Processes that have operated on the seabed of the shelves of the world in the past include waves, storms, permanent currents, subtidal currents, and turbidity currents. As a result of the wide variety of processes, sand bodies with different geometries have resulted, and they commonly contain different sedimentary structures or different sequences of sedimentary structures. On ancient shelves the most common sedimentary structures in current-deposited sandstones are planar-tangential to planar-tabular cross-beds, and current ripples. Wave-deposited sandstones are characterized by horizontal to subhorizontal laminations and symmetrical ripple forms. One of the most common shelf sequences reflects upward-increasing energy. However, a sequence reflecting upward-increasing en rgy and consequent increase in grain size is not unique to shelf sandstones.

Shelf sandstones may be classified on the basis of their position on the shelf (shoreface-attached, inner shelf, middle shelf, outer shelf) and on the basis of whether they are deposited during a transgression, regression, or a stillstand. Both vertical and lateral sequences of lithologies vary with position on the shelf, processes of deposition, position within transgressive-regressive spectra.

Cretaceous sandstones used to characterize a variety of these processes, geometries, and shelf locations include the "Gallup" (Tocito), Shannon, Fales, and Frontier sandstones.

End_of_Article - Last_Page 311------------