Selective weathering of cross-bedded layers forming shelters and small caves on Akopan Tepui (Venezuela): Field, laboratory and experimental evidence about diagenesis and weathering of the Mataui Formation arenites (Roraima Supergroup, Middle Proterozoic)

摘要

South-American table-top mountains (tepuis) are formed by quartzites and sandstones of Mataui Formation belonging to the Roraima Supergroup (Lower to Middle Proterozoic). Field observations in the northern part of Akopan Tepui in Venezuela revealed that unlike the beds with flat bedding, the cross-bedded layers are commonly affected by selective weathering, expressed by depressions, shelters, and even small caves. All the arenites are fine-grained; most of them are likely of aeolian origin. Hardness measurements confirmed that the cross bedded layers are softer than the flat-bedded or structureless beds occurring above and below them.Petrographic analysis showed that the rocks are mostly fine-grained quartzites to quartz sandstones, with original sand grains overgrown by syntaxial quartz cement and bearing signs of compaction manifested by local pressure solution. Some intergranular spaces are filled by newly-formed phyllosilicates (paragonite, locally intergrown with muscovite). In the overlying beds, no porosity was observed by microscopic study and under SEM, whereas in the underlying rocks signs of slight weathering and etching of phyllosilicates was observed, together with locally widened spaces along the grain-to-grain boundaries. The samples from cross-bedded layers display weathering, the degree of which depends on the present access of flowing water. Where water has periodical access to the arenite, it shows dissolution of quartz cement (arenization), together with complete leaching and removal of the phyllosilicates due to lateritization. The intergranular spaces are considerably widened and quartz grains bear signs of strong skeletal etching and pitting. Samples from the sites where the water access ceased show mostly leaching of phyllosilicates, with weak skeletal dissolution of quartz grains without pitting. To imitate the origin of structures observed in the field, experimental modelling was performed, using artificially layered and compacted fine-grained sands, on which sodium-silicate solution was dripped. During soaking, downward preferential penetration of the solution along the cross-bedding laminae (funnel-flow) was observed in the cross-bedded layer. The resulting structures display good lithification of horizontally layered sands and relatively poorer lithification of the cross-bedded sediment between them, showing triangular-shaped overhangs and shelters, which mimic those on the Akopan Tepui.Differential weathering of otherwise petrologically very similar arenites indicates that there had to be a difference in the initial porosity in the time of the rock denudation. A preserved porosity in the cross-bedded layers was necessary for the water/rock contact to perform dissolution. (C) 2018 Elsevier B.V. All rights reserved.