Sandstones of the Paleozoic turbidite sequences of eastern Australia show a large variation in their major element geochemistry, reflecting the distinct sedimentary provenance and tectonic setting ...

Plate Tectonics and Geochemical Composition of Sandstones

The two rock-forming polymorphs of serpentine Mg3Si2O5(OH)4, lizardite and chrysotile, occur in nature in virtually identical ranges of temperature and pressure, from surficial or near-surficial environments to temperatures perhaps as high as 400°C. Laboratory evidence indicates that lizardite is the more stable at low temperatures, but the difference in their Gibbs free energies is not more than about 2 kJ in the 300-400°C range. Above about 300°C, antigorite + brucite is more stable than both; in other words, chrysotile is nowhere the most stable. The crystal structures of lizardite and chrysotile give rise to contrasting crystallization behaviors and hence modes of occurrence. The hydration of peridotite at low temperature results in the growth of lizardite from olivine, and (commonly topotactically) from chain and sheet silicates, although the MgO-SiO2-H2O (MSH) phase diagram predicts antigorite + talc in bastite. The activity of H2O during serpentinization may be buffered to low values by the solids,...

The serpentinite multisystem revisited : chrysotile is metastable

Stratigraphic and sedimentological data from New South Wales and Queensland, eastern Australia, indicate that the Late Palaeozoic Ice Age comprised at least eight discrete glacial intervals (each 1–8 Ma in duration, here termed ‘glaciations’), separated by nonglacial intervals of comparable duration. These events spanned an interval from mid-Carboniferous ( c . 327 Ma) to the early Late Permian ( c . 260 Ma), and illustrate a pattern of increasing climatic austerity and increasingly widespread glacial ice from initial onset until an acme in the late Early Permian, followed by an opposite trend towards the final demise of glaciation in the Late Permian. The alternating glacial–nonglacial motif suggests that the Late Palaeozoic Ice Age was considerably more dynamic than previously thought. These patterns are remarkably consistent with recent interpretations of palaeofloral change, eustatic sea-level fluctuations and CO 2 –climate–glaciation relationships for this interval of time. The detailed record of alternating glacial and nonglacial climate mode disclosed herein may facilitate more closely resolved evaluations of stratigraphic records elsewhere, notably in far-field, ice-distal, northern hemisphere successions.

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Stratigraphic imprint of the Late Palaeozoic Ice Age in eastern Australia: a record of alternating glacial and nonglacial climate regime

The formation of upper Paleozoic (Visean to Sakmarian-Artinskian) Euramerican cyclothems, which resulted from base-level fluctuations of up to 100 m, commonly are attributed to large-scale waxing and waning of Gondwanan glaciers. However, evaluation of the geographic and chronostratigraphic distribution of Gondwana deposits reveals that glaciation was not the primary cause of base-level changes of that magnitude. Gondwana strata contain three non-overlapping glacial successions. Glacial I (Frasnian to possibly Tournaisian) and Glacial II (Namurian to lowermost Westphalian) rocks were deposited by alpine glaciers. Water sequestered by these glaciers was insufficient to account for the base-level changes. In contrast, Upper Carboniferous (Stephanian) to Lower Permian (Sakmarian-Artinskian) Glacial III rocks were widespread and indicate deposition by ice sheets that may have covered a total area of between 17.9 and 22.6 × 10 6 km 2 . Complete ablation of a single ice sheet of this size would produce eustatic changes of ~100 m. However, multiple ice sheets were likely present, which would have resulted in considerably smaller fluctuations in sea level during Glacial III deposition. The argument that Glacial I and II deposits were originally comparable in extent to those of Glacial III, but were eroded during the advance of Glacial III ice-sheets, is untenable. Weathered granite profiles on the pre-Glacial III unconformity occur scattered over a 1200-km length of the central Transantarctic Mountains. The profiles indicate prolonged subaerial exposure and, thus, an absence of ice cover. These and non-glacial successions in Gondwana constrain the size of ice sheets before Glacial III deposition and imply that glaciation prior to Glacial Episode III was not the primary cause of base-level changes linked to upper Paleozoic Euramerican cyclothems.

Timing of late Paleozoic glaciation in Gondwana: Was glaciation responsible for the development of northern hemisphere cyclothems?

Summary. Seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals. The symmetry and magnitude of anisotropy depend upon: (1) the volume percentages of the minerals constituting the upper mantle, (2) the degree and symmetry of preferred orientation of each mineral and (3) the alignment of the minerals’ crystallographic axes relative to one another. The nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle. Petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the Samail ophiolite, Oman, the Troodos ophiolite, Cyprus, the Bay of Islands ophiolife, Newfoundland, the Twin Sisters ultramafic, Washington, USA, the Dun Mountain ophiolite, New Zealand, the Red Hills ophiolite, New Zealand and the Red Mountain ophiolite, New Zealand. The compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent, in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle. The symmetry varies from orthorhombic to axial, with the axial symmetry axis corresponding to the olivine a-axes maxima and subparallel to spreading directions in oceanic upper mantle. Pyroxene a-, b- and c-axes maxima generally parallel olivine b-, c- and a-axes, respectively, and anisotropy decreases with increasing pyroxene content. Shear-wave splitting is predicted for all propagation directions within the upper mantle. Symmetry is also orthorhombic or axial, with the minimum difference in velocity between the two shear-waves parallel to the maximum compressional wave velocity.

