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

Abstract

The western area of the Cenozoic Madrid Basin has not been adequately studied. This, combined with the high homogeneity of detrital facies makes the stratigraphic correlation with other areas of the basin rather difficult. Consequently, only a detailed characterization of different study zones can allow subsequent correlation over this area. Over the last years there have been discovered several vertebrate fossil sites in this area that allow the dating of the sediments which host the fossil remains and provide data about palaeoclimatic trends. In this paper we present the results of the light minerals petrographic analysis carried out in one of these sites (Somosaguas paleontological site). Previous palaeontological and isotopic studies in this site indicate a climate event of cooling and rising aridity that has been described globally for the period after the Miocene Climatic Optimum. The petrographic data and indices presented here corroborate this trend towards a more arid climate through the Somosaguas sedimentary succession. Besides we study the grades of alteration of plagioclase grains as a proxy in the evaluation of palaeoclimatic variations. The increase towards the top of the succession of less altered plagioclase grains suggests a decrease in precipitations and thus more aridity. Part of the quartz and K-feldspar grains display several features like embayments and alterations pointing to palaeosoils formation and reworking processes. These characteristics and other observations suggest several sedimentary pulses in a geotectonic setting of "basement uplift" and a mixed lithological provenance for the Somosaguas deposits (granites, gneisses and minor quantities of low-grade metamorphic rocks).Keywords: middle Aragonian, Miocene Climatic Optimum, aridity, palaeosol, plagioclase alteration, sand Resumen La zona occidental de la cuenca de Madrid no ha sido adecuadamente estudiada. Esto, combinado con la alta homogeneidad de facies detríticas dificulta la correlación estratigráfica con otras áreas de la cuenca. En consecuencia, sólo una caracterización detallada de diferentes zonas de estudio puede permitir posteriores trabajos de correlación regional. En los últimos años se han descubierto varios yacimientos de fósiles de vertebrados en esta área que permiten la datación de los sedimentos que albergan estos restos y proporcionan datos paleoclimáti-cos. En este trabajo se presentan los resultados de los análisis composicionales de minerales ligeros realizados en uno de estos yacimientos (yacimiento paleontológico de Somosaguas). Estudios paleontológicos e isotópicos previos en este yacimiento registran un evento climá-tico de enfriamiento y aumento de la aridez que se ha descrito a nivel mundial para el período posterior al Óptimo Climático del Mioceno. Los datos e índices petrográficos presentados en este trabajo corroboran la tendencia hacia un clima más árido a lo largo de la sucesión sedimentaria de Somosaguas. Además, se estudian los grados de alteración de los granos de plagioclasa como prox...

Body-size structure of Central Iberian mammal fauna reveals semidesertic conditions during the middle Miocene Global Cooling Event

“…The Somosaguas deposits belong to the Intermediate Unit of the Madrid Basin [ 55 ], which records a change from sulfate to carbonate sedimentation, followed by significant progradation of detrital materials from the northern and western borders toward the centre of the basin [ 49 , 54 ]. Detrital materials were deposited by alluvial fan systems and consist of conglomerates and coarse-grained arkosic sandstones in their proximal zones, while in middle zones there are finer-grained arkosic sandstones intercalated with lutites; in distal zones the clayey levels dominate, with intercalations of micaceous sands, gypsum and carbonates [ 56 ].…”

“…Three stratigraphic levels (T1, T2 and T3) have been recognized in the Somosaguas area, corresponding to terrigenous materials derived mainly from the Central System, and representing different sedimentary environments within alluvial fans [ 49 , 58 , 59 ]. Vertebrate fossils have only been found in the lower and upper intervals (T1 and T3), while no such fossils have been discovered in the intermediate interval (T2).…”

“…In this context, the Somosaguas vertebrate fossil site, located at the Madrid Basin (Spain), has provided unprecedented evidence of the influence of MMGC and the development of open landscapes at the community scale, by means of faunal [ 24 , 39 , 45 – 47 ], mineralogical [ 48 , 49 ] and isotopic analyses [ 12 , 13 ].…”

Mortalidad excepcional en los cricétidos (Rodentia) del Mioceno medio de Somosaguas (Pozuelo de Alarcón, Madrid)

“…Los análisis paleoambientales sugieren que durante el Mioceno en Somosaguas se extendía una sabana tropical o subtropical rodeando un lago temporal, en un contexto paleoclimático global cada vez más fresco y árido como consecuencia del restablecimiento de la capa de hielo de la Antártida Hernández Fernández et al, 2006;Carrasco et al, 2008;Domingo et al, 2009;Perales et al, 2009;García Yelo et al, 2014;Fesharaki et al, 2015).…”

“…Este patrón ha sido asociado a eventos de aridez extrema en un marco macroclimático tropical semiárido (Hernández Fernández et al, 2006;Domingo, L. et al, 2012;García Yelo et al, 2014;Domingo et al, 2017). Estas condiciones paleoclimáticas fueron inferidas a partir de análisis faunísticos (Hernández Fernández et al, 2003Perales et al, 2009), así como distintos análisis geológicos del yacimiento, tanto sedimentológicos y mineralógicos (Carrasco et al, 2008;Fesharaki et al, 2015) como geoquímicos (Domingo et al, 2009). Según esta hipótesis, sequías prolongadas producirían un incremento de la mortandad, que afectaría en particular a los individuos juveniles, dando lugar a una acumulación diferencial de los restos en el área del yacimiento en épocas de lluvia Hernández Fernández et al, 2006;Martín Perea, 2015;Domingo et al, en prensa).…”