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The Penecontemporaneous Deformation of Heavy Mineral Bands in the Torridonian Sandstone of Northwest Scotland
TH E PENECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL BANDS IN THE TORRIDONIAN SANDSTONE OF NORTHWEST SCOTLAND R.
C.
SELLEY
Department of Geology, Imperial College, London (Great Britain)
INTRODUCTION
The Torridonian Formation crops out for a distance of some 320 km along the northwest coast of Scotland. It is of Precambrian age, and is composed essentially of red arkoses and feldspathic sandstones with subordinate conglomerates and shales. The maximum observed thickness is some 4 km. Many of the sandstones carry thin but conspicuous bands and laminae of heavy minerals the petrography of which has been described by PEACHet al. (1907). These heavy mineral bands commonly show disturbed bedding. The purpose of this paper is to describe the various types of contortion which occur, and to suggest that the deforniation was caused by quicksand activity of the enclosing sediment shortly after the deposition of the beds. The same mechanism has recently been attributed to larger scale sedimentary structures which are common throughout much of the Torridonian (SELLEY et al., 1963). The structures described in this paper have been observed on the islands of Skye, Scalpay and Raasay and at Stoer and elsewhere on the mainland. These places extend over a distance of some 120 km.
DESCRIPTION OF HEAVY MlNERAL BANDS
The heavy mineral bands are black, purple or grey green in colour and are composed of magnetite, haematite, ilmenite and leucoxene with accessory epidote, garnet and zircon etc. These grains are of approximately the same order of size as the quartz and feldspar of the host sandstone. These are generally fine to medium in grain-size. Where the heavy mineral bands are undisturbed they are seen to consist of alternating layers of heavy minerals and quartz-feldspar sand. The separate laminae are seldom more than 2 mni in thickness, but the bands may be up to 1 m thick depending on the number o f laminae present. The bands are inipersistent and can seldom be traced laterally for more than a few metres.
PEN'ECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL B A N D S
363
TYPES OF DEFORMATION OF HEAVY MINERAL BANDS
i n a high percentage of the examples the laminae and often the whole bands are seen to have undergone violent contortion and disruption. In some cases the tops of the resulting structures are seen to have been truncated by penecontemporaneous erosion. Thus the origin of these contortions is not tectonic. In vertical section a wide variety of deformational shapes occur but the following four modal types can be distinguished:
Type 1. The heavy mineral laminae are drawn up into sharp crested anticlines separated by broad flat synclines. Amplitudes of up to 1 m have been observed and adjacent peaks may be between 1.5-2 m apart (Fig.lc). Such structures are more commonly developed in Torridonian sandstone devoid of heavy mineral laminae and have been termed Streamers (SELLEY et al., 1963). Type 2. The heavy minerals are concentrated into droplets, a centimetre or so in diameter, each of which has two thin tails which die out upwards (Fig.2b, c). Intermediate forms [Fig.2a) suggest that the individual laminae broke up at intervals of between 5-10 cm along their length. As these segments sagged down into the underlying sand the heavy minerals coalesced to form droplets. Type 3. This kind of deformation also starts with rupturing along the length of a lamina or of several closely spaced laminae. The parts of the laminae adjacent to the ruptures are upturned and occasionally curl over onto themselves (Fig. I a, b). This gives rise to a series of shapes comparable to pseudonodules ( M A C A R , 1948). Although this similarity is morphological rather than lithological there may be some genetic connection (see next section). Type 4. The heavy minerals occur in balled-up masses some of which are 3 0 4 0 cm in diameter. In some the original laminae are still clearly deiined although highly contorted and of irregular thickness. In others the grains of the heavy minerals and quartz feldspar laminae are so intermixed as to be indistinguishable. Usually when this occurs the boundary between the balled-up mass and the enclosing sand is diffuse (Fig.3b). An example was found from which a tail of the original heavy mineral band could be traced out into the surrounding sediment (Fig.3a).
