6 
II. THE BEAU DESERT AND BOONAH AREAS. 
A. Geological Sequence. 
Mr. C. 0. Morton, of the Queensland Geological Survey, has dealt with the geology and 
the petroleum possibilities of this area in a very able sketch.* Briefly, the sequence exposed 
is as follows : — 
Formation. 
Description of Strata. 
Thickness. 
Alluvium . . 
Alluvial clays and water sands 
100 feet 
Tertiary . . 
Blue arid red clays, sometimes calcareous, with ostracods. Thin carbonaceous bed . . 
1 00 feet 
Basalts 
1 Variable, 
Acid Yolcauics 
V may reach 
Gabbroid and doleritic intrusives 
J 8,000 feet 
Probable ( ! ncouiformity. 
■ 
Upper (?) Walloon Goal Measures. Fresh water series, sandstones predominating, false- 
bedded, sometimes massive, sometimes Saggy, more or less calcareous : calcareous 
and carbonaceous shales and concretionary ironstone bands with plant remains, 
At least 2,000 
Cladophlebis australis, Taeniopteris spathulata 
f feet exposed 
( 'oal seams . . 
1 
Jurassic s 
All very lenticular . . . . . . . . . . ' 
J 
Lower (?) Bundamba Sandstone Series. Massive sandstones, false-bedded, varying from 
Approximately 
coarse siliceous grits, sometimes conglomeritic, to fine feispathic beds. Some thin 
4,000 feet 
carbonaceous shales, especially in lower beds exposed. Fossil wood preserved as 
exposed 
siliceous casts 
Dr. Jensen would place the Bundamba Series in the Upper Triassic owing to the fact that 
some of the thin carbonaceous shales in the lower part of the series contain plant remains which 
bear an Ipswich aspect. On the other hand, the Bundamba sandstones apparently merge, without 
unconformity, into the overlying Walloons, so that where the base of the Walloons happens to 
consist of massive sandstones it is impossible to draw a line between them. There is, however, 
little room for controversy, and the classification is rather one of convenience than otherwise. 
The line of division between the Upper Triassic and the Lower Jurassic beds probably comes 
somewhere in the Bundamba Sandstone Series, and since it is obviously impossible to make a 
line of division in a group of massive and mostly barren sandstones, the Series must be included 
in one or other of the divisions according to the inclinations of the observer. I have, therefore, 
adopted the classification of the Geological Survey of Queensland. 
B. Structure. 
Structurally, the country around Beaudesert and Boonah may be divided into three almost 
equal portions, a central anticlinal area, the axis of which runs almost due north and south through 
Mt. Jubbera, near Overflow, and the small town of Rathdownev, with corresponding synclinal 
depressions on either side. Beaudesert being situated near the trough of the eastern syncline. 
Boonah being a little to the east of the trough of the western syncline. This anticline is sometimes 
called the Overflow Anticline. The folding is decidedly asymmetrical, the dips on the eastern 
margin of the anticline being as high as from 40° to 65°, those on the western side averaging about 
10° before the beds flatten out in the lower portions of the syncline. The beds outcropping along 
the axis of the anticline consist of the massive sandstones of the Bundamba Series, which are badly 
cut up by erosion and weathering. The Walloon coal measures, sometimes concealed by alluvium, 
and, near Beaudesert, overlain by Tertiary deposits (lake beds'?) occupv the synclines. The anticline 
pitches to the south, where the sedimentary strata pass under immense sheets of basalt and acid 
volcanic rocks, and are thus, together with any structural features present, concealed. There is 
evidence, however, that the folding becomes less marked in this direction, the beds either flattening 
or else the forces which were at work to produce the strong simple folds to the north were dissipated 
in the formation of a series of smaller folds, which is a very common phenomenon where a strong 
folding movement is dying out. It is very evident that the Walloon measures originally passed 
right over the axis of the anticline, and have been subsequently eroded from it, so that now they 
are only found in the basins on either side, although the folding movements have been so 
comparatively recent that the streams have been able to retain their ancient channels cutting down 
through the beds as fast as they were being elevated, so they now occupy gorge-like valleys' which 
cut transversely across the axis of folding through the rough' hills and peaks formed by the archin o- 
and erosion of the massive Bundamba grits. Numerous igneous dykes and sills break the 
* “A Geological Reconnaissance of the Upper I o-mn and Albert River Watershed 3 
bilities.” — Q.G.MAT., vol. NX IV., No. 387*, 14fh July, 1023, pp. 241 240. 
Sontli -Mhrehm Wrtrfet, with special Brference to Petroleum Pn»«i- 
