OLD LAKE, NEW INTERPRETATION: A RECORD OF A POTENTIAL JURASSIC COASTAL ACID SALINE LAKE SYSTEM

The jarosite- and alunite-bearing caprock at Mollies Nipple in southern Utah, USA was formerly interpreted as the uppermost Navajo Sandstone; however, new research shows that this unit is unconformable with the Navajo Sandstone. Moreover, jarosite and alunite cements show evidence of early diagenetic/ syndepositional precipitation, which raises questions of why this cement was precipitated and why it is still there. Jarosite and alunite are abundant on the surface of Mars and laboratory stability estimates have been used to interpret these depositional and diagenetic settings as hyperacidic and/or dry. This research suggests that laboratory experiments underestimate the stability of jarosite and alunite in natural settings. New field research provides a thorough investigation of lithofacies to test earlier hypotheses regarding the precipitation of early jarosite and alunite cements within pore waters beneath an acid saline lake and provides new interpretations of the history of precipitation and dissolution of these diagenetic minerals.