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Description
From the end of the Late Cretaceous through the Eocene, southern California experienced two periods of unusual tropical climates separated and terminated by periods of semi-arid climate. Evidence of these climatic changes was compiled from nonmarine and nearshore stratigraphy of three Paleogene basins in southern California: San Diego Embayment, Santa Ana Mountains and Elsinore Trough region, and in the El Paso Mountains. Climatic parameters, such as temperature, rainfall and seasonality, were determined by the comparison of preserved paleosols with their modern counterparts and the climate by which they are formed today. Other climatic indicators used include foliage physignomy, salt weathering, the occurrence of coal, silicified wood, fresh water limestone, and sediment coloration and mineralogy. The first of the two humid tropical events began at some time in the Late Cretaceous and ended just after the beginning of the Early Paleocene. The climate then changed to semi-arid at the end of the Early Paleocene. The intervening transitional climate was typified by sub-tropical/warm temperate conditions. There is no evidence of a transitional climate at the beginning of the Late Paleocene prior to the second humid tropical event that occurred in the Late Paleocene and Early Eocene. There is inconclusive evidence that the humid tropical conditions of the Early Eocene were carried over into the Middle Eocene, but by the end of the Middle Eocene semi-arid conditions again prevailed and continued into the Oligocene. Based on the types of soils formed during these humid tropical periods, precipitation was seasonal with average annual totals of 1500 to 3000 millimeters and with average annual temperature of approximately 25° C. During the Early Paleocene transitional climate, the annual average precipitation dropped to a range of 500 to 1500 millimeters as indicated by the moderate leaf size of the vegetation existing at that time and the development of variegated red beds. Temperature and precipitation data for the semi-arid climate is very difficult to interpret because of the wide range of precipitation and temperature combinations possible. Comparison of the climatic changes seen in the southern California stratigraphy with paleotemperature curves from marine-oxygen isotope and paleobotany analysis shows a very good correlation. This comparison has also shown that the Paleocene and Eocene climatic changes in southern California were related to global climatic events.
