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Physical diagenesis; intrusive sediment and connate water
Sedimentary Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
PHYSICAL DiAGENESIS; INTRUSIVE SEDIMENT AND CONNATE WATER E. E. SWARBRICK
Geopeko Limited, Darwin, N.T. (Australia) (Received October 5, 1967)
SUMMARY
The role of connate water in diagenesis of sediment is discussed particularly with reference to the phenomena of liquefaction by thixotropy and excess hydrostatic pressure. Sediment injection structures are considered to offer evidence about depth of burial and water content of the original sediment. Processes of differentiation of intrusive sediment are documented and examples from the Carboniferous of southwest England suggest a possible connection between differentiated remobilised sediments and deposition of ore minerals.
INTRODUCTION
At least two thirds of newly deposited subaqueous sediment comprises connate (i.e., coeval) water. Whereas such water of deposition is constantly being removed in a process which may continue until the later stages of metamorphism, that which remains at any given time is a natural component of the sediment and is subject to the same conditions as the other constituents of the sediment. It also has an equal, if not greater potential for participating in post-depositional changes of the sediment. Post-depositional sediment change may involve any process which can be termed geological, chemical, or physical and is generally known in part or in whole as diagenesis. This term, meaning literally regeneration, is one of the geological science's multitude of terms which are imprecisely defined, or about which a difference of opinion exists. PETTIJOnN (1957, p.648) defined diagenesis as referring "primarily to the reactions which take place within a sediment between one mineral and another, or between one or several minerals and the interstitial or supernatant waters"; and later (p.649) " . . . . E x c e p t for compaction and expulsion of pore fluids and their dissolved solids, the physical changes are not included here." TEODOROVJCn (1961, p.9), however, states that "At present most lithologists generally use the term diagenesis (diagenesis of sediments) to indicate the combination of all processes (chemical, physical, physicochemical, biochemical and geological) that have controlled the conversion of sediments into consolidated rock (generally)without the accompaniSediment. Geol., 2 (1968) 161-175
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ment of orogenic forces or internal heat of the earth. It includes compaction and dessication of sediments, cementation, recrystallisation in part, leaching of salts, and related processes." It is pertinent to enquire whether the deformation by plastic flow of a laminated shale to produce a streaked-out mudstone, or the drying and synerectic cracking of a calcilutite leading to the formation of a dessication breccia, are as important rockforming processes as is cementation of a sandstone. Ifphysicogeologic processes can effect such radical changes as complete breakdown of a calcilutite or total destruction of original laminar structure, then they must be considered as equally diagenetic or regenerative as cementation or hydrolysis of clay minerals. Many such physical processes specifically those involving post-depositional sediment movement, depend almost entirely on connate water.
SEDIMENT INJECTION STRUCTURES
The intention of this paper is to consider intrusive sediment structures in a little more detail than is usual. Intrusive sediment is that which has been moved from its original depositional site to a new site, such movement taking place below the upper sediment interface. Many investigators of ftysch, deltaic and occasionally fluviatile deposits mention in their writings such structures as: sand dykes (DzULYNSKI and WALTON, 1965; and others); sediment pipes (FRIEND, 1965); sand volcanoes (GILL and KUENEN, 1958); air heaves (STEWART, 1956); ruptured structures (DAVIES, 1965); and many others. Load structures might also be included although they are not generally considered to be related to other intrusive sediment structures. ELLIOT (1965), attempting to classify subaqueous sedimentary structures on a genetic environment basis, classified intrusive sediment bodies as transposition structures, arising from "endokinematic" processes operating on quasi-liquid sediments. Load casts develop from identical processes acting on hydroplastic sediments. While one might beg leave to disagree with quasi-liquid rather than true liquid behaviour for some intrusive sediments, Elliot's classification has the virtue of pointing out certain "families" of structures, among them injection or intrusion structures which include load structures. In all these and similar structures the dominant factor is the amount of water present in the sediment, and the pressure to which that water is subjected. Sediments initially contain between 60 ~ and 80 ~ water (KUENEN, 1952), the greater majority of it filling pore spaces. The focus of activity arising from this water content changes with increasing depth in the sediment pile. At shallow depths the low hydrostatic pressure allows grain-to-grain contact in granular sediment thus retaining a shear strength. Argillaceous sediment, however, by virtue of its high water content is essentially liquid with negligible shear strength. With increasing depth, and therefore increasing load, hydrostatic pressure in both the more porous, i.e., granular sediment, and less porous argillaceous sediment, rises. At a critical depth and load point, an excess of hydrostatic pressure over lithostatic pressure reduces shear strength of granular sediment to zero, to permit liquid flow. At levels above this, granular Sediment. Geol., 2 (1968) 161-175
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sediment with a small clay component may flow hydroplastically. Thus at depth, granular sediment tends to become quicksand, while muddy sediment, as well as reverting to a liquid form by thixotropic phenomena also tends to flow under the influence of excess hydrostatic pressure. Sediment injection structures involving liquid or hydroplastic sediment flow, are therefore controlled to a certain extent by depth of burial.
WATER EXPULSION PIPES
Somewhat rarer structures usually called sand pipes, or some similar name (HAWLEY and HART, 1934; FRIEND, 1965) are generally not classified as sediment injections, but are ascribed (quite rightly) to the passage of water through the sediment. Connate water must find a way out of the sediment at some stage of its postdepositional history, and it seems more than probable that most of it will leave as a uniform seepage within a short period of time after deposition. The rate of outflow will be directly proportional to the rate of sedimentation, but will also be influenced to some extent by depositional lithology. Natural porosity and various surface effects will ensure that a considerable amount of water will remain in the sediment and be subjected to further burial, but even this will be eventually forced upward by increasing pressure. This secondary water outflow, however, will not be passing through unconsolidated water-logged sediment on its way to the surface, but through partly compacted sediment. Shale and mudstone in particular may have achieved some shear strength by gelling of colloidal particles which decreases permeability, and will present a formidable barrier to secondary water outflow. Such compacted fine-grained sediment also maintains a high pressure in the underlying water, by preventing the development by a hydraulic gradient toward the surface. Thus, where the secondary outflow manages to break through, it does so under the maximum available pressure. In the case of a siltstone or fine sandstone containing shale as bands or laminae, the water filling the pore spaces of the granular sediment is under pressure which increases with depth. Thus the lowest sand or silt bands or units are under the greatest pressure. If the secondary outflow breaks through the overlying confining shale lamina, it may do so with sufficient force to develop a vertical pipe-like channel through succeeding confining shale laminae. This is the most likely cause of the "rupture structure" described by DAVIES (1965, fig.7). On a larger scale the breakthrough of secondary outflow would develop the cylindrical pipes of HAWLEY and HART (1934) and FRIEND (1965). Within the granular bed the rapid movement of secondary outflow under pressure will invariably lead to the development of quicksand in the zone of flow. This "quick" character of the sediment will be maintained while the flow velocity remains above a critical point. As stated previously these structures are not generally classified as intrusive sediment structures. They do not appear in ELLIOT'S(1965) classification of sedimenSediment. GeoL, 2 (1968) 161-175
