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Below is a roadcut in central Marin County, California, in an area maps show to have Franciscan melange. I assume this is graywacke, but as you can see it varies from gray on the sides to a buff color in the center. Is the buff section due to some chert content, or did that section weather differently (and if so why)? Thanks!

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It's pretty hard to say anything about this kind of weathering without actually being there and having a closer look.

Usually, the simplest explanation for reddish or brownish colours in otherwise grey rocks is the presence of iron oxides. You can see that the brown area is on a fault or joint face. This leads me to think that maybe this was a conduit for groundwater that oxidised the rock. When the road cut (or whatever it is) was made, this part was exposed. Notice that you have some brown elsewhere - it could be that those are faults or joints cut perpendicular to the face, so that's why you only see a brown streak instead of a surface.

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