothidok Area. 101

Upper Lothidok Formation and Undifferentiated

Tertiary Deposits

The volcanic clasts in the lower part of the upper Lothidok Formation are of similar petrology to those in the lower beds. Phonolites exposed to the west and south of the

Lothidok Range are a possible source for these clasts. A second possible source might be uplifted volcaniclastic strata of the lower Lothidok Formation to the west. Proffett (1977); and Jackson and McKenzie (1983) discuss migration of listric faulting where younger generations of faults develop basinward of older ones. In the case of the Lothidok Range, the footwall of the younger fault would be comprised of the older volcaniclastic sediments and would provide a source for the later volcanic clasts. Pulses of faulting would explain the deposition of the Lokipenata conglomerates and the discrete, volcaniclastic conglomerates above. The only known fault of substantial magnitude lies approximately

30 km to the west, but there is no evidence that this fault was active at this time.

The most profound difference between the sediments of the upper and lower

Lothidok Formation is a substantial basement component in the upper beds. The source for this detritus is either the Precambrian basement and/or the arkosic sediments (Laburr

Series' of Arambourg, 1943) that overlie the basement. The angular shape of the grains, lack of quartz overgrowths and presence of abundant perthite suggest the provenance is the basement. Basement exposures along the Uganda and Turkwel Escarpments (Figure 4) are believed to have formed prior to 12.5 Ma (Bishop andTrendall, 1967), and may be an appropriate source region for the finer grained strata.

Pvroclastic Sources

Neither the mafic alkaline tephra nor the trachytic tephra of the Lothidok Range has known sources. These strata, however, reflect the close proximity of two petrologically 102

distinct volcanic centers.

In this respect the most important aspects of the strata are the relationships between

the airfall tephra and lahar deposits in both the types rocks. A common relationship is that the lahar deposits typically overlie airfall tephra. The similar mineralogy of the airfall and lahar deposits suggests origin from the same source, and soft sediment deformation features indicate a short time span between depositional events. Therefore, it is likely that the airfall tephra and lahars are deposited following a volcanic eruption that distributes abundant, coarse-grained tephra on the sides of the volcano and finer grained tephra farther away. Later, the debris on the volcano is mobilized into lahars, which are deposited directly on the airfall tephra.

The lahar deposits are useful in restricting the possible distance to the source volcano(es). Lahars rarely flow farther than 100 km (Fisher and Schmincke, 1984), and typically less than 50 km (Smith, 1986). Additional constraints can be placed on the location of the respective volcanic sources by looking at crystal sizes, clast petrologies and

associated conglomerates of the mafic alkaline tephra, and at the distribution and thickness

of trachytic lahar deposits.

Mafic Alkaline Tuffs and Lahars

The crystal component of coarse-grained tephra of the Kalodirr Tuffs is up to 3

mm diameter. Maximum distribution limits for airfall tephra particles of this size require

the source to be within 100 km (Fisher, 1964). Mafic alkaline volcanic centers along the

Kenya-Uganda border (Figure 4), 135 to 250 km west of the Lothidok Range are similar in

age to the Kalodirr Tuffs, but appear to be too far away. This implies that an unknown

mafic alkaline volcanic center lay within the Turkana Depression less than 100 km from the

Lothidok Range. In addition, the paleotransport directions at the time of deposition of the

Kalodirr Tuffs indicate a highland source to the northeast. Because lahars flow down 103 existing drainages, the source of these tephra is most likely to be to the northeast also. One possible source for these tephra is Moiti, along the east shore of Lake Turkana. Although not documented by field study, a syenite plug has been postulated to explain a Bouguer gravity anomaly at Moiti (Khan and Swain, 1978). If correct, Moiti could be the source of the syenite clasts in the lahar deposits. The evidence cited would be consistent with a mafic alkaline volcanic center to the northeast that was active from about 17.7 to 17.3 Ma., and possibly associated with Moiti. The only other known occurrence of mineralogically similar tephra is Loperot (this writer), but potassium-argon age determinations are currently in progress and the ages are not known. Similar tephra was not found at Buluk (Watkins,

1982). Moiti lies 45 km from the Lothidok Area, 95 km from Loperot, and over 125 km from Buluk (Figure 4). These distances indicate Moiti is not too far away to have supplied coarse tephra to Lothidok and Loperot, but is too far away to have deposited coarse tephra at Buluk.

Trachytic Tuffs and Lahars

Trachytic lahar deposition occurred continuously throughout the time interval between deposition of the Kalodirr Tuffs and the Loperi basalts. The altered condition of the trachytic airfall tephra make it impossible to obtain chemical analyses that might be used to identify the volcanic source(s). The lahar deposits, however, are of more use.

Although the drainage reverses, lahar deposition continued in the same region without significant change in frequency and/or distribution. The greater abundance and thickness of the deposits in the southern Lothidok Range indicate that the source may have been trachytic voncanoes both east and west of the Lothidok Range. The lahars most likely initiated near the headwaters of small drainage systems on the volcanic edifice(s) and flowed into a larger drainage system, as happened during the 1980 eruptions of Mt. St.

Helens (Janda, et al., 1981). Limits established by the lahar run-out distances imply this source is within a maximum distance of 100 km. Strata of similar age at Loperot do not contain trachytic tephra, suggesting that the source lies closer to the Lothidok Range. If the lahars originated from chain of volcanoes throughout the deposition of the Lothidok Formation, the volcanoes were probably located south of the present position of the Turkwel River. UNCONFORMITIES IN THE LOTHIDOK RANGE

Two unconformities are recognized within the Lothidok Formation, one at the base of

the Lokipenata conglomerate (between 16.6 and 13.6 Ma) and the other at the base of the

Loperi basalts (between 13.5 and 12.0 Ma). There is a strong possibility that a third

unconformity exists at the base of the undifferentiated Tertiary arkosic sediments (between

10.9 and 4.1Ma).

Base of the Lokipenata Conglomerate

A disconformity occurs at the contact between the lower and upper Lothidok

Formation at the basal contact of the Lokipenata conglomerate. The disconformable contact

consists of a broad, irregular scour surface cut into the underlying sediments. Potassium-

argon ages determined on the oldest datable bed (Kalatum basalt) above the contact indicate

that the disconformity predates 13.6 Ma. Evidence for a disconformity at this contact

includes the contrasting paleocurrent azimuths above and below the contact, the

concentration of pedogenic features at and below the contact, and the initial presence of

basement detrital material above the contact.

The strongest evidence for the presence of a disconformity at this level is the reversal

of paleocurrent azimuths in the clastic rocks above the contact relative to those below

(Figure 25). The rose diagrams represent data combined from all paleocurrent

measurements taken from the lower and upper Lothidok Formation. The reversal first

occurs in the Lokipenata conglomerate directly overlying the disconformable contact and is

maintained throughout the section above. 106

Figure 25. Paleocurrent rose diagrams illustrating the complete reversal of sediment transport direction above the disconformity relative to that below. Each diagram represents the combination of every measurement taken in the respective level and all measurements were corrected for tectonic tilt. upper Loperi Basalts Lothidok JV Fm.

68c

N = 167

Kalatum basalt (l3.6±0.2Ma)

Disconformity- lower Lothidok Fm. Naserte Tuff (l6.8±0.2Ma)

N = 2 5 8

228e

Kalakol basalts 108

The presence of pedogenic features at and below the contact provides further evidence for an unconformity. These features consist of numerous tabular calcretes interbedded with conglomerates and sandstones at this level. The calcretes range from 2 to 15 cm thick and represent up to 40% of the upper 2 m of the bed just below the disconformity. These indicate prolonged exposure and weathering suggesting a hiatus.

The final evidence for the disconformity is the first occurrence of basement detrital material in sedimentary rocks above the contact. This material consists of quartz and perthite totalling ~ 10% near the base and up to 75% near the top of the section. The weight percentages were determined by mineral separation methods, and volume percentages were determined by thin section point-counts. The presence of basement detrital material indicates exposure of a different source terrain, possibly the Cretaceous

'Labur series' or Precambrian basement.

No evidence was found to indicate tilting during uplift. Differences in the dip above and the disconformity are less than 8 °, which is certainly within measurement error.

Because of the lack of good exposures, accurate measurements could not be taken on beds less than 80 m (vertical section) above the contact. Outcrops of this contact provide little exposure of east-west variation (sections 3 and 19, Figure 3), but the contact occurs at roughly the same level.

The evidence cited strongly supports the interpretation of an unconformity at this level.

To approximate the magnitude of the time gap, average sedimentation rates were applied to the sedimentary rocks above and below the disconformity. The average rates were calculated for the intervals of sedimentary rocks of the Lothidok Formation bracketed by potassium-argon age determinations. Continuous sedimentation and constant sedimentation rates were assumed for each time interval. The youngest dated bed below the disconformity is the Naserte Tuffs (16.8 Ma) roughly 60 m below the disconformity and the oldest dated bed is the Kalatum basalt (13.6 Ma) approximately 60 m above the 109 disconformity (Figure 25). Using the average rate of 17 cm/ka for the 60 m between the

Naserte Tuffs, the disconformity represents 350 ka, and using a rate of 50 cm/ka for the 60 m below the Kalatum basalt, that interval represents 120 ka. Therefore, a hiatus exists in the section between the Naserte Tuff and the Kalatum basalt with a duration of about 2.7

Ma, from about 16.4 to 13.7 Ma.

Base of the Loperi Basalts

The second unconformity, also noted by Arambourg (1943) and by Walsh and

Dodson (1969), is apparent from the angular discordance between the Lothidok Formation and the Loperi basalts. This is observable on both a local and regional scale.

The unconformable relation between the Loperi basalts and the underlying strata is seen at Moruorot Hill. The discordance is obvious when viewed from the Kalatum River north looking along the strike of the beds. The basal contact of the Loperi basalts is not exposed but its position can be inferred from the upper extent of sedimentary-rock talus and the lowest exposures of in situ basalt along the side of the hill.

The angular discordance is also suggested by the regional relationship of the unconformable contact to key beds of the lower Lothidok Formation (Figure 8 ). This cross section is drawn along an east-west transect through sections 10.1, 26, 27 and 30. No key beds are exposed at sections 26 and 27, and therefore the total thickness of the combined sections is used. The dip of the key beds to the west relative to the unconformable contact illustrates the angular discordance between the Loperi basalts and the lower Lothidok

Formation. The discordance implies a regional westward tilt of strata prior to the erosion forming the unconformity.

The angular discordance cannot be measured in the field because no beds directly above and below the contact well were enough exposed to show dip variation. Further, the 110 exposures of strata overlying the Loperi basalts are never far enough removed from faults to be of use in determining the degree of angular discordance. The discordance can be approximated assuming that the Loperi basalts were deposited on a horizontal surface and that a complete sediment sequence existed at Moruorot Hill prior to erosion. The amount of tilt prior to deposition of the Loperi basalts can be estimated by measuring the angle between the unconformable contact and the Naserte Tuff. Because the unconformity was not planar surface, the angular discordance is approximated at 10°.

Base of the Tertiary Arkosic Sandstones

It is possible that a third unconformity exists at the base of the undifferentiated Tertiary sediments. The age of this surface, if present, is only confined to between 10.9 and 4.1

Ma. Although the unconformity is not well-defined, the following evidence suggests that it exists.

The first evidence involves the thinning of the Lothidok Formation to the north (Figure

7) as seen from the increase in both size and percentage of basement detrital material at progressively lower levels to the north. In the south, coarse pebbles and larger quartz clasts are only found in strata above the Loperi basalts. This petrologic change represents the base of the undifferentiated Tertiary sedimentary rocks. Pebbles and cobbles of quartz and feldspar first occur at 300 m above the base of the Lothidok Formation at section 1, but only 120 m above the base at section 6.2. The abundance of finer quartz and K- feldspar grains increases at approximately the same level as the first occurrence of very coarse basement material. Sublitharenites occur below this level and lithic arkoses and subarkoses occur above.

The thinning of the Loperi basalts provides a second line of evidence. Where thickest, the Loperi basalts consist of three flows totaling over 120 m. Only one flow exists north of this outcrop where the total thickness is less than 40 m. The progressive northerly thinning 111 of the Loperi basalts from Ngaletiti Hill (Plate). However, the thinning may have occurred during deposition in a confined area. The confinement could be related to the presence of a stream channel at this location.

Additional evidence for the unconformity stems from the lateral limits of the volcaniclastic conglomerate which locally overlies the Loperi basalts. This conglomerate crops out on the north end of Loperi Hill, west of Esha Hill, west of Konukuanga Hill, and above the basalts east of Konukuanga Hill (Plate). The conglomerate occurs where the

Loperi basalts are thickest and is absent where the basalts are thinnest or missing. The conglomerate is overlain by subarkosic to arkosic sandstones that directiy overlie the Loperi basalts where the conglomerate is absent Where both the conglomerate and the basalt are absent these sandstones rest on strata of the Lothidok Formation. These relationships may reflect a period of erosion prior to deposition of the arkosic sediments

Similar subarkosic to arkosic sandstones lie directly on the Kalakol basalt along the western edge of the Lothidok Range roughly 20 km north of the Kalakol-Lodwar road and

5 km west of section 6.2. At this locality, the sandstones are continuous with the Plio-

Pleistocene section of the Turkana Basin (F. H. Brown, pers. com.). The oldest, dated bed in this sequence is the Moiti Tuff, which is 4.1 Ma (McDougall, 1985). These sediments are thought to be continuous (F. H. Brown, pers. com.) with the sedimentary rocks lying above the unconformity in the southern Lothidok Range. This correlation places these strata in the late Miocene or early Pliocene, which indicates that as much as 5

Ma is not recorded in the rock record at the Lothidok Range.

The final evidence involves the lack of trachytic deposits above the unconformable contact. Although trachytic tephra are interbedded with the Loperi basalts, none is found above. No change in the sequence or frequency of trachytic tephra is noted below this level. Assuming no time gap between the deposition of the Loperi basalts and the overlying Tertiary sedimentary rocks would imply abrupt cessation of deposition of 112 trachytic tephra. Having been deposited for over 6 Ma, it is unlikely that deposition of trachytic tephra would cease coincident with the deposition of the highest basalt flow.

Assuming the unconformity does exist, uplift began after 10.9 Ma but before 4.1 Ma in the northern part of the Lothidok Range and progressed south. The presence of the arkosic sediments directly overlying the Kalakol basalt along the western edge of the

Lothidok Range implies that some uplift also occurred to the west. Although this is the least well-constrained unconformity in the Lothidok Range, temporally it is the closest to a period of regional erosion (Savage and Williamson, 1978). STRUCTURE OF THE LOTHIDOK RANGE

The dominant structures of the Lothidok Range are normal faults. Other typical rift related structures are tilted fault blocks, central grabens and horsts, and fault related folding

(Figure 26). Deciphering the temporal and spatial fault relations is beyond the scope of this thesis, but determination of the stratigraphy of the study area required cursory study of the structure.

General Structure

Regionally, the Lothidok Range gently plunges beneath Quaternary (Qal) deposits to the south (Plate). The burial of upper strata by Quaternary deposits in the south suggests the southerly plunge. A notable increase in fault density occurs to the south.

Excluding the beds exposed along the southeastern side of the Lothidok Range, all beds dip to the west. Fault blocks in the range are bounded by eastward-dipping, normal faults striking roughly north-south. The blocks tilt between 13° and 40°, averaging 18°.

Normal Faults

The normal faults can be roughly divided into two groups. One group strikes between

N 15° W and N 45° W and the other between N 15° E and N 45° E. A few faults in the southern region of the Lothidok Range strike between N 45° E and N 75° E. The crosscutting relationship between the faults suggests that many formed synchronously.

Observable fault dips range from 65° to 90°, the steeper dips attributable to block rotations by movement on younger faults. The vast majority of the faults dip eastward. West 114

Figure 26. Schematic geologic cross-sections of the Lothidok Range (no vertical exaggeration). Esha graben Esha Hill K a la tu m R iv e r QAi Quaternary deposits

Tertiary Undifferentiated

;Tlb. Loperi Basalt CD Esha graben (g ) Ngaletitl Loperi Konukuanga TIil Upper Lothidok Fm. H ill N a se rte Hill Tib Lower Lothidok Fm

Tku undifferentiated sediments 1km Jk b Kalakol basalt Tke Eragaleit beds 116 dipping faults occur as boundaries of horsts and grabens, and as antithetic faults. Faults typically terminate in folds and splay zones.

