Concretions of the Valtos Sandstone Formation of Skye: geochemical indicators of palaeo-hydrology

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Abstract
The calcite concretions of the Jurassic Valtos Sandstone Formation record the origin of the porewaters within which they grew, and the approximate flow rates. The concretions grew within meteoric porewaters, part of a Middle or Upper Jurassic meteoric water lens extending into the Sea of the Hebrides Basin from the then Hebridean landmass. Modal porefluid flow rates were low, resulting in an approximately spherical concretion geometry. The concretions are zoned due to episodic recharge of the aquifer (which is tentatively related to seismic pumping). The sandstone was open with respect to Mg, Fe, Mn and Sr, but closed with respect to Ca. The sandbody was rock-dominated with respect to Ca, Sr, C and Mn, which were derived from within the sandbody by internal redistribution; but water-dominated for Fe, Mg and O. The Valtos concretions grew by the diagenetic redistribution of aragonitic shells, and comprise only a single phase of ferroan calcite (FeCO 3 =0.89−2.7 mole %), with a small range of carbon compositions (δ 13 C = +0.25 to -0.97‰PDB). The cements have a large range of oxygen isotopic compositions (δ 18 O = −8.3 to −18.2‰ PDB) interpreted to represent changes in porewater composition, not formation temperature. Cement crystal size increases radially, reflecting decreasing degrees of supersaturation during growth. The cements have only a small range of 'bulk' Fe and Mg concentrations (as determined by ICPS), but a large range of concentrations determined by microprobe. This pattern is explained by disequilibrium trace element partitioning associated with sector zoning within the cement.