Fluid escape structures in metagabbroic ‘sediments’, SE Jersey, Channel Islands

Fluid escape structures in metagabbroic 'sediments', SE Jersey, Channel Islands

c. H. Key KEY, C. H. 1985. Fluid escape structures in metagabbroic sediments, SE Jersey, Channel Islands. Proc. Geol. Ass., 96 (2), 153-159. Layered diorites in SE Jersey have been derived fro~ an earlier layered gabbro by metasomatic recrystallization caused by granite intrusions. Relict patches of metagabbro sttl.1 possess much of the primary mineralogy and cumulate te~tures and .structures,. the m~st distinctive of which is small~scale or 'microrhythmic' layering. Dlsr~pted.mlcrorhythml~ I~yermg appears analogous to the dish structures described from some clastic sediments and ~ similar origin, by fluid escape from a compacting, unconsolidated crystal mush, IS proposed. This process of contmuous crystal compaction and return of pore fluid to the mam magma body has some Important implications for modern cumulus theory especially the ' model of in-situ crystallization from a static boundary layer.

Department of Geology, Goldsmiths' College, Rachel MacMillan Building, Creek Road, Deptford, London SE83BU

1. GEOLOGICAL SETTING In the igneous complex of SE Jersey several Cadomian post-tectonic granites have intruded and disrupted a Precambrian gabbroic pluton (Adams, 1976). The hydrothermal and chemical effects of these granites have transformed most of the basic rocks into metasomatic diorites which still often retain some chemical, mineralogical and textural evidence of their gabbroic parentage (Bishop, 1963; Key, 1974, 1977). Much of the diorite, including that which has undergone partial melting and limited rheomorphism, possesses a layered structure which is also thought to be inherited from its gabbroic precursor (Bishop & Key, 1983). At Le Nez Point (Fig. 1) there is an isolated 'island' of relatively unaltered, relict metagabbro which although showing some effects of recrystallization stili re~ains much of its original gabbroic chemistry, mineralogy, texture and structure. Within this 60 m thick sequence at Le Nez there are numerous features which are similar to those found in other well documented layered intrusions and which are currently attributed to cumulate processes. All the layering in the diorites and metagabbros of SE Jersey now dips NE at between 35° and 65°, but i~clined anti-gra.vity pipe structures orientated perpendicular to layenng in the diorites imply that all the layering originally had a near horizontal attitude ~Bi~ho~, '1963; Bishop & Key, 1983). The present inclination of the layered rocks is due to their disruption by the later granites, and possibly resulted from the rotation of large foundered blocks during cal~era collapse within this high level, sub-volcanic regime (Duff, 1981).

2. THE LE NEZ LAYERED SEQUENCE The layering in the metagabbros varies from an igneous lamination marked by the preferred orienta-

tion of plagioclase laths, to a more obvious banding or 'm~d~l lay:ring' (Irvine, 1982) produced by slight vanations m the proportions of plagioclase and pyr?xene. Within the section there are conspicuous honzons of comb layering comprised of 10-20 em thick layers of spinifex textured rock in which curved branching, mafic crystals are orientated perpendicula; to the plane of layering. These are very similar to the harrisitic or crescumulate rocks described from other layered basic intrusions (Wager & Brown, 1968). The most spectacular type of layering, however, is the sm~ll-scale or 'microrhythmic' layering (Irvine, 1982) which occurs at two horizons in the Le Nez sequence and consists of alternating 1-2 em thick layers of light and dark rock. The form and field appearance of layering in the gabbros closely parallels that in many basic plutons and it is believed that it resulted from cumulate processes operating in a basic magma chamber. The small volume and very limited 'stratigraphic' range of the relatively unaltered Le Nez gabbros makes further elucidation of the petrogenetic processes extremely tenuous, esp.e~ially in the light of current controversy over the. ongm of cumulates (McBirney & Noyes, 1979; Irvine, 1982). This paper is concerned with an unusual structure within the units of microrhythmic layering, and some possible consequences for the interpretation of cumulate rocks in general. (a) Disrupted microrhythmic layering

The .upper of. the two horizons of microrhythmic l~yenng compnses a 0.45 m thick unit of alternating light an? dark l~yers, each 1-2 em thick, and closely resembling the 'inch scale' layering of the Stillwater Complex (Hess, 1960) and the 'small scale' layering of Skaergaard (Wager & Brown, 1968). The layers are perfectly parallel and laterally continuous over the whole of the Le Nez outcrop, a distance of several tens

