Quaternary volcanic stratigraphy of Managua, : Correlation and source assignment for multiple overlapping plinian deposits

DAVID C. BICE* Department of Geology and Geophysics, University of California, Berkeley, California 94720

ABSTRACT nagua, has characterized the average largest ment have been termed the Las Sierras Group recent Nicaraguan eruptions, and has defined by Zoppis Bracci and del Guidice (1958) and A tephrochronological study that began as time-stratigraphic markers used for archaeo- the Las Sierras Formation by Kuang S. (1971). an active-fault mapping program in Mana- logic and engineering purposes. Additionally, The Las Sierras rocks consist mostly of Tertiary gua, Nicaragua, following the 1972 earth- it has provided a regional framework within to Quaternary basaltic tuffs and mudflows and quake has yielded information with broader which to interpret and correlate more de- are the oldest rocks exposed locally. Over the geologic relevance. Seven principal, wide- tailed studies of individual Nicaraguan Las Sierras "basement" at Managua lies a thin spread Quaternary air-fall deposits (the volcanoes. sequence of younger air-fall pyroclastic deposits "Managua sequence") have been defined; the interspersed with local fluvial sediments. This deposits are basaltic or rhyodacitic, span INTRODUCTION uppermost sequence is the subject of this report. probably <35,000 yr, and appear to represent the largest explosive eruptions in central Description of Area Previous and Subsequent Studies Nicaragua during this period. They were erupted from vents at the present sites of Ma- The Nicaraguan volcanic chain is located Early field work by Sapper (1925), chemical saya , Apoyo Caldera, Jiloa Caldera, close to the southern end of the active Central analyses by Burri and Sonder (1936), and exten- and Apoyeque Caldera, all within 35 km of American volcanic arc. At least 12 major active sive later work by Howell Williams (1952a, Managua. In volume, they range from <0.5 or recently active volcanic structures occur 1952b) and Alexander McBirney (1955a, 3 to >11 km (dense-rock equivalent) and are along the chain, within or along the southwest 1955b, 1956; McBirney and Williams, 1964, classified as plinian or phreatoplinian, regard- margin of the Nicaraguan Depression, a large 1965) provided the basis for the present under- less of chemical composition. Two of the graben that runs the length of the country in a standing of Nicaraguan volcanic history. Kuang units were radiocarbon dated at 6,600 yr B.P. southeasterly direction. The Depression is prob- S. (1971) delineated much of the regional stra- and 21,000 yr B.P. as a result of this study; ably late Miocene in age (McBirney and Wil- tigraphy of western Nicaragua, building on the ages of the other layers are not as well con- liams, 1964; Dengo and others, 1970) and is 45 sedimentary stratigraphic work of Zoppis Bracci strained but are estimated on the basis of soil to 50 km wide. The capital city of Managua and del Guidice (1958); his results were relied development and position in the section. (Fig. 1) sits astride the volcanic arc on a gently on heavily by later investigators of volcanic stra- Correlation and source assignment were northward-sloping, slightly gullied plain that tigraphy, such as Parsons Corporation (1972), particularly difficult because of incomplete ends at the shore of Lake Managua, a turbid and Woodward-Clyde Consultants, Inc. (1975), and 2 exposure and similarity of physical and chem- shallow fresh-water lake of 1,100-km area. Dames and Moore-LAMSA (1978). ical features, but they were accomplished One wall of the Nicaraguan Depression passes The first attempt to sort out the detailed stra- with a combination of detailed measurement 15 km to the southwest of Managua, where it is tigraphy of the Managua pyroclastic section was and sampling at Managua, reconnaissance called the Mateare Escarpment. The western made by this author and others of Woodward- mapping and sampling at potential vent areas, boundary of the city (Fig. 2) is formed by the Lundgren and Associates (Niccum, 1976; Col- and application of petrographic, mineralogic, Nejapa-Miraflores Alignment, a north-south- lins and others, 1976) in the course of active- and chemical correlation methods. The close trending line of cinder cones and collapse pits fault evaluation following the Managua earth- similarity among the rhyodacite pumices marking an offset in the arc. Six kilometres to quake of 1972 (Niccum and others, 1974; posed the greatest difficulty for correlation; the north, the low, eroded form of Chiltepe Pen- Woodward-Clyde Consultants, Inc., 1975). The cluster analysis applied to multielement XRF insula juts into Lake Managua; it contains two pyroclastic section at Managua was separated chemical analyses of pure-glass separates lake-filled , Apoyeque and Jiloa. South- from the underlying Las Sierras Group and gave the best results. east of Managua, Masaya Caldera has produced termed the "Managua Series" (Niccum, 1976). The clarification of the Managua sequence many historic basaltic and prehistoric eruptions; The term "sequence" is preferred here to "Ser- has revealed the local geologic history of Ma- Apoyo Caldera, the largest young caldera in ies," to avoid confusion with the strict definition Nicaragua, is mantled with several metres of of the term Series (American Commission on rhyodacite pumice. *Present address: 40467 Seville Court, Fremont, Stratigraphic Nomenclature, 1970). California 94539. The rocks exposed on the Mateare Escarp- This report is based on field work performed

Geological Society of America Bulletin, v. 96, p. 553-566, 15 figs., 1 table, April 1985.

553

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570.0

APOYEQUE CALDERA

580.0 Figure 2. Principal features in the immediate vicinity of Managua, Figure 1. Principal geographic and volcanic features in central Nicaragua, including locations referenced in this paper. Marginal western Nicaragua. Marginal numbers are Universal Transverse Mer- numbers are UTM grid locations, cator (UTM) grid locations.

between 1973 and 1978. Since that time, several quence (Table 1) is as evident in the field as in the designations presented here are preferred. investigators have published on aspects of Nica- the laboratory. Inasmuch as a complete section Representative chemical analyses are shown in raguan geology that involve units correlative to of the Managua sequence is nowhere exposed, it Table 1. More complete descriptions of all units the Managua sequence at their source areas. In was initially "pieced together" from scattered are given, following a discussion of correlation particular, recent studies of Masaya Caldera natural exposures and trench cuts (Woodward- methods. (Krusi and Schultz, 1979; S. Williams, 1983b), Clyde Consultants, Inc., 1975). No type locali- Apoyo Caldera (Walker, 1982; Sussman, 1982), ties for individual beds were defined, although SOURCE AREAS and the Nejapa-Miriflores Alignment (Walker, an earlier nomenclature (Niccum, 1976) used 1982) have further refined some of the volumes, designations for some units on the basis of par- Reconnaissance mapping and sample collec- ages, and interpretations of the eruptive nature ticular trench exposures. The composite column tion were carried out at the major volcanic en- for those sources. presented in Figure 3 represents average thick- ters that were considered potential sources for ness and typical layering for the units, but varia- the Managua deposits. For each source area, a The Managua Sequ ence tion across the area is as important for a unit's brief description of its physical features and the definition as is its character at any one site. results of geologic reconnaissance are given, for The term "Managua sequence," as used here, The unit designations given in Figure 3 reflect more complete results, including details of sam- is an informal designation, referring only to the the results of the correlation and mapping efforts ple collection, see Bice (1980c). major, widespread pyroclastic deposits younger of this study; therefore, for each unit, a source than the Las Sierras Group in and around the designation is followed by a descriptive grain- Apoyo Caldera vicinity of Managua. The Managua sequence in size term. In many cases, the working names most locations consists of a distinctly layered, originally applied to the units either implied no Apoyo Caldera (Fig. 1) is 35 km southeast of thin (10-20 metres total) sequence of basaltic or source or were in error as to source designation. Managua. It is a large (6.5-km diameter), deep rhyodacitic air-fall tuffs, ashes, and lapilli beds. Those earlier names that have been published or (250-300 m), circular collapse with scalloped The bimodal basalt-rhyodacite nature of the se- are in common use are also noted in Figure 3; margins. There is little evidence for a pre-

