Whether you are a diver, a geologist, or simply someone with an interest in natural history, you are likely to have a misconception about the structure of coral reefs. The error is ubiquitous in textbooks and is reinforced by media treatments of the topic. Everyone 'knows' that coral reefs have a central zone of organically bound material (the reef core), a leeward zone of flat lying sediments (the back-reef lagoonal area) and a seaward zone of steeply-dipping rubble (the reef talus). The misconception relates to the reef talus. The source of the erroneous view can be traced to Charles Darwin, who sought to follow his mentor (Charles Lyell) in explaining the past by reference to present-day processes.
"Darwin and his many followers regarded contemporary reefs as having shelf-like forms, with steep slopes facing deep water. This morphology differentiates the familiar zones of backreef, reef-crest and fore-reef. Most accounts emphasize the importance of the reef-crest, comprising the growth framework responsible for generating the reef structure. Material eroded from both the reef-crest and the upper reef-slope has been assumed to accumulate on the fore-reef, and it was argued that this provided the foundations that enabled construction to take place in waters that were otherwise too deep. This pervasive idea can be traced to Darwin (1842) and Dana (1853), although it typically only applies to windward, moderately high hydrodynamic energy, regimes. However, numerous conceptual models illustrate reefs in which the fore-reef is shown as a steep debris slope, on which depositional increments are correlated with contemporary intervals of reef growth." (from the Introduction).
An example of an educational graphic showing the fore-reef talus. (Source here)
This understanding of the fore-reef debris slope as talus is described as a "misconception", an "error", a "misleading description" - yet it has achieved widespread acceptance and is regarded as the "traditional" view and a "cherished model". This should be regarded as another example of 'consensus' thinking that owes more to the naive acceptance of Lyellian uniformitarianism than to science. We have Colin Braithwaite to thank for showing that the research findings over the past 30+ years demonstrate clearly that the textbook interpretations of "reef talus" need to be revised. After reviewing numerous papers, he writes:
"What conclusions, vis a vis Darwin's model and "talus slopes", can be drawn from these observations? Early descriptions of reefs by Darwin and others paved the way for an interpretation linking the morphology of 'the reef' to erosion and the formation of coarse debris, 'reef talus', commonly regarded as integral to conceptual models of ancient reefs. However, research over the past decades has shown that present-day processes, that include storm events at the high end of the energy spectrum, are important contributors to reef debris but do not generate large volumes of coarse debris on fore-reef slopes. Although reef erosion is a reality, transport directions generally preclude its involvement in large-scale talus formation. Neither off-reef flow nor large-scale slope failure generates debris on the reef front in the size ranges typically described as "talus"." (From the Conclusions)
Evidence amassed by Braithwaite explains that "reef talus" is a misnomer. In the main, it is not rubble from the reef core that has moved down the steeply-dipping fore-reef slope. The evidence shows that most of the reef debris caused by hurricanes and storms is moved in the other direction - into the back-reef lagoon. The mechanism is understood in this way:
"Why does such transport occur? It reflects wave set-up and the flow generated by breaking waves. However, in contrast to waves breaking on sand or gravel beaches, other than during relatively fair-weather conditions, backwash is effectively eliminated. Flow is able to continue landwards in waves of translation that decay gradually and, on a wide platform, are ultimately dissipated by surface friction. Thus, their ability to transport material is systematically reduced and is only expressed in a broadly decreasing grain-size of deposits landward of the reef margin." (From Section 3 - fore-reefs and transport)
Braithwaite argues that coral reefs in today's oceans are growing on limestone platforms that predate reef growth. The margins of those platforms are subject to a variety of forces that produce the talus slopes.
