A Survey of Tetrapod Tracksites Preserved in Pyroclastic Sediments, with Special Reference to Footprints of Hominids, Other Mammals and Birds

Abstract

Pyroclastic sediments provide an unusual and favorable medium for the recording, burial, and preservation of tetrapod tracks and other traces. Twenty-two tracksites were reviewed for the purpose of determining how these deposits contribute to track formation, burial, and preservation. These include Jurassic sites in Argentina and Mexico, Cretaceous sites in Korea, Miocene sites in Mexico and the United States, Pliocene sites in Tanzania, Pleistocene sites in Mexico, Korea, and Italy, and Holocene sites in Mexico, Turkey, Italy, Japan, New Zealand, Nicaragua, and the United States. Twelve of the sites contain hominid tracks. Tracksites occur most commonly in reworked tephra on the shorelines of rivers, lakes, and seas. They also occur on pyroclastic falls, flows, and surges, and on lahars. Most tracksites are in volcanic arcs, especially around the Pacific Rim. A few occur in continental rifts or near intraplate volcanoes. Most older tracksites (Jurassic, Cretaceous, and Miocene) occur in silicic tephras such as rhyolite and dacite. The younger tracksites are mostly associated with basaltic tephra, though other compositions are also represented. Volcanic eruptions contribute to formation of substrates suitable for recording tracks by producing abundant fine-grained pyroclasts that interact physically and chemically with water to become cohesive. Hiatuses between eruptions provide time for tracks to accumulate, and in some cases, to lithify rapidly. Both physical processes (drying and compaction) and chemical processes (mineral precipitation) appear to be involved in early lithification. Eruptions also contribute to burial of tracks through rapid sedimentation and aggradation that typically follow a pyroclastic eruption. Multiple, closely spaced track horizons are common in pyroclastic sequences. Most tracksites are buried by either fallout tephra or reworked, waterlain deposits. Others are buried by lahars, pyroclastic flows and surges, or windblown ash. Upon exhumation pyroclastic deposits contribute to the formation of bedding planes through clay drapes, friable ash layers, and biotite-rich layers. Precipitation of authigenic minerals contributes to erosional resistance within beds. Bedding plane formation and erosional resistance facilitate discovery and study of tracks.