PAPERS AND PROCEEDINGS OF THE ROYAL SOCn1TY 0" TASMANIA. VOI,UMF. IOI.
THE ORIGIN OF DEFORMED CONGLOMERATE AND PSEUDO DEFORMED CONGLOMERATE; WITH PARTICULAR REFERENCE TO THE ROCKS AT GO AT ISLAND, TASMANIA
By
A. SPRY' and K. BURNS'
(With two plates and one text figure.)
ABSTRACT DEFORMED CONGLOMERATES Rocks produced by the deformation of con There are claims in the Hteratureon stretched glomeratic sediments strongly resemble pseudo pebble cQnglomer,ates ,that various ellipsoidal ·bodies deformed conglomerates in which tectonic disrup are sedimentary pebbles which have been distQrted, tion of beds, veins and pebbles produce pebble even though, in most cases, no evidence is given ,to like bodies. establish that the objects were originally pebbles. The Goat Island Conglomerate is a tectonic It is difficult (Wilson, 1961, P. 517; Hills, 1963, p. melange containing numerous tectonically formed 127), and indeed in some cases, imposs~ble, to dis fragments but is dominantly a metaconglomerate. tinguish between true and pseudo deformed CQn glomel'ates and some ·authors seem .to be content INTRODUCTION to accept a rock as a ,true deformed conglQmerate Foliated rocks containing fiat or elong'ate frag solely on its appearance. One might, however, ments have been widely recognized as deformed question whether an object looking like a "walking conglomerates (stretched-pebble conglomerates) stick" with dimensions of 100:3:1 Qr like a plank but it is not generally appreciated that rocks of (330 x 30 x 8 cms. Kvale, 1948, II, p. 31) or like different Qrigin (pseudo deformed conglomer'ates) sheets of cardboard of grea,t size (Van Hise, 1904) can resemble them very closely. are really deformed bQulders. A deformed conglomerate is a rock of conglom Deformed conglomerates c,an be classified into eratic appearance in which the fragments are a number Qf groups depending on the internal strongly elongate or fiat with a preferred orienta structure of the fragments which in turn refiects tion. It is a deformed epiclastic rudite. the mechanism of deformation. A pseudo deformed conglomerate contains fiat, elongate or somewhat irregularly shaped fragments 1. Sedimentary. which have been formed ,by tectonic or meta DefQrmation of soft fragments in uncQnsolidated mOl'lPhic processes. It is generally foliated or sediments Qf low strength takes place by inter lineated and is acataclastic rudite. gr·anular movement; the deformation is commonly RQcksat GQat Island, near Ulverstone Qn the continuous and without fracture. The total amount central nor,th coast Qf 'Dasmania, were considered of strain may be large. by Twelvetrees (903), Dunn (926), David (1932) and others to be deformed conglomerates. The first suggestion of the pseudo-conglQmerate nature II. Tectonic. of some Qf the rocks appears ,to have been made (1) Deformation of lithified conglomerate at by ProfessQr E. S. Hills on an A.N.Z.A.A.S. excur shallow depths (e.g. of quartzite-conglomerate near sion in 1949. faults) may cause small amounts of strain. The geological environment has been given by Deformation is discontinuQus, and takes place by Burns (964) and a detailed discussion of the shear along conjugate fractures. The sense and structure will be presented elsewhere. BQth amount of mo'Vement on ,the fractures can be deformed conglomerates and pseudo deformed CQn recognized by the displacement of the outer surface glomerates are present. The "Goat Island con Qf the pebbles but the fractures are" healed ". glomerate" is a tectQnic melange (Greenly, 1919) Ca) In the shallow Beaverhead type (Tanner, derived by the deformation, under low-grade 1964) the acute bisectrix of the shears regional metamorphic conditions, of ,a group of is parallel to .the principle stress axis. conglQmera,tes, quartzites and phyllites. (b) Deformation at apparently greater con A brief review of the nature and origin of fining pressure, but still at apparently metamorphic conglomeratic rocks is presented as shallow depth in the Loch Lomond type a ba,ckground to ·the derivation of possible criteria (Ramsay, 19'64) produces cQnjugate for distinguishing between the various types. fractures similar to ,the previous type '(1) Formerly Department of Geology, Univer.sity of Tasmania, except that the Qbtu&e bisectrix of the Hobart, now Department of Physics, Monash University, Clayton, Vic. shears is parallel to the maximum stress (') School of Earth Sciences, Macquarie University, Sydney. axis. 272 THE ORIGIN OF DEFORMED CONGLOMERATE.
