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Interpreting small-scale cyclicity of halite-anhydrite precipitation within Cycle 13, Shafer No. 1 core, Paradox Basin, Utah

Kristen Marra, Russell Dubiel, Janet Pitman, Ofori Pearson, Katherine Whidden, Jennifer Eoff, and Sarah Hawkins

More than 30 salt cycles of the Paradox Formation (Hermosa Group) have been recognized within the Paradox Basin, a northwest-southeast trending asymmetric Pennsylvanian basin formed adjacent to the Uncompahgre Uplift. The cycles have previously been interpreted to reflect climatically induced marine incursions, which deposited alternating sequences comprised of halite, anhydrite, dolomite, and black shales within a restricted evaporite basin. Within individual salt sequences, rhythmically bedded halite-anhydrite couplets occur as anhedral to euhedral halite and anhydrite as 'snow-on-the-roof' texture. In order to evaluate controls on high-frequency cyclicity within salt sequences, prominent couplets were measured and counted within the 54-m thick halite sequence of Cycle 13. Thickness of halite-anhydrite couplets ranges from 1 to 18 cm, and couplets thicken within the middle 25 m of the halite section. Sylvite occurs as discrete grains, often aggregated in bands within the upper 11 m of salt, and as sylvinite, which is prominent within the lower 38 m of the salt sequence. These couplets represent periodic evaporite precipitation possibly in response to seasonal temperature variations, periodic inflow of meteoric waters during storm or monsoonal cycles, and/or the influence of spring and neap tides. Sylvite has red coloration due to hematite inclusions and its variable occurrence may reflect patterns of extreme aridity and iron dispersal associated with the proximity of the Paradox Basin to the paleo-equator during the Pennsylvanian. Detailed analysis of millimeter scale halite-anhydrite laminations are being conducted to further constrain climatic factors influencing small-scale cyclic halite-anhydrite precipitation within the Pennsylvanian salt cycles.

 

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012