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Clay diagenesis and low-grade metamorphism of Tithonian and Berriasian sediments in the Cameros Basin (Spain)

¹ Departamento de Cristalografía y Mineralogía, Universidad Complutense de Madrid, 28040 Madrid, Spain, ² Mineralogisch-Petrographisches Institut, Basel University, CH 4056 Basel, Switzerland (deceased), ³ Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, Fábrica de Armas, 45071 Toledo, Spain, and ⁴ Departamento de Estratigrafía, Universidad Complutense de Madrid, 28040 Madrid, Spain

^* E-mail: barrene{at}eucmax.sim.ucm.es

(Received 29 February 2000; revised 13 October 2000)

The clay mineral assemblages of the Tithonian and Berriasian sediments (Tera and Oncala Groups) in the eastern part of the Cameros basin are investigated at seven localities. The lowest-grade assemblage, located on the southern border of the basin, contains calcite + quartz + hematite + kaolinite + mixed-layer illite-smectite (R = 1, 65–85% illite layers) ± discrete illite (IC = 0.5–0.65°2). Systematic increases in the illite and chlorite crystallinities suggest increasing metamorphic grade from the northwest part of the basin to the southeast. This trend does not follow the pattern previously described for the overlying late Berriasian–early Aptian sediments (Urbión and Enciso Groups), which exhibit a higher metamorphic grade. This may result from local variations in sedimentary facies, as well as the circulation of hot migratory fluids. Tertiary compression occurring long after the main metamorphic event is considered to be responsible for the enhanced illite and chlorite crystallinities measured in the SE extreme of the basin.

KEYWORDS: diagenesis, low-grade metamorphism, clay minerals, illite crystallinity, chlorite crystallinity

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