Carbonate Rocks • Carbonate rocks - limestones composed of calcite (CaCO3) or dolostones composed of dolomite (CaMgCO3). • Form through biological and biochemical processes and through inorganic precipitation from seawater • Carbonate rocks widespread and in every geological period beginning with Cambrian Environment • Most carbonate sediments form in warm waters • E.g., corals and algae common in carbonate rocks, and mostly found tropical to subtropical latitudes or from about 30o N and S latitudes • Most carbonate sediments generated in photic zone • Unlike sandstones, most carbonate grains formed in environment in which they are deposited • Detrital grains in sandstones usually derived from outside the environment in which they were deposited Environment Continued • Carbonate sediments do not usually occur below depths of about 3500 -4000 meters in the oceans • Below these depths carbonate sediments will dissolve • The depths at which carbonate sediments dissolve in the ocean is called the carbonate compensation depth or CCD Classification Scheme • Two principal classification schemes used for limestones. – Folk's classification scheme, named after Robert Folk, • Dunham's classification scheme, named after Robert Dunham • Folk's scheme based on presence or absence of allochems (carbonate particles or grains), matrix (limestone clay called micrite), and type of cement (spar is a cement composed of coarse xls) • Dunham's scheme based on texture of rock. i.e. whether rock contains matrix, whether carbonate grains float in matrix or whether they are in contact with each other, and whether rock has carbonate grains Dunham’s Classification • Based on whether have more or less that 10% grains • Whether rock is mud or grain supported. • Also if original components were bound together like coral. • Tells about transport history. Folk Classification • Carbonate rocks consits of: – Allochems—grains – Intersitial material—micrite or spar cement • Micrite is "lime mud", the dense, dull-looking sediment made of clay sized crystals of CaCO3. • Spar—clear to translucent carbonate cement. Micrite and Spar Folk Continued • Name is built up by stringing together all the allochem names in order from least to most abundant, and then adding the interstitial material name ("matrix" below for short). For example, a rock like this: Oolites + Fossils + Spar matrix = Oo bio sparite The name is written as one word, Oobiosparite. Another example (again allochems from least to most abundant): Pellets + Oolites + Fossils + Micrite matrix = pel oo bio micrite The name is written as one word, Peloobiomicrite. But what if there is both micrite and spar matrix? The system is the same; just list them from least to most abundant. Fossils + Spar matrix + Micrite matrix = bio spar micrite Carbonate Grains • ooids--Ooids are spherical grains formed by calcite precipitation around a nucleus such as a shell fragment, a quartz grain, etc. They are less than 2mm in diameter and typically form in shallow, warm, agitated, and carbonate-saturated waters such as those near the Bahamas. • pisolites--Pisolites are ooids greater than 2mm in size • Similar environments as ooids • stromatolites--Stromatolites are laminated carbonate sediments composed of mats of blue-green algae and layers of sediment. The algae, which forms the mats, is a plant and plants require sunlight to survive. Therefore, stromatolites generally form in warm shallow waters. The algae is "sticky" and it grows filaments. This sticky and filamentous algae traps sediment brought in by the currents. Consequently, a layer of algae forms then a layer of sediment is deposited on top of the algae. Subsequently, the algae grows through the sediment to form another mat and the cycle begins again. Ultimately, a layered rock composed of alternating algal mats and sediments is produced. Stromatolites form in quiet, hypersaline waters with little animal life around to destroy the mats. • oncolites-- Oncolites are stromatolites rolled in a ball. • corals--Corals have a symbiotic relationship with dinoflagellate algae called zooanthellae. Algae are plants. Therefore, in order for corals to survive they generally have to be within the photic zone (the zone of maximum light penetration in the ocean). Plants require sunlight for photosynthesis. In the process of photosynthesis, plants produce oxygen and consume CO2. Corals are major reef formers today and in the past. However, keep in mind that some corals do occur in deep waters. • Intraclasts--Semi- consolidated carbonate material ripped-up and incorporated in the rock. • dolostone--A dolostone is a rock compose of 90- Launch M icrosoft O ffice Outlook.lnk 100% dolomite.
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