Sediments and Diagenesis

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Introduction: sediments and diagenesis

D. H. TARLING

Department of Geological Sciences, University of Plymouth, Plymouth PL4 8AA, UK

This introduction outlines the history of the a vast expansion in palaeomagnetic studies and palaeomagnetic study of the diagenesis of sedi- of the processes occurring within lake and ments, followed by a very general description of marine sediments, as well as how such informa- the main concepts of diagenetic process involved tion can be used to determine previous environ- as loose sediments become increasing lithified ments (Thompson & Oldfield 1986). into true rocks. This is followed by a discus- Early studies of sedimentary rocks, as sion of how this collection of articles originated opposed to sediments, were made by Graham and the basis for the way they are organized. (1949) in America, and Khramov (1958) in the A glossary of most relevant palaeomagnetic and USSR. However, the expansion in the magnetic diagenetic terms, with some minerals, is given at study of consolidated sedimentary rocks mostly the end of the book (Tarling, this volume). followed Blackett's suggestion that his new, As sediments are almost always more weakly highly sensitive astatic magnetometer (Blackett magnetized than igneous rocks, the earliest 1952) could be used for the measurement of the palaeomagnetic studies were mostly made on weak magnetization of sediments. This was igneous rocks (Delesse 1849; Folgerhaiter 1894) followed shortly by results from mainly red and fired clays (Melloni 1853; Gheradi 1862), sandstones in Britain and Europe (Creer et al. although the discovery of thermal remanence 1954; Clegg et al. 1954; Nairn 1956; Creer 1957; was made by Boyle (1691) using sedimentary Irving 1957a; Collinson et al. 1957). These were iron ores. Studies of the magnetization of then extended rapidly throughout the geological Quaternary varved sediments and marine sam- time scale and through most continents, e.g. ples were reported by McNish & Johnson Creer (1958) in South America, Irving (1957b) in (1938), followed shortly by Ising (1943) working Australia; DoeU (1955), Runcorn (1955) and on varved sediments in Sweden. Granar (1958) Balsley & Buddington (1958) in the United expanded on the Swedish work and developed States; Clegg et al. (1956) in India; Graham & the early principles of how magnetic grains, as Hales (1957), Nairn (1960) in southern Africa, they are deposited, become aligned by the etc. In general, such studies assumed that these ambient geomagnetic field and that this align- sediments carried a depositional remanence ment can be preserved after deposition. Studies acquired at the time of deposition, which could of unconsolidated sediments in the United States be isolated, using partial thermal and alternating was expanded by Johnson et al. (1948) and magnetic field demagnetization, from later further studies of the Swedish varves by Griffiths magnetic overprints, such as viscous rema- (1953) and Griffiths et al. (1960). Pioneering nences, and magnetic changes associated with magnetic studies of laboratory deposited materi- burial, uplift and exposure. It was generally als were made by King (1955), Griffiths et al. assumed that diagenetic changes were minor or (1957), Rees (1961), Griffiths et al. (1962) and occurred so soon after deposition that there was Hamilton (1963) in which the basic understand- no meaningful differences in age between deposi- ing of the origin and nature of magnetic fab- tion and diagenesis. rics in sediments were also established. These Most early concerns were whether the deposi- showed that elongated grains, when deposited, tional remanence in a sediment had been affected became slightly flattened into the bedding plane. by the depositional 'inclination error' found in As such grains tend to be magnetized along laboratory deposits. Comparison of the mag- their long axes, this depositional tilt necessarily netic properties of sedimentary rocks with coeval rotated the net magnetization towards the bed- igneous rocks largely discounted this idea, but ding plane, thereby shallowing the inclination it is still suspected to occur in unconsolidated and creating an 'inclination error' of up to sediments. The absence of this effect in lithified 4-5 ° . Improvements in coring systems, following sediments implies that any 'inclination error' is Mackereth (1958, 1969), enabled samples to be eliminated during post-depositional processes, collected with little or no disturbance, leading to i.e. essentially during diagenesis. Concurrently

