Chaco Culture National Historical Park: Paleontological resources inventory (non-sensitive version)

Varela, Phillip; Santucci, Vincent; Tweet, Justin

Scientific interest in Chaco Canyon began in the mid-19th century when United States military expeditions reported on the vast archeological resources in and around the current boundaries of the national historical park. Since then, Chaco Canyon has become one of the most important centers for archeological research in the world, and is also a World Heritage Site location. Perhaps partly due to the intense archeological focus, paleontological studies were largely ignored in the canyon, despite the obvious presence of fossils. Sporadic paleontological studies occurred throughout the 20th century, and recent paleontological surveys have begun to reveal the fossil resources of the canyon, which are much more abundant, diverse, and scientifically significant than previously realized. Cretaceous fossils at Chaco Culture National Historical Park (CHCU) represent approximately 10–15 million years of the history of life on Earth (about 90 to 75 million years ago), and document the biologically diverse populations of the ancient ecosystems. The rock units within CHCU represent repeated transgressive (marine advance) and regressive (marine retreat) depositional cycles of the Western Interior Seaway, providing a unique opportunity to study the ancient and dynamic relationships between the terrestrial on-shore and marine off-shore environments, including numerous fossil specimens that have not yet been scientifically described. From oldest to youngest, the geologic formations at CHCU include the Crevasse Canyon Formation, the Menefee Formation, the Cliff House Sandstone, the Lewis Shale, and the Pictured Cliffs Sandstone. The Crevasse Canyon, Cliff House, and Pictured Cliffs Formations primarily represent nearshore marine environments, the Lewis Shale represents an offshore marine environment, and the Menefee Formation represents a terrestrial environment. Overlying Pleistocene and Holocene alluvial deposits make up the canyon floor upon which much of the Chacoan world was built. Marine rock units contain the remains of abundant marine life, including bivalves, gastropods, cephalopods, echinoderms, sharks, bony fish, and marine reptiles. Invertebrate trace fossils and plant material, including impressions of woody debris and logs, are preserved in the marine rocks. Terrestrial rock units contain the remains of abundant petrified trees and logs, several types of turtles and dinosaurs, and crocodiles. More than 120 species of fossils have been reported from Cretaceous rocks at CHCU, the majority being fossil invertebrates from the Cliff House Sandstone. The abundance of undescribed fossils at CHCU offers an opportunity to discover and describe additional and new fossil taxa, aiding in the understanding of the evolution of life on Earth. Quaternary fossils were found from extensive archeological excavations, especially during the Chaco Project (1960s–1980s). More than 270 Quaternary species have been identified from pollen analyses, packrat midden studies, and site excavations. A notable fossil is an extinct species of rabbit brush (Chrysothamnus pulchelloides), which is thought to have gone extinct due to human practices that contributed to desertification of the environment. Understanding the differences between modern and ancient ecosystems may shed light on the human impact on the environment, the conditions that may have attracted the Ancestral Puebloans to the canyon, and what may have contributed to their eventual departure from the canyon.

Chaco Culture National Historical Park: Paleontological resources inventory (non-sensitive version)

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