5.10 Geomagnetic Excursions C. Laj, Laboratoire des Sciences du Climat et de l’Environment, Unite´ Mixte CEA-CNRS-UVSQ, Gif-sur-Yvette, France J. E. T. Channell, University of Florida, Gainesville, FL, USA ª 2007 Elsevier B.V. All rights reserved. 5.10.1 Introduction 373 5.10.1.1 History of the Polarity Timescale and Excursions 373 5.10.1.2 Nomenclature for Excursions and Polarity Intervals 375 5.10.2 Geomagnetic Excursions in the Brunhes Chron 376 5.10.2.1 Introduction 376 5.10.2.2 The Laschamp Excursion 379 5.10.2.3 The Mono Lake Excursion 383 5.10.2.4 The Blake Excursion 385 5.10.2.5 The Iceland Basin Excursion 387 5.10.2.6 The Pringle Falls Excursion 389 5.10.2.7 Excursions in the Early Brunhes Chron 391 5.10.3 Geomagnetic Excursions in the Matuyama Chron 393 5.10.3.1 Background 393 5.10.3.2 Excursions between the Gauss–Matuyama Boundary and the Re´ union Subchron 396 5.10.3.3 Huckleberry Ridge 396 5.10.3.4 Gilsa 396 5.10.3.5 Gardar and Bjorn 397 5.10.3.6 Cobb Mountain 397 5.10.3.7 Punaruu 397 5.10.3.8 Intra-Jaramillo Excursion (1r.1n.1r) 397 5.10.3.9 Santa Rosa 397 5.10.3.10 Kamikatsura 398 5.10.4 Geomagnetic Excursions in Pre-Matuyama Time 398 5.10.4.1 C5n.2n (Late Miocene) 400 5.10.4.2 Other Miocene Excursions/Subchrons 401 5.10.4.3 Oligocene and Eocene 401 5.10.4.4 Middle Cretaceous 402 5.10.5 Duration of Geomagnetic Excursions 402 5.10.6 Excursional Field Geometry 404 5.10.7 Concluding Remarks 405 References 407 5.10.1 Introduction Miocene lavas from central France (Puy de Dome). In so doing, Brunhes (1906) made first use of a field 5.10.1.1 History of the Polarity Timescale test for primary thermal remanent magnetization and Excursions (TRM) that is now referred to as the ‘baked contact’ David (1904) and Brunhes (1906) were the first to test (see Laj et al. (2002) for an account of Brunhes’ measure magnetization directions in rocks that were work). Matuyama (1929) was the first to attribute approximately antiparallel to the present Earth’s reverse magnetizations in (volcanic) rocks from field. Brunhes (1906) recorded magnetizations in Japan and China to reversal of geomagnetic polarity, baked sedimentary rocks that were aligned with and to differentiate mainly Pleistocene lavas from reverse magnetization directions in overlying mainly Pliocene lavas based on the polarity of the 373 374 Geomagnetic Excursions magnetization. In this respect, Matuyama (1929) was Proterozoic Torridonian Sandstones in Scotland, and the first person to use magnetic stratigraphy as a in rocks of Devonian and Triassic age. For the means of ordering rock sequences. Torridonian Sandstones, normal and reverse magne- The modern era of paleomagnetic studies began tizations were observed from multiple outcrops, and with the studies of Hospers (1951, 1953–54) in Iceland, an attempt was made to arrange the observed polarity and Roche (1950, 1951, 1956) in the Massif Central of zones in stratigraphic sequence. Meanwhile, Khramov France. The work of Hospers on Icelandic lavas was (1958) published magnetic polarity stratigraphies in augmented by Rutten and Wensink (1960) and Pliocene–Pleistocene sediments from western Wensink (1964) who subdivided Pliocene–Pleistocene Turkmenia (central Asia), and made chronostrati- lavas on Iceland into three polarity zones from young to graphic interpretations based on equal duration of old: N–R–N. Magnetic remanence measurements on polarity intervals. Early magnetostratigraphic studies basaltic lavas combined with K/Ar dating, pioneered by were carried out on Triassic red sandstones of the Cox et al. (1963) and McDougall and Tarling (1963a, Chugwater Formation (Picard, 1964), on the 1963b, 1964), resulted in the beginning of development European Triassic Bundsandstein (Burek, 1967, of the modern geomagnetic polarity timescale (GPTS). 1970), and on the Lower Triassic Moenkopi These studies, and those that followed in the mid- Formation (Helsley, 1969). The above studies were 1960s, established that rocks of the same age carry the conducted on poorly fossiliferous mainly continental same magnetization polarity, at least for the last few (red) sandstones and siltstones; therefore, the correla- million years. The basalt sampling sites were scattered tions of polarity zones did not have support from over the globe. Polarity zones were linked by their K/ biostratigraphic correlations. It was the early magne- Ar ages, and were usually not in stratigraphic super- tostratigraphic studies of Plio-Pleistocene marine position. Doell and Dalrymple (1966) designated the sediments, recovered by piston coring in high south- long intervals of geomagnetic polarity of the last 5 My ern latitudes (Opdyke et al., 1966; Ninkovitch et al., as magnetic epochs, and named them after pioneers of 1966; Hays and Opdyke, 1967) and in the equatorial geomagnetism (Brunhes, Matuyama, Gauss, and Pacific Ocean (Hays et al., 1969), that mark the begin- Gilbert). The shorter polarity intervals (events) were ning of modern magnetic stratigraphy. These