Maybe it's a layer of CO2-ice clathrate.... :)

Seriously, that is pretty wierd. Light colored Ash/tuff should show quartz; water-borne sediments should show carbonate.

Something high in feldspar? Like sediments of grains originally dervied from rock high in lighter colored igneous minerals other than quartz (e.g., plagioclase)? That would be unusual on earth since the feldspars weather and leave the quartz, but with out any water, that's not such a problem. Or am I way off base here, given that I've never taken a geology course in my life. :)

Salt? Talc? The concrete liner of an old martian canal?

Will TES pick up quartz even when it's in very very fine grained ash beds?

One thing to keep in mind might be that "light" is relative. The soil we are on is apparently VERY dark, so perhaps "light toned bedrock" could still be something dark grey.