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tary structures, and while Shrock does devote a short section to them they are not classed with clastic dykes, being described as "vertically transecting strata of essentially contemporaneous age and similar lithology" (my italics). According to SHROCK (1948) they should not, therefore, be classed as intrusive sediment structures. DAV1ES (1965) showed similar although smaller structures on a polished surface from the Coal Measures of Yorkshire, and called them rupture structures. Davies noted their spatial association with convolute bedding, but failed to comment on the temporal and genetic associations of water expulsion "rupture structure" with stratal flow and convolute bedding. FRIEND (1965) described similar pipes from fluvatile beds in the Wood Bay Series of Spitsbergen, and related their formation to water expulsion channels. Whereas the origin of these and similar structures is invariably recognised, their relationship to intrusive sediment structures is not. For three reasons water expulsion structures are here considered to be end members of a series, with hydroplastic shale intrusions at the other end, and sandstone dykes occupying a position in between. The three reasons are: (1) The pipes etc. are usually formed by development of quicksand in a water expulsion channel. In its "quick" condition the sand is capable of behaving as a liquid. It is possible, therefore, that some of the finer sand will tend to move with the water outflow to some extent, and thus become intrusive sediment. (2) Many of these structures are associated with sand volcanoes and similar structures (GILL and KUENEN, 1958). If sand is extruded at the surface, then at lower levels quicksand, or sand liquified by excess of hydrostatic pressure over lithostatic pressure, must move upward and thus be intruded. (3) The majority of sediments initially contain as much as 30 ~o of their solid matter as colloidal particles. The most common colloidal materials found in natural sediments are: silica; clay minerals; organic compounds; iron, calcium and magnesium hydroxides; and metal sulphides. As these gels are excessively hydrated they have specific gravities approximating to that of water and will tend to be carried by the secondary water outflow. The higher density of this water may allow it to carry sand. If any of this colloidal material is deposited from the water outflow by any one of several mechanisms (coagulation by electrolytes, aggregation, concretion, or mechanical sedimentation in a diminishing flow regime), then the deposited colloid is by definition intrusive sediment. Such colloids are frequently if not invariably deposited within the expulsion pipes. HAWLEY and HART (1934) noted that their cylindrical pipes show a colour banding "believed to be concretionary in nature". LAWLER (1923) described sandstone pipes passing vertically to chalcedony; and other examples from the Culm Measures (Carboniferous) of southwest England are described later. A complete series of intrusive sediments can, therefore, be described with water content of the mobile sediment as the distinguishing characteristic. At one end are the thick pasty intrusions of hydroplastic muddy sediment and the series passes, through liquid sands (the quasi-liquid phase of ELLIOT (1965) to completely liquid
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colloidal intrusions at the other end. At shallow depths of burial, liquid muds fall into the same category as the liquid sands from deeper layers in the sediment pile. The series could be equally well described in terms of grain size with intrusive breccias replacing hydroplastic fluids as one end member. The water content and its participation in secondary water outflow, however, is the mobilising medium, and is, therefore, the natural parameter.
THfXOTROPY Thixotropy is frequently invoked to explain many mobile sediment structures, although in some cases the actual mechanism of thixotropic yield is not fully understood. While the actual process of thixotropic yield (the rapid decrease of viscosity due to shock or shear) is generally recognised, the mechanics of the process (the breaking of loose bonds between colloidal particles--MYsEcs, 1964, p.287) is not. Thus, any sediment which yields thixotropically must have a significant colloidal component. Any sediment not possessing a sufficient colloid content can only flow when subject to an excess of hydrostatic pressure over lithostatic pressure. If thixotropy is to be given an important position in sedimentary processes, the significance of colloidal sediment must also be recognised. Liquefaction by thixotropic yield is commonly ascribed to the passage of a mass of slumping sediment or a turbidity current. Whereas such shearing movement can initiate thixotropic yield, it may have the opposite effect. At very low rates of shear, colloidal linkage is facilitated--a phenomenon known as rheopexy or rheopectic setting (MYSELS,1964, p.271). Sediments with a low water content also resist flow by the phenomenon of dilatancy which is a result of particle interaction following disturbance of an ordered particle arrangement. It can be concluded, therefore, that sediment liquefaction by excess hydrostatic pressure will be most important in sediment buried deeply in the sediment pile, and may occur in granular sediment at shallower depths. Thixotropic yield is more difficult to relate to burial depth, but general considerations would suggest that increasing depth and load pressure will induce a closer packing of colloidal sediment particles increasing the particle linkage strength and creating a higher energy barrier to thixotropic yield.
DIFFERENTIATIONOF INTRUSIVESEDIMENT Clastic or sedimentary dykes sills and related phenomena can be subdivided into two major classes: (a) those which involve the injection of plastic or hydroplastic sediment; (b) those involving the intrusion of liquid sediment. The classification of ELLIOI (1965) of quasi-liquid is not recognized here. Although he refers to "fences" (sic) between the various sediment states, such sharp divisions have not been proven. Sediment. Geol., 2 (1968) 161-175
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The addition of small amounts of water to a hydroplastic sediment will allow it more freedom and liquid behaviour. Essentially the main distinctions are the state of gelation of colloidal sediment, i.e., linked or ruptured, and water content of granular sediment. Hydroplastic in this paper is conceived as a state similar to that proposed by Elliot " . . . the particles (here taken to mean flocs or particle aggregations) are so loosely packed that they are able to revolve somewhat around their neighbours, to such a degree that the whole mass can change shape, and yet are so confined or cohesive that they cannot change their neighbours, relative movements are slight and distributed throughout the m a s s . . . " . The cohesive nature is here believed to be due to gel linkage. To this may be added that in the environment prevailing, the water component is also fixed by adsorption, and cannot leave the sediment without an increase in pressure. It is in this latter characteristic that the chief distinction between plastic (or hydroplastic) and liquid sediment lies. In liquid sediment the water is confined only by the walls of the sediment pores or cavities, and given a point of escape will flow from the sediment without any significant increase in hydrostatic pressure. Such outflowing water frequently carries with it the finer particles of the enclosing sediment. Thus, in any liquid sediment structure conditions of flow as controlled by lithology, porosity, and hydrostatic pressure, may result in differentiation of an intruded sediment body, the mobilised sediment mass tending to a finer grain size with increasing distance from the source bed. Beginning with the injection of granular material, i.e., sand as liquid, the water originally carrying the sand will, by virtue of its greater mobility, continue to flow beyond the point where the sand loses its ability to flow or where the flow can carry it. Smaller particles will continue to move with the water outflow, the coarsest ones being dropped as viscosity and flow velocity both decrease. Thus eventually the secondary outflow will be carrying only colloidal particles and molecules. The colloids will be principally silica, iron hydroxides, organic colloids, clay minerals and base metal sulphides in varying proportions, dependent upon provenance and previous sorting. These colloids will eventually be deposited by flocculation, concretion or accretion; and the secondary outflow water will continue to the upper sediment interface as a dispersed seepage carrying various salts in solution. Given suitable conditions, therefore, a sedimentary dyke could be completely differentiated, comprising sand at the base, passing up through silt, large clay particles, colloidal particles (any of which may crystallise subsequent to deposition as authigenic minerals) and possibly ending with direct precipitation or crystallisation from solution towards the surface. This of course is an idealised and complete sequence. It is unlikely that many original sediments will be so constructed as to provide a significant amount of material for each stage of the differentiation process, or that flow velocity will vary at a constant rate. Within the range of colloid deposition, various absorption-desorption phenomena may lead to complications which are not within the scope of this paper. The recognition of sediment differentiation is important in deciding where in the sediment pile sediment intrusion or injection took place. Basically injection of Sediment. GeoL, 2 (1968) 161-175
