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Pleistocene volcanic eruptions in New Zealand recorded in deep-sea sediments
EARTH AND PLANETARY SCIENCE LETTERS 4 (1968) 89-102. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM
P L E I S T O C E N E V O L C A N I C E R U P T I O N S IN NEW Z E A L A N D R E C O R D E D IN D E E P - S E A S E D I M E N T S * Dragoslav NINKOVICH Lament Geological Observatory, Columbia University, Palisades, New York, USA Received 4 April 1968
Five layers of rhyolitic ash have been identified in deep-sea cores, taken within about 1000 km east of New Zealand. Paleomagnetie stratigraphy has been established in the cores and used for dating of the ash layers. The earliest sediment penetrated is about 3 m.y. old, and the ages of the ash layers are as follows: 0.86; 0.73; 0.67; 0.31, and 0.27 m.y.B.P. The ash layers have the same age range as the New Zealand ignimbrites, suggesting a significant ash fall phase, until now little known, associated with eruptions of ignimbrites.
1. INTRODUCTION Sheets of welded rhyolitic tufts, named ignimbrites by Marshall [1], occupy the central part of the North Island of New Zealand. They cover an area of about 2500 km2 reaching a thickness of over I000 m [2]. According to Grange [3], the New Zealand ignimbrites originated in a series of eruptions, apparently of PlioPleistocene age, which created the calderas or volcanotectonic depressions of Lakes Taupe and Rotorua and probably others now buried by the ignimbrites. Recent measurements of K/A [4] and remnant magnetization [5] have shown that some of the Taupo-Rotorua ignimbrites erupted during a period from about 0.8 to 0.2 m.y.B.P., i.e., from upper Matuyama through Brunhes epochs of geomagnetic polarity. Sheets of ignimbrites probably originated in voluminous clouds of volcanic gas and vapors, overcharged with rhyolitic tephra which rushed down slopes of volcanoes [3, 6, 7]. The only evidence of an airborne ashfall phase is occurrence of small deposits of pumice [8, 22]. The present paper concerns a study of deep-sea cores taken to the east and northeast of New Zealand where five layers of rhyolitic ash occur within a radi* Lament Geological Observatory Contribution No. 1178.
us of more than 1000 km from North Island. The age of the ash layers has been inferred from a paleomagnetic stratigraphy of the cores and shows that the ashfalls have the same range as the ignimbrite sheets. The occurrence of the layers of windborne rhyolitic ash of similar age as the ignimbrite sheets is further evidence for extensive airborne ash production during the emplacement of the ignimbrite sheets.
2. DEEP-SEA CORES FROM THE NEW ZEALAND KERMADEC AND TONGA REGION Cores in this study were taken by Lament Vessels VEMA and CONRAD (fig. 1 and table 1). 2.1. Description o f cores Cores taken to the west of New Zealand (V18-227, 228 and 229) are composed entirely of foraminiferal ooze. Cores taken to the east and northeast of New Zealand are either foraminiferal ooze or light brown hitite with mineral sand and volcanic ash. Thus, cores taken in the area of the Chatham Islands are composed of foraminiferal ooze (RC9-107, I 10; also RC9111 and 112) and foraminiferal with fine mineral sand dispersed through the sediment (RC-108). Most of the cores taken from the Tonga Trench (V18-236 to
90
D, NINKOVICH /70 =
170 a
I80"
/60 °
r~/, Is. " ~ ' " Ton g o
20".
ls
~245
; "';
20 °
~b~ 2044
240~
Ca/edon/o
•
119 118
25 °
239 ~1~ 238
K E Y X • • • Q
30 °
V16 CRUISE V18 CRUISE RC8 CRUISE RC9 CRUISE CORESWITH RHYOLITIC ASH LAYER
25*
117 •
116 4'
236,237
Kermadec Is.
30 °
115
®
CORES WITH BROWN ASH LAYER 114
® 35 °
35 °
113
® 231
112
227 °
IJl
40 °
40 °
U
" I10
;"
1~4
Chatham 15
107
"8
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45 =
108
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j,
x
•
®
125
®
109 •
45 °
g
®. 80
50 •
50"
126 X 170°
/80°
170 °
160"
Fig. 1. Distribution of deep-sea cores from the area of New Zealand, Kermadec and Tonga Islands in the Lamont Geological Obser. vatoty collection.
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
91
Table 1 Location of deep-sea cores containing volcanic ash layers, shown in fig. 1. Core
Latitude S
Longitude
Depth (m)
Length (cm)
Number of ash layers colorless
v16-125 RC9-108 RC9-110 RC9-113 RC9-114 RC9-115 V18-231 V18-232 V18-234 V18-240 V18-245 V18-247
47o01 ' 45045.4 , 42052 ' 36044.2 ' 33041.4 , 31023 ' 38049 ' 36045 ' 30056 ' 22017.5 ' 20031 ' 19000 '
179o15'w 177o22.4'W 172o00.7'W 167o02.9'W 165o02.8'W 163o43.1'W 179o10'E 179o36'W 176o33'W 174o01'W 172o15'W 170o18'W
2953 4314 1917 4751 5453 5376 3530 4702 6943 8633 5662 4713
239) are composed o f coarse mineral sand, all the other cores shown in fig. 1 are composed o f light brown lutite with a small amount of carbonate (up to 5%). 2.2. Volcanic ash Most o f the cores taken to the east o f New Zealand, Kermadec and Tonga Islands contain volcanic ash dispersed throughout the sediments, sometimes forming distinct layers. Two different types o f volcanic ash have been identified: brown ash and colorless ash (fig. 2). Brown ash: Distribution o f brown ash is limited to the area near the Tonga and Kermadec Islands (fig. 1). It is dispersed throughout the sediments and forms
435
430
425
420ems RC 9 114
I00
95
90
85 cm$
V18 245
Fig. 2. Photograph of rhyolitic and brown ash in deep-sea cores east of New Zealand, Kermadec and Tonga Islands.
brown
896 1219 743 482 1032 1074 281 368 430 180 299 412
distinct layers in cores V18-234, 2 4 0 , 2 4 5 and 247 (table 2). Core V18-234 taken to the southeast of the Kermadec Islands marks the southernmost limit o f the brown ash. Colorless ash: Cores taken to the east and northeast o f New Zealand and south o f 300 south latitude contain only colorless ash (with the exception o f core V18-234, which contains b o t h brown and colorless ash). Thus, the distribution pattern suggests that the brown ash originated from volcanoes on Kermadec and Tonga Islands, and the colorless ash from eruptions on New Zealand. The origin o f a single layer of colorless ash in cores V18-240 and 245, however, is uncertain. Rhyolitic glass shards dispersed throughout the sediments and in small pockets were first recognized in the area to the east o f New Zealand in studies of b o t t o m samples and 2 to 2.5 m cores from the Chatham Rise [9, 10]. The index of refraction o f volcanic glass shards in these samples ranges from 1.498-1.504 and is similar to that o f Late Quaternary pumice deposits in the North Island o f New Zealand [11 ]. Cores examined in the present study, however, are longer and have a wider distribution. Rhyolitic ash in these cores occurs in distinct layers, up to 13 cm thick (core V16-125). Depth in deep-sea cores o f rhyolitic ash layers is shown in table 3 and fig. 3. A m a x i m u m o f five ash layers has been identified (core RC9-1 13). The ash is composed o f volcanic glass shards and minerals o f which the proportions vary throughout the layers. An average sample o f ash is composed o f about
92
D. NINKOVICH
Table 2 Volcanic ash layers in deep-sea cores from the Tonga-Kermadec region.
with the same color and cores containing only one ash layer of that color (table 3).
Depth(cm) ofashlayersin cores V18-247
Brown ash
V18-245
V18-240
V18-234
21-22 90-95
4-8 34-36 50-51
21-23 140-142
124-125
80-82
330-331
300
180
412
9- 10 30- 31 39- 40 45- 46 48- 49 90- 92 115 -120
Color~ss ash Bottom of cores
400
95% volcanic glass shards and 5% minerals. Volcanic glass shards from all samples of colorless or rhyolitic ash layers in the present study have an identical index of refraction (1.498-1.500), similar shape (fig. 4) and can be separated from heavy minerals on a Frantz isodynamic separator as a non-magnetic fraction at 1.4 amps. Three different colors of pure, rhyolitic, volcanic glass shards can be distinguished in different samples of ash: very light gray (NS); pinkish gray (SYRS/ 1), and very pale orange (10YRS/2) (after Rock Color Chart, Geol. Soc. Am., 1963). Pairs of ash layers with similar color of volcanic glass have been found in cores RC9-113 and 114. Therefore, a correlation of rhyolitic ash layers based on color of volcanic glass is difficult between cores containing pairs o f ash layers
2.3. Size analysis of rhyolitic ash Data on the size analysis are presented in table 4. The coarsest ash was found in core V18-231, taken near the eastern coast o f North Island. The maximum grain size of the ash in this core is about 0.35 mm. According to experiments [12], only shards o f vesicular acidic volcanic glass with a size less than about 0.35 m m can settle to the bottom; while the shards coarser than this size remain on the surface of the sea as floating pumice. Thus, the presence o f the maximum possible grain size of the glass shards in core V18-231 indicates that the core was taken near the source of the ash. In other cores, taken farther from North Island, the ash is finer; with a maximum grain size of 0.25 mm or less.
