SOURCING ABORIGINAL LITHIC ARTIFACTS FROM WESTERN AUSTRALIA USING EOCENE BRYOZOANS

The purpose of this project is to determine the source of Aboriginal lithic artifacts from Western Australia (WA) using fossil bryozoan-bearing chert debitage. The traditionally accepted “Offshore Hypothesis” is that the lithic source of the artifacts found in the Perth Basin of WA is located off the west coast, and was submerged by sea level rise during the early to middle Holocene, by 6 ka. Sea level curves since the Last Glacial Maximum (LGM) indicate sea level was <160 m lower during the LGM. Chert in offshore well borings are well below this, indicating the cherts were not accessible to Aborigines. This questions the “Offshore Hypothesis” and implies alternative onshore sources outside of the Perth Basin.

The lithic source tested in this study is the Middle-Late Eocene Wilson Bluff Limestone, located inland, 1,200 km to the east of the Perth Basin in the Eucla Basin. For this project 30 Aboriginal lithic artifacts as well as five subsurface well samples from the Wilson Bluff Limestone were thin sectioned. Using MacGillivray’s (1895) monograph on Australian Cenozoic bryozoans, five Eocene species of bryozoans were identified. Three are cheilostomes: Adeonellopsis sp., Cellaria rigida, C. australis and two are cyclostomes: Idmonea incurva, and I. geminata.

Three of the five species, Adeonellopsis sp., Cellaria australis, and Idmonea geminata co-occur in both the artifacts and the possible lithic source. For these three species, we compared zooecial diameter and colony branch width. T-tests indicate the mean values of the measured characters are not statistically different between the artifacts and the possible lithic source. Thus we cannot reject the hypothesis that the Aboriginal fossil bryozoan-bearing chert artifacts are from the Wilson Bluff Limestone.

Our results on the source of fossil bryozoan-bearing lithic artifacts in south WA help constrain hypotheses regarding Aboriginal adaptation to climate change-induced sea level rise. The Offshore Hypothesis needs to be re-evaluated. Understanding Aboriginal adaptation to climate change during the Holocene transgression will help us better understand prehistoric Aboriginal trade routes and trade distances in southwestern Australia. Further investigations of the geochemistry and paleontology of the offshore sources are currently underway.