PETROLOGY, PETROGRAPHY AND CONODONT BIOSTRATIGRAPHY OF THE BURLINGAME LIMESTONE AND SOLDIER CREEK SHALE MEMBERS OF THE BERN FORMATION, RULO, NEBRASKA

The upper 30 cm of the Silver Lake Shale of the Scranton Formation and the Burlingame Limestone Member and lower part of the overlying Soldier Creek Shale Member of the Bern Formation (Wabaunsee Group, Virgilian Stage, Upper Pennsylvanian) are exposed in a roadcut 3.2 km south of Rulo, Richardson County, Nebraska. The intermediate Burlingame cyclothem comprises the upper part of the Silver Lake Shale, the Burlingame Limestone and the lower part of the Soldier Creek Shale. Early transgressive deposits grade upward from sandy shales and micaceous, fossiliferous (mainly ostracodes, gastropods and clams), calcite-cemented quartz arenites in the upper part of the underlying Silver Lake Shale Member of the Scranton Formation to an arenaceous fossiliferous grainstone in the lower Burlingame Limestone. Maximum transgression and still-stand occurs about 50 cm above the base of the Burlingame Limestone in a slightly arenaceous and argillaceous lime wackestone with an abundant open marine fauna and a moderate abundance of Streptognathodus spp. conodonts (65 elements per kilogram). Regressive deposits include the upper Burlingame Limestone, a lime wackestone that grades upward to an intraclastic packstone with an intraclastic grainstone at the top. Intraclasts are rounded, up to 3 cm across and consist of arenaceous, ostracodes-rich lime wackestone. Above the main ledge of the Burlingame Limestone is about 13 cm of slightly sandy shale containing carbonized wood fragments, and an overlying 5 cm thick grainstone with abundant carbonate intraclasts and gastropod fragments. Above this lime packstone, in the base of the Soldier Creek Shale, is about 55 cm of very fossiliferous gray shale with no sand, but an open marine fauna with abundant gastropods, brachiopods and small foraminifers. This shale may represent a minor sea level rise in late stages of the Burlingame cyclothem. The top (maximum regressive deposits) of the Burlingame cycle is represented by 25 cm of light to medium gray mudstone. The upper part of the mudstone has abundant plant fossils preserved as limonite casts and may represent a poorly developed coal horizon related to the overlying Wakarusa cyclothem. Above the coaly zone, the next 1.5 m of the Soldier Creek Shale has an abundant open marine fauna with cephalopods, crinoids, bryozoan, brachiopods and conodonts.