Reading the Rock Record at the Quarry
A quarry exposes a 120 m sequence. The lower 40 m is dark, fine-grained rock with graded beds, ripple marks, and a radiometric age of 380 ± 5 Ma. Above an irregular erosional surface is 25 m of coarse, poorly sorted conglomerate with angular clasts up to 30 cm and no fossils. This is overlain by 45 m of pale limestone containing coral and brachiopod fragments and a tuff band dated at 345 ± 4 Ma. The top 10 m is red, cross-bedded sandstone with mudcracks. A nearby drill core shows the conglomerate thickens to 60 m to the east and pinches out to the west. The sequence is folded into a broad anticline whose axis trends north-south. A student concludes the conglomerate represents a quiet lake and that the whole sequence is conformable.
1. Assess the student's two conclusions using the field evidence, and place the units in relative order with justification.
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Order (oldest to youngest): dark fine-grained unit (380 Ma) → conglomerate → limestone (tuff 345 Ma) → red sandstone. The conglomerate lies above an irregular erosional surface, so it is separated from the lower unit by an unconformity - the sequence is not wholly conformable. The student's conformity claim is wrong.
The conglomerate is coarse, poorly sorted, with angular clasts up to 30 cm and no fossils - this indicates high-energy, short transport, not a quiet lake. The eastward thickening and westward pinch-out suggests a proximal source to the east, such as an alluvial fan or fault-scarp deposit. Angular clasts and poor sorting imply rapid deposition close to source.
The lower graded beds and ripples indicate a marine or turbiditic setting; the limestone with corals and brachiopods is shallow marine; the red cross-bedded sandstone with mudcracks is terrestrial/oxidising. The dates bracket the conglomerate between 380 and 345 Ma, consistent with a hiatus and erosion. The anticline postdates all units.
2. Explain how the folding and the unconformity affect interpretation of the 380 Ma and 345 Ma ages, and state what additional evidence would test whether the conglomerate is a fault-related deposit.
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The ages are radiometric crystallisation or eruption ages of the dated materials, not depositional ages of every bed. The 380 Ma age dates the lower unit; because an unconformity and erosion intervene, the conglomerate could be substantially younger than 380 Ma. The 345 Ma tuff within the limestone gives a maximum age for the limestone and a minimum age for the conglomerate, so the conglomerate is bracketed between ~380 and ~345 Ma, with the true age uncertain within that interval. Folding tilts the sequence, so apparent thicknesses measured across the anticline are exaggerated relative to true stratigraphic thickness; measurements must be corrected for dip.
To test a fault origin: map whether the conglomerate thickens abruptly across a linear north-south structure, look for slickensides, fault gouge, or a stepped basement contact, and check clast composition against the adjacent footwall. If clasts match the immediately adjacent unit and thickness changes sharply across a line, a fault-scarp fan is supported; if clasts are exotic and the base is a broad erosion surface, a regional unconformity is more likely.