Investigation of Deep-Sea Volcanic Ash Layers from Equatorial Pacific Cores

FREDERICK A. BOWLES Ocean Floor Analysis Division, U.S. Naval Oceanographic Office, Wash- ington, D.C. 20390 ROBERT N. JACK 1 Department of Geology and Geophysics, University of California, Berkeley, Cali- I.S.E. CARMICHAEL J forma 94720

ABSTRACT In 1959, Worzel identified an extensive subbottom reflector in the eastern equatorial Trace and minor element analysis of 128 Pacific as a layer of white volcanic ash. The volcanic ash samples from 56 cores taken in ash reportedly extended from about 11° N. to the eastern equatorial and southeastern Pacific 12° S., causing considerable speculation as to Ocean has allowed identification and correla- its possible global significance (Ewing and tion of individual volcanic ash layers over others, 1959). Later, Nayudu (1964) expressed wide areas of the ocean floor. Three principal doubt that the "Worzel Ash" was indeed a areas of ash deposition have been delineated, single, continuous layer. Following Worzel's with probable source areas in Guatemala, El discovery, numerous cores were taken offshore Salvador, Nicaragua, and Colombia or Ecua- of Central and South America (Fig. 1). Most dor. Analyses of continental pumice samples of these cores contain more than one ash indicate vents near the Tecpan-Chimaltenango layer. Our investigation deals with the identi- basin in the Guatemalan Highlands and the fication, correlation, and distribution of these Lake Ilopango region of El Salvador. Many, ash layers. perhaps all, of the ash layers do not date beyond 300,000 yrs. The two most extensive GEOLOGIC SETTING (400,000 and 300,000 km2) and voluminous 3 Volcanoes occur along the entire Cordillera (43 and 19 km ) ash falls have been tentatively of North, Central, and South America as part dated at 54,000 and 220,000 yrs, respectively. of an impressive volcanic system that rings Together these ash layers comprise the so- the Pacific Basin. These are typically andesitic called "Worzel Ash." The most extensive of volcanoes associated with deep-sea trenches the two layers occurs in all three ash areas and related seismic zones. The trenches are and may also correlate with an ash horizon now recognized as zones of subduction, that is, in the Gulf of Mexico dated at approximately regions where spreading oceanic plates descend 60,000 yrs. The correlation of some ash layers into the asthenosphere. The Middle America with a sequence of subbottom reflectors sug- Trench, paralleling Central America (Fig. 2), gests that the entire sequence of reflectors marks the site where the Cocos Plate is being seen in the upper 100 m of sediment off Central thrust under Central America. Likewise, the America can be attributed to discrete layers Peru-Chile Trench marks the descent of the of volcanic ash. Nazca Plate under South America. The presence of volcanoes along the Pacific coasts of INTRODUCTION Central and South America suggests that the Explosive eruptions are, by their nature, volcanism is related to these descending plates. conspicuous geologic events which are com- Some investigators have even suggested that monly recorded in both continental and the andesitic lavas may well be the direct melt marine sedimentary sequences as layers of products of the plates (McBirney, 1970). volcanic ash. Because of the generally wide- Central America is one of the most volcan- spread distribution and "instantaneous" depo- ically active regions on earth, where lavas sition of these ash layers, they are highly have been erupted from a chain of volcanoes reliable time-stratigraphic horizons. extending from Guatemala to Costa Rica.

Geological Society of America Bulletin, v. 84, p. 2371-2388, 7 figs., July 1973 2371

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Figure 1. Location of total cores taken in study area. Core locations are referred to in text by the numbers shown above. Three principal areas of ash deposition (northern, central, and southern) discussed in text are outlined with solid lines.

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30'

0° :

15C

30'

45* 115° 105° 90° 75° 60° Figure 2. Location of principal physiographic features and countries discussed in text.

The Quaternary and Holocene volcanoes are In the South American Cordillera there are confined to a narrow belt that lies close to the three areas of young volcanism (Pichler and Pacific Ocean, while comparatively few vol- Zeil, 1969): (1) a northern zone (5° N. to canoes occur on the Caribbean side of Central 2° S.) spanning southern Colombia and north- America. Presently, there is no obvious deep- ern Ecuador, (2) a central zone (15° S. to sea trench (and therefore apparently no under- 27° S.) covering southernmost Peru and thrusting) along the west coast of Panama (van northern Chile, and (3) a southern zone (33° S. Andel and others, 1971). Predictably, Panama to 44° S.) entirely in Chile. These zones is volcanically inactive. coincide well with Kelleher's (1972) "rupture

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zones," which he relates to regions of active CORRELATION BETWEEN plate underthrusting. According to Pichkr ASH LAYERS AND and Zeil (1969), volcanic activity in the SUBBOTTOM ECHOES central zone is weak compared with that ia the other two zones. Casertano (1963) has The ash layers in the cores apparently concluded that Chilean volcanoes are becom- correspond to the impressive configuration of ing extinct beginning with those in the north subbottom reflections observed in the sedi- (central zone) and progressing southward. ments of this region (Fig. 3). During May Current volcanic activity in the northern zone and June of 1970 the USNS De Steiguer con- is somewhat unusual because the Peru-Chile ducted research operations between the Gala- Trench effectively terminates as a well-defined pagos Islands and Acapulco, Mexico. Echo- feature near the Carnegie Ridge. This is also grams (3.5 kHz) of this re gion show numerous reflected by the reduced frequency of earth- highly reflective layers often revealing more quakes north of 0°30' S., indicating that than 20 distinct reflectors in the top 100 m underthrusting along Ecuador and Colombia, of sediment. The reflectors appear to be ex- if it occurs at all, is considerably less active tremely conformable and continuous except than farther south along the trench (van Andel on steep slopes where they appear to be absent. and others, 1971). Despite their apparent continuity, the number of reflectors and vertical spacing between them DESCRIPTION OF CORES change as one parallels the coast of Central AND ASH LAYERS America. Figure 3A, for example, shows a sequence of reflectors olf southern Mexico Descriptions of the cores (Fig. 1) show that and Guatemala. Farther south, a completely most of the surface sediments consist of a different configuration of reflectors occurs off homogeneous, dark-olive-green lutite. Deeper Costa Rica (Fig. 3B), and one can anticipate in the cores, lighter shades of green occur. different configurations still farther south along The biogenous content of the sediment con- the South American coast. Such an extensive sists of Radiolaria, diatoms, and Foraminifera. and complex configuration of subbottom re- The coarse fraction is made up largely of flectors is doubtless the result of many vol- volcanogenic debris. Exclusing the ash, the canoes erupting in different places at different content of coarse terrigenous debris is low, times. owing to the proximity of the Middle America Trench which traps much of the coarse detritus Worzel (1959) showed that an ash layer coming from Central America. near the sediment surface did indeed correlate The ash layers are the most distinctive with a strong subbottom reflector. Proof that feature of the cores. These range in thickness deeper reflectors are also the result of ash from <1 cm to a maximum of 46 cm and layers is provided by core and dredge samples occur in the cores at depths from 0 to 16 m. taken by the USNS De Steiguer. At location The bottom sediment-ash contact is usually 10 (Fig. 1), eight reflectors were observed in sharp, whereas the top contact frequently the top 35 m of sediment, and three closely shows a gradual change from ash to lutite. In spaced cores were taken in an attempt to many cores, thin ash layers have been disrupted identify them. The longest core (D-6) mea- by burrowing organisms. Admixtures of ash sured approximately 16.5 m and contained often occur within the lutite, and at times the five discrete layers of white volcanic ash. ash appears to be concentrated in burrows. Figure 4 shows a photographically expanded Most of the ash layers are white to grayish segment of the 3.5-kHz record at the core white in color. Microscopic examination of site and the excellent agreement between the the white ash shows mostly clear, colorless, echogram and core. For each reflector, an ash volcanic glass shards. Dark-gray ash layers, layer occurs at the appropriate depth in the consisting predominantly of "smoked" shards core. These reflectors are the same top five and black vitreous glass, are also found in reflectors appearing in Figure 3A and are many of the cores. Most of the ash occurs in identifiable over portions of the sea floor off the size range of very fine sand to about me- Guatemala. A second core (D-7) and echo- dium silt as indicated by the sieving and gram correlated equally well. settling done in preparation for chemical The ability of a layer to produce a strong analysis of the ash. subbottom reflection is dependent, in part,