3. PALEOMAGNETIC STRATIGRAPHY OF DEEPSEA CORES Measurements of the orientation of the magnetic vector in radiometrically dated sub-aerial lava flows permitted Cox et al. [13] to determine intervals of normal and reversed geomagnetic polarity during the last 4 m.y. A suggested paleomagnetic stratigraphy for the last 3 m.y. is shown on the left-hand side of fig. 3. Thus, the present polarity of the geomagnetic field has lasted since 0.70 m.y.B.P. [14] and is called
Table 3 Color of volcanic glass shards in rhyolitic ash layers from deep-sea cores east of New Zealand. Color of volcanic glass shards
RC9-113
RC9-114
Very light gray (NS)
142-145
364-368
Pinkish gray (5 YR 8/1)
175-185 370-371
425-430 831-832
Very pale orange (10YR 8•2)
404-405 471-479
Depth (cm) of rhyolitic ash layers in cores RC9-115 V18-234 V18-231 RC9-108 208-214
139-141 378-383
330-331
610-612
V16-125
RC9-110
373-386
385-390 556-559 673-684
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
M°Y.
RC9 r17
RC9 I19
VI8 2:34
RC9 115
RC9 113
RC9 I14
93
V2"~' 18I
RC9 I08
METERS 0
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ASH LAYERS RHYOLITIC • BROWN
10
!1
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12
Fig. 3. Graphic magnetic polarity logs for deep-sea cores east and northeast of New Zealand showing location of rhyolitic ash layers.
the Brunhes epoch of normal polarity. Between 0.70 and about 2.5 m.y.B.P., the earth's magnetic field was reversed most of the time and is called the Matuyama reversed polarity epoch. Two short events of normal polarity occurred during the Matuyama epoch; these are the JaramiUo (0.926 to 0.870 +-0.014 m.y. B.P.; ref. [15]), and the Olduvai (about 1.8 m.y.B.P.). The Matuyama epoch was preceded by the Gauss normal polarity epoch. A paleomagnetic stratigraphy of some of the deepsea cores in this study was shown in fig. 3, and tables 5, 6 and 7. Samples of sediments approximately 8 cm3 in size, with vertical orientation in the cores, were cut at 10 to 20 cm intervals throughout the length of each core. After a partial demagnetization
in an alternating field of 100 to 150 oersteds, the samples were measured on a spinner magnetometer described by Foster [ 16]. Cores composed of lutite and carbonate sediment with a high percentage of fine mineral sand (RC9-108) showed a sufficiently strong magnetic intensity for measurement. The magnetic intensity in pure carbonate sediment (cores RC9-110 and V16-125), however, was too feeble to be measured. 3.1. Cores containing brown ash All samples of sediments from cores containing brown ash (V18-234,240, 245 and 247) (table 2) showed a remnant magnetization of normal polarity indicating that the sediments were deposited during
94
D. NINKOVICH
Fig. 4. Microphotograph of grain size fraction 0.177-0.088 mm of rhyolitic glass shards from five ash layers in core RC9-113.
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
95
Table 4 Grain size analysis of rhyolitic volcanic ash layers in deep-sea sediments east and n o r ~ e a s t o f New Zealand. Ash layer
Core
Depth of ash layer (cm)
A 0.85 m.yo
RC9-113 RC9-115
471-479 378-383
0.73 m.y.
RC9-113
404-405
C 0.67 m.y.
RC9-113 RC9-114
370-371 831-832
D 0.31 m.y.
RC9-113 RC9-114 RC9-115
175-185 425-430 139-141
E 0.27 m.y.
V18-231 V16-125 RC9-108 RC9-113 RC9-114
208-214 373-386 610-612 142-153 364-368
350
250
Percentage coarser than size (#) 177 125 88
62
44
0.2 1.7
7.9 11.6
17.8 27.q
24.0 40.3
27.9 49.6
2.3
6.4
15.2
29.8
40.1
0.1
1.9 1.7
7.3 6.1
20,5 20.9
32.9 34.2
0.2 0.1 0.7
9.3 8.5 6.7
23.9 22.8 15.4
36.7 38.5 28.8
44.2 46.2 40.5
12.5 5.0 2.7 0.3 0.2
24.6 17.9 9.9 7.0 2.0
37.2 31.6 23.1 16.8 10.1
47.0 41.1 35.4 26.9 20.9
52.5 47.5 42.0 34.6 31.6
B
0.4
0.5
5.1 0.7
Table 5 Apparent age of rhyolitic ash layers in core RC9-113 inferred from magnetic data. Paleomagnetic stratigraphy Epoch
Event
Sediment Thickness (cm) 0-142 153- 175
Jaramillo
Volcanic ash layers
Rates of deposition Duration of interval (cm/1000y) (m.y.) 0.52 0.27 0.52 0.04
185-370 371-386
0.52 0.52
0.36 0.03
386-404 405-471
0.52 0.52
0.03 0.13
479 -482
0.52
0.005
Thickness (cm)
Apparent age (m.y.)
142-153 (E) 175-185 (D) 370-371 (C) (Reversal)
0.27 0.31 0.67 0.70
404-405 (B) 471-479 (A) (Reversal)
0.73 0.86
Table 6 Apparent age of rhyolitic ash layers in core RC9-114 inferred from magnetic data. Paleomagnetic stratigraphy Epoch
Brunhes
Matuyama
Sediment Thickness (cm) 0-364 368-425 430-831 832-873 873-1030 Bottom of core
Volcanic ash layers
Rates of deposition (cm/1000y)
Duration of interval (m.y.)
1.23 1.23 1.23 1.23
0.29 0.05 0.33 0.03
1.23
0.13
Thickness (cm)
Apparent age (m.y.)
364-368 (E) 425-430 (D) 831-832 (C) (Reversal)
0.29 0.34 0.67 0.70
96
D. NINKOVICH Table 7 Apparent age of rhyolitic ash layers in core RC9-115 inferred from magnetic data. Paleomagnetic stratigraphy
Epoch
Event
Brunhes Jaramillo Matuyama Olduvai Gauss
Volcanic ash layer
Sediment Thickness (cm) Rates of deposition (cm/1000y)
Durationof interval (m.y.)
0-139 141-314
0.45 0.45
0.31 0.39
314-378 383-418 418-644 644-690 690-907
0.45 0.70 0.28 0.30 0.33
0.14 0.05 0.80 0.15 0.65
907-1060 Bottom of core
0.30
0.50
the Brunhes epoch; therefore, the bottom of these cores is less than 0.7 m.y. old. Core V18-247 is 400 cm long and the rate of deposition is greater than 0.6 cm/1000 y. The earliest layer of brown ash at 115-120 cm in this core must be therefore less than 0.2 m.y. old. Core V18-245 is 300 cm long. The sediment was deposited with a rate greater than 0.45 cm[1000 y. The earliest brown ash at 90-95 cm is less than 0.2 m.y. old. Core I,'18-240 is 180 cm long. A rate of deposition greater than 0.25 cm indicates that the lowest brown ash in this core, too, is less than 0.2 m.y. old. Core 1118-234 contains a layer of rhyolitic ash at 330-331 cm which can be correlated with a similar layer from cores RC9-113, 114 and 115 (fig. 3), and which has an apparent age of 0.31 m.y. (tables 5, 6 and 7). Therefore, the age of the two layers of brown ash at 21-23 cm and 140-142 cm in this core is 30000 and 130000 y, respectively. Thus, the eruptions in the Tonga and Kermadec Islands, whose magnitude was sufficient to produce distinct layers of brown ash in deep-sea sediments, apparently occurred in the last 200000 y and were preceded by small eruptions indicated by scattered brown ash throughout the sediment for a period of unknown length. 3.2. Cores containing rhyolitic ash Measurement of remnant magnetization has shown that only three cores containing rhyolitic ash layers
Thickness (cm) Apparent age (m.y.) 139-141 (D) (Reversal) 378-383 (A) (Reversal) (Reversal) (Reversal) (Reversal) (Reversal)
0.31 0.70 0.85 0.90 1.70 1.85 2.50
penetrated through the 0.7 m.y. reversal. These are cores RC9-113, 114 and 115 (fig. 3). Cores V18-231, V18-234 and RC9-108 failed to penetrate the 0.7 m.y. reversal. Cores RC9-117 and 119 are also presented in fig. 3 in order to show that they penetrate the sediment in which the rhyolitic ash layers occur farther south. Absence of ash layers from these two cores indicates that the northern- and easternmost limit of the distribution of the New Zealand rhyolitic ash is about 300 south. 3.3. Dating of rhyolitic ash layers Core RC9-113: Sediment between the top and 386 cm is normally magnetized (fig. 3, table 5). Between 386 cm and the ash layer at 471-479 cm the remnant magnetization of sediment is reversed. A short section of sediment at the bottom of the core below the ash layer is normally magnetized. The paleomagnetic stratigraphy thus established suggests that the core penetrated to the middle of the JaramiUo event. The calculated rate of deposition is 0.52 cm]1000 y. Five ash layers occur in this core: three in sediment of the Brunhes epoch and two in sediment of the Matuyama epoch. The lowest ash occurs at the top of the JaramiUo event precisely at the point of reversal. The apparent ages of the five ash layers inferred from the rate of deposition are as follows: A = 0.86;B = 0.73; C= 0.67;D = 0.31 ; and E ---0.27 m.y.B.P. (table 5). Core RC9-114 penetrated to the sediment of the Matuyama series but did not reach the Jaramillo event The calculated rate of deposition is 1.23 cm/1000 y.