POSSIBLE MECHANISM OF DEFORMATION A N D GEOLOGICAL SIGNIFICANCE
In all the structures described above the field evidence indicates that the movements within the sands were dominantly vertical and that lateral movement was of only minor importance. Clearly the disturbance of the bedding was not produced by slumping of the sediment down a slope. As already mentioned (p.362) a variety of large scale sedimentary structures occur in the Torridonian rocks which are attributed to
3 64
R. C. SELLEY
a
I
100 crn-
b
I
50 cm
4
C
Fig.1. a. Heavy mineral laminae deformed to look like pseudonodules. 500 m west-northwest of Mullach nan Cam, Scalpay island, Inverness-shire. b. Heavy mineral laminae deformed to look like pseudonodules. North side of Clachtoll Bay, Stoer, Sutherlandshire. c. Heavy mineral laminae deformed to streamer structure. West coast of Raasay, 3 km north of Bagh an Inbhire, Inverness-shire.
PENECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL BANDS
365
I
'15 cm
b
I
Fig.2. a. An early stage in the deformation of laminae to droplet structure. 500 m up Allt na Criche, Scalpay, Inverness-shire. b. Droplet structure with well developed tails. West coast of Raasay, Inbhire allt Manishmore. c. Deformed heavy mineral laminae withvery well rounded droplets. North coast of Raasay, 1 km east of Manish point.
366
R. C. SELLEY
I
\
A
I
7 /
A
I 7-
I
/j.,”,
. . ,’
.
,
.
’
.I..
_ :,
..
.
__ .. .. . 30
_J-
b
CII
-
Fig.3. a. Balled up heavy mineral band showing laminae protruding from the main mass. West coast of Raasay, 400 m south of Manish island. b. Balled-up heavy mineral band. 1,100 m north of Camas na Fisteogh, east Coast of Scalpay. Note droplets at the top of the ball and the diffuse core of intermixed heavy minerals and ordinary sand (stippled).
quicksand activity within the beds shortly after deposition. Similar structures have been produced on a small scale in the laboratory (SELLEYand SHEARMAN,1962). Vessels of loosely packed water saturated sand were vibrated. This resulted in a tightening of the packing of the sediment. As the excess pore water escaped vertically it dragged up the sand laminae into shapes analogous to those seen in theTorridonian. The deformed heavy mineral bands and the large scale structures are associated in the field and show many features in common. 1t.k thus reasonable to suppose that they were produced by a similar mechanism. However in the case of the former loading must have played a significant part, the heavy minerals sinking down through the surrounding quicksand by virtue oftheir greater density. It is curious to note theway in which the heavy mineral laminae do not always disintegrate by the diffusion of the heavy mineral grains into the adjacent sand as one might expect. On the contrary the laminae typically preserve their individuality despite contortion and often even thicken as a result of the deformation. This indicates that the heavy mineral grains
PENECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL BANDS
367
were often able to retain their cohesion whilst the quartz and feldspar grains were completely disorganized by the escaping water. This cohesion might perhaps be ascribed to the magnetic attraction of the iron ore which is still retained in the rock today. Disturbed heavy mineral bands can also be observed in Holocene beach sands at Sanna bay, Ardnamurchan, and elswhere along the northwest coast of Scotland. Apart from indicating past quicksand activity in sediments, these structures could be used as criteria of order of deposition when found in rocks that have undergone a high degree of structural deformation. Furthermore they could also probably survive in metamorphic grades at which other useful sedimentary structures had been obliterated.
ACKNOWLEDGEMENTS
The author gratefully acknowledges receipt of a Department of Scientific and Industrial Research maintenance award during the period of this work. Thanks are also due to Dr. J. Watson and Mr. D. 3 . Shearman for critically reading the manuscript.
SUMMARY
Heavy mineral bands have been known from the Torridonian sandstone of Scotland for many years. Recent study of this formation shows that the heavy mineral bands often exhibit disturbed bedding. Four characteristic deforniational shapes are described. In the light of recent work on other sedimentary structures in the Torridonian and on laboratory experiments the deformation is attributed to ancient quicksand activity, the heavy mineral bands sinking down through the enclosing quicksand by virtue of their greater density.
REFERENCES
MACAR,P., 1948. Les pseudonodules du Fammenien et leur origine. Ann. SOC.GPol. Be&., Bull., 12 : B 41-15. PEACH, B. N., HORNE, J., CLOUGH, C. T. and HINXMAN, L. W., 1901. GeologicalStructureof the Northwest H&hlands of Scotland. H.M. Stationery Office, Glasgow, 285 pp. SELLEY, R. C. and SHEARMAN, D. J., 1962. Experimental production of sedimentary structures in quicksands. Proc. Geol. SOC.London, 1599 : 101-102. SELLEY, R. C., SHEARMAN, D. J., SUITON, J. and WATSON,J., 1963. Underwater disturbances in Torridonian sediments of Skye and Raasay. Geol. Map. In press.
