Transgressive Systems Tract
The transgressive systems tract is deposited during rapidly rising relative sea level (rapid increase in accommodation) that follows lowstand deposition. At this time, the rate at which new accommodation is created exceeds the rate of sediment accumulation, and shoreline transgression occurs. Commonly, the onset of transgression is abrupt, and a basal transgressive surface separates the lowstand and transgressive systems. On the shelf, the surface separates progradational highstand and aggradational lowstand incised-valley-fill deposits from overlying retrogradational deposits (fig. 2) that compose the transgressive depositional systems (fig. 4). In coastal areas of high wave energy, the transgressive surface records wave erosion of ravinement surfaces eroded into on-shelf deposits (in areas between incised valleys) that were subaerially exposed during lowstand time. In these areas, the transgressive surface is superimposed on the sequence boundary, although erosion at this surface is not evident on individual well logs.

Distal incised valleys are sites of early transgressive deposition when relative sea level rises and inundates the previously exposed shelf. Incised valleys are flooded, and estuaries are formed. Deposition within incised valleys during valley flooding includes various tidal, estuarine, and bayhead-delta sands and muds. These transgressive deposits tend to be thinner than the older, fluvial valley fill.

The sequence boundary and superimposed transgressive surface between incised valleys eventually flood, and thin, retrogradational, coastal-plain deposits and marine muds are deposited on the ravinement surface. With continued rapid rise in relative sea level, each successive coastal-plain progradational succession shifts landward, forming a retrogradational stratal stacking pattern (fig. 2). This stacking pattern, characteristic of the transgressive systems tract and well expressed on well logs, reflects the net landward shift of the shoreline. In parts of basins having limited fluvial sediment supply, the transgressive systems tract consists of shaly and silty facies represented by an upward-fining pattern on well logs (fig. 4).

Maximum transgression is recorded in the rock record by the maximum flooding surface. In the more distal portions of the shelf and basin, this surface is contained within a marine condensed section (fig. 1), which comprises shaly organic-rich facies that feature the greatest abundance and diversity of fauna within a sequence (fig. 4). The thin marine condensed section records extremely low rates of pelagic sedimentation and spans the time when transgressive shorelines stepped landward and the shelf was starved of terrigenous sediment.

The marine condensed section is most clearly recorded by the GR log, which is sensitive to radiogenic elements attached to clay minerals of pelagic origin and concentrated, in situ, organic sediments. The maximum GR value of the upward-fining, retrogradational interval above the transgressive surface marks the maximum flooding surface (fig. 4).

Highstand Systems Tract
The highstand systems tract, the youngest in a sequence, forms when the rapid relative sea-level rise that characterizes the transgressive phase slows to a rate at which sediment accumulation is now equal to, or greater than, accommodation rate. As a result, shoreline regression or stillstand occurs; the deposits that accumulate during this period of diminishing rise of relative sea level constitute the highstand systems tract. The subjacent maximum flooding surface and the next sequence boundary define the lower and upper boundaries, respectively, of the highstand systems tract.

Highstand systems tracts typically display initial aggradation and subsequent increasing progradation (fig. 1, fig. 2, and fig. 4) because of diminishing rise of relative sea level and slowing of creation of accommodation space. The resulting stratal stacking pattern is clearly expressed on GR and SP logs as a large-scale, upward-coarsening succession (fig. 3) comprising several smaller scale (higher frequency) shale-and-sandstone intervals, each of which also coarsens upward (fig. 4). The sandstone intervals at the top of each of these smaller scale units typically become successively thicker upward. All of these characteristics record a progradational, or seaward-stepping, succession of stacked deltaic or strandplain deposits.

With the onset of a new relative-sea-level cycle, lowstand conditions return (falling relative sea level), and a new sequence boundary forms at the top of the highstand systems tract. As described previously, the stratigraphic expression of rapid relative-sea-level fall in on-shelf regions is either prominent river incision of the highstand deposits (and possibly the underlying transgressive strata) (e.g., fig. 4) or subaerial exposure of nonincised intervalley areas.