NAUTICAL MILES 1 2 1800 4.5 H » 20 >< M r H— 2 M

4.75 £ PJ O O z o Cfl

60 NAUTICAL MILES l 2 H > 1200 3.0 < M Cfl 20 f- H O i-* X 40 2 H M < 60 U* W M 80 O Oz 1300 B 3.25 s Figure 3. 3.5-kHz echograms off coast of (A) Guatemala and (B) Costa Rica. on its thickness. As the thickness of a layer 1 cm. The reflective "strength" of a layer, becomes less than one-quarter of the wave- however, is not solely dependent on thickness. length of the sound energy (about 12 cm at It is also a function of the acoustic impedance 3.5 kHz), the magnitude of the reflected (velocity X density) difference—the greater energy diminishes rapidly. As a result, many the difference, the stronger the reflection. It thin layers may escape detection. Several ash is apparent that the ash layers, because of layers in D-6 are quite thin, but are never- their extreme compactness and coarseness, theless excellent reflectors of sound. Layers 2, present a large density contrast relative to the 4, and 5, for example, are only 7, 8, and 6 cm soft, homogenous lutite that surrounds them. thick, respectively. Most surprising is the top Thus, the thin nature of the layers is more ash, which is burrowed and probably was than compensated for by their high impedance. originally very thin. The equivalent layer in Unavoidable drifting on station can create D-7 is only 3 cm thick, and in D-4, taken some discordance between cores and echo- some 60 mi away (loc. 2), it measures only grams, as in the case of the third and thickest

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D-6 0-

4 •HI .- -.;,*: METERS

8 D-5 0 k-""^ / / . _ _ -20

12 X " " -30

- 4 X ^-^V —40

w t 8 / /

Figure 4. Correlation of ash layers and 3.5-kHz echogram at locatio i 10.

(43 cm) ash layer in core D-6 (loc. 10), which TRACE AND MINOR ELEMENT correlates with a relatively weak reflector. In ANALYSIS AND CORRELATION core D-5 (the third core at loc. 10), the equiva- OF ASH LAYERS lent ash layer is only 9 cm thick and is even thinner at other core locations (6, 8, 9, and Silicic or acid magmas tend to produce 11). Drifting also accounts for the fact that explosive volcanic eruptions. Usually such the top two ash layers in core D-6 are missing explosions produce glass in the form of pumice in core D-5 (Fig. 4), yet, the 3.5-kHz sound- and shards which, because of its homogenous ing recorded the identical set of flat-lying re- composition, can be successfully identified flectors shown in Figure 4. The echogram for over the entire area of its distribution. Corre- D-5 contained numerous side echoes strongly lation of ash deposits can be accomplished by suggesting that the core was taken on a slope several techniques such as refractive indices, where slumping could have removed some of mineral content, or major clement composition. the sediment (and ash layers). However, the major element compositions of silicic volcanic glasses, and therefore their re- NATURE OF THE DEEP REFLECTORS fractive indices, vary over a rather narrow Beyond coring depths the deeper reflectors range. Trace and minor ehment compositions, cannot be positively identified. However, the on the other hand, can differ between obsidians nature of the layering is so repetitious (Fig. by factors as large as 100 (Jack and Carmichael, 3A and B) that we conclude that the deep 1969). reflectors also represent ash layers. Tangible Analysis by x-ray fluorescence techniques evidence of this was obtained from a dredge for minor and trace element content was done taken on the seaward flank of the trench on 128 ash samples from 54 cores (Table 1). approximately halfway between core locations Each sample was first cleaned by repeated 11 and 12 (Fig. I). The dredge contained settling to remove clay contaminants and then several small slabs of indurated, whitish ma- by sieving to remove coarse contaminants terial consisting of clear, colorless volcanic (mica, tests, and so forth). The x-ray fluores- glass. Without question, the indurated nature cence analytical procedures used and the pre- of the slabs suggests an older ash and therefore cision obtained are essentially those reported deeper burial. by Jack and Carmichael (1969). All samples

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Sample Sample Sample depth Location Sample depth Location Core no. no. (cm) (Kg- 1) Core no. no. (cm) (Fig- 0

