Preliminary study of a Tertiary magnetic transition in Colorado

EARTH AND PLANETARY SCIENCE LETTERS 11 (1971) 333-338. NORTH-HOLLAND PUBLISHING COMPANY

PRELIMINARY A TERTIARY

MAGNETIC

STUDY OF

TRANSITION

IN COLORADO

D. YORK, D.W. STRANGWAY

Department of Physics, University of Toronto, Toronto, Ontario, Canada and E.E. LARSON

Department of Geological Sciences, University of Colorado, Boulder, Colorado 80302, USA Received 15 January 1971 Revised version received 5 May 1971

Nine of twenty-six basaltic lava flows near State Bridge, central Colorado, possess directions of remanent magnetization intermediate between older normally magnetized flows and younger reversely magnetized flows. The intermediate directions vary only slightly from flow to flow, apparently indicating that this sequence came out in rapid succession. Paleointensity studies indicate paleofields of approximately 0.50e and 0.65 Oe during the eruption of the oldest normal flows and youngest reverse flows, respectively. The intermediately magnetized flows generally indicate low field intensifies, reaching a minimum value of 0.04. Oe. The field apparently reduced considerably in strength before any changes in direction became evident, indicating that the field collapsed rather than flipped over during the reversal. We conclude from whole-rock K-Ar dating on twelve of the lavas that the lower normally magnetized section and the lavas in the transition are approximately 24.0 my old, whereas the upper reversely magnetized section is 21.5 ± 1.0 my old.

1. Introduction Paleomagnetic studies have revealed that the earth's magnetic field has commonly reversed its polarity. Precise isotopic dating has enabled establishment o f the reversal pattern during the last 4 m y [1, 2]. Beyond this time, it becomes increasingly difficult to establish the pattern unambiguously because o f imprecision in the geologic dating. One way to extend the polarity-time sequence is to date transition zones - that is, those sections o f rock that preserve the unusual, intermediate magnetic directions that are developed as the magnetic field changes from one polarity to another. Up to this time only a few transitions have been reported: (1) Pliocene, Japan [3] ;

(2) Upper Triassic, South Africa [4] ; (3) 15.1 -+ 0.3 my, Steens Mountain, Oregon [ 5 - 7 ] ; (4) 6.9 + 0.2 my, northern Nevada [81 ; (5) 13.1 to 14.3 my, Japan [9] ; and (6) 22 to 24 my, northem California [10]. The present paper describes a transition zone in central Colorado dated at about 24 my. The study illustrates some o f the difficulties which may be encountered in the whole-rock K-Ar dating o f young basalts.

2. Geology The paleomagnetic transition occurs in a sequence of basaltic lavas that total about 1000 ft in thickness. The flows overlie unconformably steeply dipping

D. York et al., Tertiary magnetic transition in Colorado

334

Table 1. Summary of paleomagnetic data. Number of samples

Flow number

1-1, 1-2 1-3, 1-4, 1-5 f 1-6, 1-7, 1-8 13 to 18 6 to 12 1 to5

~95

Paleomagnetic North Pole Long. Lat.

Limits of oval of confidence 6M 6P

99.1 10.9 5.9

9.8 19.1 16.6

181 222 141

68 64 -4

14 24 24

11 15 17

54 66

22.1 23.2

8.6 10.9

326 87

83 81

12 18

8 15

167 150 58

-51 -45 -70

24.2 18.0 62.0

15.9 14.6 4.7

230 216 138

76 62 17

21 18 8

14 R [1] 12 R [2] 7 Int. [31

3 14

57 60

26.8 33.1

7.8 9.1

324 27

87 79

ll 14

8 N [4] 10 N [5J

Direction of magnetism D I

NRM k

4 7 16

151 154 44

-61 -44 -56

14 9

5 5

5 7 16 14

After A CD at 200 Oe

I-1, 1-2 1-3, 1-4, 1-5 1-6, 1-7, 1-8 13 to 18 6 to 12 1 to5

9

"

107* I

106*

i

3. Paleomagnetic data

I

40*

"

-.,

I

I'~ . + ~

~

BRID~ \

EAGLE

\

t~

SUMM,T

PITKIN /~L~ 59*

Fig. 1. Location map in central Colorado. Paleozoic and Mesozoic sediments and are overlain conformably by Late Miocene tuffaceous sediments. Intercalated with the flows, particularly in the lower part of the section, are tuffaceous sands, gravels, and breccias. The section sampled is on the east side of Yarmony Mountain, about 2 miles north of State Bridge, Eagle County, Colorado (see fig. l).

