New Mexico Geological Society Annual Spring Meeting — Abstracts

Much of the type Wingate Sandstone in western New Mexico was reassigned to the Middle Jurassic Entrada Sandstone in the middle of the 20th century, and the stratigraphically lowest eolian sandstones in the Gallup area were assigned to the "lyanbito Member" of the Entrada in the 1970s. We have restudied the type section of the "Iyanbito Member" and conclude that, as Harshbarger et al. (1957 USGS Prof. Paper 291) argued, it more likely represents a southern tongue of the Upper Triassic-Lower Jurassic Wingate Sandstone as the unit is now construed in eastern Arizona.

The type section of the "Iyanbito Member" is 39.8 m thick and consists primarily of pale reddish brown to moderate reddish orange, very fine- to medium-grained, subangular to rounded, moderately sorted, silty quartzarenite with occasional thin (<0.03 m) shale partings. Some of these quartzarenites are locally conglomeratic, with very coarse-grained to pebbly clasts of chert, quartz, and clay. Most sandstones preserve faint, high-angle trough crossbeds and other features characteristic of eolianites (66%), whereas other sandstones are ripple laminated, laminated, or bioturbated, and thus probably are water-lain and/or water reworked (34%). In general, all "lyanbito" strata are coarser-grained and more poorly sorted than overlying rock types of the Entrada Sandstone.

We agree with previous work that demonstrated that the "lyanbito" rests disconformably on the Upper Triassic Owl Rock Formation of the Chinle Group. However, we argue that this erosional surface developed during latest Triassic through Early Jurassic time and was infilled and covered by sandstone deposits of the Wingate Sandstone (="Iyanbito Member"). The upper contact of the Wingate Sandstone is not conformable with the medial silty (Dewey Bridge) member of the Entrada Sandstone, but is everywhere disconformable and expressed as an interval of uneven color-mottling (pale reddish brown to yellowish gray and bluish gray) and irregular topography where the base of the medial silty member fills fissures in the "Iyanbito" as much as 0.3 m deep.

We note, therefore, that the "Iyanbito Member" of the Entrada is unconformity bounded and unlike overlying Entrada strata. Furthermore, there are no strata elsewhere in the Entrada lithosome, which spans at least five states, that are similar to or homotaxial with the Iyanbito and still conformable with the medial silty member. Instead, we note that the Wingate Sandstone thins southward from the Four Corners into the Gallup area, while also resting on progressively older strata. Therefore, we interpret the type "Iyanbito" as a thin but otherwise typical exposure of the Wingate Sandstone, of Late Triassic-Early Jurassic age, and abandon the term Iyanbito Member of the Entrada Sandstone. This 38-m-thick tongue is now the only horizon identified as the Wingate Sandstone in its type area.