Is there a type of amnesia that separately affects long-term memory?
If I understand your question correctly, what you are describing is retrograde amnesia.  This is further subdivided into pure, focal and isolated.

From the site:


  Typically, episodic memory is more severely affected than semantic memory, so that the patient may remember words and general knowledge (such as who their country’s leader is, how everyday objects work, colours, etc) but not specific events in their lives. Procedural memories (memory of skills, habits and how to perform everyday fucntions) are typically not affected at all.