EVOLUTION OF ILLITE SMECTITE FROM EARLY DIAGENESIS THROUGH INCIPIENT METAMORPHISM IN SEDIMENTS OF THE BASQUE-CANTABRIAN BASIN/

Prograde evolution of illite/smectite occurring in an unusually-thick (8000 m) sequence of Mesozoic-Cenozoic sediments in the Basque-Cantabr ian Basin, Spain, has been studied using XRD and TEM/AEM. The sediment s, which are only slightly tilted, cover the range from smectite to il lite, and the most deeply buried ones are unique in that they span the range from diagenesis through low-grade metamorphism (anchizone), wit h no apparent overprinting due to tectonic deformation. Pelites are ab sent from the shallow section, but smectite occurs in marls as high-ch arge, K-dominant and low-charge, K-poor anastomosing arrays of layers. At intermediate depths, authigenic clay is identified largely as R1 I /S, coexisting with packets of R much greater than 3 I/S (nearly pure illite), where illite-like and smectite-like layers can be identified by contrast in TEM images, which is consistent with XRD data. The auth igenic clay of the deepest samples consists of illite with no or almos t no expandable layers, which occurred as packets with layers largely subparallel to bedding, K-deficient composition in comparison with mus covite, 1M(d)-like SAED patterns, and 100 Angstrom mean packet thickne ss. There is no evidence of deformation stress-induced, non-bedding-pa rallel clays in the deepest samples. Detrical micas with either a phen gite-rich or a phengite-poor composition range dominate the phyllosili cate fraction of all the samples. Detrital micas show no changes over the diagenesis/metamorphism range and appear to have behaved as if iso lated from authigenic clays.Authigenic clays occur as bedding-subparal lel packets that evolved during passive burial metamorphism through di ssolution/crystallization of less-evolved clays. Where illite-like and smectite-like layers can be identified, TEM images imply a discontinu ous series in which packets of R1 I/S (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 apparen tly the potential predecessor of stress-induced, highly-evolved mica o f higher-grade, tectonically-deformed pelites.