Which Two Formations Are Separated by a Disconformity?
The short version is – you’re looking at a gap in the rock record, and the answer depends on the region you’re studying.
Ever stood on a cliff and thought the layers looked like a neatly stacked cake, only to notice a sudden jump in the pattern? Also, that “jump” is a disconformity, and it’s the geologist’s version of a missing chapter in a novel. Still, in some basins the answer is crystal‑clear – the Morrison Formation sits atop the Dakota Sandstone. In others you’ll find the Burgess Shale perched above the Ediacaran sandstones. So which two formations are you really asking about? Let’s dig into what a disconformity is, why it matters, and how you can spot the pair of strata that book‑ends it Practical, not theoretical..
What Is a Disconformity?
A disconformity is a type of unconformity where parallel sedimentary layers are separated by a gap in time. The layers above and below are still essentially horizontal, but something interrupted deposition – erosion, non‑deposition, or a brief uplift – leaving a missing slice of Earth’s history The details matter here..
Think of it like a TV series that skipped a season. The storyline (the sedimentary record) continues, but the events of that season (the missing time) are gone. In the field you’ll see evidence in three places:
- Erosional truncation – the lower beds end abruptly, often with a scoured surface.
- Soil horizons or paleosols – ancient ground surfaces that formed during the hiatus.
- Fossil assemblage shift – a sudden change in the types of fossils, indicating a jump in age.
Because the beds stay parallel, you can’t tell there’s a gap just by looking at the dip. You need to read the clues.
Why It Matters / Why People Care
If you’re a petroleum geologist, a missing interval could hide a reservoir or a seal. For a paleontologist, that break might mean an entire evolutionary episode vanished from the local record. And for anyone mapping a region, the presence of a disconformity tells you “the landscape was doing something different here” – maybe an uplift, a sea‑level fall, or a climate shift.
No fluff here — just what actually works.
In practice, recognizing a disconformity prevents you from mis‑dating rocks. Imagine you assume continuous deposition and assign a Jurassic age to a layer that actually jumps from Late Triassic to Early Jurassic. Your whole reconstruction of basin evolution goes sideways.
How It Works (or How to Identify It)
Below is the step‑by‑step approach I use when I’m in the field or staring at a core plug. It works for any region, whether you’re hunting the Morrison‑Dakota pair in the American West or the Devonian‑Carboniferous break in the UK.
1. Look for Surface Features
- Scour marks or channels – these indicate erosion before the next layer settled.
- Paleosols – thin, weathered horizons that often develop during a pause in sedimentation.
- Fossil soil horizons (root traces, burrows) – living organisms need a stable surface, so they show up when deposition stops.
2. Check the Fossil Content
If the lower unit is packed with bivalve fossils typical of a shallow marine environment, and the overlying unit suddenly bursts with trilobite assemblages, you’ve got a time gap. The key is a significant turnover in species that can’t be explained by simple ecological change Not complicated — just consistent..
3. Measure the Age Difference
Radiometric dating (U‑Pb on zircon, Ar‑Ar on volcanic ash) is the gold standard. If the lower formation yields a 150 Ma age and the overlying one clocks in at 140 Ma, you’ve got a ten‑million‑year hiatus – classic disconformity material Not complicated — just consistent..
4. Map the Extent
Disconformities can be local (a single outcrop) or regional (spanning entire basins). Use stratigraphic columns from nearby wells or sections to see if the same gap appears elsewhere. Consistency across a wide area strengthens the case And it works..
5. Identify the Formations
Now for the fun part: naming the two formations. But the answer varies by geographic setting. Below are the most frequently cited pairs that textbooks love to showcase.
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North America (Western Interior) – Dakota Sandstone / Morrison Formation
Dakota (Cretaceous) is a fluvial sandstone, while Morrison (Late Jurassic) is famous for dinosaur bone beds. The disconformity marks a regression‑transgression cycle during the Late Jurassic–Early Cretaceous. -
North America (Appalachians) – Catskill Formation / Marcellus Shale
The Catskill (Devonian) is a red‑bed fluvial deposit, and the Marcellus (Middle Devonian) is a black shale. The gap reflects a brief uplift before the sea deepened again Small thing, real impact.. -
Europe (UK) – Old Red Sandstone / Carboniferous Limestone
A classic Devonian–Carboniferous disconformity where terrestrial red beds meet marine carbonates, indicating a major sea‑level rise Which is the point.. -
Australia (Murray Basin) – Naracoorte Group / Murrumbidgee Formation
Here the Naracoorte (late Oligocene) is a limestone, and the Murrumbidgee (early Miocene) is a fluvial sandstone, separated by an erosional surface. -
South America (Patagonia) – Cerro Bayo Formation / Cerro Dorotea Formation
The volcanic‑derived Cerro Bayo (Late Cretaceous) is overlain by the marine Cerro Dorotea (Early Paleocene) after a short hiatus And that's really what it comes down to..
