PRESERVATION OF A VEGETATED LANDSCAPE IN THE EARLIEST EOCENE, POLECAT BENCH, BIGHORN BASIN, WYOMING

The Top Red A marker bed of Gingerich on Polecat Bench in the northern Bighorn Basin, Wyoming contains areas extending from the overlying unit that are interpreted as indicative of the presence of root masses of small shrub-like vegetation that once populated this Eocene paleosurface within the Willwood Formation for reasons outlined below. The paleosol profile consists of 21 cm of moderate gray (5B 6/2 to 5GY 5/2) silty mudstone overlying the 91 cm of mudstone that comprises Top Red A. The gray interval is interpreted as an A horizon, while the Top Red A bed is interpreted as a B horizon.

The areas of gray extending down into the B horizon are spherical to elongate and have roughly regular spacing. These gray areas are 15-40 cm wide and 10-70 cm long, spaced from 0.3 to 1.5 m apart (averaging 0.75 m), and typically extend <50 cm into the B horizon. However, a few extend to nearly the base of the B horizon. Irregularities in apparent size and shape may be due to either original irregularities in shape of the gray areas or, more commonly, exposure of only portions of the gray areas during the trenching process or slope erosion.

The texture, including slickensides and redoximorphic concentrations, of the gray areas more closely resembles the rest of the surrounding B horizon than the A horizon indicating that these are likely redoximorphic features within the B horizon and not irregularities in the A/B contact, such as the expression of gilgai surfaces, etc. Organic and powdery carbonate root (~0.5-1 mm) and rootlet (<0.5 mm) traces are common within the gray areas. Thin (5 mm to <1 mm) gray depletion zones radiate from the gray areas becoming smaller and less common distally. These thin depletion zones are interpreted as rhizoliths. Rhizoliths are present only sporadically away from the gray areas. These factors lead to the interpretation of the gray areas as reduction zones due to the presence of dense roots and rootlets. Ichnofossils interpreted as the burrows of soil-dwelling invertebrates within and adjacent to the gray areas support this interpretation. The relatively regular distribution and size suggests that the large gray depletion zones represent the immediate areas surrounding the root masses of small shrub-type vegetation.