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muddy sediment will be in the upper levels as liquid mud or in the intermediate and lower levels under excess hydrostatic pressure, whereas granular sediment will generally be injected under excess hydrostatic pressure at intermediate burial depths. Some granular sediment, of course, will tend to liquify by thixotropic yield in the upper levels. Sand liquified at shallow depths will probably be associated with liquid mud, and mixing rather than intrusion would seem to be the more likely result. Generally it may be stated that original liquid muds (upper level) can be distinguished from reliquified shales or mudstones by the retention to varying degrees of original structure. A consideration of clastic dyke thickness related to known compaction factors, led DZULYNSKIand WALTON (1965, p.162) to a similar conclusion. In their fig. 108A, however, they show a sandstone dyke which (p.162) "shows the liquified sand merging with (overlying) slumped material". Dzulynski and Walton suggest the liquefaction of the sand took place due to the passage of a slump bed above, and that intrusion occurred simultaneously with slumping. They, therefore, conclude that: " . . . the source beds were just below the surface." Their fig. 108A, however, does not suggest a sandstone dyke merging with the slump by intermixing, but rather by the intrusive sediment becoming fner grained upwards and eventually "disappearing" into the slump by virtue of its decreasing grain size. Intrusion at the same time as slumping would lead to dislocation of the intrusive mass, whereas in fig.108A it only tends to bend in the direction of movement, indicating later slipping at depth. Thus, whereas the dyke does "merge" into the overlying slump mass, it does so in a different sense to that of Dzulynski and Walton. If the merging is due to differentiation, there is no need to invoke a shallow burial depth at the time of intrusion. INTRUSIVE SEDIMENTS FROM THE CARBONIFEROUS OF SOUTHWEST ENGLAND
A variety of intrusive sediment structures have been recorded from the Carboniferous Culm Measure sediments of northeast Devon, England. The Culm Measures in this area comprise a lower siliceous shale horizon (Basement Formation) overlain conformably by an alternating chert and calcarenite sequence (Chert Formation). Above the Chert Formation is a sequence of pyritic shale (the Black Shale Formation) which is followed by two greywacke-mudstone assemblages (Exe Valley Greywacke Formation and Alswear Greywacke Formation) separated by a deltaic facies (Mole Valley Formation) (SWARBRIC~:, 1962; see Table I). The Chert Formation is seen as two facies: (1) a deep-water, predominantly pelagic sequence of chert and shale (Brushford Member), and (2) a flysch-like alternating sequence of pelagic chert and shale, and redeposited calcarenite and chert (Bampton Member). The redeposited cherts can be clearly demonstrated to be the result of deposition from turbidity currents, and the whole facies has been subjected to postdepositional hydroplastic and liquid flow (SwARBRICK, 1967). The deltaic or delta-front facies of the Mole Valley Formation also shows evidence of postdepositional sediment mobility. Sediment. Geol., 2 (1968) 161 175
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!ili i !!!i iii
Fig. 1. Acetate peels of undifferentiated muddy sediment intrusions. A. Thin beds and laminae of dark grey pelagic chert have risen in a pipe-like body through an overlying pale calcarenite to merge with the succeeding dark pelagic chert. B. A dark streaky chert has been squeezed upward through overlying pale calcarenite. C. Antidune structures of grey streaky chert flank downward projecting lobes of pale calcarenite. The chert has been hydroplastically deformed and squeezed upward consequent upon the localised downward motion of the calcarenite.
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Specimen CF. 212 (Fig.lA) is an acetate peel (SWARBRICK, 1964) of an alternating sequence of redeposited fine-grained calcarenite (pale), hydroplastically deformed chert (streaky grey) and pelagic chert (dark grey). At the north end of the spe:imen a dark chert lamina within the pale calcarenite is intruded upwards through the overlying calcarenite to merge with the succeeding pelagic chert. There is no differentation of the intrusion and it apparently behaved hydroplastically. Fig.lB (specimen CF.5.2. (ii) shows a peel of a similar sequence to that of Fig.lA. A dark chert, which by its streaky nature was hydroplastically deformed, has been squeezed up through calcerenite (pale) into overlying laminated pelagic chert. Fig.lC shows a common form of sediment intrusior_--the sharply pointed
TABLE I STRATIGRAPHY OF THE CULM MEASURES OF NORTHEAST DEVON
Alswear Greywacke Formation
poorly exposed turbidite sequence
Mole Valley Formation
delta or delta-front facies of arkosic sandstones and striped siltstones showing slump structures
Exe Valley Greywacke Formation turbidite sandstones exhibiting bottom structures, mega-ripples and large-scaleslump folding Black Shale Formation
pelagic pyritic shale; zone of Reticuloceras gracile (BISAT)
Chert Formation
Brusford Member, pelagic chert and mudstone
Basement Formation
pelagic cherty shale
Bampton Member, pelagic chert, turbidite limestone and chert
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masses of muddy sediment known as antidunes flanking a calcarenite load cast. No differentation occurs here and flow was hydroplastic. Fig.2A and B show differentiated muddy intrusions. In specimen CF.5.2(i) (Fig.2A) the intrusion at the southern end becomes paler upwards mainly due to decreasing amounts of dark shaly and organic material, even though it passes through dark horizons towards the top. In specimen CF.2(vii) (Fig.2B) an intruded sediment
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also b e c o m e s p a l e r a w a y from the source bed (a d a r k streaky chert), a n d eventually a p p r o x i m a t e s in c o m p o s i t i o n to an i m p u r e q u a r t z vein. Fig.2C shows a specialised case o f differentiation o f a m u d d y intrusion. T h e source bed is an u n d i s t u r b e d pelagic chert with a l t e r n a t i n g b a n d s o f organic-plusshale-rich a n d chert-rich sediments. F r o m the thickest o f the m o r e siliceous bands, two intrusive bodies rise t h r o u g h the overlying pelagic chert and pass into the succeed-
c
I
Fig.2. Acetate peels of differentiated muddy sediment intrusions. A. A streaky chert has been forced upward through an overlying pale calcarenite unit. There is a decrease in the organic-shale content of the intrusive body away from the source of the intrusive body. The sharp bend in the intrusive body indicates minor flow or other movement from left to right on the photograph. This flow is also evidenced by the slight overturning of the calcarenite unit to the left of the intrusion. B. Dark streaky chert has been intruded along an inclined axis through rapidly alternating chert and calcarenite laminae. Differentiation occurs in the direction of intrusion and the upper section of the intrusive body comprises an impure quartz vein. C. Two intrusive bodies have developed from within a laminated pelagic chert. Differentiation occurred because the organic-shale component was only partly mobilised forming gentle arches below the intrusions. The majority of the intrusive sediment was composed of colloidal silica and ferrous hydroxide, the silica being deposited first as chalcedony and the ferrous hydroxide as a limonite cap over the chalcedony.
