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H U EK R IM U A N U U H GST Prepared in cooperation with the Open-File Report 2013-1306 U.S. Department of the Interior Geological Survey of and the University of (Tanzania), Pamphlet accompanies map U.S. Geological Survey with funding from the USAID Office of Foreign Disaster Assistance 35°47´ E 810000 35°50´ Qmiu Qhl 820000 Qhl 35°55´ 830000 36°00´ E 2°30´ S 2°30´ S Qh CORRELATION OF MAP UNITS Qh Qmic Qtse 1.70±0.02 Ma 9723000 9723000 Qmic (Manega, 1993) Tsh Qtse OLDONYO LENGAI

600 Qtse Qh Approximate age, in Polarity 680 Chrons MAIN-CONE VOLCANICLASTIC ROCKS DEBRIS-AVALANCHE GELAI, KERIMASI, AND MTO WA MBAGAI millions of years* 800 1080 ALLUVIAL DEPOSITS AND LAVA FLOWS DEPOSITS NATRON- VENT FIELD ESCARPMENT STRATA (ISAAC, 1967) (nonlinear scale) 1100 Tsh 760 Qtse Ql1 Qa1 Qsd Qol Qomlc Qoda1 Qa Qtse Qa1 2 1.31±0.26 Ma Qomnl Qls Qh (Foster et al., 1997) 1000 Qtse Holocene Qh Qomnc Qh 9720000 9720000 1.41±0.04 Ma (Manega,1993) Ql2 Qmic o n Qoda2 Qoda3 0.011 Tsh a t r Qmiu N Qomlu Qoda4 1080 Brunhes Normal- e Qot Qom Qoml Polarity Chron Qh Ql1 k Tnnm 1.28±0.05 Ma Qa3Qa3 0.050 (Foster et al.,1997) a ? 2 18 Qnc Qnl Qmin Qoda5 L Qoda 3 Qomv Qomc 5 1100 12 460±75 ka 65 Qa Qntr Qnet Qh 4 Qa1 Qa QUATERNARY 3.53±0.06 Ma 5 (Manega,1993) D 0.150 Qmiu A Qmin O R 1120 338±23 ka Qoda6 O Ql3 Qpne Qtse Qmic D Tsh N O Qoda I Qoda5 Pleistocene 5 L Qomlp O

1000 L - 35°47´ E A K 340±85 ka 353±65 ka U R A G N E 0.78 0.78 Index to Notable Features Qoda5 0.988 Qoda Qvg Qvk [Shown on map by symbol ( ) and map number. Coordinates (east, north) are 5 1.072 Qoda5 1.173 UTM zone 36, New Arc (1960) datum] 800 1.1 Ma Qsk Qmiu 1.185 Qoda5 Map Qbs Qmin 1.3–1.4 Ma Feature Easting Northing Qbs Ql1 No. 50 Qoda5 Qmic Qoda5 600 QTei Matuyama Reversed- 1 Ancestral shoreline deposits atop 828963 9711815 Qoda5 1.5 Ma Qoda5 1.778 Polarity Chron debris avalanche deposits Qoda5 Qh Qhl 2°35´ QTel 2°35´ QTev am 1.8 Ma 1.81 2 Ancestral shoreline deposits atop 829307 9711731 tre 1.945 Qbs 42 Qoda5 s 2 a 600 debris avalanche deposits hut Les Qoda5 2.128 Tmc Tmc 2.148 3 Sand-and-pebble deposit (sand 826541 9708581 Ql 34 7 2 600 L195 Ql1 blow), liquefaction 1200 QTel DESCRIPTIONS OF MAP UNITS Qomc Central-crater beds (Pleistocene)—Lapilli tuff, tuff breccia, and 38 12 4 Sand-and-pebble deposit (sand 823222 9707044 Qnc tuff. Forms mass of flat-lying strata that abuts the southern L196 QTel Qoda Qoda5 QTel Qa 5 [See pamphlet for discussion of map units, radiometric ages, and references

blow), liquefaction, fig. 38C in 3 Olmoiton Camp crater wall and presumed to be the remnant fill of an old central 2.58 m

