GEOPHYSICAL CONSTRAINTS ON THE EVOLUTION OF SAND CREEK AT THE SAND CREEK MASSACRE NATIONAL HISTORIC SITE, COLORADO, USA

A near-surface seismic refraction study was conducted at Sand Creek, an ephemeral stream located in the semi-arid High Plains of southeastern Colorado. The study is located on the Sand Creek Massacre National Historic Site. In 1864, 675 U.S Army and volunteer cavalry killed approximately 130 Arapaho and Cheyenne camped along the banks of Sand Creek. The precise location of the massacre is uncertain, as it is unknown whether the channel has since changed position. The purpose of this study is to determine the historic course of the stream channel and whether it has migrated within the valley.

The survey consisted of 4 seismic refraction lines spaced approximately 100 m apart that crossed the active channel and several small terraces. The 300 m long lines were connected by a cross line parallel to the channel. A 7.25 kg sledgehammer was used as the seismic source. 2D subsurface velocity profiles obtained for each line by tomographic inversion of the first arrivals revealed 2 layers. An abrupt velocity gradient separates a 2 m-thick surface layer (Layer 1, V_p = 500 m/s) from an underlying higher velocity layer (Layer 2, V_p = 1500 m/s). In places, convex downward features up to 10 m in width produce up to 2 m relief at the top of Layer 2. These features are present beneath the modern channel and 100 m, 200 m, and 400 m north of the modern channel.

Layer 1 is interpreted as modern alluvial deposits, which overlay the Cretaceous Pierre Shale (Layer 2). The convex downward features at the top of the Pierre Shale are interpreted to be channels. These channels are older than the active channel as they are covered by modern alluvium, confirming that the channel has migrated in the past. The inferred former channel positions are consistent with the presence of a line of very mature cottonwoods 100 m north of and parallel to the active channel as well as slight topographical depressions covered with coarser sediments 100 m and 400 m north of the active channel. Except for the modern channel, all of the channels imaged in the seismic profiles are covered by modern alluvium (Layer 1). Furthermore, the cottonwoods located near the buried channel 100 m north of the modern channel have been dated to be older than 1864. Thus, although Sand Creek has changed course at least 4 times in the past, there is no evidence to indicate the stream has migrated since the massacre.