153

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Jersey Shale Form at ion

Fig. 1. Geo logical sketch map of Je rsey showing the distr ibution of the major rock types and the location of the layered metagabbros at Le Nez Point (dykes and other minor intrusions are omitted) . Published by permission of the Director , Institute of Geo logical Sciences (Nat ura l En vironm ent Research Council).

of metre s. The leucocratic layers display a wea k igneous lamination mark ed by a roughly preferred orie ntation of feldspar and mafic crystals parallel to the layering. The melanocratic layers are more variable; some are dark with shar ply defined lower boundaries, whilst others contain more plagioclase and appea r more diffuse. Thickness and composition do not vary along strike. Sharp bases to the dark layers and relatively gradational junctions with successively overlying light layers suggests some degree of mineral grading. Reverse grading has not been observed . This layering is unusu al in that at a num ber of places it has been disrupt ed . Mafic layers which are otherwise regular and continuous develop breaks, and the ruptur ed edges are deformed into an upward curve which suggests some force applied from beneath . At its most extreme the disrupted layering consists of isolated fragment s of mafic layers 2-10 em in length which are similarly deformed to produce upward curving edges. In two dimensions all these mafic fragment s are crescent-shaped (Fig. 2) and always concave upwards, and although there are no surfaces

parallel to the layering, three dimension al expos ures suggest that the se layer fragments are polygonal in plan and saucer , or dish, shaped . Their shape is stro ngly emphasised by the presence , wherever disrupti on has occurred, of a narrow 1- 3 mm thick rim of plagioclasite at the base of the mafic fragment. This felsic rim is always present when mafic layers are partly broken or completely disrupt ed , but never where they are continuous. (b) Petrography

Even the least alte red rocks in the Le Nez seque nce show some effects of metamorphi c recrystallization and many of those which are chemically gab broic and still possess a relict ophitic textur e have had almost all the primary miner als repl aced by lower temp erature hydrated minerals and sodic plagioclase. Where primary miner als are pr eserved they comprise salitic clinopyroxene (en4(jW049fsl1) and calcic plagioclase (An64) together with accessory amounts of apatite and skelet al Fe-Ti oxides . The pyroxene is seen in all

FLUID ESCAPE STRUCTURES, JERSEY

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Fig. 2. Microrhythmic layering at Le Nez Point. The partially broken layers and crescent shaped dish structures have a lower rim of plagioclasite which is absent from the continuous mafic layers (ten-pence coin as scale).

stages of alteration to green and brown hornblende and the feldspar develops turbid, saussuritized cores with clear rims of fresh oligoclase. Although most of the gabbros contain substantial amounts of normative olivine, this mineral does not occur modally. Instead, individual grains or structures such as thin layers, mafic clots and the spinifex horizons, which might originally have been olivine-rich segregations, now consist of a wholly secondary assemblage of chlorite, actinolite, cummingtonite and minor epidote in a fibrous intergrowth. The bulk chemistry of these mafic structures (Bishop & Key, 1983, Table III) can be re-cast as a mixture of olivine and plagioclase, and it would seem that its complete replacement was due to olivine being the least stable mineral and most susceptible to recrystallization under the superimposed hydrothermal conditions. The mafic layers and crescent-shaped fragments in the disrupted microrhythmic layering are composed predominantly of this chlorite-fibrous amphibole assemblage and are therefore interpreted as originally olivine-rich layers. Skeletal Fe-Ti oxides and brown hornblende with cores of clinopyroxene are also present but in subordinate amounts. The felsic rim at the base of the mafic fragments consists entirely of equant grains of plagioclase which have euhedral, turbid cores and clear margins of oligoclase. Similar plagioclase, but usually of more tabular form, together

with clinopyroxene partly or completely replaced by green and brown hornblende in a sub-ophitic texture, comprises the lighter layers and forms the matrix surrounding the disrupted mafic fragments. Boundaries between mafic fragments, feldspathic rim, and matrix are highly irregular on a microscopic scale but always very sharp (Fig. 3). There is no petrographic evidence that the layering has been tectonically deformed subsequent to the complete solidification of these gabbros.