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
The bottom of the core is about 0.83 m.y. old. Three ash layers occur in this core, all in sediment of the Brunhes epoch. Their apparent ages of 0.67, 0.34 and 0.2o m.y. (table 6) are similar to the ages of layers C, D and E from core RC9-113 (table 5, fig. 3). Layer B l'rom core RC9-113 apparently does not occur in core RCO-114 and the top of Jaramillo where layer A occurs in core RC9-113 was not reached by core RC9114. Core RC9-115 has the longest paleomagnetic stratigraphy of any core in this study. The bottom of the core apparently ends in sediment deposited during the Gauss epoch, about 3 m.y.B.P. (fig. 3). Rate of deposition of sediment above the Jaramillo event was 0.45 cm/1000 y and below this event 0.3 cm/1000 y (table 7). Two ash layers occur in this core: one in sediment of the Brunhes series, and one at the top of the JaramiUo event (as in core RC9-113, this ash layer occurs precisely at the point of reversal). The apparent ages of two ash layers in core RC9-115 are 0.85 and 0.31 m.y.B.P. (table 7). The following three cores, containing one ash layer each, failed to penetrate the 0.7 m.y. reversal (fig. 3). CoreRC9-108 is 1219 cm long. The rate of deposition in this core, therefore, must be greater than 1.75 cm/1000 y and the ash layer at 610-612 cm, less than 0.35 m.y. old. Core V18-231 is 281 cm long. Rate of deposition in this core is greater than 0.4 cm/1000 y and the layer of ash at 208-214 cm is less than 0.53 m.y. old. Core V18-234 is 412 err. long. Rate of deposition is greater than 0.6 cm/1000 y and the ash layer at 330-331 cm is less than 0.56 m.y. old. Thus, in all three cores, RC9-108, V18-231 and V18-234 which fail to penetrate the 0.7 m.y. reversal contain an ash layer with an apparent age younger than the layer C (0.67 m.y.) from core RC9-113.
4. CORRELATION OF RHYOLITIC ASH LAYERS Both color of volcanic glass shaids (tables 3 and 8) and paleomagnetic stratigraphy (fig. 3) have been used for purposes of correlation of rhyolitic ash layers. In core RC9-114, all three ash layers occur in sediment of the Brunhes epoch. In the upper layer (0.29 m.y.) the volcanic glass shards are very light gray
97
Table 8 Physical properties of rhyolitic volcanic glass shards in deepsea sediments east and northeast of New Zealand. Ash layer
Index of refraction (n)
Color
E 0.27 m.y.
1.498-1.500
Very light gray (N8)
D 0.31 m.y.
12498-1 ~500
Pinkish gray (5YR8/1)
C 0.67 m.y.
1.498-1.500
Pinkish gray (5YR8/1)
B
1.498-1.500
Very pale orange
A 0.86 m.y.
1.498-1.500
Very pale orange (10YR8/2)
0.73 m.y.
(10YR8/2)
(N8), and in both middle layer (0.34 m.y.) and lower layer (0.67 m.y.) the shards are pinkish gray (5YR8/I) (table 3). The layers correlate in age and color of volcanic glass shards with layers E, D and C respectively of the Brunhes sediment from core RC9-113. Core RC9-115 contains only one layer of ash in sediment of the Brunhes epoch. The apparent age of the layer is 0.31 m.y. and the color of the glass shards is pinkish gray (5YR8/1) (table 3). The ash therefore correlates with ash layer D from core RC9-113. The lower ash layer from core RC9-115 correlates in both age and color with layer A from core RC9-113. In core V18-234, which ends in sediment of the Brunhes epoch, volcanic glass shards in the ash layer are pinkish gray (5YR8/1) and similar to these from layers C (0.67 m.y.) and D (0.31 m.y.) from core RC9-113 (table 3). It has been shown, however, that the layer in core V18-234 is probably younger than 0.56 m.y. It therefore would correlate with layer D from core RC9-113. In cores V18-231 and RC9-108, both of which end in sediment of the Brunhes epoch (fig. 3), volcanic glass shards in the ash layer are very light gray (N8). The layer may be correlated with the layer E (0.27 m.y.) from core RC9-113. A similar ash layer has been found at 373-386 cm in carbonate core V16-125 Carbonate core RC9-110 contains three layers of rhyolitic ash; all three are composed of very light gray (N8) volcanic glass shards, similar to those from layer E (core RC9-113) (table 3). Thus, RC9-110 is the only core studied in which the ash layers cannot be correlated with layers from other cores, either on the basis
98
D. NINKOVICH
of color of glass shards or remnant magnetization of sediment, which is too feeble to be measured. 4.1. Wind directions Layer E (0.27 m.y.) occurs in cores RC9-113, RC9-114, V18-231, RC9-108 and V16-125 (figs. 1 and 3). It is, however, absent from northernmost cores V18-234 and RC9-115. The predominant components of the wind which carried the ash E were westerly and northwesterly from the assumed origin. Layer D (0.31 m.y.) occurs in cores RC9-113, RC9-114 and in the northernmost cores V18-234 and RC9-115. It is absent from southern cores RC9-I08 and V16-125. The predominant components of the winds were westerly and southwesterly. Layer C(0.67 m.y.) occurs in cores RC9-113 and RC9-114. The ash appears to have been carried by winds with a similar direction to those that transported ash from layer E. Layer B (0.71 m.y.) was found only in core RC9113. Layer A (0.86 m.y.) reached by cores RC9-113 and RC9-114 was probably deposited by winds similar to those which carried the ash D. Healy et al. [17] made a detailed and valuable study of tephra from Late Quaternary soils of the North Island. They correlated tephra layers, made isopach maps, and concluded that the tephra originated in pumice eruptions which have occurred in about the last 15 000 y. The isopach maps show that the ash was carried by westerly, northwesterly and southwesterly winds. Thus, the directions of winds in the belt which overlaps New Zealand appear to have been unchanged in the last 0.9 m.y.
5. RATES OF DEPOSITION IN DEEP-SEA CORES Rates of deposition were first established on the basis of paleomagnetic stratigraphy for cores which penetrated the 0.7 m.y. reversal (RC9-113, 114, 115, 117 and 119). In cores which failed to penetrate the reversal or in which the remnant magnetization was too feeble to be measured, the rates of sedimentation were established by correlation of rhyolitic ash layers. The diagram in fig. 5 represents depth in cores of magnetic reversals (solid points) and rhyolitic ash layers (open squares) versus paleomagnetic stratigraphy
Table 9 Rates of deposition of deep-sea sediments, inferred frompaleomagnetic data and correlation of ash layers, shown in fig. 5. Core
Rates of deposition (cm/1000 y)
RC9-117 RC9-115 RC9-113 V18-231 RC9-119 V18-234 RC9 - 114 V16-125 RC9-108
0.33 and 0.54 0.45 and 0.30 0.52 0.77 0.97 1.06 1.23 1.38 2.25
and an inferred age of five ash layers tentatively named A, B, C, D and E. Calculated rates of deposition in the cores are shown in table 9.
6. ORIGIN OF RHYOLITIC ASH LAYERS The rhyolitic ash layers have a wide distribution covering a large area of the bottom of the sea to the east and northeast of New Zealand. The distribution pattern, index of refraction of volcanic glass shards, texture and age of the ash layers suggest an origin in Pleistocene rhyolitic eruptions on North Island.
