ROCKS IN THE WESTERN PART OF THE AREA
GREENSCHIST AND PHYLLITE OF MOUNT SHUKSAN
Fine-grained thinly foliated greenschist and black phyllite occur in a north-northwest-trending belt along the west edge of the area. The rocks are particularly conspicuous near Bacon Peak and in the vicinity of Mount Shuksan.
The greenschist is a bright to dark olive-green fine-grained generally evenly foliated rock with fine-scale compositional layering. In the vicinity of Bacon Peak most of this rock is an actinolite-chlorite schist with small amounts of epidote. On Mount Shuksan, epidote is more abundant and epidote-actinolite and actinolite-epidote schists are common. Other rock types noted in the greenschist unit are tremolite schist, stilpnomelane-muscovite schist, and crossite schist. In some areas thin quartz laminae are abundant (fig. 7). We did not find any relict texture except in a narrow irregular layer on the northeast side of Bacon Peak, where we found a conglomerate made up of white quartz pebbles set in a greenschist matrix.
The phyllite, on the other hand, is considerably more contorted and crinkled, with widespread development of lineation. Lenses and stringers of crinkled quartz are abundant (fig. 8). Much of this rock is a sericite-chlorite phyllite, but it ranges in composition from a garnet-sericite phyllite to a phlogopite phyllite. These minerals, except for garnet, form in thin wavy layers in a matrix of tiny quartz grains. Although relict sedimentary features are lacking in most of the phyllite, a metamorphosed layer of sandstone in the phyllite east of Berdeen Lake still has clastic textures.
This metamorphic unit is older than other adjacent rock types (Chuckanut Formation, Hannegan Volcanics, and rocks of the Chilliwack composite batholith) as evidenced by the following: (1) The greenschist and phyllite are metamorphosed, but the younger adjacent rocks are not metamorphosed, (2) the greenschist and phyllite are unconformably overlain by the Chuckanut Formation on the north side of Bacon Peak (fig. 9) and the south side of Hagan Mountain and by the Hannegan Volcanics on the ridge east of Sulphide Creek (fig. 10), (3) fragments and pebbles of greenschist and phyllite are found in the basal conglomerate of Chuckanut Formation on the north side of Bacon Peak and in the basal part of the Hannegan Volcanics on the ridge east of Sulphide Creek, and (4) the greenschist and phyllite are intruded by rocks of the composite Chilliwack batholith in Sulphide basin (fig. 10), in the headwaters of the North Fork of the Nooksack River, and along Crystal Creek. Nowhere is the greenschist and phyllite of Mount Shuksan in contact with the metamorphic rocks of the eastern part of the study area, and their stratigraphic and structural relationship is not known.
The greenschist is distinct from the phyllite in part of the area, but in other parts the two are gradational and interlayered. Misch (1966, p. 109) divided the unit into the Shuksan Greenschist and the Darrington Phyllite. In the vicinity of Bacon Peak and Hagan Mountain, the two rock types are generally separated by faults that separate greenschist abruptly from phyllite. On Mount Shuksan, the two rock types, in addition to being faulted against one another, are interlayered and grade into one another. Misch (1966, p. 109) considered inliers of phyllite in the greenschist on the south slopes of Mount Shuksan to be the crests of tight anticlines. Many of these contacts, however, are high-angle faults. West of Mount Shuksan, just north of upper Curtis Glacier, greenschist layers occur in phyllite and phyllite layers in greenschist. Farther south, along the west branch of Sulphide Creek, rock to the west is clearly greenschist, and that to the east is clearly phyllite; between them a greenish-black rock with thin white quartz-rich layers appears to be gradational from phyllite to greenschist. Furthermore, on the ridge south of Sulphide Creek, greenschist grades into dark-gray phyllite irregularly across the strike of the foliation.
In most places the phyllite and the greenschist are of the same metamorphic grade, and the major difference between them is probably due to difference in original rock type. The greenschist may have formed from metamorphism of a sedimentary rock such as graywacke. An alternate origin is put forth by Misch (1966, p. 109), who believes that the greenschist was formed by metamorphism of basaltic rocks. The phyllite most likely was formed by metamorphism of a sandy argillite. In the vicinity of Mount Shuksan some of the irregular changes may be due to thermal metamorphism by small granitic stocks after both the greenschist and phyllite were regionally metamorphosed. The later metamorphism is suggested by the abundance of epidote in some of the gradational type rocks and by unalined crystals of amphibole minerals, such as radial needles of actinolite or tremolite that are super imposed on other alined minerals.