studies named after localities: e.g., Jaramillo (Doell and combined magnetic polarity stratigraphy and biostra- Dalrymple, 1966), Olduvai (Gromme´ and Hay, 1963, tigraphy and in so doing refined and extended the 1971), Kaena and Reunion (McDougall and GPTS derived from paleomagnetic studies of basaltic Chamalaun, 1966), Mammoth (Cox et al., 1963), and outcrops (e.g., Doell et al., 1966). Nunivak (Hoare et al., 1968). The nomenclature for Heirtzler et al. (1968) produced a polarity timescale excursions has continued this trend by naming excur- for the last 80 My using South Atlantic MMA record sions after the localities from where the earliest records (V-20) as the polarity template. They assumed constant were derived (e.g., Laschamps in France). spreading rate, and extrapolated ages of polarity chrons The fit of the land-derived polarity timescale, using an age of 3.35 Ma for the Gilbert/Gauss polarity from paleomagnetic and K/Ar studies of exposed chron boundary. This was a dramatic step forward that basalts, with the polarity record emerging from mar- extended the GPTS to 80 Ma, from 5Mabasedon ine magnetic anomalies (MMAs) (Vine and magnetostratigraphic records available at the time (e.g., Matthews, 1963; Vine, 1966; Pitman and Heirtzler, Hays and Opdyke, 1967). Heirtzler et al. (1968) made 1966; Heirtzler et al., 1968) resulted in a convincing the inspired choice of a particular South Atlantic MMA argument for synchronous global geomagnetic polar- record (V-20). Several iterations of the polarity time- ity reversals, thereby attributing them to the main scale (e.g., Labrecque et al., 1977) culminated with the axial dipole. The results relegated self-reversal, work of Cande and Kent (1992a) that re-evaluated the detected in the 1950s in the Haruna dacite from the MMArecord,andusedtheSouthAtlanticasthefun- Kwa district of Japan (Nagata, 1952; Nagata et al., damental template with inserts from faster spreading 1957; Uyeda, 1958), to a rock-magnetic curiosity centers in the Indian and Pacific Oceans. They then rather than to a process generally applicable to interpolated between nine numerical age estimates volcanic rocks. that could be linked to polarity chrons over the last The first magnetic stratigraphies in sedimentary 84 My. Eight of these ages were based on ‘high- rocks may be attributed to Creer et al. (1954) and temperature’ radiometric ages (no glauconite ages Irving and Runcorn (1957) who documented normal were included), and the youngest age (2.60 Ma) was and reverse polarities at 13 locations of the the astrochronological age estimate for the Gauss/ Geomagnetic Excursions 375 Matuyama boundary from Shackleton et al. (1990). In magnetic stratigraphy because the high sedimentation the subsequent version of their timescale, hereafter rate sequences, required to record brief (few kiloyears referred to as CK95, Cande and Kent (1995) adopted duration) magnetic excursions, were generally not tar- astrochronological age estimates for all Pliocene– geted during conventional piston coring expeditions. Pleistocene polarity reversals (Shackleton et al., 1990; Indeed, at the time, low sedimentation rate sequences Hilgen, 1991a, 1991b) and modified the age tie-point at were often preferred as they allowed the record to be the Cretaceous–Tertiary boundary from 66 to 65 Ma. pushed further back in time. The development of The astrochronological estimates for Pliocene– hydraulic piston coring (HPC) techniques, first used Pleistocene polarity chrons (Shackleton et al., 1990; in early 1979 during the Deep Sea Drilling Project Hilgen, 1991a, 1991b), incorporated in CK95, have (DSDP) Leg 64, brought high sedimentation rate not undergone major modification in the last 15 years, sequences within reach by increasing penetration by a and indicate the way forward for timescale calibration factor of 15, from 20 m for conventional piston further back in time. Subsequent to CK95, astrochro- coring to 300 m for the HPC. nological estimates of polarity chron ages have been extended into the Miocene, Paleogene, and Cretaceous 5.10.1.2 Nomenclature for Excursions and (Krijgsman et al., 1994, 1995; Hilgen et al., 1995, 2000; Polarity Intervals Abdul-Aziz, 2000, 2003, 2004). Many of these advances are included in the timescale of Lourens et al. (2004). At The International Stratigraphic Commission (ISC) present, more or less continuous astrochronologies tied has guided the use of magnetostratigraphic units to polarity chrons are available back to the Oligocene (polarity zones), their time equivalents (polarity (e.g., Billups et al., 2004). chrons), and chronostratigraphic units (polarity Recognition of brief polarity excursions as an inte- chronozones) (see Anonymous, 1977; Opdyke and gral part of the Earth’s paleomagnetic field behavior Channell, 1996). The revelation in the last 20 years has developed alongside astrochronological calibration of numerous short-lived excursions within the of the polarity timescale in the last 20 years.