In stratigraphic section described with crossbedded sandstone, ripple-marked sandstone, claystone and siltstone interbedded with thin sand lenses, the probable depositional environment is

In stratigraphic section described with crossbedded sandstone, ripple-marked sandstone, claystone and siltstone interbedded with thin sand lenses, the probable depositional environment is

• A. A dune to lagoon transition
• B. An alpine valley
• C. Deep marine shelf
• D. Braided river delta

Answer: (A)

The main idea here is how sedimentary structures and lithology reveal energy and proximity to shore. Crossbedded sandstone is a hallmark of migrating dune-form channels in an eolian (wind-blown) or nearshore setting, showing high, unidirectional transport and a strong paleo-flow direction. Ripple-marked sandstone also points to shallow-water or very dynamic conditions where water or wind moved and reworked sand. The presence of claystone and siltstone indicates a much lower-energy, finer-grained environment, typical of quiet water where mud can settle, such as a lagoon or protected back-barrier basin. The thin sand lenses interbedded with these finer sediments reflect episodic incursions of sand, likely from storm events or dune migration that briefly reworks the shoreline environment. Put together, these features describe a shore system where dunes form a barrier and a quiet back-barrier water body (a lagoon) accumulates finer sediments, with occasional sand signals from transport into the lagoon. That coastal dune-to-lagoon transition best matches the described stratigraphy. In contrast, an alpine valley would usually show dominant fluvial or glacial deposition with different sediment packages; a deep marine shelf would be dominated by muds and fine laminations with little dune-type cross-bedding; a braided river delta would emphasize extensive fluvial channel sands and deltaic coarsening-up sequences rather than a dune plus lagoon assemblage.

The main idea here is how sedimentary structures and lithology reveal energy and proximity to shore. Crossbedded sandstone is a hallmark of migrating dune-form channels in an eolian (wind-blown) or nearshore setting, showing high, unidirectional transport and a strong paleo-flow direction. Ripple-marked sandstone also points to shallow-water or very dynamic conditions where water or wind moved and reworked sand. The presence of claystone and siltstone indicates a much lower-energy, finer-grained environment, typical of quiet water where mud can settle, such as a lagoon or protected back-barrier basin. The thin sand lenses interbedded with these finer sediments reflect episodic incursions of sand, likely from storm events or dune migration that briefly reworks the shoreline environment.

Put together, these features describe a shore system where dunes form a barrier and a quiet back-barrier water body (a lagoon) accumulates finer sediments, with occasional sand signals from transport into the lagoon. That coastal dune-to-lagoon transition best matches the described stratigraphy.

In contrast, an alpine valley would usually show dominant fluvial or glacial deposition with different sediment packages; a deep marine shelf would be dominated by muds and fine laminations with little dune-type cross-bedding; a braided river delta would emphasize extensive fluvial channel sands and deltaic coarsening-up sequences rather than a dune plus lagoon assemblage.