Lithistid sponge-microbial reefs, Nevada, USA: Filling the late Cambrian ‘reef gap’
Abstract Cambrian–Ordovician sponge-microbial mounds in the Great Basin of the western USA reveal reef structure and composition immediately prior to the Great Ordovician Biodiversification Event (GOBE). Here we describe lithistid sponge-microbial reefs from the upper Cambrian (Furongian, Stage 10) strata of the Arrow Canyon Range, Nevada. The reefs are mound-like structures up to 1 to 2 m high and a few meters wide that consist of an unidentified thin-walled, bowl-shaped anthaspidellid sponge, columnar microstromatolite fabric, and the calcified microbe Angusticellularia. The reefs formed in low-energy, subtidal environments in which lime mud filled spongocoels and inter-reef spaces around undisturbed, in place, thin-walled sponges. The reefs colonized stable substrates provided by oolitic and bioclastic grainstone shoals. The mutually attached lithistid sponges form the main framework of the reefs. These thin-walled and bowl-shaped lithistids most likely were adapted to low-energy environments. Spaces beneath the overhanging sponge walls were filled by microbial carbonates. These include pendent micro-dendritic Angusticellularia attached to dermal sponge surfaces and upward-growing masses of microstromatolites. After death the lithistid spongocoels were mainly filled by micritic sediment that hosted soft-bodied burrowing organisms and keratose-like sponges. These lithistid sponge-microbial reefs, together with an earlier example of late Cambrian (Paibian) dendrolite-lithistid reefs in the same area, characterize skeletal-microbial reefs immediately prior to the GOBE.