Beidellite-nontronite clays in Neogene sediments from Cuitzeo-Charo lacustrine basin Michoacán, México. Geological setting and paleoenvironmental implications

Resumen

Lacustrine and
fluviolacustrine deposits of the Charo sequence crop out extensively
along tens of kilometres, and they overlie the Miocene volcanic
sequences of the Mil Cumbres Complex in an E-W oriented basin.
Eastwards of Morelia city (central Mexico), layers of non lithified
clayey sediments of about 3 m in thickness are overlaid by massive
diatomite deposits developed during the Miocene-Pliocene in the
Cuitzeo palaeolake. Major outcrops are located south and east of the
village of Charo at the Las Pulgas, Lomas Blancas and Tzitzimeo
sites; all having similar mineralogical facies and stratigraphic
correlation. Field studies, petrography and X-ray fluorescence
spectrometry (XRF) indicated that the parent rock was mainly of
andesitic composition with minor dacitic-rhyolitic components. The
source of the raw material was located in the upper topographic
levels of the present horst-graben system. The diagenetic clays were
identified as smectites. The central part of the Las Pulgas column
shows a beidellite-nontronite (B-N) association. Likewise, 27Al-NMR
and Mössbauer spectroscopies confirm the B-N character. Interlaid
and overlying diatomite strata indicate a lacustrine environment,
favourable to the development of secondary clays. The stability of
lacustrine conditions gave place to an incipient stratification
(smectite-illite), better developed in the middle part of the clayey
levels.