A new year of geology

Happy new year all! I thought I'd kick the new year off with some sedimentary structures in honour of my newly doctorised friend Jim. Jim studied the sedimentary rocks of Northern Tasmania which formed before the development of plant life. Jim and Mazey combined forces to foster my appreciation of all things sedimentary.

I spotted these features while capturing Ingress portals in Perth. They are located on the holocost memorial in one of Perth's lovely parks on the corner of St Georges Terrace and Barrack Street (-31.955992, 115.859792). I do not know the origin of the sandstone that forms the memorial so if any of you do, drop a comment below and educate us all!

The sedimentary structures on the right of the above image are slump features, specifically load casts and associated flame structures. They form when denser/courser material is dumped into unconsolidated fine grain sediment. In this case, medium grain sand being deposited on top of fine grain sand,. The orientation of the flame structures indicates that younging is toward the right of the above picture. All following pictures have been rotated to show younging in the upward (ish) direction.

 

Load cast - note the courser (visible) grains and the rounded shape.

 

Flame structure - as the load plops downward the finer matter is pushed upward into the 'neck' between the loads. A flame can often indicate the direction of flow as the tail will point to where the load has come from. Given the 3D nature of these structures, it is important to look at all sides if possible to determine the direction of flow as accurately as possible.

This picture is taken near to the base of the memorial (younging to the right). It is easy to see multiple flames and a jumble of casts. Given the chaotic nature of the casts, it is possible that these structures weren't caused by sedimentary transport but were caused by liquifaction. This process occurs where the finer sediments in unconsolidated  horizons are vibrated by a localised earthquake. The fine sediments start to migrate upwards allowing the heaver sediments to drop. A recent example of this phenomena is the 2011 Christchurch earthquake. Large sections of the city were built on river sediments and the liquifaction caused by the series of earthquakes caused some cars to sink over a meter and undermined many buildings.