Permian Reef Complex Virtual Field Trip
Stop II-2: Bone Spring Limestone – Basinal Source Rock
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This outcrop, faulted at its northern end, exposes Bone Spring Limestone in its basinal facies. This Leonardian limestone, the oldest unit exposed in the Guadalupe and Delaware Mountains, reaches at least 520 m (1,700 ft) thickness in this area (King, 1948). A thickness of 952 m (3,123 ft) has been measured for the combined Hueco Limestone-Bone Spring Limestone interval in the Updike well near El Capitan. This combined section thickens to greater than 1,370 m (4,500 ft) in the Delaware Mountains to the south (King, 1948, p. 13). The Bone Spring Fm. represents a gentle ramp slope which extended into water depths of the order of a hundred meters from the adjacent shelfal areas of Victorio Peak Fm. deposition.
At this locality we can see typical, dark gray to black, cherty, interbedded limestones and calcareous shales which are the dominant lithology of the basinal part of the Bone Spring Limestone. Fossils, especially small ammonites, can be found at this locality, but are generally restricted to isolated, granular or calcarenitic beds. Bedding surfaces in the basinal Bone Spring Limestone are generally wavy and show evidence of soft-sediment deformation (mainly slumping). Both the limestone and the shale units contain considerable amounts of organic matter and may have acted as source rocks within this basin. Indeed, trapped oil can be found even on outcrop in small cavities in the Bone Spring Limestone (Barker and Pawlewicz, 1987; Gawloski, 1987; Hills, 1984). It is also visible as oil-filled inclusions trapped in some of the fracture-filling calcite crystals that can be collected here (look for bands of white, follwed by black spar in the fractures). Freshly broken pieces of this rock will also emit a strong smell of hydrocarbons.