Chapter 34 Rock Units Time Units and Stratigraphic Columns

Year 2 · Stratigraphy · G224-L1

Distinguish formations biozones and time units and construct a measured stratigraphic column with contacts and uncertainty.

Relative bed order in an upright, undeformed illustrative succession. Original Geology Hub teaching diagram.

This independent lesson follows topics in the supplied University of Peshawar BS Geology curriculum labelled session 2004–05 onward. It is a learning resource, not an official university course or a statement of current degree requirements.

Learning objectives

Differentiate lithostratigraphic and biostratigraphic units.
Use relative-age principles with structural checks.
Construct a column from measured bed data.

Two different questions

Stratigraphy studies the organization and relationships of layered rock bodies through space and time. Lithostratigraphy asks which bodies have recognizable lithological characteristics. Biostratigraphy organizes rocks by their fossil content. Chronostratigraphy concerns rock bodies representing intervals of geological time, while geochronology describes the time intervals themselves. A formation boundary is therefore not automatically a time boundary.

A formation is a mappable lithostratigraphic unit defined by observable character and reference sections. It may contain several rock types and can vary laterally. Members subdivide formations; groups combine related formations. A sandstone passing laterally into mudstone can change lithostratigraphic identity even where deposition continued at the same time. A fossil biozone may cross the boundaries of more than one formation.

Relative order

Superposition places younger sediment above older sediment in an undisturbed upright sequence. Original horizontality and lateral continuity provide starting models, but cross-bedding, channel margins and reef slopes need not have been deposited flat. Cross-cutting relationships and inclusions provide additional relative-age tests. Tectonic overturning, faults and intrusion can defeat a simple bottom-to-top reading, so check way-up indicators and structural geometry.

Contacts can be sharp, gradational or erosional. An unconformity represents missing time through erosion, non-deposition or both. An angular unconformity juxtaposes differently oriented packages; a disconformity lies between approximately parallel sedimentary beds; a nonconformity places sedimentary rocks on older igneous or metamorphic material. A sharp lithological change alone is insufficient to diagnose a long hiatus.

Building a column

Choose a reference datum, measure bed thickness perpendicular to bedding and log lithology, grain size, structures, fossils, contacts and sample positions. Traverse distance on a hillside is not necessarily true stratigraphic thickness. Dip, slope and traverse direction all matter. Show covered intervals explicitly rather than filling them with guessed rock types.

A stratigraphic column is a compressed data display. Use a consistent vertical scale, a clear legend and standard grain-size or lithology conventions. Place observations such as a fossil shell bed beside the relevant level and keep interpretations such as “transgression” in a separate column. Geological time scales are revised as new evidence emerges; use a dated authoritative chart when numerical boundary ages are needed rather than reproducing an old syllabus table without checking it.

Keep rock bodies separate from time intervals

A chronostratigraphic system is a body of rocks representing a period of geological time, whereas a lithostratigraphic formation is defined by recognizable rock character. Their boundaries need not coincide. A shoreline may migrate so that sandstone deposition begins earlier at one location than another. The sandstone can remain a useful mapped unit even though its base is diachronous. Likewise, one fossil biozone can span contrasting lithologies. A correlation diagram should state whether a connecting line is a lithological boundary, a fossil datum or an interpreted time surface.

Describe the nature of a contact

A gradational contact may show progressive mixing or changing proportions over several beds. Choose a repeatable mapping criterion, such as a defined lithological change, and explain it. An erosional contact can truncate underlying beds, contain a lag of older fragments or be associated with a relief surface. A disconformity between parallel sedimentary packages may be difficult to recognize without fossils, weathering evidence or regional relationships. Missing time is not measured directly by the contact’s physical thickness: a thin surface may represent a long interval, while a thick continuously deposited package may represent a shorter one.

Correct an idealized traverse measurement

For horizontal ground crossed perpendicular to strike, the true thickness of a planar bed is horizontal outcrop width multiplied by sin dip. If width is 100 m and dip is 30°, thickness is 50 m. This shortcut assumes the traverse crosses the complete unit, the boundaries are parallel and topographic relief is negligible. A hillside traverse needs slope geometry as well as bedding orientation; a route oblique to strike needs another correction. Identify the assumptions before substituting a walking distance into a thickness formula. Fault repetition can invalidate the geometry entirely.

Test a relative-age interpretation

Consider upright sandstone A overlain by shale B, both cut by a dyke C, then eroded and overlain by conglomerate D containing clasts of C. Superposition places A before B; cross-cutting places C after both; the clasts and overlying relationship place D after emplacement and erosion of C. No numerical age is obtained. A hidden fault or overturned limb could alter a simpler interpretation, which is why field relationships are checked rather than assumed. Draw the events in separate stages so the reasoning is explicit. In a measured column, show the unconformity and any covered interval separately: an unconformity represents geological omission, while cover represents incomplete observation.

Worked activity

An invented measured section contains 4 m of sandstone, a 1 m covered interval, 3 m of shale and 2 m of limestone, in that order upward. Plot a 10 m column. Place the covered interval from 4 to 5 m and the limestone base at 8 m.

Solution: the exposed rock totals 9 m, but the section thickness including cover is 10 m. No rock type can be assigned to the covered metre. A lithological transition may support an environmental interpretation, but it does not by itself establish a numerical age or a formal formation name.

Self-check

Must a formation contain only one lithology?
Answer
No. It needs a recognizable mappable character, which may include several lithologies.
Does covered ground represent no geological time?
Answer
No. It represents missing observation, not necessarily missing strata.
Why separate observation and interpretation on a log?
Answer
A reader can reassess the interpretation without losing the underlying evidence.

Chapter 34 multiple choice questions

Choose one best answer for each question. Open each answer panel after making your choice.

A mappable sandstone passes laterally into mudstone deposited at the same time. What does this illustrate?
1. Every formation must have identical lithology everywhere
2. A lithostratigraphic boundary need not be a time boundary
3. Mudstone must always be older than sandstone
4. The geological time scale is determined by grain size
Answer and explanation
B. A lithostratigraphic boundary need not be a time boundary Lithostratigraphy groups rock character; lateral facies changes can occur within one time interval.
An upright section contains 2 m sandstone, 2 m cover, 5 m shale and 1 m limestone. What is its total thickness including cover?
1. 8 m
2. 7 m
3. 10 m
4. 12 m
Answer and explanation
C. 10 m Adding all intervals gives 2 + 2 + 5 + 1 = 10 m. Cover is missing observation, not zero thickness.
Sedimentary beds rest on an eroded older granite surface. Which unconformity term fits?
1. Angular unconformity between two sedimentary packages
2. A gradational formation boundary
3. A conformable fossil biozone
4. Nonconformity
Answer and explanation
D. Nonconformity A nonconformity places sedimentary rocks above older igneous or metamorphic material.
Which measurement is required for true bed thickness?
1. Distance measured perpendicular to bedding
2. Any walking distance along the hillside
3. The width of the bed on an oblique photograph
4. The length of the bedding strike line
Answer and explanation
A. Distance measured perpendicular to bedding Thickness is measured normal to the bed boundaries. Traverse distance must be geometrically corrected when it is oblique.
What is the best reason to place “cross-bedding observed” and “river channel interpreted” in separate log columns?
1. The two statements always mean the same thing
2. Readers can reassess the interpretation while retaining the observation
3. Interpretations are never useful in geology
4. Observations automatically determine numerical age
Answer and explanation
B. Readers can reassess the interpretation while retaining the observation Keeping evidence distinct from interpretation supports re-evaluation and makes the reasoning traceable.

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