In geology, a sequence is a stratigraphic unit which is bounded by an unconformity at the top and at the bottom.
Definition In a more rigorous and general way, a sequence is defined as a
"relatively conformable [...], genetically related succession of strata bounded by unconformities or their correlative surfaces"
Special cases and related concepts Special cases of sequences include type 1 sequences and type 2 sequences. A related concept are parasequences. Contrary to their name they are not smaller sequences.
See also Catena (soil) Sequence stratigraphy
References