Anoxic-oxic cyclical lacustrine sedimentation in the Miocene Rubielos de Mora Basin, Spain

The Rubielos de Mora Basin is a Miocene half-graben (3 × 10 km) with a sedimentary fill of over 600 m of alluvial and lacustrine sequences. Three main units make up this basin infill: lower (alluvial) and middle and upper lacustrine units. The upper lacustrine unit is mainly formed in the western part of the basin, by cyclical sequences. These sequences consist of organic-poor, non-laminated mudstones and marls cyclically alternating with thinly laminated facies such as sandy mudstones, bioclastic laminae, oil shales, rhythmites (carbonate-clay, varve-like couplets) and marls. Low-Mg calcite and non-stoichiometric (Mg-poor) dolomite are the dominant carbonate minerals in the non-laminated facies, whereas variable amounts of low-Mg calcite, high-Mg calcite, aragonite and non-stoichiometric dolomite have been recorded in most of the laminated facies. Low-Mg calcite is the main carbonate mineral in sandy mudstones and bioclastic laminae which occur at the lower part of laminated intervals in the cycles. Analysis of oil shales reveals a major macrophyte contribution of organic matter, displaying an early diagenetic evolutionary stage. Cyclical sequences record alternating oxic-anoxic bottom conditions in marginal zones of a meromictic lake, due to cyclical changes in lake water volume. This feature is recorded in the cyclical sequences of mineralogical changes and facies transitions. Strong tectonic subsidence (not balanced by sedimentation and taking place in an active rift setting) and palaeoclimate were the most striking features which could favour the establishment of a meromictic lake during this stage of the basin history. © 1988 The Geological Society.Peer Reviewe