explanatory pamphlet a Qnl cited] crater. Now largely buried by eruptive products of the northern 1400 e (Armykon hill) r 5 Tuff and sandstone between 824510 9688396 t Tmv s 30 Qoda5 crater vent. Older than debris-avalanche deposits shed from ta debris-avalanche deposits, fig. u QTel L3 h FLUVIAL, ALLUVIAL, AND COLLUVIAL DEPOSITS Eastern Chasm (Qoda1), but of uncertain age relative to main es Qoda 32 Mosonik L 5 sheet of debris-avalanche deposits (Qoda ) 3.032 Gauss Normal- 1 600 Lacustrine deposits (Holocene and Pleistocene)—Divided into: 5 6 Sand-and-pebble deposit (sand 824535 9707885 50 Qnl 2 3.116 Polarity Chron Qoda blow), liquefaction, fig. 37D Qsk Qoda 6 Ql1 Younger Lake Natron deposits (Holocene)—Unconsolidated to 3.207 5 D DEBRIS-AVALANCHE DEPOSITS 3.26 Ma A Tnnm O cohesive nonlithified sand, silt, and clay. Includes minor R 3.53 Ma 3.330

7 Subsidence pit, Holocene 822974 9709915 Qa Debris-avalanche deposits (Holocene? and 1 E evaporite in ephemeral ponds. Forms mudflats adjacent to Lake D Qsk I faulting, fig. 35D 16 Pleistocene)—Unsorted to poorly sorted, coarse chaotic tuff Tsh S Natron. Interfingers with young alluvium (Qa ) Engare Sero Qoda5 - 1 Ql3 T breccia. Forms areas of hummocky topography that ring the Tmc Pliocene 8 Thick tephra exposure west of 818623 9694749 Qa1 S 3.60 Embalulu village Qa5 A 16 Ql3 E volcano on all but the west side, where similar deposits are Tek Oldonyo Lengai Sekenge L49 Ql2 Lacustrine deposits perched by debris avalanche deposits Qbs 10 Riverside Camp likely buried against the foot of the Natron fault escarpment. Qoda5 Qoda6 22 (Holocene? and Pleistocene)—Thin-bedded, well-bedded sand TERTIARY 9 Ultramafic inclusions in east and 828741 9690837 Qoda5 8 Indicates history of repeated edifice collapse from stratovolcano 97 000 97 000 and silt. Two occurrences: a 400-m-long band (area 3.9 ha) west exposures of tephra cone 10 subsidence 10 centered at present-day Oldonyo Lengai. Divided mainly on QTel Moivaro 7 crater 6 Qoda Ql Qa5 adjacent to Sidan Indare east of Oldonyo Lengai, and a tiny Lake Natron 5 3 geographic basis (largely in keeping with the work of Kervyn 10 Good exposures, thickly bedded 819763 9710281 7 filled basin (8.4 ha) tributary to Leshuta stream west of Lake 4.187

1200 Qnc Qa and others, 2008a), though some geomorphic and age-related ? debris-avalanche deposits 5 Natron 4.300 AIRSTRIP Enarok criteria also justify the following distinctions: Tmv Qa3 11 Holocene sag (graben); fig. 36 829053 9708108 6 Kohoke 620 ? L149 GATE Ql3 Older Lake Natron deposits (Pleistocene)—Exposed on basin 4.493 S Gelai i Qoda Deposits filling modern drainage (NW) and banked against ENGAR d 1 UK school floor between 620 and 640 m altitude along east edge of map 4.631 Qa A a Qa4 3 -L 3 12 Explosion(?) crater mapped from 824174 9695877 O n 86 east midslope of Oldonyo Lengai cone (E) (Holocene or

