Chemically-precipitated quartz, confined to a thin interval at an unconformity within the Marble Falls Limestone, originated during an episode of Early Pennsylvanian subaerial exposure. The chert is principally early diagenetic chalcedony and microcrystalline quartz that formed as pore-filling cement and as replacement products for carbonate allochems. Preservation of incipient carbonate cement, stabilized at the margins of pore spaces when microquartz was precipitated in pore centers, and reworked siliceous allochems provide the principal evidence for early diagenetic silica emplacement. Corrosion surfaces in carbonates at the base of siliceous intervals indicate silica was precipitated from pore fluids undersaturated with respect to CaCO 3 . Organic matter preserved in microquartz probably played an important role in precipitation of silica by locally reducing silica solubility and promoting precipitation.