Range Front Fault

The most prominent normal fault forms the eastern range front of the Lothidok Range

(Figure 27). This is well-exposed just east of Moruorot Hill. Dip of this fault is roughly

60° east. The fault appears to be composed of numerous fault segments, a feature well- illustrated north of the Togolok and south of the Eragaleit Rivers (Figure 27).

At Moruorot Hill, footwall exposures consist of the upper Kalakol basalts through the lower Lothidok Formation dipping 20° west (Figure 4). North of the small sedimentary outcrops flanking Moruorot Hill, footwall exposures consist of the Kalakol basalts.

Hanging wall exposures of undifferentiated Tertiary strata dip up to 40° east immediately adjacent to the fault suggesting fault drag. Farther east, the beds dip 5° to 15° east.

Arambourg (1943) interpreted these beds to be the 'Labur Series'. He apparently did not discover the fault and depicts an angular unconformity between the exposures of the hanging wall and footwall.

The total thickness and exact stratigraphic position of the Tertiary sediments exposed in the hanging wall are not known. It is therefore impossible to determine the actual offset along the fault. However, the juxtaposition of the Tertiary strata against the upper Kalakol basalt flows requires a minimum dip separation of 700 m. The actual separation is probably much greater.

The fault is well-exposed only north of the Alomonet River. To the south, it appears that the range front fault is displaced 1 km west by two normal faults roughly 500 m apart that strike northeast and dip northwest. These faults bound a small fault block composed of Loperi basalts and undifferentiated Tertiary sediments dipping 10° north. South of these 117

Figure 27. Simplified structure map of the Lothidok Range illustrating location of major structural features and geologic cross sections. 118

> \_-Hill " \ \ n Konukuanga $ Area boundary ^ General strike/dip uf° Normal fault / Subsuface fault Ai—i A ' Cross section A Hill

) Paved road ■3°I0'N / / travel road / / Stream 119

faults, the range front fault is not exposed, but is inferred to continue in the subsurface. A

small, asymmetric anticline lies above the inferred positoin of this fault.

A small canyon east of Konukuanga Hill (Figure 2) lies along the crest of the anticline.

The anticline continues southward for at least 500 m to where it is extensively dissected by faulting (Plate). Strata exposed along the anticline dip 10° to 15° east. Numerous west- dipping and east-dipping faults occur throughout these strata. Fault movement produced the folding forming the anticline and east dipping beds. These anticlinal folds and east dipping drag folds to the north prompted Fuchs (1939) to interpret the structure of the

Lothidok Range to consist of folded strata and possibly reverse faults instead of normal faults as suggested by Arambourg (1943).

The fault continues southward from the Lothidok Range, and evidence for movement is seen at its intersection with the Turkwel River. The Turkwell River is more sinuous west of the intersection than east of it. The stream meander amplitude increases from 1.75 to 3 km and the wavelength decreases from 12.5 to 2 km as the river approaches the fault.

Downstream from the fault the meanders have very long, low amplitude wavelengths

(Figure 26, Plate). The abrupt changes in stream pattern may reflect gradient changes in response to fault movement (Leopold and Wolman, 1957; Schumm, 1987).

Grabens and Horsts

Grabens are referred to by the name of the dominant streamcourse within them. The

two most prominent grabens are located along the Kalodirr and Nakwel Esha Rivers

(Figure 27).

The Kalodirr graben extends for at least 10 km from the Kalakol River to about 2 km

north of the Kalakol-Lodwar highway. The southern end of the graben is buried by

Quaternary alluvium. Only undifferentiated Tertiary strata are exposed within the graben

(Plate). These beds are essentially horizontal with drag folds along boundary faults. 120

Upper Kalakol basalt flows are exposed in the western footwall and the Kalodirr Tuffs are exposed in the eastern footwall. The Kalodirr Tuffs lie approximately 50 m above the

Kalakol basalts in this area. Minimum dip separations of 520 m and 400 m occurred along the western and eastern boundary faults respectively.

The Nakwel Esha graben extends from just south of Loperi Hill northward to about 5 km south of Lothidok Hill (Figure 26). The upper Lothidok Formation in the southern end of the graben dips 20° west. Northward the graben is internally faulted, and exposes strata from the lower part of the Lothidok Formation through undifferentiated Tertiary beds that dip 10° to 45° west. A major, west dipping normal fault dissects the northern half of the graben approximately along its axis, and is referred to here as the 'axial fault'. The axial fault divides the Nakwel Esha graben into a small graben west and a small horst to the east

(Plate). East of the horst, but within the Nakwel Esha graben, lies a small tilted fault block dipping 15° west.

Throughout the length of the small graben, the sediments are folded into a series of small anticlines and synclines gently plunging west (Plate). Between the axial fault and the western boundary fault, the graben consists of west dipping fault blocks and numerous east-dipping, normal faults.

Offset on the western boundary fault decreases to the south. The fault intersects another fault west of Loperi Hill. Near the junction, the top of the Kalakol basalts is in contact with the undifferentiated Tertiary sediments, which requires at least 700 m of dip separation. About 1 km to the south, dip separation is less than 500 m. The fault terminates to the south at a fault striking N 45° E.

The eastern boundary fault bifurcates at the Kalatum River. The eastern branch continues south approximately 2.5 km to where it terminates at a fault striking about N 45°

E (Plate). Dip separation decreases to the south and the fault becomes buried by

Quaternary deposits. Between these splays are numerous, small horsts and tilted fault 121 blocks (Plate). The horsts and fault blocks are rectangular with average surface areas of less than 0.25 km2. The blocks are displaced downward to both the north and east. Dip separation is usually less than 60 m along north-south trending faults and less than 2 0 m on east-west trending faults.

South of the splays lies a small horst about 1.5 km wide that is bounded by two parallel faults striking approximately N 20° E. Kakurtua Hill is located at the southern end of the horst. Dip separation along these faults ranges from a few meters at the southern end to a few hundred meters to the north. The actual separation on these faults is difficult to determine because of an erosional unconformity present in the faulted strata. An older down-to-the-west, normal fault lies approximately in the middle of the horst (Plate). The western fault of the horst may be an associated with the fault forming the eastern boundary fault of the Nakwel Esha graben in this area.

Approximately 1 km north of the Nakwel Esha graben, two small grabens expose the same strata with a similar strike as those exposed in the northern most extent of the Nakwel

Esha graben. These are referred to as the Lataagur and Eragaleit grabens for this discussion. They appear to be the northern continuation of the Nakwel Esha graben but are separated from it by a fault zone. The adjacent grabens are distinct in that the Lataagur graben exposes a thin but complete stratigraphic sequence. The Eragaleit graben exposes only the lower strata of the section which has been truncated by a fault. In both grabens, dips are between 25° and 45° to the west, but locally, beds dip more steeply as a result of fault drag. The western boundary faults in the Lataagur graben juxtapose the Eragaleit beds in the footwall against undifferentiated Tertiary strata implying as much as 900 m of dip separation. The eastern boundary fault places the lowest sediments of the Lothidok

Formation in the hanging wall against Kalakol basalts beneath the Eragaleit beds.

Minimum dip separation is estimated to be 600 m in this area. 122

Temporal Fault Relations

The majority of the faulting in the Lothidok Range postdates deposition of the undifferentiated Tertiary sediments. Along the Lodwar-Kalakol highway, the range front fault cuts the Moiti Tuff (Brown and Feibel, 1986) and is therefore younger than 4.10 ±

0.07 Ma (McDougall, 1985).

At least one episode of faulting predates deposition of the Lothidok Formation. The fault is exposed along the Eragaleit River due south of Lothidok Hill. The fault is approximately vertical while the strata it cuts dips about 15° west. Because all strata have been rotated down to the west this fault probably dipped to the west when it formed. The

Kalakol basalt flows exposed on Lothidok Hill overlie the fault and therefore postdate it; these basalts are older than 17.7 Ma.

Fault Gouge

A calcareous fault gouge or sinter lies along many of the major faults in the Lothidok

Range. The gouge occurs most frequently where faulting has exposed the Kalakol basalt.

The gouge is pale orange to white and usually very resistant to erosion, commonly forming wall-like ridges ranging from 10 cm to 2 m wide and 20 cm to 3 m high. The presence of this fault gouge simplifies locating major faults, especially on air photos. The best exposures of fault gouge He along the western boundary fault of the Kalodirr graben. The gouge effervesces moderately to strongly in dilute (-10%) HC1, but X-ray diffraction analysis shows that the gouge is primarily dolomite with minor calcite. Fuchs (1939) described these deposits as oxidized muds resulting from fumarolic action. FAUNA OF THE LOTHIDOK RANGE

Introduction

The Kenya National Museums collected fossils at eight localities in the Lothidok

Range (Figure 28) from 1985 to 1987. Each locality is herein named after a nearby, geographic features. The names, measured section(s) and age range for each locality are listed in Table 3.

Fossil bearing strata are confined to three levels (Figure 29). The lowest level lies within the Kalakol basalts, and includes the Eragaleit localities near the base of Lothidok

Hill (#1 and #2). The middle level consists of the Kalodirr (#3), Kanukurinya (#4), and both Moruorot (#5 and # 6 ) localities, and lies in the lower Lothidok Formation. The highest level includes the Esha (#7) and Atirr (#8 ) localities, which lie within the upper

Lothidok Formation.

The faunas are very similar and roughly the same age as the faunas at several other

East African sites. Table 4 lists taxa identified from the three levels at Lothidok (Le - lower, LI - middle and Lu - upper) along with published records of taxa identified from

Bukwa, Karangu, Songhor, Rusinga, Napak, Buluk, and Fort Teman. Many taxa are common to other East African fossil localities, but a few have not been previously recorded. The vast majority of the fauna was recovered from strata between the Kalodirr

Tuffs (>17.3 ± 0.2 Ma) and the Naserte Tuffs (16.8 ± 0.2 Ma). At this writing, most of the fossils from the Lothidok, Esha and Atirr sites have not been accessioned.

None of the recently collected fossils was found in situ and few contain any form of matrix (M. G. Leakey, pers. comm.). Using fossil site markers set by the Kenya National

Museums, the fossils were placed within specific strata whenever possible. Because the 124

Figure 28. Fossil localities of the Lothidok Range: 1) Lothidok north, 2) Lothidok south, 3) Kalodirr, 4) Kanukurinya, 5) Moruorot south, 6 ) Moruorot north, 7 ) Esha, and 8 ) Atirr. 125

/Kalakol f Townshipf F e r g u s o n ,s

V Gulf

Lothidok I Hill l&taagur

Moruorot I Hi I 1 at

Konukuanqa -H ill Loperi Hill Kamurunyang

Ngaletiti Hill

Fossil locality Area boundary Kaku rtua Hill Hill Urkwe/ Paved road Gravel road Stream 126

Table 3.Measured sections, maximum and minimum ages for the fossil localities.

Fossil Locality (#) Measured Section(s) MinimumAge (Ma) Maximum Age (Ma)

Lothidok - north (1) 16 ? ? Lothidok - south (2) 7 ? ? Kalodirr (3) 1, 31, 33 16.8 + 0 .2 * 17.7 + 0.2 Kanukurinya (4) 6 . 1 , 6 .2 16.8 + 0 .2 ** 17.7 + 0.2 Moruorot - south (5) 1 0 .1 16.8 + 0 .2 ** 17.3 + 0.2 Moruorot - north (6 ) 10.3 16.8 + 0 .2 ** 17.3 + 0.2 Esha (7) 26, 27 1 2 .0 + 0 . 1 *** 13.6 + 0.2** Kamurunyang (8 ) 24, 25 13.1 + 0.1**** 13.6 + 0.2**

? Isotopic age yet to be determined. * For the majority of the fossils. ** From correlations, dated bed not exposed. *** Disconformity between fossil bearing beds and dated bed. **** Age may not represent time of deposition. Figure 29. Fossil levels and isotopic ages of the Lothidok Range. 128

Unit Age (Ma) Fossil Locality

Loperi Basalts Kamurunyang lahar > E sha (#7) - A tirr (#8) Lothidok Fm.

Kalatum basalt

Naserte Tuff K a lo d irr (#3) Kanukurinya (#4) Kalodirr Tuff M o ru o ro t (# 5 ,# 6 )

Kalakol basalts Explanation

■A11p>' V ■---- Lahors Unconformity Tephra Tuffaceous and clastic sediments Basalts \ Fault

Eragaleit beds E ra g a le it 129

Table 4. Miocene fauna of selected East African fossil localities. The locations given are listed oldest to youngest; B - Bukwa, K - Karungu , S - Songhor, Le - Lothidok (Eragaleit beds), R - Rusinga, N - Napak, LI -lower Lothidok Fm., Bu - Buluk, FT - Fort Teman and Lu - upper Lothidok Formation.

B K S Le R N LI Bu FT Lu INVERTEBRATES Mollusca Gastropoda Ampullariidae Pila ovata - - - - X - X - Lanistes carinatus ...... X - Thiaridae Melanoides tuberculata X ----- X - Lymnaeidae Lymnea natalensis ...... X - Pomatiasida LigateUa 2 spp. - - X - X - - - Stenogyridae Krapfiella angusta - - X - - - - - Homorus (Subulona) X - X - X X - - Enidae Edouardia mfwanganensis - . . . x - - - Cerastus miocenicus - - - - X - - X Achatinidae Burtoa nilotica X - - - X - X - Limnocolaria 2 spp. X - X - XXX- Heliocarionidae Trochonania 5 spp. - - X - X X - - Bloyetia 1 sp. XXX----- Thapsia X - X - - - - - Streptaxidae Tayloria 2 spp. X - X - X - - - Gulella 3 spp. X - X - X - - - Gonaxis (Marconia) 9 + spp. - - X - X - - - Primigulella - - X - - - - - Ptychotrema usiforme - . . . X - - - Edentulina rusingensis - - - - X - - - Cyclophoridae Mazania 4 spp. X - - - X X - - Bivalvia Etheria elliptica ...... x - Insec ta cf. Agromyzidae ...... x - Arthropoda Diplopoda ...... X - Myriapoda X - - - - X - -

VERTEBRATES Pisces Polyp teriformes Polypterus ...... X - Pisces (cont.) Silunformes ...... x - cf. Bagridae ...... X - Reptilia Testudines Pelomedusidae Pelusios rusingae - . . . X - - - Gen. et sp. nov...... X - 130 Table 4 continued.

B K S Le R N LI Bu FT Lu Testudinidae Impregnochelys pachytectis X ? Geochelone crassa -X- - cf. Geochelone vA Squamata _ ___ XX III Crocodilia sp. nov. - - -- _ _ X___ Crocodylidae X -

Aves X-- X- XX- - Mammalia Insec tivora Ptolemaiidae Kelba quadreemae ____ X_ .. Macroscelidae Rhynchocyon clarki __ X_ X_ _ . Rhynchocyon rusingae - - X- X--_ . _ Myohyrax oswaldi XX- - X-__ _ Pronasilio sp ------X- - - Erinaceidae Galerix africanus __ X_ X__ _ . Lanthanotherium sp -- X-. ___ . _ Amphechinus rusingensis -- X- X_ - _ . _ Gymnurechinus leakeyi ---- X___ . _ Gymnurechinus camptolophus ---- X_ . _ . _ Gymnurechinus songorensis -- X- X?___ . _ cf. Gymnurechinus ~ X“ “ - X- - - Soricidae Crocidura sp AY Tenrecidae Protenrec tricuspis X XX Erythrozootes cnamperpes -- X- - X. _ _ _ Geogale aletris -- - -X-- -- - Chrysochloridae Prochrysochloris miocaenicus - - X - Chiroptera Pteropodidae Propotto leakeyi - - X- 7 -- - -- Emballonridae Taphozous incognita - -- - X-- -- - Rodentia Phyomyidae Andrewsimys parvus X Phiomys andrewsi - - X------Thyronomyidae Parapniomys pigotti XXX_ XXX_ Paraphiomys stromeri -- X- XXX-.. Paraphiomys small sp. -- --.- X- . . Epipnyomys corydoni -- X-- X- - -- Diamantomyidae Diamantomys luederitzi - XX- XX-- -- Kenyamyidae Kenyamys mariae __ X_ __ _ . . Simonimys genovefae - X-XX- -- - Myophiomyiaae Myopniomys arambourgi __ X XX_ . . Elmerimys woodi ---- X- X- - - Bathyergoididae Bamyergiods neoteriarius - - X-- X--- - Bathyergidae Proneliophobus leakeyi -_ ? . X____ _ Bathyergidae sp. nov ...... x 131 Table 4 continued.

