Diagenetic evolution of a fractured evaporite deposit (Vilobí Gypsum Unit, Miocene, NE Spain)

Abstract

The upper Burdigalian Vilobí Gypsum Unit, located in the Vallès Penedès half‐grabben (NE Spain), consists of a 60‐m thick succession of laminated‐to‐banded secondary and primary gypsum affected by Neogene extension in the western part of Mediterranean Sea. This Tertiary extensional event is recorded in the evaporitic unit as six fracture sets (faults and joints), which can be linked with basin‐scale deformation stages. All fractures are totally or partially infilled by four types of new gypsum precipitates showing a large variety of textures and microstructures. A structural and petrological study of the unit allows us to establish the following chronology of the fracture formation and infilling processes, from oldest to youngest: (i) S1 and S2 normal faults sets formation and precipitation of sigmoidal gypsum fibres; (ii) S3 joint set formation and growing of perpendicular fibres; (iii) S4 inverse fault development, infilled by oblique gypsum fibres and deformation of the previous fillings; and (iv) S5 and S6 joint formations and later dissolution processes, infilled by macrocrystalline gypsum cements. The fractures provided the pathway for fluid circulation in the Vilobí Unit. The oxygen, sulphur and strontium isotope compositions of the host rocks and the new precipitates in the fractures suggest clear convective recycling processes from the host‐sulphates to the fracture infillings, recorded by a general enrichment trend to heavier S–O isotopes, from the oldest precipitates (sigmoidal fibres) to the youngest (macrocrystalline cements) and to the preservation of the strontium signals in the infillings.

Fibrous gypsum veins as diffuse features and within fault zones: the case study of the Pisco Basin (Ica desert, southern Peru)

“…bedding-parallel veins and fault-related discordant-to-bedding veins) have been already reported in the literature (e.g. Mathew et al 2004;Philipp 2008;Moragas et al 2013). However, we document for the first time fault-independent arrays made of layerbounded, and bedding-perpendicular, orthogonal vein sets, as well as gypsum vein remains along fault planes.…”

“…Vein remains indicate that faults are superimposed on earlier orthogonal gypsum veins and that gypsum along fault planes does not necessarily represent the product of fibrous gypsum growth within extensional openings created by slip episodes (e.g. Mathew et al 2004;Philipp 2008;Moragas et al 2013).…”

“…Fibrous gypsum veins have been documented worldwide and within sedimentary successions ranging in age from Proterozoic to Recent (Gustavson et al 1994;El Tabakh et al 1998;Testa & Lugli 2000;Mathew et al 2004;Al-Hamdani et al 2005;Philipp 2008;Neubauer et al 2010;Moragas et al 2013), and even on Mars (Showstack 2011). These veins commonly occur in association with or in the vicinity of evaporitic layers, mostly within fine-grained rocks (e.g.…”

“…mudstones and shales ;Warren 2006), although both conditions are not strictly necessary. Veins associated with fault zones (Mathew et al 2004;De Paola et al 2008;Philipp 2008;Moragas et al 2013) and folds (Neubauer et al 2010) have also been documented. Gypsum veins can reach 25 cm in thickness and are commonly bedding-parallel and/or subhorizontal, forming at very shallow depths as a result of unloading during uplift and/or tectonic compression (MacHel 1985;Gustavson et al 1994;El Tabakh et al 1998;Warren 2006;Philipp 2008;Neubauer et al 2010).…”

“…Gypsum veins can reach 25 cm in thickness and are commonly bedding-parallel and/or subhorizontal, forming at very shallow depths as a result of unloading during uplift and/or tectonic compression (MacHel 1985;Gustavson et al 1994;El Tabakh et al 1998;Warren 2006;Philipp 2008;Neubauer et al 2010). They can also be arranged in vertical or oblique sets (MacHel 1985;Gustavson et al 1994;El Tabakh et al 1998;Moragas et al 2013), or more randomly distributed as anastomosing vein networks, owing to hydraulic fracturing associated with the conversion of anhydrite to gypsum (MacHel 1985;Cosgrove 2001;Philipp 2008).…”