Pleistocene Tephra and Ash-Flow Deposits in the Volcanic Highlands of Guatemala

ALLAN J. KOCH P.O. Box 5444, Denver, Colorado 80217 HUGH McLEAN U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025

ABSTRACT rently active. The highlands lie largely STRATIGRAPHY above 1,500 m, many of the volcanoes Twenty-six Pleistocene tephra layers, reaching altitudes of more than 3,700 m. Twenty-six tephra layers, four ash-flow four ash-flow tuff deposits, and four North of the volcanoes lie a series of tuff units, four fluvial lacustrine sediment fluvial-lacustrine sediment sequences, flat-floored intermontane basins (Fig. 1). sequences, and numerous paleosols com- interstratified with paleosols, occur in an During the Pleistocene Epoch, periodic pose the surficial stratigraphic succession area of 1,500 km2 between Guatemala City eruptions deposited large volumes of silicic (Fig. 2). Formal stratigraphic nomenclature and Lake Atitlan. Most of the tephra layers pumice and ash over nearly all of the high- for these deposits has been avoided to and one of the ash flows can be individually lands and beyond. Much of the pyroclastic maintain a flexible system that may be eas- recognized in the field by mineralogy of ejecta was erupted high into the air as ily amended or modified by future workers. Fe-Mg phenocrysts, texture, pumice color, tephra (Thorarinsson, 1954), where it Major tephra units are designated with and stratigraphic position. cooled and was distributed by prevailing random capital letters (E, C, L, T, and so The texture, thickness, and distribution winds as a continuous mantle of air-fall de- forth). Tephra layers believed to be of local of the tephra layers indicate that they posits over the surrounding terrain. A large extent or that are not well exposed are originated from five separate centers: the portion of the pyroclastic material, how- given letters with numerical subscripts (Jj, Pacaya volcanic complex, Agua Volcano, ever, was erupted as hot turbulent mixtures Z2). Ash-flow tuffs carry the same letter as Acatenango or Fuego Volcanoes, the Lake of gas, pumice, and ash that moved rapidly the underlying tephra layer; for example, Atitlan area, and the Laguna de Ayarza across the ground surface, flowing preferen- the H ash-flow tuff overlies tephra layer H. area. The exact sources of the ash flows are tially down stream valleys and into basins Paleosols and sediment sequences are des- not known. Size of pumice clasts, pumice and in some cases accumulating to thick- ignated according to the youngest tephra texture, and mineral content show that the nesses of hundreds of meters. The turbulent unit they overlie (for example, post-T basin-filling ash-flow tuff units are of prim- mixtures of gas and ejecta material are here paleosol, post-H sediments). ary origin and could not have resulted from referred to as ash flows and their resulting Two surficial stratigraphic successions inwash of previously deposited pumiceous deposits as ash-flow tuffs, following the have been identified. An upland succession tephra. Each ash-flow tuff apparently was usage of Ross and Smith (1961). consisting of tephra and paleosols occurs deposited shortly after a tephra eruption. A surficial mantle of pumiceous ejecta on topography lying above the level of the Reconnaissance work indicates that the has been deposited on the north side of the basin fills. The more complex basin succes- H ash-flow tuff unit and its underlying volcanic chain, but very little pumice and sion consists of tephra, ash-flow tuff units, tephra cover a major portion of the vol- ash are preserved south of the volcanoes, and fluvial and lacustrine sedimentary canic highlands — at least 16,000 km2 and indicating prevailing southerly winds dur- rocks interstratified with paleosols (Fig. 3). 7,500 km2, respectively. Their source prob- ing eruptions. The stratigraphy of the These topographically controlled facies of ably lies close to the Lake Atitlan area. flat-floored pumice- and ash-filled basins Pleistocene pumice deposits are illustrated These units should be extremely useful as and adjacent tephra-covered uplands is well in Figure 4 and contrast the depositional time-stratigraphic marker horizons in other exposed in stream and road cuts through- history of the Guatemala City basin and its parts of the Guatemalan highlands. out a 1,500 km2 area between Guatemala surrounding highlands. The highland Radiometric dating indicates that most City and Lake Atitlan. tephra-paleosol succession is more than 30 major tephra and ash-flow deposits were The purpose of this paper is to discuss the m thick near the source volcanoes, whereas deposited between 40,000 yr and 1.84 m.y. stratigraphy, distribution, composition, due to the additional accumulation of the ago. Were similar eruptions to occur today, age, and origin of the Pleistocene tephra ash-flow tuffs and sedimentary rocks, the widespread devastation and loss of life and ash-flow deposits. This paper is based basin fill locally exceeds 100 m. could result. Key words: stratigraphy, Pleis- on field and laboratory work completed in Tephra layers are well sorted and contain tocene, pyroclastics, volcanism, absolute 1969 and 1970 by Koch (1970) and angular pumice clasts as a result of explo- age. McLean (1970). Prior to this study, no de- sive ejection and aerial transport. Particle tailed systematic examination of these de- size and thickness decrease and sorting in- INTRODUCTION posits had been attempted. However, some creases with distance from the source vent. general information is available on the dis- Two of the larger tephra sheets, E and L, The highlands between Lake Atitlan and tribution, origin, age, and mineralogy of are at least 10 m thick near their source. Guatemala City are composed mainly of pumiceous deposits in selected areas (Wil- The maximum size of pumice bombs and eroded late Tertiary volcanic rocks and a liams, I960; McBirney, 1963; Williams accidental lithic fragments reaches 23 cm strikingly linear chain of Quaternary com- and others, 1964; Bonis, 1965; Bonis and and 15 cm, respectively. Many tephra posite volcanoes, some of which are cur- others, 1966). layers are reversely graded, probably indi-

Geological Society of America Bulletin, v. 86, p. 529-541, 13 figs., April 1975, Doc. no. 50411.

