Mid-eocene climatic change, Southwestern California and Northwestern Baja California

Abstract

A widespread paleosol of Paleocene and Early Eocene age occurs in southwestern California and northwestern Baja California. The dominant quartz-kaolinite mineralogy and cation-depleted chemistry of the buried soil indicate a humid, tropical paleoclimate similar to the modern equatorial belt. Although the Paleocene—Eocene paleomagnetic latitudes are similar to the modern latitudes (36–37°N to 32–33°N, respectively), rainfall was about 125–190 cm per year and average annual temperature was about 20–25°C in marked contrast to the present annual rainfall of 25 cm and average annual temperature of 16°C. A variety of indicators in the Late Eocene sedimentary succession suggests a change to a semi-arid paleoclimate. The nonmarine portions of the Late Eocene sedimentary record are dominated by a cobble conglomerate lithosome deposited in fluvial, alluvial fan and fan delta systems. Intertongued with the conglomerate is a sandstone lithosome deposited in flood-plain and nearshore marine environments. The conglomerate clasts were transported to the depositional site via a long-distance (200–300 km), moderate gradient, braided river system mostly by flash floods. Characteristic post-depositional, in situ fracturing of conglomerate clasts probably occurred due to salt crystallization. Within the flood plain sandstones, and to a lesser degree the conglomerates, are multiple well-developed caliche horizons of probable pedogenic origin. Clay minerals from the Late Eocene rocks are dominantly vermiculite and smectite with lesser chlorite and illite; this is in marked contrast to the kaolinite that comprises the underlying Early Paleogene lateritic paleosol. The character of the Late Eocene sedimentary succession indicates a semi-arid climate. Rainfall probably did not exceed 63 cm per year; it probably was seasonal and by occasional flash floods. This paleoclimate contrasts markedly with the earlier humid tropical paleoclimate and must indicate a widespread climatic change in late Middle Eocene time.