Terminus Reservoir Geology, Paleontology, Flora & Fauna, Archeology, History

Geologic History

The Terminus Reservoir region was a part of a synclinal basin during the latter half of the Paleozoic and the early part of the Mesozoic eras. Sedimentary and igneous rocks accumulated in this basin until the late Jurassic. The history of these periods, as seen in the rocks today was complicated by intense mountain-building diastrophism, intrusion of basic magmas, regional dynamic matamorphism, and periods of erosion. This element of geologic history was brought to a close near the end of the Jurassic period by intrusion of granitic Sierra Nevada batholith, which resulted in contact metamorphism in the overlying rocks. The dynamic and contact metamorphism that accompanied the intrusion was superimposed on the already metamorphosed sediments, producing a more complicated structure and metamorphic history. A general regional uplift at this time was followed by a long period of erosion, during which, large areas of granite were exposed and the metamorphic rocks were left as roof pendants. There were subsequent elevations of the region, the greatest being at the close of the Pliocene era, when the entire Sierra Nevada was uplifted and tilted to form the present range and the present cycle of erosion was initiated.

Physiography

Kaweah River is a westerly flowing stream superimposed on the underlying formations. It is deeply entrenched in its present course and is a degrading stream, having relatively thin alluvium fill deposits on the valley floor. Its predominant pattern is dendritic, but in the lower foothills is modified to a rectangular pattern. The dendritic pattern is the result of the greater area of the drainage basin being underlain by a fairly uniform granite mass and the modifying rectangular pattern is caused by the tributary streams being structurally controlled in their erosion by metamorphic formations that have a northwest trend in the area.

In the immediate vicinity of the dam, Lime Kiln Hill is an erosional remnant of Lemon Cove Schist and Quartzite. The low saddle occupied by the auxiliary dam was formed mainly by erosion of granite starting at the contact of the granite and the more resistant metamorphics. The valley of Lime Kiln Creek is structurally formed and, for nearly 4 miles, follows the contact of the metamorphics and granite. Greasy Creek has a fairly straight southwest course and flows over granite and metamorphics. Its course appears to be controlled by a series of minor faults. The course of the river at this point may also be explained by the occurrence of marble interbedded with meta-sediments that may have allowed more rapid stream erosion.

Stratigraphy

Most of the regional rock in the Kaweah River area is granite, which covers about three-fourths of the drainage basin. The basic intrusives, meta-volcanics, and meta-sediments form prominent, resistant ridges. These rock types, due to their prominence, appear to be more extensive than they actually are. In general, the rock formations of the Terminus Reservoir region are characteristic of the bedrock complex of the western slope of the Sierra Nevada, and the rocks may be described in three groups: (1) metamorphosed sediments and volcanics (viz., rocks of the Kaweah Series, which consists of the Lemon Cove Schist, and Homer Quartzite, the Three Rivers Schist, and probably the Yokohl Amphibolite); (2) moderately metamorphosed diorite and gabbro intrusives; and (3) granite batholith and associated dikes. The younger sediments of the area include old terrace deposits and recent valley fill alluvium. Terrace deposits occur on remnants of elevated benches along both sides of the valley from Terminus Dam site upstream to Three Rivers. The deposits are generally less than 20 feet thick and are so deeply weathered that they are relatively impervious. Included granite boulders are crumbly, and the feldspar-rich sand matrix has been thoroughly decomposed. These terrace deposits are Quaternary in age and were probably formed early in the Pleistocene period. The Recent valley fill deposits are unconsolidated sands and gravels averaging about 20 feet deep at the damsite. This alluvium increases in depth downstream from the dam and extends westerly out over the river delta as alluvial fan deposits. The sands are predominantly grains of feldspar, mica, and quartz, and the cobbles and boulders are predominantly granite, quartzite, and aplite.

Regional Structure

The bedrock complex in the Terminus Reservoir region has been subjected to severe diastrophism that probably occurred in two stages. Structure of the metamorphic rocks is mostly internal within the Kaweah Series and consists of a series of large, tightly folded anticlines and synclines. Some local overturned folds occur and small scale drag folding is common. Numerous joint sets occur, but their age relationship to the intrusives is vague. The original joint system, which was probably formed during the distortion of the meta-sediments, has been modified by joint sets formed at the time of the most recent granite emplacement. Minor faulting has occurred throughout the region. It appears to be recent, cutting the meta-sediments and plutonics alike. The roof pendants have a rudimentary northwest trend and steep angle contacts with the plutonics. Bedding is usually closely spaced and is locally destroyed by deformation and recrystallization. Fracture cleavage approximately parallels axial plans of the folds. The pre-granitic plutons of hornblende, gabbro, and diorite have not been severely deformed structurally. They are cut by numerous quartz-bearing igneous dikes. The shape of these plutonics is partly controlled by the structure of the meta-sediments into which they were emplaced.

Paleontology

Earth and rock moving operations during the construction of Terminus Dam did not reveal any materials of paleontological significance. In his geological study of the area, Durrell [2] carefully searched for fossils in the places most favorable for their occurrence in order to establish the age of rocks, but found none. Discovery of fossil horse teeth in the reservoir area a number of years ago, and the recent discovery of an Ice Age mammoth tusk, estimated to be 15,000-45,000 years old, a few miles northwest of Terminus Dam, indicate there are fossil remains in the terrace gravels of the area, but it is generally concluded that the Kaweah River region is not of special paleontological importance.