(2) Deformation of conglomer'at,es during deformation is greatest near the thrust (Gold regional metamorphism of low to moderate grade schmidt, 1916; Oftedahl, 1948; Flinn, 1956, 1961; may produce moderate amounts of strain by a Kvale, 1946; 1948; Strand, 1945) but this is in continuous process involving recrystallization doubt. rather than fracture. The mechanism of deforma (3) Some normal deformed conglomerates are tion depends on the physical conditions and enclosed in strongly folded meta-sediments (com ranges from intergranular cataclasis and slip on monly isoclinal with several episodes of folding) one or more internal foliations (Goat Island type) where the pebbles are generally elongate parallel to a kind of recrystallization-low (Fetlar and to fold axes and other lineations (Stockwell and Bygdin types; Flinn, 1962; Oftedahl, 1948). Lord, 1939; Cloos, 1946; Brace, 1955; Hobbs, 1962), (3) At the high temperatures and pressures of However some examples have elongation "down high-grade regional or plutonic metamorphism. dip" or normal to the fold axes (Billings, 1954, p. conglomerates undergo large amounts of strain by 357-8; Paige, 1924; Osberg, 1952). some kind of viscous flow, (New Hampshire type,: (4) Little is known of ,the tectonic environment Walton et al, 1964). of the deformed conglomerates associated with Lithology. gneisses and granulites. Pebbles of quartz and quartzite are most common The mechanism by which near-spherical pebbles but the occurrence of gr,anite (Elwell, 1955), gneiss are changed to ellipsoids is not known but four (Lawson, 19'13), slate (Paige, 1924), limestone kinds have been discussed: (Park and Cannon, 1943) and trachyte (Stockwell 1. The simplest case is that of a sphere which and Lord, 1939) as dominant pebbles has been is converted ,to an ellipsoid by pure shear. The recorded. symmetry of the strain and of the rock is Pebbles in deformed conglomerates appear to be orthorhombic. This fi,ts the ,example of Lawson invariably surrounded by a matrix which may be (913) and Osberg (1952) where the long and siliceous, arkosic, greywacke or pelitic. The pro mean axes of the pebbles lie ina ,foliation and the portion of matrix ranges up to 40% (Billings, 1937), pebble-axes are perpendicular to the fold-axes. It 50% (Gunning, 1941), even 80% (Brace, 1955). has not been regarded as compatible with the The matrix is schistose or mylonitic and has common case where the pebble axes are parallel to apparently undergone considerable movement. the fold axes. "Flattening "by slip along con jugate shears is dominant in the Loch Lomond and The final shape of a pebble depends on its Beaverhead types and is important at Goat Island. initial shape, size, physical properties, the total strain of the enclosing rock and the proportion of 2. A spher,e may be converted to an ellipsoid by strain in the matrix. It is genemUy assumed that simple shear along a single set of shear surfaces. the original pebbles were near-spherical but the The long axis of the ellipsoid is at an angle to original shape has a profound influence on the the shear surf·aces and the symmetry of the move shape af,ter deformation. Variation in shape ment and of the rock is monoclinic. Oftedahl between closely spaced pebbles of the same com (1948) and others have regarded the foliation position is prub8!bly due to differences in original in the matrix as representing the surfaces of slip, shape rather than to inhomogeneity of strain. but they are faced with the anomaly that the long and mean axes of the pebbles 'are parallel to The amount of deformation of a pebble is the schistosity and not oblique. Oftedahl suggested related to its size because the larger pebbles in that the simple shear was followed ,by rotation of some conglomerates are less-deformed than the the pebble into the schistosity but the pebbles are smaller (Gruner, 1941; Osberg, 1952; Elwell, 1955). significantly different from the shape predicted by Mehnert (1938, p. 252) showed tha,t quartzite such a mechanism and no evidence was found of pebbles were elongated to 10 times their least a slip surface in the pebbles. Brace (955) sug diameter whereas micaceous greywacke pebbles in gested ,that the pebbles in the Vermont conglomer the same rock were elongated 45 times. Work of ate could not have rotated because of the lack of Gunning (1941), Kvale (1948, II, p. 28) and Flinn disturbance of fine sedimentary lamination. Shear (1956) showed that a given conglomerate, coarse along a single set of surfaces has operated at Goat acid~igneous pebbles are least deformed, quartzites Island where some fragments ,are oblique to that are next, then greywackes, with pelitic pebbles matrix foliation which passes through them. undergoing greatest deformation. 