TARLING, D. H. 1999. Introduction: sediments and diagenesis. In: TARLING, D. H. & TURNER, P. (eds) Palaeomagnetism and Diagenesis in Sediments, Geological Society, London, Special Publications, 151, 1-8. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

2 D.H. TARLING with such comparisons, sedimentological studies ization; the difference between shallow and deep demonstrated that many lithified 'red beds' had process depends on the characteristics of the necessarily undergone complex chemical changes sediments and their precise environment. Car- between the time of their original deposition and bonates, for example, can form well-cemented their present lithified state (Turner 1980). Con- rocks at depths which can be only centimetres sequently, hematitic reddening is generally a deep, or even at the sediment/water interface, post-depositional phenomenon, but could have while sands may exceptionally escape cementa- occurred almost immediately after deposition altogether or lithification may be delayed tion (Steiner & Helsley 1974a, b) or have been until cementing fluids pass through them at delayed, possibly for hundreds of millions of depths which could exceed a kilometre. years (Tarling et al. 1976). With increasing time, In clastic rocks, such as shales, siltstones and the importance of such chemical changes, diag- sandstones, three main phases of diagenesis are enesis, has become increasingly recognized in all generally recognized. The redoxomorphic stage fields of geology. One of the earliest meetings to is associated with compaction and dewatering in specifically consider diagenesis and palaeomag- zones that may be either oxidising or reducing. netism was held at the University of Newcastle Above the water-sediment interface, the con- (Tarling 1976). Since then, the knowledge in all ditions are largely oxic, so that organic and fields - in studies of diagenesis, rock magnetism sulphur compounds tend to react with iron, and mineral magnetism- has improved enor- destroying much, but not necessarily all, of the mously. While such studies have often served detrital ferric minerals to form soluble iron to demonstrate the complexities of diagenetic sulphates which can react and be re-deposited processes, rather than explain them, there is as the iron compounds elsewhere. Under such now considerably greater understanding of the circumstances, many detrital magnetic grains are processes. There has also been an increasing likely to be lost during prolonged diagenesis, realization by sedimentologists of how mag- and the original remanence may only be car- netic analyses can assist in the understanding ried by magnetic grain inclusions within rela- of sedimentological processes. It thus seemed tively inert grains, such as quartz and ilmenite timely to consider the current 'state of the art' (Hounslow et al. 1995). Below the interface, the in terms of the relationship between palaeomag- low oxygen concentrations tend to preserve the netism and diagenesis in sediments and sedi- organic compounds and the iron tends to link mentary rocks. with sulphur to form pyrite. Usually underly- There are numerous definitions of diagenesis. ing this phase, but not necessarily so, the loco- Most of these are only partially satisfactory morphic phase occurs where the main diagenetic because of the complexity of the interacting bio- cementation takes place. This cementation is chemico-physical processes involved. In theory, usually of silica or carbonate, depending on the diagenetic processes comprize all physical and composition of migrating fluids, and can be chemical events that affect sediments from the preceded by authigenic growth of pre-existing time they are initially deposited to when they minerals, such as quartz and calcite overgrowths. show identifiable signs of the very earliest stages Both of these predominantly chemically defined of tectonic metamorphism (ancho-metamorph- phases (redoxomorphic and locomorphic) are ism). However, the boundaries are obscure. accompanied by changes in the physical proper- While grains are still being deposited, they are ties, in particular changes in porosity. Clays, for re-acting with their environment, both physically example, are usually deposited with 50-80% and chemically, so some diagenetic processes porosity. Pore fluids are then mostly lost dur- have actually commenced prior to deposition. ing the earliest changes from clays to claystones. Similarly, the distinction between deep-burial dia- Cementation generally further reduces the per- genetic processes and physical-chemical changes meability, although some mineralogical changes, associated with the onset of tectonic defor- particularly any inversion of calcite to dolomite, mation are more semantic than real. Both may increase the permeability. Within this com- processes usually operate simultaneously. None- plex of physical effects and inorganic reactions, theless, most sedimentologists recognize two the presence of organisms and organic materials phases of diagenesis. 'Early' diagenesis (or syn- may have crucial effects. Macro-organisms do diagenesis) includes all those processes associated not merely disturb the sediment by their motions with deposition and shallow burial, while 'late' (bioturbation), but also actively comminute the diagenesis (or anadiagenesis) are those associ- material which they digest. Living organisms can ated with deeper burial. Unfortunately, 'shallow' also directly promote the formation and changes and 'deep' cannot be quantified because it is in the magnetic mineralogy. For example, the impossible to make any meaningful general- microbe, Desulfovibrio, promotes the formation Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

