Neptunian dikes and sills are conspicuous features of early-cemented limestones in Devonian reef complexes of the Canning basin. They are sedimentary fillings of fissures formed by fracturing of the limestones soon after deposition, and are most abundant in the reef subfacies, but also occur in some back-reef and marginal-slope deposits.

The dikes and sills were filled with contemporary sediment (including oolite, peloid grainstone, biomicrite, and calcareous sandstone), encrusting organisms (Renalcis and stromatolites), and early radiaxial calcite cement. Some of the sediment is dolomitized or mineralized. Some dikes contain concentrically coated balls of radiaxial calcite up to 0.3 m (1 ft) across. Sediment in the dikes and sills was cemented rapidly, and repeated refracturing and infilling occurred during brief intervals. The largest dikes are up to 6 km (3.5 mi) long, 20 m (65 ft) wide, and originally extended more than 80 m (260 ft) below the surface. However, most are less than 100 m (330 ft) long and 5 cm (2 in.) wide. Neptunian sills are up to 0.8 m (2.5 ft) thick. The principal strike directions of major dikes approximately parallel the original reef fronts of underlying basement ridges. Lateral extension through the development of fissures amounted to as much as 6.5%, and averaged about 3% in measured reef sections. The fissures allowed circulation of large volumes of seawater to considerable depth within the reef complexes, and must have played a significant role in early diagenesis.

Fissuring probably resulted from earthquake shaking, differential compaction of underlying sediments, and slippage along bedding planes, especially in the steeply dipping marginal-slope deposits.

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