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Molecap Greensand & the
Overlying Gingin Chalk
A continuous weathered sequence of shallow marine strata is
comprised of the Molecap Greensand and the overlying Gingin Chalk.
Around the townships of Dandaragan and Gingin in the
Perth Basin of
southwestern Western Australia the strata outcrop extensively. The
Molecap Greensand is mostly comprised of glauconitic sandstones with no
distinct bedding, the authors1
suggesting there may be considerable amounts of reworked material
contained in them. The extremely fine chalky sediments of the overlying
Gingin Chalk is composed of compacted planktonic skeletons. Shallow
coastal marine conditions with bottom water that was poorly oxygenated
are believed to have been the Palaeoenvironment in which both units were
deposited.
The age of the Molecap Greensand has been disputed. An unusual
mixed assemblage of plankton from the Cenomanian-Coniacian is indicated
by rich palynomorph assemblages. The authors1
suggest this may have resulted from a sediment slump, possibly
associated with the formation of the Yallalie impact structure,
a buried crater that resulted from an asteroid impact in the Late
Cretaceous. A number of ages have been proposed for the Gingin Chalk,
from the Santonian to the lowermost Campanian. The fossil record in the
Molecap Greensand/Gingin Chalk, though fragmentary, contains evidence of
marine reptiles - mosasaurs, plesiosaurs and ichthyosaurs, sharks, bony
fish and dinosaurs. In the Gingin Chalk the invertebrate record is
dominated by benthic molluscs.
Sources & Further
reading
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Kear, B.P. &
Hamilton-Bruce, R.J., 2011, Dinosaurs in Australia, Mesozoic life
from the southern continent, CSIRO Publishing.
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