1.1. The Reef Phenomenon: Definitions and History of Discovery and Research
The nature of āreefsā and āreef communitiesā has been so diverse throughout geological history that there is no general agreement on what exactly is or is not āa reefā. The reasons for this are complex but lie to a large extent in the diversity of the scientific disciplines and contrasting perspectives brought to bear on aspects of both ancient and modern structures. There have been fewer problems for biologists where the focus has been on the content rather than on the nature of the āreefā entity as a whole, but for geologists different approaches have led to a multiplicity of misinterpretations and continuing arguments. A number of attempts have been made to address the problem by proposing definitions of the āreefā (for instance, see Longman, 1981; Fagerstrƶm, 1987; Hallock, 1997; Wood, 1999; Stanley, 2001; Riding, 2002), but no consensus has been reached so far.
The existence of ācoral reefsā was well established by the time European exploration of tropical seas began in the 17th century. Although there was European speculation on the nature of corals as early as the 16th century, it was not until the 19th century that there was any serious scientific evaluation of the characteristics of reefs. One of the key outcomes of the early oceanographic exploration was the description of coral reefs as geological entities. Lyell (1832) described coral reefs in early editions of his Principles of Geology from previous observations in the Indo-Pacific regions. It was against this background that Darwin (1842) published his observations on the morphology of Polynesian Islands in The Structure and Distribution of Coral Reefs, in which he defined the genetic model of reef development, relating reef growth to subsidence (subsidence-controlled theory). The Darwinian model for the evolution of coral reefs, from fringing to barrier and atoll types, has been widely accepted, following the clear evidence provided by deep drilling through Funafuti Atoll (Cullis, 1904; Finkh, 1904; Ohde et al., 2002). The borehole encountered a substantial thickness of shallow-water limestones (339.5m), thus implying considerable subsidence. Similar results were obtained from scattered boreholes elsewhere, including the Bahamas (Field & Hess, 1933), Kita-Daito-Jima (Sugiyama, 1934, in Japanese with some results in Ladd & Schlanger, 1960; Ladd, Tracey, & Gross, 1970; Suzuki et al., 2006) and the Great Barrier Reef (Richards & Hill, 1942). Notwithstanding the dominance of the Darwinian model, there have been other explanations of reef origins and other theories. This was the starting point over one century and half of scientific controversy. In particular, Daly (1915) and Daly (1948) was prominent among the detractors and the key initiator and proponent of the so-called āGlacial Control Theoryā. He argued that far from reflecting simple subsidence, reef morphology was a reflection of changing sea levels. Part of the explanation for the formation of reef platforms was based on the pattern of reef erosion during low sea-level stands. Murray (1880) regarded dissolution of the backreef lagoon as an important part of the outward growth of atolls advancing over their own talus. Stearns (1946), Kuenen (1947), Schlanger (1963) and Ladd et al. (1970) suggested that modern barrier reefs and/or atolls formed postglacially on reef surfaces that had been subaerally eroded by dissolution during Pleistocene low sea-level stands. Glacio-eustatic sea-level changes have since been generally recognized as one of the key factors controlling reef development. Similar views were reached by Purdy (1974), Purdy and Bertram (1993) and Purdy & Winterer (2001) and Purdy & Winterer (2006). In spite of the indisputable evidence of subsidence supported by deep reef drilling, a subaerial solution-induced relief is suggested to have been accentuated by reef building to produce the typical modern barrier and atoll morphologies.
Perhaps because reefs were regarded as geological features, geologists were quick to seize on them as an explanation for the complex relationships of many ancient limestones. The only constant to emerge subsequently has been a gradual geological restriction of the term to carbonate rocks. But structures described as āreefsā have regrettably included a number of organic to inorganic carbonate deposits (Norris, 1953; Lees, 1964; Terry & Williams, 1969; Conaghan, Mountjoy, Edgecombe, Talent, & Owen, 1976) with problems of scale where accumulations on a metre (Kirtley & Tanner, 1968) or centimetre (Ager, 1963) scale have been misguidedly referred to as āreefsā.
Many of the subsequent investigations from the end of the 19th to the middle of the 20th century were based on a zoological approach. Later reviews were published by Stoddart (1969a), Lewis (1977), Dubinski (1990) and Birkeland (1997). By contrast, geology had little use for reefs until the early 1950s when a burgeoning oil industry recognized them as forming important reservoir rocks. Many of the giant fields in the Middle East are within Mesozoic so-called āreefsā, although it is fair to say that the reservoir properties of many of these examples owe as much or more to their diagenetic history than to their depositional characteristics. Cumings and Schrock (1928) had tried to clarify the reef concept by defining two new terms: ābiohermā and ābiostromeā. The main distinction here is essentially answered by the question: Does the structure have significant relief? However, the term ābiohermā in particular seems to have suffered much the same fate as āreefā and has also been misused in such ways as to raise doubts wherever it appears.
Some of the confusion was generated in the oil industry, because descriptions are commonly based on geophysical or borehole evidence. Also, even when visible in outcrop it may be difficult to differentiate between mound-like forms that had significant relief at the time they were deposited and circumscribed structures that lacked relief and were the result of the local persistence of a distinctive laterally restricted facies over a long period. From a sedimentological perspective this is an important distinction, but the problem was not formally addressed until, in 1970, Dunham proposed two new definitions. āThick laterally restricted masses of pure or largely pure carbonate rock long have been called āreefsā. Such masses ā¦ are here termed āstratigraphic reefsā in contrast to organically bound āecologic reefsā. Heckel (1974) proposed a new definition in which a āreefā is a carbonate buildup; that is a structure that has relief above the surrounding seafloor but which displays evidence of potential wave resistance or growth in turbulent water and evidence of control over the surrounding environment. Various subgroups were recognized including structures in which the principle binding agents were inorganic but these were subject to the same taxonomic inertia as others, and āreefā continues to be used in an ill-defined way.
The comparison with recent reefs that is implied by the name overlooks important questions regarding their architecture and growth history that need to be addressed. This may seem straightforward but here also there is sometimes disparity between concepts of āreefsā adopted by geologists and those by biologists, and it is fair to say that there remain differences in opinion as to what constitutes āthe reefā, in part because our un...
