When “evaporites” are not formed by evaporation: The role of temperature and pCO2 on saline deposits of the Eocene Green River Formation, Colorado, USA Robert V. Demicco† and Tim K. Lowenstein Department of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, New York 13902-6000, USA ABSTRACT INTRODUCTION rated with halite. During summer, the waters of the lake above the thermocline (at <25 m depth) Halite precipitates in the Dead Sea during Geologists studying salt deposits have noticed warm to ∼34 °C, become undersaturated with ha- winter but re-dissolves above the thermo- that very soluble minerals that occur in the cen- lite due to the temperature increase, and the bulk cline upon summer warming, “focusing” ha- ter of a basin do not extend out to the edges of of the winter-deposited halite above the thermo- lite deposition below the thermocline (Sirota the basin (Hsü et al., 1973; Dyni, 1981; Lowen- cline re-dissolves. Summer dissolution of halite et al., 2016, 2017, 2018). Here we develop an stein, 1988). This is true even where: (1) soluble occurs in the Dead Sea despite any increase in “evaporite focusing” model for evaporites salts are interpreted to have been deposited in evaporative concentration at the surface. The ha- (nahcolite + halite) preserved in a restricted a deep evaporitic lake or marine basin and (2) lite that settled into the deeper, cooler isothermal area of the Eocene Green River Formation deep-water deposits that encase salts in the cen- bottom waters, however, accumulates as an an- in the Piceance Creek Basin of Colorado, ter of the basin can be confidently traced to pe- nual layer at depths below the 25 m deep ther- USA. Nahcolite solubility is dependent on ripheral deposits. This situation is a hallmark of mocline, which “focuses” halite deposition into partial pressure of carbon dioxide (pCO2) the Parachute Creek Member of the Green River the deeper portions of the Dead Sea. Continued as well as temperature (T), so these models Formation in the Piceance Creek Basin of north- evaporative concentration, caused by a negative covary with both T and pCO2. In the lake western Colorado, USA (Fig. 1). The Parachute water balance in the basin, sets up the lake for that filled the Piceance Creek Basin, halite, Creek Member, up to ∼900 m thick, comprises the next annual cycle. nahcolite or mixtures of both could have dolomitic and siliceous mudstones, including The purpose of this paper is to apply a modi- precipitated during winter cooling, depend- oil shale, that contain bedded and disseminated fied version of the “halite focusing” model ing on the CO2 content in different parts evaporites formed in a perennial saline lake dur- to the Parachute Creek Member of the Green of the lake. Preservation of these minerals ing the early Eocene climatic optimum (EECO) River Formation in the Piceance Creek Basin. occurs below the thermocline (>∼25 m) in 52–50 Ma (Dyni 1981, 1996; Lowenstein and However, the situation for the Parachute Creek deeper portions of the basin. Our modeling Demicco, 2006; Smith et al., 2008; Tänavsuu- Member is complicated by the fact that two addresses both: (1) the restriction of evapo- Milkeviciene and Sarg, 2012; Jagniecki and evaporite minerals, nahcolite (NaHCO3) and ha- rites in the Piceance Creek Basin to the cen- Lowenstein, 2015; Jagniecki et al., 2015). The lite, occur as laminae in the center of the basin. ter of the basin without recourse to later dis- eroded remains of the lacustrine oil shale deposits Halite-nahcolite laminae and thin beds from the solution and (2) the variable mineralogy of of the Parachute Creek Member in the Piceance Parachute Creek Member have variable compo- the evaporites without recourse to changes Creek Basin cover more than 4500 km2, whereas sitions within and between layers (Fig. 2). There in lake water chemistry. T from 20 to 30 °C the evaporites in the north central portion of the are layers of pure nahcolite crystal mud; layers and pCO2 between 1800 and 2800 ppm are basin are confined to an area in the subsurface of of pure halite as millimeter-diameter cumulate reasonable estimates for the conditions in only 700 km2 (Fig. 1A). Dyni (1981) argued that cubes, surface-formed rafts together with cumu- the Piceance Creek Basin paleolake. Other the evaporites had originally extended over a con- late cubes, and larger, upward-oriented halite evaporites occur in the center of basins but siderably larger area and had been subsequently crystals that grew off the floor of the lake. Sig- do not extend out to the edges of the basin. leached by fresh groundwaters recharging the ba- nificantly, many layers contain nahcolite crystal Evaporite focusing caused by summer-win- sin (Fig. 1B), an explanation commonly invoked mud and halite cubes and rafts in