Redefinition of the base of the Cub Mountain Formation and preliminary depostional and tectonic interpretations of the Early-Middle Eocene strata in the Sierra Blanca Basin, New Mexico
Daniel J. Koning and Logan Roberts
The Sierra Blanca Sierra Blanca Basin preserves a 140–800 m-thick sequence of lower to middle Eocene fluvial strata with distinctive reddish floodplain deposits. These strata have been assigned to the Cub Mountain and the overlying Sanders Canyon Formations. We propose redefining the base of the Cub Mountain Formation to the top of an extensive paleosol that very likely coincides with the regional Cenozoic-Cretaceous unconformity. The paleosol is 3 to 8 m thick, light gray to light grayish green, and contains dark purplish black, manganese or iron oxide concretions that are 3–10 cm in diameter. The paleosol underlies reddish floodplain deposits and Eocene fossil localities. Where the paleosol is absent, the lowest occurrence of reddish floodplain deposits, or coarse channel-fills clearly associated with the reddish floodplain deposits, should serve as the base of the formation.
Near the type area of the Cub Mountain Formation, past studies have included in the basal Cub Mountain Formation a 30–60 m-thick conglomeratic sandstone interval that underlies this paleosol, but our redefinition results in this interval correlating with the Ash Canyon Member of the Crevasse Canyon Formation (Upper Cretaceous). Because imbricated gravels are relatively sparse above the paleosol near the Cub Mountain Formation type area, we suspect that much of the imbrication measurements used to support a northeast paleoflow direction for the Cub Mountain Formation actually came from the upper Crevasse Canyon Formation. Collection of new paleoflow data from strata clearly overlying the paleosol indicate southerly paleocurrents ranging from west to southwest to southeast. Stratigraphic interpretations for the 1160 m-thick Lewelling unit, designated for volcaniclastic strata overlying the Crevasse Canyon Formation in the Lewelling No. 2 well, suggest major structures (likely reverse faults) on the southwest side of the Laramide-age Sierra Blanca Basin.
- Koning, Daniel J.; Roberts, Logan, 2014, Redefinition of the base of the Cub Mountain Formation and preliminary depostional and tectonic interpretations of the Early-Middle Eocene strata in the Sierra Blanca Basin, New Mexico, in: Geology of the Sacramento Mountains region, Rawling, Geoffrey; McLemore, Virginia T.; Timmons, Stacy; Dunbar, Nelia, New Mexico Geological Society, Guidebook, 65th Field Conference, pp. 273-286.