Journal of Sedimentary Research, 2014, v. 84, 192–197 Current Ripples DOI: http://dx.doi.org/10.2110/jsr.2014.19
MEGACLASTS: PROPOSED REVISED NOMENCLATURE AT THE COARSE END OF THE UDDEN-WENTWORTH GRAIN-SIZE SCALE FOR SEDIMENTARY PARTICLES J.P. TERRY1
AND
J. GOFF2
1
Department of Geography, National University of Singapore, AS2, 1 Arts Link, Kent Ridge, Singapore 117570 2 School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney 2052, Australia e-mail:
[email protected]
ABSTRACT: The expanded Udden-Wentworth (U-W) grain-size scale is an important tool for classifying the size of sedimentary particles. However, some of the existing terminology at the coarse end of the scale is problematic. In particular, the current terms ‘‘block’’ and ‘‘slab’’ are used to define size ranges, despite having recognized shape connotations and simultaneous application in particle form (shape) diagrams. The dual usage such of terminology is undesirable and may have contributed to the lack of strict adherence by the earth science community to the upper end of the U-W scale, resulting in part in the plethora of alternative expressions that have appeared in recent years for describing very large clasts. Our solution to this conundrum is to abandon shape-related terms and replace them with an incremental system based on the prefixes meso- and macro- to classify very large boulder size ranges. In this way, both size and shape classification of very coarse sediments can be accomplished satisfactorily without unnecessary overlap or confusion in nomenclature. The term ‘‘megaclast’’ is appropriate to remain as a universal descriptor for all clasts greater than boulder size (i.e., b axis . 4.1 m), as previously defined.
INTRODUCTION
For almost a century the Udden-Wentworth (U-W) grain-size scale has been the standard system used to describe the size of sedimentary particles (Udden 1914; Wentworth 1922, 1935). In more recent times, however, the utility of the original scale has started to prove somewhat problematic since only grain sizes smaller than 4096 mm were classified. The inability to better classify larger grain sizes has become increasingly irksome as more sedimentological and geomorphological research has begun to focus on coarse clasts such as coastal ‘‘boulders’’ in storm and tsunami hazard assessments (Paris et al. 2011; Etienne and Terry 2012). While work on coastal sediments has certainly renewed attention on the problems of nomenclature and classification at the coarser end of the U-W scale, this is by no means a modern issue. Similar difficulties have been faced over several decades by researchers studying a diversity of sedimentary environments, ranging from rivers to glaciers, to terrestrial and submarine slopes failures. This dilemma was first formally addressed in 1999 when Blair and McPherson offered a valuable contribution by extending the U-W scale to include very coarse sediments with clasts of intermediate axis (i.e., b axis) length up to 1075 km (Blair and McPherson 1999). Later, partly in response to the rapid groundswell of interest in coastal deposits with coarse fabric, Paris et al. (2011) carried out a critical review of coastal boulder studies across different environmental regimes. Their review was revealing. While it was found that most of the coarse-grained clasts investigated in coastal sedimentological studies are actually boulder-sized (although some fall within larger size ranges) a wide variety of terms have been used (Table 1). Although it is unknown whether the inconsistent use of terminology stems more from accidental omission or a preference by other workers to use alternative descriptors, Paris et al. (2011) recommended that researchers of coastal deposits should be more careful in following the modified U-W grain size scheme of Blair and McPherson (1999). Published Online: March 2014 Copyright E 2014, SEPM (Society for Sedimentary Geology)
We examine the practicalities of using various versions of the extended U-W scale in the coarser clast sizes, identify continuing difficulties in terminology, and finally offer some suggestions for improvement. THE ‘‘COARSE-CLAST PROBLEM’’
On the extended U-W scale of Blair and McPherson (1999), clasts between 0.25 and 4.1 m are classified as ‘‘boulders’’ and clasts 4.1–65.5 m are ‘‘blocks.’’ Still larger clasts are classified as ‘‘slabs,’’ ‘‘monoliths,’’ or ‘‘megaliths,’’ according to the ranges given in Figure 1. The length of the intermediate axis is used as its measurement is equivalent to sieve analysis of fine-grained particles. All classes bigger than the boulder category are collectively referred to as ‘‘megaclasts.’’ The overall structure and scope of this extended U-W scale has much merit, but the names of the categories at the upper end of the scale start to become confusing when considered alongside the terminology for particle shape or form proposed by Blott and Pye (2008), where the terms ‘‘block’’ and ‘‘slab’’ represent a significant part of their classification system (Fig. 2). While it could be argued that since these terms were first included in the amended U-W scale by Blair and McPherson in 1999 and so have precedence, it is worth noting that much earlier Wentworth (1922, p. 380) stated ‘‘Angular masses of rock of the same size are commonly called blocks or slabs.’’ In a sense, this early work had already established that those expressions were more related to the shape of sedimentary clasts than to their size. We are not, however, interested in entering into a debate concerning the precedence of these terms, but rather in attempting to adopt a pragmatic approach to finding a more usable nomenclature at the coarser end of the U-W grain-size scale that will both overcome existing problems and be widely accepted (Fig. 2). The principal difficulty with employing
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FIG. 1.—Extended Udden-Wentworth grainsize scale for sedimentary particles, after Blair and McPherson (1999). Particle length is based on the intermediate axis (b axis). Key nomenclature at the coarse end of the scale of interest to this paper has been shaded in red.