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Figure 3. Simplified composite stratigraphic column ?;;;a~HviASAYA TUFF (Retiro Tuff) of the Managua sequence. Sedimentary units and minor ZhJILOA PUMICE (Apoyeque Pumice) 3MASAYA "TRIPLE LAYER" (San Judas volcanic deposits not included; scale on left side of Formation) column indicates representative thicknesses. Names Soil UPPER APOYEQUE PUMICE (Upper Apoyo used in this study shown to right of column; correlative Pumice) Rhyodacite names from earlier studies (Woodward-Clyde Consul- Pu m ice /Tuff iWiiiliJiyp—AP0Y. 0 PUMICE (Middle Apoyo Pumice) tants, Inc., 1975; Collins and others, 1976; Niccum, Basaltic ^itl 1976) in parentheses at far right. Lapilli/Ash ffiflffSLOWER APOYEQUE PUMICE (Lower Apoyo Pumice) Basaltic MASAYA LAPILLI BED Tuff/Mudflow (Fontana Lapilli)

existing volcano at the present site of the caldera, except that its west rim is considerably higher • Las Sierras Formation than its surroundings. Sapper (1925), Williams (1952a), and McBirney and Williams (1965) reported on brief visits to Apoyo. Most recently, Sussman (1982) studied the caldera and its de- TABLE 1. REPRESENTATIVE ANALYSES OF PYROCLASTIC UNITS OF THE MANAGUA SEQUENCE posits, providing the first detailed account of its Oxide Masaya Jiloa Masaya Upper Apoyo Lower Masaya features and its history. (wt%) Tuff Pumice "Triple Apoyeque(?) Pumice Apoyeque(?) Lapilli The uppermost unit (except for the Masaya (152) (130) Layer" Pumice (62) Pumice Bed (204) (128) (125) (155) Tuff, which covers most of this region), exposed all around the rim, is a very thick deposit of Si02 53.11 66.96 52.51 69.02 66.41 70.15 52.64 multilayered and crossbedded, white to pinkish AI2O3 17.01 14.67 18.36 14.04 14.99 14.87 14.95 Ti02 1.35 0.44 1.31 0.39 0.55 0.53 1.33 pumice lapilli and pumice blocks. For several Fe203 0.81 0.49 1.38 4.29 kilometres around the caldera, this deposit is so FeO 13.39 T 2.91 I2.18T 2.25 2.20 3J«t 8.20 MnO 0.19 0.12 0.22 0.09 0.12 0.12 0.22 thick that no gully or road cuts completely MgO 4.16 0.59 3.56 0.38 0.55 0.49 3.88 CaO 8.75 3.96 8.39 3.03 3.66 3.45 8.56 through it. Only in the caldera wall is the base of Na20 1.14 3.98 1.99 4.03 3.85 4.45 3.14 the unit seen, and beneath it is a sequence of K2O 0.81 2.06 1.17 2.38 2.05 2.42 1.55 P2O5 0.10 0.16 0.33 0.07 0.13 0.17 0.33 thick mafic tuffs and a few lava flows. A suite of H2O* 2.94 3.48 3.59 0.51 samples for correlation purposes was collected H2O- 0.57 0.38 0.16 0.08

from the upper walls and immediate environs of Total loo.oot 99.60* 100.00t 99.65* 99.48* loo.oot 99.60* the caldera. Element (ppm)§ Masaya Caldera PB 16.8 19.5 18.1 18.4 Th 9.2 6.6 There appear to be only two sources of re- Rb 22.8 39.1 23.7 49.1 39.4 50.1 33.8 Sr 383 297 420 258 338 246 398 cent, voluminous basaltic or andesitic eruptions Y 26.0 26.9 30.1 22.5 28.2 20.1 31.0 that could have contributed material to the Ma- Zr 86.6 144 102 161 155 160 122 Nb 6.3 6.8 8.5 5.7 nagua sequence. These are Masaya Caldera, Ni 12.0 17.7 Cu 210 21.0 299 22.7 8.4 41.2 329 24 km southeast of Managua, and the Nejapa- Zn 83.4 65.0 92.1 58.9 61.0 63.4 105 Miraflores Alignment of cinder cones on the Ga 16.6 13.9 19.2 11.6 13.4 14.4 21.8 Cs 1.7 2.4 1.7 3.1 5.6 5.0 western margin of Managua. Both sources have Ba 753 1367 838 1488 1446 1517 1079 La 6.8 14.2 8.3 10.6 7.1 8.9 5.1 been described by McBirney (1955b, 1956); as- Ce 23.9 39.2 37.9 33.5 38.7 34.1 37.8 pects of Masaya have been studied by Ui Pr 0,7 6.6 3.5 1.9 8.6 5.5 Nd 28.6 12.9 28.1 27.1 23.9 11.4 24.6 (1973), Krusi and Schultz (1979), and most re- Sm 2.6 7.5 2.4 cently S. Williams (1983b); chemical aspects of Note: Sample collection numbers (Bice, 1980c) shown in parentheses. Nejapa-Miraflores Alignment have been studied •Wet-chemical analyses by D. Kosco. by Walker (1982). txRF analyses by author; total iron as FeO; recalculated anhydrous (normalized to 100%). §XRF analyses by author. Masaya Caldera (Fig. 1) is a large, shallow, elongate caldera (6 km x 10 km) that, according to McBirney (1956), was produced by a "reces- sion of magma" on a large scale, causing a series raised northwest wall, for a pre-caldera volcano. posed in the Masaya Caldera walls were taken of coalescing collapses. Subsequent mapping Within the caldera, a post-collapse volcanic for correlation purposes. Some caldera-wall and volume calculations by this author and oth- complex (Santiago-Masaya) has grown and has lavas also were sampled. ers, however, have implied that Masaya was been historically active. formed as a result of several (Bice, 1980b, All products of the post-caldera Masaya vol- Nejapa-Miraflores Alignment 1980c) or perhaps only one (S. Williams, canoes are basaltic or andesitic (McBirney, 1983b) explosive basaltic eruption(s). Like its 1956; Ui, 1973; S. Williams, 1983b), mostly The Nejapa-Miraflores Alignment is a 15- neighbor Apoyo, Masaya Caldera shows little olivine and olivine-augite basalts. Samples of the km-long, north-south-trending line of coalesc- geomorphic evidence, other than a slightly uppermost pre-caldera unit (a thick ash) ex- ing collapse pits and small cinder cones along

