Carbonates are responsible for Karst landscapes and caves environments due to their solubility in slightly acidic water.

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3 Primarily composed of carbonate minerals, carbonate rocks demonstrate 2 main types 1. Limestone - composed of calcite or aragonite 2. Dolostone - composed of the mineral dolomite (CaMg(CO 3 ) 2 ). Carbonates are responsible for Karst landscapes and caves environments due to their solubility in slightly acidic water. 3

4 Limestones are mostly composed of calcium carbonate in the form of either calcite and aragonite. Limestones are commonly (but not exclusively) composed of the skeletal fragments of organisms. Of all sedimentary rocks limestone makes up about 10%. Aragonite is in the orthorhombic system. Almost all mollusk shells and corals (ScleracLnia) use aragonite for their hard parts. Aragonite is thermodynamically unstable and alters into calcite over Lme. At C aragonite changes into calcite. 4

5 Calcite is in the trigonal-rhombohedral system but rhombohedra are generally rare. The two common forms of calcite are low magnesium calcite and high magnesium calcite. Organisms that secrete Low Mg Calcite include brachiopods, trilobites and echinoderms (sea urchins, starfish, crinoids etc). In order of stability low magnesium calcite is the most stable form of calcium carbonate in limestones followed by high magnesium calcite and then aragonite. Over Lme most limestones will be altered to Low Mg Calcite. 5

6 CaCO 3 can also be precipitated inorganically with the the type of calcite being deposited in the oceans varying over Lme. In calcite seas low-magnesium calcite in the main precipitate. In Aragonite seas aragonite and high-magnesium calcite is precipitated. Calcite seas were common during the Early Paleozoic and the Middle to Late Mesozoic. Middle Paleozoic and Cenozoic Lmes (including today) are generally Aragonite seas. Calcite seas generally form during Lmes when the Earth was warmer than today with Aragonite seas more common during cooler periods. 6

7 Calcite seas tend to be common when seafloor spreading is aclve and global greenhouse climate condilons prevail. The reason: increased cycling of seawater through hydrothermal vents As the seawater passes through the hot basallc ocean crust calcium rich minerals are converted into magnesium-rich clays via hydrothermal metamorphism. This reduces the Mg/Ca ralo of the sea water returning to the oceans favoring the produclon of calcite over aragonite. The associalon of calcite seas with greenhouse condilons also relates to increased spreading rates which would also be related to increased volcanism and increased CO 2 delivery to the atmosphere. Warm condilons and warmer oceans also favour the precipitalon of calcite over aragonite. 7

8 These are the basic chemical steps of the precipitalon of calcite an important part of the carbon cycle. 8

9 In addilon to the catalyzing aclvity of organisms, the concentralon of dissolved CO 2 is a very important factor in calcium carbonate precipitalon. Reduced CO 2 concentralon means you have lower produclon of carbonic acid (which dissolves calcite) and higher CaCO 3 precipitalon. CO 2 concentralon is effected in 3 principle ways: 1. At lower pressures CO 2 is released from water. As a result lower pressure (shallower condilons) will be more conducive to the precipitalon of calcite. 2. CO 2 has lower concentralon in warm waters. 3. Increased water agitalon leads to diffusion of CO 2 to the atmosphere, making condilons more hospitable to CaCO 3 deposilon Overall therefore, high temperatures, lower pressure & breaking waves favor carbonate precipitalon 9

10 AccumulaLon and precipitalon of CaCO 3 below 5000 meters is uncommon. The depth below which it cannot accumulate is referred to as the Carbonate CompensaLon Depth (CCD), which varies between m depending on water temperature. At depth more CO 2 is dissolved in the water making the ocean more acidic (carbonic acid see equalon on figure above) which dissolves the CaCO 3. The posilon of the sea bed relalve to the CCD therefore will determine which type of sediment can be deposited. If above the CCD calcareous oozes can form which when lithified form a type of fine grained limestone we know as chalk. It is possible to find calcareous oozes in oceans deeper than the CCD if a layer of clay or siliceous ooze is deposited on top of calcareous oozes that form on a ridge that is above the CCD. As the ocean floor moves away from the ocean ridge it cools and falls below the CCD (see diagram) any sediment that accumulated at the ridge (above the CCD) and capped by the clay or siliceous oozes will be protected in the deeper ocean. 10

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13 Modern limestones are mostly deposited in reefs, carbonate sand bodies close to reef and in lagoons. In the past (especially during warm greenhouse periods) limestones were more common due to warmer condilons and greater area of shallow shelf seas. 13

14 Allochems are equivalents of grains in claslc sediments. Material between the allochems is equivalent to the matrix or cements in claslcs. 14

15 Skeletal grains are any biological component of a limestone. The character of the skeletal grains with vary depending on the environment AND geological period of deposilon of a rock. 15

16 For example crinoids (stalked echinoderms) were very important components of Devonian and Carboniferous shallow marine segngs and significant limestone producers. They are much less important today and generally restricted to deep marine segngs in relalvely low numbers. You can see some of the important reef producers in this figure - no, you don t have to learn all of this figure for the exam. 16

