Gingin (Molecap Hill; Poison Hill; MacInytre Gully), Gingin Shire, Western Australia, Australiai

Regional Level Types
Gingin (Molecap Hill; Poison Hill; MacInytre Gully)	- not defined -
Gingin Shire	Shire
Western Australia	State
Australia	Country

(Map- Poison Hill locality)

Gingin is a small farming town, situated forty miles north of Perth.
Gearksutite associated with phosphatic nodules.

Based on one of the references (Geology of the Northern Perth Basin), three localities are listed, Poison Hill, Molecap Hill, MacIntyre Gully, which are studied as examples of this greensand formation. However, glauconite bearing greensands stretch for 150 kms between Gingin and Eneabba. It averages 15 kms wide between the Darling Fault and Gingin Scarp, in the Dandaragan Trough between. It is a potential huge source of potash.

Poison Hill is 6 kms north north-west of Gingin, MacIntyre Gully 2.5 kms north and Molecap Hill 1 km south of Gingin. All are on wheat farms, and therefore private property. Due to the extensive nature of the deposit, examples exist on public land elsewhere.

In general the greensand is poorly exposed, and found mainly on the surface in gullies along the scarp, and classified in the Coolyena Group. The Molecap and Poison Hill Greensand units contain dinocysts and spore-pollen assemblages. Molecap Greensand unit lowest and the Poison Hill Greensand uppermost. The greensands are an anomalous association of coarse to grain sized quartz and medium sized dark green glauconite. Inbetween the two greensand units is the Gingin Chalk containing foraminferal fauna of the Santonian-Campanian age. The basal unit is 1-2cm chalk clasts set in glauconite-quartz-chalk matrix. Then 5-15 metres of bioturbated chalk with scattered fragments of large inoceramus shells deposited when the water depth was 100-200 metres.

The Molecap Hill locality is a quarry where 6510 tonnes of glauconite was mined between 1932-1962, used as a water softening agent. The quarry exposes the Molecap Greensand unit, 11 metres thick, flat-lying greensand and glauconitic sandstone with phosphate (apatite, dufrenite and vivianite) nodules near the top.

18 metres of fossilferous friable slightly glauconitic Gingin Chalk overlies the greensand at this locality. Macrofossils have been found in the very top and bottom of the phosphate rich nodule beds at the quarry. The bottom band is no longer exposed. These contain theropod dinosaur bones and rarely shark teeth and bones.

E.S. Simpson in 1920 reported the occurrence of a gearksutite horizon at the locality, the third location for the species at that time. The horizon was 30cm thick, but did not appear to be laterally extensive. It also contained abundant glauconite and phosphate nodules, 1.8 metres below the surface, near the base of the upper greensand.

The Poison Hill location contains medium grained to granule glauconite-quartz sandstone, bioturbated in the lower 5 metres, and strongly ferruginized in the upper section. It contains abundant spore, pollen and microplankton, and the unit here is thought to be about 50 metres thick.

At MacIntyre the glauconite is 7 metres thick, dark green, rounded, and crossed by shrinkage cracks. Individual quartz grains within this come in a variety of forms. Also found in this unit is shark teeth, scales and bones, monasaur remains and small saurius limb bones in the upper 2 metres of the unit. Rare ichthyosaur and plesiosaur remains have been found in the unit west of Dandaragan.

The fossils in general are thought to be Cenomanian to Coniacian aged. There is a mix of terrestrial plants, marine plankton, and mixed land and marine fauna in the units. The reference suggests this may have resulted from slumping caused by the Yallalie impact crater (another story) 110 kms to the north.

Other Regions, Features and Areas containing this locality

References