D-3 1 125 11 66 1420 2 142 V-21-212 68 280 42 3 165 69 746 4 292 V-21-214 70 398 38 5 460 71 962 6 560 V-24-36 72 71 34 7 747 73 238 8 840 74 450 D-4 9 400 2 75 550 D-5 10 30 10 V-24-37 76 135 30 11 220 V-15-17 77 285 32 12 1008 V-15-18 78 360 24 D-6 13 820 10 79 1380 14 1010 V-15-19 80 0-3 23 15 1023 81 265 16 1245 V-15-21 82 4 22 17 1538 V-15-22 83 112 15 D-7 18 770 10 84 168 19 1029 V-15-26 85 25 14 20 1057 86 75 21 1065 87 270 D-8 22 20 4 88 475 RC-10-247 23 19 19 89 580 RC-10-248 24 1030 20 V-15-28 90 320 RC-10-249 25 195 28 91 340 RC-10-250 26 582 35 V-15-30 92 156 37 RC-12-29 27 520 93 612 28 760 V-15-32 94 604 45 29 810 V-15-51 95 200 49 RC-12-30 30 344 16 96 500 31 505 97 650 RC-12-31 32 0-12 13 V-15-54 98 1050 RC-12-32 33 50 13 99 1160 34 112 V-15-57 100 435 51 35 190 V-17-42 101 775 39 37 242 V-17-43 102 820 41 38 252 V—17-44 103 676 46 39 265 V-18-333 104 187 6 40 300 105 550 41 385 V-18-334 106 410 7 42 480 107 700 RC-12-33 43 18 12 108 1000 44 22 V-18-335 109 45 8 45 70 V-18-337 110 590 3 RC-12-34 46 600 1 111 630 RC-12-36 47 440 5 V-18-340 112 68 9 48 454 113 157 49 810 114 323 V-15-17 50 550 115 557 V-15-18 51 1127 116 579 52 1180 117 801 V-15-26 53 510 V-18-346 118 693 29 54 539 V-l8-348 119 50 31 V-15-27 55 15 25 V-18-349 120 710 56 40 V-19-25 121 791 40 57 110 122 1200 58 356 V-l9-28 123 875 43 59 542 V-l9-29 124 1226 47 60 582 V-19-30 125 1280 44 V-15-33 61 1035 48 126 1290 V—15-54 62 310 50 V-20-16 127 375 27 V-18-349 63 282 36 V-20-17 128 225 21 64 801 V-20-18 129 250 17 V-20-19 65 1324 18 130 1600

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were analyzed for fifteen elements, but only 117 from core V18-340 and samples 37 and 40 nine are reported. The following elements from RC12-32, because o:: their high values of were omitted: Nb because of rather low con- Ti, Fe, or Mn, and light gray color, show an centrations (5 to 50 ppm) and high statistical affinity to the central area ashes but cannot be variability; Y because of very low, relatively correlated with any specific one (sample 115 constant concentrations (5 to 15 ppm); Ga does resemble sample 88 in the central area). because of small over-all variations (15 to 25 Ash type C also shows the same affinity but ppm); and Zn, Cu, and Ni because of highly occurs stratigraphically too high for the central variable and occasionally abnormally high area ashes. The remaining unclassified samples values, possibly the result of contamination are all similar in chemistry to ash D, but be- on the sea floor or perhaps during laboratory cause of their clearly different stratigraphic processing (for example, sieving). The analyses positions and slight differences in chemical for the remaining elements are based on the composition, cannot be classified as type D, U.S. Geological Survey standard granite G-2, nor correlated with each cither. These ash falls assuming the concentrations in the granite are present in cores taken relatively close to given by Carmichael and others (1968, Table shore, and therefore probably represent weak III). No mass absorption correction was applied eruptions of limited areal extent. to the results because the complete major The top ash layers in cores D-6 and D-7 element composition was not determined. Such were not sampled (because: of burrowing), but a calculation did not seem necessary since the their correlation with the ash in D-4 is based purpose of the analysis was for identification on examination of the 3.5-kHz records and the and correlation. identical nature of the sediment surrounding It would seem that the chemical signature the ash layers. In core VI8-337, the two ash of the ash would be destroyed by devitrifica- layers are so close chemically that we labeled tion, chemical alteration by sea water, or mask- one G' for convenience. ing of the composition by contamination and It is apparent from Figure 5 that rates of mechanical mixing. Individual ash layers, how- sediment accumulation have varied considera- ever, were found to be quite uniform in com- bly from one core location to the next. Such position for the elements chosen as shown by variability seems to be characteristic of por- the successful identification and correlation of tions of the equatorial Pacific, particularly the 102 ash samples. central part where evidence indicates faster A northern, a central, and a southern deposi- sedimentation rates in depressions than on tional area have been outlined in Figure 1. hills (Menard, 1964; Moore, 1969). Relatively Each is characterized by a unique suite of rough topography in areas traversed by the ash types which have been designated by De Steiguer may explain, in part, some of the capital letters. Correlations made within each variability shown by Figure 5. However, we area are based primarily on chemical similari- lack sufficient control of variables (principally ties between ash samples but also on the strati- topography) throughout most of the area to graphic position of samples and at times on the make a strong statement concerning the ob- locations of cores. served differences in sedimentation rates. Northern Area Central Area The ash types of the northern area are Aside from the appearance of ash D, an clearly defined by distinctive trace element entirely different ash suite is found in the chemistries (Table 2). A few stand out rather central area (Table 3; Fig. 6). The I and J prominently. Ash type D, for example, is ashes, which clearly fall into two groups, are easily distinguished by its low values of Zr, Sr, easy to distinguish from the other ash types Ti, Fe, and Ca. Ash H is quite similar to D but because of their usually high values of Mn, is differentiated by its lower stratigraphic posi- Ti, Fe, and Ca. Correlations are also aided by tion (Fig. 5). High values for Ti and Fe easily the fact that all the J ashes are dark gray in identify ash C. The compositions for C show color, whereas the I ashes are light gray or some mismatch, but the stratigraphic position occasionally white. The color of the I-} ashes of the samples warrants their correlation. in most cases reflects their more basic (ande- Seventeen samples in Table 2 are not classi- sitic?) composition relative to the acid (da- fied. Several of these simply do not fit any of citic?) white ashes. the designated ash types. Samples 114, 115, and Subdivisions within each I and J group are

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Ash type A B B B B C C D D D D Sample no. 9 13 1 18 34 112 4 104 106 47 48

Ba 1060 1080 1070 1100 1115 1585 1105 1135 1145 1130 1115 Zr 190 135 125 135 140 150 210 75 80 80 95 Sr 175 220 185 220 210 185 220 105 95 100 100 PPm Rb 110 115 115 120 115 100 110 135 145 145 145 Mn 765 515 570 540 530 780 700 720 640 610 620 Ti 1550 1555 1250 1575 1625 2450 2145 965 785 815 810 Fe 1.18 1.22 1.09 1.23 1.31 2.54 1.52 1.02 0.60 0.71 0.66 % Ca 0.98 1.20 1.06 1.23 1.18 1.14 1.27 0.61 0.52 0.56 0.56 K 2.85 2.94 3.05 2.86 3.04 2.50 2.69 3.27 3.27 3.34 3.29

