Sequence stratigraphy: sequences, surfaces, and systems tracts

Relative sea level, accommodation and supply

Relative sea level is sea level measured against a datum fixed to the basement: eustasy plus subsidence, or minus uplift. A rise adds accommodation, room for sediment below base level; a fall takes it away. The rocks record relative sea level, never eustasy alone. Write $A$ for the rate at which relative sea level rises and $S$ for the rate at which sediment builds the ground up at the shoreline. Their balance sets how the shoreline moves (Catuneanu et al. 2009):

• Forced regression: relative sea level falls ($A < 0$), so the shoreline moves basinward and down whatever the supply, and the shelf it leaves dry is exposed and cut into a subaerial unconformity.
• Normal regression: relative sea level rises, but more slowly than sediment fills ($A/S < 1$): the shoreline advances basinward as it climbs (progradation, turning to aggradation as $A/S$ nears 1).
• Transgression: relative sea level rises faster than sediment fills ($A/S > 1$): the shoreline retreats landward and the beds step back (retrogradation).

Through one cycle the curve falls, then rises slowly, fast and slowly again, so the shoreline goes through forced regression, normal regression, transgression and normal regression once more. Those four phases are the four systems tracts, and the moments between them are the key surfaces.

The four systems tracts

• Falling-stage systems tract (FSST): laid down while relative sea level falls, by forced regression (Hunt and Tucker 1992). Its shoreface and delta deposits step down basinward and come loose from the shelf behind them, which is exposed and eroded, so its layers are truncated or toplap at the top. On a margin with a shelf break, sand that bypasses the shelf builds slope and basin-floor fans of the same age. It rests on the basal surface of forced regression and lies under the sequence boundary.
• Lowstand systems tract (LST): laid down early in the rise, while sediment still outpaces the slow rise. The shoreline advances and climbs from its lowest position, coastal deposits onlap the sequence boundary landward, and valleys cut during the fall begin to fill. It lies on the sequence boundary and under the maximum regressive surface.
• Transgressive systems tract (TST): laid down while the rise outpaces sediment. The shoreline retreats, each bed steps landward of the one below (retrogradation) and onlaps landward, and the waves may plane off the top of the older shoreface as the shoreline crosses it (a ravinement). Its marine part is often thin, the upper part condensed and rich in organic matter. It lies between the maximum regressive and the maximum flooding surfaces.
• Highstand systems tract (HST): laid down late in the rise, when sediment once more outpaces the slowing rise. The shoreline advances again, aggradation dies away toward the highstand, and the clinoforms downlap the maximum flooding surface. The next fall truncates its top.

Older models fold the falling stage into the other tracts. Posamentier and Vail (1988) put the sequence boundary at the onset of the fall and call its deposits the early lowstand, basin-floor fans included; the figure follows the depositional sequence of Hunt and Tucker (1992), one of the models Catuneanu et al. (2009) set side by side: the fall is a tract of its own, under a boundary placed at the end of the fall. Say which model you use: it moves the boundary.

The key surfaces

• Sequence boundary (SB): landward of the lowstand shoreline, the subaerial unconformity cut while relative sea level falls; basinward, its correlative conformity, the sea floor at the end of the fall (Hunt and Tucker 1992). Landward it has truncation below and onlap above, and time is missing across it; basinward the record is continuous, and the surface is found only by tracing it from the shelf. It is the surface that divides the rock record into sequences; a basin may hold five to twenty mappable ones.
• Basal surface of forced regression (BSFR): the sea floor when the fall begins, the base of the falling-stage deposits, whose clinoforms downlap it.
• Transgressive surface (TS), or maximum regressive surface (MRS): the surface at the end of regression, when the shoreline stands farthest basinward; landward it is the ground the transgressing sea floods, often reworked into a ravinement. Above it the beds retrograde.
• Maximum flooding surface (MFS): the surface at the end of transgression, when the shoreline stands farthest landward. The highstand clinoforms downlap it, and it is often a condensed, organic-rich interval and a continuous reflector.

The first two come from the curve, at the start and at the end of the fall. The last two come from the shoreline’s turns, so they move when the supply changes, and with enough sediment they do not form at all.

