Clay mineral genesis and chemical evolution in the Miocene sediments of Somosaguas, Madrid Basin, Spain

Abstract

AB ST R ACT : A mineralogical and microtextural study of Somosaguas Miocene deposits, located in the Madrid Basin (western Madrid, Spain), was carried out using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and optical microscopy, whereas crystal chemistry data were obtained by analytical electron microscopy-transmission electron microscopy and electron icroprobe analysis. Four stratigraphic sections were studied, compising detrital rocks representing intermediate and distal facies from alluvial fan deposits. The predominant source area of these sediments was the granitic rocks of the Spanish Central System with a lesser contribution of metamorphic rocks. Clayey arkoses are the most abundant rocks of these sections, typical of granite alteration under warm, semi-arid climates. The mineralogy is characterized by phyllosilicates, followed by feldspars and quartz. The data obtained reveal mineral mixtures of detrital (quartz, feldspars, kaolinite, micas and chlorite), transformed (illite and beidellite) and neoformed (montmorillonite) origin. Clay minerals resulted from interactions between detrital minerals and meteoric waters. Two trends of degradation of micas are detected. The first shows a transition from muscovites and dioctahedral illites, to beidellites. The other trend is defined by the biotite degradation to beidellites with different layer charge and octahedral Fe content. Montmorillonites were neoformed from the hydrolysis and weathering of primary minerals (feldspars and muscovite). Magnesian clay minerals such as sepiolite, palygorskite and trioctahedral smectites, extremely abundant in the centre of the basin, were not detected in Somosaguas sediments.

Composition of clastic sediments in the Somosaguas area (middle Miocene, Madrid Basin): insights into provenance and palaeoclimate

“…These levels are homogenous and some of them show good lateral continuity. Their mineral composition indicates phyllosilicate contents of up to 80% (Fesharaki et al, 2007). In addition, they are the richest levels in organic matter but lack fossil remains (López Martínez et al, 2000).…”

“…Climbing ripples are fre- quent and are replaced with high-energy parallel lamination towards the top Fesharaki, 2005). This unit contains as much as 65% phyllosilicates (di and trioctahedral micas, beidellites and montmorillonites, kaolinite, and scarce illite/smectite mixed layers; Fesharaki et al, 2007). According to Mínguez , its lithology is equivalent to that of the so-called "Guadalajara Facies" of the Madrid Basin.…”

“…Sands are matrix supported, mainly clayey (Fesharaki, 2005). According to Fesharaki et al (2007), this unit contains as much as 70% phyllosilicates (di and trioctahedral micas, beidellites and montmorillonites, kaolinite, and scarce illite/smectite mixed layers). The unit's microfossil content makes it one of the most productive sites in the Madrid Basin (López Martínez et al, 2000).…”

“…These climate perturbations coincide in time with the formation of the Somosaguas palaeontological site (Domingo et al, 2009(Domingo et al, , 2012aGarcía Yelo et al, 2014;see Fesharaki et al, 2012 for a complete bibliographic references list about this palaeontological site). Previous palaeoclimatic and palaeoenvironmental inferences based on the abundance and variety of fauna and its characteristics, together with sedimentological, mineralogical and isotope data obtained for the Somosaguas site, have assigned this site to a period of worldwide climate changes (López Martínez et al, 2000;Hernández Fernández et al, 2006;Fesharaki et al, 2007;Carrasco et al, 2008;Domingo et al, 2009;Perales et al, 2009;García Yelo et al, 2014). This available data make it an ideal location to extend palaeoclimate studies to other scientific fields such as light mineral petrology.…”

“…This paper addresses sand petrofacies formed during the Middle Aragonian (14.2Ma) at the Somosaguas palaeontological site (Madrid Basin), the evolution of their source area (Spanish Central System) and the climate that prevailed over the middle Miocene in this area. To this end, we also used data reported for recent materials arising from the same source area as the sands of the Somosaguas site (Palomares et al, 1990;Palomares and Arribas, 1993;Tortosa et al, 1988Tortosa et al, , 1989Tortosa et al, , 1991 as well as data emerging from sedimentological and clay mineralogy studies performed at the site itself Fesharaki, 2005;Fesharaki et al, 2007). Finally, this paper contributes to the existing database on the provenance of sands filling intracratonic basins.…”