NDLI: Calcite cement successions in Middle Devonian (Givetian) carbonate mud buildups of the southern Ahnet basin (Algerian Sahara)

Content Provider	Springer Nature Link
Author	Kaufmann, Bernd Wendt, Jobst
Copyright Year	2000
Abstract	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 δ$^{18}$O=−2.6 (±0.4)%. PDB and δ$^{13}$C=+2.6(±0.4)%. PDB are assumed to represent the Givetian stable-isotopic seawater signature of the Northwest-African carbonate province. The exceptional high δ$^{18}$O 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. The$^{87}$Sr/$^{86}$Sr 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 CO$_{2}$ 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 δ$^{18}$O 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. δ$^{13}$C 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 radiogenic$^{87}$Sr/$^{86}$Sr 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.
Starting Page	149
Ending Page	161
Page Count	13
File Format	PDF
ISSN	08912556
Journal	Carbonates and Evaporites
Volume Number	15
Issue Number	2
e-ISSN	18785212
Language	English
Publisher	Springer Netherlands
Publisher Date	2000-01-01
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Geology Mineral Resources Mineralogy
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology

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