3. A sphere may be converted to a cylindrical The longest and mean axes of the pebbles are shape by rolling, e.g. a 'ball of putty rolled between generally parallel to the foliation in the matrix the hands takes on a cigar-shape (Hills, 1940; (although this is not always the case) and the Elwell, 1955). Such an hypothesis allows correla long axes are parallel giving a pronounced line tion of pebble axes with the fold axes and with the ation. fabric of the l'ocks, but pebbles such as Kvale's 100: 3: 1 "walking sticks" would require a great Deformed conglomerates ,belong to a number of deal of rolling. The hypothesis is inadequate to tectonic environments: ac'count for the very fia,t pebbles which are common (1) The Beaverhead 'and Loch Lomond types in deformed conglomerates. with their fractured pebbles are related to steep 4. Various complex combinations of "rolling and fauLts. flattening" (,Flinn, 1962) may be closer to the truth (2) Some normal deformed conglomerates, (e.g. and many authors have considered a rather vague Bygdin type) occur in regions of alpine tectonics. process of rock flow and viscous drag. This ,concept it has been said that the elongation is related to was recognized as long ago as 1862 when Scrope movements parallel to large thrusts, and that discussed the elongation of vesicular cavities in A. SPRY AND K. BURNS 273
lavas; Harker (1885) applied it to the deformation are characterized by orthorhombic symmetry of of fossils &c., in metamorphic rocks and Flinn (1956, form, pointed ends, incompletely disrupted boudins, p. 503 showed experimentally its application to the lines of closely spaced fragments of similar size, deformation of conglomerates. This hypothesis shape and composition, and by a foliation wrapping requires thwt the main flow in the rocks is parallel around the boudin. Some examples contain internal to fold axes and this is not accepted by many shears which are symmetrically developed ,as two workers. sets at 40° to each other with the long axis of the PSEUDO DEFORMED CONGLOMERATES boudin as the acute bisectrix. These rocks contain fiat, elongate or somewhat Asymmetrical 'boudins formed by the intersection irregularly shaped fragments in a foliated matrix of compositional layering and a foliation are related and resemble deformed conglomerates very closely. to the cleav·age mullions of Wilson (1953). Shear They may result from the following processes:- ing takes pl-ace along the bounding surface of the mullions. Rotation of the cleavage mullions on the (1) Mechanical disruption (cataclasis) of various limbs of folds produce "lozenge-shaped" cleavage elements which 'were originally planar ('beds, veins, boudins (Rast, 1956>. Baird (1962) showed that this dykes) ,cylindrical (mullions, quartz rods) or process produced pseudo deformed conglomerates in ellipSOidal (pebbles, boudins). the cores of folds where curved and intersecting (2) Growth of ellipsoidal bodies, e.g. elongate, near-axial fracture-cleavage sliced quartzite layers pebble-like blebs of quartz or pegmatite in schist into pebble-like rods. All gradations from undis (Quirke and Collins, 1930; Wilson, 1953; Reynolds, sected folds to tectonic breccias with relict isola·ted 1961) . fold hinges were found. Matley described similar (3) Accumulation of elongate clastic fragments rocks as long ago as 1899 and noted that the harder under normal sedimentary conditions. Lineated layers were drawn out into lenses, rolled into strings quartzites commonly weather into cigar-shaped or broken into fragments. ellipsoidal fragments which might pack together to Christensen (1963) showed the importance of give a conglomerate