C.
SELLEY
Department of Geology, Imperial College, London (Great Britain)
INTRODUCTION
The Torridonian Formation crops out for a distance of some 320 km along the northwest coast of Scotland. It is of Precambrian age, and is composed essentially of red arkoses and feldspathic sandstones with subordinate conglomerates and shales. The maximum observed thickness is some 4 km. Many of the sandstones carry thin but conspicuous bands and laminae of heavy minerals the petrography of which has been described by PEACHet al. (1907). These heavy mineral bands commonly show disturbed bedding. The purpose of this paper is to describe the various types of contortion which occur, and to suggest that the deforniation was caused by quicksand activity of the enclosing sediment shortly after the deposition of the beds. The same mechanism has recently been attributed to larger scale sedimentary structures which are common throughout much of the Torridonian (SELLEY et al., 1963). The structures described in this paper have been observed on the islands of Skye, Scalpay and Raasay and at Stoer and elsewhere on the mainland. These places extend over a distance of some 120 km.
DESCRIPTION OF HEAVY MlNERAL BANDS
The heavy mineral bands are black, purple or grey green in colour and are composed of magnetite, haematite, ilmenite and leucoxene with accessory epidote, garnet and zircon etc. These grains are of approximately the same order of size as the quartz and feldspar of the host sandstone. These are generally fine to medium in grain-size. Where the heavy mineral bands are undisturbed they are seen to consist of alternating layers of heavy minerals and quartz-feldspar sand. The separate laminae are seldom more than 2 mni in thickness, but the bands may be up to 1 m thick depending on the number o f laminae present. The bands are inipersistent and can seldom be traced laterally for more than a few metres.
PEN'ECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL B A N D S
363
TYPES OF DEFORMATION OF HEAVY MINERAL BANDS
i n a high percentage of the examples the laminae and often the whole bands are seen to have undergone violent contortion and disruption. In some cases the tops of the resulting structures are seen to have been truncated by penecontemporaneous erosion. Thus the origin of these contortions is not tectonic. In vertical section a wide variety of deformational shapes occur but the following four modal types can be distinguished:
Type 1. The heavy mineral laminae are drawn up into sharp crested anticlines separated by broad flat synclines. Amplitudes of up to 1 m have been observed and adjacent peaks may be between 1.5-2 m apart (Fig.lc). Such structures are more commonly developed in Torridonian sandstone devoid of heavy mineral laminae and have been termed Streamers (SELLEY et al., 1963). Type 2. The heavy minerals are concentrated into droplets, a centimetre or so in diameter, each of which has two thin tails which die out upwards (Fig.2b, c). Intermediate forms [Fig.2a) suggest that the individual laminae broke up at intervals of between 5-10 cm along their length. As these segments sagged down into the underlying sand the heavy minerals coalesced to form droplets. Type 3. This kind of deformation also starts with rupturing along the length of a lamina or of several closely spaced laminae. The parts of the laminae adjacent to the ruptures are upturned and occasionally curl over onto themselves (Fig. I a, b). This gives rise to a series of shapes comparable to pseudonodules ( M A C A R , 1948). Although this similarity is morphological rather than lithological there may be some genetic connection (see next section). Type 4. The heavy minerals occur in balled-up masses some of which are 3 0 4 0 cm in diameter. In some the original laminae are still clearly deiined although highly contorted and of irregular thickness. In others the grains of the heavy minerals and quartz feldspar laminae are so intermixed as to be indistinguishable. Usually when this occurs the boundary between the balled-up mass and the enclosing sand is diffuse (Fig.3b). An example was found from which a tail of the original heavy mineral band could be traced out into the surrounding sediment (Fig.3a).
POSSIBLE MECHANISM OF DEFORMATION A N D GEOLOGICAL SIGNIFICANCE
In all the structures described above the field evidence indicates that the movements within the sands were dominantly vertical and that lateral movement was of only minor importance. Clearly the disturbance of the bedding was not produced by slumping of the sediment down a slope. As already mentioned (p.362) a variety of large scale sedimentary structures occur in the Torridonian rocks which are attributed to
3 64
R. C. SELLEY
a
I
100 crn-
b
I
50 cm
4
C
Fig.1. a. Heavy mineral laminae deformed to look like pseudonodules. 500 m west-northwest of Mullach nan Cam, Scalpay island, Inverness-shire. b. Heavy mineral laminae deformed to look like pseudonodules. North side of Clachtoll Bay, Stoer, Sutherlandshire. c. Heavy mineral laminae deformed to streamer structure. West coast of Raasay, 3 km north of Bagh an Inbhire, Inverness-shire.
PENECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL BANDS
365
I
'15 cm
b
I
Fig.2. a. An early stage in the deformation of laminae to droplet structure. 500 m up Allt na Criche, Scalpay, Inverness-shire. b. Droplet structure with well developed tails. West coast of Raasay, Inbhire allt Manishmore. c. Deformed heavy mineral laminae withvery well rounded droplets. North coast of Raasay, 1 km east of Manish point.
366
R. C. SELLEY
I
\
A
I
7 /
A
I 7-
I
/j.,”,
. . ,’
.
,
.
’
.I..
_ :,
..
.
__ .. .. . 30
_J-
b
CII
-
Fig.3. a. Balled up heavy mineral band showing laminae protruding from the main mass. West coast of Raasay, 400 m south of Manish island. b. Balled-up heavy mineral band. 1,100 m north of Camas na Fisteogh, east Coast of Scalpay. Note droplets at the top of the ball and the diffuse core of intermixed heavy minerals and ordinary sand (stippled).
quicksand activity within the beds shortly after deposition. Similar structures have been produced on a small scale in the laboratory (SELLEYand SHEARMAN,1962). Vessels of loosely packed water saturated sand were vibrated. This resulted in a tightening of the packing of the sediment. As the excess pore water escaped vertically it dragged up the sand laminae into shapes analogous to those seen in theTorridonian. The deformed heavy mineral bands and the large scale structures are associated in the field and show many features in common. 1t.k thus reasonable to suppose that they were produced by a similar mechanism. However in the case of the former loading must have played a significant part, the heavy minerals sinking down through the surrounding quicksand by virtue oftheir greater density. It is curious to note theway in which the heavy mineral laminae do not always disintegrate by the diffusion of the heavy mineral grains into the adjacent sand as one might expect. On the contrary the laminae typically preserve their individuality despite contortion and often even thicken as a result of the deformation. This indicates that the heavy mineral grains
PENECONTEMPORANEOUS DEFORMATION OF HEAVY MINERAL BANDS
367
were often able to retain their cohesion whilst the quartz and feldspar grains were completely disorganized by the escaping water. This cohesion might perhaps be ascribed to the magnetic attraction of the iron ore which is still retained in the rock today. Disturbed heavy mineral bands can also be observed in Holocene beach sands at Sanna bay, Ardnamurchan, and elswhere along the northwest coast of Scotland. Apart from indicating past quicksand activity in sediments, these structures could be used as criteria of order of deposition when found in rocks that have undergone a high degree of structural deformation. Furthermore they could also probably survive in metamorphic grades at which other useful sedimentary structures had been obliterated.
ACKNOWLEDGEMENTS
The author gratefully acknowledges receipt of a Department of Scientific and Industrial Research maintenance award during the period of this work. Thanks are also due to Dr. J. Watson and Mr. D. 3 . Shearman for critically reading the manuscript.
SUMMARY
Heavy mineral bands have been known from the Torridonian sandstone of Scotland for many years. Recent study of this formation shows that the heavy mineral bands often exhibit disturbed bedding. Four characteristic deforniational shapes are described. In the light of recent work on other sedimentary structures in the Torridonian and on laboratory experiments the deformation is attributed to ancient quicksand activity, the heavy mineral bands sinking down through the enclosing quicksand by virtue of their greater density.
REFERENCES
MACAR,P., 1948. Les pseudonodules du Fammenien et leur origine. Ann. SOC.GPol. Be&., Bull., 12 : B 41-15. PEACH, B. N., HORNE, J., CLOUGH, C. T. and HINXMAN, L. W., 1901. GeologicalStructureof the Northwest H&hlands of Scotland. H.M. Stationery Office, Glasgow, 285 pp. SELLEY, R. C. and SHEARMAN, D. J., 1962. Experimental production of sedimentary structures in quicksands. Proc. Geol. SOC.London, 1599 : 101-102. SELLEY, R. C., SHEARMAN, D. J., SUITON, J. and WATSON,J., 1963. Underwater disturbances in Torridonian sediments of Skye and Raasay. Geol. Map. In press.