Reflection terminations: onlap, downlap, toplap, truncation

Why terminations matter

Reflectors don’t just stop randomly. When a reflector ends on a seismic section, something geological is telling you its story:

• The bed STOPPED DEPOSITING when it ran up against a topographic obstacle — onlap.
• The bed PROGRADED OUT onto a deeper surface below — downlap.
• The depositional system RAN OUT OF accommodation space and simply stopped adding material to the top — toplap.
• The bed was DEPOSITED and then ERODED AWAY by a later surface — truncation.

Learn to see these four terminations and you can read the history of a basin directly from the seismic, without ever looking at a single well log. This is the foundation on which sequence stratigraphy (§4.2), depositional systems (§4.3–§4.5), and seismic geomorphology (§4.6) all build.

Exercise — learn the four terminations

• The widget starts with Onlap. Notice the BASIN-MARGIN SURFACE (red line) dipping from upper-left to lower-right. Horizontal beds (the basin fill) terminate against the margin, each younger one reaching farther up the slope. The yellow dots mark the actual termination points. This is the hallmark signature of a transgression filling a basin.
• Switch to Downlap. Now the key surface (red) is HORIZONTAL at the base. INCLINED clinoforms above dip down-to-the-right and terminate where they meet the flooding surface. Each clinoform progrades a bit farther out than the previous one. This is a deltaic or lobe progradation signature.
• Switch to Toplap. The key surface (red) is HORIZONTAL at the top. Inclined foresets terminate against it. Crucially, the toplap surface is NOT erosional — the beds just couldn’t be built any higher. This is what happens when a depositional system is stuck at base level (wave base, sea level, lake level): topset deposition is starved, and foresets prograde beneath.
• Switch to Truncation. Tilted OLDER beds terminate against an overlying erosional surface. Younger beds drape conformably ABOVE the unconformity. This records uplift + erosion + subsequent deposition — the classic angular unconformity.
• Compare toplap and truncation carefully. Both involve a bed terminating AGAINST a surface ABOVE. The difference: toplap = NON-deposition (conformable), truncation = EROSION (unconformity with a time gap). In real seismic, a small time gap may make them hard to distinguish, but the geometry gives it away: truncation shows clear angularity and cross-cutting of older beds by the unconformity.

The four terminations in detail

• Onlap: a bed terminates AGAINST an UNDERLYING surface that dips up-section. The bed itself is roughly horizontal or gently dipping. Records deposition that FILLED a pre-existing topographic low — a basin, a half-graben, a canyon. Each successively younger onlapping bed reaches farther up the margin, creating the characteristic "pinch-out" geometry. Common in transgressive settings (marine flooding onto a shelf) and in syn-rift and post-rift basin-fill sequences.
• Downlap: an INCLINED bed (a clinoform) terminates AGAINST an underlying surface that is horizontal or gently dipping. Records PROGRADATION — a depositional system building out into deeper water. The underlying surface is usually a flooding surface or a condensed interval (the maximum flooding surface, MFS — see §4.2). Clinoforms come from deltas, shelf-margin wedges, and turbidite fans; downlap is a ubiquitous termination style in the world’s prograding margins.
• Toplap: an INCLINED bed terminates UPWARD against a surface where deposition simply STOPPED — no erosion, just starvation. Records a depositional system AT BASE LEVEL: the top of a delta building out into the sea, where the upper shoreface is at wave base and nothing accretes above. Topset beds are absent or very thin. This is the most pedagogically subtle of the four terminations because it LOOKS like erosion but isn’t; the key is that the stratigraphic relationship is CONFORMABLE across the toplap surface — there’s no time gap above.
• Truncation: a bed terminates UPWARD against an EROSIONAL unconformity that cuts ACROSS underlying stratigraphy. Records UPLIFT + EROSION (sometimes with subsequent subsidence and deposition of younger beds). The unconformity is a time gap — often millions of years. Truncation is the most DIAGNOSTIC termination for sequence boundaries (§4.2): wherever you see clear truncation, you have identified a surface with real geological significance.

How to spot terminations on seismic

• Look for reflector endings. Follow a reflector laterally. If it doesn’t continue across the whole section — if it ends — that ending is a termination.
• Note whether it ends against a surface above or below. This splits the four into two families: against-below (onlap, downlap) or against-above (toplap, truncation).
• Check the relative dip. In the against-below family: onlap has the reflector roughly horizontal and the surface dipping; downlap has the reflector dipping and the surface horizontal. In the against-above family: toplap has a conformable UPPER relationship; truncation has the overlying surface cutting across older beds angularly.
• Follow into 3D. Terminations that are cleanly expressed along one line may disappear along strike. Use the cube-viewing discipline from Part 2: cycle inline / crossline / time-slice to confirm terminations are real and not just artifacts of a single 2D line.
• Distinguish terminations from fault cuts. A fault also terminates reflectors, but the signature is a straight, often dipping plane that offsets reflectors rather than a depositional pattern. Fault terminations belong in structural interpretation (Part 3); stratigraphic terminations belong here.

Why this matters: the stratigraphic eye

The structural eye asks: is this reflector bent, faulted, or folded? The stratigraphic eye asks: is this reflector ending, and if so, what kind of ending?

Structural interpretation reveals the FRAMEWORK — the geometric trap. Stratigraphic interpretation reveals the HISTORY — what was deposited, when, where it came from, where it pinches out. A modern interpreter uses both eyes simultaneously: identify structure first, then overlay stratigraphic architecture on top. This is why §4.6 (Seismic geomorphology) will eventually tie Part 4 back into Part 3 — you start to see paleo-rivers, paleo-shorelines, paleo-fan lobes as physical features of the rock that your structural interpretation has already placed in a tectonic framework.

For now, master the four terminations. Every concept for the rest of Part 4 is built on recognizing these four patterns.

Pitfalls in termination identification

• Confusing toplap with truncation. Both terminate against an upper surface. Rule of thumb: if the overlying section is conformable (no angular discordance, no inferred time gap), it’s toplap. If there’s visible angularity OR a distinct younger drape that overlies the terminations, it’s truncation. Use well ties or biostratigraphy to confirm when ambiguous.
• Mistaking apparent terminations for processing artifacts. Migration artifacts, acquisition footprint, and data gaps can create false terminations. Always cross-check along strike (inline/crossline) and use volume attributes (§6.4 coherence) to validate suspicious terminations.
• Ignoring the 2D-vs-3D problem. A reflector that appears to terminate on a 2D line may actually continue in a direction perpendicular to that line. Only 3D data allows confident termination identification. If you only have 2D, annotate terminations with low confidence and seek 3D or well data to confirm.
• Resolution limits near pinch-outs. An onlap or downlap where the terminating bed is thinner than the seismic resolution (¼ wavelength, typically 8–40 m) will NOT be resolvable. The termination is there geologically but invisible seismically. This is the "stratigraphic trap vs seismic resolution" trade-off that drives much of exploration at modern shelf margins.
• Over-interpreting in poor data. If the seismic quality is poor, terminations can be imagined rather than real. Calibrate your termination confidence against the overall seismic SNR and tie to any available wells.