Sespe Formation

Sespe Formation Stratigraphic range: Oligocene, upper Eocene
Weathered, poorly sorted conglomerate from the lower member of the Sespe Formation, Santa Ynez Mountains, California. The clasts range in size from boulders to small pebbles.
Type	sedimentary
Underlies	Vaqueros Formation, Monterey Formation
Overlies	Coldwater Formation ("Coldwater Sandstone", "Coldwater Shale")
Thickness	0-7,500 ft
Lithology
Primary	sandstone, conglomerate
Other	mudstone, occasional shale
Location
Region	Southern and south central California
Country	United States
Type section
Named for	Sespe Creek
Named by	Watts (1897); redefined by Kew (1924)

The Sespe Formation is a widespread fossiliferous sedimentary geologic unit in southern and south central California in the United States. It is of nonmarine origin, consisting predominantly of sandstones and conglomerates laid down in a riverine, shoreline, and floodplain environment between the upper Eocene Epoch (around 40 million years ago) to the end of the Oligocene Epoch (around 24 million years ago). It is often distinctive in appearance, with its sandstones weathering to reddish-brown, maroon, pinkish-gray, tan, and green. Since many of its sandstones are more resistant to erosion than many other regional sedimentary units it often forms dramatic outcrops and ridgelines in many local mountain ranges.

The type locality of the Sespe is along Sespe Creek in the Topatopa Mountains, about four miles (6 km) north of Fillmore, where it was first named in 1897 and redefined in 1924.

The Sespe Formation has been divided into three clearly differentiable subunits, designed Lower, Middle, and Upper.

During the Oligocene Epoch, the granitic and volcanic mountain ranges in the present-day vicinity of San Diego County eroded over many millions of years, with their alluvium, ranging in size from microscopic particles to pebbles and even larger rocks, deposited in an extensive coastal floodplain intersected with rivers. The sedimentary rocks which resulted from the millions of years of deposition included mudstones, siltstones, and abundant sandstones and conglomerates. During this time the seashore gradually regressed to the west, as the large mountains eroded and filled the floodplain with alluvium. Over time the depositional environment changed from continental to marine, due to changes in surface elevation and sea level; the marine sedimentary formation equivalent to the Sespe is known as the Alegria Formation, and is more often found to the west, for example in southwestern Santa Barbara County. A peculiarity of the Sespe Formation is the presence of an unconformity throughout much of the geographic distribution of the unit, representing a gap of millions of years and including most or all of the early Oligocene; in the vicinity of Simi Valley, the gap corresponded to an erosional event lasting about 8 million years.