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the western margin of the city of Managua. It layer and is, in places, extremely pisolite-rich. hidden sources to explain the geologic history stretches from south of Nejapa Crater to north of Samples of the pumice from the caldera rim and represented by the Managua sequence, although Miraflores Station (Fig. 2). The alignment flanks were taken for correlation. it is impossible to discount the likelihood o:: such marks a right-lateral offset in the Quaternary a source; neither has it been necessary to invoke volcanic chain at :his point, which may be re- Apoyeque Caldera eruptions from more distant sources than Apoyo lated to segmentation of the subducting Cocos to the southeast or Apoyeque to the northwest. Plate (Stoiber and Carr, 1974). Collapse features The steep-walled, rather deep (250 m) outnumber vents along the alignment, and to- Apoyeque Caldera is located in the center of the CORRELATION AND SOURCE ward the north end they form an intricate inter- Chiltepe Peninsula, at the site of its former high- ASSIGNMENT METHODS secting pattern and are partially filled with est elevation. The nearly circular caldera mea- sediment, leaving, in places, separated "monad- sures 2.75 km in diameter; its southeast rim is None of the units of the Managua sequence nocks" of bedded ash standing above their flat < 1 km from the northwest rim of Jiloa Caldera. nor their potential source-area correlatives pos- surroundings (Fig. 2). Lavas have been erupted, Sapper (1925) claimed that the walls of sess particularly unique chemical or physical but rhythmically Ijedded, coarse ash/fine ash, Apoyeque Caldera were composed of amphi- properties. Chemically and mineralogically, they basaltic tuff sequences greatly predominate. bole- and pyroxene-andesite lavas, as did Wil- are typical of Central American calc-alkalic Olivine-pyroxene basalts are most common, but liams (1952a) and McBirney (1955b), based on bimodal suites (McBirney, 1969), although andesitic and even dacitic lavas have been pro- boulders scattered about the rim. Kuang S. somewhat more mafic and lower in potassium duced (Bice, 1980c; Walker, 1982). Both the (1971) reported basalt flows on the lower walls (Walker, 1982); the basalts are perhaps \ransi- oldest (post-Las Sierras) and the youngest erup- and pyroclastics (including dacite tuff) in the tional toward tholeiitic (Parsons Corporation, tions that have lefl significant deposits in Mana- upper part. Both Williams (1952a) and McBir- 1972; Bice, 1980c). Physically, the Managua se- gua have come from the Nejapa-Miraflores ney (1955b) noted deposits of "white dacite quence units are products of moderate to large Alignment, although the total volume is small. pumice" covering the outer slopes of the plinian or phreatoplinian eruptions with grain- Laguna Tiscapa, the remains of a small cinder volcano. size distributions and bedding as expected. Ac- cone surrounding a lake-filled crater, stands 5 Most of the steep caldera wall is inaccessible, cordingly, few laboratory correlation methods km to the east atop the major active surface fault and thick vegetation hides its features. The cal- were of use in identifying, separating, or corre- that traverses Managua (Brown and others, dera rim and its southern and western flanks are lating the units (particularly the rhyodacite pum- 1973; Woodwaid-Clyde Consultants, Inc., covered by a few metres of massive to multi- ices), and no combination of any methods was 1975). The volcanic products of Tiscapa are vis- layered white pumice lapilli. A several-metre- diagnostic in all cases. Color, major-element ually and chemically indistinguishable from thick, pisolite-bearing, massive to laminated fine composition, glass refractive index, and (:micro- those of the Nejapa-Miraflores Alignment. white tuff (weathered light tan) overlies the probe) mineral composition were of no use for Samples of lavas and pyroclastics from several pumice to the east and north. For correlation correlation; variations within a single unit were points along the alignment and from the walls of purposes, pumice samples were collected from often greater than mean differences between Laguna Tiscapa w ere collected for correlation. several points around the rim of Apoyeque and units. Comparison of relative mineral abun- on its flanks. No in-place pumice (or lava) was dance was not successful, except in one special Jiloa Caldera seen in the walls of the caldera below its rim, but case (below). Trace-element composition, by use exposure was extremely poor. At one location of variation diagrams, was also insufficient for The Chiltepe Peninsula, 11 km in diameter on the northwest outer flanks, however, a lower resolution. Only through a statistical compari- and 6 km north of the city of Managua (Fig. 2), pumice layer outcrops beneath the capping unit; son routine could reasonable separation of units is an approximately circular protrusion into this one pumice outcrop is of particular interest be achieved. Even with this, however, one par- Lake Managua. It is apparently the eroded re- for correlation (Sample 177, below). ticular deposit could not be successfully differen- mains of a low shield volcano and contains sev- tiated on a chemical basis from its (composi- tionally) closest neighbors. Fortunately, field eral cinder cones and two calderas, Jiloa and Other Potential Sources Apoyeque. relations of stratigraphic position, thickness, and appearance, coupled with the successful chemi- Laguna Jiloa, on the eastern flanks of the pen- The largest Quaternary pumice deposit in cal and petrographic correlations, allowed posi- insula, is a deep lake in a circular caldera with Nicaragua is 60 km northwest of Managua, near tive identification of all units in question. scalloped margins. The caldera is 2.5 km in di- the towns of Malpaisillo and La Paz Centra. ameter, and its low rim rises from just above The existence of the deposit was first noted by lake level on the southeast to 220 m on the side McBirney and Williams (1965). Potassium- Field Appearance closest to Apoyec;ue Caldera. Williams (1952a) argon dates (Bice, 1980c) imply that the Malpai- called Jiloa Caldera a "low-rimmed explosion sillo/La Paz Centro pumice is possibly one-half In some instances, gross field appearance and pit"; both he and McBirney considered Jiloa to million years old, that is, significantly older than obvious stratigraphic relations can be sufficient be the source of the pumice beds exposed at the Managua sequence and thus an unlikely oar- for establishing correlation and source assign- Managua. relative. In addition, the Malpaisillo and La Paz ment. For example, the uppermost tuff through- The outer flanks of Jiloa and its rim, where Centro samples are chemically distinct from the out Managua can be correlated on the basis of accessible, are seen to be topped by a deposit of Managua pumices. position and appearance and can be traced al- white, multilayeied pumice lapilli ~2 metres McBirney (1955b) speculated on the possible most continuously to its source, Masaya Cal- thick, which is, in turn, overlain by a laminated existence of a dacite pumice source beneath the dera. For most other units, however, exposure is to crossbedded mixture of pumice lapilli and waters of Lake Managua which might have much less continuous, and subtle variations in fine white tuff. The tuffaceous layer is four or erupted some of the pumice layers seen in Ma- bedding or color were used initially to distin- five times as thick as the underlying pumice nagua. It has not been necessary to postulate any guish between unlike units or to tie like units

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Figure 4. Typical thin sections of Managua basaltic units (plane-polarized light), (a) Masaya Lapilli Bed at Villa Fontana. Phenocryst-poor pale brown glass with separate, rounded vesicles; representative of all Masaya-derived basalts at Managua, (b) Basaltic ash from Cerro Motastepe. Opaque black glass with irregular, intersecting vesicles, rich in mafic phenocrysts (here, mostly olivine); representative of all products of the Nejapa-Miraflores Alignment.

together. Consideration of changes in thickness cles; phenocrysts are abundant, especially py- XRF Composition/Cluster Analysis and bedding character of a unit toward its roxene and olivine. By contrast, the uppermost source, together with available stratigraphie con- tuff unit on the wall of Masaya Caldera appears Fifty-nine pure-glass separates of pumice straints at each outcrop, allowed tentative con- in section as a vitrophyre with clear, pale brown samples were prtpared for chemical analysis by struction of the initial Managua sequence glass showing rounded, separate vesicles; pheno- X-ray fluorescence. The separates were pow- column; subsequent laboratory correlations then crysts are small and sparse and predominantly dered, pressed into disks, and analyzed in an were used to test the postulated field plagioclase. Most caldera-wall and post-caldera energy-dispersive XRF spectrometer for Pb, Th, assignments. Masaya lavas are similar in section. These same Rb, Sr, Y, Zr, Nb, Fe, Co, Ni, Cu, Zn, Ga, CI, K, features distinguish all samples of the Masaya Ca, Ti, Mn, Ba, Na, Mg, Al, and Si. Variation Petrography Tuff at Managua and all identified samples of diagrams were prepared for various element the Masaya Lapilli Bed and the Masaya "Triple pairs, triplets, and pairs of element ratios, of The difficulty of preparing thin sections of Layer." Petrography alone, then, distinguishes which Figure 6 is one example. The dashed lines very friable pumice lapilli precluded extensive Masaya products from those of the Nejapa- in the figure enclose the compositional fields for use of pétrographie correlations of pumice units. Miraflores Alignment and further implies that certain "known," or identified units; individual Glass textures are similar for all pumices, phe- all basaltic layers in the Managua sequence symbols represent glass composition of certain nocrysts are sparse, and mineralogy is essentially come from Masaya, not Nejapa-Miraflores. "unknowns." Figure 6 will be discussed further identical in kind, abundance, and composition in connection with the Jiloa Pumice (below), Mineral Separates (using pétrographie methods), with one excep- but it can be seen immediately that the dashed tion noted below. For basaltic units, however, Seventy-eight mafic mineral separates were compositional fields for some of the known units pétrographie characteristics were diagnostic. prepared from pumice samples for the determi- overlap, preventing positive correlation for any Thin sections were made of identified samples of nation of relative abundance of heavy minerals. unknown whose composition falls into the over- the Masaya Lapilli Bed (Fig. 4a) from locations Line counting (described in Galehouse, 1969) lap region. Overlaps of one or more fields of around Managua; other sections were made of was performed on 50 of the samples; -1,000 knowns occurred on all variation diagrams pre- various thick basaltic cinder beds. Their posi- grains were counted on each slide for a probable pared for all elements or ratios selected. tions in the section were unclear. Sections were precision of ~l% for a mineral with 10% abun- To resolve those compositional overlaps, clus- also made of the Masaya Tuff at Managua, the dance. Mineral proportions for the remaining 28 ter analysis on the glass compositions was em- basaltic mudflows at the human footprint site at samples were estimated visually, using as stand- ployed in an attempt to consider simultaneously Acahualinca (see below), and the cinders and ards the slides already counted. Minerals all useful chemical information rather than just tuff that cap the northwest wall of Masaya Cal- counted, in average order of abundance, were two or three elements at a time. The method dera. Another suite of sections was prepared hypersthene, augite, magnetite, olivine, and chosen is described in Davis (1973); the routine from cinders and flows along the Nejapa- greenish black hornblende. Of these, only horn- was written by the author for use on a small Miraflores Alignment (Fig. 4b), including La- blende was diagnostic, and it was useful in iden- dedicated computer (DEC LSI-11) which was guna Tiscapa. tifying only one deposit. Figure 5 is a bar graph part of the automated XRF analytical system AH samples from the Nejapa-Miraflores of relative hornblende abundance for the 78 from which the chemical data were obtained. Alignment show common features. The glass heavy-mineral samples. The figure is discussed Briefly, the cluster analysis proceeds as fol- (most samples are vitrophyric to intersertal) is below in connection with the Upper Apoye- lows. For a set of m objects (samples), each opaque black with irregular, intersecting vesi- que(?) Pumice. containing a value for n variables (elements), a

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/4/553/3445115/i0016-7606-96-4-553.pdf by guest on 28 September 2021 Figure 5. Bar graph of relative hornblende abundance for min- 50 eral separates of 78 Nicaraguan pumice samples. Upper Apoyeque(?) Pumice at Managua (right) correlates with various 1601 thick pumice layers along Nejapa-Miraflores Alignment and at Tiscapa Crater (left). See text for explanation of sample 177.

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NEJAPA-MIRAFLORES AND UPPER APOYEQUE l?l TISCAPA THICK PUMICES PUMICE AT MANAGUA MAFIC MINERAL SEPARATES

Figure 6. Variation diagrams for glass separates of Nicaraguan pumices for selected elements. Trace elements in ppm, major elements in dimensionless X-ray intensity (K-ratio). Dashed l ines enclose compositional fields of four defined units: JILOA (pum- ice at rim of Jiloa Caldera); APOYO (pumice at rim of Apoyo Caldera); U. APOYEQUE (identified exposures of Upper

Cl- Rb Apoyeque(?) Pumice at Managua); L. APOYEQUE (identified exposures of Lower Apoyeque(?) Pumice at Managua). Symbols in this figure represent samples being tested for correlation with the Jiloa Pumice. Solid circles = pumice from flanks and rim of Apoyeque Caldera. Open circles = Jiloa Pumice at Managua. Open squares = pumice overlying human footprints at Acahua- linca. All samples (except no. 177; see text for explanation) plot close to or witlnn the compositional field of the pumice at Jiloa Caldera. W

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UPPER LOWER

Figure 7. Cluster analysis dendrogram for all analyzed pumice samples (except those older than the Managua sequence). Clustering was performed on 13-element XRF analyses of pure glass separates. Jiloa Pumice, Apoyo Pumice, and Upper Apoyeque Pumice and their respective correlatives are clearly identified and separated: Lower Apoyeque(?) Pumice samples cluster with either Apoyo or Upper Apoyeque(?) Pumices and cannot be separated by this method.

Figure 8. Ouster analysis dendrogram for mafic pyroclastics near MASAYA TUFF Managua. Clustering performed on mqjor-element XRF analyses of I MASAYA 'TRIPLE LAYER' whole-rock samples. Products of Masaya Caldera, together with all 1 \ MASAYA LAPILLI BED (KUIUliSwUOM ut O III ui <9 nuyor basaltic units in the Managua sequence, are clearly separated OfcjWQ® & « o from products of the Nejapa-Miraflores Alignment. u L^j [u^ M u i^r-

(m) x (n) raw-data matrix is created, normalized to weight all variables equally and then con- [MASAYA] verted to a (m) x (m) similarity matrix which 9 'C [NEJAPA-MIRAFLORES] ® « rates each sample to every other sample on the basis of the n variables. Similarity coefficients I <0 (n-dimensional Euclidian distances) range be- tween 0 and 1, where 1 implies identical similar- ity. The clustering routine (unweighted-link clustering gave the best results) then operates on particular combination. The specific correlations this matrix, forming groups (pairs) of the mutu- yielded by this method are discussed below, but ally most similar samples. The first groups are in general, the cluster analysis, by looking at then joined by samples or by other groups at n-dimensional chemical space rather than the progressively lower similarities until finally, at two dimensions of variation diagrams, was al- THE MANAGUA SEQUENCE the lowest similarity, a single group consisting of most completely successful in separating the all samples is the result. Correlation in this case known pumice units from each other. The clus- The principal units of the Managua sequence is accomplished by noting the order in which ter analysis thus allowed confident correlation in are presented from the top downward, so that samples and clusters join with each other. the case of most unknowns. Only in the case of the first-described layers are the ones about Clustering can be represented graphically by a the Lower Apoyeque(?) Pumice did the method which most is known. Discussion then proceeds dendrogram, of which Figure 7 is the example fail to produce a consistent separation. from greater to lesser certainty as deeper levels for all analyzed pumices. The elements chosen Cluster analysis was also carried out for the are considered. for clustering were Pb, Rb, Sr, Y, CI, K, Ca, Ti, basaltic units (Fig. 8) and yielded the same two Deposit volumes were calculated from circu- Fe, Ba, Na, Mg, and Si. The greatest separation groupings that were inferred on the basis of pe- lar or piecewise elliptical integration of best-fit among known groups was achieved with this trography (above). exponential thickness functions regressed from