Ash type D D D D D D D D D D E Sample no. 109 113 10 14 15 5 19 20 21 35 49 Ba 1145 1110 1125 1125 1140 990 1145 1125 1175 1060 1195 Zr 70 90 85 85 75 70 90 75 75 90 195 Sr 105 110 100 105 105 125 100 95 110 135 120 PPm Rb 150 145 145 135 135 125 135 140 140 130 105 Mn 625 640 620 620 620 760 620 620 620 610 680 Ti 830 845 815 885 870 705 855 810 1005 1110 1625 Fe 0.72 0.69 0.60 0.71 0.69 0.64 0.67 0.64 0.87 0.99 1.59 % Ca 0.53 0.57 0.56 0.59 0.56 0.60 0.60 0.56 0.61 1.02 0.84 K 3.42 3.15 3.27 3.16 3.34 3.41 3.17 3.17 3.39 3.07 3.47

Ash type E E E E F F F G G G G Sample no. 22 11 16 7 116 8 41 107 110 111 12 Ba 1045 1185 1205 1170 1240 1340 1135 1130 1080 1050 1060 Zr 175 165 155 185 150 270 265 115 125 115 120 Sr 135 130 120 150 175 180 185 195 190 195 200 PPm Rb 120 110 120 120 110 115 120 120 140 125 125 Mn 685 700 670 725 1060 755 715 565 460 480 470 Ti 1395 1620 1495 1400 2085 2010 1895 1350 1195 1350 1310 Fe 1.14 1.23 1.17 1.21 2.04 1.77 1.73 1.34 0.89 1.04 0.98 % Ca 0.94 0.94 0.82 0.87 1.20 1.06 1.27 1.02 0.96 1.04 1.05 K. 3.40 3.56 3.69 3.54 3.14 3.43 3.25 2.96 3.16 2.96 3.17

Ash type G H H ? ? ? ? ? ? ? ? Sample no. 17 105 108 2 3 6 32 33 37 38 39 Ba 1090 1050 1025 1165 1210 1090 1105 910 1045 1085 1080 Zr 125 75 60 115 125 105 105 145 85 85 115 Sr 195 125 120 95 105 120 115 47 180 125 130 PPm Rb 125 130 130 135 140 110 135 135 125 130 145 Mn 465 850 865 640 645 620 610 1065 655 635 625 Ti 1265 810 725 960 1095 1130 990 845 1720 1080 1050 Fe 1.06 0.97 0.78 0.86 0.93 0.86 0.80 0.82 1.64 0.96 0.89 % Ca 1.01 0.59 0.56 0.61 0.66 0.76 0.70 0.36 1.49 0.85 0.78 K 3.18 3.25 3.48 3.84 3.74 3.40 3.05 3.60 2.93 3.16 3.10

Ash type ? ? ? ? ? ? ? ? ? Sample no. 40 42 43 44 45 46 114 115 117 Ba 945 805 1025 1040 1025 985 1850 1265 1515 Zr 160 135 125 115 125 145 135 100 115 Sr 315 115 170 160 145 255 225 215 175 PPm Rb 90 180 120 120 115 100 105 45 95 Mn 705 585 755 765 780 600 640 1455 1235 Ti 2250 985 1345 1145 1210 1610 2690 4390 2730 Fe 2.53 0.92 0.99 0.83 0.87 1.52 2.57 3.80 2.76 % Ca 4.78 0.66 1.04 0.89 0.79 1.57 1.27 2.81 1.22 K 2.13 3.75 3.14 3.29 3.18 3.08 2.51 1.86 2.73

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NORTHERN AREA 12 3 4 5 10 13 9 11 10 2 10 7 6 8

Figure 5. Correlation of ash layers in cores from locations shown in Figure 1. northern ash area. Numbers in top row refer to core in some instances determined on stratigraphic to treat J4 as another eruption because of its position of the sample rather than chemistry. occurrence on the Cocos Ridge. For example, J and Ji in core RC12-29 share a Ash type K appears in three of the most similar chemistry but obviously cannot share distant cores (V20-18, V18-346, and RC10- the same identity. 248). The I and J groups of ashes apparently Three cores (Fig. 6) containing I and J ashes do not occur this far offshore, whereas nearer occur on the Cocos Ridge (Fig. 2) and one shore, ash K may be present at depths not (V15-17) on the flank. These ash layers are reached by the central area cores. Except for not only separated from the other I and J ash slightly more potassium and less strontium, ash layers by an area in which only ash D occurs K is quite similar to ash G in the northern but also differ slightly in chemistry (Table 3). area and may correlate with it. If so, then the Both facts suggest that the Cocos Ridge ashes time of eruption of the I and J ashes (excluding do not correlate with the more northerly I and the Cocos Ridge cores) is nicely bracketed be- I ash layers and perhaps involve a different tween layers D and G of the northern ash source. Ash J4 in core V15-17 is chemically area. Interestingly, four of the unidentified similar to ash Ji farther north, but we prefer ashes in the northern area (40, 114, 115, and

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Ash type D D D D D D D D D D D Sample no. 129 23 128 30 80 82 27 78 55 56 76 Ba 1140 1120 1165 1160 1110 1135 1120 1100 1105 1120 1125 Zr 85 105 75 95 85 75 85 90 95 80 90 Sr 115 105 105 130 115 110 110 105 100 105 100 ppm Rb 135 130 125 145 130 135 145 135 130 135 145 Mn 620 665 650 605 615 620 625 645 635 645 625 Ti 980 905 965 1035 935 870 835 860 880 910 770 Fe 1.06 0.90 1.30 0.81 0.99 0.74 0.67 0.76 0.69 0.73 0.61 % Ca 0.66 0.64 0.61 0.69 0.61 0.59 0.57 0.58 0.61 0.59 0.55 K. 3.07 3.08 3.20 3.31 3.20 3.25 3.40 3.20 3.31 3.20 3.32

