SILURIAN DOLOMITE IN PORT BYRON, IL: A COMPARISON OF TWO LOCAL QUARRIES

In Port Byron, IL, the Racine Member of the Niagaran Formation consists of relatively pure, massively bedded dolomite. Due to the purity of the dolomite, the area was extensively quarried in the 19th and early 20th centuries. Bioherm structures originating in eastern Iowa extend into this area of northwestern Illinois. The lithologic features and structures in two closely spaced quarries in Port Byron are being compared in this project.

The first quarry, which belongs to the city of Port Byron and is labeled as Site A, consists of an upper and lower section. While the bedding is fairly massive (~15-30 cm on average, max. 45cm), the layers in the lowest three meters of the lower section are thinner (7-10 cm). Only two or three fossils were found in the entire site, although the texture of the dolomite is very vuggy, with five large vugs in the lower section. Some of the large vugs contain quartz sand, believed to have washed in from younger Pennsylvanian deposits.

Site B, privately owned, lies a block to the south of Site A. The exposure here correlates with the lower section of Site A, with a distinction between the thinner-bedded dolomite overlain by more massive layers. Solution weathering is more prevalent here, with a .5m gap following the line of contact between the different bedded layers. Fossils are extremely abundant at this site, including brachiopods, crinoid debris, stromatoporoids, and a cephalopod cast. Presumably, the vuggy texture at Site A is the result of the cast and molds of the brachiopods found in the rock that have eroded away. Though there are many differences between the two sites, due to the similarities in the way that the lower layers are bedded and the relations of their depth, both sites must connect to the same structure.