A PRELIMINARY STUDY OF THE CENOZOIC HISTORY OF THE KERN RIVER, SIERRA NEVADA MOUNTAINS, CALIFORNIA

This study of the Kern River (southern Sierra block--SSB), shows that the course changes of the river can be related to tectonic events affecting the SSB of the Sierra Nevada. The nature of the course changes reveals some of the dates and nature of the tectonic activity affecting the SSB. Deposits from the Kern River during ~70-40 Ma occur in the El Paso Mountains of the southwestern Basin and Range province; distal deposits less than ~20 Ma occur in the well documented San Joaquin Basin. These deposits provide the primary source of data for the courses of the Kern River.

Between ~70 and ~20 Ma, the combined blocks of the Sierra Nevada were reduced to a low-lying, low-relief terrain. Until ~40 Ma, the Kern River delivered sediment to a basin east of the Sierras; this sediment indicates that the river dates from ~70 Ma. At ~20 Ma and approximately contemporaneous with the beginning of the current San Andreas Fault and the San Joaquin Basin, tectonic events separated the SSB from the rest of the Sierra mountains. This included the development of the Garlock and San Andreas Faults (south side), Kern Front Fault system (west side), and the Sierra Nevada Fault system (east side). A fault, bounding the north side of the SSB, occurred at about 36^o N latitude and is marked by an offset of the drainage, a remnant of an earlier drainage, later basaltic volcanism, and a distinct change in topography. At this time, the drainage of the upper Kern River shifted and that of the lower Kern River changed from east to west. The block moved vertically upward along the Kern Front Fault system by at least 10’s of meters, and the SSB became a fault-bounded block distinct and separate from the central-northern block. At ~12 Ma, movement on a reactivated part of the Kern Canyon fault system tilted a small block of the central part of the SSB 1-2 degrees northwest, and redirected the river into its present channel. This faulting also created two ancient lake basins which were emptied by the end of the Pleistocene Ice Ages. Starting at ~5 Ma, additional uplifts plus late discharge changes related to the Pleistocene Ice Ages created several nick points and valley-shape changes in the Kern River Gorge. Sometime between 9 and 3 Ma, the central-northern Sierra Block tilted (east side up), rejoining the two Sierra blocks as well as the northern most drainage of the Kern River.