Stromatolitic Weather Vanes in Western Pangea; Current Oriented Stromatolites in the Pennsylvanian Hermosa Group, Western Shelf of the Paradox Basin, SE Utah

Current-aligned digitate stromatolites provide indicators of Pennsylvanian (Desmoinesian) shallow marine, highstand circulation patterns on the western shelf of the Paradox Basin (southeastern Utah) during the deposition of the Akah interval of the Hermosa Group. The 2.5-3.5m meter-thick bed of club-shaped digitate stromatolites can be traced for 25km along the San Juan River in SE Utah, which provides a dip -oblique 18 km view of the shelf. This bed occurs in a fourth to fifth order sequence (Sequence 2.10, Giannny, 1995) within the third order lower Akah sequence. During lowstands, the Akah interval contains the most wide spread evaporites in the basin, however these stromatolite beds and other shelf carbonates are highstand deposits. Composite stromatolitic heads are formed by 8-10 individual vase-shaped , bifurcating columns or “digits” which nucleate from a shared base. Within the stromatolitic head, individual fingers branch from the nucleation center for the first 30 cm and become wider. From 30 cm to 63 cm, there is a clear shift from branching fingers to a zone with interlaminated grey sediment. At the top of each head, some fingers become lobate, to form laterally linked hemispheroids, while others continue bifurcating upwards. Most vase-shaped heads grow asymmetrically and are elongate in a preferred growth orientation to the southeast, with a range of azimuths from 77⁰ to 310

⁰. The orientation of well-exposed, digitate stromatolite heads on bedding planes were measured at four sites along a 3km section river. Sample size ranged from 31 to 158 at each site, with 309 samples total. In each site more than 60% of the measured samples have a clear southeast orientation. We have not found independent current indicators such as bedforms in the same bed or in surrounding strata.

These current-oriented stromatolites may constrain Pennsylvanian highstand marine circulation patterns on the western margin of Pangaea. Here we assume these stromatolites grew into the current, as they do in the modern. If their morphology was controlled by tides, then they indicate a more complex shelf than a simple northeast-southwest shoreline. If their growth was influenced by wind-driven shallow oceanic circulation, then equatorial trade wind-driven waves and longshore current from the SE would be plausible. The orientation data are not consistent with currents flowing from the northwest, as would be expected from a monsoon over western Pangaea.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California