529

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021 530 KOCH AND McLEAN

eating a progressive increase with time in post-E soil is believed to be more than volume. The average lithic fragment con- the intensity of the eruptions. Other units 40,000 years old. tent for the L, R, and T tuffs is about 20 to have alternating fine- and coarse-textured Gray andesitic tephra are much more 30 percent by volume. horizons suggesting cyclic changes in inten- numerous among younger deposits, and The H ash-flow tuff, in contrast to the sity of the eruptions. Few of the tephra only a few are interbedded with older white three older units, is a unique deposit with layers contain evidence of intralayer weath- tephra. The majority of the younger cinder many identifiable characteristics. The upper ering, suggesting that the eruptions that layers in the Guatemala City—Antigua areas portion of the layer is usually pink (10R produced the layers were short lived. Where were derived from Agua and Pacaya Vol- 7/4). The pink tint tends to be quite uniform tephra rained down upon slopes of 30 per- canoes. Some cinder layers extend more and in many outcrops forms a sharp color cent grade or more, larger particles rolled than 20 km from the parent volcanoes. contrast with the lower light-gray (10YR downhill over smaller particles, resulting in Field criteria found most useful in dif- 7/1) portion of the layer. In other areas, the stratification and a thicker accumulation at ferentiating the tephra layers include (1) the pink tint is patchy. No textural or composi- the base of the slope. The term "roll bed- character of the loose Fe-Mg crystals within tional change has been observed with the ding" is proposed for this type of the ash fraction of the layer; (2) color of color change. Unlike the older ash-flow stratification. Tephra particles deposited in pumice and ash; (3) bedding characteristics, tuffs, the lithic fragments in H include lake waters are well stratified, and some are such as alternating fine and coarse layers or plutonic as well as volcanic rock types. The reversely graded. well-developed reverse-graded bedding; plutonic lithic fragments consist largely of Ash-flow tuff units compose the bulk of and (4) nature and abundance of accessory coarse-grained biotite-bearing granite, the flat-floored basin fills. Where unre- rock fragments. These criteria, in conjunc- quartz monzonite, and granodiorite and worked, they are characterized by lack of tion with the relative maturity of associated compose about 15 percent of the total lithic sorting and stratification (Fig. 5). They typ- paleosols and a comparison of the layer's component. Microcline is the dominant ically have irregular lower boundaries and stratigraphie position with known marker feldspar in some of the granite examined. flat upper contacts. A matrix of fine to horizons, usually permit on-the-spot The volcanic fragments are composed coarse ash, pumice, and lithic fragments identification. largely of porphyritic augite—hypersthene predominates over lapilli- and bomb-sized Some tephra layers have highly charac- andesite and lesser amounts of andesitic pumice and lithic fragments. Maximum teristic features that help determine the crystal-vitric tuffs. diameters of pumice bombs and lithic stratigraphie order and simplify the tracing fragments reach 60 cm and 40 cm, respec- of units within the basin fills. For example, COMPOSITION tively. In many localities, the ash-flow tuffs tephra Y is distinctive in the field, the upper contain charcoal logs and branches, inclu- one-third to one-half of the layer being dark Tephra pumice is composed of glass, sion trains of lithic fragments, and a mea- gray, in contrast to a lower white portion phenocrysts of plagioclase, minor amounts surable preferential orientation of elongate (Fig. 7, 8). Tephra L is characterized in of quartz, and a variety of Fe-Mg pheno- pumice lapilli and blocks. All four of the most exposures by a central band that is crysts including magnetite, biotite, ash-flow tuff units are poorly consolidated finer than the tephra both above and below. hornblende, hypersthene, olivine, clino- and show no signs of welding. Each Tephra H is white, has abundant glass pyroxene, and in one case, traces of cum- ash-flow tuff was preceded by an air-fall shards and contains distinctive elongate mingtonite. The relative abundance of bio- event and no weathering of the underlying lapilli. It overlies a mature paleosol, here tite, hornblende, and hypersthene is par- tephra occurred before deposition of the named the Molino paleosol for numerous ticularly useful in identification of tephra ash-flow tuff. exposures in the Molino River valley on the layers. Of the 26 tephra layers, 14 have At least four sequences of fluvial and (or) southwest edge of Guatemala City. Tephra hornblende as their dominant Fe-Mg lacustrine sediments can be recognized in E is the youngest major tephra layer in the phenocryst, 8 have biotite, and the remain- the Guatemala City basin: the post-H, Guatemala City area, and its stratigraphie ing 4 are hypersthene rich. In most cases, post-L, post-R, and post-T successions. position makes it easy to recognize. the dominant phenocrysts can be estab- Each sequence directly overlies an ash-flow Identification of the underlying tephra lished in the field. Magnetite occurs in all tuff and probably resulted from the forma- appears to be the easiest method of identify- layers and is significant only with regard to tion of lakes and readjustment of streams ing the overlying ash-flow tuff. Ash-flow its abundance. Potassium feldspar is nota- following infilling of the basin by an ash tuff units L, R, and T are similar in color, bly lacking from most layers. Pumice flow. The fluvial sedimentary deposits are mineralogy, and texture. Lithic fragments mineralogy is sufficiently constant within a mainly composed of weakly indurated, in these ash flows are mainly gray, fine- to given tephra to be an excellent criterion for well-stratified beds of pumiceous ash and medium-grained porphyritic andesite. identification over large areas. rounded gravel-sized pumice. Fluvial beds However, unit T contains a light-gray Although the mineral content is generally contain sedimentary features such as crystal-vitric tuff with black hornblende quite uniform, tephra layers E, Y, T, L, and large-amplitude tabular cross-bedding, ir- phenocrysts. Fragments of this hornblende H show intralayer vertical mineralogical regular and lenticular bedding, and crystal- tuff locally compose up to 10 percent of the and compositional changes. The amount of and lithic-rich layers. Lacustrine deposits lithic fragments contained in unit T. Angu- hypersthene increases and biotite gradually are composed of white, very well bedded lar and subangular lithic fragments in the L, decreases from the base to the top of layer E tuffaceous and diatomaceous mudstone in- R, and T ash-flow tuffs range in size from in a 9-m exposure near the source volcano, terbedded with a few thin cinder layers (Fig. less than 1 mm to more than 40 cm, with as shown in Table 1 and Figure 9. The color 6). the majority between 5 and 15 mm. The av- contrast in tephra Y, from dark gray at the The thickness and maturity of paleosols erage volume of accessory volcanic lithic top to white at the bottom, coincides with show that periods of quiescence between fragments in ash-flow tuffs L, R, and T is an abrupt compositional change. eruptions were of variable duration. Ma- generally three to four times greater than in Biotite:hornblende ratios in tephra L in- ture paleosols indicating long periods of tephra. Each ash-flow tuff is characterized crease from 3:1 at the base to 9:1 at the top. nondeposition of pyroclastic ejecta were by great lateral variation in lithic fragment Tephra T has about 2.5 times as much developed on tephra R1; R2, R3, L, T, and E. concentration. Unit T contains the highest hornblende, relative to biotite, at the top as In the Guatemala City area, where tephra E percentage of lithic fragments, with local at the bottom. Tephra H differs in composi- was the last major eruptive deposit, the concentrations up to 60 or 70 percent by tion from the overlying H ash-flow tuff.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021 PLEISTOCENE TEPHRA AND ASH-FLOW DEPOSITS IN THE VOLCANIC HIGHLANDS OF GUATEMALA 531

Bonis and others, 1970).