Age of the greenschist and phyllite of Mount Shuksan is not known. It is the oldest rock unit in the western part of the study area and is older than the Chuckanut Formation of Cretaceous and Paleocene age. Two exploratory potassium-argon dates on crossite schist are reported by Misch (1966, p. 109). An age of 259±8 m.y. (million years) on a whole rock sample and an age of 218±40 m.y. on crossite are interpreted by Misch to indicate metamorphism no later than Late Permian time.
The Chuckanut Formation is composed mainly of plagioclase arkose and has lesser amounts of dark-gray argillite and conglomerate. Though widespread in northwest Washington, the Chuckanut occurs only in scattered patches in the study area, and these are mainly near the northwest and southwest corners. In the southwest, around Hagan Mountain and Bacon Peak, the Chuckanut lies with angular unconformity on the greenschist and phyllite of Mount Shuksan (pl. 1; fig. 9) and is intruded by quartz diorite of the Chilliwack batholith. In the northwest, in the Silesia Creek area, it is overlain by the Hannegan Volcanics and intruded by the Chilliwack batholith (pl. 1), which has baked the rocks to a spotted hornfels. In the Silesia Creek area, dikes and sills of the Hannegan Volcanics intrude the upper part of the Chuckanut Formation. About halfway between these two principal areas, on the ridge between Crystal and Pass Creeks, a small roof pendant (not shown on pl. 1) of metamorphosed Chuckanut surrounded by batholith suggests that the Chuckanut was once continuous along the west side of the study area.
The name Chuckanut Formation, herein adopted, was applied by McLellan (1927, p. 136-137) to rocks along Chuckanut Drive on the shore of Bellingham Bay. The formation has been traced east by Weaver (1937, fig. 11) and by Misch (1966, p. 136) to the west side of the Cascade Mountains.
Plagioclase arkose (fig. 11), the dominant rock type in the Chuckanut Formation, is typically gray and weathers to a light gray, but locally has a brownish tint. It is mostly medium grained, generally moderately well sorted, and commonly shows graded bedding and crossbedding. Poorly preserved fragmental plant remains appear on some bedding planes. Locally, concentrated layers of quartz are present, but all the sandstone seen was arkosic. Microscopic examination of the arkose shows it to consist of plagioclase (30-75 percent), quartz (15-50 percent), cherty and slaty rock fragments, brown detrital biotite, chlorite, and traces of zircon. No detrital potassium feldspar was found. Quartz, feldspar, and rock fragments are mostly angular to subangular.
Argillite layers (fig. 11) ranging in thickness from a fraction of an inch to more than 100 feet make up less than 10 percent of the Chuckanut Formation. The argillite is very fine grained, generally thin bedded, and black, dark gray, or dark grayish brown. Locally, it appears to be carbonaceous.
Conglomerate layers that range in thickness from a few inches to as much as 15 feet compose about 1 percent of the formation. Maximum pebble or boulder diameter is generally less than 4 inches, and in many layers it is less than 1 inch. Most of the conglomerate layers contain a high proportion of distinctive angular finely laminated chert pebbles, and all the layers are well cemented. Graded bedding is widespread. Basal conglomerate of variable thickness, exposed on the north side of Bacon Peak, consists principally of pebbles of the underlying greenschist, of quartz, and of a few pebbles of Custer Gneiss in a matrix of small greensehist fragments. Thus, the conglomerate was largely derived from the underlying greenschist.
At its type locality near Samish Bay, the Chuckanut Formation is over 11,000 feet thick (Weaver, 1937, p. 85-88), but in the western part of the study area most of the unit has been removed by erosion, and only about 700 feet remains of a much greater original thickness.
Fragmental plant fossils seen in the Chuckanut Formation in the study area are not diagnostic. However, fossil plants collected near the Puget Sound and south of the town of Glacier are believed to date the rocks as Late Cretaceous and Paleocene (Weaver, 1937; Miller and Misch, 1963). Thus, the Chuckanut is probably correlative with the Swank Formation of the central Cascade Mountains and with part of the Nanaimo Group on Vancouver Island (Miller and Misch, 1963, p. 170).
The Hannegan Volcanics (name adopted in this report) are present in several patches that extend from near the center of the area north westerly for 17.5 miles to the Canadian border (pl. 1). The largest exposure has an outcrop area of 9.5 square miles and extends for about 6 miles from Hannegan Peak on the north to the headwater of Crystal Creek on the south. The unit was named by Misch (1952, p. 18) for the exposures in Hannegan Pass, which is in the northern part of the largest volcanic area; this location is the type area.
The upper part of the Hannegan Volcanics has been removed by erosion. The thickest remnant is along the headwaters of the Chilliwack River, where at least 2,500 feet of the unit is exposed.
