Geochemical conditions of exogenous minerals formation of the weathering zone in the upper jurassic depositions of the middle Volga region

Abstract

© 2019, International Multidisciplinary Scientific Geoconference. All rights received. Upper Jurassic deposits on the territory of the Tatarstan Republic are represented by the coastal-marine, terrigenous complex. In its composition, the predominant rocks are dark gray calcareous clays, among which lie thick (up to 5.0 m) layers of combustible shale and thin (up to 1.0 m) layers of silt and sandstones with signs of wave ripples. All the rocks of the complex contain inclusions of authigenic pyrite (1–4%). Jurassic sediments on the Middle Volga did not experience deep dives. Their diagenesis ended at the stage of diagenesis. Starting from the end of the Mesozoic to this day, the area of development of these rocks was in the phase of tectonic uplift. As a result, the Jurassic clay-terrigenous sediments were exposed to factors of weathering for a long time. An analysis of the transformations of the Upper Jurassic terrigenous complex allowed establishing peculiar zoning in the profile of the weathering zone. The most intensive transformation of the Jurassic sediments takes place in the upper part of the weathering profile, in the zone of the vertical drainage of precipitation. Weeping through the rocks, infiltration waters activated pyrite oxidation processes. First of all, the destruction of pyrite aggregates occurred in the aeration zone. Under the action of oxygen-containing pore waters, sulfur was removed from the structure of FeS2 in the form of H2SO4. The released iron is partially bound to hydroxides, forming the aggregates of goethite (α-FeOOH). Part of the acidified solutions containing Fe2+ migrated along the cracks. Interacting with microcline, they were enriched with K+ ions, and lower down the section were connected into jarosite (KFe3[SO4]2(OH)6). In the walls of the exposures of the Jurassic age, the yellowish aggregates of jarosite throughout the section were traces of acidified infiltration water migration. The lower part of the jarosite zone, where the cracks disappear, is marked by the area of intensive deposition of gypsum. Here, acidic mineralized solutions enriched in SO42-, interacting with clay rocks with carbonates, were enriched with Ca2+ ions. It precipitated gypsum mineralization. In some cases, the area of development of calcite aggregates in the form of concretion and streaks is noted under the gypsum zone. Concretions are confined to the boundaries between clay layers that are heterogeneous in composition and physical properties. The formation of calcite aggregates is due to the accumulation of Ca2+ and HCO3-ions in the composition of groundwater, which, when the acidity of the medium drops, are bound into calcite. The source of Ca2+ and HCO3-ions was biogenic calcite located in Upper Jurassic clays. Thermodynamic calculations made it possible to establish that exogenous mineral associations actually mark zones in the profile of weathering with various acid-base parameters. The upper part of the section is dominated by minerals resistant in a strongly acidic medium (pH>2), in the lower part-in a neutral and slightly alkaline medium (pH = 7-8).