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Comments on ‘palaeomagnetism of the Cambrian Purple Sandstone from the Salt-Range, W. Pakistan’
EARTH AND PLANETARY SCIENCE LETTERS 15 (1972) 215-217. NORTH-HOLLANDPUBLISHINGCOMPANY
COMMENTS ON 'PALAEOMAGNETISM OF THE CAMBRIAN PURPLE S A N D S T O N E F R O M T H E S A L T - R A N G E , W. P A K I S T A N ' b y M.W. M c E l h i n n y , E a r t h and P l a n e t a r y Science L e t t e r s 8 ( 1 9 7 0 ) 1 4 9 - 1 5 6 R.N. ATHAVALE * National Geophysical Research Institute, Hyderabad, India
Received 28 October 1971 Revised version received 29 March 1972
In this paper, McElhinny [1] has obtained a palaeomagnetic pole position at 28°S, 32°E, for the Purple Sandstone of Lower Cambrian age from the Salt Range located in West Pakistan and has assumed that this result represents the Lower Cambrian palaeomagnetic pole for the entire Indian peninsular shield, which is considered to be a constituent member of the ancient supercontinent of Gondwanaland. On the basis of this Lower Cambrian palaeomagnetic pole position, McElhinny [1] has suggested that India should be placed adjacent to East Africa in a reconstruction of the Gondwanaland and has claimed that "the new Lower Cambrian results from the Indian sub-continent make it possible to fit the final pieces of the jigsaw puzzle of Gondwanaland into place". Simultaneously with the publication of this work, McElhinny and Luck [2] have published a paper on "a unique reconstruction of Gondwanaland" and McElhinny [3] has published a paper tracing the history of "formation of the Indian Ocean". The configuration of Gondwanaland proposed in these three papers is essentially one and the same. It follows from McElhinny's statement quoted above that although other palaeomagnetic data from the continents constituting Gondwanaland have been analysed in these studies, the result crucial for the conclusions drawn and that which made these studies feasible is that coming from the Purple Sandstone * Present address: Laboratoriet for anvendt Geofysik og Palaeomagnetisme, Universitetets Mineralogisk-Geologiske Instituter, Ostervoldgade 7, 1350 Copenhagen K, Denmark.
from Salt Range in West Pakistan. In view of the far-reaching conclusions which have been drawn from this data, it is necessary to examine this work [1] in detail and establish its credibility in proper perspective. McElhinny [1] has collected samples from only two sites in the Purple Sandstone and has reported that thirteen samples collected from one of these sites showed unstable magnetisation while ten samples collected from the other site showed stable magnetisation. The palaeomagnetic pole position obtained on the basis of results given by these ten samples coming from a single site has been used [1-3] in drawing the far-reaching conclusions mentioned above. These data however appear to be inadequate to meet the normal statistical requirement accepted by palaeomagnetists for obtaining a reliable and representative mean stable palaeomagnetic direction for a formation. In addition, the palaeomagnetic data is obtained on samples from a region which is known to have had a complex tectonic history [4,5]. While describing the geology of the Salt Range in his paper, McElhinny [1] does not refer at all to this well-known fact. Fig. 1, reproduced from Wadia [4], is a geological section of the Salt Range. It passes through Khewra, the locality from where McElhinny [1] obtained his samples of the Purple Sandstone. The section clearly shows that the formations have been folded, faulted overthmst and inverted. In his paper, McElhinny [1] does not give any details of the structural setting of his sampling sites, other
216
R.N. Athavale, Comments
S.S.E.
N.N.W.
( l l ~ miles S.W. D a n d o t Rly Stn) , " " ~.
Pt. 2061' ~-~ ~ ' r ~ . ~ ~ I I ~" ~ , ~ \ \ ~ ,,, I~.B. Do ndot ~ ~ " 9 . ~ ~ - - ~ . ~ ~ .~ , ~ - . ~ .
8
23567
8
7
G a n d h a l a gorge
( l m i l e E. of W a u l a )
/
~"t
~ ~ ~
3
7
4 7 .
2/_.. - - ~
8
1,6km
76
54
3
2
Pt. 2900' ~ ~ '
-
8
/ * -
~
7
65
E.R. Gee 1.
Lower
2.
Nummulitic (Laki
Siwaliks and
Dandot 3.
Speckled
Ranikot)
coal
Salt
seam
Sandstones.
boulder-bed 4.
(Kamlial
stage).
5.
Magnesian
6.
Neobolus
near the base.
;L
Purple Sandstones.
with
8.
Saline
limestones
and shales
including Talchir
Sandstones. Shales.
series.
at the base.
Pseudomorph
beds (thin
and
irregular).