3. ORIGIN OF DISRUPTED LAYERING The mechanism of formation of microrhythmic layering in basic plutons is not clearly understood. The scale of this feature and, in many instances, its lateral persistence seem to preclude an origin by magma convection and crystal settling. At the time when layered gabbroic rocks were almost universally accepted as products of crystal settling Wager and Brown (1968) suggested that regular layering on such a small scale must have been produced by some other mechanism such as supercooling and rhythmic nucleation. More recent elaborations on the mechanism of cumulate formation suggest that in-situ crystallization at the floor of the magma chamber might play an even greater role in the formation of layered rocks (Campbell, 1978; McBirney & Noyes,

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5mm. Fig. 3. Photomicrograph of a single dish structure in cross section . The crescent shaped mafic fragment , its lower rim of plagioclasite , and the surrounding matr ix are all separated by an irregular , but sharp, junction (outlined in black to the left of the dashed line, for clarity). A-fibrous intergrowth of actinolite , cummingtonite and chlorite ; B-brown and green hornbl ende with relict patches of clinopyroxene ; C---skeletal Fe-Ti oxides with minor amount s of sphene ; D-epidote ; E-rim of plagioclasite-cquant grains of calcic plagioclase have saussuritized cores and clear rims of oligoclase. Th e matrix surr oundin g the dish structure is comp osed of a more even distribu tion of the same minerals . (Taken in P.P .L. )

1979), and comparisons have been made between the disrupti ve force , which he called sedimentary small-scale rhythmic layering and Liesegang crystal- stoping, comes from the release of a horizontal body of water trapped bene ath the disrupted bed. Whether lization . Whatever the mechanism , whether crystal settling one or both of the se processes oper ate, it appea rs that or in-situ crystallization , it seems likely that at the time the upward movement of water can be by slow of formation cumulates consisted of a framework of seepage , or more rapid and result in liquefaction when solid crystals and interstitial pore liquid, and physically the sediment becomes temporarily fluid-supported. resembled a grain-supported clastic sediment. Esti- The escape of water is counterbalanced by settling and mates of the porosity vary between 35 and 50 percent compaction of the sediment and it is this contraflow (Wager & Brown, 1968). The strongly zoned margins which disrupts the more coherent argillaceous laminae of many cumulate primocryst minerals is taken as into fragments which become deformed around their evidence for the final solidification of this trapped edges by the upward drag of the fluid to form dish interstitial magma. Further analogies can be drawn structures. The trigger mechanism has been attributed with unconsolidated clastic sediments from the to either mass flow downslope of an unstable pile of frequent occurrence in many intrusions of post sediment or earthquake shock waves. It is suggested here that a similar mechanism was depositional, soft sediment deformation structures such as slumping , convolute bedding and load responsible for the disruption of microrhythmic structures (for example, Wadsworth , 1973; Parsons, layering in the gabbro at Le Nez. Shortl y after deposition (or in-situ crystallization) the units of 1979; Lee , 1981; Parsons & Butterfield, 1981). A relat ively recently recognized structure in clastic microrhythmic layering consisted of alternating mafic sediments was termed 'dish structure' by Wentworth and felsic layers which prob ably had differing (1967) and interpreted by Lowe and LoPiccolo (1974) porositie s due either to different textures, as a result and Lowe (1975) as resulting from the escape of pore of crystal shape and habit, or to different degrees of fluid from laminated sands and silts which caused crystallization . It is possible that the olivine-rich originally coherent layers to rupture and form layers , now replaced by hydrated alteration products , polygonal , concave upwards 'dishes'. Allen (1982) were almost completely solid due to adcumulate prop osed a modification to this mechanism in which growth of a mafic 'crust' on the surface of the sediment

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FL U ID ESCAP E ST RUCT U RES . J ERSE Y

pile . Shock induced liquefaction of this unconsolidated magmati c sediment would cau se compaction and result in the upward movement of th e interstitial magma or pore fluid . Fluid escape would only occur when the relatively impermeable mafic layers became disrupted. The fact that the y are broken into lar ge fragments rather than dispersed crystals is evidence that the mafic layers were more coherent than the feldspar-rich layers. Compared with water satura ted sediments the pore fluid in ga bbroic cumulate s would have had conside rab ly greate r viscosity and , whilst this would initia lly tend to inhibit liquefaction , once initiated th e greater dr ag effect would be mor e likely to cau se disruption and produce curvatur e of the mafic fragm ent s to form dish shapes. Th e pre sence of felsic rims of plagiocl asite on the lower conv ex surfaces of the mafic dish es, and their occurrence only where the layering is disrupted, could be due to the crystallization of upw ard-moving pore fluid. If thi s interstitial magma was enriched in plag ioclase co mponent due to adcumulate crystallization of olivine, its contact with the undersurface of the dish structures could have caused het erogeneous nucleat ion and crystallization of plagioclase. If this po re fluid was also crystallizing pyroxene with, or with out , olivine their formulation at the lower surfaces

