PEDOGENIC AND GROUNDWATER PROCESSES IN A CLOSED MIOCENE BASIN (NORTHERN SPAIN)

The Sepulveda-Ayllon Basin (SAB) was an episodically closed Miocene basin adjacent to the Tertiary Duero Basin (northern Spain). The sediments studied here represent a depositional system developed from the margin to the centre of the basin by alluvial fan, mud flat and saline mud flat/ephemeral lacustrine environments. An assemblage of authigenic lithotypes was formed in the basin, their distribution being influenced by that of the depositional environments within the basin. Authigenesis took place mainly before sedimentation of the next set of beds. In the distal alluvial fan, authigenesis of smectite (outer part), palygorskite (inner part) and calcrete predominates, and the original muddy deposits are still recognizable. In the mud flat, a more intense transformation of the sediments took place, with predominant formation of palygorskite (outer part), interstitial gypsum, calcrete (outer part), dolocrete (inner part) and brown silcrete. In the saline mud flat/ephemeral lake, original sediment was extensively disturbed and replaced by magnesian clays (sepiolite and magnesian smectite), interstitial gypsum, dolocrete and cream and white silcrete. Authigenic alteration within each cycle varied in time and place, and was both pedogenic and groundwater-related. The authigenic assemblages point to a closed basin under evaporation in a seasonally dry climate. At the end of this period the SAB became connected with the Duero Basin to the northeast, thus losing the endoreic regime. Consequently, the authigenic evaporitic assemblages were then unstable and were partially replaced by a late sparry calcrete.