Phases Of Reef Growth Research Articles

The evolution of the Great Barrier Reef during the Last Interglacial Period

Reef response to Last Interglacial (LIG) sea level and palaeoenvironmental change has been well documented at a limited number of far-field sites remote from former ice sheets. However, the age and development of LIG reefs in the Great Barrier Reef (GBR) remain poorly understood due to their location beneath modern living reefs. Here we report thirty-nine new mass spectrometry U-Th ages from seven LIG platform reefs across the northern, central and southern GBR. Two distinct geochemical populations of corals were observed, displaying activity ratios consistent with either closed or open system evolution. Our closed-system ages (~129–126ka) provide the first reliable LIG ages for the entire GBR. Combined with our open-system model ages, we are able to constrain the interval of significant LIG reef growth in the southern GBR to between ~129–121ka. Using age-elevation data in conjunction with newly defined coralgal assemblages and sedimentary facies analysis we have defined three distinct phases of LIG reef development in response to major sea level and oceanographic changes. These phases include: Phase 1 (>129ka), a shallow-water coralgal colonisation phase following initial flooding of the older, likely Marine Isotope Stage 7 (MIS7) antecedent platform; Phase 2 (~129ka), a near drowning event in response to rapid sea level rise and greater nutrient-rich upwelling and; Phase 3 (~128–121ka), establishment of significant reef framework through catch-up reef growth, initially characterised by deeper, more turbid coralgal assemblages (Phase 3a) that transition to shallow-water assemblages following sea level stabilisation (Phase 3b). Coralgal assemblage analysis indicates that the palaeoenvironments during initial reef growth phases (1 and 2) of the LIG were significantly different than the initial reef growth phases in the Holocene. However, the similar composition of ultimate shallow-water coralgal assemblages and slow reef accretion rates following stabilisation of sea level (phase 3b) suggest that reefs of both ages developed in a similar way during the main phase of relatively stable sea level.

Successive phases of Holocene reef flat development: Evidence from the mid- to outer Great Barrier Reef

A re-examination of 46 recently published U/Th reef flat ages from Heron and One Tree reefs in the southern Great Barrier Reef (GBR) identified several distinct Holocene reef growth phases with a clear 2.3-kyr hiatus in lateral reef accretion from 3.9ka to 1.5ka. An analysis of all available published radiocarbon reef flat ages (165) from 27 other mid-outer platform reefs revealed a similar hiatus between 3.6ka and 1.6ka for the northern, south-central and southern GBR. However, no hiatus in reef flat growth was observed in reefs from the central GBR with ages ranging from 7.6ka to 0.9ka. Increased upwelling, turbidity and cyclone activity in response to increased sea-surface temperature (SST's), precipitation and El-Nino Southern Oscillation variability have been ruled out as possible mechanisms of reef turn-off for the mid-outer platform reefs. Rather, a fall (~0.5m) in relative sea level at 4–3.5ka is the most likely explanation for why reefs in the northern and southern regions turned off during this time. Greater hydro-isostatic adjustment of the central GBR and long term subsidence of the Halifax-Basin may have provided greater vertical accommodation for the mid-outer reefs of the central GBR, thus allowing these reefs to continue to accrete vertically despite a fall in sea level~4–3.5ka. Further evidence for greater subsidence in this region includes the lack of senile reefs and dominance of incipient and juvenile reefs in the central GBR. This suggests that these reefs approached sea level considerably later than the northern and southern reefs, consistent with their deeper antecedent substrates. Thus, these results not only provide important information about possible reef flat demise in response to natural environmental factors, but also provide insights into regional subsidence that affected relative sea level along the east Australian margin during the Holocene.

Recurrent patterns of coral community and sediment facies development through successive phases of Holocene inner-shelf reef growth and decline

Coral community decline and resultant ‘turn-off’ of reef accretion potential are commonly predicted outcomes of the increasing environmental stresses experienced by many coral reefs. The potential for coral reef communities to re-establish once environmental conditions ameliorate or new substrates for colonisation become available are, however, less well understood. Here we discuss data from a fringing reef at Dunk Island, an inner-shelf high island on Australia's Great Barrier Reef (GBR). Although this reef is spatially contiguous, detailed coring and dating reveal it was constructed in two discrete temporal phases. The inner part of the reef flat formed first, during the late stages of the post-glacial marine transgression-early sea level highstand (~ 6.9–4.5 k cal yr B.P.), whereas the outer fringe developed after the late-Holocene sea level regression, from ~ 1.6 k cal yr B.P. to present. A hiatus of almost 3000 years separates the two reef growth phases. Data from this reef thus provide a same-site perspective on the degree of temporal continuity that exists where an actively growing reef slows and eventually ceases to accrete for an extended period, before re-establishing and growing again—a temporal transition equivalent to reef ‘turn-off’ followed by renewed reef ‘turn-on’. The depositional architectures of the two phases of reef growth are very similar, as are patterns of facies and coral assemblage development. This study suggests that reefs, over the timescales at which cycles of reef ‘turn-on’ and ‘turn-off’ operate (the centennial to millennial timescale), can not only re-establish successfully, but also that regrowth may be characterised by the establishment of near identical reef-building communities and the development of near identical depositional structures. These findings thus have direct relevance to the on-going debate about whether, and in what form, reef communities will be able to re-establish themselves following periods of suppressed growth, such as those projected for the near-future.

A new coral reef province in the Gulf of Carpentaria, Australia: Colonisation, growth and submergence during the early Holocene

Multibeam sonar mapping, drill cores and underwater video data have confirmed the existence of a previously unknown coral reef province in the Gulf of Carpentaria, Australia. Seven reefs, comprised of coral limestone that support living corals have been mapped so far and as many as 50 other reefs may exist in the region. U/Th ages show that reef growth commenced shortly after limestone pedestals were submerged by rising sea level around 10.5 kyr BP, making them the oldest Holocene reefs known in Australia. Reef growth persisted for ~ 2.0 kyr but it had ceased at most locations by ~ 7.0 kyr BP. Measurements of reef growth rates (0.95 to 4 m kyr − 1 ), indicate that the reefs were unable to keep pace with contemporaneous rapid sea level rise (> 10 m kyr − 1 ), which is consistent with a “give up” reef growth history. Core samples from reef platforms demonstrate that Pleistocene limestone is exposed at depths of 27 and 30 m below present mean sea level. These depths represent regionally significant phases of reef growth during a prolonged sea level still stand. We conclude that the reefs are therefore mostly relict features, whose major phase of growth and development relates to an earlier, pre-Holocene sea level still stand.

Facies distribution and coral-microbialite reef development on a low-energy carbonate ramp (Chay Peninsula, Kimmeridgian, western France)

The Chay Peninsula (western France) succession corresponds to a Kimmeridgian shallow and low-energy ramp system that developed in the northeast of the Aquitaine Basin. The sedimentation shows a transition from a carbonate-dominated to a mixed carbonate–siliciclastic depositional environment that takes place at the base of a major, third-order sea-level transgression. Facies partitioning and coral-microbialite reef levels (i.e., reef windows) suggest higher-frequency (fourth-order) sea-level oscillations. Reef-growth phases (i.e., high-frequency reef windows) are interpreted to have been controlled by fifth-order sea-level oscillations. In the mixed siliciclastic–carbonate system, successive prograding units of shoreface sediments pass into thick marl–limestone alternations over a distance of tens of metres. Such remarkable short-distance lateral facies variations suggest a complex sedimentary system made of juxtaposed deposits that rapidly shift through time. Terrigenous fluxes, in modifying the light intensity and the nutrient level in the water column strongly controlled the facies distribution and notably the reef composition and growth. Identified coral genera suggest mixed photo-heterotrophic mode of nutrition and low-mesotrophic conditions in marine waters. For each reef-growth phase, nutrient increase favoured large amounts of microbialites to develop, forming a crust that entirely capped the coral reef surface. Thus, high-frequency fluctuations in nutrient input in tune with fifth- (or higher) order sea-level fluctuations controlled the reef development at a millennial scale. The Chay Peninsula reef distribution and development provide insight into understanding the response of reef communities under climatically-induced environmental changes.

Reef development at high-latitudes during multiple interglacial cycles: New evidence from Lord Howe Island, southwestern Pacific

Reef development during past Interglacial periods, when sea level and sea surface temperatures were higher than today, provide unique insights into how reef systems may respond to projected human-induced global warming. Lord Howe Island currently represents the southernmost limit of reef development in the Pacific. Reef growth of Pleistocene age has been inferred to have occurred around the island, and this paper provides the first detailed descriptions on the character of this development. Two phases of reef growth are identified, which occurred as isolated fringing reefs along the edge of the basaltic hills of the island. Uranium-series dating indicates that the upper part of the sequence is of Last Interglacial age, however extensive calcite recrystallisation meant the lower part of the sequence does not yield reliable ages. Calcite cements suggest that several phases of recrystallisation have occurred meaning the lower part of the sequence is most likely to represent reef of Penultimate Interglacial age. Component analysis of the sedimentary matrix within the reef indicates coralline algae dominated sands which are very similar to the modern reef environment. This suggests that the environment at Lord Howe Island has remained at or close to the environmental limits for reef growth during the past few interglacials, despite lithospheric plate motion moving this island further north into reef building seas.