Ash type D D D D D D D Ii h I2 I2 Sample no. 25 127 90 91 119 72 77 58 81 59 51 Ba 1125 1115 1145 1125 1100 1115 1145 1555 1510 1625 1515 Zr 90 90 65 85 70 70 70 180 290 260 275 Sr 105 105 105 95 105 125 100 310 410 365 405 PPm Rb 140 150 135 145 140 140 135 85 85 75 80 Mn 640 640 655 955 655 600 695 830 1030 1075 1055 Ti 810 870 915 875 855 805 970 1780 5075 4940 4820 Fe 0.65 0.74 0.79 0.77 0.86 0.77 0.86 1.71 3.61 3.77 3.55 % Ca 0.55 0.56 0.59 0.64 0.55 1.29 0.59 1.62 2.52 2.78 2.47 K 3.26 3.20 3.56 3.22 3.35 3.41 3.44 2.78 3.02 2.76 2.98

Ash type h la Is I. Is Is I. J Ji Ji Sample no. 89 84 60 79 52 63 26 75 28 31 87 Ba 1710 1660 1760 1830 1935 1485 1440 1485 770 1140 900 Zr 180 175 175 170 160 265 265 355 70 70 65 Sr 240 270 220 210 205 440 435 535 285 275 250 PPm Rb 55 55 50 55 50 90 95 95 30 25 30 Mn 900 925 1205 1265 1315 805 905 830 1290 1515 1620 Ti 2575 2600 2760 2840 3045 4150 4740 5025 5225 5365 6155 Fe 2.22 2.27 3.23 3.36 3.83 2.99 3.43 2.55 6.39 6.72 7.41 % Ca 1.81 1.94 2.17 1.88 1.94 6.28 4.69 3.69 5.05 5.27 5.11 K 2.04 1.93 2.42 2.35 2.28 3.04 2.98 3.33 1.02 1.15 1.27

Ash type J. Ji h Js Js J4 Js Js J. K K Sample no. 29 57 53 54 83 50 120 74 64 130 24 Ba 995 965 915 825 895 1005 985 1015 965 1050 1160 Zr 70 65 80 75 80 70 130 135 125 130 120 Sr 275 275 295 320 305 270 495 510 525 170 165 PPm Rb 30 20 30 30 30 25 45 50 35 125 135 Mn 1660 1620 1455 1300 1310 1815 1320 1340 1210 585 625 Ti 5370 5465 5285 5085 4805 5690 6010 6330 5685 1310 1235 Fe 6.80 6.93 5.82 5.63 5.27 7.15 5.15 5.52 4.89 1.09 0.96 % Ca 5.40 5.08 4.58 4.94 4.15 6.00 6.73 5.46 4.05 1.06 1.11 K 1.13 1.11 1.19 1.10 1.16 1.15 1.67 1.70 1.59 3.51 3.51

Ash type K Ki ? ? ? ? ? Sample no. 118 73 65 66 85 86 88 Ba 1200 1180 1270 7120 1185 1475 1190 Zr 95 110 205 195 155 140 130 Sr 155 155 140 315 270 265 295 PPm Rb 130 130 175 65 50 45 40 Mn 690 645 610 1315 775 960 1510 Ti 1155 1365 1615 3245 1920 2890 4120 Fe 0.92 1.22 1.35 3.47 1.87 3.11 3.92 % Ca 1.00 1.32 0.84 1.52 1.77 2.52 3.58 K 3.56 3.27 3.94 2.21 1.87 1.74 1.46

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CENTRAL AREA

18 29 20 19 17 21 22 16 14 15 25 26 24 23 27 28 30 31 33 32 34 36 35

Figure 6. Correlation of ash layers in cores from locations shown in Figure 1. central ash area. Numbers in top row refer to core

117) having chemical affinities with those in Southern Area the central area lie between ash layers D and G. Ash type K' (V24-36) is nearly identical to The last and simplest area to describe is the ash K but is found some distance away on the southern one in which only two ash types are Cocos Ridge. If these correlate, then the I and identified, namely D and L (Table 4; Fig. 7). J ash layers in cores V24-36, RC10-250, and The occurrence of D in core V15-30 is fortu- VI8-349 would predate the other I and J ashes nate, because it completes the stratigraphic of the central area. As in the northern area, link between the three ash areas. Unfortunately, sedimentation rates in the central area hi.ve the available data do not allow determination also varied considerably. If ash K and ash K' of the stratigraphic position of ash L relative do indeed correlate, then core V24-36 indi- to the other ash layers below ash D. cates a remarkably slow sedimentation rate on Cores V21-212 and V21-214 each have an the ridge compared with core V20-18 on the ash layer above ash L, but their chemistry does deeper portion of the sea floor. not allow them to be correlated with ash D or Ash samples 65, 66, 85, 86, and 88 remain among themselves. Also, one ash layer occurs unidentified. Samples 65 and 66 occur in care below ash L in core VI9-25. V20-19, which is the most distant from shore The southern area contrasts markedly with of all the cores. The fact that these layers are the other ash areas in two ways: (1) correlations not identified in other central (or northern) are much simpler by virtue of fewer ash layers, area cores suggests that they are quite old and and (2) the depth to layer L appears to be occur at depths greater than that penetrated remarkably uniform over a large portion of by the other cores. Samples 85, 86, and 88 the southern area, indicating a rather uniform (V15-26) occur close to shore and probably rate of sedimentation. represent weak eruptions. Ash layers from three cores taken in the

southeastern Pacific were sampled and analyzed is 25 km3 (Williams, 1972, written commun.) (Table 5). Chemically they are not unlike and Mount Katmai, Alaska, is 19 km3 (Wil- many of the ash layers examined, but because liams, 1952). If 43 km3 is anywhere near cor- of the extreme southerly location, they obvi- rect, then ash D represents an eruption of ously cannot be correlated with the ash layers monumental proportions. in the southern area. Two samples (98 and Based on the correlations, it is obvious that 100) appear to correlate perfectly, while each no single ash layer blankets the entire region of the others has its own peculiar chemical as Worzel (1959) assumed. Instead, the signature. We have included the analyses "Worzel Ash" consists of at least two ash chiefly for interest and as a prelude to future layers (D and L) of different age. In the central investigation. area, ash D is usually uppermost, while in most of the southern area the same is true of EXTENT OF ASH LAYERS ash L. This accounts for their being mistaken Ash layers D (400,000 km2) and L (300,00 as a single, continuous layer. km2) constitute the two most widespread ash falls. These are of moderate size when com- SOURCES OF ASH LAYERS pared with the Mount Mazama eruption in The sources of the ash layers lie among the Crater Lake, Oregon, which covered an area volcanoes of Central and South America. Some of at least 106 km2 with volcanic ash (Fryxell, probable source areas have been isolated 1965). If ash types K, K', and G can be corre- simply by observing the areal distribution and lated, then they would comprise an ash fall chemical character of the ash layers. Our nearly equal in area to ash D. Using the average results show no systematic thickening of ash thickness of ash layers D (10.6 cm) and L (6.5 layers in the direction of source, and no at- cm), we calculate the total volume of each ash tempt was made to determine the direction of fall to be 43.6 km3 and 19.5 km3, respectively. source vents by size analysis of specific ash The volume of ash D is impressive and com- layers. parable to 50 km3, the most quoted figure for Although the distribution and deposition the volume of ejecta of the great eruption of of the ash is subject to the action of currents Cosequina in Nicaragua (Williams, 1952). How- (both surface and bottom), the depositional ever, Williams thinks that 50 km3 is much too patterns of the ash layers still clearly reflect high and that 10 km3 may be closer to the the direction of the transporting winds. The truth. For further comparison, Mount Mazama prevailing surface winds in the equatorial