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The magnitude, symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites

Fragments of Jurassic ophiolite having U-Pb zircon ages narrowly grouped at 160 to 170 m.y. are widespread over parts of northwest Washington. The Haystack thrust fault is inferred to mark the base of the ophiolite in the San Juan Islands and adjacent Cascade foothills; other bodies of mafic and ultramafic rock in the western Cascades may be klippen of the Haystack thrust plate. The Haystack thrust fault is probably the structurally highest and possibly most extensive thrust yet recognized within a family of Late Cretaceous thrust faults in northwest Washington. The ophiolite and its time of emplacement (bracketed between about 100 and 88 m.y.) suggest a similarity with the Coast Range thrust of California which thrust Upper Jurassic ophiolite and the Great Valley sedimentary sequence over the Franciscan assemblage. However, relations in the Cascades are complicated by the extraordinarily diverse character of lower plate rocks, of which very few resemble the Franciscan. We conclude that an original subduction system was modified by later tectonic activity so that a variety of terranes was juxtaposed as a family of rootless thrusts, with the ophiolite forming, at least in some areas, the uppermost structural unit. Perhaps the emplacement of Wrangellia, an allochthonous microcontinent west of the San Juan Islands, caused the thrusting.

Allochthonous Jurassic ophiolite in northwest Washington

Summary
Phyllosilicates and zeolites grew in Columbia River sediments during hydrothermal experiments at relatively low temperatures and pressures. Although the new minerals may not be equilibrium assemblages, our results strongly support the idea that matrix minerals in graywackes may be the result of alteration of components thermodynamically unstable in the environment of diagenesis. Scanning electron micrographs show that the new minerals have formed as a mesh-like coating on original grains. The textural relationship of the new minerals to the original minerals resembles graywacke texture.

Diagenetic origin of graywacke matrix minerals

Geology of st. croix, u.s. virgin islands

The Chiwaukum graben in central Washington contains two unconformity-bounded fluvial and lacustrine units, here named the “Chumstick” and “Wenatchee” Formations. The Chumstick is dated at 45 m.y. (middle Eocene), and the Wenatchee is dated at 34 m.y. (early Oligocene) by the fission-track method on zircons from tuffs. Previously, both formations were thought to be part of the Swauk Formation, which is older. The Chumstick Formation rests on weathered crystalline basement and is several thousands of metres thick. Fanglomerate occurs at the base and along the margins. Most of the formation consists of feldspathic sandstone and pebbly sandstone of fluvial origin; but within the upper part, there is a lacustrine unit, herein designated the Nahahum Canyon Member. Tuff is common in the lower part of the formation. Both of the bounding faults, the Leavenworth and the Entiat, were active during deposition of the Chumstick Formation, but relief was greatest on the northeast (Entiat) side. The Wenatchee Formation occurs in the vicinity of Wenatchee, Washington, and is ≤300 m thick. It unconformably overlies the Chumstick Formation within the graben but overlaps the northeast side of the graben, where it lies directly on weathered metamorphic basement. The Wenatchee Formation, like the Chumstick, was deposited primarily in fluvial and lacustrine environments; unlike the Chumstick, much of the sediment is mature quartz sandstone. Relief in the source area probably was very subdued during its deposition.

Stratigraphy and age of the Chumstick and Wenatchee Formations: Tertiary fluvial and lacustrine rocks, Chiwaukum graben, Washington: Summary

Glacial and probably fluvioglacial rocks are present in the Currabubula Formation, an 8500-foot unit of Carboniferous age (Westphalian) exposed in the Werrie Basin of the New England geosyncline, New South Wales, Australia. Approximately 70 per cent of the formation is conglomerate and coarse conglomeratic sandstone, approximately 10 per cent is mudstone and fine sandstone, and approximately 20 per cent is tuff. The Rosedale Member, about 60 feet thick at the type locality, occurs in the middle of the Currabubula Formation and consists largely of varved mudstone with abundant ice-rafted pebbles. The best evidence for Carboniferous glaciation in eastern Australia is from the Rosedale Member and similar units exposed in other areas of northeastern New South Wales.

The most common rock types in the Currabubula Formation are poorly sorted pebble conglomerates and lithic wackes composed largely of mineral grains and lithic fragments from flows and tuffs. Nearly all such rocks are stratified to some degree, and were probably deposited by streams, perhaps as glacial outwash. Rhyolitic and andesitic tuffs were deposited subaerially, in shallow lakes, and as ignimbrites.

The Currabubula Formation was probably deposited near a mountain range undergoing glaciation. Much of the clastic material is volcanic, and includes pyroclastic fragments similar in composition to the contemporaneously deposited tuffs. This suggests that the clastic sediments were locally derived, and supports the idea that glaciation was restricted to the alpine variety. It is not necessary to infer either widespread Carboniferous climatic cooling or a relationship with the more widely occurring Permian glacial rocks in other parts of Australia.

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John T. Whetten

Papers

Allochthonous Jurassic ophiolite in northwest Washington

Diagenetic origin of graywacke matrix minerals

Geology of st. croix, u.s. virgin islands

Stratigraphy and age of the Chumstick and Wenatchee Formations: Tertiary fluvial and lacustrine rocks, Chiwaukum graben, Washington: Summary

Carboniferous Glacial Rocks From the Werrie Basin, New South Wales, Australia