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

Candice Burnette, James Dittrick, Rustin Freeman, Kelsey Swanson, and

Jay Van Tassell

Science Department, Eastern Oregon University,

One University Boulevard, La Grande, OR 97850; jvantass@eou.edu

ABSTRACT

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 complete Lupherella valves, one additional bivalve (unknown genus), and 3 cephalopods (unknown genera). This suggests an average of ~2140 complete Lupherella valves 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.

INTRODUCTION

The bivalve Lupherella boechiformis is found in the Stikine, Wallowa, Izee, and Northern Sierra terranes of the western United States.  It is an important part of the paleobiogeographic evidence suggesting that Stikinia was in the tropics well south of its present position during the late Pliensbachian (Aberhan, 1998, 1999; Figure 1).  Two species of Lupherella, one of which is comparable to Lupherella boechiformis, have been found in the upper Sinemurian and lower Pliensbachian strata of eastern Guangdong, China.  This discovery suggests that the planktotrophic larvae of Lupherella may have dispersed eastward across the Pacific during the global eustatic sea-level rise during the Sinemurian and Pliensbachian (Yin and others, 2004).

Figure 1.  Paleogeographic reconstruction showing the locations of Wrangellia (W) and Stikinia (S) during the Pliensbachian.  Note the eastward flowing currents that may have carried Lupherella larvae from China to Stikinia during this time of high eustatic sea level.  Modified from Aberhan (1998).

Imlay (1967) named the genus Lupherella based on his examination of specimens from partially metamorphosed siltstone of from the Sailor Creek Formation of east-central California, mudstone from the Hurwal formation of the northern Wallowa Mountains of northeastern Oregon, and calcareous concretions and mudstones from the Nicely Formation of east-central Oregon.

Kingdom      Metazoa Heckel 1874

Phylum         Mollusca Linneaus 1758

Class                       Bivalvia Linnaeus 1758

                       Subclass      Autolamellibranchiata Grabben 1894

Order                       Pterioda Newell 1965

Genus                      Lupherella Imlay 1967

Species       boechiformis (Hyatt)

 The Lupherella specimens from the Wallowa Mountains described by Imlay (1967) came from three localities on the northeast side of Sheep Ridge in Wallowa County, Oregon (Figure 2, 3).

Figure 2.  Lupherella localities in the United States (Imlay, 1967, and this study).

Figure 3.  Western U.S. terranes (from Dorsey, http://www.uoregon.edu/~rdorsey/Blue Mountains.html.).  The Wallowa Mountains Lupherella specimens come from the Wallowa terrane, which is inferred to be an equivalent of the Stikine terrane.

DEPOSITIONAL SETTING

Lupherella boechiformis is found in the Wallowa Mountains in the ~450 m thick late Triassic to early Jurassic Hurwal Formation, which was deposited in a collisional basin between the Wallowa and Olds Ferry volcanic arcs (Figure 4).  The occurrence of the bivalve Halobia, the belemnite Aulacoceras, and the trace fossil Chondrites in the lower part of the formation near Excelsior Gulch along Eagle Creek suggests that the carbonate mudstones of the Hurwal Formation were derived from local volcanic activity and carried downslope by turbidity currents into a shallow subsiding basin (Follo, 1994.)  According to Imlay (1967, p. B9), "Lupherella boechiformis (Hyatt), on the basis of lithologic facies and faunal association, probably lived in a fairly shallow sea in which organic mud was accumulating and which locally in Oregon contained a considerable variety of mud-dwelling, attached, and free-swimming organisms." 

Figure 4.  Eastern Oregon Mesozoic terranes, stratigraphy, and depositional setting during the early Jurassic.  Note that stratigraphic position of the Hurwal Formation (modified from Dorsey, http://www.uoregon.edu/rdorsey/Blue Mountains.html)

SITE DESCRIPTION

The Lupherella abundance zone along Eagle Creek was discovered by Jay Van Tassell in September 1998.  It is located at latitude 45°00'30.47"N, longitude 117°26'18.4"W in the SW¼, SW¼, Sect.27, R43E, T6S on the Bennett Creek 1:24,000 quadrangle in Baker County, Oregon, 1.6 km southeast of Tamarack Campground on the northeast side of USFS Road 77 at an elevation of ~1317 meters (Figure 5).

