Sunday, November 20, 2016

Marbug Sandstones - Dougal Johnston

The tipped up big sandstone boulder in the photos was part of the scree rubble fallen from the Marburg Sandstones in the cliffs and steep slopes either side of Spinach Creek.

These Marburg Sandstones underlie most of the Darling Downs, the Lockyer Valley and are even under the range at Toowoomba. Basalt caps remain on most high ridges and the Dividing Range.

The attached map is cut out from the 1:500,000 scale Moreton Geology (downloadable from the Geoscience Australia website). The map is rather old, but allows us to see the rocks are present over a large area. I have brought in the relevant bit of the legend and added some annotations.

The Field Naturalists have seen these sandstones at Murphy's Creek, Cooby Dam, Mt Sylvia, Hartman Park (Crows Nest) and now at Spinach Creek. Everyone driving east sees them in the highway road cuttings from Withcott to Black Soil.

These sandstones have been considered to be river deposits, mostly as overbank, flood plain and ox-bow lake fill. As such they are not as clean as marine sands, and contain variable amounts of mud, clay sometimes as mud pellet fragments, as well as part weathered feldspars and rock fragments mixed with the quartz grains. The proportion of non quartz grains and grain size were very dependent on the water current.

Slow current meant lots of mud and fine grain size, while fast currents only deposited relatively clean coarse quartz, sometimes even just pebble or gravel beds.

The fine mud filled beds had lots of small spaces between tiny grains (high porosity) but the spaces were not connected (low permeability). In contrast the coarser sands to gravels had connected spaces where water could move and thus high permeability.

The dark red-brown layer in the photos was such a pebble or gravel bed.

During weathering water had been moving through this pebble bed. At depth ground water is reduced, but becomes oxidised as it comes out to the surface (e.g. along a permeable bed outcropping in a cliff).
Reduced iron is easily carried in solution, but as the water becomes oxidised the iron precipitates out (commonly as thin concentric layers in cavities or as coatings on pebbles). These precipitates were probably hydrated ferric hydroxides, but on drying and baking in the sun they convert to hematite (or a variant called maghemite). A characteristic of hematite is the fine powder from scratching (called streak) is bright red.

[Photos provided by Mike Ford]

No comments: