According to John Tvedtnes, most of Mormon's abridgment (Mosiah through Mormon) gives precise years for the various events, even when they are "flashbacks." Such precision implies that the records which Mormon possessed were very precise on chronology. It is likely that the historical records he consulted were in the form of annals.
However, sometimes Mormon listed years without recording any events for them. Thus, in 4 Nephi 1:6 he wrote, "And thus did the thirty and eighth year pass away, and also the thirty and ninth, and forty and first, and the forty and second, yea, even until forty and nine years had passed away, and also the fifty and first, and the fifty and second; yea, and even until fifty and nine years had passed away." Note also 4 Nephi 1:14: "And it came to pass that the seventy and first year passed away, and also the seventy and second year, yea, and in fine, till the seventy and ninth year had passed away; yea, even an hundred years had passed away."
Such rambling seems to be a waste of precious space on the plates. Mormon's run-down of dates in these verses is probably intended to record the years of the annals he consulted, even though he did not feel to write the history of each. [John A. Tvedtnes, "Mormon As an Abridger of Ancient Records," in The Most Correct Book, pp. 14-15]