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

Abstract

In the Ahnet Basin of southernAlgeria, carbonate mud buildups were establishedin a deeper subtidal environmentduring Givetiantimes. The primarycavity system(irregularcavities,stromatactis) of thesebuildupsexhibitsa calcitecement succession precipitated in marine-phreatic to deep-burialenvironments.Cementationstartedin the marine environment. Cloudy,radiaxial-fibrouscalcites(RFCs)of originallyhighMg-calcitecomposition coat the walls ofthe cavitiesand occupy 50-80%ofthe pore space. The carbon-and oxygen-isotopic signatureof these cements correspondsto that of co-occurring, nonluminescentbrachiopodshells and to that of coevalRFCs from the Mader Basin of Morocco. Average valuesof 8 180 = -2.6 (± 0.4)%0 PDBand 8 13C = +2.6 (± 0.4)%0 PDB are assumedto representthe Givetianstable-isotopicseawatersignatureof theNorthwestAfricancarbonateprovince. The exceptionalhigh 8 180 values,comparedto MiddleDevoniandata ofNorth America,are interpretedto result from the mid-latitudinal (about 40 0 S), colder-water settings of the Northwest-African carbonate buildups. The 87Srj86Sr ratios of RFCs (0.70802±17)and nonluminescentbrachiopodshells (0.70780±3) fit well into establishedSr-isotopiccurves of Devonian seawater.Cementation in the shallow-burial realm is characterizedby 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 influenceon this cement sequencecan be excluded.Deep-burialcementationfollowinga ratherlongperiodofnon-cementation is representedbystrong-ferroan, dull-luminescentblockycements. Alternatively to pressure solution, outgassingof CO 2 by decreasingpore-fluidpressure caused by Variscan fracturingcan be consideredas carbonatesourcefor ferroancalciteprecipitationin the remainingpore space. Strong8 18 0 depletion(-10.6to -14.8%0 PDB) of thesecements point to the influenceof hydrothermalfluidsrather than to a pure depth-related temperaturecontrol. 8 13 C values show a trend from a nearly marinesignatureto more negativevalues(upto -9.0%0 PDB)which suggeststhe inputof methaneby hydrocarbonmigrationduringVariscan compression. More radiogenic8 7 Srj86Sr ratios(0.70826to 0.71684)with respectto the marine compositionare probably causedby the input of radiogenicstrontiumfrom K-feldspardissolutionand/or clay recrystallization from adjacent siliciclasticunits.

Mesozoic (Lower Jurassic) red stromatactis limestones from the Southern Alps (Arzo, Switzerland): calcite mineral authigenesis and syneresis-type deformation

“…Indeed, and as confirmed with the application of advanced geochemical methods such as carbon and oxygen isotope (e.g. Boulvain 1993Boulvain , 2001Bourque and Raymond 1994) and strontium isotope chemistry (e.g., Kaufmann and Wendt 2000), or rare earth element patterns (this paper), Bathurst's (1982) view that stromatactis remains to be explained in terms of early diagenesis under normalmarine conditions stands untouched.…”

Middle Devonian (Givetian) coral-stromatoporoid reefs in West Sahara (Morocco)

“…As already discussed in Kaufmann (1997) and Kaufmann and Wendt (2000), the mean oxygen-isotope values are about 1.0& heavier than even the heaviest d 18 O values from North America (d 18 O = À3.7&, compare Popp et al, 1986). According to Friedman and O'Neill (1977), such a shift would correspond to 5°C lower seawater temperatures of the Northwest-African carbonate province.…”

“…Additional data allowing a more precise calibration of the sections and the temporal evolution of individual reefs (Kaufmann, 1997) and from the Ahnet Basin of Algeria (Kaufmann and Wendt, 2000), respectively. is a challenge for the future which depends largely on an easier accessibility of the entire area.…”

“…Stromatactis cavities of the Uein Terguet mud mounds (1st reef cycle) are largely occluded by radiaxial-fibrous calcites (RFCs) which are regarded as altered marine cements of originally high Mg-calcite composition. Their petrographic features, including cathodoluminescense, correspond to those of Givetian mudmounds in the Mader Basin (eastern AntiAtlas) of Morocco (Kaufmann, 1997) and in the Ahnet Basin of southern Algeria (Kaufmann and Wendt, 2000). Only 9 samples from the first reef cycle (mud mounds) have been available for the analysis of the C-and O-isotopic composition.…”

Stromatactis cavities in sediments and the role of coarse-grained accessories

“…After an initial stage of calm cavity microenvironments (e.g., Dieken 1996, Aubrecht et al 2002a, the early diagenetic creation of pipes and cracks in the sealing layers (compare Hladil 2005b, Fig. 11) commonly allowed the circulation of marine pore water and relatively fast cementation (e.g., Kaufmann & Wendt 2000). This partial breakage caused by the aging of the sediment lead to the deposition of the next internal sediment.…”