Late Carboniferous (Moscovian) Microbial “Shrubbery” Intricate Digitate Stromatolites of the Baker Creek Interval, Western Paradox Basin, SE Utah

Unique microbial limestones with intricate shrub-like heads composed of tens of individual centimeter-diameter digitate stromatolites occur in one horizon within the lower Desmoinesian Barker Creek interval of the Hermosa Group on the western shelf of the Paradox basin, southeastern Utah. These deposits occur within in Sequence 2.5 of Gianniny (1995) in the lower portion of the third order Barker Creek sequence (Ritter, 2002). The deposits are found in the canyons of the San Juan River in Raplee anticline and in the Goosenecks area, where they can be traced for 26 km across the shelf.

It appears that dip position and limited accommodation space during mid-Barker Creek deposition controls the lateral variation within the stromatolitic strata. From the western and up-dip portion of the basin, the 10 cm deposit thickens to 70 cm in the eastern down-dip shelf position, with three distinct layers of stromatolites interbedded with transported stromatolite fragments. In six locations in the up-dip portion of the shelf 12 km to the southwest, the 10 cm microbial deposit contains rare vase-shaped heads 1-10 cm thick with digits as thin as 0.4-1.5 cm but is dominated by elongate transported stromatolite fragments. Down dip on the shelf in the Raplee anticline exposure, the 70 cm thick deposit has numerous complex and well developed branching stromatolite heads with centimeter diameter “fingers” which produce composite shrub-shaped buildups. The composite heads have widths of 10-30 cm and heights ranging from 10-40 cm. From a side view the stromatiolites are vase-shaped and grew on top of one another creating an undulating topography on the sea floor. Between, and interbeded with the heads, are channels filled with transported stromatolite fragments and pelloidal packstones. Bedding plane views reveal both radial and directional growth in the heads, but no dominant growth direction. Bedding plane views also show that some heads maintain 0.5-1.5 cm diameter digits to the top, while others have arcuate blade-shaped digits that form discontinuous concentric layers of dark grey to black stromatolites in a light gray matrix. Below the microbial layer in the Barker Creek Interval are centimeter scale lamiae with bi-directional ripples in a peloidal grainstone attributed to tides. It is overlain by an evaporite-bearing sahbka dolomite. Analogous, but larger, shrub-like digitate stromatolites occur in freshwater lakes in British Columbia (Laval et al. 2000).

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