Oracle Trace File Interpretation: enq: TX - index contention
OBJ#=-1 means either means that the object number is missing or it's not related to an object.
The best idea is to go into V$active_session_history and find the SQL_ID then get the SQL text via the SQL_ID and look at the object sin the SQL and what the SQL was doing.

As far as people suggesting INIT TRANS, that is unlikely as there is a separate wait event for INIT TRANS.  

USN, SLOT and SEQUENCE  map the UNDO entry for the transaction.
They could be used to map back to activity in the UNDO but it's probably easier to get the transaction id the XID and use VERSIONS_XID on the objects involved.

see http://www.slideshare.net/khailey/ukoug-oracle-transaction-locks

Example ASH query

col object for A15
col otype for A10
select
      substr(event,0,20)    lock_name,
      ash.session_id        waiter,
      mod(ash.p1,16)        lmode,
      --ash.p2                p2,
      --ash.p3                p3,
      o.object_name           object,
      o.object_type           otype,
      CURRENT_FILE#           filen,
      CURRENT_BLOCK#          blockn,
      ash.SQL_ID             waiting_sql,
      BLOCKING_SESSION        blocker
      --,ash.xid
from
        v$active_session_history ash,
        all_objects o
where
          event like 'enq: TX - index contention'
  and o.object_id (+)= ash.CURRENT_OBJ#
/