Reflection terminations: onlap, downlap, toplap, truncation

The vocabulary of Mitchum, Vail and Thompson

Seismic stratigraphy reads a reflection as a stratal surface, a bedding plane that was once a depositional surface and so is close to a time line. Mitchum, Vail and Thompson (1977) defined the depositional sequence as a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities, and classified how strata meet those surfaces. Lapout is the lateral end of a stratum where it was originally laid down; at the base of a sequence it is baselap.

• Onlap: initially horizontal strata end against an initially inclined surface, or inclined strata end updip against a surface of greater inclination. The strata climb the surface.
• Downlap: initially inclined strata end downdip against an initially horizontal or inclined surface. The strata descend onto it.
• Toplap: inclined strata end updip against the surface above, mainly by nondeposition, with sediment bypassing the top and at most minor erosion.
• Erosional truncation: strata end against the surface above because erosion removed them beyond it. It is more extreme than toplap and shows as erosional relief or an angular unconformity.
• Concordance: strata run parallel to the surface, and nothing ends.

The terminations sort into two families by the surface they end against: onlap and downlap against the surface below, toplap and truncation against the surface above. Where later deformation makes onlap and downlap hard to tell apart, Mitchum and his co-authors used the general term baselap.

Reading the figure

The figure opens on onlap along the dip line. Ten reflections rise gently east and end against a strong reflection beneath them, an older surface that climbs 2.5° to the east; they meet it at 2.2°. Each younger stratum ends higher and farther east, and in (c) the gap above the old surface, the time when nothing was laid down there, widens toward the margin. The surface is older than everything that rests on it, and the basin filled upward against it.

• Onlap records space filled upward against older relief: a basin margin, the flank of a high, the wall of a valley. Where the onlapping strata were laid down at the shoreline, each step up the surface is a rise of relative sea level, and Vail, Mitchum and Thompson (1977) built their curves of relative sea-level change from the vertical climb of this coastal onlap. Marine onlap in deep water records the basin filling, and does not by itself show that base level rose.
• Downlap: nine clinoforms build east, and their toes downlap a flat surface at 2.0°. Their topsets climb from each clinoform to the next, so base level was still rising while they built out: these are sigmoid clinoforms, which aggrade as they prograde (Mitchum, Vail and Sangree, 1977). In (c) the gap beneath them widens basinward, because the floor received almost no sediment while the clinoforms advanced across it. A floor starved like that gathers only a thin condensed section, and the surface that clinoforms downlap is most often the maximum flooding surface.
• Toplap: thirteen oblique clinoforms, with no topsets, end updip against a flat surface at 2.7°, each farther east than the last, while their toes downlap the surface below. Base level held still, so there was no room to build the top up: sediment bypassed it and the clinoforms built out beneath it. In (c) no rock is missing that was ever laid down; the gap above them is bypass and nondeposition.
• Truncation: thirteen strata tilted 3° are cut by a flat erosion surface. They are parallel beds cut at their full dip, not clinoforms converging downward, and the faint band in (c) is rock that was laid down and later eroded. That is the difference from toplap: truncation removes rock, toplap only stops adding it.
• Concordance: the strata run parallel to the surface above and below it. Nothing ends, so the geometry alone cannot say whether time is missing: the surface may be a conformity, or a gap with no angle across it that only ages from wells can prove.

Near its end every stratum is thinner than a quarter wavelength, the tuning thickness, so its reflection merges with the surface’s before it truly ends (Widess, 1973). On the dip line that takes the last 550 m of each onlapping reflection: $\lambda/4$ is 21 m at 30 Hz, and $d = (\lambda/4)/\tan\theta'$ with $\theta'$ = 2.2°. The table gives $d$ for every line.

How terminations define surfaces

Terminations are the evidence for the surfaces that divide the record. Mitchum, Vail and Thompson (1977) drew a sequence boundary where strata beneath it end by toplap or truncation and strata above it begin by onlap or downlap; traced basinward, where nothing ends against it, the same surface continues as its correlative conformity.