with elong.ate fragments. With bedding-cleavage intersections in producing len slight deformation a cleavage might be produced ticular layering which simulated flattened and and the pebbles rotate to become parallel to the stretched pebbles. The limbs of folds were dis cleavage (Pettijohn, 1943; Glenn, Donner and West, rupted by slip on closely spaced c1eavage and the 19'57) . fragments were rotated to form lenticular frag Pseudo-conglomerates produced by the first ments lying with their long and mean axes parallel (tectonic) process above, have been 'found in regions to the cleavage, and their long axes paranel to the where there are alternations of thin layers of differ bedding-cleavage intersection. Cores of isoclinal ing competency. They are commonly associated folds were detached and rolled into cylinders. with isoclinal folds in rocks which have been These processes have all operated at Goat Island folded several times. The long-and mean-·axes of and Picnic Point. Quartzite layers and quartz veins the pseudo-pebbles are parallel to the foliation of have been sliced through by an oblique, penetratiye - the rock, and .the long axes impart a lineation foliation leaving lines of en echelon lenses resembl which is parallel to adjacent fold axes and other ing pebbles. Large fragments (some of which were lineations. Such rocks are not uncommon and have proba,bly boulders but some certainly detached been described by Paige (1924); Wegmann (1934); fragments of beds) and fold mullions (with folded Engel (1949); Bailey and Tilley (1952); Sutton and cores visible) have been sliced through 'by inter Watson (1952) ; Wilson (1953, 1961) ; King and Rast secting foliation surfaces. The smaller fragments (1956); Ramsay (956); Back (961); Baird are still in contact in many places hut others are (1962); Hills (1963); and Christensen (963). slightly or greatly displaced. These commonly have Pseudo-pebbles area type of boudin. Pseudo concave surfaces. Nes·ted fragments and .. tad deformed conglomerates have ,been called poles" are also commonly formed in this way and "boudinees" (Wegmann, 1934) and the deforma many thin wisps and flakes have been sliced off tion processes are rela.ted to ·boudinage. Two main larger bodies. genetic types ofboudins may be recognized; the flrst is produced by non-rotational strain and could GOAT ISLAND CONGLOMERATE be called the symmetrical type (Ramberg, 1955) Abundant exposures of the rock known as .. Goat and the se,cond is produced by rotational strain and Island Conglomerate" on the 'beach and wave-cut could be called the asymmetrical type (Hills, 1940; p·laUorm between Goat Island and Picnic Point Wilson, 1953; Rast, 1956; Baird, 1962; and (Burns. 1964) are composed of detached, rootless Christensen, 1963). quartzite bodies of various sh·apes and sizes in a The most common, simple, symmetrical boudins sparse, foliated quartz-mica (phyllite) or quartzose are sausage-like with one long axis, or barrel matrix. The quartzite bodies include many pebble shaped (Rast, 1956) or approximatelyequidimen lik·e or boulder-like objects which range from a sional (chocolate-block). They are formed by the centimetre or so to a metre in length, with the flow of less-competent past more-competent layers dimensions 30 x 15 x 7 cms. being common. Most so that ,the more viscous or brittle layers are necked fragments are composed of medium ,to fine-grained or ruptured to give lenticular, oval or blocky cross quartzite, mainly white but some pink or grey, some sections. Ramberg (1955) produced this kind of folia-ted and some massive. Grey flaky fragments of boudinage experimentally ,by flattening. phyllite are common in some places. Many quartzite layers, quartz veins, quartzite The rock is monolithologic and a comprehensive fold-mullions and pebbles at Picnic Point and Goat search has failed to locate a range of rock types Island have been fragmented by this process. They among the fragments; this would have established
