Debris flow is another possible source of stray larger rocks in a
fine-grained setting. I vaguely recall a paper on such material from
Jamaica, probably with reference to the Bowden (Pliocene).
Also, it seems likely that a few pebbles can be transported by rare
episodes of higher energy in a normally quiet setting. Two weekends ago I
found a small pebble in fine-grained fossiliferous Yorktown Formation
deposits here in North Carolina. It was close enough to the ancient
shoreline to have relatively easy transport of an occasional river pebble,
but I do not know what to look for that would be diagnostic of its history.