New Mexico Geological Society Annual Spring Meeting — Abstracts

The oldest Jurassic strata across New Mexico have long been I assigned to the Entrada Sandstone of Middle Jurassic (Callovian) age. In New Mexico, the J-2 unconformity separates Entrada strata from the underlying Upper Triassic Chinle Group, and the Entrada is mostly overlain disconformably (= J-3 unconformity?) by the Todilto Formation of Callovian age.

Two informal members of the Entrada Sandstone--medial silty and upper sandy--have long been recognized in western NM. These members are identical in lithology and stratigraphic position to the Dewey Bridge and overlying Slick Rock Members of the Entrada in Utah-Colorado. Therefore, we apply these formal names to the Entrada members in New Mexico. The Dewey Bridge Member in New Mexico is as much as 20 m thick and is mostly reddish brown, very fine grained sandstone and siltstone with laminated or bioturbated bedding. The Slick Rock Member is as much as 122 m thick and consists mostly of yellowish gray, trough-crossbedded sandstone. The Dewey Bridge and Slick Rock Members are present throughout west-central New Mexico, in the southern San Juan Basin and in the Hagan basin. Elsewhere, the Dewey Bridge Member is absent, so in western and central NM the Entrada consists entirely of the Slick Rock Member.

In west-central New Mexico, the Dewey Bridge Member rests disconformably on a 12-to-44-m-thick unit of coarse-grained eolian sandstone, red-bed fine-grained sandstone and siltstone and conglomerate. These strata have been termed the "Iyanbito Member of the Entrada Sandstone" but have no stratigraphic equivalent elsewhere in the vast Entrada lithosome. Like Harshbarger et al. (1957, USGS Prof. Paper 291), we correlate the "Iyanbito" to the Wingate Sandstone, and thus remove it from the Entrada Sandstone. In eastern NM, the Entrada Sandstone consists of the Slick Rock Member and an upper tongue, the Exeter Member, that overlies the Todilto Formation. Therefore, in NM there are three Entrada Members: (1) Dewey Bridge Member in the western and central parts of the state; (2) Slick Rock Member throughout the Entrada outcrop belt; and (3) Exeter Member in eastern NM.