New Mexico Geological Society Annual Spring Meeting — Abstracts

The Jurassic Todilto Formation in northern New Mexico is typically composed of a basal limestone member (Luciano Mesa) and an overlying gypsum member (Tonque Arroyo); however, a 0.5-2.0 m thick limestone bed generally caps the gypsum member of the Todilto Formation in the southern Chama Basin. Furthermore, a pale green, moderately sorted, fine-grained sandstone with subangular to subround quartz grains is interbedded with the gypsum at two localities northwest of the village of Arroyo del Agua. The Todilto Formation is overlain by 8-12 m of white to light green and red, fine- to very fine-grained quartzose sandstone that has been assigned by previous workers to the basal part of the Jurassic Summerville Formation. This thin-bedded sandstone has ripple marks and gypsum blade casts. West of Ghost Ranch, thin limestone beds appear in the sandstone unit; the limestone beds become thicker and more numerous toward the west. The white sandstone unit is overlain by variegated maroon and pinkish-gray quartzose to subarkosic siltstone. The thin-bedded white sandstone unit is more like the underlying Todilto Formation than the overlying maroon siltstones, particularly near Arroyo del Agua.

The thin-bedded white sandstone beds above the gypsum member of the Todilto Formation contain two types of fluid release features. One set of features formed during the earliest phase of burial, before the sediments were lithified. A breccia composed of both 1-5 m long blocks of the gypsum-capping limestone and <1 m long blocks of the thin-bedded sandstone characterizes the second type of fluid release structure. The breccia, which obviously formed after lithification, may have developed when gypsum converted to anhydrite at temperatures of about 60°C. Simple 1-D thermal models suggest that these temperatures could have occurred at burial depths of 1.3 to 1.5 km during deposition of the Cretaceous Lewis Shale.