The Hannegan Volcanies are composed mainly of volcanic breccias (fig. 12A) and tuffs ranging from rhyolite to andesite. Flows and (or) sills of rhyodacite, dacite, and andesite are common in many places. Dikes of similar rock cut the earlier volcanics in some places and are well exposed along the headwaters of the Chilliwack River A small metaporphyry stock about 1 mile long on the Canadian border has also been included in the Hannegan.
The volcanic breccias and tuffs are partly ash flow and partly air fall, as in the area east of Hannegan Pass. The pyroclastic rocks are white, light gray, dark gray, greenish gray, or brown. Rock fragments make up 10-60 percent of the unit. In most places the fragments, which are as much as 1.5 feet in diameter, are of light- and dark-gray volcanic rocks, but in some placessuch as the south side of Hannegan Peak, west of Sulphide Creek, and on Pioneer Rangethey are mainly of the surrounding country rock. Small fragments of plagioclase (by far the most common), quartz, hornblende, and chlorite also may be present in the tuffs and breccias, the matrix of which is ash. The matrix of a welded rhyolite tuff on Hannegan Peak is composed almost entirely of flattened glass shards. Later alteration has formed clays in the matrix and epidote along fractures.
The flows are light to dark gray and are most commonly rhyodacite or dacite. They consist of as much as 50 percent small phenocrysts, mostly white rectangular crystals of plagioclase, set in a glassy or finely crystalline matrix. Magnetite, hornblende, and biotite are found in most rocks; quartz and clinopyroxene are sparse. The matrix is made up mainly of tiny crystals of quartz and feldspar. Chlorite, calcite, epidote, and prehnite are common secondary minerals in some of these rocks. Stain tests on some of the rocks show that potassium feldspar also occurs in the groundmass.
The small metaporphyry stock on the Canadian border on the west side of Middle Peak (pl. 1) may have been a feeder for the extrusive volcanic rocks to the west. The lenticular shape of this stock and the presence of near-vertical flow banding suggest that the stock is intrusive into metamorphosed granodiorite of the Chilliwack batholith that is exposed to the east (pl. 1). The stock is metamorphosed by a younger quartz diorite pluton of the Chilliwack that lies along its north and west sides. The metaporphyry is a black to dark-gray rock with altered plagioclase and hornblende phenocrysts set in a fine-grained matrix of quartz, plagioclase, potassium feldspar, and muscovite. Accessory minerals are apatite, sphene, and magnetite. Hornblende phenocrysts are generally replaced by aggregates of chloritized biotite and quartz. In one place adjacent to the quartz diorite, intense metamorphism has developed andalusite in the metaporphyry.
The Hannegan Volcanics overlie the Custer Gneiss along Pioneer Ridge, the greenschist and phyllite of Mount Shuksan on the east side of Sulphide Creek (fig. 10) and west of Hannegan Pass, and the Chuckanut Formation on both sides of Silesia Creek, and hence they are younger than these rocks. Furthermore, cobbles and fragments of these three units are found in the basal part of the volcanics (fig. 12A). Locally, as in the area adjacent to Silesia Creek, though, the lower parts of the volcanic rocks are interbedded with arkoses and argillites that may be part of the Chuckanut Formation. The Hannegan Volcanics are in part older and in part younger than the rocks of the Chilliwack composite batholith. An older age is indicated on the east side of the headwaters of Silesia Creek (near Egg Lake) and at the head of Picket Creek (north of Pioneer Ridge) where the volcanic rocks are cut by quartz diorite dikes. A younger age is indicated south of Hannegan Pass, east of Sulphide Creek, and on Pioneer Ridge where volcanic breccia contains fragments of quartz diorite. The volcanic rocks also unconformably overlie granodiorite and quartz diorite on Ruth Mountain, on the ridge north and northeast of Copper Creek (a tributary of the Chilliwack River east of Hells Gorge), and near the upper end of Hells Gorge (fig. 12B).
The age of the Hannegan Volcanics is Tertiary. As the base of the volcanic rocks in the Silesia Creek area is interbedded with the upper part of the Chuckanut Formation, the lower part of the volcanics in this area is most likely Paleocene.
The Hannegan Volcanics are apparently about the same age as the Skagit Volcanics, for both are in part older and in part younger than the adjacent granitic rocks. We have discussed them separately because, although they contain the same types of rocks, the proportion of air-fall to ash-flow deposits differs greatly from one to the other. Further detailed work on these two groups of volcanic rocks is needed to determine their relationship.
Last Updated: 28-Mar-2006