17 a) Ooids = < 2 mm in diameter and spheroidal. They are referred to as "coated sedimentary grains meaning they are concentrically layered. They are most commonly composed of calcium carbonate, but can also be formed by iron or phosphate-based minerals. Environment of deposilon - usually in shallow tropical seas (eg Bahamas or Persian Gulf). Pisoids are ooids >2 mm in diameter. 17

18 Ooid form as concentric layers of material are precipitated around a nucleus such as a shell fragment or grain of sand. Today, most ooids are composed of aragonite. Most ancient ooids are composed of calcite forming either by direct precipitalon during periods of calcite seas or via the alteralon of ooids that were originally aragonite. The formalon of an ooid is thought to require the elevalon of the saturalon of calcium carbonate increases this can occur when water is warm and / or agitated which will drive off CO 2 and help precipitate calcite. 18

19 b) Oncoids - spherical concentrically layered coated grains formed by photosynthelc cyanobacteria that grow around a nucleus such as a shell fragment. Oncoids, like stromatolites are layered associalons of cyanobacteria but form spherical bodies rather than columns or domes of stromatolites. As oncolites form from photosynthelc cyanobacteria they form within the pholc zone (depth to which sunlight can penetrate to the ocean floor). Oncolites rarely exceed 10 cm in diameter. 19

20 c) Peloids are thought to be the fecal pellets (poop) of organisms. If they occur in high concentralons and become a liple squashed they can give the impression of a rock composed enlrely of micrite (carbonate mud). 20

21 d) Intraclasts are irregularly-shaped allochems. Form by erosion of parlally lithified sediment which is then re-deposited as irregularly shaped grains (syndeposilonal). e) Limestones may also contain non carbonate rock fragments, quartz and clays but colleclvely these must be less than 50% of the rock if it slll to be classified as a limestone. 21

22 Micrite = CaCO 3 mud composed of clay sized crystals. In todays oceans micrite commonly forms as calcareous algae skeletons breakdown. The exact mode of formalon of ancient micrite is uncertain. 22

23 We will deal with limestone cements in more detail during the lecture on diagensis. 23

24 Developed by Robert L. Folk uses a descriplon of the three main components of carbonate rocks: allochems (grains), matrix (mostly micrite), and cement (sparite). A two part naming system is used the first name refers to the grains and the second refers to the matrix or cement. The folk scheme is commonly used when describing thin seclons of carbonate rocks. You can have more than one type of allochem named in the Folk classificalon. EG: Oolites + Fossils + Spar matrix = Oo bio sparite = Oobiosparite. Pellets + Oolites + Fossils + Micrite matrix = pel oo bio micrite = Peloobiomicrite. The system can also be used if there is both matrix and cement EG: Fossils + Spar matrix + Micrite matrix = bio spar micrite = Biosparmicrite 24

25 The Dunham system classifies using deposilonal textures. Rocks are divided into four main groups based proporlons of allochems and whether or not they were originally in contact with each other. If not originally in contact = mud-supported. If originally in contact = grain supported. As the Dunham scheme deals with texture rather than grain composilon it is more useful when describing hand samples. - Mudstone: <10% grains supported by a lime mud. - Wackestone: >10% grains, supported by a lime mud. - Packstone: contains lime mud, but is grain supported. - Rudstone: coarse limestones supported by grains >2 mm. - Grainstone: lacks mud, and is grain supported. - Boundstones: original components have been bound together arer deposilon such as in a coral reef 25

26 Limestone deposilon has a complex and varied history. Will here provide a VERY brief overview of some of the environments of deposilon of limestone and some of the main limestone types that characterize them. 26

27 Although carbonates can form in a number of terrestrial environments, we will mostly concentrate on the lacustrine environments here. i. Inorganic carbonates: limestones (micrites) precipitated by loss of CO 2 due to evaporalon (water warms) in a shallow environment (low pressure). Fresh water entering into a saline lake can also help precipitate carbonates. If the water is agitated, ooids may also form. ii. Algal carbonates. Can form when you get blooms of calcareous algae in a lake but also may form in stromatolites as you can see in the image above iii. Ooids and skeletal sands. Fragments of algae or larger carbonate secrelng organisms like mollusks. A common facies papern in carboante precipitalng lakes would be stromatolite reefs in shallow areas (with ooids and skeletal sands in the more agitated area) passing into finer micrites in the deeper / quieter segngs 27

28 Ancient marine carbonate environments were much more extensive than they are today. In general though carbonate facies paperns can be divided into a broad papern of shore (Ldal flat in the above) platorm or shelf (both deep and shallow) basin. 28

