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Constraining the Uplift History of the Montezuma Hills, Sacramento Delta Region, California

The Montezuma Hills are an area of broad relief reaching ~90 m elevation amid the mostly flat sea-level landscape of the Sacramento Delta. Their plateau-like geomorphology suggests a relict surface that has been uplifted and tilted northward. This deformation would be related to the Great Valley fault system which runs along the east side of the California Coast Ranges, but the precise causative structure(s) remain unidentified. Little is known about faults in the area other than crude locations of a few; slip rates, earthquake frequency, geometry and connectivity are unknown. Improving knowledge of these faults is vital both for understanding the local tectonics and for clarifying the hazards posed to the Delta region.

We approach this problem by investigating the uplift history of the Montezuma Hills. The hills are capped by the near-horizontal Montezuma Formation, largely composed of fine-grained sediments that are presumed to be deltaic deposits. This formation is generally identified as early Pleistocene, but the only quantitative age constraint comes from the 3.3 Ma Putah Tuff in the underlying Tehama Fm., giving an uplift rate of >0.03 mm/yr. We have identified a series of wind gaps (former distributary channels of the Delta) that indicate a locus of uplift along the western crest of the hills. We excavated a series of pits in and near these wind gaps, all of which revealed similar massive silt deposits below modern soil. The similarity of the sediment inside and outside the wind gaps suggests that it is all part of the Montezuma Fm. which was deposited prior to the incision of the channels and that no sediments associated with channel flow are preserved in the wind gaps. We are in the process of luminescence dating which will at least place a lower bound on the age of the deposits. If our samples prove too old for luminescence ages, we will pursue other techniques such as analysis of microfossils or pedogenic carbonate. Establishing an age for the Montezuma Fm. will greatly refine the uplift rate of the Montezuma Hills.

Session: Advances in Earthquake Geology: Spatiotemporal Variations in Fault Behavior From Geology and Geodesy [Poster]

Type: Poster

Room: Evergreen Ballroom

Date: 4/22/2022

Presentation Time: 08:00 AM Pacific

Presenting Author: Belle Philibosian

Student Presenter: No

Additional Authors