Getting the last data update time of a specific table in Oracle

You can do this with auditing, but it incurs a performance overhead and does not work retrospectively.

Read the section Verifying Security Access with Auditing in the Oracle® Database Security Guide 11g Release 2 (11.2).

I'll add that in the majority of cases, somebody in the organisation thinks that this type of auditing is required, but more often than not it is just a misguided requirement.

First you have to find out what the current sequence value is and then reset it. I would write a script which does the following tasks:

1. Get the max value from the table
2. update/recreate the sequence

The script could look like this:

declare
  maxval number;
begin
  select max(id) into maxval from <table_1>;
  execute immediate 'ALTER SEQUENCE <sequence_name_1> START WITH '|| maxval+1 ||';';

  select max(id) into maxval from <table_2>;
  execute immediate 'ALTER SEQUENCE <sequence_name_2> START WITH '|| maxval+1 ||';';
end;
/

I'm pretty sure there are better scripts but if you have to do this task frequently this might do the job without much development time.