Elsevier

Gondwana Research

Volume 19, Issue 2, March 2011, Pages 535-541
Gondwana Research

Global mass wasting during the Middle Ordovician: Meteoritic trigger or plate-tectonic environment?

https://doi.org/10.1016/j.gr.2010.07.001Get rights and content

Abstract

Mass wasting at continental margins on a global scale during the Middle Ordovician has recently been related to high meteorite influx. Although a high meteorite influx during the Ordovician should not be neglected, we challenge the idea that mass wasting was mainly produced by meteorite impacts over a period of almost 10 Ma. Having strong arguments against the impact-related hypothesis, we propose an alternative explanation, which is based on a re-evaluation of the mass wasting sites, considering their plate-tectonic distribution and the global sea level curve. A striking and important feature is the distribution of most of the mass wasting sites along continental margins characterised by periods of magmatism, terrane accretion and continental or back-arc rifting, respectively, related to subduction of oceanic lithosphere. Such processes are commonly connected with seismic activity causing earthquakes, which can cause downslope movement of sediment and rock. Considering all that, it seems more likely that most of this mass wasting was triggered by earthquakes related to plate-tectonic processes, which caused destabilisation of continental margins resulting in megabreccias and debris flows. Moreover, the period of mass wasting coincides with sea level drops during global sea level lowstand. In some cases, sea level drops can release pore-water overpressure reducing sediment strength and hence promoting instability of sediment at continental margins. Reduced pore-water overpressure can also destabilise gas hydrate-bearing sediment, causing slope failure, and thus resulting in submarine mass wasting. Overall, the global mass wasting during the Middle Ordovician does not need meteoritic trigger.

Research highlights

►The research highlights of our paper are the presentation of alternative explanations for the global mass wasting during the Middle Ordovician, which has recently been related to high meteorite influx. We argue that nearly all of these deposits can be related to terrestrial processes (i.e. plate tectonics, sea level changes) and give a short summary of each location with its alternative explanation.

Introduction

The Earth's geological record shows mass wasting deposits from the Precambrian to Recent (e.g., Bugge et al., 1987, Hine et al., 1992, Hampton et al., 1996, Spence and Tucker, 1997, Keller et al., 1998, Hoffman and Hartz, 1999, Keller, 1999, Woodcock and Morris, 1999, Weaver et al., 2000, Cooper et al., 2001, Piper et al., 2003, Clift et al., 2004, Maslin et al., 2004, Locat and Lee, 2005, Wendorff, 2005, Ryu et al., 2005, Moscardelli et al., 2006, Valverde-Vaquero et al., 2006, Gee et al., 2007, Ratzov et al., 2007, Callot et al., 2008, Lee, 2009, Strozyk et al., 2009, Hornbach et al., 2010).

The present study focuses on Middle Ordovician mass wasting deposits, comprising mainly sedimentary megabreccias with maximum clast sizes from 1 m to > 1 km, which were deposited along the margins of different palaeo-continents, e.g., Avalonia, and on volcanic arcs (Fig. 1). This mass wasting at continental margins on a global scale has recently been related to earthquake-driven slope failure following meteorite impacts (Parnell, 2009). This hypothesis is based on the observation that these megabreccias overlap in time with enhanced occurrence of extraterrestrial chromite and a shift to lower 187Os/188Os values in an essentially continuous sequence of Middle Ordovician shallow marine limestones of southern Sweden and several thousand kilometres away in central China (Schmitz et al., 2001, Schmitz et al., 2003, Schmitz et al., 2008). They were deposited a few million years after the disruption of the L-chondrite parent body in the asteroid belt at about 470 ± 6 Ma ago (Korochantseva et al., 2007) (Fig. 2). Using semiquantitative calculations, Parnell (2009) suggested that up to 500 impactors of 100 m in diameter, including 250 impactors if only landward impacts are considered, fell within about 30 km of the 20,000 km long Iapetus coastline. The disruption of a parent body in the asteroid belt will lead to enhanced meteorite influx on Earth in less than one or a few million years (e.g., Schmitz et al., 2001) and may affect the Earth's surface with a number of impact craters. Table 1 lists the known impact structures of Ordovician age but interestingly only three craters within the age range of the megabreccias (468–461 Ma) have been found so far. Furthermore, shatter cones, microscopic planar deformation features (PDFs) in quartz, high-pressure mineral phases and high-temperature glasses and melts (or their relics) related to impact events (e.g., Dypvik and Jansa, 2003, French and Koeberl, 2010) have not yet been found in 468–461 Ma-old strata, although that may simply be because these features have been overlooked or may have been destroyed by terrestrial processes (e.g., erosion, subduction) since their formation (French and Koeberl, 2010). Two exceptions are PDFs in quartz grains from Darriwilian breccias of the Osmussaar area in northwestern Estonia and from a polymict breccia of the Granby structure in Sweden. The former are likely recycled ejecta material from the nearby early Cambrian Neugrund crater (Suuroja et al., 2003, Ainsaar et al., 2007), whereas the latter may be real impact-related features of Middle Ordovician age (Alwmark, 2009).

