SITE 01: LARGE FAULT

In accompaniment to our Geology Basecamp Feild Guide (March 2025)

This ‘very obvious’ fault juxtaposes thickly bedded, medium- to coarse-grained sandstone channel succession against thinly bedded, low energy, mudstone (occasionally highly carbonaceous) and finegrained sandstone flood plain deposits. The amount of throw is indeterminable since marker beds across the fault are absent. The orientation bedding into the fault suggests normal fault movement. Evidence of post-depositional extensional tectonics is recorded by micro-structural horst and graben structures in some of the sandstone units.

The lower part of the cliff section comprises ~3 m low energy flood plain deposits that is truncated by ~3 m of high energy channel sandstones. The contact is irregular and scoured. Participants should consider the time taken for the channel sandstone deposition versus flood plain deposits.

Note the ‘mushroom-shaped’ sedimentary structures that occur along a ~50 cm thick section across the cliff section. These appear to be load casts due to the result of density inversion of sandstones and mudstones. It has been argued that the mud is sinking into the sand, as opposed to classic turbidite structures where there is rapid loading of denser sediment (sandstone) onto a less dense sediment (mudstone).

One school of thought suggests the ‘mushroom-shaped’ sedimentary structures are formed in response to freezing and thawing of permafrost soils during the annual winter-summer cycle (aka cryoturbation features). Others argue they are the result of liquefaction during earthquakes (injectites). The features have been identified at 4 other spots along this stretch of coastline.