6. I. Structure and volcanicity o f New Zealand Volcanicity on North Island has been discussed by Grange [3]; Kear [18]; Grindley [19];Grindley et al. [20]; Healy et al. [17, 21]; Thompson et al. [22] and others. Data summarized by Grindley and Harrington [23] on volcanism and structure of both South and North Islands are reproduced, with some modifications, in fig. 6 of the present paper. Two regions of Late Tertiary and Quaternary volcanism in New Zealand are separated by a non-volcanic region. On South Island the volcanoes are basaltic with trachytes and phonolites. Activity lasted from late Miocene to early Quaternary. No pumice eruptions have been recognized on South Island [18]. The volcanic activity on North Island has been related to two different belts: one, striking northwest from the central part of the island toward New Caledonia, and the other, striking northeast across the central part of the North Island, aligned with the Ker-
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
99
3.0
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0.8
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200
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400
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600
700
800
900
I000
I100
1200
(CMS)
Fig. 5. Depth versus age of magnetic polarity reversals and rhyolitic ash layers in deep-sea cores. The solid points represent points of magnetic reversals and the open squares, rhyolitic ash layers.
100
D. NINKOVICH
-34" I
I
I
170 °
I
~
I
174e
/F LEGEND
m
eo.~,i~ m
/
x.d..i~c ITTTI~olltl.
/
/
/
,/
o
/
/
/
.
\ C f
G X.
o /,
O
~
o
$0
aoo •
2S t
~
Scael
o I
o
174t
I
I
~o J
,, o
I00 I
--j j km
ISO J milts loo
I70Q
j
I
Fig. 6. Structure and volcanicity of New Zealand during Quaternary (after Grindley and Harrington, 1961, modified). Sheets of ignimbrites in the central part of the North Island originate in eruptions which occurred between 0.8 and 0.2 m.y.B.P. (refs. [4, 5]). They correlate in age with rhyolitic ash layers in deep-sea sediments (figs. 1 and 2).
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND madec-Tonga volcanic arc (figs. 1 and 6). The northwest belt is divided into different structural and petrographic zones, characterized by eruptions of andesites and basalts. The andesitic volcanic activity occurred during Miocene and Pliocene time with occasional eruptions of pumice. These eruptions which may have caused some smaller concentrations of ash in deep-sea sediments are apparently older than sediments reached by the studied cores. The basaltic activity has continued to the present. Volcanic activity in the central part of the North Island originated in an uplift of the northeast belt which started in Miocene and which culminated in eruptions of Taupo-Rotorua ignimbrites (fig. 4), 0.8 -0.2 m.y. ago [4, 5]. The rhyolitic ash layers in deep-sea cores (figs. 1 and 3) correlate in age with these ignimbrites. The oldest identified post-ignimbrite deposits of rhyolitic tephra are considered to be approximately 40000 years old [24]; the eruptions have lasted until the present. According to the research by Healy et al. [ 17], an average thickness o f the ash layers on the eastern coast of North Island, about 100-150 km from the source, is 1-2 cm, and maximum 15 cm. These post-ignimbrite, late Quaternary eruptions have produced only small concentrations of ash identified in b o t t o m samples and 2-2.5 m cores from Chatham Rise, about 400 km from the source [9,10], but have not been recorded in the co~es of the present study.
7. CONCLUSIONS Five distinct layers o f windborne rhyolitic ash have been identified in deep-sea cores east of New Zealand. Paleomagnetic stratigraphy has been established in the cores and used for both correlation and dating o f the ash layers. The earliest sediment penetrated is about 3 m.y. old and the apparent ages o f the ash layers, tentatively named A, B, C, D andE, are: 0.86; 0.73; 0.67; 0.31, and 0.27 m.y.B.P., respectively. Distribution pattern suggests that the ash falls originated in eruptions in New Zealand. Late Tertiary and Quaternary history ofvolcanicity in New Zealand shows that the eruptions culminated in emplacement o f sheets of welded rhyolitic tufts or ignimbrites in the Taupo-Rotorua region o f the North Island. The ash layers in deep-sea cores have
101
the same age range as the ignimbrites [4, 5] suggesting a significant ash-fall phase, until now little known, associated with the emplacement of ignimbrites.
ACKNOWLEDGEMENTS Professor Morice Ewing, Director of Lament Geelogical Observatory, provided the cores and offered encouragement. Paleomagnetic measurements of the cores were made in association with Dr. N. Opdyke. Drs. J. J. Stipp and A. Cox offered their unpublished data on K/A and paleomagnetic measurements of New Zealand ignimbrites, respectively. Drs. W. S. Broecker, A. Ewart, G. W. Grindley, J. D. Hays, J. Healy and B. C. Heezen offered valuable suggestions and helped in redaction of the manuscript. This study was supported by the U.S. Navy and the National Science Foundation under Grants G.A.1193 and G.A.-824.
REFERENCES [ 1] P. Marshall, Acid rocks of the Taupo-Rotorua volcanic district, Trans. Roy. Soc. New Zealand 64 (1935) 323. [2] R.C. Martin, Stratigraphy and structural outline of the Taupe volcanic zone, New Zealand J. Geol. Geophys. 4 (1961) 449. [3] L.I. Grange, The geology of the Rotorua-Taupo subdivision, New Zealand Geol. Sure., Bull. 37 (N.5) (1937) 138. [4] J.J. Stipp, K/A Ages of the Coromandel and Central Volcanic Region, North Island, New Zealand ( in preparation); K/A History of the Western Volcanics, North Island, New Zealand (in preparation). [5] A. Cox, Paleomagnetic Stratigraphy of the New Zealand ignimbrites (in preparation). [6] R.L. Smith, Ash flows, Bull. Geol. Soc. Am. 71 (1960) 795. [7] A. Steiner, Origin of ignimbrites of the North Island, New Zealand: A new petrogenetic concept, New Zealand Geol. Sure., Bull. 68 (1960) 72. [ 8] A. Ewart, Mineralogy and petrogenesis of the Whakamaru ignimbrite in the Maraetai areas of the Taupe volcanic zone, New Zealand, New Zealand J. Geol. Geophys. 8,4 (1965) 611. [9] J.J. Reed and N. deB. Hornibrook, Sediments from the Chatham Rise, New Zealand J. Sci. Tech. B. 34 (3) (1952) 173. [10] R.M. Norris, Sediments of Chatham Rise, New Zealand Dep. Sci. Indust. Res. Bull. 159 (1964) 1.
102 [ 11 ]
D. NINKOVICH
A. Ewart, Petrology and petrogenesis of the Quaternary pumice ash in the Taupo area, New Zealand, J. Petrol. 4 (1963) 392. [12] R.V. Fisher, Settling velocity of glass shards, Deep-sea Res. 12 (1965) 345. [13] A. Cox, R. R. Doell and G. B. Dalrymple, Geomagnetic polarity epochs: Sierra Nevada II, Science 142, 3590 (1963) 382. [14] R.R. Doell and G. B. Dalrymple, Geomagnetic polarity epochs: A new polarity event and the age of the Brunhes-Matuyama boundary, Science 152 (1966) 1060. [15 ] N. Opdyke, The Jaramillo event as recorded in oceanic cores, N.A.T.O. conference on Internal Constitution of Earth and Planets, ed. S. K. Runcorn (Wiley, London), in press. [ 16 ] J.H. Foster, A paleomagnetic spinner magnetometer using a fluxgate gradiometer, Earth Planet. Sci. Letters 1, 6 (1966) 463. [17] J. Healy, C. B. Vucetich and W. A. Pullar, Stratigraphy and chronology of Late Quaternary volcanic ash in Taupo, Rotorua and Gisborn districts, New Zealand Geol. Surv., Bull. 73 (1964) 1.
[18] [ 19]
[20]
[21]
[22]
[23]
[24]
D. Kear, Pumice chronology in New Zealand, New Zealand J. Sci. Tech. 38B (1957) 862. G. W, Grindley, Geological map of New Zealand, 1 : 250000, Sheet 8-Taupo, New Zealand Geol. Surv., Dept. of Sci. and Industr. Res. (1960). G.W. Grindley, H. J. Harrington and B. L. Wood, The geological map of New Zealand, 1 : 2 000 000, New Zealand Geol. Surv., Bull. 66 (1961). J. Healey, J. C. Schofield and B. N. Thompson, Geological map of New Zealand, 1 : 250000, Sheet 5-Rotorua, New Zealand Geol. Surv., Dept. of Sci. and Industr. Res. (1964). B.N. Thompson, L. O. Kermode and A. Ewart (editors), New Zealand volcanology, central volcanic region, New Zealand Dep. Sci. Industr. Res. Inf. Ser. N. 50 (1966) 1. G.W. Grindley and H. J. Harrington, Late Tertiary and Quaternary volcanicity and structure in New Zealand (abstract). The Proceedings of the 9th Pacific SCI. Congress, 1957, 12 (1961) 198. A. Ewart and J. Healy, personal communication, 1967.