B K S Le R N LI Bu Anomaluridae Paranomalurus bishopi X X Paranomalurus soniae -- X- XX___- Paranomalurus walkeri -- X-- X____ Zenkerella wintoni --X------Pedetidae Megapedetes pentadactylus X_X__ X Megapedetes sp -- X Cricetodontidae Afrocrictodon songhori __X__ X Paratarsomys macmnesi ---- X- ____ Notocricetodon petteri - - X- X- ____ ?gen et sp. nov. ------X- -- Cricetida Leakeymys temani ------X- Sciuridae Vulcanisciurus africanus _ _ X_ XX_ Sciuridae sp “- -- X- - - - - Lagomorpha Ochotonidae Kenyalagomys minor X X ?Lagomorpha X- -X------Primates Lorisidae Komba minor . X XX Komba robustus - - X- XX__ __ Progalago songhorensis -- X- X- _ ___ Progalago dorae -- X-- X. ___ Mioeuoticus sp nov -- X- X- -_ __ Mioeuoticus bishopi ---- - X-_ __ Lorisinae ?gen. et sp nov. X- Hominoidea Incertae sedis Afropithecus turkanensis _ .. XX Turkanapithecus kalakolensis ------X-__ Simiolus enjiessi ------X- -- Pongidae Proconsul africanus _ - X_ X_ X Proconsul nyanzae - X-- X__ ___ Proconsul major -- X?_ X____ Proconsul gordini - - X- X-_ ___ Proconsul vancouveringi - - X- X_ ____ Limnopithecus legetet X- X- XX_ ___ Limnopithecus evansit -- X- - _ . ___ Micropithecus clarki - -X - Hylobatidae Dendropithecus macinnesi - XX- X- - - -- Ramapithiidae Kenyapithecus wickeri -- - - _ _ _ X_ X Cercopithecoidea “- ”“-- - - - X Creodonta Hyaenodontidae Metasinopa napaki . X Anasinopa leakeyi - X- - X- X_ 7 _ Dissopsalis pyroclasticus --- - - X-_ __ Teratodon spekei -- X- ____ _ Teratodon enigamae - - X- -- -_ __ Leakiherium hiwegi - -- - X. - __ Pterodon africanus ---- XX_ ___ Pterodon kaiseri - X- - X_ ___ Pterodon zadoki -- - - X- --_ _ Hyaenodon sp. ---- . X- X. _ Hyaenodon andrewsi -- X- X- X_ _ _ Hyaenodon matthewi -- X- X_ . ___ Hyaenodon pilgrimi - - X- X- _ - __ Hyainailourus nyanzae ---- XX--__ 132 Table 4 continued.

B K s Le R N LI By FT Lu Hyaenodontidae cont Hyaenolurus africanus -- X - - - - - cf. Hyaenodon : X Carnivora ' Canidae Hecubides macrodon _ XX _ X XX __ Hecubides euryodon -- X - X X ___ Viverridae Kichechia zamanae - - X _ X X X _ __ Hyracoidea --- X -- ___ Megalohyrax championi X X XX X - X X _ _ Meroehyrax batae X --- X -_ ___ Deinotherioidea Prodeinotherium hobleyi XX - X XX X X X _ Deinotherium sp. ---_ - ____ X Proboscidea Platybelodon sp. -- - - - __ X __ Gomphotheriidae --- . _ ____ Gomphotherium angustidens -- ? X ? XX ___ Gomphotherium pygmaeus ----- X ---_ Protanancus macinnesi -- ? - - XX X X _ Eozygodon morotoensis -- - X - - - - Perissodactyla Chalicotheriidae Chalicotherium rusingense -- X - X X - ___ Rhinocerotidae Brachypotherium X --- X X - ___ cf. Aceratherium acutirostratus X X -- X XX X _ _ Dicerorhinus leakeyi ? --- X X _ _ Chilotherium sp X - X _ X ___ _ Paradiceros murkirii - ______XX Artiodactyla Anthracotheriidae Masritherium aequitoralis X X X X X _ X __ _ Brachyodus aequitoralis X --- . - _ ___ Hyoboops palaeindicus X - cf. Hyoboops africanus X - - X - - X - Anthracotheriidae sm. X X X Suidae Kenyasus rusingensis - X -- X - X ___ Libycochoerus jeanneli X XX - X - X ___ Lopholistriodon moruoroti ------X X _ _ Diamantohyus africanus X XX - X _ X X _ _ Hyotherium dartevelli - X X - X X ___ Hyotherium kijivium -- X - X X ____ Kubanochoerous jeannelli -- X - X X -__ Listrodon akatidogus ---- X - ____ Xenochoerus africanus X XX _ X __ _ Palaeomercidae Climacoceras africanus _ _ x cf. Climacoceras --- ___ X _ X Canthumerx sirtensis _ _ . ___ X X Paleomeryx africanus ---- _ X _ . Giraffidae cf. Paleotragus primaevus -- -- - _ X __ _ Giraffoidea sp nov. --- - __ X __ _ Tragulidae --- X - ___ _ Dorcatherium chappuisi X -- - XX - X _ Dorcatherium pigotti X X - - X - XX __ Dorcatherium parvum XX- - X - X --- Table 4 continued.

B K S Le R N LI Bu FT Lu Tragulidae cont. Dorcatherium songhorensis - - X - - x - - - - Bovidae sm ...... X Bovidae lg...... X

Source of information: Andrews and Van Couvering (1975); Bishop (1968); Kenya National Museums (pers. comm., 1987); Leakey and Leakey (1986a); Leakey and Leakey (1986b); Leakey, R. E. and Leakey, M. G., 1987; Madden (1972); Maglio and Cooke (1978); Meylan and Auffenberg (1986); Van Damme and Gautier (1972); Walker (1969); and P. Williamson (pers. comm., 1988). 134 specimens were collected at different times than this study was conducted, most of the placements of fossil within specific beds should be regarded as approximate.

Eragaleit Beds. Kalakol Basalts

The lowest fossil-bearing strata in the Lothidok Range belong to the Eragaleit beds, and the sites from which fossils were collected are here named the Eragaleit localities

(Figure 30). The northern site (#1) is located along a small tributary to the Nathuraa River and the southern site (#2) is located on the Eragaleit River (Figure 29). Faunas from both sites are listed together under the heading 'Le' in Table 4.

The Eragaleit beds are bounded by flows of the Kalakol basalts. Isotopic ages for these basalts and an interbedded tephra sequence are currently being determined. The majority of the fauna is associated with the fluvial sediments of beds 18 through 22, section

16 and bed 12, section 17.

Lower Lothidok Formation

The lower fossil level of the lower Lothidok Formation comprises the Kalodirr,

Kanukurinya and Moruorot localities. Fossils collected from these sites are listed together under the heading 'LI' in Table 4.

The Kalodirr locality (#3, Figure 29; Leakey and Leakey, 1986a; Leakey and

Leakey, 1986b) is located at the headwaters of the Kalodirr River (3° 20' N, 35° 45' E) between Lokipenata Ridge and the basalt hills to the east. The locality extends from just south of the Kalakol-Lodwar road to the northern end of Lokipenata ridge.

The majority of the fossil sites in the Kalodirr locality occur within the continuous stratigraphic sequence between the Kalodirr Tuffs (17.3 ± 0.2 - 17.7 ± 0.2 Ma) and

Naserte Tuffs (16.8 ± 0.2 Ma; Figure 30). A few fossil sites lie above the Naserte Tuffs but none lies more than 20 m above this level. The majority of the fossils are from beds 135

22, 34 and 52 of section 31 and equivalent beds in sections 1, 1.2 and 3 (Appendix B;

Boschetto, 1988). Although problems exist with ages from pumice clasts from lahar deposits, those from the Naserte Tuff are believed reliable. Not only are there airfall tephra related to the lahars, but in addition ages from multiple samples collected from different localities are indistinguishable.

The most significant fossil discoveries at the Kalodirr locality are two new genera of primates, Afropithecus turkanensis (Leakey and Leakey, 1986a) and Turkanapithecus kalakolensis (Leakey and Leakey, 1986b). Five of 21 A. turkanensis fossil sites are placed within specific beds (beds 16 and 34, section 31) and all of these sites are located between the Kalodirr (17.3 ± 0.2 Ma) and Naserte Tuffs. Only one of four fossil sites that yielded T. kalakolensis is placed in a specific bed (bed 52, section 31). This bed lies directly below the Naserte Tuffs. Additional primate specimens include Simiolus enjiessi,

Proconsul africanus and Hominoidea indet. found in strata between the Kalodirr and

Naserte Tuffs.

The Kanukurinya locality (3° 25' N, 35° 45' E) is referred to as the Kalodirr Leaf

Site by the Kenya National Museum (#4, Figure 29). Nearly all of the specimens recovered here lie within the Kanukurinya tuff beds of the Kalodirr Tuffs (beds 3 - 19, section 6.1; beds 3-17, section 6.2).

The Moruorot localities lie on the south (#5) and north (#6) flanks of Moruorot Hill

(3° 17' N, 35° 50' E, Figure 29 and are stratigraphically equivalent to the Kalodirr and

Kanukurinya localities. The fossil bearing strata at the Moruorot localities were discovered by C. Arambourg in 1932 (Arambourg ,1943). Leakey and Leakey (1986a) and Madden

(1972) provide lists of fauna previously identified from the Moruorot localities.

The Naserte Tuff forms the top of section 10.1 at the southern site (#4) and therefore all strata exposed and all fauna collected are older than 16.8 ± 0.2 Ma (Figure

29). The majority of the fauna collected by the Kenya National Museums derive from the 136

Alomonet tuff beds of the Kalodirr Tuffs (bed 8, 9 and 10, section 10.1, bed 4, 6, 7, and

8, section 10.3), or from immediately underlying strata. This level should be equivalent to the lowest fossil-bearing strata noted by Arambourg (Figure 12, p. 179, Arambourg,

1943). Another fossil bearing stratum lies roughly 42 m above the Alomonet tuff beds and

19 m below the Naserte Tuffs (bed 36, section 10.1). This bed lies at the same stratigraphic level as bed 34, section 31 and is probably the same bed as Arambourg's highest fossil-bearing stratum (Figure 12, p.179, Arambourg, 1943).

Upper Lothidok Formation

The upper fossil bearing level of the upper Lothidok Formation comprises the Atirr and Esha localities. Taxa identified from these localities are listed together under the heading under 'Lu' in Table 4.

The age of these strata is not tightly constrained because of the lack of datable material in the section, poorly constrained correlations of lahar deposits and the unconformity between the fossil-bearing strata and the Loperi basalts. In addition, the potassium-argon age determinations of the Kamurunyang lahar represent only the time the volcanogenic minerals formed and not the age of the lahar deposition. Minimum ages for this level are established by the Loperi basalts at 12.0 ±0.1 Ma. The Kalatum basalt, correlated to a level below the fossil-bearing strata, provides a maximum age of 13.6 ± 0.2

Ma. Possible tighter constraints for each locality are discussed below.

The Esha locality (#7, Figure 29) is located along the Nakwel Esha River just north of Esha Hill (3° 17' N, 35° 48' E). The site is referred to as Moruorot-Kalatum north by the Kenya National Museums.

Provided the eroded remnants of a lahar deposit at this locality are equivalent to the

Kamurunyang lahar at section 29, the specimens are older than 13.7 + 0.2 Ma (13.4 Ma, if the possibly anomalous date of 14.0 Ma is excluded, Figure 30). However, the uncertainty 137

involved in correlating lahar deposits, as previously discussed, precludes a positive

correlation. Because the Loperi basalts unconformably overlie the fossil-bearing strata, the

specimens may be closer to the older age.

Primates collected by the Kenya National Museums include Kenyapithecus wickeri,

and unidentified hominoid and cercopithecoid specimens.

The Atirr locality (#8, Figure 29) is located approximately 6 km south of Moruorot

Hill on the east side of the Lothidok Range (3°13' N, 35° 5 l'E). The area is referred to as

Moruorot - Kalatum south by the Kenya National Museums. The fossil bearing strata here

are directly overlain by the type Kamurunyang lahar dated at 13.1 ± 0.2 Ma (Figure 30).

The strata can be securely placed between 12.0 and 13.6 Ma, and possibly predate 13.1

Ma, the eruptive age of pumice clasts in the lahar. NGAKORINGORA RIDGE STRATA

Description

The least understood feature of the Lothidok Range is the anomalous section exposed at Ngakoringora Ridge, located 100 m south of the Kalatum River and 2.5 km west of Loperi Hill (Plate). The deposits at Ngakoringora Ridge, informally referred to as the 'Carbonate Ridge', are herein referred to as the Ngakoringora Ridge strata .

Although previously interpreted as a calc-tufa deposit associated with volcanic and fault activity (Fuchs, 1939; Walsh and Dodson, 1969), leaf-fossils discovered during this study suggest the Ngakoringora Ridge strata are considerably older than the Lothidok

Formation and their presence is not straightforward.

The entire outcrop is roughly 300 m long by 100 m wide (Plate) and ~100 m of strata is exposed (section 11). The outcrop is bounded by faults to the north and south, and lithologically similar strata do not crop out at sections exposing this stratigraphic level. Deposits comprising the lower 86 m consist of siliceous and dolomitic limestones striking 300° and dipping 40° to 45° W; micrite and quartzite, also striking 300°, compose the upper 14 m of the section, but dip 21° W. The section appears to be continuous and this dip variation may have resulted from fault-related folding of the upper strata.

About 9 m of the altered limestones (from the 55 to 64 m level of section 11) actually form Ngakoringora Ridge. These very well-indurated deposits are medium to dark grey with dark brown blotches and layers. Fresh samples are white, pale yellow and tanish grey. The rock is 40-60% silica, occurring as stringers and void fillings, and

10-15% dolomite. Diagenesis of the rock nearly obliterated all depositional textures. 139

Remnant textures, typically visable only on weathered exterior surfaces and some polished slabs, include wavy, irregular, thinly laminated to thinly bedded strata that appear stromatolitic. Additional textures resemble mudcracks and small concretions.

No macro-fauna were found within these strata during this or previous studies (Walsh and Dodson, 1969). Thin needles of dolomite, observable in thin section, resemble algal filaments but were not positively indentified.

The 30 m of section above the ridge-forming limestone is very poorly exposed.

Near the 86 m level, 1 m of white to light grey, micritic limestone crops out. This moderately indurated and very sandy limestone is overlain by approximately 40 cm of dark brown to black quartzite. The quartzite appears to have been formed from a moderately to well-sorted, fine- to medium-grained sandstone. Remnant stratification suggests the original rock was thinly laminated and possibly ripple stratified.

Interbedded pockets of micritc limestone are common. No faunal remains were recovered but leaf molds occur along many of the bedding planes. The most prevalent are 1 to 3 mm wide, 2 to 15 cm long blades of a grass- or palm-like plants. Only two other types of leaf molds were found; one has been tentatively identified as a cycadaceae

(cf. cycas) and the other belongs to either Progymnospermopsida or Filicosida (B. F.

Jacobs, pers comm.).

The top of the section consists of a 1 m thick paleo-weathering horizon overlying the quartzite. The lower 70 cm is black to brown, very sandy, poorly indurated and has been altered to montmorillinite. The upper 30 cm consists of a white, chalky soil carbonate. Calcareous tubes, ranging from 2-8 mm diameter and up to 10 cm long, are common in float and in the small stream bed to the west; none was found in situ. These tubes occur separately and grouped, and possibly represent tufa deposition. 140

The Ngakoringora Ridge strata crop out at no other localities in the Lothidok

Range. Lithologically and diagenetically similar rocks, up to boulder size, occur in recent alluvial deposits of the Eragaleit and Nathuraa Rivers (Figure 4), but the source was not located. Unlike the Ngakoringora Ridge strata, these extensively altered rocks contain poorly preserved gastropod and unidentifiable fossils, which may include brachiopods, crinoids and sponges, but these have not been recognized with certainty.

Interpretation

Although the contacts are buried, exposures of upper Lothidok Formation sandstones occur east of the Ngakoringora Ridge strata and a thin, single Loperi basalt flow overlain by undifferentiated Tertiary strata crops out to the west. The occurrence of these deposits gives the appearance that the Ngakoringora Ridge strata are interbedded within the upper Lothidok Formation. Although the anomalous strata appear to lie within the Lothidok Formation, the following evidence suggests otherwise.

