Stratigraphy of Quaternary Dunes by Sand Mineralogy and Pedogenic Features, Los Osos, California

• Main Author: Cousineau, Lyssa A
• Format: Others
• Published: DigitalCommons@CalPoly 2012
• Subjects: Stratigraphy
• Online Access: https://digitalcommons.calpoly.edu/theses/827; https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1872&context=theses

The goal of this study was to assess mineralogy and pedogenic features of sand dunes in a stratigraphic sequence. The purpose was to determine whether these features significantly differ to reflect age differences with depth within the sequence. This study was conducted in Montaña de Oro State Park, located on the central Californian coast eighteen kilometers northwest of San Luis Obispo in San Luis Obispo County. Samples were collected from the vertical exposure of one dune face by stratified random sampling at 1.0-m vertical intervals. Particle size distribution was determined through particle-size analysis by hydrometer and sieve. Electrical conductivity and pH were determined using a 1:1 soil/water paste. Total soil carbon and nitrogen contents were determined by combustion. Pedogenic iron oxides were extracted by ammonium oxalate in the dark and citrate-bicarbonate-dithionite, and then quantified by flame atomic absorption spectrometry. Sand mineralogy of fifteen thin sections was analyzed by polarized light microscopy. Grain counts quantified the sand mineralogy of the thin sections. Total carbon significantly decreases with soil depth and age reflecting modern development of soil at 0 to 1 meters within the stratigraphic sequence. Certain morphologic and mineralogic features, including an increase in nitrogen content and the presence of fossilized fungal hyphae, suggest that a buried A horizon may be preserved 9 to 10 meters below the crest of the modern dune complex. Using relative dates compiled from previous research, it was determined that the soil at 9 to 10 meters depth was developed 15-ka to 30-ka after sand deposition during a eustatic sea level lowstand. The presence of fossils in general suggests that the ancient soil was rapidly covered.