Metapelites of the Valyukhta Formation as indicators of the Riphean residual weathering crust,Patom Highland

Bimodal metapelites of complex (sedimentary-epigenetic) origin were found at the base of the Upper Riphean Zhuya Group, which disconformably overlies the Lower-Middle Riphean Kadalikan Group. The substrate of these rocks is represented by relatively low-aluminous mudstones of the Valyukhta Formation, which bear evidence of physical disintegration replaced upsection by chemical decomposition. The altered mudstones compose a weathering crust with the typical fissure, clastic, rubble-grus (lithomarge), and fine-clastic (dispersed) zones. Rocks of the lithomarge zone are enriched in silica, which was removed from the dispersed zone during the formation of the chemical weathering crust. During partial erosion of this crust, detrital quartz and fine-dispersed (mature) clay material penetrated the fissured mudstones. The bulk of detrital quartz was precipitated in the lithomarge and clastic zones. Fine-dispersed clay matter was accumulated in lower parts of the fissure zone. Mudstones subjected to different-depth epigenetic alterations are transformed into sericite-bearing shales of in the fissure zone, quartz-bearing shales in the clastic zone, siliceous-quartz shales and clay-like siliceous-quartz rocks in the rubble-grus zone, and sericite phyllites in the fine-clastic zone. These rocks, developed after mudstones of the Valyukhta Formation, represent different-depth indicators of the weathering crust.