Type 1 and type 2 sequence

In sequence stratigraphy, a sub discipline of geology, type 1 sequences and type 2 sequences are special sequences that are defined by having distinct types of sequence boundaries. In modern literature, the distinction in type 1 sequences and type 2 sequences was abandoned.^[1][2]

Definition

Type 1 and 2 sequence boundary

A type 1 sequence boundary is defined to be a sequence boundary "characterized by subaerial exposure and concurrent subaerial erosion associated with stream rejuvenation, a basinward shift of facies, a downward shift in coastal onlap, and onlap of overlying strata".^[3] Similarly, a type 2 sequence boundary is "marked by subaerial exposure and a downward shift in coastal onlap landward of the depositional-shoreline break; however, it lacks both subaerial erosion associated with stream rejuvenation and a basinward shift in facies"^[3]

So the main distinction between type 1 and type 2 sequence boundaries is the amount of subaerial exposure. Type 2 sequence boundaries display, in contrast to type 1 sequence boundaries, only little subaerial exposure.

Type 1 and 2 sequence

With the notions introduced above, a type 1 sequence is a sequence that is bounded by a type 1 sequence boundary below and a type 1 or a type 2 sequence boundary above.^[3]

Similarly, a type 2 sequence is a sequence that is bounded by a type 2 sequence boundary below and a type 1 or a type 2 sequence boundary above.^[3]

Usage

The definition of a type 1 and type 2 sequence was first introduced by Vail et al. (1984).^[4] Since they were hard to recognize, they were redefined in 1990 by Van Wagoner et al.. However even with this new definition, type 2 sequence boundaries were hard to recognize in the field due to their lack of subaerial exposure. Therefore, the abandonment of this nomenclature was recommended by Posamentier and Allen in 1999.^[5]