SEQUENCE STRATIGRAPHY 165 Figure Physical expressions of (A) type and (B) type sequence boundaries The type boundary exhibits an abrupt downward shift in facies; the type boundary exhibits more a gradual downward shift (Adapted from Posamentier HW and Vail PR (1988) Eustatic controls on clastic deposition II: sequence and systems tract models In: Wilgus CK, Hastings BS, Kendall CStCC, et al (eds.) Sea Level Changes: An Integrated Approach, pp 125 154 Special Publication 42 Tulsa: Society of Economic Paleontologists and Mineralogists.) boundary may be of either type depending on local accommodation and sedimentation rates The subaerially produced unconformity that occurs at a sequence boundary passes at some basinwards point into a genetically related conformable stratigraphic contact Some workers contend that a sequence boundary should be recognized only where an unconformity exists However, many workers correlate this as the same chronostratigraphic surface, potentially allowing a sequence to be recognized over an entire basin Transgressive surface The transgressive surface is the first marine flooding surface of significant areal extent landwards of the offlap break (Figure 5) In a complete sequence, it marks a change from progradational or aggradational parasequence stacking at the bottom of the sequence to retrogradation in the middle of the sequence It may be somewhat diachronous, with the onset of transgression appearing earlier in more basinward areas and reaching more landward areas later Maximum-flooding surface The maximum-flooding surface reflects the maximum landward extent of transgression and is marked by a stacking-pattern change from retrogradational in the middle part of a complete sequence to progradational in the upper part (Figure 5) On seismic profiles, it is commonly marked by the downlap of younger horizons onto it and, as such, is sometimes termed the downlap surface The maximum-flooding surface may have an associated condensed section characterized by strongly burrowed intervals or hardgrounds, in some cases with an associated marine hiatus, as well as enrichment of authigenic minerals such as glauconite or phosphate, a high organic content, and a peak in the abundance of deeper-marine fossils The condensed section reflects slow sedimentation rates in basinward areas when the peak of the transgression focuses sedimentation in the heads of estuaries and in other landward areas The elevated concentration of fossils in condensed sections commonly makes them important intervals for the occurrence of age-significant fossils such as ammonites, planktonic foraminifera, and calcareous nannofossils Systems Tracts Lowstand systems tracts Three systems tracts can be deposited during a lowstand of relative sea-level: lowstand fan, lowstand wedge, and shelf-margin wedge (Figure 5) These lowstand systems tracts overlie the sequence boundary and express a basinwards shift in facies produced during periods of relative sealevel fall The lowstand fan systems tract is the most basinward of the lowstand systems tracts and forms by the accumulation of clastic deposits in a deepbasin setting During times of relative sea-level fall, large areas of the basin margin are exposed and subjected to erosion Sedimentation mostly bypasses the basin margin and is fed directly to the basin through incised valleys and submarine canyons As a result, lowstand fans are commonly detached from the depositional system that built the preceding highstand complex upslope They may onlap the underlying sequence boundary in the landwards direction; the relief built during fan formation may also produce bidirectional downlap in other directions (Table 1) Aggradational to slightly retrogradational stacking patterns are the most common The top of the fan may be marked by a shift in deposition to the overlying lowstand wedge systems tract, which produces a downlap surface between the units In some deepwater systems, the lowstand fan can be divided into two parts: a basin-floor fan, which occurs on the basin floor and may be detached from the depositional system that built the preceding highstand complex, and a slope fan, which develops along the middle or lower part of the slope