A Riverbank Cut and the Fossils Within It
A river has cut a 12 m bank exposing layered rock. From bottom to top: a coarse sandstone with rounded pebbles, a shale with abundant shell fossils, a limestone with reef-like structures, a tilted layer of sandstone separated from the limestone by an irregular surface, and a loose gravel cap. A student finds a fish skull in the shale and a shell fragment in the gravel cap. The tilted sandstone contains the same shell species as the shale. Nearby, a basalt dike cuts vertically through the shale and limestone but stops below the gravel cap. The class must reconstruct the sequence of events and decide which fossils can date which layers.
1. Order the events recorded in the bank, including the dike, the tilted layer, and the irregular surface. Justify each placement with a principle.
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Using superposition and cross-cutting relationships, the sequence is: (1) deposition of sandstone, then shale, then limestone, since each overlies the previous in an undisturbed stack; (2) tilting of the sandstone layer, because it is inclined while the layers around it are not, indicating deformation after deposition; (3) erosion producing the irregular surface, an unconformity that removes rock and separates older tilted beds from later deposits; (4) intrusion of the basalt dike, which cuts shale and limestone and therefore postdates them; (5) deposition of the gravel cap, which lies above the dike's top and is not cut by it, so the gravel postdates the dike.
The dike could be older than the tilting only if it were itself deformed, which is not reported. The safest ordering places the dike after the sedimentary layers it cuts and before the gravel that buries it. Each step rests on a specific principle: superposition for the stack, original horizontality for tilting, cross-cutting for the dike, and the unconformity for the erosion surface.
2. The same shell species appears in the shale and the tilted sandstone. What does this imply, and which fossils are useful for dating?
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The shared shell species indicates the tilted sandstone and the shale are close in age, and the sandstone was probably deposited, then tilted, before or during the interval represented by the unconformity. The species acts as a correlative marker linking the two units even though their orientations differ.
Dating usefulness depends on the fossil. The shell species, if it is a short-ranging index fossil, can correlate the shale and sandstone to a narrow interval. The fish skull in the shale is a body fossil that can also support correlation if its range is known. The shell fragment in the loose gravel cap is not useful for dating the cap's deposition because it is a reworked clast eroded from older rock; it tells you about the source area, not the age of the gravel. The reef structures in the limestone indicate a shallow marine environment rather than a precise age. The class should use the shale and sandstone fossils for correlation and treat the gravel fossil as recycled material.
3. A classmate concludes the area was always underwater because marine fossils are present throughout. Evaluate this inference.
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The inference is too strong. Marine fossils in the shale and limestone do indicate deposition in water at those times, and reef structures support a shallow marine setting for the limestone. But the coarse sandstone with rounded pebbles suggests a higher-energy setting, possibly fluvial or shoreline, and the gravel cap is a terrestrial or river deposit. The basalt dike is igneous and unrelated to water depth.
Presence of marine fossils in some layers shows those layers formed underwater; it does not show the whole sequence, or the region since, stayed submerged. The unconformity and the tilting record uplift and erosion, which require the rocks to have been above conditions of continuous deposition. The defensible conclusion is that the area experienced alternating marine and non-marine conditions with episodes of deformation and erosion. Generalizing from fossils in two layers to 'always underwater' ignores the facies changes and the structural evidence in the same outcrop.