Carbonate production within the orcadian basin, northern Scotland: A petrographic and geochemical study

Abstract

A number of primary phases and textures are described for the Debonian lacustrine carbonates of the Orcadian Basin. Textural and mineralogical variations record changes in the original lake water chemistry. Stable isotopic and trace element studies of the primary carbonate phases support the conclusion that the Orcadian Basin was the site of a lacustrine system that fluctuated between two end-member conditions: (1) one or a number of shallow evaporative playa-lacustrine systems; and (2) a deep, probably stratified, freshwater lake or lakes. The basin was receiving 18O-depleted meteoric groundwaters derived from the adjacent Caledonian mountains. From relatively fresh waters with a δ 18O composition of −7% or less primary precipitates of low-Mg calcite or pure calcite are preserved as calcite. At times of low lake stand with a water composition of −3% or greater, high-Mg calcite, aragonite or protodolomite was precipitated and dolomitized. Sodium contents of the carbonates generally increase with dolomite content, and the grain size of primary calcite precipitates was greater at higher lake levels.