Wave-Worked Conglomerates — Depositional Processes and Criteria for Recognition

Abstract Wave-worked conglomerates are preserved in progradational and transgressive sequences typically associated with technically active coastlines and high-energy wave climates. These settings are commonly subjected to sporadic, intense storm activity, leaving a record that may either eliminate fair-weather deposits or may juxtapose sediments deposited in the shifting dynamic zones of the nearshore. Biogenic features associated with wave-worked conglomerates not only help to distinguish marine from non-marine conglomerates, but they also help elucidate the processes that operate in nearshore zones. The study of fossil hard parts, including their taphonomy, can contribute to detailed paleoecologic analysis of conglomeratic sequences. Trace fossils are commonly a good indicator of the depositional and erosional intensity of the nearshore zone. Beach gravels and conglomerates, deposited in the swash zone, are typified by well-defined layers of well-sorted, imbricated, disc-shaped pebbles. Detailed knowledge of sorting by size and shape of beach clasts, both across shore and along shore, has yet to be applied to (ancient) beach conglomerates. Upper shoreface conglomerates are typified by crudely graded, tabular beds and trough cross-bedded pebbly sandstone. Lower shoreface conglomerates are characterized by low-angle-stratified pebbly sandstone and lenticular conglomerates. Breaking waves, surf, rip currents and longshore currents are the major processes that control their deposition, but the interaction and effect of these processes, as well as of tides, river mouths, and biologic activity, on nearshore gravels, are not yet well understood.