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

In a transgressive sequence, the expected order of rock units from oldest to youngest typically reflects the transition of environments as sea levels rise. This process results in a sequence where progressively finer-grained sediments are deposited as you move from older to younger layers.

Starting with the oldest unit in a transgressive sequence, sandstone usually represents a high-energy environment like a beach or river delta, where coarser particles settle first as sea levels began to rise. As the transgression continues, the energy of the environment decreases, allowing finer sediments like shale to be deposited in quieter, deeper water.

Finally, the youngest unit, limestone, often forms in even quieter, more stable conditions in deeper marine environments where calcium carbonate precipitates from seawater or accumulates from biological debris like shells and coral.

The sequence of sandstone, followed by shale, and then limestone accurately represents this progression from high-energy, coarser sediments to low-energy, finer-grained sediments in a transgressive setting. Thus, understanding the depositional environments clarifies why this sequence is expected.