Figure 5.  Location of the Lupherella abundance zone along Eagle Creek.  Air photos are from Google Earth.

10 bivalve specimens and 1 specimen of an unknown cephalopod were collected at the site in October 2004 by George Stanley and Angela Sanderson of the University of Montana, Missoula.  They identified the bivalves as Lupherella boechiformis.

The site consists of an ~5 meter-thick sequence of light brownish-gray, very thinly-to thinly-bedded silty shales and siltstones which contain abundant external molds and casts of Lupherella boechiformi and rare cephalopods (Figure 6 ).  The fossils occur in concentrations along bedding planes.  Most of the slabs lack sedimentary structures.  A few have parallel laminations and holes that once contained pyrite cubes.

Figure 6.  Photos of Eastern Oregon Univesity students collecting fossils at the Lupherella abundance zone along Eagle Creek.

METHODS

The Eagle Creek Lupherella site has been sampled many times over the past ten years by Eastern Oregon University students.  8 slabs were chosen for detailed analysis from this collection.

First we counted the number of bivalves and cephalopods on the top and bottom of each slab using a hand lens and a Leica Zoom 2000 binocular microscope.  Only complete Lupherella valves were counted; if all the partial valves were included, the numbers would have been about 3 times higher.  Then we measured the total area of the top and bottom of each slab in using a grid divided into 1 cm2 units.  The data from all the slabs was combined and used to calculate the total number of complete Lupherella valves per unit area.

Next we chose the best preserved valves on the slabs (18 left valves and 19 right valves) and measured their lengths, heights, and lengths of their dorsal margins using digital calipers.  This data was used to calculate the ratio of the height to length.  We also counted the number of concentric ribs and identified the percentage of left valves with posterior wings and the percentage of right valves with byssal ears.  Because of the orientation of the valves parallel to the bedding planes, it was not possible to measure the widths of the Lupherella fossils.  The number of radial ribs present also proved difficult to measure due to the poor preservation of the outer margins of many of the fossils.

EAGLE CREEK LUPHERELLA

Almost every rock slab at the Lupherella abundance zone site on Eagle Creek contains abundant fossils.  The 8 slabs that we examined in detail had a total surface area (top and bottom) of 630 cm2 and a total of 135 complete Lupherella valves, plus one additional bivalve of an unknown genus and 3 cephalopods.  The number of Lupherella valves would have tripled if we had chosen to include incomplete valves in our counts.  This data suggests an average of ~2140 complete Lupherella valves per m2 on bedding surfaces at the site.  Approximately 50% of the fossils are convex up and ~50% are concave down.  Four sets of butterfly valves (left and right valves that are still connected) were found (Figures 7-10).

Figure 7.  Right Lupherella valves, including a cast (left) and an external mold (right).  Scale bars are 1 cm in length.

Figure 8.  Cast of left Lupherella valve.  Scale bar is 1 cm long.

Figure 9.  Possible Lupherella butterfly valve.  According to Imlay (1967) butterfly valves are rarely found at other Lupherella localities.  Scale bar is 1 cm long.

Figure 10.  Cephalopods from the Eagle Creek Lupherella site.  Further work is needed to identify which genera these belong to.  Scale bars are 1 cm long.

MEASUREMENTS

Measurements of the best 19 left and 18 right Lupherella valves on the 8 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) but the maximum heights, dorsal margin lengths, height to length ratios, and the number of concentric ribs are less (Table 1).

Table 1.  Comparison of the Eagle Creek Lupherella and samples described by Imlay (1967).

The left Lupherella valves from 8 slabs from the Eagle Creek site have an average length of 8.15 mm, an average width of 6.44 mm, and an average dorsal margin width of 2.21 mm.  The right Lupherella valves averaged 8.68 mm in length, 6.83 mm in width, and 2.41 mm in dorsal margin length.  The most common lengths of both left and right valves falls in the range of 6-8 mm and the most common width falls in the range of 4-6 mm.  The most common dorsal margin lengths are 1-2 mm for left valves, and 2-3 mm for right valves.  The size distributions of the Eagle Creek Lupherella shell dimensions suggest that most of the bivalves are young individuals (less than a year old) that were buried by sediment before they could grow to larger sizes (Figure 11).

