Question

The diagram below represents a cross section of the Earth's crust showing rock units and a fault. The rock layers are not overturned.

Which rock unit is the youngest?
Shale
Sandstone
Basalt
Limestone

Answer 1

In geological terms, the principle of superposition states that in undisturbed rock layers, the oldest layers are at the bottom and the youngest layers are at the top. Since you've mentioned a fault and that the rock layers are not overturned, the rock unit that is directly above the fault or on top of the other layers is likely to be the youngest.

If we assume a typical scenario based on common geological principles, and if basalt is present, it is often an igneous intrusion that can sometimes be younger than sedimentary layers (like shale, sandstone, and limestone) if it has intruded or flowed over them. However, without the visual aid of the actual cross-section diagram you mentioned, I cannot determine definitively which of the provided units is the youngest.

If basalt intrudes into the shale, sandstone, and limestone, it's usually the youngest. If you see any of the sedimentary layers on top of the others without any intrusions from basalt, then the layer on top would be the youngest.

So generally, if basalt is present and not covered, it's most likely the youngest rock unit. If it's not there, then look for the layer that is on top of the others.