Read erosion surfaces and missing time

A surface can record missing history

In section 0, erosion surface E cuts the upper part of bed B and the intrusion I. Younger sediment C was then deposited on that eroded surface. E represents an interruption in the local preserved record. Such a gap is associated with an unconformity; the surface is a contact, not an extra layer of sediment.

A thin line can represent a long interval of nondeposition and erosion. The missing material is not drawn because it is no longer preserved. Conversely, a thick preserved layer need not represent a longer time than a thin one. The sketch has no time scale and supplies no erosion or sedimentation rates.

Explain the evidence for truncation

Choose I compared with E in section 0. The supplied interpretation states that E demonstrably truncates I, so intrusion preceded erosion. C also contains fragments identified as derived from I, independently supporting I older than C: the source material existed before it became fragments within the younger deposit.

The fragment identity is an explicit premise. Similar-looking specks would not by themselves establish that they came from this particular intrusion. Real correlation could require mineralogy, texture, chemistry and spatial context. If fragment provenance were uncertain, the corresponding inference would need to be qualified.

Simply seeing a dike end below C is weaker than establishing erosional truncation. An intrusion can terminate underground without ever crossing an overlying bed. The lesson therefore gives contact interpretation and fragment provenance instead of asking you to infer chronology from an endpoint alone.

Change the intrusion relationship

Switch to section 1. I cuts across the existing erosion surface and intrudes C. E predates deposition of C, and C predates I. The relation I versus E reverses. The erosion surface still represents missing local record, but the intrusion now belongs later in the history.

Compare E with C in both sections. That relation remains unchanged because C is still deposited on the previously eroded surface. The changed intrusion relation does not overturn every other observation. A revised geological explanation should preserve constraints that remain supported.

Record the two local chains. Mark which links depend on superposition, which on cross-cutting and which on identified inclusions. An arrow path can combine distinct kinds of evidence; it should not hide them behind a single unsupported story.

Separate type, order and duration

An erosional gap between broadly parallel sedimentary beds is commonly described as a disconformity. An angular unconformity involves a different orientation of older and younger strata; a nonconformity places sedimentary rocks over older igneous or metamorphic basement. These categories describe relationships, not fixed gap lengths. Our simplified bed-parallel section does not include a tilting episode or a complete basement contact.

The event E is treated as one ordering milestone for the exercise. Actual erosion can occupy an interval, recur or vary across a landscape. The model does not supply the start and end of that interval. “E happened before C” is a relative constraint, not a claim that erosion everywhere stopped at one instantaneous boundary.

Design a check using preserved records

Suppose you want to estimate how much time is absent. Propose observations above and below the surface that could constrain ages, and state what those observations date. A suitable dated ash bed can constrain deposition if it is primary and its dated minerals record the appropriate event. Reworked grains may be older than the deposit containing them. A date from an intrusion supplies a different relationship from a date on a deposited ash bed.

If sediment below the gap was deposited at 80 Ma and sediment above at 60 Ma, under the stated event interpretations the interval between those dated deposits is 20 million years. It is not automatically 20 million years of active erosion. Some of that interval could include nondeposition or deposition later removed. This distinction matters even when the numerical ages are accurate.

Transfer to another contact

An upright bed K is cut by intrusion J. An erosion surface removes the tops of both, and younger bed L contains fragments verified as J. A student concludes that J is older than K because J extends deeper, and that the erosion took one year because its line is thin. Diagnose both errors and give the supported sequence.

Check the unconformity transfer

K existed before J intruded it. Erosion then truncated both, and L was deposited afterward with fragments of J. The supported order is K → J → erosion → L. Depth does not override an intrusive contact, and the drawn line gives no erosion duration. A separate age or rate argument would be required for a numerical estimate.

Save a comparison that changes the intrusion order while preserving E before C. Explain which pieces of evidence support each statement and which missing-time claims remain unresolved.

References: Utah Geological Survey: relative dating, inclusions and unconformities and Physical Geology: relative dating methods. The fictional sections, age example and transfer exercise are original.