Consulta SQL para agrupar los resultados según la secuencia

WITH numbered AS (
  SELECT
    ID, Seq, Amt,
    SeqGroup = ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Seq) - Seq
  FROM atable
)
SELECT
  ID,
  Start = MIN(Seq),
  [End] = MAX(Seq),
  TotalAmt = SUM(Amt)
FROM numbered
GROUP BY ID, SeqGroup
ORDER BY ID, Start
;