Exercise, read the figure

1. The figure opens on the finished cycle. In (a), find where the curve starts and stops falling: those two moments are the basal surface of forced regression and the sequence boundary. In (b), find where the shoreline turns: the maximum regressive and the maximum flooding surfaces.
2. Press Play the cycle and watch (c): the falling-stage deposits step down basinward while the shelf behind them is cut into the subaerial unconformity; the lowstand wedge climbs from the lowest shoreline; the transgressive sheet steps landward; the highstand builds out. Plate (d) shows the same history against time, with the gap the fall left on the shelf.
3. Use Pick out a tract on each tract in turn. The figure strengthens the surfaces that bound it and marks the terminations that diagnose them: truncation under the sequence boundary, onlap onto it, downlap onto the maximum flooding surface, the patterns of Section 4.1.
4. Drag the supply from starved to abundant and read the transgression in the table. With little sediment the shoreline turns landward soon after the lowstand and retreats far; with abundant sediment it turns later and retreats only a few kilometres, and against a gentler swing of 10 m it never retreats more than a few hundred metres, so neither the maximum regressive nor the maximum flooding surface forms.
5. With the supply back at its starting value, shrink the eustatic swing to 6 m and raise subsidence past the mark on its slider. Relative sea level no longer falls: no falling-stage tract, no subaerial unconformity, no sequence boundary, while the shoreline’s turns remain.

Every tract and surface in the figure is read from the curve and from the shoreline of the section the model built; none is drawn by hand for a given state.

What sequence stratigraphy gives an interpreter

• Correlation where wells are sparse. Key surfaces are often regional reflectors, so a framework mapped on seismic carries a few well ties across a basin. The ages still come from wells, fossils and other dating; the framework says which rocks are the same age, not how old they are.
• Prediction of reservoir positions. Falling-stage and lowstand deposits hold the lowstand deltas, slope fans and basin-floor fans; highstand shoreface sands lie updip of the clinoforms that downlap the maximum flooding surface; valleys cut into the subaerial unconformity hold their own fills. Knowing which tract a package belongs to narrows where the sand should be before a well is drilled.
• Source, reservoir and seal. The condensed shales at the maximum flooding surface are common source rocks and seals; transgressive shales commonly seal lowstand reservoirs. Sequence stratigraphy maps the petroleum system onto the seismic.
• A hypothesis about base level. Local sequences can be compared with global sea-level charts (Haq et al. 1987; Miller et al. 2005), but a match is a hypothesis to test against independent ages, and how far such charts are global is debated (Miall 1992).

Pitfalls

• Applying the model too mechanically. Not every section is a perfect sequence. Tectonics, climate and autogenic processes (river avulsion, delta-lobe switching) make patterns that do not fit the template. Use the framework as a guide, not a straitjacket.
• Confusing the SB with the MFS. Both can be strong regional reflectors. Tell them apart by their terminations: the SB has truncation below and onlap above, the MFS has downlap above. Beds that drape parallel above a surface, with neither downlap nor onlap, may mark a minor flooding surface instead. And the SB is hardest to see where it is a correlative conformity.
• Over-interpreting poor data. A sequence interpretation is only as good as the seismic. Where the data are poor, pick only the most robust boundaries and flag the uncertainty.
• Ignoring tectonics. Relative sea level is eustasy plus subsidence. Where subsidence is faster than the fastest eustatic fall, relative sea level never falls, and the cycle leaves no subaerial unconformity and no sequence boundary, only transgressions and regressions; tie such sections to global cycles with care.
• Scale confusion. Sequences exist at many scales: third-order sequences of about 1 to 10 Myr (the classic Vail scale), fourth-order ones of about 0.1 to 1 Myr, and higher frequencies. A parasequence is something else: a shoaling-upward succession bounded by marine flooding surfaces (Van Wagoner et al. 1988), the building block inside a systems tract. Map the scale your objective needs.
• Model dependence. Where the sequence boundary sits, and whether a falling-stage tract is separated, depends on the model (Posamentier and Vail 1988; Hunt and Tucker 1992; Catuneanu et al. 2009). State the one you use.