ROCKS WEST OF THE STRAIGHT CREEK FAULT

West of the Straight Creek Fault, a belt of serpentinite melange, the Helena-Haystack melange, and a coincident zone of major disruption and faulting, the Darrington-Devils Mountain Fault Zone, separates two distinct assemblages, or superterranes: the Northwest Cascade System and the western and eastern mélange belts (Tabor and others, 1989; Tabor, 1994). Major supracrustal units in the Sauk River quadrangle west of the Straight Creek Fault have been referred to the Northwest Cascades System by Brown (1987) and Brown and others (1987), following the structural framework established by Misch (1966). The Northwest Cascades System is characterized by rocks of oceanic and arc origin that have been metamorphosed at low temperatures (T) and at pressures (P) ranging from low to high. The structural complexity of the Northwest Cascades System has led Brown (1987) to characterize it as a regional mélange. Following the lead of Tabor and others (1989), we restrict the Northwest Cascades System in the Sauk River quadrangle to the units northeast of the Darrington-Devils Mountain Fault Zone.

North of the Sauk River quadrangle, the following units of the Northwest Cascades System are exposed in a well-developed regional thrust stratigraphy from structurally highest to lowest (Misch, 1966; Brown and others, 1987; Tabor, 1994; Tabor and others, in press): the Easton Metamorphic Suite, the Bell Pass mélange, the Chilliwack Group of Cairnes (1944) and the Cultus Formation of Daly (1912), and the Nooksack Group and Wells Creek Volcanics of Misch (1966).

In the Sauk River quadrangle, the Nooksack Group, Wells Creek Volcanics, and Cultus Formation are not exposed and only scraps of the Bell Pass mélange crop out. Figure 4 is a simplified map of the rocks in the Northwest Cascades System and the thrust stratigraphy exposed in the Sauk River quadrangle and the Mount Baker quadrangle to the north.

Figure 4. Nappes of the North West Cascade System exposed in the Mount Baker and Sauk River quadrangles

The Paleozoic Chilliwack Group of Cairnes (1944) is composed of marine sedimentary rocks, mafic to intermediate volcanic rocks, and widespread but minor limestone and marble. Locally Chilliwack rocks are strongly foliate, mostly with a low dip. Chilliwack strata probably were deposited in an arc setting in the late Paleozoic (Blackwell, 1983; Brown, 1987). The Easton Metamorphic Suite comprises the Shuksan Greenschist and Darrington Phyllite. The Shuksan Greenschist is a fine-grained but well-recrystallized metamorphic rock, commonly containing sodic amphiboles. Its oceanic basalt protolith formed in the Middle and Late Jurassic and it was metamorphosed in the Early Cretaceous (Brown, 1987). The Shuksan protolith was overlain by oceanic shale and sandstone, protoliths of the Darrington Phyllite. Most of the Darrington is graphitic quartz-albite-sericite phyllite, but locally it is well-recrystallized, fine-grained muscovite schist, commonly with albite porphyroblasts and well-developed lawsonite.

The Bell Pass mélange consists of various tectonic slices of crystalline schist, gneiss, and meta-igneous rocks in a disrupted phyllite and semischist matrix. Much of the disrupted clastic matrix appears to be derived from the Elbow Lake Formation of Brown and others (1987), which north of the quadrangle, characteristically contains chert and Ti-rich metabasalt (Brown and others, 1987; Sevigny and Brown, 1989). In the Sauk River quadrangle, small slices of probable Bell Pass mélange are commonly associated with high-angle faults. These fault slices rarely contain chert and Ti-rich greenstone, but are associated with a variety of metamorphic and meta-igneous rocks. The most prominent slices in the mélange are the Yellow Aster Complex of Misch (1966) composed of tonalitic to granitic gneisses that yield Proterozoic zircon ages (Mattinson, 1972, p. 3775-3776). Other exotic slices are the Vedder Complex of Armstrong and others (1983) composed of quartzose amphibolite and other schists yielding late Paleozoic metamorphic ages.

Misch (1966) considered the deformed rocks of the Bell Pass mélange and the exotic blocks to be a thick imbricate zone beneath the Shuksan Thrust Fault, a regional overthrust which separates the Easton Metamorphic Suite from the Chilliwack Group. Except for the low-angle fault on Suiattle and Prairie Mountains (see below), most contacts between the Chilliwack Group, Easton Metamorphic Suite, and Bell Pass mélange in the Sauk River quadrangle are high-angle faults that some workers have interpreted to be the Shuksan Thrust Fault (see Silverberg, 1985, for further discussion of the faults).