ing pale chert. The l a m i n a e in the d a r k chert are b e n t u p w a r d s n e a r the c o n t a c t d e m o n s t r a t i n g the intrusive n a t u r e ; a n d the lower b o u n d a r y o f the overlying pale chert is forced u p over the two masses. The two intrusions are differentiated, c o n t a i n ing less d a r k s h a l y - o r g a n i c m a t e r i a l and m o r e silica u p w a r d s , until t o w a r d the t o p they consist o f p u r e m i c r o c r y s t a l l i n e (chalcedonic) silica. I m m e d i a t e l y a b o v e this Sediment. Geol., 2 (1968) 161-175
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pure chert are lopolith-shaped lenses of limonite. The limonite is so intimately associated with the intrusive masses that the primary and contemporaneous character of the structures is undeniable. The limonite, therefore, represents the last deposit from the ascending intrusion. When the dilute ferruginous gel encountered the overlying impermeable pale chert, it was forced to spread laterally, losing flow velocity and depositing the colloidal iron. These intrusive sediments, therefore, developed from secondary water outflow which permitted liquid rather than hydroplastic flow. As a result of the greater particle freedom differentiation occurred. Fig.3 shows evidence of a somewhat similar process in the granular sediments of the Mole Valley Formation. In this specimen secondary water outflow developed and maintained an extrusion piFe in the fine sand. The sharp delineation of the structure suggests that the liquefying agent was probably excess pressure in this case rather
Fig.3. Acetate peel of differentiated arenaceous intrusive sediment. A water expulsion pipe passes through fine-grained sandstone with the arenaceous component being intruded to a minor degree (as evidenced by upturning of the laminae near the margin of the pipe). Differentiation of the intrusive sediment caused a concentration within the pipe of limonite originating in the bedded sediment. Sediment. Geol., 2 (1968) 161-175
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than thixotropy. Flow in the pipe continued while a certain critical excess pressure was maintained. The fine silty laminae within the sandstone are bent upwards and abruptly terminated at the contact with the pipe indicating removal of cohesive material from the wall of the pipe and suggesting that liquefaction occurred only in the water channel. The structure is, therefore, related only to water outflow. All through the pipe are thin twisted filaments of [imonite, again probably deposited originally in colloidal form perhaps by the decreasing flow velocity within the pipe. This pipe may be closely compared with the "sandstone pipe with concretion" of HAWLEY and HART (1934). Differentiation of granular intrusive sediments in such pipes would be limited due to the preponderance of granular material in the original sediment. The muddy sediment intrusions from the Chert Formation have a further significance with reference to the relative role of Radiolaria in bedded cherts. The Chert Formation has been classed by earlier workers such as HrNDE and Fox (1895) as radiolarian cherts, and lateral equivalents to the west (Codden Hill Cherts) are also considered by present workers (PRENTICE, 1960) as radiolarian cherts. Within the Chert Formation there are certainly many Radiolaria-like masses of cryptocrystalline silica. They do not, however, reveal any typically radiolarian structures and the fact that they deform in anomalous ways (SwARBRICK, 1967) leads to suspicions as to their origin. In the source beds of the differentiated muddy intrusions of Fig.2A and 2B Radiolaria-like bodies are common, appearing as spheroidal masses of cryptocrystalline silica mosaic. In the undisturbed sediment immediately surrounding each intrusion and in the walls of the intrusions themselves are other Radiolaria-like bodies identical to the others in all respects except that instead of being spherical they are elongate. The long axes of all such bodies parallel the intrusions. Further, although the dark shale and organic content of the intrusions decrease away from the source bed, the concentration of Radiolaria-like bodies does not. Two questions immediately arise: (a) why are the Radiolaria-like bodies in the walls deformed ?, and (b) if the ascending liquid sediment could not carry the relatively light and fine-grained shale and organic fraction, how did it carry the much larger and presumably heavier Radiolaria-like bodies ? In considering the first question, it must be recognised that the only evidence of shear movement parallel to the long axes of deformed bodies is the relatively insignificant upward movement of the intrusive sediment. The only answer which effectively satisfies both questions is that the Radiolaria-like bodies were not crystalline prior to sediment intrusion. They were, therefore, either bodies of silica in solution or masses of silica gel. If the silica was in solution, there should be evidence of considerable volume decrease accompanying crystallisation. There is no such evidence, and it must be concluded that the bodies were originally composed of silica gel. A third question must, therefore, be considered: namely how did such relatively pure colloidal masses develop and become emplaced in these sediments ? The answer may lie in a process described by MYSELS (1964). Mysels (p.87) Sediment. Geol., 2 (1968) 161-175
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showed that the mobilising of colloidal particles by hydrodynamic forces may bring particles sufficiently close together to produce a net attractive particle force as opposed to the more normal repulsive force. Thus in a suitable shear regime, colloidal particles may be induced to aggregate into large accretions. The source beds of the intrusions in Fig.2A and B show evidence of shearing leading to a streaking of the original laminated pelagic sediment. In the original cherty sediment dispersed colloidal silica would be an important constituent, and the individual particles in a suitable shear regime would tend to form large aggregates. Such aggregates in the gel form would be cohesive and yet easily deformed due to the high water content. The high water content of the silica gel aggregates would also reduce the specific gravity of the aggregates, allowing them to be carried by the secondary water outflow. With later dehydration, the aggregates would tend to crystallise to microcrystalline silica. A more detailed consideration of aggregation of colloids and preservation of the results in the geological record is given by ELLISTON(1963).
CONCLUSION
Intrusive sediment structures merit much more attention than they have been given so far. They may provide a complete spectrum of information concerning depth of burial, water content, hydrostatic pressure, lithostatic pressure and various other factors involved in diagenesis. The deposition of primary iron-rich minerals in association with intrusive sediments as demonstrated in Fig.2C and 3 leads naturally to the possibility of larger scale structures (which have been recorded from various areas of the world and sections of the geological column) being associated with mineralisation on an economic scale. The colloidal nature of many sulphide deposits has long been recognised, although such deposits have usually been interpreted as developing initially from hydrothermal sources. The clear evidence of deposition of colloidal iron hydroxide in association with intrusive sediments should prompt a more critical examination of ore genesis theory, particularly for deposits occurring in association with mobile sediments.
ACKNOWLEDGEMENTS
The author is indebted to Mr. G. R. Ryan for critical reading of the manuscript, and to Mr. L N. Elliston who directed the author's attention to the importance of colloidal processes in natural sediments and whose personal research partly prompted the writing of this paper. This paper is published with kind permission of the Board of Directors of Peko Wallsend Investments Limited of Sydney, Australia. Sediment. Geol., 2 (1968) 161-175
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REFERENCES DAVIES, H. G., 1965. Convolute lamination and other structures from the Lower Coal Measures of Yorkshire. Sedimentology, 5 : 305-325. DZULYNSKI, S. and WALTON, E. K., 1965. Sedimentary Features of Flysch and Greywackes. Elsevier, Amsterdam, 274 pp. ELmOT, R. E., 1965. A classification of subaqueous sedimentary structures based on rheological and kinematical parameters. Sedimentology, 5 : 193-209. ELLISTON, J.N., 1963. The diagenesis of mobile sediments. In: S. WARREN CAREY (Convener and Editor), Syntaphral Tectonics and Diagenesis. University of Tasmania, Hobart, pp. Q1-QI 1. FRIEND, P. F., 1965. Fluvatile structures in the Wood Bay Series (Devonian) of Spitsbergen. Sedimentology, 5 : 39-68. G1LL, W. D. and KUENEN, P. H., 1958. Sand volcanoes on slumps in the Carboniferous of County Clare, Ireland. Quart. J. Geol. Soc. London, 113:441-460. HAWLEY, J. E. and HART, R. C., 1934. Cylindrical structures in sandstones. Bull. Geol. Soc. Am., 45 : 1017-1034. HINDE, G. J. and Fox, H., 1895. On a well marked horizon of radiolarian rocks in the Lower Culm Measures of Devon, Cornwall, and Somerset. Quart. J. Geol. Soc. London, 51 : 609-668. KUENEN, PH. H., 1952. Estimated size of the Grand Banks turbidity current. Am. J. Sci., 250 : 874-884. LAWLER, T. B., 1923. On the occurrence of sandstone dykes and chalcedony veins in the White River, Oligocene. Am. J. Sci., 5 : 160-172. MYSELS, K. J., 1964. Introduction to ColloM Chemistry. Wiley, New York, N.Y., 475 pp. PETT1JOHN, F. J., 1957. Sedimentary Rocks. Harper, New York, N.Y., 718 pp. PRENTICE, J. E., 1960. Dinantian, Namurian, and Westphalian rocks of the district southwest of Barnstaple, north Devon. Quart. J. Geol. Soc. London, 115 : 261-290. SHROCK, B. R., 1948. Sequence in Layered Rocks. McGraw-Hill, New York, N.Y., 507 pp. STEWART, H. B., 1956. Contorted sediments in modern coastal lagoon explained by laboratory experiments. Bull. Am. Assoc. Petrol. Geologists, 40 : 153-179. SWARBRlCK, E. E., 1962. Stratigraphy and Sedimentology of the Culm Measures between the Rivers Exe and Mole, North Devon. Thesis, Univ. London, 238 pp. (unpublished). SWARBR~CK, E. E., 1964. A peel technique for the study of sedimentary structures. Sedimentology, 3 : 75-78. SWARBRICK,E. E., 1967. Turbidite cherts from northeast Devon. Sediment. Geol., 1 (2) : 145-158. TEODOROVICH, G. 1., 1961. Authigenic Minerals in Sedimentary Rocks. Consultants Bureau, New York, N.Y., 120 pp.