varying pro- ter T changes in the solubility of the miner- for the Parachute Creek Member (cf. plates 1 and portions (see Jagniecki and Lowenstein, 2015, als should be considered for such deposits 2 in Johnson and Brownfield, 2015). for further details). The addition of nahcolite to and variable pCO2 within the evaporating Recent work on halite deposition in the mod- the evaporites in the Parachute Creek Member brines also needs to be considered if pCO2 ern Dead Sea by Sirota et al. (2016, 2017, 2018) adds two complexities. First, nahcolite and ha- sensitive minerals are found. has documented what those authors refer to as lite share a common component, Na, so if ei- “halite focusing.” The precipitation of halite is ther halite or nahcolite precipitates, the activity triggered by falling water temperatures (T) dur- (concentration) of Na in the brine will decrease ing the winter months when the entire Dead Sea and affect the saturation state of the other min- †[email protected]. becomes isothermal at ∼24 °C and supersatu- eral. Second, unlike halite, nahcolite solubility GSA Bulletin; Month/Month 2019; 0; p. 1–16; https://doi.org/10.1130/B35303.1; 11 figures; 1 table. For permission to copy, contact [email protected] 1 © 2019 Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B35303.1/4860087/b35303.pdf by SUNY Binghamton Science Library user on 04 November 2019 Demicco and Lowenstein is notably dependent on both T and partial pres- atmospheric pCO2 at >680 ppm depending on minerals) might be affected. Here we develop a sure of carbon dioxide (pCO2) in the brine. This the temperature of the brine. modified “evaporite focusing” model using nu- fact formed the basis of the “nahcolite proxy” The fact that the solubility of carbonate miner- merical simulations with the programs EQL/EVP (Eugster, 1966; Lowenstein and Demicco, als is dependent on the pCO2 in the brine as well (Risacher and Clement, 2001) and FREZCHEM 2006; Jagniecki et al., 2015). In these studies, as temperature also suggests that, if different parts (Marion, 2001). We show that, in the Piceance the finely laminated, finely crystalline nahcolite of a lake with very nearly the same brine chem- Creek Basin, natural variations of total dissolved in the Piceance Creek Basin was interpreted to istry come into equilibrium with different pCO2 CO2 in different parts of the ancient lake coupled have formed from a brine in equilibrium with or temperature, the solubility of those carbonate with summer-winter variations in temperature atmospheric pCO2 which constrained EECO minerals and halite (which shares Na with those of these different parts of the lake were capable A er te Riv Whi Yellow Creek o 30’ o Rangely 108 108 C0196 Meeker 40o C0034 C0008 C0179 C0116 C0158 Figure 1 (on this and following page). Eocene Green River For- C0306 mation deposits in the Piceance RioBlanco C0204 Creek Basin of northwestern Colorado, USA. (A) Map outlin- C0434 ing the extent of the Mahogany Bed, an organic-rich carbonate C0230 C0077 mudstone (“oil shale”) found at 39o30’ C0360 the top of the Parachute Creek Member of the Green River For- Rifle mation. The much smaller area C0098 outlined by the red line shows the extent of the evaporites in ints l Po the subsurface (modified from R vi oa An Brownfield et al., 2010a). Sur- n C Parachute face outcrops (labeled) and wells re e k (C0 numbers) comprise the er iv R C0183 cross section in B. o ad or ol C DeBeque 01020km Extent of theMahoganyOil ShaleBed Restored extent of Mahogany forarea calculation C0008 Core (or outcrop)locationand identifier Extent of evaporites in thesubsurface 2 Geological Society of America Bulletin, v. 130, no. XX/XX Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B35303.1/4860087/b35303.pdf by SUNY Binghamton Science Library user on 04 November 2019 The role of temperature and pCO2 on Eocene Green River saline deposits B 4 H H 8 0 10 RT 20 km CO03 9 CO00 SOUT NO 100 m s Mahogany Zone CO17 6 oint 7 0 6 0 8 4 6 BGroove lP 11 vi CO30 CO07 CO23 CO19 CO36 0 CO15 CO CO20 An CO434 reek AGroove wC llo CO183 Ye R-6 L-5 R-5 L-4 R-4 L-3 R-3 L-2 R-2 L-1 R-1 “Rich”oilshales zones Basin center evaporites “Lean” oilshalezones U.S. GeologicalSurvey dissolutionboundary Marginal lake deposits Figure 1 (Continued). (B) Roughly north to south cross section through the Parachute Creek Member (and laterally equivalent basin margin deposits) in the Piceance Creek Basin (modified from Johnson and Brownfield, 2015). Evaporites (halite and nahcolite) shown in yellow. Rich oil shale zones are commonly interpreted as deep, anoxic lake deposits. In the interpretation shown here, evaporites once extended an un- known distance out away from where they are now preserved and were removed later in the history of the basin by groundwater dissolution.