3.4 - Geology

The upper Strawberry Creek watershed consists mostly of the geologically very recently uplifted Berkeley Hills which hinge along the northwest trending Hayward fault zone. In general, the steep hill area is unstable and bedrock is close. to the surface, resulting in numerous landslides and extensive soil erosion. The hill area is part of a very complex volcanic vent structure that has been truncated and displaced northward by the Wildcat Fault. West of the Hayward fault zone is a gently westward sloping older alluvial plain which has been altered in the Central Campus and LBL areas by cut and fill construction activity. The subsoil in the Central Campus consists of soft, highly erodable stream sediments grading from clayey silts to cobbles and boulders. These sediments exhibit very poor stability along the banks of Strawberry Creek. The geologic units of the watershed are presented in Table 3 and shown on Figure 4

The bedrock in the Canyon grades time-wise from Upper Cretaceous marine sediments through Miocene sediments (Claremont Formation) to the late Pliocene sediments and volcanics of the Orinda, Moraga, Grizzly and Bald Peak Formations. A series of earth movements extensively folded and faulted these formations and massive volcanic intrusion has further altered the bedrock. The resultant formational contacts are steeply sloping to vertical and the time sequence is thus commonly horizontal rather than lying in vertical succession. The character of the rocks varies widely, ranging from fairly hard sandstones in the Cretaceous series to soft and clayey semi-shales of the late Pliocene deposits. The volcanic members vary from extremely hard but generally intensely fractured basalts to soft tuffaceous sediments. 

The Upper Cretaceous rocks underlie about 16% (186 ac) of the watershed between the Hayward and Wildcat Faults bordering both sides of Strawberry Creek. These sediments are the oldest rocks in the hill area, consisting of fragments of older rocks which have been significantly altered and fractured by tectonic movements. These rocks are moderately to highly weathered, soft to medium hard, and locally folded. This unit weathers fairly easily, prcxlucing thick residual soils that often migrate downhill by landslides or by colluvial processes. 

Most of the northern area of the watershed between the Hayward and Wildcat Faults is underlain by the Moraga-Grizzly Peak-Bald Peak Formations (180 ac). These are extrusive igneous rocks that have been altered by shearing and weathering, and are highly fractured. These formations are permeable enough to permit rapid and extensive water circulation. Generally, the rocks are medium to completely weathered, soft to medium hard and very thickly bedded. The Grizzly Peak unit is slightly weathered and very hard ,with medium-spaced (8-24 inches) fractures. 

East of the Wildcat Fault lies the Claremont Formation (163 ac) which consists mainly of thin bedded shales and siltstone of moderate hardness. The rocks are generally fairly siliceous or have an appreciable calcite content and are thus relatively hard, resistant to erosion, and stable at quite steep slopes. Although porous, these rocks are not very permeable because pore openings are very small and poorly interconnected. 

The Orinda Formation (154 ac) lies to the east of the Claremont Formation and also occurs in pockets between the Hayward and Wildcat Faults. These rocks are soft and relatively easily ercxled so they are unstable at steep slopes, especially when saturated. These rocks have been extensively degraded by both tectonic shearing and intense surf ace weathering. The majority of the numerous large landslides in the canyon have been based in Orinda Formation materials. Hillslopes are commonly covered with a mantle of landslide debris that will easily slide when undercut or flow when saturated. Debris or mudflows in the canyon pose a major hazard because obstructions may clog inlets to the storm drain system and divert flow out of normal channels, resulting in extensive damage. Much of upper Centennial Drive and parts of the Lawrence Hall of Science are located on the Orinda Formation, as well as most LBL facilities. 

The last major geologic unit in the watershed is the Moraga Formation ( 126 ac) which is located mainly in the steep northeastern hill area. These rocks are generally hard, but intensely fractured due to both natural shrinkage processes upon cooling and tectonic movements which occurred after deposition. Permeability is therefore generally high. The Moraga Formation is stratigraphically and topographically the highest bedrock unit in the hill area, capping the upper hills and the Grizzly Peak ridgeline on the northeastern boundary of the watershed. 

