PEBBLY MUDSTONE

If you jumped in at soon as you saw the link it may be a little premature but I am not to sure how best to handle this unit. The pebbly mudstones are approx. 80 metres thick. They are a series of debris flows with beds commonly two to three metres thick. Mud layers are very much the minor component. More than 99% of the pebbles are mudstone, siltstone, or arenite, and the most common texture is quartz wacke. Therefore the unit is overwhelming derived from a continental provenance. The major problem with this is that the pebbly mudstones rest on several hundred metres of andesite [some of it tending towards basalt]. My interpretation is that the pebbly mudstones almost entirely pre-date magmatism and that the andesites underneath the debris flow beds are high-level intrusives. The one per cent of pebbles not accounted for by the statements above are igneous, so the deposition of the pebbly mudstones overlapped the start of magmatic activity in the region. Overall the pebbly mudstone unit is wedge shaped and was perhaps deposited in a half-graben generated by the extensional teconics that went on to cause the magmatism. Another significant feature of the pebbly mudstone unit is that it contains metric-scale sulphide mounds conformable with the layering which suggests that sulphide exhalation started during the deposition of the debris flows ahead of the magmatism which led me to suggest that the IPB deposits had more in common with sedimentary-hosted massive sulphides than had previously been considered.

Back to Corta Atalaya Overview

In the photo below of one of the pebbly mudstone beds you will notice many bedded clasts.