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isopach maps (E'.ice, 1980c). These volumes are S. Williams (1983b) calculated a significantly pachs are drawn on the pumice layer only). The approximately as certain as the isopach maps are larger volume (7-9 km3), including the ignim- source region is clearly defined as Jiloa and/or complete. A summary of the radiometric dates brite facies. Grain size and dispersal area classify Apoyeque Caldera. The dual-layer nature of the and other considerations on which the age esti- it as "salic equivalent of surtseyan" (Walker, deposit (tuff over lapilli) is preserved in all out- mates given below are based is also presented in 1973) or phreatoplinian (Walker, 1980). crops. If both pumice and overlying tufi' are in- Bice (1980c). Deposit classifications based on Deposits with radiocarbon ages of 2,250 and cluded, estimated dense-rock equivalent volume texture and dispersal area follow the method of 6,400 yr (Woodward-Clyde Consultants, Inc., for the Jiloa Tuff is 0.6 km3. Texturally, the Walker (1973, 1980). 1975; Bice, 1980c) bracket the Masaya Tuff. deposit is a combination of plinian (pumice) and There is questionable evidence, based on an un- phreatoplinian (tuff). Masaya Tuff certain correlation, that it may be older than Charcoal found at the base of the Jiloa Tuff at 6,000 yr (J. Johnson, 1978, personal commun.). the caldera rim yielded a radiocarbon age of This widesprea d deposit, the result of the last It is estimated here to be between 3,000 and 6,590 yr (Bice, 1980c). This dates the eruption major explosive activity to reach the Managua 6,000 yr old. and forms an upper bound to the age of the area, is easily traced throughout the region (be- Masaya Tuff as well as a lower bound to the age neath 0.5 m to 1.5 m of modern soil); thickening Jiloa Pumice (Footprint Pumice) of the footprints at Acahualinca. unquestionably psints to the site of the present Masaya Caldera as its source. This unit at Managua is a thin, double-layered Masaya "Triple Layer" The tuff at Managua is a poorly to moderately deposit that is commonly separated from the indurated, faintly layered to massive, fine air-fall overlying Masaya Tuff by a few inches of brown This thin but distinctive, multilayered basaltic ash. Its weathered color (most common) is light soil. The upper part consists of 10-25 cm of very unit is found throughout Managua closely be- tan, but where fresh, it is a dark olive gray. fine, white pisolitic tuff that most commonly has neath the Masaya Tuff, whether or not the inter- Within the limits of Managua, the tuff varies been completely weathered to soil. The lower vening Jiloa Pumice is present. It typically gives from ~ 10 cm to 50 cm in thickness. Toward part, directly beneath the tuff, consists of the appearance of two thin (1-2 cm), indurated Masaya, as the thickness increases, current bed- 5-10 cm of white pumice lapilli and coarse ash. gray tuffs sandwiched among three thicker ding features gradually appear until a thick base- This lower layer commonly is strongly weath- (5-10 cm), loose layers of coarse black ash (be- surge section (Krusi and Schultz, 1979) replaces ered also, so that typically only a thin line of cause of its distinct appearance, its original the air-fall tuff veiy close to the caldera. At the scattered pumice lumps remains to mark the po- working name "Triple Layer" is retained here, caldera wall itself, the tuff section is continuous sition of the unit. At its freshest exposures (in preceded by a source designation). Its total downward with approximately 15 m of cross- northwestern Managua), this pumice-tuff de- thickness in Managua is 10-30 cm; the unit bedded basaltic ash and cinders. S. Williams posit rests directly on the next older air-fall unit thickens toward Masaya and becomes more (1983b) has most recently studied this unit in (Masaya "Triple Layer") without visible inter- complex, due to the presence of extra layers. The connection with the history of Masaya Caldera vening paleosol. easily recognizable features of the Masaya Tuff and has separated ignimbrite and surge facies Petrographically, the Jiloa Pumice is typiciil and Triple Layer and their close association from air-fall portions of the Masaya Tuff. of other Nicaraguan rhyodacites, consisting of formed the original core around which the stra- The Masaya Tuff at Managua is composed very frothy clear glass with scattered glomeritic tigraphy of the Managua sequence was con- structed. predominantly of pale brown sideromelane phenocrysts of plagioclase (An45-An55), hyper- fragments and minor, small, free crystals of pla- sthene, augite, and magnetite. Xenocrystic(?) oli- Petrographically and chemically, the Masaya gioclase, pyroxene, and olivine. Accessory lithic vine and scattered hornblende are present in Triple Layer is similar to the Masaya Tuff, con- fragments are present but minor; accretionary minor amounts. sisting of predominantly phenocryst-poor pale pisolites are abunda nt at some locations. Whole- The original working name for this unit at brown glass with a silica content near 53% rock analyses for this study show the Masaya Managua was not Jiloa Pumice; that name has (Table 1). This and the direction of thickening Tuff to be a basalt or basaltic andesite with a since been applied because both field evidence both imply a Masaya source for the Triple silica content near 53% (Table 1); this is more and correlation results have demonstrated that Layer. Petrography and chemistry of the human silicic than many other Nicaraguan basalts. this pumice-tuff layer beneath the Masaya Tuff footprint-bearing mudflow at Acahualinca Petrography and cluster analysis corroborate at Managua is the same unit as the thick pumice (Sample 204; Fig. 8), as well as its stratigraphic the source assignment made from field evidence: and tuff deposits that mantle the rim of Jiloa position relative to the Jiloa Pumice, indicate the Masaya Tuff was erupted from the site of the Caldera 6 km north of the city. It is, further- that the mudflow and the complex of mudllows present Masaya Caldera. In Figure 8, the cluster- more, the same unit that closely overlies paleo- and ash falls immediately overlying it are equiv- ing of the Masaya Tuff at Managua (Samples lithic human footprints at an excavation in the alents of the Masaya Triple Layer. The prehis- 153, 154) with samples from the rim of Masaya Barrio of Acahualinca (Williams, 1952a; Bryan, toric Managua residents who impressed their Caldera (Samples 1:51, 152) is evident. 1973; Bice, 1979). Figure 6 shows the chemical record into the still-warm volcanic mud of Aca- The Masaya Tuff and its near-vent facies ap- correspondence of both the thin pumice-tuff' hualinca were thus fleeing an eruption at distant parently represent the last pre-caldera eruption layer in Managua and the pumice overlying the Masaya, not at the much closer Nejapa- at Masaya and are probably genetically related footprints with the thick deposit at Laguna Jiloa. Miraflores Alignment. to the collapse and f armation of the caldera as it Cluster analysis also confirms this (Fig. 7); the Thickness data for the Masaya Triple Layer exists presently (Bice, 1980b; S. Williams, thin Managua pumice-tuff (Sample 192), the are shown in Figure 11. The isopachs are very 1983b). The Masaya Tuff displays the most footprint pumice (Sample 130), and the Jiloa incomplete; nevertheless, they strongly suggest, complete isopachs of any of the Managua se- Caldera pumice samples all cluster with each both by thickening and direction of curvature, a quence units (Fig. 9) because of its uppermost other, separate from all other pumices. source at (or toward) Masaya. Based on this position in the section. The probable total de- Figure 10 shows isopachs drawn on the Jiloa limited information, an estimated volume for posit volume is 4.3 k;m3 (dense-rock equivalent). Pumice and its equivalents in Managua (iso- the Masaya Triple Layer is 1.7 km3 dense-iock

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58 6 61 5700

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Figure 10. Isopach map for the Jiloa Pumice (pumice portion only). Figure 9. Isopach map for the Masaya Tuff. Data points (dots) from Data points (dots) from Bice (1980c); thickness in centimetres. Bice (1980c); thickness in cm. For further interpretation of this and other Masaya-related units, see S. Williams (1983b).

equivalent (S. Williams, 1983b, presents a re- stratigraphie level is also the position of the pumice lapilli. Internal bedding is absent; the vised volume estimate of 0.35 km3). Its multiple most prominent paleosol in the Managua sec- lapilli are grossly subangular to subrounded but layers alternate between plinian (coarse beds) tion; where best developed, it is a very dark are very irregular in surface detail. Small, angu- and phreatoplinian (fine beds) in character. brown, 50-cm-thick, waxy clay vertisol with a lar accessory fragments of mafic lava are scat- The general absence of a paleosol or an well-developed blocky structure. By contrast, tered throughout the deposit. eroded interval between the Jiloa Pumice (6,600 the modern soil at the top of the section, which Apoyeque Caldera is considered to be this yr) and the Triple Layer indicates that their age has been developing for a minimum of 2,250 yr, unit's probable source because identified out- difference is small. Beneath the Triple Layer, the is a far less mature, tan to medium brown sandy crops thicken toward the Chiltepe Peninsula, Apoyo Pumice is dated at 21,000 yr (see loam. and because one outcrop of probably equivalent below), but an unconformity and a thick paleo- Both features signify that there was a consid- pumice has been found on the caldera's flanks sol between these units suggests that the Masaya erable time period, after eruption of the Upper (Sample No. 177; Figs. 6 and 7). This does not, Triple Layer is much closer in age to the Jiloa Apoyeque(?) Pumice and before deposition of of course, prove a source at Apoyeque Caldera Pumice. It is estimated here at 7,000-9,000 yr. the Masaya Triple Layer, during which no rather than at Jiloa Caldera, but Apoyeque is a A minor unconformity, not evident in all out- major eruptions left deposits at Managua. This reasonable candidate; the query in the source crops, separates the Masaya Triple Layer from depositional gap appears to be larger than any designation is retained, however, to reflect this deeper deposits of the Managua sequence. The other within the Managua sequence. uncertainty. unconformity is most visible on the shoulders of The pumice is a white to gray, pearly, ex- the minor fault scarps that cross Managua Upper Apoyeque(?) Pumice tremely frothy, tubular vesiculated glass with (Woodward-Clyde Consultants, Inc., 1975; Col- small, scattered glomeritic phenocrysts of plagi-

lins and others, 1976). There, the upper units The Upper Apoyeque(?) Pumice is a promi- oclase (An45-An50), hornblende, hypersthene, follow the surface topography, truncating the nent layer in Managua, consisting of a metre or augite, and magnetite. This pumice layer is the flatter underlying units at a low angle. This more of slightly reverse-graded white air-fall only one in the Managua region containing sig-

