A significant change in the composition of turbidites deposited at active tectonic settings is seen at the Archean/post-Archean transition. In comparison to post-Archean active margin turbidites, Archean greenstone turbidites exhibit more uniform Th/Sc ratios (but with a similar average), greater HREE-depletion ($$Gd_{N}/Yb_{N} >2$$), less Eu-depletion (Eu/Eu* mostly >0.85) and an absence of low Th/U ratios (Th/ U >3). These data indicate that differing mantle sources and/or conditions of mantle melting existed during the Archean and that intracrustally differentiated rocks (older cratons or young differentiated arcs) were relatively unimportant as provenance components for Archean turbidites. In contrast, some sedimentary rocks preserved in Archean platformal sequences of high-grade terranes display significant negative Eu-anomalies similar to the majority of post-Archean shales, indicating that the general process of cratonization, including intracrustal differentiation, has taken place since at least 3...

Sedimentary Rocks and Crustal Evolution: Tectonic Setting and Secular Trends

The bulk chemical compositions, mineralogy, and mineral proportions of sands and muds of the Mallacoota Basin in southeastern Australia reflect the composition of weathering profiles mantling source rocks, rather than bedrock. Muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles. Sands are strongly enriched in quartz relative to source rocks, even in the headwaters of the fluvial system, demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system. K-feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or K-feldspar in weathering profiles. Subsequent erosion and sorting produce sands enriched in quartz, with high K-feldspar:plagioclase ratios relative to source rocks. The composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat...

Effects of Chemical Weathering and Sorting on the Petrogenesis of Siliciclastic Sediments, with Implications for Provenance Studies

Pyoclastic density currents are awesome volcanic phenomena that can wreak destruction on a regional scale and can impact global climate. They deposit ignimbrites, which include vast impact lansdscape-modifying sheets with volumes exceeding 1000 km3.This book takes stock of our understanding of pyroclastic density currents and presents a new conceptual framework for investigating how ignimbrites are deposited. It integrates the results of field-based studies, laboratory experiments and numerical modelling, including work on clastic sedimentologym and industrial particle transport. Topics covered include the behaviour or particulate currents, mechanisms of clast support and segregation, interpreting ignimbrite lithofacies and architectures, and future research directions. The new approach focuses on processes and conditions within the lower flow-boundary zone of currents. Superb diagrams explain many new concepts, while the 95 photographs make an explanatiry atlas of deposit types. This is essential reading for workers investigating volcanic hazards, and for anyone wishing to interpret modern or ancient ignimbrites, as well as other catastrophically emplaced sediments.

“Given the depth of scholarship that they have brought to the subject, the power of their arguments, and the degree of synthesis with other fields, this would seemto qualify as a seminal work… I think that this will be the paper on the topic that others will have to contend with for many years to come.” Marcus Bursik, State University of New York

Pyroclastic density currents and the sedimentation of ignimbrites

A review of the Superior Province of the Canadian Shield, a product of Archean accretion

Shape is a fundamental property of all objects, including sedimentary particles, but it remains one of the most difficult to characterize and quantify for all but the simplest of shapes. Despite a large literature on the subject, there remains widespread confusion regarding the meaning and relative value of different measures of particle shape. This paper re-examines the basic concepts of particle shape and suggests a number of new and modified methods which are widely applicable to a range of sedimentological problems; it is shown that the most important aspects of particle form are represented by the I/L ratio (elongation ratio) and S/I ratio (flatness ratio). A combination of these two ratios can be used to classify particles in terms of 25 form classes. A method of obtaining a quantitative measure of particle roundness using simple image analysis software is described, and a new visual roundness comparator is presented. It is recommended that measurements of both roundness and circularity (a proxy measure of sphericity) are made on grain images in three orthogonal orientations and average values calculated for each particle. A further shape property, irregularity, is defined and a classification scheme proposed for use in describing and comparing irregular or branching sedimentary particles such as chert and coral.