INTRODUCTION 3

of iron sulphides in anoxic conditions and has a shallow, sub-tidal depths, under conditions of profound influence on the magnetic mineralogy high marine salinity in silled-basins, or at sea- of lake and marine sediments, particularly if level where such higher density brines interface organic material is present. The knowledge of with fresh-water lenses, such as in sabkha much of the magnetic mineralogy is relatively conditions. Such Mg-enriched brines can also new; the existence of the iron sulphide mineral, enter into the evolving system at somewhat later greigite, was largely unknown until the studies stages, resulting in new or further dolomitiza- of Skinner et al. (1964), partially because of its tion; this later, but still 'early' diagenetic instability on exposure to air, yet this mineral dolomite may be recognized because it is usually provides a major link in the pathway of iron related to more permeable horizons, such as minerals during the evolution of many lake and unconformities, or to tectonic features that cut marine sediments. The biochemical significance across depositional surfaces. Conversely, if rela- of wide variety of such organisms similarly tively fresh water, i.e. with low Mg/Ca, pene- remains poorly understood, even when they are trates the system, de-dolomitization can result, a direct agent in affecting the magnetic miner- accompanied by a decrease in porosity as the alogy. Magnetotactic bacteria are now known to dolomite crystals invert to calcite which is up to produce chains of single domain magnetite 13% more voluminous. This also occurs if sea grains, apparently in order to determine 'way level fell and exposed the rocks to subaerial up' in the interface between fluid and sediment or partial subaerial conditions. It is also pos- (Blakemore 1975; Kirschvink & Lowenstam sible that silica-rich fluids can enter the system 1979; Frankel & Blakemore 1990). On death, resulting in the formation of a silica cement such chains or the individual grains may be and formation of cherts. Within this complex, preserved to provide a high stability magnetic organic material can drastically change the local mineral whose remanence may be locked in as geochemical environment and can accumulate to the grains become cemented. However, bacteria form major anoxic organic deposits such as those can fix iron minerals within their structure in that form future hydrocarbon sources. Within many different ways (Vali & Kirschvink 1990) such deposits, complex biochemical reactions and are currently being cultivated for their will occur in a variety of reactions - as outlined potential ability to fix heavy metal pollutants. above for clastic deposits. In carbonates, how- The current knowledge of the magnitude of ever, the reactions are even more complex the effects of these and other bacteria is still because the deposits themselves are generally poor, but they are likely to be major factors in more chemically active than those of the clastic numerous diagenetic processes in anoxic and deposits. Such reactions with organic matter sub-oxic environments, even at depths down to usually convert iron oxides and hydroxides into 6-7 km. However, in more oxidizing environ- sulphides or highly soluble sulphates. Again, as ments, such biogenetic grains are likely to be for the clastic sediments, the role of micro- oxidized. organisms is fundamental, yet poorly under- In carbonates, diagenetic processes are far stood; even dolomitization may well be enhanced more complex and rapid because the initial in the presence of such organisms. It seems deposits have a high porosity, 40-70%, and probable that magnetite-bearing bacteria may much of the initial carbonate mineralogy may be also be of far greater importance than in clastic in the hexagonal form, aragonite, which is rocks. This is partially because of the very low metastable under non-marine conditions and concentrations of any other magnetic minerals inverts readily to the more stable orthorhombic in carbonates, which are often diamagnetic, but form, calcite. This crystallization results in a also because the chemical reactivities in the car- decrease in the primary porosity. In high sal- bonates are far greater and such grains are far inity conditions, such as in restricted marine more likely to become cemented in the sediment basins, the higher concentration of magnesium very shortly after death. This not only locks in in bottom waters can react with the aragonitic any magnetic orientation, but also preserves the muds, forming early diagenetic ('syngenetic') grains against subsequent oxidation. However, dolomite. These complex carbonate interactions even now, little is known of any of the complex between aragonite, calcite and dolomite (poly- inorganic and organic re-actions in such circum- morphism) are also accompanied by re-crystal- stances. Finally, drastic chemical changes can lization, resolution and growth of pre-existing also occur as a result of the migration of, for carbonates, all forming the neomorphic phase of example, hydro-carbon-rich fluids through the diagenesis. Such reactions can take place in a system at any stage (Machel 1995). variety of locations, depending on the source for During even greater burial of all lithol- the Mg-enriched fluids. This can occur at very ogies, clastics and carbonates, the increasing Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