Whereas tephra H pumice contains only (Table 1). These properties alone or in of specific mineral species occur. The three traces of cummingtonite, the mafic fraction combination with the Fe-Mg phenocryst as- older ash-flow tuffs (L, R, and T), like their from pumice in the overlying tuff locally semblage are generally sufficient to charac- underlying tephra, are rich in hornblende may contain up to 50 percent cummingto- terize a given layer. and biotite phenocrysts and contain lesser nite. The 0.5-mm to 0.062-mm fraction of amounts of hypersthene, olivine, and The relative amounts and variety of Fe- crushed pumice lapilli or coarse ash was clinopyroxene. Although the different un- Mg phenocrysts that characterize each used for laboratory analysis. Mafic and fel- derlying tephra layers may be differentiated tephra layer are shown in Figure 2 and sic phenocrysts were isolated from glass by from one another by biotite:hornblende Table 1 and plotted on a ternary diagram in magnetic and heavy-liquid separations. ratios, the amounts of these two minerals Figure 9. Also shown in Figure 9 is the vari- Percentages of the phenocrysts were deter- are too variable in ash-flow pumice to be of ation of layer composition for different mined with a pétrographie microscope by much value in differentiating the tuffs. source volcanoes. Some volcanoes appar- counting all grains within a number of ran- Unlike the older ash-flow tuffs, the H ently produced numerous tephra layers domly selected fields of view in grain ash-flow tuff is easily distinguished from with similar proportions of Fe-Mg pheno- mounts of the mafic mineral concentrate. the older deposits by its mineralogy. Pumice crysts, whereas others (for example, Pacaya Felsic phenocrysts were identified using the samples analyzed from the H unit contain Volcano) erupted many layers with quite staining technique of Chayes (1952) and biotite, cummingtonite, and magnetite as different Fe-Mg mineral assemblages. Laniz and others (1964). principal Fe-Mg phenocrysts. Discounting Other derived properties of tephra The mineral composition of pumice in magnetite, the cummingtonite ranges from pumice include weight percent magnetite the ash-flow tuff units is generally similar to trace amounts to nearly 50 percent of the and Fe-Mg phenocrysts, refractive index of that of their respective underlying tephra remaining mafic phenocrysts. Hornblende, glass, and composition of plagioclase layers, although differences in abundance hypersthene, and clinopyroxene occur in

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021 MINIMUM AREAL MAJOR F E-MG PUMICE MAXIMUM THICKNESS DISTRIBUTION SOURCE VOLCANO SPECIAL FEATURES PHENOCRYSTS COLOR (METERS) AND LOCATION (KM2)

textural and compositional layering r <53 hyp, cpx gry to blk 1.0 NW flank V. Acatenango 160 Acatenango or Fuego TEPHRAG1-G3 |G2 hornbl, hyp white 0.5 NE 80 // MM -1 very prominent hornblende white 1.0 NE 300 J phenocrysts to 2 mm in length nAKwti iww t J3 biotite white 0.4 "l Km 33, Hwy CA9 125 Pacaya R aasixM TEPHRA J1-J3 | ^ WwJJTOww biotite, cpx white 0.4 J Amatitla'n 125 hornbl, biotite •.•VV white 2.0 Laguna de Calderas 110 TEPHRA B biotite, hornbl white 0.5 E. Guat. City basin 780 Ayarza area mostly ash in Guat. City area S 0 0 0 0 0 c O <5 O O 0 n 0 0 0 °o ô 0 0 « 0 > 0 hornbl, hyp, biotite TEPHRA E white 10.0 N. side of , 1330 Pacaya reversely graded bedding 0 0 00 0 0 <0 « ®« 4> Lake Amatitlan 0 « 0.®» z 0 0«« • A « TJ .S — hornbl, hyp 0.3 Antigua area TEPHRA A A white 550 fine ash in Guat. City area V 2 bftoJtiMaeoMr s» biotite, hornbl white 0.3 Antigua area 550 "O 0 0 0 0 0 "Ö 0 « 0 0 ° S _ Km 23 Hwy CA9 in part deposited in paleo- TEPHRA C 0 0 0 0 0 « hornbl white 1000 Agua «'«»O » 0 Amatitlan lake in S. Guat. City basin c 3 > POST - H SEDIMENTS 5 3E pink color, granitic T lithic fragments, biotite, cummingtonite pink Lake Atitlan area ? 100 Patzun - Los 16,000 charcoal logs white Chocoyos area H ASH-FLOW TUFF -

TEPHRA H biotite, tr. cummingtonite white 2.3 Lake Atitlan area 7,500 Lake Atitlan area ? elongate blade-shaped lapilli

MOLINO PALEOSOL

POST-T SEDIMENTS

charcoal logs and hornbl, biotite v. It. gry — Guat. City basin - - branches T ASH-FLOW TUFF

0 - ' • « « • 9 • « 0 • - Km 24 Hwy CA9 TEPHRA T # hornbl, biotite white 2100 Pacaya abundant lithic fragments 00 • 0 u Amatitlan 0 0 % 0 TFAVXWAW. z 0 0 0 0 O 5 0 * « 0 hyp white 3.0 U.N. Park Amatitlan 225 Pacaya

Z hornbl, biotite gry-white 2.0 Pastores Km 64 550 TEPHRA Z.-Z- ' * Acatenango or Fuego - 1 8 z 3 '^'O hyp, hornbl It. gry 1.0 Pastores Km 64 450

z2 0 0 0 0 0 0 hornbl It. gry 2.0 U.N. Park, Amatitlan 100 Pacaya L Q • 0 « 0 Z1 irvTvy ' hornbl white 0.5 Pastores, Km 64 e ® « « 0 white 2.0 Pastores, Km 64 Agua abundant lithic fragments TEPHRAS « 0 4 0 0 hornbl 550 M « • « TEPHRA Y hornbl, biotite d.K.äW 2.0 Km 59.6 Hwy CA1 1000 Acatenango or Fuego upper one half of layer is gray • • « 0 white 2.0 Km 59.6 Hwy CA 1 - TEPHRAX • • « 0 • hornbl, biotite 900 - 0QWAV/AW • 0 0 0 0 basal cinder layer in TEPHRA W « » 0 0 0 biotite white 4.0 Km 131 Hwy CA1 2700 Lake Atitlan area P V 0 « Atitlan area c 00 O

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021 y«w*www l 0 o o « o " *5 0 • « • hyp white 3.0 U.N.Park Amatitlan 225 Pacaya TWTOAMWA z 4 hornbl, biotite gry-white 2.0 Pastores Km 64 550 TEPHRA Z,-Z5 Acatenango or Fuego - hyp, hornbl It. gry 1.0 Pastores Km 64 450

z2 O O O 0 o o hornbl It. gry 2.0 U.N. Park, Amatitla'n 100 L « « « » o Pacaya Wl'tf'«' hornbl white 0.5 Pastores, Km 64 « O o t> « TEPHRAS * % ° hornbl white 2.0 Pastores, Km 64 550 Agua abundant lithic fragments dkary TEPHRA Y hornbl, biotite 2.0 Km 59.6 Hwy CA1 1000 Acatenango or Fuego upper one half of layer is gray white • • « o TEPHRA X • •% O « hornbl, biotite white 2.0 Km 59.6 Hwy CA 1 900 - - WNMWMW • O o o basal cinder layer in TEPHRA W biotite white 2700 Lake Atitlan area « „ O ° ° 4.0 Km 131 Hwy CA1 Atitlan area O p O Q a® o «y O « POST-L SEDIMENTS -• 0

pale brown pfí biotite, hornbl - - Guat. City basin - < - 0 L ASH-FLOW TUFF , - — _ o 'o 0 - 0- * OQOCtt « « « • O • • fleo» e

0 O O « « O .9Hífi2 distinctive shower bedding TEPHRA L W v.»<• *»e hornbl, biotite It. brownish gry 10.0 S. Quat. City basin 1100 Pacaya usually with central layer 0 O O OOOO which is finer in texture t O « O « o 0 0 o o o « 0«««®Ofl

POST-R SEDIMENTS

white to It. gry biotite, hornbl — Guat. City basin {weathers - - . - o * brn) R ASH-FLOW TUFF a Tephra pumice and ash

