Day 1 : Stop 2

DAY 1 (9/26/2025) - STOP 2

"Black Bedded Micritic Limestone"

COORDINATES:  31.81594° N, 104.82929° W

    Our second stop brought us to a roadside stop where we examined a form
ation of black bedded micritic limestone (fig. 3). This formation is important as it shows deposition in the deep waters of the ancient Delaware Basin, a low-energy environment that existed long before the Capitan Reef system formed around the basin margins. Unlike other fossil-rich reef limestones nearby, the dark beds at this site formed offshore, where fine carbonate mud slowly settled through the water column and accumulated in oxygen-poor conditions.

Fig. 4 Calcite Veins (Secondary Process)

    When observing some hand samples broken off from the formation, we saw that the limestone breaks into smooth, dark pieces that reveal extremely fine calcite crystals (CaCO3), typical of micrite, which forms when carbonate mud is lithified. When the rocks are struck, the rock releases a distinctly strong odor, the result of bitumen, a viscous form of petroleum, preserved within the rock’s organic-rich matrix. This smell is a reminder that these deep-basin sediments once contained high concentrations of organic material that partially decayed before burial. The presence of bitumen also explains the limestone's dark grey to black color. Some pieces also contain thin calcite veins (fig. 4), occurring from a secondary process where later fluid circulation filled fractures after the original sediment lithified into rock.

Fig. 3 Deep Ocean Black Micritic Limestone

    Although the interior of the limestone is nearly black, the surface is noticeably lighter in color. This is the result of weathering processes over long exposure at Earth’s surface, during which oxidation and chemical alteration altered the original organic richness. Beneath that outer layer, however, the rock still preserves the conditions of its formation: calm, deep water, low oxygen, steady accumulation of carbonate mud, and the burial of organic material.

    This unit represents an essential part of the Permian formation. Understanding the formation and geomorphology of this site helps us understand the geological events during the deeper, quieter parts of the sea while the Capitan Reef was actively growing along the margins.