END - MEMBER MIXING ANALYSIS (EMMA) APPLIED TO SEDIMENT GRAIN SIZE DISTRIBUTIONS TO CHARACTERIZE FORMATIONAL PROCESSES OF THE MAIN EXCAVATION BLOCK, UNIT 2, OF THE RIMROCK DRAW ROCKSHELTER (35HA3855), HARNEY BASIN, EASTERN OREGON (USA)

摘要

End-member mixing analysis (EMMA) was conducted on the grain size distributions (GSD) of 13 sediment samples collected from the main excavation block, Unit 2, of the Rimrock Draw Rockshelter, a deeply stratified, multi-component Palaeoindian rockshelter in the Harney Basin of eastern Oregon, USA. EMMA confirmed the three stratigraphic units (SU) observed in the field and identified three sedimentary end- members (EM) that account for 95.52% of Unit 2 site formation. EM 1 comprises 70.72% of the total measured grain-size variation and represents fluvial deposition, correlating with the bottom stratigraphic unit, SU 3. EM 2 comprises an additional 17.74% of the total measured grain-size variation and represents fluvially reworked aeolian deposition, correlating with the upper stratigraphic unit, SU 1. EM 3 comprises the remaining 7.06% of the total measured grain-size variation and represents possibly a two-component aeolian deposition of suspension and short-term saltation, correlating with the middle unit, SU 2. The results presented demonstrate the ability of EMMA as a technique to (1) unbiasedly distinguish between the SUs identified in the field using only the measured grain size distributions, (2) provide genetically meaningful and quantitative grain-size end-members to identify multiple major depositional processes of site formation, and (3) provide a geologically meaningful interpretation of site formation that is capable of facilitating the formulation of more focused hypotheses regarding human and environment interactions at the site.