Particle shape: a review and new methods of characterization and classification

Sand and Sandstone

For many centuries Merapi volcano has generated hot avalanches of blocks, lapilli and ashes, derived from the destruction of partially solidified, viscous lava domes (Merapi-type nuees ardentes). On 15 June 1984, at least four nuees ardentes came down the southwest slope of the Merapi, the first and the last being responsible for more than 99% of the deposits which are now exposed. The first nuee ardente, a Merapi-type nuee ardente, was produced by the destruction of the dome, travelled 7 km from the crater, leaving a measured deposit, 2.7 m thick, 4 km from the crater, near its upper depositional limit, regularly increasing to a maximum measured thickness of 12 m at the front of the deposit. The lower contact is sharp, non-erosive, with pines still rooted in the underlying paleosol. The deposit consists of 50% ash, 33% lapilli, and 17% blocks, with two subpopulations (one Rosin and one normal), and is finespoor, with less than 4% of fine ash (d finer than 4 φ). The deposit displays reverse population grading of both vesiculated and massive clasts, and of the maximum grain size. The maximum size significantly increases regularly down-current over most of the exposed length of unit 1, and bed thickness increases for the entire length of the deposit. The deposit of the second nuee ardente is only 6–21 cm thick, and of very limited lateral extent. It is a normally graded, coarse to fine ash, with a finespoor base. The third unit consists of fines-poor, normally graded coarse ash, exposed in low-amplitude (20–40 cm), 12-m-wavelength dunes. The deposit of the fourth nuee ardente rests in sharp erosive contact on the underlying unit, increasing in thickness down-flow. It consists of transitional coarse and fine-grained strata, 6–130 c cm thick, dipping 5–10° down-flow. The deposit, made up of two subpopulations (one Rosin and one normal), is normally graded over the entire bed, but coarsegrained strata are reversely graded. The relative content of vesiculated clasts increases up-bed in both strata types, from 12% at the base to 40% at the top. The characteristics of unit 1 suggest that it accumulated from a concentrated suspension of cohesionless solids exhibiting non-Newtonian behavior, where dispersive pressure played an important role in the suspension of the clasts. Units 2 and 3 were probably deposited from dilute turbulent suspensions, whereas the upper unit (4) is a classic example of deposition from a high-density turbulent suspension leading to the formation of multiple traction carpets driven by the overlying, lower-density, surge. The horizontal distance travelled by a hot rock avalanche may be influenced by its transport mechanism. Debris flows are mobile on very low slopes-as low as 1°-whereas grain flows, even density-modified grain flows, require relatively high slopes-more than 6° at Merapi-to maintain their mobility. If the present Merapi dome were to collapse and produce a debris flow, its present volume coupled with the minimal 1.5 km vertical drop could travel a distance ranging between 15 and 30 km. However, if transport were by grain flow mechanisms, the mass could come to rest as it reaches a 5–10° slope.

The 1984 nuée-ardente deposits of Merapi volcano, Central Java, Indonesia: stratigraphy, textural characteristics, and transport mechanisms

Twenty-six detrital zircon grains from the late Archaean Pontiac sediments of the Abitibi greenstone belt were dated individually by the U–Pb method in order to identify possible elements of an ancient (pre-2.7 Ga) shield. The results show that at least two components were present at the source areas of these sediments: (1) materials of about 2700–2725 Ma, which were most likely derived from contemporaneous acid igneous rocks; and (2) significantly older materials, parts of which are at least 2940 Ma old, which were either part of an uplifted sedimentary basin or a sialic basement complex. These results can be used to suggest a close link between igneous activity and basin subsidence and that the development of the Abitibi volcanic belt was closely associated with the presence of some older sialic blocks.

U–Pb systemstics in single zircons from the Pontiac sediments, Abitibi greenstone belt

Archean pyroclastic rocks at Reneault and D'Alembert in the Noranda region are composed of crystals and lithic, vitric and pumiceous fragments. The pyroclastic rocks contain two types of beds, whos...

Jean Lajoie

Papers

Sand and Sandstone

The 1984 nuée-ardente deposits of Merapi volcano, Central Java, Indonesia: stratigraphy, textural characteristics, and transport mechanisms

U–Pb systemstics in single zircons from the Pontiac sediments, Abitibi greenstone belt

The anatomy and interpretation of an Archean volcaniclastic sequence, Noranda region, Quebec

The 1902 Peléean deposits in the Fort Cemetery of St. Pierre, Martinique: a model for the accumulation of turbulent nuées ardentes