Type 1 and Type 2 Sequences
Not all relative falls in sea level occur at a fast enough rate to expose the continental shelf. For example, during a eustatic fall, a rapidly subsiding margin may still experience a relative rise in sealevel, provided the rate of eustatic fall is less than the rate of subsidence. Early seismic studies recognized two types of sequences reflecting the case of sea-level fall below the shelf-slope break (type 1) and the case where sea level does not fall below this break (type 2). Although there has been much subsequent confusion about the application of these two types to outcrop studies, their definitions have been modified such that a type 1 sequence now refers to one in which there is a relative fall in sea level below the position of the present shoreline and a type 2 sequence refers to a sequence in which the relative fall in sea level does not force a shift in the position of the shoreline. Type 1 sequences were discussed above in the preceeding sections; type 2 sequences are discussed below.
Type 2 sequences (shown below) are similar to type 1 sequences (shown above) in nearly all regards except for the extent of the sequence-bounding unconformity and its expression in the marine realm. In addition, the two sequences differ in the name of the systems tract lying above the sequence boundary but below the transgressive surface .
In a type 2 sequence, the extent of the sequence-bounding unconformity can reach seaward only to the position of the previous shoreline, but no further. In other words, none of the marine areas of the previous highstand are subaerially exposed during a type 2 sequence boundary. Updip of these areas, the sequence bounding unconformity is expressed as for a type 1 sequence, but no incised valley forms as sealevel does not fall far enough for incision. In the marine realm, no basinward shift of facies occurs as in a type 1 sequence, and the type 2 sequence boundary is characterized only by a slight change in stacking patterns from increasingly progradational in the underlying highstand to decreasingly progradational (possibly aggradational) above the sequence boundary. Detecting this subtle transition in marine sections may be difficult to impossible and many type 2 sequence boundaries probably go undetected.
The shelf margin systems tract in a type 2 sequence is equivalent in stratigraphic position to the lowstand systems tract of a type 1 sequence. As stated above, the shelf margin systems tract is characterized by aggradational stacking. Like the lowstand systems tract, the shelf margin systems tract is capped by the transgressive surface.
In general, far more type 1 sequences have been reported than type 2 sequences, possibly in part reflecting their comparative difficulty or ease of detection. Some workers have gone so far as to question the existence of any type 2 sequences.
Next . . . Application to Outcrops