ABSTRACT

The Pillara Formation (Devonian), Canning Basin, Western Australia contains 3 members: 3 -- Menyous Member (420 m max. thickness), consisting of cyclical carbonate sediments, overlies 2 -- Red Bull Member (270 m max. thickness), consisting of cyclically interbedded carbonate and terrigenous sediments, overlies 1 -- Big Spring Member (over 200 m thick), consisting of cyclical terrigenous sediments.

Carbonate cycles of the Menyous Member are of 2 types: Type A carbonate cycles contain in descending order

2. Pellet/intraclast limestone with tubular fenestral and cryptalgal fabrics

1. Biostromes up to 3 m thick, containing in descending order

(iii) Amphipora limestone

(ii) Stachyodes limestone

(i) subspherical-stromatoporoid limestone

Type B carbonate cycles contain in descending order

3. Pellet/intraclast limestone with cryptalgal and fenestral fabrics

2. Biostromes 3 to 10 m thick, containing in descending order

(iii) Stachyodes limestone

(ii) subspherical-stromatoporoid limestone

(i) tabular-stromatoporoid limestone

1. Skeletal-fragment limestone.

Carbonate cycles are shoaling-upward sequences formed by periodic rapid submergence followed by shoaling under stable conditions. Limestones with cryptalgal and fenestral fabrics are tidal-flat facies which provide approximate sea-level datums for estimation of water depths of other cycle lithofacies. During deposition of Type A carbonate cycles, water depths were shallow (3 m or less) whereas maximum water depths during deposition of Type B carbonate cycles reached 10 m or more. Upward shoaling was accompanied by decreasing energy conditions together with biotic changes.

Distinctive cryptalgal and fenestral fabrics in pellet/intraclast limestones occur in a definite vertical succession. Thick sequences (2 to 10 m) contain repeated successions and minor reversals. The fabric sequences reflect shoaling and seaward progradation of algal mats (intertidal), and halophytes (mainly supratidal). In many supratidal areas internal sediments, leach fabrics and colourmottles were developed in sediments under vadose zone conditions.

Carbonate-terrigenous cycles (over 1 to 6 m thick) of the Red Bull Member consist of, in descending order,

2. Massive or flat-bedded to cross-bedded sandstone, minor siltstone and pellet intraclast limestone

1. Subspherical- and cyclindrical-stromatoporoid biostrome.

Carbonate-terrigenous cycles are shoaling-upward sequences formed by periodic rapid submergence of deltaic plain/tidal-flat environments, followed by shoaling

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by biostrome growth under stable sea-level conditions. Maximum water depths were probably 2 to 3 m. Shoaling caused seaward progradation of deltaic sediments over biostromes.

Terrigenous cycles of the Big Spring Member are fining-upward sequences (2.5 to 20 m thick), containing

2. Red calcareous siltstone, interbedded dolomitic limestone and thin sandstones (in Type A cycles) or Interbedded green, flat-laminated siltstone, shale and thin sandstone (in Type B cycles), overlies

1. Festoon cross-bedded coarse sandstone.

Thick, coarse sandstones are channel deposits whereas siltstones and thin sandstones are overbank deposits (levee and flood-plain deposits). Type A terrigenous cycles probably formed in alluvial plain/upper deltaic plain environments; Type B terrigenous cycles were formed in lower deltaic-plain environments.

Pillara Formation members are an onlapping sequence formed during marine transgression from south to north and west to east. Thick clastic sections in the northeast indicate that the northern Kimberley Block was a paleo-landmass. Direction of clastic transport was to the southeast. Tidal current directions were probably normal to the present margin of the Kimberley Block, suggesting that the paleo-shoreline paralleled this trend.