help deleting obsolete records

I don't know how to write an SQL for the following:

Table "teamwork"
  Attribute |           Type       |  Modifier
-----------+----------------------+-------------
  team      | character varying(8) | not null
  emp       | character varying(2) | not null

Table "timesheet"
  Attribute |           Type           |  Modifier
-----------+--------------------------+-------------
  emp       | character varying(8)     | not null
  team      | character varying(2)     | not null
  lo_shift  | timestamp with time zone |
  hi_shift  | timestamp with time zone |

Consider a subset of data from timesheet which meets the condition:
         lo_shift > (now()-'30days'::interval)
Now I want to delete all rows from teamwork where the data pair "emp,team"
is *not* in the subset of data retrieved from timesheet.  It is possible
that timesheet will contain multiple rows for any given emp,team pair.

Frank