Countertop geology: Orbicular granite, Boogardie Station, Mt Magnet, Western Australia

 

Orbicular granite featuring a large vein which has bisected a number of the orbs. The vein in the upper right seems to have experienced a significant level of movement resulting in the offset of a previous creamy coloured up/down vein.

 

A closer look at the orbs and their growth patterns. SHRIMP U-Pb analysis of the matrix titanite is 2687 ± 5 Ma (one of the oldest known orbicular granites)

A brief outline of the Boogardie Granite for the National Rock Garden and the mindat entry.

Photos were taken in the Melbourne Museum on the  May 7th, 2015 (yes, I know, this post took a while. There's a good reason for that!).

Places I've been: Umpherston Sinkhole, Mt Gambier, South Australia

 

While I was in Mt Gambier a while ago, I got the opportunity to visit the Umpherston Sinkhole. Located within 5 minutes of the CBD, the sinkhole is very approximately 50m deep. Originally  beautified by James Umpherston in 1886, the sinkhole has been replanted to restore it's original format. In the past I've seen both tame possums begging for apple scraps and a clutch of random guinea pigs racing around the base of the sinkhole.

 

The sinkhole originally formed through the dissolution of calcite from the limestone. This slowly created a cave. The roof eventually fell in on the northern side allowing meteoritic water into the sinkhole. This aided further dissolution resulting in a slope to the base of the sinkhole. Further collapses opened up the hole to the size seen today. The tourist signs around the sinkhole indicate there was originally a lake that took up a good portion of the hole but this has disappeared with the lowering of the local water table. The lowest point of the sinkhole seems to remain damp which suggests additional collapses may be on the horizon.

 

The gardens are lovely, and the early morning sunlight allowed me to take the above image. It looks so much like a nature documentary!

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.