Southwest of the Helena-Haystack mélange and the Darrington-Devils Mountain Fault Zone, the western and eastern mélange belts contain rocks of probable submarine fan and deep oceanic origin that are characterized by extreme disruption on an outcrop scale and low P and T metamorphic mineral assemblages. The western mélange belt is mostly clastic, characterized by commonly thick-bedded, volcanic subquartzose sandstone. A regional antiform exposes a pelitic facies, characterized by pervasive phyllitic cleavage. The eastern mélange belt is locally less penetratively deformed and is characterized by mafic volcanic rocks, chert, and ultramafic rocks. The Trafton terrane appears to be structurally high and folded over the regional antiform expressed in the western mélange belt. We consider the eastern mélange belt and Trafton terrane to be in fault contact with the underlying western mélange belt, although the disruption between the two units along this inferred fault is no more severe than that within the mélange or the Trafton units themselves (Tabor and Booth, 1985). In the western mélange belt, fossil ages are Late Jurassic and Early Cretaceous except for limestone which is Permian and may be olistostromal. Meta-igneous blocks in the eastern mélange belt are Late Triassic in age, whereas fossils range in age from Late Triassic to Late(?) Jurassic. Fossils in the Trafton terrane range from Mississippian to Early Jurassic in age, and a metatonalite block is at least as old as Pennsylvanian (Table 1; Frizzell and others, 1987, p. 135; Whetten and others, 1980b).

Within the Darrington-Devils Mountain Fault Zone, the Helena-Haystack mélange is composed of a variety of lithologies in a serpentinite matrix (Vance and others, 1980). Components of the eastern mélange belt and the Easton Metamorphic Suite are represented in the Helena-Haystack mélange as well as foliated silicic volcanic rocks, hornblende tonalite, and fine-grained schistose amphibolite, rock types not commonly present in the adjoining units. A hornblende tonalite block has a probable crystallization age of 150 Ma (Late Jurassic) as revealed by a U-Pb age of zircon, whereas a metarhyolite block has a metamorphic age of about 90 Ma (Late Cretaceous) as shown by a K-Ar muscovite age and a two point Rb-Sr "isochron". Clearly, rocks of great diversity in age and origin have been mixed together in the mélange in Late Cretaceous and (or) early Tertiary time (Tabor, 1994).

The conditions of low-grade metamorphism differ between rocks north of the Helena-Haystack mélange and coincident Darrington-Devils Mountain Fault Zone and rocks to the south. To the north rocks are characterized by development of minerals representative of high P/T, specifically blue amphibole and lawsonite in the Easton Metamorphic Suite and lawsonite and aragonite in the Chilliwack Group (Brown and others, 1981). South of the Darrington-Devils Mountain Fault Zone, blue amphiboles have not been found in the western and eastern mélange belts, but prehnite and pumpellyite are common, and, although aragonite is present in veins, lawsonite has not been positively identified (Tabor, 1994). Many rocks within the Darrington-Devils Mountain Fault Zone, that is in the Helena-Haystack mélange, were also metamorphosed at high P/T. Brown and others (1987) and (Reller, 1986) considered metavolcanic rocks in the Helena-Haystack mélange underlying Big and Little Deer Peaks to have been metamorphosed at about the same P, but at slightly lower T than rocks in the Easton Metamorphic Suite.

The Helena-Haystack mélange may have formed in Late Cretaceous to middle Eocene time when the western and eastern mélange belts were obducted onto the Northwest Cascades System (Tabor, 1994)

TERTIARY AND QUATERNARY ROCKS AND DEPOSITS

The Mount Pilchuck and Granite Falls stocks and associated small bodies intruded and thermally metamorphosed rocks of the western mélange belt in the early middle Eocene. At the same time the rhyolite of Hanson Lake erupted. The similarity in age and the occurrence of garnet in both intrusive and extrusive rocks suggests that the rhyolite was an eruptive phase of the granite of the Mount Pilchuck stock (Wiebe, 1963 p. 36-39). The nearby Bald Mountain pluton appears to be older than the Mount Pilchuck stock based on its sill-like shape and strongly deformed margins, but it yields discordant U-Th-Pb ages that suggest a middle Eocene age. The Bald Mountain pluton contains garnet and cordierite and shows some of the same S-type characteristics as the Mount Pilchuck stock. The chemical features and the older Pb component may reflect contamination by underlying continental basement, such as the Swakane Biotite Gneiss (Haugerud and others, 1994, p. 2E19-20).

A widespread eruptive event produced the middle and late Eocene Barlow Pass Volcanics of Vance (1957a, b). Within the thick pile of predominantly basalt, basaltic andesite, and rhyolite of the Barlow Pass are interbeds of fluviatile feldspathic sandstone and conglomerate, which in many areas dominate the section.