3.1. M a g n e t i c d i r e c t i o n s

Bulk samples were collected in the field and oneinch cores were drilled from them in the laboratory. Measurements were made with a spinner magnetometer and the results were analyzed by standard Fisher statistics. The directions of the natural remanent magnetization (NRM) and those after alternating field (A.F.) demagnetization in a peak field of 200 Oe are presented in table 1. Fig. 2 shows the manner in which the inclination and declination vary from flow to flow. The 12 lowest flows (nos. 1 - 1 2 ) are all normal in direction. The next 9 flows (nos. 1 3 - 1 8 and 1-6 to 1-8) possess an intermediate polarity with a mean upward inclination of - 70 ° and a declination of 30 °. All nine flows display essentially the same direction: there is no gradational pattern such as has been found in other transition-zone studies, e.g. Watkins [5] and Goldstein et al. [7]. Since reversals are believed to occur geologically rapidly the similarity in direction is taken as an indication that the transition flows came out in a very short time interval. The uppermost 5 flows (nos. l-I to 1-5) are reversely magnetized.

D. York et al., Tertiary magnetic transition in Colorado

INCLINATION

335

DECLINATION

FLOW NO (3

'-' [ ,'5,, AGES ~-2 > 2,9 220

I-4

18 7

I-5 I-6

22 5 206

I-8

245

16

I-7

,

~J

13

230

--

II IO •

230

=E 243

4 3 2 I 9O

~,0

d. .50

.2

-90

21'0

350

90

190

27'0

Fig. 2. Graph showing inclination and declination through the sequence of flows - heavy lines represent normal and reversed directions.

180

Table 2. Paleo field data. Sample no.

In Oe

1-1-1

0.61

1-2-1

0.63

1-3-1

0.43

1-5-1

0.43

1-6-1

0.15

18A

0.04

0

14A

0.18

Fig. 3. Mean pole positions from the five groups of flows at Yarmony Mountain. The + sign indicates the present mean dipole axis. Equal area projection used.

12A

0.03

5A

0.18

2A

0.46

90W

336

D. York et al., Tertiary magnetic transition in Colorado

Table 3. Potassium-argon data, Yarmony Mountain, Colorado. Sample no.

1-3-1 1-4-1 1-5-1 1-5-2 1-6-1 1-7-1 1-8-1 16A 16A 10A 8A 4A

%K

1.35 1.58 1.03 1.49 0.59 1.19 1.33 1.01 1.01 0.29 0.87 0.90

'*OAr* (cm 3 at NTP/g) X 10 -6

% Atmos. contam.

1.18 1.39 0.81 1.04 0.50 1.07 1.10 0.97 1.02 0.27 0.80 0.88

25.5 35.1 56.9 57.8 57.0 54.8 60.1 39.5 46.3 94.4 53.0 71.4

I-I ,i-2 iI-3L

Age (my)

-5i I-SP

21.9 22.0 19.9 17.5 21.0 22.3 20.6 24.0 25.0 23.0 23.0 24.3

The ages are considered to have a precision of 3% at 1o. Sample 10A is an exception and could be in error by 10%. Systematic error is considered to be less than 1.5%. * Radiogenic component, he = 0.584 X 10 -I° yr -I , KG= 4.72 X 10-1°yr -I , 4°K = 0.0119 percent potassium, atomic. The mean paleopole positions of the five groups are given in table 1 and plotted in fig. 3.

4. Paleointensity analysis The procedure we used for determining paleointensity has bi~en described previously [ 11 ] . Briefly, it consists of comparing the alternating field demagnetization curve o f the natural remanence with the curve acquired by allowing the sample to cool from above its Curie temperature and then demagnetizing. In general, it is required that the values in the range of 200 to 800 oersteds be essentially consistent. In the present study, eleven sampies were tested: ten appeared suitable for the paleointensity determinations. Suitability is determined by running alternating field demagnetization curves after heating to 600°C, 700°C and 800°C. If all these runs give similar decay curves, we assume that little change has taken place in the minerals by heating to 600°C. The calculated paleofield values are given in table 2 and plotted in fig. 4 as a function o f flow number in the sequence.

r'r" uJ

!Tt ~5~-

z 0

' F2

IO

?f ' ' " I . 0

.

.

.

02

I

04

PttLEOFIELD

06 -

0'8

IO

OERSTEDS

Fig. 4. Plot of intensity vs. sequence of flows studied.

The peak value of the paleofield was about 0.63 Oe as indicated in the youngest flows, a value slightly larger than the present field. In the transition, the paleofield values are considerably less, the minimum being 0.04 Oe. In the normal rocks below the transition, the paleofield decreases from a maximum o f 0.46 Oe close to the base o f the section to 0.03 Oe just below the transition flows. Apparently, the field started to decrease before there was much change in direction, which is in agreement with the conclusions drawn by Smith [12] about the nature of the reversal process. The paleointensity data seem to indicate that the transition zone is associated with the lower normal section. It may or may not be associated with the upper reverse section.

