Abstract

Lower Permian Abo–Tubb subsurface strata of northeastern New Mexico, U.S.A. consist of 30 to 120 m of fluvial and eolian deposits, containing numerous dolomitic paleosols. Dolomite in the Abo–Tubb interval occurs in discrete intervals, interpreted as paleosols on the basis of recognizable horizonation and the presence of associated features such as ped structures and rhizoliths. The dolomite commonly forms veins, rhizoliths, and discrete and coalesced nodules.

Petrographic analysis indicates four dolomite types present in paleosol horizons: microsparitic, pseudospherulitic, clear cement, and cloudy (inclusion-rich) cement. Microsparitic dolomite constitutes most of (70%) the dolomite, and is stoichiometric, with relatively low Sr and Na and elevated Fe and Mn. Pseudospherulitic dolomite forms 15% of the dolomite, and consists of stoichiometric microspar and spar, with abundant fluid inclusions, low Sr, Na, and Fe, and elevated Mn. Clear dolomite cement constitutes approximately 12% of the dolomite and is finely to coarsely crystalline with a relatively calcian composition, low Sr and Na, and elevated Fe and Mn. The cloudy cement forms a minor (2%) phase of very coarsely crystalline rhombs with a calcian composition, low Sr and Na values, and relatively high Fe and Mn. All dolomite types exhibit a mottled texture of moderate to bright orange and red luminescence. Fluid-inclusion analysis was not possible on the most common (microsparitic) dolomite type, but analyses on the coarser varieties reveal homogenization temperatures typically ranging from 70°C to 110°C and final melting temperatures of ice ranging from −20°C to −34°C, suggesting fluids of high (25% by weight) salinity. Bulk-rock carbon and oxygen isotopes of carbonate nodules and rhizoconcretions from four paleosol profiles most commonly exhibit mean values typically ranging between~ −3 to −5.5‰ (VPDB) and −0.5 to 2.8‰ (VPDB), respectively. Sulfur isotopes were analyzed for the overlying Cimarron Anhydrite interval and an anhydrite nodule near the base of the study interval. Both samples yielded similar results, suggesting that brines that precipitated the Cimarron Anhydrite also influenced the study interval.

The dolomite in the Abo–Tubb interval is interpreted to have precipitated in multiple stages. Solid calcite microinclusions in microsparitic and pseudospherulitic dolomite suggest that pedogenensis and groundwater penecontemporaneous with Early Permian deposition led to precipitation of calcite rather than dolomite. Oxygen and sulfur isotope data indicate dolomitization, from later reflux of brines associated with deposition of the overlying (Leonardian) Cimarron Anhydrite. Petrographic relations and elevated temperatures and salinities recorded by the fluid inclusions in the coarse dolomite are consistent with recrystallization of the earlier dolomite and precipitation of dolomite cement from warm saline fluids of a possible Tertiary hydrothermal origin, just predating migration of mantle-derived gas into the Bravo Dome reservoir.

These findings offer insight into questions about the climate significance of dolomite in paleosols, origin of dolomite in hydrothermally altered hydrocarbon reservoirs, and the long-term effect of CO₂ sequestration in saline aquifers. The results show that dolomitic nodules in paleosols may form as replacements of paleosol calcite long after the paleosol formed. Primary pedogenic dolomite would undoubtedly have climatic and paleoenvironmental significance, but recognizing it as forming during soil formation, as opposed to later dolomitization, is key in establishing that significance. Also, this study provides an example of high-temperature recrystallization of low-temperature dolomite, a process hotly debated in discussions of dolomitized hydrocarbon reservoirs with evidence for hydrothermal fluid flow. Finally, as a natural CO₂ reservoir, the Bravo Dome system shows that the long-term result of dissolution of CO₂ into brines is predominantly carbonate mineral dissolution.