Yes, that is a possibility. Another is tectonic thickening. A friend of
mine was involved in a study of Cretaceous coals on the Kaparowitz Plateau.
There the same seam of coal thickened and thinned over short aerial
distances, ranging from inches thick to over 24 feet thick. This of course
is a mining geologist's nightmare, but is of interest with respect to the
mode of origin. Here tectonic thickening was ruled out by the planar nature
of the bedding and the undeformed caprock sandstone. In Australia there is
a seam reported to be 750 meters thick, which I believe is in a synclinal
structure, and thus may well represent tectonic thickening.
Art
http://chadwicka.swau.edu