4 D.H. TARLING pressure and temperature induce further diage- deposition. Poor results are usually associated netic changes, termed the phyllomorphic phase, with bioturbated sites and those with higher which involves the recrystallization of clays, in organic content. Pisarevky (this volume) con- particular, and the recrystallization of less stable siders experiments on a variety of Quaternary phyllosilicates, such as muscovite and biotite to continental sediments to simulate the natural form chlorites. While the net orientation of these wetting and drying that will have occurred to flaky minerals remains close to the horizontal, them as a result of changes in the local height of such changes are still recognized as diagenetic, water table. Not surprisingly, these confirm but if the crystal growths are under the influence the models suggested by Noel (1980) and of oblique-to-horizontal, tectonic forces, they confirm that the magnetic properties of perme- are recognized as an early metamorphic process, able sediments of any age may have been ancho-metamorphism. In reality, the boundaries drastically altered by this process. More impor- between diagenesis and metamorphism are tantly, any sediment samples that have dried poorly defined and partially lithologicallydepen- out, or been soaked, since collection may have dent. At any stage, accumulating sediments may had their properties changed. In some sense, well be raised, by tectonic forces, from one such processes are similar to weathering, but regime to another. This can cause a reversal of without chemical changes. Therefore the effects some of the diagenetic processes as minerals that of weathering and viscous remanences are con- are stable at greater pressures equilibrate with sidered next as these can affect the results of the new conditions. Such reactions are termed all of the techniques used to isolate the orig- epidiagenesis as they are superimposed on pre- inal, primary magnetization (depositional or vious diagenetic features. As, or sometimes diagenetic). Such methods involve fundamental more, important is the entry of new fluids into assumptions concerning the properties of vis- the sedimentary basins due to tectonic activity cous remanence acquired by rocks while simply on their margins. The actual pathways and lying in the Earth's magnetic field prior and composition of these basinal fluids are largely subsequent to collection. Viscous remanence controlled by the permeability characteristics of in limestones is considered by Borradaile (this the sediments. These fluids may completely volume), although such behaviour may well destroy all pre-existing iron-bearing minerals, occur in other lithologies. He demonstrates, although new authigenic ferromagnetic minerals using stones from buildings, that viscous rema- may then date the time that such fluids have nences are not necessarily so readily removed passed. Many of the Jurassic limestones in during standard demagnetization tests as is Britain, for example, appear to have been altered generally supposed, and poses the problem of by prolonged water circulation from the over- how such stable remanences can arise and be lying Cretaceous seas. Only very rarely can recognized. (It is interesting to note that the minute specks of blue limestone be seen in deep technique used by Borradaile was originally quarries. Such limestones still show, superim- used by Melloni in 1853 who used stones from posed on this deep 'weathering', a few centi- the Roman amphitheatre at Pompeii to test the metres of present-day weathering. Such chemical magnetic stability of lavas from Vesuvius.) The activity will be enhanced if the fluids are at importance of biologically produced magnetite higher temperatures, commonly up to 200- is considered by Maher & Hounslow (this 300°C according to fluid inclusion studies in volume), predominantly for Quaternary sedi- many sedimentary basins. If such temperatures ments and soils. Such magnetite is potentially are maintained for a few million years, then they of great palaeomagnetic importance as organ- can physically cause the complete remagnetision isms have optimized the grain-size to be that of of the ferromagnetic minerals in the basin, single-domain sizes with very high magnetic irrespective of any chemical effects although stability. When present, they may well carry a these are usually more locally deleterious. remanence relating to the time that the grain In this book, the evidence from sediments became locked into the rock, usually during deposited over archaeological time scales are cementation. As is pointed out, the separation considered first by Batt (this volume) who shows and identification of such materials is not that many undisturbed archaeological sediments simple. Where ferromagnetic grains of detrital have acquired remanence directions related to origin are present, these may well swamp the the Earth's magnetic field direction only shortly signal of the biogenic material, but in areas of after deposition, suggesting that such rema- very low detrital input, as on some carbonate nences are probably not depositional, but related platforms, these may well be a significant carrier to post-deposition rotations that have taken of remanence. However, as with all other iron- place from a few months to a few years after bearing minerals, they are subject to potential Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

