1) Identify Major Stratal Stacking Patterns
In this exercise cross section, the progradational successions display an upward trend of increasingly coarser sandstone beds (SP and GR curves shift to the left). These beds are interstratified with siltstones and shales within intervals 50 to 70 ft thick. Each sandstone bed exhibits an upward-coarsening log motif (fig. 3). The gradual shift of the SP and GR log curves to the right in the section immediately above the progradational patterns identifies a retrogradational zone that exhibits an overall upward-fining log trend.

The progradational and retrogradational successions represent highstand and transgressive systems tracts, respectively. Note that many of these upward-fining, retrogradational zones are dominated by shale and are commonly much thinner than the progradational zones, which typically contain more and thicker sandstones (fig. 4). In these shaly zones, look for the horizon with the maximum GR value; this horizon generally delineates the marine condensed section at the top of the retrogradational package. In contrast, blocky and blocky-serrate, aggradational sections marked by stacking of sandstones of nearly equal SP and GR values represent lowstand incised-valley-fill deposits.

2) Identify and Correlate Key Sequence-Stratigraphic Surfaces
Sequence Boundary and Transgressive Surface
The most striking lateral variation in stratal stacking patterns on the cross section occurs in the sandstone-bearing zone at about 10,800 to 10,900 ft. In wells 1, 4, and 5, the zone is characterized by well-developed, blocky-serrate sandstone beds with sharp, abrupt bases. Regional mapping and whole-core data indicate that these sandstones are fluvial in origin and are incised into prodeltaic and marine shales and sandstones. They have regional extent as well, filling broad (>1 mi wide) erosional valleys. The bases of these fluvial deposits mark a regional erosion surface that formed during a fall in relative sea level of such magnitude that it enabled river systems to extend seaward across a subaerially exposed shelf platform. Fluvial deposits accumulated directly above prodeltaic and marine sediments on the exposed shelf. The erosion surface is a sequence boundary.

However, no such fluvial incised-valley fills are evident laterally in the same interval in wells 2 and 3. Instead, you can see a strongly developed progradational pattern that records deltaic or strandplain deposition, as interpreted from whole-core data. Careful correlation of marker beds within the progradational succession reveals that it is this highstand interval into which the fluvial incised-valley deposits are incised. The top of this progradational succession is therefore the surface of exposure developed in the intervalley areas during fluvial incision. This exposure surface is the sequence boundary in wells 2 and 3. It occurs approximately at the horizon where the first deposits of the transgressive systems tract are developed (the base of the retrogradational interval). However, in the incised valleys, the boundary between the lowstand fluvial deposits and the overlying transgressive systems tract is the transgressive surface. Where no valley fill is developed in the intervalley areas, the sequence boundary and the transgressive surface coincide.

Maximum Flooding Surface
The maximum flooding surface marks the culmination of retrogradation. This surface is most easily recognized on the GR log because this log is sensitive to compositional variations in shales. Higher clay and organic content generally produces higher GR values. The highest GR value at the top of an overall upward-fining, retrogradational section coincides with the maximum flooding surface (fig. 4) within a marine condensed section. Whole cores of this interval are characterized by black shale, abundant and diverse marine fossils, high organic content, and low silt content—in marked contrast to the underlying and overlying shales. The upward-coarsening, progradational succession above the maximum flooding surface represents highstand deposition.

3) Designate Systems Tracts
The accurate construction of a sequence-stratigraphic framework is in part dependent on the ability to distinguish between incised-valley fills and locally developed channel fills, such as those of delta-distributary channels. The following characteristics of incised-valley fills collectively serve to distinguish them from other channel-fill types (Van Wagoner and others, 1990). Erosion of a single incised-valley system occurs along a single stratigraphic surface, whereas distributary channels from a single delta system are commonly stacked in multiple local horizons. The regional extent (width) of incised valleys (as much as several miles) and their maximum depth of incision (as much as 300 ft or more) are markedly greater than those of any other channelized physiographic features that occur in a shelf setting. The strata that are truncated by incised valleys are commonly distal-marine facies; shelf mudstones and thin sandstones. In contrast, distributary-channel fills are typically encased in sandy stream-mouth bar deposits of deltas.