Thermal evolution of the Tertiary Shimanto Belt, Muroto Peninsula, Shikoku, Japan

Abstract The Shimanto accretionary complex on the Muroto Peninsula of Shikoku comprises two major units of Tertiary strata: the Murotohanto Sub-belt (Eocene-Oligocene) and the Nabae Sub-belt (Oligocene-Miocene). Both sub-belts have been affected by thermal overprints following the peak of accretion-related deformation. Palaeotemperatures for the entire Tertiary section range from ～ 140 to 315°C, based upon mean vitrinite reflectance values of 0.9–5.0%R_m. Values of illite crystallinity index are consistent with conditions of advanced diagenesis and anchimetamorphism. Illite/mica b₀ lattice dimensions indicate that burial pressures were probably no greater than 2.5kbar. In general, levels of thermal maturity are higher for the Murotohanto Sub-belt than for the Nabae Sub-belt. The Eocene-Oligocene strata also display a spatial decrease in thermal maturity from south to north and this pattern probably was caused by regional-scale differential uplift following peak heating. Conversely, the palaeothermal structure within the Nabae Sub-belt is fairly uniform, except for the local effects of mafic intrusions at the tip of Cape Muroto. There is a paleotemperature difference of ～ 90°C across the boundary between the Murotohanto and Nabae Sub-belts (Shiina-Narashi fault), and this contrast is consistent with approximately 1200 m of post-metamorphic vertical offset. Subduction prior to Middle Miocene probably involved the Kula or fused Kula-Pacific plate and the background geothermal gradient during the Eocene-Oligocene phase of accretion was ～ 30–35°C/km. Rapid heating of the Shimanto Belt evidently occurred immediately after a Middle Miocene reorganization of the subduction boundary. Hot oceanic lithosphere from the Shikoku Basin first entered the subduction zone at ～ 15 Ma; this event also coincided with the opening of the Sea of Japan and the rapid clockwise rotation of southwest Japan. The background geothermal gradient at that time was ～ 70°C/km. Whether or not all portions of the inherited (Eocene-Oligocene) palaeothermal structure were overprinted during the Middle Miocene remains controversial.     

Teaching GIS in Geography*

Although numerous courses in geographic information systems (GIS) have been introduced into the geography curriculum over the past few years, there has been remarkably little debate over the issues involved. This paper first discusses the arguments for geography as an appropriate disciplinary setting for GIS teaching at the undergraduate level. This is followed by a discussion of the nature of GIS courses and their appropriate place in the undergraduate geography curriculum. The final section of the paper describes the NCGIA Core Curriculum project and examines its significance in this broader context.     

Data quality in applied geochemistry: the requirements, and how to achieve them

To obtain a reliable interpretation of geochemical data, we need to ensure that the measurements are of sufficiently high quality. To do this we need to be able to specify quantitatively the data quality that we need, and then to be certain that we are getting it. The latter can be achieved only if properly validated analytical methods are employed, and well-designed quality control schemes are used as a matter of routine.     

Reevaluation of the lake-sediment chronology in the Dead Sea basin, Israel, based on new Th/U dates

The Dead Sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes, Lake Samra and Lake Lisan. The sedimentary history of these lakes was recon-structed by means of ²³⁰Th/²³⁴U ages of 30 samples, mostly of argonite laminae, from 8 columnar sections up to 110 km apart. The general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency, agreement with stratigraphic order, and concordance with ¹⁴C ages. In the south, only the part of the Samra Formation older than 170,000 yr is exposed, while the aragonite-detritus rhythmites found in the central and northern region are generally younger than 120,000 yr. The Lisan Formation started accumulating about 63,000 yr B.P., with the clay and aragonite beds in the south-central area reflecting a rise in water level to at least −280 m. The upper part of the Lisan Formation, the aragonite-rich White Cliff Member, started accumulating about 36,000 yr B.P. The lake probably reached its highest level sometime after this, based on the ages of Lisan sediments preserved in the southernmost reaches of the basin.     

High chromium contents in Tertiary coal deposits of northwestern Washington — A key to their depositional history

Chromium contents obtained from 20 coal and 5 associated rock samples collected from the basal part of the Eocene Chuckanut Formation, in Skagit and Whatcom counties, northwest Washington, range between 30 and 300 ppm (mean 120 ppm whole-coal basis). The lenticular coals, ranging in rank from subbituminous to anthracite, and with an ash content of 12–46%, crop out along the western flank of the Cascade Range. Results of X-ray diffraction analysis of low-temperature ash show that the mineral matter in the coal samples consists predominantly of quartz and clay (kaolinite, illite and chlorite group). However, accessory minerals, isolated from the coal samples and analyzed by X-ray diffraction, scanning electron microscope and optical methods, contain angular fragments and euhedral crystals of the spinel group (chromite, magnetite and trevorite ), kaolinite-serpentine group (antigorite and chrysotile), chlorite group, amphibole group and pyroxene group minerals (augite, diopside and enstatite), all of which are commonly enriched in chromium.Although associated primarily with the inorganic fraction of the coal, concentrations of chromium in the samples show no statistically significant correlation with ash content. Localized concentrations of chromium in the coal are the result of natural contamination from the alteration of detrital chromium-bearing mineral grains introduced into the peat-forming mires from nearby Jurassic ophiolite bodies. The coals formed in the early Eocene, in rapidly subsiding small basins that developed during the uplift and erosion of the pre-Tertiary ophiolite terrain. Scattered bodies of source rock, random distribution of chromium-bearing minerals within the coal and sample heterogeneity account for the variation in Cr contents of the samples.