Paleoenvironmental and paleogeomorphological implications of the combined Delmar-Ardath Middle Eocene pollen and spore flora (Delmar formation and Ardath Shale of the La Jolla Group), San Diego, California
The Delmar formation-Ardath Shale combined pollen assemblage yields a climatic determination, for the middle Eocene San Diego Embayment, of "Aw" for the coast and immediate interior and "Caw" for the interior uplands. The minimum average elevation of the Caw site was about 620 m (2,000 feet) above sea level. The mean annual temperature of the coast was probably 25°C, with the coldest month averaging above 13°C. The uplands' climate was similar to that of the coast in the summer, but the coldest month averaged closer to 0°C. Rainfall was at least 100 cm annually and was concentrated mainly in the summer. A well-drained, xerophytic microenvironment on the barrier beach (Torry sandstone to be) may have harbored drought tolerant plants like Ephedra. The Eocene Delmar lagoon was well isolated from the marine environment, and probably received most of its contained pollen and spores from a distributary channel of the same large river that deposited the extensive, terrestrial Eocene conglomerates of the San Diego area. Evidence of some communication between the sea and the lagoon is indicated by the exhumed tidal scour channel preserved at the Delmar sample locality (refer to stereograms at the end of this report). The tidal orifice migrated down the barrier beach after its erosive currents had cut a channel into the soft lagoonal muds. The resulting isolated depression in the lagoon recieved much organic matter, which ultimately became the pollen-rick, locally developed, lignitic siltstone bed which is exposed today at the sample site.