"Contemporary reefs are shedding sediment into deeper waters, but there is also evidence of larger-scale margin collapse and gravity-driven slope failure of the platforms beneath them. Blocks of kilometre dimensions have been described on the west Florida margin, the Bahamas, and bounding the Nicaraguan Rise." (From Section 5 - slope deposits and platform shedding)
For those with an interest in the geological issues, Braithwaite's discussion is informative and thought-provoking, but this will not be considered further here. Suffice to say that it incorporates plenty of examples from fossil 'reefs' that confirm the proposed model.
"The premise that "reef talus" is an expression of the erosional history of the underlying platform rather than an integral product of a living reef can be illustrated by examples from rocks of a variety of ages." (From Section 6 - Ancient analogues)
Braithwaite=s paper is suggestive of a distinction that can be made between uniformitarianism and actualism. Darwin illustrates the former - although he claimed to be saying that the present is the key to the past, he invoked only gradualist processes that he thought were operative in the present and failed to test his hypotheses rigorously. Those who have followed him appear to have lacked the will to formulate and test hypotheses and to consider the viability of alternative models. Although there is an appeal to contemporary processes, uniformitarians tend to favour those characterised by small incremental effects. By contrast, Braithwaite illustrates actualism, with an evaluation of a much wider range of processes. He considers hurricane-driven tidal flows, tsunamis, and even waves generated by a meteorite impact capable of lifting blocks weighing 100 tons onto cliffs 33m above present sea level. He cites a case of sediment accumulation over about 500 years but all of it being transported and dispersed by a hurricane-induced current in about 5 hours. He shows how alternative models can be tested and how evidence can be used to falsify hypotheses. This approach to science is much more healthy, for there is a willingness to challenge cherished models and an openness to the operation of different mechanisms.
Why is this worthy of our attention? The principles in evidence here are relevant to a large number of topics that relate to the past. Unfortunately, these origins issues often are characterised by excessive appeals to consensus and cherished models, and not enough attention is given to the weight of evidence. Lyell's and Darwin's uniformitarianism still have an undue influence on our educational system and the media. Attempts to increase the level of critical scrutiny are met with emotive responses rather than reasoned arguments. To help us think through our methodology for dealing with these tensions, Braithwaite's approach to the "reef talus" model may provide a useful case study.
Reef Talus: A popular misconception
Colin J.R. Braithwaite
Earth-Science Reviews, Volume 128, January 2014, pages 169?180.
Abstract: Reef fronts have traditionally been regarded as comprising debris derived by contemporaneous erosion of 'the reef'. However, evidence from wave transport indicates that on present-day reefs the bulk of the debris generated in this way accumulates in the back-reef area, with only finer-grained sediment carried off-reef by retreating flows or by overwash. Nevertheless, in contrast to this observation, 'fore-reef' debris slopes are commonly considered "characteristic" of Phanerozoic reefs. This apparent error reflects the conflation of processes defining contemporary growth and accretion of the reef, and the corresponding long-term accretion of the carbonate platform on which it rests. Present-day reefs are commonly (although not exclusively) additions to long-lived carbonate platforms. Growth of the latter is intermittent and has been moderated by changes in sea-level that, for recent reefs, have been on time scales of less than 100 ka. During low sea-level stands, growth ceases or is translated downslope and earlier deposits are subject to lithification and subaerial erosion. Similar changes are applied on a larger scale to the aggrading growth of carbonate platforms, but the bulk accretion of these includes quite different processes and reflects far longer timescales. During low sea-level stands, the margins of platforms commonly become unstable, with instability reflected in slope failure and in the shedding of blocks, ranging from metres to kilometres in diameter, associated with the generation of debris flows and turbidites. It is argued that these are the materials that are commonly described as 'reef talus' in ancient structures, although their formation is largely independent of any contemporary reef growth. Difficulties arise where 'the reef' and 'the platform' are treated as a single functional entity. It is important to recognize the conceptual distinction between them, 'reef talus' is a misleading description of the debris predominantly generated by platform erosion and slope failure.
Tyler, D. The unscientific hegemony of uniformitarianism, ARN Literature Blog (16 May 2011)
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