Novel reef fabrics from the Devonian Canning Basin, Western Australia

Large cement-filled cavities (0.2 to 1.5 m wide) are well developed in slope-margin sediments of the spectacular Upper Devonian (Frasnian) reefs of the Canning Basin, Western Australia, where they account for up to 50% of the primary porosity. These are here interpreted as primary reef framework cavities that formed beneath a variety of domal, tabular or laminar stromatoporoid sponges. Of particular note are those created by unusual, very thin (2 to 8 mm), laminar stromatoporoids (mainly Stachyodes australe), that formed arching, hollow domes up to 0.3 m in height and 1.5 m in diameter over the sediment surface to enclose flat-based cavities. The free undersurface of these stromatoporoids often supported a hitherto unrecognised cryptic community, dominated by pendent growth of the putative calcified cyanobacterium Renalcis, with rare intergrown lithistid sponges and spiny atrypid brachiopods. The uneven growth surface of the cryptos imparts an irregular, stromatactis-like texture to the upper surface of the remaining cavity, which is filled by early marine, finely banded, fibrous cements (mainly radiaxial calcite) interbedded with often multiple generations of geopetal sediment containing peloids and ostracod debris. This ecology yields the tabular stromatoporoid-Renalcis fabric described ubiquitously from the Canning Basin reef complex. Such unusual reef fabrics are a consequence of the ecology of shallow marine midPalaeozoic reefs which were quite unlike that of modem coral reefs. The frequent preservation of relatively delicate, in situ communities wets due to (1) rapid and pervasive early cementation, (2) growth under non-energetic conditions, and (3) the relative insignificance of bioeroders associated with reefs at this time. (C) 1998 Elsevier Science B.V. All rights reserved.