Sediment. Geol., 2 (1968) 161-175
PHYSICAL DiAGENESIS; INTRUSIVE SEDIMENT AND CONNATE WATER E. E. SWARBRICK
Geopeko Limited, Darwin, N.T. (Australia) (Received October 5, 1967)
SUMMARY
The role of connate water in diagenesis of sediment is discussed particularly with reference to the phenomena of liquefaction by thixotropy and excess hydrostatic pressure. Sediment injection structures are considered to offer evidence about depth of burial and water content of the original sediment. Processes of differentiation of intrusive sediment are documented and examples from the Carboniferous of southwest England suggest a possible connection between differentiated remobilised sediments and deposition of ore minerals.
INTRODUCTION
At least two thirds of newly deposited subaqueous sediment comprises connate (i.e., coeval) water. Whereas such water of deposition is constantly being removed in a process which may continue until the later stages of metamorphism, that which remains at any given time is a natural component of the sediment and is subject to the same conditions as the other constituents of the sediment. It also has an equal, if not greater potential for participating in post-depositional changes of the sediment. Post-depositional sediment change may involve any process which can be termed geological, chemical, or physical and is generally known in part or in whole as diagenesis. This term, meaning literally regeneration, is one of the geological science's multitude of terms which are imprecisely defined, or about which a difference of opinion exists. PETTIJOnN (1957, p.648) defined diagenesis as referring "primarily to the reactions which take place within a sediment between one mineral and another, or between one or several minerals and the interstitial or supernatant waters"; and later (p.649) " . . . . E x c e p t for compaction and expulsion of pore fluids and their dissolved solids, the physical changes are not included here." TEODOROVJCn (1961, p.9), however, states that "At present most lithologists generally use the term diagenesis (diagenesis of sediments) to indicate the combination of all processes (chemical, physical, physicochemical, biochemical and geological) that have controlled the conversion of sediments into consolidated rock (generally)without the accompaniSediment. Geol., 2 (1968) 161-175
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ment of orogenic forces or internal heat of the earth. It includes compaction and dessication of sediments, cementation, recrystallisation in part, leaching of salts, and related processes." It is pertinent to enquire whether the deformation by plastic flow of a laminated shale to produce a streaked-out mudstone, or the drying and synerectic cracking of a calcilutite leading to the formation of a dessication breccia, are as important rockforming processes as is cementation of a sandstone. Ifphysicogeologic processes can effect such radical changes as complete breakdown of a calcilutite or total destruction of original laminar structure, then they must be considered as equally diagenetic or regenerative as cementation or hydrolysis of clay minerals. Many such physical processes specifically those involving post-depositional sediment movement, depend almost entirely on connate water.
SEDIMENT INJECTION STRUCTURES
The intention of this paper is to consider intrusive sediment structures in a little more detail than is usual. Intrusive sediment is that which has been moved from its original depositional site to a new site, such movement taking place below the upper sediment interface. Many investigators of ftysch, deltaic and occasionally fluviatile deposits mention in their writings such structures as: sand dykes (DzULYNSKI and WALTON, 1965; and others); sediment pipes (FRIEND, 1965); sand volcanoes (GILL and KUENEN, 1958); air heaves (STEWART, 1956); ruptured structures (DAVIES, 1965); and many others. Load structures might also be included although they are not generally considered to be related to other intrusive sediment structures. ELLIOT (1965), attempting to classify subaqueous sedimentary structures on a genetic environment basis, classified intrusive sediment bodies as transposition structures, arising from "endokinematic" processes operating on quasi-liquid sediments. Load casts develop from identical processes acting on hydroplastic sediments. While one might beg leave to disagree with quasi-liquid rather than true liquid behaviour for some intrusive sediments, Elliot's classification has the virtue of pointing out certain "families" of structures, among them injection or intrusion structures which include load structures. In all these and similar structures the dominant factor is the amount of water present in the sediment, and the pressure to which that water is subjected. Sediments initially contain between 60 ~ and 80 ~ water (KUENEN, 1952), the greater majority of it filling pore spaces. The focus of activity arising from this water content changes with increasing depth in the sediment pile. At shallow depths the low hydrostatic pressure allows grain-to-grain contact in granular sediment thus retaining a shear strength. Argillaceous sediment, however, by virtue of its high water content is essentially liquid with negligible shear strength. With increasing depth, and therefore increasing load, hydrostatic pressure in both the more porous, i.e., granular sediment, and less porous argillaceous sediment, rises. At a critical depth and load point, an excess of hydrostatic pressure over lithostatic pressure reduces shear strength of granular sediment to zero, to permit liquid flow. At levels above this, granular Sediment. Geol., 2 (1968) 161-175
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sediment with a small clay component may flow hydroplastically. Thus at depth, granular sediment tends to become quicksand, while muddy sediment, as well as reverting to a liquid form by thixotropic phenomena also tends to flow under the influence of excess hydrostatic pressure. Sediment injection structures involving liquid or hydroplastic sediment flow, are therefore controlled to a certain extent by depth of burial.