T. F.-- T h r u s t - fault. E F. -- F o l d - Fault.
Fig. 1. Section across the Dandot scarp from Khewra to Gandhala showing the structural setting of Cambrian and younger rocks from the Salt Range (after Wadia [4] ). than the local value for the angle of dip, which changes from 5 ° at one site to 36 ° at the other (rejected) nearby site. In view of the highly disturbed nature of the formations, even after the raw palaeomagnetic data is corrected for the local structural inclination, a basic doubt as to whether the samples from Khewra could be considered 'in situ', remains to be answered. Demonstration of an agreement in the palaeomagnetic data obtained on adequate number of sites, located at appreciable distance from each other in the Purple Sandstone, could have helped in clearing this doubt. Even if we assume that the paleomagnetic pole obtained by McElhinny [1] truly represents the palaeomagnetic pole for the Lower Cambrian of Salt Range, y e t another fundamental question remains to be answered, before this data is used for fixing the position of India in the reconstructed Gondwanaland. The northern boundaries of the Indian peninsular shield are not clearly defined. This is because the Peninsular shield is separated from the Himalayas and the Salt Range region by the vast stretch of the Indo-Gangetic basin which is known to have an immense thickness o f sedimentary strata of upper Tertiary age. The separation between the Salt Range and the nearest Precambrian exposures in the main mass o f the shield in the
south is of the order of 400 km. The Indo-Gangetic depression has been variously considered as a rift, a deep trough and a deep crustal warping [4]. According to Gansser [6], the sub-Himalayas and the low and high Himalayas are composed of elements which once belonged to the boundary part of the Indian shield, and the northernmost (Tibetan) Himalayas were formed out ofgeosynclinal sediments deposited in the Tethys sea. On the basis of available geological evidence, Wadia [7] has pointed out that the Tibetan (Tethyan) geosyncline overspread into Kashmir and the Hazara territory and the Salt Range. According to him, the Salt Range sediments were laid down in a distant bay of the Tethys geosyncline. The sediments depos. ited in this geosyncline were uplifted and folded [8] into a mountain system, due to tile thrust of the Indian shield, which drifted northwards after the break-up of Gondwanaland.
References [ 1] M.W. McElhinny, Palaeomagnetism of the Cambrian Purple Sandstone from the S~t Range, West Pakistan, Earth Planet. Sci. Letters 8 (1970) 149. [2] M.W. McElhinny and G.R. Luck, Palaeomagnetism and Gondwanaland, Science 168 (1970) 830.
R.N. Athavale, Comments [3] M.W. McElhinny, Formation of the Indian Ocean, Nature 228 (1970) 977. [4] D.N. Wadia, Geology of India (Macmillan, London, 1966) facing p. 141 and pp. 388-392. [5] M.S. Krishnan, Geology of India and Burma (Higginbothams, Madras, 1968) 198.
[6] A. Gansser, Geology of the Himalayas (Interscience, London, 1964) 253. [7] D.N. Wadia, The Himalayan Geosyncline, Proc. Nat. Inst. Sci. India 32 (1966) 527. [8] A. Holmes, Principles of Physical Geology (Thomas Nelson and Sons, London 1966) 1188.
217
COMMENTS ON 'PALAEOMAGNETISM OF THE CAMBRIAN PURPLE S A N D S T O N E F R O M T H E S A L T - R A N G E , W. P A K I S T A N ' b y M.W. M c E l h i n n y , E a r t h and P l a n e t a r y Science L e t t e r s 8 ( 1 9 7 0 ) 1 4 9 - 1 5 6 R.N. ATHAVALE * National Geophysical Research Institute, Hyderabad, India
Received 28 October 1971 Revised version received 29 March 1972
In this paper, McElhinny [1] has obtained a palaeomagnetic pole position at 28°S, 32°E, for the Purple Sandstone of Lower Cambrian age from the Salt Range located in West Pakistan and has assumed that this result represents the Lower Cambrian palaeomagnetic pole for the entire Indian peninsular shield, which is considered to be a constituent member of the ancient supercontinent of Gondwanaland. On the basis of this Lower Cambrian palaeomagnetic pole position, McElhinny [1] has suggested that India should be placed adjacent to East Africa in a reconstruction of the Gondwanaland and has claimed that "the new Lower Cambrian results from the Indian sub-continent make it possible to fit the final pieces of the jigsaw puzzle of Gondwanaland into place". Simultaneously with the publication of this work, McElhinny and Luck [2] have published a paper on "a unique reconstruction of Gondwanaland" and McElhinny [3] has published a paper tracing the history of "formation of the Indian Ocean". The configuration of Gondwanaland proposed in these three papers is essentially one and the same. It follows from McElhinny's statement quoted above that although other palaeomagnetic data from the continents constituting Gondwanaland have been analysed in these studies, the result crucial for the conclusions drawn and that which made these studies feasible is that coming from the Purple Sandstone * Present address: Laboratoriet for anvendt Geofysik og Palaeomagnetisme, Universitetets Mineralogisk-Geologiske Instituter, Ostervoldgade 7, 1350 Copenhagen K, Denmark.