L Qoda6 area, about 4 km south of modern shoreline. Includes sandstone, I I

O subsidence n Gilbert Reversed- air photos QTel N Qa1 Ql3 Waterfall d Pleistocene) 4.799 D crater 11 siltstone, and carbonate (stromatolite?). Beds are highly Polarity Chron Camp O a Qoda5 r 793±63 ka R Qoda5 4.896 O e deformed and disheveled, likely a consequence of having been A Tek Qls Qa3 D Qoda2 East-northeast-directed deposits (Holocene or Pleistocene) Qa4 plowed by an early debris avalanche from Oldonyo Lengai 4.997 Qoda5 6 1240 680 20 Alluvium (Holocene and Pleistocene)—Sand, silt, and gravel of Qoda QTel L310 Qoda5 Qa4 3 North-northeast-directed deposits (Holocene or Pleistocene) 5.235 23 Endukai R Qoda5 active and inactive streams and alluvial fans. Divided into: 5.33 Kiti Endonyo 4 Qoda Naado L67 4 South- and southeast-directed deposits (Pleistocene)—Mass to Tek Qa1 Young alluvium (Holocene)—Deposits of active or sporadically south is well exposed in upper reach of Sinja Ndare. Terminates Miocene 30 700 Qa active drainages and adjacent floodplains. Drainage floors AFRICA 2 Qnc Esoit abruptly farther south, on lower flank of Kerimasi volcano. grained. No primary volcanic bombs or blocks recognized. typically graded to Lake Natron. Interfluvial surfaces have Qoda5 Qoda5 27 Ondulali Southeast mass possesses subdued, flattish topographic expres- Ashy component, described as olivine melilitite by Neukirchen 1000 Qoda5 poorly developed soils and are sparsely vegetated, owing to 720 sion, but every hillock has scattered, heterogeneous blocks of and others (2010), contains about 1 percent or less primary Qol flooding and scouring. Includes colluvium where mapped on porphyritic phonolite and nephelinite magmatic phlogopite, rarely as large as 7 mm but mostly less 6.03 steep slopes of the Natron escarpment. Shown stippled are than 5 mm across. Forms stratigraphically highest volcanic alluvial fans north of Engare Sero village that are rich in angular Qoda *Time scale from Gradstein and others (2004) o Qoda5 5 Main avalanche sheet (Pleistocene)—Equivalent to zebra strata along escarpment at latitude of Engare Sero village er 680 to subangular blocks owing to flashy discharge of streams S debris-avalanche deposits of Kervyn and others (2008a). Age Qoda5 Normal polarity (shaded) and reversed e 1000 draining the escarpment r Qnet older than 10,000 years and possibly older than 50,000 years on Qbs a Qa2 Basaltic sedimentary strata (Pleistocene) polarity (white) shown for subchrons g n basis of stromatolite and beach deposits that overlie it (Hillaire- through end of Gilbert time 680 2°40´ E 7 2°40´ Qa2 Deposits of areas rarely alluviated in past 200–2,000 years or 30˚E 34˚ 38˚ 760 Marcel and others, 1987) Volcanic rocks of the Natron escarpment (Pleistocene and SOUTH SUDAN Qoda Qvg Qnet more (Holocene)—Lithologically similar to but older than ETHIOPIA 5 Qoda Pliocene)—Chiefly basalt, basanite, and tephrite in composition 5 L238 young alluvium (Qa ) on basis of geomorphic characteristics. Qpne Qoda5 1 Qoda6 Blocks in ancestral lake deposits (Pleistocene)—Avalanche (Neukirchen and others, 2010). Divided into: Qpne Found in areas where stream channelization protects higher- Qoda5 emplacement age likely in the range 240,000–135,000 years Y standing areas from frequent stripping Lateral margins and snouts of lobate geomorphic features (on Qntr ago, which is a period of Lake Natron highstands estimated QTel Lava flows O Qoda5 27 Oldonyo Lengai)—Narrow breadths and convex-downslope 800 from U/Th ages from stromatolite associated with a OF CONGO UGANDA 800 Embalulu Qvg Qa Perched sand and gravel (Pleistocene)—Deposits result from the snouts probably mark the boundaries of debris flows. More QTel Qsd N 3 655-m-altitude paleoshoreline (Hillaire-Marcel and others, QTev Vent deposits—Scoriaceous lapilli and ash. Bold lines show dikes Equator 24 Olkujita temporary damming and subsequent overflow of Engare Sero 1987). We presume that lacustrine beds, in which the avalanche extensive areas, including those that open downslope from ke 27 Qvg a ia Qoda3 and Leshuta streams by Oldonyo Lengai-derived debris- upslope apices, show sectors of the volcano that are now active L or blocks are embedded, formed during those highstands QTei Dikes—Broadly basaltic in composition. Intrude vent deposits ct Qntr depositional slopes that abut slightly higher-standing, less active i Qoda5 avalanche deposits, upon which they were commonly emplaced. (QTev) and, locally, underlying lava flows (QTel) V Found on west side of map area slopes. Estimated relative age indicated by Y (younger) and O RWANDA 840 Embalulu ROCKS OF NATRON-ENGARUKA VOLCANIC FIELD REPUBLIC (older) Qoda3 Ondoriko Tek Lava of Endukai Kiti (Pliocene)—Thin lava flows and tuff 720 Qa Gelai sandstone (Pleistocene)—Light grayish-red sandstone and [Scattered vent deposits and lesser lava flows in the area from Lake n 24 4 breccia (agglomerate) of very fine-grained basanite. Thickness 4˚S a BURUNDI Qoda3 Qnet e an overlying basalt-cobble veneer that extends outward from Natron south for 40 km to village of Engaruka, as defined by Fault—Solid where precisely or approximately located; short dashed c 60 m, but base not exposed. Includes 20 m of fallout tuff and Qoda where inferred; dotted where buried. May show dip. Ball and O 3 Gelai volcano (east side of map area). The “Gelai fans” overlie Dawson and Powell (1969). Lithology and emplacement mecha- Qoda 5 Endonyo E breccia (probably all near-vent) and an overlying sequence of