0 - * 0 o o o o •; Fine to coarse ash ' R3 O O O ft 9 biotite, hornbl" gry-white 3.0 Río Pensativo Pacaya ? » o o °co Antigua area o O O O O O"" o o o o o • ' • 0 « 0 O 0 TEPHRA R,-R3 „° « ° R2 white « o o o o hyp, hornbl - 0 o o o Ash-flow tuff 4 ' t>

o « o o R 1 O O 0„ © j? 8.0 Rio Idolos bridge Patzun Fluvial sediments KAWAWW QUATERNARY-TERTIARY V * V t' ACIDIC TO INTERMEDIATE hornbl, biotite white j8—^ Lacustrine sediments FLOWS

Paleosol 1.5 Km 39 Hwy CA1 Cinders

Figure 2. Proposed informal stratigraphie COMPOSITE SECTION OF SURFICIAL PUMICEOUS nomenclature and basic descriptive information for surficial pumiceous deposits. DEPOSITS IN THE CENTRAL VOLCANIC HIGHLANDS

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021 534 KOCH AND McLEAN

trace amounts. The presence of abundant cern because many units were either eroded cummingtonite in the H ash-flow tuff from the steep volcano flanks or buried by makes the layer uniquely traceable, because younger pyroclastics. no other tephra or ash-flow deposits ex- The 26 tephra layers are believed to be amined contain the mineral in more than derived from five separate volcanic centers: trace amounts. the Pacaya volcanic complex, Agua Vol- cano, Acatenango and (or) Fuego twin DISTRIBUTION AND INFERRED composite volcanoes, the Atitlàn volcanic SOURCE AREAS complex, and possibly the Ayarza caldera area. Within these five source areas, only Source areas for tephra were determined Pacaya and Fuego Volcanoes are currently mainly from texture and thickness of the active. Since the beginning of the Quater- Figure 3. Faulted basin succession deposits deposits. The layers are thickest and the nary Period, the Pacaya complex has been exposed in a road cut south of Guatemala City thè most active, erupting at least eight near San Cristobal and the Molino River. particle size of pumice and lithic fragments Weathered zone between tephra E and C con- is coarsest adjacent to and on the flanks of tephras (L, Z2, Z5, T, E, Jl5 J2, and J3) andpos- sibly the T, L, and R ash flows. The twin tains At and A2, which cannot be seen in this the source volcanoes. The source areas of photo. older tephra layers are more difficult to dis- composite volcanoes, Acatenango and

UPLAND SUCCESSION

Figure 4. Diagrammatic relation between basin and upland succession surficial deposits. Upland deposits are generally thinner than 30 m, whereas basin deposits are commonly as thick as 100 m.

"caldera forming" eruption in the Laguna de Ayarza area (Williams, 1960) postdates H and may be correlative with tephra B. Reconnaissance work on the west side of Laguna de Ayarza shows that at least four tephra eruptions antedate deposition of tephra H. The minimum areal distribution of tephra sheets ranges from 600 km2 to more than 7,500 km2 (Fig. 10). Some of the younger tephra sheets, including E and C, were amenable to isopach mapping. The Figure 5. Nonsorted texture of the H larger tephra sheets H and W need more ash-flow tuff 25 km west of Guatemala City. Figure 7. Upland succession (X, Y, T, and H) work before their patterns of distribution tephra (light units) and their overlying paleosols can be accurately delineated. Fuego, have erupted six tephra sheets (X, Y, (dark units) exposed in a road cut at Km 59, Z4, G,, G2, and G3). Agua Volcano has pro- The patterns of tephra distribution sug- Highway CA-1 west of Guatemala City. duced two tephra layers (S and C). gest that most of the pyroclastic ejecta was Reconnaissance work indicates that blown to the north. Tephra lobes generally Mixco, pumice and ash could have flowed tephra layers H and W were derived from trend northward, with little air-fall pumice into the Guatemala City and Santiago the Lake Atitlan area. Two smaller tephra and ash preserved south of the volcanoes, Sacatepequez basins, and possibly into the sheets overlie H east of Lake Atitlan and although some waterlaid and reworked area northeast of Antigua and the eastern probably were derived either from Atitlan, pumice has been observed in parts of the portion of the Chimaltenango basin. If, Toliman, or San Pedro Volcanoes. In road coastal plain south of the Quaternary vol- however, considerable subsidence has low- cuts along Highway CA9 in the Nahuala canic chain (H. Williams, 1974, written ered the southern end of the Guatemala area, four thick white tephra layers from an commun.). Because the present annual av- City basin during or since the deposition of unknown source or sources overlie layer H. erage wind direction is consistently from the ash-flow tuffs, then the Pacaya volcanic In the Quezaltenango basin near Ostun- the north-northeast, this preservation pat- complex, from which the tephra beneath calco, a mature paleosol separates layer H tern of the tephra appears anomalous. The the ash-flow tuffs were erupted, is a likely from the underlying 10 to 12 m of tephra pattern observed is possibly due to a change source for the ash flows. presumably derived from Siete Orejas Vol- in wind directions during Pleistocene time The H ash-flow tuff has a remarkably cano (Williams, 1960). or to southerly winds at higher altitudes. large areal distribution in comparison with No spatial pattern of eruptive activity is The latter explanation is supported by the the older ash-flow deposits. Reconnaissance evident along the volcanic chain during the eruption of Fuego Volcano on November 9, work suggests that it is the most extensive past 1.8 m.y. The activity appears to have 1962. At lower elevations, the eruptive layer of pumice and ash in Guatemala, cov- moved at random from one part of the cloud was blown toward the south; how- ering at least 16,000 km2. The layer occurs chain to another. Tephra ejected during the ever, between 9,000 and 15,000 m, the dust in most of the major drainage basins in an and ash were blown by winds toward the area bounded by the cities of northwest. Fallout of ash and cinders was Huehuetenango, Coban, Nueva Santa later reported 150 km northwest of Rosa, and Quezaltenango (Fig. 12). It ex- Huehuetenango (Bohnenberger and others, tends beyond the volcanic highlands into 1966). the drainages of the Sierras of Central The three older ash-flow tuff units (T, L, America, encompassing an area larger than and R) are best exposed in the northern and the underlying H tephra. It apparently is the southern portions of the Guatemala City only voluminous Pleistocene pumice and basin (Fig. 11). Outside the basin, one or ash deposit north of the Motagua River. more of the ash-flow tuffs compose the valley fill near Santiago Sacatepéquez, 3 km northeast of Antigua and northeast of Guatemala City (Fig. 1). Ash-flow tuffs L and R also crop out along the Rio Guaca- late valley, 3.5 km east of Parramos. These ash-flow tuff units become difficult to identify beyond the borders of their underlying tephra. The source or sources of the T, L, and R ash-flow units are unknown. Possible sources include volcanoes, a single vent in the upland area, or a system of fissures now blanketed by younger air-fall pumice. The upland area between Santiago Figure 8. Tephra Y at Km 60, Highway CA-1 Sacatepéquez and Mixco could have been a west of Guatemala City. Color change from source area for units T, L, and R, assuming white to gray is due to a more basic pumice composition and increased Iithic fragment content in that the present topography is similar to Figure 6. Tephra layers C, A,, A2, and E in the upper half of the layer. Change in composi- quarry exposure 1 km north of Amatitlan. Layer that existing prior to deposition of the tion and reversely graded bedding suggest pro- C is interbedded with lacustrine sedimentary ash-flow tuffs and that the ash flows moved gressive evacuation of a compositionally rocks in this area. Dark layer between tephra E downslope into topographic lows. From a stratified magma reservoir as the intensity of the and A2 is an unnamed weathered horizon. location just a few kilometers southwest of eruption increased.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021 536 KOCH AND McLEAN