from Salt Range in West Pakistan. In view of the far-reaching conclusions which have been drawn from this data, it is necessary to examine this work [1] in detail and establish its credibility in proper perspective. McElhinny [1] has collected samples from only two sites in the Purple Sandstone and has reported that thirteen samples collected from one of these sites showed unstable magnetisation while ten samples collected from the other site showed stable magnetisation. The palaeomagnetic pole position obtained on the basis of results given by these ten samples coming from a single site has been used [1-3] in drawing the far-reaching conclusions mentioned above. These data however appear to be inadequate to meet the normal statistical requirement accepted by palaeomagnetists for obtaining a reliable and representative mean stable palaeomagnetic direction for a formation. In addition, the palaeomagnetic data is obtained on samples from a region which is known to have had a complex tectonic history [4,5]. While describing the geology of the Salt Range in his paper, McElhinny [1] does not refer at all to this well-known fact. Fig. 1, reproduced from Wadia [4], is a geological section of the Salt Range. It passes through Khewra, the locality from where McElhinny [1] obtained his samples of the Purple Sandstone. The section clearly shows that the formations have been folded, faulted overthmst and inverted. In his paper, McElhinny [1] does not give any details of the structural setting of his sampling sites, other
216
R.N. Athavale, Comments
S.S.E.
N.N.W.
( l l ~ miles S.W. D a n d o t Rly Stn) , " " ~.
Pt. 2061' ~-~ ~ ' r ~ . ~ ~ I I ~" ~ , ~ \ \ ~ ,,, I~.B. Do ndot ~ ~ " 9 . ~ ~ - - ~ . ~ ~ .~ , ~ - . ~ .
8
23567
8
7
G a n d h a l a gorge
( l m i l e E. of W a u l a )
/
~"t
~ ~ ~
3
7
4 7 .
2/_.. - - ~
8
1,6km
76
54
3
2
Pt. 2900' ~ ~ '
-
8
/ * -
~
7
65
E.R. Gee 1.
Lower
2.
Nummulitic (Laki
Siwaliks and
Dandot 3.
Speckled
Ranikot)
coal
Salt
seam
Sandstones.
boulder-bed 4.
(Kamlial
stage).
5.
Magnesian
6.
Neobolus
near the base.
;L
Purple Sandstones.
with
8.
Saline
limestones
and shales
including Talchir
Sandstones. Shales.
series.
at the base.
Pseudomorph
beds (thin
and
irregular).
T. F.-- T h r u s t - fault. E F. -- F o l d - Fault.
Fig. 1. Section across the Dandot scarp from Khewra to Gandhala showing the structural setting of Cambrian and younger rocks from the Salt Range (after Wadia [4] ). than the local value for the angle of dip, which changes from 5 ° at one site to 36 ° at the other (rejected) nearby site. In view of the highly disturbed nature of the formations, even after the raw palaeomagnetic data is corrected for the local structural inclination, a basic doubt as to whether the samples from Khewra could be considered 'in situ', remains to be answered. Demonstration of an agreement in the palaeomagnetic data obtained on adequate number of sites, located at appreciable distance from each other in the Purple Sandstone, could have helped in clearing this doubt. Even if we assume that the paleomagnetic pole obtained by McElhinny [1] truly represents the palaeomagnetic pole for the Lower Cambrian of Salt Range, y e t another fundamental question remains to be answered, before this data is used for fixing the position of India in the reconstructed Gondwanaland. The northern boundaries of the Indian peninsular shield are not clearly defined. This is because the Peninsular shield is separated from the Himalayas and the Salt Range region by the vast stretch of the Indo-Gangetic basin which is known to have an immense thickness o f sedimentary strata of upper Tertiary age. The separation between the Salt Range and the nearest Precambrian exposures in the main mass o f the shield in the
south is of the order of 400 km. The Indo-Gangetic depression has been variously considered as a rift, a deep trough and a deep crustal warping [4]. According to Gansser [6], the sub-Himalayas and the low and high Himalayas are composed of elements which once belonged to the boundary part of the Indian shield, and the northernmost (Tibetan) Himalayas were formed out ofgeosynclinal sediments deposited in the Tethys sea. On the basis of available geological evidence, Wadia [7] has pointed out that the Tibetan (Tethyan) geosyncline overspread into Kashmir and the Hazara territory and the Salt Range. According to him, the Salt Range sediments were laid down in a distant bay of the Tethys geosyncline. The sediments depos. ited in this geosyncline were uplifted and folded [8] into a mountain system, due to tile thrust of the Indian shield, which drifted northwards after the break-up of Gondwanaland.
References [ 1] M.W. McElhinny, Palaeomagnetism of the Cambrian Purple Sandstone from the S~t Range, West Pakistan, Earth Planet. Sci. Letters 8 (1970) 149. [2] M.W. McElhinny and G.R. Luck, Palaeomagnetism and Gondwanaland, Science 168 (1970) 830.
R.N. Athavale, Comments [3] M.W. McElhinny, Formation of the Indian Ocean, Nature 228 (1970) 977. [4] D.N. Wadia, Geology of India (Macmillan, London, 1966) facing p. 141 and pp. 388-392. [5] M.S. Krishnan, Geology of India and Burma (Higginbothams, Madras, 1968) 198.
[6] A. Gansser, Geology of the Himalayas (Interscience, London, 1964) 253. [7] D.N. Wadia, The Himalayan Geosyncline, Proc. Nat. Inst. Sci. India 32 (1966) 527. [8] A. Holmes, Principles of Physical Geology (Thomas Nelson and Sons, London 1966) 1188.
217