WATER EXPULSION PIPES
Somewhat rarer structures usually called sand pipes, or some similar name (HAWLEY and HART, 1934; FRIEND, 1965) are generally not classified as sediment injections, but are ascribed (quite rightly) to the passage of water through the sediment. Connate water must find a way out of the sediment at some stage of its postdepositional history, and it seems more than probable that most of it will leave as a uniform seepage within a short period of time after deposition. The rate of outflow will be directly proportional to the rate of sedimentation, but will also be influenced to some extent by depositional lithology. Natural porosity and various surface effects will ensure that a considerable amount of water will remain in the sediment and be subjected to further burial, but even this will be eventually forced upward by increasing pressure. This secondary water outflow, however, will not be passing through unconsolidated water-logged sediment on its way to the surface, but through partly compacted sediment. Shale and mudstone in particular may have achieved some shear strength by gelling of colloidal particles which decreases permeability, and will present a formidable barrier to secondary water outflow. Such compacted fine-grained sediment also maintains a high pressure in the underlying water, by preventing the development by a hydraulic gradient toward the surface. Thus, where the secondary outflow manages to break through, it does so under the maximum available pressure. In the case of a siltstone or fine sandstone containing shale as bands or laminae, the water filling the pore spaces of the granular sediment is under pressure which increases with depth. Thus the lowest sand or silt bands or units are under the greatest pressure. If the secondary outflow breaks through the overlying confining shale lamina, it may do so with sufficient force to develop a vertical pipe-like channel through succeeding confining shale laminae. This is the most likely cause of the "rupture structure" described by DAVIES (1965, fig.7). On a larger scale the breakthrough of secondary outflow would develop the cylindrical pipes of HAWLEY and HART (1934) and FRIEND (1965). Within the granular bed the rapid movement of secondary outflow under pressure will invariably lead to the development of quicksand in the zone of flow. This "quick" character of the sediment will be maintained while the flow velocity remains above a critical point. As stated previously these structures are not generally classified as intrusive sediment structures. They do not appear in ELLIOT'S(1965) classification of sedimenSediment. GeoL, 2 (1968) 161-175
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tary structures, and while Shrock does devote a short section to them they are not classed with clastic dykes, being described as "vertically transecting strata of essentially contemporaneous age and similar lithology" (my italics). According to SHROCK (1948) they should not, therefore, be classed as intrusive sediment structures. DAV1ES (1965) showed similar although smaller structures on a polished surface from the Coal Measures of Yorkshire, and called them rupture structures. Davies noted their spatial association with convolute bedding, but failed to comment on the temporal and genetic associations of water expulsion "rupture structure" with stratal flow and convolute bedding. FRIEND (1965) described similar pipes from fluvatile beds in the Wood Bay Series of Spitsbergen, and related their formation to water expulsion channels. Whereas the origin of these and similar structures is invariably recognised, their relationship to intrusive sediment structures is not. For three reasons water expulsion structures are here considered to be end members of a series, with hydroplastic shale intrusions at the other end, and sandstone dykes occupying a position in between. The three reasons are: (1) The pipes etc. are usually formed by development of quicksand in a water expulsion channel. In its "quick" condition the sand is capable of behaving as a liquid. It is possible, therefore, that some of the finer sand will tend to move with the water outflow to some extent, and thus become intrusive sediment. (2) Many of these structures are associated with sand volcanoes and similar structures (GILL and KUENEN, 1958). If sand is extruded at the surface, then at lower levels quicksand, or sand liquified by excess of hydrostatic pressure over lithostatic pressure, must move upward and thus be intruded. (3) The majority of sediments initially contain as much as 30 ~o of their solid matter as colloidal particles. The most common colloidal materials found in natural sediments are: silica; clay minerals; organic compounds; iron, calcium and magnesium hydroxides; and metal sulphides. As these gels are excessively hydrated they have specific gravities approximating to that of water and will tend to be carried by the secondary water outflow. The higher density of this water may allow it to carry sand. If any of this colloidal material is deposited from the water outflow by any one of several mechanisms (coagulation by electrolytes, aggregation, concretion, or mechanical sedimentation in a diminishing flow regime), then the deposited colloid is by definition intrusive sediment. Such colloids are frequently if not invariably deposited within the expulsion pipes. HAWLEY and HART (1934) noted that their cylindrical pipes show a colour banding "believed to be concretionary in nature". LAWLER (1923) described sandstone pipes passing vertically to chalcedony; and other examples from the Culm Measures (Carboniferous) of southwest England are described later. A complete series of intrusive sediments can, therefore, be described with water content of the mobile sediment as the distinguishing characteristic. At one end are the thick pasty intrusions of hydroplastic muddy sediment and the series passes, through liquid sands (the quasi-liquid phase of ELLIOT (1965) to completely liquid
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colloidal intrusions at the other end. At shallow depths of burial, liquid muds fall into the same category as the liquid sands from deeper layers in the sediment pile. The series could be equally well described in terms of grain size with intrusive breccias replacing hydroplastic fluids as one end member. The water content and its participation in secondary water outflow, however, is the mobilising medium, and is, therefore, the natural parameter.
THfXOTROPY Thixotropy is frequently invoked to explain many mobile sediment structures, although in some cases the actual mechanism of thixotropic yield is not fully understood. While the actual process of thixotropic yield (the rapid decrease of viscosity due to shock or shear) is generally recognised, the mechanics of the process (the breaking of loose bonds between colloidal particles--MYsEcs, 1964, p.287) is not. Thus, any sediment which yields thixotropically must have a significant colloidal component. Any sediment not possessing a sufficient colloid content can only flow when subject to an excess of hydrostatic pressure over lithostatic pressure. If thixotropy is to be given an important position in sedimentary processes, the significance of colloidal sediment must also be recognised. Liquefaction by thixotropic yield is commonly ascribed to the passage of a mass of slumping sediment or a turbidity current. Whereas such shearing movement can initiate thixotropic yield, it may have the opposite effect. At very low rates of shear, colloidal linkage is facilitated--a phenomenon known as rheopexy or rheopectic setting (MYSELS,1964, p.271). Sediments with a low water content also resist flow by the phenomenon of dilatancy which is a result of particle interaction following disturbance of an ordered particle arrangement. It can be concluded, therefore, that sediment liquefaction by excess hydrostatic pressure will be most important in sediment buried deeply in the sediment pile, and may occur in granular sediment at shallower depths. Thixotropic yield is more difficult to relate to burial depth, but general considerations would suggest that increasing depth and load pressure will induce a closer packing of colloidal sediment particles increasing the particle linkage strength and creating a higher energy barrier to thixotropic yield.
