Paleomagnetic results have been obtained from the Late Cretaceous‐early Tertiary igneous complexes of the north‐central Montana alkalic province. Data from 94 sites in Eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82.0°N, 170.2°E (A95 = 3.5°; k = 18.6), while 36 sites in Paleocene intrusions yield a paleomagnetic pole at 81.8°N, 181.4°E (A95 = 5.4°; k = 20.2). These poles differ by only 1.6° and are not significantly different statistically. The 130‐site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander (APW) during the period of magnetization of the igneous centers. Postmagnetization structural complications in these rocks are minimal. The presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration. The northcentral Montana data together with other early Tertiary, Cretaceous, and mid‐Tertiary paleomagnetic results require modification of our earlier APW chronology [Diehl et al., 1980]. It now appears that APW relative to North America since the Early Cretaceous consists of a polar still‐stand during much of the Cretaceous (120–75 m.y. B.P.), a period of rapid movement in latest Cretaceous time (75–65 m.y. B.P.), and a period of slow polar movement thereafter. The onset of this period of rapid APW correlates well with a major change in plate motions at ∼80 m.y. and the beginning of the Laramide Orogeny.
Floodplain sediments deposited along the lower course of the Morava River (eastern Czech Republic), were studied in the Strážnické Pomoraví region to describe the alluvial history of the river over the last millennium. The sediments exposed in up to 5 m high erosional river banks were analysed using mineral magnetic, geochemical and chemical approaches. The age model of the sedimentary sequences was constructed from radiocarbon dates in association with 206Pb/207Pb and POP (DDT, PCB) analysis and 137Cs activity data. The Cu-trien method was used for stratigraphically correlating these deposits based on the variation of expandable clay minerals in the sediments. The resulting stratigraphic pattern reveals the alluvial history of the currently active river channel system since the end of the first millennium AD. Fine overbank clayey sediments deposited during the `Mediaeval Warm Period' were eroded from cultivated fields newly formed during Mediaeval colonization between 1250 and 1450. These fine deposits are overlain by coarser floodplain sediments of the `Little Ice Age', indicating a change in the sediment source since the sixteenth century AD, and a substantial increase in the sediment load in the second half of twentieth century. The Strážnické Pomoraví floodplain deposits represent a valuable palaeoenvironmental archive of the last millennium, containing records of fluvial processes considerably altered by human activities.
This initial study investigates the possible use of hysteresis parameters and electron spin resonance (ESR) spectroscopy to identify and correlate volcanic ash. ESR and hysteresis properties are sensitive to characteristics such as the chemical composition, mineralogy, and grain size and shape. These characteristics are determined by the tectonic setting of the volcano and by the magmatic and eruptive history of the volcanic ash. Hysteresis properties and ESR spectra, therefore, should be distinct for each ash eruption and may help to identify the eruptive source of the ash and to correlate ash from unknown sources. We conducted ESR spectroscopy at room temperature and magnetic hysteresis measurements on 19 samples of a single ash, the 1974 October 14 eruption of the Fuego volcano, Guatemala, and on single samples of ash obtained from eight different volcanoes. The Fuego ash samples were obtained at increasing distances from the volcano. For the single Fuego ash, ESR spectra and hysteresis parameters become increasingly similar as the distance from the volcano increases. At distances greater than 30 km, ESR spectra and hysteresis properties are uniform. The variability of magnetic and ESR properties with distance from Fuego is due to the preferential fall‐out of phenocrysts closer to the volcano. At large distances, the ash is more uniform, containing more glass and microcrystals. All eight ash samples from the different volcanoes can be distinguished from the distal Fuego 1974 October 14 ash using ESR spectra and hysteresis parameters. These results suggest that ESR and hysteresis measurements have a potential to be used as tools to identify distal ash when used in conjunction with geochemical, mineralogical and/or other types of data.
Difficulty in removing the effects of grain size dependence have hampered interpretations of magnetic property changes during low‐temperature oxidation of titanomaghemite. For the present study, samples were collected perpendicular to cracks in seafloor basalts to produce subsamples of essentially constant grain size but varying degree of low‐temperature oxidation within a few centimeters. Coarse‐grained samples and those showing evidence of high‐temperature oxidation were excluded. Increasing Curie temperature was used as the most reliable measure of increasing maghemitization. Intensity of natural remanent magnetization (JNRM) consistently decreased with increasing low‐temperature oxidation indicating that any increase in JNRM with age of oceanic crust must be due to other factors. Coercive force (HC) increased until Curie temperatures reached 250°C and then, in samples exhibiting single‐domain behavior, decreased drastically. Because coercive force depended on degree of oxidation in this complex manner, chemical remanent magnetization (CRM) in even slightly oxidized grains should have a different coercivity than the original thermal remanent magnetization (TRM). Most Deep Sea Drilling Project samples in this study have experienced tectonic rotations or field reversals within 1.0 m.y. of extrusion. Thus, field‐controlled CRM, if present, should have different coercivities and a discrete direction and be discernible during alternating field (AF) demagnetization. Alternating field demagnetizations showed no evidence for significant acquisition of field‐controlled CRM. Viscous remanent magnetization (VRM) was the dominant secondary magnetization in samples with several components of remanence. In very fine‐grained basalts, VRM was difficult to remove by AF demagnetization and may be the source of increased dispersion in highly oxidized, oceanic basalts. Mechanisms which may, in part, be responsible for coercivity and VRM changes are (1) an increase in the critical size of domain transitions during low‐temperature oxidation, and (2) incipient phase separation into Fe‐rich and Ti‐rich areas, the Fe‐rich areas being superparamagnetic. The latter mechanism explains the increase in saturation magnetization (JS) and concomitant decrease in JNRM when Curie temperatures are above 360°C.
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