P L E I S T O C E N E V O L C A N I C E R U P T I O N S IN NEW Z E A L A N D R E C O R D E D IN D E E P - S E A S E D I M E N T S * Dragoslav NINKOVICH Lament Geological Observatory, Columbia University, Palisades, New York, USA Received 4 April 1968
Five layers of rhyolitic ash have been identified in deep-sea cores, taken within about 1000 km east of New Zealand. Paleomagnetie stratigraphy has been established in the cores and used for dating of the ash layers. The earliest sediment penetrated is about 3 m.y. old, and the ages of the ash layers are as follows: 0.86; 0.73; 0.67; 0.31, and 0.27 m.y.B.P. The ash layers have the same age range as the New Zealand ignimbrites, suggesting a significant ash fall phase, until now little known, associated with eruptions of ignimbrites.
1. INTRODUCTION Sheets of welded rhyolitic tufts, named ignimbrites by Marshall [1], occupy the central part of the North Island of New Zealand. They cover an area of about 2500 km2 reaching a thickness of over I000 m [2]. According to Grange [3], the New Zealand ignimbrites originated in a series of eruptions, apparently of PlioPleistocene age, which created the calderas or volcanotectonic depressions of Lakes Taupe and Rotorua and probably others now buried by the ignimbrites. Recent measurements of K/A [4] and remnant magnetization [5] have shown that some of the Taupo-Rotorua ignimbrites erupted during a period from about 0.8 to 0.2 m.y.B.P., i.e., from upper Matuyama through Brunhes epochs of geomagnetic polarity. Sheets of ignimbrites probably originated in voluminous clouds of volcanic gas and vapors, overcharged with rhyolitic tephra which rushed down slopes of volcanoes [3, 6, 7]. The only evidence of an airborne ashfall phase is occurrence of small deposits of pumice [8, 22]. The present paper concerns a study of deep-sea cores taken to the east and northeast of New Zealand where five layers of rhyolitic ash occur within a radi* Lament Geological Observatory Contribution No. 1178.
us of more than 1000 km from North Island. The age of the ash layers has been inferred from a paleomagnetic stratigraphy of the cores and shows that the ashfalls have the same range as the ignimbrite sheets. The occurrence of the layers of windborne rhyolitic ash of similar age as the ignimbrite sheets is further evidence for extensive airborne ash production during the emplacement of the ignimbrite sheets.
2. DEEP-SEA CORES FROM THE NEW ZEALAND KERMADEC AND TONGA REGION Cores in this study were taken by Lament Vessels VEMA and CONRAD (fig. 1 and table 1). 2.1. Description o f cores Cores taken to the west of New Zealand (V18-227, 228 and 229) are composed entirely of foraminiferal ooze. Cores taken to the east and northeast of New Zealand are either foraminiferal ooze or light brown hitite with mineral sand and volcanic ash. Thus, cores taken in the area of the Chatham Islands are composed of foraminiferal ooze (RC9-107, I 10; also RC9111 and 112) and foraminiferal with fine mineral sand dispersed through the sediment (RC-108). Most of the cores taken from the Tonga Trench (V18-236 to
90
D, NINKOVICH /70 =
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I80"
/60 °
r~/, Is. " ~ ' " Ton g o
20".
ls
~245
; "';
20 °
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•
119 118
25 °
239 ~1~ 238
K E Y X • • • Q
30 °
V16 CRUISE V18 CRUISE RC8 CRUISE RC9 CRUISE CORESWITH RHYOLITIC ASH LAYER
25*
117 •
116 4'
236,237
Kermadec Is.
30 °
115
®
CORES WITH BROWN ASH LAYER 114
® 35 °
35 °
113
® 231
112
227 °
IJl
40 °
40 °
U
" I10
;"
1~4
Chatham 15
107
"8
e78 •
45 =
108
" ~ J 1 2 2
j,
x
•
®
125
®
109 •
45 °
g
®. 80
50 •
50"
126 X 170°
/80°
170 °
160"
Fig. 1. Distribution of deep-sea cores from the area of New Zealand, Kermadec and Tonga Islands in the Lamont Geological Obser. vatoty collection.
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
91
Table 1 Location of deep-sea cores containing volcanic ash layers, shown in fig. 1. Core
Latitude S
Longitude
Depth (m)
Length (cm)
Number of ash layers colorless
v16-125 RC9-108 RC9-110 RC9-113 RC9-114 RC9-115 V18-231 V18-232 V18-234 V18-240 V18-245 V18-247
47o01 ' 45045.4 , 42052 ' 36044.2 ' 33041.4 , 31023 ' 38049 ' 36045 ' 30056 ' 22017.5 ' 20031 ' 19000 '
179o15'w 177o22.4'W 172o00.7'W 167o02.9'W 165o02.8'W 163o43.1'W 179o10'E 179o36'W 176o33'W 174o01'W 172o15'W 170o18'W
2953 4314 1917 4751 5453 5376 3530 4702 6943 8633 5662 4713
239) are composed o f coarse mineral sand, all the other cores shown in fig. 1 are composed o f light brown lutite with a small amount of carbonate (up to 5%). 2.2. Volcanic ash Most o f the cores taken to the east o f New Zealand, Kermadec and Tonga Islands contain volcanic ash dispersed throughout the sediments, sometimes forming distinct layers. Two different types o f volcanic ash have been identified: brown ash and colorless ash (fig. 2). Brown ash: Distribution o f brown ash is limited to the area near the Tonga and Kermadec Islands (fig. 1). It is dispersed throughout the sediments and forms
435
430
425
420ems RC 9 114
I00
95
90
85 cm$
V18 245
Fig. 2. Photograph of rhyolitic and brown ash in deep-sea cores east of New Zealand, Kermadec and Tonga Islands.
brown
896 1219 743 482 1032 1074 281 368 430 180 299 412
distinct layers in cores V18-234, 2 4 0 , 2 4 5 and 247 (table 2). Core V18-234 taken to the southeast of the Kermadec Islands marks the southernmost limit o f the brown ash. Colorless ash: Cores taken to the east and northeast o f New Zealand and south o f 300 south latitude contain only colorless ash (with the exception o f core V18-234, which contains b o t h brown and colorless ash). Thus, the distribution pattern suggests that the brown ash originated from volcanoes on Kermadec and Tonga Islands, and the colorless ash from eruptions on New Zealand. The origin o f a single layer of colorless ash in cores V18-240 and 245, however, is uncertain. Rhyolitic glass shards dispersed throughout the sediments and in small pockets were first recognized in the area to the east o f New Zealand in studies of b o t t o m samples and 2 to 2.5 m cores from the Chatham Rise [9, 10]. The index of refraction o f volcanic glass shards in these samples ranges from 1.498-1.504 and is similar to that o f Late Quaternary pumice deposits in the North Island o f New Zealand [11 ]. Cores examined in the present study, however, are longer and have a wider distribution. Rhyolitic ash in these cores occurs in distinct layers, up to 13 cm thick (core V16-125). Depth in deep-sea cores o f rhyolitic ash layers is shown in table 3 and fig. 3. A m a x i m u m o f five ash layers has been identified (core RC9-1 13). The ash is composed o f volcanic glass shards and minerals o f which the proportions vary throughout the layers. An average sample o f ash is composed o f about
92
D. NINKOVICH
Table 2 Volcanic ash layers in deep-sea cores from the Tonga-Kermadec region.
with the same color and cores containing only one ash layer of that color (table 3).
Depth(cm) ofashlayersin cores V18-247
Brown ash
V18-245
V18-240
V18-234
21-22 90-95
4-8 34-36 50-51
21-23 140-142
124-125
80-82
330-331
300
180
412
9- 10 30- 31 39- 40 45- 46 48- 49 90- 92 115 -120
Color~ss ash Bottom of cores
400
95% volcanic glass shards and 5% minerals. Volcanic glass shards from all samples of colorless or rhyolitic ash layers in the present study have an identical index of refraction (1.498-1.500), similar shape (fig. 4) and can be separated from heavy minerals on a Frantz isodynamic separator as a non-magnetic fraction at 1.4 amps. Three different colors of pure, rhyolitic, volcanic glass shards can be distinguished in different samples of ash: very light gray (NS); pinkish gray (SYRS/ 1), and very pale orange (10YRS/2) (after Rock Color Chart, Geol. Soc. Am., 1963). Pairs of ash layers with similar color of volcanic glass have been found in cores RC9-113 and 114. Therefore, a correlation of rhyolitic ash layers based on color of volcanic glass is difficult between cores containing pairs o f ash layers
2.3. Size analysis of rhyolitic ash Data on the size analysis are presented in table 4. The coarsest ash was found in core V18-231, taken near the eastern coast o f North Island. The maximum grain size of the ash in this core is about 0.35 mm. According to experiments [12], only shards o f vesicular acidic volcanic glass with a size less than about 0.35 m m can settle to the bottom; while the shards coarser than this size remain on the surface of the sea as floating pumice. Thus, the presence o f the maximum possible grain size of the glass shards in core V18-231 indicates that the core was taken near the source of the ash. In other cores, taken farther from North Island, the ash is finer; with a maximum grain size of 0.25 mm or less.