5. Potassium-argon data Whole-rock K-Ar analyses were carried out on twelve samples. Potassium concentrations were determinated b y flame p h o t o m e t r y and radiogenic argon volumes were measured relative to 3SAr spikes with an A.E.I. MS10 mass spectrometer. The experimental methods followed were similar to those de-

D. York et al., Tertiary magnetic transition in Colorado

scribed by Baksi et al. [6]. The data are presented in table 3, and the dates found for the various lavas are presented in fig. 2. It is immediately apparent from the figure that the magnetic and geochronometric results are not in complete agreement. The isotopic data indicate a split transition zone composed of two sections, one older than the other by 2 to 3 my. This, however, is inconsistent with the conclusions based on paleomagnetism which indicate that all flows with intermediate directions came out rapidly and are part of a single transition. To resolve this paradox other features were sought which might correlate with the age break. In general, all samples are fresh, exhibiting little if any weathering and alteration. The amount of calcite in fractures and vesicles is randomly variable. Deuteric alteration of olivine to iddingsite has occurred in nearly all samples. The one significant difference seems to be that almost all the rocks above the break in the K-Ar ages have a worm-like filling of voids and replacement of some feldspar by SiO2, or what is interpreted to be SiO2 - it is too fine-grained to be positively identified in thin section. The samples showing the most discordant ages (1-5-1 and 1-5-2) exhibit the most highly developed replacement. However, the uppermost flow dated (1-3-1) is apparently free o f calcite, glass devitrification, and SiO2 alteration; yet it still gave a young age of 22 my. We consider it most probable, therefore, that this is the true age of this flow. The most reasonable conclusion, synthesizing the paleomagnetic, geochronologic, and petrographic data appears to be that there is a chronologic break in the flow sequence of about 2 - 3 my. The transition zone and the lower normal section with which the transition seems magnetically associated are considered to be 24.0 + 1.0 flay and are overlain by a reversed section 21.5 -+ 1.0 m y old. It is, of course, remotely possible that a chronologic gap of about 2 my is present within the transition zone. If so, the earth's field must either have been in a stable transitional state for 2 my or two partial transitions, each with the same magnetic direction are recorded in this section. Neither of these two possibilities seems very likely. All of the paleomagnetic data indicate that transitions are short-lived events, lasting approximately 103 to

337

104 y [1]. The probability of finding a transition recorded in a volcanic section is small; the probability of finding two adjacent to each other is essentially nit.

6. Conclusions Nine flows in a 26-flow sequence were found to possess an intermediate direction of magnetization. The lower normal section and the transitional flows are concluded to be 24.0 -+ 1.0 my in age. The uppermost reverse section of 5 flows is dated at 21.5 -+ 1.0 my. The intensity of the field varied from 0.43 Oe in the lower normal section to 0.04 Oe in the transition and back to 0.63 Oe in the upper section.

Acknowledgements This research was supported by NASA as part of the lunar samples program under contract no. NAS 9-7973. The dating was supported by the National Research Council at the University of Toronto. Many of the magnetic measurements were done by D. Enggren whom we thank along with R.J. Doyle and W.J. Kenyon for their assistance in the K-At program.

References [ 1 ] G.B. Dairymple, A. Cox, R.R. Doeli and C.S. Groining, Pliocene geomagnetic polarity epochs, Earth Planet. Sci. Letters 2 (1967) 163. [2] G.B. Dalrymple and R.R. Doell, Comment on paper by M. Ozima, M. Kono, I. Kaneoka, H. Kinoshita, Kazuo Kobayashi, T. Nagata, E. Larson and D. Strangway, J. Geophys. Res. 73 (1968) 1502. [ 3 ] K. Momose, Studies on the variation of the Earth's field during Pliocene time, Bull. Earthquake Res. Inst. 41 (1963) 487. [4] J.S. Van Zijl, K.W.T. Graham and A.L. Hales, The paleomagnetism of the Stormberg lavas 1 and I1, Geophys. J. 7 (1962) 23 and 169. [5] N.D. Watkins, Frequency of extrusion of some Miocene lavas in Oregon during an apparent transition of the polarity of the geomagnetic field, Nature 206 (1965) 4986, 801. [6] A.D. Baksi, D. York and N.D. Watkins, Age of the Steens Mountain geomagnetic polarity transition, J. Geophys. Res. 72 (1967) 6299.

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D. York et aL, Tertiary magnetic transition in Colorado

[7] M.A. Goldstein, E.E. Larson and D.W. Strangway, Paleomagnetism of a Miocene transition zone in Southeastern Oregon, Earth Planet. Sci. Letters 7 (1969) 231. [ 8 ] D.F. Heinrichs, Paleomagnetism of the Plio-Pleistocene Lousetown Formation, Virginia City, Nevada, L Geophys. Res. 72 (1967) 3277. [9] N. Kawai and K. llirooka, (abstract only) Anomalous direction of NRM in Miocene volcanic rocks in Jzp~.n, Proc. of U.S.-Japan scientific cooperation meeting, Kyoto, Oct. 1966.

[ 10] S. Gromm~, Anomalous and reversed paleomagnetic field direction from the Miocene Lovejoy basalt, northern California, J. Geomag. Geoelect. 17 (1965) 445. [ I 1 ] D.W. Strangway, B.E. McMahon and E.E. Larson, Magnetic paleointensity of a recent basalt from Flagstaff, Arizona, J. Geophys. Res. 73 (1968) 7031. [ 12] P.J. Smith, The intensity of the ancient geomagnetic field: a review and analysis, Geophys. J. 12 (1967) 321