Which pair you’re after depends on the region you’re mapping. If you’re in the Rocky Mountains, the Dakota–Morrison combo is the go‑to answer. If you’re in the UK, it’s Old Red Sandstone over Carboniferous Limestone.
Common Mistakes / What Most People Get Wrong
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Calling any parallel contact a disconformity – Not every flat surface is a time gap. A conformable contact can be flat if deposition was steady. Look for the erosional or paleosol clues.
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Mixing up disconformity with non‑conformity – A non‑conformity is where sedimentary rocks overlie igneous or metamorphic basement. The two are often confused in textbooks, but the key difference is rock type.
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Assuming the missing time is short – Some disconformities span hundreds of millions of years. The “jump” can be massive, especially across major eustatic sea‑level changes Most people skip this — try not to..
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Ignoring lateral facies changes – Sometimes the lower formation thins out laterally, giving the illusion of a gap. A careful facies analysis can reveal continuous deposition, just in a different environment That's the part that actually makes a difference..
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Relying solely on visual inspection – A subtle disconformity may have no obvious scour marks. Geochemical signatures (e.g., a shift in carbon isotopes) can be the decisive evidence The details matter here..
Practical Tips / What Actually Works
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Carry a hand lens and a field notebook – The tiniest fossil or soil horizon can be the giveaway. Sketch the contact, note grain size changes, and photograph the surface.
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Sample both sides for dating – Even a small ash layer in the lower unit can give you a precise age. Pair it with a volcanic ash in the upper unit for a clean bracket.
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Use well logs when out in the field – Gamma‑ray and resistivity logs often show a sharp change at a disconformity, even if the outcrop is weathered And that's really what it comes down to..
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Cross‑check with regional stratigraphic charts – Most geological surveys publish basin‑wide columns. If you see a known disconformity on the chart, you’re probably looking at the same surface.
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Don’t overlook the “soft” evidence – Soil structure, root traces, and even a change in color (oxidized red beds below, gray shales above) are low‑tech but high‑impact clues.
FAQ
Q: How can I tell the difference between a disconformity and a simple erosional surface?
A: A disconformity specifically separates two sedimentary units of different ages. Look for age constraints (fossils, radiometric dates) on each side; a simple erosional surface may still be within the same depositional sequence.
Q: Are all disconformities associated with sea‑level change?
A: Not always. While many are tied to eustatic fluctuations, tectonic uplift, climate‑driven sediment supply changes, or even river avulsion can create a hiatus.
Q: Can a disconformity occur within a single formation?
A: Yes. If a formation is thick enough, a later erosional event can truncate its lower part, creating an internal disconformity. It’s then called a “formation‑scale disconformity.”
Q: Do fossils always disappear across a disconformity?
A: Not necessarily. Some resilient taxa survive across the gap, but you’ll usually see a distinct shift in assemblage composition or a sudden appearance of index fossils.
Q: What’s the best way to illustrate a disconformity on a map?
A: Use a dashed line for the contact, label the two formations, and add an inset showing a cross‑section with the erosional surface and any paleosol That's the part that actually makes a difference..
So, which two formations are separated by a disconformity? Worth adding: the answer lives in the map you’re holding. In the Rockies, it’s the Dakota Sandstone below and the Morrison Formation above. Practically speaking, in the UK, it’s the Old Red Sandstone and the Carboniferous Limestone. Everywhere else, the pair changes, but the process—erosion, non‑deposition, and a missing slice of time—remains the same Worth keeping that in mind..
Next time you stand on a cliff and notice that sudden break in the pattern, you’ll know you’re looking at a disconformity, and you’ll be ready to name the two formations that book‑end it. Happy hunting!
Conclusion
Disconformities are more than just gaps in the rock record—they are windows into the dynamic history of our planet. They reveal moments when erosion, climate shifts, or tectonic activity interrupted sedimentation, leaving behind a preserved record of time. By combining field techniques like well logs and soft evidence with regional stratigraphic data, geologists can decode these surfaces and reconstruct the sequences of events that shaped the Earth’s crust. Whether you’re mapping the Dakota Sandstone-Morrison Formation boundary in the Rockies or tracing the Old Red Sandstone-Carboniferous Limestone contact in the UK, each disconformity offers a unique story of environmental change. Recognizing these surfaces not only sharpens your geological skills but also deepens your understanding of how landscapes evolve over millennia. In a world where time is often compressed into layers of rock, disconformities remind us that the past is not always continuous—it is fragmented, eroded, and waiting to be pieced together. So, as you venture into the field, keep your eyes open for those subtle breaks in the strata, for they hold the key to unlocking the Earth’s ancient narratives Worth keeping that in mind..