The elevation at which the deposit occurs Quezaltenango-Totonicapan area, where it to charcoal logs and branches imbedded in increases progressively from east to west. It is thickest and texturally coarsest. In these valley-filling ash-flow tuff units. Two sam- is found at elevations of 600 to 1,300 m in areas the deposit has not been adequately ples of charcoal from H ash-flow tuff out- the eastern part of its areal extent, increases examined, except on a reconnaissance crops were dated by Bonis and others to 1,900 m at Huehuetenango, and reaches basis, and therefore the exact source has (1966). One sample exposed 1 km east of its maximum elevation of 2,400 to 2,500 m not been delineated. Detailed inspection of San Juan Ostuncalco in the Quezaltenango in the Quezaltenango-Totonicapan area the distribution, texture, and thickness basin yielded an age of 35,000 ± 3,000 yr and north of Lake Atitlan. Texturally, the relations of the ash-flow tuff for possible B.P. The second sample from east of Lake H ash-flow tuff is 90 to 95 percent ash on vents in the Lake Atitlan and Atitlan on the Guatemala City—Atitlan its perimeter and becomes coarser toward Quezaltenango-Totonicapan areas should highway at Km 92 near Los Chocoyos was the area of its highest elevation where indicate the origin of the unit. dated at 31,000 ± 3,000 yr B.P. A more re- pumice blocks and lithic fragments have cent unpublished date from a charcoal- maximum diameters of 60 cm and 40 cm, AGE bearing pumiceous diamicton 5 km south- respectively. west of San Jose Pinula (lat 14°31' N., long The H ash flow probably originated Radiometric dating of pumice deposits in 90°27' W.) yielded an age of approximately somewhere in the Lake Atitlan or Guatemala was restricted prior to this study 41,000 yr B.P. (M. Carr, 1969, personal commun.). Three samples of charcoal were collected AMPHIBOLES and dated during our field work in Guatemala. Two samples came from the H ash-flow tuff, the first from the Ostuncalco locality sampled by Bonis and others (1966) and the second from Rio Molino valley near San Cristobal on the southwest edge of Guatemala City. A third sample of charcoal was collected from the upper few centime- ters of the paleosol at the base of tephra E near San Cristobal. All three samples (UW-141, UW-142, UW-143) yielded C14 ages of approximately 40,000 yr B.P. From these results, it would appear that earlier C14 ages are too young and that all of the ash-flow tuffs and probably all of the major tephra in the study area were deposited more than 40,000 yr ago. Phenocryst concentrates from tephra layers C, T, and L were dated by K-Ar analyses. The dating results conflict in that the stratigraphically youngest tephra layer (C) yielded the oldest radiometric date (3.2 OLIVINE ± 1.3 and 4.1 ± 0.8 m.y. B.P.). Tephra T was dated at -0.3 ± 0.3 and 0.0 ± 0.3 m.y.

AMPHIBOLES B.P. and tephra L at 1.27 ± 0.57 m.y. B.P. We feel that the C hornblende date is probably too old due to contamination. The source of the contamination is unclear; perhaps older hornblende may have mixed with C magma at depth before it was erupted. If the date on layer C is dis- counted, most of the important tephra and ash-flow tuff units were deposited more than 40,000 yr and less than 1.84 m.y. ago. All pumice deposits overlying tephra T were deposited within the past 300,000 yr. Figure 13 summarizes the significant radiometric age results for pumice and ash in the volcanic highlands. Because the H tephra and ash-flow tuff are the most extensive deposits in the Guatemalan highlands and their total areal extent is not known, possibly one or both may be present offshore as deep-sea ash deposits. If the H tephra, or ash flow, has been preserved in marine sediments, its proper- OLIVINE ties may make it a valuable marker horizon. Figure 9. Ternary diagrams depicting Fe-Mg phenocryst composition in tephra pumice (upper). Numerous volcanic ash layers have been Lower ternary relates the Fe-Mg phenocryst composition to source volcano. correlated in the area offshore of southern

Guatemala using trace and minor element late with the ash layers in these two above Lake Amatitlan. More radiometric analysis by Bowles and others (1973). The offshore areas. dates from pumice or rock units associated most areally extensive unit found in the Near Km 23 on Highway CA-1 west of with these magnetically reversed rocks may offshore was "layer D," which may corre- Guatemala City, approximately 10 m of indicate the period of reversal to which they late with the most extensive tephra deposit dark-gray pyroxene andesite is overlain by belong. in the highlands, tephra H. Bowles and tephra L and underlain by tephra R^ (Fig. Pollen studies in cores of bottom others (1973) estimated the age of layer D 13). The contact between the andesite flow sedimentary deposits in Lake Amatitlan to be 50,000 to 60,000 yr, which agrees and the underlying tephra is marked by a suggest that the age of the oldest sediment with our onshore age bracketing of 40,000 baked zone of pink pumice several cen- so far studied is about 3,000 yr (Tsukada to 300,000 yr for tephra H. timeters thick. An oriented sample of the and Deevey, 1967). No interbedded layers Based on refractive index, age relations, andesite from Km 23 analyzed by John of pumice and ash were found in the lake and relative magnitude of the eruption, H Whitney (Univ. Washington) was found to sediments cored. If any of the J tephra are might also be the most likely tephra layer of have a reversed magnetic field. This represent in the lake sedimentary deposits, all those studied to be correlated with one versed volcanic rock may be broadly cor- they must be more than 3,000 yr old. of three ash layers reported in the Gulf of relative with rocks exposed along the north Mexico (Ewing and others, 1958). Other shore of Lake Amatitlan, where DISCUSSION eruptions judged to be of sufficient mag- paleomagnetic analyses by Eggers (1972) nitude to deposit ash in the offshore are indicate that the Lake Amatitlan volcanic Previous interpretations of the origin of those that resulted in tephra W, L, T, E, and succession is reversed at the base and nor- the surficial pumiceous deposits were partly B. More work will be required to confirm mal at the top. This volcanic sequence is incorrect because detailed knowledge of the that tephra H or other tephra layers corre- overlain by tephra L at United Nations Park tephra stratigraphy was lacking. In the

TABLE 1. QUANTITATIVE COMPOSITIONAL INFORMATION FOR TEPHRA AND ASH-FLOW PUMICE

Mafic phenocrysts Felslc phenocrysts Glass

Quantitative Total Magnetite Quantitative Plagloclase Wt % Refractive Silica Layer mineralogy (wt %) (wt %) mineralogy (anorthlte %) Index content

G, HystCis07Hotr 6.6 4.0 58-62 4.0 1.525 61

G2 Ho8iHyi9 25.3 5.1 Pioo 48-52 20.8 1.506 69

GI Ho9«Hyio 19.2 4.9 Pioo 40-44 22.0 1.504 70 J) Bi oo 2.2 2.0 Pioo 27-31 4.0 1.497 73