First, the leaf fossils suggest the Ngakoringora Ridge strata are significantly older than the adjacent strata. Based on the two large leaf fossils, the age of the quartzite is placed at pre-Cenozoic with 90% confidence and pre-Mesozoic with 60% confidence (B. F. Jacobs, pers. comm.). Second, the Ngakoringora Ridge strata strike and dip differently with respect to proximal exposures of the upper Lothidok Formation and Loperi basalts. The Ngakoringora Ridge strata strike 35° to 40° closer to west and dip up to 25° steeper. Additionally, Ngakoringora Ridge strata underwent siliceous alteration and cementation; all other strata of the Lothidok Range, including the sedimentary beds adjacent to the ridge, are cemented with CaCC>3.

The evidence disscussed above suggests the Ngakoringora Ridge strata do not lie within the Lothidok Formation. At the present, Ngakoringora Ridge is interpreted as a local erosional remnant or paleotopographic high. Exposure or uplift of this small 141 feature occurred prior to deposition of the Loperi Basalt (12.0 Ma), but does not appear to have affected sedimentation of the deposits below the Kalatum basalt (13.6 Ma). No deposits crop out between the Kalatum basalt and Ngakoringora Ridge to further constrain the timing of uplift. The ridge may have been exposed during the uplift of the

Lothidok Range between ~13.0 and 12.0 Ma.

The suggestion of Paleozoic carbonate deposits in the Turkana Basin warrants further study. Detailed paleontological studies could provide greater confidence in the age of the strata. Petrographic work on the rocks found in the Eragaleit and Nathuraa alluvium could reveal their relation to the Ngakoringora Ridge strata and may help constrain the stratigraphic positon of these sedimentary rocks. GEOLOGIC HISTORY AND PALEOGEOGRAPHY

Introduction

Interpretations of geologic history and paleogeography of the Lothidok region are

inferred from evidence found in the strata of the Lothidok Range. The following synopsis

of the geologic history is based on evidence descibed in previously in this text. The

discussion is divided into the following time intervals: -25 Ma to 17.7 Ma, 17.7 to 17.3

Ma, 17.3 to -16.4 Ma, -16.4 to -13.7, -13.7 to -13 0 Ma, -13.0 to 10.9 Ma, and 10.9 to

-5 Ma. Dates not preceeded by the symbolwere determined isotopically and those with

the symbol were extrapolated from determined ages.

-25 to 17.7 Ma

From -25 to 17.7 Ma, a minimum of 785 m of basalt, with minor intercalated

sedimentary rocks, was deposited in the Lothidok region. These basalts are similar in age

as the Samburu basalts, roughly 130 km to the southeast, although no direct correlations

have been made (King and Chapman, 1972).

The earliest record of deposition in the Lothidok region is the Eragaleit beds, which

predate the oldest dated basalt (17.7 Ma). The exact age of these beds is not known but

deposition is believed to have occurred in the early Miocene or late Oligocene Epoch.

Tephra from an unknown volcanic source formed the Nathuraa tuffs synchronously with

deposition of this sedimentary sequence.

Presence of at least one fault predating 17.7 Ma indicates active tectonism at this time but it is not clear if this tectonism formed a highland and/or an adjacent basin. 143

Volcanic flows are known to construct highland regions in Kenya (Williams, 1972) and sedimentary deposits accumulate on their flanks. If a subsiding basin was present, the paucity of sedimentary rock relative to basalts indicates that it was continualy filled with basalt, which did not allow substantial sediment accumulation.

The first period of substantial sedimentation began -18 Ma (Figure 30), with the deposition of coarse, alluvial conglomerates coeval with the deposition of the highest

Kalakol basalt flow. The anomalous thickness of this generally thin conglomerate at

Moruorot suggests the presence of a channel. The very coarse clasts of these conglomerates, up to 2.5 m diameter, indicate the source is adjacent to the Lothidok region.

Paleocurrent transport directions imply the provenance of these and later sedimentary rocks is a highland east of the Lothidok region. Intertonguing of the lowest conglomerates of the

Lothidok Formation with those below the highest Kalakol basalt flow indicates sedimentation continued as basalt deposition ceased. Deposition of the lower Lothidok

Formation followed the end of the early period of basalt deposition, persisting until at least

~16.4 Ma. Accumulation of the sedimentary deposits either represents the initial formation of a basin, or indicates that an earlier formed basin was no longer being filled with basalts.

17.7- 17.3 Ma

By about 17.7 Ma, mafic- alkaline, pyroclastic volcanism resulted in deposition of the airfall tephra and lahars of the Kalodirr Tuffs (Figure 31). This mafic-alkaline volcanism continued for about 400 ka and was synchronous with nephelinitic and carbonatitic volcanism southwestern Kenya and eastern Uganda (Bishop, et al., 1969).

Evidence discussed above suggests the source of these deposits is in the vicinity of Moiti.

The Kalodirr Tuffs and underlying sedimentary rocks provide the first constraints on the geometry of the volcanic highland and its distance from the Lothidok region. The upper conglomerate in the Kalakol basalts and the Basal Conglomerate of 144

Figure 30. Paleogeographic reconstruction of the Lothidok region at -18 Ma and explanation of symbols used for paleogeographic reconstructions of the Lothidok region (Figures 31-36). EXPLANATION

Perennial Stream Bed Ephemeral Stream Bed Alluvial Fan H -H ia h f 'y

L- Low Highland Margin Active Volcanic Center / j V Inactive Volcanic Center Volcanic Center-Uncertain if Active Inferred Normal Fault 'x '° Bed Dip Lahar Deposit • V v ' Basalt Deposit ''''■i- : Basement Exposure 146

0 2 5 50km 147

Figure 31. Paleogeographic reconstruction of the Lothidok region at 17.7 Ma. 148

Mafic Alkaline Volcanic Center

50k m 149 the Lothidok Formation crop out over the entire Lothidok Range. This requires the highland to have been at least as long as the Lothidok Range, which is 30 km. The airfall and lahar deposits of the Kalodirr Tuffs establish a limit of 100 km for the maximum distance to the volcanic highland. The width of the highland must be less than this because similar strata are absent at Buluk (Figure 4), roughly 150 km in the direction of the inferred location of the highland, although the strata at Buluk are of the same age (Watkins, 1982).

The highland was not large enough to contribute sediment to both the Lothidok and Buluk areas, and therefore its width was probably less than 50 km.

17.3 - -16.4 Ma

Sedimentation at this time continued into the basin at rates of 10 to 30 cm/ka. The depositional features of these sedimentary rocks imply deposition by braided streams on an alluvial plain or fan. Sediment transport directions during this time varied from the south to the west suggesting that a major stream system continued draining the volcanic highland to the east. The presence of Etheria elliptica fossils in situ indicates that at least some streams were perennial.

Deposition of trachytic tephra and lahars is first recorded in strata after 17.3 Ma.

Following the initial eruptions, deposition of trachytic tephra and lahars becomes quite frequent. These deposits include the Naserte Tuffs and Nakwel Esha beds. The source(s) of the trachytic tephra in also unknown, but because the lahar deposits are commonly in direct association with airfall tephra, they are interpreted to originate from the same volcano or volcanic system. Presence of lahar deposits suggests the volcanic center was located near the Lothidok region at 16.8 Ma (Figure 32). Because southern outcrops contain the thickest, coarsest and most frequent lahars, and because the streams sytems drained to the west, the source is interpreted to have been southwest of the Lothidok region. Only minor fluvial deposition is recorded in southern sections following initial deposition of trachytic 150

Figure 32. Paleogeographic reconstruction of the Lothidok region at -16.8 Ma. 151 1 5 2 lahars. Fluvial deposition dominated the sedimentary processes to the north. The lack of fluvial beds suggests that the southern area was topographically higher than the north, possibly as a result of continual lahar deposition and/or the presence of a trachytic volcanic highland to the south.

-16.4 --13.7 Ma

The disconformity within the Lothidok Formation represents a hiatus of as much as

2.7 Ma. It is not clear if this disconformity reflects a change in base level, regional uplift, or merely an extended period of nondeposition.

-13.7--13.0 Ma

The disconformity is followed by rejuvenated basin subsidence and accumulation of the sedimentary rocks that comprise the upper Lothidok Formation. Sedimentation rates were as high as 60 cm/ka. A complete reversal in paleocurrent directions in all sediments above the disconformity reflects a sediment source to the west at this time. The volcanic highland to the east was no longer a sediment or volcanic source, and was either eroded during the period represented by the disconformity, or buried in a subsiding basin.

Initially (-13.7 Ma), the strata represent braided stream deposition on alluvial fans at -13.7 Ma (Figure 33). The upper sedimentary rocks, however, reflect deposition by meandering streams at - 13.0 Ma (Figure 34). Fossils of Etheria elliptica indicate perennial flow in some streams.

Trachytic airfall tephra and lahars were deposited throughout the upper Lothidok

Formation. These deposits are indistinguishable from those of the lower Lothidok

Formation and appear to originate from the same source(s). However, the lahars must have used eastward-draining stream systems and therefore could not have originated from the same volcano(es) as the lahars of the lower Lothidok Formation. This implies at least 1 5 3

Figure 33. Paleogeographic reconstruction of the Lothidok region at ~13.7 Ma. 1 5 4 1 5 5

Figure 34. Paleogeographic reconstruction of the Lothidok region at -13.0 Ma. 1 5 6 1 5 7

one other trachytic volcanic center or system existed near the Lothidok region. Similar to those of the lower Lothidok Formation, the lahar deposits are thickest and more numerous in southern outcrops, and therefore the source is thought to have been to the southwest of the Lothidok region (Figure 35).

-13.0 to 10.9 Ma

Faulting between ~13.0 and 12.0 Ma rotated the Lothidok strata down to the west and approximately 10° (Figure 35). The fault(s) must have been parallel to the present orientation of the Lothidok Range because the strike of the erosion surface is roughly parallel to the strike of the overlying strata. Erosion beveled the uplifted and tilted strata, producing an irregular erosional surface. Uplift and exposure of the Ngakoringora Ridge

strata are interpreted to have occurred at this time. Successive flows of the Loperi basalts

then buried the southern Lothidok region. Because of limited outcrop, the northern extent

of the basalt flows is not known. To the south, basalt deposition at Loperot (Figure 4)

began at 13.9 Ma and continued through this time interval. Thin trachytic tuffs,

interbedded within the Loperi basalts, are the last record of trachytic volcanism in the

Lothidok region.

10.9 to ~5 Ma

Deposition of very coarse volcaniclastic conglomerates followed deposition of the

Loperi basalts (Figure 36). Finer-grained, arkosic sandstones overlie the conglomerates but because of the unconformity within this sequence, the time of deposition is unclear.

The unconformity may have resulted from erosion of a southward-progressing uplift. This period of erosion occurred at the roughly the same time as at Baragoi where a mid-Miocene planation surface is recorded (Baker, et al., 1972).

The very coarse volcaniclastic deposits overlying the Loperi basalts indicate a 1 5 8

Figure 35. Paleogeographic reconstruction of the Lothidok region at -12.0 Ma. 1 5 9 1 6 0

Figure 36. Paleogeographic reconstruction of the Lothidok region between 10.9 and ~5 Ma. 161 proximal source. Potential source terrains are the phonolite and basalt flows to the west

(Smith, 1938; Walsh and Dodson, 1969). Provenance of the arkosic sediments is either the Precambrian basement or the Cretaceous arkosic sediments to the west. Faulting is known to have been active at 4.1 Ma approximately 30 km to west of the Lothidok Range, where Precambrian basement is presently exposed (F. H. Brown, pers. comm ). CONCLUSIONS

Recent discoveries of three new hominoid species prompted the interest in defining the stratigraphy of the Lothidok range of northern Kenya. The stratigraphy of the Lothidok

Range consists of the Kalakol basalts (new informal name), the Lothidok Formation (new stratotype), the Loperi basalts (new informal name) and undifferentiated Tertiary sedimentary strata. The total thickness of the strata of the Lothidok Range is about 1500 m.

The Kalakol basalts consist of a mimimum of 785 m of olivine-augite basalts and intercalated fluvial, sedimentary rocks. One sequence of fluvial strata contains abundant vertebrate remains, named the Eragaleit beds. The base of these basalts is not exposed in the Lothidok Range.

The Lothidok Formation consists of a minimum of 540 m of interstratified fluvial sedimentary rocks, lahars, mafic alkaline and trachytic tephra and minor basalt flows. The formation is informally divided into the lower and upper Lothidok Formation. Three units in the lower Lothidok Formation, formally defined as members, are the Basal

Conglomerate Member, the Kalodirr Tuff Member and the Naserte Tuff Member. Five informally named units are the Nakwel Esha beds, the Lokipenata conglomerate, the

Kalatum basalt, the Akwang’a basalt and the Kamurunyang lahar. The boundary between the lower and upper units is a disconformity, estimated to represent 2.7 Ma. of non­ deposition.

The Lothidok Formation is overlain by 0 - 121 m of the Loperi basalts with minor intercalated fluvial and pyroclastic rocks. The thickest exposures occur in the south and thin progressively to the north. The contact between the Loperi basalts and the Lothidok 164

Formation is taken at an angular unconformity representing as much as 1.1 Ma. The angular discordance between the Lothidok Formation and the overlying unit is approximately 10°-

A minimum of 80 m of undifferentiated Tertiary strata lies above the Loperi basalts in southern exposures and above the Lothidok Formation to the north. The base of the arkosic sediments of this sequence may also be an unconformity although this is not proven.

The sedimentary rocks of the lower Lothidok Formation consist predominantly of conglomerates and conglomeratic litharenites. These are interpreted to have been deposited by braided streams on an alluvial complex draining to the southwest. The upper Lothidok

Formation consists of conglomerates, conglomeratic litharenites and feldspathic litharenites at its base interpreted as braided stream, alluvial fan deposits. Near the top, the formation is dominated by finer grained lithic arkoses, arkoses and subarkoses representing meandering stream deposition. Paleocurrents in the upper Lothidok Formation indicate sediment transport to the northeast marking a complete reversal in drainage during the period of the disconformity.

New potassium-argon dates were determined for 19 samples of volcanogenic minerals and basalts associated with this section. Sedimentation of the Lothidok Formation occurred from -17.7 to ~ 12.0 Ma.

Two unconformities are recognized within the strata of the Lothiodok Range. The first is disconformity at the boundary between the lower and upper Lothidok Formation that represents a time gap from -16.4 to ~13.7 Ma. The second is an angular unconformity between the Lothidok Formation and the Loperi basalts. A third unconformity may exist at the base the undifferentiated Tertiary arkosic sandstones.

The structure of the Lothidok Range is dominated by normal faults formed in the Late

Tertiary. Only one fault was found to predate deposition of the Lothidok Formation. This 1 6 5

fault is older than 17.7 Ma. The remaining faults postdate the undifferentiated Tertiary

arkosic sandstones, which are younger than 10.9 Ma. Movement along the range front

fault in the northern Lothidok Range postdates 4.1 Ma. The fault is subsurface in the

southern Lothidok Range and the overlying strata are folded.

The stratigraphy now established for the Lothidok Range provides a context for placing

the recent and previous fauna collections. Based on potassium-argon ages, the fauna can

be divided into two groups. The first is between 17.7 to 16.4 Ma (equivalent to Rusinga)

and the second is between 13.7 to 12.0 Ma (slightly younger than Fort Teman).

