Spectrum of Ancient Shelf Sandstones: ABSTRACT

Abstract

Abstract A wide variety of processes have operated on the shelves of the world in the past, including waves, storms, permanent currents, subtidal currents and turbidity currents. A variety of sand bodies with different geometrical characteristics has resulted and they commonly contain different sequences of sedimentary structures. On ancient shelves the most common sedimentary structures in sandstones are planar-tangential to planar-tabular cross-beds, horizontal to subhorizontal laminations, current and wave ripples, and bioturbated (>75% burrowed) sandstones. Sequences indicating upward-increasing energy are common on shelves. This sequence may also reflect an increase in grain size, however, it is not unique to shelf sandstones; a similar coarsening-upward pattern is reflected in subsurface log patterns in both river- and wave-dominated deltas and in beach-barrier-dominated shorelines. 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, and position within transgressive-regressive spectra. In the middle and outer shelf, shelf sandstones are almost always surrounded by shale. On the inner shelf, and where attached to the shoreface, shelf sandstone overlies a variety of lithotypes (sandstone, siltstone and shale) depending on whether they were deposited during a transgression, regression or stillstand. The lithotypes deposited lateral to shelf sandstones also vary with the position of the sand body within the spectra of transgression-regression. Lateral sequences of lithotypes are most variable in the inner shelf. Local topography may also affect the distribution of shelf sandstones. Winnowing of topographically high areas may concentrate sand into sand ridges. Shallow depressions in the sea floor from erosion during a fall in sea level, or a retreat of the shoreface, may be filled with coarse to fine-grained sand. Several of these processes, shelf locations and differing geometrical characteristics are documented for the “Gallup” (Tocito), Shannon, Fales, Hygiene, Teckla and Frontier Sandstones.