J2 B77C,2Hy70, 3.0-5.0 0.8 P.ooQtr. 29-33 ' 9.0 1.501 72

Ji HOioB.tCsO, 1.6-1.9 0.3-0.5 PiooQtr' 33-39 6.7 1.504-1.505 70-73

B B7iHo22CiHy, 2.0-2.7 0.4-0.5 P9sS2Q2 12-16 12.6 1.496-1.498 72-73

Ec Ho6oHy2oB2o 9.0-10.1 1.5-2.1 PIOO 25-29 26.0-32.0 1.501-1.502 72

E„ HOtoHystBj 9.9 1.9 PiooQtr 27-31 30.6 1.502 72

E, Ho8!Hy23Bi2 9.6 1.6 24-28 1.502 72

E2 Ho52HytB32 8.9 2.2 24-28 1.501 72

Ei HossHytr B,t 7.8 1.4 Pioo 22-26 26.3 1.500 72

A2 Hos9Hy2iC»0»B2 6.4 1.8 P97Str Qs 29-33 1.7 1.504 70

AI Bs»Ho,CsHy, 1.3 0.2 P75S2,Qtr 20-24 0.5 1.506 69 C Ho,sHy»Btn 10.6-11.8 2.0-2.9 Pioo 40-48 14.4 1.514-1.518 65-66 H Ash-flow tuff BsoCusoHytr.Hotr. 1.0 0.2 P 8 aQ 17 22-26 8.8 1.496 ' 73 Ctr.» BiooCtr.

H B9»HosCtr Cutr 0.7 0.2 PsoOio 20-26 4.0-7.0 1.496 73

T Ash-flow tuff HOsO-7lB25_i,BHyo-2 4.1 2.0 23 24 72 (0 + C)o-2 T Ho„B,«(0 + C), 5.6 2.1-2.4 23 15 1.502 72

zs Hy,,(o + C), 3.5 2.0 27 1.502 72

Z, Ho9sB,Hy2(0 + C)2 1.16 3.0 27 1.500 72

z, HoioHyss(0 + C)s 3.0 50 1.502 72

Z2 HoioBi(0 + C)i 2.0

Z, Ho92Bä(0 + C)5 1.2 50 s HosaHyi(0 + C), 2.5 1.5 38 14 1.508 68 69 Y Hos»BioHy2(0 + C), 2.3 2.1-2.5 38 10 1.506

X Ho,sHy,o(0 + C)s 2.1 1.0 40 11 1.503 71 W BssHo.Hy! 3.8 1.0 11 10 1.496 73 70 L Ash-flow tuff HOie-2flBs2-72Hyo-3 4.5 1.9 34 8 (upper) (0 + C)o_io 72 L Ash-flow tuff Ho(to-eoB3o-"(2Hyo-2 4.0 1.7 42 11 (lower) (0 + C)o-is

L B7OHO20(0 + C)io 5.4 1.5-2.4 34 15 1.506 69 R Ash-flow tuff HOso-seBso-eoHyo-a 3.8 1.5 23 24 74 (0 + C)o_5

Rs HO77B22(0 + C)i 5.0 2.2 23 1.500 72 23 1.500 72 R2 HossHyas 0.55 0.6 1.504 70 RI HOj»BtiHy2(0 + C)j 3.44 4.0 34

Note: Hy = hypersthene, Cu = cumm1ngton1te, 0 = olivine, Ho = hornblende, B = biotlte, C » cHnopyroxene, P = plagloclase, Q - quartz. and S = sanldlne.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021 538 KOCH AND McLEAN

Guatemala City basin, Williams (1960) re- wash of tephra deposits from surrounding partly filled the basin, flattening the rugged ported that only one pumice deposit had hills could not have produced the coarse topography produced by erosion of older been deposited as a "glowing avalanche" particle size found in the ash-flow tuffs. rocks. As the basin became filled with (H ash-flow tuff). He considered the older, 2. The ash-flow pumice has elongate, ash-flow pumice, the drainage was tem- thick, nonstratified, nonsorted pumice de- finely cellular, parallel vesicles, whereas porarily blocked, and shallow lakes formed posits (the L, R, and T ash-flow tuff units) vesicles of tephra pumice typically have no south of the present continental divide. The to be "laid down by streams and torrential discernible fabric. unconsolidated, unwelded nature of the floods (lahars) which stripped air-borne 3. Fe-Mg phenocryst content is consist- ash-flow tuffs in all localities implies that pumice falls from the surrounding hills" ent within each ash-flow tuff. Composition they were deposited under conditions simi- (Williams, 1960, p. 1). McBirney (1963) of lithic fragments is generally constant. lar to those of ash flows at Crater Lake suggested an air-fall origin for the pumice This uniformity of composition would not (Williams, 1942); Komagatake (Kozu, and ash of the H ash-flow tuff in the large be expected in lahars derived from upland 1934); Krakatau (Williams, 1941); the Val- valleys north of the Chuacus Mountains, areas covered with various tephra layers of ley of 10,000 Smokes, Alaska; and Tepic, but he noted that in some exposures, the differing composition. Furthermore, the Mexico (H. McLean, 1972, personal com- pumice deposits were not unlike those Fe-Mg phenocrysts in ash-flow tuff units do mun.). known to have originated by nuée ardente. not differ greatly from those in associated Thick cooling units that show little or no The following evidence is presented in sup- underlying tephra. welding or crystallization are considered by port of a primary ash-flow origin of units 4. The lack of any soil or weathered Smith (1960) to be low-temperature de- H, L, R, and T, as opposed to inwash of horizon at the contact between the tuff and posits. A possible mechanism for low- previously deposited air-fall pumice and tephra suggests that the ash flow was de- temperature emplacement of ash flows is ash: posited soon after the tephra. In addition, the eruption of large quantities of ash and 1. In all outcrops examined, pumice the thick tephra layers E and C, although at pumice in a vertical column above the clasts in the ash-flow tuff units are three to least 40,000 yr old, have no pumiceous source vent and partial cooling by mixing four times the size of the largest particles diamictons associated with them in the val- with air before falling to the surface where found anywhere in the underlying tephra. ley deposits. it forms an ash flow. This method of erup- The largest tephra bomb found in the Distribution patterns of the three tion has been inferred by Hay (1959) for Guatemala City basin area was 23 cm in ash-flow tuffs in the Guatemala City basin observed glowing avalanches on St. Vin- diameter, whereas pumice blocks in the indicate that the basin is at least as old as cent, British West Indies, and for the 1929 diamictons are commonly 40 to 60 cm. In- ash-flow tuff R. Each successive ash flow eruption of Komagatake (Kozu, 1934).

TEPHRA LAYER T TEPHRA LAYERS C aw

N / > sv w 2 2700 Km \ xs L.ATITLXN / i \ ( 2100 Km2 \ ] V.T0URF.>8 $ GUATEMALA CITY J w )o r V. ATITLAN / • «000 Km" . V. ACATENANGO / / V. FUEGO® L/AMATITLAN ® %// V. AGUA @

C0n ^V.PACAYA (SOURCE) SOURCE

\ \ PHYSIOGRAPHIC BOUNDARY OF THE VOLCANIC HIGHLANDS \ V TEPHRA LAYERS LftY \ TEPHRA LAYER E

1100 S

fcv V 1,500 Km2 ( j^iT2 r @ \ \ J Ty-1100 Km W f^' ^ f ®v@ v SOURCE SOURCE

N \ 0 20 \ ^ KM \

Figure 10. Estimated minimum areal distribution and thickness of selected tephra layers in the central volcanic highlands. Unlabeled boundaries enclose area where tephra layer is exposed.

