Overview

Little Panoche Detention Dam

Panoche Creek is incised 10 to 30 feet in a broad, alluvium filled valley that trends northwest across a deformed sequence of Upper Cretaceous and lower to middle Tertiary marine sedimentary rocks. The Cretaceous and Tertiary rocks are unconformably overlain by slightly deformed late Tertiary fluvial. deposits known as the Tulare formation and by several tilted terraces of latest Tertiary and Pleistocene to Recent age.

The valley crosses a major structural flexure in the sequence of deformed Cretaceous and Tertiary beds. North of the valley these beds strike northwest and dip northeast along the eastern flank of the Panoche Hills, a strong uplift of Pleistocene to Recent age. South of the valley, in the Tumey Hills, the beds are folded into a series of southeast-plunging anticlines and synclines. These structures have been prospected on several occasions for oil and gas but no successful strikes have been reported. The Tumey Hills are not as strongly uplifted as the Panoche Hills and are partly covered by moderately deformed fluvial sediments of the Tulare formation.

The Cretaceous and Tertiary sediments have been strongly eroded more or less along strike north of Panoche Creek, producing a terrain of spurs and valleys. South of the creek, however, the beds dip generally into the hills south of the valley and here erosion proceeds along joints that dip steeply northwesterly and strike northeasterly. The joints are spaced one to two feet apart in sandstone and from one to three inches in shales. Intermediate spacings are noted in siltstone. High water losses were noted in drilling core hole S-Z 509 and it is believed that much of the loss was to joint planes. Some joint planes are partly healed by gypsum.

North of Panoche Creek the east-northeast joint direction is reflected by steep draws eroding more or less normal to the strike of the beds; however, the principal erosion in this area has been parallel to the strike of the beds, along weak shale and siltstone units.

Slopes south of Panoche Creek average 2:1 but show abundant evidence of instability, even under present semiarid conditions.

Numerous landslides, some still active, are found mostly along the south side of the valley. The slide debris is very heterogeneous, but consists chiefly of sand and silt derived from the Tertiary sediments mixed with varying amounts of gravel contributed by exposures of the Tulare formation high on the rim of the valley. In addition to well-defined slides, the south side of the valley shows widespread surface soil creep and the formation of cracks and small caverns reflecting the solution of gypsum along open fractures.

Temporary slopes in the Cretaceous and Tertiary formations Of 1.5:1 or steeper depending on height of cut and other land conditions, should be stable during construction if cuts are made more or less normal to the strike of the beds. Care should be taken to avoid undercutting dipping beds, and provision to protect temporary slopes during rainy weather will be necessary. Permanent slopes of 3:1 or flatter probably will be required for efficient operation. Landslides can be anticipated in the reservoir, particularly south of Panoche Creek and east of Silver Creek.

Little Panoche Dam is located at a topographic constriction in the creek valley about 2 miles upstream from where the creek enters the San Joaquin Valley.

The damsite is in the low foothills along the western margin of the San Joaquin Valley, a large northwesterly-trending structural trough. These hills are the flank of the Diablo Range. The range consists of a complex of varied sedimentary and metamorphic rocks of the Franciscan formation which have been uplifted along the Ortigalita fault. Northeast of the fault, marine sedimentary rocks of Cretaceous and Tertiary age strike northwesterly and dip to the northeast.

An ancient erosional surface called the Las Aquilas land surface has cut the tilted Cretaceous and Tertiary beds and local accumulations of late Tertiary fluvial deposits. This old land surface has been uplifted and gently warped between San Luis Creek, about 30 miles to the northwest, and the edge of the Panoche Hills, about one mile south of the damsite. The Panoche Hills are a later, stronger uplift and contain some remnants of the Las Aquilas surface.

The valley of Little Panoche Creek is incised into the Las Aquilas land surface near the center of a gentle synclinal warping. Within the trough, Cretaceous rocks of the Panoche group are poorly exposed beneath varied thickness of late Tertiary fluvial deposits known as the Tulare formation.

At the Little Panoche damsite there are Cretaceous marine sedimentary rocks of the Panoche group. These consist of interbedded sandstones, clay shales, and silty shales that are weakly cemented except for scattered lenses of hard limestone and carbonate-cemented sandstone. Rocks exposed on the lower slopes of the hills north and south of Little Panoche Creek strike north-northwest and dip 30 to 45 degrees easterly.

The rocks are mostly weathered to loose sand and shale chips. Scattered carbonate cemented lenses persist, revealing the attitudes of the beds. Weathered rocks show oxidation effects and contain traces of gypsum; altered rocks have a greenish-blue color and are frequently pyritic. Because of the cover of the continental sediments, it was not anticipated that the weathered and altered portions of the Cretaceous beds would pose any special problems to foundation stability on the abutments.

Beneath the damsite the Cretaceous erosion surface forms a ridge which rises to approximate elevation 630 feet and is buried beneath continental sediments and fluvial terrace deposits. The buried topography falls away to the south-southwest and north-northeast. The trend of the ridge reflects the presence of resistant sandstone beds. The buried surface on the Cretaceous rocks falls away to an unknown depth both upstream and downstream from the damsite.

Three distinct terrace levels have been noted within the Little Panoche Valley. All three levels are slightly tilted northeastward toward the mouth of the valley, the oldest most steeply due to recent uplift of adjacent portions of the Coast Range. Little Panoche Creek is entrenched 10 to 20 feet below the floor of its valley.

General

Dimensions

Hydraulics & Hydrology