The Earth’s history, recorded in rock layers, often contains significant gaps, much like missing pages in a vast book. This geological discontinuity is known generally as an unconformity, representing a period when sediment deposition ceased or when previously formed rock layers were eroded away. These missing time intervals, or hiatuses, are fundamental to understanding the planet’s past, revealing major events like changes in sea level or tectonic activity. Studying these surfaces helps geologists piece together the sequence of events that shaped a region over millions of years.
Defining the Disconformity
A disconformity is a specific type of unconformity defined by the relationship between the rock layers, or strata, above and below the boundary. Crucially, the sedimentary layers immediately above and below the erosional surface remain parallel to one another. This parallel orientation makes the discontinuity difficult to spot at first glance, as the rock beds appear to follow a continuous sequence.
The disconformity surface itself is a buried erosional contact, often uneven or jagged, indicating that a significant amount of time and material is absent from the rock record. This gap can span millions of years, representing time during which no sediments were deposited or a prior sequence of rock was removed. The defining characteristic is the evidence of erosion, such as channeling or irregularities, separating the two parallel units.
The existence of a disconformity means that the area was subjected to a significant change in environmental conditions. It separates an older sedimentary rock unit from a younger one, with the contact surface showing signs of having been exposed to weathering forces. While the layers are parallel, the fossils or other time-sensitive markers within the upper and lower units reveal a measurable jump in geologic time.
How Disconformities Form
The formation of a disconformity requires a sequence of geological processes, beginning with the initial deposition of horizontal sedimentary layers. These layers accumulate, often in a marine environment, over a period of time. The key next step involves a change in relative sea level or uplift of the landmass, which exposes the newly formed rock to the air.
Once exposed, the rock layers undergo subaerial erosion, meaning they are weathered and worn down by wind, water, or ice. This process removes some of the upper layers, creating the irregular, uneven erosional surface that characterizes the disconformity. The time represented by the removed rock and the exposure period constitutes the hiatus.
Following the period of erosion, the region must subside, or the sea level must rise again, allowing the area to be submerged once more. This transgression initiates a new phase of sedimentation, where younger layers of rock are deposited horizontally on top of the previously eroded surface. The resulting boundary between the older, eroded layer and the younger, deposited layer is the disconformity.
Identifying a Disconformity
Because the rock beds above and below a disconformity are parallel, geologists must look for specific physical clues at the contact surface.
- Basal conglomerate: This is a layer of coarse, rounded rock fragments derived from the underlying older unit, deposited as the sea transgressed over the exposed surface.
- Paleosol: Ancient soil developed on the older rock surface before the younger layers were deposited. Paleosols are identified by features like root traces or distinct soil horizons showing chemical weathering.
- Truncation evidence: Geologists look for features like burrows, mud cracks, or channels that are abruptly cut off at the boundary.
- Abrupt changes: The disconformity may be marked by an abrupt change in rock type or by the presence of iron-stained or altered rock fragments.
- Fossil gap: A missing sequence of expected index fossils across the boundary can be used to calculate the length of the time hiatus.
Context: Disconformity Compared to Other Unconformities
The disconformity is one of several types of unconformities, each defined by the angular relationship between the rock units.
Angular Unconformity
It is distinct from an angular unconformity, where the older layers below the break have been tilted, folded, or deformed before the younger, horizontal layers were deposited on top. This contrast in angle makes the angular unconformity visually obvious in the field.
Nonconformity
The disconformity also differs from a nonconformity, which is a contact surface separating sedimentary rock from much older igneous or metamorphic rock below it. The younger sedimentary layers are deposited directly onto a crystalline basement that was brought to the surface and eroded. This boundary represents a major shift in rock type and indicates a substantial time gap.
Paraconformity
A paraconformity, while similar to a disconformity in having parallel beds, is characterized by a lack of clear erosional features. The time gap is due primarily to non-deposition without significant erosion, making it the most subtle and challenging type of unconformity to identify visually. The disconformity, by contrast, is specifically marked by the physical evidence of an irregular, weathered, or scoured surface.