You can use Workspace Manager events to accomplish this. See chapter 2 of the user manual for additional details. First you would need to enable events:
SQL> execute dbms_wm.SetSystemParameter ('ALLOW_CAPTURE_EVENTS', 'ON');
Then you specify which events you are interested in using dbms_wm.SetCaptureEvent. In your case, you would want to enable the WORKSPACE_MERGE* and TABLE_MERGE events. See the wm_events_info view for supported events. They are also listed in the user manual.
Then when you dequeue the message you can get the time from the message_properties parameter. The payload parameter(which is of type wmsys.wm$event_type) would have details about the workspace, table_name, etc.