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580.0 580.0 Figure 11. Isopach map for the Masaya "Triple Layer." Data points Figure 12. Isopach map for the Upper Apoyeque(?) Pumice. Data (dots) from Bice (1980c); thickness in centimetres. For further inter- points (dots) from Bice (1980c); thickness in centimetres. pretation of this and other Masaya-related units, see S. Williams (1983b).

nificant hornblende, which, in some cases, con- Pumice (see Fig. 2 for locations). The pumice older than the Jiloa Pumice (6,600 yr) and suffi- stitutes more than half the total mafic mineral that outcrops near the waterline at Laguna Tis- ciently younger than the underlying Apoyo content. Xenocrystic(?) olivine occurs in some capa, the uppermost of three successive pumices Pumice (21,000 yr) to have allowed formation samples. All analyzed samples contain several at Villa Fontana, and the uppermost of three of the moderately developed paleosol that sep- percent water; recalculated water-free silica con- pumices exposed in the wall of Nejapa crater are arates the two. By analogy with the modern soil tent for the Upper Apoyeque(?) Pumice com- each correlated with the Upper Apoyeque(?) at Managua, it is estimated that the Upper monly exceeds 71% (Table 1). A minor amount Pumice. This unit, being the only pumice on the Apoyeque(?) Pumice is no more than a few of compositional zoning (and significant minera- order of a metre thick in the Managua area, lis thousand years younger than the Apoyo Pum- logical zoning) is indicated by some analyses. the one most commonly seen in natural expo- ice, or -16,000 to 20,000 yr old. Hornblende content (Fig. 5) was the most di- sures and artificial excavations. As explained agnostic correlation tool for the Upper Apoye- above, however, it is not the pumice that over- Apoyo Pumice que(?) Pumice, and results from mineralogical lies the human footprints at Acahualinca. correlation were in every case substantiated by Isopachs on the identified exposures of the At a few exposures around Managua (for ex- cluster analysis on glass composition (Fig. 7). Upper Apoyeque(?) Pumice are shown in Fig- ample, at Villa Fontana in Nejapa Crater and Thick pumice layers within the bedded basaltic ure 12. Assuming average thickness distributions just south of Valle Ticomo), a 30-70-cm pumice ash of the Nejaipa-Miraflores Alignment at similar to the exposed parts, the dense-rock layer appears, stratigraphically below the Upper Punto de los Libros, at Cuesta del Plomo, and equivalent volume for the unit is 2.0 km3. Apoyeque(?) Pumice and above the l^ower capping "Cerro Frawley" and "Cerro km 14" Texturally, it is plinian. Apoyeque(?) Pumice (at most other exposures are correlated with the Upper Apoyeque(?) The Upper Apoyeque(?) Pumice is much the two Apoyeque(?) pumices are separated

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Figure 13. Isopach map for the Apoyo Pumice. Data points (dots) from Bice (1980c); other data (open circles) interpreted from Dames and Moore-LAMSA (1978). For further interpretation of this unit and Apoyo Caldera, see Sussman (1982).

only by a brown paleosol). This rare, third pum- ice layer typically has a slight pinkish cast and is crudely bedded in alternating fine and coarse layers. Medium brown fine sandy loam paleo- sols 20-50 cm thick separate it from the Apoyeque(?) pumices above and below. Like other Managua pumices, this third pum- ice layer consists of light, frothy glass with glomerocrystic plagioclase, hypersthene, augite, magnetite, and rare ilmenite. Its composition also is typical of other pumices; minor chemical zoning in the magma chamber prior to eruption is implied by some analyses. This unit initially had no working name or source assignment because it is seen at so few locations in Managua. As field work progressed, however, the significance of the pinkish color, the crude bedding, and the fact that both thick- ness and complexity (number of beds) of the unit increased in a southeasterly direction be- came apparent. Correlation of the unit with the very thick and complex, pinkish pumice deposit on the rim of Apoyo Caldera, suspected on the basis of field evidence, was confirmed by the cluster analysis on glass separates (Samples 132, 133, 186; Fig. 7). The stratigraphic position of this pumice layer confirms Williams' (1952a) age of 20,920 yr for the eruption (Bice, 1980c). unit is missing at many locations, and there are conclusion, based on morphology alone, that Similar ages have been obtained from subse- few data points on which to base its characteris- Apoyo Caldera is older than Masaya Caldera. quent collections (Dames and Moore-LAMSA, tics. Petrographically and mineralogically, it is Exposures of the Apoyo Pumice beyond the 1978). similar to other Managua pumices; it differs immediate vicinity of the caldera are few; iso- from the Upper Apoyeque(?) Pumice only in its pachs were constructed along a northwest- Lower Apoyeque(?) Pumice lack of hornblende and perhaps in a slightly trending radius from the vent (Fig. 13), and a lower silica content (Table 1). calculated dense-rock equivalent volume of 4.3 A thin (10-20 cm) layer of unbedded, uni- In cluster analysis (Samples 125, 131, 136, km3 was obtained for the pumice deposit as a formly graded, fine white pumice lapilli to 143; Fig. 7), the Lower Apoyeque(?) Pumice whole. This makes the Apoyo the largest of all coarse ash commonly occurs at Managua -20 grouped with either the Apoyo or Upper recent Nicaraguan pumice deposits, as Williams to 100 cm beneath the Upper Apoyeque(?) Apoyeque(?) Pumices in an almost random (1952a) surmised; furthermore, the recent work Pumice, a medium brown, sandy paleosol (and fashion, depending on the elements chosen for by Sussman (1982) on Apoyo Caldera has indi- perhaps the Apoyo Pumice as described above) clustering and the specific samples of Apoyo or cated that the total volume of the Apoyo Pumice intervening. This layer appears only in associa- Upper Apoyeque(?) present. The variation dia- may be substantially greater, sufficient to ac- tion with the Upper Apoyeque(?) Pumice and gram (Fig. 6) indicates that for all element pairs count for the entire (11 km3) volume of Apoyo thickens in the same direction, hence its designa- shown, the compositional field of the Lower Caldera. Grain size and dispersal factors show tion. The query following the source name is Apoyeque(?) Pumice overlaps one or the other the deposit as a whole to be plinian, but in de- made for the same reason as for the Upper (or both) of the other two pumices. In this case, tail, it is a complex collection of several fall Apoyeque(?) Pumice; it is reasonably certain cluster analysis was not able to resolve the over- units. that the source area is Chiltepe Peninsula, but it lap. Fortunately, because all of the other units of Charcoal deposits near the base of the Apoyo is not clear from which caldera it was erupted. the Managua sequence were susceptible to both Pumice at Diria, on the caldera rim, yielded an Because of its thinness and fine grain size, this chemical and field correlation, the Lower

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580.0 Figure 14. Isopach map for the Lower Apoyeque(?) Pumice. Data Figure 15. Isopach map for the Masaya Lapilli Bed. Data points points (dots) from Bice (1980c); thickness in centimetres. (dots) from Bice (1980c); thickness in cm. For further interpretation of this and other Massiya-related units, see S. Williams (1983b).

