Megaclasts: Term Use and Relevant Biases

Abstract

The term “megaclast” started circulation near the beginning of the 21st century. The present review is aimed at examination of the use of this term in the modern geoscience literature. The main method is bibliographical survey of the articles published during 2000–2017 with the on-line bibliographical database “Scopus”. The main findings are as follows. The term “megaclast” has not been used extensively, but the number of the articles employing this term increased in the mid-2000s and in the early 2010s. The majority of the papers deal with megaclasts of Quaternary age. The megaclast research focuses on five regions, namely West Europe, Australia and New Zealand, Western North America, Southern South America, and the Pacific and circum-Pacific. The most studied are megaclasts occurring on coasts influenced by tsunamis and storms; significant attention has been paid also to those clasts transported by volcanism-triggered debris flows and slope failures, both continental and submarine. There are three serious biases relevant to the use of the term “megaclast” in the geoscience literature, namely stratigraphical, geographical, and genetic biases. Due to this incompleteness in the knowledge of megaclasts, this term should be either used more actively, which is preferable, or abandoned.

Geomorphology of a Holocene Hurricane Deposit Eroded from Rhyolite Sea Cliffs on Ensenada Almeja (Baja California Sur, Mexico)

“…Various conventions exist for the differentiation of sedimentary clasts but the definition for a boulder followed in this exercise is that proposed by Wentworth (1922) for a clast equal to or greater than 256 mm in diameter [10]. No upper limit for this category is found in the geological literature, although Ruban et al (2019) championed the term "megaclast" for boulders of extraordinary size [11]. That term is an appropriate descriptor for some of the boulders in the Almeja CBB.…”

“…Ruban et al (2019) compiled a representative collection of 58 published studies concerning Earth-bound processes capable of producing megaclasts [11]. From this sample, more than half (53%) are limited to boulder deposits that formed during Quaternary time, 20 of which represent coastal depositional settings.…”

“…An equally clear-cut study by Cox et al (2018) relates to blocks with an estimated weight as much as 620 metric tons pealed back from Carboniferous limestone layers exposed at the top of high sea cliffs in western Ireland [28] that are unequivocally linked to major sea storms. Not included in the data base of Ruban et al (2019) are other studies on massive carbonate megaclasts from the Bahamas and Bermuda interpreted ambiguously as either tsunami or storm-related [29] or unequivocally as storm related [30]. Current literature on Quaternary CBBs appears to be skewed towards studies on carbonate megaclasts, such as the work by Biolchi et al (2019) from the northern Adriatic Sea [31].…”

Coastal Boulder Deposits of the Neogene World: A Synopsis

“…The objective of the present paper is to offer an overview of the literature on Neogene coastal boulder deposits with some inferences on the current state of research. The importance of such a bibliographical analysis in megaclast research was demonstrated earlier [40]. The peculiarities of this paper are triplicate.…”

Finding Coastal Megaclast Deposits: A Virtual Perspective

“…Coastal sediment dynamics is always highly complex, and, thus, it provides a lot of issues for geoscience investigations. An interest in nomenclature development for large clasts that started near the end of the 20th century [1] and attention to coastal hazards facilitated by the Indian Ocean Tsunami of 2004 [2] have shaped a new international research direction, namely megaclast studies [3]. Megaclasts of ocean and sea coasts have been investigated most intensively.…”