Paper No. 4
Presentation Time: 8:00 AM-5:00 PM
REVISED STRUCTURE AND STRATIGRAPHY OF THE CHILLIWACK GROUP ON BLACK MOUNTAIN, WASHINGTON STATE
The stratigraphy of the Upper Paleozoic Chilliwack Group is best exposed on Black Mountain, a 25 km2 area of rugged topography approximately 40 km northeast of Bellingham, Washington State. New field work indicates that the five currently named informal units are not in stratigraphic order here and that a previously unrecognized unit should be added to the Chilliwack Group on Black Mountain. These units, in structural order, are (1) the lowermost Silver Lake clastic unit, (2) the newly recognized Blue Creek unit, (3) the Red Mountain limestone unit, (4) the Old Goat clastic unit, conformable with (5) the Black Mountain limestone unit, and (6) the uppermost volcaniclastic unit, called the Liumchen nappe. Contacts between these units, where visible, are low angle faults. An ammonite fragment and fossil plant material in the Silver Lake unit indicate an age younger than mid-Permian. Middle Devonian corals were found in a bioherm intercalated within the Liumchen nappe. Fusulinids in the Blue Creek unit range from Fusulinella through Parafusulina indicating a mid-Pennsylvanian through lower Permian age. The Black Mountain unit contains younger fusulinids, such as Parafusulinella and Parafusulina. The Red Mountain limestone contains endothyroid foraminifers in the lower beds and early Pennsylvanian Millerella in the uppermost. Foraminiferal succession within the Blue Creek, Red Mountain unit, and Black Mountain units indicates that each is right side up. These units appear to be fault slices imbricated between the youngest but lowest Silver Lake unit, and the oldest but uppermost Liumchen nappe.