Compositional changes within tephra Probably the most interesting deposit in layers and between the tephra and their the volcanic highlands is the H ash-flow overlying ash-flow tuff are believed to rep- tuff. The unique presence of cumming- resent vertical compositional and tonite, lateral and vertical intralayer sorting mineralogical zonation within the source of pumice and lithic fragments, striking magma. By this interpretation the base of pink top, and vast areal distribution all the tephra layer should contain minerals combine to make the ash-flow sheet an ex- that were formed near the top of the cellent subject for further study. The origin magma chamber. Tephra E was probably of the pink color that tints the upper por- derived by progressive evacuation of a tion of the H ash-flow tuff is yet to be re- magma reservoir that contained more bio- solved conclusively. Williams (1960) for- tite near its top due to higher water pres- merly believed that the pink coloring was sures and was richer in hypersthene with caused by hematite dust sublimated from depth. The gray upper portion of tephra Y fumarolic vapors during cooling of a glow- probably also indicates a compositionally ing avalanche. Likewise, Smith (1960, p. stratified reservoir, with less silicic magma 831), in discussing low-temperature ash occurring at greater depth. Tephra H may flows, stated, "The presence of a red zone represent the upper fraction of a magma caused by oxidation of iron probably indi- chamber that was progressively enriched in cates a higher temperature." McBirney cummingtonite with depth. Changes in (1963) contended that in the San Miguel hornblende:biotite ratios, such as occur in Chicaj area, the absence of thermal effects, tephra layers L and T, may reflect the texture of the material, and the distance nonuniform phenocryst concentrations from any likely source made it likely that within magma reservoirs. The overall in- the deposit here referred to as the H ash crease in the amount of tephra of inter- flow originated as a wind-borne ash; there- mediate composition (cinders) within the fore, the pink tint was due to weathering. youngest deposits, especially from Agua The extremely sharp lower boundary of the Figure 11. Basin succession tephra, ash-flow Volcano, may represent a long-term change pink zone and uniformity of the pink tint in tuff units, and sedimentary rocks exposed in headwall of the canyon of Rio Guacamaya 3.2 in composition or in conditions of magma many exposures of H ash-flow tuff is km northwest of Guatemala City. generation at depth. difficult to explain by weathering. We be-

Coban ( 1300m)

tí» Huehuef§nango —\. ..T^SjjjWjjffi^—•—^.j, I" i-0' „.„ .ÍSÍ^WCTtC No pumic« ' Mataeatancifof ^ •—ß&ifö " mapped V X --rsr ¿m ~y>\ x />. T ; rrx ri iti V I \ / V / V. v -nz—at Salami ( 1000m ) Son,a Cruz // JSBN/ L®'(\Ì3S» delQuiche < 2000m) \ . <£' } " i® ¡íjííSC/fiíSh» . / y i

iS/teto

-— MINIMUM AREAL DISTRIBUTION OF TEPHRA LAYER H \ ^ /"f^0^ ? DISTRIBUTION OF H-ASH-FLOW TUFF UNKNOWN" . / j

@ VOLCANO

iff' H ASH-FLOW TUFF

( m) APPROXIMATE MEAN ALTITUDE OF BASIN FILL/

20 Km

Figure 12. Known and inferred distribution of the H ash-flow tuff and the minimum distribution of tephra H in south-central Guatemala.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021 540 KOCH AND McLEAN

lieve the formation of the pink color is the Guatemala City basin after traversing cated by fabric measurements; (3) the probably due to a primary temperature- the basins arrayed in a stair step fashion to origin of cummingtonite in a volcanic envi- oxidation cooling process, the nature of the west (Fig. 12). The ash flow may have ronment; and (4) the enigmatic origin of the which is not clear, especially in the absence entered the southern portion of the basin by pink top. of evidence of fumaroles and vapor phase way of the Santiago Sacatepequez valley. crystallization products. Although some This access route would explain its rela- VOLCANIC HAZARDS older Guatemalan ash-flow tuff units con- tively coarse texture in the southwestern tain minor pink zones, the development of a portion of the Guatemala City basin. Addi- Although ash-flow deposits studied in uniform, sharply bounded pink horizon tional study of the ash-flow sheet itself this investigation are all more than 40,000 appears unique to the H ash-flow tuff and should provide important information yr old and their recurrence interval is not could represent special conditions of com- bearing on (1) sorting mechanisms operable known, similar ash flows conceivably could position and cooling. in ash flows; (2) flow directions, as indi- recur at any time from active Guatemalan In the absence of a map showing com- plete distribution of the H ash-flow tuff unit and of more detailed thickness data, the dimensions and volume of the deposit can only be approximated. A reasonable esti- mate is that the H ash-flow tuff covers TEPHRA E 2 about 16,000 km and has a volume of at C14 CHARCOAL1" least 20 and possibly more than 50 km3. V/.sX».W'/.vVl» •//aWAV/AY'.V The maximum distance of travel of the ash >40,000 YEARS B.P. flow from the inferred general source area is TEPHRA C about 125 km. This size of ash-flow deposit K-Ar HORNBLENDE3 ranks as a fifth-order ash flow (Smith, 3.2 +1.3 m.y. B.P. 1960), comparable to the ash-flow deposits 4.1 ± 0.8 m.y. B.P. of Krakatau (Williams, 1941) and Crater Lake (Williams, 1942). lb ASH FLOW H C14 CHARCOAL The plutonic lithic fragments of granite, >40,000 YEARS B.P. quartz monzonite, and granodiorite are 10 most likely xenoliths derived from the C14 CHARCOAL TEPHRA H country rocks beneath the source vent. In >40,000 YEARS B.P. addition to granitic rock fragments, some clasts of mica schist and phyllite occur in the ash-flow tuff in road cuts along High- way CA-1 near the Motague River and in the San Miguel Chicaj area. Since such ASH FLOW T rocks commonly crop out in these areas, the metamorphic rock fragments in the tuff — ' <0 probably represent surface stones incorpo- O 2 < rated during the turbulent passage of the — ' o ash flow. ° ö ° Further study of the ash-flow deposits TEPHRA T o « o o K-Ar SANIDINE4 will be needed to determine detailed flow O o o directions and the mechanism required to -0.3 ± 0.3 m.y. generate a deposit of such wide areal ex- 0.0 + 0.3 m.y. tent. Intralayer textural trends of lithic fragments and pumice, together with fabric ASH FLOW L measurements, may furnish important data bearing on the source of the ash flows. Some ash-flow deposits occur in basins that seem inaccessible to an ash flow strictly confined to topographic lows; possibly such deposits resulted from air-fall deposition O O O O o from the cloud portion of the ash flow. The TEPHRA L K-Ar BIOTITE2 0 O o o O ® dust cloud that must have accompanied an .0 o o o C. ash flow of this size may have been carried 1.27 ± .57 m.y. B.P. * V V A A by winds considerable distances beyond the PYROXENE A * v t. * t, l> 4 limits of the flow itself. If significant ash- ANDESITE cloud deposition did take place, these de- /AVA * A > v 1 REVERSED POLARITY posits should differ texturally from the flow " 1 >* A deposits. j" <. y N The distribution of the H ash-flow tuff TEPHRA Ri 1. University of Washington Radiocarbon Lab Sample can be explained mainly by flowage along UW-141(1a). 142(1b) & 143(1c), 1969 previously established stream courses or by 2. Teledyne Isotopes No. 3-1491-212, June, 1970 3. Mobil Field Research Lab. No. 2254, November, 1972 ANDESITE flowage from topographically higher basins 4. Mobil Field Research Lab. No. 2596, July, 1973 into progressively lower basins away from the source. The ash flow probably entered Figure 13. Radiometric dates from pumiceous deposits in the volcanic highlands.