TABLE 4. TRACE (PPM) AND MINOR (%) ELEMENT ANALYSES OF ASH SAMPLES IN SOUTHERN ASH AREA

Ash type D L L L L L L L L L L Sample no. 92 93 69 71 101 121 102 103 123 124 125 Ba 1245 1195 1110 1090 1070 1065 1110 1145 1075 1075 1085 Zr 75 170 195 190 190 190 170 175 175 180 175 Sr 95 245 260 265 265 275 235 250 245 255 250 PPm Rb 140 175 175 175 185 175 180 175 180 175 175 Mn 725 650 520 445 430 445 430 450 405 455 450 Ti 860 1820 1755 1775 1815 1810 1640 1780 1640 1790 1800 Fe 0.84 1.59 1.27 1.22 1.30 1.34 1.14 1.35 1.20 1.29 1.33 % Ca 0.71 1.11 1.12 1.20 1.26 1.19 0.97 1.41 1.48 1.14 1.21 K 3.28 3.31 3.34 3.39 3.27 3.21 3.51 3.27 3.30 3.32 3.25

Ash type L L L ? F Sample no. 126 61 94 68 70 122

Ba 1100 1110 1075 1455 1340 660 Zr 180 175 185 85 135 90 Sr 255 250 255 175 345 155 80 PPm Rb 180 185 190 125 150 Mn 455 420 435 925 630 445 1770 Ti 1785 1770 1805 935 955 Fe 1.33 1.41 1.27 1.06 1.46 0.81 % Ca 1.31 1.03 1.21 2.20 4.28 1.87 K 3.30 3.39 3.33 3.21 2 69 3.81

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Figure 7. Correlation of ash layers in cores from locations shown in Figure 1. southern ash area. Numbers in top row refer to core Pacific are easterly and maintain a high degree presence of ash L near volcanically active of constancy in direction (Riehl, 1954). As a Ecuador and Colombia clearly indicates a vent first approximation, then, the ash areas shown in one of these countries, but the distribution in Figure 1 should lie essentially offshore from of the ash does not show preference. The their respective source areas (Fig. 2). The absence of ash L in cores nearer the Galapagos

TABLE 5. TRACE (PPM) AND MINOR (%) ELEMENT ANALYSES OF ASH SAMPLES IN CORES FROM SOUTHEASTERN PACIFIC

Ash type ? ? ? ? ? ? ? Sample no. 98 100 62 95 96 97 99 Ba 700 735 490 3085 785 665 740 Zr 190 200 95 60 145 85 465 Sr 55 60 280 455 150 265 195 PPm Rb 190 190 30 30 140 75 120 Mn 770 745 1305 415 715 625 795 Ti 1545 1590 7840 2350 1785 2645 4415 Fe 1.15 1.11 6.29 2.65 1.59 2.43 2.93 % Ca 0.56 0.59 4.79 16.03 1.58 5.99 1.50 K 3.86 3.69 1.15 1.24 3.41 2.48 3.61

Islands effectively rules out these islands as an from Boquerón Volcano near San Salvador. alternative source area. Furthermore, the Together these deposits form the impressive Galapagos "pumiceous ash" is of trachytic "Tierra Blanca" of central El Salvador. Con- composition and hence markedly different ceivably, then, Boquerón and (or) vents in the from the rhyolitic, dacitic, and andesitic ashes Lake liopango region are the source of the I of the Central American and Andean volcanoes group of ash layers and probably of the basic (Williams, 1972, written commun.). ash layers found in the northern ash area. The source for the "white" siliceous ash The more basic composition of the J group layers located off Guatemala and southern of ash layers projects their source farther to Mexico seems to be found in the Guatemala the south into Nicaragua or possibly even Highlands where volcanoes have discharged Costa Rica. Nicaragua is a likely candidate. immense volumes of dacitic pumice. As a The volcanoes here consist chiefly of frag- result, "white" deposits of ash and pumice are mental ejecta (Williams, 1952), attesting to a striking feature of the landscape (Williams, their highly explosive history, and most of the 1960). The volcanoes of Mexico are located Quaternary volcanic rocks are iron-rich ande- somewhat farther north and are separated from sitic basalts (McBirney and Williams, 1965). the Guatemalan volcanoes by the barren sec- A number of pumice samples from volcanoes tion of the Isthmus of Tehuantepec. in Guatemala and the Lake Ilopango region, El Outside the Guatemalan Highlands, accord- Salvador, were analyzed for their trace and ing to McBirney (1969), andesites become minor element content. The samples represent progressively more basic in composition until, eruptions of such proportion that they must in Nicaragua, many are indistinguishable from have deposited ash in the adjacent deep-sea basalts. Rhyolites, in turn, become less sili- areas. Two tentative correlations are shown in ceous. This gradation is reflected offshore by Table 6. One sample from the Lake Ilopango the presence of the "white" siliceous ash layers region compares quite favorably with the I off Guatemala and of more basic ash layers to ashes in general, strengthening our opinion the south off El Salvador, Honduras, Nicara- that this region is the source for the I group gua, and Costa Rica. of ashes. The pumice sample agrees best with Pumice samples of Quaternary age are con- the average composition for ash I3. spicuously absent in Honduras (Williams and The average composition for ash D is re- McBirney, 1969). El Salvador, on the other markably close to that for a sample of airborne hand, appears to be the source for the I group pumice from the Tecpán-Chimaltenango basin of ashes in the central area (except perhaps in the Guatemalan Highlands. The sources of those on the Cocos Ridge). The I ash layers, the pumice are vents now represented by a although quite basic in composition, are light cluster of Pelean domes which form the Tecum gray to white in color. Vast deposits of "white" Umán Ridge along and near the continental ash and pumice dating from the Pleistocene divide in Guatemala (Williams, 1972, written are present in the Lake Ilopango region of El commun.). The pumice is overlain by glowing Salvador (Williams and Meyer-Abich, 1955). avalanche deposits dated at 31,000 ± 3,000 yrs Coeval with their deposition and quite similar (Bonis and others, 1966), thereby approximate- in composition was a long series of eruptions ly establishing the time of the explosive volcan-

