MIDDLE CAMBRIAN FOSSILS FOUND NEAR CLEAR LAKE, SPOKANE COUNTY, WASHINGTON

On a clear April day in 2002, Dr. Robert Derkey, a geologist with the Wahington Department of Natural Resources, and his assistant Mike Hamilton discovered trilobites and other fossils in a reddish shale on a hill near the water tank located at the Fairchild Air Force Base recreational site on the southeast shore of Clear Lake. The two men had been looking at the older basement rocks that were once the higher peaks, now called steptoes, which are surrounded by the Miocene Columbia River basalts. The discovery of fossils in the rocks near Clear Lake came as a surprise, since these rocks had previously been assigned to much older formations which predate multicellular life.

The fossil collection was turned over to Dr. Linda McCollum, a paleontologist in the Geology Department at Eastern Washington University who specializes in trilobite faunas. She instantly recognized the fauna as being diagnostic of the Middle Cambrian Glossopleura Biozone, and that this fauna had not been reported from the state of Washington previously. Dr. McCollum immediately put in for a permit to excavate on the Air Force site, and received a Northwest Institute grant to study the faunas. Permission from the Air Force was granted to pursue a study of the geology of the recreation site and to excavate for more of the fossil fauna.

In the fall of 2002, Dr. McCollum and her husband Mike began a study of the geology of the Air Force recreation site. The high hill with the water tank on the north side of the recreational site consisted of a medium bedded, pure quartz sandstone with ripple marks and dolomitic layers, which they assigned to the Lower Cambrian Addy Quartzite. This sandstone had been tilted to the east, with a dip of 40 degrees, and a stratigraphic thickness of just over 100 meters, with the base of the measured section on the shore of Clear Lake. At the north end of the exposure is a reclaimed borrow pit, the material being used as the subbase for the Clear Lake road, and a northwest-trending fault separates the Addy Quartzite from the shales and limestones on the south flank of the hill.

Roadcuts into the hillside on the Clear Lake road expose a light gray to pale orange phyllitic shale, just to the south of the blocky rubble of the Addy Quartzite. A one to two meter thick, medium dark gray limestone found in the shales produced inarticulate brachiopods and silicified trilobite fragments when dissolved in acid. However, no organic traces were found in the phyllitic shales themselves.

The fossiliferous red shale was not exposed in the roadcut, and the only specimens recovered were from the military recreation site. The red shale is soft and not very resistant to erosion, so much of it is covered by soil and flood deposits from the Ice Age Lake Missoula. It was the resculpting of the land surface by the military that had removed the soil, and the excavation for utilities and a sewage containment system that had temporarily exposed the fossiliferous shale. The material that was excavated was later used as road metal on the base, including the area near the water tank where the original discovery of fossils by Derkey and Hamilton was made. Therefore, the only fossils available to study at the surface were either transported from the original excavation sites or were found at the base of telephone poles or in rodent burrows.

It was decided that mechanical excavation would be needed to procure any in situ fossils, and Bob Derkey solicited the help of the US Soil Service and their jeep-mounted backhoe to dig a few exploratory holes at the site. This was done on October 10, 2002, but with limited results. The soil was found to be about a meter thick over the red shale in most places, and it was clear that a large excavation was not going to be compatable with the recreational function of this site. A small group of students under the direction of Dr. McCollum did make a hand-dug pit near the end of the picnic grounds that did yield the only in situ fossils.

Dr. McCollum studied the Clear Lake trilobites the following year, and found that there were at least four species, Glossopleura boccar, Zacanthoides libertyensis, Ehmaniella sp., and Amecephalus sp. That summer, Dr. McCollum began a study of the Glossopleura Biozone faunas found along the shore of Pend Orielle Lake near Lakeview, in order to make a comparison with the Clear Lake faunas. She found that there were few if any species in common between the two localities, suggesting that although the faunas were the same age, they probably represented different environments. In the fall, Mike Hamilton, Bob Derkey, and Linda McCollum presented the results of their research at the national meeting of the Geological Society of America, held in Seattle, Washington.

During the summer of 2004, Dr. McCollum and her husband Mike began a regional study of the bedrock exposed in the steptoes between Cheney and Reardan. Subesquently, they decided it was worthwhile to make a detailed geologic map of the steptoes within the vicinity of Medical, Clear, and Silver Lakes. In September, they discovered a second Middle Cambrian trilobite locality south of Salnave Road, about a mile south of the original discovery at Clear Lake. Several trilobites were found in a yellow shale along an old drainage ditch that was originally dug in 1904. Permission to do mechanical excavation on the new site was kindly granted by the property owners, Eugene and Lorene Robinette. Further excavations are planned at this new site for early next summer. In addition, Dr. McCollum has received a Northwest Institute grant aimed at preparing a detailed geologic map of the Medical Lake 7.5' quadrangle.

The basement geology within the steptoes under study can be divided into two separate geologic terranes, whose boundary is not exposed, but is presumed to be structural. The terrane boundary is fairly well constrained just to the west of West Medical Lake and the east flank of Fancher Butte, and the trend of the contact appears to be about 20-30 degrees west of due north. The terrane to the east of this structural line consists of Mesozoic hornblende-biotite granites and metamorphic rocks assigned to the Precambrian Wallace Formation of the Belt Supergroup. The western terrane consists of exposures of the Precambrian Deer Trail Group and overlying Lower Cambrian Addy Quartzite and Middle Cambrian phyllites, shales, and limestones.

The basement rocks in these two terranes are easily accessible. Exposures of the granitic and metamorphic eastern terrane can be seen in roadcuts along Interstate 90 just to the south of Granite Lake, along the Medical Lake-Four Lakes highway from Riddle Hill to just west of the causeway across the north end of Silver Lake, along the Medical Lake road from Bartholomew Road to Graham Road, and along the west side of Olson Hill, and the granites are extremely well exposed along the west side of Medical Lake and on the roads up to the Eastern Washington State Hospital.

Precambrian and Cambrian rocks of the western terrane are exposed in the higher buttes, including Magnison Butte, west of Waukon; Fancher Butte, west of Medical Lake; Booth Hill, west of Lakeland Village; along the Clear Lake road adjacent to the Fairchild AFB recreational facility; a small roadcut at the intersection of Salnave and Blue Heron roads; and a low-lying roadcut along Interstate 90 in the southbound lanes, about half a mile south of the Salnave Road interchange. At this last exposure, the whitish outcrop of the Addy Quartzite contrasts sharply with the normally dark-colored Columbia River Basalt. By this time next year, we hope to have posted a preliminary geologic map of the Medical Lake quadrangle, which will provide a graphical depiction of the extent of these rocks.

1/15/05