What would be the expected sequence of rock units from oldest to youngest for a transgressive sequence?

• A. Sandstone, shale, limestone
• B. Limestone, shale, sandstone
• C. Shale, sandstone, limestone
• D. Sandstone, limestone, shale

Answer: (A)

In a transgression, sea level rises and the shoreline shifts landward, so the depositional environment moves to deeper water with time. That means the oldest rocks record the nearshore, higher-energy setting where sand dominates, while younger rocks reflect progressively deeper, quieter water where fine sediments and carbonates accumulate. So the bottom layer starts as sandstone deposited in the shoreface, then a deeper-water shale forms as conditions deepened, and finally limestone appears higher in the sequence where conditions are suitable for carbonate deposition. This pattern—sandstone first, then shale, then limestone—fits a transgressive rise in sea level, with each successive unit representing a deeper-water environment established later in time.