Geology and Geological Forces of the Woodward Reservoir Area near Oakdale, California

Abstract

Woodward Reservoir is located near Oakdale, California within the northern San Joaquin Valley and is in a region that has been sculpted by numerous geological processes over the last 3 million years. This involves the processes of tectonism, sedimentation, and climate that have coalesced to shape this region's terrain and geology. This document encompasses a wide range of geologic formations, significant attributes and possible hazards in regard to this area. The Pleistocene epoch and subsequent fractures, play a key role in the geological history of the locality, are of particular interest. The paper also touches upon prominent hazards including land subsidence and seismicity, with the goal of building a solid geologic framework to clarify the genesis and active development of the Woodward Reservoir locality.

Introduction

Woodward Reservoir is located within the Great Valley Geomorphic Province of Stanislaus County, California, which has a basin-like flat topography. So this area has long acted as a natural sedimentary carrier for material eroded from the Sierra Nevada and Coast Ranges, carried and laid down by an assortment of fluvial processes over geologic time. However, this necessarily limits your knowledge of potential relevant impediments due to factors like geophysics to the region in which your data training was conducted (and the mineralogy and chronology of that terrain) affecting things like drainage or urban development and degradation, relating specifically to the challenges of land subsidence and ground water management. By learning about Woodward Reservoir geology, land planners and natural conservationists in this important agricultural area can make use of that information.

Geologic Setting

Geomorphic Province

The reservoir is located in the Great Valley Geomorphic Province of the forearc basin from a tectonically complex system of subduction processes. All of these sediments from the Sierra Nevada and Coast Ranges were deposited into this basin, resulting in a geological quilt reflecting the dramatic geological past of the area.

Stratigraphy

Pleistocene Modesto and Riverbank Formations dominate the surface geology of the Woodward Reservoir area and are critical to understanding the stratigraphic framework of the area:

Modesto Formation: Characterized by granitic sands, silts, and more recent alluvial deposition processes, including eolian dune formation. Data up until the year of 2023.

Riverbank Formation: Beneath the Modesto Formation is the Riverbank Formation, which is comprised of older, gravel-rich alluvial sediments. These sediments date back to older geological eras and are of great importance for comprehending the sedimentary processes that have contributed to the evolution of the valley throughout history.

The two formations are underlain by older marine sediments, a testament to the area's rich tectonic history and a time when the region was part of a shallow embayment.

The Geologic Forces that Sculpted the Region

Tectonics

Although no active surface faults are located within on outlying within a 50 km proximity of the reservoir, the tectonic setting reflects distant fault systems, e.g. Ortigalita and San Andreas faults. These tectonic features and their related moderate seismic ground motions may cause secondary hazards in the region. Knowledge of these tectonic drivers is crucial in evaluating the potential for seismic hazards.

Erosion and Deposition

Extensive fluvial deposition in the relatively flat terrain of the Woodward Reservoir area, straddling the Sierra Nevada, has aided the process. The Modesto and Riverbank formations have sedimentary deposits that suggest the event of flooding and the process of soil formation are cyclical. Such geological movements have massive repercussions on the suitability of land towards agriculture due to their impact on soil and topographical diversity.

Subsidence

Groundwater extraction is the main driver for a critical problem in the region: land subsidence. Some regions have seen alarming rates of subsidence, with one site measuring declines as great as 30 feet. This is especially problematic for local infrastructure and agricultural practices. The pressure on groundwater has been somewhat reduced due to the importation of surface water.

Soil and Seismic Hazards

Soils

The two most common soil types in the Woodward Reservoir area are Atwater Loamy Sand and Delhi Sand. These soils have well-drained properties that support agriculture. [Land of unificar] However, their structure becomes unconsolidated and makes them prone to subsidence, with difficulties in the land.

Liquefaction and Landslide Risk

While liquefaction or landslide hazards are generally low, localized conditions can pose vulnerabilities. The deep water table and flat topography result in a stable environment; yet localized instability is observed in places where intermittent high clay lenses exist, increasing the risk of saturation due to irrigation or rainfall.

Seismic Risk

The seismic hazard model for the Woodward Reservoir area is characterized by low to moderate risk, with an estimated PGA (peak ground acceleration) value of around 0.239g (g=acceleration due to gravity) for near-surface alluvium (Tinsley et al., 1985). On the bright side there is little risk of rupture at the surface, which helps limit the impact of a catastrophic earthquake event that may affect the local population and infrastructure.

Conclusion

Sedimentation originating from the Sierra Nevada runoff and tectonic activity within the Great Valley has shaped Woodward Reservoir's terrain and geology. Soil formation and agricultural potential for the region is greatly influenced by the predominance of the Modesto and Riverbank formations, along with the interaction between eolian and alluvial processes. The geology of the area presents a lot of conditions where they might have some challenges such as land subsidence and small earthquakes.

References

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Ireland, R.L. (1986). Land Subsidence in the San Joaquin Valley, California as of 1981. U.S. Geological Survey Professional Paper 437-H.

California Department of Water Resources (DWR). (2004). California’s Groundwater Bulletin 118 – San Joaquin Valley Groundwater Basin.

U.S. Geological Survey. (2006). Quaternary Fault and Fold Database of the United States. https://earthquake.usgs.gov

National Resources Conservation Service (NRCS). (2003, 2006, 2012). Soil Survey of Stanislaus and Merced Counties. USDA Soil Data Mart.

California Geological Survey. (2011). Fault Activity Map of California. DOC State Mapping Program.

Hart, E.W. & Bryant, W.A. (2007). Fault-Rupture Hazard Zones in California: Alquist-Priolo Earthquake Fault Zoning Act with Index to Earthquake Fault Zones Maps. California Geological Survey Special Publication 42.

California Building Code (CBC). (2010). Building Standards for Seismic Design.