Apoyeque(?) Pumice could be correlated with B.P. Using the modern soil at Managua as an Commonly, thin, whitish bands of what appear reasonable assurance by elimination wherever it analogy, the Lower Apoyeque(?) Pumice may to be plant ash or cemented volcanic dust run appeared, as long ¡is other known units could be be between 22,000 and 26,000 yr old. through the deposit horizontally, like ghost bed- identified in the same outcrop. This was the case ding. At Managua, the lapilli bed is 1-2 m thick; for all except a few isolated exposures of thin Masaya Lapilli Bed thickness increases toward Masaya to >4 m be- pumice within the bedded basaltic ash of the fore the unit is lost beneath younger deposits. Nejapa-Miraflores Alignment, and these remain This is the lowest unit in the Managua se- The Masaya Lapilli Bed is petrograpaically unidentified. quence. It rests on a variably developed gray to identical in appearance to the Masaya Tuff (ex- Figure 14 shows isopachs for the Lower tan paleosol as much as 1 m in thickness which cept for the size of individual fragments), Apoyeque(?) Pumice. The thickening direction formed on the mostly deeply weathered basaltic consisting largely of pale brown sideromelane and isopach curvature both point to a Chiltepe mudflows and tuffs of the Las Sierras Group. A with rounded vesicles and sparse, small pheno- Peninsula source. Using Apoyeque Caldera as medium brown sandy paleosol is developed on crysts of plagioclase, clinopyroxene, and olivine the assumed source point, an estimated dense- the top of the unit, separating it from the overly- (Fig. 4a). Its chemical composition is similar 3 rock equivalent volume of 0.4 km was calcu- ing Lower Apoyeque(?) Pumice. also; it is a basalt with -53% Si02 (Table 1). lated. It is classified on grain size as plinian. The Masaya Lapilli Bed consists of uniform, Cluster analysis (Fig. 8) also confirms that the The age of the unit is not well constrained. It black basaltic cinders of medium- to coarse- Masaya Lapilli Bed is like the Masaya Tuff and is sufficiently older than the Apoyo Pumice to lapilli size, ungraded and unbedded except for a like other products of Masaya Caldera/Volcano. have developed a moderate soil by 21,000 m.y. distinctive basal black ash layer ~5 cm thick. Owing to its chemical and petrographical

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similarity to both the Masaya Tuff and Masaya restricted occurrence. The layer was sampled, and dating of the Apoyo Pumice foi this study Triple Layer, as well as to other Masaya prod- however, and is correlated (Sample 28; Fig. 8) provided the initial basis for a more in-depth ucts, and because it thickens in the direction of with other deposits from Masaya Caldera. examination of that vent (Sussman, 1982) and Masaya, the Masaya Lapilli Bed is considered to has also yielded the unexpected observation that be a product of pre-caldera . SUMMARY AND IMPLICATIONS the two most voluminous sources of Quaternary As the lowest unit in the Managua sequence, rhyodacites in central Nicaragua, Chiltepe and the Masaya Lapilli Bed is exposed over only a Seven principal air-fall tephra units make up Apoyo, had their major eruptive activity small portion of its dispersal area. The isopachs the Managua sequence as defined by this study; during the same time period, although they are (Fig. 15) show that the unit thickens directly four are plinian rhyodacite pumices and three >35 km apart: the 21,000-yr-old Apoyo Pumice toward Masaya; on that basis, nothing more are basalts, either plinian or phreatoplinian. is bracketed at Managua by the two major conclusive about its source can be said. A prob- They were erupted from 4 different source areas Apoyeque pumices (Fig. 3). able dense-rock equivalent volume for the unit, over the past 35,000 yr, and each ranges in vol- The largest Quaternary deposit in the Mana- calculated from these isopachs, is 2.6 km3; S. ume from 0.4 km3 to 11 km3 (or more) of gua area, and still the least well understood, is Williams (1983b) gave a revised volume of 3.4 erupted magma. They may represent the largest the Las Sierras Group of basaltic lahars, ignim- km3. Grain size and dispersal area clearly show Nicaraguan eruptions over that time range, al- brites, and minor silicic pumices (Kuang S., this basaltic deposit to be plinian. though larger plinian deposits have been re- 1971), upon which the Managua sequence rests. The only radiometric constraints on the age of ported in the Quaternary chain (for example, at With a probable volume of >450 km3 (Bice, the Masaya Lapilli Bed are that it is older than Malpaisillo). 1980c), an age for its upper layers that could be the Apoyo Pumice (21,000 yr) and younger The correlations, source assignments, and age > 1 m.y. or <30,000 yr and source area(s) as yet than the upper Las Sierras Group, which is dates developed during this study, besides yield- undiscovered, the Las Sierras presents perhaps probably > 100,000 yr old at Managua and pos- ing a local geologic history and giving a picture the greatest remaining challenge for understand- sibly >1 m.y. old (based on low-precision of the average largest eruptions for this area, ing the Quaternary evolution of Nicaraguan potassium-argon dates on its upper layers; Bice have defined a set of widespread time-strati- volcanism. 1980c). S. Williams (1983b), however, feels that graphic markers that are of use for archaeologic the uppermost Las Sierras south of Managua and engineering purposes. Correlation and dat- ACKNOWLEDGMENTS may be as young as 30,000 yr, based on a radio- ing of the Jiloa Pumice has better defined the carbon date and tentative field correlation. age of prehistoric human footprints at Acahua- I am grateful to the many individuals and Using the modern soil as an analogy again, and linca (Williams, 1952a; Bryan, 1973; Bice, organizations who have been of aid during the accounting for the fact that the paleosol formed 1979), and careful mapping of offsets in distinc- extended course of this project. Partial financial on the Masaya Lapilli Bed is somewhat better tive dated tephra layers crossing Managua has support during field work and research was pro- developed than is the modern soil but is signifi- been useful in documenting fault movements vided by Woodward-Clyde Consultants, Inc. cantly less well developed than is the paleosol and determining earthquake recurrence intervals (San Francisco, California), California Energy above the Upper Apoyeque(?) Pumice, the Ma- (Niccum and others, 1974; Woodward-Clyde Company (Santa Rosa, California), Dames and saya Lapilli Bed is estimated to be between Consultants, Inc., 1975; Dames and Moore- Moore (San Francisco, California), ROTO WA, 25,000 and 35,000 yr old. LAMSA, 1978). The dated sequence itself pro- S. A. (Managua, Nicaragua), United Scientific vides a measure of the environmental hazard to Corporation (Mountain View, California), and Other Deposits within the Section the city of Managua from large explosive erup- Eureka Resource Associates, Inc. (Berkeley, tions (extremely low relative to seismic hazard; California); departmental grants from the Uni- Several other units occur within the Managua Bice, 1980c). In addition, volume calculations versity of California Department of Geology stratigraphic section in restricted areas and have for the rhyodacite pumices at Managua, com- and Geophysics, and National Science Founda- not been included in the Managua sequence as bined with reconnaissance mapping of a large tion grants to I.S.E. Carmichael. Valuable presented here. First, all sedimentary deposits part of the active chain, have permitted an initial ideas on Managua stratigraphy came from con- have been excluded; the best known are a thick estimate of the rate of magma production for sultations with many individuals, including 3 gravel sequence throughout much of southern this part of (10 km /km/m.y.; M. Niccum, M. Cline, J. Johnson, D. Collins, Managua and a very complex mudflow and Bice, 1980a). D. Schwartz, E. Utush, and others of Wood- flood-deposit sequence interfingered with the The units of the Managua sequence also pro- ward-Clyde; Dr. J. Espinosa E., J. Garayar, uppermost pyroclastic deposits in the northern vide a framework for deeper study of individual and D. Jerez of Nicaragua; and H. Williams, part of the city (Collins and others, 1976). Vol- vent areas. The recognition that the Masaya Tuff L. Hintze, and A. L. Bryan. Wet-chemical rock canic deposits from the nearby Nejapa-Mira- is related to the formation of Masaya Caldera analyses were performed by D. Kosco; extensive flores Alignment (including Laguna Tiscapa) (Bice, 1980b) and that two other Masaya- XRF analyses were facilitated by J. Hampel and occur at all levels but are extremely local in related units are of plinian style, unusual for ba- R. Vane; potassium-argon analyses were per- distribution; they have been discussed above as saltic deposits (Bice, 1980c, 1983; S. Williams, formed by R. Drake and B. Turrin. S. Williams they relate to individual units of the Managua 1983a, 1983b), has revised certain concepts of and J. Walker kindly provided copies of their sequence. One thin air-fall basaltic layer in the caldera formation and explosive volcanism and theses. Manuscript preparation was aided southeast part of the city, called "upper black has helped to understand better the detailed evo- greatly by N. Tannaci; valuable editorial sugges- lapilli" in some reports (Collins and others, lution of this unusual caldera complex (S. Wil- tions were made by reviewers F. Brown and 1976), has not been included because of its very liams, 1983b). Correlation, source-assignment, D. Feisinger.

