TRILOBITE FAUNA CORRECTS AGE ASSIGNMENT FOR LOWER METALINE TO ALBERTELLA BIOZONE

Albertella Biozone trilobites are present in about 130 m of thin- to medium-bedded black bioclastic limestone and fine- to medium-grained gray and black limestone, interbedded with lamina to thin-bedded gray to black shale in the lower member of the Metaline Formation of northeastern Washington. The fauna was assumed to be Bathyuriscus Elrathina/ Bolaspidella Biozone, due to long ranging Middle Cambrian trilobites being found earlier. Distortion from deformation may have contributed to diagnosis problems. The study area is in the Lafarge quarries adjacent to the town of Metaline Falls, Washington.

The lower thin-bedded member of the Metaline Formation is about 325 meters thick in the Metaline Falls area. Fair to well preserved trilobites occur in the top 130 meters of this member. Trilobites discovered in earlier work were from a six- to ten-meter thick shaly unit near the base of the fossiliferous section. Poorly preserved fragmental trilobites have been found in phyllitic shales near the bottom of the lower member, but require additional collecting to define a zonal age determination.

Some deformation has occurred in the Metaline Formation. Many multiple specimen slabs were collected to compare features, also some were oriented and the strain measured to get a better understanding of the ratio changes on the trilobites. This helped identify Poliella.

Recent discoveries of genera including Fieldaspis, Paralbertella, Plagiura, Poliella, Ptarmigania, Ptarmiganoides, Vanuxemella, and Yohoaspis are found with genera that are not restricted to the Albertella Biozone. Over 1500 specimens were collected, comparisons were made, and Middle Cambrian specialists consulted to make these conclusions. To date: Fieldaspis and Poliella are only found in the bottom thirty meters of the fossiliferous section, Paralbertella is found scattered through the whole section, and Ptarmiganoides dominates the top seventy meters.

These findings revise the age of the lower member of the Metaline Formation to early Middle Cambrian from upper Middle Cambrian. This section time correlates to most of the Cathedral, lower Gordon Shale, Twin Knobs, Naomi Peak, and other early Middle Cambrian Cordilleran lithologies.