INTRODUCTION 5 oxidation or sulphidization during later diagen- tions, particular if these can be combined with esis, particularly if organic matter is present. palaeontological and geochemical data. The two Unconsolidated lake sediments have usually previous papers have clearly demonstrated the accumulated over time scales that cover con- complexities of diagenesis, particularly when ventional archaeological studies and extend organic materials are involved, and suggest back to even earlier times. However, most any statistical parameters attempting to iden- lacustrine environments are complicated by the tify different diagenetic zones must consider a effects of high organic activity in them and host of physico-chemico-biological properties. would require an entire publication on such However, standard variate analyses of various aspects alone. Consequently, the next contribu- partially related palaeomagnetic, rock-magnetic, tion considers a lacustrine situation where geochemical and palaeontological parameters biological activity can be largely excluded, in are obviously tedious and most such statistical this case because of extremely high salinities. techniques require class boundaries to be defined Marco et al. (this volume) present studies of the before assessing the means and variances. Such 'chemical' deposits of the Lisan Formation in sharp classifications seems inappropriate for Israel. They show that the remanences preserve properties that merge and overlap over a range short-term features (secular variations) of the of scales. Urbat & Dekkers (this volume) geomagnetic field from shortly after deposition, propose that fuzzy-c statistics appear to be possibly within a few weeks or months. How- particularly appropriate to such a situation. ever, being in a highly active seismic region close The examples which they use relate mostly to the the Dead Sea Fault, violent seismic shocks early diagenetic situations, but the techniques can briefly fluidize such sediments, causing a re- are equally applicable to later stages of dia- magnetisation down to depths of about 1 m genesis. Fuzzy-c statistical analysis are shown below which the sediments were presumably to be particularly appropriate when consider- already sufficiently consolidated that their rema- ing overlapping diagenetic groupings which, nences were preserved. M6rner (1996) has in nature, are very fuzzy! There is also the attributed similar seismically induced fluidiza- advantage of speed of comparative analysis and tion effects in the Swedish Quaternary varves the avoidance of subjectively selecting classes, as where locally some 2 to 4 m may be affected in required by more routine multivariate ana- an region generally regarded as seismically quiet! lyses - although outliers still have to be manu- The reason for this difference is, presumably, ally excluded! (While this may mean starting on because the Lisan Formation is likely to become yet another initial learning curve, the potential more cohesive because of its composition than does seem high!) the more inert varved silts and clays in Sweden. Wilson & Roberts (this volume) consider an Such studies also highlight the hazards of unusual situation where sediments have been handling poorly consolidated sediments which repeatedly re-cycled through erosion, diagenesis could clearly result in remagnetization. On the and re-deposition within sulphate-reducing con- bright side, however, it is interesting to note that ditions. Such processing essentially removed the magnetic effects of palaeo-seismic distur- most of the 'standard' magnetite and hema- bances provides a new method for studying tite that normally carry the remanence in palaeo-seismicity. sedimentary rocks, demonstrating the extreme Turning to somewhat more lithified sedi- of repeated early diagenetic processes. They ments, Dinares-Turell & Dekkers (this volume) demonstrate that the remanence appears to be consider palaeomagnetic results from the lower carried almost entirely by hemo-ilmenites, with Pliocene Trubi Marls in Sicily. They show that it a very minor contribution from chromite. This can be extremely difficult to assess the extent seems strange as such ilmenite would normally to which exposed sediments are truly pristine be considered to have paramagnetic properties and also demonstrate the drastic differences that at room temperature, but it suggests that such can occur as a result of different depositional remanence-carrying hemo-ilmenites may be a environments. In this particular case, identifica- significant component in other sediments where tion of the correct components is also vital to diagenetic changes have largely removed most determining the structural evolution of the area. magnetite and hematite. Turner et al. (this Further evidence of the particular complexity in volume) demonstrate how palaeomagnetic tech- the diagenetic evolution of organic matter is niques, combined with petrological studies, given by Vigliotti et al. (this volume) in both can be used to evaluate the reliability in bore- lacustrine and marine sediments. They show cores from sediments that cannot be studied in that careful rock magnetic analyses may provide outcrop. This restricts many of the standard a vital key to assessing environmental condi- tests used in palaeomagnetic studies, but such Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