Numerous landslides occur in the hill area, especially in the vicinity of the LBL complex. These slides are composed of substantial soil and rock masses that have slid downslope along a failure plane. They may occur rapidly in a single major event or slowly through repeated small failures. New or old slides may be precipitated by high groundwater levels, ground shaking, or changes in slope geometry and loading. Slides can often be recognized by a bulging, cracked "toe" at the lower end, and by arched headscarps and topographic depressions at the upper end. In the Berkeley Hills some thick mobile accumulations of colluvium (soil and rock fragments transported downslope by gravity) closely resemble shallow landslides, making it difficult to distinguish between them. Most older landslides are marked by heavy brush and tree cover because the vegetation prefers the relatively soft wet soil found there. 

The Hayward fault rone is a member of the San Andreas Fault system which is a major geologic feature and plate boundary along which massive continental drift is presently occurring. The Hayward fault zone forms a distinct geologic break between the Central Campus area to the west and the hill area to the east. The Wildcat Fault is another significant geologic feature in the watershed, and is probably a secondary member of the San Andreas system. This fault traverses the Canyon in the vicinity of the Botanical Garden. 

The Hayward fault exhibits right-lateral movement, with the westerly side moving to the northwest in comparison to the easterly side. Slow tectonic creep (movement) is presently continuing along the fault at the rate of about 0.1 inch per year in the area of Memorial Stadium. The "Little Inch" Strawberry Creek drainage culvert under the stadium has been previously damaged by this movement. 

Tectonic creep has prcxiuced major tension and compression faults east of the Hayward fault zone and the tectonic movements which caused the uplift of the Berkeley Hills has produced thrust faulting as well. The result is a complex system of cross-faults with both vertical and lateral movement. One such secondary fault has formed the canyon of the South Fork of Strawberry Creek above the stadium. These faults range in size from very small breaks to the Hayward fault zone, which is several hundred feet wide. 

The Hayward fault rone can be assumed to the active along its entire length. Severe earthquakes were caused by movement along faults within the Hayward fault zone in 1836 and 1868. Ground rupture was reported across the west side of the Clark Kerr campus and northwestward between Prospect and Warring Streets. Future movement within the Hayward fault zone may or may not follow the specific fault trace shown on Figure 4. The trace should not be construed as indicating the only line within the zone where movement has taken place in the past, nor is it necessarily the line where movement will occur in the future. 

Structures which lie within or across the Hayward fault zone may not only be damaged by sudden movement, offset, and rupture along a fault in the event of an earthquake originating in the fault zone, but may also be subject to constant strain and damage due to opposite sides of faults within the zone continuously moving very slowly in opposite directions. 

The Wildcat Fault is also a right-lateral fault with apparently local vertical movements. This constitutes another major structural non-conformity in the watershed. This fault has undoubtedly been active in relatively recent geologic time. Whether creep is currently occurring along the fault or whether there is currently any hazard of displacement is not known with any degree of certainty. East of the Wildcat Fault the secondary fault pattern continues with less expression, apparently reflecting lesser total long­term movement. These secondary faults are presumably currently inactive. The fractured rocks along any of these faults form numerous passages for groundwater. 

The most recent geologic studies of the complex hill area (Converse Assoc., 1984) suggest that a single deep groundwater table may exist in combination with numerous perched groundwater tables. The perched groundwater areas may exist seasonally or only after periods of heavy rainfall. The local presence of groundwater in the hill area is also strongly influenced by the presence of seepage barriers such as faults and the numerous Orinda-Moraga formational contacts. The primary sources of groundwater in the hill area were deduced to be surficial (runoff and infiltration) as well as the volcanic flow rocks east of the Wildcat Fault. 

Groundwater may move relatively freely through the highly fractured Moraga rocks, but is impeded by the relatively impermeable contact zone and less permeable nature of the Orinda rocks. This restriction of groundwater flow results in an accumulation of water at and above the contact zone of these two formations. Flow occurs along these contact zones when gradients are sufficient and often exits the hillsides in the form of springs or seeps. A strong correlation exists between spring locations and Orinda-Moraga contacts. These contacts are highly irregular because of the interbedded nature of these formations, so springs do not occur at any given elevation on the hillsides.