3. PALEOMAGNETIC STRATIGRAPHY OF DEEPSEA CORES Measurements of the orientation of the magnetic vector in radiometrically dated sub-aerial lava flows permitted Cox et al. [13] to determine intervals of normal and reversed geomagnetic polarity during the last 4 m.y. A suggested paleomagnetic stratigraphy for the last 3 m.y. is shown on the left-hand side of fig. 3. Thus, the present polarity of the geomagnetic field has lasted since 0.70 m.y.B.P. [14] and is called
Table 3 Color of volcanic glass shards in rhyolitic ash layers from deep-sea cores east of New Zealand. Color of volcanic glass shards
RC9-113
RC9-114
Very light gray (NS)
142-145
364-368
Pinkish gray (5 YR 8/1)
175-185 370-371
425-430 831-832
Very pale orange (10YR 8•2)
404-405 471-479
Depth (cm) of rhyolitic ash layers in cores RC9-115 V18-234 V18-231 RC9-108 208-214
139-141 378-383
330-331
610-612
V16-125
RC9-110
373-386
385-390 556-559 673-684
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
M°Y.
RC9 r17
RC9 I19
VI8 2:34
RC9 115
RC9 113
RC9 I14
93
V2"~' 18I
RC9 I08
METERS 0
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4
5
u 6 < ,!
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&o
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ASH LAYERS RHYOLITIC • BROWN
10
!1
\
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12
Fig. 3. Graphic magnetic polarity logs for deep-sea cores east and northeast of New Zealand showing location of rhyolitic ash layers.
the Brunhes epoch of normal polarity. Between 0.70 and about 2.5 m.y.B.P., the earth's magnetic field was reversed most of the time and is called the Matuyama reversed polarity epoch. Two short events of normal polarity occurred during the Matuyama epoch; these are the JaramiUo (0.926 to 0.870 +-0.014 m.y. B.P.; ref. [15]), and the Olduvai (about 1.8 m.y.B.P.). The Matuyama epoch was preceded by the Gauss normal polarity epoch. A paleomagnetic stratigraphy of some of the deepsea cores in this study was shown in fig. 3, and tables 5, 6 and 7. Samples of sediments approximately 8 cm3 in size, with vertical orientation in the cores, were cut at 10 to 20 cm intervals throughout the length of each core. After a partial demagnetization
in an alternating field of 100 to 150 oersteds, the samples were measured on a spinner magnetometer described by Foster [ 16]. Cores composed of lutite and carbonate sediment with a high percentage of fine mineral sand (RC9-108) showed a sufficiently strong magnetic intensity for measurement. The magnetic intensity in pure carbonate sediment (cores RC9-110 and V16-125), however, was too feeble to be measured. 3.1. Cores containing brown ash All samples of sediments from cores containing brown ash (V18-234,240, 245 and 247) (table 2) showed a remnant magnetization of normal polarity indicating that the sediments were deposited during
94
D. NINKOVICH
Fig. 4. Microphotograph of grain size fraction 0.177-0.088 mm of rhyolitic glass shards from five ash layers in core RC9-113.
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
95
Table 4 Grain size analysis of rhyolitic volcanic ash layers in deep-sea sediments east and n o r ~ e a s t o f New Zealand. Ash layer
Core
Depth of ash layer (cm)
A 0.85 m.yo
RC9-113 RC9-115
471-479 378-383
0.73 m.y.
RC9-113
404-405
C 0.67 m.y.
RC9-113 RC9-114
370-371 831-832
D 0.31 m.y.
RC9-113 RC9-114 RC9-115
175-185 425-430 139-141
E 0.27 m.y.
V18-231 V16-125 RC9-108 RC9-113 RC9-114
208-214 373-386 610-612 142-153 364-368
350
250
Percentage coarser than size (#) 177 125 88
62
44
0.2 1.7
7.9 11.6
17.8 27.q
24.0 40.3
27.9 49.6
2.3
6.4
15.2
29.8
40.1
0.1
1.9 1.7
7.3 6.1
20,5 20.9
32.9 34.2
0.2 0.1 0.7
9.3 8.5 6.7
23.9 22.8 15.4
36.7 38.5 28.8
44.2 46.2 40.5
12.5 5.0 2.7 0.3 0.2
24.6 17.9 9.9 7.0 2.0
37.2 31.6 23.1 16.8 10.1
47.0 41.1 35.4 26.9 20.9
52.5 47.5 42.0 34.6 31.6
B
0.4
0.5
5.1 0.7
Table 5 Apparent age of rhyolitic ash layers in core RC9-113 inferred from magnetic data. Paleomagnetic stratigraphy Epoch
Event
Sediment Thickness (cm) 0-142 153- 175
Jaramillo
Volcanic ash layers
Rates of deposition Duration of interval (cm/1000y) (m.y.) 0.52 0.27 0.52 0.04
185-370 371-386
0.52 0.52
0.36 0.03
386-404 405-471
0.52 0.52
0.03 0.13
479 -482
0.52
0.005
Thickness (cm)
Apparent age (m.y.)
142-153 (E) 175-185 (D) 370-371 (C) (Reversal)
0.27 0.31 0.67 0.70
404-405 (B) 471-479 (A) (Reversal)
0.73 0.86
Table 6 Apparent age of rhyolitic ash layers in core RC9-114 inferred from magnetic data. Paleomagnetic stratigraphy Epoch
Brunhes
Matuyama
Sediment Thickness (cm) 0-364 368-425 430-831 832-873 873-1030 Bottom of core
Volcanic ash layers
Rates of deposition (cm/1000y)
Duration of interval (m.y.)
1.23 1.23 1.23 1.23
0.29 0.05 0.33 0.03
1.23
0.13
Thickness (cm)
Apparent age (m.y.)
364-368 (E) 425-430 (D) 831-832 (C) (Reversal)
0.29 0.34 0.67 0.70
96
D. NINKOVICH Table 7 Apparent age of rhyolitic ash layers in core RC9-115 inferred from magnetic data. Paleomagnetic stratigraphy
Epoch
Event
Brunhes Jaramillo Matuyama Olduvai Gauss
Volcanic ash layer
Sediment Thickness (cm) Rates of deposition (cm/1000y)
Durationof interval (m.y.)
0-139 141-314
0.45 0.45
0.31 0.39
314-378 383-418 418-644 644-690 690-907
0.45 0.70 0.28 0.30 0.33
0.14 0.05 0.80 0.15 0.65
907-1060 Bottom of core
0.30
0.50
the Brunhes epoch; therefore, the bottom of these cores is less than 0.7 m.y. old. Core V18-247 is 400 cm long and the rate of deposition is greater than 0.6 cm/1000 y. The earliest layer of brown ash at 115-120 cm in this core must be therefore less than 0.2 m.y. old. Core V18-245 is 300 cm long. The sediment was deposited with a rate greater than 0.45 cm[1000 y. The earliest brown ash at 90-95 cm is less than 0.2 m.y. old. Core I,'18-240 is 180 cm long. A rate of deposition greater than 0.25 cm indicates that the lowest brown ash in this core, too, is less than 0.2 m.y. old. Core 1118-234 contains a layer of rhyolitic ash at 330-331 cm which can be correlated with a similar layer from cores RC9-113, 114 and 115 (fig. 3), and which has an apparent age of 0.31 m.y. (tables 5, 6 and 7). Therefore, the age of the two layers of brown ash at 21-23 cm and 140-142 cm in this core is 30000 and 130000 y, respectively. Thus, the eruptions in the Tonga and Kermadec Islands, whose magnitude was sufficient to produce distinct layers of brown ash in deep-sea sediments, apparently occurred in the last 200000 y and were preceded by small eruptions indicated by scattered brown ash throughout the sediment for a period of unknown length. 3.2. Cores containing rhyolitic ash Measurement of remnant magnetization has shown that only three cores containing rhyolitic ash layers
Thickness (cm) Apparent age (m.y.) 139-141 (D) (Reversal) 378-383 (A) (Reversal) (Reversal) (Reversal) (Reversal) (Reversal)
0.31 0.70 0.85 0.90 1.70 1.85 2.50
penetrated through the 0.7 m.y. reversal. These are cores RC9-113, 114 and 115 (fig. 3). Cores V18-231, V18-234 and RC9-108 failed to penetrate the 0.7 m.y. reversal. Cores RC9-117 and 119 are also presented in fig. 3 in order to show that they penetrate the sediment in which the rhyolitic ash layers occur farther south. Absence of ash layers from these two cores indicates that the northern- and easternmost limit of the distribution of the New Zealand rhyolitic ash is about 300 south. 3.3. Dating of rhyolitic ash layers Core RC9-113: Sediment between the top and 386 cm is normally magnetized (fig. 3, table 5). Between 386 cm and the ash layer at 471-479 cm the remnant magnetization of sediment is reversed. A short section of sediment at the bottom of the core below the ash layer is normally magnetized. The paleomagnetic stratigraphy thus established suggests that the core penetrated to the middle of the JaramiUo event. The calculated rate of deposition is 0.52 cm]1000 y. Five ash layers occur in this core: three in sediment of the Brunhes epoch and two in sediment of the Matuyama epoch. The lowest ash occurs at the top of the JaramiUo event precisely at the point of reversal. The apparent ages of the five ash layers inferred from the rate of deposition are as follows: A = 0.86;B = 0.73; C= 0.67;D = 0.31 ; and E ---0.27 m.y.B.P. (table 5). Core RC9-114 penetrated to the sediment of the Matuyama series but did not reach the Jaramillo event The calculated rate of deposition is 1.23 cm/1000 y.
