[valcker]

Crimea was an "oblast" (or region) of Ukrainian SSR and the referendum was about re-establishing the Crimean Autonomous republic which was the case and was signed by Ukrainian SSR council: http://zakon1.rada.gov.ua/laws/show/712-12

During the separation of republics from USSR, Crimea stayed as a part of Ukraine (and voted for the independence of Ukraine, btw) which was a logical move in terms of infrastructure.