Micromorphology of opaline features in soils on the sediments of the ex-Lago de Texcoco, México

The former Texcoco Lake (ex-Lago de Texcoco) is situated in the closed Basin in Mexico. The lake was artificially drained in the 20th century. Six soil profiles on the eastern border of the former lake were analysed for mineralogical and microfabric characteristics in order to differentiate between sedimentary and pedogenic processes. Large quantities of silica liberated as a result of weathering of the surrounding volcanic mountains partly precipitated in the lake. As a result, layers of an exceptional sediment, locally called “jaboncillo”, were deposited. They are characterised by a high Si-content and a very high (up to 500%) water retention. In addition several amorphous silica features were formed in these lacustrine conditions. In the centre of the lake opaline nodules, mainly composed of opal-A, formed in the jaboncillo. Near the former coast line concentric silico-calcitic nodules (ooliths) were formed as a result of fluctuations of the input of fresh water and biological activity. Several superposed layers of in situ silicified roots point to frequent changes in the shore line position. After emergence of the sediments, about 100 years ago, a translocation of silica, mainly caused by an alkalisation of the sediment, gave rise to the formation of different pedogenic types of opaline coatings and infillings, resulting locally in a duripan. The micromorphological and mineralogical properties of these silica features are discussed and sedimentary and pedogenic processes considered.