Miocene silcretes in argillaceous playa deposits, Madrid Basin, Spain: petrological and geochemical features

Abstract

Four regressive sequences are present in the opaline rocks and related deposits of the Miocene Intermediate Unit of the Madrid Basin. The sequences consist of silty mudstones and argillaceous opals, separated by transitional facies. The silty mudstone consists mainly of dioctahedral smectites, whereas the argillaceous opal is principally opal‐CT and variable amounts of sepiolite. In the transitional facies, lamina of dioctahedral smectite co‐exist with neoformed opal‐CT and sepiolite. Petrological and geochemical features (major, trace and REE elements) indicate that the opaline levels and the transitional facies are related and are a consequence of silcrete formation in an argillaceous playa deposit. The isocon method was used to calculate changes in element concentration associated with silcrete formation. The geochemical data suggest silicification in an arid environment. The silcrete profile occurs four times, possibly as a result of highstand–lowstand fluctuations of the lake level caused by climatic changes. Structures and cements in the silcretes indicate that, although silicification may have commenced at the top of a groundwater table, it continued in the unsaturated zone above the water table.

Dolomite–silica stromatolites in Miocene lacustrine deposits from the Duero Basin, Spain: the role of organotemplates in the precipitation of dolomite

“…The inferred rapid precipitation of silica and the predominance of opal rather than quartz in Miocene stromatolites indicate that the opal formation was possibly encouraged by the evaporation processes. The evaporation mechanisms provide the high concentrations of silica in the water and the rapidity required for the precipitation of opal (Bustillo & Bustillo, 2000). Papers by Wopfner (1978) and Ambrose & Flint (1981) suggested that shorelines of regressive lakes were the preferred sites for silica precipitation owing to the high evaporation rates.…”

“…This interpretation is based mainly on the transitional relationships between dolomite and silica beds, the typical pustular mat morphologies preserved in some of the silica facies, the relatively light 18 O values for associated dolomite, and the higher density of bioturbation traces in silica than in dolomite beds. Bioturbation tubes create further organic micro‐environments suitable for silica nucleation, as described by several authors (Gardner & Hendry, 1995; Bustillo & Bustillo, 2000). The drop in water level in the lake is interpreted as possibly having led to a rapid silicification of microbial mats developed in the marginal fringes which prevented their degradation and the subsequent dolomite precipitation.…”

Pattern of sedimentary infilling of fossil mammal traps formed in pseudokarst at Cerro de los Batallones, Madrid Basin, central Spain

“…Preservation of the morphology of the buttes was strongly conditioned by the occurrence of chert horizons at several levels within the upper portion of the Intermediate Unit of the Miocene. These chert horizons formed mainly by early diagenetic replacement of calcareous and/or mudstone deposits (Bustillo & Bustillo, ; Bustillo & Alonso‐Zarza, ).…”

Cherts with Moganite in Continental Mg-Clay Deposits: An Example of "False" Magadi-Type Cherts, Madrid Basin, Spain

“…The silcretes are very common, and many petrological types have been described in relation to their mineralogical composition, textures, and host rocks (Bustillo 1976). Lacustrine-palustrine limestones, calcretes, dolocretes, gypsum, and Mg-clay deposits are the host rocks of these groundwater or vadose silcretes (Bustillo 1984;Bustillo and Bustillo 1987;Bustillo and Bustillo 2000). At the south of the Madrid basin, near the villages of Cabañas and Yunclillos (Fig.…”