Illite-smectite-rich clay parageneses from Quaternary tunnel valley sediments of the Dutch southern North Sea – mineral origin and paleoenvironment implications

Branimir Šegvić, Antonio Benvenuti, Andrea Moscariello
2016 Clays and clay minerals  
12 The Pleistocene sediment infill of elongated glacial incisions of the Southern North Sea (SNS) 13 often is referred to as tunnel valleys (TV). Its depositional environment is not yet fully understood and 14 present study addresses this challenge from a perspective of clay mineral transformation (illite to I-S) 15 reported from the largest Elsterian TV of SNS. Material acquired from the K14-12 borehole in the 16 Dutch offshore was analyzed by X-ray diffraction, electron microscopy, electron
more » ... roscopy, electron microprobe analyses, 17 and laser particle-size analysis. Illite and illite-smectite appeared as dominant clays along with minor 18 amounts of kaolinite, kaolinite-smectite, and chlorite. Highest amount of I-S is recognized in TV main 19 portion, while in pre-glacial and uppermost deposits I-S is less abundant. The XRD peak fitting and 20 deconvolution suggest I-S consists of several intermediatesordered (well-crystallized illite + R3 I-S) 21 and disordered (R0 I-S + R0 I-SS). Given the average particle sizes (> 2 µm) and Kübler index values 22 (0.415-0.341°Δ2θ), illite as well as chlorite and kaolinite were interpreted as detrital. On the basis of I-23 S distinctive distribution, grain sizes, and compositional variations its formation by way of early 24 diagenetic in-situ smectitization of illite under a cold climate is proposed. The process operated via a 25 series of mixed-layer intermediates derived from an illite component being progressively converted to 26 low-charged smectite. The reaction is marked by a significant net loss of K and Al with replacement 27 by Si in a tetrahedral coordination. Layer charge imbalance is accommodated by Fe 3+ and Mg entering 28 an octahedral layer, whereas Ca partly fills the interlayer sites. Smectitization rates were controlled by 29 illite grain sizes. The results of this study strongly support the existence of an ice-marginal fresh water 30 depositional environment at the glacial maximum in SNS in which early diagenesis at low 31 temperatures resulted in incomplete illite conversion into smectite. 32 33 Manuscript Click here to download Manuscript MS_CCM_1032_Revised_Version_Clean.docx This is a 'pre-publication' version of an accepted article for Clays and Clay Minerals. This version may be subject to change during the production process. The DOI given, which may be used for citation purposes, though which will not be active until the version of record is published, is
doi:10.1346/000986016820500438 fatcat:sahfub5di5bufm7l7ktfyfxhmy