Along much of the Darrington-Devils Mountain Fault Zone, the Barlow Pass Volcanics overlie the Helena-Haystack mélange. The Tertiary rocks are locally highly faulted, and locally have been incorporated into the mélange. Locally, sheet-like fault slivers of serpentinite from the underlying mélange have been implaced into the sandstone facies of the unit and recrystallized as metaperidotite (Vance and Dungan, 1977). Clasts in Tertiary conglomerates are highly stretched, roughly horizontally, parallel to the faults, suggesting strike-slip movement along the Darrington-Devils Mountain Fault Zone in post-late middle Eocene time. The Barlow Pass Volcanics erupted in a time of regional extension (Ewing, 1980; Heller and others, 1987) and probable strike-slip faulting. Waning strike-slip movement along the Straight Creek Fault may have occurred during this time as well, but large displacement in post-Barlow Pass time is precluded by the presence of the Barlow Pass Volcanics on both sides of the fault south of the Sauk River quadrangle (Tabor and others, 1993).

With establishment of the Cascade arc in the early Oligocene along what is now the north-trending backbone of the Cascade Range, a series of intrusive events followed the eruption of the Barlow Pass Volcanics (Vance and others, 1986). Stocks at Vesper Peak, Squire Creek, and Granite Lakes are the northern outliers of the 34-Ma (early Oligocene) Mount Index batholith exposed south of the quadrangle. Magmas of the Index family (Tabor and others, 1989) appear to have come up along the southwest side of the northwest-trending Darrington-Devils Mountain Fault Zone. The plutons cut the fault zone and are not displaced by it.

In the latest Oligocene (about 25 Ma), small outlying satellitic stocks and plugs (including the Monte Cristo stock and Dead Duck pluton) of the Grotto batholith (a plutonic body of the Snoqualmie family), exposed south of the Sauk River quadrangle, invaded the north-trending Straight Creek Fault Zone. About 22 to 23 Ma, in Miocene time, the Cloudy Pass batholith and satellitic bodies of the Cascade Pass family apparently invaded northeast-trending structures in the higher grade metamorphic rocks east of the Straight Creek Fault. The northeast-trending Cascade Pass dike and Mount Buckindy pluton were emplaced at 18 and 15-16 Ma, respectively.

The Cool Glacier stock, a small tonalite body, intruded at about 4 Ma, and the associated volcanic rocks of Gamma Ridge erupted at around 1.8 Ma. These late Pliocene magmas were precursors to the Quaternary volcanism that built Glacier Peak volcano at the same eruptive center. A major eruption of Glacier Peak 11,250 years ago mantled much of the country east of the Sauk River quadrangle with dacitic pumice. Deposits of other eruptions (Beget, 1981), form extensive terraces and fills in the valleys of the Suiattle, White Chuck, Sauk, and North Fork of the Stillaguamish Rivers. Alluvium enriched in ash presumably derived from Glacier Peak has been identified as far west as Whidbey Island, 40 km west of the quadrangle and over 100 km west of Glacier Peak (D.P. Dethier, written commun., 1985).

PLEISTOCENE GLACIAL DEPOSITS

Glaciations in the Sauk River quadrangle are represented by deposits of both alpine and ice-sheet glaciers. Valley-bottom and valley-wall deposits in the upland trunk drainages (such as Downey Creek, Sulphur Creek, Illabot Creek, and Clear Creek [18]) include till and outwash from alpine glaciers that originated at the drainage headwalls. Most of these deposits probably date from the Evans Creek stade of the Fraser glaciation (Armstrong and others, 1965), about 20,000 yr B.P. Additional deposits have been derived from lesser expansions of these same glaciers in latest Pleistocene and Holocene time.

In the west half of the quadrangle, deposits derived from the Puget lobe of the Cordilleran ice sheet fill many of the lower valleys and mantle the upland surfaces. Virtually all deposits date from the Vashon stade of the Fraser glaciation culminating at about 15,000 yr BP (Booth, 1987). Ice tongues from the Puget lobe probably advanced up each of the major river valleys into areas previously occupied by downvalley-advancing alpine ice. In general, the surface altitude of the ice sheet increased to the north, reflecting the major source area in British Columbia (Booth, 1986). In the northeastern part of the quadrangle, the projected high altitude of the ice-sheet surface indicates that the Cascade glaciers in the headwaters of each drainage probably merged with the Puget lobe, creating a near-continuous ice cover across this part of the Cascade Range. Most of these upvalley deposits lack good exposure or diagnostic non-local clast types, so mappable deposits of the ice sheet are found only west of the Sauk River.