Stromatoporoid response to muddy substrates in Silurian limestones

Stromatoporoids grew on both soft and hard substrates, and recent work revealed primary cavities in Devonian reef systems. We present evidence that stromatoporoids also developed primary cavities in muddy sediments. Stromatoporoids initiated on two categories of material: (1) fine-grained sediment, usually covering large bioclasts; (2) uncovered bioclasts, forming a small area of clean substrate. In both, subsequent growth was previously usually interpreted to be across, and in direct contact with, adjacent fine-grained sediment, itself presumed to be unlithified in most cases. However, stromatoporoid basal surfaces show two types of morphology: (A) smooth, generally curved and flat, and (B) corrugated, with concentric growth rings as minor ridges projecting downwards. Both may be present in different stromatoporoid individuals of the same species. Smooth bases are interpreted in most cases to indicate growth on unlithified sediment, and are common in category 1 above. For corrugated bases, rings are interpreted to result from growth to form minor primary cavities by the stromatoporoid skeleton growing laterally a few mm above the substrate. The base then likely settled into the soft sediment as skeletal mass increased. Primary cavities are interpreted to be similar to those in muddy Devonian facies, but both differ from those in Devonian reefs (e.g. Canning Basin) where cavities may be tens of cm high, and were permanent. In the Silurian samples, a third morphology of concentric basal ring arrangement containing sediment wedges is interpreted as due to episodic sedimentation. Also, stromatoporoids grew on lithified and eroded intraclasts, indicating patchy early lithification of the sea floor in these open shelf settings. Evidence presented from the Silurian examples emphasises the dynamic response of stromatoporoids to their substrate, consistent with sediment-avoiding habits of modern calcified sponges.