Figure 11.  Lengths, widths, and dorsal margin lengths of the best Lupherella specimens on the 8 slabs from the Eagle Creek site that were chosen for detailed analysis.

The lengths and heights of the Lupherella valves are clearly related, with a linear correlation coefficient of 0.84.  The valves with lengths greater than 12 mm show more variation in height than valves that have lengths less than 12 mm.

Figure 12.  The relationship between length and height of the 37 Lupherella valves selected for detailed measurements.  The 95% confidence range on the linear correlation coefficient is 0.72-0.94.

DISCUSSION AND CONCLUSIONS

The Lupherella fossils at the Eagle Creek site are mostly young individuals that are probably less than one year in age.  The Eagle Creek Lupherella lived in a shallow marine environment where rapid accumulation of organic mud provided an abundant food source.  The thinness of the shells of Lupherella described by Imlay (1967) and the overall shape of Lupherella valves suggest that these bivalves were shallow burrowers.  It is possible that the paucity of sedimentary structures in the Main Eagle Creek siltstones and shales is due to bioturbation by Lupherella. 

Lupherella fossils found in the Nicely Formation of central Oregon are associated with several genera of bivalves (Ostrea, Entolium, Camptonectes, Oxytoma, Modiolus, and Pinna).  We have not found any of the rhynconellid brachiopods, belemnites, fish remains, and small gastropods that are associated with Lupherella in the Nicely Formation.  We have found only one specimen on the 8 slabs from the Eagle Creek site that could be a genus different from Lupherella.  The Lupherella-bearing Hurwal Formation exposed in Sheep Creek on the north side of the Wallowas contains the ammonites Arieticeras, Canavaria, Harpoceras, and Prodactylioceras (Imlay, 1968).  It is possible that the cephalopods found at the Eagle Creek locality belong to some of the same genera.  We have not found any of the rhynconellid brachiopods, belemnites, fish remains, and small gastropods that are associated with Lupherella in the Nicely Formation.

The low diversity and high abundance of the Eagle Creek Lupherella fossils suggest that Lupherella was an opportunistic species that rapidly colonized hostile environments characterized by high sediment.  The presence of butterfly valves suggests that these fossils were not transported long distances before they were buried by sediment.

More work needs to be done to identify the cephalopods present in the Lupherella abundance zone at the Eagle Creek site and to identify similar sites elsewhere in the southern Wallowa Mountains.

REFERENCES CITED

Aberhan, M., 1998, Paleobiogeographic patterns of Pectinoid bivalves and the Early Jurassic tectonic evolution of western Canadian terranes:  Palaios, v. 13, p. 129-148.

Aberhan, M., 1999, Terrane history of the Canadian Cordillera: estimating amounts of latitudinal displacement and rotation of Wrangellia and Stikinia:  Geological Magazine, v. 136, no. 5, p. 481-492.

Follo, M.F., 1994, Sedimentology and stratigraphy of the Martin Bridge Limestone and Hurwal Formation (upper Triassic to lower Jurassic) from the Wallowa terrane, Oregon:  U.S. Geological Survey Professional Paper 1439, p. 1-27.

Imlay, R.W., 1967, The Mesozoic pelecypods Otapiria Marwick and Lupherella Imlay, new genus, in the United States:  U.S. Geological Survey Professional Paper 573-B, p. B1-B11.

Imlay, R.W., 1968, Lower Jurassic (Pliensbachian and Toarcian) ammonites from eastern Oregon and California:  U.S. Geological Survey Professional Paper 593-C, p. C1-C51.

Yin, J., Yao, H., and Sha, J., 2004, First record of the Early Jurassic Lupherella fauna (Bivalvia) in eastern Guangdong, southeast China:  New Zealand Journal of Geology & Geophysics, v. 47, p. 321-326.