Page 147 - Carbonate Facies in Geologic History

134 The Advent of Framebuilders in the Middle Paleozoic

REFERENCE
PALAEOZOIC. MESOZOIC
AND RECENT
DEVONIAN REEF
COMPLEXES

EJ PRECAMBRIAN

o SCAI..E OF MILO eo
.. 0

Fig.lV -23. Locality map, northern Canning basin, northwestern Australia. From Playford
and Lowry (1966, Fig. I). Middle and Late Devonian reef trend lies along the Lennard shelf
margin

Fitzroy clastic trough (Napier Range), fringing reefs with lower relief lying within
the shelf, carbonate ramps, bordering islands and peninsulas of Precambrian
"basement" (Oscar Range), and shallow lagoon atolls or far os in the southeastern-
most shelf exposures in Emanuel Range (Bugle Gap). Despite the steep shelf
margin, immense amount of forereef debris, and proximity to an eroding land
mass-all of which are differences from the Alberta buildups, the biota developed
in the two areas is very similar. It is instructive to compare two such widely
separated areas and fortunately researchers such as P. E. Playford, E. Mountjoy,
lL. Wray and A.l Wells have been able to do field work in both areas.
The following descriptions derive principally from Playford's work on the
steeper barrier reef along the Napier Range. Later some comments are made on
the atolls and low relief banks known farther southeastward. The stratigraphic
subdivision given on Fig. IV-24 is for the southeastern ranges but serves as well
for the northwestern end where the total forereef facies is not subdivided and is
included in the Napier Formation.
The forereef and the interreef areas were deposited in water ranging to more
than 200 m in depth. Spectacular depositional dipping strata of 25-30 degrees
have been mapped over horizontal distances of some kilometers. These have
extensively built out basinward from the linear reef trend (Fig. IV -25). Geometric
reconstructions from such dips and subsurface indications from a few wells are
that the Devonian floor of the basin could have been hundreds of meters deep.
This floor is now buried beneath 4000 m of Late Paleozoic sediment filling the
Fitzroy trough. The forereef sediments contain blocks of reef limestone as much
as 10-100 m across. These megabreccias occur in Iithoclastic limestone of float-