GeoResources - Fluid Evolution in the La Vid Group during Burial Diagenesis and Subsequent Uplift, Cantabrian Zone, N-Spain


	Fluid Evolution in the La Vid Group During Burial Diagenesis and Subsequent Uplift (Cantabrian Zone, Northern Spain)
	Abstract





	Scientists Jochen Schneider, Hydroisotop, Munich, formerly University of Heidelberg Thilo Bechstädt, GeoResources and University of Heidelberg Ronald Bakker, University of Leoben Hans Machel, University of Alberta, Edmonton

	Abstract The Lower Devonian La Vid Group (Cantabrian Zone, Northern Spain) forms the first carbonate-clastic interval in the Devonian succession. The succession was deposited in a rifted continental margin setting, which in the Carboniferous evolved into a foreland basin, affected by Variscan thin-skinned folding and thrusting. Soon thereafter the originally linear orogenic belt underwent secondary curvature, forming the Variscan Ibero-Armorican Arc. This bending caused extension, crustal thinning and strike-slip movements in the outer part of the orocline, where the area of our investigation belongs to. Subsequently the Variscan orogen subsided in Meso- and Cenozoic time, was covered by sediments of unknown thickness and was affected by Alpidic tectonics. The Paleozoic Cantabrian Basin is an outcrop analogue for km-scale differences in type and degree of porosity and cementation that can be expected in foreland basins of other areas as well. Origin of cements and related fluids in the La Vid carbonates can be described by three distinct fluid evolution mechanisms: local in-situ fluid generation, mixing of external with internally generated fluids and the incursion of exotic fluids. Local in-situ fluid generation – The oldest recorded fluid belongs to the basin stage in pre-Variscan times. Upward migration of an in-situ generated fluid from underlying organic-rich shales through iron-sandstones into the carbonates of the La Vid Group was accompanied by the precipitation of various Fe-carbonates in different stratigraphic positions of this succession. Among the cements are ferroan saddle dolomites with inclusions of solid bitumen in the lower part and mature petroleum in the upper part of the succession. Assuming a common origin of petroleum and aqueous fluid inclusions in the ferroan saddle dolomite, trapping conditions of 114 to 130°C and a pressure of about 36.5 to 40.5 MPa were determined: this corresponds to a burial depth of about 3650 to 4050 m. The La Vid Group reveals a covariance of carbon and oxygen isotope values with lowfield magnetic susceptibility (MS). Also, strontium isotopes, and to a minor degree Fe, follow this trend. The main carriers of the magnetic susceptibility appear to be diagenetic Fe-carbonates, i.e., siderite, ferroan dolomite, and ankerite, which occur as cements in primary and secondary voids, as well as in fractures. In some layers, especially at the top of the succession, secondary Fe-chlorite and pyrite additionally occur. The La Vid carbonates reveal distinct variations in MS from base to top, due to different amounts and contents in paramagnetic minerals. There is good correlation between these variations and bulk Fe-content. Their predominance is evidenced by optical methods, temperature-dependent MS-measurements and high-field magnetisation behaviour. The Fe-carbonates were formed during upward migration of a reducing, iron-bearing, petroliferous ﬂuid that was depleted in 13C and carried radiogenic Sr. We interpret the variation in MS to reflect two stages of Fe-bearing mineral precipitation with an earlier Fe-carbonate and a later Fe-chlorite crystallisation; the latter restricted to interbedded carbonates and shales. Furthermore, porosity has an additional influence on the MS-signature, with high values in coarse-grained sandstones and carbonates, and lower values in fine-grained dolostones and limestones. This highlights the influence of diagenetic mineral formation on MS-variations and stable isotope values in carbonate-bearing successions. Similar geochemical covariance and/or correlations can be expected in other sedimentary successions influenced by the migration of petroliferous formation ﬂuids. Mixing of external with internally generated fluids – The second important fluid event exhibits mixing of a low salinity internally generated local fluid with an exotic high salinity fluid of widespread occurrence. The latter fluid caused impressive dolomitizations in Carboniferous successions a few km´s further to the north, but also in Devonian rocks some tens of km´s to the west. This fluid is attributed to the post-Variscan time, probably the formation of the Cantabrian orocline, based on several tectonic features occurring in the cements and on indirect dating of the mentioned epigenetic dolomites. Clear saddle dolomite precipitated during this fluid event in the dolostone unit of the La Vid Group only. These dolomites exhibit a distribution of zoned fluid inclusions from low salinity in the core to high salinity in the outer part of the crystals. Minimum trapping temperatures are 160 to 170 °C at the core and temperatures of 110 to 120 °C at the rim. Higher temperatures in the rim (up to 210 °C) appear to have resulted from post entrapment re-equilibration (stretching) of the inclusions during tectonism. Incursion of exotic fluids – The last cementation phase was caused by a cool and oxidizing fluid, related to tectonic activity, most probably of Alpidic age. This fluid precipitated calcite, celestite and kaolinite 2 and recrystallized former Fe-carbonate cements to calcite and to Fe-oxyhydroxides. Fluid inclusions in celestite have low salinities, ruling out basinal brines and pointing to a meteoric or marine origin. Occurrences of these cements in various locations all over the Cantabrian Zone imply a large-scale fluid event.

	Publications Schneider, J., Bechstädt, T. & Machel, H. (2004): Covariance of C- and O-isotopes with magnetic susceptibility as a result of burial diagenesis of sandstones and carbonates – an example from the Lower Devonian La Vid Group, Cantabrian Zone, NW Spain.- Intern. J. Earth Sci., 93/6, 990-1007. Schneider, J., de Wall, H., Kontny, A. & Bechstädt, T. (2004) Magnetic susceptibility variations in carbonates of the La Vid Group (Cantabrian Zone, NW-Spain) related to burial diagenesis.- Sedimentary Geology, 166, 73-88. Schneider, J., Bakker, R.J., Bechstädt, T. & Littke, R. (2008): Fluid evolution during burial diagenesis and subsequent uplift: the La Vid Group (Cantabrian Zone, NW-Spain).- J. Sediment. Res., 78/4, 282-300.

	Funding Organizations German Research Fund (DFG) German Academic Exchange Fund (DAAD)

GeoResources - Fluid Evolution in the La Vid Group during Burial Diagenesis and Subsequent Uplift, Cantabrian Zone, N-Spain

Fluid Evolution in the La Vid Group During Burial Diagenesis and Subsequent Uplift (Cantabrian Zone, Northern Spain)