MILLS, MICHAEL, Pacific Soils Engineering, Inc., Cypress, CA, and ROY J. SHLEMON, Consultant, Newport Beach, CA

ABSTRACT: Trench Exposures and Tectonic-Geomorphic Evidence for Latest Pleistocene and Holocene Uplift of North Signal Hill, Long Beach, California

Recent investigations of the Cherry Hill fault (CHF), a segment of the Newport-Inglewood fault system, indicate that the North Signal Hill area has been significantly warped and uplifted in the latest Pleistocene and Holocene. Trench exposures and interpretation of boring logs show that the northeast-dipping CHF has undergone at least 25 m of near-surface reverse displacement within about the last 125,000 yr. Displaced and upwarped fluvial and near-shore sediments that were once part of the "Long Beach plain" now cap and flank the uplifted North Signal Hill. A post-125,000 yr age is assigned to most of these sediments on the basis of site soil stratigraphy, association with the marine isotope stage chronology, and likely coeval deposition with sediments elsewhere on Signal Hill and in the adjacent Palos Verdes Hills dated by uranium-series and amino-acid racemization methods.

A high rate of latest Pleistocene and Holocene uplift is also indicated by tectonic-geomorphic features, in particular, a wind gap incised about 25 m into the axial crest of North Signal Hill. As shown on pre-development maps of 1896, the wind gap was expressed by a topographic saddle about 30 m wide, and was a former channel of a now-abandoned, low-order, through-flowing stream. The old wind gap stream was probably tributary to the lower Los Angeles River which, itself, occupies a major gap separating the Dominguez Hills on the northwest from North Signal Hill on the southeast. With the apparent post-125,000 yr accelerated rise of Signal Hill and the resultant "tectonic defeat" of through-flowing drainage, ephemeral underfit streams began to flow from the axial-crest wind gap into pr -development swampy terrain, particularly on the north side of Signal Hill.

The approximately 25 m offset of the CHF and the presence of the water gap, as a tectonic-geomorphic indicator, suggest that the latest Pleistocene and Holocene uplift rate of the North Signal Hill areas is conservatively about 0.2-0.4 mm/yr. We also suggest that most regional compression resulting in uplift of North Signal Hill started not earlier than about 100,000-200,000 yr ago, and may well have been accelerating throughout the Holocene.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.