MAP n N 860 9 older Lake Natron beds (unit Ql ) and debris-avalanche blocks nisms discussed in detail by Mattson and Tripoli (2011); geochemis- bar on downthrown side G a Olboloti Qnc 3 1400 orange-brown-weathering, moderately bedded volcaniclastic TANZANIA i A 6 AREA d R L26B in ancestral lake deposits (unit Qoda6) try is described by Keller and others (2006). Tuff rings and L26A U Fault with known Holocene or latest Pleistocene offset n sandstone 6–8 m thick. Corresponds to waterfall sequence of I K Qnl 5 fine-grained tephra cones (tuff cones) are common across the length 35 Qoda5 A

DEMOCRATIC Neukirchen and others (2010). Forms oldest strata exposed - 1000 L Fault with Pleistocene offset L118 O N Qa5 High-standing alluvial fan (Pleistocene)—Pebbly sandstone. of the area, whereas coarser-grained tephra cones are found mainly 8˚ L along face of Lake Natron escarpment between Leshuta stream I L117 900 O Forms surface planed onto older Lake Natron beds (Ql3). in the divide between the modern Lake Natron basin and a large QTev Qoda1 N and Oldonyo Lengai volcano. Reversed polarity magnetization D Qoda5 Lineament Qa2 ZAMBIA O Surface topographic gradient projects higher into basin than top closed topographic depression south of it]

R O of Gelai sandstone (unit Qa4) A Qnc Tephra cones (Holocene? and Pleistocene)—Near-vent fallout Tmc Mosonik conglomerate (Pliocene)—Cobble conglomerate rich in D Gaping crack Quaternary and Tertiary Lakes and Rift faults QTei 760 tephra that built cone-shaped landforms. Many cones are built L33 OL184 volcanic stones, with sparse clasts of volcaniclastic sedimentary volcanic rocks ocean 9700000 Qnc 9700000 Qsd Sand dune (Holocene)—Accumulation of sand on north flank of

Nasira Qoda5 chiefly of fine-grained strata (tuff cones), but some are lapilli rocks. No Precambrian clasts seen. Clast size typically 10–15 Dikes (unit QTei) associated with escarpment vent deposits Qomnl Qnc e Embalulu Ondoriko tuff ring near Sidan Indare stream