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REFERENCES CITED Galehouse, J. S., 1969, Counting grain mounts: Number percentage vs. number Volcanologique, v. 36, no. 1, p. 174-190. frequency: Journal of Sedimentary Petrology, v. 39, p. 812-815. Walker, G.P.L., 1973, Explosive volcanic eruptions—A new classification American Commission or Stratigraphic Nomenclature, 1970, Code of Krusi, A., and Schultz, J., 1979, Base surge deposits of the Nicaragua volcano scheme: Geologische Rundschau, v. 62, no. 2, p. 431-466. stratigraphic nomenclature: Tulsa, Oklahoma, American Association of Masaya: Geological Society of America Abstracts with Programs, v. 11, 1980, The Taupo pumice: Product of the most powerful known (ultra- Petroleum Geologists, 17 p. no. 3, p. 87-88. plinian) eruption?: Journal of Volcanology and Geo thermal Research, Bice, D. C., 1979, Tepbra correlation and the age of human footprints near Kuang S., J., 1971, Estudio geologico del Pacifico de Nicaragua: Nicaragua, v. 8, p. 69-94. Managua, Nicaragua: Geological Society of America Abstracts with Catastro e Inventario de Recursos Naturales, Informe Geologico, no. 3, Walker, J. A., 1982, Volcanic rocks from the Nejapa and Granada tinder cone Programs, v. 11, no. 7, p. 338. 101 p. alignments, Nicaragua, Central America [Ph.D. thesis]: New Brunswick, — 1980a, Eruption ratt in Central America estimated from volumes of McBimey, A. R., 1955a, Thoughts on the eruptions of the Nicaraguan Volcano New Jersey, Rutgers University, 143 p. pumice deposits: American Geophysical Union Transactions, v. 61, Las POas: Bulletin Volcanologique, Series II, v. 17, p. 113-117. Williams, H., 1952a, Geologic observations on the ancient humar footprints no. 6, p. 70. 1955b, The origin of the Nejapa Pits near Managua, Nicaragua: Bulletin near Managua, Nicaragua: Contributions to American Anthropology —— 1980b, Origin of Mi saya Caldera, Nicaragua: Geological Society of Volcanologique, Series II, v. 17, p. 145-154. and History, no. 52, p. 5-31. America Abstracts with Programs, v. 12, no. 3, p. 98. 1956, The Nicaraguan volcano Masaya and its caldera: American Geo- 1952b, The great eruption of Coseguina, Nicaragua, in 1835, with notes 1980c, Tephra stratigraphy and physical aspects of recent volcanism physical Union Transactions, v. 37, no. 1, p. 83-96. on the Nicaraguan volcanic chain: University of California P jblications near Managua, Nicaragua [Ph.D. thesis]: Berkeley, California, Univer- 1969, Compositional variations in Cenozoic calc-alkaline suites of Cen- in the Geological Sciences, v. 29, p. 21-45. sity of California, 422 p. tral America: Oregon Department of Geology and Mineral Industries Williams, S. N., 1983a, Plinian airfall deposits of basaltic composi ion: Geol- 1983, Comment on 'Plinian airfall deposits of basaltic composition': Bulletin 65, p. 185-189. ogy, v. 11, p. 211-214. Geology, v. 11, p. 61Í-617. McBirncy, A. R., and Williams, H., 1964, The origin of the Nicaraguan ¡De- 1983b, Geology and eruptive mechanisms of Masaya Calder i complex, Brown, R. D., Ward, P. L, mid Ptafker, G., 1973, Geologic and seismologic pression: Bulletin Volcanologique, v. 27, p. 63. Nicaragua [Ph.D. thesis]: Hanover, New Hampshire, Dartmouth Col- aspects of the Managua, Nicaragua, earthquake of December 23,1972: 1965, Volcanic history of Nicaragua: University of California Publica- lege, 169 p. U.S. Geological Survey Professional Paper 838. tions in the Geological Sciences, v. 55, p. 1-69. Woodward-Clyde Consultants, Inc., 1975, Summary report of investigation of Bryan, A. L., 1973, New light, on ancient Nicaraguan footprints: Archaeology, Niccum, M., 1976, Regional tectonic, geologic and seismic data with hazard active faulting in Managua, Nicaragua, and vicinity: Managt a, Nicara- v. 26, no. 2, p. 146-1*7. classifications: Appendix A, unpublished report for use with gua, Viœ Ministerio de Planification Urbana. Bum, G, and Sonder, R. A., 1936, Beitrage zur Geología and Petrographie des Woodward-Clyde Consultants, inc. reports in Managua, Nicaragua: Zoppis Bracci, Luigi, and Daniele del Guidice, 1958, Geologia de Iti Costa de jungtertiaren und reanten vulcanismus in Nicaragua: Zeitschrift fur San Francisco, California, Woodward-Clyde Consultants, Inc. Pacifico de Nicaragua: Nicaragua, Serviào Geologico National, Boletin Vulkanologie, Bd. 17, p. 34-92. Niccum, M., Bice, D., Cline, K., Cluff, L., and Schwartz, D., 1974, Active faults no. 2, p. 19-68. Collins, D. E., Niccum, M. R.and Bice, D. C., 1976, Preliminary summary of in Managua, Nicaragua: Geological Society of America Abstracts with late Pleistocene and Hi Eocene volcanic and sedimentary stratigraphy of Programs, v. 6, no. 7, p. 889-890. the Managua area, Nicaragua: Publicaciones Geológicas del Instituto Parsons Corporation, 1972, The geology of western Nicaragua: Nicaragua, 1'ax Centroamericano de Investigación y Technologia Industrial, no. V, Improvement and Natural Resources Inventory Project, Final Technical p. 105-112. Report, Volume IV. Dames and Moore-LAMSA, 1978, Estudio geologico de tas ciudades del sis- Sapper, C., 1925, Los Volcanes de America Central: Verlag, Halle. Max Nie- tema metropolitano: Managua, Nicaragua, Vice-Ministerio de Planifi- meyer, 116 p. (translation into Spanish). cación Urbana, 143 p. Stoiber, R. E., and Carr, M. J., 1974, Quaternary volcanic and tectonic segmen- Davis, J. C„ 1973, Statistics ;ind data analysis in geology: New York, John tation of Central America: Bulletin Volcanologique, v. 37, no. 3, Wiley & Sons, 550 p. p. 304-325. Dengo, G., Bohnenberger, O., and Bonis, S., 1970, Tectonics and volcanism Sussman, D., 1982, The geology of Apoyo Caldera, Nicaragua (M.S. thesis): MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 12,1984 along the Paciñc marginal zone of Central America: Geologise he Rund- Hanover, New Hampshire, Dartmouth College, 116 p. REVISED MANUSCRIPT RECEIVED SEPTEMBER 24,1984 schau, v. 59. no. 3, p. 1215-1232. Ui, T., 1973, Recent volcanism in Masaya-Granada area, Nicaragua: Bulletin MANUSCRIPT ACCEPTED SEPTEMBER 27,1984

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