of dish structures would be inhi bited by elutriation , Th is would, on a sma ll scale , tend to sepa rate buoya nt plagioclase fro m denser mafic miner als. A ltho ugh not part of the disrupted layer ing, th ere are closely adjacent rocks within th e Le Nez seq uence which appear to show more direct evide nce of fluid escape. Sever al horizon s with othe rwise regular microrhythm ic, mod al layerin g possess irregular feld spath ic veins and stringers oriented ro ughly perpend icular to , and cutt ing, th e layered roc ks (Fig. 4). Th ese veins consist alm ost enti rely of relat ively sodic plagioclase with a little chlorit ized bio tite . Th e way in which, at any particular hor izon , th ey all appear to origina te within one layer and termina te several centime tres higher up as diffuse , inte rgra nular wisps, suggests that they formed as an integral part of the layer ed rocks, rather than as a distinctly sepa ra te, later, intrusive phase . Veins of the latt er type do occur at Le Nez but they are composed of qu artz and K-feldspar, are usually parallel-sided , and cut across all pre-existing str uctur es in a completely ra ndo m mann er. Th ese intrus ive quartz-K-feldspar veinlets are rel ate d to the widesprea d injection of gra nite pegmat ites in SE Jersey, whereas the plagioclasite stringers ap pear more likely to have res ulted fro m the expulsion of fraction at ed pore fluid fro m unconsolidat ed cumulates. Donaldson (1982) described similar feld spathic veins from th e Rhum intrusion which he att ributed to cum ulate compaction , filte r pressing, and upward expulsion of pore fluid. The migratin g int erstitial liquid in th at case apparently 'ponded' ben eath relative ly impermeable layers and crystallized to produce pods and layer s of harri site .

4. DISCUSSION

Fig. 4. Microrh ythmic layering showing bot h modal and grain size varia tion. The wispy, feldspat hic veins oriented perpend icular to the layering have resulte d from the upward seepage of intercumulus liquid .

Dish struc tures have not previou sly bee n described from igneo us 'sediments' but there are num erou s accoun ts of defo rma tional structures in laye red gabbroic rocks which appear to be due to the mov em ent of an unconsolidated crystal mush and interstitial pore liquid . Wadsworth (1973) described a variet y of depositional and post-de positional structures in 'magmatic sediments' which he suggests indicate an uncon solidated crysta l mush 2- 3 m thick . He restricts post-depositional struc tures to slumping caused by lateral movement and sta tes that comp action and load structures due to vertical displacem ent of mat erial have not bee n posi tively ident ified . McBirn ey & Noyes (1979) are also of the opinion that there is no evidence of gra vita tiona l sett ling or compact ion within the crysta l mush of the Skaer gaard intrusion and cite thi s as part of their evidence for a 'sta tic bo unda ry layer '. In their mod el , at depths below abo ut one metre , and certa inly where the crystals were in gra in to grain contact , the sta tic boundary layer is envisage d as having prop erties close to those of solid rocks. Th ey

