The sedimentology and stratigraphy of the permo-carboniferous Grant group, Barbwire Terrace, Canning Basin, Western Australia.
The Canning Basin is a large intra-cratonic basin which underlies an
onshore area of 430,000sq. km. The study area, located on the Barbwire
Terrace, contains a series of stratigraphic boreholes drilled by Western
Mining Corporation Ltd., which provide fully cored sections through the
previously poorly exposed Grant Group.
From this core, integrated with seismic data and wireline logs, the
Grant Group has been divided into three new formations, each containing a
number of distinctive and intimately related facies types.
The basal Hoya Formation comprises a complex suite of interbedded
diamictites, sandstones and mudstones. The diamictites are interpreted as
lodgement tills, melt-out tills and flow tills, deposited from the
retreating ice sheet. Interbedded with the diamictites are massive and
laminated mudstones, deposited under fluctuating marine and lacustrine
conditions. Stacked cross-bedded sandstone units are restricted to the
west of the study area, forming subsurface linear mounded features, clearly
displayed on the regional seismic. These sandstones are interpreted to be
deposited from braided fluvial outwash systems. However, the majority of
sandstones are massive and normally graded, of mass-flow origin, deposited
from a series of subaqueous fans fed by meltwater from the ice sheet.
The overlying Calytrix Formation contains a thin basal sandstone
unit, rich in marine fauna, but is characterised by a thick sequence of
basinal mudstones. It is overlain by the Clianthus Formation, which has a
basal fluvial sandstone unit, capped by heterolithic sandstones, siltstones
and mudstones, interpreted to be shallow marine shelf deposits.
The Grant Group sediments record the gradual deglaciation of the
basin, and indicate that the ice sheet was extensive during the Perm-
Carboniferous. The Hoya Formation contains all the glaciogenic sediments,
and provides evidence for periodic ice advance and retreat. The mudrock
dominated Calytrix Formation is interpreted to reflect the rise in sea
level subsequent to the main deglaciation phase, and the regressive package
of sediments that form the Clianthus Formation result from isostatic uplift
and basin fill under post glacial conditions.