High-frequency palaeoenvironmental fluctuations recorded in Jurassic coral- and sponge-microbialite bioconstructions

During the Jurassic, coral and sponge reefs were particularly widespread along the northwestern Tethys. These bioconstructions occurred from shallow- to deep-shelf settings, in pure carbonate, carbonate-dominated or clay-rich mixed carbonate-siliciclastic sedimentary contexts. According to the depositional environment, the amount of microbialites strongly fluctuates, to form up to 70% of the reef volume. This study focuses on bioconstructions where microbialites constitute at least 15% of the reef volume. Analysis of the close relationships between the reef structures and the stacking pattern of laterally-deposited sediments reveals analogous architectures and growth modes between coral- and sponge-microbialite bioconstructions. Firstly, corals or sponges erected a primary framework more or less projected above the sea floor (constratal or superstratal growth fabrics). Contemporaneously, a thin microbialite layer encrusted the local dead parts of corals or sponges. In a second stage, microbialites largely developed and entirely covered the reef surface, becoming the main reef builders. The whole development of coral- and sponge-microbialite reefs corresponds to a ‘low-frequency reef-growth phase’. These bioconstructions also display surfaces of reef growth interruption delimiting ‘medium-frequency reef-growth phases’. They include several ‘high-frequency reef-growth phases’ (or elementary units) corresponding to coral or sponge frameworks plus microbialite crusts. At the scale of the elementary unit, microbialites are interpreted to reflect nutrient-richer conditions and developed to the detriment of phototrophic-, mixed- or heterotrophic-dominated assemblages of corals or sponges. The development of Jurassic coral and sponge bioconstructions was punctuated by high-frequency (millennial time-scale) ecological crises during which microbialites occurred at the reef surface. Such ecological shifts from skeletal reef metazoans (i.e. corals or sponges) to microbialite crusts were recorded whatever the palaeogeographical position (shallow-proximal platform or deep-distal shelf) along the northwestern Tethys margin. Three main orders of climatic oscillations, correlated to the ‘high-, medium- and low-frequency reef-growth phases’, could have directly regulated the amount of terrigenous materials and nutrients delivered to the ocean, which in turn controlled platform carbonate production and accumulation, as well as reef development.

Paleocene coralgal reefs of the western Pyrenean basin, northern Spain: New evidence supporting an earliest Paleogene recovery of reefal ecosystems

The rate of recovery of photic reefal ecosystems after the Cretaceous–Tertiary crisis is controversial. Most reports from earlier authors concluded that Paleogene reef systems did not completely recover from that crisis at least until Oligocene–Miocene times. Several other authors, however, have pointed out that such conclusion was biased by poor preservation and/or inaccessibility of many early Paleogene successions and that the recovery of reefs might have been a faster process. The latter possibility is here strongly supported with data from the Pyrenean basin, where Paleocene shallow-water carbonate deposits are thickly developed and outcrop widely. Five growth phases of reef development have been recognized and age dated with calcareous nannofossils, which together define a complete sequence of expansion-reduction of reef ecosystems during the Paleocene epoch. Reef growth phase 1 (early Danian, lower–middle NP3 calcareous nannofossil Zone), the initial step of that sequence, demonstrates that coral-dominated reef systems came back into existence less than 2 Ma after the K/T boundary, a comparatively short time considering the magnitude of the end-Cretaceous biological crisis. The climax of the sequence is recorded by growth phase 3 (late Danian, NP4 Zone), when a thick barrier-reef complex was developed, which was at least 200 km in length (but probably much larger) and made up by a well diversified coralgal assemblage. Reef growth phases 4 and 5 (early and middle Thanetian, NP6 to NP8 Zones) were characterized by low-relief reef banks dominated by encrusting calcareous algae and had less well-diversified coral assemblages, features that point to a deterioration of environmental conditions, tentatively interpreted in terms of a climatic cooling. Reef bioconstruction almost disappeared in the Pyrenean basin during late Thanetian times, although in all likelihood this is a regional effect of no evolutionary relevance. The new data suggest that the Cenozoic recovery of reefs was rapid but punctuated, with a phase of important reef expansion already in the late Danian.

