Abstract
The facies and clay mineral study of clay/marl-rich levels from the Torrelapaja Formation (latest Hauterivian–early Barremian, NW Iberian Range, NE Spain) allowed to establish the palaeoclimatic and palaeoenvironmental conditions under they were generated. The muddy levels and pisoids contained therein of two logs were sampled and studied by X-ray diffraction and optical and electron microscopy. A similar mineralogical upwards trend is recorded in both logs, with a decrease in calcite coupled with an increase in quartz and orthoclase content and constant proportions in goethite, hematite, diaspore, anatase, rutile, ilmenite, and clay mineral content. The lower muddy levels have higher kaolinite content than the upper levels, where illitic phases are the dominant clay minerals. Smectite and intergrowths of illitic phases and kaolinite are also detected upwards. The kaolinite and smectite textures indicate an authigenic origin, whereas the illitic phases are former phases acting as a substrate for kaolinite crystallization. Pisoids mineralogy and texture show an in-situ origin, but some are fractured, indicating reworking processes. The mineral association found in the muddy levels is characteristic of oxisols formed under warm and humid conditions. The upward decrease in kaolinite content is coeval with an increase in the illitic phases and quartz content, related to siliciclastic input, but is also coeval with the presence of authigenic smectite. This indicates a decrease in chemical weathering, not fully registered due to the siliciclastic contribution, which was possibly associated with a change to colder, drier conditions during the latest Hauterivian–early Barremian in the studied area.
Highlights
The study of the soils that formed in the landscapes of the past provides key data for palaeoclimatic and palaeoenvironmental reconstructions (Mack et al, 1993)
The mudstones to calcareous mudstone of the Las Cañadas (LC) and VJ logs are mainly formed by quartz, calcite, and clay minerals together with orthoclase, goethite, hematite, diaspore, anatase, rutile and ilmenite (Fig. 4)
In the < 2 μm fraction, abundant kaolinite and illitic phases are identified in the X-ray diffraction (XRD) patterns (Fig. 4b and d)
Summary
The study of the soils that formed in the landscapes of the past (i.e., palaeosols) provides key data for palaeoclimatic and palaeoenvironmental reconstructions (Mack et al, 1993). The formation of soils takes place near the Earth’s surface in the contact among lithosphere, atmosphere, hydrosphere, and biosphere (Tabor et al, 2017), and the weathering processes that generate them are subject to strong climatic control For this reason, the study of the mineral phases formed during soil formation, such as clay minerals and oxides, is of great interest, since they directly provide information about the climate and environment (Chamley, 1989; Do Campo et al, 2018; Laita et al, 2020; Sheldon & Tabor, 2009; Varela et al, 2018). Gleysols are formed under dysoxic or anoxic conditions that give rise to redoximorphic features and gleyed horizons generated by the influence of the shallow or fluctuating groundwater table, which inhibits drainage and creates low-oxygen conditions (Tabor et al, 2017)
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