Rincon Formation

Rincon Formation Stratigraphic range: Miocene
Weathered outcrop of the Rincon Formation, Sycamore Canyon, Santa Barbara, California.
Type	sedimentary
Underlies	Monterey Formation
Overlies	Vaqueros Formation
Thickness	500 to 1700 feet
Lithology
Primary	shale
Other	siltstone, mudstone, with minor sandstone beds
Location
Region	Coastal southern California
Country	United States
Type section
Named for	Rincon Mountain (east slope, Los Sauces Canyon), Ventura County
Named by	Kerr, 1936

The Rincon Formation (or Rincon Shale) is a sedimentary geologic unit of Lower Miocene age, abundant in the coastal portions of southern Santa Barbara County, California eastward into Ventura County. Consisting of massive to poorly bedded shale, mudstone, and siltstone, it weathers readily to a rounded hilly topography with clayey, loamy soils in which landslides and slumps are frequent. It is recognizable on the south slopes of the Santa Ynez Mountains as the band at the base of the mountains which supports grasses rather than chaparral. Outcrops of the unit are infrequent, with the best exposures on the coastal bluffs near Naples, in the San Marcos Foothills, at the Tajiguas Landfill, and in road cuts. The geologic unit is notorious as a source of radon gas related to its high uranium content, released by radioactive decay.

The type locality of the unit is in Ventura County, on the east side of Rincon Mountain, where the formation is exposed in the south-flowing Los Sauces Canyon, north of U.S. Highway 101 and in the vicinity of the Rincon Oil Field. The unit is mapped from Ventura County westward along the south slope of the Santa Ynez Mountains, as well as on portions of the north slope. It also occurs in a few places north of the Santa Ynez Fault, but outcrops nowhere north of the Santa Ynez River. In the subsurface it frequently is found in well cores, underlying the Monterey Formation and overlying the Vaqueros Formation. At the ground surface, the contact with the Vaqueros is obvious along the south slope of the mountains, for it is almost always defined by the line dividing the rounded, grassy foothills from the more rugged, chaparral-covered sandstones upslope.