Borehole Correlation, a Truncated Dike, and a Conflicting K–Ar Age
A regional geologic map shows a faulted sedimentary basin. Two exploratory boreholes lie 2.4 km apart along a north–south transect. Borehole A, near the basin margin, intersects 95 m of sandstone, 150 m of limestone with Early Devonian conodonts, and 165 m of graptolite-bearing shale before reaching granite at 410 m. The limestone rests on a 3 m conglomerate containing shale clasts. A basaltic dike cuts the shale between 300 and 310 m and is truncated by the limestone. Borehole B, nearer the depocenter, intersects 310 m of limestone and 145 m of shale before granite at 455 m; no sandstone is present and no fossils were recovered. The dike yields a whole-rock K–Ar age of 280 ± 6 Ma. A seismic reflection profile at B images the top of granite at 0.28 s two-way travel time; the processing velocity is 3.25 km/s. The contact between limestone and shale is sharp in both cores.
1. Construct a relative-age sequence for the events recorded at borehole A and evaluate whether the 280 ± 6 Ma dike age is consistent with the field and biostratigraphic evidence.
Read the worked answer
Relative sequence, oldest to youngest. (1) Granite basement. (2) Ordovician shale deposited on the granite; graptolites give the biostratigraphic age. (3) Dike intruded the shale, so it postdates shale lithification. (4) Uplift and erosion produced the conglomerate with shale clasts. (5) Early Devonian limestone was deposited across the eroded shale and dike, truncating the dike. (6) Sandstone was deposited last at A.
Age consistency. A Permian 280 Ma dike cannot be truncated by Early Devonian limestone roughly 400 million years old; the dike must be older than the limestone. The field relation and fossils are consistent with an Ordovician-to-Early Devonian intrusion, but not with 280 Ma. Whole-rock K–Ar ages on altered basalt commonly record argon loss and therefore a minimum age. Before rejecting the field relation, check for a faulted contact, sheared limestone, or a weathered dike margin. The most defensible conclusion is that the K–Ar age is reset or unreliable; cross-cutting relations and conodonts should control the relative chronology.
2. Calculate the seismic depth to granite at borehole B and evaluate whether the absence of sandstone at B is better explained by erosion or by non-deposition. What single observation would most strengthen your interpretation?
Read the worked answer
Depth = (3.25 km/s × 0.28 s) / 2 = 0.455 km = 455 m, matching the borehole. That agreement supports the velocity model at this scale, but it does not by itself distinguish erosion from non-deposition.
At A the limestone is 150 m and shale 165 m; at B the limestone is 310 m and shale 145 m, and sandstone is absent. Basinward thickening of limestone with thinning of shale suggests a deeper-water facies and greater subsidence at B, not simply removal of sandstone. If erosion had stripped sandstone at B, the top of the limestone should show a regional truncation surface, and any overlying unit should contain limestone clasts. The sharp limestone–shale contact is not enough. The strongest single observation would be seismic geometry: downlap or onlap of the limestone onto the shale, or truncation of underlying reflectors. A core across the contact showing a weathered surface, paleosol, or karst would support exposure; a conformable, fossiliferous transition would support non-deposition.
3. A colleague proposes a normal fault between A and B to explain the thickness changes. What data would test this hypothesis, and what would falsify it?
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A fault should produce measurable stratigraphic separation and structural disruption. Test it with: (1) seismic offset or flexure of the granite reflector and internal reflectors; (2) core evidence of slickensides, fault breccia, or repeated section; (3) a map-scale linear valley, spring line, or offset marker bed; (4) stratigraphic separation inconsistent with a simple dip. The first check is whether the same marker beds are present on both sides and whether their thicknesses change abruptly at a fault plane. If the seismic reflector is continuous, the thickness change is gradual, and no fault-rock fabrics appear in cores, the fault hypothesis is weakened. A fault is falsified if the limestone–shale contact can be traced continuously across the transect with no offset and if the thickness changes are depositional rather than structural.