Content
Contact
Email
Thilo Bechstädt (Info)
Rainer Zühlke (Info)
Phone
+49 6221 54-8292
+49 6221 54-6055
Fax
+49 6221 54-5503
Thilo Bechstädt (Info)
Rainer Zühlke (Info)
Phone
+49 6221 54-8292
+49 6221 54-6055
Fax
+49 6221 54-5503
Project Login
Projects / Fundamental Research / Diagenesis and Fluid Flow / Late Burial Dolomitization, Láncara Fm., NW Spain
Late Burial, Hydrothermal Dolomitization of the Cambrian Láncara Fm. (Cantabrian Zone, NW Spain): Origin of the Dolomitizing Fluids and Relation to the Geodynamic Setting |
||
| Abstract | ||
 |
||
| Scientists Fabio Lapponi, University of Heidelberg (now: Statoil) Thilo Bechstädt, GeoResources and University of Heidelberg Ronald Bakker, University of Leoben |
||
| Abstract Several petrographic and geochemical methods have been applied in order to distinguish and characterize the late epigenetic dolomitization event occurring in the Cambrian Carbonate succession of the Láncara Formation, Cantabrian Zone, NW Spain. The contact between dolomitized and undolomitized lithologies is very sharp in the outcrops, usually crosscutting the bedding surface. The dolomitization affects mostly the calcareous members of the Láncara Formation (glauconitic packstone, birdseye limestone). Rarely, the stromatolitic dolostone, which forms the upper part of the lower member of the Láncara Formation, is also replaced by the epigenetic event. At the hand specimen scale, two different generations of dolomite can be easily distinguished: a matrix-replacive dolomite (Dol A), which is usually grey to beige in colour, and a void-filling coarser, white saddle dolomite (Dol B). Usually, Dol B is volumetrically less important relative to matrix-replacive Dol A, but locally it may reach 30-35 % of the dolomitized bodies. The transition between the matrix-replacive Dol A and the void-filling Dol B is gradual, never corresponding to a reaction boundary. The polimodal grain size distribution allows subdivision of Dol B in different sub-generations, with the last one (Dol B II) consisting of a clear inclusion-free rim. In CL, Dol B is characterised by the same red to red-dull luminescence of Dol A, with the exception of the last Dol B II generation (the clear rim), which is non luminescent. Millimetre-scale pores are abundant in the dolomitized bodies whereas they are absent in the undolomitized lithologies. This dolomite generation has a burial origin, as indicated by coarse cristallinity, abundant saddle dolomite cement, zebra structures, high Fe-Mn concentrations and low Sr concentrations, low δ18O, radiogenic 87Sr/86Sr and fluid inclusions TH between 80 and 120°C. Crush-leach analyses reveal that the dolomitizing fluids are highly evaporated seawater, which has been modified through water-rock interaction only at a minor degree. For comparison, crush-leach tests were performed on samples from the dolomitized Namurian and Westphalian limestones. The data reveal that a possible coeval fluid flow with similar composition affected both the Cambrian and the Carboniferous carbonate successions, favouring an external fluid source, which flooded both rock types. This fluid flow event affected the Cantabrian Zone presumably during the late stages of the Variscan orogeny, indicated by indirect geological evidence like temporal relationship with dated ore minerals and main Variscan structures. This period was characterized by an extensional tectonic regime and by a high geothermal gradient, which might have favoured the onset of convective cells in the study area. |
||
| Publications Lapponi. F., Bechstädt, T., Boni, M., Banks D. and Schneider, J., submitted, Burial dolomitization in the Cantabrian Lancara Formation, Cantabrian Zone: Sedimentology. Lapponi, F., Bakker R., and Bechstädt T., 2007, Low temperature behaviour of natural saline fluid inclusions in saddle dolomite (Paleozoic, NW Spain): Terra Nova, v. 19/6, p. 440-444. |
||
| Funding Organizations German Research Fund (DFG) |
||