Coralline algae indicate Pleistocene evolution from deep, open platform to outer barrier reef environments in the northern Great Barrier Reef margin

Coralline algae from a drill core in Ribbon Reef 5 have been used to interpret changes in the depositional palaeoenvironments in the northern Australian Great Barrier Reef over the last 790 ka. Three main coralline algal assemblages, each dominated by members of a particular subfamily, have been distinguished by quantitative analysis: (1) mastophoroid assemblages, usually occurring as crusts on corals, are typical of the shallowest reef settings; (2) lithophylloid assemblages within algal nodules may represent shallow-water, cooler environments or deeper reef subenvironments; and (3) melobesioid assemblages are characteristic of deeper-water platform areas. The algal assemblages between 96 and 210 m b.s.f. (metre below sea floor) record a fluctuating but progressive shallowing-upwards from deep, outer-platform to shallower, non-reefal depositional environments. Two intervening episodes dominated by mastophoroids represent two phases of reef growth. The section above 96 m b.s.f. comprises several stacked reefs in which mastophoroid crusts similar to the present-day shallow-water assemblages predominate.

Palaeoenvironmental control on sponge-microbialite reefs and contemporaneous deep-shelf marl-limestone deposition (Late Oxfordian, southern Germany)

The Late Oxfordian (Bimammatum Zone) deep-shelf deposits of southern Germany are characterised by abundant reefs that laterally pass into marl-limestone alternations. The bioconstructions are made from three main components, sponges, microbialites and allochthonous sediment. A detailed morphological analysis of sponge-microbialite reefs in the Plettenberg section (Swabian Alb, southern Germany) is combined with investigation of calcareous nannofossil assemblages in laterally correspondent marl-limestone alternations in order to trace the evolution of palaeoenvironmental conditions during reef development. Fluctuations of CaCO3 content in deep-shelf deposits provides evidence of a succession of small-scale lithological units, generally formed by five to six marl-limestone couplets. Within a small-scale lithological unit, a close link is observed between growth patterns of sponge-microbialite reefs and the changes in the nannofossil assemblage composition. Phases of expansion of the build-ups correspond to carbonate deposits, characterised by a relative dominance of large, oligotrophic nannoplankton species (large Watznaueria britannica, Watznaueria manivitiae and Schizosphaerella spp.). Marly intervals reveal a high relative abundance of small-sized taxa (mainly Lotharingius hauffii, small W. britannica and subordinated Biscutum dorsetensis), and bioconstructions show a net reduction in size, or a total demise. This suggests that the same palaeoenvironmental parameters controlled, at the same time, marl versus limestone deposition, reef growth-phases, and calcareous nannofossil assemblage composition. Successive reef growth-phases recognized within a bioherm are correlated to the limestone hemi-couplets. Each reef growth-phase is composed of several stacked elementary sequences, comprising the following succession: (i) sponge, (ii) microbialites and (iii) carbonate mud. In turn, the microbialitic crust is formed of three successive microbial layers: (1) a fine crust of dense to clotted micrite; (2) a centimetre-scale to multicentimetric columnar layer of clotted to peloidal micrite; (3) a centimetre-scale stromatolitic crust. The passage from an initial dense microbial layer, with numerous microencrusters, to a final stromatolitic crust is interpreted as resulting from an increase of the microbial growth-rate, mainly driven by changes in the accumulation rate. Deep-shelf deposits resulted from changes in platform carbonate production and export basinwards controlled both by sea-level and climate changes. Climate controlled the intensity of carbonate production in shallow-platform environments and export towards the deep-shelf, as well as the trophic conditions of the deep-shelf marine-waters. Low trophic conditions favoured platform carbonate production and export basinwards, and a large development of the sponge-microbialite reefs in the deep-shelf. However, a high carbonate accumulation rate in the deep-shelf was also commonly responsible for the demise of sponge-microbialite reefs. During unfavourable and higher trophic conditions (during marl deposition), reduced development and frequent demise of the sponge-microbialite reefs is observed.

