According to the answer for this question, that was not accepted, alluvial fans might have water and thus a IVc category mission would be required to visit such an area.
Curiosity meets only the IVa category criteria and thus would not be allowed to enter an alluvial fan. Only a IVc category mission is allowed to enter a special region, the definition of which is further clarified and updated in this report.
Special regions are defined there as areas within which sufficient water activity may exist, among other criteria, and alluvial fans are not mentioned.
According to these criteria, alluvial fans should not be considered special in comparison to other regions, for instance because they both have frozen water in the subsurface.
See also Appendix B about MEPAG findings, revisions and updates.
So are Peace Vallis and it's alluvial fan classified or do they have the conditions to be classified as special regions or not ?