In a stratigraphic section bottom to top, with cross-bedded sandstone, ripple-marked sandstone, claystone interbedded with thin sand lenses, what depositional environment is most probable?

• A. A braided river channel
• B. A deltaic plain
• C. A dune to lagoon transition
• D. A deep marine basin

Answer: (C)

The sequence shows a clear drop in energy from wind-blown sand to quieter water deposition behind a coastal barrier, which points to a dune to lagoon transition. Cross-bedded sandstone at the base indicates high-energy, unidirectional flow typical of aeolian dunes (or very active braided-channel sands). If the environment were purely fluvial, you’d expect more persistent channel features and less sudden shifts to quiet water deposits. The next layer, ripple-marked sandstone, suggests a shallow, nearshore or shoreface setting where water motion creates ripple structures, still relatively energetic but distinctly subaqueous. Moving upward, claystone interbedded with thin sand lenses records very low-energy deposition with occasional sand incursions—classic for a protected, mud-dominated setting like a lagoon or back-barrier environment where fine sediments accumulate and sand lenses reflect episodic reworking or brief incursions of higher-energy water.

Together, the vertical progression from dunes to shallow-water ripple bedding to quiet mud with occasional sand lenses matches a coastal dune system transitioning into a lagoon behind a barrier, rather than a braided river, a deltaic plain, or a deep marine basin.