Palaeoenvironmental control on sponge-microbialite reefs and contemporaneous deep-shelf marl-limestone deposition (Late Oxfordian, southern Germany)

The Late Oxfordian (Bimammatum Zone) deep-shelf deposits of southern Germany are characterised by abundant reefs that laterally pass into marl-limestone alternations. The bioconstructions are made from three main components, sponges, microbialites and allochthonous sediment. A detailed morphological analysis of sponge-microbialite reefs in the Plettenberg section (Swabian Alb, southern Germany) is combined with investigation of calcareous nannofossil assemblages in laterally correspondent marl-limestone alternations in order to trace the evolution of palaeoenvironmental conditions during reef development. Fluctuations of CaCO 3 content in deep-shelf deposits provides evidence of a succession of small-scale lithological units, generally formed by five to six marl-limestone couplets. Within a small-scale lithological unit, a close link is observed between growth patterns of sponge-microbialite reefs and the changes in the nannofossil assemblage composition. Phases of expansion of the build-ups correspond to carbonate deposits, characterised by a relative dominance of large, oligotrophic nannoplankton species (large Watznaueria britannica, Watznaueria manivitiae and Schizosphaerella spp.). Marly intervals reveal a high relative abundance of small-sized taxa (mainly Lotharingius hauffii, small W. britannica and subordinated Biscutum dorsetensis), and bioconstructions show a net reduction in size, or a total demise. This suggests that the same palaeoenvironmental parameters controlled, at the same time, marl versus limestone deposition, reef growth-phases, and calcareous nannofossil assemblage composition. Successive reef growth-phases recognized within a bioherm are correlated to the limestone hemi-couplets. Each reef growth-phase is composed of several stacked elementary sequences, comprising the following succession: (i) sponge, (ii) microbialites and (iii) carbonate mud. In turn, the microbialitic crust is formed of three successive microbial layers: (1) a fine crust of dense to clotted micrite; (2) a centimetre-scale to multicentimetric columnar layer of clotted to peloidal micrite; (3) a centimetre-scale stromatolitic crust. The passage from an initial dense microbial layer, with numerous microencrusters, to a final stromatolitic crust is interpreted as resulting from an increase of the microbial growth-rate, mainly driven by changes in the accumulation rate. Deep-shelf deposits resulted from changes in platform carbonate production and export basinwards controlled both by sea-level and climate changes. Climate controlled the intensity of carbonate production in shallow-platform environments and export towards the deep-shelf, as well as the trophic conditions of the deep-shelf marine-waters. Low trophic conditions favoured platform carbonate production and export basinwards, and a large development of the sponge-microbialite reefs in the deep-shelf. However, a high carbonate accumulation rate in the deep-shelf was also commonly responsible for the demise of sponge-microbialite reefs. During unfavourable and higher trophic conditions (during marl deposition), reduced development and frequent demise of the sponge-microbialite reefs is observed.

Growth Dynamics of Neoproterozoic Calcimicrobial Reefs, Mackenzie Mountains, Northwest Canada

ABSTRACT Neoproterozoic buildups of the Little Dal Group grew in a deep-water epicratonic basin. These kilometer-scale reefs display aggradational and progradational geometries comparable to those described from Phanerozoic reefs. Four phases of reef growth correspond to four regional shale-to-carbonate packages in laterally equivalent off-reef strata. The lower, shaly part of each package is interpreted to reflect transgressive to highstand deposition. The upper carbonate part of each package reflects carbonate precipitation in the water column as a result of postulated basin restriction and increased salinity during sea-level lowstand. Reefs nucleated at the beginning of the first major transgressive event. Reefs typically aggraded during transgressive intervals, but could also prograde or contract, likely depending on the rate of relative sea-level rise versus reef growth rate. Reefs prograded during regressions, probably owing to reduction of accommodation space. They shed talus at lowstand in response to increased erosion and/or progradation over unstable substrates. Reef growth ended with return of shallow-water conditions. The overall architecture of aggradation and progradation demonstrates that the Neoproterozoic calcimicrobial reef ecosystem was capable of responding to environmental changes in the same way as the more extensively studied, ecologically complex, and faunally diverse buildups of the Phanerozoi .