LOCATION OF MAP AREA Qomnl Qa1 r cones; agglutinate and spatter occur locally cm, rarely to 1.4 m. Unit is 80–100 m thick along (QTev)

L28 a Qol 1000 d north-trending segment of Leshuta stream, where beds dip 2° Qom n I 30 Qls Landslide deposit (Holocene or Pleistocene)—Coarse blocky Qomnc L30 n Qnl Lava flows—Sparse ‘a‘ā and block flows ranging in composition eastward. Overlies Mosonik volcanic rocks. Late Pliocene and Crater rims—Enclose intact and breached volcanic craters of small Qoda5 a N 26 d deposit. Mantles lower flank of escarpment just south of the i from olivine melilitite to nephelinite. Issued from tephra cones (or) early Pleistocene age on basis of stratigraphic position

S to moderate size. Hachures point into crater. Not shown for L161 mouth of Engare Sero stream and extends downslope into gully 800 (unit Qnc). See explanatory pamphlet for chemical analyses and above Mosonik volcanic beds and apparent discordance with 35°30´ E 35°45´ 36° 36°15´ E Qomnc 340±85 ka major craters, which are portrayed adequately by topographic 2°15´ S 1600 Qvg 13 L31 Oremit evidence bearing on relative ages that unit contours on the map. Rim labeled “subsidence crater” 7 km and Qol 820 Qnc ROCKS AND DEPOSITS OF OLDONYO LENGAI VOLCANO OL642 Qvg south of Lake Natron is probably related to liquefaction during a QTev L9 Qntr Tuff rings (Pleistocene)—Tuff and lapilli tuff. Commonly Tmv Mosonik volcanic rocks (Pliocene or Miocene)—Debris-flow or LAMUNYANI BININI LAKE NATRON Qnl DOWNWIND AND OTHER BEYOND-CONE DEPOSITS basin earthquake. Another small crater at 2,340-m altitude on TRAIL Qnc 30 fine-grained to very fine-grained, more so than tuff rings from lithic pyroclastic-flow deposits. Clast population varies little north flank of Oldonyo Lengai was recognized in air-photo 27/3 27/4 EAST QTei Qnc L164 Y volcanic arcs worldwide. The grain size distinction may arise among outcrops: typically aphyric in hand sample, with 1600 Y O Qnc mapping and distant views but not visited. It seemingly lacks 28/3 1200 O Qa1 Qot Tephra (Holocene and Pleistocene)—Ash beds. Shown only where from a greater role played by CO2 as an explosive agent microphenocrysts of clinopyroxene <0.5 mm visible with hand substantial mounding of material around the crater rim and may 2°30´ L299 Qoda2 Qnc Qvg sufficiently thick to obscure underlying rocks over large areas. TRAIL Y (compared to lake water or groundwater in many volcanic arcs) lens. Those samples are phonotephrite on basis of two analyses O Ingaimurunya 2 have formed by minor CO explosions during a summit eruption MAP AREA OL247 Ql2 Includes young ash that mantles lower west flank of modern 2 1400 860 5 (Mattson and Tripoli, 2011). Age ranges widely, to judge from (table 1). See explanatory pamphlet for discussion of age L300 14 N cone and a thick tephra blanket that has accumulated downwind 2008 lithic L97 degree of erosional state, though