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

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