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TABLE 6. COMPARISON OF TRACE (PPM) AND MINOF. yrs). Neither was found. Thus, the ash layers (%) ELEMENT ANALYSIS OF PUMICE SAMPLE FROM THI: in D-6 and all the ash layers examined in the LAKE ILOPANGO REGION OF EL SALVADOR WITH THI; northern area, with the possible exceptions of AVERAGE COMPOSITION OF ASH TYPE L, AND ANALYSI:. G and H, are younger than 300,000 yrs. As- OF PUMICE SAMPLE FROM TECPAN-CHIMALTENANGO suming that core D-6 stopped just short of BASIN, GUATEMALA, WITH ASH TYPE D 300,000 yrs, then the calculated minimum sedimentation rate of 5.4 cm per 1,000 yrs A VW^fUl Ash type D (Guatemala) gives an extrapolated age of about 190,000 yrs for ash layer D. Because: we cannot precisely Ba 1123 1140 Zr 83 75 date the bottom of core D-6, this age is very Sr 108 110 tentative and probably much too old. Further- ppm Rb 138 150 more, there is evidence that ash D may be Mn 648 605 younger by nearly a factor of four. Ti 895 705 Fe 0.79 0.55 Ash layer L, found in the two Carnegie % Ca 0.64 0.55 Ridge cores mentioned, has been dated at K 3.26 3.31 approximately 220,000 y::s (Burckle, 1972, oral commun.). Again, an element of uncertainty Lake Ilopango exists because this is an extrapolated age based Ash Type I3 (El Salvador) Ba 1740 1410 on the identification of Termination II at Zr 175 165 127,000 yrs in both cores (Broecker and van Sr 235 285 ppm Donk, 1970). However, the ash is found at Rb 53 55 nearly the same depth in both cores, indicating 1400 Mn 1073 that the rates of sedimentation may not have Ti 2693 3485 fluctuated significantly. Hence, we suppose Fe 2.77 2.90 % Ca 1.95 1.97 that 220,000 yrs may be very close to the K 2.18 2.20 actual age for ash L. Accepting 220,000 yrs for ash L results in an extrapolated age of ism. Correspondence of the samples (Table 6) about 54,000 yrs for ash D (V15-30), which strongly indicates that a vent near the Tecpan- seems more reasonable than 190,000 yrs in Chimaltenango basin is the source of ash D. view of the generally shallow stratigraphic However, it will be shown in the following position of ash D in cores from the northern section that 31,000 yrs is probably too recent and central areas. Its unusually deep burial in for ash D. There are, though, numerous exam- cores D-5 and D-7 is explained by the fact ples of volcanoes that have erupted identical that both cores are located in a deep which lavas through time. For example, the Mono apparently has received more sediment after Craters in California, ranging in age from 600 deposition of ash D than nearby areas. to 30,000 yrs, have deposited over 27 indi- The central area ash layers are all obviously vidual lavas, each having the same composition older than ash D. Interestingly, an age of (Jack and Carmichael, 1969). Therefore we 50,000 to 60,000 yrs for ash D places the I suggest that an earlier eruption from a vent in group of ash layers in about the proper time the Tecpan-Chimaltenango area is the source frame for the explosive eruptions of Boqueron of ash D. and the Lake Ilopango vents, estimated to have begun approximately 160,000 yrs ago (Williams AGE OF THE ASH LAYERS and Meyer-Abich, 1955). Tentatively we conclude that most, if riot Two other points ¡should be considered. all, of the ash layers examined are not older First is the possibility that some ash layers in than 300,000 yrs. This conclusion is based the northern area correlate with three dissemi- primarily on a faunal analysis of core D-6 in nated ash zones identified in the Gulf of the northern ash area and total carbonate Mexico (Ewing and others, 1958; Kennett analysis of cores V19-29 and V19-30 from the and others, 1972). The Gulf of Mexico is Carnegie Ridge (Fig. 2) in the southern ash within easy range of the Guatemalan volcanoes area. as shown by a recent eruption of Santa Maria Samples of core D-6 were examined for :he Volcano (Eaton, 1964), and all three ash zones presence of Stylatractus (extinction at 400,000 fit the time frame imposed by the Pacific yrs) and Druppatractus (extinction 300,000 ashes. Presently, we cannot be certain which,

