REVIEWS OF GEOPHYSICS AND SPACE PHYSICS, VOL. 20, NO. 2, PAGES 171-192, MAY 1982 Magnetic Properties of Marine Limestones WILLIAM LOWRIE AND FRIEDRICH HELLER Institut fiir Geophysik, ETH-H6nggerberg, CH-8093 Ziirich, Switzerland Numerous paleomagnetic investigations have been carried out on limestones, but their rock magnetic properties have often been neglected.In this review, geologicaland rock magneticfactors which determine whether marine limestones are suitable for paleomagnetic study are summarized, and laboratory techniques for the identificationof the magneticmineralogy are evaluated. The conditions under which pelagiclimestones form are especially favorable for the acquisitionof a stable primary natural remanence(NRM), probably by a postdepositional alignment process.The effectiveness and timing of acquisitionof this remanenceare influencedby the thoroughness of bioturbation. Other remanencecomponents can be acquired during diagenesis, in the soft sedimentright after deposition or much later even in the indurated limestone. The magneticmineralogy of a limestone is difificult to describe optically because of the smallgrain size and low concentration; magnetictechniques are more convenient. Direct analysis of extracted magnetic minerals is handicapped by the difiSculty of obtaininga representative extract. A particularly useful techniqueinvolves combinedobservations of isothermal remanent magnetization (IRM) acquisition in strong fields up to at least 4 T and the subsequentdestruction of this IRM during stepwise or continuous thermal demagnetization. The magnetic minerals most commonly identified by these techniquesare magnetite, goethite, hematite, and maghemite.These magneticminerals occur in different combinations in the marine limestonesof central Europe and the Mediterranean realm. Pyrite is commonin certain limestones,where it can be the precursorof an unwelcomelate goethite. Pyrrhotite may be responsible for unstablemagnetization in the Swiss Helvetic limestones but otherwise is rare. Thermal demagnetization of some limestones causes changesof susceptibility and coercivity at quite low temperatures (300ø-400øC) due to the breakdown of a metastablemineral such as maghemite,goethite, or pyrrhotite. At high temperatures (above 500øC), pronouncedchangesof magnetic mineralogy occur in all limestones because of the growth of new magnetite. This necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observationaldifiSculties due to viscous remanent (VRM) behavior of the newly formed magnetite. VRM is a widespreadcomponentof NRM in many limestones, but it can usually be removed effectively by standard magnetic cleaning techniques. The susceptibility servesas a simple control of mineralogical changeduring heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles. The susceptibilityis anisotropic and shows a classical 'depositional' pattern of the distribution of principal axes. However, the probable postdepositional mechanism of NRM acquisition (especially in bioturbatedsediments) implies that the anisotropypattern resultsfrom postdepositional compactionof the sediment. INTRODUCTION Carbonate rocks constitute approximately 10% of the sedimentary record exposed on land [Blatt et al., 1980]. Limestonesare more abundant in somesystems than others but are represented in all periods of the Phanerozoic. Marine limestonespredominateover freshwaterlimestones and de- velopeprincipallyin tropicalandtemperate seas (Figure la). However, the presentlatitudinal distributionof ancientcoral reef limestones which formedin equatorial watersis bimodal with peaks at intermediate to high latitudes (Figure lb). Restoration of the limestones to the paleolatitudesdeter- mined from theft paleomagnetic inclinations [Briden and Irving, 1964; Irving, 1964] results in a distribution with latitude which resembles that of moderncarbonates (Figure lc). The limestoneevidencesupports the uniformitarianism principle and indicates the effects of continental drift. Limestonesrangingin agefrom Cambrianto Tertiary have been the subject of paleomagnetic research by numerous investigators since the initiation and development of the modern scienceof paleomagnetism in the late 1940'sand the 1950's. Graham [1949] demonstrated that the natural rema- nent magnetizations (NRM) of the Silurian McKenzie lime- stone in the Appalachians partly failed his tectonic fold test Copyright¸ 1982 by the American Geophysical Union. Paper number 1R1845. 0034-6853/82/001R- 1845515.00 but reflected that the directions were not completely unsta- ble and may have been affected by deformation. He later investigatedthe NRM of the Ordovician Trenton limestone and found well-grouped directions steeplyinclinedtoward the south; the directions in slumped beds werethe same asin undistortedbeds, indicatingresetting of the NRM after slumping occurred [Graham, 1954, 1956]. Nairn [1960] in- cluded a few Liassiclimestones in his paleomagnetic study of rocks from Britain, France, and Luxembourg. Reversed magnetizations were observed at four of five limestone sites. Hargraves and Fischer [1959] found normal polarities in Jurassic (Pliensbachian)limestonesfrom the Northern Cal- careous Alps in Austria.The NRM directions wereinterpret- ed as evidencefor 45 ø of large-scale counterclockwise rota- tion of a tectonic block. These studies were based on uncleaned NRM directions and would be considered unacceptable by modern paleomag- netic standards, which requirethorough analysis of direc- tional components during systematic demagnetization [Zij- derveld, 1967; Halls, 1976; Hoffman and Day, 1978]. However, they demonstrate that limestones can possess suitablemagnetic properties for paleomagnetic studies; the cleanedremanence components often preserve accurately the paleomagnetic field direction. The stable remanence in limestones canpersist overgeologically long periods of time unless it suffers localintense deformation or strong diagenet- ic remagnetization. The principalreasonwhy limestones 171