Contribución de la mineralogía de arcillas a la interpretación de la evolución paleogeografica del sector occidental de la Cuenca del Guadalquivir

Abstract

Marine tertiary materials of the Guadalquivir basin consist of four mayor depositional sequences of great continuity in the whole basin which are separate by unconformities. The lower outcroping unit (unit 2) ineludes glauconite-rich sandstones and marly limestones (Fm. Calcarenitas de Niebla, Tortonian-Messinian) and Olisthostromic materials. This unit lies discordantly over the Paleozoic, and was sedimented in a shaUow marine platform. Olisthostromic materials (Mesozoic and Miocene), composed of sand and elayey sand, were deposited during Tortonian-Messinian times. As a consequence of a transgressive pulse, the sedimentation of a thick marly series (Fm. Arcillas de Gibraleón) ofthe Messinian-Low Pliocene age took place. In the last stages of the filling of the basin (Pliocene) a regression began, and sands and silts were deposited (Fm. Arenas de Huelva and Fm. Arenas de Bonares). Also, and as a final consequence of the regression stage, lacustrine sedimentary deposits (Upper Pliocene) occur at the south of the basin, which are composed of marly limestone and greenish palygorskite and sepiolite marls. From a mineralogical point of view, autochthonous materials from this marine detritalcarbonate series, are mainly composed of calcite, quartz and phyUosilicates with minor dolomite and feldspars, and secondary gypsum. The elay minerals present are illite and smectite with minor kaolinite and chlorite. The evolution of the elay minerals associations varies from bottom to top as foUows: Illite or Clauconite >> Smectite > Chlorite/Kaolinite (Fm. Calcarenitas de Niebla); Smectite ≥ Illite > Kaolinite (Lower Fm. Arcillas de Gibraleón); Illite > Smectite ≥ Kaolinite/Chlorite) (Upper Fm. Arcillas de Gibraleón), and Smectite > Illite > Kaolinite (Fm. Arenas de Huelva y Fm. Arenas de Bonares). This vertical variation confirms a stratigraphical discontinuity (paraconformity) into the Fm. Arcillas de Gibraleón, which was observed by others from stratigraphical and paleontological data, and supports a new mineralogical change between the Fm. Arcillas de Gibraleón y Fm. Arenas de Huelva. Sedimentation of those materials above described took place in a shallow marine environment which was more superficial upward, according to palaeontological data. The illite crystal-chemistry characteristics, suggest that metamorphic and igneous rocks from the South Iberian Massif should be the source area for these sediments. The mineralogical composition of allochthonous materials are quartz, opal-A and opalCT, phyllosilicates, and calcite. The clayminerals are predominantly smectites and minor amounts of illite and kaolinite. The microfauna associations together with the absence of detrital minerals, may support that sedimentation of these materials took place in a rather shallow marine basin (< 150 m deepth). The presence of opal A and CT, seem to indicate that sediments have only undergone an early diagenesis. The source area for these materials could be the Triassic materials of the Betic Cordillera.