PALEOCENE - EOCENE THERMAL MAXIMUM IN SAN DIEGO

The impact of the Paleocene-Eocene Thermal Maximum (PETM) may be observed on an uppermost Paleocene conglomerate in San Diego. This key 11-m-thick section is in a relict hill that stands up within younger abutting Eocene rocks in the cliffs at Torrey Pines State Beach. The most striking aspect of the conglomerate is its weathering profile. Upper horizons of this paleosol consist only of kaolinite and siliceous particles such as quartz grains and quartzite clasts. Poway rhyolite clasts well below the most intensely weathered part of the section are still recognizable but even these once ultradurable clasts are reduced to ghost-like outlines and their only unaltered parts are their original quartz phenocrysts. Although the effects of chemical weathering are pervasive, many original sedimentary structures are still recognizable and indicate deposition within the nonmarine part of a fan delta. Lithofacies, as well as nearly identical conglomerate-clast compositions, allow correlation with marine conglomerates at Well Canyon on Santa Cruz Island where calcareous nannofossils bracket these unique conglomerates between CP8a and CP9a zones at the Paleocene-Eocene boundary. The age-equivalent heavily weathered, kaolinized conglomerate in outcrop at Indian Trail Canyon would then be no older than 57.2 MYA (CP8a zone) and no younger than 54 MYA (CP9a zone), an age range that encompasses the PETM (56 MYA). The 0.75 meter-thick interval with high purity kaolinite at the top of the section records the maximum impact of chemical weathering under extremely high thermal conditions near the Paleocene-Eocene boundary.