Mapping geological stages of climate-dependent iron and clay weathering alteration on lithologically uniform sedimentary units using Thematic Mapper imagery (Tertiary Duero Basin, Spain)
Weathering processes are responsible for slight surface mineralogical differences allowing the distinction between lithologically similar geological units using Thematic Mapper (TM) data. Two different stages throughout time of overlying iron alteration are notoriously distinctive on the imagery and laboratory spectra. Their diverse spectral behaviour follows the dominant iron hydroxide with kaolinite and carbonate crusts on the Pliocene Ochre Alteration typical of a humid warm climate, compared with the dominant nonhydratated iron oxides with smectite on the Miocene Red Alteration developed under a mediterranean dry climate. Iron materials with carbonate hinder appearance of the typical iron absorption features in the visible wavebands. Therefore, the iron weathering alteration coatings will be obscured on the imagery when it is developed on carbonate sediments or detritic sediments with carbonate cement or matrix. The presence of carbonate within the sediment as cement or alteration product decreases the overall reflectance of laboratory nonconsolidated rocks and the clay size fraction from rocks, apart from smoothing the 2200 nm absorption typical of OH-bearing minerals. The presence of carbonate cement and carbonate crusts favours the differentiation of some units. Digital mapping through image processing of different series of digital data leads to a sequential masking of classes to produce a final map. The sequence of masking produces different maps which can be used as a tool to model aspects of the sedimentary basin and geological processes throughout time.