all are thought to be younger SANJAN MOSONIK GELAI 7 to the west atop the Natron escarpment over thousands of years Levee in lava flows, lahar deposits, and debris-avalanche 17 pyroclastic flow than Gelai and Kerimasi volcanoes and therefore younger than 1 AREA OF MTO WA MBAGAI 39/1 39/2 40/1 deposits—Hachures point into body of flow L259 1600 Qnc “Dorobo” Qoda2 Ma. Phlogopite samples from tuff rings of Loolmurwak and Qomlc Qol Mica-rich lahars (Holocene)—Fine-grained lahar deposits rich in O cone Qnc Kisitei craters have K-Ar weighted-mean ages of about 0.37 Ma [Area not mapped by us. Depiction from Isaac (1967) and fitted to the Mosonik phlogopite. Located on north and northwest alluvial apron of Isopach—Showing 5–10-cm thickness limit for 2007–08 explosive 2°45´ Y Qvg (Nos. 3 and 4, table 2; data of Macintyre and others, 1974) topographic map for this map] volcano. Separate exposures may not be age-equivalent deposits deposit T R 12 A Y Lalarasi O L304 Qtse I 2400 2200 Tuff and sandstone atop escarpment (Pleistocene)—Ill-defined 2°45´ L L303 Qnet Extensive tuff deposits (Pleistocene)—Yellowish gray to light ANGATA SALE OLDONYO KITUMBEINE OL789 L156 Eledoi 2°45´ CONE-BUILDING VOLCANICLASTIC ROCKS AND LAVA FLOWS Latest Pleistocene shoreline—Extant between about 13,900 and 1600 TRAIL OL788 33 Qomv L154 L15 Qnc stratigraphic unit likely composed of sand and gravel deposited 1400 OL801 Qnc brownish gray tuff and lapilli tuff. Chiefly pyroclastic-flow and L305 OL821 11,500 cal yr B.P. (from Hillaire-Marcel and others, 1986, 1987) 39/3 LENGAI WEST 1800 OL803 Qntr 30 QTel 2600 L306 OL824 debris-flow deposits that originated from tuff cones and rings of before escarpment had reached its current level of structural 39/4 40/3 Qomlu Qoda Qomv Qom Main-cone volcaniclastic rocks and lava flows (Holocene and L274 sm 1 offset. Includes distal volcanic ash from unknown sources ern Cha the Engaruka-Natron volcanic field. Beds 0.5–3.0 m thick. East 35 L236 re Pleistocene)—Stippled on northwest flank where pyroclastic Strike and dip of beds 3° OL230 L276 da Qa OL503 n Qa Qnc Includes some thinner fallout lapilli tuff, lapillistone (ash-size 2 8 2 18 I 2 L172 flow(?) deposits of 2007–08 can be delineated from oblique OL7/88 40 353±65 ka an Peninj Group (Pleistocene)—Consists of: Inclined—Showing dip 0 20 40 KILOMETERS Qomlc Qomc Embalulu d content minimal), and fine- to medium-grained sandstone i photography. Mapped separately are: L271 S 24 Qnet summit 2952 m L275B Oltatwa 32 Qa Horizontal INDEX TO 1:50,000-SCALE 2800 1 OL806 Qpne Phonolite lava flow 10 km northeast of summit Moinik Formation—Lacustrine deposits and lesser interbedded OL478 Qoml Lava flows (Holocene and Pleistocene)—Chiefly nephelinite and TRAIL Qnc Qnc lava flows. Exposed in upthrown block of Natron fault. Age TOPOGRAPHIC QUADRANGLE MAPS OL795 (Pleistocene)—Slightly porphyritic lava flow or flat dome.

