When “evaporites” are not formed by evaporation: The role of temperature and pCO2 on saline deposits of the Eocene Green River Formation, Colorado, USA

2019 ◽  
Vol 132 (7-8) ◽  
pp. 1365-1380 ◽  
Author(s):  
Robert V. Demicco ◽  
Tim K. Lowenstein
Keyword(s):  
Carbon Dioxide ◽  
Dead Sea ◽  
Green River Formation ◽  
Green River ◽  
Restricted Area ◽  
Lake Water Chemistry ◽  
Summer Warming ◽  
Different Parts ◽  
Winter Cooling

Abstract Halite precipitates in the Dead Sea during winter but re-dissolves above the thermocline upon summer warming, “focusing” halite deposition below the thermocline (Sirota et al., 2016, 2017, 2018). Here we develop an “evaporite focusing” model for evaporites (nahcolite + halite) preserved in a restricted area of the Eocene Green River Formation in the Piceance Creek Basin of Colorado, USA. Nahcolite solubility is dependent on partial pressure of carbon dioxide (pCO2) as well as temperature (T), so these models covary with both T and pCO2. In the lake that filled the Piceance Creek Basin, halite, nahcolite or mixtures of both could have precipitated during winter cooling, depending on the CO2 content in different parts of the lake. Preservation of these minerals occurs below the thermocline (>∼25 m) in deeper portions of the basin. Our modeling addresses both: (1) the restriction of evaporites in the Piceance Creek Basin to the center of the basin without recourse to later dissolution and (2) the variable mineralogy of the evaporites without recourse to changes in lake water chemistry. T from 20 to 30 °C and pCO2 between 1800 and 2800 ppm are reasonable estimates for the conditions in the Piceance Creek Basin paleolake. Other evaporites occur in the center of basins but do not extend out to the edges of the basin. Evaporite focusing caused by summer-winter T changes in the solubility of the minerals should be considered for such deposits and variable pCO2 within the evaporating brines also needs to be considered if pCO2 sensitive minerals are found.

Stratigraphic Architecture of a Fluvial-lacustrine Basin-fill Succession at Desolation Canyon, Uinta Basin, Utah: Reference to Walthers’ Law and Implications for the Petroleum Industry

2016 ◽  
Vol 53 (1) ◽  
pp. 5-28 ◽  
Author(s):  
Grace Ford ◽  
David Pyles ◽  
Marieke Dechesne
Keyword(s):  
Cross Section ◽  
Oil Shale ◽  
Large Scale ◽  
Green River Formation ◽  
Sand Content ◽  
Fluvial System ◽  
Uinta Basin ◽  
Green River ◽  
Lacustrine Basin ◽  
Basin Fill

A continuous window into the fluvial-lacustrine basin-fill succession of the Uinta Basin is exposed along a 48-mile (77-kilometer) transect up the modern Green River from Three Fords to Sand Wash in Desolation Canyon, Utah. In ascending order the stratigraphic units are: 1) Flagstaff Limestone, 2) lower Wasatch member of the Wasatch Formation, 3) middle Wasatch member of the Wasatch Formation, 4) upper Wasatch member of the Wasatch Formation, 5) Uteland Butte member of the lower Green River Formation, 6) lower Green River Formation, 7) Renegade Tongue of the lower Green River Formation, 8) middle Green River Formation, and 9) the Mahogany oil shale zone marking the boundary between the middle and upper Green River Formations. This article uses regional field mapping, geologic maps, photographs, and descriptions of the stratigraphic unit including: 1) bounding surfaces, 2) key upward stratigraphic characteristics within the unit, and 3) longitudinal changes along the river transect. This information is used to create a north-south cross section through the basin-fill succession and a detailed geologic map of Desolation Canyon. The cross section documents stratigraphic relationships previously unreported and contrasts with earlier interpretations in two ways: 1) abrupt upward shifts in the stratigraphy documented herein, contrast with the gradual interfingering relationships proposed by Ryder et al., (1976) and Fouch et al., (1994), 2) we document fluvial deposits of the lower and middle Wasatch to be distinct and more widespread than previously recognized. In addition, we document that the Uteland Butte member of the lower Green River Formation was deposited in a lacustrine environment in Desolation Canyon. Two large-scale (member-scale) upward patterns are noted: Waltherian, and non-Waltherian. The upward successions in Waltherian progressions record progradation or retrogradation of a linked fluvial-lacustrine system across the area; whereas the upward successions in non-Waltherian progressions record large-scale changes in the depositional system that are not related to progradation or retrogradation of the ancient lacustrine shoreline. Four Waltherian progressions are noted: 1) the Flagstaff Limestone to lower Wasatch Formation member records the upward transition from lacustrine to fluvial—or shallowing-upward succession; 2) the upper Wasatch to Uteland Butte records the upward transition from fluvial to lacustrine—or a deepening upward succession; 3) the Uteland Butte to Renegade Tongue records the upward transition from lacustrine to fluvial—a shallowing-upward succession; and 4) the Renegade Tongue to Mahogany oil shale interval records the upward transition from fluvial to lacustrine—a deepening upward succession. The two non-Waltherian progressions in the study area are: 1) the lower to middle Wasatch, which records the abrupt shift from low to high net-sand content fluvial system, and 2) the middle to upper Wasatch, which records the abrupt shift from high to intermediate net-sand content fluvial system.

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