VARIANCE IN DOLOMITE MINERALOGY WITHIN THE PERMIAN-TRIASSIC LYKINS FORMATION, NORTHERN COLORADO: IMPLICATIONS FOR CONSTRAINING REGIONAL DEPOSITIONAL PROCESSES THROUGHOUT THE LATE PERMIAN

The Middle Permian to Early Triassic Lykins Formation outcrops continuously throughout the Colorado Front Range and is an important indicator of the Colorado paleoenviroment throughout the Permian-Triassic transition. The Lykins Formation consists of siltstone and shale redbeds which bound 6 discrete dolostone and gypsum members, suggesting deposition occurred through multiple regimes. Although, the specifics of each depositional regime remain enigmatic. The Late Permian Forelle Member of the Lykins Formation is one such dolostone member which represents the product of a largely unconstrained depositional regime. Preliminary work uses variances in dolomite mineralogy within an outcrop of the Forelle Member in Golden, Colorado to constrain the regime responsible for deposition of the Forelle Member, with implications for other members of the Lykins Formation. The Forelle Member consists of 5 dolomite beds and 1 siltstone bed; a paleokarst surface exists on the uppermost dolomite bed. Dolomite within the lower portion of the Forelle is stromatolitic. Energy dispersive X-ray spectroscopy reveals dolomite within the Forelle Member is of variable composition with a bulk chemical composition ranging from Ca_0.96Mg_1.03(CO₃)₂ to Ca_0.79Mg_1.20(CO₃)₂; dolomite composition varies inconsistently throughout each bed. Powder X-ray diffraction indicates dolomite cation ordering increases with ascension throughout each bed.

The presence of near-stoichiometric, stromatolitic dolomite suggests the Forelle Member was deposited within a shallow hypersaline basin. Dolomite matured and ordered at rates correlating to the temperature and salinity of the surrounding hypersaline fluid; high-frequency alterations to accommodation space resulted in the development of hypersaline fluids with varying ionic concentrations and temperatures. Therefore, increases in accommodation space resulted in the formation of relatively poorly ordered dolomite, while decreases in accommodation space resulted in the formation of relatively well-ordered dolomite. Such data suggests each dolomite bed represents one parasequence within a larger depositional sequence, as represented by the entirety of the Forelle Member. The presence of a paleokarst surface on the uppermost dolomite bed further supports such a hypothesis.