the terms ‘‘block’’ and ‘‘slab’’ to define particle size ranges is that they are also common as generic shape descriptors in the earth sciences, for example in landslide investigations (e.g., Varnes 1978; UNESCO 1993; Stock et al. 2012). More broadly conceived, the vernacular use of the word ‘‘block’’ also implies a degree of angularity or ‘‘blockiness’’ of form and ‘‘slab’’ implies a degree of ‘‘flatness,’’ regardless of true shape (or size). The existing terminology therefore exerts certain constraints on the use of ‘‘block’’ and ‘‘slab’’ outside their designated size ranges. For example, an angular and cuboid ‘‘coral reef block’’ (Fig. 4A) cannot strictly be referred to as such if it falls outside the size range 4.1–65.5 m, while a flat-shaped piece of detached beachrock (Fig. 4B) should not be called a ‘‘beachrock slab’’ if it is smaller than the range 65.5–1048.6 m. Subsequently, Blair and McPherson (2009), in a comprehensive study of alluvial-fan deposits, revisited their earlier terminology for coarse particles
and replaced the size category ‘‘slab’’ with ‘‘megablock.’’ Although this commendable attempt at revision was carried out with good intention, several challenges nonetheless still exist. First, the term ‘‘block’’ remains present in the nomenclature, with its attendant problems regarding connotations of shape as mentioned above. Second, the 2009 modified scale does not yet enjoy such a wide usage within sedimentological research as the earlier 1999 version, although of course it is a newer publication and so this situation might change over time. Third, the introduction of the term ‘‘megablock’’ might be misleading. The reason is that the prefix ‘‘mega’’ is recognized as a modifier for standard SI units of measure. Therefore, there is an implication that a ‘‘megablock’’ is 3106 times the size of a ‘‘block,’’ which is in reality not the case on the clast size scale. On this point, it is also worth noting that some researchers have voiced concern about the proliferation of the use of newer terms like ‘‘mega-
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TABLE 1.— Sample publications and terminology of various authors used to describe very coarse sediment grain sizes (after Terry et al. 2013). Grain-Size Term
Publication
Study Area
Size of Clast/Comment
Block
Simkin and Fiske (1983) Yu et al. (2004) Leroy (2012)
Boulder and Megaclast
Lorang (2011)
Anyer, Java coast Nansha area, South China Sea Between Anyer and Carita, Java, Indonesia Kalalau Beach, Hawaii
Coarse Debris and Megaclasts Bloc Cyclope´en Mega-Boulder
Scheffers et al. (2009) Bourrouilh-Le Jan and Talandier (1985) Fichaut and Suanez (2008) Kelletat et al. (2007)
Ireland and Scotland Rangiroa, French Polynesia Banneg Island, Brittany, France West coast of Thailand
Megablock
Frohlich et al. (2011)
– (Encyclopedia entry for megablock)
Megaclast
Noormets et al. (2002, 2004)
Oahu, Hawaii
Williams and Hall (2004)
North Atlantic
block’’ and ‘‘megaclast’’ without an agreed definition of what they really mean. For instance, there is a mismatch between the intended use of ‘‘megablock’’ by Blair and McPherson (2009) to mean clasts with sizes 65.5–1048.6 m and the entry for ‘‘megablock’’ in the Encyclopaedia of Coral Reefs to describe ‘‘intact blocks or boulders, often composed of coral and occasionally with dimensions of 10 m or greater’’ (Frohlich et al. 2011, p. 679). Similarly, even the lead author here is culpable in this regard, having previously applied the term ‘‘megaclast’’ inappropriately to large coral reef boulders in Fiji (Terry and Etienne 2011). Finally, the term ‘‘monolith’’ (1.0–33.6 km) appearing in both 1999 and 2009 versions of the Blair and McPherson extension to the U-W scale is problematic, because it is used in other disciplines to refer to a carved rock that is still connected to bedrock (M. Rygel, personal communication). In the most recent addition to the literature on the subject of
300 m3, height 6.5 m, 600 tons 1–2.5 m in height No dimensions given but included a photo of the block Developed mathematical equations from previous studies, with a conceptual example from the study area Up to . 250 tons Largest size: 15 m 3 10 m 3 5 m Largest size: 5.3 m 3 3.9 m 3 0.5 m ‘‘Mega-boulder is no set term’’ (p. 423) Boulders from 50 tons (indicate size by weight) Lists ‘‘boulders or megaboulder deposits’’ as a synonyms of megablocks (p. 679) Largest weight 96 tons (8.8 m 3 5.5 m 3 2.5 m) Largest size 6 m 3 5 m 3 3.2 m
particle sizes, Blott and Pye (2012) advocated removing all distinctions above ‘‘boulder’’ size and merging them instead into one all-encompassing size grouping ‘‘megaclasts.’’ Although we appreciate that this is one possible solution to the issues identified, we feel that at the same time it would be a pity to lose the richness that is afforded by having a number of separate divisions for the larger clast sizes above the boulder range. SUGGESTED REVISIONS
Clearly, it is not unreasonable to anticipate that investigators of coarse sediments will need to describe simultaneously both the size and form of large clasts under scrutiny. In response, to avoid confusion between ‘‘block’’ and ‘‘slab’’ while retaining a consistency with regards to the widely-used term ‘‘boulder,’’ we propose a revised terminology for coarse
FIG. 2.—Particle form (shape) terminology suggested by Blott and Pye (2008) based on the degree of elongation and flatness, plotted on the A) Sneed and Folk and B) Zingg diagrams (Zingg 1935; Sneed and Folk 1958). Key nomenclature of interest to this paper is shaded in red.
