In a regressive sequence followed by delta development, what is the order from oldest to youngest?

• A. Limestone, shale and sandstone overlain by sand clay and march deposits
• B. Sandstone, limestone, and shale overlain by sandstone and clay deposits
• C. Shale and sandstone overlain by limestone and clay deposits
• D. Sandstone and shale overlain by limestone and march deposits

Answer: (A)

In a regressive sequence, the shoreline moves seaward, so the layers record deposition that shifts from deeper-water, quieter settings to progressively shallower, higher-energy environments. Limestone forms in relatively clear, shallow marine conditions, which often sit lower in the sequence. Over that, shale represents a continued but finer-grained deposition as energy remains low or gradually decreases. As the regression progresses and energy decreases or increases toward shore, sandstone appears, reflecting nearshore or beach environments with coarser material. After this regressive phase, delta development occurs as rivers deliver sediment to the coast, building a deltaic sequence of sand, clay, and marsh deposits on top of the older shallow-water units.

So the oldest to youngest progression is limestone at the base, overlain by shale, then sandstone, with deltaic sand, clay, and marsh deposits on top.