Abstract

Clastic dikes and solution cavities filled with Devonian sandstone and shale are common features in Upper Silurian strata of northern Indiana and adjacent midwestern states. The clastics-filled dikes and solution cavities occur just beneath the Silurian- Devonian unconformity surface, where they are concentrated in the flank beds of large Silurian reefs and reef bank strata, and also are found in interreef strata.

The dike crevices probably originated as gravity induced tension fractures and appear to have been infixed from above, some subaqueousiy. The solution cavities probably originated as sinks, grikes, caves, and bedding plane solution features associated with karstification of the Silurian bedrock surface. Some dikes and solution cavities are filled with well sorted white quartz sandstones and arenaceous pale green shales; others are filled with poorly sorted grey-green quartz sandstones containing carbonate clasts, and with dark green or black shales.

The quartz sandstones found in dikes and solution cavities in northern Indiana probably were derived from Cambro-Ordovician sandstones to the north or northwest. Some evidence suggests that the well sorted sandstones were transported into this area by the wind during Early Devonian time, but it is also possible that they were transported here by streams in Early Devonian time or by marine shoreline processes during early Middle Devonian time. The poorly sorted sandstones found in some dikes, and the sands of the basal sandy bed of the Middle Devonian sequence in northern Indiana probably were reworked and deposited in a littoral environment during the early Middle Devonian marine transgression that affected this area.