Calcite cement successions in Middle Devonian (Givetian) carbonate mud buildups of the southern Ahnet basin (Algerian Sahara)
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In the Ahnet Basin of southern Algeria, carbonate mud buildups were established in a deeper subtidal environment during Givetian times. The primary cavity system (irregular cavities, stromatactis) of these buildups exhibits a calcite cement succession precipitated in marine-phreatic to deep-burial environments.
Cementation started in the marine environment. Cloudy, radiaxial-fibrous calcites (RFCs) of originally high Mg-calcite composition coat the walls of the cavities and occupy 50-80% of the pore space. The carbon-and oxygen-isotopic signature of these cements corresponds to that of co-occurring, nonluminescent brachiopod shells and to that of coeval RFCs from the Mader Basin of Morocco. Average values of δ18O=−2.6 (±0.4)%. PDB and δ13C=+2.6(±0.4)%. PDB are assumed to represent the Givetian stable-isotopic seawater signature of the Northwest-African carbonate province. The exceptional high δ18O values, compared to Middle Devonian data of North America, are interpreted to re from the mid-latitudinal (about 40°S), colder-water settings of the Northwest-African carbonate buildups. The87Sr/86Sr ratios of RFCs (0.70802±17) and nonluminescent brachiopod shells (0.70780±3) fit well into established Sr-isotopic curves of Devonian seawater.
Cementation in the shallow-burial realm is characterized by a typical non-bright-dull cathodoluminescence sequence of nonluminescent scalenohedral cements (dog-tooth cements) with bright-luminescent outer margins overgrown by slight-ferroan, moderate-luminescent cements. Meteoric influence on this cement sequence can be excluded.
Deep-burial cementation following a rather long period of non-cementation is represented by strong-ferroan, dull-luminescent blocky cements. Alternatively to pressure solution, outgassing of CO2 by decreasing pore-fluid pressure caused by Variscan fracturing can be considered as carbonate source for ferroan calcite precipitation in the remaining pore space. Strong δ18O depletion (−10.6 to −14.8%. PDB) of these cements point to the influence of hydrothermal fluids rather than to a pure depth-related temperature control. δ13C values show a trend from a nearly marine signature to more negative values (up to −9.0%. PDB) which suggests the input of methane by hydrocarbon migration during Variscan compression. More radiogenic87Sr/86Sr ratios (0.70826 to 0.71684) with respect to the marine composition are probably caused by the input of radiogenic strontium from K-feldspar dissolution and/or clay recrystallization from adjacent siliciclastic units.
KeywordsCalcite Middle Devonian Calcite Cement Cement Succession Carbonate Buildup
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