The Terry Sandstone Member, which lies within the middle part of the Upper Cretaceous Pierre Shale in the Denver basin of northeastern Colorado, has been interpreted as a marine-shelf depositional sequence. Such interpretation has been based mainly on the sedimentological character of the sandstone units within the depositional sequence and on their position seaward of correlative Lower Cretaceous shorelines. This study conducted a process sedimentological analysis of the Terry Sandstone sequence in an area the size of two townships approximately 8 mi downdip from the sandstone outcrop. Four core sequences, two of which are illustrated herein, and wire-line logs from over 200 wells were utilized to construct a series of subsurface maps and cross sections with which to evaluate sandstone body geometry, trend, and sedimentological character.

The Terry Sandstone Member thickens over a distance of 11 mi from less than 60 ft thick in the northwestern corner of the study area to greater than 120 ft thick in the southeastern corner of the study area. An upper, sandy mudstone sequence forms a blanketlike unit that ranges from 50 to 70 ft thick throughout the study area. The lower sandstone unit of the Terry Sandstone is composed of two distinct northwest-trending, elongate, lens-shaped sandstone bodies (designated herein as the "northeastern" and "southwestern" sandstone bodies) that display a shinglelike overlapping relationship across a 1- to 2-mi wide, northwest-trending strip. The two units are separated by a shale permeability barrier unit (designated herein as the shale "notch").

Within the lower unit of the Terry Sandstone Member, the northeastern sandstone body displays a symmetrical lens-shaped cross section up to 70 ft thick and 4-5 mi in maximum width. It is characterized by a gradational base, a coarsening-upward vertical sequence, and a sharp upper contact with overlying marine mudstone. The shale "notch" appears to be composed of silt-poor clay shale; it "migrates" from the top to the base of the lower sandstone unit of the Terry Sandstone within 1.5 mi and effectively separates the two sandstone bodies. The Southwestern Sandstone Body displays an asymmetrical lens-shaped cross section (with more rapid thinning along the northeastern flank), is up to 50 ft thick and 5-6 mi in maximum width, and is characterized by a sharp, erosional basal contact, fining-upward vertical sequence, and gradational top.

Evidence suggests that both sandstone bodies of the lower sandstone unit of the Terry Sandstone were deposited in a shallow marine-shelf setting. This evidence includes the general paleogeographic setting and features such as the extensively burrowed to bioturbated sandstone and mudstone sequences, the abundance of glauconite, and the extensive marine mudstone intervals of the Pierre Shale that completely encase the Terry Sandstone sequence. The northeastern sandstone body formed as an offshore marine sand bar, whereas the southwestern sandstone body apparently represents a channelized deposit composed of active channel-fill sand and overlying inactive channel-fill sandy mudstone. Of economic significance is the segregation of hydrocarbon types in the sandstone bodies; noncommercial volumes of gas are present in the northeastern sandstone body and commercial volumes of oil in the southwestern sandstone body.