Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021 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 deposi- tion 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,