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

In a transgressive sequence, the expected rock units from oldest to youngest typically reflect a progression from sedimentary environments that are influenced by rising sea levels. The correct order, sandstone, shale, limestone, corresponds with this idea.

During transgression, as sea levels rise, the shoreline moves inland, and different sediment types are deposited in response to changing water depths. Initially, in a more terrestrial environment, you would find sandstone forming as rivers and streams transport sediments into shallow marine areas. As sea levels continue to rise, the environment shifts to deeper waters where finer sediments accumulate, leading to the formation of shale. Finally, as deeper marine conditions prevail, limestone, which is primarily formed from biological processes and precipitation in clear, warm, shallow marine waters, becomes prevalent.

Thus, in a transgressive sequence, you can expect to see sandstone deposited first, followed by shale in deeper waters, and finally limestone reflecting the maximum transgressive phase. This spatial arrangement of rock units illustrates the effects of sea level changes over geological time.