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do, however , accept filter pressing as a means of rocks existed for a period of time as a crystal mush and expelling pore fluid into dilational cracks to produce interstitial pore liquid prior to their compl ete 'pegmatoidal layering' . solidification, whether the crystal mush was produced In the Klokken gabbro-syenite complex Parsons by gravity controlled deposition or in-situ crystalliza(1979) described 'disturbed layering' and lamination tion . This situation is analogous to the physical disrupted by 'swirl structures', and Parsons and conditions existing in unconsolidated , grain-supported , Butterfield (1981) cite load structures as evidence of clastic sediments and many of the structures in layer ed gravitational comp action and expulsion of pore fluid. basic rocks can be explained by th e same postOn this evidence Parsons and Butterfield (op . cit. ) depositional processes which 'are kno wn to deform suggest that the static boundary layer of Campbell clastic sediment s. The se processes involve either (1978) and McBirne y and Noyes (1979) is not later al movement , which result s primarily in mass flow applicable to the formation of the Klokken layered and slumping, or vertical movement which results in rock s. Some leucos yenit e hori zons in the Klokken loadin g and comp action of the solid and upward intrusion are thought by Parsons and Butterfield to displacement of the pore fluid. The latter can be represent intercumulus mate rial expelled from ultra- slowly continuous (seepage) or, under favourable mafic layers at lower levels by filter pre ssing. conditions, more rapid due to liquefaction which can Pegmatitic textures and hyd rou s mafic miner als in the disrupt and deform originally regul ar lamination. It is upper parts of flame structures lead them to believe believed that this process of rapid fluid escap e was that the intercumulus liquid was more akin to a responsible for the disruption of microrhythmic hydrothermal fluid than a silicate melt. layer ing in the Jersey gabbros at Le Nez and produced Some of the most convincin g post-depositional fragment s of deformed mafic layers analogous to the sedimentary structures are illustrated and described by dish structures in clastic sediments. This rapid fluid escape is prob ably relati vely rare Lee (1981) from the Bush veld intrusion. Some of the se are attributed to crystal mush compaction and fluid and its disrupti ve effects might only be detectable in escape , an interpretation supported by the pre sence of small-scale or microrhythmic layering which is itself an coars e-grained , plagioclase-rich stringers oriented uncommon feature of layered basic plutons. Slower perpendicular to the layerin g and apparently resulting seepage of pore fluid due to gravitational loadin g and from the seepage of intercumulus liquid from below . compaction of the crysta l mush is probably much more Occasionally there are better defined liquid escape common , if not inevitable, in crystallizing basic structures analogous to the pillar structures in clastic plutons and might be difficult Of impossible to detect. sediments and which in the Bushveld chromitite Accentuation of igneous lamination and deformation horizons clearly disrupt the fine-scale layering and of individual crystals might be the only obvious produce upward curving edges to the mafic layers petrographic effects of compacti on. The upward moving pore liquid would presumably undergo parti al (Le e, fig. 4). The Muskox intrusion pro vides geochemical evi- crystallization and hen ce subject the interstitial magma dence for the compaction of crystal mushe s and to a second stage of fractionation (Irvine, 1980). The upward movement of pore liquid . Here , Irvine (1978; main effects of these post-depositional proce sses are 1980) has shown that the displacement of composi- threefold: tional discont inuities in a rhythmically layered (i) Ad cumulu s growth would be controlled by the composition and rate of percolation of interstitial sequence can be explained by the upward migration of liquid from lower stratigraph ic levels. Pre viously pore liquid causing infiltra tion metasomatism at higher applied meth ods of estimating the composition of stratigraphic levels where cumulus crystals were not in contemporaneous magmas from the intercumulus equilibrium with the modified intercumulus liquid. material (Brown , 1956; Henderson , 1970) may be Irvine estimates that the pore liquid was displaced vertically at least 100 m and possibly as much as 250 m, invalid for rocks which have undergone comp acand suggests that the po ssibility of continuously tion . returning as much as 50 percent of the pore liquid to (ii) The possibilit y of continuously returning some the magma bod y could 'significantly affect the primary pore liquid to the magma body by gravity stage of crystal fractionation ' (Irvine, 1978, p. 751). induced filter pressing may not affect the cour se Irvine also points out that the migrating pore fluid of fraction al crystallization but would increa se would probably undergo so me crystallization and the proportion of strongly fractionated residual hence a secondary stage of fraction ation which would liquid . be one of the principal factors in controlling the (iii) The return of the expelled pore liquid to the degree and type of adcumulus growth. crystal mush-magma interface invalidates the static boundary layer model of McBirney and 5. CONCLUSIONS Noyes (1979) in those intru sions where comp acTheoretical considerations supported by petrographic tion has taken place . Furthermore , since this evidenc e suggests that in layered basic intrusions the 'returned' liquid would have und ergone secon-

FLUID ESCA PE STRUCTUR E S, JE RSEY

dary fr actionation due to adcumulus cr ystallization , a nd hence be enriched in low melting temperature components , its diffusion to the cry st al mush-magma interface would ha ve the effect of inhibiting any in- situ cr yst allization .

ACKNOWLEDGEMENTS I th ank Drs. A. C. Bishop , D . G . Helm and P. J .

