RECOGNIZING EXTREME ENVIRONMENTS IN THE SEDIMENTARY RECORD

Western Laurentia during the Jurassic was a time of arid climate where large deserts existed and persisted. As Laurentia moved north during the late Jurassic, the climate changed rapidly, and the river systems of the Morrison Formation were deposited. The arid to semi-arid environments during this time produced both alkaline and acid saline lake systems that are preserved: Lake T’oo’dichi’ within the Brushy Basin Member of the Morrison, the Wanakah Formation in western Colorado, and a recently documented, acid saline lake system within the Carmel Formation near the intertonguing with the Page Sandstone that extends from at least near Boulder, UT to the border of Utah/Arizona. Paragenesis shows that early hyper-acidic suites of cements (jarosite and alunite) were precipitated syndepositionally within this unit. The presence of water-lain deposits interbedded with eolian facies suggest a marginal marine to coastal dune depositional environment with a fluctuating groundwater table that drove ferrolysis and evolved the groundwater to a low pH. The lack of carbonate buffer to this acidic fluid likely is a result of substantial detrital input from the south that limited the formation of carbonate minerals – minerals that are observed in places within the formation to the west. Additionally, microbial interaction was likely a large contributor to the evolution of this acidity, as this could speed ferrolysis and/or sulfide oxidation reactions.