SQL> with
  2    data as (
  3      select val, 
  4             case when nvl(lag(val) over(order by val),val)!=val-1
  5                  then val
  6             end grp
  7      from t
  8    )
  9  select min(val) start_number, max(val) end_number
 10  from (select val, max(grp) over(order by val) grp from data)
 11  group by grp
 12  order by grp
 13  /
START_NUMBER END_NUMBER
------------ ----------
           1          5
          10         12
          50         52
          55         57
         100        100

5 rows selected.

select min(val) seq_start, max(val) seq_end from
  (select val from test1 order by val)
group by val-rownum
order by seq_start;