Australia: The Land Where Time Began

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Gingin Area - Northern Perth Basin

Poor exposures of strata of Cretaceous age are present along part of the Gingin Scarp and Dandaragan Plateau. Gullies in the scarp are the sites of most outcrops, of rocks of the Coolyena Group. Direct correlation to the standard chronostratigraphic scale is possible only in the Gingin Chalk, as it is the only unit containing fossils. A comprehensive set of illustrations of the macrofossils found in the Gingin Chalk has been provided (McNamara et al., 1993). Dinocyst and spore-pollen assemblages have been found in the Osborne Formation, Molecap Greensand and Poison Hill Greensand that allow correlation with local palynomorph zonations (Marshall, 1985). These are also widely used in petroleum exploration of the West Australian North West Shelf. They have been more widely defined (Helby et al., 1987). Some of the contacts are apparently irregular, as indicated by thickness differences between the units, that are subhorizonal, over small distances.

There is an abundant foraminiferal fauna in the Gingin Chalk with succession datum levels that are very consistent with that of the Southern Carnarvon Platform, that is 600-1000 km to the north, suggesting that similar oceanic conditions, as well as sea level histories, prevailed in both areas in the Late Cretaceous. 

The type section of the Gingin Chalk is of Santonian-Campanian age, based on the presence of Uintacrinus socialis, a crinoid, below about 4 m, and between 4 and 6 m in the type section, Marsupites, including the typical M. testudinarius (Withers, 1924, 1926; Feldman, 1963). Also nannofossils, at 5.1 m and above, Calculites obscurus and at 20.5 m and above, Broinsonia parca (Shafik, 1990).

B. parca first appears in the Gingin Chalk type section above the last appearance of Marsupites. In successions in Europe and North America these 2 organisms also appear in the same order (Gale et al., 1995). Of the 6 biostratigraphic events recognised for the Late Santonian - earliest Campanian (Gale et al., 1995), the first appearance of C. obscurus above the first appearance of U. socialis was the only one to be supported by evidence from the Gingin type section, the first known appearance of C. obscurus in the Gingin type section being within the Marsupites testudinarius Zone, and not within the lower, Uintacrinus socialis Zone. According to Burnett (1998), in northern Europe and the Indian Ocean, the first appearance of consistent C. obscurus was in her Zone UC11c, correlating this level with the Lower Santonian, below the first appearance of U. socialis, that she believes marks the Middle Santonian. The first appearance of B. parca has been used to indicate the base of Zone UC14 in the Indian Ocean, being placed in the 'Lower Lower Campanian' (Burnett, 1998), the extinction of Marsupites being taken by her to mark the boundary between the Santonian and the Campanian.

It has been provisionally recommended that the extinction level of Marsupites testudinarius, a crinoid, be used to mark the Santonian-Campanian boundary (Hancock & Gale, 1996). It has also been suggested that the first appearance of Uintacrinus socialis be used to mark the base of the Upper Santonian (Lamolda & Hancock, 1996). Based on these criteria the lowest 6 m of the type section of the Gingin Chalk would be within the Upper Santonian, the rest of the deposit being within the Lower Campanian (see Locality 4, Kalbarri Area, Source 1).

According to unpublished zonation of dynacists (Marshall, 1985), in Ginginup 1, the underlying Molecap Greensand ranges between the Cenomanian and the lower Santonian. The boundary between the Molecap Greensand and the Gingin Chalk is very irregular over short distances, to the point where the Molecap Greensand is missing completely in places, the Gingin Chalk lying directly on the Leederville Formation (Playford et al., 1976), though close to Gingin, the Molecap Greensand is directly above the Leederville Formation (Mory et al., 2005).

It has been suggested that the Yallalie structure, that is 100 km north of Gingin, is possibly an impact crater, the impact occurring after the deposition of the Molecap Greenstone and before that of the Gingin Chalk  (Dentith et al., 1999). It has been suggested that this may be the cause fo the undulating topography of the boundary between these formations. It was also suggested that it may have been the cause of other characteristics of the Molecap Greensand, such as the composition, lack of bedding, and mixture of fossils. The authors suggest an impact on a broad continental shelf of low gradient, in water that may have been 100 m deep, the waves propagated by the impact generating currents that mixed siliclastic and authigenic sediment (deep sea sediment formed in place on the sea floor) from different facies and rapidly depositing them to produce an undulating sea floor.

Locality 35 - McIntyre Gully. About 2.5 km north of Gingin. Gingin Chalk, Upper Cretaceous, type section of fossiliferous marine facies.

Locality 36 - Molecap Hill. About 1.4 km south of Gingin. Molecap Greensand, Upper Cretaceous, type section of shallow marine facies in a quarry.

Locality 37 - Poison Hill. Poison Hill Greensand, Upper Cretaceous, the type section of shallow marine facies is in a ridge.

Sources & Further reading

  1. Geology of the Northern Perth Basin, Western Australia - a field guide
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 21/10/2016

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading