Saturday, 1 June 2013

The Koukandowie Formation has a cool Name

Previously I’ve completed blogs on the stratigraphy of the upper units of the Clarence-Moreton Basin. These upper units have been the Grafton Formation (youngest at Late Jurassic), Woodenbong Beds, Kangaroo Creek Sandstone and Walloon Coal Measures including the Maclean Sandstone Member of that unit (Middle Jurassic). Now, as we get towards the middle units of the basin we get into the Early Jurassic with much more complexity to the mode of formation of the geological units. Because of this the stratigraphic units have been divided into groups, subgroups, formations and members. The First one that I will tackle is the Koukandowie Formation, which is part of the Marburg Subgroup which in turn is part of the Bundamba Group.

The Koukandowie Formation actually is made up of an additional three members known as the Heifer Creek Sandstone Member, Ma Ma Creek Member and Towallum Basalt. But I’ll focus on these individually in future posts, for the time being it is worth noting that the Towallum Basalt is a very important unit for understanding the relative age relationships of all of the units in the Bundamba Group. For the time being I’ll focus on the Koukandowie Formation specifically.

Bundamba Group with conglomerate and abundant organic fragments - Tabulam
Unfortunately I do not have a photo specifically of the Koukandowie Formation. But I have attached a photograph a similar type of rock as the Koukandowie of the an undifferentiated part of the Bundamba Group.

The Koukandowie Formation was deposited in a dominantly fluvial (riverine) environment. Essentially the unit is comprised of sets of channel lithic sandstone (sandstone made from fragments of older rock) with some finer grained rock such as siltstone and even shale. But the formation also thin layers of conglomerate or occasional woody fragments that have turned into coal. The way the lithic sandstone was deposited means that a feature known as cross-bedding is very common and this structure is further evidence of its fluvial origin. The exact nature of these particular cross-bedding structures it interpreted by Wells & O'Brien 1994 as meaning that the river system that created the Koukandowie Formation was not a meandering stream but fairly straight. A modern example might be the middle reaches of the modern day Clarence River.

A rough stratigraphic guide to the Bundamba Group
(Walloon Coal Measures are above and Gatton Sandstone under)
The Koukandowie Formation was considered an important formation for gas and oil exploration in the region. The Koukandowie was thought to be a generally an impermeable unit, that is, stops the migration of fluids and gases such as oil and natural gas. This means that the underlying units of the Bundamba Group which are more conducive to forming and storing these gases and fluids may retain them in structural traps (such as folds in the earth or faults). How effective this unit has been seems to be a bit hit and miss. I understand that the NSW Government and some companies during the 1970’s and 1980’s had some success with this model but not enough to make it viable financially at the time. More recent work by exploration companies has shown this to be on its own not-viable. But when combined with similar modes of gas source rocks in the overlying Walloon Coal Measures, other more deeply buried organic rich units and other sources of gas directly from coal seams the economics seem to have looked good for some companies.

As for ground water sources, like the overlying Walloon Coal Measures the Koukandowie Formation does not contain much in the way of useful fresh groundwater. This is for two reasons:

  1. the finer grained components of the formation tend to contain more salt due to the some of the sedimentary depositional environment; and
  2. the Koukandowie Formation tends to show very little lateral porosity. This means that the water is stored in smaller localised aquifers of low long term yield. 

These reasons also imply the nature of recharge of what aquifers to occur in the area. Essentially vertical percolation from surface water through fractures is the main driver of aquifer recharge. Though there are exceptions due to the location of the sub-units in the Formation.

As for the name the Koukandowie Formation takes its name from Mount Koukandowie which is located near Nymboida. The formation outcrops in a relatively thin band in from the margins of the Clarence-Moreton Basin. I don't think that the formation outcrops anywhere in the middle areas the Clarence-Moreton Basin but certainly occurs at within the basin depth. The formation tends to weather and erode easily and therefore most of the outcrop of the unit shows relatively subdued landforms of rolling hills.

References/bibliography:

*McMahon, G.A. & Cox, M.E. 1996. The relationship between groundwater chemical type and Jurassic sedimentary formations: The example of the Sandy Creek Catchment, Lockyer, southeast Queensland. Mesozoic 96 Conference at Brisbane - extended abstracts.
*O’Brien, P.E. & Wells, A.T. 1994. Sedimentology of the Bundamba Group. In Wells, A.T. & O’Brien, P.E. 1994. Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Bulletin 241. Australian Geological Survey Organisation
*Wells, A.T. & O’Brien, P.E. 1994. Lithostratigraphic Framework of the Clarence-Moreton Basin. In Wells, A.T. & O’Brien, P.E. 1994. Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Bulletin 241. Australian Geological Survey Organisation.
*Wells, A.T., O’Brien, P.E. Willis, I.L. & Cranfield, L.C. 1990. A new lithostratigraphic framework of the Early Jurassic units in the Bundamba Group, Clarence-Moreton Basin, Queensland and New South Wales. B.M.R. Journal of Australian Geology and Geophysics. V11.

1 comment:

  1. Thanks for sharing such a detailed and comprehensive article on the rock formation.

    ReplyDelete