None of the sources for the sediments or the pyroclastic rocks is known, but the

sedimentary and pyroclastic rock record allows the location of these sources to be inferred,

and provides information from which to interpret the geologic history of the Lothidok

Range. These rocks suggest that an early Miocene volcanic highland and mafic alkaline

volcanic center lay in the present location of Lake Turkana, and that a middle Miocene

trachytic center(s) was located south of the Lothidok Range. APPENDIX A

LIST OF MEASURED SECTIONS IN THE LOTHIDOK RANGE 1 6 7

Section # Strike and Dip Thickness fm) Exposed unitis) 1 334, 26 W 518.0 lower Lothidok Formation 1.2 341, 11 W 132.9 lower Lothidok Formation 3 340, 11 W 288.6 lower Lothidok Formation 4 328, 20 W 68.8 lower Lothidok Formation 6.1 348, 14 W 72.2 lower Lothidok Formation 6.2 345, 15 W 248.4 lower Lothidok Formation 7 348, 12 W 448.5 Kalakol basalts 8 357, 40 W 265.6 upper Lothidok Formation 10.1 340, 14 W 144.0 lower Lothidok Formation 10.3 327, 26 W 95.1 lower Lothidok Formation 11 300,21-45 W 100.0 Ngakoringora Ridge strata 15 -10, 0-5 W 22.2 undiff. Tertiary deposits 16 20, 10 W 54.4 Kalakol basalts (Eragaleit beds) 17 330, 25 W 166.7 + -200 Kalakol basalts (Eragaleit beds) 18 325, 19 W 27.3 lower Lothidok Formation 19 320, 20 W 82.7 lower Lothidok Formation 20 346, 11 W 50.6 & 11.8 lower Lothidok Formation 21 285, 10W 145.0 lower Lothidok Formation 22 330, 15 W 186.7 lower Lothidok Formation 23 350, 15 W 17.0 lower Lothidok Formation 24 30, 22 E 118.0 upper Lothidok Formation 25 25, 20 E 82.2 upper Lothidok Formation 26 315, 20 W 208.5 upper Lothidok Formation 27 325, 20 W 58.0 upper Lothidok Formation 28 355, 30 W 92.6 upper Lothidok Formation (gap in sect. 8) 29 340, 40 W 400.1 upper Lothidok Formation 30 330, 24 W 552.0 Lothidok Formation 31 345, 15 W 285.5 lower Lothidok Formation 32 345, 15 W 152.7 Kalakol basalts (below sect. 1) 33 341, 11 W 12.6 Kalodirr Tuffs 34 345, 20 W 164.0 lower Lothidok Formation 35 320, 12 W 69.1 upper Lothidok Formation 36 320, 12 W 62.6 upper Lothidok Formation 37 330, 18 W 319.7 upper Lothidok Formation (above sect. 4) 38 330, 18 W 134.3 upper Lothidok Formation (above sect. 3) 39 0, 13 E 51.6 lower Lothidok Formation 40 327, 26 W 90.1 Kalakol basalts (below sect. 10.3)

Average Strike = 340° Average dip = 18° APPENDIX B

FIELD DESCRIPTIONS OF THE COMPOSITE STRATIGRAPHY OF

THE LOTHIDOK RANGE 1 6 9 Top of composite section

Section 29: S 320°, D 40° W

UNDIFFERENTATED TERTIARY DEPOSITS

End of continuous section 49. Sandstone, pale yellow to white, weathers dark grey, fine- to very coarse-grained, abundant scattered fine to very coarse pebbles, poorly sorted to conglomeratic, clasts: angular to subrounded, 60% quartz, 15-20% kspar, minor lithics and rip-up clasts, locally extremely well-cemented, very poorly cemented overall, medium-scale trough and planar crossbeds, weathers to unconsolidated quartz and kspar pebbles. Sample K87-3442 8.5 m/ 1538.5 m

No exposure 1.7 m/ 1530 m

48. Sandstone, pale red, fine- to medium-grained, moderately sorted, 65-70% quartz, 5­ 10% kspar, <2% pyroxenes, <2% biotite, moderately to very well-cemented, calcite cement, interbedded 10-20 cm thick, red siltstones, poorly consolidated, interbedded 15-30 cm thick, extensively burrowed and poorly defined trough cross-stratified horizons, burrows commonly filled with muddy sandstone, generally vertical, solitary, rarely branch, ,4-.8 cm diameter, extensive hematite staining. Sample K87-3441. 2.2 m/ 1528.3 m

No exposure 6.8 m/ 1526.1 m

47. Mudstone, tan to dark brown, pale red, very silty, abundant very fine-grained sand, massive, poorly consolidated, abundant calcareous root casts, 1-10 cm thick white, chalky, soil carbonate, interbedded with sandstones (see below). 1.9 m/ 1519.3 m

No exposure 2.9 ml 1517.4 m

46. Sandstone, pale reddish grey to pink, very fine- to coarse-grained, subrounded to rounded, 65% quartz, 15% kspar, <5% lithic grains, <2% pyroxenes/amphiboles, calcite cement locally very well-cemented, poorly cemented overall, occasionally scattered quartz and kspar clasts, fine pebbles, fines upward, poorly sorted to conglomeratic at base, poorly defined small- to medium-scale trough crossbeds. Sample K87-3440. 3 m /1514.5 m

45. Sandstone, grey, weathers dark grey, very fine- to very coarse-grained, abundant fine to medium pebbles, subangular to subrounded, 45-55% quartz, 25-30% lithic grains, minor kspar, calcite cement, crudely bedded, very poorly sorted, poorly exposed. Sample K87-3439. .6 m/ 1511.5 m 1 7 0 ...... UNCONFORMITY ?------

44. Conglomerate, very poorly exposed, clasts: small cobbles to small boulders, phonolite and basalt, Fe stained up to 40-80% surface area of clasts, unconsolidated, no matrix exposed 30.6 m /1510.9 m

BASE OF UNDIFFERENTATED TERTIARY DEPOSITS

TOP OF LOPERI BASALTS

43. Basalt, grey, fine plagioclase phyric, abundant calcite amygdules. 53.7 mJ 1480.3 m

42. Basalt, dark grey, coarse plagioclase phyric, minor altered olivine, abundant chlorite, minor calcite amy dules. 43.9 m/ 1426.6 m

41. Basalt, dark grey, augite phyric, augite up to 6 mm diameter, abundant calcite amygdules, extensivley altered. 30.6 ml 1382.7 m

BASE OF LOPERI BASALTS Section 29: S 320°, D 40° W

------ANGULAR UNCONFORMITY------

Section 25: S 25°, D 20° E TOP OF THE UPPER LOTHIDOK FORMATION

17. Sandstone, grey, interbedded well-sorted, fined-grained, poorly consolidated, very poorly sorted to conglomeratic, non- to well-consolidated, subangular to subrounded, 55% quartz, 10% kspar, 15% lithic grains, 5% pyroxenes/amphiboles, calcite cement, small-scale trough crossbeds. 5.1 m/ 1352.1 M

No exposure 13.6 ml 1347 M

Kamurunyang lahar 16. Lahar, brownish yellow, massive, matrix support, clay to coarse sand grade matrix,~20% pebble to boulder clasts, clasts: phonolite and basalt, angular to subrounded, 15-25% pumice, pumice: altered to clay/zeolite and or calcite, clayey pumice weathers to produce a vesicular texture on surface of bed, clast support lenses common near base, calcified pumice weathers from upper surface, up tp 12 cm diameter and 20 cm in long dimension, crude reverse grading of pumice, Samples K87- 3410, K87-3411 (pumice). 10.2 ml 1333.4 M 171 Sharp, irregular, nonerosional contact

15. Mudstone, dark brown, massive, well-consolidated, -45% silt to very fine-grained sand (lithic, quartz?, amphibole grains). .4 m1 1323 m

14. Tuff, pale olive green to grey, massive?, -55% pumice (altered to clay/zeolite), up to 4 mm diameter, randomly oriented, clay/zeolite matrix, minor sand grade fragments, decreasing pumice upwards, no pumice in upper .5 m. 1.7 m/ 1322.8 m

13. Mudstone, weathered outcrop, brown, moderately to very sandy. 1.8 m/ 1321.1 m

Gradational contact

12. Sandstone, pale olive to grey, very fine grained, abundant silt/clay, moderately to poorly sorted, 45% quartz, 25% pyroxene, poorly to moderately consolidated, ripple stratified, grades to sandy mudstone, massive, well-consolidated. I.6 m/ 1319.3

II. Sandstone, bed 9 continued ,9 m/ 1317.7 m

No exposure .8 m/ 1316.8 m

10. Conglomerate, clast support, basalt, quartz, phonolite clasts, average small cobbles, up to medium cobbles, nonconsolidated, lateraly discontinous, massive(?), matrix: rarely exposed, medium- to very coarse-grained sand (lithic, quartz grains), rounded, abundant pyroxene grains, calcite cement, grades to sandstone. .7 m/ 1316.0 m

Gradational contact

9. Sandstone, grey to reddish grey, dark brown, well-consolidated, poorly sorted to conglomeratic, fine- to coarse-grained, subangular, -55% quartz, -20% kspar, 5 % pyroxenes, calcite cement, medium-scale trough crossbeds, abundant vertebrate fossils, interbedded siltstones up to .8 m thick, tan to grey, ripple stratified, poorly consolidated. 6.0 m/1315.3 m

No exposure .6 m/ 1309.3 m

8. Mudstone, olive with yellow to orange limonite streaks, very thinly laminated, abundant salt, very poor exposure. Sample K87-3413. .7 ml 1308.7 m

No exposure .6 m/ 1308 m 172 7. Tuffs/ashes/reworked tuffs, pale yellow to yellowish tan, abundant fine to coarse pumice lapilli, minor very fine biotite, thinly laminated to thinly bedded massive to ripple stratified, occasionally horizontally laminated, ashes occasionally altered to mudstones. 1.9 ml 1307.4 m

6. Sandstone, grey to reddish grey, weathers dark brown,fine- to medium-grained, subangular to angular, moderately sorted, minor clay/silt in matrix, 60-70% quartz, 5­ 15% amphiboles/pyroxenes, calcite cement, minor biotite, minor kspar, small scale trough crossbeds, very poorly exposed. 5.1 ml 1305.5 m

5. Tuffs, pale yellow to dark yellow, massive, coarse pumicous bed at base, very fine, thinly laminated beds above, grades into tan to grey mudstone, very sandy, abundant amphiboles and lithic sand grains, medium to very coarse pumice lapilli. .6 m/ 1300.4 m

4. Siltstone, grey, ripple stratified, well-consolidated, occasional pumiceous beds, pumice: very coarse-grained, grades into tan to brown pumiceous mudstone (altered tuff?). .3 ml 1299.8 m

3. Reworked tuffs/tuffs, tan to yellowish grey, very fine- to very coarse-grained, grains: primarily lithic, minor quartz, sanidine, medium- to coarse-grained pumice, very poorly sorted, ash grade matrix, ripple stratified to small-scale trough crossbeds in reworked tuffs, well- consolidated. 3.7 ml 1299.5 m

2. Tuffs?, pale yellow to dark yellow, slightly sandy, abundant accretionary lapilli, moderately consolidated, massive, occasionally ripple stratified grading into small-scale trough crossbeds, overlain by thin lahar: tan, abundant medium lithic clast pebbles, abundant medium pumice. .8 m7 1295.8 m

1. Siltstone, grey to olive grey, well-consolidated, ripple stratified?, grades to very fine, ripple stratified sandstone, occasionally grades to mudstone, abundant biotite. Sample K87-3412. .5 ml 1295 m

No exposure 1.5 m/ 1294.5 m Section 25: S 25°, D 20° E

Section 24: S 30° D 22o E Akwang'a basalt 5. Basalt, dark grey, extensively weathered, coarsely phyric, pyroxene phenocrysts, abundant olivine? (altered), abundant calcite amygdules, abundant chlorite and clay?, abundant cavities. .9 ml 1293 m 1 7 3 4. Sandstone, grey, very fine- to very coarse-grained, very poorly sorted to conglomeratic, subangular to angular, occasionally rounded, 60% quartz, 5-15% amphiboles/pyroxenes, minor olivine, garnet?, kspar, 15-25% calcite cement, < 10% lithic grains, abundant scattered kspar, quartz, lithic pebbles, occasionally moderately sorted, beds coarsen laterally, medium-scale trough crossbeds, veiy low-angle to planar crossbedding, very poorly to well-consolidated occasional interbedded siltsones, tan to tannish red, well-consolidated, ripple stratified, occasional interbedded mudstones, mudstones: brown, abundant mudstone rip-up clasts, poorly exposed. Sample K87-3402. 31.4 m/ 1292.1

No exposure 1.7 m/ 1260.7 m

Sandstone, bed 2 continued, very poor exposure. 1.7 m/ 1259 m

3. Mudstone, brown, massive, well-consolidated, very sandy, occasional FeC>2 spots and streaks, pinches out into sandstone above. 1.7 m/ 1257.3 m

2. Sandstone, pale red, very fine- to very coarse-grained, rare to abundant scattered pebble clasts, angular to subangular, 55% quartz, 15% pyroxenes, calcite cement, apparent medium-scale trough crossbeds, poorly exposed, lenticular, matrix support, abundant hematite (clay?) in matrix, clasts average fine to medium pebbles, moderately to well- cemented, coarsens up, grades into conglomerate locally, clasts: up to large cobbles, ave medium pebbles, primarily basalt and phonolite, quartz clasts up to small cobbles, extensive FeC>2 staining, poorly imbricated, occasionally clast supported, poorly consolidated. Sample K87-3401. 5.4 m/ 1255.6 m

1. Sandstone, grey, very fine- to very coarse-grained, very poorly sorted, subangular to angular, occasionally subrounded, 45% quartz, 10% amphibole, 10% lithic grains, abundant clay, 25% calcite cement, medium- to large-scale trough crossbeds, occasional scattered quartz pebbles, poorly exposed. .4 m/ 1250.2 m Section 24: S.3Q0, D 220 E

Section 30: S 330°, D 220 W

59, cont., Sandstone, grey to pink, red to dark red, dark grey, fine -o very coarse-grained, angular, 45-55% quartz, 15-20% lithic grains, 5-8% kspar, 5-10% euhedral pyroxenes, very poorly sorted to conglomeratic, clasts: fine to coarse pebbles, FeC>2 coating common, predominantly basalt and phonolite, minor quartz, bioturbated, medium- to large-scale trough crossbeds, moderately cemented, 15-20% calcite cement, basal conglomerate, nonconsolidated (float only), predominantly phonolite and basalt cobbles, FeC>2 coating common, 10-15% quartz, no matrix exposed. 34 2m/ 1249.8 m 174 No exposure 10 m/1217.6 m

59. Sandstone continued .4 m/ 1207.6

Sharp, scour contact

58. Tuff, pale red, massive, very well-consolidated, 10-15% euhedral, medium to coarse, amphiboles/pyroxenes, 10-15% medium to coarse lithic grains, ash to very fine tuff matrix, abundant calcified plant debris. .3 ml 1207.2

Sharp contact

57. Reworked tuff, light grey, very fine- to very coarse-grained, poorly sorted, subrounded to subangular, 30-40% pumice clasts (altered to calcite), 35-45% lithic grains, 10-15% quartz, 10-15%, calcite cement, 5-8% pyroxenes, small-scale trough crossbeds, grades to red sandstone 7.8 my 1206.9

Sharp contact

56. Tuff, pale yellow, thickly laminated to thinly bedded, ripple stratified, deformed bedding, vertical burrows, filled with dark brown mudstone and tuffaceous material, meniscate fill. 4.4 m/1199.1m

Sharp contact

55. Sandstone, dark grey to pale red, medium- to fine-grained, normally graded, very poorly sorted to conglomeratic, 1-3 m upward fining sequences, clasts average small pebbles, abundant silt, abundant mud, interbedded brown mudstones. 10.4 m/ 1195 m

No exposure 3.6 ml 1184.6 m

54. Reworked tuff, pale yellow, very fine to coarse grained, 20-30% silt, 5-10% lithic grains, 2-4% biotite, 4-6% amphibole/pyroxenes, 50-60% very fine to very coarse pumice clasts (altered to calcite), thinly laminated, ripple stratified to small-scale trough crossbeds, local pale red to orange stain. 1.5 ml 1181 m

53. Sandstone, red to dark red, fine to coarse grained, poorly to well-sorted, angular to subrounded, 55-65% quartz, 10-12% lithic grains, 5-8% pyroxene, 15% fine to medium pebbles, grades laterally to very coarse, poorly sorted conglomeratic sandstone, clasts: predominantly quartz, basalt, phonolite, rare kspar, ignimbrite, medium-scale trough crossbeds, upper 1.5 m ~30-40% fine to very coarse pumice clasts (alterd to calite), grades to reworked tuff, moderately very poorly cemented, 10­ 25% calcite and hematite cement 1.8 ml 1179.4 m 1 7 5

52. Tuff, pale yellow, very poorly exposed, fine to medium, interbedded thinly laminated and massive beds, extensively burrowed, burrows filled with dark brown mud. 1.5 m/ 1174.6 m

No exposure 1.5m/ 1173.1 m

51. Sandstone, red to dark red, very fine- to medium-grained, subangular to angular, poorly sorted, 20-25% sanidine, 35-45% quartz, 4-6% pyroxene, 5-18% calcite cement, 5-8% hematite cement, non- to very poorly cemented, extremely well-cemented locally, appears massive, abundant burrows(?). .3 m/ 1173.1 m

No exposure 3.0 m/ 1171.3

50. Sandstone, dark red, very fine- to fine-grained, subangular, 45-65% quartz, 15-25% silt, pyriboles 5-100%, nonconsolidated. .2 m/ 1168.1 m

49. Reworked tuff, light grey, interbedded very fine-grained, moderately sorted and fine- to coarse-grained poorly sorted, 20-45% very fine to very coarse pumice clasts (altered to calcite), ineversely graded, abundance increases upwards, 15-40% matrix, 15-25% very fine, subrounded pyriboles, rounded lithic grains, minor euhedral sanadine crystals, moderately consolidated, very thinly bedded to thickly laminated, poorly exposed. 1.6 m/ 1168.1 m

No exposure 10.8 m/ 1166.5 m

Sandstone continued, extremely poor exposure. 3.7 m1 1155.7 m

Sandstone continued, moderately to poorly sorted, grains medium- to very coarse-grained, occasionally conglomeratic, 5-8% quartz and volcanic pebbles. 3.4 m /1152 m

Sandstone, continued. 1.2 m/ 1148.6 m

No exposure 6.8/41.1

48. Sandstone, red to dark red, very fine- to fine-grained, subangular, well-sorted, 65­ 75% quartz, 5-10% pyriboles, -2% biotite, moderately cemented, 5-15% calcite cement, 5-10% hemitite cement, very poorly exposed. 1.2 m/1142 m

No exposure 1.5 m/1140.6 m 1 7 6

47. Lahar, pale yellow, minor red staining, massive, 35-45% pumice clasts (altered to calcite), ~ 15-20% fine to very coarse, angular pebbles, minor euhedral biotite and sanadine, vesicular weathering texture, very poorly exposed, primarily cobble/boulder float, abundant coarse angular pebbles. 1.7 m/ 1139.1 m

No exposure 2.9 m/1137.4 m

46. Conglomerate, grey, weathers dark grey, very poorly sorted, occasional mud matrix, clasts (in situ): fine to coarse pebbles, average medium, angular, 60-65% quartz, 5-8% pyriboles, 5-10% kspar, 5-10% mud, clasts (float): coarse pebble to small cobbles ~60- 65%, 20-25% phonolite 10-20% basalt, 5-10% quartz, <5% kspar, <2% ignimbrite, moderately cemented, medium-scale trough crossbeds,/, thinly interbedded sandstones, common, non- to moderately consolidated. 9.3 mI 1134.5 m

No exposure 3m/l 125.2 m

Kalatum basalt 45. B. Basalt, dark green, aphyric at base, grades? into very coarse plagioclase and amphibole phyric, aphanitic groundmass. 8 ml 1122.2 m

45. A. Basalt, dark green, basal flow: agglomerate, blocks: up to 2 m diameter, very coarse plagioclase phyric in aphanitic groundmass. 12 m /1114.2 m

Contact covered

Section 30: S 330°, D 22° W

BASE OF UPPER LOTHIDOK FORMATION

------DISCONFORMITY------

TOP OF LOWER LOTHIDOK FORMATION

Section 19: S 325°, D 19° W

Lokipenata conglomerate 34. Conglomerate, red, clast support, clasts primarily phonolite and basalt, Fe02 stained, medium to large cobbles, average coarse to very coarse pebbles, matrix: very fine- to very coarse-grained sand, rounded to subangular, abundant pyroxene/amphiboles, moderately consolidated. 5.1 ml 1102.2 m 1 7 7 33. Conglomerate, red, matrix support, abundant 20-40 cm white, soil carbonates at base, clasts: phonolite, basalt, up to medium cobbles, average very coarse pebbles, majority are stained with Fe02, poorly imbricated, poorly consolidated, matrix: white, clayey, soil carbonate?, rare rip-up clasts from unit below. 2.8 m/ 1097.1 m

32. Sandstones, mudstones and conglomerates (as below), sandstone: yellow, medium-to coarse-grained, poorly sorted, subrounded to rounded, abundant pumice, abundant clay in matrix, moderately consolidated, mudstones: dark brown, sandy, abundant coarse grade pumice, average 10 cm thick, moderately consolidated. 4.6 m/ 1094.3 m

31. Conglomerate, green clasts in yellow matrix, matrix support, clasts: up to medium boulders, average large cobbles to small boulders, primarily phonolite with rare basalt, laterally discontinous, fines upward, abundant clast supported boulder lenses. 1.2 m/ 1089.7 m

30. Conglomerate, yellow, matrix support, clasts up to small cobbles, average medium to coarse pebbles, 80% phonolite, 10% pumice, very crudely bedded, matrix: medium- to very coarse-grained lithic, pumice and pyroxene/amphibole grains, rounded to subrounded. 5.8 m/ 1088.5 m

No exposure 5.8 m/ 1082.7 m

Sharp erosional contact

29. Sandstone,/conglomerate, sandstone: yellowish tan, very fine to very coarse-grained, subrounded to angular, primarily lithic grains, 25% pink and yellow pumice, pumice increases to 50% up and and to 1 cm diameter upwards, poorly sorted, grades into conglomerate: yellow to pinkish yellow, clast support, clasts up to medium to large cobbles, average medium pebbles, primarily phonolite, minor basalt, fines upward into very fine- to coarse-grained conglomeratic sandstone, abundant scattered small pebbles in very fine- to very coarse-grained, very poor sorting abundant clay/altered ash? in matrix, abundant yellow & pink pumice up to 1 cm diameter, medium-scale trough crossbeds, poorly defined, normal and reversely graded, upward fining sequences average 10-15 cm thick. 5.4 m/ 1076.9 m

No exposure 1.7 m/ 1071.5 m

Poor exposure

2 m/ 1069.8 m

28. Sandstone, pale yellow, fine- to medium-grained, well-consolidated, rounded to subrounded, some amphibole(?), occasionally very coarse-grained, rare small pebbles, poorly sorted, abundant clay/silt in matrix, massive, coarsens upwards into conglomeratic sandstone, moderately consolidated. .5 m/ 1067.8 m 178

Sharp erosional contact (pinch and swell)

27. Tuff(?)/altered ash, ash matrix with very fine-grained sand, occasionally very coarse pumice lapilli. .2 m/ 1067.3 m

26. Tuff(?), tan, yellow & pink pumice lapilli up to 1 cm diameter, randomly dispersed throughout bed, 15-20% very fine to coarse lithic grains, rounded, layers occasionally 90% pumice, pumice usually flattered, elongate parallel to bedding. Sample K87-3310. .7 ml 1067.1 m

Sharp, irregular contact

Nakwel Esha beds 25. J. Conglomerate, yellow, clast support, clasts: small to medium cobbles, primarily phonolite, fines upward to very coarse-grained sandstone, crude graded bedding, crudely graded overall, abundant pumice, clasts occasional large rip-up clasts from bed below. Sample K87-3311. .6 ml 1066.4 m ‘

Erosional contact

I. Sandstone, reworked tuff, yellow grey, medium- to coarse-grained, moderately sorted, moderately consolidated, thinly bedded, abundant pumice. .2 ml 1065.8 m

Sharp contact

H. Tuffs, ashes?, laminated and massive, abundant 1 cm long pink, medium grade, yellow pumice (altered to clay/zeolite), waxy/soapy texture, very poor exposure, upper 2 cm is a brown mudstone. .4 ml 1065.6 m

G Tuff/reworked tuff, coarse-grained, pink to yellow, mostly pumice. .3 ml 1065.2 m

F. Claystone?/altered ash?, grey thinly laminated, moderately to well-consolidated, spreroidal weathering. .1 ml 1064.9 m

Sharp contact

E. Tuffs?, reworked tuffs?/ashes?, interbedded pink and yellow very coarse tuffs and grey altered ashes, thinly to thickly laminated, 40% pumice lapilli, abundant subangular to subrounded lithic grains, moderately sorted, ashes altered to clay/zeolite. .2 m/ 1064.8 m 179 Sharp erosional contact

D. Mudstone, dark brown, moderately consolidated, abundant black MnC>2 speckles, minor yellow & pink pumice, interbedded thin, discontinuous, 5 cm thick, very coarse­ grained sandstone. .6 m/1064.6 m

Sharp contact

C. Tuff?/Lahar?, tan to brown, massive, matrix support, crudely inversely graded, clasts predominantly large yellow and green pumice, green pumice up to 20 cm diameter, yellow pumice up to 8 cm diameter, average 2 cm, matrix: clay?/altered ash, occasional medium cobble grade phonolite clasts. Samples K87-3309, K87-3311. .6 ml 1064 m

B. Mudstone?/altered ash?, dark brown, grades into overlying bed(?). 1.3 ml 1063.4 m

Sharp, irregular, nonerosional contact

A. Tuffs/ashes, grey, grey with yellow speckles, alternating 10-18 cm thick upward fining sequences, abundant very fine to coarse grade pumice lapilli layers, pumice: yellow, up to 2 cm long, 1 cm diameter, parallel to bedding, not graded, commonly in clasts support, matrix: fine to medium grade, very thin planar laminations, minor very low-angle crossbedding, crudely bedded, drapes underlying topograghy, reworked tuffs: tan, grey, fine- to medium-grained, moderately sorted, rounded, abundant clay?/altered ash? matrix, fine to coarse pumice, crude planar bedding, overall bed coarsens upward, pumice clasts up to very coarse pebble grade, occasional fine lithic pebbles, up to 60% pumice at top of bed. 1 m/ 1062.1 m

No exposure 9.4 m/ 1061.1 m

24. Sandstone, greyish yellow, very fine- to coarse-grained, poorly sorted, abundant clay/silt in matrix, primarily subrounded, volcanic grains, <5% quartz & kspar, <5% coarse pumice grains, .3 ml 1051.7 m

23. Mudstone, dark brown to tannish grey, moderately to very, sandy, occasionally abundant pumice, moderately to well-consolidated, massive. 1.6 m/ 1051.4 m

22. Reworked/altered tuff, pale yellow to yellow, 20% pumice, up to very coarse grade, abundant mudstone rip-up clasts, massive moderately to well-consolidated. .3 ml 1049.8 m

21. Mudstone, brown with brownish yellow and black mottle, 15-20% very fine- to coarse-grained, moderately consolidated, up to 90% pumice in some layers, interbedded 10-20 cm thick medium- to coarse-grained reworked tuffs. 2.7 m/ 1049.5 m 1 8 0 Sharp contact

20. Sandstone (altered/reworked tuff?), tannish yellow, fine- to coarse-grained, subangular to subrounded, 30% white, chalky matrix. .1 m, 1046.8 m

Sharp contact

19. Mudstone?, brownish yellow, granular texture, poorly consolidated. .2 m/ 1046.7 m

No exposure 3 m/ 1046.5 m

18. Altered ashes, pale yellow, grity, abundant altered pumice lapilli, pumice: up to 3 cm diameter, vesicular/porous texture (due to weathering of altered pumice), thinly laminated horizontal/planar beds, small-scale trough crossbeds, climbing ripples, thinly to thickly laminated ashes, coarsens upwards into coarse tuff, reworked upper beds, small- to medium-scale low-angle crossbeds, tuffs commonly bound by 1 mm thick ashes. 1.9 m/ 1041.5 m

Contact covered

17. Conglomerate, clast support, clasts: up to medium to large cobbles, average coarse pebbles, very coarse-grained matrix, calcite cement, calcite rinds cover clasts where matrix is absent, clasts extremely weathered, primarily phonolite and basalt, rare mudstone rip-up clasts. 1.4 m/ 1041.6 m

Sharp erosional contact.

16. Mudstone, dark brown, 10% fine-grained sand, well-consolidated, <5% pumice?, <4 mm diam, grades to dark greyish tan mudstone in upper 25 cm. 1.1 m/ 1040.2 m

Sharp contact

15. Mudstone?, altered tuff?, dark brown, matrix, 60% very coarse pumice, pumice: up to 1 cm diameter, mostly altered to clay/zeolite,bed occasionally grades to grey to yellow, reworked sandy tuff (upper 10 cm), 40-60% medium- to coarse-grained sand, rounded, moderately sorted. .4 m/ 1039.1 m

Contact covered - possibly gradational

14. Ash beds, tan to grey, 20%-70% fine to medium pumice, <15% detrital grains, interbedded massive and thinly laminated ash layers, poorly exposed. .5 m/ 1038.7 m 181 13. Mudstone, brown, massive, moderately consolidated, <5% detrital grains, 25% very coarse pumice. .1 ml 1038.1 m

Sharp contact

12. Tuff/altered ash?, yellow, massive, abundant very coarse mudstone(?)/altered pumice clasts, <5% detrital material, ash matrix. .1 m/ 1038.1 m

Sharp contact

11. Mudstone, dark brown, moderately consolidated, 5-10% medium-grained sand. .6 m/ 1038 m

Sharp contact

10. Tuffs/ash beds, greyish white, very thinly laminated ashes and very fine tuffs, massive and ripple stratified to very small trough crossbedded coarse tuffs, climbing ripples, < 15% detrital material, reworked upper bed, gradual increase in detrital material 35%, primarily well-rounded, fine- to medium-grained, abundant coarse grade pumice lapilli, pumice generally altered to clay/zeolite. .4 m/ 1037.4 m

Sharp contact

9. Mudstone, dark brown, <5% sand grains, well-consolidated .9 m/ 1037 m

Sharp contact

8. Altered/reworked tuffs, base: grey, very fine-grained, massive, 5-60% lithic sand grains, silt to very fine-grained sand matrix, rounded, grades to medium-grained sandstone, thickly laminated, occasional very coarse mudstone rip-up clasts, small- scale trough crossbeds, very poorly exposed, well-consolidated, abundant calcite filled fractures. .1 m/ 1036.1 m

7. Mudstone/altered ash?, brown, 20-25% very fine- to very coarse-grained sand, <5% fine to very coarse pumice, massive, moderately consolidated. .9 m/ 1036 m

6. Sandstone, yellowish grey, medium- to very coarse-grained, occasional fine to medium pebbles, subangular to subrounded, 45-55% pumice (up to .3 cm diameter), beds to coarsen upwards, moderately consolidated, moderately sorted to conglomeratic, mudstone rip up clasts common, crudely bedded, small-scale trough crossbeds, occasional interbedded fine- to medium-grained sandstone. .8 ml 1035.1 m 1 8 2 5. Mudstone, brown to dark brown, <10% very fine-to coarse-grained sand, occasional rounded, dark brown mudstone rip-up clasts?/altered pumice? up to .8 cm long. 1 m/ 1034.3 m

Conglomerate cont., very poor exposure. 3.4 ml 1034.3

No exposure .6 m/ 1029.9 m

4. Conglomerate, reddish brown, grain support, clasts/grains, up to medium pebbles, grains subangular to subrounded, clasts: primarily phonolitic, some basalt, crudely bedded, interbedded very coarse, poorly sorted sandstones, interbedded pumice clast conglomerates, pumice: yellow, average 6-8 mm diameter, 6 cm in length, altered to clay/zeolite, coarsens upwards, abundant calcite replaced pumice, up to 6-8 cm diameter, 16 cm long (in float), lithic clasts coarsen to medium cobbles laterally, extremely poor outcrop. 1.8 m/ 1029.3 m

Sharp erosional contact

3. Altered ash bed, poor expousre, yellowish tan, abundant detrital material, fine- to very coarse-grained sand to small pebble size, primarily phonolitie and basalt, minor quartz, subangular to subrounded, 10% amphiboles, abundant altered yellow pumice up to 4cm long and 2-5 cm diameter, pumice contains lithic grains and sanidine crystals, crudely aligned to parallel to beding, commonly flattened. .8 mJ 1027.5 m

2. Altered ash, pale tan to yellow, abundant very fine-grained sand and amphibole grains, extensively altered. 2.2 mJ 1026.7 m

1. Ash, brown, yellow, altered to mudstone, poorly consolidated, abundant altered pumice, 20% medium- to coarse-grained lithic grains, rounded, abundant amphiboles, poor exposure. .3 m/ 1024.5 m

Section 19: S 325°, D 19° W

Section 21: S 310°, D 5° W Naserte Tuffs 25. Lahar, orange to pale yellowish tan, massive to inversely graded, 15-25 % phonolite and basalt clasts, average cobble with rare boulders, 15-30% pumice, very fine to very large (up to 35 cm long and 25 cm in diameter) finer pumice generally altered to clay/zeolite, weathers to produce vesiclar texture, larger pumice replaced by calcite, generally weathered from upper surface, minor rip-up clasts of underlying tuffs (up to 1 m long and 10 cm thick), bed has variable surface hardening, discontinuous, basal pebble grade conglomerate: up to 3 cm thick, angular clasts, well-sorted. Sample K87- 3351. 11.9 m/ 1024.2 m 183

