A Paleocurrent Analysis of a Portion of the Chuckanut Depositional Basin near Bellingham, Washington

The Chuckanut Formation is an upper Cretaceous to lower Tertiary sequence of conglomerate, sandstone, siltstone, and some coal deposited by streams near Bellingham, in western Whatcom County, Washington. Paleocurrent analysis of an area in the northwestern portion of the Chuckanut Formation shows two stream directions: to the south or southeast for the lower and middle sections and to the west or northwest for the upper section of the unit. Distinct differences in the lithology of clasts in the conglomerates support this conclusion. Angular phyllitic and quartz clasts in the lower and middle sections probably were derived locally from the Barrington Phyllite, which underlies the Chuckanut Formation. No outcrops of the Barrington Phyllite north of the study area are known; however a westward extension of the Boulder Creek Fault may have been active during Chuckanut time, forming a structural high to the north that could have introduced sediment into the Chuckanut depositional basin. Pink granitic clasts, present only in the upper portion of the Chuckanut Formation could have been derived from the Okanogan Highlands to the east. Red chert in the upper portion of the Chuckanut Formation could have been derived from petrographically similar chert now exposed to the west in the San Juan and Gulf Islands; no other source of red chert is known. The lack of volcanic clasts throughout the Chuckanut Formation implies that the formation was deposited prior to the onset of volcanism in the Tertiary magmatic arc to the east. Data collected during the course of this study suggest the Chuckanut Formation has no association with either the Puget Group to the south or the Nanaimo Group to the northwest, but was instead a structurally bounded basin of accumulation. The Swauk Formation to the southeast may have been closely associated with the Chuckanut Formation prior to the onset of movement along the Straight Creek Fault.