Expression of high-frequency cyclicity in different lithofacies of the Paradox Formation, Paradox Basin, Utah
Katherine Whidden, Russell Dubiel, Jennifer Eoff, Sarah Hawkins, Kristen Marra, Ofori Pearson, and Janet Pitman
The Pennsylvanian Paradox Basin is an asymmetric basin, with the deepest part to the northeast, adjacent to the ancestral Uncompahgre uplift. During the Middle Pennsylvanian (Desmoinesian), a thick succession of halite, anhydrite, and black shales accumulated in the basin trough and penesaline and normal-marine carbonates were deposited along the gently dipping south margin. Low-frequency and very high-frequency cyclicity have been described previously in the Paradox Formation. A detailed examination of the Shafer-1 core, drilled in the northwest part of the basin, reveals five different scales of nested cyclicity defined by changes in mineralogy and sediment fabric. The five cycles consist of: 1) widely recognized low-frequency alternation of salt and non-salt cycles on a scale of tens of meters; 2) intermediate-frequency cyclicity within the salt and non-salt intervals identified by mineralogic changes on the meter scale; 3) intermediate-frequency changes in mineralogy on a 10- to 20- cm scale; 4) high-frequency changes in mineralogy on the 2- to 5- cm scale; and 5) highest-frequency cycles defined by thin laminae of changing mineralogy on the mm scale. All scales of cyclicity are recognized throughout the core, although not in every interval. Four main lithologies were identified in the core: 1) halite/sylvite; 2) anhydrite; 3) dolomitic siltstone; and 4) gray to black dolomitic shale. Each lithology contains the 2-to 5- cm scale cycles, which are expressed in a variety of ways -- halite/sylvite intervals have thin drapes of anhydrite, anhydrite laminae are differentiated by color, dolomitic siltstone sequences display thin nodular halite laminae, and dolomitic shale beds have partings. During relative sea level fall in the basin, the expressed cyclicity in the Shafer-1 core represents periodic increases in meteoric water input (e.g., anhydrite drapes on halite crystals), in accordance with previous models. The same scale of cyclicity during relative sea level rise indicates brief intervals of increased restriction (e.g., halite nodules in siltstone). The evidence suggests that this 2- to 5- cm scale of cyclicity in all lithologies is due to rainfall variability associated with local to regional climatic patterns. Paleo-reconstructions place the Paradox Basin in the Northern Hemisphere, within 15 deg of the paleoequator, suggesting paleolatitude played a role in controlling the amount of rainfall.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012