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