Abstract

The Glass Mountains have long been a classic exposure of the Permian stratigraphy in the Southern Delaware Basin. Due to limited access, no modern (post 80’s) work has been done on those outcrops that expose the entire section from folded Ordovician through Pennsylvanian to nearly undeformed Early Permian (Leonardian) to Late Permian (Castille Fm. and younger equivalent).the RCRL long term goals in the Glass Mountains is to better understand the controls (if any) of platform development, margin geometry and trajectory, and slope system for the delivery of deep water to basinal sediment. This is particularly relevant for the current exploration effort in the southern Delaware Basin. In addition, the Glass Mountain paleogeographic and tectonic setting provide a unique opportunity to understand the development of carbonate platforms, and slope and basins in an active compressional tectonic setting.

Our initial aerial reconnaissance and field mapping of the Wolfcampian through Early Capitanian slope sections in the Glass Mountains shows 5 types of slope systems:

• A grainy crinoid/fusulinid dominated upper slope in the late Wolfcampian

• A grainy/debris dominated slope system during the Leonardian

• A muddy to silty dominated slope during the Leonard /Guadalupian time with small lenticular debris flows.

• A sandy to silty slope with microbial mounds during the Goat Seep equivalent with large reef olistholiths probably equivalent to the “get away” debris

• A “classic” deep reef and debris dominated Capitanian slope, very similar to the one in the Guadalupes

The first two are beautifully exposed in the Leonard Mountains. There, the deformed and eroded upper Wolfcampian slope is characterized by 5-20 m thick polymict conglomerate with large blocks overlain by medium bedded, faintly graded, 5-10 m thick set of crinoid and fusulinid grain-dominated packstone to grainstone with lenticular (channelized) massive bed of grainstone up to 8 meter thick. The lower conglomerates are interpreted as syntectonic mega-breccia whereas the upper grainy interval is interpreted as concentrated density flows (aka grainflow). The top of the Wolfcampian is marked by an angular unconformity. Above this surface, the initial Leonardian platform consists of thickly bedded dolomitized crinoidal, fusulinid grainstone. The margin of this platform is scalloped by a steep erosional surface onto which younger slope deposits onlap. The leonardian slope system displays from bottom to top, large reefal blocks up to 10 m high within a megabreccia overlain by dolomitized concentrated density flows and debris flows. Then, a couple of 5-10 m thick grainy flow complexs are overlain by a medium bedded graded crinoid packstone interpreted as carbonate turbidites interbedded with thin-bedded siltstone. The upper part of the slope section is dominated by conglomerates and soft sediment deformed conglomerate that are commonly rich in quartz. The amount of deformation and erosion in the upper part of the section is spectacular. This slumped and channel complex is overlain by a thick stack of thin bedded silt intervals with a few sandy debris flows and thin sandy carbonate turbidites throughout.

Further up section, the Cathedral Mountain Fm., the Road Canyon Fm., and the base of the Word Fm. consists of a thick (~150 m) stack of thinly bedded siltstone intercalated with thin sandstone and carbonate turbidites with several 8-20 m thick debris flows. The Leonardian Guadalupian boundary has been established at the base of those debris flows in the Road Canyon Fm., making this interval a time equivalent of the Cut Off Fm. in the Guadalupe Mountains. This silty slope section is overlain by approximately 50 m of crinoidal sandy grainstone grading upward into sand-rich well-sorted peloidal grainstone. This shallower water interval is overlain by a thick section of silt to medium grained sandstone inter-bedded with rare thin carbonate gravity flows. This interval contains however, massive (up to 30 m thick and 100 m wide) blocks of bedded dolomitized boundstone floating in a conglomerate matrix. These blocks are interpreted as a massive failure of probably an aggrading dolomitized reefal margin. The sand-rich slope grades upward into an interval of thin-bedded silty dolomitized mud to wakestone, with rare fine-grained sandstone beds and coarse faintly graded crinoid brachiopod rudstone interpreted as thin carbonate gravity flow deposits. This thin bedded silty slope interval gradually changes into an interval with similar thin-bedded silty dolomitized mud-wackestone, interbedded with more massive dolomitized boundstone forming 1 to 6 m bioherms. Gradually the bioherm interval becomes more massive to form clinothem that make up the massive cliff Vidrio Fm. at the top of the section.

The stratigraphy exposed in this part of the Glass Mountains is somewhat different to the one established in the Diablo and Guadalupe Mountains. Based on our works, existing literature and the available biostratigraphic data, we are proposing an initial correlation to the well-established stratigraphy of the Guadalupe Mountains using the sequence architecture and nomenclature established in the Guadalupe Mountains.