Question regarding formations in and around San Francisco

[LexonTheDragon]

Is Pacifica part of the Purisima Formation or the Merced Formation? If possible, could someone tell me areas around Pacifica that are Purisima, and some that are Merced? sorry if I'm asking a lot, I am just genuinely confused about where each formation is because they are so close to each other. 

[Crusty_Crab]

I'm not sure why they are mutually exclusive. When I go to a beach with sea cliffs, you can see exposures that can reveal layers from different formations. The Purisima is upper Miocene and Pliocene https://pubs.usgs.gov/pp/2007/1740/ while the Merced is Plio-Pleistocene https://ngmdb.usgs.gov/Geolex/UnitRefs/MercedRefs_13684.html. I don't see why you can't be in Pacifica and see layers that correspond to both. I quote from the latter link, "In Santa Cruz region the Merced rests on Purisima formation. It is considered older than Berkeley group. (See USGS San Francisco folio, no. 193.)"

[DPS Ammonite]

@Boesse This is right up your alley.

 

I believe that the Merced Formation crops out on the east side of the San Andreas Fault and well east of the Pacifica area since the San Andreas is east of Pacifica.

 

See pages 176-178:

https://www.researchgate.net/profile/John-Wakabayashi/publication/249552115_Stepovers_that_migrate_with_respect_to_affected_deposits_Field_characteristics_and_speculation_on_some_details_of_their_evolution/links/555f60b408ae9963a118b12a/Stepovers-that-migrate-with-respect-to-affected-deposits-Field-characteristics-and-speculation-on-some-details-of-their-evolution.pdf?origin=publication_detail

Edited May 18, 2022 by DPS Ammonite

[Boesse]

The base of the Merced Formation is somewhere around the garbage dump at the north end of Pacifica - there's a section of cliffs that stretches for several miles and are difficult to survey owing to lack of access points, but I spent a lot of time visiting when I was in high school. The Merced Formation exposures are continuous from here all the way up to Fort Funston, where the deposits transition from marine to nonmarine.

 

The San Andreas Fault goes offshore at mussel rock, which is Fransiscan basement, and the Merced is deposited right up against the fault.