Patterns of reef sedimentation and diagenesis in the Miocene of Cyprus

An Early Tertiary regressive sedimentary succession developed along an active plate margin in Cyprus. A varied clastic and carbonate succession of the Miocene Pakhna Formation and then Messinian evaporites culminate the regression. Within the Pakhna Formation, two phases of reef growth were developed: the Aquitanian—Burdigalian, Terra Member, and the Tortonian, Koronia Member. The Terra Member is exposed only in west and southeast Cyprus, mainly between pelagic carbonates. The Koronia Member is exposed around the margins of the Troodos Massif in north, south and west Cyprus and on the Akamas Peninsula in northwest Cyprus. There was little tectonic influence on the growth pattern of the Terra Member reefs. The Koronia Member reefs, however, are developed on up-faulted blocks. Most of the Koronia Member fore-reef facies and associated clastic input was shed into downfaulted basinal depocentres. There are differences between the first and second Miocene reef phases in Cyprus in primary reef construction and in the level of background carbonate pelagic sedimentation. The Terra Member reefs are dominantly framestones, comprising faviids and domal poritids with several types of secondary reef-dwelling corals. There is no indication of a connection with the Indo-Pacific coral faunal province. Benthonic foraminifera and fragments of crustose coralline algae are abundant in the off-reef. By contrast, the Koronia Member is a bindstone comprising virtually monospecific, laminar poritid corals but with coralline algae also playing a role in encrustation. The Koronia Member off-reef facies comprises decimetre-thick beds of bioclastic reef detritus. In north Cyprus however, beds of similar detritus (centimetres in thickness), are overlain by progressively thicker (up to several metres) debris-flow sheets. The Terra Member reefs have limited acicular fringe cement (probably aragonitic). This is overlain by an abundant, marine, bladed, formerly high-Mg calcite cement. Later cements present in the reef facies include micritised calcite spars, perhaps indicating microboring in cements during their precipitation. Neomorphism was extensive, converting most cements to low-Mg calcite. Stable isotopic data for all the Cyprus calcites exhibit depletion in the heavier isotopes, 18O and 13C, indicative of meteoric alteration. The Koronia Member limestones show abundant indication of early lithification with fringing acicular aragonite and micrite cements. Stable isotopic data of the botryoidal aragonite cements indicate a marine origin. The dolomite is thought to result from marine—freshwater mixing and to be of Pliocene age, on the basis of stratigraphical, solution-replacement fabric and stable isotopic data. Sr isotopic data do not contradict a Pliocene age. Dolomite characteristically replaces the Pliocene fissure fill contents in southeast Cyprus and off-reef horizons in the Terra Member of west Cyprus and Koronia Member of north Cyprus. The growth and burial history of Miocene reefs contribute to the understanding of the tectonic history and palaeogeographical evolution of Cyprus.

Geologic History of Reefs: ABSTRACT

A reef, whether fossil or modern, is the physical expression of a community of calcium carbonate-secreting organisms which grew in one place for an extended period of time, forming either isolated structures or cores of complex buildups. These sea-floor highs are not only sites of rapid carbonate fixation and accretion but also locales of internal sedimentation, synsedimentary lithification, and active bioerosion--attributes which set them apart from most other sedimentary deposits. The core facies of large and complex fossil reefs generally illustrate a succession of growth stages, each of which is characterized by specific lithologies and invertibrate taxa. These stages, now recognized in reefs throughout the geologic record, are generally referred to as the pioneer (stabilization), colonization, diversification, and domination (climax) phases of reef growth. They record the transition from shoals of skeletal sand populated by small, rooted invertebrates and/or algae to thickets of branching or lamellar organisms to complexes made up of many different taxa with varied growth forms and life habits to a cap comprising only a few, generally lamellar to encrusting skeletons. Just as the development of any one reef is dependent upon inherently biological factors so the history of reefs in general reflects the evolution of Phanerozoic marine invertebrates. For a reef illustrating all stages to develop, a prerequisite is the existence of metazoans capable of secreting large skeletons of variable growth form. At those times in geologic history when only small, prone, branching sessile invertebrates occur, buildups called reef mounds are present that illustrate only the first two stages of growth. Thus there are times when no reefs occur, periods when reef mounds are the norm, and periods when complex reefs dominate. Although the attributes that characterize all reefs are present in the very earliest Cambrian bioherms, against the backdrop of geologic time, there are two general cycles of reef growth. Each begins with reef mounds which are subsequently populated by sponges and then gradually transformed into complex reefs by the appearance of corals and/or stromatoporoids. The first cycle begins in Early Cambrian time and culminates in the Late Devonian (240 m.y.); the second cycle begins in the Mississippian and has continued to the present (340 m.y.). During the early phases of these cycles reef mounds occur on or around shoal-rimmed platforms; in the latter stages reefs commonly form the rim facies, and control platform evolution. End_of_Article - Last_Page 1681------------