L279 E QTel TRAIL 960 phonolite, but includes recent carbonatite on west flank. Shown OL226 2600 Qnc N between about 1.1 and 1.5 Ma according to assessment of Vertical

(NAME AND SHEET NUMBER) G Overtopped by debris-avalanche deposits of unit Qoda3, but age T are those lava flows for which mapping is sufficient to approxi- R Qomlp A several radiometric ages from lava flows within and beneath A L110 R of those deposits poorly known. Relation to Oldonyo Lengai’s

I 338±23 ka U mate their extent; other less well-defined flows are contained L Oldonyo Lengai 34 Notable site—Places likely of interest to future Earth scientists Qntr TRAIL L103 K unit (see explanatory pamphlet). Divided by Isaac (1967) into A magma system unknown: if lava flow from central vent, then L47 L46 within the unit of main-cone volcaniclastic rocks and lava flows 5 - 2000 OL450 L working in the area but not flagged by other symbols. See table L106 O sufficiently old that erosion has made it topographically Embalulu L (Qom). Subdivided into: I Qmiu Upper tuff (this sheet) for index of features Qa 2200 OL440 O distinct; if peripheral dome, it stands well beyond the 6-km Qot Kirurum 1 1800 N OL442 Qoml? D O radius that encompasses all other vents associated with that Qomv? Qomlc Carbonatite lava flows (Holocene)—Pāhoehoe and ‘a‘ā of Qot Qomv R Qmin Nephelinite lava flow—Potassium-argon ages of 1.31±0.26 Ma Paleomagnetic determination—Measured by portable fluxgate Qoda4 O volcano. May be a randomly emplaced rift-basin phonolite or, A 2006 eruption N Qom Y D (Foster and others, 1997) and 1.41±0.04 Ma (Manega, 1993) magnetometer. Normal-polarity magnetization (N) at three sites 1000 conceivably, an unusually silicic component of the O along east side of map area thought to be in lava flows from Qomv? Qomlu Nephelinite of upper flanks (Pleistocene)—Remnants of lava Natron-Engaruka volcanic field 1600 Qmic Claystone and tuff Gelai volcano (unit Qvg) and another site in lava erupted from a Qa1 Location? flow, highest part of which is sometimes called “Pearly Gates” vent in Natron-Engaruka volcanic field. Reversed-polarity 0.15±0.02 Ma ROCKS OF THE ESCARPMENT AND GELAI AND KERIMASI 35°45´ E 36°00´ L82 (Bagdasaryan for the slot traversed through it by westside climbers’ trail Qh Humbu Formation—Deltaic sandstone and interbedded lava magnetization (R) from intrusion within Natron escarpment lava 1800 Qom Qnc VOLCANOES 2°30´ S et al., 1973) L79 flows. Age between about 1.5 and 1.8 Ma according to assess- of Endukai Kiti (unit Tek) Qomlp Phonolite lava flow on lower east-southeast flank Qvg Volcanic rocks of Gelai volcano (Pleistocene)—Chiefly lava flows Qomv? O Qnc ment of widely scattered radiometric ages from lava flows 9 (Pleistocene) and thin interbedded tephra. As described by Dawson (2008), above, within, and beneath unit. Divided locally to show: Sample locations for new radiometric ages and geochemical QTel Y Qnc on 28 Ildonyo forms shield volcano that ranges in composition from alkali tr 1600 a 30 Qnc analyses (this report) N L77 Loolmurwak Nasira tephra cones and lava flows (Holocene)—Extrusive basalt to trachyte, and upper slopes have scattered outcrops of ke Qnc 1000 Qhl Lava flows 338±23 ka 40Ar/39Ar ages—Showing age in thousands of years (ka). Com- La volcanic rocks on lower north flank of Oldonyo Lengai. trachyte. Sparse published chemical analyses are basanitic, Isaac, 1967 plete data in appendix 3 of explanatory pamphlet 1400 Qntr Qvk Undated. Divided into: (table 1, this map; Paslick and others, 1996). Radiometric age is Y L37 Tnnm 2°35´ 96 000 96 000 Nephelinite lava north of Mosonik volcano 90 O 90 about 1 Ma on basis of two gas extractions from a sample Qnl (Pliocene)—Radiometric ages of 3.26±0.08 Ma (Isaac and L37 7 Qomnc Tephra cones and spatter mounds collected on south flank (table 3, No. 5; Evans and others, 1971) Geochemical analyses—Showing sample number (short version) THIS MAP Loolmurwak Curtis, 1974) and 3.53±0.06 (Manega, 1993) crater Previously analyzed rock samples—Showing sample number Qomnl Lava flows Qvk Volcanic rocks of Kerimasi volcano (Pleistocene)—Volcaniclastic OL184 1200 Tsh Sambu and Hajaro Basalts (Pliocene) Neukirchen Qoda4 beds. Forms southern limit of our mapping; deposits not studied (Klaudius and Keller, 2006) and others, 2010 Qomv Qnc Flank vent deposits (Holocene and Pleistocene)—Forms chiefly by us. As discussed by Hay (1976, p. 137), age older than 0.37 L210 1040 1020 Previously dated samples—Showing age and analytical error, with 2°40´ Dawson, 1962 tephra cones. Several supposed tephra cones are shown queried Ma, the average of several ages from mica-rich tuff of EXPLANATION OF MAP SYMBOLS Qnc 1200 0.15±0.02 Ma publication source in parentheses. Sites with “Location?” have owing to burial by tephra of past several decades that has Loolmurwak and Kisitei craters (Bagdasaryan Klaudius and Keller, 2006 5 7 Qvk Kisitei large positional uncertainty Loluni thoroughly mantled underlying deposits. Some of these have et al., 1973) 1100 Contact—Solid where precisely or approximately located; short crater obvious cone morphology and likely are buried vent deposits; Qsk Sekenge tuff ring deposit (Pleistocene)—Slightly micaceous tuff, Mosnonik Kervyn and others, 2008a dashed where inferred; dotted where buried Springs—Showing those verified during mapping: (1) in Sidan Kervyn and others, whereas others are more subdued, and their semicircular lapilli tuff, and tuff breccia. Has more bedrock rubble, including quadrangle Engatoto Qntr Qa1 Indare, (2) near Engare Sero village, (3) near mouth of Leshuta 2008b downslope edges may be underlain instead by the snouts of conglomeratic clasts, in its block component than any other tuff o village Internal contact n stream a debris-flow deposits or by landslide or debris-avalanche blocks ring or tuff cone in the area, most of which are notably fine 2°45´ lc o Qa i v 1 Oldonyo a 96 000 96 000 ng 87 87 Lengai Le 1400 1200 o Qvk quadrangle ny ldo O 2°50´ S 2°50´ S 1600 1600 1600

