THE EOLIAN DEPOSIT ON MARINE TERRACES – A POTENTIAL DATING TOOL FOR COASTAL NORTH AMERICA

An eolian deposit along a narrow strip of coastal northwestern California is thought to be a time marker dating from the last glacial maximum(LGM). A numerical age range should allow this unit to be a reliable stratigraphic marker in the latest Pleistocene. The source of the material is believed to be from offshore during the sea level fall related to the onset of the LGM. The deposit has been identified when as thin as 30 cm and as thick as 150 cm, it is commonly found on the lower marine terrace(s). It has been reported in marine terrace studies as the A horizon of that surface. However, the eolian nature of the deposit, based upon the extremely well sorted fine sand requires that the origin is quite distinct from the underlying sands and gravels. The physical nature of the eolian cap near McKinleyville, CA was studied in detail by Smith (1983), but a detailed analysis of the materials and efforts to define the age(s?) of the deposit have been missing until now. This present study was conducted immediately south of Eureka, CA on Humboldt Hill, where it lies unconformably upon Quaternary sands and gravels of varying ages.

The soil development of the eolian material clearly supports the hypothesis of a singular sediment pulse that buries a variety of substrates, some of which appear to be stripped well-cemented Btq horizons of much older materials. The basal contact of the eolian cap is always an abrupt to very abrupt soil boundary. The soil color varies from 10YR5/3 to 10YR3/2 and the upper portion of the deposit is an A horizon. The darkness is apparently due to the high CEC of the organics produced since vegetation of the deposit. There is usually a Bw horizon based on the development of structure, and this tops a buried soil.

Soil parameters are supportive of ages in the 20-30 ka range. Studies on terraces known to be Holocene in age have no record of the eolian materials in support of pre-Holocene eolian deposition. The extent of the sea level fall during the onset of the LGM is believed to be required to produce the volume of material represented in the eolian cap.