A DANCE OF CURRENTS IN THE LATE PALEOZOIC

Pennsylvanian to early Permian pebbly sandstone/conglomerate (Sst/Cgl) with interbedded sandstone. (Sst). The sandstone with slightly asymmetrical current ripples (arrows) displays erosional scour casts on the underside. Observable is a possible siltstone rip-up clast (Slst). The long arrow indicates gradation.

Typically, erosional scour structures are preserved as casts on the bases of coarser-grained beds overlying finer-grained beds. However, it is possible for erosional scour structures to form on the base of a sandstone overlying a coarser-grained sediment like pebbly sandstone or conglomerate, as the formation of scours depends on the flow energy of the current. During events such as storms, high-energy currents can erode into coarser-grained sediments, creating depressions that are subsequently filled with finer-grained material like sand.

Here’s an example of Pennsylvanian to Early Permian pebbly sandstone/conglomerate interbedded with siltstone/sandstone displaying erosional scour casts and ripple marks with low-angle cross-lamination, possibly truncated or eroded at the top. The slightly asymmetrical, curved nature of the ripples suggests they were formed by a unidirectional current, perhaps in a deltaic or shallow marine environment. The combination of erosional scour casts and ripple marks within the same layer is a strong indicator of unidirectional flow, as erosional scours are a hallmark of unidirectional currents due to their requirement for consistent flow in one direction to produce the elongated, channel-like form.

The observed features likely formed when a high-energy, unidirectional current scoured the underlying surface, possibly during an event flow such as a strong storm surge. As current energy decreased, sediment was deposited, forming erosional scour casts at the base of the layer. With further reduction in flow energy, ripples developed atop the deposit.

A siltstone slab, possibly a rip-up clast, embedded within pebbly sandstone/conglomerate may represent a piece of semi-consolidated or lithified siltstone that was ripped from a substrate by a high-energy current and subsequently redeposited within the pebbly sandstone/conglomerate. This interpretation aligns with the depositional environment suggested by the erosional scour casts and ripple-like structures, where intermittent high-energy conditions would favor the formation of such clasts.