24. Tuffs, pale yellow to pale orange with red, purple and dark orange staining, very fine to coarse, massive to thinly laminated, thinly to thickly interbedded, 10-25 % very fine to very coarse lithic fragments,minor sanidine, abundant accretionary lapilli, normally and inversely graded, irregular bed thickness (pinch and swell), very well- consolidated. Sample K87-3351. .2 m/ 1012.3 m

Section 21 :S 310°, D 5° W

Section 31: S 345°, D 15° W

Sharp contact

52. Sandstone, reddish brown, very poorly sorted, very fine- to very coarse-grained, 60­ 65% lithic grains, 5-8% pyroxene/amphibole, 3-4% pumice, 15-20% calcite cement, minor hematite stain/cement, moderately to well-cemented, burrowed, mud lined burrows, basal conglomerate bed, matrix support, scattered clasts up medium cobbles, grades to siltstone, tan to reddish tan, massive?, abundant mud matrix, interbedded massive, tan, sandy mudstones, up to 1 m thick, ripple stratification?, extremely weathered, vertebrate fossils. Sample K87-3464, Fossil site 650. 5.4 m/ 1012.1 m

Sharp(?) contact

51. Conglomerate, reddish grey, very poor sorting, clast support, clasts: up to large cobbles, phonolite, basalt, rare lahar (rip-up) clasts, matrix: 60-75% lithic grains, 5% pyroxenes, 15-20% calcite cement, moderately cemented, interbedded very coarse, poorly sorted sandstones (of matrix material), grades into siltstone/mudstone, thickens laterally. 2.9 m/ 1006.7

Sharp basal contact

No exposure 1 m/ 1003.8 m

50. Tuff (reworked?), pale yellow to yellowish grey, interbedded coarse and very fine tuffs, coarse tuffs: 2-5 cm thick 40-45% subrounded, medium lithic grains, 35-40% medium to coarse pumice lapilli: yellow, calcareous, grades to very fine tuffs/ashes (altered), abundant large root casts, capped by 45 cm thick massive bed, 25-30% fine to coarse lithic grains, abundant coarse pumice, grades to coarse-grained reworked tuff, abundant fine to medium grade pumice clasts, clast support, calcified pumice, very well-cemented with calcite, crudely graded, overlain by lahar/massive tuff?, occasionally yellow, poorly to well-consolidated, extensively altered, surface hardening. 2.1m / 1002.8 m 184 Contact covered

49. Tuff, yellowish brown, massive, very poorly consolidated, 60-85% pumice: calcareous, yellow, extensively altered. .9 my 1000.7 m

No exposure 1.7 m/999.8 m

48. Reworked tuffs, dark yellow to white, coarse pumiceous beds, 2-6 cm thick, ripple stratified, fine to coarse pumaice lapilli, minor lithic grains, massive to ripple stratified, poorly exposed. .9 ml 184.2

No exposure .9 ml 997.2 m

47. Tuffs, pale yellow to dark yellowish tan, interbedded very fine and medium to coarse tuffs, ripple to planar stratified and massive beds, coarse tuffs: small-scale trough crossbedding, 5-15% fine pumice lapilli, 15-30% lithic grains, ash matrix, deformed underlying beds, fine ashes: ripple stratified, extremely deformed, vertical synerisis? cracks, bedding planes commonly wavy/undulatoiy, desiccation cracks common to upper surfaces. 1.2 mJ 996.3 m

Sharp contact

46. Reworked tuff, yellow to dark yellow, fine- to coarse-grained, subangular to subrounded, 15-25% lithic fragments, 60-65% pumice: medium to very coarse, 15­ 20% calcite cement, crudely bedded, possible small-scale trough crossbeds, planar stratification, pumice increase to 75-80% upwards, grades to brown sandy mudstone: waxy texture, massive, 15-20% lithic grains, 10-15% pumice, thins laterally. ,5 m /995.1 m

Sharp scoured? contact.

45.Tuff? lahar?, yellowish tan, massive, 5-25% angular to subangular lithic fragments, minor pumice, up to 2 mm diameter, calcareous, yellow, well-consolidated, upper 15cm weathered to brown mudstone, abundant white, chalky, calcareous root casts. 1.0 m/ 994.6 m

Sharp contact

44. Sandstone, pale red, weathers dark red, poorly sorted, average fine- to medium- grained, abundant fine to medium pebbles, abundant silt, 15-25% lithic grains, extremely well-cemented at outcrop, grades to siltstone vertically, fine to medium pebble basal conglomerate, grades to coarse pebble conglomerate laterally, up to 4 m thick, extensive bioturbation in upper 1 meter, abundant calcareous root casts, .4 cm- 3cm diameter, burrows: .4-.6 cm diameter, vertical to subvertical. Sample K87-3463 (gastropod). 2.2 ml 993.6 m 1 8 5 Deeply scoured contact

43. Siltstone? altered ash?, tan to grey, 10-15% very fine to fine-grained sand, very muddy, 15-25% pumice, massive, bioturbated? upper 30 cm , abundant white, chalky, calcareous root casts. 1.8 m /991.4 m

Gradational? lower contact

42. Sandstones, siltstones, light grey, very fine- to fine- grained sandstones, very silty, poorly to moderately sorted, lithic grains: subrounded, thinly bedded, 1-4 cm thick, grade to sandy siltstones, ripple to planar stratified, grades to mudstone/claystone, grey to olive, brown, abundant very fine sand and silt, massive, sequences .2-.4 m thick, occasionally up to 40-50% pumice, yellow, white, calcareous. 1.6 m1 989.6 m

No exposure 3.9 ml 998 m

41. Reworked tuff, yellowish white, medium- to coarse-grained, subangular, 55-65% pumice: calcareous, yellow, 15-20% lithic grains, 15-25% calcite cement, well- consolidated, poorly defined bedding, planar to small-scale trough crossbeds, fine- to medium-grained sandstone interbeds, planar to ripple stratified, .3-4 cm thick beds, interbedded siltstones & claystones, planar to ripple stratified. 1.2 m/ 984.1 m

40. Tuffs/reworked tuffs, yellowish grey, very fine to fine, poorly sorted, very silty, 2-6 cm thick, ripple stratified, interbedded thinly laminated altered ashes and very coarse, reworked tuffs: medium- to very coarse-grained, small-scale trough crossbeds, subangular to subrounded lithic grains, rain drop imprints & mudcracks common, occasionally bright yellow to brownish yellow stain on upper bedding surfaces, soft sediment deformation folds. Samples K87-3458, K87-3459, K87-3460, K87-3461, K87-3462. .9 ml 982.9 m

Contact buried

39. Mudstone see below .8 ml 982 m

38. Tuff, pale yellow, alternating very fine to coarse, .2 -10 cm bedding, massive, fine tuffs contain 5-15% fine to medium, subangular grains, beds commonly drape small topographic features, predominantly ripple stratified, interbedded planar beds, thinly to thicldy laminated, small-scale trough crossbeds, minor burrows with meniscate fill, minor vertical syneresis? cracks. 1.2 m/981.2 m

37. Tuff, pale reddish tan, massive, 5-10% fine very coarse lithic grains, subangular to subrounded, 20-30% accretionary lapilli up to .7 cm diameter, .4 cm average, minor euhedral sanidine, very fine matrix, well-consolidated, grades to tan mudstone (altered tuff?), moderately sandy, 10-15% medium to coarse grains, subangular to subrounded, 2 poorly defined pumice layers with yellow calcareous, hard pumice up to 3cm long 186 and 1.5 cm diameter, slightly flattened, aligned roughly parallel to bedding, these layers consist are the thickness of the pumice lapilli, also abundant scattered pumice throughout bed, massive, moderately to well-consolidated, bed continues with interbedded reworked, ripple stratified, thinly laminated and laminated and massive tuffs, pale yellow tan, extensive bioturbation, extensive soft sediment, abundant calcareous root casts. 2.9 ml 980 m

Sharp contact

36. Tuff/reworked tuff, yellow to pale red, normally graded, 80-85% pumice, 1-5 mm diameter, 5-10% lithic grains, subrounded, medium- to coarse-grained, 30-35% fine pumice lapilli, 55-60% rounded to subrounded lithic grains, pumice: calcareous, yellow. .2 ml 977.1 m

35. Siltstone, tan to brownish grey, very sandy, 15-25% very fine- to medium-grained sand, angular to subrounded, rare scattered pumice grains, l-3mm diameter, calcareous, moderately consolidated. .9 ml 976.9 m

Contact covered

34. Conglomerate, red to brownish orange, matrix to clast support, grades to coarse, poorly sorted sandstones, clasts: 65-75% phonolite and basalt, up to medium cobbles, average very coarse pebbles to small cobbles, matrix (when present): fine- to coarse­ grained, 90-95% lithic grains, 5-10% pyroxenes/amphiboles, up to 20% silt, 10-30% calcite cement, occasionally no matrix around clasts, interstitial voids are filled only with calcite cement, crudely graded bedding, very poorly sorted with widely scattered fine to very coarse pebbles, numerous interbedded/interfingering coarse, poorly sorted sandstones, laterally variable bed thickness, grades to siltstones and mudstones. Fossil sites 727, 730, 686, 731, 685, 742. 6.8 ml 976 m

Sharp, poorly exposed scour contact

33. Lahar, tan, massive, medium-grained to coarse pebbles, subrounded to subangular, 40-50% clay to silt matrix, abundant brown pumices, 1-3 cm long, concentrated near top, glass replaced by zeolite, vesicles filled with calcite, smaller pumices: altered to dark yellowish brown clay, weather to produce vesicular texture in bed. 1.2 ml 969.2 m

Sharp planar contact

32. Reworked tuff, tan, fine- to medium-grained, moderately sorted, abundant silt & clay, ripple laminated. 1 m/ 968 m - 187 Sharp, erosional contact

31. Tuff(?), tan, massive, moderately consolidated, 15-20% fine to coarse lithic grains, pumice: <5% near base of exposure, in upper 10cm 35-45%, 1.5 cm diameter, matrix: ash to very fine tuff, pumice contains fine, euhedral sanidine crystals. .4 m! 967 m

No exposure 4.5 mi 966.6 m

30. Conglomerate, red, weathers dark red, clast support, clasts: average fine to coarse pebbles, maximum small cobbles, subrounded, predominantly basalt and phonolite, matrix: medium to very coarse lithic grains, subrounded, minor pyroxene/amphiboles, subhedral, well-cemented, 15-20% calcite cement, 5-8% hematite cement, crude graded bedding, abundant calcareous root casts. Sample K87-3457, Fossil site 662. 1.5 ml 962.1 m

29. Conglomerate, pale brownish yellow, reversely graded, crudely bedded, trough crossbeds, scoured basal contact, interbedded coarse, poorly sorted sandstones, siltstones, sandstones: fine- to medium-grained, poorly to moderately sorted, moderately to very silty, planar to ripple stratified, thickly laminated to thinly bedded, laterally gradational. 1.0 ml 960.6 m

Sharp, erosional basal contact

28. Lahar, pale brownish yellow, massive, 30-40% medium grains to medium pebbles, angular to subrounded, abundant calcified wood debris (.5 cm diam), predominantly basalt and phonolite grains. .5 ml 959.6 m

Sharp, nonerosional contact

27. Reworked tuff, pale reddish yellow to reddish white, medium- to coarse-grained, moderately sorted, 40-45% lithic grains, 35-65% pumice(?) altered to clay/zeolite, 5­ 10% pyroxene/amphiboles, 10-15% calcite cement, 5-10% hematite cement, 60-75% lithic/volcanic grains, well-consolidated, planar to ripple stratification grading to small- scale trough crossbeds, grades upwards to fine- to medium-grained sandstone, planar to very small ripple stratified, grades to siltstone, pale brownish yellow, minor compaction deformation. .2 ml 959.1 m

Sharp contact

26. Conglomerate, orange to brown, matrix support, matrix: medium-to very coarse­ grained, angular to subrounded, 65-70% lithic grains, 5-10% pyroxenes/amphibles, minor silt, 10-20% calcite cement, occasionally grades to clast support, clasts: up to large cobbles, average coarse pebbles to small cobbles, -40-55% basalt/phonolite clasts, occasional crudely graded bedding, occasionally inversely graded, small- to medium-scale trough crossbeds, grades locally into coarse, poorly sorted sandstone near base, minor hematite cement, abundant hematite stain, grades vertically into sandstone: pale reddish yellow to reddish tan, very poorly sortted, fine- to very coarse- 188 grained, subrounded to rounded, 70-80% lithic/volcanic grains, 2-4% pyroxenes/amphiboles, 15-20% calcite cement, abundant scattered medium to coarse pebbles, interbedded, conglomeratic lenses, discontinuous with crude grading to sandstone, deep internal scours with pebble lags, very crudely bedded, trough crossbeds?, extensive bioturbation, calcareous root casts, minor calcite, root casts and wood fragments, possible burrows, poorly preserved burrow fills, abundant vertebrate fossils, calcite nodules, moderately to well-consolidated, interbedded fine sandstones/siltstones/mudstones, yellow, pale red (mudstones), gradational upper contacts, minor 3-8 cm thick mudstones, extensive bioturbation, poorly exposed. Fossil site 687. 8.9 ml 958.9 m

Sharp scour contact

25. Tuff(?) Reworked tuff, reddish yellow, thinly interbedded ashes to very fine tuffs, generally sharp upper & lower boundaries, also coarsens to very fine reworked tuffs, commonly contain 1-3 cm rip-up clasts of altered ashes, finer beds: ripple to climbing ripple stratified, coarser tuffs: ripple stratified to small-scale trough crossbedded, minor compaction deformation, few interbedded thin altered ash beds, planar stratification. 2.2 m/ 950 m

Sharp basal contact

24. Tuff(?), pale reddish tan, massive, 3-6% fine to medium pyroxenes/amphiboles, subhedral to euhedral, 10-15% fine to medium lithic grains, angular to subrounded, 10­ 20% coarse to very coarse lithic grains (altered pumice?), 65-75% ash matrix, interbedded 1 cm thick, pale red claystones (altered ashes?), continuous, spheroidal weathering, well-consolidated. Sample K87-3456. .5 m/ 947.8 m

Sharp basal contact

23. Sandstone, yellowish grey, medium-grained, well-sorted, subrounded, 75-85% lithic grains, 2-5% pyroxene/amphiboles, 10-15% calcite cement, very well-cemented, planar to occasional ripple stratified, minor compaction deformation in upper 10 cm, rare vertical burrows?. .2 m/ 947.3 m

Sharp scour contact

22. Sandstone, pale reddish yellow, very fine- to fine-grained, predominantly lithic grains, moderately cemented, calcite cement, abundant silt, grades to siltstones and mudstones: massive, mudstones extensively bioturbated, occasional well-preserved burrows: 2-5 mm diameter, filled with sandstone, horizontal to vertical, common in upper sequences, abundant calcareous root casts, calcrete in upper 40 cm. Fossil sites 733, 687, 735, 736, 737, 738, number not discemable). 1.5 m/ 947.1 m 189 Contact buried

21. Siltstone?, altered ash?, pale yellowish brown, ripple stratified, climbing ripples, abundant soft sediment deformation, grades to very fine tuff?, abundant calcareous root casts, occasional interbedded rippled mudstones?/ashes?. Sample K87-3455, Fossil sites 735, ? (number not discernible). 1.6 m1 945.6 m

Sharp basal contact

20. Sandstone/reworked tuff, pale red, weathers dark red, medium- to coarse-grained, moderately sorted, angular to subrounded, 45-50% silt grade matrix, 5% very fine lithic grains, rarely up to medium pebbles, 15% pyroxene/amphiboles, 15-20% calcite cement, hematite cement, ripples to small-scale trough crossbeds, deformed bedding: pinch and swells, folds, laterally continuous. . 1 ml 944 m

Irregular scour basal surface

19. Siltstone? altered ash?, pale olive yellow, massive, 5-10% very fine lithic grains, 2-4% very fine pyroxenes/amphiboles, poor outcrop, abundant calcareous root casts. .5 ml 943.9 m

18. Tuff?, red to pale red, massive, 15-20% medium to coarse grains in very fine matrix, extensive altered to mudstone, minor analcime lined cavities, abundant calcite roots(?), 5-8% pyroxenes/amphiboles, euhedral to sudhedral, extremely poor outcrop. Fossil sites 739, 662 1.5 ml 943.4 m

No exposure 1.6 ml 941.9 m

17. Siltstone/sandstones, pale brownish red, interfingering very fine-grained sandstones and siltstones, sandstones: very fine- to very coarse-grained, poorly sorted to conglomeratic, 50-65% lithic grains, 5-8% pyroxenes/amphiboles, 10-15% rip-up clasts, 10-15% calcite cement, crudely bedded, small-scale trough crossbeds, ripple stratified, climbing ripples, laterally discontinuous, poorly exposed, grades to siltstone, upward fining sequences 30-80 cm thick, siltstones and mudstones are moderately to extremely well-consolidated, massive to ripple straified. Fossil site 758. 2.4 m/ 940.3 m

Sharp contact

16. Sandstone, yellowish grey to grey, very coarse-grained, poorly sorted, rounded to subrounded, 20%-30% lithic grains, 5-10% rip-up clasts?, 20-25% pyroxenes/amphiboles, abraded, calcite cement, small-scale trough crossbeds. .4 m/ 937.9 m 1 9 0 Sharp scour contact

Kalodirr Tuffs (upper) Alomonet tuff beds 15. Tuff, yellowish brown, very fine, ripple stratified, abundant very fine biotite, biotite flakes lie parallel to bedding, interbedded reworked tuffs, grey, weathers dark grey, very fine- to coarse-grained, scattered fine pebbles, normally and inversely graded, 30­ 40% pyroxenes/amphiboles, abraded, fine- to coarse, 25-35% rounded to angular lithic grains, planar to ripple stratified, interfmgers with fine tuffs (as above) with very small ripple stratification, occasional very thin < 2mm thick red muds (altered ashes?), desiccation cracks, leaf imprints generally parallel to stratification, entire margins, grass imprints and small calcite wood debris also common, analcime crystals in voids, artiodactyl and bird foot prints,. Samples K87-3452, K87-3453, K87-3454. 2.3 ml 937.5 m

14. Sandstone, yellow to tan, very coarse-grained, poorly sorted to conglomeratic, abundant medium to coarse pebbles, 55-60% lithic/volcanic, 10% pyroxene/amphiboles, 10-15% rip-up clasts, 15-20% calcite cement, small- to medium- scale trough crossbeds, crudely graded, occasional inverse grading, interbedded pale red, lenticular, bioturbated mudstones, up to .3 m thick, abundant calcareous root casts. 2.9 m/ 935.2 m

Sharp, scour contact

Section 31: S 345°, D 15° W

Section 6.1: S 295°, D 12° W

Kalodirr Tuffs (lower) Kanukurinya tuff beds 21. Tuff, pale red to grey, olive, bedded, massive, calcite cement, amphibole, biotite common. .5 ml 932.3 m

20. Sandstone, reworked tuff, clast support, clasts: fine to medium pebbles, matrix: coarse-grained lithic grains, trough crossbeds, abundant volcanic mineral grains. 1.9 ml 931.8 m

19. Reworked tuff, light greyish green to olive, abundant volcanic mineral grains. 1.7 m7 929.9 m

18. Tuff, light greyish green to olive, pale red, medium to coarse, euhedral volcanic minerals: amphibole, biotite, pyroxene, thickly laminated to thinly bedded,planar bedding, calcite cement. .7 ml 928.2 m

17 Sandstone, pale orange, very fine grained to medium pebbles, very poor sorting, small- scale trough crossbeds. 1.6 my 927.5 m 191 Sharp, erosional contact

16. Tuff, olive, alternating massive coarse and fine tuffs, thinly interbedded, planar bedding, biotite parallel to bedding, calcite cement. .2 ml 925.9 m

Sharp contact

15. Tuff, light tan, scattered very fine amphiboles/pyroxenes, well-consolidated, very poorly exposed. 1.2 m/ 925.7 m

14. Mudstone, tan to pink, well-consolidated, blocky, massive, slightly silty, occasional scattered lithic fragments. .7 m/ 924.5 m

13. Tuff, pale red to pink, weathers dark red, fine crystalline, abundant amphibole/pyroxene, veiy well-cemented, thinly to thickly laminated, laminae drape topographic features. .2 m/ 923.8 m

Sharp, nonerosional contact

12. Tuff, pale pink to tan, very fine, widely scattered medium crystals (amphibole, biotite, pyroxene), very well-consolidated, few scattered very fine pebbles, poorly exposed. .7 m1 923.6 m

Sharp contact

11. Sandstone, tan to light brown, medium- to coarse-grained, rare fine pebbles, lithic and volcanic clasts, poorly sorted, very poor consolidated, interbedded very fine-grained, grey sandstones, moderately sorted, moderately to well-consolidated, lenticular, minor interbedded soil carbonates. .9 m/ 922.9 m

Sharp, erosional contact

10. Lahar, dark olive to greyish green, mud to clay matrix, clasts: up to large boulders, average small boulders, phonolite, basalt, syenite, moderately to well-consolidated. 2.3 m/ 922 m

Sharp contact

9. Tuffs, olive to green, pale to dark red, very fine to coarse crystal and lithic grains, euhedral volcanic minerals: amphibole, pyroxene, biotite, biotites commonly parallel to bedding, horizontally bedded, thinly laminated to thickly bedded, calcite cement, moderately to well-consolidated, calcite gastropods, common, occasionally reworked, occasional interbedded altered ashes?/mudstones. 4.4 m/ 919.7 m 192 Sharp contact

8. Lahar, dark olive green, very coarse volcanic crystals (amphiboles/pyroxenes up to 3 cm diameter, biotites up to 2 cm diameter), biotites randomly oriented, leaf imprints very common, leaves generally have entire margins, abundant calcite casts of wood debris with very well-preserved exterior surfaces. 4.7 ml 915.3 m

Sharp contact

7. Tuffs, yellowish tan, pale to dark red, very fine to coarse crystal and lithic grains, euhedral volcanic minerals: amphibole, pyroxene, biotite, biotites commonly parallel to bedding, horizontally bedded, thickly bedded, calcite cement, moderately to well- consolidated, compaction deformation common, rare leaf imprints. .9 m/ 910.6 m

Sharp contact

6. Mudstone, dark brown, weathers red, occasional scattered very fine-grained lithic fragments, extremely altered. 1 m/ 909.7 m

Sharp contact

5. Tuff?, purplish red, abundant very fine amphibole/pyroxene crystals, minor lithic fragments. .4 ml 908.7 m

Sharp contact

4. Tuff?,dark red to purple, abundant very fine to coarse amphibole/pyroxene crystals, abundant lithic fragments, upper 10 cm reworked. .7 ml 908.3 m

Sharp contact

3. Tuff?,dark red, rare scatttered biotite flakes parallel to bedding, occasional euhedral fine amphibole/pyroxene crystals. .3 m/ 907.6 m

Section 6.1: S 295°, D 12° W

Section 31: S 345°, D 15° W

Contact covered

4. Sandstone, purplish grey, moderately sorted, very fine- to very coarse-grained, aundant scattered pebble and minor rip-up clasts at base, 55-65% lithic grains, subrounded to angular, 10-12% pyroxenes, 10-15% calcite cement, 5-8% rip-up clasts, moderately to very well-cemented, abundant calcite plant fragments, extensively burrowed, very 193 poorly defined small-scale trough crossbedding and ripple stratification, calcite gastropods, coarsens upwards, poorly sorted with rare large cobbles, laterally variable Siickness. Sample K87-3451. 1.8 m//907.3 m

Sharp contact

3. Sandstone, grey, poorly sorted, locally , abundant rip-up clasts from bed below, predominantly lithic grains, crudely bedded, normally graded, widely scattered pebbles throughout, predominantly lithic rounded to subrounded, 25% calcite cement, laterally discontinuous, basal fine to coarse pebble conglomerate. .2 m/ 905.5 m

Sharp, scoured basal contact

2. Sandstone, olive to tan, very fine-grained, poorly sorted, moderately consolidated, massive, abundant calcareous, grades to thinly laminated siltstone. .4 m/ 905.3 m

No exposure .4 ml 904.9 m

Basal conglomerate 1. Conglomerate, grey to purplish grey, matrix to clast support, clasts: phonolite and basalt, up to small boulders, average medium pebbles to small cobbles, commonly coated with FeC>2, medium- to large-scale trough crossbeds, low-angle crossbeds common, internal scour and fill sequences common, subhorizontal erosional surfaces common, very poor exposure, interbedded sandstones: pale red to reddish grey, very poorly sorted, fine- to very coarse-grained, small to medium trough crossbeds, mudstones: brown, massive slightly to very sandy, moderately consolidated, abundant calcareous root casts and nodules. 1.8 ml 904.5

No exposure 105.9 m /902.7 m

Section 31: S 345°, D 15° W

BASE OF LOWER LOTfflDOK FORMATION

TOP OF KALAKOL BASALTS Section 7: S 348°, D 12° W

19. Basalt, spheriodal weathering, olivine phyric, olivines altered to limonite, limonite also occurs along weathered surfaces. 9.4 m /796.8 m

18. Mudstone, pale red, orange, very sandy, weathered (paleoweathering horizon?). 5 ml 787.4 m 194 17. Basalt, black to grey, aphanitic, occasionally abundant calcite vesicles, rare coarse feldspar phenocrysts. Samples B 37, B 37A. 24 ml 782.4 m

16. Mudstone, pale to very dark red, abundant lithic and volcanic mineral grains, very poorly consolidated, grades to muddy sandstone, mudstone rip-up clasts common, weathered. 17.4 m/ 758.4 m

15.Basalt, black to dark grey, coarse olivine phyeric. 14.8 m /741 m

14. Basalt, black to dark grey, weathers dark purplish grey, abundant olivine, extensively weathered upper surface, abundant calcite filled vesicles and amydules. 14.4 ml 726.2 m

13. Basalt, black to dark grey, aphanitc with rare widely scattered olivine phenocrysts, minor pyroxene, calcite filled voids common to upper few meters. 17.6 m/ 711.8 m

12. Basalt, black to grey, aphanitc, rare fine olivine, minor pyroxene phenocrysts. 8 ml 694.2 m

11. Basalt, black to dark grey, abundant extremely altered olivine phenocrysts at base, rare pyroxene phenocrysts, aphanitic near top. 59.2 m! 686.2 m

10. Basalt, black to dark grey, abundant plagioclase, olivine, pyroxene microphenocrysts, aphanitic groundmass, extensively fractured, altered, grades to pale to dark red mudstone (paleoweathering horizon?). 9.1 ml 627 m

9. Basalt, purplish grey, extremely weathered/altered, scattered medium-grained olivine phenocrysts, vesicular upper surface, vesicles filled with calcite, calcite stringers common. 8 ml 617.9 m

8. Basalt, black to dark grey, mostly buried, rare medium-grained olivine phenocrysts, commonly altered to iddingsite? with limonite rims. 4 m/ 609.9 m

7. Mudstone, dark brown to red, non- to very sandy, primarily lithic grains and mudstone rip-up clasts, grades to muddy sandstone. 16 m/ 605.9 m

6. Basalt, black to dark grey, aphanitic, upper surface grades to mudstone (see above), extensive calcite fracture and void filling. 24 ml 589.9 m 1 9 5 5. Interbedded sandstones, mudstones, sandstones: dark brown to grey, pale red, beds appear massive, abundant basalt clasts, abundant mudstone rip-up clasts, very poorly exposed, grades to sandy mudstones: pale red, extremely weathered, moderately consolidated, blocky fractures common, interbeded white, chalky soil carbonates, very calcareous. 5 mJ 565.9 m

4. Basalt, black to dark grey, abundant olivine phenocrysts (decrease in abundance upwards), extensively weathered, calcite filled fractures common. 54.2 m1 560.9 m

3. Basalt, black to dark grey, abundant coarse to veiy coarse olivine and pyroxene phenocrysts, extensivey fractured, calcite filled voids and fractures. Sample B35 20.3 m7 506.7 m

2. Basalt, dark grey, aphanitic groundmass, fine to coarse olivine phenocrysts, silicic fracture filling, occasionally vesicular. Sample B36 34.1 ml 486.4 m

I. Basalt, dark purplish grey, autobrecciated, blocks up to 30 cm diameter, increase in size upwards, aphyric, silicic fracture and void filling, extremely weathered, thin interbedded, pale red mudstone near top of bed. 68.8 m/ 452.3 m

Section 7: S 348°, D 12° W FAULT ...... —Uncertain Correlation------

Section 17: S 330°, D 25° W 14. Basalts, not measured or examined in detail. ~ 200 m/ 383.5 m

Eragaleit beds 12. Conglomerate, dark red with purple streaks and yellow blotches, average fine to medium pebbles, matrix: coarse-grained sand, subrounded to rounded, pyroxenes/amphiboles common, clasts predominantly lithic and rip-ups, crudely bedded, medium-scale trough crossbeds, scour and fill structures common, grades vertically into coarse-grained sandstone, crudely graded, sharp erosional lower contacts, moderately to well-consolidated, calcite cement, forms ridges, predominantly matrix support, upward fining sequences average ~ 1 m thick, minor inverse grading, crude overall upward coarsening, grades laterally and vertically to sandstones: medium to very coarse lithic grains, abundant clay, calcite cement, plagioclase grains, small- scale trough crossbeds, rare ripple laminations, very poorly sorted, abundant vertebrate fossils, interbedded siltstones, dark red, well-consolidated, massive appearance. 12.9 ml 183.5

II. Conglomerate, redish grey, clast support, clasts: basalt and phonolite, up to coarse pebbles, fines upward in 1-3 m sequences, very poorly exposed. 11.1 m /170.6 m 196 Graditional contact

Nathuraa tuffs 10. Interbedded/interfingering altered and reworked tuffs, reworked tuffs: pale green to yellow, very fine to coarse, poorly sorted, thinly laminated to thickly bedded, abundant altered pumice (clay/zeolite), martix support common, very fine matrix, tuffs: brown to pale reddish brown, massive, slightly to very sandy, altered, abundant medium to coarse pumice: altered to zeolite/clay, randomly oriented, dispersed throughout beds, commonly elongate parallel to bedding, flattened, rare interbedded lithic conglomerates and sandstones. 14.2 m/ 159.5 m

9. Claystone, yellowish green to pale olive grey, massive. 3 m1 145.3 m

8. Conglomerate, red weathers yellow, clast support, clasts: average fine pebbles, up to coarse pebbles, predominantly claystone/sitstone rip-up clasts, basalt and phonolite common, abundant pumice, abundant clay/silt in matrix, interbedded 2-4 cm thick, lenticular lithic clast conglomerate lenses common, locally very well-cemented, grade to lithic conglomerate; interbedded 2-4 cm thick sandstones common, well-sorted very crudely bedded, abundant clay in matrix, grades to a silty claystone, reddish brown, coarse-grained sand common. 9.3 ml 142.3 m

Gradational contact

7. Sandstone, pale red with yellow interbeds, very fine- to fine-grained, veiy silty, 2-4 cm thick coarse yellow interbeds, moderately sorted, abundant clay, grades to sandy claystone, moderately to well-cemented, interbeds veiy well-cemented, limited exposure. 3.1 ml 133. 0

6. Conglomerate, grey to dark reddish grey, clast support, clasts: predominantly phonolite, up to small boulders, average medium cobbles, imbricated, inversely graded, matrix: medium- to coarse-grained, very large internal scour and fill structures, up to 1.3 m, calcite cement, interfingers with mudstones and sandstones. 4.8 m / 129.9 m

Erosional contact

5. Basalt, Samples K87-3100, K87-3101 14.3 m/ 125.1 m

4. Conglomerate, purplish grey, clasts: coarse pebbles to large cobbles, predominantly phonolite, interbedded mudstone(?), extremely poor outcrop 7.1m/110.8 m

3. Basalt, greenish grey, very fine to fine plagioclase crystals, medium olivine crystals, altered, weathers green with brown coating, also weathers dark purplish red, upper 1.5 m of flow contains abundant quartz stringers, spheroided weathering. Sample Los 17-3 39.3 ml 103.7 m 197 2. Basalt, light grey, weathers red to brown, medium plagioclase phenocryts, in aphanitic grounds mass. Sample Los 17-2. 30.4 m/ 64.4 m

1. Basalt, dark grey, medium crystalline, plagioclase, olivine (?), fairly resistant, olivines altered, small calcite amygdules, quartz veins. Sample Los 17-1. 34 m/ 34 m

FAULT Section 17: S 330°, D.250 W

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