29 Intermipently covered with water with week currents and wave aclon. Sediments are micrite dominated and may contain pellets. The sediments in these environments may exhibit fenestrae, gaps within the sediment that may represent desiccalon and shrinkage of material with the sediment or the trapping of gas bubbles. These spaces are then filled wither with sediment or cement. Mud cracks may also be a feature of these sediments. The fossils in these environments will generally be of restricted diversity as Ldal environments are a stressful environment to inhabit given daily changes in water level and potenlally salinity. In part due to this (as there will be fewer grazing organisms) stromatolites and algal mats may be common. In very arid environments, intense evaporalon may permit the development of evaporites in this environment. In addilon, this evaporalon may draw water through the sedimentary pile and cause the dolomilzalon of some of the calcium carbonate (See later). 29

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31 Similar lithologically to the previous environment but without evidence of exposure to the air. Green calcareous algae are the main sediment contributors in this environment. Such lagoonal limestones are common in the geological record. 31

32 Occur in areas of high Ldal aclvity and current / wave aclon. Depth of deposilon generally less than 10m forming beaches and shoals composed of ooids and rounded skeletal fragments. 32

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34 These areas are generally poorly developed today when compared to the geological past when extensive shallow (epeiric) seas where more common. Modern examples would include the Yucatan shelf and the eastern Gulf of Mexico. Carbonates in this environment are deposited in standard salinity / well oxygenated condilons. The sediments change in character from more grainstone towards the shoreline to more micrite rich ocean-ward. These sediments are commonly heavily bioturbated. The skeletal components are diverse. Patch reefs may be common, parlcularly towards the outer shelf area. 34

35 Reef is a term that is used in many contexts. For us we will consider a reef to be a topographic wave resistant structure. Reef rocks are commonly classed as biolithotes, framestones and bindstones and may contain considerable primary porosity in the form of large caviles represenlng the complex structure of the framework of a reef. These caviles may be filled with debris and cement. This potenlal porosity and permeability makes them great petroleum reservoirs. In front of this, bioclaslc material broken off from the reef will be deposited down the reef as talus. In this area rudstones and floatstones may be found associated with slump structures. It is also in this area that carbonate turbidites may be generated that pass down the fore-reef and into the basin. 35

36 Today corals and coralline algae are the most common reef producing organisms. In the past the main reef producing organisms have been very different. During the Precambrian and Cambrian stromatolites were the main reef producers. Stromatoporoids ( a kind of sponge), rugose and tabulate corals are characterislc of lower paleozoic reefs see the figure earlier in this lecture for more informalon. 36

37 Reefs can roughly be broken down into 3 zones: 1. Fore Reef and reef crest: The steep slope of the fore reef can be almost verlcal with organisms construclng the reef at the reef crest. Tallus will fall from the reef and be deposited downslope somelme in the form of turbidites. 2. Reef Flat: No more than 1 2m of water depth, Major area of growth of reef organisms 3. Back Reef: Will receive some debris from the reef flat and pass into lagoon deposits 37

38 The following taken from: hpp:// geologicformalons.htm Through much of the early and middle Permian much of modern-day New Mexico and Texas occupied the western edge of Pangaea near the equator. A vast ocean surrounded Pangea, but a narrow inlet, the Hovey Channel, connected the ocean with the Permian Basin, an inland sea which covered parts of what is now northern Mexico and the southwestern United States. The Permian Basin had three arms: the Marfa, Delaware, and Midland Basins. The middle arm (the Delaware Basin) contained the Delaware Sea which covered and area 150 miles long and 75 miles wide over what is now Western Texas and Southeastern New Mexico. For several million years the Capitan Reef expanded and thrived along the rim of the Delaware Basin unll events altered the environment crilcal to its growth approximately 260 million years ago. The outlet conneclng the Permian Basin to the ocean became restricted and the Delaware Sea began to evaporate faster than it could be replenished. Evaporites began to precipitate out of the vanishing waters and drir to the sea floor, forming thin, alternalng bands of mineral salts and mud. 38

39 Pelagic sedimentalon takes over in these systems. Too deep for significant benthic (bopom dwelling) organisms to thrive. May get some tallus and trubidites closer to the reef front. Lower limit of deposilon of sediment set by the CCD. 39

40 Modern carbonate pelagic organisms are pteropods, coccoliths and formainifera. 40

41 Good example of extensive pelagic limestone produclon: Cretaceous chalk seas. Perfect for pelagic carbonates: warm climalc condilons, shallow seas flooded conlnents providing extensive shelf environments far from any claslc sources. 41

42 Dolomite: calcium magnesium carbonate CaMg(CO 3 ) 2. Rocks composed of dolomite = dolostones. The proporlon of dolomite in a sediment will determine its overall classificalon: Limestone: 0 10% dolomite in the rock DolomiLc limestone: 10 50% dolomite CalciLc Dolomite: 50-90% dolomite Dolomite: % dolomite 42

43 Although the possibility of primary sedimentalon of dolomite has been proposed it is mostly regarded as forming as the alternalon of lime (CaCO 3 ) sediments during early diagenesis or the alteralon of limestones in deep burial segng. In this image, diamond-shaped crystals of dolomite have grown arer deposilon, during diagenesis. They replace the fine calcium carbonate mud (dark material in the photo) that makes up the rest of the rock. Because of the coarse nature of dolomite crystals much of the original textures of limestones are lost during dolomilzalon. 43

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