Hargis + Associates Inc

Fold geometry model of Coyote hills segment of Puente hills blind-thrust system applied to interpretation of long-term water level data

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Courtesy of Hargis + Associates Inc

Recent work by others along the Coyote Hills segment of the Puente Hills blind-thrust system has imaged the near-surface structural features generated by activity of that intrabasin fault.

Published shallow seismic profiles near the south flank of the Coyote Hills indicate the blindthrust system is expressed by south-facing folding of Quaternary terrestrial sediments underlying the hills. Monitoring wells installed at a former aerospace facility on the south flank of the Coyote Hills between 1997 and 2004 were completed in highly interbedded sediments of the regional aquifer to various depths, ranging from approximately 165 feet to 500 feet.

Correlation of strata based on drill cuttings and core samples was attempted, but was of limited confidence due to pervasive interbedding. Stratigraphic continuity and geometry were evaluated using long-term water level monitoring of wells completed to various depths.

Selected wells were fitted with pressure transducers / dataloggers, recording water levels nearly continuously over periods of months to years. A conceptual site model (CSM) was developed based on differential water level responses as well as on subsurface soil sampling,published surface geologic mapping and hydrogeologic cross sections of the Orange County groundwater basin. The varied and apparently depth-independent response of water levels across the site to pumping events in a production well approximately 1-2 miles distant was a significant input to the CSM. Given these regional hydrogeologic constraints, refinement of the CSM necessarily incorporated a south-dipping monocline at the foot of the Coyote Hills. The published model of fault-bend folding above an active thrust ramp based on seismic profiling is consistent with a hydrogeological CSM based on variable water level responses to pumping in the Orange County basin.

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