DIFFERENTIATIONOF INTRUSIVESEDIMENT Clastic or sedimentary dykes sills and related phenomena can be subdivided into two major classes: (a) those which involve the injection of plastic or hydroplastic sediment; (b) those involving the intrusion of liquid sediment. The classification of ELLIOI (1965) of quasi-liquid is not recognized here. Although he refers to "fences" (sic) between the various sediment states, such sharp divisions have not been proven. Sediment. Geol., 2 (1968) 161-175
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The addition of small amounts of water to a hydroplastic sediment will allow it more freedom and liquid behaviour. Essentially the main distinctions are the state of gelation of colloidal sediment, i.e., linked or ruptured, and water content of granular sediment. Hydroplastic in this paper is conceived as a state similar to that proposed by Elliot " . . . the particles (here taken to mean flocs or particle aggregations) are so loosely packed that they are able to revolve somewhat around their neighbours, to such a degree that the whole mass can change shape, and yet are so confined or cohesive that they cannot change their neighbours, relative movements are slight and distributed throughout the m a s s . . . " . The cohesive nature is here believed to be due to gel linkage. To this may be added that in the environment prevailing, the water component is also fixed by adsorption, and cannot leave the sediment without an increase in pressure. It is in this latter characteristic that the chief distinction between plastic (or hydroplastic) and liquid sediment lies. In liquid sediment the water is confined only by the walls of the sediment pores or cavities, and given a point of escape will flow from the sediment without any significant increase in hydrostatic pressure. Such outflowing water frequently carries with it the finer particles of the enclosing sediment. Thus, in any liquid sediment structure conditions of flow as controlled by lithology, porosity, and hydrostatic pressure, may result in differentiation of an intruded sediment body, the mobilised sediment mass tending to a finer grain size with increasing distance from the source bed. Beginning with the injection of granular material, i.e., sand as liquid, the water originally carrying the sand will, by virtue of its greater mobility, continue to flow beyond the point where the sand loses its ability to flow or where the flow can carry it. Smaller particles will continue to move with the water outflow, the coarsest ones being dropped as viscosity and flow velocity both decrease. Thus eventually the secondary outflow will be carrying only colloidal particles and molecules. The colloids will be principally silica, iron hydroxides, organic colloids, clay minerals and base metal sulphides in varying proportions, dependent upon provenance and previous sorting. These colloids will eventually be deposited by flocculation, concretion or accretion; and the secondary outflow water will continue to the upper sediment interface as a dispersed seepage carrying various salts in solution. Given suitable conditions, therefore, a sedimentary dyke could be completely differentiated, comprising sand at the base, passing up through silt, large clay particles, colloidal particles (any of which may crystallise subsequent to deposition as authigenic minerals) and possibly ending with direct precipitation or crystallisation from solution towards the surface. This of course is an idealised and complete sequence. It is unlikely that many original sediments will be so constructed as to provide a significant amount of material for each stage of the differentiation process, or that flow velocity will vary at a constant rate. Within the range of colloid deposition, various absorption-desorption phenomena may lead to complications which are not within the scope of this paper. The recognition of sediment differentiation is important in deciding where in the sediment pile sediment intrusion or injection took place. Basically injection of Sediment. GeoL, 2 (1968) 161-175
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muddy sediment will be in the upper levels as liquid mud or in the intermediate and lower levels under excess hydrostatic pressure, whereas granular sediment will generally be injected under excess hydrostatic pressure at intermediate burial depths. Some granular sediment, of course, will tend to liquify by thixotropic yield in the upper levels. Sand liquified at shallow depths will probably be associated with liquid mud, and mixing rather than intrusion would seem to be the more likely result. Generally it may be stated that original liquid muds (upper level) can be distinguished from reliquified shales or mudstones by the retention to varying degrees of original structure. A consideration of clastic dyke thickness related to known compaction factors, led DZULYNSKIand WALTON (1965, p.162) to a similar conclusion. In their fig. 108A, however, they show a sandstone dyke which (p.162) "shows the liquified sand merging with (overlying) slumped material". Dzulynski and Walton suggest the liquefaction of the sand took place due to the passage of a slump bed above, and that intrusion occurred simultaneously with slumping. They, therefore, conclude that: " . . . the source beds were just below the surface." Their fig. 108A, however, does not suggest a sandstone dyke merging with the slump by intermixing, but rather by the intrusive sediment becoming fner grained upwards and eventually "disappearing" into the slump by virtue of its decreasing grain size. Intrusion at the same time as slumping would lead to dislocation of the intrusive mass, whereas in fig.108A it only tends to bend in the direction of movement, indicating later slipping at depth. Thus, whereas the dyke does "merge" into the overlying slump mass, it does so in a different sense to that of Dzulynski and Walton. If the merging is due to differentiation, there is no need to invoke a shallow burial depth at the time of intrusion. INTRUSIVE SEDIMENTS FROM THE CARBONIFEROUS OF SOUTHWEST ENGLAND
A variety of intrusive sediment structures have been recorded from the Carboniferous Culm Measure sediments of northeast Devon, England. The Culm Measures in this area comprise a lower siliceous shale horizon (Basement Formation) overlain conformably by an alternating chert and calcarenite sequence (Chert Formation). Above the Chert Formation is a sequence of pyritic shale (the Black Shale Formation) which is followed by two greywacke-mudstone assemblages (Exe Valley Greywacke Formation and Alswear Greywacke Formation) separated by a deltaic facies (Mole Valley Formation) (SWARBRIC~:, 1962; see Table I). The Chert Formation is seen as two facies: (1) a deep-water, predominantly pelagic sequence of chert and shale (Brushford Member), and (2) a flysch-like alternating sequence of pelagic chert and shale, and redeposited calcarenite and chert (Bampton Member). The redeposited cherts can be clearly demonstrated to be the result of deposition from turbidity currents, and the whole facies has been subjected to postdepositional hydroplastic and liquid flow (SwARBRICK, 1967). The deltaic or delta-front facies of the Mole Valley Formation also shows evidence of postdepositional sediment mobility. Sediment. Geol., 2 (1968) 161 175
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!ili i !!!i iii
Fig. 1. Acetate peels of undifferentiated muddy sediment intrusions. A. Thin beds and laminae of dark grey pelagic chert have risen in a pipe-like body through an overlying pale calcarenite to merge with the succeeding dark pelagic chert. B. A dark streaky chert has been squeezed upward through overlying pale calcarenite. C. Antidune structures of grey streaky chert flank downward projecting lobes of pale calcarenite. The chert has been hydroplastically deformed and squeezed upward consequent upon the localised downward motion of the calcarenite.
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Specimen CF. 212 (Fig.lA) is an acetate peel (SWARBRICK, 1964) of an alternating sequence of redeposited fine-grained calcarenite (pale), hydroplastically deformed chert (streaky grey) and pelagic chert (dark grey). At the north end of the spe:imen a dark chert lamina within the pale calcarenite is intruded upwards through the overlying calcarenite to merge with the succeeding pelagic chert. There is no differentation of the intrusion and it apparently behaved hydroplastically. Fig.lB (specimen CF.5.2. (ii) shows a peel of a similar sequence to that of Fig.lA. A dark chert, which by its streaky nature was hydroplastically deformed, has been squeezed up through calcerenite (pale) into overlying laminated pelagic chert. Fig.lC shows a common form of sediment intrusior_--the sharply pointed
TABLE I STRATIGRAPHY OF THE CULM MEASURES OF NORTHEAST DEVON
Alswear Greywacke Formation
poorly exposed turbidite sequence
Mole Valley Formation
delta or delta-front facies of arkosic sandstones and striped siltstones showing slump structures
Exe Valley Greywacke Formation turbidite sandstones exhibiting bottom structures, mega-ripples and large-scaleslump folding Black Shale Formation
pelagic pyritic shale; zone of Reticuloceras gracile (BISAT)
Chert Formation
Brusford Member, pelagic chert and mudstone
Basement Formation
pelagic cherty shale
Bampton Member, pelagic chert, turbidite limestone and chert
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masses of muddy sediment known as antidunes flanking a calcarenite load cast. No differentation occurs here and flow was hydroplastic. Fig.2A and B show differentiated muddy intrusions. In specimen CF.5.2(i) (Fig.2A) the intrusion at the southern end becomes paler upwards mainly due to decreasing amounts of dark shaly and organic material, even though it passes through dark horizons towards the top. In specimen CF.2(vii) (Fig.2B) an intruded sediment
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also b e c o m e s p a l e r a w a y from the source bed (a d a r k streaky chert), a n d eventually a p p r o x i m a t e s in c o m p o s i t i o n to an i m p u r e q u a r t z vein. Fig.2C shows a specialised case o f differentiation o f a m u d d y intrusion. T h e source bed is an u n d i s t u r b e d pelagic chert with a l t e r n a t i n g b a n d s o f organic-plusshale-rich a n d chert-rich sediments. F r o m the thickest o f the m o r e siliceous bands, two intrusive bodies rise t h r o u g h the overlying pelagic chert and pass into the succeed-
c
I
Fig.2. Acetate peels of differentiated muddy sediment intrusions. A. A streaky chert has been forced upward through an overlying pale calcarenite unit. There is a decrease in the organic-shale content of the intrusive body away from the source of the intrusive body. The sharp bend in the intrusive body indicates minor flow or other movement from left to right on the photograph. This flow is also evidenced by the slight overturning of the calcarenite unit to the left of the intrusion. B. Dark streaky chert has been intruded along an inclined axis through rapidly alternating chert and calcarenite laminae. Differentiation occurs in the direction of intrusion and the upper section of the intrusive body comprises an impure quartz vein. C. Two intrusive bodies have developed from within a laminated pelagic chert. Differentiation occurred because the organic-shale component was only partly mobilised forming gentle arches below the intrusions. The majority of the intrusive sediment was composed of colloidal silica and ferrous hydroxide, the silica being deposited first as chalcedony and the ferrous hydroxide as a limonite cap over the chalcedony.
