Page 146 - Carbonate Facies in Geologic History

Canning Basin, Western Australia 133

(e.g., on the southeast sides ofWapiabi Creek of Miette Bank (Noble, 1970), and
at the Ancient Wall complex). These banks consist mostly of interior facies, light-
colored, well-bedded limestones and dolomites with restricted marine faunas
(Amphipora beds and peloid mudstones with radiosphaerid calcispheres). Their
margins of massive light-colored, coarse, sucrose dolomite must represent organic
growths of algae and stromatoporids. In the mountains these facies are termed
Southesk Formation. The slope deposits are of more interest. Within a few
hundred meters the obvious bank facies gives way to a variety of calcarenitic beds
with onkoids, coarse bioclastic fragments and in some places, coral heads. Crinoi-
dal beds and 1hamnopora debris are known in such strata at Wapiabi Creek; at
Miette the detritus is lithoclastic with calcirudite pieces composed of cemented
lime sand in a calcarenite matrix whose grains were originally uncemented. This
interstitial calcarenite was later much compacted compared to the fragments of
coarser debris included within it as clasts. Such beds may represent sand flows
down the slope. Spectacular megabreccias are described at Ancient Wall resulting
from debris mud flows which moved large blocks up to some tens of meters in
diameter. These blocks occur chaotically in a micritic matrix and are derived from
various facies on the bank and its margin. No grading is obvious in the megabrec-
cias but some finer beds composed of locally derived shelf grains are graded
(allodapic limestone of Meischner). The megabreccias occur in beds from 3 to
25 m thick, some with channelled but generally planar bases (see illustration in
Cook et aI., 1972; Mountjoy et aI., 1972).
The megabreccias extend a few km into the basin where they are interbedded
with dark shale and siltstone. Slopes no greater than 2 degrees were necessary for
the processes of slumping and flowage which created these wholely submarine
deposits. Presumably they were caused by storm wave or tsunami phenomenon or
possibly by earthquakes, although regionally the environment appears to have
been tectonically stable. Such coarse sedimentary deposits are unknown in the
low-relief banks drilled in the Alberta subsurface.

Canning Basin, Western Australia

The Middle and Late Devonian (Givetian and Frasnian stages) of semiarid north-
western Australia, contain some of the finest exposures of ancient carbonate
buildups in the world. They are arranged along a Devonian to Late Paleozoic
fault system which trends within and borders the narrow Lennard shelf on the
northeastern side of the Fitzroy trough. This shelf separated the Kimberley Pre-
cambrian block of the Australian continent from a deep water basin, later filled by
Permian terrigenous and glacial sediments and now buried in the subsurface. We
owe our present clear understanding of this paleogeography and the related reefs
to an excellent publication of the Geological Survey of Western Australia (Play-
ford and Lowry, 1966, see Fig. IV-23).
The exposed Lennard shelf trends about 400 km in a southeasterly direction,
inland from Derby, Western Australia. It contains about 500 m of Devonian
strata with classical interreef, forereef, reef, and backreef facies. The buildups
along the shelf take many forms: a barrier with steep slopes directly into the deep