A CARBONIFEROUS SPONGE FOREST ON THE CARBONATE RAMP OF THE ANTLER FORELAND BASIN, CENTRAL IDAHO

Chert nodules and concretions are common features in the Carboniferous mixed carbonate and siliciclastic ramp deposits of central Idaho. The majority are diagenetic in origin, although a recently discovered assemblage of 150 large silicified cylinders at Leslie Buttes provides macro- and microscopic evidence that support a poriferan affinity. The cylinders range in diameter from 10-35 cm and attain lengths of up to 250 cm, with more than half oriented perpendicular to bedding plane surfaces, and the remainder lying subhorizontally. The base of vertical specimens typically is curved and forms a complex lobate structure, while the upper portion of the cylinders may branch upward at approximately 45°. Regardless of depositional orientation, internal morphology of the cylinders is consistently layered with 3-5 concentric light-to-dark bands, roughly corresponding to zones of high and low levels of silicification. Detrital laminated mudstone alternates with siliceous layers, suggesting that layers were originally separated by open cavities. Petrographic analysis of the siliceous layers demonstrates a complex diagenetic history of early chertification followed by dolomitization. Monaxial and possible branched spicules are preserved by chert in some specimens, but most preserve euhedral quartz prisms within a lime and dolomudstone matrix. Fragmented pelmatozoans and intact productid brachiopods are common bioclasts in the mudstones and wackestones surrounding the cylinders. If the poriferan affinity suggested for these silicified cylinders is correct, they would represent a novel Carboniferous equivalent to stromatoporoid (aulacerid) sponge forests common in the Late Ordovician.