A 48-year-old woman was injured yesterday when the snowmobile on which she was riding with her husband overturned while traversing deep powder in the Shirley Site area.  State Parks officers responded to the accident, as they investigate these incidents by agreement with federal officials on whose land the accident occurred.  Chaffee Search and Rescue South volunteers, along with Chaffee EMS paramedics, responded to her location and transported her to an ambulance site.  The woman, visiting from Missouri, was taken to Heart of the Rockies Regional Medical Center where she was treated for a broken ankle.