EARLY JURASSIC (PLIENSBACHIAN) LUPHERELLA ABUNDANCE ZONE, HURWAL FORMATION, SOUTHERN WALLOWA MOUNTAINS, NORTHEAST OREGON

An ~5 meter-thick sequence of light brownish-gray, very thinly-to thinly-bedded silty shale and siltstone located adjacent to Eagle Creek 1.6 km east of Tamarack Campground at an elevation of ~1317 m in the SW¼SW¼Sect. 27, R43E, T6S of the Bennett Creek 1:24,000 quadrangle in Baker County, Oregon, contains abundant external molds and casts of Lupherella boechiformis. The fossils occur in concentrations along bedding planes. Counts of the fossils on 8 slabs with a total surface area of 630 cm² (top and bottom) yielded a total of 135 Lupherella, one additional bivalve (unknown genus), and 3 cephalopods (unknown genera). This suggests an average of ~2140 Lupherella fossils per m² on bedding surfaces at the site. ~50% of the fossils are convex up and ~50% are concave down. 4 sets of butterfly valves were found. Detailed measurements of 19 left and 18 right Lupherella valves on the slabs showed that 53% of the left valves have posterior wings and 50% of the right valves have byssal ears. The maximum lengths of the right and left Lupherella valves from the Eagle Creek abundance zone are similar to those reported by Imlay (1967; USGS Prof. Paper 573-B), but the maximum heights, dorsal margin lengths, height to length ratios, and the number of concentric ribs are less. The size distributions of the Eagle Creek Lupherella suggest that the bivalves are young individuals that were buried by sediment before they could grow to larger sizes.