6 D.H. TARLING combined studies enable a magnetostratigraphy only briefly interrupted by rare exposures to a to be established, indicating different rates of meteoric regime when relative sea-level dropped deposition in different parts of the Jupiter oil at the top of the more major planetary orbital field. Hailwood et aL (this volume) then shows cycles. The palaeomagnetic properties show how the fluid permeability anisotropy of such same cyclicities as the sedimentary parameters, bore-samples can be determined more quickly implying that the Earth's magnetic field is also and efficiently after impregnation by high affected by such perturbations. They also show magnetic susceptibility fluids. Such a technique the patterns and rates of change that suggest is not, of course, confined to bore-cores but that geomagnetic secular variations have been could be invaluable when, as for Turner et al. preserved. Such observations mean that the (this volume), there are no outcrop exposures to remanence must have been locked-in during enable three-dimensional assessment of the very early diagenesis of these sediments. The routes through which fluids have passed. Such combined sedimentological and palaeomagnetic fluids can obviously drastically affect the mag- evidence can therefore also be used to date netic properties of the rocks through which they diagenetic features with a remarkably high pass, whether they are oxic or reducing, but, precision, averaging some 360 years. Mfirton conversely, the remaining minerals provide a (this volume) describes an unusual example at clue to the nature of such fluids, while their the Triassic/Jurassic boundary where Triassic magnetization enables the date of their passage carbonates were exposed prior to being infilled to be assessed. and succeeded by Jurassic pelagic carbonates. The problem of defining the change from deep Careful examination of these exposures and diagenesis to tectonic metamorphism is consid- comparison with the data from a bore-core ered by Hrouda & Jezek (this volume). A variety and neighbouring localities show how the late of sediments bordering the Alps have fabrics Triassic remanences had been re-set, without that range from almost entirely sedimentary to evident chemical change, during the Jurassic- strongly tectonized. As the distinction between a re-setting that would have been almost deep diagenetic and tectonic processes is based impossible to distinguish under normal circum- fundamentally on whether the phyllosilicate stances. In Ordovician and Silurian carbonates minerals have bedding-parallel or cross-bedding in Estonia, Shogenova (this volume) finds that orientation, it is clearly fabric techniques that dolomites have different ages and genesis in define which process is dominant. Hrouda and different part of the region. Geochemical obser- Jezek demonstrate how magnetic anisotropy vations combined with palaeomagnetic analyses studies pick out the earlier phases of grain show that many are of very early diagenetic alignments by these two different orientation origin, but others are different, both geochemi- process faster, cheaper and more effectively cally and magnetically. These atypical dolo- than most standard petrological techniques. mites are spatially related to deep basement Borradaile (this volume) finds that it is still faults that provided conduits for the fluids possible to isolate fabrics with sedimentary that caused the dolomitization at later times. characteristics, by separating the fabrics asso- In Texas, Hauboid (this volume) found analo- ciated with diamagnetic quartz from those of gous results in similarly aged carbonates, again ferromagnetic minerals. This clearly requires combining geochemical and magnetic observa- considerable care in undertaking the analyses tions. In this case, he was able to palaeomag- in the correct sequence, but also indicates how netically date the effects of the passage of the strictly diagenetic fabrics can occur together fluids responsible for the changes. These were with tectonic fabrics - thereby demonstrating the clearly orogenic fluids generated in the nearby arbitrary nature of the distinction between very Ouachita Orogen which then passed through the late diagenesis and ancho-metamorphism. more permeable beds and fissures. Palaeomag- The carbonates are considered last as having netic dating enabled both primary (early diagen- different reactivities from clastic rocks. D'Ar- esis) and secondary re-magnetizations to be genio et al. (this volume) deal with Cretaceous dated, showing that impermeable beds, and shallow-water carbonates which, on sedimento- other lithologies more distant from the orogenic logical grounds, have retained their early diage- front, still retained their original properties. netic features. These carbonates show cyclical depositional as well as early diagenetic changes The following people, in alphabetical order, were asked related to periodic variations in the Earth's to referee one or more of the submitted papers: C. M. orbital parameters. Early diagenetic features Batt, G. Borradaile, J. Dinares-Turrell, N. Hamilton, developed almost immediately after deposition H. Haubold, R. Ixer, S. Marco, E. Marton, M. No~l, in a shallow, sub-tidal marine realm which was S. Openshaw, W. Owens, S. Pisarevsky, G. Potts, Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

