Linda and Roger's Bunhybee Grasslands

Geology

This is a page within Roger and Linda's Bunhybee Grasslands Web-Site.
Bunhybee Grasslands is a 49ha/120-acre conservation property 35km south of Braidwood, in sthn N.S.W.

This is a first-cut attempt to build some information about the geology underlying the property.
The purpose is not abstract science, but part of a process of understanding about the soil-types.
The content was developed from an hour or two's web-surfing, and at this stage nothing more.

• An area map should be available from the Geoscience Portal and/or Geosciences Australia
• This report from the mid-1950s says that:
  • "quartz felsite [a very fine grained volcanic rock that may or may not contain larger crystals] forms the valleys of the Jerrabattgulla Creek" (p.6)
  • "The 'Upper Shoalhaven Granite' is a biotite [a common phyllosilicate mineral within the mica group] granite [a common type of felsic intrusive igneous rock that is granular and phaneritic in texture] and has a mapped length of sixty-four miles.
    "Its most northerly outcrop is two miles east of Lake Bathurst Railway Station and it extends to the southern edge of the Canberra 4-mile sheet; its average width is eight miles. It is obscured by the Devonian volcanics north of Krawaree and by the Devonian lavas and Tertiary sediments north-west of Lambert.
    "A notable pendant or screen of Silurian(?) strata extends of average width two miles ... south from Mount Fairy for a distance of twenty-four miles.
    "The granite is cut by the Jerrobattgulla Fault which,is fifty miles long. It is strongly sheared in parts especially alongside contacts with sediments, and is probably of Bowning age" (p.10)
  • "Granite to the north-west of the [Jerrabattgulla Creek] fault exposed in a quarry is sheared and mineralised"
• This report from 1980 says that:
  • "At Jerrabattgulla Creek, basalts dated at 19.1±0.4 m.y. were extruded along a 5 km length of the active Shoalhaven Faultline; about, 30m of vertical movement has taken place along the fault since the basalts were deposited". Presumed translation. There was a volcano close by c.20m years ago
• A map from a study of the Shoalhaven catchment identifies the area as "Goulburn Fractured Rock"
• This map shows that the nearest entries on the NSW soil and land information database are 1500m SW (Gilston?) and 500m N (Di Izzard's property) – with:
  • 6 inches of "fine sandy clay loam" (!? – maybe well down the slope towards the creek?) and
  • "highly weathered rock rhyolite layer" [an igneous, volcanic rock, of felsic (silica-rich) composition] from 1m deep downwards"
• Within the Land History page, we have that an Appraisement of 5 C.P.'s on the eastern side of the road [Bunhybee and its neighbour Parlour?] was undertaken on 12 October 1907 – p.1, p.2, p.3. ...
  The physical characteristics of the 5 Lots (as a whole) was "Reasonably level land, with a gradual Western Slope". The character of the soil and sub-soil was Poor clayey soil. Stoney to Rockey. Clay"
• This report says that, in the area to the S and/or E, "The granodiorite [a phaneritic texture intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar] is probably Early Devonian [400m years ago] and has been deformed in the Carboniferous"
• We've observed on the property (with no geological competency whatsoever):
  • white, limestone-looking (clay?) scrapes
    (Clay is a pretty generic term: "a fine-grained natural rock or soil material that combines one or more clay minerals with traces of metal oxides and organic matter")
  • multiple small granite-looking outcrops
  • a small number of conglomerate rocks (fallen/tossed from Bunhybee Peak?)
• A Geological Study about 10km South of Bunhybee, from Uni of Adelaide, and another output from that study

This is a page within the Bunhybee Grasslands Web-Site, home-page here, and site-map here
Contact: Linda or Roger
Created: 17 October 2015; Last Amended: 18 November 2015