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E. J. Helley

Publications and source records attributed to E. J. Helley.

31 records · Page 2Linked to original sources
USGS1973

Map showing evidence for recent fault activity in the vicinity of Antioch, Contra Costa County, California

The U.S. Geological Survey, in cooperation with other federal, state, and local agencies, is studying active and potentially active faults in the counties surrounding San Francisco Bay as past of a program to define and clarify geologic hazards in the region. Much more detailed information about known faults has been collected; in addition, poorly known or previously unknown zones of demonstrable or suspected active faulting have been delineated. Documentation of these zones is of immediate importance because of both their scientific and social impacts.

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USGS1973

Liquefaction potential of unconsolidated sediments in the southern San Francisco Bay Region, California

A preliminary map of liquefaction potential of unconsolidated sediments has been made for the southern part of the San Francisco Bay region. Map zones are established on the basis of recent detailed geologic mapping. Liquefaction potential for each zone is estimated from an analysis of lithologic, water-table and standard penetration test data. Due to empirical aspects of the evaluation method, approximations in establishing parameters and the generalizations of limited test data to entire map zones, the liquefaction potential map must be considered as~preliminary and approximate. Nevertheless, the map should serve the intended purpose of generally delineating areas where the probability for liquefaction during a major earthquake is greatest and, hence, areas where special attention should be given to the possibility of liquefaction. Sediments with the highest potential for liquefaction are clean granular layers within the younger bay sediments. These layers are not everywhere present but commonly occur locally near present and former stream channels. Clean granular beds and layers immediately beneath the younger bay sediments generally have a lesser but significant potential for liquefaction. Younger (Holocene) alluvial deposits have an even lower potential, however, the potential can be locally high in recent channel and overbank deposits. Part of the Holocene unit is normally above water-table and thus would have only a seasonal potential for liquefaction. Older (Pleistocene) alluvial fan deposits have a generally low liquefaction potential. Lateral spreading landslides are commonly associated with liquefaction beneath gentle slopes. Thus. during a major earthquake. slides of this type would probably be the most pervasive damaging effect of liquefaction on the broad alluvial plain surrounding San Francisco Bay.

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USGS1971

Geologic map of Late Cenozoic deposits, Santa Clara County, California

This map is the first of several in the San Francisco Bay region showing the distribution and differentiation of the late Cenozoic alluvial, estuarine, and volcanic deposits. The sedimentary deposits of gravel, sand, silt, and clay were separated into geologic map units on the basis of their post-depositional soil development, texture, and geomorphology. Some of the geologic units are associated with different landforms having recognizable topographic expression such as alluvial fans, natural stream terraces, levees, and interfluvial basins. The relative ages of these unites were established on the basis of intensity of soil profile development, stratigraphic position, and geomorphic expression. The older deposits exhibit strongly developed soil profiles with strong horizon differentiation whereas younger deposits display minimal soil profile development, consisting primarily of organic matter accumulations near the land surface. Geomorphic expression and degree of erosion and dissection were additional criteria used to aid in the age determinations. For example, younger deposits form well-defined morphologic features such as levees, terraces, and broad, undissected alluvial fans along the margin of the bay basin and are related to present drainage patterns. The oldest deposits shown on this map (QTs) are structurally deformed by folding and faulting and therefore exhibit no original depositional geomorphic features. These deposits are not related to present drainage patterns but suggest earlier patterns much different from those existing today.

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