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USGS CMG InfoBank: Active Fault Scarp

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Comment: 22:08 - 23:25 (01:17)
Source: Annenberg/CPB Resources - Earth Revealed - 17. Sedimentary Rocks: The Key to Past Environments
Keywords: "Cathy Busby-Spera", "sedimentary rock", "clastic sediment", "sedimentary structure", "Ridge Basin", "basin margin", "San Gabriel Fault", "active fault scarp", "coarse grained sediment", "grain size", "grain shape", "poor sorting", sand, cobble, angularity, transportation, sedimentologist, crust, "tectonic history"

Our transcription: At the margin of the basin, next to the San Gabriel Fault, evidence suggests that the fault was active at the time the basin formed.
We're looking at coarse grained sedimentary rocks at the basin margin.
You can see that the clasts are large and very angular and that the rocks are very poorly sorted; that is, it's a mixture of all different grain sizes from sand on up to cobbles.
The angularity and the poor sorting indicate that this material has not traveled very far from the source area and was probably shed from the active fault scarp as the basin was down dropped.
Sedimentologists have concluded that this fault activity could itself have created the Ridge Basin.
Owing to a bend in the San Gabriel Fault movement would have stretched the crust in the area of the basin causing it to sag downward.
Through careful fieldwork combining the study of sedimentary rocks with local tectonic history, geologists have reconstructed a detailed view of the ancient Ridge Basin.

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