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a nd suggesti ng im prove me nts, a nd Dr. K . T . Pick ering for helpful discu ssion on the sedimentary as pec ts of th ese rock s. I am indebted to Mr. T om East er for photogr aphic ass ista nce , and to U rs ula Wood who kindly prepar ed severa l versions of the typescript. Financi al suppo rt for the field work from G oldsmiths' College R ese arch Committee is gra tefu lly appreciate d .

Will iams for criticall y reading th e o rigina l manuscript

References AD AMS, C. J. D. 1976. Geo chron ology of the Channel Islands and adjacent Fren ch mainland . 1. geol. Soc. London , 132, 233-50. ALLEN, J . R. L. 1982. Sedimentary Structures: Their character and physical basis, Vol . 11. Elsevier, Amsterdam, 663 pp. BISH OP , A . C. 1963. Dark margins at igneo us contacts: a critical study with special refer ences to those in Jersey, C.I. Proc. Geo l. A ss. L ondon, 48,247-75. - - & C. H. KEY , 1983. Natur e and origin of layering in the diorites of SE Jersey, Chann el Islands. J. geol. Soc . L ondon , 140, 921- 937. BROWN , G . M. 1956. The layered ultrabasic rocks of Rhurn , Inner Hebrides . Phil. Trans. R. Soc ., B240, 1- 53. CA MPBELL, I. H . 1978. Some problems with the cumulus theory. L ithos, 11, 311- 23. DO NALDSO N, C. H. 1982. Origin of some of the Rhum harrisite by segregation of intercumulus liquid. Min eral. Mag ., 45, 201-9 . DUFF , B. A . 1981. Scattered palaeomagnetic directions acquired during dioritization and stoping of the dioritemet agabbr o complex, Jerse y, Cvl , J. geol. Soc . London , 138, 485-92. HE NDERSON , P. 1970. The significance of the mesostasis of basic layered igneous rocks. J. Petrol., 11,463-73. HE SS, H. H . 1960. Stillwater Igneous Complex, Montan a: a quantitative mineralogical study. Mem . geol. Soc . A m ., 80. IRVINE , T. N. 1978. Infiltrat ion metasomatism, adcumulus growth , and secondary different iat ion in the Muskox Intru sion . Yb . Carnegie Instn . Wash ., 77, 743-51. - - 1980. Magmatic infiltration metasomatism, doublediffusive fractional crystallization, and adcumulus growth in the Muskox Intrusion and other layered intrusions. In

(Hargraves, R. B.; ed .), Phy sics of Magm atic Processes. Princeton , N.J.: Princeton Universit y Press, 325-83 . - - 1982. Te rminology for layered intrusions. J. Petrol., 23, 127-62. KEY, C. H. 1974. The layered diorites of Jersey, Channel Island s. Thesis, Ph.D ., Univ. London (unpubl.) . - - 1977. Origin of appinitic pockets in the diorites of Jersey, Channel Islands . Mineral. Mag., 41, 183-92. LE E , C. A. 1981. Post-deposition structures in the Bushveld Complex mafic sequence. J. geol. Soc. London, 138, 327-41. LOWE , D. R. 1975. Wate r escape structures in coarsegrained sediments. Sedime ntology, 22, 157-204. - - & R. D. LoPICCOLO , 1974. The characteris tics and origins of dish and pillar structures. 1. sedim . Petrol., 44, 483- 501. McBIRNEY, A. R. & R. M. NOYES, 1979. Crystallization and layering of the Skae rgaard Intru sion . J. Petrol., 20, 487-554. PARSONS, I. 1979. The Klokken gabbro-syenite complex, South Greenland : cryptic variation and origin of inversely graded layering. J. Petrol., 20, 653-94. - - & A. W. BUTTERFIELD , 1981. Sedimentarv featur es of the Nunarssuit and Klokken syenites, S. Gre~nland . J. geol. Soc. Lo ndo n, 138, 289- 306. WADSWORTH , W. J. 1973. Magmatic sediments. Miner. Sci. Eng., 5, 25-35 . WAGER, L. R. & G. M. BROWN , 1968. Layered Igneous Rocks . Edinburgh : Oliver & Boyd Ltd . WENTWORTH , C. M. 1967. Dish structure , a primary sediment ary structure in coarse turb idites. A m . Assoc. Petroleum Geo logists Bull., (abs.), 51, p. 485.