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City of Pleasanton—Hidden Canyon Residences and Preserve Draft EIR Geology, Soils, and Seismicity FirstCarbon Solutions 3.6-15 https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx Lateral Spreading Lateral spreading is a secondary hazard generated from strong seismic shaking and liquefaction that occurs on nearly horizontal slopes comprised of coherent material overlaying saturated and loose sediments that are susceptible to liquefaction. Primary hazards such as strong seismic shaking induces liquefaction to occur within the lower saturated soils causing the upper cohesive material to fracture and entrain slowly downslope. As previously mentioned, subsurface conditions on the project site consists of mostly the Diablo Clay and Millsholm silt series that are composed of a clayey texture. The Updated Geotechnical and Geologic Report determined that the project site is underlain by colluvium and residual soils comprised of clays that are cohesive in texture that are thin at higher elevations and increase in depth at the lower elevations and drainage swales on the project site. The residual soils and colluvium overlay shallow bedrock consisting of Miocene siltstone and clay shale (Tmc), and the Miocene Clairmont Shale member of the Monterey Formation (Tm). In addition, subsurface conditions underlaying Devany Creek consist of young alluvial soils consisting of sands and gravels. Since the colluvium and residual soils on the project site overlay shallow bedrock that are not loosely deposited sediments, the potential for lateral spreading is determined to be low. However, development along Devany Creek may be susceptible to lateral movement due to containing younger alluvial sandy and gravely deposits that face the Devany Creek channels.22 Paleontological Resources Fossils are nonrenewable resources which can consist of body and trace fossils or can be petrified through replacement of the original fossil with minerals. Geomorphological areas that tend to have higher paleontological sensitivity are areas of lower elevation where deposition has been greater such as basins, valleys, and floodplains. Rock units that have high paleontological sensitivity are classified within the Society of Vertebrate Paleontology (SVP) Standard Procedures for the Assessment and Mitigation of Adverse Impacts to Paleontological Resources as middle-Holocene or older sedimentary formations lithologically suitable for the preservation of fossils such as fine- grained fluvial, marine, and cross-bedded point bar sandstones, or argillaceous paleosols, some volcaniclastic formations, and low-grade metamorphic rocks that have yielded significant paleontological resources within their geographical extent, while rock units characterized as having low potential are those that poorly represent fossil specimens in institutional collections, such as basalt flows and recently deposited colluvium. Most of the project site’s surficial conditions consist of colluvium and residual soils with shallow depths from 0.5-foot to 2 feet on the tops of ridges that increases approximately 5 to 11 feet in depth within the troughs and drainage swales, with the exception of recently deposited Quaternary alluvium terrace deposits on the southeastern portion of the project site along Devaney Creek. The colluvium and residual soils overlay marine sedimentary rock units of Miocene age including sandstone, siltstone, and siliceous shale comprised of the Clairmont Shale Member (or Formation) that is part of the Monterey Formation (or Group) (Tm), along with an undifferentiated part of the Monterey Formation (Tmc). As previously discussed, there are historic variations in the nomenclature of the geology of the sediments classified as the Monterey Formation or Group. For 22 Cornerstone Earth Group. 2020. Updated Preliminary Geotechnical and Geologic Report. March 31.