2°50´ 35°50´ 816000 820000 35°55´ 830000 36°00´ E Deeti cone 0 5 10 KILOMETERS Base from Surveys and Mapping Division, Ministry of Lands, SCALE 1: 50 000 Geology mapped by D. Sherrod, M. Magigita, and 0°9 United Republic of Tanzania. Mosonik (sheet 39/2), 1990; ' MN 1 0 1 2 3 4 5 6 KILOMETERS S. Kwelwa, February–March 2010. 1000 Oldonyo Lengai (sheet 39/4), 1990 INDEX TO PREVIOUSLY MAPPED AREAS GN Assisted by Kisana Mollel and Olemelok Nantatwa Transverse Mercator projection, UTM zone 36. 0°8' 1 0 1 2 3 4 MILES New (1960) Arc datum GIS database and digital cartography by D. Sherrod 3000 0 3000 Geologic Map of Oldonyo Lengai Volcano and Surroundings, 6000 9000 12000 15000 18000 21000 FEET Edited by Claire Landowski Any use of trade, product, or firm names in this publication is for descriptive purposes only and APPROXIMATE MEAN Manuscript approved for publication Dec. 23, 2013 DECLINATION AND does not imply endorsement by the U.S. Government UTM GRID CONVERGENCE, 2012 CONTOUR INTERVAL 20 METERS This map is published as an online-only version. If printed, then dimensional calibration may vary Region, United Republic of Tanzania between electronic plotters and between X and Y directions on the same plotter, and paper may EXCEPT 40-METER INTERVAL BETWEEN 1800- and 2800-METER ALTITUDE change size due to atmospheric conditions; therefore, scale and proportions may not be true on ON CONE OF OLDONYO LENGAI plots of this map. By Digital files available at http://pubs.usgs.gov/of/2013/1306 VERTICAL DATUM UNSTATED Suggested Citation: Sherrod, D.R., Magigita, M.M., and Kwelwa, S., 2013, Geologic map of 1 2 3 Oldonyo Lengai (Oldoinyo Lengai) and surroundings, Arusha Region, United Republic of Tanzania: David R. Sherrod, Masota M. Magigita, and Shimba Kwelwa 1U.S. Geological Survey; 2 U.S. Geological Survey Open-File Report 2013-1306, pamphlet 65 p., scale 1:50,000 Geological Survey of Tanzania; [http://dx.doi.org/10.3133/ofr20131306]. 3 2013 University of Dar es Salaam ISSN 2331-1258