Paper No. 8
Presentation Time: 10:15 AM
The first macroborings known from quartzite substrates: Trypanites in boulders from the Upper Cambrian Deadwood Formation, Black Hills of South Dakota
Shallow borings attributed to Trypanites occur as clusters with average densities of 1-3.5 borings per square centimeter on quartzite boulders from the basal conglomerate of the Cambrian-Ordovician Deadwood Formation in the east-central Black Hills of South Dakota. Borings with lower densities also occur on vein quartz. These are the first macroborings recorded from quartzite and vein quartz. The largest quartzite surface with abundant borings measures about 1 square meter in area. Some borings are superimposed on primary but enigmatic semi-circular structures with a 2.5 to 5 cm diameter that originated prior to quartzite formation. The boulders were eroded from Paleoproterozoic quartzites from different stratigraphic units exposed as steeply inclined beds. The oldest overlying conglomerate grades upwards or abruptly changes to sandstone through a layer less than or equal to 2 meters thick in the Marjuman transgression (regionally correlated to the Cedarina dakotaensis trilobite zone). This transgression occurred prior to the start of the globally recognized Upper Cambrian Paibian Stage. Physically similar rocky-shore settings are widely known from quartzite islands of Cambrian age in Wisconsin, Middle Ordovician age on Ontario's Manitoulin Island, Ordovician-Silurian age in Manitoba, and Devonian age in Western Australia, but the Black Hills of South Dakota are the first known to have been excavated into quartzite. We do not know how these Trypanites-producing organisms bored into this siliceous substrate. We can only at this point suggest that they used a mechanical boring technique to exploit surfaces already chemically bioeroded by microorganisms. This process is known today in basalts and volcanic glasses.