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]
