Five depositional models have been utilized during the last 30 years to explain the deposition of the Upper Cretaceous Gallup and Tocito Sandstone and Tocito Sandstone Lentil in the San Juan Basin in New Mexico. It has generally been recognized that most of the true Gallup sandstones were deposited as strand plain and beach deposits. There is a continuing controversy as to the relationship of the sand ridge (offshore bar) deposits to the Gallup shoreline sandstones. The offshore deposits have been designated as Gallup by some writers and as the Tocito Sandstone Lentil of the Mancos Shale by others. Scenarios such as described in this paper for the Gallup Sandstones may be more common for shelf sandstones than is presently recognized.
The Gallup Sandstone may be divided into two major depositional units. Most of the Gallup Sandstone is a strand plain deposit with a typical transition zone, shoreface, foreshore vertical sequence. Hummocky cross stratification marks the base of the Gallup sequence in some areas. Most of the fluvial portions of the Gallup are designated as the Torrivio Sandstone Member.
Some of the earliest correlations (Model I) suggested that the Gallup Sandstone was younger than the Tocito. Other correlations (Model II) indicated that the Gallup consisted of a series of synchronous shoreline and offshore deposits.
The third, fourth and fifth models include a major unconformity which separates the shoreline Gallup sandstones from offshore-bar sandstones designated as the Tocito Sandstone Lentil of the Mancos Shale. The third model stresses the importance of
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Shelf sandstone reservoirs are becoming a more and more common exploration target. What they are, how they may be characterized, and how they differ from shoreline and deep-water deposits in the subject of this publication. Shelf sands and sandstone reservoirs are among the more poorly understood types of sandstones. Continental, shoreline and deep water sandstones have all been studied in much more depth than have shelf sands and sandstones. However, during the last fifteen years significant progress has been made in understanding shelf sands and sandstones. Studies of modern sediments have allowed us to understand many of the depositional processes active on the shelf. This book is intended to be an up-to-date summary of shelf processes and products. The papers are intended for those new to shelf sands and sandstones as well as the shelf specialist.