INJECTED BRECCIA AT THE DECORAH IMPACT STRUCTURE

The Decorah impact structure is an almost entirely subsurface Middle Ordovician impact crater, approximately half of which is located beneath the City of Decorah, northeast Iowa. The 5.6 km diameter structure is buried by Middle-Upper Ordovician St. Peter through Galena Group strata and Quaternary alluvium of the Upper Iowa River. Only one small surface exposure of crater-fill deposit is known to exist, that of the uppermost preserved crater-fill, the Winneshiek Shale. Polymict breccia containing impact generated shocked quartz grains occurs beneath the Winneshiek Shale, but this unit is not exposed and is known solely from subsurface samples.

Several small exposures of the youngest preserved target formation, the Lower Ordovician Prairie du Chien Group Shakopee Formation, are estimated to be within 100 m of the buried crater rim along the structure’s eastern edge. One of these exposures exhibits deformation features consistent with its location proximal to the crater rim where target strata typically experience deformation during transient crater excavation. Within this area a few samples of polymict breccia containing possible shocked quartz have been recovered from a poor exposure that is obscured by colluvium, soil cover and anthropogenic garbage. This represents the only known occurrence of polymict breccia from outside the crater. Considering the currently hypothesized deeply eroded condition of the crater, this occurrence of breccia is interpreted as having been emplaced into the crater walls by injection during crater excavation, rather than being a remnant of an ejecta blanket.