Grading a Stratigraphic Column That Contains a Detrital Date
A road cut exposes, from bottom to top: (1) Precambrian gneiss dated 1.1 Ga; (2) a conglomerate containing gneiss clasts; (3) a sandstone with a volcanic ash bed dated 88 ± 2 Ma; (4) a marine shale containing ammonites of Late Cretaceous affinity (roughly 100-66 Ma); (5) an angular unconformity; (6) flat-lying limestone. A basalt dike dated 62 ± 1 Ma cuts the shale and the ash bed but is truncated by the limestone. A fault offsets the gneiss and the conglomerate but not the sandstone. A student submits a report asserting that the shale is the oldest sedimentary unit because a whole-rock shale sample gave 145 Ma, that the limestone is older than the dike because the dike stops at its base, and that the fault is the youngest feature because faults are usually late. The teacher must grade the report and turn the errors into a lesson on dating methods.
1. Construct the correct sequence of events from oldest to youngest, citing the specific field relationship or date that justifies each step.
Read the worked answer
- Gneiss forms and crystallizes - dated 1.1 Ga, the only date that directly records a crystallization event here.
- Conglomerate is deposited on it, containing gneiss clasts, so it postdates the gneiss.
- Faulting cuts the gneiss and conglomerate but not the sandstone, so faulting is younger than the conglomerate and older than the sandstone.
- Sandstone is deposited on the eroded faulted surface; its ash bed gives 88 ± 2 Ma, a direct eruption age that dates deposition.
- Shale with Late Cretaceous ammonites accumulates above the sandstone, so it must be younger than 88 Ma, consistent with the ammonite assemblage.
- Folding and tilting affect the shale, producing the angular unconformity.
- Basalt dike intrudes at 62 ± 1 Ma, cutting the shale and ash but not the limestone.
- Erosion bevels the dike and the folded beds, creating the unconformity surface.
- Flat-lying limestone is deposited on that surface, so it is younger than 62 Ma - the reverse of what the student wrote.
2. Explain why the 145 Ma whole-rock shale date cannot be the depositional age of the shale, and say which materials in this column give reliable ages and why.
Read the worked answer
A whole-rock shale date is a mixed signal. Shale is a mechanical mixture of detrital clay minerals, silt-sized rock fragments, possibly reworked older zircons, and later authigenic cements. An isotopic analysis of the bulk rock averages these components, so the result reflects the provenance of the detritus or a mixture of detrital and authigenic phases - not the moment the mud was deposited. Even if the 145 Ma number is analytically sound, it is at best a maximum age for the sediment and cannot be a depositional age.
The internal contradiction settles the argument regardless of method: the shale lies above a sandstone dated 88 ± 2 Ma, so a 145 Ma shale would have to be older than the rock beneath it, which requires a thrust that is not observed.
The reliable constraints here are the ash bed, because volcanic ash crystallizes quickly and its mineral date approximates eruption and deposition, and the ammonites, which give an independent biostratigraphic age. The basalt dike is also reliable as an intrusion age and therefore as a cross-cutting constraint.
3. The student also claims that the limestone is older than the dike because the dike stops at its base, and that the fault is youngest because faults are usually late. Correct both statements and explain the principle that governs each.
Read the worked answer
Both claims reverse or ignore cross-cutting relationships. A dike that terminates against the base of a layer has been truncated by the surface on which that layer was deposited: the dike was emplaced, then the top of it was eroded away, then the limestone was laid down over the beveled top. Truncation means the intruded rock is older than the rock doing the truncating. Therefore the limestone is younger than 62 ± 1 Ma, not older.
The fault claim substitutes a generalization for evidence. The only relevant observation is that the fault displaces the gneiss and conglomerate but not the sandstone. A structure can only displace rocks that already existed, so the fault predates the sandstone. If the fault were the youngest feature, it would cut every unit, including the limestone.
The governing principle is that cross-cutting features are younger than the rocks they cut, and that a date is only as good as the material it comes from. Students should be asked to state, for every contact, which unit is cut and which unit cuts.