562 SEDIMENTARY ENVIRONMENTS/Reefs (‘Build-Ups’) Reefs (‘Build-Ups’) B W Sellwood, University of Reading, Reading, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Reefs are biologically generated accumulations of carbonate (see Sedimentary Rocks: Limestones) that grow upwards from the adjacent seafloor and are entities of their own making They are generally considered to be robust structures (i.e., rigid or wave-resistant), although biogenic mud mounds are treated as reefs by many authors There have been periods in the geological past when the main contributors to biogenic mounds appear to have been organisms lacking a rigid frame, and their constructions have had lime mud (matrix-rich) compositions Thus a wide spectrum of structures have been loosely classified as reefs It has therefore generally been considered useful to draw a distinction between reefs with a rigid calcareous frame and those without (i.e., mud mounds) As communities growing on the accumulated debris of their own forebears, reefs are essentially unbedded constructions The debris derived from the reef-dwellers fills the interstices between in situ constructors and also forms aprons of bedded resedimented material on the flanks of the edifice, and in adjacent hollows Such aprons provide critical evidence that biogenic activity produced an elevated mound in the first place, and may indicate that upward growth of the mound had taken place into shallow waters, where strong currents and waves were active Growing reefs are often strongly zoned ecologically and usually have a core, the main carbonate factory within the system, showing a distinct vertical succession from initial colonization through to culmination The vertical zonation can frequently be related to upward growth in response to the filling of accommodation space (i.e., formed in response to ‘transgression’ with the bulk of the reef pile accumulating during a ‘highstand’) Reef successions are often stacked one on top of the other, separated by lowstand emersion surfaces (often ‘karstic’ in character) (‘framestone’(Figure 2)), sheeted encrustations generate ‘bindstone’(Figures and 4), whereas meadows of upstanding but flimsy (generally noncoral) organisms may passively trap fines to form mud mounds (‘bafflestone’ after lithification (Figure 5)) Attached Figure Modern patch reefs with intervening grass anchored carbonate sands as seen from the air Great Bahamas Bank, field of view approximately km across Modern Reefs Modern reefs (Figure 1), dominated by scleractinian corals (see Fossil Invertebrates: Corals and Other Cnidaria), reflect a balance between growth and destruction Coral growth produces a rigid framework Figure Framestone with in situ coral heads and unbedded character Key Largo Limestone, Quaternary, Florida, USA Height of outcrop about m