No abstract

No abstract

No abstract

No abstract

The Early Eocene Fur Formation is a clayey diatomite interbedded with layers of volcanic ash. The diatomite is non-calcareous, laminated or structureless, and thin sections show that the proportion of clay-sized particles to diatom frustules varies between laminae. The clay mineral phase has been isolated and its chemical composition has been determined by X-ray fluorescence (XRF). Clay mineralogical analyses based on X-ray diffraction (XRD) show that the dominant phase is smectite. A general formula for the clay mineral phase is: X0.1K0.2(Mg0.3Fe(III)0.3–0.7Al1.1–1.4)Si4O10(OH)2.The investigation indicates that the clayey diatomite of the Fur Formation comprises three components: diatom frustules, clay minerals and volcanic dust. The chemical compositions of bulk samples have been determined. The clay mineral content is calculated to be 30–45% (by weight). Volcanic dust constitutes c. 10% of the diatomite, and the rest is diatoms (opal-A). The diatom frustules contain some Al2O3 that is either of primary or diagenetic origin.

Latest Mesozoic to earliest Cenozoic deformation affected SE Asia's Sundaland core. The deformation event bridges the Mesozoic SE Asian fusion with the Cenozoic era of rifting, translation, basin formation, and the creation of modern SE Asian oceans. Southern Cambodia and Vietnam are central to this shift, but geological investigations of the region are in their infancy. Based on apatite and zircon fission track analyses (AFTA and ZFTA), stratigraphic and structural observations, seismic data, thermal maturity, and igneous rock dating, the geological evolution of southern Cambodia and Vietnam is investigated. Diverse depositional styles, igneous activity, structural deformation and subsurface unconformities testify to a highly variable Phanerozoic tectonic setting. Major latest Cretaceous to Paleocene thrusting and uplift affected the Kampot Fold Belt and surrounding regions and the associated up to ~11 km exhumation probably exceeds earlier denudation events since at least Permian time. The present relief of the Bokor Mountains rising high above the Kampot Fold Belt represents an artifact after differential erosion and only 2.5–4.5 km of erosion affected this area. The latest Cretaceous to Paleocene orogenesis affected much of greater Indochina probably owing to plate collision along eastern Sundaland or a combination of collisions along both east and west Sundaland. AFTA and ZFTA data document protracted cooling of Cretaceous granites and locally elevated thermal gradients persisting a few tens of million years after their emplacement. The thermal gradient had stabilized by early Miocene time, and Miocene cooling probably reflects a renewed denudation pulse driven by either regional tectonism or climate‐enhanced erosion.

No abstract

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Kalsbeek, F., Nutman, A. P., Escher, J. C., Friderichsen, J. D., Hull, J. M., Jones, K. A., & Schack Pedersen, S. A. (1999). Geochronology of granitic and supracrustal rocks from the northern part of the East Greenland Caledonides: ion microprobe U–Pb zircon ages. Geology of Greenland Survey Bulletin, 184, 31-48. https://doi.org/10.34194/ggub.v184.5228 _______________ Granitoid rocks from different settings within the northern part of the East Greenland Caledonian fold belt have yielded U–Pb zircon dates between 2000 and 1730 Ma, confirming the Palaeoproterozoic origin of the crystalline basement. Widespread sandstone sequences occur both in the Caledonian fold belt and in the foreland to the west; all of these have been assigned to the Independence Fjord Group, and attributed a Mesoproterozoic age on geological maps. However, metarhyolitic rocks associated with the sandstones in the Caledonian fold belt have yielded an age of 1740 ± 6 Ma, significantly older than anticipated. Zircon ages for a sandstone sample in the same area suggest deposition after the end of Palaeoproterozoic orogenic events, but in part prior to emplacement of the rhyolitic rocks at 1740 Ma; sandstone from another locality may have been deposited before emplacement of the latest Proterozoic granite sheets. Field relations suggest that some granitic veins and sheets might be Caledonian in age, but, with one possible exception, all those analysed proved to be Proterozoic. The apparent absence of Caledonian granites in the northern part of the East Greenland Caledonides, despite regional high-grade metamorphism, may be related to the lack of major occurrences of pelitic supracrustal rocks within the crystalline basement complexes.