Dolomite Perspectives on a Perplexing Mineral
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03 dolomite perspectives on a perplexing mineral
Marine-Meteoric Mixing Model—The strati-
graphic position, related fossil assemblages and lack of associated evaporite indicate that some dolomites do not form within a restricted-marine, supratidal setting. Instead, they are found in areas where Mg-rich saline waters mix with fresh meteoric water. Modern and ancient dolomite formations around the world support variations on this theme. One such variation is grounded in widespread dolomitic facies associated with shallow epicon- tinental shelves or structural highs, where— unlike the previous models—evidence of saline brine evaporation is not seen. 28 The origin of these dolomites is explained by the dorag model, in which dolomitization occurs in the brackish zone that forms when fresh groundwater is mixed with seawater. 29 In this zone, seawater supplies Mg 2+ ions, and dissolution of CaCO 3 occurs as the two waters mix. Calculations show that mixing meteoric groundwaters with 5% to 30% seawater can cause undersaturation with respect to cal- cite, while dolomite becomes supersaturated. Within this range, calcite can be replaced by dolomite. In general, the dolomitization process can be expressed by the chemical equation: 2CaCO 3 + Mg 2+ => CaMg(CO 3 ) 2 + Ca 2+ . This model is based on Mifflin carbonate outcrops of the Platteville Formation, in the Middle Ordovician Champlain Series of Wisconsin, USA. Here, the carbonates appear homogeneous over a broad area, and the thickness of the unit and general structure suggest a broad, shallow open-marine environment. The open-marine fossils, along with a lack of mud cracks, algal mats and evaporites, preclude supratidal deposi- tion and dolomitization in a physically restricted lagoonal environment. Dolomitization of the Mifflin Member was the result of a relatively early diagenetic process fol- lowing subaerial exposure of uplifted limestone and subsequent establishment of freshwater lenses. Dolomitization occurred in the brackish zone where seawater and fresh waters mixed, with a dolostone-limestone boundary established along the lower margin of the groundwater lens. 30 In a somewhat different mode, dolomite may be created through the circulation of saline groundwaters deep within a carbonate platform. In southern Florida, USA, cold, dense seawater is drawn through the platform margin from the deep Straits of Florida. Geothermally driven circulation causes the Mg-rich seawater to rise into the inte- rior of the Florida carbonate platform, where it mixes with fresh meteoric water before discharg- ing through an extensive aquifer system. The interplay of fresh and saline waters with geothermal heat flow is known as Kohout convection. 31 In this scenario, the resulting pore waters become undersaturated with respect to calcite and aragonite but still saturated with respect to dolomite, which is precipitated in the permeable aquifers. Another environment for mixing of fresh and saline waters is found along the coastal plains of southeastern Australia. From the present, and extending throughout the Quaternary Period, microcrystalline dolomite and other carbonate minerals have been forming in shallow ephemeral lakes of the Coorong region. These lakes develop along a 100-km [62-mi] belt, in an interdune cor- ridor located immediately inland from the present coastline, behind a calcareous sand barrier. The lakes are considered to be outcrops of the water table, and free water, resulting from rainfall and regional or local aquifer recharge, is found at their surface only during winter and spring. 32 Modern dolomite is found only in lakes sub- jected to an annual desiccation phase. Those lakes occur mainly in areas receiving less than 500 mm > Sabkha reflux environment. This schematic of peritidal sediments on a Qatar peninsula sabkha shows another variation on the reflux theme. Seawater is pushed onshore during storm surges, becomes concentrated through evaporation, then seeps into the underlying sediment to reflux to its source. (Adapted from Warren, reference 2.) MattV_ORAUT09_Fig_9 Subtidal Water level Intertidal Supratidal High water Low water Evaporation Seepage reflux Storm flo od 26678schD5R1.indd 7 11/5/09 3:53 PM Autumn 2009 39 [19.7 in.] of rainfall per year, and typically fill to a water depth of 0.5 to 1 m [1.6 to 3.3 ft]. When filled, these lakes have a carbonate-mud bottom that contains algae and other organic matter. As lake levels fall, the waters become increasingly saline before eventually exposing the mud bottom to sunlight and consequent desiccation. Ensuing brines form during the drying phase and are refluxed out of the system into seaward-flowing groundwaters. Fine-grained dolomites and other carbonates remain behind, while saline and sul- fate evaporite minerals are flushed out of the sys- tem. This dolomite is thought to precipitate from a carbonate gel suspension, not through replace- ment of a preexisting carbonate. The dolomites in this system accumulate above shallow continental groundwaters that flow toward the sea. During their coastward migration, the groundwaters traverse large volumes of predomi- nantly carbonate aquifer sediments. The source of the Mg is poorly understood but is believed either to be supplied by a local Quaternary volcanic prov- ince or to be scavenged by groundwater flow from other sources. Download 2.33 Mb. Do'stlaringiz bilan baham: |
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