• A sequence boundary has truncation or toplap beneath it and onlap above it. Truncation proves erosion; the onlap above shows the next sequence filling the relief that erosion left.
• A maximum flooding surface is the surface the overlying clinoforms downlap. It lies in a condensed section, which often makes a strong reflection that can be followed far, and a regional seal.
• A conformity shows concordance on both sides. It can be traced from where the same surface shows terminations, but it cannot be found from its own geometry.

Section 4.2 builds a whole sequence from these surfaces and the systems tracts between them.

Apparent and true terminations on a 2D line

Terminations are defined in the direction of dip, and a 2D line rarely runs exactly along it. A plane that dips $\theta$ shows an apparent dip $\theta'$ on a section at $\varphi$ to its dip direction, $\tan\theta' = \tan\theta\cos\varphi$, and the same cosine cuts the difference between the slopes of the strata and the surface they end against. Turn the line in the figure and three things happen:

• The angle falls. The onlap’s 2.2° is 1.1° on a line 60° from dip, and the distance over which each reflection merges with the surface grows past a kilometre.
• The ends move off the line. The line runs more nearly along the lapout lines, so it crosses fewer of them: ten onlaps on the dip line, four at 60°. Within about 6° of strike no end can be resolved, and a strike line shows apparent concordance where the truth is onlap, downlap, toplap or an angular unconformity.
• Downlap can read as onlap. The clinoforms of the downlap example were tilted 0.75° south after they were laid down, so they dip obliquely to the lines along which they end. Turn the line toward the north, the side the tilt raised, and they descend less and less toward their ends: from about 63° from dip by less than a quarter wavelength over half the line, so they look flat where they meet the surface, and beyond their strike, about 69° and dotted in (b), they rise toward their ends. Flat or rising strata that end against an inclined surface read as onlap, and so they do on every line in the hatched wedge of (b); a line turned the other way still shows downlap. Onlap can read as downlap in the same way, where the onlapping strata dip along strike.

So read terminations on lines along dip, name the type only where the line is close to dip, and call an end against the surface below baselap where its type cannot be told. A 3D survey removes the problem: cut the section along the dip of the terminations, wherever that runs.

How to spot terminations on seismic

• Follow each reflection to its end. Where it stops before the edge of the data, ask what it ends against.
• Find the surface it ends against. Against the surface below, it is onlap or downlap; against the surface above, toplap or truncation.
• Read the relative dip. Onlapping strata climb the surface, flat or rising toward their ends; downlapping strata descend onto it. Toplapping strata are clinoforms that converge downward and downlap their own base; truncated strata are cut at their full dip, often parallel beds, beneath a surface with erosional relief or an angular discordance.
• Check the line’s direction. On a line oblique to dip the angles shrink and onlap and downlap can trade places. In 3D, cut the section along the dip of the terminations, and confirm each one on the crossline and the time slice.
• Tell terminations from faults. A fault offsets reflections along a plane and the same succession continues on its far side; a termination ends a reflection against another reflection, a depositional or erosional surface. Fault cuts belong to Part 3.

Pitfalls in termination identification

• Toplap or truncation. Both end against the surface above. Toplap is nondeposition with little or no erosion; truncation removes rock. Truncation shows as erosional relief or as beds that once continued cut at an angle, while toplapping clinoforms converge downward to their own downlap. Where the geometry is ambiguous, ages from wells decide: missing rock that was once laid down means erosion.
• Toplap is not conformable. It marks a hiatus at the top of the sequence, even if a short one, and the surface above toplap is often a sequence boundary.
• Apparent terminations. Migration artifacts, acquisition footprint, noise and gaps in the data end reflections falsely, and a 2D line oblique to dip shrinks or reverses real terminations. Check every termination along dip, on the crossline, and with coherence (Section 6.4).
• Resolution near the end. Within $(\lambda/4)/\tan\theta'$ of its end a stratum is below the tuning thickness and its reflection merges with the surface’s, so the seismic end is not the geological end. At a low angle, or on an oblique line, that distance is a kilometre or more.
• Terminations imagined in poor data. In noisy data the eye finds endings that are not there. Weigh each termination by the data quality around it and by whether its neighbours tell the same story, and tie it to wells where you can.