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
The bottom of the core is about 0.83 m.y. old. Three ash layers occur in this core, all in sediment of the Brunhes epoch. Their apparent ages of 0.67, 0.34 and 0.2o m.y. (table 6) are similar to the ages of layers C, D and E from core RC9-113 (table 5, fig. 3). Layer B l'rom core RC9-113 apparently does not occur in core RCO-114 and the top of Jaramillo where layer A occurs in core RC9-113 was not reached by core RC9114. Core RC9-115 has the longest paleomagnetic stratigraphy of any core in this study. The bottom of the core apparently ends in sediment deposited during the Gauss epoch, about 3 m.y.B.P. (fig. 3). Rate of deposition of sediment above the Jaramillo event was 0.45 cm/1000 y and below this event 0.3 cm/1000 y (table 7). Two ash layers occur in this core: one in sediment of the Brunhes series, and one at the top of the JaramiUo event (as in core RC9-113, this ash layer occurs precisely at the point of reversal). The apparent ages of two ash layers in core RC9-115 are 0.85 and 0.31 m.y.B.P. (table 7). The following three cores, containing one ash layer each, failed to penetrate the 0.7 m.y. reversal (fig. 3). CoreRC9-108 is 1219 cm long. The rate of deposition in this core, therefore, must be greater than 1.75 cm/1000 y and the ash layer at 610-612 cm, less than 0.35 m.y. old. Core V18-231 is 281 cm long. Rate of deposition in this core is greater than 0.4 cm/1000 y and the layer of ash at 208-214 cm is less than 0.53 m.y. old. Core V18-234 is 412 err. long. Rate of deposition is greater than 0.6 cm/1000 y and the ash layer at 330-331 cm is less than 0.56 m.y. old. Thus, in all three cores, RC9-108, V18-231 and V18-234 which fail to penetrate the 0.7 m.y. reversal contain an ash layer with an apparent age younger than the layer C (0.67 m.y.) from core RC9-113.
4. CORRELATION OF RHYOLITIC ASH LAYERS Both color of volcanic glass shaids (tables 3 and 8) and paleomagnetic stratigraphy (fig. 3) have been used for purposes of correlation of rhyolitic ash layers. In core RC9-114, all three ash layers occur in sediment of the Brunhes epoch. In the upper layer (0.29 m.y.) the volcanic glass shards are very light gray
97
Table 8 Physical properties of rhyolitic volcanic glass shards in deepsea sediments east and northeast of New Zealand. Ash layer
Index of refraction (n)
Color
E 0.27 m.y.
1.498-1.500
Very light gray (N8)
D 0.31 m.y.
12498-1 ~500
Pinkish gray (5YR8/1)
C 0.67 m.y.
1.498-1.500
Pinkish gray (5YR8/1)
B
1.498-1.500
Very pale orange
A 0.86 m.y.
1.498-1.500
Very pale orange (10YR8/2)
0.73 m.y.
(10YR8/2)
(N8), and in both middle layer (0.34 m.y.) and lower layer (0.67 m.y.) the shards are pinkish gray (5YR8/I) (table 3). The layers correlate in age and color of volcanic glass shards with layers E, D and C respectively of the Brunhes sediment from core RC9-113. Core RC9-115 contains only one layer of ash in sediment of the Brunhes epoch. The apparent age of the layer is 0.31 m.y. and the color of the glass shards is pinkish gray (5YR8/1) (table 3). The ash therefore correlates with ash layer D from core RC9-113. The lower ash layer from core RC9-115 correlates in both age and color with layer A from core RC9-113. In core V18-234, which ends in sediment of the Brunhes epoch, volcanic glass shards in the ash layer are pinkish gray (5YR8/1) and similar to these from layers C (0.67 m.y.) and D (0.31 m.y.) from core RC9-113 (table 3). It has been shown, however, that the layer in core V18-234 is probably younger than 0.56 m.y. It therefore would correlate with layer D from core RC9-113. In cores V18-231 and RC9-108, both of which end in sediment of the Brunhes epoch (fig. 3), volcanic glass shards in the ash layer are very light gray (N8). The layer may be correlated with the layer E (0.27 m.y.) from core RC9-113. A similar ash layer has been found at 373-386 cm in carbonate core V16-125 Carbonate core RC9-110 contains three layers of rhyolitic ash; all three are composed of very light gray (N8) volcanic glass shards, similar to those from layer E (core RC9-113) (table 3). Thus, RC9-110 is the only core studied in which the ash layers cannot be correlated with layers from other cores, either on the basis
98
D. NINKOVICH
of color of glass shards or remnant magnetization of sediment, which is too feeble to be measured. 4.1. Wind directions Layer E (0.27 m.y.) occurs in cores RC9-113, RC9-114, V18-231, RC9-108 and V16-125 (figs. 1 and 3). It is, however, absent from northernmost cores V18-234 and RC9-115. The predominant components of the wind which carried the ash E were westerly and northwesterly from the assumed origin. Layer D (0.31 m.y.) occurs in cores RC9-113, RC9-114 and in the northernmost cores V18-234 and RC9-115. It is absent from southern cores RC9-I08 and V16-125. The predominant components of the winds were westerly and southwesterly. Layer C(0.67 m.y.) occurs in cores RC9-113 and RC9-114. The ash appears to have been carried by winds with a similar direction to those that transported ash from layer E. Layer B (0.71 m.y.) was found only in core RC9113. Layer A (0.86 m.y.) reached by cores RC9-113 and RC9-114 was probably deposited by winds similar to those which carried the ash D. Healy et al. [17] made a detailed and valuable study of tephra from Late Quaternary soils of the North Island. They correlated tephra layers, made isopach maps, and concluded that the tephra originated in pumice eruptions which have occurred in about the last 15 000 y. The isopach maps show that the ash was carried by westerly, northwesterly and southwesterly winds. Thus, the directions of winds in the belt which overlaps New Zealand appear to have been unchanged in the last 0.9 m.y.
5. RATES OF DEPOSITION IN DEEP-SEA CORES Rates of deposition were first established on the basis of paleomagnetic stratigraphy for cores which penetrated the 0.7 m.y. reversal (RC9-113, 114, 115, 117 and 119). In cores which failed to penetrate the reversal or in which the remnant magnetization was too feeble to be measured, the rates of sedimentation were established by correlation of rhyolitic ash layers. The diagram in fig. 5 represents depth in cores of magnetic reversals (solid points) and rhyolitic ash layers (open squares) versus paleomagnetic stratigraphy
Table 9 Rates of deposition of deep-sea sediments, inferred frompaleomagnetic data and correlation of ash layers, shown in fig. 5. Core
Rates of deposition (cm/1000 y)
RC9-117 RC9-115 RC9-113 V18-231 RC9-119 V18-234 RC9 - 114 V16-125 RC9-108
0.33 and 0.54 0.45 and 0.30 0.52 0.77 0.97 1.06 1.23 1.38 2.25
and an inferred age of five ash layers tentatively named A, B, C, D and E. Calculated rates of deposition in the cores are shown in table 9.
6. ORIGIN OF RHYOLITIC ASH LAYERS The rhyolitic ash layers have a wide distribution covering a large area of the bottom of the sea to the east and northeast of New Zealand. The distribution pattern, index of refraction of volcanic glass shards, texture and age of the ash layers suggest an origin in Pleistocene rhyolitic eruptions on North Island.
