The Use of Statistical End-Member Mixing Analysis (EMMA) of Grain Size Distributions to Characterize Site Deposition of a Deeply Stratified Paleoindian Rock Shelter, Harney Basin, Eastern Oregon

Abstract:

Sedimentological investigations were conducted on Excavation Unit 2 of Rimrock Draw Rockshelter (35HA3855), a deeply stratified, multi-component Paleoindian site located along the western margin of the Harney Basin, eastern Oregon. Field descriptions and end-member mixing analysis (EMMA) of grain-size distributions (GSD) of 13 sediment samples identified six stages of site formation: three stratigraphic units (SU), two unconformities, and a relict Bt soil horizon. EMMA resulted in the characterization of three end-members (EM) that correlate with field descriptions. EM's 1 and 2 represent 88.4% of the total variance among samples and are present within the upper-most stratigraphic unit, SU1, and the lower-most stratigraphic unit, SU3. EM 3 correlates with the poorly developed Bt horizon within the middle stratigraphic unit, SU2. EM 1 is a well-sorted coarse to medium sand interpreted as fluvial deposition. EM 2 is a trimodal deposit of poorly sorted sand, silt, and clay, interpreted as a predominantly aeolian deposition occasionally punctuated by colluvium derived from the colluvial wedge to the east of the Unit and from the brow of the rockshelter. EM 3 is a bimodal distribution of medium sand to fine silts and clays that represent a predominately aeolian deposit overprinted by a Bt horizon. The results of this study demonstrate the applicability of EMMA to distinguish between depositional units within an archaeological context and provide geologically meaningful interpretations of paleoenvironments for the development of hypotheses related to human environmental interactions.