Addressing a Phanerozoic carbonate facies conundrum - sponges or clotted micrite? Evidence from Early Silurian reefs, South China Block

We describe Early Silurian carbonate reef facies containing amalgamated micritic masses, commonly layered, interpreted to have formed by bacterial processes creating clotted fabrics. However, some curved structures in these masses resemble published images of interpreted sponges, raising the question of their nature, relevant to many carbonate studies including reefs and mud mounds throughout the Phanerozoic. Many lithistid sponges are well-established but others are open to interpretation. For keratose sponges, Cambrian examples are known, but several interpreted cases in later rocks are not confirmed; one example in Devonian and Triassic rocks using 3D imaging did not lead to firm verification. Thus criteria to distinguish sponges and clotted micrites remain problematic. A careful approach to interpretation of such sponges is needed, they might instead be microbially-mediated clotted micritic masses. The difficult process of 3D reconstruction is likely needed to resolve this interesting issue of interpretation.