6. I. Structure and volcanicity o f New Zealand Volcanicity on North Island has been discussed by Grange [3]; Kear [18]; Grindley [19];Grindley et al. [20]; Healy et al. [17, 21]; Thompson et al. [22] and others. Data summarized by Grindley and Harrington [23] on volcanism and structure of both South and North Islands are reproduced, with some modifications, in fig. 6 of the present paper. Two regions of Late Tertiary and Quaternary volcanism in New Zealand are separated by a non-volcanic region. On South Island the volcanoes are basaltic with trachytes and phonolites. Activity lasted from late Miocene to early Quaternary. No pumice eruptions have been recognized on South Island [18]. The volcanic activity on North Island has been related to two different belts: one, striking northwest from the central part of the island toward New Caledonia, and the other, striking northeast across the central part of the North Island, aligned with the Ker-
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND
99
3.0
~rss.
a
2.0
~n n,.
Id OLDUVAI Id n,
a,v
'4~
IJ_l (_9
>.
I-
1.0
X
-o-~
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~
.~_~,6,~ '~
0.8
0
0
I00
200
~
400
~ I)EPTH
600
700
800
900
I000
I100
1200
(CMS)
Fig. 5. Depth versus age of magnetic polarity reversals and rhyolitic ash layers in deep-sea cores. The solid points represent points of magnetic reversals and the open squares, rhyolitic ash layers.
100
D. NINKOVICH
-34" I
I
I
170 °
I
~
I
174e
/F LEGEND
m
eo.~,i~ m
/
x.d..i~c ITTTI~olltl.
/
/
/
,/
o
/
/
/
.
\ C f
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o /,
O
~
o
$0
aoo •
2S t
~
Scael
o I
o
174t
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~o J
,, o
I00 I
--j j km
ISO J milts loo
I70Q
j
I
Fig. 6. Structure and volcanicity of New Zealand during Quaternary (after Grindley and Harrington, 1961, modified). Sheets of ignimbrites in the central part of the North Island originate in eruptions which occurred between 0.8 and 0.2 m.y.B.P. (refs. [4, 5]). They correlate in age with rhyolitic ash layers in deep-sea sediments (figs. 1 and 2).
PLEISTOCENE VOLCANIC ERUPTIONS IN NEW ZEALAND madec-Tonga volcanic arc (figs. 1 and 6). The northwest belt is divided into different structural and petrographic zones, characterized by eruptions of andesites and basalts. The andesitic volcanic activity occurred during Miocene and Pliocene time with occasional eruptions of pumice. These eruptions which may have caused some smaller concentrations of ash in deep-sea sediments are apparently older than sediments reached by the studied cores. The basaltic activity has continued to the present. Volcanic activity in the central part of the North Island originated in an uplift of the northeast belt which started in Miocene and which culminated in eruptions of Taupo-Rotorua ignimbrites (fig. 4), 0.8 -0.2 m.y. ago [4, 5]. The rhyolitic ash layers in deep-sea cores (figs. 1 and 3) correlate in age with these ignimbrites. The oldest identified post-ignimbrite deposits of rhyolitic tephra are considered to be approximately 40000 years old [24]; the eruptions have lasted until the present. According to the research by Healy et al. [ 17], an average thickness o f the ash layers on the eastern coast of North Island, about 100-150 km from the source, is 1-2 cm, and maximum 15 cm. These post-ignimbrite, late Quaternary eruptions have produced only small concentrations of ash identified in b o t t o m samples and 2-2.5 m cores from Chatham Rise, about 400 km from the source [9,10], but have not been recorded in the co~es of the present study.
7. CONCLUSIONS Five distinct layers o f windborne rhyolitic ash have been identified in deep-sea cores east of New Zealand. Paleomagnetic stratigraphy has been established in the cores and used for both correlation and dating o f the ash layers. The earliest sediment penetrated is about 3 m.y. old and the apparent ages o f the ash layers, tentatively named A, B, C, D andE, are: 0.86; 0.73; 0.67; 0.31, and 0.27 m.y.B.P., respectively. Distribution pattern suggests that the ash falls originated in eruptions in New Zealand. Late Tertiary and Quaternary history ofvolcanicity in New Zealand shows that the eruptions culminated in emplacement o f sheets of welded rhyolitic tufts or ignimbrites in the Taupo-Rotorua region o f the North Island. The ash layers in deep-sea cores have
101
the same age range as the ignimbrites [4, 5] suggesting a significant ash-fall phase, until now little known, associated with the emplacement of ignimbrites.
ACKNOWLEDGEMENTS Professor Morice Ewing, Director of Lament Geelogical Observatory, provided the cores and offered encouragement. Paleomagnetic measurements of the cores were made in association with Dr. N. Opdyke. Drs. J. J. Stipp and A. Cox offered their unpublished data on K/A and paleomagnetic measurements of New Zealand ignimbrites, respectively. Drs. W. S. Broecker, A. Ewart, G. W. Grindley, J. D. Hays, J. Healy and B. C. Heezen offered valuable suggestions and helped in redaction of the manuscript. This study was supported by the U.S. Navy and the National Science Foundation under Grants G.A.1193 and G.A.-824.
REFERENCES [ 1] P. Marshall, Acid rocks of the Taupo-Rotorua volcanic district, Trans. Roy. Soc. New Zealand 64 (1935) 323. [2] R.C. Martin, Stratigraphy and structural outline of the Taupe volcanic zone, New Zealand J. Geol. Geophys. 4 (1961) 449. [3] L.I. Grange, The geology of the Rotorua-Taupo subdivision, New Zealand Geol. Sure., Bull. 37 (N.5) (1937) 138. [4] J.J. Stipp, K/A Ages of the Coromandel and Central Volcanic Region, North Island, New Zealand ( in preparation); K/A History of the Western Volcanics, North Island, New Zealand (in preparation). [5] A. Cox, Paleomagnetic Stratigraphy of the New Zealand ignimbrites (in preparation). [6] R.L. Smith, Ash flows, Bull. Geol. Soc. Am. 71 (1960) 795. [7] A. Steiner, Origin of ignimbrites of the North Island, New Zealand: A new petrogenetic concept, New Zealand Geol. Sure., Bull. 68 (1960) 72. [ 8] A. Ewart, Mineralogy and petrogenesis of the Whakamaru ignimbrite in the Maraetai areas of the Taupe volcanic zone, New Zealand, New Zealand J. Geol. Geophys. 8,4 (1965) 611. [9] J.J. Reed and N. deB. Hornibrook, Sediments from the Chatham Rise, New Zealand J. Sci. Tech. B. 34 (3) (1952) 173. [10] R.M. Norris, Sediments of Chatham Rise, New Zealand Dep. Sci. Indust. Res. Bull. 159 (1964) 1.
102 [ 11 ]
D. NINKOVICH
A. Ewart, Petrology and petrogenesis of the Quaternary pumice ash in the Taupo area, New Zealand, J. Petrol. 4 (1963) 392. [12] R.V. Fisher, Settling velocity of glass shards, Deep-sea Res. 12 (1965) 345. [13] A. Cox, R. R. Doell and G. B. Dalrymple, Geomagnetic polarity epochs: Sierra Nevada II, Science 142, 3590 (1963) 382. [14] R.R. Doell and G. B. Dalrymple, Geomagnetic polarity epochs: A new polarity event and the age of the Brunhes-Matuyama boundary, Science 152 (1966) 1060. [15 ] N. Opdyke, The Jaramillo event as recorded in oceanic cores, N.A.T.O. conference on Internal Constitution of Earth and Planets, ed. S. K. Runcorn (Wiley, London), in press. [ 16 ] J.H. Foster, A paleomagnetic spinner magnetometer using a fluxgate gradiometer, Earth Planet. Sci. Letters 1, 6 (1966) 463. [17] J. Healy, C. B. Vucetich and W. A. Pullar, Stratigraphy and chronology of Late Quaternary volcanic ash in Taupo, Rotorua and Gisborn districts, New Zealand Geol. Surv., Bull. 73 (1964) 1.
[18] [ 19]
[20]
[21]
[22]
[23]
[24]
D. Kear, Pumice chronology in New Zealand, New Zealand J. Sci. Tech. 38B (1957) 862. G. W, Grindley, Geological map of New Zealand, 1 : 250000, Sheet 8-Taupo, New Zealand Geol. Surv., Dept. of Sci. and Industr. Res. (1960). G.W. Grindley, H. J. Harrington and B. L. Wood, The geological map of New Zealand, 1 : 2 000 000, New Zealand Geol. Surv., Bull. 66 (1961). J. Healey, J. C. Schofield and B. N. Thompson, Geological map of New Zealand, 1 : 250000, Sheet 5-Rotorua, New Zealand Geol. Surv., Dept. of Sci. and Industr. Res. (1964). B.N. Thompson, L. O. Kermode and A. Ewart (editors), New Zealand volcanology, central volcanic region, New Zealand Dep. Sci. Industr. Res. Inf. Ser. N. 50 (1966) 1. G.W. Grindley and H. J. Harrington, Late Tertiary and Quaternary volcanicity and structure in New Zealand (abstract). The Proceedings of the 9th Pacific SCI. Congress, 1957, 12 (1961) 198. A. Ewart and J. Healy, personal communication, 1967.