Swaley Cross-Stratification and Associated Features, Upper Cretaceous Western Interior Seaway of United States

Roderick W. Tillman

Swaley cross-stratification (SCS) occurs in upper shoreface sequences in the Upper Cretaceous. SCS, which is composed primarily of concave-downward laminations, was defined by Leckie and Walker in 1982 and was contrasted by them with hummocky cross-stratification (HCS) and trough cross-bedding. SCS is significant as a depth marker within the upper shoreface on strand-plain, delta, and barrier-island coastlines.

The position of SCS relative to other shallow marine sedimentary features is documented in five vertical sequences: (1) Lewis Sandstone, beach, SCS, beach; (2) Haystack Mountains Formation, transition zone (including HCS), SCS, marine shale; (3) Shannon (Cody) Sandstone, marine shale, SCS, marine shale; (4) Panther Sandstone; sequence A--transition zone, horizontal lamination, SCS; sequence B--horizontal lamination, SCS, trough cross-beds.

Where it occurs in a shallowing sequence, SCS is always shallower than HCS and deeper than beach (foreshore) laminations. SCS is similar to HCS in that both are commonly very fine grained (less than 125 µm). It also shows no preferred three-dimensional orientation and contains very thin, slightly curved lamina sets, which truncate underlying sets. It occurs immediately above or below beds with Rosselia, Ophiomorpha, and Asterosoma trace fossils. Traces of sand-filled burrows are observed within SCS, but more commonly it is not burrowed. SCS is an upper shoreface indicator and presumably was deposited and preserved above fair-weather wave base.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.