In our opinion, earthquake-driven slope failure producing the Middle Ordovician mass wasting was not necessarily caused by bombardment of the Earth's surface with large meteorites over a period of almost 10 Ma. The most striking and important feature, which led to our alternative hypothesis, is the distribution of the mass wasting sites along and close to active continental margins, arc terranes, and rift basins in Middle Ordovician palaeotectonic reconstructions (Fig. 1). Note that in the Quaternary, for instance, all active continental margins (e.g., Gee et al., 2007, Ratzov et al., 2007, Strozyk et al., 2009, Hornbach et al., 2010, and references therein) and passive margins (e.g., Bugge et al., 1987, Piper et al., 2003, Lee, 2009, and references therein) have large mass wasting deposits. Thus, finding them in the Middle Ordovician sedimentary record is not surprising.

In this paper, we present an alternative explanation for the global Middle Ordovician mass wasting without the need of extraterrestrial support. We propose that destabilisation of continental margins causing this global mass wasting was simply due to earthquakes and instability of slopes, related to plate-tectonic processes. Global sea level changes may also have triggered destabilisation of carbonate platforms and continental margin sediments. Nonetheless, we want to emphasise that we do not neglect the enhanced influx of extraterrestrial material on the Earth during the Ordovician. This influx has probably been responsible for local mass wasting (e.g., Hummeln structure: Lindström et al., 1999; Kärdla structure: Lindström, 2003) rather than global.

Section snippets

Mass wasting

Mass wasting is a general term describing down slope movement of sediment and rock. Common mass wasting deposits are debris flows, slides and slumps (e.g., Bugge et al., 1987, Einsele, 1993, Weaver et al., 2000, Locat and Lee, 2005, Lee, 2009). The formation of mass wasting deposits depends on the configuration of the terrain and relies on soil and rock mechanics parameters and failure criteria (Locat and Lee, 2005). Submarine slides, for instance, are most common in fjords, active river deltas

Earth's extraterrestrial influx

The Earth has suffered meteorite bombardment ever since its formation with fluctuating intensity but only under certain circumstances has it been recorded (e.g., Peucker-Ehrenbrink and Schmitz, 2001, Schmitz et al., 2001, Dypvik and Jansa, 2003, Lindström, 2003, Spray, 2009, French and Koeberl, 2010). About 40,000 tonnes of extraterrestrial material fall to the Earth each year (Brownlee, 2001). For example, about 14,000 meteorite fragments have been collected from a ~ 2500 km2 large area of the

Ordovician mass wasting localities

Parnell (2009) described 12 megabreccia localities from various sites along the margins of the Iapetus Ocean and other parts of the Middle Ordovician globe (Fig. 1, Fig. 2). In addition, debris flow deposits from NW Argentina and Western Yunnan were mentioned. Some of these breccias and debris flows belong to carbonate platforms, which were deposited during relative sea level lowstand (Fig. 2). In general, the megabreccias contain clasts up to kilometre size embedded in a fine-grained

Plate tectonics

Throughout the Ordovician period, magmatism, terrane accretion and back-arc rifting, related to subduction of oceanic lithosphere, chiefly of the Iapetus Ocean, occurred along the margins of Avalonia, Ganderia, Laurentia and Gondwana (e.g., Bird and Dewey, 1970, Dewey and Mange, 1999, Friedrich et al., 1999, Soper et al., 1999, Cocks, 2001, Stampfli and Borel, 2004, Thomas and Astini, 2003, Clift et al., 2004, Valverde-Vaquero et al., 2006, Ryan, 2008, von Raumer and Stampfli, 2008, Fergusson,

Summary of alternative explanations for the mass wasting sites

As outlined above, most of the mass wasting during the Middle Ordovician can simply be explained by seismic-induced slope instability related to plate-tectonic processes. Global sea level changes (in particular sea level falls) may have also had a significant influence. Possible explanations for the formation of the mass wasting on different palaeo-continents during the Middle Ordovician are given below, whereby the numbers in brackets refer to the mass wasting localities discussed, as shown in

Conclusions

Although meteorite impacts caused modifications of the Earth's surface ever since its formation, in the case of the mass wasting during the Middle Ordovician, they may have had only regional impact rather than global. Furthermore, preserved meteorite fragments or relict minerals (e.g., chromite: Schmitz et al., 2001, Schmitz et al., 2003, Schmitz et al., 2008) in Middle Ordovician shallow marine limestone of southern Sweden and in central China do not necessarily indicate that an impact event

Acknowledgments

We gratefully thank Jürgen F. von Raumer for his stimulating discussions and support during the preparation of the manuscript. Thanks go also to David J. W. Piper for his comments on an earlier version of the manuscript and to Robert Scott for polishing the English text. Reviews by two anonymous reviewers and editorial handling by Damian Nance are gratefully acknowledged. This paper is a contribution to the IGCP projects 497 and 503.

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