DETERMINING THE ORIGIN OF CAVE SEDIMENTS BY COMPARISON TO MODERN SURFACE STREAM SEDIMENTS AT THE OREGON CAVES NATIONAL MONUMENT

The Oregon Caves contain thick deposits of glacio-fluvial sediments. Today, Upper Cave Creek (UCC) feeds into the Oregon Caves, but there are other creeks in the area that could have fed into the cave in the past such as Lake Creek (LC), which flows from the Bigelow Lakes, two glacial lakes near the top of Mt. Elijah up gradient from the caves.

UCC and LC flow over different rock types. LC nearest Bigelow Lakes flows primarily over diorite from the Jurassic Greyback Pluton. UCC flows over Triassic quartzite and chert and some argillite. The composition of the cave sediments were investigated as a possible reflection of the source composition.

To determine whether or not LC may have once fed into the Oregon Caves, sediment samples were collected from several off-trail locations within the cave, where sediments are least likely to have been disturbed or reworked by cave visitors. Six samples were taken from four sample locations in the cave and compared to four sediment samples from LC and three from UCC. For each sample, a grain size analysis was completed by wet sieving. Using only the portion of the sample larger than 2mm (#10 sieve), the clasts were sorted by composition to determine a weight percent distribution.

The cave sediments did not resemble the LC samples, which contained on average 63.9% diorite clasts whereas the cave sediments contain on average only 2.12% diorite. The UCC Samples contain on average 54.03% meta-pelite, 42.35% meta-basalt and 3.23% diorite. In comparison, the cave sediments were 50.56% meta-pelite, 19.91% meta-basalt, 2.12% diorite and 10.74% quartzite/chert. It is possible that older sediments buried under flowstone deposits may have a different composition, but could not be sampled. Based on the sediment composition, the cave sediments more closely reflect the UCC drainage as the source of cave sediments.