if any, of the northern area ashes correlate Burckle, Lamont-Doherty Geological Observa- with those in the Gulf of Mexico. The tentative tory, for examining the faunal content of core age of 54,000 yrs for ash D, however, is very D-6; William Ruddiman, Troy Holcombe, close to the 60,000 yrs suggested by Ewing and Willard Moore, and Herb Eppert of the Naval others (1958) for the youngest Gulf of Mexico Oceanographic Office (Ocean Floor Analysis ash. The wide distribution of ash D indicates Division) for critically reviewing the manu- a tremendous explosion easily capable of strew- script; and Gloria Garner, Carol Fruik, and ing ash in the Gulf of Mexico (and Caribbean). Barbara Grosvenor for their help. Also, the siliceous character of the youngest ash The National Science Foundation (GA- (I.R. = 1.498; Ewing and others, 1958) in 32445X) contributed substantially to the the Gulf of Mexico tentatively precludes the chemical analysis of the ash layers. Mexican volcanoes as its source. The Quat- ernary and Holocene volcanic activity of REFERENCES CITED Mexico produced mostly cinder cores, lavas, Bonis, S., Bohnenberger, O., Stoiber, R. E., and and pyroclastics of chiefly olivine basaltic com- Decker, R. W., 1966, Age of pumice deposits position (Guzman and Cserna, 1963). in Guatemala: Geol. Soc. America Bull., v. The second consideration pertains to three 77, p. 211-212. ash samples analyzed in this study but previ- Broecker, W. S., and van Donk, J., 1970, In- 18 ously dated by K-Ar analyses (Garlick and solation changes, ice volumes, and the 0 Dymond, 1970). One sample from core V15-28 record in deep-sea cores: Rev. Geophysics (327 cm), which we have identified as ash D, and Space Physics, v. 8, p. 169-198. Carmichael, I.S.E., Hampel, J., and Jack, R. N., is dated at 0.50 + 0.05 m.y. The other two 1968, Analytical data on the U.S.G.S. stan- samples, both identified as ash L, are from dard rocks: Chem. Geology, v. 3, p. 59-64. cores VI5-32 (607 cm) and V15-33 (1,037 cm) Casertano, L., 1963, General characteristics of and dated at 0.85 ± 0.05 and 0.90 ± 0.05 m.y., active Andean volcanoes and a summary of respectively. Based on our evidence, all three their activities during recent centuries: dates are considerably too old. The absence of Seismol. Soc. America Bull., v. 53, p. 1415- the Stylatractus and Druppatractus extinction 1433. levels in core D-6 clearly rules out any date Eaton, G. P., 1964, Windborne volcanic ash: A older than 300,000 yrs for ash D. In the case possible index to polar wandering: Jour. of the two Carnegie Ridge cores containing Geology, v. 72, p. 1-35. ash L, tight control was provided by the good Ewing, W. M., Ericson, D. B., and Heezen, B. C., agreement between the carbonate maxima and 1958, Sediments and topography of the Gulf of Mexico, in Weeks, L. G., ed., Habitat of oil minima identified in the two cores. The dis- —A symposium: Am. Assoc. Petroleum cordance between the faunally determined Geologists, p. 995-1053. dates and the K-Ar dates can most likely be Ewing, W. M., Heezen, B. C., and Ericson, D. B., attributed to contamination of the ash by 1959, Significance of the Worzel deep-sea ash: atmospheric Ar or perhaps incomplete out- Natl. Acad. Sci. Proc., v. 45, p. 355-361. gassing of the lava (Funkhouser and others, Fryxell, R., 1965, Mazama and Glacier Peak 1966). In either case, both cause excess volcanic ash layers—Relative ages: Science, v. radiogenic Ar to be present and therefore 147, p. 1288-1290. yield anomalously old ages. Funkhouser, J. G., Barnes, I. L., and Naughton, J. J., 1966, Problems in the dating of volcanic ACKNOWLEDGMENTS rocks by the potassium-argon method: Bull. Volcano!., v. 29, p. 709-718. Appreciation is extended to Howel Williams, Garlick, G. D„ and Dymond, J. R„ 1970, Oxgyen Department of Geology and Geophysics, Uni- isotope exchange between volcanic materials versity of California, Berkeley, for kindly pro- and ocean water: Geol. Soc. America Bull., v. viding samples of pumice from Central America 81, p. 2137-2142. Guzman, E. J., and Cserna, Z. de, 1963, Tectonic and for critically reviewing the manuscript; history of Mexico, in Childs, O. E., and Beebe, Roy R. Capo, Curator of the Lamont-Doherty B. W., eds., Backbone of the Americas—A Geological Observatory Core Laboratory (sup- symposium: Am. Assoc. Petroleum Geologists ported by Office of Naval Research N-000- Mem. 2, p. 113-129. 14067-A-0108-004 and National Science Foun- Jack, R. N., and Carmichael, I.S.E., 1969, The dation GA-294-60) for furnishing the majority chemical "fingerprinting" of acid volcanic of deep-sea ash samples analyzed; Lloyd rocks: California Div. Mines and Geology,

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Spec. Rept. 100, p. 17-32. Chilean Andes, in Proceedings of the Andesite Kelleher, J. A., 1972, Rupture zones of large South Conference: Internat. Upper Mantle Project American earthquakes and some predictions: Sci. Rept., no. 16, p. 165-174. Jour. Geophys. Research, v. 77, p. 2087-2103. Riehl, H., 1954, Tropical meterology: New York, Kennett, J. P., Huddlestun, P., and Clark, H. C„ McGraw-Hill Book Co., 392 p. 1972, Associations between late Pleistocene van Andel, Tj. H., Heath, G. R., Malfait, B. T., paleo-climatic history, volcanism, paleomag- Heinricks, D. F., and Ewing, J. I., 1971, Tec- netism, and faunal extinctions and reactions, tonics of the Panama tasin, eastern equatorial western Gulf of Mexico [abs.]: EOS (Am. Pacific: Geol. Soc. America Bull., v. 82, p. Geophys. Union Trans.), v. 53, p. 423. 1489-1508. McBirney, A. R., 1969, Compositional variations Williams, H., 1952, The great eruption of Cose- in Cenozoic aclc-alkaline suites of Central quina, Nicaragua, in 1835: California Univ. America, in Proceedings of the Andesite Con- Pubs. Geol. Sci., v. 29, p. 21-46. ference: Internat. Upper Mantle Project Sci. — 1960, Volcanic history of the Guatemalan Rept. no. 16, p. 185-189. Highlands: California Univ. Pubs. Geol. Sci., 1970, Some current aspects of volcanology: v. 38, p. 1-87. Earth-Sci. Review, v. 6, p. 337-352. Williams, H., and McBirney, A. R., 1969, Volcanic McBirney, A. R., and Williams, H., 1965, Volcanic history of Honduras: «California Univ. Pubs. history of Nicaragua: California Univ. Pubs. Geol. Sci., v. 85, p. 101. Geol. Sci., v. 55, p. 1-65. Williams, H., and Meyer-Abich, H., 1955, Vol- Menard, H. W., 1964, Marine geology of the canism in the southern part of El Salvador: Pacific: New York, McGraw-Hill Book Co., California Univ. Pubs. Geol. Sci., v. 32, p. 271 p. 1-64. Moore, T. C., 1969, Abyssal hills in the centra!, Worzel, J. L., 1959, Extensive deep-sea subbottom equatorial Pacific: Sedimentation and stra- reflections identified as white ash: Natl. Acad. tigraphy: Deep-Sea Research, v. 17, p. 573— Sci. Proc., v. 45, p. 349-355. 593. Nayudu, Y. R., 1964, Volcanic ash deposits in the Gulf of Alaska and problems of correlation of deep-sea ash deposits: Marine Geology, v. MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEM- 1, p. 194-212. BER 19, 1972 Pichler, H., and Zeil, W., 1969, Andesites of the REVISED MANUSCRIPT RECEIVED JANUARY 1, 1973

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