R.S.-20 274 THE ORrGlN OF DEFORMED CONGLOMERATE. the existence of primary conglomerate. True con The main fabric-elements consist of a girdle of glomerate is known from the Ulverstone Meta quartz [0001] normal to the long axis of pebbles morphics at Spalford, 10 miles to the south (Burns, a·nd boudins and to the lineation of interbedded 1964), and conglomerate on the beach midway quartzites. A small-circle concentration with an between Goat Island and Picnic Point is only very axis perpendicular to the major folia,tion is most slightly deformed (Plate D. Much of the Goat notable in the boudins where its axis is parallel to Island rock was probably originally conglomerate. the shortest axis of the boudin. The girdle normal The shapes of the fragments are not generaHy to the lineation is strongest in the quartzites. geometrically simple and it is difficult to classify Examination of petrofaibric diagrams for quartz them but the following twelve types are most in deformed pebbles from other conglomerates by common at Goat Island:- Strand (194,5), Brace (1955), Flinn (1956) and 1. Sub-spherical simple ellipsoids. Koark (961) and in boudins by Delitsin, (1964) 2. Simple triaxial ellipsoids-prolate (cigar sug'gests some features in common. Most important shaped) . is the persistence of the plane of symmetry normal 3. Simple triaxial ellipsoids-oblate (plate- to the long axis of the pebble. The common lack of shaped) . quartz optic axes near the long axis of the pebble 4. Platy flakes or wisps. may be ,expressed as a single strong girdle (Strand, 5. Asymmetrical twisted ellipsoids. 1945), as -two girdles, symmetrically disposed about 6. Concave-convex ellipsoids. the long axis (Brace, 1955) or as a cleft girdle 7. Tadpole-shapes (asymmetrical ellipsoids with whose axis is the long axis of the pebble. The one rounded bulbous end and a sharp fabrics at Goat Island show all three tendencies. " tail ") . A general lack of optic axes near the mean pebble 8. Sub-quadrate ellipsoids. axis causing a symmetrical break in the main girdle 9. Single-boudinaged ellipsoids (with one neck) . pattern is also seen in the diagmms of Strand. 10. Multiple-'boudinaged ellipsoids (with several The tendency for optic axes to be concentrated necks) . near the short axis (with perhaps a broken small 11. Nested groups. circle near this axis) is also shown by Flinn. 12. Irregular shapes which cannot be named. In general, the Goat Island fragments have fab This classification is somewhat arbitrary but rics which are most similar to Flinn's Fetlar some outcrops mid-way between Goat Island and specimens. The degree of preferred orientation is Picnic Point are composed mainly of ,types 1 and not nearly as high as Brace's Vermont specimens in 2; strongly deformed conglomerates at Goat Island which no diagram contains a maximum weaker than cont,ain abundant examples of types 2 to 8; pseudo 5%, with abundant 8% maxima and one of 16%. deformed conglomera.tes from Picnic Point are Maxima of 3, 4 and 5% are found in diagrams rich in types 9 to 12. from Bygdin, Fetlar and Goat Island. At the Goat Island outcrop, the long axes of the parallel ellipsoidal fragments constitute a Neither Brace BED OR VEIN PEBBLE (0) (C) RAMBEl1G (1955) ------1 BED OR VEIN PEBBLE (e) (d) RAST (1956) RAMSAY (1956) CHRISTENSEN (1963) HILLS (1963) PAPERS AND PROCEEDINGS OF THE ROYAL SOCIETY OF TASMANIA, VOLUME 101. 1 2 3 4 5 6 PLATE 1.- No. 3.-Boudinaged qUartzite layer, strongly necked. No. I.-Thick quartzite, Picnic Point. The foliation No. 4.-Isolated quartz lenses in phyllite due to dis in enclosing phyllite is S, parallel to S2' The ruption of a quartz vein. boudin node is milky quartz. No. 5.-Boudinaged isoclinal folds in quartzite, Picnic No. 2.-Necked pebble, Picnic Point. A quartzite Point. layer just above the fragment has been sliced No. 6.-Pebbles disrupted at a shear surface crossing into pebble-like fragments. the foliation at a low angle. F.P.278 PAPERS AND PROCEEDINGS OF THE ROYAL SOCIETY OF TASMANIA, VOLUME 101. 1 2 3 4 5 6 PLATE II.- No. 4.-Folded pebbly layer in phyllite, eastern end No. 1.-Almost undeformed conglomerate, If central of marine platform, Goat Island. The principal outcrop", (mid-way between Goat Island and foliation in the phyllite, 8 2 is axial-surface Picnic Point). with subspherical boulders in a to the near-isoclinal folds. phyllite matrix. 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