Diagenesis of carbonatesGEOL 342: Sedimentation and StratigraphyLecture 14: carbonate environments5 April 2005Assoc. Prof. A. Jay KaufmanCarbonate environmentsCarbonate platforms have often been considered as “factories” due to the rapid buildup of carbonate in appropriate warm and shallow marine environments. Carbonate factories frequently shallow to the surface where they maybe exposed due to sea level fluctuations on various time scales, resulting inerosion and karstification. To continue carbonate sedimentation sea levelwould have to rise (or the platform subside), or the facies would have tomigrate out to sea.Material from the carbonate factory is constantly redistributed aroundthe platform and off into the abyssal deep. Facies are subdivided into depth-restricted zones including 1) supratidal, 2) peritidal, and 3) subtidalenvironments where there are extreme variations in water depth, salinity,and organisms.SupratidalSupratidal refers to environments that are generally above the tidal rangeand are thus wet with seawater only during storm events. These areotherwise dominated by brackish or fresh water sources. A typicalenvironment would be a marsh where it is likely to find high abundances oforganic matter and where coal is likely to form. The organisms that inhabitthis environment must be able to tolerate brackish conditions, and underextreme evaporitic conditions may be absent altogether.1PeritidalPeritidal environments commonly form in the tidal range and hence aredaily exposed to seawater. Deposits are laterally persistent, evenly beddedlimestone and dolomite that is often preserves flat laminations andshallowing upward cycles that are often repeated to great thickness. Thisenvironment is seen in modern and ancient tidal flats similar to those in thePersian Gulf.Due to the daily alternation of the tides peritidal environments oftencontain carbonate (dolomite) mudflats, evaporites, herringbone x-stratification, shell debris, and rip-up clasts or plate breccias, mud cracks andalgal mats. Fenestral (sheet like) porosity or birds-eye structures are causedby the dissolution of evaporites or from voids originally filled with gas.SubtidalIn our own backyard the Bahama Banks is an excellent laboratory of asubtidal carbonate factory. Subtidal environments with normal marine waters2and a limited range of salinities produce thick carbonate blankets that can beup to 1000s of meters in thickness, depending on the rate of subsidencealong the continental margin. These muddy subtidal environments have a marine fauna which ismore diverse than in peritidal environments, and sediments are oftenbioturbated.Subtidal environments can have a wide distribution of facies associated with both water depth and wave energy. 1. reefs in high energy environment as well as ooid and skeletal shoals2. mud and pellet carbonate sands in protected lagoonal environments3. grapestones where ooids congregate and cement together before being coated againRepetitive changes in sea level cause cyclic deposition on carbonate platforms, which canbe readily identified in the field. For subtidal facies, relative water depth may be inferred by the relative abundance of shaley interbeds and the thickness of carbonate layers.3ReefsReefs are sediment systems built entirely from the organisms that call it a home. It is a wave resistant framework. Modern reefs primarily exist in oligotrophic environments and these rival the rainforests for biodiversity. Reefs, which form at the edges of carbonate banks, can be excellent oil traps.The architects of reefs (framework builders) include scleractinian coral,coralline algae, bryozoans and sponges, but in the past even microbial matscould built up reefs. However, framework builders are generally only 10% ofthe total volume of the reef, the remainder is composed of skeletalfragments, micrite, breccia and cements, which fill in the interstitial spaces ofthe reef frameworkCorals are tiny marine animals (polyps) which live in small cone-like cells, commonly inwarm, tropical waters. The animals have tentacles to assist feeding, and may seal the end of theircells with an operculum (lid). They often live in colonies, behaving either independently asindividuals or with a degree of specialization of function so that the whole colony operates, tosome extent, as an organism. Their skeletons often accumulate in vast quantities, sometimes asreefs, which may become consolidated as various types of limestone. There are many hundredsof different living species-700 alone in the Indo-Pacific region, and similar numbers of extinctspecies. Two extinct types of corals which are frequently preserved in limestones are the rugoseand the tabulate corals, both of which arose in the Ordovician Period (434 to 490 million yearsago) and became extinct at the end of the Permian Period (251 million years ago).4Thus largely due to mass extinction, the types of framework builders inreefs have changed through time.Diagenesis of carbonatesDue to their solubility, evaporites and carbonates can be readilymodified by diagenetic processes (including anything that happens afterdeposition of the sediment). Dissolution of these minerals creates porosity,which might result in a hydrocarbon trap. Selective dissolution in carbonatescreates some interesting textures, including fenestrae, oomoldic porosity, andstylolites. Stylolites are jagged irregular seams dividing limestone into twoparts that are interpenetrating “tooth and socket” textures usually due topressure solution (dissolution perpendicular to the plane of the stylolite).Carbonates may be rapidly cemented, especially in coastalenvironments, as a result of rapid changes in water chemistry. Beachrock,5for example, forms quickly due to the evaporation of seawater on beachesthat result in a loss of CO2. Shallow subtidal areas with high rates ofevaporation also result in hardgrounds, which are unconformities formedduring times of non-depositionVarious types of carbonate cement have been observed and described,including drusy, blocky, and rim types.Syntaxial – if cement is in optical continuity with the pre-existing grain (goes to