ing pale chert. The l a m i n a e in the d a r k chert are b e n t u p w a r d s n e a r the c o n t a c t d e m o n s t r a t i n g the intrusive n a t u r e ; a n d the lower b o u n d a r y o f the overlying pale chert is forced u p over the two masses. The two intrusions are differentiated, c o n t a i n ing less d a r k s h a l y - o r g a n i c m a t e r i a l and m o r e silica u p w a r d s , until t o w a r d the t o p they consist o f p u r e m i c r o c r y s t a l l i n e (chalcedonic) silica. I m m e d i a t e l y a b o v e this Sediment. Geol., 2 (1968) 161-175
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pure chert are lopolith-shaped lenses of limonite. The limonite is so intimately associated with the intrusive masses that the primary and contemporaneous character of the structures is undeniable. The limonite, therefore, represents the last deposit from the ascending intrusion. When the dilute ferruginous gel encountered the overlying impermeable pale chert, it was forced to spread laterally, losing flow velocity and depositing the colloidal iron. These intrusive sediments, therefore, developed from secondary water outflow which permitted liquid rather than hydroplastic flow. As a result of the greater particle freedom differentiation occurred. Fig.3 shows evidence of a somewhat similar process in the granular sediments of the Mole Valley Formation. In this specimen secondary water outflow developed and maintained an extrusion piFe in the fine sand. The sharp delineation of the structure suggests that the liquefying agent was probably excess pressure in this case rather
Fig.3. Acetate peel of differentiated arenaceous intrusive sediment. A water expulsion pipe passes through fine-grained sandstone with the arenaceous component being intruded to a minor degree (as evidenced by upturning of the laminae near the margin of the pipe). Differentiation of the intrusive sediment caused a concentration within the pipe of limonite originating in the bedded sediment. Sediment. Geol., 2 (1968) 161-175
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than thixotropy. Flow in the pipe continued while a certain critical excess pressure was maintained. The fine silty laminae within the sandstone are bent upwards and abruptly terminated at the contact with the pipe indicating removal of cohesive material from the wall of the pipe and suggesting that liquefaction occurred only in the water channel. The structure is, therefore, related only to water outflow. All through the pipe are thin twisted filaments of [imonite, again probably deposited originally in colloidal form perhaps by the decreasing flow velocity within the pipe. This pipe may be closely compared with the "sandstone pipe with concretion" of HAWLEY and HART (1934). Differentiation of granular intrusive sediments in such pipes would be limited due to the preponderance of granular material in the original sediment. The muddy sediment intrusions from the Chert Formation have a further significance with reference to the relative role of Radiolaria in bedded cherts. The Chert Formation has been classed by earlier workers such as HrNDE and Fox (1895) as radiolarian cherts, and lateral equivalents to the west (Codden Hill Cherts) are also considered by present workers (PRENTICE, 1960) as radiolarian cherts. Within the Chert Formation there are certainly many Radiolaria-like masses of cryptocrystalline silica. They do not, however, reveal any typically radiolarian structures and the fact that they deform in anomalous ways (SwARBRICK, 1967) leads to suspicions as to their origin. In the source beds of the differentiated muddy intrusions of Fig.2A and 2B Radiolaria-like bodies are common, appearing as spheroidal masses of cryptocrystalline silica mosaic. In the undisturbed sediment immediately surrounding each intrusion and in the walls of the intrusions themselves are other Radiolaria-like bodies identical to the others in all respects except that instead of being spherical they are elongate. The long axes of all such bodies parallel the intrusions. Further, although the dark shale and organic content of the intrusions decrease away from the source bed, the concentration of Radiolaria-like bodies does not. Two questions immediately arise: (a) why are the Radiolaria-like bodies in the walls deformed ?, and (b) if the ascending liquid sediment could not carry the relatively light and fine-grained shale and organic fraction, how did it carry the much larger and presumably heavier Radiolaria-like bodies ? In considering the first question, it must be recognised that the only evidence of shear movement parallel to the long axes of deformed bodies is the relatively insignificant upward movement of the intrusive sediment. The only answer which effectively satisfies both questions is that the Radiolaria-like bodies were not crystalline prior to sediment intrusion. They were, therefore, either bodies of silica in solution or masses of silica gel. If the silica was in solution, there should be evidence of considerable volume decrease accompanying crystallisation. There is no such evidence, and it must be concluded that the bodies were originally composed of silica gel. A third question must, therefore, be considered: namely how did such relatively pure colloidal masses develop and become emplaced in these sediments ? The answer may lie in a process described by MYSELS (1964). Mysels (p.87) Sediment. Geol., 2 (1968) 161-175
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showed that the mobilising of colloidal particles by hydrodynamic forces may bring particles sufficiently close together to produce a net attractive particle force as opposed to the more normal repulsive force. Thus in a suitable shear regime, colloidal particles may be induced to aggregate into large accretions. The source beds of the intrusions in Fig.2A and B show evidence of shearing leading to a streaking of the original laminated pelagic sediment. In the original cherty sediment dispersed colloidal silica would be an important constituent, and the individual particles in a suitable shear regime would tend to form large aggregates. Such aggregates in the gel form would be cohesive and yet easily deformed due to the high water content. The high water content of the silica gel aggregates would also reduce the specific gravity of the aggregates, allowing them to be carried by the secondary water outflow. With later dehydration, the aggregates would tend to crystallise to microcrystalline silica. A more detailed consideration of aggregation of colloids and preservation of the results in the geological record is given by ELLISTON(1963).
CONCLUSION
Intrusive sediment structures merit much more attention than they have been given so far. They may provide a complete spectrum of information concerning depth of burial, water content, hydrostatic pressure, lithostatic pressure and various other factors involved in diagenesis. The deposition of primary iron-rich minerals in association with intrusive sediments as demonstrated in Fig.2C and 3 leads naturally to the possibility of larger scale structures (which have been recorded from various areas of the world and sections of the geological column) being associated with mineralisation on an economic scale. The colloidal nature of many sulphide deposits has long been recognised, although such deposits have usually been interpreted as developing initially from hydrothermal sources. The clear evidence of deposition of colloidal iron hydroxide in association with intrusive sediments should prompt a more critical examination of ore genesis theory, particularly for deposits occurring in association with mobile sediments.
ACKNOWLEDGEMENTS
The author is indebted to Mr. G. R. Ryan for critical reading of the manuscript, and to Mr. L N. Elliston who directed the author's attention to the importance of colloidal processes in natural sediments and whose personal research partly prompted the writing of this paper. This paper is published with kind permission of the Board of Directors of Peko Wallsend Investments Limited of Sydney, Australia. Sediment. Geol., 2 (1968) 161-175
PHYSICAL DIAGENESIS; INTRUSIVE SEDIMENT AND CONNATE WATER
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