New Mexico Geological Society Annual Spring Meeting — Abstracts

As part of a larger study of cyclic sedimentation, a locally restricted karst surface was identified in the lower (Morrowan) portion of the Horquilla Formation (Pennsylvanian-Permian) in the New Well Canyon of the Big Hatchet Mountains, southwestern New Mexico. The surface is located about 40 feet above the top of the Paradise Formation (Mississippian). Although other examples of subaerial exposure are known from the upper Pennsylvanian and Permian portions of the Horquilla, this is the lowest known example.

Over a distance of 2000 feet, karst features range from non-existent to accentuation of joints to a depth of two feet suggesting a surface of low relief. The surface is developed on crinoidal mudstone, with underlying rocks containing abundant micrite and a mixed restricted and normal marine biota suggesting a shallow marine setting.

A 2-5 foot thick bioclast, intraclast packstone with small scale cross-bedding is deposited above the surface. At the base of this is a lag, containing numerous mudstone clasts and an unusual abundance of phosphatic clasts (bone?) and fish teeth. The lag infills the joints developed at and below the karst surface.

Above the packstone is a 3-6 foot intraclast ooid packstone containing large scale trough cross stratification up to 2 feet thick. Ooids within individual beds show considerable variation in size and some sheltered pores show evidence of early marine cements. This sequence culminates with a crinoid -bearing wackestone/mudstone interval that contains a biota (ostracods and molluscs) suggesting eurytopic conditions.

We conclude that inundation by normal marine waters ended a brief interval of subaerial exposure. The initial phase of marine conditions was occasioned by low rates of sedimentation during which there was a thin accumulation of skeletal fish debris at the karst surface and in the eroded joints. As carbonate production commenced in occasionally agitated waters, bioclast, intraclast and ooid production contributed to the buildup of a mud rich shoal. With decreasing depth, the effect of agitation and circulation was decreased, and conditions returned to a restricted, mud dominated style of shoaling upward sedimentation completing the cycle.