Reef talus: A popular misconception
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- 作者：Colin J.R. Braithwaite
- 关键词：Coral reefs ; Reef accretion ; Talus ; Slopes ; Megabreccias ; Carbonate platforms
- 刊名：Earth-Science Reviews
- 出版年：January, 2014
- 年：2014
- 卷：128
- 期：Complete
- 页码：169-180
- 全文大小：2286 K
文摘
Reef fronts have traditionally been regarded as comprising debris derived by contemporaneous erosion of 鈥榯he reef鈥? 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, 鈥榝ore-reef鈥?debris slopes are commonly considered 鈥渃haracteristic鈥?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 鈥榬eef talus鈥?in ancient structures, although their formation is largely independent of any contemporary reef growth. Difficulties arise where 鈥榯he reef鈥?and 鈥榯he platform鈥?are treated as a single functional entity. It is important to recognize the conceptual distinction between them, 鈥榬eef talus鈥?is a misleading description of the debris predominantly generated by platform erosion and slope failure.