Finding contiguous regions in a sorted MS Access query

I didn't see any purpose for the someVal1, someVal2, and someVal3 fields, so I left them out. I used an autonumber as the primary key instead of your date/time field; but this approach should also work with your date/time primary key. This is the data in my version of your table.

pkey_field target
1          A
2          A
3          A
4          B
5          B
6          A

I used a correlated subquery to find the previous pkey_field value for each row.

SELECT
    m.pkey_field,
    m.target,
    (SELECT Max(pkey_field)
        FROM YourTable
        WHERE pkey_field < m.pkey_field)
    AS prev_pkey_field
FROM YourTable AS m;

Then put that in a subquery which I joined to another copy of the base table.

SELECT
    sub.pkey_field,
    sub.target,
    sub.prev_pkey_field,
    prev.target AS prev_target
FROM
    (SELECT
        m.pkey_field,
        m.target,
        (SELECT Max(pkey_field)
            FROM YourTable
            WHERE pkey_field < m.pkey_field)
        AS prev_pkey_field
    FROM YourTable AS m) AS sub
    LEFT JOIN YourTable AS prev
    ON sub.prev_pkey_field = prev.pkey_field
WHERE
    sub.prev_pkey_field Is Null
    OR prev.target <> sub.target;

This is the output from that final query.

pkey_field target prev_pkey_field prev_target
1          A  
4          B      3               A
6          A      5               B

Here is a first attempt,

SELECT t1.Row, t1.target
FROM t1 WHERE (((t1.target)<>NZ((SELECT TOP 1  t2.target FROM t1 AS t2 WHERE t2.DateTimeId<t1.DateTimeId ORDER BY t2.DateTimeId DESC),"X")));