Robert Giegengack

Libyan Desert glass: A review

Abstract Libyan Desert Glass occurs as a concentration of glass fragments of a broad range of sizes strewn across the desert surface between the NNW-SSE-trending linear sand dunes along the southwestern margin of the Great Sand Sea in western Egypt; it was discovered by the international scientific community in 1932. Despite the difficulty of access to the region where the glass occurs, at least 20 geologists have examined its field context in the course of at least 10 expeditions since 1932. Many more individuals have undertaken laboratory analyses of materials brought back by the various field parties. These analyses have established that the gross chemical composition of Libyan Desert Glass approximates that of both the sandstone of the Nubia Formation that underlies the Great Sand Sea and the quartz aggregate of the dunes. A fissiontrack age of 28.5×10 6 y has been assigned to the glass. The quality of internal equilibration among various components distributed through the glass supports the interference that the fusion process that presumably formed the glass occurred at a high temperature: that temperature was sustained for a long time; and that the glass cooled to ambient temperature slowly. The 1981 expedition, described herein, established that the present mass of glass exceeds 1.4×10 9 g; the original mass of glass may have been 10 000 times greater. The glass has been dispersed across a limited area of the desert surface by Earth-surface processes during a period of erosional lowering that is still in progress. No glass fragment representing an edge of the original mass of glass or a transition zone with the precursor material from which it presumably formed has yet been found. The origin of Libyan Desert Glass remains as much a physical and geologic enigma today as it was in 1932.

Apatite fission-track evidence for Laramide and post-Laramide uplift and anomalous thermal regime at the Beartooth overthrust, Montana-Wyoming

The crystalline rocks of the Beartooth overthrust in southwest Montana and northwest Wyoming were uplifted and thrust over the northwest margin of the Bighorn basin during the Laramide orogeny. The timing and geometry of that event is well documented in the synorogenic Paleocene and early Eocene sedimentary rocks that were deposited ahead of the rising Beartooth basement block and partially over-ridden as the block advanced. The absence of post-Laramide sedimentary rocks on the block and in the adjacent Bighorn Basin has, however, precluded the reconstruction of the post-orogenic tectonothermal history of this region. We report here the results of fission-track analysis of apatite from the three components of the Beartooth over-thrust with the aim of reconstructing the Laramide and post-Laramide tectono-thermal history of the southeast margin of the Beartooth Mountains. Apatite fission-track ages and corresponding horizontal confined track length distributions (HCTLDs) from Precambrian basement rocks constituting the upper plate of the Beartooth overthrust indicate that from 7 to 12 km of uplift of the Red Lodge corner of the Beartooth block has occurred since early Paleocene time. This amount of uplift occurred in two stages, with an intervening mid-Tertiary period of tectonic quiescence. The latter was a period of either (1) Oligocene and/or Miocene deposition or (2) tectonic quiescence. Uplift of 4 to 8 km occurred during the first phase of cooling, which lasted from early Paleocene time (∼61 Ma) to early Eocene time (∼52 Ma). During the second phase, which began in late Miocene-early Pliocene time and continues to the present day, about 4 km of uplift occurred. Our fission-track data suggest that the thermal regime in rocks above the Beartooth overthrust was relatively stable during Tertiary time. The maximum geothermal gradient permitted by model thermal histories generated from our observed fission-track data during post-Laramide time is 17 °C/km. This value is the same as that of the present-day geothermal gradient measured in the Amoco Beartooth well and suggests that a low geothermal gradient prevailed throughout Tertiary time. Apatite fission-track ages and HCTLDs from Jurassic and Cretaceous sedimentary rocks beneath the Beartooth overthrust indicate that these rocks have remained 10-20 °C cooler than overlying rocks in the shear zone and the lowermost part of the upper plate since ∼61 Ma, an observation that is not consistent with the stratigraphic position of these rocks. We interpret this temperature to be the result of a persistent thermal regime in which ground water circulating through the sedimentary rocks of the lower plate cooled them and insulated them from conductive heat transfer from hotter overlying rocks; this ground-water circulation may have been responsible for flushing hydrocarbons out of the rock column explored by the Beartooth well. Shear-zone rocks experienced a higher temperature during Cenozoic time than did rocks of the upper and lower plates; this condition was maintained by flow through the shear zone of ground water heated to higher temperatures at deeper levels along the thrust.

Petrogenesis of Artifact-Bearing Fossil- Spring Tufa Deposits from Kharga Oasis, Egypt

Petrographic and geochemical study of artifact-bearing fossil-spring tufas in stratigraphic contexts associated with Upper Acheulean and Middle Stone Age-to-historic artifacts provides a basis for characterizing and comparing Quaternary tufa deposition and diagenesis across the Kharga Oasis region of south-central Egypt. Analysis of tufa deposits at Refuf Pass, Midauwara Pass, A‘in ‘Amur, and Umm el Dabadib suggest that the low Mg-carbonates were precipitated by similar inorganic and biogeochemical processes operating within freshwater spring-fed alkaline stream environments throughout Quaternary time. Detailed petrographic studies suggest that the tufas are relatively pristine, with the original rock textures well preserved with minimal postdepositional alteration. Microstratigraphic details indicate that a variety of interformational facies were present within the former stream environments; understanding these contexts is valuable for interpreting prehistoric human activities.

