New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1946, Kelley and Wood (USGS Oil and Gas Inv. Prelim. Map 47) named the Missourian-Virgilian Atrasado Member of the Madera Formation in the Lucero uplift, but did not describe a type section. An excellent lectostratotype section of the Atrasado Formation of the Madera Group is exposed in secs. 35-36, T6N, R3W. Here, the Atrasado Formation is ~100 m thick and can be divided into five lithologically distinct units.

Unit A (31.5 m) starts with a 0.5-m thick conglomerate and sandstone that rests on limestone at the top of the Gray Mesa Formation. A covered interval is followed by gray, fossiliferous, bedded limestone forming several ledges up to 2 m thick. Limestones in the lower part of this unit contain abundant fusulinaceans; some beds contain brachiopods, bryozoans, calcareous algae and crinoid fragments. Unit B (22 m) consists of 4 cycles, each composed of (from base to top): (1) massive gray algal limestones, (2) dark gray, micritic, bioclastic limestones, and (3) covered intervals (shale).

A polymict conglomerate forms the base of unit C (18.4 m). It is overlain by gray, bedded fossiliferous limestone that in the lower part is indistinctly wavy bedded, bioturbated, and contains chert nodules. Overlying limestones are fossiliferous, with abundant bryozoans. The topmost bed is a dark gray, micritic limestone. Above this follow 14.1 m of greenish-grayish shale, mostly covered, with two intercalated, fine-grained, sandstone beds in the middle (unit D). The uppermost unit, E (13.6 m), is composed of wavy to irregularly bedded gray limestone, locally containing calcareous algae, crinoids or fusulinaceans. In the upper part, nodular shale and pale green shale are present. The base of the overlying 2.5-m thick sandstone interval is the base of the Red Tanks Member of the Bursum Formation.

Limestones of the type Atrasado Formation are characterized by muddy textures and diverse biota indicating deposition in an open shelf environment below the wave base within the photic zone under normal marine salinity. Coarse clastics most probably document tectonic events.