I agree with R.L.
The rocks looked like angular chunks of igneous rocks such as previously seen by Pathfinder and Viking. These rocks are probably ejecta from meteor impacts. Sedimentary rocks (if they exist) maybe buried below the regolith and inaccesible to MER. Unlike the Hematite landing site which is guaranteed to be interesting due to its surface spectra, Gusev might not pan out as a noteworthy landing site (I was disappointed when Gusev was selected). However even igneous rocks will tell us many interesting things about Mars.
Gary