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Carbonates and Evaporites

, Volume 15, Issue 2, pp 149–161 | Cite as

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

  • Bernd Kaufmann
  • Jobst Wendt
Article

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 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.

Keywords

Calcite Middle Devonian Calcite Cement Cement Succession Carbonate Buildup 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer 2000

Authors and Affiliations

  • Bernd Kaufmann
    • 1
  • Jobst Wendt
    • 1
  1. 1.Geologisch-Paläontologisches InstitutUniversität TübingenTübingenGermany

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