Variations in the period of the sunspot cycle

Using a wavelet analysis approach, subtle variations in the solar cycle period were extracted from the yearly means of the sunspot numbers. For the nominal eleven-year solar cycle, the resulting estimates of the cycles period are in good agreement with those given by Friis-Christensen and Lassen. Longer period structures in the solar-cycle, including the Gleissberg cycle, are also delineated by the wavelet analysis.

Analyses of fluid inclusions in Neoproterozoic marine halite provide oldest measurement of seawater chemistry

We analyzed primary fluid inclusions in halite from marine evaporites in the ca. 830 Ma Browne Formation of the Officer Basin in Western Australia using the cryogenic scanning electron microscopy– energy dispersive X-ray spectrometry analysis technique. The concentrations of the major ions, except K + and possibly SO 4 2– , fall within the range of Phanerozoic seawater. This is the first direct measurement of the composition of mid-Neoproterozoic seawater, and extends present-day knowledge of seawater chemistry by ∼300 m.y. Our estimates suggest that mid-Neoproterozoic marine sulfate concentrations were lower (∼90%) than modern values. By the terminal Neoproterozoic, fluid inclusions in halite and evaporite mineralogy indicate seawater sulfate levels rose significantly, to 50%–80% of modern concentrations, which parallels increases in atmospheric and oceanic oxygen.

Modern spectral climate patterns in rhythmically deposited argillites of the Gowganda Formation (Early Proterozoic), southern Ontario, Canada

Abstract Rhythmically deposited argillites of the Gowganda Formation (ca. 2.0–2.5 Ga) probably formed in a glacial setting. Drop stones and layered sedimentary couplets in the rock presumably indicate formation in a lacustrine environment with repeating freeze–thaw cycles. It is plausible that temporal variations in the thickness of sedimentary layers are related to interannual climatic variability, e.g. average seasonal temperature could have influenced melting and the amount of sediment source material carried to the lake. A sequence of layer couplet thickness measurements was made from high-resolution digitized photographs taken at an outcrop in southern Ontario, Canada. The frequency spectrum of thickness measurements displays patterns that resemble some aspects of modern climate. Coherent periodic modes in the thickness spectrum appear at 9.9–10.7 layer couplets and at 14.3 layer couplets. It is unlikely that these coherent modes result from random processes. Modern instrument records of regional temperature and rainfall display similar spectral patterns, with some datasets showing significant modes near 14 yr in both parameters. Rainfall and temperature could have affected sedimentary layering in the Gowganda argillite sequence, and climate modulation of couplet thickness emerges as the most likely explanation of the observed layering pattern. If this interpretation is correct, the layer couplets represent predominantly annual accumulations of sediment (i.e. they are varves), and the thickness spectrum provides a glimpse of Early Proterozoic climatic variability. The presence of interannual climate patterns is not unanticipated, but field evidence presented here may be of some value in developing a climate theory for the Early Proterozoic.

Brecciation of clasts in diamictites of the Gowganda Formation, Ontario, Canada

Igneous boulders in the Gowganda Formation were brecciated prior to the induration of the diamictites in which they occur. The breccias evidently represent the results of the maximum stresses imposed on a lodgment till in the final stages of emplacement. The stresses necessary to fracture granitoid clasts are likely to occur at the base of glaciers of unremarkable size, and could easily be imposed by small glaciers if the pore pressure in the underlying till dropped as a result of sudden leakage of pore fluid.

Piracy on the High Desert: the Long-Range Desert Group 1940–1943

From June 1940, when General Sir Archibald Wavell, Commander in Chief of British Forces, Middle East, authorised then Major Ralph Bagnold to form the Long Range Desert Group (LRDG), until near the end of the North African campaign in April 1943, the highly mobile, self-contained desert patrols of the LRDG maintained close watch on the movement of enemy troops and vehicles and attacked enemy military installations hundreds of kilometres behind their lines. This spirited ‘piracy on the high desert’ not only cost the Axis forces dearly in men and equipment, but tied up great numbers of troops and large quantities of arms and supplies to protect Axis positions from the threat of slashing hit-and-run attacks of LRDG patrols that seemed to materialise out of the desert suddenly and without warning. Key to the success of the LRDG was its skilful and imaginative utilisation of the terrain of the Libyan Desert, based on the experience and insight gained by Bagnold and a few close associates during their private expeditions in the region during pre-war years.