Evolution of Illite/Smectite from Early Diagenesis Through Incipient Metamorphism in Sediments of the Basque-Cantabrian Basin

Abstract

Abstract--Prograde evolution of illite/smectite occurring in an unusually-thick (8000 m) sequence of Mesozoic-Cenozoic sediments in the Basque-Cantabrian Basin, Spain, has been studied using XRD and TEM/AEM. The sediments, which are only slightly tilted, cover the range from smectite to illite, and the most deeply buried ones are unique in that they span the range from diagenesis through low-grade metamorphism (anchizone), with no apparent overprinting due to tectonic deformation.Pelites are absent from the shallow section, but smectite occurs in marls as high-charge, K-dominant and low-charge, K-poor anastomosing arrays of layers. At intermediate depths, authigenic clay is identified largely as R1 US, coexisting with packets of 1t>>3 I/S (nearly pure illite), where illite-like and smectitelike layers can be identified by contrast in TEM images, which is consistent with XRD data. The authigenic clay of the deepest samples consists of illite with no or almost no expandable layers, which occurred as packets with layers largely subparallel to bedding, K-deficient composition in comparison with muscovite, 1Mr-like SAED patterns, and 100 ]k mean packet thickness. There is no evidence of deformation stress-induced, non-bedding-parallel clays in the deepest samples. Detrital micas with either a phengiterich or a phengite-poor composition range dominate the phyllosilicate fraction of all the samples. Detrital micas show no changes over the diagenesis/metamorphism range and appear to have behaved as if isolated from authigenic clays.Authigenic clays occur as bedding-subparallel packets that evolved during passive burial metamorphism through dissolution/crystallization of less-evolved clays. Where illite-like and smectite-like layers can be identified, TEM images imply a discontinuous series in which packets of R1 US (50% I) transform to packets of nearly-pure illite, that is, an Ostwald-step-rule-like sequence. Such immature illite remains unmodified with further burial, and is apparently the potential predecessor of stress-induced, highlyevolved mica of higher-grade, tectonically-deformed pelites.

Thermal models and clay diagenesis in the Tertiary-Cretaceous sediments of the Alava block (Basque-Cantabrian basin, Spain)

“…The interpretation of XRD profiles in this study is consistent with transmission electron microscopy observations made by Nieto et al (1996) of a composite stratigraphic sequence, to the north of the area, which covered the classic R0-R1-R3-illite range. The presence of illite as a discrete phase is generalized in all the samples (Fig.…”

“…Smectite-to-illite evolution with progressive diagenesis can be assigned to two basic schemes, according to the literature: (1) continuous transformation of smectite layers to illite as burial diagenesis progresses, with intermediate illite-rich mixed-layer phases and the larger order in the more diagenetically evolved samples (Hower et al, 1976;Drits et al, 1997;Bauluz et al, 2000); (2) discontinuous step-by-step transformation, with variations only in the proportion of coexisting phases, the composition of which basically remains constant (Dong & Peacor, 1996;Nieto et al, 1996;Dong et al, 1997). (a) Refers to the temperature for disappearance of smectite, (b) of R1 and (c) of kaolinite.…”

Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

“…Incorporation of detrital phyllosilicates to the ~2 mm fraction is a well-known feature (Kübler et al 1991;Warr et al 1996;Nieto et al 1996;Lanson et al 1998;Gharrabi et al 1998); therefore, when IC results contradict those obtained by other methods, they should be considered with caution. In order to confirm the detrital character of illite in thẽ 2 mm fraction, two smaller fractions were separated (~1 and ~0.5 mm).…”

Low-grade metamorphism of Cambro-Ordovician successions in the Famatina belt, Southern-Central Andes: Burial-inversion history linked to the evolution of the proto-Andean Gondwana margin

“…Quantitative AEM analyses were obtained from thin edges using a 1000x200Å scanning area. Counting times of 15 s and 100 s minimized alkali-loss problems as short counting times improve reproducibility for K + and Na + (Nieto et al, 1996). Standards used to obtain the K-factors for the transformation of intensity ratios to concentration were albite, biotite, spessartine, muscovite, olivine, and titanite (following the methods of Lorimer, 1975 andChampness et al, 1981).…”