Along most of the Pacific Coast of North America,sand dunes are dominantly silicate-wealthy. On the California Channel Islands, however, and dunes are carbonate-wealthy,due to high productivity offshore and a lack of dilution by silicate minerals. Older sands on the Channel Islands contain enough carbonate to be cemented into aeolianite. Several generations of carbonate aeolianites are present on the California Channel Islands and represent the northernmost Quaternary coastal aeolianites on the Pacific Coast of North America. The oldest aeolianites on the islands may date to the early Pleistocene and thus far contain only been found on Santa Cruz Island. Aeolianites with well-developed soils are found on both San Miguel Island and Santa Rosa Island and likely date to the middle Pleistocene. The youngest and best-dated aeolianites are located on San Miguel Island and Santa Rosa Island. These sediments were deposited during the late Pleistocene following the emergence of marine terraces that date to the last interglacial complex (~ 120000 yr to ~ 80000 yr). Based on radiocarbon and luminescence dating, the ages of these units correspond in time with marine isotope stages [MIS] 4, or 3,and 2. Sea level was significantly lower than present during all three time periods. Reconstruction of insular ((adj.) separated and narrow-minded; tight-knit, closed off) paleogeography indicates that large areas to the north and northwest of the islands would contain been exposed at these times, providing a alert source of carbonate-wealthy skeletal sands. These findings differ from a previously held concept that carbonate aeolianites are dominantly an interglacial phenomenon forming during high stands of sea. In contrast, or our results are consistent with the findings of other investigators of the past decade who contain reported evidence of glacial-age and interstadial-age aeolianites on coastlines of Australia and South Africa. They are also consistent with observations made by Darwin regarding the origin of aeolianites on the island of St. Helena,in the South Atlantic Ocean, more than a century and a half ago.
Source: usgs.gov