ABSTRACT

The Devonian Pillara Formation in the Canning Basin, Western Australia, contains carbonate cycles of 2 basic types:

Type A cycles --- consist of, in descending order:

• 2) limestone with cryptalgal and fenestral fabric,
• 1) stromatoporoid biostrome (up to 3 metres thick) containing
  • (iii) Amphipora limestone at the top,
  • (ii) Stachyodes limestone,
  • (i) subspherical-stromatoporoid limestone; rests on cryptalgal and fenestral limestone of previous cycle.

Type B cycles --- consist of, in descending order:

• 3) limestone with cryptalgal and fenestral fabric,
• 2) stromatoporoid biostrome (up to 10 metres thick) containing
  • (iii) poorly developed Stachyodes limestone at the top,
  • (ii) subspherical-stromatoporoid limestone,
  • (i) tabular-stromatoporoid limestone,
• 1) skeletal-fragment limestone.

Type B cycles differ from Type A in having skeletal-fragment and tabular-stromatoporoid limestone, and lacking Amphipora limestone.

The type section of the Pillara Formation contains approximately 70 cycles. Type A cycles are typical of the lower part of the formation whereas Type B cycles occur in the upper part of the unit. Cycles reflect periodic rapid subsidence (or sea-level rise) followed by shoaling sedimentation under relatively stable sea-level conditions. Upward shoaling is accompanied by increasing biotic restriction and decreasing energy conditions.

Limestone with cryptalgal and fenestral fabrics are tidal-flat facies which provide approximate sea-level datums that may be used to estimate maximum water depths of other cycle lithofacies. Water depths during deposition of Type A cycles were relatively shallow (less than 3 m) whereas maximum water depths during deposition of Type B cycles reached 10 m or more.

1 Manuscript received July 20, 1972.

2 Department of Geology, The University of Western Australia, Nedlands, Western Australia.

This paper is an early result of investigations of Devonian limestones in the Canning Basin, being carried out in the Department of Geology, University of Western Australia as Research Project 52402572MT. The project is supervised by Brian W. Logan and is funded by grants from West Australian Petroleum Pty. Ltd. The author acknowledges the use of field maps, age data and background information made available by WAPET, and helpful discussions with R. I. Alexander Jr. and D. A. Lyons. The manuscript was critically read by Brian W. Logan and V. Semeniuk.

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