Douglas W. Kirkland: Role of Hydrogen Sulfide in the Formation of Cave and Karst Phenomena in the Guadalupe Mountains and Western Delaware Basin, New Mexico and Texas

Abstract

This monograph provides a theory for multiple stages of speleogenesis related to production, transportation, and oxidation of hydrogen sulfide in the western Delaware Basin and along the margin of the basin in the vicinity of the Guadalupe Mountains (southeastern New Mexico, USA). Large caves in the Guadalupe Mountains formed during the late Miocene and early Pliocene (~12-4 Ma ago). They originated dominantly from sulfuric acid (H2SO4), a powerful cave-forming agent that dissolved both limestone of an ancient sponge-algal reef—the Capitan Formation (Middle Permian; ~270-260 Ma ago)—and limestone and dolomite of age-equivalent, near-backreef (shelfal) strata. The reef-front formed the boundary between the Guadalupe Mountains to the northwest and the Delaware Basin to the southeast.. The strong acid was produced as dissolved oxygen (O2) from the earth’s atmosphere reacted with aqueous hydrogen sulfide (H2S) from the adjacent basin. The H2S originated within and migrated from the Castile Formation (earliest Late Permian, ~260.0-259.8 Ma ago) because of the influence of two overlapping Late Tertiary events: - transient high-heat flow, particularly in the western Delaware Basin, and - eastward uniform tilting (ultimately by 1° to 2°) of the huge paleo-Guadalupe tectonic block (including the mountains and much of the basin). The Castile before extensive Late Tertiary dissolution extended throughout the basin and consisted of thick (tens of meters), remarkably persistent, alternating beds of halite (NaCl) and anhydrite (CaSO4). Late Miocene artesian groundwater flowed within Permian aquifers eastward down the sporadically rising tectonic block. Much groundwater then rose along fractures generated or regenerated during the tilting and dissolved Castile evaporites at hundreds of local sites up to ≈30 km east of the shelf edge. Free convective flow resulted. Castile halite, in particular, became the vehicle of its own dissolution. To replace the sinking brine, the least dense, least saline, most solutionally aggressive groundwater persistently rose to the highest accessible elevation. The groundwater dissolved chambers vertically upward through thick-bedded halite until the voids contacted the smooth, intact base of a bed of Castile anhydrite (solubility ~1/140 of that of halite), all beds of which dipped uniformly eastward by <1-2°. The conduits, except for their anhydritic ceiling, were confined to the uppermost parts of halite beds and they are hypothesized to have been narrow (< ≈30 m), low (< ≈2 m), and elongated (≈30 km). They advanced westward by convective dissolution directly up the slight slope of the paleo-Guadalupe tectonic block. Many conduits eventually terminated at the nearly vertical face of the youngest-most Capitan paleo-reef or at the steepto- shallow face of the youngest-most paleo-forereef, both of which were in side-by-side contact with beds of Castile anhydrite and halite. Basinal stratal temperatures transiently increased shortly before and as the conduits were forming resulting in generation of billions of cubic meters of methane (CH4). Much gaseous CH4 ascended into the Castile evaporites at the same localities at which groundwater convectively rose and sank. The gas progressively dissolved within ambient water beneath a thick (~1 km) sealing cover of strata (chiefly red beds, carbonates, and evaporites) and reacted with SO4 2- derived from dissolution of Castile anhydrite. The reaction, aided by enzymes of anaerobic microbes, generated many millions of metric tons of both aqueous H2S and aqueous CO2. The CO2 reacted instantaneously with Ca2+, liberated as CaSO4 dissolved, replacing laminated, nodular, massive, and brecciated Castile anhydrite with permeable limestone. The anhydrite-encased limestone bodies, commonly with dimension, in plan, >30 m, formed at ~1000 scattered localities. Pressurized artesian groundwater transported the H2S from the carbonate bodies into the conduits within overlying Castile halite. The groundwater then flowed up the homoclinal slope and by forced convection moved through fractures and pores of the Capitan Formation and adjacent shelfal carbonates, and descended to low levels because of a relatively high density imparted by dissolved halite. The H2S-charged, saline groundwater flowed sluggishly throughout the late Miocene and early Pliocene within a basin-margin carbonate aquifer that formed a narrow (~6 km) northeast-trending belt across the eastward-dipping paleo-Guadalupe tectonic block. The highest part of the belt, therefore, was to the far southwest. Here, west-to- east-trending erosion initially removed the impermeable cover of mainly Salado and Rustler evaporitic strata (Late Permian; ~259.8-250.0 Ma ago) and groundwater initially fell allowing atmospheric O2 to enter the uppermost level of incipient caves. H2S-H2SO4 speleogenesis occurred when H2S degassed from cave pools and when atmospheric O2 moved into the caves. The gaseous O2 probably entered permeable carbonates that cropped out in southwestern highlands; it then descended laterally through fractures beneath sealing evaporites. The H2S and O2 dissolved within subaerial water on carbonate wall rocks and reacted completely (aided by bacterial enzymes) to form H2SO4. Then, over a span of ~8 Ma, each episodic uplift of the tectonic block resulted in further erosion of the cover, deeper descent of the groundwater table, further progression of speleogenesis southeastward along the belt, and deeper penetration of speleogenesis within carbonates of the cave belt. Within 12 to ~50 km southeast of the cave belt, genetically related karstic processes formed deposits of native sulfur dispersed within biogenic limestone and encased within Castile and Salado anhydrite. The caves and the sulfur deposits owe their origin to a coincidence of essentially the same stratigraphic, tectonic, thermal, and biogenic events. The sulfur deposits occur along graben-bounding faults that breached both the Castile (~30% halite; ~0.5 km thick) and the directly overlying Salado (~85% halite; ~0.5 km thick) and extended to the surface. The faults guided hypogenic groundwater upward by forced convection, and during subsequent free convection, the returning brine locally increased the permeability of the steep fracture pathways through bedded anhydrite. Gaseous CH4 migrated upward along the same pathways. It dissolved within water and reacted with SO4 2- to generate porous CaCO3 and, within at least three deposits, > 1,000,000 metric tons of H2S. Simultaneously, meteoric water flowing down the same pathways dissolved Salado halite (and gypsum) into which overlying Permian and Mesozoic strata collapsed forming large (up to many hectares), closed, karstic depressions. The dolines focused enormous volumes (up to many cubic kilometers) of saline, O2-saturated (~2 to >4 mg/l) groundwater into the subsurface. The brine descended through the faulttracking pathways along an inverted density gradient and discharged into underlying channel-fill sandstone. Where saline, O2-bearing groundwater sinking along one course contacted relatively fresh H2S-bearing groundwater rising along an adjacent course, elemental sulfur precipitated.