volcanoes. The initial pumice fall might REFERENCES CITED 133-174. have disastrous effects within a few tens of Laniz, R. V., Stevens, R. E., and Norman, M. B., kilometers of the eruptive vent, but a sub- Bohnenberger, O., Bengoeche, A. J., Dóndoli, C., 1964, Staining of plagioclase and other sequent ash flow might devastate vast areas and Marroquin-Castro, A., 1966, Report minerals with F. D. and C. Red no. 2: U.S. Geol. Survey Prof. Paper 501-B, p. within the intermontane basins, resulting in on active volcanoes in Central America during 1957 to 1965: Segunda Reunion de Ë152-B153. a nearly total loss of life and property. The Geologos de America Central, 35 p. McBirney, A. R., 1963, Geology of a part of the presence of a distinctive air-fall tephra be- Bonis, Samuel, 1965, Geología del area de central Guatemalan cordillera: California neath each major prehistoric ash-flow tuff Quezaltenango, República de Guatemala: Univ. Pubs. Geol. Sci., v. 38, no. 4, p. suggests that an interval of time, possibly Ministerio de Comunicaciones y Obras 177-242. brief, separated the initial tephra eruption Públicas [Guatemala] Inst. Geog. Nac., McLean, Hugh, 1970, Stratigraphy, mineralogy, from the ensuing devastating ash-flow 84 p. and distribution of the Sumpango group event. Immediate evacuation of populated Bonis, Samuel, Bohnenberger, Otto, Stoiber, R. pumice deposits, Guatemala [Ph.D. lowland areas would be advisable in the E., and Decker, R. W., 1966, Age of pumice dissert.]: Seattle, Univ. Washington, 90 p. event of future major pumiceous tephra deposits in Guatemala: Geol. Soc. America Ross, C. S., and Smith, R. L., 1961, Ash-flow tuffs — Their origin, geologic relations, and eruptions in the volcanic highlands. To this Bull., v. 77, p. 211-212. identifications: U.S. Geol. Survey Prof. end, government officials concerned with Bonis, Samuel, Bohnenberger, Otto, and Dengo, Gabriel, 1970, Mapa Geologico de la Paper 366, 81 p. volcanic hazards might use the geologic República de Guatemala (1st ed.): Minis- Smith, R. L., 1960, Ash flows: Geol. Soc. record of such events in evolving suitable terio de Comunicaciones y Obras Públicas America Bull., v. 71, p. 795-841. plans to cope with potential disasters. [Guatemala] Inst. Geog. Nac. Thorarinsson, S., 1954, The tephra-fall from Bowles, F. A., Jack, R. N., and Carmichael, Hekla on March 29, 1947, Pt. 2, in The I.S.E., 1973, Investigation of deep-sea vol- eruption of Hekla 1947-48: Reykjavik, Soc. Sci. Islandica, p. 68. ACKNOWLEDGMENTS canic ash layers from equatorial Pacific cores: Geol. Soc. America Bull., v. 84, p. Tsukada, Matsuo, and Deevey, E. S., Jr., 1967, 2371-2388. Pollen and analyses from four lakes in the We extend our appreciation to the In- Chayes, Felix, 1952, Notes on the staining of southern Maya area of Guatemala and El stituto Geogrâfico Nacional, Guatemala potash feldspar with sodium cobaltinitrite Salvador, in Wright, H. E., Jr., and Frey, D. City, for financial and logistic support. in thin section: Am. Mineralogist, v. 37, p. G., eds., Quaternary paleoecology: New Gabriel Dengo and Otto Bohnenberger of 337-340. Haven, Conn., Yale Univ. Press, p. ICAITI graciously permitted use of their Eggers, Albert A., 1972, The geology and pe- 303-331. laboratory in Guatemala City and were trology of the Amatitlán quadrangle, Williams, Howel, 1941, Calderas and their helpful in obtaining additional samples for Guatemala [Ph.D. dissert.]: Hanover, origin: California Univ., Pubs. Geol. Sci., v. dating. Discussions of Guatemalan geology N. H., Dartmouth College, 221 p. 25, no. 6, p. 239-346. 1942, The geology of Crater Lake National and geological problems with Howel Wil- Ewing, W. M., Ericson, D. B., and Heezen, B. C., 1958, Sediments and topography of the Park, Oregon, with a reconnaissance of the liams and Alexander McBirney were greatly Gulf of Mexico, in Weeks, L. G., ed., Cascade Range southward to Mount appreciated. Ray Wilcox confirmed the Habitat of oil — A symposium: Tulsa, Shasta: Carnegie Inst. Washington Pub. cummingtonite identification. Okla., Am. Assoc. Petroleum Geologists, p. 540, 162 p. John T. Whetten and Stephen C. Porter 995-1053. 1960, Volcanic history of the Guatemalan suggested the project, supervised field and Hay, R. L., 1959, Formation of the crystal-rich highlands: California Univ. Pubs. Geol. laboratory work, and critically reviewed glowing avalanche deposit of St. Vincent, Sci., v. 38, no. 1, p. 1-86. the manuscript. Travel expenses were pro- B.W.I.: Jour. Geology, v. 67, p. 540-562. Williams, Howel, McBirney, A. R., and Dengo, vided by a grant to Porter and Whetten Koch, Allan J., 1970, Stratigraphy, petrology, Gabriel, 1964, Geologic reconnaissance of southeastern Guatemala: California Univ. from the Organization for Tropical Studies, and distribution of Quaternary pumice deposits of the San Cristobal group, Pubs. Geol. Sci., v. 50, p. 1-56. Inc. Guatemala City area, Guatemala [Ph.D. Two radiometric age determinations dissert.]: Seattle, Univ. Washington, 80 p. MANUSCRIPT RECEIVED BY THE SOCIETY were made by the Field Research Labora- Kozu, Shukusuke, 1934, The great activity of FEBRUARY 11, 1974 tory, Mobil Research and Development Kamagatake (Japan) in 1929: Tschermaks REVISED MANUSCRIPT RECEIVED AUGUST 26, Corporation, Dallas, Texas. Mineralog. u. Petrog. Mitt., v. 45, p. 1974

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/529/3443996/i0016-7606-86-4-529.pdf by guest on 25 September 2021