active tectonics
-- The Canon 10-22 lens is surprisingly fun for geology. We love context and for that wide is better. For closeups you just put it a foot or so from the outcrop and click away. This is one of those "notes to self" as I often figure out cool things and then struggle to remember the details when I go back years later --
I sort of stumbled across this on my day off. The sedimentary deposit was pretty cool all by itself but its relationship to active tectonics in California raises it to another level.
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An outcrop of "Año Nuevo fan" deposit in a down-dropped block between the Frijoles and Coastways splays of the San Gregorio-Hosgri fault system. This is an active strike-slip fault system outboard of the San Andreas on the Santa Cruz County Coast. The fan deposit is one of our best records of it's activity and movement.
Subtle extension along this section of the fault zone has resulted in subsidence and accommodation of several 10-15 foot thick young alluvial deposits along the axis of the fault system on the Año Nuevo terrace. These deposits are found cut-off and dragged north relative to their respective source streams. This fact can be used to constrain the direction and rate of local fault movement -- a right lateral offset of 5 to 11 mm / year during the Holocene along the San Gregorio-Hosgri fault system -- perhaps 10 to 20 percent of the total right-lateral strain in central California.
Charcoal fragments date the base of this particular deposit to the early Holocene (12,000 years before present). Lower units in the graben fill include clean beach sands and organic rich estuarine muds (seen at the base of the outcrop here) with erosive meandering conglomerate filled channels. These indicate deposition at or near sea-level. The upper units (making up the bulk of the outcrop seen here) consist of tabular conglomerates , thin discontinuous shales, and mudflow deposits. It is interpreted to be a terrestrial braid plain filling the down-dropped block on the terrace between subsidence events.
Clasts in the conglomerates consist largely of local Miocene shale (probably from the underlying Purisima Formation or perhaps the Santa Cruz Mudstone found to the east of the San Gregorio Fault). The thick proximal mudflow unit half way up the outcrop behaved plastically during and following emplacement -- injecting down into underlying gravels and erupting through overlying gravels to the "paleo-surface". Cobbles in the upper unit show strong imbrication and indicate westward paleocurrents. Channels in the lower unit run generally westward. Although the deposit is now cross-cut by Año Nuevo Creek, the age and nature of the deposit are consistent with derivation from Finney Creek, 2000 feet to the southwest.
The San Gregorio Fault system shows no evidence of aseismic creep and is expected to generate a large earthquakes in the future. 5-9 separate past surface-rupture events have been inferred from relationships on the Año Nuevo marine terrace.
active tectonics
-- The Canon 10-22 lens is surprisingly fun for geology. We love context and for that wide is better. For closeups you just put it a foot or so from the outcrop and click away. This is one of those "notes to self" as I often figure out cool things and then struggle to remember the details when I go back years later --
I sort of stumbled across this on my day off. The sedimentary deposit was pretty cool all by itself but its relationship to active tectonics in California raises it to another level.
------------------------------------------------
An outcrop of "Año Nuevo fan" deposit in a down-dropped block between the Frijoles and Coastways splays of the San Gregorio-Hosgri fault system. This is an active strike-slip fault system outboard of the San Andreas on the Santa Cruz County Coast. The fan deposit is one of our best records of it's activity and movement.
Subtle extension along this section of the fault zone has resulted in subsidence and accommodation of several 10-15 foot thick young alluvial deposits along the axis of the fault system on the Año Nuevo terrace. These deposits are found cut-off and dragged north relative to their respective source streams. This fact can be used to constrain the direction and rate of local fault movement -- a right lateral offset of 5 to 11 mm / year during the Holocene along the San Gregorio-Hosgri fault system -- perhaps 10 to 20 percent of the total right-lateral strain in central California.
Charcoal fragments date the base of this particular deposit to the early Holocene (12,000 years before present). Lower units in the graben fill include clean beach sands and organic rich estuarine muds (seen at the base of the outcrop here) with erosive meandering conglomerate filled channels. These indicate deposition at or near sea-level. The upper units (making up the bulk of the outcrop seen here) consist of tabular conglomerates , thin discontinuous shales, and mudflow deposits. It is interpreted to be a terrestrial braid plain filling the down-dropped block on the terrace between subsidence events.
Clasts in the conglomerates consist largely of local Miocene shale (probably from the underlying Purisima Formation or perhaps the Santa Cruz Mudstone found to the east of the San Gregorio Fault). The thick proximal mudflow unit half way up the outcrop behaved plastically during and following emplacement -- injecting down into underlying gravels and erupting through overlying gravels to the "paleo-surface". Cobbles in the upper unit show strong imbrication and indicate westward paleocurrents. Channels in the lower unit run generally westward. Although the deposit is now cross-cut by Año Nuevo Creek, the age and nature of the deposit are consistent with derivation from Finney Creek, 2000 feet to the southwest.
The San Gregorio Fault system shows no evidence of aseismic creep and is expected to generate a large earthquakes in the future. 5-9 separate past surface-rupture events have been inferred from relationships on the Año Nuevo marine terrace.