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FIG. 3.—Our proposed amended grain-size scale nomenclature for sedimentary particles, with key nomenclature changes shaded in blue.
clasts at the top end of the U-W scale. Our system abandons shape (or potentially ambiguous) terms entirely and instead adopts the incremental size references ‘‘meso’’ and ‘‘macro’’ that are added as prefixes to ‘‘boulder.’’ Similarly, we propose ‘‘mesolith’’ and ‘‘macrolith’’ for the two largest clast sizes. This is in order to maintain an association with ‘‘lith’’ at the top end of the earlier Blair and McPherson schemes (1999, 2009). We further propose adopting ‘‘megaclastic sediment’’ and ‘‘megaclastic rock’’ as the collective terms to represent the unlithified and lithified fractions respectively (Fig. 3). This retains the advantages of both Blair and McPherson (1999) and Blott and Pye (2012) to allow the inclusive term ‘‘megaclast’’ for materials greater than boulder size, but at the same time avoids any difficulties with the prefix ‘‘mega’’ being specifically assigned to any individual quantitative clast size range. In other words,
this conveniently means that ‘‘megaclast’’ may still be applied to all size classes exceeding 4.1 m, which seems to correspond with the growing de facto usage of this term in much of the current literature. Importantly, in a practical way, the proposed scheme also means that grain-size and grain-shape classification systems can be used concurrently without overlap or consequent misunderstanding. To illustrate, Figure 4 provides examples of the Boulder–Block–Slab (Blair and McPherson 1999) and corresponding Boulder–Mesoboulder– Macroboulder (this paper) clast sizes in association with the Block–Slab terminology for shape (Blott and Pye 2008). Under the existing nomenclature, it is seen that clasts of different size and shape in Figure 4C–F would be classified confusingly as ‘‘block-sized block,’’ ‘‘slab-sized block,’’ ‘‘block-sized slab’’ and ‘‘slab-sized slab,’’ respectively.
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FIG. 4.—Examples of coarser sediment types and their definitions according to our proposed nomenclature and existing nomenclature on the extended UddenWentworth grain-size scale: A) Taveuni, Fiji; B) Lu Dao, Taiwan; C) Fahefa, Tongatapu, Tonga; D) Mermaids Inlet, NSW, Australia; E) Beer, Devon, UK; F) Earlsferry, Scotland. Original photos by authors except Part E by Dr. Ian West (modified from West 2011; http://www.soton.ac.uk/,imw/Beer.htm) and Part F by Mr. James Allan (http://www.geolocation.ws/v/E/1410827/precarious-rock/en). ‘‘Existing’’ terminology is based upon Blair and McPherson (1999).
Under our proposed nomenclature these would become ‘‘mesobouldersized block,’’ ‘‘mesoboulder-sized slab,’’ ‘‘macroboulder-sized block,’’ and ‘‘macroboulder-sized slab,’’ i.e., all terms that satisfactorily and simultaneously describe clast size and shape without difficulty. Furthermore, any large clast can sensibly be called a ‘‘block’’ or a ‘‘slab,’’ according to its shape. CONCLUSIONS
In order to improve and simplify the nomenclature at the coarse end of the Udden-Wentworth scale for sediment grain size, a more intuitive terminology has been proposed to replace some existing terms. The previously ambiguous size terminology of ‘‘block’’ and ‘‘slab’’ that has recognized shape connotations may have in part contributed to the proliferation of alternative non-standard names for very large clasts within the earth science community. Our new terms retain key useful elements of proposed earlier terminology (Blair and McPherson 1999; Blott and Pye 2012) while combining more effectively with existing shape terminology. This will hopefully lead to a more vigilant adherence to the modified U-W grain-size scheme from now on. ACKNOWLEDGMENTS
The authors are grateful for the constructive comments of Dr. Rapha¨el Paris, an anonymous referee, and the Associate Editor (Dr. M. Rygel). Their insights greatly assisted in revising the original manuscript. REFERENCES
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