INTRODUCTION 7

A. P. Roberts, J. Sahota, J. Shaw, A. A. Shogenova, FOLGERHAITER, G. 1894. Origine del magnetismo Helle Stephenson, A. Turner, L. Vigliotti, H. Vizan and roccie vulcaniche del Lazio et aL Attai. della Reala G.S. Wilson. Accadema Lincei, 3, 53, 117 & 165. Their help is very gratefully acknowledged. I also FRANKEL, R. B. & BLAKEMORE, R. P. (eds) 1990. Iron wish to acknowledge my indebtedness to all of my Biominerals. Plenum Press, New York colleagues in the department for releasing me from GHERADI, S. 1862. Title to be supplied in proof Nuovo virtually all teaching and administrative duties while Cimento, 16, 384. undertaking the task of editing- and to all those GRAHAM, J. W. 1949. The stability and significance contributors who not only followed the recommenda- of magnetism in sedimentary rocks. Journal of tions of the referees, but also tolerated my extensive Geophysical Research, 54, 131-167. re-writes. Bruno D'Argenio and Marina Iorio also GRAHAM, K. W. & HALES, A. L. 1961. Preliminary helped to check the introduction and glossary. Not palaeomagnetic measurements on Silurian sedi- least, I thank my wife, Nanette, for her tolerance ments from South Africa. Geophysical Journal and support. Royal Astronomical Society, 5, 318-325. GRANAR, L. 1958. Magnetic measurements on Swedish varved sediments. Arkivefur Geofysik, 3, 1-40. GRIFFITHS, D. H. 1953. Remanent magnetism of varved clays from Sweden. Nature, 172, 539. References --, KING, R. F. & REES, A. I. 1962. The relevance of magnetic measurements on some fine grained silts BALSLEY, J. R. & BUDDINGTON, A. F. 1958. Iron- to the study of their depositional process. titanium oxide minerals, rocks and aero-magnetic Sedimentology, 1, 134-144. anomalies of the Adirondack area, New York. & WRIGHT, A. E. 1960. The remanent Econic Geology, 53, 777-805. magnetism of some recent varved sediments. BLACKETT, P. M. S. 1952. 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